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  • C12N15/907—Stable introduction of foreign DNA into chromosome using homologous recombination in mammalian cells
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  • C12N2800/00—Nucleic acids vectors
  • C12N2800/80—Vectors containing sites for inducing double-stranded breaks, e.g. meganuclease restriction sites

Definitions

• the present invention relates to methods of obtaining Cas proteins suitable for use as single effector CRISPR system-associated nucleases, i.e., class II Cas proteins, which are not isolatable from recognized microbial sources.
• the invention provides reconstructed ancestral sequences derived by evolutionary tracing from a phylogenetic tree compiled using Cas protein sequences of existing species.
• Such reconstructed proteins are thus synthetic proteins in the sense that they are not isolatable from modern day sources but can be utilized in the same way as naturally-occurring Cas proteins in class II CRISPR systems which are now widely used for gene-editing.
• the Inventors have coined the term “ancestral Cas” or “AnCas” for such reconstructed sequences.
• CRISPR-Cas systems provide immunity to prokaryotes responding to invading nucleic acids from infectious genetic elements.
• Cas proteins guided by CRISPR- encoded RNA molecules gRNAs
• CRISPR-Cas9 system Since the first CRISPR-Cas9 system was repurposed as a gene editing tool, such CRISPR systems, and other class II CRISPR-Cas systems, have revolutionized the field of genome engineering. Nevertheless, CRISPR is not ready for implementation as a therapeutic tool due to limitations such as generation of unwanted mutations at similar loci, production of multiple alleles leading to genetic mosaicism, low efficiency and possible induction in the host of an immune response.
• Bacterial Cas9 proteins were the first studied Cas proteins and Spycas9 remains the most extensively studied Cas9 and much used for gene-editing. Such proteins are characterized as type II Cas9 proteins by containing two nuclease domains, both an HNH-like and RuvC-like nuclease domain, and associated catalytic residues required for double-strand DNA endonuclease cleavage resulting in blunt ends. Since 2012, Cas endonucleases have been isolated from many different bacteria and archaea.
• class II Cas nucleases The latest classification of class II Cas nucleases includes 3 types and 17 sub-types as reviewed in Makarova et al. , 2020 (Nature Rev Microbiol. 18(2):67-83).
• class II CRISPR-Cas systems currently include types II, V and VI systems with type VI systems being the first and so far, only variety of CRISPR-Cas systems that exclusively cleave RNA.
• type V systems fundamentally differ from type II systems by the domain architecture of their effector Cas proteins.
• type II effectors (Cas9 nucleases) contain two nuclease domains that are each responsible for the cleavage of one strand of the target DNA, with the HNH nuclease inserted inside the RuvC-like nuclease domain sequence
• type V effectors (Cas 12 nucleases) by contrast only contain a RuvC-like domain that cleaves both strands.
• Type VI effectors (Casl3 nucleases) are unrelated to the effectors of type II and type V, as they contain two HEPN domains and apparently target transcripts of invading DNA genomes in their native environment. Casl3 proteins also display collateral, non-specific RNase activity that is triggered by target recognition.
• V-U effectors a number with smaller RuvC-like domains are currently classified as sub-type V-U effectors. These show high sequence similarity to TnpB proteins (predicted RuvC-like nucleases) encoded by IS605-like transposons and are thought to be intermediates on the evolutionary path from TnpB to fully fledged type V effectors.
• CRISPR-Cas systems evolved from different groups of TnpB on multiple, independent occasions, as has been shown by phylogenetic analysis of the TnpB family. Analysis of the interference activity of four subtype V-U effectors has more recently resulted in one such variant being upgraded to a separate subtype V-F.
• Casl4f The subtype V-F effector, Casl2f (originally denoted Casl4), has been shown to cleave single-stranded DNA (ssDNA).
• ssDNA single-stranded DNA
• phylogenetic analysis of type V Cas enzymes has only been used as a means of classifying isolated naturally-occurring such nucleases with a single RuvC-like nuclease domain.
• SpyCas9 has been widely-adapted for genome editing and as a fusion enzyme for transcriptional control, epigenome-editing, base-editing and prime-editing. Despite its versatility, SpyCas9 is still limited for certain such applications by its “NGG” PAM recognition requirement.
• Phylogenetic Ancestral Sequence Reconstruction has been used to generate variants of bacterial Cas9 predicted to have been present in organisms that lived billions of years ago.
• Ancestral enzymes have greater stability and efficiency, exhibit chemical promiscuity and are more versatile than their modern descendants.
• a benefit of looking to ancestral enzymes for gene therapy is that the host’s pre-existing immunity against these proteins can be potentially dismissed.
• the inventors have designed and tested, for example, ancestral Firmicutes , Bacilli and Streptococci Cas9 forms. They show high level of expression, non-specific tracrRNA binding, and high efficiency gene editing in cells of the human HEK293T cell line.
• the invention has been founded on use of phylogenetic information for a diverse population of Cas9 enzymes from the phylum Firmicutes and within the bacterial classes of Clostridia and Bacilli, including many species of Streptococcus encompassing for example, Streptococcus pyogenes, plus some Cas9 sequences from the phylum of Actinobacteria, it will be appreciated that the same approach may be employed to obtain ancestral versions of Cas single nuclease effectors of other classification types, e.g. an ancestral Type V or Type VI Cas enzyme. The ancestral version may be of the same type but of a different sub-type.
• the present invention provides a method of phylogenetic ancestral reconstruction for obtaining a functional, single effector Cas protein nuclease (commonly referred to as Class II Cas protein), e.g., a functional Cas9 variant, comprising:
• step (a) providing a phylogenetic tree from sequence analysis of a population of Cas sequences comprising naturally-occurring single effector Cas nuclease sequences of the same classification type, e.g., a population of Type II Cas9 sequences, and derived from a plurality of existing species, preferably of more than one genus, still more preferably of more than one class and possibly spanning more than one phylum; (b) selecting an ancestral variant sequence by tracing back an evolutionary route from the phylogenetic tree, wherein the highest probability amino acid for each amino acid of the selected ancestral variant is determined, and (c) producing said variant, wherein said variant is capable of exhibiting Cas protein endonuclease and/or nickase activity.
• the starting population of Cas sequences for provision of the phylogenetic tree of step (a) may include one or more predetermined ancestral variant sequences obtained by prior application of such a method.
• step (b) may comprise:
• an ancestral sequence which is compiled as an ancestor sequence just for all or at least a large proportion of available Bacilli sequences spanning a plurality of genera, preferably further (iii) compiling at least one inter-class ancestor sequence able to trace back to starting species of more than one class.
• Such an ancestral variant may be a preferred selection for production, but a variety of ancestral variants thus compiled may be found to have beneficial properties.
• FIG. 1 One such evolutionary route map is shown in Figure 1 leading to compilation of such an inter-class ancestral variant sequence (or common phylum ancestor sequence) starting from a population of Cas9 sequences as noted above, including Cas9 sequences of existing bacterial species of both the Bacilli and Clostridia classes.
• the starting sequences of bacterial species of the Bacillus class include many known Cas9 sequences of Streptococci including SpyCas9 (28 in number).
• Use of such a diverse population of starting sequences, including Cas9 sequences from a diverse range of bacteria belonging to the Bacillus class, including a substantial number of existing Streptococci Cas9 sequences, e.g., 25 or more, will be recognized as highly desirable for such evolutionary map construction.
• step (c) will normally be by providing a nucleic acid sequence for expression in a suitable host cell, e.g., E. coli.
• the coding sequence may be codon- optimized.
• the exemplification illustrates how desired cleavage activity may be tested for even in the absence of knowledge of any PAM requirement. Desirably where such activity is observed initially by in vitro test, it will be maintained in further testing in human cells.
• the activity of the selected ancestral variant may be tested under conditions in vitro and in a human cell line known to be suitable for endonuclease activity of a Cas9 sequence from an existing species, e.g., SpyCas9.
• human codon-optimized sequences for the Cas enzymes will desirably be employed in expression vectors suitable for Cas protein expression in the chosen cells.
• the initially selected variant is a Cas endonuclease
• it may be subsequently converted to a nickase or converted to a deadCas (dcas) in known manner for amino acid mutagenesis of relevant nuclease catalytic sites and/or fused to a non-nuclease effector.
• dcas deadCas
• novel Cas enzymes obtained by an ancestral reconstruction method as described above and nucleic acid sequences encoding the same, e.g., provided in an expression vector for expression in a host cell.
• the Cas nucleases or Cas nuclease variants described herein are hereby interchangeably referred to as “ancestral Cas” or “AnCas”.
• AnCas variant enzymes which, compared to SpyCas9 under conditions for linearization of a dsDNA plasmid target by SpyCas as a reference nuclease, will exhibit time separable nickase and endonuclease activity reflected by a higher ratio of nicked template to linearized template. That is to say that the AnCas variant enzymes described herein may produce a greater percentage of nicked plasmid DNA template compared to SpyCas9 under the same conditions or may produce a lower percentage of double stranded breaks in a plasmid DNA template compared to SpyCas9 under the same conditions.
• SpyCas9 is not recognized as a nickase enzyme under commonly employed conditions of use except when one nuclease site is removed.
• AnCas enzymes obtained as Cas9 ancestral variants having a ratio of linearized DNA plasmid template to nicked DNA plasmid template of between at least 2.3:1 to at least 1:4 under conditions whereby SpyCas9 results in a ratio of linearized DNA plasmid target to nicked DNA plasmid template of at least 4:1.
• AnCas enzymes obtained in accordance with the invention such as LFCA, LBCA and LSCA may nick at least 30% of the DNA template up to at least 70%, e.g. about 80%, of the DNA template whereas under the same conditions SpyCas9 nicks about 10% of the DNA template in the same amount of time (see Figure 11).
• the AnCas nuclease has a higher nick rate and lower linearization rate on a dsDNA plasmid target under conditions whereby SpyCas9 results in substantially exclusively linearization or almost exclusively linearization while the AnCas nuclease and variants of interest provide under the same conditions observable nicked target.
• LFCA Cas, LBCA Cas and LSCA Cas have been shown to exhibit a higher nick rate and lower endonuclease (double stranded cleavage) rate than SpyCas9 under conditions suitable for SpyCas9 dsDNA cleavage.
• the ratio of these activities has been found to be a function of ancestral age with LFCA Cas having the highest ratio observed to date.
• Figure 1 illustrates ancestral Cas9 reconstruction and characterization. Shown is a phylogenetic tree of Cas9 enzymes from the Clostridia and Bacilli classes of the phylum Firmicutes , plus Cas9 enzymes from some Actinobacteria. The evolutionary route from
• Last Firmicute Common Ancestor SEQ ID NO 1 following the ancestors of Bacilli (LBCA; SEQ ID NO 2), Streptococci (LSCA; SEQ ID NO 3) and several streptococcus species ancestors to modern S. pyogenes (LPCA; SEQ ID NO 4 and LPCDA; SEQ ID NO 5) is indicated by the white dashed arrow.
• LPCA Last Firmicute Common Ancestor
• Figures 2a-c illustrate testing of AnCas endonuclease activity as exemplified by testing of LFCA.
• Figure 2a shows a DNA library containing 7 random nucleotides right after a target DNA; these 7 N represent all possible PAM sequences.
• Figure 2b shows Cas9 activity assay comparing LFCA with SpyCas9.
• LFCA Cas cuts the PCR target amplified from the DNA library, producing two fragments with the expected sizes.
• Figure 2c shows Cas9 activity assay using the S. pyogenes PAM sequence.
• LFCA is able to recognize the NGG PAM sequence as SpyCas9 does.
• Figures 3a-d illustrate demonstrating nicking and endonuclease activity of LFCA.
• Figure 3a shows a DNA plasmid containing a TGG PAM sequence after the DNA target. Cas9 can cut one or both strands of the DNA.
• Figure 3b shows a 1 % agarose gel of the DNA plasmid after 1 hour contact with 30 nM Cas9 resulting in endonuclease activity.
• LFCA under the same conditions shows nicking activity after 10 minutes of incubation and double strand cleavage after 1 hour.
• SpyCas9 exhibits mostly double strand cutting activity.
• Figure 3c shows total cleavage expressed in % (nicking and endonuclease activity) from both LFCA and SpyCas9 as a function of time.
• Figure 3d shows cleavage expressed in % with a distinction made between nicking and endonuclease activity.
• LFCA cuts one strand of DNA and, after 1 hour of incubation, starts to cleave the other strand.
• SpyCas9 has mostly endonuclease activity (i.e., cuts both strands).
• Figures 4a-b illustrate PAM determination for LFCA.
• Figure 4a shows the PAM wheel from LFCA PAM assessment with provision of 3 nucleotide PAMs.
• LFCA does not show specificity of recognition for any particular PAM with 3 nucleotides. Similar results were obtained with 7 nucleotide PAMs.
• Figure 4b shows in vitro cleavage assay results of plasmid DNA with different PAM sequences comparing both LFCA and SpyCas9.
• LFCA (10 nM) nicked all the plasmids with different PAMs within only 10 minutes of reaction.
• FIGS 5a-d show thermal and pH stability testing of LFCA.
• Figure 5a shows total Cas enzyme activity assay using plasmid DNA with a TGG PAM for 1 hour with 30 nM of Cas enzyme at pH 7.9 and different temperatures ranging from 4°C to 60°C.
• LFCA shows higher activity than SpyCas9 at 4°C and from 53-60°C.
• Figure 5b shows nicking and endonuclease activity of both Cas enzymes at different temperatures.
• Figure 5c shows total Cas enzyme activity assay using plasmid DNA with a TGG PAM for 1 hour and with 30 nM of Cas enzyme at 37°C and different pH ranging from 4 to 9.5.
• LFCA showed higher activity at acidic pH (4-5.5) in comparison with SpyCas9.
• Figure 5d shows nicking and endonuclease activity of both Cas enzymes at different pH.
• Figures 6a-f illustrate a comparison of LFCA and SpyCas9 genome-editing in HEK293T cells.
• Figure 6a shows humanized LFCA and SpyCas9 coding sequences cloned in expression plasmid pCDNA 3.1 for transfection with gRNA into HEK293T cells for targeting the AAVS1 locus.
• Figure 6b shows an immunofluorescent image from cells transformed with either Cas enzyme. Cells expressing the hCas coding sequence are stained in orange, the nucleus is stained with DAPI.
• Figure 6c shows results of a T7 assay for Cas enzyme activity.
• Figure 6d shows hCas9, gRNA and donor DNA carrying eGFP gene transfected into HEK293T cells for knock-in of eGFP into the AAVS llocus.
• Figure 6e shows confocal microscopy images of cells expressing eGFP.
• Figure 6f shows relative fluorescent measured in the images from cells expressing eGFP after hCas enzyme transfection.
• FIG 7 illustrates a comparison of LFCA and SpyCas9 knock-in in HEK293T cells targeting a TTC PAM. Also shown is an electrophoresis gel with the extracted gDNA from the cells and amplified locus. The bands with the expected size in all the samples are seen on the gel apart from the TTC PAM targeted with Spy Cas9.
• Figure 8 shows agarose gel test results illustrating the ability of LFCA to use a sgRNA in which the targeting sequence is linked to a tracRNA component corresponding to the Cas9 tracrRNA component of one of a wide variety of existing bacterial species having a Cas9 ortholog.
• Figure 9 provides a cladogram constructed using the sequences listed in Table 1. Each node represents an ancestral state with a sequence shown in the sequence listing.
• Figures lOa-c show nicking and endonuclease Cas9 activity of LFCA, LBCA and LSCA in comparison to SpyCas9.
• Figure 10a shows total cleavage (both nicking and endonuclease activity) for LFCA, LBCA, LSCA and SpyCas9. All the Cas9 enzymes reached total cleavage within around 10 minutes of incubation.
• Figure 10b shows plasmid linearization rate of LFCA, LBCA, LSCA and SpyCas9.
• Figure 10c shows nick rate of LFCA, LBCA, LSCA and SpyCas9.
• the linearization and nick rate are shown plotted against AnCas age for all of LFCA, LBCA and LSCA compared to SpyCas9.
• a negative value for the nick rate is shown.
• This is a conversion of the percentage of cleaved (nicked or linearized) plasmid template as shown in Figure 11 into time units which provides a negative lambda parameter of the exponential decay shown in Figure 10.
• LBCA and LSCA have a higher linearization rate than LFCA, but still lower than SpyCas9 and hence the trend is for linearization rate to decrease with ancestral age. In contrast, the nick rate increases with ancestral age.
• Figures 13a-b illustrate PAM determination for LBCA and LSCA.
• Figure 13a shows a PAM wheel from LBCA and LSCA PAM sequencing.
• LBCA and LSCA do not show specificity of recognition for any 3 nucleotide PAM. Similar results were obtained using 7 nucleotides.
• Figure 13b shows results of an in vitro cleavage assay of plasmid DNA with different PAM sequences comparing LFCA, LBCA, LSCA and SpyCas9.
• LBCA (10 nM) nicked all the plasmids with different PAM within 10 minutes of reaction.
• LSCA showed similar preference but has higher linearization rate cleavage.
• Figure 14 shows results of testing the same ancestral Cas9 enzymes for endonuclease activity on single-stranded DNA. It is shown that the three ancestral enzymes cleave single-stranded DNA with or without gRNA. As expected, SpyCas9 was unable to cleave the same single stranded DNA.
• Figures 15A-E show the activity of AnCas endonucleases on a supercoiled DNA substrate.
• Figure 15A shows in vitro cleavage assay for SpCas9 and all AnCas on a 4007-bp substrate at different reaction times showing nicked and linear fractions.
• Figure 15B shows the quantification of total cleavage at different reaction times and exponential fits (lines).
• Figure 17D shows in vitro cleavage assay on a 60-nt ssRNA at different incubation times for LFCA [FCA], LBCA [BCA] and SpCas9.
• Figure 17E shows the quantification of fraction cleavage of ssDNA at different times and exponential fits for determination of kinetics parameters.
• the control lane is the same for the three proteins.
• Figure 17F shows the quantification of fraction cleavage of ssRNA at different times and exponential fits for determination of kinetics parameters. All kinetics parameters are summarized in Table 2.
• Figures 19A-D show the activity of LFCA [FCA] H838A endonucleases on a supercoiled DNA substrate.
• Figure 19A shows in vitro cleavage assay for LFCA [FCA] H838A on a 4007-bp substrate at different reaction times showing nicked and linear fractions.
• Figure 19B shows the quantification of total cleavage fraction at different reaction times and exponential fits (lines).
• Figure 19C shows the quantification of fraction nicked at different times.
• Figure 19D shows the quantification of DSB cleavage. Single-exponential fits were used to obtain kcieave and maximum fraction cleaved (amplitude).
• Figure 23 shows traffic light reporter cleavage assay.
• the relative NHEJ frequency is estimated by the number of RFP-positive cells and is normalized to SpCas9.
• the starting sequence population for obtaining a functional ancestral Cas variant by the strategy now taught may preferably be, as exemplified, a population of Cas9 sequences from bacterial species in existence, whereby a phylogenetic tree can be constructed based on sequence alignment information.
• Computer-implemented methods for constructing such trees are well known in the field involving sequence alignment and recognition of conserved regions.
• the phylogenetic tree may be constructed of sequences of another class II Cas enzyme type.
• the starting sequences will span more than one genus.
• a plurality of Cas9 sequences may be selected from two or more of Streptoccocus , Enterococcus , Listeria , Clostridium , Pelagirhabdus , Halolactibacillus , Floricoccus, Vagococcus, Urinacoccus , Vagococcus, Dorea, Ruminococcus , Lachnospira ,
• the starting population of sequences will span more than one class of a phylum of interest.
• the starting population of sequences may desirably comprise at least multiple sequences derived from different species of Streptococcus , multiple sequences derived from different species of Enterococcus , multiple sequences derived from different species of Listeria and multiple sequences derived from species of Clostridium.
• the diversity of the starting population may be further expanded to cross phyla as illustrated by inclusion of some Actinobacteria sequences in the starting population of Cas9 sequences employed in the Example section.
• the starting population of Cas sequences may span a plurality of sub-types.
• evolutionary routes to predicted ancestral forms may be compiled, which may equate with many millions of years predating today.
• a selected ancestral variant sequence obtained in accordance with the invention may equate with an evolutionary period of at least 500 million years from the present, for example at least 700-800 million years, or even 1000 million years or more.
• the evolutionary period may equate with as long as 2-3 Bys, e.g ., about 2.2 to 2.4 Bys.
• LFCA Cas is such a reconstructed ancestor of Cas enzymes derived from evolutionary route analysis off a phylogenetic tree of Cas9 sequences derived from a population of existing bacterial species spanning Clostridia and Bacilli (both bacterial genera of the Firmicutes phylum) and supplemented with some Actinobacteria.
• LSCA is in turn an ancestor to a reconstructed ancestor of a smaller selection (8 out of 28) of Cas enzymes of Streptococcus origin (the reconstructed ancestor designated LPCA Cas, with SEQ ID NO: 4) and LPCA is in turn an ancestor of Streptococcus pyogenes and Streptococcus dysgalactiae sequences (the reconstructed ancestor LPDCA Cas, with SEQ ID NO: 5).
• LFCA Cas is represented by node 63 of the illustrated evolutionary route and has the amino acid sequence shown in SEQ ID NO: 1. It shows high production level as well as high efficiency targeting and editing of DNA both in vitro and in human cells.
• a Cas nuclease comprising or consisting of the LFCA Cas having the amino acid sequence of SEQ ID NO: 1, representing a preferred example of a functional ancestral Cas obtained by adoption of the strategy taught herein for identification of such novel Cas enzymes.
• the LFCA Cas is deemed evolutionarily related to SpyCas9 but with a number of advantageous differences which render it an especially preferred AnCas nuclease.
• LFCA Cas is not known in nature and has only 54 % of sequence identity with SpyCas9. Nevertheless, it can employ a sgRNA with the 3' end of a SpyCas sgRNA for guide RNA/Cas protein interaction as shown in the Example section;
• LFCA Cas shows cleavage activity for a single-stranded DNA substrate as shown in Figure 13. As is well known, this is not an activity of SpyCas9 under normal usage conditions in the gene modification field.
• LFCA Cas has been shown to be capable of driving indel formation at a targeted locus (exemplified herein with the AAVS1 locus) when expressed in such cells with a suitable gRNA. Furthermore, ability to drive knock-in genetic modification at the same locus has been confirmed as shown in
• a sgRNA in which the targeting sequence is linked to a tracrRNA component wherein said tracrRNA component is selectable from the tracrRNA components of Cas9 gRNAs employed by a plurality of existing bacterial species, e.g., including all of Streptococcus pyogenes , Streptococcus thermophilus , Enterococcus faceium , Clostridium perfringens and Finegoldia magna.
• an AnCas with the above-noted relaxed PAM specificity, possibly in combination with one or both of characteristics (a) and (d) or one or both of characteristics (a) and (e) or possibly in combination with all of (a), (d) and (e).
• the selected AnCas may, for example, provide a ratio of linearized DNA plasmid target to nicked DNA plasmid template of between at least about 2.3 : 1 to at least 1 :4 under conditions whereby SpyCas9 results in a ratio of linearized DNA plasmid target to nicked DNA plasmid template of at least about 4:1.
• Such a method may further comprise converting such an AnCas nuclease to a variant which is either a nickase only or a deadCas with no nuclease activity and/or provide linkage to a non-nuclease effector, e.g., in a fusion protein.
• a variant which are either a nickase only or a deadCas with no nuclease activity and/or a fusion protein are also contemplated as product per se in the present invention.
• variants of the exemplified AnCas nucleases noted above which retain one or more of the distinguishing characteristics (a) to (e) above compared to SpyCas9.
• especially preferred may be retention of the relaxed PAM specificity as exhibited by, for example, LFCA Cas, possibly in conjunction with one, two or all characteristics specified in (a), (b), (d) and (e) above, e.g., production of higher amounts of nicked template and/or lower amounts of linearized template (amount of double stranded breaks) compared with SpyCas9 as noted above and/or higher ratio of nick rate to linearization rate compared with SpyCas9 as noted above and/or ability to cleave single-stranded DNA.
• all these characteristics will be retained.
• LFCA Cas and variants thereof which are functionally equivalent, i.e., maintain all the characteristics of LFCA Cas (i) to (viii) listed above.
• LFCA Cas variants which retain at least relaxed PAM specificity and/or flexible tracrRNA usage as discussed above are, however, deemed highly favourable additions to the Cas enzyme toolbox.
• linear activity and “endonuclease activity” are used synonymously herein to refer to nuclease activity for cleaving both strands of a double stranded DNA where it is provided in the form of plasmid.
• cleavage buffer e.g, 100 mM NaCl, 50 mM Tris-HCl, 10 mM MgCk, 100 pg BSA, pH 7.9
• target DNA e.g., a plasmid
• the preferred Cas nucleases of the invention may produce a ratio of linearized DNA plasmid target to nicked DNA plasmid template of between at least 2.3:1 to at least 1:4 under conditions whereby SpyCas9 results in a ratio of linearized DNA plasmid target to nicked DNA plasmid template of at least 4:1.
• the percentage of DNA template with double stranded breaks (DSB) (i.e ., linearized template) formed by the preferred Cas nucleases of the invention may be from 10 % up to about 70 %.
• the percentage of DNA template with double stranded breaks (DSB) formed by the preferred Cas nucleases of the invention may be at most 70 %, 60 %, 50 %, 40 %, 30 %, 20 %, or 10 %.
• the percentage of DNA template with double stranded breaks (DSB) formed by the preferred Cas nucleases of the invention may be from 15 % up to about 65 %.
• the percentage of double stranded breaks (DSB) formed in a DNA template by the preferred Cas nucleases of the invention may be from 19 % up to about 62 %.
• LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas and LPCDA Cas and variants thereof as discussed above are seen as highly useful novel additions to the toolbox of Cas proteins, especially LFCA Cas with the highest observed nick rate.
• LFCA Cas LBCA Cas, LSCA Cas, LPCA Cas and LPCDA Cas or such variants may be linked, e.g. , fused, with an effector protein for gene modification, e.g. , a base editor such as a deaminase for base editing or a reverse transcriptase for prime-editing.
• a base editor such as a deaminase for base editing or a reverse transcriptase for prime-editing.
• variants of any of LFCA Cas, LBCA Cas, LSCA Cas, LPCA and LPCDA Cas which have one or more amino acid changes by way of substitution or deletion, e.g., one or more conservative substitutions, may be similarly employed as a Cas9 endonuclease or Cas9 nickase provided endonuclease and/or nickase activity is retained.
• variants that also retain the relaxed PAM specificity as shown for LFCA Cas, LBCA Cas and/or LSCA Cas.
• any of LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas and LPCDA Cas or variants thereof as noted above may also be converted to a dCas with no nuclease activity by catalytic site mutagenesis and may further be linked, e.g. , fused, with a non-nuclease effector protein.
• LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas and LPCDA Cas, variants thereof as discussed above (and functional equivalents thereof, e.g. , those represented by a node on the evolutionary route of Figure 9 or others obtained in accordance with the ancestral reconstruction strategy of the invention) may be employed in the whole panoply of genetic modification techniques envisaged for naturally-occurring Cas9 nucleases of existing species and modified versions thereof. These extend to use in combination with an effector for gene modification or regulation such as a base editor where linkage is via an RNA extension of a guide and an RNA-binding domain as taught in WO 2017/011721 (Rutgers University, licensed to Horizon Discovery). See also Collantes etal, 2021. CRISPRJ.. 4(l):58-68).
• the invention additionally provides nucleic acids for expression of the AnCas proteins described herein, including variants and functional equivalents thereof, e.g., expression vectors for expression of such proteins.
• Such vectors may be employed with a guide RNA, or guide RNA expressed from DNA.
• a combination of vectors providing an AnCas nuclease as herein taught or variant or functional equivalent thereof, and a suitable guide RNA may be provided for transfection into cells.
• the Cas protein may be LFCA Cas or a corresponding nickase.
• the corresponding nickase of any of the ancestral enzymes herein taught may be so provided, e.g. , an LBCA nickase, LSCA nickase, LPCA nickase or LPCDA nickase.
• the present invention further relates to a nucleic acid capable of expressing a Cas nuclease or Cas nuclease variant according to the invention.
• the nucleic acid is a DNA or an RNA molecule.
• the nucleic acid is a DNA molecule, e.g. , a complementary DNA molecule.
• the nucleic acid is an RNA molecule, e.g. , a messenger RNA molecule.
• the nucleic acid is single stranded or double stranded. In some embodiments, the nucleic acid is single stranded. In some embodiments, the nucleic acid is double stranded.
• the present invention further relates to a combination of a vector comprising the nucleic acid according to the invention as described hereinabove, and a guide RNA for targeting the Cas nuclease or variant or functional equivalent thereof to a target DNA sequence, or a vector capable of expressing the guide RNA.
• a novel AnCas nuclease as herein taught such as LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas or LPCDA Cas or a variant or functional equivalent thereof, e.g ., a corresponding nickase as discussed above, may be provided as a ribonucleoprotein (RNP) complex with a guide RNA for transfection into cells, e.g.
• RNP ribonucleoprotein
• the present invention further refers to a ribonucleoprotein complex comprising a Cas nuclease or Cas nuclease variant according to the invention, or a Cas nuclease or Cas nuclease variant and a guide RNA for targeting the Cas nuclease or Cas nuclease variant to a target DNA sequence.
• guide RNA may be a single molecule targeting RNA (sgRNA) or, if suitable as for a naturally-occurring Cas9, a dual sequence RNA comprising (i) a DNA targeting segment comprising a nucleotide sequence complementary to the target sequence (the crRNA) and (ii) a protein-binding segment that interacts with the Cas protein (the tracrRNA).
• sgRNA single molecule targeting RNA
• Cas9 a dual sequence RNA comprising (i) a DNA targeting segment comprising a nucleotide sequence complementary to the target sequence (the crRNA) and (ii) a protein-binding segment that interacts with the Cas protein (the tracrRNA).
• the invention provides a method for modifying or regulating a target nucleic acid sequence, e.g. , a target DNA sequence, the method comprising contacting the target sequence with a complex comprising (i) a Cas protein as taught, e.g., LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas or LPCDA Cas, or a variant or functional equivalent thereof as discussed above, and (ii) a guide RNA for targeting the Cas protein to the target sequence, wherein either:
• said contacting is in vitro on an isolated target nucleic acid sequence or in a cell ex vivo, preferably with the proviso that methods of modifying the germ line identity of a human being are excluded;
• the method is not a method of medical treatment practiced on the human or animal body.
• the complex may further comprise a nucleic acid molecule encoding a transgene of interest, e.g. , for introduction of this transgene of interest int the target DNA sequence.
• the Cas protein may be, for example, LFCA Cas, LBCA Cas, or another exemplified AnCas nuclease as noted above, which retains the same relaxed PAM requirement.
• the Cas protein may be such an AnCas but modified to present only a nickase activity or no nuclease activity in the form of a fusion protein.
• Cas proteins as now taught e.g., LFCA Cas, LBCA Cas and other AnCas, including variants and functional equivalents thereof, may also find use in relation to genetic modification in plants, e.g, by modifying target sequences, possibly but not exclusively, in protoplasts.
• the invention also extends to a combination for use in therapeutic treatment by modifying or regulating a target nucleic acid sequence, for example a DNA sequence, wherein the combination comprises: (i) a Cas protein as taught herein, e.g, LFCA Cas, LBCA Cas, LSCA Cas, LPCA Cas, or LPCDA Cas, or a variant or functional equivalent thereof as discussed above, or a polynucleotide capable of expressing the same, and (ii) a guide RNA for targeting the Cas protein to a target nucleic acid sequence or a polynucleotide capable of expressing the same.
• therapeutic treatment may include the prevention and/or treatment of genetic diseases.
• the combination may then further comprise a nucleic acid molecule encoding a transgene of interest, wherein said transgene of interest may, e.g, compensates a gene defect responsible for the genetic disease.
• the low sequence identity of, for example LFCA Cas to SpyCas9 is considered advantageous in relation to contemplating such use.
• Such use may embrace for example Cas action in pathogenic bacteria or for manipulation of the gut microbiome or skin microbiome.
• the following exemplification illustrates the invention with reference to both obtaining and testing of the Cas enzymes LFCA Cas, LBCA Cas and LSCA Cas, but as noted above it is envisaged that other ancestral Cas proteins with advantageous properties may be obtained by the same strategy depending on the choice of starting population of Cas enzyme sequences providing the phylogenetic tree for the evolutionary analysis.
• the predicted resurrection may be as old as 3 Bys.
• Sequences were collected of the gene cas9 from the Uniprot database from several Firmicutes bacterial species using as query the sequence SpyCas9 (Uniprot code: Q99ZW2). The search confirmed the existence of hundreds of sequences of cas9 genes from the phylum Firmicutes within the classes Bacilli and Clostridia. Some sequences from Actinobacteria were also found. After downloading 59 sequences (Table 1), a sequence alignment was constructed that confirmed the common origin of the Cas9 sequences with a portion of the sequences showing significant conservation. Using Bayesian inference (BEAST software), a phylogenetic tree was compiled to confirm the phylogenetic relationship of the sequences.
• BEAST software Bayesian inference
• a DNA fragment containing the S. pyogenes PAM was cloned and incubated with LFCA Cas or SpyCas9 at different times ranging from 5 to 160 minutes. Both enzymes were incubated with gRNA and target DNA and the reaction stopped by adding loading buffer and EDTA. The samples were run on a 1 %-agarose gel to detect supercoiled, nicked and linear DNA (Fig. 3a). On the agarose gel (Fig. 3b), the different DNA conformations after Cas enzyme activity were observed. The band intensity was measured and the total cleavage by both enzymes at different times was calculated (Fig. 3c).
• the AnCas genes were synthesized and cloned into pBAD/glll expression vectors, carrying an arabinose inducible promoter and a gill-encoding signal that directs the AnCas to the periplasmic space. All AnCas were expressed at high levels in Escherichia coli BL21 cells.
• H838A LFCA AnCas mutant was tested (H840A with respect to the wild-type SpCas9 amino acid sequence).
• the mutant was able to produce nicked and, surprisingly, linear products, showing a profile practically identical to that obtained with wild type LFCA AnCas (Fig. 19A-D).
• the cleavage activity of the AnCas enzymes could be seen to follow a trend which is shown in Fig. 11.
• the percentage of double stranded breaks after an incubation time of 30 minutes can be seen to be highest for SpyCas9 and decreases with the age of ancestral Cas enzymes (i.e., the percentage of double stranded breaks can be seen to decrease with the older the ancestral enzyme as follows: %DSB for SpyCas9 > %DSB for LSCA Cas > %DSB for LBCA Cas > %DSB for LFCA Cas).
• a DNA library containing a target sequence followed by seven random nucleotides (NNNNNNN) that corresponded to all possible PAMs was designed.
• An sgRNA was designed using the scaffold of S. pyogenes and 20 nucleotides complementary to the target sequence.
• PCR primers were designed to amplify an 844 bp-fragment containing both the target and PAM sequence, which was used as a substrate for AnCas and SpCas9. In vitro digestion using the purified Cas protein, and the transcribed sgRNA, was performed with the PCR target.
• Fig. 16A summarizes the results of PCR cleavage assay in the form of PAM wheels (Krona plot) for the five AnCas and SpCas9.
• NGS Next-generation Sequencing
• LFCA AnCas is the first fully PAMless Cas9 endonuclease ever reported to our knowledge.
• the fragment was sequenced by Illumina sequencing and the reads were mapped to the reference sequence using Geneious Prime (2020 version).
• Illumina miSeq reads were aligned against amplified sequence with minimap2 for short reads to filter unspecific sequences. Then, reads with 3 nucleotides before the PAM region were selected from the aligned reads. The nucleotides in the region of interest were extracted using a custom script. Finally, logo plots of the PAM region were obtained with ggseqlogo and the PAM wheel of each sample was graphically represented with KronaTools.
• sgRNAs were selected following previous studies on sgRNA classification and function, in which sgRNAs were divided into seven clusters. These distinct sgRNAs were contrasted against S. pyogenes guides containing spacers of two sizes, 18 and 20-nucleotide long, referred as “18 sgRNA” and “20 nt sgRNA”, respectively.
• SpCas9 and the five AnCas were incubated for 10 minutes at 37°C with a target plasmid DNA and TGGPAM recognition. From the agarose gel of cleavage products in Fig. 17A, it can be observed that, as expected, SpCas9 only linearized plasmid DNA when using its own sgRNA, although more efficiently when using the 20 nt spacer version, and sgRNAs from other species mostly resulted in nicked products leaving most supercoiled DNA substrate intact. On the contrary, LFCA Cas and LBCA Cas were able to nick and linearize plasmid DNA with all sgRNAs, the A.
• faecium sgRNA showing better efficiency for LFCA Cas, and the 18 nt sgRNA from S. pyogenes preferred for LBCA Cas.
• the other AnCas were also tested, observing that mostly LFCA Cas and LBCA Cas had a marked promiscuity for sgRNA. All other AnCas and SpCas9 seemed to work best with a 20 nt sgRNA from S. pyogenes (Fig. 17B).
• LFCA Cas In another line of experiments, using the same in vitro plasmid cleavage assay as noted above, the ability of LFCA Cas to use sgRNAs with a targeting sequence linked to various tracrRNA sequences was investigated. TracrRNA sequences were employed corresponding to the tracrRNA components employed by Cas9 gRNAs of various existing bacterial species. Thus, a plasmid was provided including the S. pyogenes PAM TGG.
• LFCA Cas9 has very flexible gRNA use. It was able to nick or linearize the plasmid DNA regardless of the tracrRNA element of the gRNA employed. Indeed, improved cleavage was seen with some sgRNAs other than a conventional S. pyogenes sgRNA. Such gRNA flexibility is not shown for SpyCas9 and, as indicated above, is believed to be another novel property of LFCA Cas. Thermal and pH stability
• LFCA Cas The thermal stability of LFCA Cas was studied by performing a cleavage reaction for 1 hour at pH 7.9 and at different temperatures ranging from 4°C to 60°C.
• LFCA Cas showed higher activity than SpyCas9 at low temperatures (4°C and 20°C) and presented higher thermal stability from 53°C to 60°C (Fig. 5a).
• the nicking and endonuclease activities were calculated and it was observed that LFCA Cas had nicking activity at lower temperatures; at higher temperatures, the two activities were equally distributed (Fig. 5b).
• AnCas in particular LFCA AnCas, might share some commonalities with type V effector nucleases lacking a HNH domain (e.g, Cpfl (Casl2a), Casl4 (Casl2f) or Cas ⁇ E> (Casl2j)27-30).
• Cpfl Casl2a
• Casl4 Casl2f
• Cas ⁇ E> Casl2j
• the oldest AnCas such as LFCA Cas, LBCA Cas and LSCA Cas showed high activity at pH values below 7, unlike SpCas9 and newer AnCas, where activity drops abruptly.
• AnCas endonucleases outperformed SpCas9 at low and high temperatures, below 10°C and above 50°C.
• HEK293T cell genome editing by LFCA Cas HEK293T cells were transfected with an expression plasmid carrying an LFCA Cas humanized gene to study the ancestral enzyme effectiveness at editing genomic DNA.
• a gRNA to target the AAVS1 locus with S. pyogenes PAM was designed.
• the expression plasmid with encoded LFCA Cas was co-transfected with another plasmid to express the gRNA (Fig. 6a). Then, the genomic DNA was extracted to study the insertion and deletion events (indels). Intracellular LFCA Cas expression was confirmed by making an immunofluorescent image of the cells using an anti-Cas9 antibody (orange) (Fig. 6b).
• the cell nucleus was dyed blue by DAPI. Cells were observed that expressed the LFCA Cas in the nucleus, in the same way as SpyCas9.
• the genomic DNA was extracted from the HEK293T cells after 72 hours of transfection and a fragment of the AAVS1 locus was amplified where the Cas enzyme cleavage was targeted.
• T7E1 endonuclease assay was performed with these fragments to confirm genome editing (Fig. 6c). After T7E1 incubation, the two expected fragments were observed, confirming indel formation after LFCA Cas transfection. The same was observed with intracellular expression of SpyCas9 as control.
• FIG. 7 A similar knock-in experiment targeting the AAVS1 region was carried out but using a different PAM than that of SpyCas9.
• the TTC PAM was targeted (Fig. 7).
• Cells were transfected with a gRNA and LFCA Cas or SpyCas9. Fluorescence was observed after 72 hours in all the samples from the DNA template (some transient fluorescence in the TTC sample with SpyCas9).
• the gDNA was extracted and the A A VS I locus was amplified.
• the PCR amplicons were run on a gel. The expected band in all the LFCA Cas samples was observed but not in samples with SpyCas9.
• the gDNA was extracted and amplified at the AAVS1 locus.
• ssDNA cutting activity has been suggested to be an ancestral trait present in smaller Cas9 such as subtype II-C Cas9. This could also be reflected in the nickase activity of the ancestral forms from subtype II-A, such as AnCas.
• Earlier forms of Cas9 with smaller catalytic domains might have been the origin of this ssDNA cutting activity that was still present in larger ancestral nucleases, which then gradually evolved towards DSB activity over time as part of a specialization process.
• the genome editing activity of these ancestral nucleases was tested in mammalian cells (HEK293T) in culture, to answer the question whether these synthetic ancestral Cas can perform DNA cleavage; double strand breaks (DSB), and trigger editing in cells by non-homologous end joining (NHEJ) under similar conditions as those associated with the standard SpCas9.
• the cells were co-transfected with plasmid vectors containing the humanized versions of AnCas or SpCas9, as well as the corresponding sgRNAs (standard sgRNA from S. pyogenes carrying a 20-nt spacer target with SEQ ID NO: 237 to 239). Seventy-two hours after co-transfection, cells were collected and the genomic DNA extracted. In vitro site-specific editing was measured in the HEK293T cells by Next- generation Sequencing (NGS) using advanced analysis with Mosaic finder software.
• NGS Next- generation Sequencing
• TLR Traffic Light Reporter
• gDNA was extracted from cells using DNAzol Reagent (ThermoFisher) according to the manufacturer’s protocol.
• DNA target was amplified by PCR using Phusion® Hot Start Flex DNA Polymerase (NEB) using primers (F' TATTGTTCCTCCGTGCGTCAG (SEQ ID NO: 8) and R GACGAGAAACACAGCCCCA ( SEQ ID NO: 9)) from gDNA.
• the T7EI assay was performed using as substrate these PCR amplicons to confirm indel formation.
• the T7E1 endonuclease (NEB) was used according to the manufacturer’s protocol. Reaction was stopped by adding 6x loading dye (NEB) with EDTA and the final reaction products run on a 2% agarose gel. Gels were dyed with SYBR gold (ThermoFisher) and imaged with ChemiDoc XRS + System (Bio-Rad).
• DNA plasmid carrying TGG PAM was used.
• the cleavage assay was performed in cleavage buffer (100 mM NaCl, 50 mM Tris-HCl, 10 mM MgCh, 100 pg/BSA, pH 7.9) at 37 °C. 3 nM of AnCas and SpCas9 were incubated for 15 min with 3 nM sgRNA of each species at 1:1 ratio in cleavage buffer and 3 nM DNA plasmid was added. After 10 min, the reaction was stopped by adding 6X loading dye (NEB) with EDTA and run 2% agarose gel.
• 6X loading dye NEB
• ssDNA or ssRNA was added and incubated for different time intervals (0, 5, 10, 30 and 60 min).
• reaction was stopped by adding 6X loading dye (NEB) with urea. Samples were boiled for 10 min at 80 °C and were resolved by 2.5% denaturing urea agarose gel.
• ssRNA target reaction was stopped by adding 2X RNA gel -loading buffer (NEB) with urea. Samples were boiled for 10 min at 95 °C and were resolved by 15% denaturing urea polyacrylamide gel electrophoresis.
• ELISA test Elisa test was performed by using a modified protocol described elsewhere 60 . Briefly, 1 pg/well of SpCas9, LFCA AnCas, LBCA AnCas and bovine serum albumin (BSA, Sigma Aldrich) were diluted in lx bicarbonate buffer and coated onto 96-well plates (ThermoFisher Scientific) overnight at 4 °C. Plates were washed with IX wash buffer (TBST, ThermoFisher Scientific) and blocking with 1% BSA blocking solution for 1 hour at room temperature. Anti-Cas9 rabbit antibody (Rockland, 600-401 -GKO) was diluted 1:25000 in 1% BSA blocking solution and plates were incubated for 2 hours at room temperature.
• BSA bovine serum albumin
• HEK293T cells Functional validation of ancestral Cas nucleases was carried out in human HEK293T cells, as described elsewhere (Harms, D.W. et al. Human Genetics 83, 2014).
• Cells were grown in DMEM medium (Dulbecco’ s Modified Eagle Medium, Gibco), supplemented with sterile-filtered 10% fetal bovine serum (FBS), 10 mM HEPES pH 7.4, 2 mM L-glutamine and penicillin (100 IU/ml)-streptomycin (lOO pg/ml) and handled under aseptic conditions using a sterile hood.
• HEK293T cells were cultured in incubators at 37°C, 95% humidity and 5% CO2.
• SEQ ID NOs: 1 to 5 correspond to the ancestral Cas proteins exemplified above.

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