Patent Application
20120028912
The present invention provides the three-dimensional structure of a histone acetyltransferase bromodomain. The interaction between bromodomains and their binding partners play a crucial role in various cellular functions, including in the regulation/modulation of DNA transcription. Therefore, the present invention provides methods for modulating the interaction of bromodomains and their binding partners by such agents as, for example, small molecules.
In recent years great strides have been made in the elucidation of the steps involved in intercellular and intracellular signaling. Indeed, the individual steps of the cascade of events involved in a number of cellular signal transduction processes have been determined. For example, intercellular signal transduction generally begins with an intercellular ligand binding the extracellular portion of a receptor of the plasma membrane. The bound receptor then either directly or indirectly initiates the activation of one or more cellular factors. An activated cellular factor may act as transcription factor by entering the nucleus to interact with its corresponding genomic response element, or alternatively, it may interact with other cellular factors depending on the complexity of the process. In either case, one or more transcription factors ultimately bind to one or more specific genomic response elements. This binding plays a crucial role in the up and/or down regulation of the transcription of the specific genes that are under the control of these genomic response elements. However, the process of re-organizing the chromatin of eukaryotic cells, which is a prerequisite for the binding of the transcription factor to the genomic response elements, has remained a mystery.
Chromatin contains several highly conserved histone proteins including: H3, H4, H2A, H2B, and H1. These histone proteins package eukaryotic DNA into repeating nucleosomal units that are folded into higher-order chromatin fibers (Luger and Richmond, Curr. Opin. Genet. Dev. 8:140-146 (1998)). A portion of the histone that comprises roughly a quarter of the protein protrudes from the chromatin surface, and is thereby sensitive to proteolytic enzymes (van Holde, in Chromatin (Rich, A., ed., Springer, N.Y.) pages 111-148 (1988); Hect et al., Cell 80:583-592 (1995)). This portion of the histone is known as the “histone tail”. Histone tails tend to be free for protein-protein interaction, and are also the portion of the histone most prone to post-translational modification. Such post-translational modification includes acetylation, phosphorylation, methylation, ubiquitination, and ADP-ribosylation (van Holde, in Chromatin (Rich, A,. ed., Springer, N.Y.) pages 111-148 (1988)).
Of all classes of proteins, histones are amongst the most susceptible to post-translational modification. Perhaps the best studied post-translational modification of histones is the acetylation of specific lysine residues (Grunstin, M., Nature, 389:349-352 (1997)). Indeed, acetylation of histone lysine residues has been suggested to play a pivotal role in chromatin remodeling and gene activation. Consistently, distinct classes of enzymes, namely histone acetyltransferases (HATs) and histone deacetylases (HDACs), acetylate or de-acetylate specific histone lysine residues (Struhl, Genes Dev. 12:599-606 (1998)).
Nearly all known nuclear HATs contain an approximately 110 amino acid sequence known as the bromodomain (Jeanmougin et al., Trends in Biochemical Sciences, 22:151-153 (1997)), a protein motif that was initially discovered in Drosophila brahma protein. Bromodomains are found in a large number of chromatin-associated proteins and have now been identified in approximately 42 human proteins, often adjacent to other protein motifs (Jeanmougin et al., Trends in Biochemical Sciences, 22:151-153 (1997); Tamkun et al., Cell, 68:561-572 (1992): Hanes et al., Nucleic Acids Research, 20:2603 (1992); Sanchez, R. & Zhou, M.-M., Current Opinion in Drug Discovery & Development, 12(5):659-665 (2009)). Proteins that contain a bromodomain often contain a second bromodomain. However, despite the wide occurrence of bromodomains and their likely role in chromatin regulation, their three-dimensional structure and binding partners heretofore have remained unknown.
The bromodomain, present in chromatin associated proteins and histone lysine acetyltransferases,6a is an acetyl-lysine binding domain.6b Bromodomain/AcK binding plays an important role in control of chromatin remodeling and gene transcription.6c BRDs adopt the highly conserved structural fold of a left-handed four-helix bundle (αZ, αA, αB and αC), as first shown in the PCAF BRD (Zeng, et al., FEBS Letters (2002) 513, 124-8) (
Therefore, there is a need to identify binding partners for a bromodomain. In addition, there is a need to identify agonists or antagonists to the bromodomain-binding partner complex. Since a preferred method of drug-screening relies on structure based drug design, there is also a need to determine the three-dimensional structure of a bromodomain. In this case, once the three dimensional structure of bromodomain is determined, potential agonists and/or potential antagonists can be designed with the aid of computer modeling (Bugg et al., Scientific American, December: 92-98 (1993); West et al., TIPS, 16:67-74 (1995); Dunbrack et al., Folding & Design, 2:27-42 (1997). However, heretofore the three-dimensional structure of the bromodomain has remained unknown. Therefore, there is a need for obtaining a form of the bromodomain that is amenable for NMR analysis and/or X-ray crystallographic analysis. Furthermore, there is a need for the determination of the three-dimensional structure of the bromodomain. Finally, there is a need for procedures for related structural based drug design predicated on such structural data.
The replication cycle of the human immunodeficiency virus (HIV) presents several viable targets for anti-HIV chemotherapy. The current anti-HIV drugs specifically target the viral reverse transcriptase, protease and integrase (Garg, et al., Chem. Rev. (1999) 99, 3525-601) However, because of the development of viral drug resistance from mutations in the targeted proteins, continuous viral production by chronically infected cells contributes to HIV-mediated immune dysfunction (Ho, et al., Nature. Med. (2000) 6, 757-61; Wei et al., Nature (1995) 373, 117-22) and there is still no cure for AIDS. A rapid growing AIDS epidemic calls for new therapeutic strategies targeting different steps in the viral life cycle. Therapeutic intervention at the stage of HIV gene expression can complement the existing therapy to interfere with virus production. Transcription of the integrated HIV provirus is regulated by the concerted action between cellular transcription factors and a unique viral trans-activator Tat. Tat binds to a viral RNA TAR and recruits cyclin T1 and cyclin-dependent kinase 9 that hyper-phosphorylates and enhances elongation efficiency of the RNA polymerase II (Keen et al., J. EMBO. J. (1997) 16, 5260-72; Karn, J. Mol. Biol. (1999) 293, 235-54; Jones, Genes. Dev. (1997) 11, 2593-99; Kao, et al., Nature (1987) 330, 489-93).
Tat transactivation requires acetylation of its lysine 50 and recruitment of histone lysine acetyltransferase transcriptional coactivators for remodeling nucleosome that contains the integrated proviral DNA (Ott et al., Curr. Biol. (1999) 9, 1489-92). Our recent study shows that Tat coactivator recruitment requires its acetylated lysine 50 (AcK50) binding to the bromodomain (BRD) of the coactivator PCAF (Mujtaba, et al., Mol. Cell. (2002) 9, 575-86), and microinjection of anti-PCAF BRD antibody blocks Tat transactivation (Dorr et al., EMBO. J. (2002) 21, 2715-33). These data suggest that Tat/PCAF recruitment via a BRD-AcK binding is essential for HIV transcription, and this interaction serves as a new therapeutic target for intervening HIV replication.
Small molecule inhibitors that block Tat/PCAF binding by targeting the BRD of PCAF are needed. Targeting a host cell protein essential for viral reproduction, rather than a viral protein, may minimize the problem of drug resistance due to mutations of the viral counterpart as observed with protease inhibitors.
The citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.
The present invention is based in part on the discovery that bromodomains bind to acetyl-lysine residues of proteins. The present invention provides the three-dimensional structure of a bromodomain as well as the three-dimensional structure of a bromodomain-acetyl-histamine complex. The structural information provided can be employed in methods of identifying drugs that can modulate the cellular processes that involve bromodomain-acetyl-lysine interactions. These interactions include chromatin remodeling, which is a required step in eukaryotic transcription. In a particular embodiment, the three-dimensional structural information is used in the identification and/design of an inhibitor of human leukemia. In another embodiment, the three-dimensional structural information is used in the identification and/design of an inhibitor of HIV-1 infection and/or AIDS.
The present invention provides an isolated nucleic acid that encodes a peptide of about 21 to 40 amino acids that contains all or a part of a ZA loop of a bromodomain. In a preferred embodiment the peptide is about 23 to 34 amino acids. The isolated nucleic acid may further contain a heterologous nucleotide sequence.
In a preferred embodiment the peptide contains the amino acid sequence of SEQ ID NO:3. In another embodiment the peptide contains the amino acid sequence of SEQ ID NO:43. In particular embodiments the ZA loop is obtained from the bromodomain having the amino acid sequence of SEQ ID NO:7, or SEQ ID NO:8, or SEQ ID NO:9, or SEQ ID NO:10, or SEQ ID NO:11, or SEQ ID NO: 12, or SEQ ID NO: 13, or SEQ ID NO: 14, or SEQ ID NO:15, or SEQ ID NO:16, or SEQ ID NO:17, or SEQ ID NO:18, or SEQ ID NO:19, or SEQ ID NO:20, or SEQ ID NO:21, or SEQ ID NO: 22, or SEQ ID NO:23, or SEQ ID NO:24, or SEQ ID NO:25, or SEQ ID NO:26, or SEQ ID NO:27, or SEQ ID NO:28, or SEQ ID NO:29, or SEQ ID NO:30, or SEQ ID NO: or SEQ ID NO:31, or SEQ ID NO:32, or SEQ ID NO: 33, or SEQ ID NO:34, or SEQ ID NO:35, or SEQ ID NO:36, or SEQ ID NO:37, or SEQ ID NO:38, or SEQ ID NO: or SEQ ID) NO:39, or SEQ ID NO:40, or SEQ ID NO:41, or SEQ ID NO:42.
The present invention further provides a recombinant DNA molecule that is an isolated nucleic acid of the present invention, as described above, with or without a heterologous nucleotide sequence. Such a recombinant DNA molecule can be operatively linked to an expression control sequence and can be part of an expression vector. The present invention further provides a cell that comprises such an expression vector. The cell can be either a eukaryotic or a prokaryotic cell. The present invention further provides a method of expressing the peptides of the present invention or fragments thereof in this cell. One such method comprises culturing the cell in an appropriate cell culture medium under conditions that provide for expression of the peptide by the cell.
The present invention further provides a peptide of about 21 to 40 amino acids that contains all or a part of a ZA loop of a bromodomain. In a preferred embodiment the peptide is about 23 to 34 amino acids. The present invention also provides fusion proteins or peptides having these peptides.
In a preferred embodiment the peptide contains the amino acid sequence of SEQ ID NO:3. In another embodiment the peptide contains the amino acid sequence of SEQ ID NO:43. In yet another preferred embodiment the peptide comprises the amino acid sequence of SEQ ID NO:48.
In particular embodiments the ZA loop is obtained from the bromodomain having the amino acid sequence of SEQ ID NO:7, or SEQ ID NO:8, or SEQ ID NO:9, or SEQ ID NO: 10, or SEQ ID NO: 11, or SEQ ID NO: 12, or SEQ ID NO: 13, or SEQ ID NO: 14, or SEQ ID NO:15, or SEQ ID NO:16, or SEQ ID NO:17, or SEQ ID NO:18, or SEQ ID NO:19, or SEQ ID NO:20, or SEQ ID NO:21, or SEQ ID NO: 22, or SEQ ID NO:23, or SEQ ID NO:24, or SEQ ID NO:25, or SEQ ID NO:26, or SEQ ID NO:27, or SEQ ID NO:28, or SEQ ID NO:29, or SEQ ID NO:30, or SEQ ID NO: or SEQ ID NO:31, or SEQ ID NO:32, or SEQ ID NO: 33, or SEQ ID NO:34, or SEQ ID NO:35, or SEQ ID NO:36, or SEQ ID NO:37, or SEQ ID NO:38, or SEQ ID NO: or SEQ ID NO:39, or SEQ ID NO:40, or SEQ ID NO:41, or SEQ ID NO:42.
In another aspect, the present invention provides antibodies raised against the peptides/proteins of the present invention, or raised against an antigenic fragment of these proteins/fragments. In a particular embodiment an antibody is raised against a fragment of the ZA loop of a bromodomain. In another embodiment an antibody is raised against a fragment of a protein or peptide that comprises an acetyl-lysine, so that the protein or peptide can bind to a bromodomain. Such fragments can be conjugated to a carrier protein or be part of a fusion protein. In one embodiment the antibody is a polyclonal antibody. In another embodiment, the antibody is a monoclonal antibody. A hybridoma that makes the monoclonal antibody is also part of the present invention. In a particular embodiment the antibody is a chimeric antibody. Antibodies that can specifically recognize acetyl-lysine residues involved bromodomain binding are also part of the present invention.
In another aspect the present invention provides a method for identifying a compound that modulates the affinity of a bromodomain for a ligand (and/or protein) that comprises an acetylated lysine or an analog of an acetylated lysine. In one embodiment the method features contacting the bromodomain and the ligand in the presence of a compound under conditions such that the bromodomain and the ligand bind in the absence of the compound. The affinity of the bromodomain for the ligand may then be determined or measured. A compound is identified as a compound that modulates the affinity of a bromodomain for a ligand when there is a change in the affinity of the bromodomain for the ligand in the presence of the compound. When the affinity of the bromodomain for the ligand increases in the presence of the compound, the compound is identified as a promoting agent for the bromodomain-ligand complex. When the affinity of the bromodomain for the ligand decreases in the presence of the compound, the compound is identified as an inhibitor of the bromodomain-ligand complex. In a preferred embodiment, the compound to be tested is pre-selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-6 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference. More preferably the selecting is performed in conjunction with computer modeling. In a particular embodiment, the compound is selected by performing rational drug design with the set of atomic coordinates obtained from a set of atomic coordinates defining the three-dimensional structure of a bromodomain consisting of the amino acid sequence of SEQ ID NO:7 alone or with acetyl-histamine.
In another aspect, the present invention provides a method of identifying a compound that modulates the stability of a bromodomain-ligand binding complex. Preferably the ligand comprises either an acetyl-lysine or an analog of acetyl-lysine. One such embodiment comprises contacting the bromodomain-ligand binding complex in the presence of the compound under conditions in which the bromodomain-ligand binding complex forms in the absence of the compound. The stability of the bromodomain-ligand binding complex is then determined (e.g., measured). A compound is identified as a compound that modulates the stability of the bromodomain-ligand binding complex when there is a change in the stability of the bromodomain-ligand binding complex in the presence of that compound. When the stability of the bromodomain-ligand binding complex increases in the presence of the compound, the compound is identified as a stabilizing agent. When the stability of the bromodomain-ligand binding complex decreases in the presence of the compound, the compound is identified as an inhibitor. In a preferred embodiment, the compound to be tested is pre-selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-6 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference. More preferably the selecting is performed in conjunction with computer modeling. In a particular embodiment, the compound is selected by performing rational drug design with the set of atomic coordinates obtained from a set of atomic coordinates defining the three-dimensional structure of a bromodomain consisting of the amino acid sequence of SEQ ID NO:7 alone or with acetyl-histamine.
As anyone having skill in the art of drug development readily understands, the potential drugs selected by the above methods can be refined by retesting in appropriate drug assays, including those disclosed herein. Chemical analogs of such potential drugs can be obtained (either through chemical synthesis or drug libraries) and be analogously tested. Therefore, methods comprising successive iterations of the steps of the individual drug assays, as exemplified herein, using either repetitive or different binding studies, or transcription activation studies or other such studies are envisioned in the present invention. In addition, potential drugs may be identified first by rapid throughput drug screening, as described below, prior to performing computer modeling on a potential drug using the three-dimensional structure of the bromodomain.
The present invention further provides potential, selected, and putative compounds (drugs) identified by the methods of the present invention, as well as the final drugs themselves identified using the methods of the present invention.
The present invention further provides methods for identifying potential binding partners for a protein (e.g., a histone) comprising an acetyl-lysine. One such embodiment comprises contacting the protein with a polypeptide comprising a bromodomain. In a preferred embodiment the bromodomain comprises the amino acid sequence of SEQ ID NO:3. In particular embodiments the bromodomain has the amino acid sequence of SEQ ID NO:7, or SEQ ID NO:8, or SEQ ID NO:9, or SEQ ID NO:10, or SEQ ID NO: 11, or SEQ ID NO: 12, or SEQ ID NO: 13, or SEQ ID NO: 14, or SEQ ID NO:15, or SEQ ID NO:16, or SEQ ID NO: 17, or SEQ ID NO:18, or SEQ ID NO:19, or SEQ ID NO:20, or SEQ ID NO:21, or SEQ ID NO: 22, or SEQ ID NO:23, or SEQ ID NO:24, or SEQ ID NO:25, or SEQ ID NO:26, or SEQ ID NO:27, or SEQ ID NO:28, or SEQ ID NO:29, or SEQ ID NO:30, or SEQ ID NO: or SEQ ID NO:31, or SEQ ID NO:32, or SEQ ID NO: 33, or SEQ ID NO:34, or SEQ ID NO:35, or SEQ ID NO:36, or SEQ ID NO:37, or SEQ ID NO:38, or SEQ ID NO: or SEQ ID NO:39, or SEQ ID NO:40, or SEQ ID NO:41, or SEQ ID NO:42.
The present invention further provides methods for identifying a protein having a bromodomain. One such embodiment comprises contacting a cellular extract with a peptide comprising an acetyl-lysine and/or an acetyl-lysine analog.
The present invention further provides agents that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine (AcK). In one embodiment the agent is ISYGR-AcK-KRRQRR (SEQ ID NO:4). In another embodiment the agent is ARKSTGG-AcK-APRKQL (SEQ ID NO:5). In still another embodiment the agent is QSTSRHK-AcK-LMFKTE (SEQ ID NO:6). In yet another embodiment the agent is an analog of acetyl-lysine (see
The present invention further provides compounds such as small molecules that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine. In a related aspect, the present invention provides methods for preventing or inhibiting the binding of bromodomains to acetyl-lysine residues of proteins comprising the step of administering a therapeutically effective amount of a pharmaceutical composition comprising a compound such as a small molecule.
Some useful small molecules of the invention that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine may be represented by the following general formula (I) wherein:
Other useful small molecules of the invention that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine may be represented by general formula (II) wherein:
The general formula (H) includes every stereoisomer, epimer and diastereoisomer, as a mixture or in isolated form. In especially preferred embodiments, R1 R2 and R3 are independently selected from the group consisting of hydrogen, lower alkyl, NH3+, OH, SH, and halogen. Also in especially preferred embodiments, R4 is selected from the group consisting of lower alkyl and aryl.
Other useful small molecules of the invention that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine may be represented by general formula (III) wherein
The general formula (III) includes every stereoisomer, epimer and diastereoisomer, as a mixture or in isolated form.
Other useful small molecules of the invention that can inhibit the binding of a bromodomain with a protein comprising an acetyl-lysine may be one of the following:
The present invention further provides an apparatus that comprises a representation of a bromodomain or a bromodomain-ligand complex (e.g., the Tat-P/CAF complex). One such apparatus is a computer that comprises the representation of the bromodomain or a bromodomain-ligand complex in computer memory. In one embodiment, the computer comprises a machine-readable data storage medium which contains data storage material that is encoded with machine-readable data which comprises the atomic coordinates from a bromodomain or a bromodomain-ligand complex. Preferably the computer comprises a machine-readable data storage medium which contains data storage material that is encoded with machine-readable data which comprises a portion or all of the structural coordinates contained in Tables 1-6 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference. In one embodiment, the computer comprises a machine-readable data storage medium which contains data storage material that is encoded with machine-readable data which comprises the structural coordinates for the Tat-P/CAF complex. More preferably the computer further comprises a working memory for storing instructions for processing the machine-readable data, a central processing unit coupled to both the working memory and to the machine-readable data storage medium for processing the machine readable data into a three-dimensional representation of the Tat-P/CAF complex, for example. In a preferred embodiment, the computer also comprises a display that is coupled to the central-processing unit for displaying the three-dimensional representation.
In addition, the present invention provides methods of identifying compounds that modulate the affinity of P/CAF for Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In one such embodiment the method comprises contacting the bromodomain of P/CAF or a fragment thereof with a binding partner in the presence of the compound under conditions in which the bromodomain of P/CAF and the binding partner bind in the absence of the compound. The affinity of the bromodomain of P/CAF and the binding partner is then determined (e.g., measured). When there is a change in the affinity of the bromodomain of P/CAF for the binding partner in the presence of the compound, the compound is identified as a modulator. In one embodiment of this type the binding partner is Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In a preferred embodiment the binding partner is a fragment of Tat comprising an acetyl-lysine at position 50. In still another embodiment the binding partner is an analog of the fragment of Tat comprising an acetyl-lysine at position 50. When the affinity of the bromodomain of P/CAF for the binding partner increases in the presence of the compound, the compound is identified as a Tat-P/CAF complex promoting agent, whereas when the affinity of the bromodomain of P/CAF for the binding partner decreases in the presence of the compound, the compound is identified as an inhibitor of the Tat-P/CAF complex.
In a preferred embodiment the compound is selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-5 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference. More preferably the selection is performed in conjunction with computer modeling. Compounds selected by these methods are also part of the present invention. Preferably the compound is a small organic molecule. More preferably the compound is an analog of acetyl-lysine. Even more preferably, the compound is not included in
The present invention also provides methods of identifying a compound that modulates the stability of the binding complex formed between P/CAF and Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In one such embodiment the method comprises contacting the bromodomain of P/CAF or a fragment thereof with a binding partner in the presence of the compound under conditions in which the bromodomain of P/CAF and the binding partner bind in the absence of the compound. The stability of the bromodomain of P/CAF and the binding partner is then determined (e.g., measured). When there is a change in the stability of the binding complex between the bromodomain of P/CAF and the binding partner in the presence of the compound, the compound is identified as a modulator. In one embodiment of this type the binding partner is Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In a preferred embodiment the binding partner is a fragment of Tat comprising an acetyl-lysine at position 50. In still another embodiment the binding partner is an analog of the fragment of Tat comprising an acetyl-lysine at position 50. When the stability of the bromodomain of P/CAF for the binding partner increases in the presence of the compound, the compound is identified as a stabilizing agent, whereas when the stability of the bromodomain of P/CAF for the binding partner decreases in the presence of the compound, the compound is identified as an inhibitor of the Tat-P/CAF complex. In a preferred embodiment the compound is selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-5 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference. More preferably the selection is performed in conjunction with computer modeling. Compounds identified by these methods are also part of the present invention. Preferably the compound is an analog of acetyl-lysine. More preferably the compound is a small organic molecule not included in
The present invention also provides agents that can modulate the binding of P/CAF and Tat. In a preferred embodiment the agent is a small organic molecule. Preferably the agent inhibits and/or destabilizes the binding of P/CAF with Tat. The agent may be, for instance, an analog of acetyl-lysine or any of the small molecules described herein.
Another aspect of the present invention provides methods of preventing, retarding the progression and treating HIV infection in an individual. One embodiment features administering to the individual a compound that modulates the Tat-P/CAF complex. In some embodiments the compound administered may be an acetyl-lysine analog or a small molecule. The compound may in some instance either de-stabilize or inhibit the Tat-P/CAF complex.
In another aspect, the present invention features methods for treating viral infection by administering a therapeutically effective amount of a pharmaceutical composition comprising a compound that inhibits interaction of Tat and P/CAF. In some preferred embodiments, the viral infection is HIV infection.
Accordingly, it is a principal object of the present invention to provide the three-dimensional coordinates of a bromodomain. It is a further object of the present invention to provide the three-dimensional coordinates of a bromodomain complexed with acetyl-histamine. It is a further object of the present invention to provide the three-dimensional coordinates of the Tat-P/CAF complex. It is a further object of the present invention to provide an assay for identifying proteins that contain bromodomains that bind proteins that comprise acetyl-lysine. It is a further object of the present invention to provide methods of identifying drugs that can modulate the bromodomain-acetyl-lysine binding complex. It is a further object of the present invention to provide methods of identifying drugs that can inhibit the binding of a bromodomain to a protein containing acetyl-lysine. It is a further object of the present invention to provide methods of identifying drugs that can modulate the Tat-P/CAF binding complex. It is a further object of the present invention to provide methods of identifying drugs that can inhibit the binding/formation of the Tat-P/CAF binding complex. It is a further object of the present invention to provide methods that incorporate the use of rational design for identifying such drugs. It is a further object of the present invention to provide a method of identifying drugs that can treat leukemia. It is a further object of the present invention to provide a method of identifying drugs that can treat, retard the progression, prevent and/or cure AIDS.
These and other aspects of the present invention will be better appreciated by reference to the following drawings and detailed description.
The present invention identifies a general binding partner (ligand) for the protein motif known as the bromodomain. Indeed, by combining structural and site-directed mutagenesis studies the present invention demonstrates that bromodomains can interact specifically with acetyl-lysine (AcK), making them the first protein modules known to exhibit such interactions. Like other modular domains, such as Src homology-2 (SH2) and phosphotyrosine binding (PTB) domains, which specifically interact with phosphotyrosine-containing proteins, the bromodomain/acetyl-lysine recognition provides a means to regulate protein-protein interactions via protein lysine acetylation. The nature of the acetyl-lysine recognition by the bromodomain is similar to that of histone acetyltransferase interaction with acetyl-CoA. The present invention therefore couples for the first time, the functionality of the bromodomain with the HAT activity of coactivators in the regulation of gene transcription.
The present invention further provides both a nuclear magnetic resonance (NMR) structure of the bromodomain from the HAT coactivator P/CAF (p300/CBP-associated factor) as well as the structure for the P/CAF bromodomain in complex with acetyl-histamine. The structure reveals an unusual left-handed up-and-down four-helix bundle.
The results disclosed herein explain prior deletion experiments which showed that the bromodomain is indispensable for the function of GCN5 in yeast. Bromodomain-AcK binding also appears to be important for the assembly and activity of multiprotein complexes in transcriptional activation. The results reported herein therefore form the foundation for identifying specific biological ligands and for defining the molecular mechanisms by which the extensive family of bromodomains participate in chromatin remodeling and transcriptional activation
As disclosed herein, the binding partner for the bromodomain is a peptide or protein comprising an acetyl-lysine (AcK). Interestingly, whereas a free acetyl-lysine does not appear to bind the bromodomain, an analog of the acetyl-lysine, acetyl-histamine, does. This is most likely due to the additional charge present in the free amino acid. Consistently, free acetyl-histidine also does not to bind the bromodomain.
In addition, as disclosed herein, the gene transactivation of HIV-1 Tat protein requires lysine-acetylation at amino acid residue 50 of Tat (see SEQ ID NO:45) by the transcription co-activator p300/CBP and the subsequent formation of a binding complex between the Tat having the acetylated lysine with P/CAF. The binding complex between P/CAF and Tat is mediated via the bromodomain of P/CAF and the acetylated lysine of Tat. Indeed, this binding is required for the gene transactivation activity of Tat and thus, for HV-1 expression and replication.
The present invention further provides a key region of the bromodomain for the interaction with its acetyl-lysine binding partner, the ZA loop. The amino acid sequence of the ZA loop is defined in
more preferably the ZA loop has about 23 to 34 amino acid residues and comprises the amino acid sequence:
In a specific embodiment, the ZA loop has between about 20 and 64 amino acid residues comprising the amino acid sequence:
(1) The single letter amino acid code is used in this description, i.e., “F” for phenylalanine; “P” for proline; “Y” for tyrosine; and “D” for aspartic acid.
(2) “X” indicates any amino acid (an undesignated amino acid); and X, X2, X2-3, X5, and X5-8 indicates one undesignated amino acid, two consecutive undesignated amino acids, two or three consecutive undesignated amino acids, five consecutive undesignated amino acids, and five to eight consecutive undesignated amino acids respectively.
(3) “J” indicates that identity of the amino acid is restricted to a particular group, again the one letter code is used
(i) JP/K/H is either proline, lysine or histidine.
(ii) JY/F/H is either tyrosine, phenylalanine or histidine.
(iii) JM/I/V is either methionine, isoleucine, or valine.
(iv) JI/L/M/Vis either isoleucine, leucine, methionine, or valine
Since this region of the bromodomain is important in binding its acetyl-lysine binding partner, antibodies specifically raised against this region are also included in the present invention. In a particular embodiment, the antibody is a humanized chimeric antibody that can be used in therapeutic treatment. Thus monoclonal, chimeric, and polyclonal antibodies raised against bromodomains, preferably against amino acid residues in the ZA loop region are part of the present invention. In a specific embodiment the antibody is raised against a peptide, fusion peptide or conjugated peptide consisting of amino acid residues 746 to 765 of SEQ ID NO:2, i.e., WPFMEPVKRTEAPGYYEVIR (SEQ ID NO:44). In another embodiment the antibody is raised against a peptide, fusion peptide or conjugated peptide consisting of amino acid residues 748 to 809 of SEQ ID NO:2 (which is SEQ ID NO:49).
Such antibodies can be used in the treatment of leukemia or AIDs for example. Alternatively, these antibodies can be used in drug discovery assays.
Analogously, the present invention provides peptides derived from the HIV-1 Tat protein. In one such embodiment the peptide comprises 7 to 21 amino acid residues comprising the amino acid sequence
In a specific embodiment the peptide fragment of Tat has ten amino acid residues and the amino acid sequence:
Preferably the lysine corresponding to lysine 50 of Tat (see SEQ ID NO:45) is acetylated. These peptide fragments can be used in the drug assays of the present invention and/or as antigens for antibodies that specifically interfere with the interaction (e.g., binding) of Tat with P/CAF interaction.
The present invention provides the first detailed structural information regarding a bromodomain and a bromodomain complexed with its acetylated binding partner. The present invention therefore provides the three-dimensional structure of the bromodomain and a bromodomain acetylated binding partner complex. Since the interaction of the bromodomain with a histone for example, can play a significant role in chromatin remodeling/regulation, the structural information provided herein can be employed in methods of identifying drugs that can modulate basic cell processes by modulating the transcription. In a particular embodiment, the three-dimensional structural information is used in the design of a small organic molecule for the treatment of cancer or as disclosed below, HV-1 infection and/or AIDs. In addition, the present invention provides a critical structural feature for a class of inhibitors (acetyl-lysine analogs) of the interaction between bromodomains and their protein binding partners which contain an acetylated-lysine (e.g., Tat with P/CAF), see
Indeed, the bromodomain and lysine-acetylated protein interaction can now be implicated to play a causal role in the development of a number of diseases including cancers such as leukemia. For example, chromatin remodeling plays a central role in the etiology of viral infection and cancer (Archer, et al., Curr. Opin. Genet. Biol. 9:171-174 (1999); Jacobson, et al., Curr. Opin. Genet. Biol. 9:175-184 (1999)). Both altered histone acetylation/deacetylation and aberrant forms of chromatin-remodeling complexes are associated with human diseases. Furthermore, chromosomal translocation of various cellular genes with those encoding HATs and subunits of chromatin remodeling complexes have been implicated in leukomogenesis. The MOZ (monocytic leukemia zinc finger) and MLL/ALL-1 genes are frequently fused to the gene encoding the co-activator HAT CBP (Sobulo et al., Proc. Natl. Acad. Sci. USA 94:8732-8737(1997)). The resulting fusion protein MLL-CBP contains the tandem bromodomain-PHD finger-HAT domain of CBP. It also has been shown that both the bromodomain and HAT domain of CBP are required for leukomogenesis, because deletion of either the bromodomain or the HAT domain results in loss of the MLL-CBP fusion protein's ability for cell transform. These results indicate that the CBP bromodomain, and more particularly, the ZA loop of the CBP bromodomain, is an excellent target for developing drugs that interfere with the bromodomain acetyl-lysine interaction that can be used in the treatment of human acute leukemia. In addition, an antibody (e.g., a humanized antibody) raised specifically against a peptide from the ZA loop of the CBP bromodomain could also be effective for treating these conditions.
In addition, it is now known that the human immunodeficiency virus type 1 (HIV-1) trans-activator protein, Tat, is tightly regulated by lysine acetylation (Kiernan et al., EMBO Journal 18:6106-6118 (1999)). HIV-1 Tat transcriptional activity is absolutely required for productive HIV viral replication (Jeang, et al., Curr. Top. Microbiol. Immunol., 188:123-144(1994)). Therefore, the interaction of the acetyl-lysine of Tat with one or more bromodomain-containing proteins associated with chromatin remodeling could mediate gene transcription. More particularly, it is disclosed herein that acetylated lysine50 of Tat specifically binds to the bromodomain of P/CAF. Therefore, this particular bromodomain/lysine-acetylated Tat interaction serves as a drug target for blocking HIV replication in cells. As indicated above, an antibody raised specifically against a peptide from the ZA loop of the P/CALF bromodomain could also be effective for treating and/or preventing HIV infections including those that lead to AIDs.
In addition, based on the new structural information disclosed herein, the key amino acid residues for the binding of a given bromodomain and its binding partner can be identified and further elucidated using basic mutagenesis and standard isothermal titration calorimetry, for example. Indeed, both the critical amino acids for the bromodomain and the binding partner (i.e., apart from the acetyl-lysine) can be readily determined and are also part of the present invention.
Therefore, the results obtained from the structural and functional studies disclosed herein provide the foundation for both high throughput drug screening and structure-based rational drug design. The agents identified by this procedure are useful for ameliorating conditions involving chromatin remodeling/regulation, and/or in the treatment of cancer and/or AIDS, as indicated above.
Structure based rational drug design is the most efficient method of drug development. However, heretofore, no information has been disclosed regarding the structure of the bromodomain or more importantly, its interaction with the acetyl-lysine of its binding partner. Obtaining detailed structural information requires an extensive NMR or X-ray crystallographic analysis. By determining and then exploiting the detailed structural information of the bromodomain and of the bromodomain/acetyl-histamine (exemplified by NMR analysis below) the present invention provides novel methods for developing new drugs through structure based rational drug design.
Thus the present invention provides representative sets of the atomic structure coordinates of the free form of the P/CAF bromodomain (Table 5 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference), of the P/CAF bromodomain-acetyl-histamine complex (Table 6 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference) and of the Tat-P/CAF complex (Table 10 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference) which were all obtained by NMR analysis. A Ribbon diagram of the three-dimensional structure of the P/CAF bromodomain is depicted in
In addition, Tables 1-6 and/or 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference, are also capable of being placed into a computer readable form which is also part of the present invention. Furthermore, methods of using these coordinates and chemical shifts and related information (including in computer readable forms) either individually or together in drug assays are also provided. More particularly, such atomic coordinates can be used to identify potential ligands or drugs which will modulate the binding of a bromodomain with its binding partner.
In a particular aspect of the present invention, the lysine-acetylated Tat is shown herein to specifically bind to the bromodomain of the p300/CBP-associated factor (P/CAF) in vitro and in vivo. Structural and mutational analyses provides the identification of key amino acid residues on both the bromodomain and Tat that are important for the binding complex. The identification of these important amino acid residues further demonstrates the biological importance of this interaction for Tat transactivation activity. Together, the findings disclosed herein indicate a novel mechanism by which the lysine-acetylated Tat recruits P/CAF via a bromodomain interaction, leading to chromatin remodeling-mediated transcriptional activation of HIV-1. Furthermore, the extreme specificity of the Tat-P/CAF binding (see e.g.,
Therefore, the three-dimensional structural information provided by the present invention allows the identification and/or design of specific compounds that can act as modulators of crucial processes. In the case of the Tat-P/CAF interaction, such compounds can be used as drugs to inhibit HIV-1 expression in a cell and/or subsequent infection of other cells. Therefore, the inhibitors identified and/or designed by the methods disclosed can be used to prevent, treat, retard the progression, and potentially cure HIV-1 infections and AIDS.
Therefore, if appearing herein, the following terms shall have the definitions set out below.
As used herein a “bromodomain-acetyl-lysine binding complex” is a binding complex between a bromodomain or fragment thereof and either a peptide/polypeptide comprising an acetyl-lysine (or an analog of acetyl-lysine), or a free analog of acetyl-lysine, such as acetyl-histamine disclosed in the Example below. Preferably, the peptide comprises at least six amino acids in addition to the acetyl-lysine. A fragment of a bromodomain preferably comprises a ZA loop as defined below. The dissociation constant of a bromodomain-acetyl-lysine binding complex is dependent on whether the lysine residue or analog thereof is acetylated or not, such that the affinity for the bromodomain and the peptide comprising the lysine residue (for example) significantly decreases when that lysine residue is not acetylated. One example of a bromodomain-acetyl-lysine binding complex is that formed between P/CAF with Tat (the “Tat-P/CAF complex”) as exemplified below.
As used herein the term “acetyl-lysine analog” is used interchangeably with the term “analog of acetyl-lysine” and is a compound that contains the acetyl-amine-like structure as depicted in
As used herein a “ZA loop” of a bromodomain is a key portion of a bromodomain that is involved in the binding of the bromodomain to the acetyl-lysine. The structure of the actual ZA loop of the bromodomain of P/CAF is depicted in
A “polypeptide” or “peptide” comprising a fragment of a bromodomain, such as the ZA loop, or a peptide or polypeptide comprising an acetyl-lysine, as used herein can be the “fragment” alone, or a larger chimeric or fusion peptide/protein which contains the “fragment”.
As used herein the terms “fusion protein” and “fusion peptide” are used interchangeably and encompass “chimeric proteins and/or chimeric peptides” and fusion “intein proteins/peptides”. A fusion protein comprises at least a portion of a protein or peptide of the present invention, e.g., a bromodomain, joined via a peptide bond to at least a portion of another protein or peptide including e.g., a second bromodomain in a chimeric fusion protein. In a particular embodiment the portion of the bromodomain is antigenic. Fusion proteins can comprise a marker protein or peptide, or a protein or peptide that aids in the isolation and/or purification of the protein, for example.
As used herein, and unless otherwise specified, the terms “agent”, “potential drug”, “compound”, “test compound” or “potential compound” are used interchangeably, and refer to chemicals which potentially have a use as an inhibitor or activator/stabilizer of bromodomain-acetyl-lysine binding. Therefore, such “agents”, “potential drugs”, “compounds” and “potential compounds” may be used, as described herein, in drug assays and drug screens and the like.
As used herein a “small organic molecule” is an organic compound, including a peptide (or organic compound complexed with an inorganic compound (e.g., metal)) that has a molecular weight of less than 3 Kilodaltons. Such small organic molecules can be included as agents, etc. as defined above.
As used herein the term “binds to” is meant to include all such specific interactions that result in two or more molecules showing a preference for one another relative to some third molecule. This includes processes such as covalent, ionic, hydrophobic and hydrogen bonding but does not include non-specific associations such as solvent preferences.
As used herein the term “about” signifies that a value is within twenty percent of the indicated value i.e., a peptide containing “about” 20 amino acid residues can contain between 16 and 24 amino acid residues.
General Techniques for Constructing Nucleic Acids That Encode the Bromodomains and Fragments Thereof (Including, ZA Loops), and the Bromodomain Binding Partners of the Present Invention.
In accordance with the present invention there may be employed conventional molecular biology, microbiology, and recombinant DNA techniques within the skill of the art. Such techniques are explained fully in the literature. See, e.g., Sambrook, et al. Molecular Cloning: A Laboratory Manual, Third Edition (2001) Vols. I-E, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (herein “Sambrook and Russell, 2001”), Sambrook, et al., Molecular Cloning: A Laboratory Manual, Second Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (herein “Sambrook et al., 1989”); DNA Cloning: A Practical Approach, Volumes I and II (D. N. Glover ed. 1985); Oligonucleotide Synthesis (M. J. Gait ed. 1984); Nucleic Acid Hybridization (B. D. Hames & S. J. Higgins eds. (1985)); Transcription And Translation (B. D. Hames & S. J. Higgins, eds. (1984)); Animal Cell Culture (R. I. Freshney, ed. (1986)); Immobilized Cells And Enzymes (IRE Press, (1986)); Perbal, A Practical Guide To Molecular Cloning (1984); Ausubel et al. (eds.), Current Protocols in Molecular Biology, John Wiley & Sons, Inc. (1994).
As used herein, the term “gene” refers to an assembly of nucleotides that encode a polypeptide, and includes cDNA and genomic DNA nucleic acids.
A “vector” is a replicon, such as plasmid, phage or cosmid, to which another DNA segment may be attached so as to bring about the replication of the attached segment. A “replicon” is any genetic element (e.g., plasmid, chromosome, virus) that functions as an autonomous unit of DNA replication in vivo, i.e., capable of replication under its own control.
A “cassette” refers to a segment of DNA that can be inserted into a vector at specific restriction sites. The segment of DNA encodes a polypeptide of interest, and the cassette and restriction sites are designed to ensure insertion of the cassette in the proper reading frame for transcription and translation.
A cell has been “transfected” by exogenous or heterologous DNA when such DNA has been introduced inside the cell.
A “nucleic acid molecule” refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine or cytidine; “RNA molecules”) or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; “DNA molecules”), or any phosphoester analogues thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix. Double stranded DNA-DNA, DNA-RNA and RNA-RNA helices are possible. The term nucleic acid molecule, and in particular DNA or RNA molecule, refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear or circular DNA molecules (e.g., restriction fragments), plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences may be described herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the nontranscribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA). A “recombinant DNA molecule” is a DNA molecule that has undergone a molecular biological manipulation.
A nucleic acid molecule is “hybridizable” to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength (see Sambrook et al., 1989 supra, Sambrook and Russell, 2001). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. For preliminary screening for homologous nucleic acids, low stringency hybridization conditions, corresponding to a Tm of 55°, can be used, e.g., 5.times.SSC, 0.1% SDS, 0.25% milk, and no formamide; or 30% formamide, 5×SSC, 0.5% SDS). Moderate stringency hybridization conditions correspond to a higher Tm, e.g., 40% formamide, with 5.times.or 6.times.SCC. High stringency hybridization conditions correspond to the highest Tm, e.g., 50% formamide, 5× or 6×SCC. Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see Sambrook et al., 1989 supra, 9.50-10.51, Sambrook and Russell, 2001). For hybridization with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see Sambrook et al., 1989 supra, 11.7-11.8, Sambrook and Russell, 2001). Preferably a minimum length for a hybridizable nucleic acid is at least about 12 nucleotides; preferably at least about 18 nucleotides; and more preferably the length is at least about 27 nucleotides; and most preferably 36 nucleotides.
In a specific embodiment, the term “standard hybridization conditions” refers to a Tm of 55° C., and utilizes conditions as set forth above. In a preferred embodiment, the Tm is 60° C.; in a more preferred embodiment, the Tm is 65° C.
A DNA “coding sequence” is a double-stranded DNA sequence which is transcribed and translated into a polypeptide in a cell in vitro or in vivo when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding sequence can include, but is not limited to, prokaryotic sequences and synthetic DNA sequences. If the coding sequence is intended for expression in a eukaryotic cell, a polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding sequence.
Transcriptional and translational control sequences are DNA regulatory sequences, such as promoters, enhancers, terminators, and the like, that provide for the expression of a coding sequence in a host cell. In eukaryotic cells, polyadenylation signals are control sequences.
A “promoter sequence” is a DNA regulatory region capable of binding RNA polymerase in a cell and initiating transcription of a downstream (3′ direction) coding sequence. For purposes of defining the present invention, the promoter sequence is bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter sequence will be found a transcription initiation site (conveniently defined for example, by mapping with nuclease S1), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase.
A coding sequence is “under the control” of transcriptional and translational control sequences in a cell when RNA polymerase transcribes the coding sequence into mRNA, which is then trans-RNA spliced and translated into the protein encoded by the coding sequence.
A DNA sequence is “operatively linked” to an expression control sequence when the expression control sequence controls and regulates the transcription and translation of that DNA sequence. The term “operatively linked” includes having an appropriate start signal (e.g., ATG) in front of the DNA sequence to be expressed and maintaining the correct reading frame to permit expression of the DNA sequence under the control of the expression control sequence and production of the desired product encoded by the DNA sequence. If a gene that one desires to insert into a recombinant DNA molecule does not contain an appropriate start signal, such a start signal can be inserted in front of the gene.
As used herein, the term “homologous” refers to the relationship between proteins that possess a “common evolutionary origin,” including proteins from superfamilies (e.g., the immunoglobulin superfamily) and homologous proteins from different species (e.g., myosin light chain, etc.) (Reeck et al., Cell, 50:667 (1987)). Such proteins have sequence homology as reflected by their high degree of sequence similarity.
Accordingly, the term “sequence similarity” in all its grammatical forms refers to the degree of identity or correspondence between nucleic acid or amino acid sequences of proteins that may or may not share a common evolutionary origin (see Reeck et al., supra). However, in common usage and in the instant application, the term “homologous,” when modified with an adverb such as “highly,” may refer to sequence similarity and not a common evolutionary origin.
Two DNA sequences are “substantially homologous” when at least about 60% (preferably at least about 80%, and most preferably at least about 90 or 95%) of the nucleotides match over the defined length of the DNA sequences. Sequences that are substantially homologous can be identified by comparing the sequences using standard software available in sequence data banks, or in a Southern hybridization experiment under, for example, stringent conditions as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art (See, e.g., Sambrook et al., 1989 supra; DNA Cloning, Vols. I & II, supra; Nucleic Acid Hybridization, supra., and Sambrook and Russell, 2001)
As used herein an amino acid sequence is 100% “homologous” to a second amino acid sequence if the two amino acid sequences are identical, and/or differ only by neutral or conservative substitutions as defined below. Accordingly, an amino acid sequence is 50% “homologous” to a second amino acid sequence if 50% of the two amino acid sequences are identical, and/or differ only by neutral or conservative substitutions.
As used herein, DNA and protein sequence percent identity can be determined using MacVector 6.0.1, Oxford Molecular Group PLC (1996) and the Clustal W algorithm with the alignment default parameters, and default parameters for identity. These commercially available programs can also be used to determine sequence similarity using the same or analogous default parameters.
The term “corresponding to” is used herein to refer similar or homologous sequences, whether the exact position is identical or different from the molecule to which the similarity or homology is measured. Thus, the term “corresponding to” refers to the sequence similarity, and not the numbering of the amino acid residues or nucleotide bases.
As used herein a “heterologous nucleotide sequence” is a nucleotide sequence that is added to a nucleotide sequence of the present invention by recombinant methods to form a nucleic acid which is not naturally formed in nature. Such nucleic acids can encode fusion proteins or peptides, including chimeric proteins and peptides. Thus the heterologous nucleotide sequence can encode peptides and/or proteins which contain regulatory and/or structural properties. In another such embodiment the heterologous nucleotide can encode a protein or peptide that functions as a means of detecting the protein or peptide encoded by the nucleotide sequence of the present invention after the recombinant nucleic acid is expressed. In still another such embodiment the heterologous nucleotide can function as a means of detecting a nucleotide sequence of the present invention. A heterologous nucleotide sequence can comprise non-coding sequences including restriction sites, regulatory sites, promoters and the like.
The present invention also relates to cloning vectors containing nucleic acids encoding analogs and derivatives of the bromodomains of the present invention and polypeptides/peptides that can bind a bromodomain when a lysine of the polypeptide/peptide is acetylated, including modified fragments, that have the same or homologous functional activity as the individual fragments, and homologs thereof. The production and use of derivatives and analogs related to the fragments are within the scope of the present invention.
Due to the degeneracy of nucleotide coding sequences, other DNA sequences which encode substantially the same amino acid sequence as a nucleic acid encoding a protein comprising bromodomain or bromodomain binding partner (i.e., when post-transcriptionally acetylated) of the present invention for example, may be used in the practice of the present invention. These include but are not limited to allelic genes, homologous genes from other species, which are altered by the substitution of different codons that encode the same amino acid residue within the sequence, thus producing a silent change. Likewise, the peptides and polypeptides of the present invention include, but are not limited to, those containing, as a primary amino acid sequence, analogous portions of their respective amino acid sequences including altered sequences in which functionally equivalent amino acid residues are substituted for residues within the sequence resulting in a conservative amino acid substitution. For example, one or more amino acid residues within the sequence can be substituted by another amino acid of a similar polarity, which acts as a functional equivalent, resulting in a silent alteration. Substitutes for an amino acid within the sequence may be selected from other members of the class to which the amino acid belongs. For example, the nonpolar (hydrophobic) amino acids include alanine, leucine, isoleucine, valine, proline, phenylalanine, tryptophan and methionine. Amino acids containing aromatic ring structures are phenylalanine, tryptophan, and tyrosine. The polar neutral amino acids include glycine, serine, threonine, cysteine, tyrosine, asparagine, and glutamine. The positively charged (basic) amino acids include arginine, and lysine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid.
Particularly preferred conserved amino acid exchanges are:
(a) Lys for Arg or vice versa such that a positive charge may be maintained;
(b) Glu for Asp or vice versa such that a negative charge may be maintained;
(c) Ser for Thr or vice versa such that a free —OH can be maintained;
(d) Gln for Asn or vice versa such that a free NH2 can be maintained;
(e) Ile for Leu or for Val or vice versa as roughly equivalent hydrophobic amino acids; and
(f) Phe for Tyr or vice versa as roughly equivalent aromatic amino acids.
A conservative change generally leads to less change in the structure and function of the resulting protein. A non-conservative change is more likely to alter the structure, activity or function of the resulting protein. The present invention should be considered to include sequences containing conservative changes which do not significantly alter the activity or binding characteristics of the resulting protein. Specific amino acid residues for the P/CAF bromodomain have been identified that are important for binding, indicating a potential lower stringency for the substitution of the remaining amino acids residues.
All of the peptides/fragments of the present invention can be modified by being placed in a fusion or chimeric peptide or protein, or labeled e.g., to have an N-terminal FLAG-tag, or H6 tag. In a particular embodiment the P/CAF bromodomain fragment can be modified to contain a marker protein such as green fluorescent protein as described in U.S. Pat. No. 5,625,048 filed Apr. 29, 1997 and WO 97/26333, published Jul. 24, 1997 each of which are hereby incorporated by reference herein in their entireties.
The nucleic acids encoding peptides and protein fragments of the present invention and analogs thereof can be produced by various methods known in the art. The manipulations which result in their production can occur at the gene or protein level (Sambrook et al., 1989, supra; Sambrook and Russell, 2001, supra). The nucleotide sequence can be cleaved at appropriate sites with restriction endonuclease(s), followed by further enzymatic modification if desired, isolated, and ligated in vitro. In addition a nucleic acid sequence can be mutated in vitro or in vivo, to create and/or destroy translation, initiation, and/or termination sequences, or to create variations in coding regions and/or form new restriction endonuclease sites or destroy preexisting ones, to facilitate further in vitro modification. Any technique for mutagenesis known in the art can be used, including but not limited to, in vitro site-directed mutagenesis (Hutchinson et al., J. Biol. Chem., 253:6551 (1978); Zoller, et al, DNA, 3:479-488 (1984); Oliphant et al., Gene, 44:177 (1986); Hutchinson et al., Proc. Natl. Acad. Sci. U.S.A., 83:710 (1986)), use of TABφ linkers (Pharmacia), etc. PCR techniques are preferred for site directed mutagenesis (see Higuchi, “Using PCR to Engineer DNA”, in PCR Technology: Principles and Applications for DNA Amplification, H. Erlich, ed., Stockton Press, Chapter 6, pp. 61-70 (1989)).
The identified and isolated nucleic acids can then be inserted into an appropriate cloning vector. A large number of vector-host systems known in the art may be used.
A bacterial protein expression system can be used to make various stable isotopically labeled (13C, 15N, and 2H) protein samples that are useful for a three-dimensional NMR structural determination of a protein complex. For example a pET14b (Novagen) bacterial expression vector can be constructed which expresses the recombinant P/CAF bromodomain as an amino-terminal His-tagged fusion protein.
Protein expression and purification can be conducted using standard procedures for His-tagged proteins (Zhou et al., J. Biol. Chem. 270:31119-31123 (1995)). To optimize the level of protein expression, various bacterial growth and expression conditions can be screened, which include different E. Coli cell lines, and growth and protein induction temperatures. Generally, it is preferred to obtain the maximum amount of soluble protein while still inducing protein expression with a relatively low IPTG concentration e.g., .about.0.2 mM (final concentration) at 16° C. As exemplified below, the bromodomain of P/CAF (residues 719-832 of SEQ ID NO:2 which is SEQ ID NO:7) was subcloned into the pET14b expression vector (Novagen) and expressed in Escherichia coli BL21(DE3) cells. Uniformly 15N- and 15N/13C-labeled proteins were prepared by growing bacteria in a minimal medium containing 15NH4Cl with or without 13C6-glucose. A uniformly 15N/13C-labeled and fractionally deuterated protein sample was prepared by growing the cells in 75% 2H2O. The bromodomain was purified by affinity chromatography on a nickel-IDA column (Invitrogen) followed by the removal of poly-His tag by thrombin cleavage. The final purification of the protein was achieved by size-exclusion chromatography. The acetyl-lysine-containing peptides were prepared on a MilliGen 9050 peptide synthesizer (Perkin Elmer) using Fmoc/HBTU chemistry. Acetyl-lysine was incorporated using the reagent Fmoc-Ac-Lys with HBTU/DIPEA activation. NMR samples contained approximately 1 mM protein in 100 mM phosphate buffer of pH 6.5 and 5 mM perdeuterated DTT and 0.5 mM EDTA in H2O/2H2O (9/1) or 2H2O.
One major advantage of using the heteronuclear multidimensional approach, as exemplied herein, is that the NMR resonance assignments of a protein are obtained in a sequence-specific manner which assures accuracy and greatly facilitates data analysis and structure determination (Clore, et al., Meth. Enzymol. 239:249-363 (1994)). In addition, the signal overlapping problems in the protein spectra are minimized by the use of multidimensional NMR spectra, which separates the proton signals according to the chemical shifts of their attached hetero-nuclei (such as 15N and 13C). This NMR approach has been proven very powerful for structural analysis of large proteins (Clore, et al., Meth. Enzymol. 239:249-363 (1994)). To facilitate sequence-specific resonance assignments for the structural study, a uniformly 13C, 15N-labeled and fractionally (75%) deuterated protein sample of the bromodomain can be prepared by growing bacterial cells in 75% 2H2O as exemplified below. Such protein samples can be used for triple-resonance NMR experiments. A triple-labeled protein sample is useful for high-resolution NMR structural studies. Because of the favorable 1H, 13C, and 15N relaxation rates caused by the partial deuteration of the protein, constant-time triple-resonance NMR spectra can be acquired with higher digital resolution and sensitivity (Sattler, et al., Structure 4:1245-1249 (1996)). In addition, various stable-isotopically labeled (15N and 13C/15N) proteins can also be prepared using this procedure.
The term “polypeptide” is used in its broadest sense to refer to a compound of two or more subunit amino acids, amino acid analogs, or peptidomimetics. The subunits are linked by peptide bonds. The terms “polypeptide”, “protein”, and “peptide” are used interchangeably herein, though preferably as used herein a “peptide” refers to a compound of at least two but less than fifty subunit amino acids, and a polypeptide or protein refers to compound of fifty or more amino acids. The polypeptides of the present invention may be chemically synthesized or as detailed above, genetically engineered or isolated from natural sources.
In addition, potential drugs or agents that may be tested in the drug screening assays of the present invention may also be chemically synthesized. When the peptide is to be modified, e.g., acetylated, the modification can be at any time during the peptide synthesis, including using an acetyl-lysine as a starting material or acetylating a lysine residue of a peptide after the peptide has been synthesized. In the Example below, the acetyl-lysine-containing peptides were prepared on a MilliGen 9050 peptide synthesizer (Perkin Elmer) using Fmoc/HITU chemistry. Acetyl-lysine was incorporated using the reagent Fmoc-Ac-Lys with BBTUIDIPEA activation.
Thus, synthetic polypeptides, prepared using the well known techniques of solid phase, liquid phase, or peptide condensation techniques, or any combination thereof, can include natural and unnatural amino acids. Amino acids used for peptide synthesis may be standard Boc (Nalpha.-amino protected Nalpha.-t-butyloxycarbonyl)amino acid resin with the standard deprotecting, neutralization, coupling and wash protocols of the original solid phase procedure of Merrifield (J. Am. Chem. Soc., 85:2149-2154 (1963)), or the base-labile Nalpha.-amino protected 9-fluorenylmethoxycarbonyl (Fmoc) amino acids first described by Carpino and Han (J. Org. Chem., 37:3403-3409 (1972)). Both Fmoc and Boc Nalpha.-amino protected amino acids can be obtained from Fluka, Bachem, Advanced Chemtech, Sigma, Cambridge Research Biochemical, Bachem, or Peninsula Labs or other chemical companies familiar to those who practice this art. In addition, the method of the invention can be used with other Nalpha-protecting groups that are familiar to those skilled in this art. Solid phase peptide synthesis may be accomplished by techniques familiar to those in the art and provided, for example, in Stewart and Young (Solid Phase Synthesis, Second Edition, Pierce Chemical Co., Rockford, Ill. (1984)) and Fields and Noble (Int. J. Pept. Protein Res., 35:161-214 (1990)), or using automated synthesizers, such as sold by ABS. Thus, polypeptides of the invention may comprise D-amino acids, a combination of D- and L-amino acids, and various “designer” amino acids (e.g., β-methyl amino acids, Ca-methyl amino acids, and Nα-methyl amino acids, etc.) to convey special properties. Alternative synthetic amino acids that can be used include ornithine for lysine, fluorophenylalanine for phenylalanine, and norleucine for leucine or isoleucine. Other synthetic amino acids include 2-aminoadipic acid, beta-alanine, beta-aminopropionic acid, 2-aminobutyric acid, 4-aminobutyric acid, piperidinic acid, 6-aminocaproic acid, 2-aminoheptanoic acid, 2-aminoisobutyric acid, 3-aminoisobutyric acid, 2-aminopimelic acid, 2,4 diaminobutyric acid, desmosine, 2,2′-diaminopimelic acid, 2,3-diaminopropionic acid, N-ethylglycine, N-ethylasparagine, hydroxylysine, allo-hydroxylysine, 3-hydroxyproline, 4-hydroxyproline, isodesmosine, allo-isoleucine, N-methylglycine, sarcosine, N-methylisoleucine, 6-N-methyllysine, and N-methylvaline. Additionally, by assigning specific amino acids at specific coupling steps, α-helices, β turns, β sheets, γ-turns, and cyclic peptides can be generated.
In a further embodiment, subunits of peptides that confer useful chemical and structural properties will be chosen. For example, peptides comprising D-amino acids will be resistant to L-amino acid-specific proteases in vivo. In addition, the present invention envisions preparing peptides that have more well defined structural properties, and the use of peptidomimetics, and peptidomimetic bonds, such as ester bonds, to prepare peptides with novel properties. In another embodiment, a peptide may be generated that incorporates a reduced peptide bond, i.e., R1—CH2—NH—R2, where R1 and R2 are amino acid residues or sequences. A reduced peptide bond may be introduced as a dipeptide subunit. Such a molecule would be resistant to peptide bond hydrolysis, e.g., protease activity. Such peptides would provide ligands with unique function and activity, such as extended half-lives in vivo due to resistance to metabolic breakdown, or protease activity. Furthermore, it is well known that in certain systems constrained peptides show enhanced functional activity (Hruby, Life Sciences, 31:189-199 (1982); Hruby et al., Biochem J., 268:249-262 (1990)); the present invention provides a method to produce a constrained peptide that incorporates random sequences at all other positions.
Constrained and cyclic peptides. A constrained, cyclic or rigidized peptide may be prepared synthetically, provided that in at least two positions in the sequence of the peptide an amino acid or amino acid analog is inserted that provides a chemical functional group capable of crosslinking to constrain, cyclise or rigidize the peptide after treatment to form the crosslink. Cyclization will be favored when a turn-inducing amino acid is incorporated. Examples of amino acids capable of crosslinking a peptide are cysteine to form disulfides, aspartic acid to form a lactone or a lactam, and a chelator such as γ-carboxyl-glutamic acid (Gla) (Bachem) to chelate a transition metal and form a cross-link. Protected γ-carboxyl glutamic acid may be prepared by modifying the synthesis described by Zee-Cheng and Olson (Biophys. Biochem. Res. Commun., 94:1128-1132 (1980)). A peptide in which the peptide sequence comprises at least two amino acids capable of crosslinking may be treated, e.g., by oxidation of cysteine residues to form a disulfide or addition of a metal ion to form a chelate, so as to crosslink the peptide and form a constrained, cyclic or rigidized peptide.
The present invention provides strategies to systematically prepare cross-links. For example, if four cysteine residues are incorporated in the peptide sequence, different protecting groups may be used (Hiskey, in The Peptides: Analysis, Synthesis, Biology, Vol. 3, Gross, et al., eds., Academic Press: New York, pp. 137-167 (1981); Ponsanti et al., Tetrahedron, 46:8255-8266 (1990)). The first pair of cysteines may be deprotected and oxidized, then the second set may be deprotected and oxidized. In this way a defined set of disulfide cross-links may be formed. Alternatively, a pair of cysteines and a pair of chelating amino acid analogs may be incorporated so that the cross-links are of a different chemical nature.
Non-classical amino acids that induce conformational constraints. The following non-classical amino acids may be incorporated in the peptide in order to introduce particular conformational motifs: 1,2,3,4-tetrahydroisoquinoline-3-carboxylate (Kazmierski et al., J. Am. Chem. Soc., 113:2275-2283 (1991)); (2S,3S)-methyl-phenylalanine, (2S,3R)-methyl-phenylalanine, (2R,3S)-methyl-phenylalanine and (2R,3R)-methyl-phenylalanine (Kazinierski, et al., Tetrahedron Lett. (1991)); 2-aminotetrahydronaphthalene-2-carboxylic acid (Landis, Ph.D. Thesis, University of Arizona (1989)); hydroxy-1,2,3,4-tetrahydroisoquino-line-3-carboxylate (Miyake et al., J. Takeda Res. Labs., 43:53-76 (1989)); β-carboline (D and L) (Kazmierski, Ph.D. Thesis, University of Arizona (1988)); HIC (histidine isoquinoline carboxylic acid) (Zechel et al., Int. J. Pep. Protein Res., 43 (1991)); and HIC (histidine cyclic urea) (Dharanipragada).
The following amino acid analogs and peptidomimetics may be incorporated into a peptide to induce or favor specific secondary structures: LL-Acp (LL-3-amino-2-propenidone-6-carboxylic acid), a β-turn inducing dipeptide analog (Kemp et al., J. Org. Chem., 50:5834-5838 (1985)); β-sheet inducing analogs (Kemp et al., Tetrahedron Lett., 29:5081-5082 (1988); β-turn inducing analogs (Kemp et al., Tetrahedron Lett., 29:5057-5060 (1988)); .varies.-helix inducing analogs (Kemp et al., Tetrahedron Lett., 29:4935-4938 (1988)); γ-turn inducing analogs (Kemp et al., J. Org. Chem., 54:109:115 (1989)); and analogs provided by the following references: Nagai, et al., Tetrahedron Len., 26:647-650 (1985); DiMaio et al., J. Chem. Soc. Perkin Trans., p. 1687 (1989); also a Gly-Ala turn analog (Kahn et al., Tetrahedron Lett., 30:2317 (1989)); amide bond isostere (Jones et al., Tetrahedron Lett., 29:3853-3856 (1988)); tretrazol (Zabrocki et al., J. Am. Chem. Soc., 110:5875-5880 (1988)); DTC (Samanen et al., Int. J. Protein Pep. Res., 35:501:509 (1990)); and analogs taught in Olson et al., J. Am. Chem. Sci., 112:323-333 (1990) and Garvey et al., J. Org. Chem., 56:436 (1990). Conformationally restricted mimetics of beta turns and beta bulges, and peptides containing them, are described in U.S. Pat. No. 5,440,013, issued Aug. 8, 1995 to Kahn.
Protein structural analysis using NMR spectroscopy has several unique advantages. In addition to high-resolution three-dimensional structural information, the chemical shift assignments for the protein obtained in the structural study further provides a map of the entire protein at the atomic level, which can be used for structure-based biochemical analysis of protein-protein interactions. For example, the information generated from the NMR structural analysis can also serve to identify specific amino acid residues in the peptide-binding site for complementary mutagenesis studies. Specific focus can be placed on those residues that display long-range NOEs (particularly the side-chain NOEs in the 13C-NOESY data) between the bromoomain and a peptide comprising an acetyl-lysine.
To ensure mutant proteins are valid for functional analysis, it can be determined as to whether a mutation results in any significant perturbation of the overall conformation of the bromodomain, particularly the effects of mutation on the acetyl-lysine binding sites. NMR spectroscopy is a powerful method for examining the effects of such a mutation on the conformation of the protein. One can readily obtain information about the global conformation of a mutant protein from the proton (1H) ID spectrum, by examining the chemical shift dispersion and peak line-width of NMR signals of amide, aromatic and aliphatic protons. Moreover, 2D 1H-15N HSQC spectra reveal details of the effects of a mutation on both local and global conformation of the protein, since every single 1H/15N signal (both the chemical shift and line-shape) in the NMR spectrum is a “reporter” for a particular amino acid residue. Thus, to assess how mutations affect protein stability and the overall protein conformation, the 15N HSQC spectra of mutated proteins can be compared to that of the wild-type protein bromodomain.
Chemical-shift perturbations due to ligand binding have proven to be a reliable and sensitive probe for the ligand binding site of the protein. This is because the chemical-shift changes of the backbone amide groups are likely to reflect any changes in protein conformation and/or hydrogen bonding due to the peptide/ligand binding. To examine the effects of a mutation on the ligand binding (in this case the ligand is a peptide comprising an acetyl-lysine), peptide titration experiments can be conducted by following the changes of 1H/15N signals of the mutant proteins as a function of the peptide concentration. These experiments indicate whether the acetyl-lysine binding site remains the same or changes in the mutants relative to the wild type protein. The effects of the mutation on the peptide binding affinity can also be examined by NMR spectroscopy. If the mutated proteins result in the reduction of the binding affinity, a change of the exchange phenomenon between the free and the ligand-bound signals should be observed in NMR spectrum. If the reduction in binding affinity causes the peptide binding to change from a slow exchange rate to a fast exchange rate, on the NMR time scale, then the peptide binding affinity can be determined from the NMR titration experiment. From these mutation analyses key amino acid residues that are important for binding a peptide comprising the acetyl-lysine can be identified. Such analysis has been exemplified below.
The NMR results from the present invention are summarized by the atomic structure coordinates of the free form of the P/CAF bromodomain (Table 5 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference), of the P/CAF bromodomain-acetyl-histamine complex (Table 6 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference), and the Tat-P/CAF complex (Table 10 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference). The NMR chemical shift assignments of the P/CAF bromodomain are included in the chemical shift table (Table 1 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference) for the 1H-15N HSQC spectrum of P/CAF bromodomain. The unambiguous NOE-derived Inter-proton Distance Restraints for the P/CAF bromodomain are in Table 2 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference, the ambiguous NOE-derived Inter-proton Distance Restraints are in Table 3, and the 1H bonding restraints are disclosed in Table 4 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference. The NMR chemical shift assignments of the Tat-P/CAF complex are included in the chemical shift table (Table 11 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference) for the 1H-15N HSQC spectrum of Tat-P/CAF complex The unambiguous NOE-derived Inter-proton Distance Restraints for the Tat-P/CAF complex are in Table 13, the ambiguous NOE-derived Inter-proton Distance Restraints are in Table 14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference, and the 1H bonding restraints are disclosed in Table 12 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference.
Backbone and Side-chain Assignments: Sequence-specific backbone assignment can be achieved by using a suite of deuterium-decoupled triple-resonance 3D NMR experiments which include UNCA, HN(CO)CA, HN(CA)CB, HN(COCA)CB, HNCO, and HN(CA)CO experiments (Yamazaki, et al., J. Am. Chem. Soc. 116:11655-11666 (1994)). The water flip-back scheme is used in these NMR pulse programs to minimize amide signal attenuation from water exchange. Sequential side-chain assignments are typically accomplished from a series of 3D NMR experiments with alternative approaches to confirm the assignments. These experiments include 3D 15N TOCSY-HSQC, HCCH-TOCSY, (H)C(CO)NH-TOCSY, and H(C)(CO)NH-TOCSY (see Clore, et al., Enzymol. 239:249-363 (1994); Sattler et al., Prog. in Nuclear Magnetic Resonance Spec. 4:93-158 (1999)).
Stereospecific Methyl Groups: Stereospecific assignments of methyl groups of Valine and Leucine residues can be obtained from an analysis of carbon signal multiplet splitting using a fractionally 13C-labeled protein sample, which can be readily prepared using M9 minimal medium containing 10% 13C-/90% 12C-glucose mixture (see Neri, et al., Biochemistry 28:7510-7516 (1989)).
Dihedral Angle Restraints: Backbone dihedral angle (.PHI.) constraints can be generated from the.3JHNHα coupling constants measured in a HNHA-J experiment (see Vuister, et al., J. Am. Chem. Soc. 115:7772-7777 (1993)). Side-chain dihedral angles (chi.1) can be obtained from short mixing time 15N-edited 3D TOCSY-HSQC (see Clore, et al., J. Biomol. NMR 1:13-22 (1991)) and 3D HNH experiments (see Matson et al., J. Biomol. NMR 3:239-244 (1993)), which can also provide stereospecific assignments of β methylene protons.
Hydrogen Bonds Restraints: Amide protons that are involved in hydrogen bonds can be identified from an analysis of amide exchange rates measured from a series of 2D1H/15N HSQC spectra recorded after adding 2H2o to the protein sample.
NOE Distance Restraints: Distance restraints are obtained from analysis of 15N, and 13C-edited 3D NOESY data, which can be collected with different mixing times to 30 minimize spin diffusion problems. The nuclear Overhauser effect (NOE)-derived restraints are categorized as strong (1.8-3 .ANG.), medium (1.8-4 A) or weak (1.8-5. ANG.) based on the observed NOE intensities. A recently developed procedure for the iterative automated NOE analysis by using ARIA (see Nilges et al., Prog. NMR Spectroscopy 32:107-139 (1998)) can be employed which integrates with X-PLOR (Brunger, X-PLOR Version 3.1: A system for X-Ray crystallography and NMR, Yale University Press, New Haven, Conn., (1993)) for structural calculations. To ensure the success of ARIA/X-PLOR-assisted NOE analysis and structure calculations, the ARIA assigned NOE peaks can be manually confirmed.
Intermolecular NOE Distance Restrains: For the structural determination of a protein/peptide complex, intermolecular NOE distance restraints can be obtained from a 13C-edited (F,) and 15N, and 13C-filtered (F3) 3D NOESY data set collected for a sample containing isotope-labeled protein and non-labeled peptide.
Structure Calculations and Refinements: Structures of the protein can be generated using a distance geometry/simulated annealing protocol with the X-PLOR program (see Nilges, et al., FEBS Lett. 229:317-324 (1988); Kuszewski, et al., J. Biolmol. NMR 2:33-56 (1992); Brtinger, A. T. X-PLOR Version 3.1: A system for X-Ray crystallography and NMR (Yale University Press, New Haven, Conn., 1993)). The structure calculations can employ inter-proton distance restraints obtained from 15N- and 13C-resolved NOESY spectra. The initial low-resolution structures can be used to facilitate NOE assignments, and help identify hydrogen bonding partners for slowly exchanging amide protons. The experimental restraints of dihedral angles and hydrogen bonds can be included in the distance restraints for structure refinements.
Once the three-dimensional structure of the bromodomain and the bromodomain-acetyl-lysine binding complex are determined, a potential drug or agent (antagonist or agonist) can be examined through the use of computer modeling using a docking program such as GRAM, DOCK, or AUTODOCK (Dunbrack et al., 1997, supra). This procedure can include computer fitting of potential agents to the bromodomain, for example, to ascertain how well the shape and the chemical structure of the potential ligand will complement or interfere with the interaction between the bromodomain and the acetyl-lysine (Bugg et al., Scientific American, December: 92-98 (1993); West et al., TIPS, 16:67-74 (1995)). Computer programs can also be employed to estimate the attraction, repulsion, and steric hindrance of the agent to the dimer-dimer binding site, for example. Generally the tighter the fit (e.g., the lower the steric hindrance, and/or the greater the attractive force) the more potent the potential drug will be since these properties are consistent with a tighter binding constant. Furthermore, the more specificity in the design of a potential drug the more likely that the drug will not interfere with related proteins. This will minimize potential side-effects due to unwanted interactions with other proteins.
Initially a potential drug could be obtained by screening a random peptide library produced by recombinant bacteriophage for example, (Scott, et al., Science, 249:386-390 (1990); Cwirla et al., Proc. Natl. Acad. Sci., 87:6378-6382 (1990); Devlin et al., Science, 249:404-406 (1990)) or a chemical library. In particular, based on the NMR structural analysis provided herein, compounds that comprise an “acetyl-amine-like” structure as depicted in
An agent selected in this manner could be then be systematically modified (if necessary) by computer modeling programs until one or more promising potential drugs are identified. Such analysis has been shown to be effective in the development of HIV protease inhibitors (Lam et al., Science 263:380-384 (1994); Wlodawer et al., Ann. Rev. Biochem. 62:543-585 (1993); Appelt, Perspectives in Drug Discovery and Design 1:23-48 (1993); Erickson, Perspectives in Drug Discovery and Design 1:109-128 (1993)).
Such computer modeling allows the selection of a finite number of rational chemical modifications, as opposed to the countless number of essentially random chemical modifications that could be made, any one of which might lead to a useful drug. Each chemical modification requires additional chemical steps, which while being reasonable for the synthesis of a finite number of compounds, quickly becomes overwhelming if all possible modifications needed to be synthesized. Thus, through the use of the three-dimensional structural analysis disclosed herein and computer modeling, a large number of these compounds can be rapidly screened on the computer monitor screen, and a few likely candidates can be determined without the laborious synthesis of untold numbers of compounds.
Once a potential drug (agonist or antagonist) is identified it can be either selected from a library of chemicals as are commercially available from most large chemical companies including Merck, GlaxoWelcome, Bristol Meyers Squib, Monsanto/Searle, Eli Lilly, Novartis and Pharmacia UpJohn, or alternatively the potential drug may be synthesized de novo. As mentioned above, the de novo synthesis of one or even a relatively small group of specific compounds is reasonable in the art of drug design.
The potential drug can then be tested in any standard binding assay (including in high throughput binding assays) for its ability to bind to the ZA loop of a bromodomain. Alternatively the potential drug can be tested for its ability to modulate the binding of a bromodomain to acetylated histamine, for example. When a suitable potential drug is identified, a second NMR structural analysis can optionally be performed on the binding complex formed between the bromodomain-acetyl-lysine binding complex, or the bromodomain alone and the potential drug. Computer programs that can be used to aid in solving such three-dimensional structures include QUANTA, CHARMM, INSIGHT, SYBYL, MACROMODE, and ICM, MOLMOL, RASMOL, AND GRASP (Kraulis, Appl Crystallogr. 24:946-950 (1991)). Most if not all of these programs and others as well can be also obtained from the WorldWideWeb through the internet.
Using the approach described herein and equipped with the structural analysis disclosed herein, the three-dimensional structures of other bromodomain-acetyl-lysine binding complexes can more readily be obtained and analyzed. Such analysis will, in turn, allow corresponding drug screening methodology to be performed using the three-dimensional structures of such related complexes.
For all of the drug screening assays described herein further refinements to the structure of the drug will generally be necessary and can be made by the successive iterations of any and/or all of the steps provided by the particular drug screening assay, including further structural analysis by NMR, for example.
Phage libraries for Drug Screening: Phage libraries have been constructed which when infected into host E. coli produce random peptide sequences of approximately 10 to 15 amino acids (Parmnley, et al., Gene 73:305-318 (1988), Scott, et al., Science 249:386-249 (1990)). Specifically, the phage library can be mixed in low dilutions with permissive E. coli in low melting point LB agar which is then poured on top of LB agar plates. After incubating the plates at 37° C. for a period of time, small clear plaques in a lawn of E. coli will form which represents active phage growth and lysis of the E. coli. A representative of these phages can be absorbed to nylon filters by placing dry filters onto the agar plates. The filters can be marked for orientation, removed, and placed in washing solutions to block any remaining absorbent sites. The filters can then be placed in a solution containing, for example, a radioactive bromodomain. After a specified incubation period, the filters can be thoroughly washed and developed for autoradiography. Plaques containing the phage that bind to the radioactive bromodomain can then be identified. These phages can be further cloned and then retested for their ability to bind to the bromodomain as before. Once the phage has been purified, the binding sequence contained within the phage can be determined by standard DNA sequencing techniques. Once the DNA sequence is known, synthetic peptides can be generated which are encoded by these sequences. These peptides can be tested, for example, for their ability to modulate the affinity of the bromodomain for its binding partner (e.g., Tat or a fragment of Tat containing the acetyl-lysine corresponding to position 50 of SEQ ID NO:45).
The effective peptide(s) can be synthesized in large quantities for use in in vivo models and eventually in humans to treat certain tumors. It should be emphasized that synthetic peptide production is relatively non-labor intensive, easily manufactured, quality controlled and thus, large quantities of the desired product can be produced quite cheaply. Similar combinations of mass produced synthetic peptides have been used with great success (Patarroyo, Vaccine, 10:175-178 (1990)).
The drug screening assays of the present invention may use any of a number of means for determining the interaction between an agent/drug (e.g., an acetyl-lysine analog) and a peptide comprising an acetyl-lysine and/or a bromodomain. Thus, standard high throughput drug screening procedures can be employed using a library of low molecular weight compounds, for example that can be screened to identify a binding partner for the bromodoamin. Any such chemical library can be used including those discussed above.
In a particular assay, a bromodomain (e.g., from P/CAF) is placed on or coated onto a solid support. Methods for placing the peptides or proteins on the solid support are well known in the art and include such things as linking biotin to the protein and linking avidin to the solid support. An agent is allowed to equilibrate with the bromodomain to test for binding. Generally, the solid support is washed and agents that are retained are selected as potential drugs. Alternatively, a peptide comprising an acetyl-lysine is placed on or coated onto a solid support. In a particular embodiment of this type, the peptide comprises the amino acid sequence of SEQ ID NO:4. In a preferred embodiment, the peptide comprises the amino acid sequence of SEQ ID NO:46.
The agent may be labeled. For example, in one embodiment radiolabeled agents are used to measure the binding of the agent. In another embodiment the agents have fluorescent markers. In yet another embodiment, a Biocore chip (Pharmacia) coated with the bromodomain is used, for example and the change in surface conductivity can be measured.
In addition, since a number of proteins have been identified that contain bromodomains, and the binding partners of many of these proteins are known, the fact that the bromodomain specifically binds to an acetylated lysine as disclosed herein allows the identification and preparation of a number of potential modulators of the bromodomain-acetyl-lysine binding complex based on the amino acid sequences of the binding partners to the proteins. Such potential modulators include: ISYGR-AcK-KRRQRR (SEQ ID NO:4), ARKSTGG-AcK-APRKQL (SEQ ID NO:5) and QSTSRHK-AcK-LMFKTE (SEQ ID NO:6) which bind to the P/CAF bromodomain as shown in the Example, below. Such peptides also can be used, for example, as a starting point for the design of an inhibitor of the bromodomain-acetyl-lysine binding complex.
Alternatively, a drug can be specifically designed to bind to the ZA loop of a bromodomain for example, such as the P/CAF bromodomain, and be assayed through NMR based methodology (Shuker et al., Science 274:1531-1534 (1996) hereby incorporated by reference in its entirety.) In a particular embodiment, analogs of the binding partner of the bromodomain can be used in this analysis. One such peptide has the amino acid sequence of SEQ D NO:4. In another embodiment of this type, the peptide has the amino acid sequence of SEQ ID NO:5. In another such embodiment of this type, the peptide has the amino acid sequence of SEQ ID NO:6.
The assay begins with contacting a compound with a 15N-labeled bromodomain. Binding of the compound with the ZA loop of the bromodomain can be determined by monitoring the 15N- or 1H-amide chemical shift changes in two dimensional 15N-heteronuclear single-quantum correlation (15N-HSQC) spectra upon the addition of the compound to the 15N-labeled bromodomain. Since these spectra can be rapidly obtained, it is feasible to screen a large number of compounds (Shuker et al., Science 274:1531-1534 (1996)). A compound is identified as a potential ligand if it binds to the ZA loop of the bromodomain. In a further embodiment, the potential ligand can then be used as a model structure, and analogs to the compound can be obtained (e.g., from the vast chemical libraries commercially available, or alternatively through de novo synthesis). The analogs are then screened for their ability to bind the ZA loop of the bromodomain thus to obtain a ligand. An analog of the potential ligand is chosen as a ligand when it binds to the ZA loop of the bromodomain with a higher binding affinity than the potential ligand. In a preferred embodiment of this type the analogs are screened by monitoring the 15N- or 1H-amide chemical shift changes in two dimensional 15N-heteronuclear single-quantum correlation (15N-HSQC) spectra upon the addition of the analog to the 15N-labeled bromodomain as described above.
In another further embodiment, compounds are screened for binding to two nearby sites on the bromodomain. In this case, a compound that binds a first site of the bromodomain does not bind a second nearby site. Binding to the second site can be determined by monitoring changes in a different set of amide chemical shifts in either the original screen or a second screen conducted in the presence of a ligand (or potential ligand) for the first site. From an analysis of the chemical shift changes the approximate location of a potential ligand for the second site is identified. Optimization of the second ligand for binding to the site is then carried out by screening structurally related compounds (e.g., analogs as described above). When ligands for the first site and the second site are identified, their location and orientation in the ternary complex can be determined experimentally either by NMR spectroscopy or X-ray crystallography. On the basis of this structural information, a linked compound is synthesized in which the ligand for the first site and the ligand for the second site are linked. In a preferred embodiment of this type the two ligands are covalently linked. This linked compound is tested to determine if it has a higher binding affinity for the bromodomain than either of the two individual ligands. A linked compound is selected as a ligand when it has a higher binding affinity for the bromodomain than either of the two ligands. In a preferred embodiment the affinity of the linked compound with the bromodomain is determined monitoring the 15N- or 1H-amide chemical shift changes in two dimensional .sup.15N-heteronuclear single-quantum correlation (15N-HSQC) spectra upon the addition of the linked compound to the 15N-labeled bromodomain as described above.
A larger linked compound can be constructed in an analogous manner, e.g., linking three ligands which bind to three nearby sites on the bromodomain to form a multilinked compound that has an even higher affinity for the bromodomain than the linked compound.
By disclosing that protein bound acetyl-lysine is a binding partner for bromodomains, the present invention provides a method of identifying novel proteins that contain bromodomains. In short, a protein fragment or analog thereof comprising an acetyl-lysine or an acetyl-lysine analog can be used as bait to identify a binding partner that comprises a bromodomain. Any one of a number of procedures can be carried out to identify such a binding partner. One such assay comprises passing a cell extract over the bait peptide which is attached to a solid support. After washing the solid support to remove any non-specific binders, the bromodomain containing protein can be eluted from the solid support with an appropriate eluant. In a particular embodiment, the free bait peptide can be used in the elution. Other methodology includes the use of a yeast two-hybrid system, a GST pull down assay, ELISA, immunometric assays, and a modification of the CORT procedure of Schlessinger et al., (U.S. Pat. No. 5,858,686, Issued on Jan. 12, 1999 which is hereby incorporated by reference in its entirety) for use with the bromodomain-acetyl-lysine binding complex.
Suitable labels include enzymes, fluorophores (e.g., fluorescein isothiocyanate (FITC), phycoerythrin (PE), Texas red (TR), rhodamine, free or chelated lanthamide series salts, especially Eu.3+, to name a few fluorophores), chromophores, radioisotopes, chelating agents, dyes, colloidal gold, latex particles, ligands (e.g., biotin), and chemiluminescent agents. When a control marker is employed, the same or different labels may be used for the test and control marker gene.
In the instance where a radioactive label, such as the isotopes 3H, 14C, 32P, 35S, 36Cl, 51Cr, 57Co, 58Co, 59Fe, 90Y, 125I, 131I, and 186Re are used, known currently available counting procedures may be utilized. In the instance where the label is an enzyme, detection may be accomplished by any of the presently utilized colorimetric, spectrophotometric, fluorospectrophotometric, amperometric or gasometric techniques known in the art.
Direct labels are one example of labels which can be used according to the present invention. A direct label has been defined as an entity, which in its natural state, is readily visible, either to the naked eye, or with the aid of an optical filter and/or applied stimulation, e.g. U.V. light to promote fluorescence. Among examples of colored labels, which can be used according to the present invention, include metallic sol particles, for example, gold sol particles such as those described by Leuvering (U.S. Pat. No. 4,313,734); dye sole particles such as described by Gribnau et al. (U.S. Pat. No. 4,373,932 and May et al. (WO 88/08534); dyed latex such as described by May, supra, Snyder (EP-A 0 280 559 and 0 281 327); or dyes encapsulated in liposomes as described by Campbell et al. (U.S. Pat. No. 4,703,017). Other direct labels include a radionucleotide, a fluorescent moiety or a luminescent moiety. In addition to these direct labeling devices, indirect labels comprising enzymes can also be used according to the present invention. Various types of enzyme linked immunoassays are well known in the art, for example, alkaline phosphatase and horseradish peroxidase, lysozyme, glucose-6-phosphate dehydrogenase, lactate dehydrogenase, urease, these and others have been discussed in detail by Eva Engvall in Enzyme Immunoassay ELISA and EMIT in Methods in Enzymology, 70:419-439 (1980) and in U.S. Pat. No. 4,857,453.
Suitable enzymes include, but are not limited to, alkaline phosphatase, β-galactosidase, green fluorescent protein and its derivatives, luciferase, and horseradish peroxidase.
Other labels for use in the invention include magnetic beads or magnetic resonance imaging labels.
In addition, the present invention provides a computer that contains a representation of a bromodomain (or a bromodomain-ligand complex, e.g., the Tat-P/CAF complex) in computer memory that can be used to screen for compounds that will or are likely to inhibit the bromodomain-ligand interaction. In a particular embodiment of the present invention the bromodomain-ligand complex is the Tat-P/CAF complex and the compound identified by the screen can used to prevent, retard the progression, treat and/or cure AIDS.
In a related embodiment, the computer can be used in the design of altered bromodomains that have either enhanced, or alternatively diminished binding activity activity. Preferably, the computer comprises portions of and/or all of the information contained in Tables 1-6 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference. In a particular embodiment, the computer comprises: (i) a machine-readable data storage material encoded with machine-readable data, (ii) a working memory for storing instructions for processing the machine readable data, (iii) a central processing unit coupled to the working memory and the machine-readable data storage material for processing the machine-readable data into a three-dimensional representation, and (iv) a display coupled to the central processing unit for displaying the three-dimensional representation.
Thus the machine-readable data storage medium comprises a data storage material encoded with machine readable data which can comprise portions and/or all of the structural information contained in Tables 1-6 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference. One embodiment for manipulating and displaying the structural data provided by the present invention is schematically depicted in
Input hardware 12, coupled to the computer 2 by input lines 10, may be implemented in a variety of ways. Machine-readable data may be inputted via the use of one or more modems 14 connected by a telephone line or dedicated data line 16. Alternatively or additionally, the input hardware 12 may comprise CD-ROM or disk drives 5. In conjunction with the display terminal 6, the keyboard 7 may also be used as an input device. Output hardware 22, coupled to computer 2 by output lines 20, may similarly be implemented by conventional devices. Output hardware 22 may include a display terminal 6 for displaying the three dimensional data. Output hardware might also include a printer 24, so that a hard copy output may be produced, or a disk drive 5, to store system output for later use, see also U.S. Pat. No. 5,978,740, Issued Nov. 2, 1999, the disclosure of which is hereby incorporated by reference in its entirety.
In operation, the CPU 3 (i) coordinates the use of the various input and output devices 12 and 22; (ii) coordinates data accesses from mass storage 5 and accesses to and from working memory 4; and (iii) determines the sequence of data processing steps. Any of a number of programs may be used to process the machine-readable data of this invention.
Antibodies to Portions of the Bromodomain that Interact with Acetyl-Lysine
According to the present invention, the bromodomains, and more particularly the ZA loops of the bromodomains and fragments thereof can be produced by a recombinant source, or through chemical synthesis, or through the modification of these peptides and fragments; and derivatives or analogs thereof, including fusion proteins, may be used as an immunogen to generate antibodies that specifically interfere with the formation of the bromodomain-acetyl-lysine binding complex. Similarly, antibodies can be raised against peptides that comprise one or more acetyl-lysine residues which also interfere with the formation of the bromodomain-acetyl-lysine binding complex. Such antibodies include but are not limited to polyclonal, monoclonal, chimeric, single chain, Fab fragments, and a Fab expression library.
Various procedures known in the art may be used for the production of the polyclonal antibodies. For the production of antibody, various host animals can be immunized by injection with the peptide having the amino acid sequence of SEQ ID NO:3, for example, or a derivative (e.g., or fusion protein) thereof, including but not limited to rabbits, mice, rats, sheep, goats, etc. In one embodiment, the peptide can be conjugated to an immunogenic carrier, e.g., bovine serum albumin (BSA) or keyhole limpet hemocyanin (KLH). Various adjuvants may be used to increase the immunological response, depending on the host species, including but not limited to Freund's (complete and incomplete), mineral gels such as aluminum hydroxide, surface active substances such as lysolecithin, pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpet hemocyanins, dinitrophenol, and potentially useful human adjuvants such as BCG (bacille Calmette-Guerin) and Corynebacterium parvum.
For preparation of monoclonal antibodies directed toward the peptides or protein fragments of the present invention, or analog, or derivative thereof, any technique that provides for the production of antibody molecules by continuous cell lines in culture may be used. These include but are not limited to the hybridoma technique originally developed by Kohler, et al. (Nature, 256:495-497 (1975)), as well as the trioma technique, the human B-cell hybridoma technique (Kozbor et al., Immunology Today, 4:72 (1983); Cote et al., Proc. Natl. Acad. Sci. U.S.A., 80:2026-2030 (1983)), and the EBV-hybridoma technique to produce human monoclonal antibodies (Cole et al., in Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc., pp. 77-96 (1985)). In an additional embodiment of the invention, monoclonal antibodies can be produced in germ-free animals utilizing technology described in PCT/US90/02545. In fact, according to the invention, techniques developed for the production of “chimeric antibodies” (Morrison et al., J. Bacteriol., 159:870 (1984); Neuberger et al., Nature, 312:604-608 (1984); Takeda et al., Nature, 314:452-454 (1985)) by splicing the genes from a mouse antibody molecule specific for the peptide having the amino acid sequence of SEQ ID NO:3, for example, together with genes from a human antibody molecule of appropriate biological activity can be used; such antibodies are within the scope of this invention. Such human or humanized chimeric antibodies are preferred for use in therapy of human diseases or disorders (described infra), since the human or humanized antibodies are much less likely than xenogenic antibodies to induce an immune response, in particular an allergic response, themselves.
According to the invention, techniques described for the production of single chain antibodies (Huston, U.S. Pat. Nos. 5,476,786 and 5,132,405; U.S. Pat. No. 4,946,778) can be adapted to produce specific single chain antibodies. An additional embodiment of the invention utilizes the techniques described for the construction of Fab expression libraries (Huse et al., Science, 246:1275-1281(1989)) to allow rapid and easy identification of monoclonal Fab fragments with the desired specificity.
Antibody fragments which contain the idiotype of the antibody molecule can be generated by known techniques. For example, such fragments include but are not limited to: the F(ab′)2 fragment which can be produced by pepsin digestion of the antibody molecule; the Fab′ fragments which can be generated by reducing the disulfide bridges of the F(ab′)2 fragment, and the Fab fragments which can be generated by treating the antibody molecule with papain and a reducing agent.
In the production of antibodies, screening for the desired antibody can be accomplished by techniques known in the art, e.g., radioimmunoassay, ELISA (enzyme-linked immunosorbant assay), “sandwich” immunoassays, immunoradiometric assays, gel diffusion precipitin reactions, immunodiffusion assays, in situ immunoassays (using colloidal gold, enzyme or radioisotope labels, for example), western blots, precipitation reactions, agglutination assays (e.g., gel agglutination assays, hemagglutination assays), complement fixation assays, immunofluorescence assays, protein A assays, and immunoelectrophoresis assays, etc. In one embodiment, antibody binding is detected by detecting a label on the primary antibody. In another embodiment, the primary antibody is detected by detecting binding of a secondary antibody or reagent to the primary antibody. In a further embodiment, the secondary antibody is labeled. Many means are known in the art for detecting binding in an immunoassay and are within the scope of the present invention. For example, to select antibodies which recognize a specific epitope of a ZA loop of a bromodomain, for example, one may assay generated hybridomas for a product which binds to a bromodomain fragment containing such an epitope and choose those which do not cross-react with bromodomain fragments that do not include that epitope.
In a specific embodiment, antibodies that interfere with the formation of the bromodomain-acetyl-lysine complex can be generated. Such antibodies can be tested using the assays described and could potentially be used in anti-cancer therapies.
Compounds of the present in invention may function by modulating the stability of the binding complex formed between P/CAF and Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In one such embodiment the the present invention features contacting the bromodomain of P/CAF or a fragment thereof with a binding partner in the presence of the compound under conditions in which the bromodomain of P/CAF and the binding partner bind in the absence of the compound. The stability of the bromodomain of P/CAF and the binding partner is then determined (e.g., measured). When there is a change in the stability of the binding complex between the bromodomain of P/CAF and the binding partner in the presence of the compound, the compound is identified as a modulator. In one embodiment of this type the binding partner is Tat that is acetylated at the lysine residue at position 50 of SEQ ID NO:45. In a preferred embodiment the binding partner is a fragment of Tat comprising an acetyl-lysine at position 50. In still another embodiment the binding partner is an analog of the fragment of Tat comprising an acetyl-lysine at position 50. When the stability of the bromodomain of P/CAF for the binding partner increases in the presence of the compound, the compound is identified as a stabilizing agent, whereas when the stability of the bromodomain of P/CAF for the binding partner decreases in the presence of the compound, the compound is identified as an inhibitor of the Tat-P/CAF complex. In a preferred embodiment the compound is selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-5 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference. More preferably the selection is performed in conjunction with computer modeling. Compounds identified by these methods are also part of the present invention. Preferably the compound is an analog of acetyl-lysine. More preferably the compound is a small organic molecule not included in
Compounds of the present invention may function by modulating the binding of P/CAF and Tat. In a preferred embodiment the agent is a small organic molecule. Preferably the agent inhibits and/or destabilizes the binding of P/CAF with Tat. The agent may be an analog of acetyl-lysine.
Compounds of the present in invention are useful for modulating preventing, and/or retarding the progression and/or treating HIV infection in an individual. One such method employs administering to the individual compounds that modulate the Tat-P/CAF complex selected by performing rational drug design with the set of atomic coordinates obtained from one or more of Tables 1-5 and 10-14 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, the disclosure of which is hereby incorporated by reference. In a preferred embodiment the compound administered is an acetyl-lysine analog. In a particular embodiment this compound is a small organic molecule contained in
As used herein the following terms are defined as follows:
As used herein, and unless otherwise specified, the terms “agent”, “potential drug”, “compound”, “test compound” or “potential compound” are used interchangeably, and refer to chemicals which potentially have a use as an inhibitor or activator/stabilizer of bromodomain-acetyl-lysine binding. Therefore, such “agents”, “potential drugs”, “compounds” and “potential compounds” may be used, as described herein, in drug assays and drug screens and the like.
As used herein a “small organic molecule” is an organic compound, including a peptide or organic compound complexed with an inorganic compound (e.g., metal) that has a molecular weight of less than 3 Kilodaltons. Such small organic molecules can be included as agents, etc. as defined above.
As used herein the term “binds to” is meant to include all such specific interactions that result in two or more molecules showing a preference for one another relative to some third molecule. This includes processes such as covalent, ionic, hydrophobic and hydrogen bonding but does not include non-specific associations such as solvent preferences.
According to the invention, the component or components of a therapeutic composition, e.g., an agent of the invention that interferes with the bromodomain-acetyl-lysine binding complex such as the peptide having the amino acid sequence of SEQ ID NOs:4, 5, 6, 46, or 47, or an acetyl-lysine analog as defined by
In a preferred aspect, the agent of the present invention can cross cellular and nuclear membranes, which would allow for intravenous or oral administration. Strategies are available for such crossing, including but not limited to, increasing the hydrophobic nature of a molecule; introducing the molecule as a conjugate to a carrier, such as a ligand to a specific receptor, targeted to a receptor; and the like.
The present invention also provides for conjugating targeting molecules to such an agent. “Targeting molecule” as used herein shall mean a molecule which, when administered in vivo, localizes to desired location(s). In various embodiments, the targeting molecule can be a peptide or protein, antibody, lectin, carbohydrate, or steroid. In one embodiment, the targeting molecule is a peptide ligand of a receptor on the target cell. In a specific embodiment, the targeting molecule is an antibody. Preferably, the targeting molecule is a monoclonal antibody. In one embodiment, to facilitate crosslinking the antibody can be reduced to two heavy and light chain heterodimers, or the F(ab′)2 fragment can be reduced, and crosslinked to the agent via the reduced sulfhydryl. Antibodies for use as targeting molecule are specific for a cell surface antigen.
In another embodiment, the therapeutic compound can be delivered in a vesicle, in particular a liposome (see Langer, Science, 249:1527-1533 (1990); Treat et al., in Liposomes in the Therapy of Infectious Disease and Cancer, Lopez-Berestein, et al. (eds.), Liss: New York, pp. 353-365 (1989); Lopez-Berestein, ibid., pp. 317-327; see generally ibid.).
In yet another embodiment, the therapeutic compound can be delivered in a controlled release system. For example, the agent may be administered using intravenous infusion, an implantable osmotic pump, a transdermal patch, liposomes, or other modes of administration. In one embodiment, a pump may be used (see Langer, supra; Sefton, CRC Crit. Ref Biomed. Eng., 14:201 (1987); Buchwald et al., Surgery, 88:507 (1980); Saudek et al., N. Engl. J. Med., 321:574 (1989)). In another embodiment, polymeric materials can be used (see Medical Applications of Controlled Release, Langer, et al (eds.), CRC Press: Boca Raton, Fla. (1974); Controlled Drug Bioavailability, Drug Product Design and Performance, Smolen, et al. (eds.), Wiley: New York (1984); Ranger, et al., J. Macromol. Sci. Rev. Macromol. Chem., 23:61 (1983); see also Levy et al., Science, 228:190 (1985); During et al., Ann. Neurol., 25:351 (1989); Howard et al., J. Neurosurg., 71:105 (1989)). In yet another embodiment, a controlled release system can be placed in proximity of the therapeutic target, i.e., the bone marrow, thus requiring only a fraction of the systemic dose (see, e.g., Goodson, in Medical Applications of Controlled Release, supra, vol. 2, pp. 115-138 (1984)). Other controlled release systems are discussed in the review by Langer (Science, 249:1527-1533 (1990)).
Pharmaceutical Compositions. In yet another aspect the present invention provides pharmaceutical compositions of the above. Such pharmaceutical compositions may be for administration for injection, or for oral, pulmonary, nasal or other forms of administration. In general, the invention provides pharmaceutical compositions comprising effective amounts of a low molecular weight component or components, or derivative products, of the invention together with pharmaceutically acceptable diluents, preservatives, solubilizers, emulsifiers, adjuvants and/or carriers. Such compositions include diluents of various buffer content (e.g., Tris-HCl, acetate, phosphate), pH and ionic strength; additives such as detergents and solubilizing agents (e.g., Tween 80, Polysorbate 80), anti-oxidants (e.g., ascorbic acid, sodium metabisulfite), preservatives (e.g., Thimersol, benzyl alcohol) and bulking substances (e.g., lactose, mannitol); incorporation of the material into particulate preparations of polymeric compounds such as polylactic acid, polyglycolic acid, etc. or into liposomes. Hylauronic acid may also be used. Such compositions may influence the physical state, stability, rate of in vivo release, and rate of in vivo clearance of the present proteins and derivatives. See, e.g., Remington's Pharmaceutical Sciences, 18th Ed. (1990, Mack Publishing Co., Easton, Pa. 18042) pages 1435-1712 which are herein incorporated by reference. The compositions may be prepared in liquid form, or may be in dried powder, such as lyophilized form.
Oral Delivery. Contemplated for use herein are oral solid dosage forms, which are described generally in Remington's Pharmaceutical Sciences, 18th Ed.1990 (Mack Publishing Co. Easton Pa. 18042) at Chapter 89, which is herein incorporated by reference. Solid dosage forms include tablets, capsules, pills, troches or lozenges, cachets or pellets. Also, liposomal or proteinoid encapsulation may be used to formulate the present compositions (as, for example, proteinoid microspheres reported in U.S. Pat. No. 4,925,673). Liposomal encapsulation may be used and the liposomes may be derivatized with various polymers (e.g., U.S. Pat. No. 5,013,556). A description of possible solid dosage forms for the therapeutic is given by Marshall, K. In: Modern Pharmaceutics Edited by G. S. Banker and C. T. Rhodes Chapter 10, 1979, herein incorporated by reference. In general, the formulation will include an agent of the present invention (or chemically modified forms thereof) and inert ingredients which allow for protection against the stomach environment, and release of the biologically active material in the intestine.
Also specifically contemplated are oral dosage forms of the above derivatized component or components. The component or components may be chemically modified so that oral delivery of the derivative is efficacious. Generally, the chemical modification contemplated is the attachment of at least one moiety to the component molecule itself, where said moiety permits (a) inhibition of proteolysis; and (b) uptake into the blood stream from the stomach or intestine. Also desired is the increase in overall stability of the component or components and increase in circulation time in the body. An example of such a moiety is polyethylene glycol.
For the component (or derivative) the location of release may be the stomach, the small intestine (the duodenum, the jejunum, or the ileum), or the large intestine. One skilled in the art has available formulations which will not dissolve in the stomach, yet will release the material in the duodenum or elsewhere in the intestine. Preferably, the release will avoid the deleterious effects of the stomach environment, either by protection of the protein (or derivative) or by release of the biologically active material beyond the stomach environment, such as in the intestine.
The therapeutic can be included in the formulation as fine multi-particulates in the form of granules or pellets of particle size about 1 mm. The formulation of the material for capsule administration could also be as a powder, lightly compressed plugs or even as tablets. The therapeutic could be prepared by compression.
One may dilute or increase the volume of the therapeutic with an inert material. These diluents could include carbohydrates, especially mannitol, a-lactose, anhydrous lactose, cellulose, sucrose, modified dextrans and starch. Certain inorganic salts may be also be used as fillers including calcium triphosphate, magnesium carbonate and sodium chloride. Some commercially available diluents are Fast-Flo, Emdex, STA-Rx 1500, Emcompress and Avicell.
Disintegrants may be included in the formulation of the therapeutic into a solid dosage form. Materials used as disintegrates include but are not limited to starch, including the commercial disintegrant based on starch, Explotab. Binders also may be used to hold the therapeutic agent together to form a hard tablet and include materials from natural products such as acacia, tragacanth, starch and gelatin.
An anti-frictional agent may be included in the formulation of the therapeutic to prevent sticking during the formulation process. Lubricants may be used as a layer between the therapeutic and the die wall. Glidants that might improve the flow properties of the drug during formulation and to aid rearrangement during compression also might be added. The glidants may include starch, talc, pyrogenic silica and hydrated silicoaluminate.
In addition, to aid dissolution of the therapeutic into the aqueous environment a surfactant might be added as a wetting agent. Additives which potentially enhance uptake of the protein (or derivative) are for instance the fatty acids oleic acid, linoleic acid and linolenic acid.
Nasal Delivery. Nasal delivery of an agent of the present invention (or derivative) is also contemplated. Nasal delivery allows the passage of a peptide, for example, to the blood stream directly after administering the therapeutic product to the nose, without the necessity for deposition of the product in the lung. Formulations for nasal delivery include those with dextran or cyclodextran.
Transdermal administration. Various and numerous methods are known in the art for transdermal administration of a drug, e.g., via a transdermal patch. Transdermal patches are described in for example, U.S. Pat. No. 5,407,713, issued Apr. 18, 1995 to Rolando et al.; U.S. Pat. No. 5,352,456, issued Oct. 4, 1004 to Fallon et al.; U.S. Pat. No. 5,332,213 issued Aug. 9, 1994 to D'Angelo et al.; U.S. Pat. No. 5,336,168, issued Aug. 9, 1994 to Sibalis; U.S. Pat. No. 5,290,561, issued Mar. 1, 1994 to Farhadieh et al.; U.S. Pat. No. 5,254,346, issued Oct. 19, 1993 to Tucker et al.; U.S. Pat. No. 5,164,189, issued Nov. 17, 1992 to Berger et al.; U.S. Pat. No. 5,163,899, issued Nov. 17, 1992 to Sibalis; U.S. Pat. Nos. 5,088,977 and 5,087,240, both issued Feb. 18, 1992 to Sibalis; U.S. Pat. No. 5,008,110, issued Apr. 16, 1991 to Benecke et al.; and U.S. Pat. No. 4,921,475, issued May 1, 1990 to Sibalis, the disclosure of each of which is incorporated herein by reference in its entirety.
It can be readily appreciated that a transdermal route of administration may be enhanced by use of a dermal penetration enhancer, e.g., such as enhancers described in U.S. Pat. No. 5,164,189 (supra), U.S. Pat. No. 5,008,110 (supra), and U.S. Pat. No. 4,879,119, issued Nov. 7, 1989 to Aruga et al., the disclosure of each of which is incorporated herein by reference in its entirety.
Pulmonary Delivery. Also contemplated herein is pulmonary delivery of the pharmaceutical compositions of the present invention. A pharmaceutical composition of the present invention is delivered to the lungs of a mammal while inhaling and traverses across the lung epithelial lining to the blood stream. Other reports of this include Adjei et al. (Pharmaceutical Research, 7:565-569 (1990); Adjei et al., International Journal of Pharmaceutics, 63:135-144 (1990) (leuprolide acetate); Braquet et al., Journal of Cardiovascular Pharmacology, 13(suppl. 5):143-146 (1989) (endothelin-1); Hubbard et al., Annals of Internal Medicine, Vol. 111, pp. 206-212 (1989) (al-antitrypsin); Smith et al., J. Clin. Invest., 84:1145-1146 (1989) (a-1-proteinase); Oswein et al., “Aerosolization of Proteins”, Proceedings of Symposium on Respiratory Drug Delivery II, Keystone, Colo., March, (1990) (recombinant human growth hormone); Debs et al., J. Immunol., 140:3482-3488 (1988) (interferon-γ and tumor necrosis factor alpha); Platz et al., U.S. Pat. No. 5,284,656 (granulocyte colony stimulating factor)). A method and composition for pulmonary delivery of drugs for systemic effect is described in U.S. Pat. No. 5,451,569, issued Sep. 19, 1995 to Wong et al.
A subject in whom administration of an agent of the present invention is an effective therapeutic regiment for cancer, for example, is preferably a human, but can be any animal. Thus, as can be readily appreciated by one of ordinary skill in the art, the methods and pharmaceutical compositions of the present invention are particularly suited to administration to any animal, e.g., for veterinary medical use, particularly for a mammal, and including, but by no means limited to, domestic animals, such as feline or canine subjects, farm animals, including bovine, equine, caprine, ovine, and porcine subjects, wild animals (whether in the wild or in a zoological garden), research animals, such as mice, rats, rabbits, goats, sheep, pigs, dogs, cats, avian species, such as chickens, turkeys, and songbirds.
The present invention may be better understood by reference to the following non-limiting Examples, which are provided as exemplary of the invention. The following examples are presented in order to more fully illustrate the preferred embodiments of the invention. They should in no way be construed, however, as limiting the broad scope of the invention.
Sample preparation: The bromodomain of P/CAF (residues 719-832 of SEQ ID NO:2) was subcloned into the pET14b expression vector (Novagen) and expressed in Escherichia coli BL21(DE3) cells. Uniformly 15N- and 15N/13C-labelled proteins were prepared by growing bacteria in a minimal medium containing 15NH4Cl with or without 13C6-glucose. A uniformly 15N/13C-labelled and fractionally deuterated protein sample was prepared by growing the cells in 75% 2H2O. The bromodomain was purified by affinity chromatography on a nickel-IDA column (Invitrogen) followed by the removal of poly-His tag by thrombin cleavage. The final purification of the protein was achieved by size-exclusion chromatography. The acetyl-lysine-containing peptides were prepared on a MilliGen 9050 peptide synthesizer (Perkin Elmer) using Fmoc/HBTU chemistry. Acetyl-lysine was incorporated using the reagent Fmoc-Ac-Lys with HBTU/DIPEA activation. NMR samples contained approximately 1 mM protein in 100 mM phosphate buffer of pH 6.5 and 5 mM perdeuterated DTT and 0.5 mM EDTA in H2O/2H2O (9/1) or 2H2O.
NMR spectroscopy: All NMR spectra were acquired at 30° C. on a Bruker DRX600 or DRX500 spectrometer. The backbone assignments of the 1H, 13C, and 15N resonances were achieved using deuterium-decoupled triple-resonance experiments of HNCACB and HN(CO)CACB (Yamazaki et al., J. Am. Chem. Soc. 116:11655-11666 (1994)) recorded using the uniformly 15N/13C-labeled and fractionally deuterated protein. The side-chain atoms were assigned from 3D HCCH-TOCSY (Clore, et al., Meth. Enzymol. 239:249-363 (1994)) and (H)C(CO)NH-TOCSY (Logan et al., J. Biolmol. NMR 3:225-231 (1993)) data collected on the uniformly 15N/13C-labeled protein. Stereospecific assignments of methyl groups of the Val and Leu residues were obtained using a fractionally 13C-labeled sample (Neri et al., Biochemistry 28:7510-7516 (1989)). The NOE-derived distance restraints were obtained from 15N- or 13C-edited 3D NOESY spectra. φ-angle restraints were determined based on the 3JHN,H coupling constants measured in a 3D HNHA spectrum (Clore, et al., Meth. Enzymol. 239:249-363 (1994)). Slowly exchanging amide protons were identified from a series of 2D 15N-HSQC spectra recorded after the H2O buffer was changed to a 2H2O buffer. The intermolecular NOEs used in defining the structure of the bromodomain/Ac-histamine complex were detected in 13C-edited (F1), 13C/15N-filtered (F3) 3D NOESY spectrum (Clore, et al., Meth. Enzymol. 239:249-363 (1994)). All NMR spectra were processed with the NMRPipe/NMRDraw programs and analyzed using NMRView (Johnson, et al., J. Biomol., NMR 4:603-614 (1994)).
Structure calculations: Structures of the bromodomain were calculated with a distance geometry/simulated annealing protocol using the X-PLOR program (Brunger, X-PLOR Version 3.1: A system for X-Ray crystallography and NMR, Yale University Press, New Haven, Conn., (1993)). A total of 1324 manually assigned NOE-derived distance restraints were obtained from the 15N- and 13C-edited NOE spectra. Further analysis of the NOE spectra was carried out by the iterative automated assignment procedure using ARIA (Nilges, et al., Prog. NMR Spectroscopy 32:107-139 (1998)), which integrates with X-PLOR for structure calculations. A total of 1519 unambiguous and 590 ambiguous distance restraints were identified from the NOE data by ARIA, many of which were checked and confirmed manually. The ARIA-assigned distance restraints were in agreement with the structures calculated using only the manually assigned NOE distance restraints, 28 hydrogen-bond distance restraints for 14 hydrogen bonds, and 54□-angle restraints. The final structure calculations employed a total of 3515 NMR experimental restraints obtained from the manual and the ARIA-assisted assignments, 2843 of which were unambiguously assigned NOE-derived distance restraints that comprise of 1077 intra-residue, 621 sequential, 550 medium-range, and 595 long-range NOEs. For the ensemble of the final 30 structures, no distance and torsional angle restraints were violated by more than 0.3 Å and 5□, respectively. The total, distance violation, and dihedral violation energies were 178.7±2.4 kcal mol−1, 41.6±0.9 kcal mol−1, and 0.50±0.06 kcal mol−1, respectively. The Lennard-Jones potential which was not used during any refinement stage, was −526.2±16.8 kcal mol−1 for the final structures. Ramachandran plot analysis of the final structures (residues 727-828) with Procheck-NMR (Laskowski et al., J. Biolmol. NMR 8:477-486 (1996)) showed that 71.0±0.6%, 23.8±0.6%, 3.5±0.2%, and 1.7±0.2% of the non-Gly and non-Pro residues were in the most favorable, additionally allowed, generously allowed, and disallowed regions, respectively. The corresponding values for the residues in the four -helices (residues 727-743, 770-776, 785-802, and 807-827) were 88.9±0.4%, 11.0±0.4%, 0.1±0.1%, and 0.0±0.0%, respectively. The structure of the bromodomain/acetyl-histamine complex was determined using the free form structure and additional 25 intermolecular and 5 intra-ligand NOE-derived distance restraints.
Site-directed mutagenesis: Mutant proteins were prepared using the QuickChange site-directed mutagenesis kit (Stratagene). The presence of appropriate mutations was confirmed by DNA sequencing.
Ligand titration: Ligand titration experiments were performed by recording a series of 2D 15N- and 13C-HSQC spectra on the uniformly 15N-, and 15N/13C-labelled bromodomain (˜0.3 mM), respectively, in the presence of different amounts of ligand concentration ranging from 0 to approximately 2.0 mM. The protein sample and the stock solutions of the ligands were all prepared in the same aqueous buffer containing 100 mM phosphate and 5 mM perdeuterated DTT at pH 6.5.
The full length nucleic acid sequence of the human p300/CBP-associated factor (P/CAF) was obtained from GenBank. Accession No: U57317.2 (SEQ ID NO:1). The full length protein sequence of the human p300/CBP-associated factor (P/CAF) was obtained from GenBank. Accession No: U57317.2, (SEQ ID NO:2).
The full length nucleic acid sequence of the human p300/CBP-associated factor (P/CAF) was obtained from GenBank. Accession No: U57317.2 (SEQ ID NO: 1):
The full length protein sequence of the human p300/CBP-associated factor (P/CAF) was obtained from GenBank. Accession No: U57317.2, (SEQ ID NO:2):
Results. The P/CAF bromodomain represents an extensive family of bromodomains (FIG. 1). A large number of long-range nuclear Overhauser enhancement (NOE)-derived distance restraints were identified in the NMR data of the P/CAF bromodomain, yielding a well-defined three-dimensional structure (
The modular bromodomain structure supports the idea that the bromodomain can act as a functional unit for protein-protein interactions. The observation that bromodomains are found in nearly all known nuclear HATs (A-type) that are known to promote transcription-related acetylation of histones on specific lysine residues, but not present in cytoplasmic HATs (B-type), prompted the determination of whether bromodomains can interact with acetyl-lysine (AcK). The NMR titration of the P/CAF bromodomain were performed with a peptide (SGRGKGG-AcK-GLGK) derived from histone H4, in which Lys8 is acetylated (Lys8 is the major acetylation site in H4 for GCN5, a yeast homologue of P/CAF). Remarkably, the bromodomain could indeed bind the AcK peptide. Moreover, this interaction appeared to be specific, based on the 15N-HSQC spectra which showed that only a limited number of residues underwent chemical shift changes as a function of peptide concentration (
Intriguingly, the bromodomain residues that exhibited the most significant 1H and 15N chemical shift changes on peptide binding are located near the hydrophobic pocket between the ZA and BC loops (
To identify the key residues involved in bromodomain-AcK recognition, the NMR structure of the P/CAF bromodomain in complex with acetyl-histamine was elucidated. As anticipated, the acetylated moiety binds in the bromodomain hydrophobic pocket (
>10,000 d
a The effects of mutations on the structural integrity of the bromodomain were assessed by using the 15N-HSQC spectra. The amide 1H/15N resonances of the mutant proteins were compared to those of the wild-type bromodomain to determine if the particular mutations lead to global or local structure disruption. Severe line-broadening of the amide resonances would indicate protein conformational exchange due to a decrease of structure stability resulting from point mutations. Structural integrity of the mutant proteins is expressed here relative to that of the wild-type, using the signs of “++++” for as stable as the wild-type, “+++” for mildly destabilized, “++” for moderately destabilized, and “−” for completely unfolded.
b The ligand binding affinity (KD) of the bromodomain proteins was estimated by following chemical shift changes of amide peaks in the 15N-HSQC spectra as a function of the ligand concentration.
c No detectable ligand binding observed in the NMR titration.
d Ligand binding affinity was significantly reduced and beyond the limit for reliable measurements by NMR titration.
Substitution of Ala for Tyr809 completely abrogated the bromodomain binding to the lysine-acetylated H4 peptide, while the Tyr802Ala, Tyr760Ala, and Val752Ala mutants had significantly reduced ligand binding affinity. To assess whether these mutations disrupted the overall bromodomain fold, the 15N-HSQC spectra of the mutants was compared to that of the wild-type protein. For the Tyr809Ala mutant, the amide chemical shifts were only affected for a few residues near the mutation site. However, mutations of the other residues in the hydrophobic binding pocket perturbed the local protein conformation to greater extents, particularly the ZA loop (Table 1; Table 7 of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001, now U.S. Pat. No. 7,589,167, herein incorporated by reference). Thus, the NMR structural analysis and the mutagenesis studies show that Tyr809, which is structurally supported by Trp746 and Asn803 (
Whereas the life cycle of HIV is still being elucidated, it is currently accepted that HIV binds to CD4 protein of a host T cell or macrophage and with the aid of a chemokine receptor (e.g., CCR5 or CXCR4) enters the host cell. Once in the host cell, the retrovirus, HIV-1, is converted to a DNA by reverse transcriptase and the expression of the HIV-1 genome is dependent on a complex series of events that are believed to be under the control of two viral regulatory proteins, Tat and Rev (Romano et al., J. Cell Biochem. 75(3):357-368 (1999)). Rev controls post-translational events, whereas, Tat (the trans-activator protein) functions to stimulate the production of full-length HIV transcripts and viral replication in infected cells. The Tat protein transactivates the transcription of HIV-1 starting at the 5′ long terminal repeat (LTR) (Romano et al., J. Cell Biochem. 75(3):357-368 (1999)) by recruiting one or more carboxyl-terminal domain kinases to the HIV-1 promoter. More specifically, Tat stimulates transcription from the LTR at a hairpin element, the transactivation responsive region (TAR) (Kiernan et al., EMBO J. 18:6106-6118 (1999)) at least in part by interacting with and thereby recruiting the carboxyl-terminal domain kinase, i.e., the positive transcriptional elongation factor (P-TEFb) to the TAR RNA element (Garber et al., Mol. Cell. Biol. 20(18):6958-6969 (2000)). P-TEFb is a muti-subunit kinase that minimally comprises a heterodimer consisting of the regulatory cyclin T1 and its corresponding catalytic subunit, cyclin-dependent kinase 9 (CDK9). P-TEFb acts by phosphorylating the carboxyl-terminal domain of RNA polymerase II (Peng et al., J. Biol. Chem. 274 (49):34527-34530 (1999); Romano et al., J. Cell Biochem. 75(3):357-368 (1999)).
Recently, it has been shown that HIV-1 Tat transcription activity is regulated through lysine acetylation by, and association with the histone acetyltransferases (HATs) p300/CBP and the p300/CBP-associating factor (P/CAF), which specifically acetylate Lysine 50 (K50) and Lysine 28 (K28) of the Tat protein, respectively (Kiernan et al., EMBO J. 18:6106-6118 (1999); Ott et al., Curr. Biol. 9:1489-1492 (1999)). Notably, the acetylation of K50 by the transcriptional co-activator p300/CBP is on the C-terminal arginine-rich motif (ARM) of Tat, which is essential for its binding to the TAR RNA element and for nuclear localization, (Kiernan et al., EMBO J. 18:6106-6118 (1999); Ott et al., Curr. Biol. 9:1489-1492 (1999)). Acetylation of K28 of Tat by P/CAF enhances Tat binding to P-TEFb, whereas acetylation of K50 of Tat by P300/CBP promotes the dissociation of Tat from the TAR RNA element. This dissociation of Tat from the TAR RNA element occurs during early transcription elongation (Kiernan et al., EMBO J. 18:6106-6118 (1999)). However, heretofore, little else was known regarding the relationship of these HATs with Tat after the acetylation has occurred.
Sample preparation: The bromodomain of P/CAF (residues 719-832) was subcloned into the pET14b expression vector (Novagen) and expressed in Escherichia coli BL21(DE3) cells. Uniformly 15N- and 15N/13C-labeled proteins were prepared by growing bacteria in a minimal medium containing 15NH4Cl with or without 13C6-glucose. A uniformly 15N/13C-labeled and fractionally deuterated protein sample was prepared by growing the cells in 75% 2H2O. The bromodomain was purified by affinity chromatography on a nickel-IDA column (Invitrogen) followed by the removal of poly-His tag by thrombin cleavage. The final purification of the protein was achieved by size-exclusion chromatography. The acetyl-lysine-containing peptides were prepared on a MilliGen 9050 peptide synthesizer (Perkin Elmer) using Fmoc/HBTU chemistry. Acetyl-lysine was incorporated using the reagent Fmoc-Ac-Lys with HBTU/DIPEA activation. NMR samples contained ˜0.5 mM protein in complex with the lysine-acetylated Tat peptide in 100 mM phosphate buffer of pH 6.5 and 5 mM perdeuterated DTT and 0.5 mM EDTA in H2O/2H2O (9/1) or 2H2O. The bromodomain-containing constructs from P/CAF, CBP and TIF-1 were cloned into pGEX4T-3 vector (Pharmacia). These recombinant GST-fusion proteins were expressed in BL21 (DE3) codon plus cell line, and purified by using glutathione sepharose column.
NMR spectroscopy: All NMR spectra were acquired at 30° C. on a Bruker DRX600 or DRX500 spectrometer. The backbone assignments of the 1H, 13C, and 15N resonances were achieved using deuterium-decoupled triple-resonance experiments of HNCACB and HN(CO)CACB (Yamazaki et al., J. Am. Chem. Soc. 116:11655-11666 (1994)) recorded using the uniformly 15N/13C-labelled and fractionally deuterated protein. The side-chain atoms were assigned from 3D HCCH-TOCSY (Clore, et al., Meth. Enzymol. 239:249-363 (1994)) and (H)C(CO)NH-TOCSY (Logan et al., J. Biolmol. NMR 3:225-231 (1993)) data collected on the uniformly 15N/13C-labeled protein. Stereospecific assignments of methyl groups of the valine and leucine residues were obtained using a fractionally 13C-labeled sample (Neri et al., Biochemistry 28:7510-7516 (1989)). The NOE-derived distance restraints were obtained from 15N- or 13C-edited 3D NOESY spectra (Clore, et al., Meth. Enzymol. 239:249-363 (1994)). □-angle restraints were determined based on the 3JHN,H coupling constants measured in a 3D BNHA spectrum (Clore et al., Meth. Enzymol. 239:249-363 (1994)). Slowly exchanging amide protons were identified from a series of 2D 15N-HSQC spectra recorded after the H2O buffer was changed to a 2H2O buffer. The intermolecular NOEs used in defining the structure of the bromodomain/Ac-histamine complex were detected in 13C-edited (F1), 13C/15N-filtered (F3) 3D NOESY spectrum (Clore et al., Meth. Enzymol. 239:249-363 (1994)). All NMR spectra were processed with the NMRPipe/NMRDraw programs and analyzed using NMRView (Johnson, et al., J. Biomol., NMR 4:603-614 (1994)).
Ligand titration experiments were performed by recording a series of 2D 15N-HSQC spectra on the uniformly 15N-labelled bromodomain (˜0.3 mM), respectively, in the presence of different amounts of ligand concentration ranging from 0 to ˜2.0 mM. The protein sample and the stock solutions of the ligands were all prepared in the same aqueous buffer containing 100 mM phosphate and 5 mM perdeuterated DTT at pH 6.5.
Structure calculations. Structures of the bromodomain were calculated with a distance geometry/simulated annealing protocol using the X-PLOR program (Brunger, X-PLOR Version 3.1: A system for X-Ray crystallography and NMR, Yale University Press, New Haven, Conn., (1993)). A total of 1324 manually assigned NOE-derived distance restraints were obtained from the 15N- and 13C-edited NOE spectra. Further analysis of the NOE spectra was carried out by the iterative automated assignment procedure by using ARIA (Nilges, et al., Prog. NMR Spectroscopy 32:107-139 (1998)), which integrates with X-PLOR for structure calculations. The ARIA-assigned distance restraints were in agreement with the structures calculated using only the manually assigned NOE distance restraints, hydrogen-bond distance restraints, and 54 φ-angle restraints. The final structure calculations employed a total of 2903 NMR experimental restraints obtained from the manual and the ARIA-assisted assignments. For the ensemble of the final 30 structures, no distance and torsional angle restraints were violated by more than 0.3 Å and 5 Å, respectively. The Lennard-Jones potential which was not used during any refinement stage, and stereochemistry of the final structures was validated with Ramachandran plot analysis by using Procheck-NMR (Laskowski et al., J. Biolmol. NMR 8:477-486 (1996)).
Site directed mutagenesis. Site directed mutagenesis was performed on selected residues of P/CAF Bromodomain using quick-change kit (Stratagene). The mutants were confirmed by sequencing and proteins were expressed and purified as above.
Peptide binding assay. Equal amount (10 μM) of GST, GST-P/CAF bromodomain and its mutant proteins, as well as various GST-fusion bromodomains from CBP and TIF1 were incubated for at least two hours at room temperature with the N-terminal biotinylated and lysine-acetylated Tat peptide (50 μM) in a 50 mM Tris buffer of pH 7.5, containing 50 mM NaCl, 0.1% BSA and 1 mM DTT. Streptavidin agarose (10 μL) was added to mixture and the beads were washed twice in the Tris buffer with 500 mM NaCl and 0.1% NP-40. Proteins were eluted from the argarose beads in SDS buffer and separated on a 14% SDS-PAGE. The resolved proteins were transferred onto nitrocellulose membrane (Pharmacia), and the membrane was blocked overnight with 5% non-fat milk in washing buffer of 20 mM Tris, pH 7.5, plus 150 mM NaCl and 0.1% Tween-20 at 4° C. Western blotting was performed with anti-GST antibody (Sigma) and goat anti-rabbit IgG conjugated with horseradish-peroxidase (Promega) and developed by chemiluminescence. Peptide competition experiments were performed by incubating various non-biotinylated and mutant Tat peptide with the P/CAF bromodomain and the biotinylated and wild type Tat peptide. The molar ratio of the wild type and mutant Tat peptides in the mixture were kept at 1:2. The binding results were analyses by using the procedure as described above. The full length protein sequence of the Human Immunodeficiency Virus type 1 Tat was obtained from GenBank, Accession No: AAA83395 (SEQ ID NO:45).
Results. To test whether or not the bromodomains of these HATs can bind to the lysine-acetylated Tat, in vitro binding assays were performed by using recombinant and purified bromodomains and lysine-acetylated peptides derived from the acetylation sites in Tat. While the bromodomains of CBP and TIF1 did not show any binding, the P/CAF bromodomain binds tightly only to the Tat peptide containing AcK50 (where AcK stands for an N-acetyl lysine residue) (
To determine how the P/CAF binding affects Tat function in vivo, transactivation activity of Tat was measured. Superinduction of Tat transactivation activity exhibited as much as a 30-fold increase upon P/CAF stimulation (
To further understand the molecular basis of the P/CAF bromodomain recognition of the lysine-acetylated Tat, the three-dimensional structure was determined for the P/CAF bromodomain in complex with an 11-residue Tat peptide containing AcK50. A total of 2,903 NMR-derived distance and dihedral angle restraints were used. The structure of the bromodomain in the peptide-bound form consists of an up-and-down four-helix bundle (helices Z, A, B, and C) with a left-handed twist, and a long intervening loop between helices Z and A (termed the ZA loop) (
a Of the total 2903 NOE-derived distance restraints, only 341 were obtained by using ARIA program, of which 122 are classified as ambiguous NOEs. The latter resonance signals in the spectra match with more than one proton atom in both the chemical shift assignment and the final NMR structures.
b The Lennard-Jones potential was not used during any refinement stage.
c None of these final structures exhibit NOE-derived distance restraint violations greater than 0.5 Å or dihedral angle restraint violations greater than 5°.
The Tat AcK50 peptide adopts an extended conformation and lies between the ZA and BC loops (
To identify the amino acid residues of the P/CAF bromodomain that are important for complex formation, mutant proteins were tested for binding to the biotinylated and lysine-acetylated Tat peptide that is immobilized onto streptavidin agarose (
To further determine Tat sequence preference for P/CAF interaction, various mutant peptides were synthesized and their binding to the P/CAF bromodomain tested in a competition assay by using a western blot with the antibody against the GST-fusion bromodomain (
The HIV-1 Tat is a versatile protein and elicits many cellular functions. In addition to its lysine-acetylation and interaction with P/CAF as disclosed herein, this portion of arginine-rich motif (named ARM) has also been shown to interact with the TAR RNA element as well as protein nuclear localization, particularly involving arginine52 and arginine53. The findings disclosed herein that are based on the detailed structural and mutational analyses indicate that the lysine-acetylated Tat specifically is associated with P/CAF via a bromodomain interaction in vivo, and that this interaction is important for transactivation activity of Tat in cells. Further, the data disclosed herein reveal that in addition to the acetylated-lysine (K50) the flanking residues, tyrosine (AcK−3) and glutamine at (AcK+4) positions in Tat are also uniquely important for the specificity of the Tat and P/CAF bromodomain recognition, but not with its other functions. This new information is extremely useful in applying mutational analysis in in vivo studies to further elucidate the biological importance of the Tat-P/CAF association in molecular mechanisms by which Tat transactivates gene transcription of HIV-1 via chromatin remodeling.
Sample preparation. The PCAF bromodomain (residues 719-832) was expressed in E. coli BL21(DE3) cells using the pET14b vector (Novagen) (Dhalluin, et al., Nature (1999) 399, 491-496). Isotope-labeled proteins were prepared from cells grown on a minimal medium containing 15NH4Cl with or without 13C6-glucose in either H2O or 75% 2H2O. The protein was purified by affinity chromatography on a nickel-IDA column (Invitrogen), followed by the removal of poly-His tag by thrombin cleavage. GST-fusion PCAF bromodomain was expressed in E. coli BL21 (DE3) codon plus cells using the pGEX4T-3 vector (Pharmacia), and purified with a glutathione sepharose column. The lysine-acetylated peptide was ordered from Biosynthesis, Inc.
Protein structure determination by NMR. NMR samples contained the bromodomain (0.5 mM) in complex with a chemical ligand (˜2 mM) in 100 mM phosphate buffer of pH 6.5, containing 5 mM perdeuterated DTT and 0.5 mM EDTA in H2O/2H2O (9/1) or 2H2O. All NMR spectra were acquired at 30° C. on a Bruker 500 or 600 MHz NMR spectrometer. The backbone 1H, 13C and 15N resonances were assigned using 3D HNCACB and HN(CO)CACB spectra. The side-chain atoms were assigned from 3D HCCH-TOCSY and (H)C(CO)NH-TOCSY data. The NOE-derived distance restraints were obtained from 15N- or 13C-edited 3D NOESY spectra. The 3JHN,H═ Coupling constants measured from 3D HNHA data were used to determine □-angle restraints. Slowly exchanging amide protons were identified from a series of 2D 15N-HSQC spectra recorded after H2O/2H2O exchange. The intermolecular NOEs used in defining the structure of the PCAF bromodomain/ligand complex were detected in 13C-edited (F1), 13C/15N-filtered (F3) 3D NOESY spectra (Clore et al., Meth. Enzymol. (1994) 239, 249-363). Protein structures were calculated with a distance geometry-simulated annealing protocol with X-PLOR (Brunger, X-PLOR Version 3.1: A system for X-Ray crystallography and NMR. version 3.1 ed. 1993, New Haven, Conn.: Yale University Press). Initial structure calculations were performed with manually assigned NOE-derived distance restraints. Hydrogen-bond distance restraints, generated from the H/D exchange data, were added at a later stage of structure calculations for residues with characteristic NOEs. The converged structures were used for iterative automated NOE assignment by ARIA for refinement (Nilges et al., Prog. NMR Spectroscopy (1998) 32, 107-139). Structure quality was assessed by Procheck-NMR (Laskowski et al, J. Biomol. NMR (1996) 8, 477-486). The structure of the protein/ligand complex was determined using intermolecular NOE-derived distance restraints.
Chemical screening by NMR. A chemical library was constructed with small-molecules obtained from Chembridge Corp. (San Diego, Calif.), which were selected on the basis of molecular weight (<250 Da), non-reactivity, drug-like chemical framework (i.e. ring systems, linker atoms, side-chain atoms and framework), functional moieties (for hydrogen-bond or electrostatic interactions, but not reactive) and good solubility in aqueous solution. All the stock solutions of the chemical compounds are prepared in predeuterated DMSO. NMR-based screening was conducted with compounds (˜1 mM) and the PCAF bromodomain (50-200 μM) using methods including 1D NOE-pumping (Chen et al., J. Am. Chem. Soc. (1998) 120, 10258-10259) and saturation transfer difference (Kwak et al., J. Biol. Chem. (1995) 270, 1156-1160; Klein et al., J. Am. Chem. Soc. (1999) 121, 5336-5337), as well as 2D 15N-HSQC spectra (Hajduk et al., Quarterly Reviews of Biophysics (1999) 32, 211-240; Moore, Curr. Opin. in Biotech. (1999) 10, 54-58). The latter is particularly helpful for selective screening to identify compounds that bind to a specific site of the target protein.
Ligand binding to the PCAF bromodomain. The ELISA assay was carried on a 96-well microplate (Nunc) that was pre-coated with anti-GST antibody (Sigma-Aldrich) overnight at 4° C. in 100 μL carbonate/bicarbonate buffer, and then washed with PBS buffer supplemented with 1% Tween-20. Non-specific binding sites were minimized by treatment with PBS buffer containing 2% BSA and 1% Tween-20 for 2 hours at room temperature. GST-PCAF bromodomain (1 μg per well) was added to the plate and incubated for two hours at room temperature for binding to anti-GST antibody. The plate was washed and blocked with PBS buffer containing 10% BSA and 1% Tween-20. The biotinylated HIV Tat-AcK50 peptide (Biotin-GISYGR-AcK-KKRRQRRRP) (5 μM) and increasing concentrations of a given compound were added and allowed to bind to the PCAF bromodomain overnight at 4° C. Plate was washed with washing buffer, and bromodomain-bound peptide was determined by incubating 100 μL of a neutravidin-conjugated HRP (Pierce) solution (0.1 μg/ml) for 1 hour at room temperature, followed by washes and incubation with 100 μL of tetramethyl benzidine (Pierce) as an HRP substrate. The reaction was stopped by addition of 100 μL of 2.0 M sulfuric acid. The absorbance of the colored product was measured at 450 nm. Absorbance in each well was corrected for the blank obtained in a corresponding well subjected to the complete procedure but containing no PCAF bromodomain.
Chemistry. Melting points were recorded on an XT-4 micro-melting point apparatus and were uncorrected. 1H and 13C NMR spectra were recorded on a 300 MHz Bruker NMR spectrometer using tertramethylsilane as internal standard and the data were reported as the following: chemical shifts in ppm (δ), number of protons, multiplicity (s, singlet; d, doublet; t, triplet; m, multiplet), coupling constants in hertz. IR spectra were measured on Bruker EQUINOX55 spectrometer. Mass spectra were recorded on HRMS (LC-TOF spectrometer (micromass)) (EI/CI). Elemental analyses were recorded on Elementar Vario EL-III spectrometer. All chromatographic purifications were performed with silica gel (100-200 mesh). All purchased materials were used without further purifications. All solvents were reagent grades.
N1-(2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 78%; mp >169° C.; FTIR (KBr)
N1-(4-methyl-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 60.8%; mp 184-186° C.; FTIR (KBr)
N1-(4-ethyl-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 72.7%; mp 183-185° C.; FTIR(KBr)
N1-(3-methyl-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 81.5%; mp 180-182° C.; FTIR (KBr)
N1-(5-methyl-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 67.1%; mp >215° C.; FTIR (KBr)
N1-(3-nitro-biphenyl-4-yl)-propane-1,3-diamine monohydrochloride. mp >190° C.; FTIR (KBr)
N1-(4-cyano-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. mp >230° C.; FTIR (KBr)
N1-(5-cyano-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride. mp >185° C.; FTIR (KBr)
N1-(2-methyl-5-nitro-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 81%; mp >152° C.; FTIR(KBr)
N1-(2-carboxyl-phenyl)-propane-1,3-diamine monohydrochloride. Yield: 51%; mp >135° C.; FTIR (KBr)
N1-(2-carboxymethyl-phenyl)-propane-1,3-diamine monohydrochloride. mp 141-143° C.; FTIR(KBr)
3-(2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 79%; mp 160-162° C.; FTIR (KBr)
3-(4-methyl-2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 83%; mp 158-160° C.; FTIR(KBr)
3-(4-methoxy-2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 73.5%; mp 154-156° C.; FTIR (KBr)
3-(4-chloro-2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 82%; mp 184-186° C.; FTIR (KBr)
3-(5-methyl-2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 80.6%; mp 211-213° C.; FTIR (KBr)
3-(3-methyl-2-nitro-phenoxy)-1-propylamine monohydrochloride. Yield 77%; mp 160-162° C.; FTIR (KBr)
4-(2-nitro-phenyl)-butylamine monohydrochloride. Yield 8%; oil; FTIR (film)
N1-(2-nitro-phenyl)-butane-1,4-diamine monohydrochloride. Yield: 50.4%; mp 173-176° C.; FTIR (KBr)
N1-(4-nitro-phenyl)-ethane-1,2-diamine monohydrochloride. Yield: 67%; mp 198° C.; FTIR (KBr)
N1-(4-nitro-phenyl)-butane-1,4-diamine monohydrochloride. Yield: 70.5%; mp 183-185° C.; FTIR (KBr)
Structure coordinates: Coordinates for the three-dimensional NMR structures of the PCAF bromodomain in complex with the lead compounds N1-(2-nitro-phenyl)-propane-1,3-diamine monohydrochloride or N,-(4-methyl-2-nitro-phenyl)-propane-1,3-diamine monohydrochloride have been deposited in the Brookhaven Protein Data Bank under accession numbers 1WUM and 1WUG, respectively.
a None of these final structures exhibit NOE-derived distance restraint violations greater than 0.3 Å or dihedral angle restraint violations greater than 5°.
b The Lennard-Jones potential was not used during any refinement stage.
c Protein residues 723-830. The residues in the secondary structure of the protein are 727-741, 772-777, 785-802 and 808-826.
d Protein and ligand complex.
The following compounds represented in Table 5 were prepared according to synthesis techniques well known to those of skill in the art.
To develop selective small-molecule inhibitors for blocking Tat/PCAF association, we conducted NMR-based chemical screening for the BRD by monitoring ligand-induced protein signal changes in 2D 15N-HSQC spectra (Hajduk, et al., Q. Rev. Biophys. (1999) 32, 211-40). We placed an emphasis on identifying compounds that bind selectively the BRD near but not just the AcK binding pocket, as the former may be more selective for this BRD. From screening of a few thousands of small-molecules from commercial libraries, we discovered several compounds including 1 that meet this criterion. Compound 1 binds the PCAF BRD with an affinity comparable to that of the Tat-AcK50 peptide (see below)(Mujtaba, et al., Mol Cell. (2004) 13, 251-63). Importantly, these compounds do not bind the structurally similar BRDs from CBP and TIF1β at millimolar concentration.
We next synthesized a series of analogs of compound 1 to probe the structure-activity relationship (SAR) (Table 6). We assessed their binding to the PCAF BRD by measuring an IC50 in an ELISA assay, in which a compound competes against a biotinylated Tat-AcK50 peptide for binding to the GST-fusion BRD immobilized to glutathione-coated 96-well microtiter plate. The SAR study reveals salient features of BRD recognition of 1. First, the BRD prefers a 4-methyl group on the aniline ring, which improves IC50 by 3-fold to 1.6 μM (2 vs. 1). While substitution of a 4-ethyl, 3- or 5-methyl group on the aniline ring slightly weakens the binding (3-5 vs. 1), addition of a 4-phenyl group nearly abolishes the binding (6 vs. 1). Adding a 4- or 5-cyano group weakens the binding by ˜7-12-fold (7 and 8 vs. 1). Second, a 2-nitro group on the aniline ring is vital for the binding. Swapping of 2-nitro and 5-methyl causes a 7-fold reduction in binding (9 vs. 5). Surprisingly, substitution of 2-nitro with 2-caroxylate or 2-caroxyl ester abrogates the binding (10 and 11 vs. 1). Third, the functional importance of the 2-nitro is further supported by the effects of changing the NH to an O linkage in the aniline, which severely compromises the binding to the PCAF BRD (12-17 vs. 1-5). Moreover, changing to a carbon linkage eliminates the binding (18 vs. 1). Fourth, the BRD prefers an amino three-carbon aliphatic chain in 1—a four-carbon chain reduces the binding by 30-fold (19 vs. 1) and a two-carbon chain nearly loses the binding (20 vs. 1). Alteration of 1 by two key elements, i.e. changing to a four-carbon chain and 4-nitro, abolishes the binding (21 vs. 1). Finally, the terminal amine group is also an important functional moiety for the BRD binding (22-24 vs. 1).
n
To understand ligand selectivity of the PCAF BRD, we solved the 3D structures of the protein bound to 1 and 2. The two ligands are bound in the protein structure in nearly the same manner. For clarity, only the 2-bound structure is reported here, which is similar to the free structure except for the ZA and BC loops that move closer to each other by clamping onto the ligand (
The present invention therefore provides a class of novel small molecules that can effectively inhibit the PCAF BRD/Tat-AcK50 association in vitro by selectively binding to the BRD (
Bromodomain. The present invention utilizes detailed structural information regarding a bromodomain and a bromodomain complexed with its acetylated binding partner. The present invention therefore provides the three-dimensional structure of the bromodomain and a bromodomain acetylated binding partner complex. Since the interaction of the bromodomain with a histone for example, can play a significant role in chromatin remodeling/regulation, the structural information provided herein can be employed in methods of identifying drugs that can modulate basic cell processes by modulating the transcription. In a particular embodiment, the three-dimensional structural information is used in the design of a small organic molecule for the treatment of cancer or as disclosed below, HIV-1 infection and/or AIDs. In addition, the present invention provides a critical structural feature for a class of inhibitors (acetyl-lysine analogs) of the interaction between bromodomains and their protein binding partners which contain an acetylated-lysine (e.g., Tat with P/CAF), see
Indeed, the bromodomain and lysine-acetylated protein interaction can now be implicated to play a causal role in the development of a number of diseases including cancers such as leukemia. For example, chromatin remodeling plays a central role in the etiology of viral infection and cancer (Archer, et al., Curr. Opin. Genet. Biol. 9:171-174 (1999); Jacobson, et al., Curr. Opin. Genet. Biol. 9:175-184 (1999)). Both altered histone acetylation/deacetylation and aberrant forms of chromatin-remodeling complexes are associated with human diseases. Furthermore, chromosomal translocation of various cellular genes with those encoding HATs and subunits of chromatin remodeling complexes have been implicated in leukomogenesis. The MOZ (monocytic leukemia zinc finger) and MLL/ALL-1 genes are frequently fused to the gene encoding the co-activator HAT CBP (Sobulo et al., Proc. Natl. Acad. Sci. USA 94:8732-8737(1997)). The resulting fusion protein MLL-CBP contains the tandem bromodomain-PHD finger-HAT domain of CBP. It also has been shown that both the bromodomain and HAT domain of CBP are required for leukomogenesis, because deletion of either the bromodomain or the HAT domain results in loss of the MLL-CBP fusion protein's ability for cell transform. These results indicate that the CBP bromodomain, and more particularly, the ZA loop of the CBP bromodomain, is an excellent target for developing drugs that interfere with the bromodomain acetyl-lysine interaction that can be used in the treatment of human acute leukemia. In addition, an antibody (e.g., a humanized antibody) raised specifically against a peptide from the ZA loop of the CBP bromodomain could also be effective for treating these conditions.
In addition, it now known that the human immunodeficiency virus type 1 (HIV-1) trans-activator protein, Tat, is tightly regulated by lysine acetylation (Kiernan et al., EMBO Journal 18:6106-6118 (1999)). HIV-1 Tat transcriptional activity is absolutely required for productive HIV viral replication (Jeang, et al., Curr. Top. Microbiol. Immunol., 188:123-144(1994)). Therefore, the interaction of the acetyl-lysine of Tat with one or more bromodomain-containing proteins associated with chromatin remodeling could mediate gene transcription. More particularly, it is disclosed herein that acetylated lysine50 of Tat specifically binds to the bromodomain of P/CAF. Therefore, this particular bromodomain/lysine-acetylated Tat interaction serves as a drug target for blocking HIV replication in cells. As indicated above, an antibody raised specifically against a peptide from the ZA loop of the P/CALF bromodomain could also be effective for treating and/or preventing HIV infections including those that lead to AIDs.
In addition, based on the new structural information disclosed herein, the key amino acid residues for the binding of a given bromodomain and its binding partner can be identified and further elucidated using basic mutagenesis and standard isothermal titration calorimetry, for example. Indeed, both the critical amino acids for the bromodomain and the binding partner (i.e., apart from the acetyl-lysine) can be readily determined and are also part of the present invention.
Compounds may be active to bind to two nearby sites on the bromodomain. In this case, a compound that binds a first site of the bromodomain does not bind a second nearby site. Binding to the second site can be determined by monitoring changes in a different set of amide chemical shifts in either the original screen or a second screen conducted in the presence of a ligand (or potential ligand) for the first site. From an analysis of the chemical shift changes the approximate location of a potential ligand for the second site is identified. Optimization of the second ligand for binding to the site is then carried out by screening structurally related compounds (e.g., analogs as described above). When ligands for the first site and the second site are identified, their location and orientation in the ternary complex can be determined experimentally either by NMR spectroscopy or X-ray crystallography. On the basis of this structural information, a linked compound is synthesized in which the ligand for the first site and the ligand for the second site are linked. In a preferred embodiment of this type the two ligands are covalently linked. This linked compound is tested to determine if it has a higher binding affinity for the bromodomain than either of the two individual ligands. A linked compound is selected as a ligand when it has a higher binding affinity for the bromodomain than either of the two ligands. In a preferred embodiment the affinity of the linked compound with the bromodomain is determined monitoring the 15N- or 1H-amide chemical shift changes in two dimensional 15N-heteronuclear single-quantum correlation (15N-HSQC) spectra upon the addition of the linked compound to the 15N-labeled bromodomain as described above. A larger linked compound can be constructed in an analogous manner, e.g., linking three ligands which bind to three nearby sites on the bromodomain to form a multilinked compound that has an even higher affinity for the bromodomain than the linked compound.
The PCAF bromodomain (BRD) recognizes small molecule compounds MIB and NP1 at two proximal sites in the acetyl-lysine (Kac) binding pocket between the ZA and BC loops (
The HIV Tat/PCAF association via the PCAF BRD is highly selective as compared to the bromodomains of human CBP or transcriptional intermediate factor TWO (
To test functional efficacy of small-molecule BRD ligands in the cell, we developed an efficient HIV-LTR luciferase reporter gene assay similar to that reported previously (Bieniasz et al., EMBO J, 1998. 17: p. 7056-7065; Madore et al., J. Virol., 1993. 67: p. 3703-3711). Specifically, in this assay, we first propagated human 293T cells in Dulbecco's modified Eagle's medium with 10% fetal calf serum and then transfected the cells with DNA plasmids encoding HIV Tat (pcTat) and HIV LTR-luciferase gene construct (pHIV-LTR-Luc) by using the calcium phosphate co-precipitation method. The transfected 293T cells were incubated in the cell culture media in the presence of a specified amount of a small-molecule BRD as listed in Table 1. The cells were incubated with the ligand for 24 hours and then lysed and assayed for luciferase activity of the cell extracts using a luciferase-based assay system (Promega). Luciferase activities derived from HIV-1 LTR were normalized to a co-transfected vector expressing beta-galactosidase. As illustrated in
Inhibition of Tat-mediated transactivation by small-molecule inhibitors that block the PCAF bromodomain interaction with HIV-1 Tat-AcK50. The effect was assessed by a microinjection study as described previously by Dorr et al. (EMBO J. 21; 2715-2723, 2002). In this microinjection assay, HeLa-Tat cells were grown on Cellocate coverslips and microinjected at room temperature with an automated injection system (Carl Zeiss). Samples were prepared as a 20 μl injection mix containing the LTR-luciferase (100 ng/ml) and CMV-GFP (50 ng/ml) constructs together with 5 mg/ml a chemical compound or pre-immune IgGs. Live cells were examined on a Zeiss Axiovert microscope to determine the number of GFP-positive cells. Four hours after injection, cells were washed in cold phosphate buffer and processed for luciferase assays (Promega).
Effect of PCAF bromodomain inhibitors on Tat transactivation.
Effect of the PCAF Bromodomain inhibitors on viral infection, using a procedure described by Pagans et al. (PLoS Biol. 3(2): e41, 2005).
The present invention is not to be limited in scope by the specific embodiments described herein. Indeed, various modifications of the invention in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.
It is further to be understood that all base sizes or amino acid sizes, and all molecular weight or molecular mass values, given for nucleic acids or polypeptides are approximate, and are provided for description.
Various publications are cited herein, the disclosures of which are hereby incorporated by reference herein in their entireties.
The present application is a continuation-in-part of U.S. Ser. No. 10/209,201, filed Jul. 31, 2002 which is a division of U.S. Ser. No. 09/784,553, filed Feb. 16, 2001 which is a continuation in part of U.S. Ser. No. 09/510,314, filed Feb. 22, 2000, the disclosures of which are hereby incorporated by reference in their entireties. Applicants claim the benefits of these applications under 35 U.S.C. §120.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 09784553 | Feb 2001 | US |
| Child | 10209201 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12154158 | May 2008 | US |
| Child | 12590069 | US | |
| Parent | 10209201 | Jul 2002 | US |
| Child | 12154158 | US | |
| Parent | 09510314 | Feb 2000 | US |
| Child | 09784553 | US |
