Patent Application
20210301269
Type II CRISPR-Cas9 enzymes are RNA-programmable endonucleases that have been used in diverse DNA-targeting applications, including gene knock-out and knock-in, mutagenesis, gene activation and inhibition, base editing, and CpG methylation (Jinek et al., 2012). Cas9 enzymes, including the most commonly used S. pyogenes Cas9 (Cas9), recognize target DNA sequences that are complementary to their guide RNA spacer and that contain a protospacer adjacent motif (PAM). Although mismatches between the target DNA and a portion of the guide RNA can be tolerated, the presence of the PAM is a strict requirement, which imposes a limit on the number of targetable genomic loci (Hsu et al., 2013). While the availability of PAM sites (such as NGG for Cas9) is typically not a problem for CRISPR-mediated gene knockout because nearly all protein-coding exons can be targeted (Meier et al., 2017), the optimal targeting space for transcriptional modulation (inhibition or activation) is usually smaller, between 50 and 100 nucleotides (Sanson et al., 2018). Other common genome editing tasks, such as homology-directed repair or base editing, require an even narrower window for Cas9 positioning, with the desired target site placed at a precise position from the PAM sequence (e.g. 10-20 nucleotides for homology-directed repair or 13-17 nucleotides for base editing) (Findlay et al., 2014; Komor et al., 2016).
To address this problem, several Cas9 orthologs and other CRISPR nucleases from different bacterial species have been characterized, such as S. aureus Cas9 and Cas12a/Cpf1 (Kim et al., 2016; Ran et al., 2015). However, none of them have a simpler PAM requirement than Cas9. Initial attempts at developing more PAM-flexible Cas9 variants through structure-based design or directed evolution yielded enzymes recognizing slightly altered PAMs but still requiring a three-nucleotide motif (Kleinstiver et al., 2015). Recently, two Cas9 variants capable of recognizing an NG PAM were generated, one through phage-assisted continuous evolution (xCas9), and the other through structure-guided design (Cas9-NG) (Hu et al., 2018; Nishimasu et al., 2018). These Cas9 variants were characterized primarily in terms of their nuclease activity at several endogenous genomic loci, and their relative performance at NG sites was highly variable. One of these PAM-flexible variants, xCas9, led to superior CRISPR activation (CRISPRa) when fused to VP64-p65-Rta (VPR) over wild-type dCas9-VPR, with higher transcriptional activation for all sgRNAs tested. This is presumably due to the directed evolution selection pressure—transcriptional activation and not nuclease activity—used to derive xCas9.
What is needed are Cas9 variants with altered PAM specificity.
Provided herein in a first aspect, is a recombinant or engineered Cas9 protein. The Cas9 protein has an amino acid sequence that is at least 90% identical to the amino acid sequence of SEQ ID NO: 2. The Cas9 protein has at least one mutation in an amino acid residue selected from 262, 324, 409, 480, 543, 694, of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence, and at least one mutation in an amino acid residue selected from 1111, 1135, 1218, 1219, 1322, 1335, and 1337, of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. The amino acid sequence of the recombinant Cas9 protein is not identical to the amino acid sequence of a naturally occurring Cas9 protein.
In one embodiment, the mutations are selected from X262T, X324L, X4091, X480K, X543D, X694I, X1111R, X1135V, X1218R, X1219F, X1219V, X1322R, X1335V, and X1337R of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence, wherein X represents any amino acid. In another embodiment, the mutations are selected from A262T, R324L, S409I, E480K, E543D, M694I, L1111R, D1135V, G1218R, E1219F, E1219V, A1322R, R1335V, and T1337R of the amino acid sequence provided in SEQ ID NO: 2. In one embodiment, the Cas9 protein has the sequence of SEQ ID NO: 1.
In another aspect, a fusion protein is provided. The fusion protein includes a recombinant Cas9 protein as described herein, fused to a heterologous functional domain, with an optional intervening linker, wherein the linker does not interfere with activity of the fusion protein.
In another aspect, a nucleic acid encoding a Cas9 protein or fusion protein as described herein, is provided. In yet another aspect, a vector comprising a nucleic acid encoding a Cas9 protein or fusion protein as described herein, is provided. In another aspect, a host cell comprising nucleic acid encoding a Cas9 protein or fusion protein as described herein, is provided.
In another aspect, a method of altering the genome of a cell is provided. The method includes expressing in the cell, or contacting the cell with, the recombinant Cas9 protein or fusion protein of as described herein, and a guide RNA having a region complementary to a selected portion of the genome of the cell.
In yet another aspect, a method of evaluating a CRISPR-Cas system is provided. The method includes a) obtaining a sgRNA library comprising multiple sgRNA sequences which target sites in the genome, b) cloning said library into a lentiviral plasmid comprising a nucleic acid sequence encoding a Cas protein and, optionally, a barcode, c) producing lentivirus containing said plasmid, d) transducing mammalian cells with said lentivirus, e) culturing said cells for a sufficient time period, and f) evaluating said cells for CRISPR activity.
Other aspects and advantages of the invention will be readily apparent from the following detailed description of the invention.
A key limitation of the commonly-used CRISPR enzyme S. pyogenes Cas9 is the strict requirement of an NGG protospacer-adjacent motif (PAM) at the target site, which reduces the number of accessible genomic loci. This constraint can be limiting for genome editing applications that require precise Cas9 positioning. Recently, two Cas9 variants with a relaxed PAM requirement (NG) have been developed (xCas9 and Cas9-6NG) but their activity has been measured at only a small number of endogenous sites. See, US Patent Publication No. 2019/0106687 and US Patent Publication No. 2019/0225955, both of which are incorporated herein by reference.
Given the utility of PAM-flexible Cas9 enzymes for precise genome engineering, we designed an unbiased, massively-parallel competition assay to compare Cas9 enzyme variants at thousands of target sites in the human genome. We benchmarked both PAM-flexible enzymes head-to-head with Cas9 for nuclease-driven loss-of-function, gene activation and gene repression. Across all 3 modalities, we found that PAM flexibility comes at the cost of markedly lower activity. Wild-type Cas9 outperformed both PAM-flexible variants at NGG sites for every modality tested. At NGH PAMs (H=A, C or T), we found that Cas9-NG is universally better than xCas9 and that xCas9 is often indistinguishable from the wild-type enzyme.
We were able to partially rescue xCas9 nuclease activity by adding Cas9-NG mutations to create a new Cas9 variant, xCas9-NG. For gene activation, we found that xCas9-NG outperforms both xCas9 and Cas9-NG at both NGG and NGH PAMs. We expect that this novel PAM-flexible Cas9 will be useful for a multitude of genome-engineering applications where precise Cas9 positioning is required.
Recombinant Cas9 Variants with Altered PAM Specificities
Provided herein are recombinant or engineered Cas9 variants. The Cas9 variants described herein greatly increase the range of target sites accessible by wild-type Cas9. The Streptococcus pyogenes (sp) Cas9 wild type sequence is as follows, and is set forth in SEQ ID NO: 2 (Uniprot Q99ZW2-1):
In one aspect, the recombinant Cas9 protein (also called Cas9 variant) has a variation in at least one amino acid selected from residues 262, 324, 409, 480, 543, 694, of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence, and at least one mutation in an amino acid residue selected from 1111, 1135, 1218, 1219, 1322, 1335, and 1337 of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. The recombinant or engineered Cas9 protein is not identical to the amino acid sequence of a naturally occurring Cas9 protein.
In some embodiments, the Cas9 variant is at least 80%, e.g., at least 85%, 90%, 95%, 96%, 97%, 98% or 99% identical to the amino acid sequence of SEQ ID NO: 2, e.g., has differences at up to 1%, 2,%, 3%, 4%, 5%, 10%, 15%, or 20% of the residues of SEQ ID NO: 2, replaced, e.g., with conservative mutations. In one embodiment, the Cas9 protein is at least 95% identical to SEQ ID NO: 2. In preferred embodiments, the variant retains desired activity of the parent, e.g., the nuclease activity (except where the parent is a nickase or a dead Cas9), and/or the ability to interact with a guide RNA and target DNA), although not necessarily at the same level.
To determine the percent identity of two amino acid sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in one or both of a first and a second amino acid or nucleic acid sequence for optimal alignment and non-homologous sequences can be disregarded for comparison purposes). The length of a reference sequence aligned for comparison purposes is at least 80% of the length of the reference sequence, and in some embodiments is at least 90% or 100%. The residues at corresponding amino acid positions are then compared. When a position in the first sequence is occupied by the same residue as the corresponding position in the second sequence, then the molecules are identical at that position (as used herein amino acid “identity” is equivalent to amino acid “homology”). The percent identity between the two sequences is a function of the number of identical positions shared by the sequences, taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences. Percent identity between two polypeptides or nucleic acid sequences is determined in various ways that are within the skill in the art, for instance, using publicly available computer software such as Smith Waterman Alignment (Smith, T. F. and M. S. Waterman (1981) J Mol Biol 147:195-7); “BestFit” (Smith and Waterman, Advances in Applied Mathematics, 482-489 (1981)) as incorporated into GeneMatcher Plus™, Schwarz and Dayhof (1979) Atlas of Protein Sequence and Structure, Dayhof, M. O., Ed, pp 353-358; BLAST program (Basic Local Alignment Search Tool; (Altschul, S. F., W. Gish, et al. (1990) J Mol Biol 215: 403-10), BLAST-2, BLAST-P, BLAST-N, BLAST-X, WU-BLAST-2, ALIGN, ALIGN-2, CLUSTAL, or Megalign (DNASTAR) software. In addition, those skilled in the art can determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the length of the sequences being compared. In general, for proteins or nucleic acids, the length of comparison can be any length, up to and including full length (e.g., 5%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95%, or 100%). For purposes of the present compositions and methods, at least 80% of the full length of the sequence is aligned using the BLAST algorithm and the default parameters.
In some embodiments, the SpCas9 variants include mutations at one, two, three, four, five, or all six of the following positions: 262, 324, 409, 480, 543, and 694 of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. In some embodiments, the SpCas9 variants include mutations at one, two, three, four, five, six, or all seven of the following positions: 1111, 1135, 1218, 1219, 1322, 1335, and 1337 of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. In some embodiments, the SpCas9 variants include mutations at two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, or all thirteen of the following positions: 262, 324, 409, 480, 543, 694, 1111, 1135, 1218, 1219, 1322, 1335, and 1337 of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence.
In some embodiments, the mutations are selected from X262T, X324L, X4091, X480K, X543D, X694I, X1111R, X1135V, X1218R, X1219F, X1219V, X1322R, X1335V, and X1337R of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence, wherein X represents any amino acid. However, substitutions for the mutated residue may be selected, especially amongst conservative residues. Conservative substitutions typically include substitutions within the following groups: glycine, alanine; valine, isoleucine, leucine; aspartic acid, glutamic acid, asparagine, glutamine; serine, threonine; lysine, arginine; and phenylalanine, tyrosine. For example X262T or X262F.
In another embodiment, the mutations are selected from A262T, R324L, S409I, E480K, E543D, M694I, L1111R, D1135V, G1218R, E1219V, A1322R, R1335V, and T1337R of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. In another embodiment, the mutations are selected from A262T, R324L, S409I, E480K, E543D, M694I, L1111R, D1135V, G1218R, E1219F, A1322R, R1335V, and T1337R of the amino acid sequence provided in SEQ ID NO: 2 or the corresponding residue of an aligned sequence. In one embodiment, the Cas9 protein has the sequence of SEQ ID NO: 1, sometimes referred to herein as xCas9-NG. In another embodiment, the Cas9 protein has the sequence of SEQ ID NO: 3.
The Cas9 proteins exemplified herein are derived from S. pyogenes (Sp), which wild type sequence can be found in SEQ ID NO: 2. This wild type sequence is used herein, for simplicity as the base sequence on which the variants are described. However, all of the Cas9 variants described herein can be utilized with other previously described improvements to the Cas9 platform (e.g., truncated sgRNAs (Tsai et al., Nat Biotechnol 33, 187-197 (2015); Fu et al., Nat Biotechnol 32, 279-284 (2014)), nickase mutations (Mali et al., Nat Biotechnol 31, 833-838 (2013); Ran et al., Cell 154, 1380-1389 (2013)), dimeric FokI-dCas9 fusions (Guilinger et al., Nat Biotechnol 32, 577-582 (2014); Tsai et al., Nat Biotechnol 32, 569-576 (2014)); and high-fidelity variants (Kleinstiver et al. Nature 2016). Each of these documents is incorporated herein by reference. That is, in one embodiment, the starting Cas9 is a variant from the wild type Cas9 shown in SEQ ID NO: 2.
In some embodiments, in addition to the mutations described above, the Cas9 variants also include mutations at one of the following amino acid positions, which reduce or destroy the nuclease activity of the Cas9: D10, E762, D839, H983, or D986 and H840 or N863, e.g., D10A/D10N and H840A/H840N/H840Y, to render the nuclease portion of the protein catalytically inactive (also referred to as dead Cas9 or dCas9)(SEQ ID NO:5). Substitutions at these positions could be alanine (as they are in Nishimasu al., Cell 156, 935-949 (2014), which is incorporated by reference herein), or other residues, e.g., glutamine, asparagine, tyrosine, serine, or aspartate, e.g., E762Q, H983N, H983Y, D986N, N863D, N863S, or N863H (see WO 2014/152432). In some embodiments, the variant includes mutations at D10A or H840A (which creates a single-strand nickase). The sequence of Cas9D10A is shown in SEQ ID NO: 4. In one embodiment, the Cas9 variant has at least one mutation in a residue selected from 262, 324, 409, 480, 543, 694, of the amino acid sequence provided in SEQ ID NO: 4, and at least one mutation in an amino acid residue selected from 1111, 1135, 1218, 1219, 1322, 1335, and 1337 of the amino acid sequence provided in SEQ ID NO: 4. In another embodiment, the Cas9 variant has at least one mutation in a residue selected from 262, 324, 409, 480, 543, 694, of the amino acid sequence provided in SEQ ID NO: 4, and at least one mutation in an amino acid residue selected from 1111, 1135, 1218, 1219, 1322, 1335, and 1337 of the amino acid sequence provided in SEQ ID NO: 4.
Also provided herein are isolated nucleic acids encoding the Cas9 variants, vectors comprising the isolated nucleic acids, optionally operably linked to one or more regulatory domains for expressing the variant proteins, and host cells, e.g., mammalian host cells, comprising the nucleic acids, and optionally expressing the variant proteins.
The variants described herein can be used for altering the genome of a cell; the methods generally include expressing the variant proteins in the cells, along with a guide RNA having a region complementary to a selected portion of the genome of the cell. Methods for selectively altering the genome of a cell are known in the art, see, e.g., U.S. Pat. No. 8,697,359; US2010/0076057; US2011/0189776; US2011/0223638; US2013/0130248; WO/2008/108989; WO/2010/054108; WO/2012/164565; WO/2013/098244; WO/2013/176772; US20150050699; US20150045546; US20150031134; US20150024500; US20140377868; US20140357530; US20140349400; US20140335620; US20140335063; US20140315985; US20140310830; US20140310828; US20140309487; US20140304853; US20140298547; US20140295556; US20140294773; US20140287938; US20140273234; US20140273232; US20140273231; US20140273230; US20140271987; US20140256046; US20140248702; US20140242702; US20140242700; US20140242699; US20140242664; US20140234972; US20140227787; US20140212869; US20140201857; US20140199767; US20140189896; US20140186958; US20140186919; US20140186843; US20140179770; US20140179006; US20140170753; Makarova et al., “Evolution and classification of the CRISPR-Cas systems” 9(6) Nature Reviews Microbiology 467-477 (1-23) (June 2011); Wiedenheft et al., “RNA-guided genetic silencing systems in bacteria and archaea” 482 Nature 331-338 (Feb. 16, 2012); Gasiunas et al., “Cas9-crRNA ribonucleoprotein complex mediates specific DNA cleavage for adaptive immunity in bacteria” 109(39) Proceedings of the National Academy of Sciences USA E2579-E2586 (Sep. 4, 2012); Jinek et al., “A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity” 337 Science 816-821 (Aug. 17, 2012); Carroll, “A CRISPR Approach to Gene Targeting” 20(9) Molecular Therapy 1658-1660 (September 2012); U.S. Appl. No. 61/652,086, filed May 25, 2012; Al-Attar et al., Clustered Regularly Interspaced Short Palindromic Repeats (CRISPRs): The Hallmark of an Ingenious Antiviral Defense Mechanism in Prokaryotes, Biol Chem. (2011) vol. 392, Issue 4, pp. 277-289; Hale et al., Essential Features and Rational Design of CRISPR RNAs That Function With the Cas RAMP Module Complex to Cleave RNAs, Molecular Cell, (2012) vol. 45, Issue 3, 292-302.
The variant proteins described herein can be used in place of the Cas9 proteins described in the foregoing references with guide RNAs that target sequences that have the following PAM sequences: NGG and NGH, where N is A, G, C, or T, and where H is A, C, or T. As described herein, xCas9-NG has been shown to outperform previously described variants xCas9 and Cas9-NG at both NGG and NGH PAMs. In one embodiment, the PAM has the following sequence: AGG, GGG, CGG, TGG, AGA, GGA, CGA, TGA, AGC, GGC, CGC, TGC, AGT, GGT, CGT, or TGT.
Also provided herein are fusion proteins comprising the Cas9 variants described herein. In one embodiment, the Cas9 protein is fused to a heterologous functional domain, with an optional intervening linker, wherein the linker does not interfere with activity of the fusion protein. The variants described herein can be used in fusion proteins in place of the wild-type Cas9 or other Cas9 mutations (such as the dCas9 or Cas9 nickase described above) as known in the art, e.g., a fusion protein with a heterologous functional domain as described in WO 2014/124284. For example, the N or C terminus of the Cas9 variant can be fused to a heterologous functional domain.
In one embodiment, the heterologous functional domain is a transcriptional activation domain. Transcriptional activation domains include VP16, VP64, rTA, NF-κB p65, or the composite VPR (VP64-p65-rTA). In another embodiment, the functional domain is a transcriptional silencer or transcriptional repression domain. Transcriptional repression domains include Krueppel-associated box (KRAB) domain, ERF repressor domain (ERD), and mSin3A interaction domain (SID), and others, e.g., amino acids 473-530 of the ets2 repressor factor (ERF) repressor domain (ERD), amino acids 1-97 of the KRAB domain of KOX1, or amino acids 1-36 of the Mad mSIN3 interaction domain (SID); see Beerli et al., PNAS USA 95:14628-14633 (1998), which is incorporated herein by reference. Transcriptional silencers include Heterochromatin Protein 1 (HP1, also known as swi6), e.g., HP1α or HP1β. Other heterologous functional domains include proteins or peptides that could recruit long non-coding RNAs (lncRNAs) fused to a fixed RNA binding sequence such as those bound by the MS2 coat protein, endoribonuclease Csy4, or the lambda N protein. In another embodiment, the functional domain is an enzyme that that modifies the methylation state of DNA such as a DNA methyltransferase (DNMT) or TET protein. In another embodiment, the functional domain is an enzyme that modifies a histone subunit, such as histone acetyltransferases (HAT), histone deacetylases (HDAC), histone methyltransferases (e.g., for methylation of lysine or arginine residues) and histone demethylases (e.g., for demethylation of lysine or arginine residues).
In some embodiments, the heterologous functional domain is a base editor. Suitable base editors include a deaminase that modifies cytosine DNA bases, e.g., a cytidine deaminase from the apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like (APOBEC) family of deaminases, including APOBEC1, APOBEC2, APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3D/E, APOBEC3F, APOBEC3G, APOBEC3H, and APOBEC4 (see, e.g., Yang et al., J Genet Genomics. 2017 Sep. 20; 44(9):423-437); activation-induced cytidine deaminase (AID), e.g., activation induced cytidine deaminase (AICDA); cytosine deaminase 1 (CDA1) and CDA2; and cytosine deaminase acting on tRNA (CDAT). Each of these documents is incorporated by reference herein. The following provides exemplary sequences with GenBank Accession Nos.; other sequences can also be used.
hAID/AICDA NM_020661.3 isoform 1 NP 065712.1 variant 1 NM_020661.3 isoform 2 NP 065712.1 variant 2
APOBEC1 NM_001644.4 isoform a NP_001635.2 variant 1 NM_005889.3 isoform b NP_005880.2 variant 3
APOBEC2 NM_006789.3 NP_006780.1
APOBEC3A NM 145699.3 isoform a NP_663745.1 variant 1 NM_001270406.1 isoform b NP_001257335.1 variant 2
APOBEC3B NM_004900.4 isoform a NP_004891.4 variant 1 NM_001270411.1 isoform b NP_001257340.1 variant 2
APOBEC3C NM_014508.2 NP_055323.2
APOBEC3D/E NM_152426.3 NP_689639.2
APOBEC3F NM 145298.5 isoform a NP_660341.2 variant 1 NM_001006666.1 isoform b NP_001006667.1 variant 2
APOBEC3G NM_021822.3 (isoform a) NP_068594.1 (variant 1)
APOBEC3H NM_001166003.2 NP_001159475.2 (variant SV-200)
APOBEC4 NM_203454.2 NP_982279.1 CDA1* NM_127515.4 NP_179547.1
In some embodiments, the heterologous functional domain is a deaminase that modifies adenosine DNA bases, e.g., the deaminase is an adenosine deaminase 1 (ADA1), ADA2; adenosine deaminase acting on RNA 1 (ADAR1), ADAR2, ADAR3 (see, e.g., Savva et al., Genome Biol. 2012 Dec. 28; 13(12):252); adenosine deaminase acting on tRNA 1 (ADAT1), ADAT2, ADAT3 (see Keegan et al., RNA. 2017 September; 23(9):1317-1328 and Schaub and Keller, Biochimie. 2002 August; 84(8):791-803); and naturally occurring or engineered tRNA-specific adenosine deaminase (TadA) (see, e.g., Gaudelli et al., Nature. 2017 Nov. 23; 551(7681):464-471) (NP_417054.2 (Escherichia coli str. K-12 substr. MG1655); See, e.g., Wolf et al., EMBO J. 2002 Jul. 15; 21(14):3841-51). Each of these documents is incorporated by reference herein. The following provides exemplary sequences with GenBank Accession Nos; other sequences can also be used.
ADA (ADA1) NM_000022.3 variant 1 NP_000013.2 isoform 1
ADA2 NM_001282225.1 NP_001269154.1
ADAR NM_001111.4 NP_001102.2
ADAR2 NM_001112.3 variant 1 NP_001103.1 isoform 1 (ADARB1)
ADAR3 NM_018702.3 NP_061172.1 (ADARB2)
ADAT1 NM_012091.4 variant 1 NP_036223.2 isoform 1
ADAT2 NM_182503.2 variant 1 NP_872309.2 isoform 1
ADAT3 NM_138422.3 variant 1 NP_612431.2 isoform 1
In another embodiment, the heterologous functional domain is a prime editor. Prime editors have recently been shown to insert genetic information into a specified DNA site using a catalytically impaired Cas9 endonuclease fused to an engineered reverse transcriptase, programmed with a prime editing guide RNA (pegRNA) that both specifies the target site and encodes the desired edit. Thus, in one embodiment, the Cas9 variant is based on a Cas9 nickase, and is fused to a reverse transcriptase domain. This fusion protein then complexes with the guide RNA (pegRNA) to form the Prime Editing complex. In another embodiment, the heterologous functional domain is a reverse transcriptase. See, Anzalone et al, Search-and-replace genome editing without double-strand breaks or donor DNA, Nature, 576:149-157 (Dec. 5, 2019), which is incorporated herein by reference.
In some embodiments, the heterologous functional domain is an enzyme, domain, or peptide that inhibits or enhances endogenous DNA repair or base excision repair (BER) pathways. Such enzymes, domains, or peptides include thymine DNA glycosylase (TDG; GenBank Acc Nos. NM_003211.4 (nucleic acid) and NP_003202.3 (protein)) or uracil DNA glycosylase (UDG, also known as uracil N-glycosylase, or UNG; GenBank Acc Nos. NM_003362.3 (nucleic acid) and NP_003353.1 (protein)) or uracil DNA glycosylase inhibitor (UGI) that inhibits UNG mediated excision of uracil to initiate BER (see, e.g., Mol et al., Cell 82, 701-708 (1995); Komor et al., Nature. 2016 May 19; 533(7603)); or DNA end-binding proteins such as Gam, which is a protein from the bacteriophage Mu that binds free DNA ends, inhibiting DNA repair enzymes and leading to more precise editing (less unintended base edits). See, e.g., Komor et al., Sci Adv. 2017 Aug. 30; 3(8):eaao4774. See, e.g., Komor et al., Nature. 2016 May 19; 533(7603):420-4; Nishida et al., Science. 2016 Sep. 16; 353(6305). pii: aaf8729; Rees et al., Nat Commun. 2017 Jun. 6; 8:15790; or Kim et al., Nat Biotechnol. 2017 April; 35(4):371-376) as are known in the art can also be used. Each of these documents is incorporated by reference herein. In another embodiment, the heterologous functional domain is a recombinase. In another embodiment, the heterologous functional domain is a nickase.
A number of sequences for domains that catalyze hydroxylation of methylated cytosines in DNA are known. Exemplary proteins include the Ten-Eleven-Translocation (TET)1-3 family, enzymes that converts 5-methylcytosine (5-mC) to 5-hydroxymethylcytosine (5-hmC) in DNA.
Sequences for human TET1-3 are known in the art and are shown below with GenBank Accession Nos:
TET1 NP_085128.2 NM_030625.2
TET2* NP_001120680.1 (var 1) NM_001127208.2 NP_060098.3 (var 2) NM_017628.4
TET3 NP_659430.1 NM_144993.1
*Variant (1) represents the longer transcript and encodes the longer isoform (a). Variant (2) differs in the 5′ UTR and in the 3′ UTR and coding sequence compared to variant 1. The resulting isoform (b) is shorter and has a distinct C-terminus compared to isoform a.
In some embodiments, all or part of the full-length sequence of the catalytic domain can be included, e.g., a catalytic module comprising the cysteine-rich extension and the 2OGFeDO domain encoded by 7 highly conserved exons, e.g., the Tea catalytic domain comprising amino acids 1580-2052, Tet2 comprising amino acids 1290-1905 and Tet3 comprising amino acids 966-1678. See, e.g.,
In some embodiments, the heterologous functional domain is a biological tether, and comprises all or part of (e.g., DNA binding domain from) the MS2 coat protein, endoribonuclease Csy4, or the lambda N protein. These proteins can be used to recruit RNA molecules containing a specific stem-loop structure to a locale specified by the dCas9 gRNA targeting sequences. For example, a dCas9 variant fused to MS2 coat protein, endoribonuclease Csy4, or lambda N can be used to recruit a long non-coding RNA (lncRNA) such as XIST or HOTAIR; see, e.g., Keryer-Bibens et al., Biol. Cell 100:125-138 (2008), that is linked to the Csy4, MS2 or lambda N binding sequence. Alternatively, the Csy4, MS2 or lambda N protein binding sequence can be linked to another protein, e.g., as described in Keryer-Bibens et al., supra, and the protein can be targeted to the dCas9 variant binding site using the methods and compositions described herein. In some embodiments, the Csy4 is catalytically inactive. In some embodiments, the Cas9 variant, preferably a dCas9 variant, is fused to FokI as described in WO 2014/204578.
In some embodiments, the fusion proteins include a linker between the Cas9 variant and the heterologous functional domain. Linkers that can be used in these fusion proteins (or between fusion proteins in a concatenated structure) can include any sequence that does not interfere with the function of the fusion proteins. In preferred embodiments, the linkers are short, e.g., 2-20 amino acids, and are typically flexible (i.e., comprising amino acids with a high degree of freedom such as glycine, alanine, and serine). In some embodiments, the linker comprises one or more units consisting of GGGS (SEQ ID NO:2) or GGGGS (SEQ ID NO:3), e.g., two, three, four, or more repeats of the GGGS (SEQ ID NO:2) or GGGGS (SEQ ID NO:3) unit. Other linker sequences can also be used.
Delivery and Expression Systems
To use the Cas9 variants described herein, it may be desirable to express them from a nucleic acid that encodes them. In another aspect, provided herein is a nucleic acid encoding any of the Cas9 variants or fusion proteins described herein. In one embodiment, the nucleic acid encoding the Cas9 variant is cloned into an intermediate vector for transformation into prokaryotic or eukaryotic cells for replication and/or expression. Intermediate vectors are typically prokaryote vectors, e.g., plasmids, or shuttle vectors, or insect vectors, for storage or manipulation of the nucleic acid encoding the Cas9 variant for production of the Cas9 variant. The nucleic acid encoding the Cas9 variant can also be cloned into an expression vector, for administration to a plant cell, animal cell, preferably a mammalian cell or a human cell, fungal cell, bacterial cell, or protozoan cell.
To obtain expression, a sequence encoding a Cas9 variant is typically subcloned into an expression vector that contains a promoter to direct transcription. Suitable bacterial and eukaryotic promoters are well known in the art and described, e.g., in Sambrook et al., Molecular Cloning, A Laboratory Manual (3d ed. 2001); Kriegler, Gene Transfer and Expression: A Laboratory Manual (1990); and Current Protocols in Molecular Biology (Ausubel et al., eds., 2010). Bacterial expression systems for expressing the engineered protein are available in, e.g., E. coli, Bacillus sp., and Salmonella (Palva et al., 1983, Gene 22:229-235). Kits for such expression systems are commercially available. Eukaryotic expression systems for mammalian cells, yeast, and insect cells are well known in the art and are also commercially available.
The promoter used to direct expression of a nucleic acid depends on the particular application. For example, a strong constitutive promoter is typically used for expression and purification of fusion proteins. In contrast, when the Cas9 variant is to be administered in vivo for gene regulation, either a constitutive or an inducible promoter can be used, depending on the particular use of the Cas9 variant. In addition, a preferred promoter for administration of the Cas9 variant can be a weak promoter, such as HSV TK or a promoter having similar activity. The promoter can also include elements that are responsive to transactivation, e.g., hypoxia response elements, Gal4 response elements, lac repressor response element, and small molecule control systems such as tetracycline-regulated systems and the RU-486 system (see, e.g., Gossen & Bujard, 1992, Proc. Natl. Acad. Sci. USA, 89:5547; Oligino et al., 1998, Gene Ther., 5:491-496; Wang et al., 1997, Gene Ther., 4:432-441; Neering et al., 1996, Blood, 88:1147-55; and Rendahl et al., 1998, Nat. Biotechnol., 16:757-761).
In addition to the promoter, the expression vector typically contains an expression cassette that contains all the additional elements required for the expression of the nucleic acid in host cells, either prokaryotic or eukaryotic. A typical expression cassette thus contains a promoter operably linked, e.g., to the nucleic acid sequence encoding the Cas9 variant, and any signals required, e.g., for efficient polyadenylation of the transcript, transcriptional termination, ribosome binding sites, or translation termination. Additional elements of the cassette may include, e.g., enhancers, and heterologous spliced intronic signals.
The particular expression vector used to transport the genetic information into the cell is selected with regard to the intended use of the Cas9 variant, e.g., expression in plants, animals, bacteria, fungus, protozoa, etc. Standard bacterial expression vectors include plasmids such as pBR322 based plasmids, pSKF, pET23D, and commercially available tag-fusion expression systems such as GST and LacZ.
Expression vectors containing regulatory elements from eukaryotic viruses are often used in eukaryotic expression vectors, e.g., SV40 vectors, papilloma virus vectors, and vectors derived from Epstein-Barr virus. Other exemplary eukaryotic vectors include pMSG, pAV009/A+, pMTO10/A+, pMAMneo-5, baculovirus pDSVE, and any other vector allowing expression of proteins under the direction of the SV40 early promoter, SV40 late promoter, metallothionein promoter, murine mammary tumor virus promoter, Rous sarcoma virus promoter, polyhedrin promoter, or other promoters shown effective for expression in eukaryotic cells.
The vectors for expressing the Cas9 variants can include RNA Pol III promoters to drive expression of the guide RNAs, e.g., the H1, U6 or 7SK promoters. These human promoters allow for expression of Cas9 variants in mammalian cells following plasmid transfection.
Some expression systems have markers for selection of stably transfected cell lines such as thymidine kinase, hygromycin B phosphotransferase, and dihydrofolate reductase. High yield expression systems are also suitable, such as using a baculovirus vector in insect cells, with the gRNA encoding sequence under the direction of the polyhedrin promoter or other strong baculovirus promoters. The elements that are typically included in expression vectors also include a replicon that functions in E. coli, a gene encoding antibiotic resistance to permit selection of bacteria that harbor recombinant plasmids, and unique restriction sites in nonessential regions of the plasmid to allow insertion of recombinant sequences.
Standard transfection methods are used to produce bacterial, mammalian, yeast or insect cell lines that express large quantities of protein, which are then purified using standard techniques (see, e.g., Colley et al., 1989, J. Biol. Chem., 264:17619-22; Guide to Protein Purification, in Methods in Enzymology, vol. 182 (Deutscher, ed., 1990)). Transformation of eukaryotic and prokaryotic cells are performed according to standard techniques (see, e.g., Morrison, 1977, J. Bacteriol. 132:349-351; Clark-Curtiss & Curtiss, Methods in Enzymology 101:347-362 (Wu et al., eds, 1983).
Any of the known procedures for introducing foreign nucleotide sequences into host cells may be used. These include the use of calcium phosphate transfection, polybrene, protoplast fusion, electroporation, nucleofection, liposomes, microinjection, naked DNA, plasmid vectors, viral vectors, both episomal and integrative, and any of the other well-known methods for introducing cloned genomic DNA, cDNA, synthetic DNA or other foreign genetic material into a host cell (see, e.g., Sambrook et al., supra). It is only necessary that the particular genetic engineering procedure used be capable of successfully introducing at least one gene into the host cell capable of expressing the Cas9 variant.
Alternatively, the methods can include delivering the Cas9 variant protein and guide RNA together, e.g., as a complex. For example, the Cas9 variant and gRNA can be overexpressed in a host cell and purified, then complexed with the guide RNA (e.g., in a test tube) to form a ribonucleoprotein (RNP), and delivered to cells. In some embodiments, the variant Cas9 can be expressed in and purified from bacteria through the use of bacterial Cas9 expression plasmids. For example, His-tagged variant Cas9 proteins can be expressed in bacterial cells and then purified using nickel affinity chromatography. The use of RNPs circumvents the necessity of delivering plasmid DNAs encoding the nuclease or the guide, or encoding the nuclease as an mRNA. RNP delivery may also improve specificity, presumably because the half-life of the RNP is shorter and there's no persistent expression of the nuclease and guide (as you'd get from a plasmid). The RNPs can be delivered to the cells in vivo or in vitro, e.g., using lipid-mediated transfection or electroporation. See, e.g., Liang et al. “Rapid and highly efficient mammalian cell engineering via Cas9 protein transfection.” Journal of biotechnology 208 (2015): 44-53; Zuris, John A., et al. “Cationic lipid-mediated delivery of proteins enables efficient protein-based genome editing in vitro and in vivo.” Nature biotechnology 33.1 (2015): 73-80; Kim et al. “Highly efficient RNA-guided genome editing in human cells via delivery of purified Cas9 ribonucleoproteins.” Genome research 24.6 (2014): 1012-1019.
As the CRISPR technology landscape develops, it is useful to have a means for evaluating various Cas enzyme variants and other variations to the CRISPR machinery. Thus, provided herein is a method of evaluating a CRISPR-Cas system. The method includes obtaining a guide RNA library which includes multiple gRNA sequences which target sites in the genome. The library is cloned into a plasmid comprising a nucleic acid sequence encoding a Cas protein and, optionally, a barcode, and the virus is produced. Host cells, preferably mammalian cells, are transduced with the virus containing the Cas plasmid. The cells are cultured for a time period sufficient to allow the CRISPR reaction to occur and the cells are then evaluated for CRISPR activity.
The provided method allows for evaluation of one or multiple variables in the CRISPR system. For example, as demonstrated herein, a single high-throughput competition assay was able to test three Cas9 variants across different PAM sites and different genome engineering tasks. Thus, in one embodiment, the method includes evaluation of multiple Cas proteins. Such proteins may be variants of the same Cas wild type protein, such as wild-type [WT] Cas9, Cas9-NG and xCas9, as shown herein. In one embodiment, one, two, three, four, five, six, seven, eight, nine, ten or more Cas variants are evaluated simultaneously. In one embodiment, the Cas proteins are Cas9 proteins.
Plasmids are designed to express each Cas protein to be tested. For example, as described herein, human codon optimized Cas9 from lentiCRISPR v2 plasmid (Addgene 52961, Sanjana et al., 2014) as background for xCas9 and Cas9-NG mutations. xCas9 (also known as xCas3.7) mutations are as follows: A262T, R324L, S409I, E480K, E543D, M694I and E1219V (Hu et al., 2018). Cas9-NG mutations are: L1111R, D1135V, G1218R, E1219F, A1322R, R1335V, T1337R (Nishimasu et al., 2018). xCas9-NG mutations are: A262T, R324L, S409I, E480K, E543D, M694I, L1111R, D1135V, G1218R, E1219F, A1322R, R1335V and T1337R. For transcriptional modulation, Cas9 variants contained additional D10A and H840A mutations to make them catalytically inactive. KRAB domain was derived from pHAGE EF1α dCas9-KRAB (Addgene 50919, Kearns et al., 2014). VPR complex was derived from lenti-EF1a-dCas9-VPR-Puro (Addgene 99373, Ho et al., 2017) and modified to abolish BsmBI restriction sites. sgRNA scaffold was modified to improve its stability and Cas9 binding (F+E modification, Chen et al., 2013). Finally, we inserted a six-nucleotide barcode between the sgRNA scaffold and EFS promoter to act as an identifier for Cas9 variant and CRISPR modality (
In another embodiment, the method includes evaluation of one or more CRISPR modalities or genetic perturbations. Such perturbations include nuclease activity, transcriptional activation (CRISPRa), and transcriptional repression (CRISPRi). For CRISPRa and CRISPRi, dCas9 variants may be used. In addition, Cas fusion proteins as described herein may be used. For example, for transcriptional activation (CRISPRa), nuclease-null versions of each Cas9 variant (D10A/H840A) may be fused to a transcriptional activation domain, such as VPR. VPR and other synergistic activators with multiple activation domains, such as SAM and SunTag, outperform single domain activators (Chavez et al., 2016). For transcriptional repression (CRISPR inhibition, CRISPRi), the nuclease-null Cas variants are fused to a transcriptional repression domain, such as the KRAB repressor domain (Kearns et al., 2014).
In another embodiment, the method includes evaluation of PAM specificity or flexibility of the Cas protein(s). In this embodiment, the sgRNA library is designed so that it targets sites spanning all possible three-nucleotide PAM combinations in the binding area of the selected gene. Such target sites may include coding exons (CDS) or a region within 3 kb of the transcription start site (TSS) of the selected gene. When evaluating CRISPR nuclease activity, the target sites may include CDS. When evaluating CRISRPi or CRISPRa activity, or both, the target sites may include TSS.
In one embodiment, the plasmid contains a barcode. The barcode is a short nucleotide sequence used to identify the particular Cas variant or specific modality being tested (functional domain present), or both. The barcode can be identified by sequencing, e.g., high-throughput sequencing. In one embodiment, the barcode is 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 nucleotides.
After culturing, the cells are evaluated for CRISPR activity, which may be done in various ways known to the person of skill in the art. The effect on targeted genes can be evaluated by FACS for cell surface proteins or by western blot or ELISA for any cellular protein. In addition, the effects can be evaluated at DNA level and/or mRNA transcript level by any form of DNA/transcription sequencing methods. It is desirable, in some embodiments, to select a gene that encodes a cell surface marker, which allows antibody staining and detection of expression via FACS or similar method. In another embodiment, high-throughput single-cell RNA sequencing is used to detect expression of the selected gene. See, e.g., Mimitou et al, Multiplexed detection of proteins, transcriptomes, clonotypes and CRISPR perturbations in single cells, Nature Methods, 16:409-12 (May 2019), which is incorporated herein by reference.
The following examples are illustrative only and are not intended to limit the present invention. The publication Legut, M. et al, High-Throughput Screens of PAM-Flexible Cas9 Variants for Gene Knockout and Transcriptional Modulation, Cell Reports, 30(9):2589-2868, ES, March 3, 2020 is incorporated herein by reference in its entirety.
To compare Cas9 variants across different PAM sites and different genome engineering tasks, we designed a high-throughput competition assay to test three Cas9 variants (WT Cas9, Cas9-NG, and xCas9) and three different genetic perturbations (nuclease, transcriptional activation, and transcriptional repression) at thousands of target sites in the human genome (
To build a sufficiently large dataset, we selected single-guide RNAs (sgRNAs) at thousands of target sites spanning all possible three-nucleotide PAM combinations. Specifically, we designed three sgRNA libraries targeting the genes CD45, CD46 and CD55, which encode cell surface markers that can be detected by antibody labeling, and are expressed in human K562 cells (
The libraries were cloned into a lentiviral plasmid containing a Cas9 variant (WT, Cas9-NG or xCas9) and a six-nucleotide barcode specific for the particular Cas9 variant and given modality (nuclease, repression or activation). This plasmid design allowed us to determine simultaneously the sgRNA and Cas9 effector (barcode) identities by high-throughput Illumina sequencing (
Following puromycin selection of transduced cells, we pooled together an equal number of cells transduced with different enzymes (WT, Cas9-NG or xCas9), performed antibody staining for each cell-surface protein, and sorted them by target expression via fluorescence-assisted cell sorting (FACS) (
Cas9-NG Targets NGH PAMs with 2- to 4-Fold Lower Nuclease Activity than Cas9 at NGG PAMs
We first performed the CRISPR competition screens using catalytically-active nucleases and compared the fold-change of sgRNAs targeting coding exons (n=2,107 sgRNAs). Across all three cell-surface proteins, we observed the greatest fold-change for target sites with the canonical NGG PAM using the WT Cas9 enzyme (
To further dissect Cas9 variant activity at specific PAMs and to discover potentially targetable non-NG PAMs, we next examined all possible nucleotide combinations at PAM positions 2 and 3 (
To further validate our pooled comparison, we targeted the CD46 gene in K562 cells with 18 individual sgRNAs at NGG and NGH PAMs using all 3 enzymes and quantified protein expression via FACS. To minimize bias due to sgRNA nucleotide composition, we designed sgRNAs targeting NGH PAMs to be shifted one nucleotide downstream from the corresponding NGG PAM-targeting sgRNAs. Following lentiviral transduction and selection, we measured the knockout efficiency by flow cytometry (
Interestingly, we noticed a difference in knockout kinetics between wild-type Cas9 and Cas9-NG. While knockout efficiency of Cas9-NG (at both NGG and NGH PAM sites) sharply increased between days 4 and 14 post-transduction, wild-type Cas9 activity reached levels close to saturation already at day 4 (
We also measured the editing efficiency at the DNA level by high-throughput amplicon sequencing and we observed that the frequency of alleles with insertions or deletions (indels) correlated well with protein expression from flow cytometry (r2=0.93,
Cas9-NG, but not xCas9 or WT Cas9, Efficiently Modulates Gene Expression at NGH PAMs
CRISPR nuclease activity is a two-step process: first, the Cas9-sgRNA complex binds the target DNA and second, it undergoes a conformational change which enables double-strand break formation (Nishimasu et al., 2018; Wu et al., 2014). In contrast, CRISPR transcriptional modulation only requires Cas9 sgRNA binding in the target region to enable recruitment of transcriptional repressors or activators. We hypothesized that xCas9, which showed suboptimal performance as a nuclease, might perform better in context of CRISPRi and CRISPRa because it was evolved via selection for DNA binding without cleavage. In the phage-based evolution and selection assay used to derive xCas9, nuclease-null Cas9 (dCas9) was fused to an E. coli RNA polymerase and targeted upstream of an essential gene for phage replication (Hu et al., 2018). In that study, xCas9 was shown to have, on average, a 12-fold increase in activity in human cells over WT Cas9 when fused to the VPR transcriptional activator (Hu et al., 2018). Given our previous results with xCas9 nuclease, we wanted to determine if dCas9 variants of the 3 enzymes fused to transcriptional activators and repressors would result in greater activity at NGH PAMs.
For this purpose, we first examined sgRNAs for all NGG PAMs tiling the 3 kb region surrounding the gene's primary TSS to identify the optimal target region for subsequent analysis and comparison across all PAMs. In general, we found that the optimal CRISPRi window was shifted downstream of the optimal CRISPRa window by ˜120 bp, possibly resulting from the interference of the bound Cas9 complex with the assembly of transcriptional machinery at the TSS (
Overall, we observed that WT dCas9 produced the strongest effect on transcriptional modulation at NGG PAMs (
To further validate the pooled competition screen results, we targeted CD45 gene expression using 23 individual sgRNAs in two CD45neg cell lines, A375 (
We next computed the relative activity of all three Cas9 enzymes at NGG and NGH PAMs, across all three modalities tested (nuclease, transcriptional activation, transcriptional repression), integrating data from nine separate CRISPR competition screens (
Our high-throughput CRISPR pooled competition screens and arrayed sgRNA validation data indicated that Cas9-NG is active for all modalities at NGN PAMs, albeit to a lesser extent than WT Cas9 at NGG sites. We also found that xCas9 had the poorest performance at virtually all PAMs and for all modalities. Due to this marked difference in Cas9-NG and xCas9 activity, we examined the position of the mutations in both Cas9 variants (
Using Cas9-NG as a baseline, we compared xCas9 and xCas9-NG nucleases using several sgRNAs to target CD46 in K562 cells at both NGG and NGH PAMs (
Taken together, we performed nine independent CRISPR competition screens, spanning three endogenously expressed human genes and three CRISPR modalities, to assess the efficacy of recently-described PAM-flexible Cas9 variants at different PAM sites. These are the first pooled CRISPR screens using xCas9 or Cas9-NG, testing thousands of endogenous genomic loci in a massively-parallel manner. By combining cells transduced with all three Cas9 variants prior to FACS, we were able to perform a pooled comparison where each variant competes against other variants. This high-throughput CRISPR competition screen provides a general method of assessing relative efficacies of PAM-flexible Cas9 variants and provides a far richer dataset than previous work with only a few target sites (Hu et al., 2018; Nishimasu et al., 2018). While this screen was not designed to discover sequence features determining the on-target efficiency of PAM-flexible Cas9 enzymes, that could be achieved by scaling up the number of assayed sgRNAs.
We showed that the mutations that increase PAM flexibility of Cas9 lead to decreased activity of these enzymes at NGG target sites. This observation applies to both catalytically active and inactive Cas9 variants. When comparing Cas9 variants at target sites with NGH PAMs, we were surprised to discover that while Cas9-NG maintains a similar level of activity as for target sites with NGG PAMs, the activity of xCas9 was profoundly diminished. In fact, at target sites with NGH PAMs, xCas9 did not perform better than wild-type Cas9 across all modalities tested (nuclease, activation, and inhibition). The discrepancies between the results reported in this study and in the original xCas9 publication could potentially stem from differences in accessibility of the target sites, thus highlighting the need to test endogenous loci for meaningful comparisons. Recent studies in plants (Ge et al., 2019; Hua et al., 2019; Negishi et al., 2019; Wang et al., 2019; Zhong et al., 2019) have shown that the overall efficiency of indel formation and base editing at non-NGG sites is much higher for Cas9-NG than for xCas9, supporting our findings in the mammalian context. Furthermore, David Liu and colleagues recently demonstrated that Cas9-NG base editors outperform xCas9 base editors at target sites with NGH PAMs and observed very low or no editing at the vast majority of loci tested when using xCas9 (Huang et al., 2019).
Structural studies have shown that the mechanisms behind relaxed PAM recognition by xCas9 and Cas9-NG are considerably different. In case of Cas9-NG, NG PAM recognition is enabled by mutating both the R1335 residue interacting with the third nucleobase of the PAM (dG3), and E1219, which stabilizes R1335. The remaining five mutations are introduced to enhance Cas9-NG binding to the now smaller, two-nucleobase PAM (Nishimasu et al., 2018). Conversely, in xCas9 the R1335-dG3 interaction is disrupted indirectly, by abrogating the E1219-R1335 interaction and allowing R1335 to adopt multiple conformations (Guo et al., 2019). The remaining xCas9 mutations are located in the recognition (REC) lobes and result in the conformational change of Cas9 binding to DNA.
Given these differences, we investigated how the change of REC lobes conformation (xCas9 mutations) would affect the editing activity of the enzyme when combined with enhanced binding to the two-nucleobase PAM (Cas9-NG mutations). This new Cas9 variant, termed xCas9-NG, showed improved nuclease activity compared to xCas9, presumably due to stronger interactions with the PAM, although it did not fully rescue nuclease activity to the Cas9-NG level. In contrast, we also found that xCas9-NG was superior to both xCas9 and Cas9-NG for transcriptional modulation, possibly indicating that a more relaxed REC lobe interaction with target DNA allows for easier access of the recruited transcriptional machinery. Over the entire human exome and functional non-coding regions, the relaxed PAM constraints of xCas9-NG enable a significantly larger target space (
As none of the three PAM flexible Cas9 mutants were capable of matching the efficacy of wild-type Cas9 at NGG PAM sites, relaxing PAM interactions through these mutations likely incurs a fitness cost in enzyme performance. New strategies are needed for designing efficient, PAM-flexible (or perhaps even PAM-independent) Cas9 enzymes. The CRISPR competition screen presented here provides a robust and scalable platform for future benchmarking of different genome editing enzymes prior to their implementation in research, clinical or industrial applications.
K562 and A375 cell lines were obtained from ATCC. HEK 293FT cells were obtained from Thermo Scientific. K562 cells were cultured in Iscove's Modified Dulbecco's Medium (IMDM); A375 and HEK293FT cells were cultured in Dulbecco's Modified Eagle Medium (DMEM). All media were from Caisson Labs. Media were supplemented with 10% Serum Plus II Medium Supplement (Sigma-Aldrich). Cells were regularly passaged and tested for presence of mycoplasma contamination (MycoAlert Plus Mycoplasma Detection Kit, Lonza).
In order to enable a meaningful comparison between different Cas9 variants, we used the human codon optimized Cas9 from lentiCRISPR v2 plasmid (Addgene 52961, Sanjana et al., 2014) as background for xCas9 and Cas9-NG mutations. xCas9 (also known as xCas3.7) mutations are as follows: A262T, R324L, S409I, E480K, E543D, M694I and E1219V (Hu et al., 2018). Cas9-NG mutations are: L1111R, D1135V, G1218R, E1219F, A1322R, R1335V, T1337R (Nishimasu et al., 2018). xCas9-NG mutations are: A262T, R324L, S409I, E480K, E543D, M694I, L1111R, D1135V, G1218R, E1219F, A1322R, R1335V and T1337R. For transcriptional modulation, Cas9 variants contained additional DMA and H840A mutations to make them catalytically inactive. KRAB domain was derived from pHAGE EF1α dCas9-KRAB (Addgene 50919, Kearns et al., 2014). VPR complex was derived from lenti-EF1a-dCas9-VPR-Puro (Addgene 99373, Ho et al., 2017) and modified to abolish BsmBI restriction sites. sgRNA scaffold was modified to improve its stability and Cas9 binding (F+E modification, Chen et al., 2013). Finally, we inserted a six-nucleotide barcode between the sgRNA scaffold and EFS promoter to act as an identifier for Cas9 variant and CRISPR modality (
Lentiviral sgRNA Library Design nd Cloning
The sgRNAs targeting the 3 kb region surrounding the TSS and constitutive protein-coding exons were chosen to include all possible 20-mer sequences upstream of an NG PAM sequence, and equal numbers of 20-mer sequences upstream of NH PAM sequences. Primary TSS and exon annotations were obtained from the UCSC Genome Browser based on the hg38 genome assembly. We also included 250 non-targeting sgRNAs from the GeCKO v2 library (Sanjana et al., 2014) as a negative control in each library.
Lentivirus was produced by polyethylenimine linear MW 25000 (Polysciences 23966) transfection of HEK 293FT cells with the transfer plasmid containing a barcoded Cas9 effector and sgRNA library, packaging plasmid psPAX2 (Addgene 12260) and envelope plasmid pMD2.G (Addgene 12259). After 72 h post-transfection, cell media containing lentiviral particles was harvested and filtered through 0.45 μm filter Steriflip-HV (Millipore SE1M003M00). Each sgRNA library and Cas9 effector combination was transduced into K562 cells individually, to avoid barcode swapping, and thus Cas9 misidentification, during lentiviral integration (Xie et al., 2018). In total we produced 27 individual lentiviral libraries and transduced them into separately into K562 cells. The transduction was performed at a multiplicity of infection (MOI) ˜0.4 to minimize the fraction of cells with multiple sgRNAs. We maintained 1,000× coverage of each sgRNA library. Transduced cells were selected with 1 μg ml−1 puromycin for at least 7 days after transduction. During the course of the screen the cells were maintained at numbers ensuring >1,000× representation of the library. Transduced cells were maintained as 27 separate cell cultures for 14 days. At day 14 post-transduction, cells transduced with the sgRNA library targeting the same gene and the same CRISPR modality (but different Cas9 variants) were combined in equal numbers, resulting in 9 separate cell pools for screening, and then analyzed and sorted via FACS. All cell counting was done using a Cellometer Auto T4 counter (Nexcelom).
For arrayed CD46 knockout validation in K562 and A375 cells, sgRNAs targeting exons 2 and 3 of CD46 gene were designed in benchling software as 20-mers upstream of an NGG PAM, or by shifting +1 bp upstream, as 20-mers upstream of an NGH PAM (
For arrayed CRISPR inhibition validation, we selected guide RNAs from sequences included in the screen library (NG PAMs) or designed to target within close proximity to NG PAM sgRNAs (NH PAM). The sequences of sgRNAs are listed in
For arrayed CRISPR activation validation, sgRNA-specifying oligos were either obtained from sgRNA sequences included in the screen library (NG PAMs) or designed to target within close proximity to NG PAM sgRNAs (NH PAM). The sequences of sgRNAs are listed in
HEK293FT cells were transiently transfected with equal amounts of Cas9 variants expression vectors. At 24 hours post-transfection, the cells were collected, lysed with THE buffer (10 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1 mM EDTA, 1% Nonidet P-40) supplemented with protease inhibitor cocktail (Bimake B14001) for 1 hour on ice. Cells lysates were spun for 10 min at 10,000 g, and supernatants were used to determine the protein concentration for each sample using the BCA assay (ThermoFisher 23227). Equal amounts of whole cell lysates (20 μg protein per sample) were denatured in Tris-Glycine SDS Sample buffer (ThermoFisher LC2676), and loaded on a Novex 4-20% Tris-Glycine gel (ThermoFisher XP04205BOX). PageRuler pre-stained protein ladder (ThermoFisher 26616) was used to determine the protein size. The gel was run in 1× Tris-Glycine-SDS buffer (IBI Scientific IBI01160) for 20 min at 80V, and then for additional 100 min at 120V. Proteins were transferred on a nitrocellulose membrane (BioRad 1620112) in presence of prechilled 1× Tris-Glycine transfer buffer (FisherSci LC3675) supplemented with 20% methanol for 100 min at 100V. Immunoblots were blocked with 5% skim milk dissolved in 1× PBS+1% Tween 20 (PBST), washed well with PBST and incubated overnight at 4° C. separately with the following primary antibodies: mouse anti-2A peptide, clone 3H4 (1 μg/mL, Millipore MABS2005); rabbit anti-GAPDH 14C10 (0.1 μg/mL, Cell Signaling 2118S). Following the primary antibody, the blots were incubated with IRDye 680RD donkey anti-rabbit (0.2 μg/mL, LI-COR 926-68073) or with IRDye 800CW donkey anti-mouse (0.2 μg/mL, LI-COR 926-32212). The blots were imaged using Odyssey CLx (LI-COR). Band intensity quantification was performed using ImageJ version 1.51.
For CRISPR library sorting, >108 cells were taken for antibody staining (˜10,000×library representation). We set aside 107 cells for the pre-sort control (˜1,000×coverage). After harvesting the cells and removing leftover medium by washing with PBS, the cells were stained for 5 minutes at room temperature with LIVE/DEAD Fixable Violet Dead Stain Kit (ThermoFisher L34864). Subsequently, the cells were stained with antibodies for 20 minutes on ice. The following antibodies were used: CD45-PE (clone 2D1), CD46-APC (clone TRA-2-10) or CD55-APC (clone JS11). All antibodies were purchased pre-conjugated from BioLegend. Cells were washed with PBS to remove unbound antibodies prior to sorting. Cell acquisition and sorting was performed using a Sony SH800S cell sorter.
Sequential gating was performed as follows: 1) exclusion of debris based on forward and side scatter cell parameters, 2) doublet exclusion, and 3) dead cell exclusion (
The sgRNA library preparation was performed as described before (Shalem et al., 2014). Briefly, gDNA was extracted using GeneJET DNA Purification Kit (Thermo Fisher Scientific). All of the extracted gDNA was then used in the first PCR reaction, in multiple reactions not exceeding 10 μg gDNA per 100 uL PCR reaction. Samples were then subjected to a second PCR to add sequencing adaptors and to barcode the samples. All PCR primers are listed in
The sgRNA sequences present in the sorted samples (read 1) as well as their corresponding barcodes indicating the Cas9 variant and CRISPR modality (read 2) were enumerated. sgRNA sequences were mapped to the reference sgRNA library with one mismatch allowed (bowtie -v 1 -m 1). Read numbers were normalized to the total number of reads per sample (with a pseudocount added to all sgRNAs) and loge-transformed. The median of non-targeting sgRNAs was calculated for each of the three Cas9 variants present in a sample. The median of non-targeting (NT) sgRNAs associated with each Cas9 was then used to normalize the sgRNA read counts associated with that Cas9. Finally, the fold-change of each NT-normalized sgRNA-Cas9 pair in top 15% bin was calculated over the NT-normalized sgRNA-Cas9 pair in the bottom 15% bin. Statistical significance was determined by two-sided Student's t-test with Bonferroni correction (RStudio). For CRISPRi and CRISPRa screens, we needed to determine optimal windows around the TSS to pick the sgRNAs for subsequent analyses (i.e. to compare Cas9 variants across NGN PAMs, and to identify new functional NHN PAMs). Windows were selected to capture the peak region identified from the LOESS fit for all three enzymes, using only the NGG sgRNAs for strongest signal. The following parameters were chosen for LOESS fitting using the Gviz package (Hahne and Ivanek, 2016) in RStudio: span=0.2, evaluation=500, degree=10.
For validation of arrayed CD46 knockout, genomic DNA was isolated using QuickExtract DNA Extraction Solution (Epicentre). Two sets of PCR primers were designed: first set was flanking the exons to be amplified and contained handles for the second PCR. The primers for the second PCR were handle-specific, and added Illumina sequencing adaptors and indexes (
Illumina single-end reads for CD46 genomic amplicons were analyzed using CRISPResso2 software (Clement et al., 2019) to quantify the fraction of reads containing editing at expected sites, and to determine the editing outcome in terms of indel type and size. Flow cytometry data was analyzed using FlowJo software. Visualization of Cas9 protein structures was performed in PyMOL software (PDB IDs: 4un3; 6ai6). All other data analysis was performed in GraphPad Prism 8 and RStudio. All correlation coefficients (r) and coefficients of determination (r2) are Pearson's correlation. DNase I hypersensitivity (HS) sites in the K562 cell line were downloaded from ENCODE DNase Uniformly Processed Peaks from UCSC based on hg19 genome build.
In all boxplots, boxes indicate the median and interquartile ranges, with whiskers indicating either 1.5 times the interquartile range, or the most extreme data point outside the 1.5-fold interquartile. All transfection experiments show the mean of three replicate experiments, with error bars representing the standard error of mean.
All publications cited in this specification are incorporated herein by reference. While the invention has been described with reference to particular embodiments, it will be appreciated that modifications can be made without departing from the spirit of the invention. Such modifications are intended to fall within the scope of the appended claims.
1. Anders, C., Niewoehner, O., Duerst, A., and Jinek, M. (2014). Structural basis of PAM-dependent target DNA recognition by the Cas9 endonuclease. Nature 513, 569-573.
2. Chavez, A., Tuttle, M., Pruitt, B. W., Ewen-Campen, B., Chari, R., Ter-Ovanesyan, D., Haque, S. J., Cecchi, R. J., Kowal, E. J. K., Buchthal, J., et al. (2016). Comparison of Cas9 activators in multiple species. Nature Methods 13, 563-567.
3. Chen, B., Gilbert, L. A., Cimini, B. A., Schnitzbauer, J., Zhang, W., Li, G.-W., Park, J., Blackburn, E. H., Weissman, J. S., Qi, L. S., et al. (2013). Dynamic Imaging of Genomic Loci in Living Human Cells by an Optimized CRISPR/Cas System. Cell 155, 1479-1491.
4. Clement, K., Rees, H., Canver, M. C., Gehrke, J. M., Farouni, R., Hsu, J. Y., Cole, M. A., Liu, D. R., Joung, J. K., Bauer, D. E., et al. (2019). CRISPResso2 provides accurate and rapid genome editing sequence analysis. Nature Biotechnology 37, 224-226.
5. Feldman, D., Singh, A., Garrity, A. J., and Blainey, P. C. (2018). Lentiviral co-packaging mitigates the effects of intermolecular recombination and multiple integrations in pooled genetic screens. BioRxiv.
6. Findlay, G. M., Boyle, E. A., Hause, R. J., Klein, J. C., and Shendure, J. (2014). Saturation editing of genomic regions by multiplex homology-directed repair. Nature 513, 120-123.
7. Ge, Z., Zheng, L., Zhao, Y., Jiang, J., Zhang, E. J., Liu, T., Gu, H., and Qu, L. (2019). Engineered xCas9 and SpCas9-NG variants broaden PAM recognition sites to generate mutations in Arabidopsis plants. Plant Biotechnology Journal.
8. Guo, M., Ren, K., Zhu, Y., Tang, Z., Wang, Y., Zhang, B., and Huang, Z. (2019). Structural insights into a high fidelity variant of SpCas9. Cell Research 29, 183-192.
9. Hahne, F., and Ivanek, R. (2016). Visualizing Genomic Data Using Gviz and Bioconductor. In Statistical Genomics, E. Mahe, and S. Davis, eds. (New York, NY: Springer New York), pp. 335-351.
10. Hegde, M., Strand, C., Hanna, R. E., and Doench, J. G. (2018). Uncoupling of sgRNAs from their associated barcodes during PCR amplification of combinatorial CRISPR screens. PLoS ONE 13, e0197547.
11. Hill, A. J., McFaline-Figueroa, J. L., Starita, L. M., Gasperini, M. J., Matreyek, K. A., Packer, J., Jackson, D., Shendure, J., and Trapnell, C. (2018). On the design of CRISPR-based single-cell molecular screens. Nature Methods 15, 271-274.
12. Ho, S.-M., Hartley, B. J., Flaherty, E., Rajarajan, P., Abdelaal, R., Obiorah, I., Barretto, N., Muhammad, H., Phatnani, H. P., Akbarian, S., et al. (2017). Evaluating Synthetic Activation and Repression of
13. Neuropsychiatric-Related Genes in hiPSC-Derived NPCs, Neurons, and Astrocytes. Stem Cell Reports 9, 615-628.
14. Hsu, P. D., Scott, D. A., Weinstein, J. A., Ran, F. A., Konermann, S., Agarwala, V., Li, Y., Fine, E. J., Wu, X., Shalem, O., et al. (2013). DNA targeting specificity of RNA-guided Cas9 nucleases. Nature Biotechnology 31,827-832.
15. Hu, J. H., Miller, S. M., Geurts, M. H., Tang, W., Chen, L., Sun, N., Zeina, C. M., Gao, X., Rees, H. A., Lin, Z., et al. (2018). Evolved Cas9 variants with broad PAM compatibility and high DNA specificity. Nature 556,57-63.
16. Hua, K., Tao, X., Han, P., Wang, R., and Zhu, J.-K. (2019). Genome Engineering in Rice Using Cas9 Variants that Recognize NG PAM Sequences. Molecular Plant 12,1003-1014.
17. Huang, T. P., Zhao, K. T., Miller, S. M., Gaudelli, N. M., Oakes, B. L., Fellmann, C., Savage, D. F., and Liu, D. R. (2019). Circularly permuted and PAM-modified Cas9 variants broaden the targeting scope of base editors. Nature Biotechnology 37,626-631.
18. Jinek, M., Chylinski, K., Fonfara, I., Hauer, M., Doudna, J. A., and Charpentier, E. (2012). A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity. Science 337, 816-821.
19. Kearns, N. A., Genga, R. M. J., Enuameh, M. S., Garber, M., Wolfe, S. A., and Maehr, R. (2014). Cas9 effector-mediated regulation of transcription and differentiation in human pluripotent stem cells. Development 141,219-223.
20. Kim, D., Kim, J., Hur, J. K., Been, K. W., Yoon, S., and Kim, J.-S. (2016). Genome-wide analysis reveals specificities of Cpf1 endonucleases in human cells. Nature Biotechnology 34,863-868.
21. Kleinstiver, B. P., Prew, M. S., Tsai, S. Q., Topkar, V. V., Nguyen, N. T., Zheng, Z., Gonzales, A. P. W., Li, Z., Peterson, R. T., Yeh, J.-R.J., et al. (2015). Engineered CRISPR-Cas9 nucleases with altered PAM specificities. Nature 523,481-485.
22. Komor, A. C., Kim, Y. B., Packer, M. S., Zuris, J. A., and Liu, D. R. (2016). Programmable editing of a target base in genomic DNA without double-stranded DNA cleavage. Nature 533,420-424.
23. Meier, J. A., Zhang, F., and Sanjana, N. E. (2017). GUIDES: sgRNA design for loss-of-function screens. Nature Methods 14,831-832.
24. Negishi, K., Kaya, H., Abe, K., Hara, N., Saika, H., and Toki, S. (2019). An adenine base editor with expanded targeting scope using SpCas9-NG v1 in rice. Plant Biotechnology Journal.
25. Nishimasu, H., Shi, X., Ishiguro, S., Gao, L., Hirano, S., Okazaki, S., Noda, T., Abudayyeh, O. O., Gootenberg, J. S., Mori, H., et al. (2018). Engineered CRISPR-Cas9 nuclease with expanded targeting space. Science 361,1259-1262.
26. Ran, F. A., Cong, L., Yan, W. X., Scott, D. A., Gootenberg, J. S., Kriz, A. J., Zetsche, B., Shalem, O., Wu, X., Makarova, K. S., et al. (2015). In vivo genome editing using Staphylococcus aureus Cas9. Nature 520,186-191.
27. Sanjana, N. E., Shalem, O., and Zhang, F. (2014). Improved vectors and genome-wide libraries for CRISPR screening. Nature Methods 11,783-784.
28. Sanson, K. R., Hanna, R. E., Hegde, M., Donovan, K. F., Strand, C., Sullender, M. E., Vaimberg, E. W., Goodale, A., Root, D. E., Piccioni, F., et al. (2018). Optimized libraries for CRISPR-Cas9 genetic screens with multiple modalities. Nature Communications 9.
29. Shalem, O., Sanjana, N. E., Hartenian, E., Shi, X., Scott, D. A., Mikkelsen, T. S., Heckl, D., Ebert, B. L., Root, D. E., Doench, J. G., et al. (2014). Genome-Scale CRISPR-Cas9 Knockout Screening in Human Cells. Science 343, 84-87.
30. Wang, J., Meng, X., Hu, X., Sun, T., Li, J., Wang, K., and Yu, H. (2019). xC as9 expands the scope of genome editing with reduced efficiency in rice. Plant Biotechnology Journal 17, 709-711.
31. Wu, X., Scott, D. A., Kriz, A. J., Chiu, A. C., Hsu, P. D., Dadon, D. B., Cheng, A. W., Trevino, A. E., Konermann, S., Chen, S., et al. (2014). Genome-wide binding of the CRISPR endonuclease Cas9 in mammalian cells. Nature Biotechnology 32, 670-676.
32. Xie, S., Cooley, A., Armendariz, D., Zhou, P., and Hon, G. C. (2018). Frequent sgRNA-barcode recombination in single-cell perturbation assays. PLOS ONE 13, e0198635.
33. Zhang, Y., Ge, X., Yang, F., Zhang, L., Zheng, J., Tan, X., Jin, Z.-B., Qu, J., and Gu, F. (2015). Comparison of non-canonical PAMs for CRISPR/Cas9-mediated DNA cleavage in human cells. Scientific Reports 4.
34. Zhong, Z., Sretenovic, S., Ren, Q., Yang, L., Bao, Y., Qi, C., Yuan, M., He, Y., Liu, S., Liu, X., et al. (2019). Improving Plant Genome Editing with High-Fidelity xCas9 and Non-canonical PAM-Targeting Cas9-NG. Molecular Plant 12, 1027-1036.
This application claims the benefit under 35 USC §119(e) of the priority of U.S. Patent Application No. 62/964,483, filed Jan. 22, 2020. This application is hereby incorporated by reference in its entirety.
This invention was made with government support under HG010099 awarded by the National Institutes of Health and D18AP00053 awarded by DARPA. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62964483 | Jan 2020 | US |
