Patent Application
20060045873
The present invention relates to methods for methylating CpG sequences in DNAs using dsRNAs, and relates to methylation-inducing agents that comprise dsRNAs. The present invention also relates to methods for using dsRNAs to suppress gene expression by inducing the methylation of CpG sequences in promoter regions, and relates to gene expression-suppressing agents that comprise dsRNAs.
Double-stranded RNAs (dsRNAs) induce post-transcriptional gene silencing through RNA interference (RNAi) in animals and plants (Fire A. et al. Nature 391, 806-811 (1998); Hammond, S. M. et al. Nature Rev. Genet. 2, 110-119 (2001); Hutvagner, G., & Zamore, P. D. Curr. Opin. Genet. Dev. 12, 225-232 (2002)). In this system, small interference RNAs (siRNAs) produced by RNaseIII Dicer are incorporated into RNAi-induced silencing complexes (RISCs) (Tuschl, T. et al. Genes Dev., 13, 3191-3197 (1999); Hammond, S. M. et al. Nature, 404, 293-296 (2000); Zamore, P. et al. Cell 101, 25-33 (2000); Bernstein, E. et al. Nature 409, 363-366 (2001); Elbashir, S. M. et al. Genes Dev. 15, 188-200 (2001)). The siRNA-RISC complexes thus obtained promote mRNA degradation in the cytoplasm (Tuschl, T. et al. Genes Dev., 13, 3191-3197 (1999); Hammond, S. M. et al. Nature, 404, 293-296 (2000); Elbashir, S. M. et al. Nature, 411, 494-498 (2001); Hutvagner, G., & Zamore, P. D. Science 297, 2056-2060 (2002); Zeng, Y., & Cullen, B. R. RNA, 8, 855-860 (2002); Kawasaki, H., & Taira, K. Nucleic Acids Res. 31, 700-707 (2003)). It has also been reported that, in plants, dsRNAs can induce sequence-specific DNA methylation and regulate gene expression at the transcriptional level (Pelissier, T., & Wassenegger, M. RNA 6, 55-65 (2000); Mette, M. F. et al. EMBO J. 19, 5194-201 (2000); Vaucheret, H., & Fagard, M. Trends Genet. 17, 29-35 (2001); Jones, L. et al. Curr. Biol. 11, 747-757 (2001); Aufsatz, W. et al. Proc. Natl. Acad. Sci. USA 99, 16499-16506 (2002); Hamilton, A. et al. EMBO J. 21, 4671-4769 (2002); Aufsatz, W. et al. EMBO J. 21, 6832-6841 (2002)). In other words, both long and short dsRNAs can induce sequence-specific DNA methylation, namely RNA-directed DNA methylation (RdDM), in plants. Furthermore, introduced genes can also induce sequence-specific DNA methylation (Vaucheret, H., & Fagard, M. Trends Genet. 17, 29-35 (2001); Morel, J. B. et al. Curr. Biol. 10, 1591-1594 (2000); Beclin, C. et al. Curr. Biol. 12, 684-688 (2002)). These systems have been thought to serve as defense systems against RNA and DNA viruses. However, it was unclear whether such mechanisms existed in mammalian cells. Further studies are required to clarify whether defense systems related to the above-described systems exist in human cells via RdDM or introduced genes.
The DNA methylation of gene promoters has an important role in inducing tumor genesis and development by regulating the expression of specific genes (Li, E. Nature Rev. Genet. 3, 662-673 (2002); Esteller, M. Oncogene 21, 5427-5440 (2002)). Many proteins involved in DNA methylation, such as proteins belonging to the families methyl-CpG binding protein 2 (MeCP2), methyl-CpG-binding domain (MBD), and DNA methyltransferase (DMNT), have been identified and well characterized in human cells. However, it remains unclear as to how and by what these proteins are directed, as well as how the methylatation of specific CpG target sites of cognate genes is induced during tumor genesis or development. Synthetic siRNAs and tRNA vector-based siRNAs are localized predominantly in the cytoplasm, where siRNA-mediated degradation of mRNAs also occurs. One possibility is that a small fraction of siRNA-protein complexes might be transported to the nucleus. Alternatively, siRNAs might gain access to genomic DNAs during cell division, since the nuclear membrane disappears at this time.
The present inventors demonstrated that both synthetic small interfering RNAs (siRNA) and vector-based siRNAs induce sequence-specific DNA methylation in human cells. Synthetic siRNAs (E-cadherin-siRNAs) targeted to CpG islands on E-cadherin promoters induced significant DNA methylation in MCF-7 cells. As a result, these siRNAs suppressed the expression of the E-cadherin gene at the transcriptional level. In addition, E-cadherin-siRNA-directed DNA methylation disappeared on the disruption of DNMT1 by specific siRNAs. Furthermore, vector-based siRNAs targeted to the erbB2 promoter also induced DNA methylation and transcriptional gene silencing in MCF-7 cells. Thus, in human cells, siRNAs targeted to CpG islands on the promoters of genes of interest can induce gene silencing at the transcriptional level by means of DNMT1-dependent or DNMT3b-dependent DNA methylation, and may result in new types of gene therapeutic agents. The present invention is based on the above findings, and specifically includes:
[1] A DNA methylation-inducing agent that comprises a dsRNA targeted to a site comprising CpG or CpNG (where N is any one of A, T, C, and G) in a DNA in a mammalian cell.
[2] A DNA methylation-inducing agent that comprises an expression vector comprising a DNA encoding a dsRNA targeted to a site comprising CpG or CpNG (where N is any one of A, T, C, and G) in a DNA in a mammalian cell.
[3] The DNA methylation-inducing agent of [1] or [2], wherein the dsRNA consists of 30 nucleotides or less.
[4] The DNA methylation-inducing agent of [1] or [2], wherein the dsRNA consists of 30 to 5,000 nucleotides.
[5] The DNA methylation-inducing agent of any one of [1] to [4], wherein the dsRNA comprises a hairpin structure.
[6] The DNA methylation-inducing agent of [2], wherein the vector further encodes Dicer.
[7] The DNA methylation-inducing agent of [2], wherein an exon-intron-exon cassette is operably linked downstream of a promoter and the DNA encoding the dsRNA is inserted into an intron.
[8] The DNA methylation-inducing agent of [7], wherein the exon-intron-exon cassette is derived from an immunoglobulin gene.
[9] The DNA methylation-inducing agent of [2], wherein the vector comprises two promoters, wherein the DNA encoding one strand of the dsRNA is placed downstream of one of the promoters and the DNA encoding the other strand of the dsRNA is placed downstream of the other promoter.
[10] The DNA methylation-inducing agent of [9], wherein the two promoters have the same orientation.
[11] The DNA methylation-inducing agent of [9], wherein the DNA encoding the dsRNA is placed between the two facing promoters.
[12] The DNA methylation-inducing agent of any one of [2] to [11], wherein the DNA encoding the dsRNA is operatively linked to a PolII or PolIII promoter in the expression vector.
[13] The DNA methylation-inducing agent of [12], wherein the PolIII promoter is any one of tRNA promoter, U6 promoter, and H1 promoter.
[14] The DNA methylation-inducing agent of [12], wherein the PolII promoter is any one of SV40 promoter, CMV promoter, and SRa promoter.
[15] The DNA methylation-inducing agent of any one of [2] to [14], wherein the expression vector is a viral expression vector.
[16] The DNA methylation-inducing agent of any one of [2] to [15], wherein the viral expression vector is of retroviral, adenoviral, or lentiviral origin.
[17] The DNA methylation-inducing agent of any one of [2] to [16], wherein the DNA encoding the dsRNA is operatively linked to a promoter comprising a tetracycline operator sequence (TetO) in the expression vector.
[18] A DNA methylation method that comprises the step of introducing the methylation-inducing agent of any one of [1] to [17] into a mammalian cell.
[19] The DNA methylation method of [18], wherein the mammal is human.
[20] A gene expression-suppressing agent that comprises a dsRNA targeted to a site comprising CpG or CpNG (wherein, N is any one of A, T, C, and G) in a gene promoter in a mammalian cell.
[21] A gene expression-suppressing agent that comprises an expression vector encoding a dsRNA targeted to a site comprising CpG or CpNG (where N is any one of A, T, C, and G) in a gene promoter in a mammalian cell.
[22] The gene expression-suppressing agent of [20] or [18], wherein the types of dsRNA differ and the each of the dsRNAs target a different site comprising CpG or CpNG (where N represents any one of A, T, C, and G).
[23] The gene expression-suppressing agent of any one of [20] to [22], wherein the dsRNA consists of 30 nucleotides or less.
[24] The gene expression-suppressing agent of any one of [20] to [22], wherein the dsRNA consists of 30 to 5,000 nucleotides.
[25] The gene expression-suppressing agent of any one of [20] to [24], wherein the dsRNA comprises a hairpin structure.
[26] The gene expression-suppressing agent of [24], wherein the vector further encodes Dicer.
[27] The gene expression-suppressing agent of [21], wherein an exon-intron-exon cassette is operably linked downstream of a promoter and the DNA encoding the dsRNA is inserted into the intron.
[28] The gene expression-suppressing agent of [27], wherein the exon-intron-exon cassette is derived from an immunoglobulin gene.
[29] The gene expression-suppressing agent of [21], wherein the vector comprises two promoters, wherein the DNA encoding one strand of the dsRNA is placed downstream of one of the promoters and the DNA encoding the other strand of the dsRNA is placed downstream of the other promoter.
[30] The gene expression-suppressing agent of [29], wherein the two promoters have the same orientation.
[31] The gene expression-suppressing agent of [29], wherein the DNA encoding the dsRNA is placed between the two facing promoters.
[32] The gene expression-suppressing agent of any one of [21] to [31], wherein the DNA encoding the dsRNA is operatively linked to a PolII or PolIII promoter in the expression vector.
[33] The gene expression-suppressing agent of [32], wherein the PolIII promoter is any one of tRNA promoter, U6 promoter, and H1 promoter.
[34] The gene expression-suppressing agent of [32], wherein the PolII promoter is any one of SV40 promoter, CMV promoter, and SRα promoter.
[35] The gene expression-suppressing agent of any one of [21] to [34], wherein the expression vector is a viral expression vector.
[36] The gene expression-suppressing agent of [35], wherein the viral expression vector is of retroviral, adenoviral, or lentiviral origin.
[37] The gene expression-suppressing agent of any one of [21] to [36], wherein the DNA encoding the dsRNA is operatively linked to a promoter comprising a tetracycline operator sequence (TetO) in the expression vector.
[38] The gene expression-suppressing agent of [20] to [37], wherein the gene is a disease-associated gene whose expression is involved in a disease.
[39] The gene expression-suppressing agent of [38], wherein the disease is a tumor.
[40] The gene expression-suppressing agent of [38], wherein the gene is erbB2.
[41] A cell proliferation-suppressing agent that comprises the gene expression-suppressing agent of [40] as an active ingredient.
[42] A gene expression-suppressing method that comprises the step of introducing the gene expression-suppressing agent of any one of [21] to [40] into a cell.
[43] The gene expression-suppressing method of [42] that further comprises the step of introducing dsRNAs targeted to a coding region of a gene into a cell.
[44] The gene expression-suppressing method of [42] or [43], wherein the cell is derived from a human.
In the first embodiment, the present invention provides methylation-inducing agents that use dsRNA for methylating CpG or CpNG sequences in a sequence-specific manner in mammalian cells. The methylation-inducing agents of the present invention comprise dsRNAs targeted to DNA sites comprising CpG or CpNG sequences in mammalian cells, or vectors encoding the dsRNAs.
In the present invention, the “CpG or CpNG in DNAs” may be any CpG or CpNG sequences selected from those in DNAs to be methylated. Methylation has the function of regulating various biological activities in vivo. For example, methylating CpG sequences or such in promoter regions suppresses downstream gene expression. Thus, in the present invention, CpG or CpNG sequences in the promoter regions of mammalian genes can be preferably selected. Herein, the “N” of CpNG represents any of A, G, C, and T.
The “sites” of the present invention may comprise at least one unit of CpG or CpNG, and preferably, a CpG island comprising two or more CpG units can be selected as the site. By using a site comprising two or more CpG units as a target, two or more methyl groups can be introduced into the site. This can enhance the biological effects of methylation.
The phrase “dsRNAs targeted to” refers to dsRNAs that are complementary to double-stranded DNAs of the above-described CpG- or CpNG-comprising sites. More specifically, the term refers to double-stranded RNAs formed by pairing two single-stranded RNAs complementary to the (+) and (−) strands of an above-described site comprising CpG, where the double-stranded DNAs consist of complementary (+) and (−) strands. The term “targeted dsRNAs” preferably refers to dsRNAs formed by pairing RNA strands each complementary to a double-stranded DNA site comprising CpG or CpNG sequences to which methyl groups are to be introduced. However, as described in the Examples, methyl groups can be also introduced into CpG or CpNG sequences that are near sites corresponding to the dsRNAs. Thus, the phrase “dsRNAs targeted to” also includes not only dsRNAs directly corresponding to the “DNA sites comprising CpG or CpNG sequences” as described above, but also includes dsRNAs complementary to DNA sequences not corresponding to sites to which methyl groups are to be introduced. The distance between the dsRNAs and the DNA sequence sites to which methyl groups are to be introduced may range, for example, from about 50 to 60 nucleotides. In other words, CpG or CpNG sequences as the targets can be methylated by using dsRNAs complementary to a position about 50 to 60 nucleotides away from the site comprising the CpG or CpNG to which the methyl groups are to be introduced. As described below, the target DNA sequences are not necessarily perfectly complementary to the RNA sequences, and either or both of the strands of a dsRNA may contain non-complementary nucleotides.
Any dsRNA constructs used for RNA interference (RNAi) can be used as the “dsRNAs (double-stranded RNAs)” of the present invention. The present inventors found that dsRNAs for RNAi, which have been widely used in recent years, induced DNA methylation in mammalian cells. The present inventors then developed DNA-methylation agents for mammals based on this finding. At present, dsRNAs for RNAi in mammalian cells include both artificially synthesized dsRNAs, and dsRNAs expressed in mammalian cells using expression vectors. Thus, both types of dsRNAs can also be used as the methylation-inducing agents of the present invention.
The cytotoxicity of long dsRNAs to mammalian cells must be considered in cases where artificially synthesized dsRNAs are introduced into mammalian cells. Thus, short dsRNAs are preferably used as the artificially synthesized dsRNAs (hereinbelow referred to as “synthetic dsRNAs”). The short dsRNAs may range, for example, from 15 to 49 base pairs, preferably from 15 to 35 base pairs, and more preferably from 21 to 30 base pairs.
Further, dsRNAs produced from expression vectors (hereinbelow also referred to as “vector-based dsRNAs” or the like) are also preferably short dsRNAs in consideration of cytotoxicity. However, long dsRNAs can be expressed by using constructs designed to produce short dsRNAs by cleavage of the long dsRNAs immediately after expression. The cytotoxicity is caused by enzymes that work to eliminate long dsRNAs when they invade the cytoplasm. Thus, not only short dsRNAs but also long dsRNAs can be used when directly expressed in a nucleus in which genomic DNAs exist. Accordingly, vector-based dsRNAs may range, for example, from 15 to 50,000 base pairs, preferably from 21 to 10,000 base pairs, and more preferably from 30 to 5,000 base pairs. The short dsRNAs described above are preferably used to methylation of specific target CpGs or CpNGs. In contrast, the long dsRNAs are suited to methylate CpGs or CpNGs in a greater range of DNAs.
An example of a construct capable of expressing long dsRNAs using expression vectors, and then cleaving them to short dsRNAs immediately after expression, is a construct for expressing enzymes such as Dicer, which can cleave long dsRNAs to short dsRNAs simultaneously on expression of the long dsRNAs. More specifically, such an expression vector comprises DNA encoding long dsRNAs and the Dicer gene. When both are simultaneously expressed in cells, Dicer can cleave the long dsRNAs to short dsRNAs immediately after their expression. More specific constructs can be prepared with reference to the Examples herein below.
An exemplary construct for using expression vectors to directly express dsRNAs in a nucleus comprising genomic DNAs is a construct in which DNAs encoding dsRNAs are inserted into an intron in an expression cassette comprising an exon-intron-exon. Eukaryotic introns are excised from RNA precursors by processing in the nucleus. Therefore, when expression cassettes comprising the above exon-intron-exon are expressed in mammalian cells, the introns are excised from the RNA precursors in the nucleus, and the dsRNAs inserted into the introns induce methylation at target sites in the genomic DNA, without activating cytoplasmic enzymes and others. In the Examples described below, it was indeed confirmed that long dsRNAs expressed using an expression cassette comprising an exon-intron-exon induced DNA methylation, but produced no cytotoxic response.
An expression cassette comprising an exon-intron-exon may be derived from any gene, as long as the expression products do not have negative effects in cells and do not inhibit the DNA methylation that is an objective of the present invention. The “exon-intron-exon” units that can be used in the present invention include, for example, cdk inhibitor p16 gene, Bcl2 gene, and the like, and preferable examples are exon-intron-exon units derived from immunoglobulins. The exon-intron-exon unit may contain at least one intron flanked by two exons. Units comprising two or more introns, such as exon-(intron-exon-intron)n-exon (wherein, n represents an integer of one or greater) for example, may also be used. By using units comprising introns encoding dsRNAs targeted to different DNA sites, two or more sites can be simultaneously methylated.
Artificially synthesized dsRNAs and dsRNAs expressed using a vector may or may not have a hairpin structure with a loop at one end (sometimes referred to as a “stem-loop structure”).
The two loopless ends of dsRNAs that do not have loop at their ends may be blunt or sticky (protruding) ends. The structures of sticky (protruding) ends include not only the structures with a 3′ protruding end reported for siRNA (Elbashir, S. M et al. Duplexes of 21-nucleotide RNAs mediate RNA interference in cultured mammalian cells. Nature 411, 494-498 (2001); Caplen, N. J. et al. Specific inhibition of gene expression by small double-stranded RNAs in invertebrate and vertebrate systems. Proc Natl Acad Sci USA 98, 9742-9747 (2001)), but also structures with a 5′ protruding end, as long as they induce DNA methylation. The number of protruding nucleotides is not limited to two to three as reported in the above references, and may be any number of nucleotides, as long as an RNAi effect can be induced. The number of nucleotides may range, for example, from one to eight, and preferably from two to four.
The term “dsRNAs having a hairpin structure” refers to dsRNAs comprising a stem-loop structure, which is linked to one end of a double-stranded RNA by a linker RNA or the like. The length of hairpin dsRNAs is based on the double-stranded RNA portion (the stem portion). Thus, when measured as the length of the double-stranded RNA portion (the stem portion), synthetic dsRNAs may range, for example, from 15 to 49 base pairs, preferably from 15 to 35 base pairs, and more preferably from 21 to 30 base pairs; while vector-based dsRNAs may range, for example, from 15 to 50,000 base pairs, preferably from 21 to 10,000 base pairs, and more preferably from 30 to 5,000 base pairs, as described above.
The linker lengths are not limited, as long as they are a length that does not inhibit stem portion pairing. For example, a cloverleaf tRNA structure may be used as a linker portion to stabilize stem portion pairing and to suppress recombination between DNAs encoding the stem portion. Alternatively, even when the length of a linker inhibits stem portion pairing, the construct can comprise a structure such that, for example, an intron is included in the linker portion, the intron is excised during the processing of precursor RNAs to mature RNAs, and stem portion pairing is thus made possible.
The double-stranded RNA portions formed by RNA pairing in dsRNAs include not only perfectly complementary double-stranded RNAs, but also double-stranded RNAs containing unpaired portions such as mismatches (where corresponding nucleotide are not complementary) and bulges (where one strand has no corresponding nucleotides), in their sense RNA strand. A double-stranded RNA may comprise such unpaired portions as long as they inhibit neither the formation of dsRNAs, nor the induction of methylation.
“Bulges” as described above preferably consist of one to two unpaired nucleotides, and the number of bulges varies depending on the length of dsRNAs. For example, for dsRNAs comprising about 30 nucleotides, the sense RNA strands comprise one to seven bulges, and preferably about one to five bulges, within the double-stranded RNA region. The number of the above-described “mismatches” also varies depending on the length of the dsRNA. For example, for dsRNAs comprising about 30 nucleotides, the sense RNA strand comprises one to seven mismatches, and preferably about one to five mismatches, in the double-stranded RNA region. dsRNAs may comprise both bulges and mismatches. In such cases, the number of bulges and mismatches ranges, for example, from one to seven in total, and preferably from about one to five in total.
As described above, the structure of vector-based dsRNAs may or may not have a hairpin structure at one end. When dsRNAs with two loopless ends are expressed using vectors, the vector system may contain two promoters. In an embodiment of such a double-promoter system, DNAs encoding each strand of the dsRNAs (hereinbelow, one strand is called the “sense RNA strand” and the other is called the “antisense RNA strand”) are placed in tandem (in the same orientation) in the vectors, and promoters are arranged upstream of each DNA. In another embodiment of the invention, since the ssDNAs encoding sense- and antisense-RNA strands are obviously complementary to each other, complementary sense and antisense RNAs may be prepared by attaching two promoters, each in an inward direction, to each end of a dsDNA formed by the two ssDNAs, where one promoter uses one strand in the dsDNA as a template, and the other promoter uses the other dsDNA strand as a template. When any of the double-promoter expression vectors for dsRNA is introduced into cells, the respective sense and antisense RNA strands are expressed and form a dsRNA by annealing in the cells. RNAs formed in cells through a double-promoter system may be short or long RNAs consisting of from 15 to 5,000 nucleotides. Promoters such as those described below can be selected appropriately according to the length of the RNAs to be expressed. When long dsRNAs are generated, Dicer or the like may be simultaneously expressed, as required.
By introducing vectors constructed as described above into cells, each of the RNA strands are expressed and annealed to form dsRNAs in the cells.
Meanwhile, expression vectors for hairpin dsRNAs can be constructed by constructing units in which DNAs encoding antisense and sense strands are ligated with inverse orientations via DNAs encoding linker RNAs (linker DNAs). The units comprise “sense RNA strand—loop—antisense RNA strand”, preferably in this order. A promoter can be ligated at one end of such units to construct the expression vectors for hairpin dsRNAs. There is no limitation on the length and sequence of linker DNAs used to construct the expression vectors for hairpin dsRNA, as long as the sequence is not a termination sequence or the like which inhibits the formation of siRNAs, and as long as the sequence and the length of the linker allows pairing of the stem portion of the mature RNAs. For example, the DNAs encoding tRNAs as described above can be used as the linker DNAs.
As the promoters for expressing these dsRNAs, PolII and PolIII promoters can be preferably used. PolIII promoters include, for example, U6 promoters, tRNA promoters, adenovirus VA1 promoter, 5S rRNA promoter, 7SK RNA promoter, 7SL RNA promoter, and H1 RNA promoter. When a U6 promoter is used, four uridine residues are added to the 3′ end of the RNAs; however, the 3′ protruding ends of the siRNAs ultimately produced can be freely adjusted to be four, three, two, one, or zero nucleotides by adding zero, one, two, three, or four adenines to the antisense-coding DNAs and sense-coding DNAs. When other promoters are used, the number of protruding nucleotides can also be freely adjusted by the same procedure.
When PolIII promoters are used, terminators are preferably added at the 3′ ends of sense-coding DNAs and antisense-coding DNAs, to express only short RNAs and appropriately terminate the transcription. The type of terminator is not limited, as long as its sequence allows the termination of transcription driven by the promoters. For example, a sequence comprising four or more consecutive Ts (thymine) or As (adenine), or a sequence that can form a palindromic structure, can be used as the terminator. Sequences comprising four or more consecutive Ts (thymine) are especially preferred.
PolIII promoters include retroviral LTR promoters, cytomegalovirus promoters, T7 promoters, T3 promoters, SP6 promoters, RSV promoters, EF-1α promoters, α-actin promoters, γ-globulin promoters, SRα promoters, and others. These PolII promoters are preferably used to express moderately long dsRNAs. When generating short dsRNAs using a PolII promoter, it is preferable to also use a means for RNA cleavage by self-processing, such as a ribozyme. Antisense or sense RNAs of interest can be produced by the combined use of the means for RNA cleavage by self-processing to trim moderately long RNA strands expressed by the PolII promoter. Stem-loop RNAs can be also generated by inserting a stem-loop sequence immediately downstream of the PolII promoter, and placing a polyA addition signal downstream of the sequence. Such ribozyme-based units for generating antisense or sense RNAs can be constructed, for example, with reference to WO 03/046186. Ribozymes with self-processing activity include hammerhead ribozymes, hairpin ribozymes, HDV ribozymes, and ribozymes derived from Tetrahymena (Biochem. Biophys. Res. Commun., Vol. 186, pp. 1271-1279 (1992); Proc. Natl. Aca d. Sci. USA, Vol. 90, pp. 11302-11306 (1993); BIO medica, Vol. 7, pp. 89-94 (1992); Gene, Vol. 122, pp. 85-90 (1992)).
siRNAs can be expressed at a preferred time by using an inducible promoter as the above-described promoter. Such inducible promoters include tRNA promoters and U6 promoters to which a tetracycline operator (TetO) sequence is added to allow transcriptional induction by tetracycline (Ohkawa, J. & Taira, K. Control of the functional activity of an antisense RNA by a tetracycline-responsive derivative of the human U6 snRNA promoter. Hum Gene Ther. 11, 577-585 (2000);
“Vectors” of the present invention may be selected appropriately according to the type of cells into which the vectors are to be introduced. For example, when mammalian cells are used, the vectors may include, but are not limited to, viral vectors such as retroviral vectors, adenoviral vectors, adeno-associated viral vectors, vaccinia viral vectors, lentiviral vectors, herpes viral vectors, alpha viral vectors, EB viral vectors, papilloma viral vectors, and foamy viral vectors; and non-viral vectors such as cationic liposomes, ligand-DNA complexes, and gene guns (Y. Niitsu et al., Molecular Medicine 35:1385-1395 (1998)). Dumbbell DNAs (Zanta M. A. et al., Gene delivery: a single nuclear localization signal peptide is sufficient to carry DNA to the cell nucleus. Proc Natl Acad Sci USA. 1999 Jan. 5; 96(1):91-6), nuclease-resistant modified DNAs, and naked plasmids can also be preferably used in addition to viral vectors (Liu F, Huang L. Improving plasmid DNA-mediated liver gene transfer by prolonging its retention in the hepatic vasculature. J. Gene Med. 2001 Nov-Dec;3(6):569-76).
Each of the RNA strands of a vector-based dsRNA is not necessarily expressed from the same vector. The dsRNAs may be designed so that the antisense and sense RNAs are expressed from different vectors.
In the second embodiment, the present invention provides methods for methylating DNAs. The methylation methods of the present invention comprise the step of introducing mammalian cells with vectors directing the expression of dsRNAs or dsRNAs that serve as the above-described DNA methylation-inducing agents.
The type of mammalian cell is not particularly limited, and the cells may be derived from humans, monkeys, dogs, mice, rats, rabbits, etc. There is also no particular limitation on the type of method for introducing dsRNAs or vectors into such mammalian cells. The methods can be selected from calcium phosphate methods (Virology, Vol. 52, p. 456 (1973)), electroporation (Nucleic Acids Res., Vol. 15, p. 1311 (1987)), lipofection methods (J. Clin. Biochem. Nutr., Vol. 7, p. 175 (1989)), viral infection-mediated introduction methods (Sci. Am., p. 34, March (1994)), gene gun methods, and others. Methods for introduction into plant cells include electroporation (Nature, Vol. 319, p. 791 (1986)), polyethylene glycol methods (EMBO J., Vol. 3, p. 2717 (1984)), particle gun methods (Proc. Natl. Acad. Sci. USA, Vol. 85, p. 8502 (1988)), Agrobacterium-mediated introduction methods (Nucleic. Acids Res., Vol. 12, p. 8711 (1984)), and the like. Alternatively, for convenience dsRNAs or vectors may be introduced using commercially-available kits, as described in the Examples.
The methylation can be confirmed by methylation-specific PCR analysis or bisulfite sequencing methods, as in the Examples. The specific procedures for these analyses are described in the Examples. When long dsRNAs are used, their cytotoxicity can be evaluated according to elfα activity, which is described in detail in the Examples.
In cells into which the above-described DNA methylation-inducing agents are introduced, DNAs are methylated in a sequence-specific manner. Herein, the term “sequence-specific manner” refers to the methylation of CpG or CpNG in target sites for dsRNAs. The findings in the Examples suggest that histone H3 methylation and DNA methyltransferase 1 are involved in dsRNA-directed DNA methylation.
DNA methylation is a mechanism for controlling various biological activities. Thus, the above-described DNA methylation-inducing agents and the methods for inducing DNA methylation are useful to elucidate in vivo mechanisms regulated by methylation.
In the third embodiment, the present invention provides agents for suppressing gene expression, and gene expression methods using these agents.
DNA methylation is a mechanism for regulating gene expression. Specifically, the methylation of CpG islands in gene promoter regions causes an alteration in DNA conformation, resulting in suppression of gene expression. Thus, gene expression can be suppressed by using the above methylation-inducing agents to methylate CpG islands in gene promoter regions. Unlike siRNAs, which suppress the translation of mRNAs into proteins, the gene expression-suppressing agents of the present invention suppress the transcription of DNAs into mRNAs. Accordingly, the gene expression-suppressing agents of the present invention can induce transcriptional silencing (or gene knockdown at a transcriptional step).
When the above-described methylation-inducing agents are used as gene expression-suppressing agents, dsRNAs, or vectors encoding the dsRNAs, are constructed to target a site comprising a CpG or CpNG (where N is any one of A, T, C, and G) of a gene promoter in a mammalian cell. Furthermore, the greater the degree of methylation by the gene-expression suppressing agents, the more effectively the expression of genes is suppressed. Thus, the number of target sites is preferably not one, but two or more. When the above-described methylation-inducing agents target two or more sites, they preferably use a combination of dsRNAs targeting different sites, and these dsRNAs can be the above-described vector-based long dsRNAs (for example, dsRNAs derived from a co-expression system for long dsRNAs and Dicer, or a system where DNAs encoding dsRNAs are inserted into the intron of an exon-intron-exon) or short dsRNAs.
As described above, the gene expression-suppressing agents of the present invention suppress expression at a transcriptional level. Thus, for more intensive suppression of gene expression, dsRNAs for RNAi may be used in combination with the gene expression-suppressing agents of the present invention. Specifically, gene expression-suppressing agents may be prepared by combining dsRNAs (siRNAs) targeted to a coding region and dsRNAs targeted to CpG islands of a gene promoter region. By using siRNAs combined with the gene expression-suppressing agents of the present invention as described above, the gene expression-suppressing agents of the present invention will suppress gene expression at a transcriptional level, and even if mRNAs are transcribed, the siRNAs will suppress their translation. Thus, strong gene-knockdown agents can be prepared by combining the gene expression-suppressing agents of the present invention with siRNAs that target identical genes.
Genes whose expressions are to be suppressed according to the present invention are not particularly limited, and may be any genes as long as they comprise a CpG or CpNG in their promoter regions. When genes with enhanced expression in a particular phenotype are selected as target “genes” for expression suppression, their expression is suppressed by methylating the gene promoters using the above-described gene-suppressing agents. Thus, by selecting genes involved in a particular phenotype, the gene-suppressing agents of the present invention are useful as reagents for analyzing that particular phenotype, or as tools for preparing knockdown animals, and thus are extremely useful in biochemistry.
When selecting genes with enhanced expression in diseases as the above-described “genes”, the above-described gene expression-suppressing agents are used to suppress their expression by methylation of the gene promoters involved in the diseases. In fact, by selecting a gene associated with tumor genesis, for example, erbB, and then introducing cancer cells with dsRNAs targeted to CpG islands in the erbB promoter, the proliferation of cancer cells can be suppressed by suppressing the expression of erbB. Thus, the gene expression-suppressing agents targeted to cancer-associated genes can be used as reagents for studying diseases such as cancer, or therapeutic agents for cancers in the medical field.
The present inventors found for the first time that synthetic siRNAs and vector-based siRNAs can induce DNMT1-dependent RMDM in a sequence-specific manner in human cells. Thus, these siRNAs can sequence-specifically regulate gene silencing at a transcriptional level.
According to the present invention, the expression level of specific genes in mammalian cells can be regulated by siRNAs not only through disruption of cognate mRNAs, but also by suppression of transcription. The tRNA-shRNAs in the present invention may be useful as therapeutic agents.
Any patents, published patent applications, and publications cited herein are incorporated by reference.
Herein below, the present invention will be specifically described using Examples, however, it is not to be construed as being limited thereto.
The “siRNAs” refer solely to short dsRNAs in all of the Examples described below, and short dsRNAs, such as for the purpose of RNA interference, were used to induce DNA methylation. Thus, unless otherwise specified, herein, siRNAs refer to short dsRNAs used to induce DNA methylation.
The present inventors synthesized siRNA targeted to CpG islands of E-cadherin promoter (E-cadherin-siRNA) to test whether synthetic siRNAs can induce DNA methylation in mammalian cells.
It has been reported that in some tumor cell lines E-cadherin is silenced by aberrant methylation of its promoter (Herman, J. G. et al. Proc. Natl. Acad. Sci. USA. 93, 9821-9826 (1996); Graff, J. R. et al. J. Biol. Chem. 272, 22322-22329 (1997); Corn, P. G. et al. Clin. Cancer Res. 6, 4243-4248 (2000)). The present inventors used MCF-7 cells in the present study because CpG sites in the E-cadherin promoter in MCF-7 cells are not methylated. As a control the inventors used HL-60 cells, whose E-cadherin promoter CpG sites are methylated (Corn, P. G. et al. Clin. Cancer Res. 6, 4243-4248 (2000)). Ten CpG sites in the E-cadherin promoter (sites 1 to 10) were used as siRNA targets (
The target sequences for E-cadherin-siRNA are as follows:
Then, to prepare siRNAs of 25 nucleotides against these target sites, the respective synthetic sense and antisense RNA strands were annealed by a conventional method (Elbashir, S. M. et al. Genes Dev. 15, 188-200 (2001)).
MCF-7 cells were cultured in Dulbecco's modified Eagle medium (DMEM) containing 10% fetal bovine serum (FBS). Each of the above E-cadherin-siRNAs (200 nM) was introduced into the MCF-7 cells using Oligofectamine™ (Invitrogen, CA, USA) according the supplier's protocol. After introducing the siRNAs, the cells were cultured for 96 hours. Then, total DNAs were collected from the cells, and genomic DNAs were isolated from the DNAs.
PCR-based methylation analysis was then carried out. Specifically, the isolated genomic DNAs described above were digested with methylation-sensitive enzymes (HpaI, Acil, AccI, and HhaI;
This analysis is based on the principle that the methylation-sensitive enzymes described above cleave un-methylated CpG islands in promoters, and thus the DNAs are not amplified (Qian, X. et al. Am. J. Pathol. 153, 1475-1482 (1998)). Based on this principle, the methylation of CpG islands in promoters can be detected by detecting the amplified products of this analysis.
In a control experiment, intact genomic DNA of E-cadherin promoter was treated with PstI, which is capable of cleaving E-cadherin promoter, and HindIII, which is incapable of cleaving the promoter. All samples were digested by each of the enzymes in two independent experiments, to exclude the possibility of incomplete digestion. PCR amplification was repeated at least twice for each of the two lots of digestion products. To prevent over-cycling in the PCR reaction, the cycle curve number method was used to determine the number of cycles required for each primer set, using an intact template and the template digested with restriction enzyme PstI (which gives no PCR products).
Primers specific to E-cadherin are listed below.
GAPDH was used as a control gene. After enzymatic treatment with HpaI, Acil, AccI, HhaI, PstI, and HindIII, which are incapable of digesting the GAPDH gene, DNA fragments were amplified by PCR using the specific forward primer ((SEQ ID NO: 63) 5′-GTC TTC ACC ACC ATG GAG AAG GCT-3′) and reverse primer ((SEQ ID NO: 64) 5′-GCC ATC CAC AGT CTT CTG GGT GGC-3′). The amplification levels of partial GAPDH gene in the respective samples were thus found to be comparable, and to fall within the limits of experimental error.
As shown in
The present inventors carried out RT-PCR using primers specific to the E-cadherin promoter to test whether E-cadherin-siRNA-directed DNA methylation had a correlation with expression of the E-cadherin gene.
Total RNAs were isolated from MGF-7 cells using ISOGEN™ (Nippon Gene; Toyama, Japan) according the supplier's protocol. RT-PCR was carried out using RNA PCR Kit ver. 2 (TaKaRa) and the following primers:
The PCR products amplified using the above primers were analyzed by electrophoresis in a 2% agarose gel.
As shown in
In contrast, the E-cadherin mRNA level in MCF-7 cells containing a mixture of all E-cadherin-siRNAs was significantly lower than those in MCF-7 cells to which any one of the siRNAs had been transfected. This suggests the existence of an additive effect (the “sites 1 to 10” columns in the Figure). It is noteworthy that treatment with 5-aza-2′-dC almost completely stopped the activities of E-cadherin-siRNAs (
The level of E-cadherin protein was determined by Western blotting analysis using an antibody specific to E-cadherin.
Western blotting analysis was carried out by previously reported methods (Kawasaki, H. et al. Nature, 393, 284-289 (1998); Kawasaki, H. et al. Nature 405, 195-200 (2000)). MCF-7 cells comprising each of the E-cadherin-siRNAs targeting the E-cadherin promoter were individually harvested. Total proteins (20 μg each) were separated by SDS-PAGE (10% polyacrylamide gel) and then transferred to a polyvinylene difluoride (PVDF) membrane (Funakoshi; Tokyo, Japan) by electroblotting. The resulting immune complexes were visualized with polyclonal antibodies specific to E-cadherin (Santa Cruz; CA, USA) and to actin (UBI; CA, USA) using an ECL kit (Amersham Co.; Buckinghamshire, UK). The relative levels of E-cadherin were normalized based on the actin levels.
As shown in
It is known that DNA methylation in mammalian cells is typically caused by DNA methyltransferase (DNMT) (Bestor, T. H. Human Mol. Genet. 9, 2395-2402 (2000)). The present inventors tried to suppress DNMT gene expression using siRNAs targeted to DNMT mRNA, to examine whether DNMT is involved in siRNA-directed DNA methylation in human cells. DNMT1 is a major DNA methyltransferases in human cells (Rhee, I. et al. Nature 416, 552-556 (2002); Robert, M. F. et al. Nature Genet. 33, 61-65 (2003)). Thus, the present inventors synthesized siRNA targeted to DNMT1 mRNA (Robert, M. F. et al. Nature Genet. 33, 61-65 (2003)) (
The sequences of DNMT1-siRNA and mutant DNMT1-siRNA are shown in
The above-described DNMT1-siRNA and mutant DNMT1-siRNA (200 nM each) were each introduced into MCF-7 cells using Oligofectamine™ by the same procedure as described in Example 2. 96 hours after introduction, total RNAs were collected using ISOGEN™ (Nippon Gene; Toyama, Japan). The levels of DNMT1 mRNA were determined by RT-PCR.
RT-PCR was carried out using RNA PCR Kit ver. 2 (TaKaRa) and the following primers: DNMT1 forward primer ((SEQ ID NO: 25) 5′-ATG GCT CGC GCC AAAACA GTC ATG A-3′) and reverse primer ((SEQ ID NO: 26) 5′-CTC GGG ACT GGG ATC CAT GAG AAT-3′). Control experiments were carried out using GAPDH forward primer ((SEQ ID NO: 27) 5′-ATG GGG AAG GTG AAG GTC GGA GTC-3′) and reverse primer ((SEQ ID NO: 28) 5′-TGG AAT TTG CCA TGG GTG GA-3′) by the same procedure. The resulting PCR products were analyzed by electrophoresis in a 2% agarose gel.
As shown in
The level of DNMT-1 protein in the MCF-7 cells to which DNMT1-siRNA was introduced was also determined by Western blotting analysis using a DNMT1-specific antibody. The resulting data is not shown in the drawings herein. The introduction of DNMT1-siRNA was confirmed to decrease DNMT-1 not only at the mRNA level, but also at the protein level.
Then, a mixture of the above E-cadherin-siRNAs (sites 1 to 10) was introduced into MCF-7 cells containing DNMT1-siRNA to evaluate the influence of the reduced level of DNMT-1 gene expression on siRNA-directed DNA methylation. The siRNAs were introduced using Oligofectamine™. After 96 hours, the total DNAs were collected from the cells, and genomic DNAs were isolated from the DNAs. The degree of methylation in the E-cadherin promoter region was analyzed using the isolated genomic DNAs and the PCR-based analytical method for methylation described in Example 1.
As shown in
Furthermore, whether E-cadherin-siRNA influenced the expression of E-cadherin in MCF-7 cells containing DNMT1-siRNA was also examined. Specifically, DNMT1-siRNA (or the mutant DNMT1-siRNA) was introduced into MCF-7 cells, and then E-cadherin-siRNA was further introduced into the same cells. These siRNAs were introduced using the kit described in Example 1. 96 hours after introduction of E-cadherin-siRNA, total RNAs were collected from the MCF-7 cells and the levels of E-cadherin mRNA were determined by RT-PCR. The RT-PCR was carried out by the procedure described in Example 2.
As shown in
The erbB2 gene is known to be over-expressed and also unmethylated in some tumor cell lines such as MCF-7 cell line. Meanwhile, the erbB2 gene is silenced upon methylation of its promoter in various normal cell lines (Hattori, M. et al. Cancer Lett. 169, 155-164 (2001)). The present inventors constructed expression vectors for short hairpin RNA (shRNA) targeted to the above erbB2 promoter, taking into consideration future gene therapy by regulating DNA methylation using siRNA.
The present inventors have previously demonstrated that shRNA generated by tRNAval induces siRNA-directed gene silencing in human cells (Kawasaki, H., & Taira, K. Nucleic Acids Res. 31, 700-707 (2003)). Hence, the inventors constructed an expression vector for tRNA-shRNA targeted to the erbB2 promoter using pPUR-tRNA plasmid which comprises a pPUR (Clontech, CA, USA) backbone and a synthetic human gene promoter for tRNAval inserted between the EcoRI and BamHI sites (Kawasaki, H., & Taira, K. Nucleic Acids Res. 31, 700-707 (2003)). Five CpG islands (sites 1 to 5) in the erbB2 promoter were selected as the target sequences for tRNA-shRNA targeted to the erbB2 promoter. The target sequences (sites 1 to 5) are shown below.
A synthetic oligonucleotide encoding dsRNA, which is targeted to the above-described erbB2 promoter and comprises a loop motif, was prepared as a double-stranded sequence by PCR. After digestion with SacI and KpnI, the resulting fragment was cloned into the above-described pPUR-tRNA, downstream of the tRNA gene promoter.
Then, whether shRNAs of interest were generated from the expression plasmids for shRNA in MCF-7 cells was confirmed experimentally. The shRNA expression plasmids were introduced transiently into MCF-7 cells by the same method described in Example 1. 72 hours after introduction, total RNAs were extracted and analyzed by Northern blotting. The results showed that proper processing had occurred to generate shRNA in cells expressing tRNA-shRNA (
It was then tested whether the degree of DNA methylation in the erbB2 promoter was altered by the shRNA generated using each of the expression vectors as described above. This analysis was carried out by the PCR-based method for analyzing methylation, as described in Example 2. Specifically, each pPUR-tRNA-shRNA expression vector was introduced into MCF-7 cells using Effectin™ reagent (QIAGEN, Hiddeln, Germany) according to the attached supplier's protocol. 96 hours after introduction, genomic DNAs were collected from the cells, and the degree of methylation in the erbB2 promoter was determined.
The genomic DNA (500 ng) was digested with the methylation-sensitive enzymes, HpaI and HhaI (
As in Example 1, all samples were digested by each of the enzymes in two independent experiments, to exclude the possibility of incomplete digestion. PCR amplification was repeated at least twice for each of the two lots of digestion products. To prevent over-cycling in the PCR reaction, the cycle curve number method was used to determine the number of cycles required for each primer set, using an intact template and the template digested with restriction enzyme PstI (which gives no PCR products). Primers specific to earbB2 promoter are listed below:
GAPDH was used as a control gene. After treatment with HpaI, Acil, AccI, HhaI, PstI, and HindIII, which are incapable of cleaving the gene, the fragment DNAs were amplified by PCR using a specific forward primer ((SEQ ID NO: 63) 5′-GTC TTC ACC ACC ATG GAG AAG GCT-3′) and reverse primer ((SEQ ID NO: 64) 5′-GCC ATC CAC AGT CTT CTG GGT GGC-3′). The levels of amplified partial GAPDH gene in the respective samples were found to be comparable, and to fall within the limits of experimental error.
As shown in
Then, the present inventors used RT-PCR to determine the level of erbB2 mRNA to test whether the vector-based siRNA suppressed erbB2 gene expression. RT-PCR was carried out by the same procedure as described in Example 2, except that an erbB2 forward primer ((SEQ ID NO: 23)5′-ATG GAG CTG GCG GCC TTG TGC CGC-3′) and reverse primer ((SEQ ID NO: 24)5′-TTG TTC TTG TGG AAG ATG TCC TTC C-3′) were used.
As shown in
A tRNA-shRNA targeted to the erbB2 gene-encoding region (site 6) was transfected in combination with all the above tRNA-shRNAs (1 to 5 sites) to MCF-7 cells and then RT-PCR was carried out by the same procedures as described above. As a result, the expression of the target gene was suppressed more efficiently, as shown in
Furthermore, the present inventors investigated the growth rates of various cell lines to assess the phenotypes of cells expressing tRNA-shRNA targeted to erbB2 gene promoter. The growth rate of each cell line was determined using a Cell Proliferation Kit II (Roche Ltd., Switzerland) according to the supplier's instructions (Kawasaki, H., & Taira, K. Nucleic Acids Res. 31, 700-707 (2003)).
As shown in
Sequencing is carried out to confirm the sequences of constructed expression vectors for the hairpin (stem-loop) RNA molecules described in this Example; however, since the stem-loop structures are rigid the sequences are difficult to analyze. Even when the structure of the stem-loop RNA molecules is changed to one for ease of sequence analysis, by introducing one to ten mismatch sequences or bulges into the sense strand of the stem sequences, the molecules still exhibit activity comparable to that of a perfectly complementary stem-loop RNA molecule with respect to DNA methylation or RNAi-mediated suppression of target genes (data not shown).
The expression of long (30 nucleotides or more) hairpin (or stem-loop) RNA molecules (“long hRNAs”) results in PKR activation and non-specific translation suppression in animal cells. Accordingly, to induce RNAi effect using hRNAs, stem-loop RNA molecules comprising 30 nucleotides or less must be prepared. Meanwhile, since the RNAi effect depends on the sequence of a target gene, many types of shRNAs should be prepared in order to select effective target sequences.
Thus, the present inventors devised a co-expression system for long hRNAs (25 to 500 nucleotides) and Dicer gene. Specifically, a vector was constructed to have both a Dicer expression cassette comprising Dicer gene linked downstream of a PolII promoter, and an HRNA expression cassette comprising short stem-loop RNA molecule encoding DNA linked downstream of a PolIII promoter, and this was then introduced into cells. Firstly, the expression-suppressing effect produced by the co-expression of Dicer gene and long hRNAs targeted to the coding region of erbB2 gene was found to be stronger than the effect obtained using conventional hRNAs (
Furthermore, eIF2α phosphorylation was used as an indicator of PKR activation to analyze whether PKR was activated in this system. Specifically, Western blotting was carried out using an antibody which specifically recognizes phosphorylated eIF2α, and the amount of phosphorylated eIF2α relative to that of the internal control, β-actin, was estimated using the NIH Image program.
eIF2α phosphorylation was hardly observed in the results (
This suggests that, in this system, Dicer was expressed and the co-expressed long stem-loop RNAs were rapidly processed to siRNAs in cells. Furthermore, PKR activation can be prevented more effectively by introducing mismatch sequences (G-T pairs and the like) and bulges (two to ten nucleotides) into the sense strands of the stem sequence at 1 to 100 positions.
In the Examples as described above, the sequence-specific methylation of a target gene promoter region, which was mediated by synthetic siRNAs or siRNAs expressed from an expression vector, was analyzed by methylation-specific PCR. In this Example, the present inventors used bisulfite sequencing, which can confirm siRNA-directed methylation at a sequence level. The bisulfite reagent used in the analysis converts unmethylated cytosine, but not methylated cytosine, to uracil. Accordingly, the methylation of cytosine can be assessed by recovering DNAs from cells into which siRNAs or the like have been introduced, reacting the DNAs with the bisulfite reagent, analyzing their sequences, and confirming the presence or absence of substituted uracil. Treatment with the bisulfite reagent was carried out using a CpGenome DNA-modification kit (Intergen; USA).
Specifically, as described in Example 1, synthetic siRNAs (siRNAs each corresponding to sites 1 to 10) targeted to E-cadherin promoter were each introduced into human breast cancer cells (MCF-7 cells) using Oligofectamine (Invitrogen; USA). The genomic DNAs were recovered 96 hours after introduction and treated with bisulfite reagent using a CpGenome DNA-modification kit (Intergen; USA). After incubation with the bisulfite reagent, the promoter DNA was amplified by PCR. The amplified promoter DNA was inserted into a plasmid vector using a TA cloning kit (Invitrogen; USA). The resulting plasmid vector was introduced into Escherichia coli for cloning. The cloned plasmids were sequenced to monitor methylation frequency.
Some results are shown in
The above-described experiments using the bisulfite sequencing assay showed that synthetic siRNAs targeted to E-cadherin promoter induced sequence-specific methylation.
In addition, the bisulfite sequencing assay was also used to test whether DNA methylation in the erbB2 promoter was induced in a site-specific manner by the tRNA promoter-based expression vector for siRNA. The introduction of the expression vector and recovering of genomic DNAs were achieved by the same procedures as the above-described Examples.
The bisulfite sequencing assay showed that, like the synthetic siRNAs, the siRNAs expressed by the tRNA promoter induced methylation in a sequence-specific manner (
siRNAs are known to induce methylation of Lys9 on histone H3 in plants and Schizosaccharomyces (Zilberman D, Cao X, Jacobsen SE.ARGONAUTE4 control of locus-specific siRNA accumulation and DNA and histone methylation. Science. 2003 Jan. 31; 299(5607):716-9; Volpe T A, Kidner C, Hall I M, Teng G, Grewal S I, Martienssen RA.Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi.Science. 2002 Sep. 13; 297(5588):1833-7).
Thus, whether the above siRNAs targeted to E-cadherin or erbB2 promoter also induced the methylation of Lys9 on histone H3 in animal cells was also tested.
A mixture of siRNAs, each targeted to sites 1 to 10 of E-cadherin promoter, was introduced into MCF-7 cells. After 96 hours, the cells were harvested, treated with 1% formaldehyde, and then lysed in a lysis buffer. An antibody against methylated Lys9 of histone H3 was added to the lysate and gently mixed for eight hours. Protein A Sepharose was added to the mixture for the pull-down reaction. After elution with 200 μl of a solution containing 1% SDS and 0.1 M NaHCO3, the eluted material was cross-linked using formaldehyde. Then, PCR was carried out using primers specific to E-cadherin promoter. The amplified products were electrophoresed and the resulting bands were stained using ethidium bromide for detection.
The above-described chromatin immunoprecipitation (ChIP) experiments using antibody against methylated Lys9 of histone H3 revealed that the methylation of Lys9 on histone H3 is induced by the siRNAs in a sequence-specific manner (
The experiments described herein showed that synthetic siRNAs and siRNAs expressed by tRNA promoters or U6 promoters suppress the expression of target genes at a transcriptional level by inducing the methylation of CpG and CpNG sequences in a sequence-specific manner. However, the siRNAs used in the experiments consisted of 21 to 30 nucleotides, and thus can be predicted to induce partial methylation resulting in only partial suppression of target gene expression at a transcriptional level. In general, CpG islands in gene promoter regions range from approximately 1 kbp to several kbp in length. Various siRNAs covering the entire region are required to achieve efficient suppression of target gene expression at the transcriptional level. However, it is inefficient to construct many types of synthetic siRNAs or siRNA expression vectors.
Long dsRNAs commonly enhance non-specific suppression of gene expression by activating interferon-mediated signaling pathways. However, there are reports that long dsRNAs localized in the nucleus do not activate interferon-mediated signaling pathways.
Accordingly, to localize long dsRNAs in the nucleus, the present inventors conceived of inserting genes encoding long dsRNAs into beta globulin introns whose expression is driven by a PolII promoter (
In fact, the present inventors constructed a vector pSV-BGI, in which a gene encoding a dsRNA consisting of 500 nucleotides targeted to erbB2 promoter was inserted into a beta globulin intron whose expression is driven by PolII promoter. In addition, they also constructed vectors in which the gene encoding dsRNA was inserted downstream of an expression vector for tRNA promotor or U6 promoter. Each vector was introduced into MCF-7 cells. 96 hours after introduction, genomic DNAs were recovered and analyzed for methylation in the erbB2 promoter region using the method described in Example 4.
The results showed that pSV-BGI induced more efficient and more extensive methylation than the tRNA promoter and U6 promoter (
Western blotting was carried out using an antibody that specifically recognized erbB2 protein. Then, the amount of phosphorylated eIF2α relative to that of the internal control, β-actin, was estimated using the NIH Image program. It was revealed that erbB2 gene expression was markedly suppressed as a result of the methylation described above (
The cytotoxicity caused by the introduction of each vector was also evaluated. The cytotoxicity of dsRNAs can be assessed based on the degree of eIF2α phosphorylation by dsRNA-dependent protein kinase (PKR). Thus, Western blotting was carried out using an antibody that specifically recognized phosphorylated eIF2α, and the amount of phosphorylated eIF2α relative to that of the internal control β-actin was estimated using the NIH Image program.
It was found that the vector pSV-BGI was not cytotoxic, while both the expression vectors for tRNA promoter- and U6 promoter-based expression vectors exhibited cytotoxicity (
The results described in Example 8 showed that the vector (pSV-BGI), in which a long dsRNA had been inserted into a beta globulin intron expressed under the control of the PolII promoter, caused transcriptional gene silencing by efficiently and extensively inducing methylation. Furthermore, vector systems suited to expressing long dsRNAs were also developed. When a dsRNA expression system is used, sense-strand and antisense-strand RNAs are transcribed under the control of two independent adjacent promoters placed in the same gene locus (
To demonstrate that the above-described system functions, a DNA fragment encoding a dsRNA comprising about 540 nucleotides targeted to erbB2 promoter was amplified by PCR using the primers of SEQ ID NOs: 31 and 32, and then subcloned into pDP (pDP-erbB2 vector). pDP-erbB2 vector was introduced into MCF-7 cells using Effectene™ reagent (QIAGEN, Hiddeln, Germany) according to the attached supplier's protocol. 96 hours after introduction, genomic DNAs were extracted from the cells to determine the degree of methylation in the erbB2 promoter.
The genomic DNAs (500 ng) were digested with the methylation-sensitive enzymes HpaI, HhaI, and AccII (
Specifically, the primer sequences are as follows:
The results showed that the introduction of each pDP-erbB2 vector caused methylation of erbB2 promoter in the cells (
| Number | Date | Country | |
|---|---|---|---|
| 60449860 | Feb 2003 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP04/02448 | Feb 2004 | US |
| Child | 11209044 | Aug 2005 | US |
