Rationally-designed meganucleases with altered sequence specificity and dna-binding affinity   

Abstract: Rationally-designed LAGLIDADG meganucleases and methods of making such meganucleases are provided. In addition, methods are provided for using the meganucleases to generate recombinant cells and organisms having a desired DNA sequence inserted into a limited number of loci within the genome, as well as methods of gene therapy, for treatment of pathogenic infections, and for in vitro applications in diagnostics and research.

This application is a continuation of U.S. patent application Ser. No. 13/223,852 filed Sep. 1, 2011, which is a continuation of U.S. patent application Ser. No. 11/583,368 filed Oct. 18, 2006, now U.S. Pat. No. 8,021,867, which claims the benefit of priority to U.S. Provisional Patent Application No. 60/727,512, filed Oct. 18, 2005, the entire disclosures of which are incorporated by reference herein.

The invention was supported in part by grants 2R01-GM-0498712, 5F32-GM072322 and 5 DP1 OD000122 from the National Institute of General Medical Sciences of National Institutes of Health of the United States of America. Therefore, the U.S. government may have certain rights in the invention.

FIELD OF THE INVENTION

The invention relates to the field of molecular biology and recombinant nucleic acid technology. In particular, the invention relates to rationally-designed, non-naturally-occurring meganucleases with altered DNA recognition sequence specificity and/or altered affinity. The invention also relates to methods of producing such meganucleases, and methods of producing recombinant nucleic acids and organisms using such meganucleases.

BACKGROUND OF THE INVENTION

Genome engineering requires the ability to insert, delete, substitute and otherwise manipulate specific genetic sequences within a genome, and has numerous therapeutic and biotechnological applications. The development of effective means for genome modification remains a major goal in gene therapy, agrotechnology, and synthetic biology (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73; Tzfira et al. (2005), Trends Biotechnol. 23: 567-9; McDaniel et al. (2005), Curr. Opin. Biotechnol. 16: 476-83). A common method for inserting or modifying a DNA sequence involves introducing a transgenic DNA sequence flanked by sequences homologous to the genomic target and selecting or screening for a successful homologous recombination event. Recombination with the transgenic DNA occurs rarely but can be stimulated by a double-stranded break in the genomic DNA at the target site. Numerous methods have been employed to create DNA double-stranded breaks, including irradiation and chemical treatments. Although these methods efficiently stimulate recombination, the double-stranded breaks are randomly dispersed in the genome, which can be highly mutagenic and toxic. At present, the inability to target gene modifications to unique sites within a chromosomal background is a major impediment to successful genome engineering.

One approach to achieving this goal is stimulating homologous recombination at a double-stranded break in a target locus using a nuclease with specificity for a sequence that is sufficiently large to be present at only a single site within the genome (see, e.g., Porteus et al. (2005), Nat. Biotechnol. 23: 967-73). The effectiveness of this strategy has been demonstrated in a variety of organisms using chimeric fusions between an engineered zinc finger DNA-binding domain and the non-specific nuclease domain of the FokI restriction enzyme (Porteus (2006), Mol Ther 13: 438-46; Wright et al. (2005), Plant J. 44: 693-705; Urnov et al. (2005), Nature 435: 646-51). Although these artificial zinc finger nucleases stimulate site-specific recombination, they retain residual non-specific cleavage activity resulting from under-regulation of the nuclease domain and frequently cleave at unintended sites (Smith et al. (2000), Nucleic Acids Res. 28: 3361-9). Such unintended cleavage can cause mutations and toxicity in the treated organism (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73).

A group of naturally-occurring nucleases which recognize 15-40 base-pair cleavage sites commonly found in the genomes of plants and fungi may provide a less toxic genome engineering alternative. Such “meganucleases” or “homing endonucleases” are frequently associated with parasitic DNA elements, such as group 1 self-splicing introns and inteins. They naturally promote homologous recombination or gene insertion at specific locations in the host genome by producing a double-stranded break in the chromosome, which recruits the cellular DNA-repair machinery (Stoddard (2006), Q. Rev. Biophys. 38: 49-95). Meganucleases are commonly grouped into four families: the LAGLIDADG family, the GIY-YIG family, the His-Cys box family and the HNH family. These families are characterized by structural motifs, which affect catalytic activity and recognition sequence. For instance, members of the LAGLIDADG family are characterized by having either one or two copies of the conserved LAGLIDADG motif (see Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774). The LAGLIDADG meganucleases with a single copy of the LAGLIDADG motif form homodimers, whereas members with two copies of the LAGLIDADG motif are found as monomers. Similarly, the GIY-YIG family members have a GIY-YIG module, which is 70-100 residues long and includes four or five conserved sequence motifs with four invariant residues, two of which are required for activity (see Van Roey et al. (2002), Nature Struct. Biol. 9: 806-811). The His-Cys box meganucleases are characterized by a highly conserved series of histidines and cysteines over a region encompassing several hundred amino acid residues (see Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774). In the case of the NHN family, the members are defined by motifs containing two pairs of conserved histidines surrounded by asparagine residues (see Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774). The four families of meganucleases are widely separated from one another with respect to conserved structural elements and, consequently, DNA recognition sequence specificity and catalytic activity.

Natural meganucleases, primarily from the LAGLIDADG family, have been used to effectively promote site-specific genome modification in plants, yeast, Drosophila, mammalian cells and mice, but this approach has been limited to the modification of either homologous genes that conserve the meganuclease recognition sequence (Monnat et al. (1999), Biochem. Biophys. Res. Commun. 255: 88-93) or to pre-engineered genomes into which a recognition sequence has been introduced (Rouet et al. (1994), Mol. Cell. Biol. 14: 8096-106; Chilton et al. (2003), Plant Physiol. 133: 956-65; Puchta et al. (1996), Proc. Natl. Acad. Sci. USA 93: 5055-60; Rong et al. (2002), Genes Dev. 16: 1568-81; Gouble et al. (2006), J. Gene Med. 8(5):616-622).

Systematic implementation of nuclease-stimulated gene modification requires the use of engineered enzymes with customized specificities to target DNA breaks to existing sites in a genome and, therefore, there has been great interest in adapting meganucleases to promote gene modifications at medically or biotechnologically relevant sites (Porteus et al. (2005), Nat. Biotechnol. 23: 967-73; Sussman et al. (2004), J. Mol. Biol. 342: 31-41; Epinat et al. (2003), Nucleic Acids Res. 31: 2952-62).

The meganuclease I-CreI from Chlamydomonas reinhardtii is a member of the LAGLIDADG family which recognizes and cuts a 22 base-pair recognition sequence in the chloroplast chromosome, and which presents an attractive target for meganuclease redesign. The wild-type enzyme is a homodimer in which each monomer makes direct contacts with 9 base pairs in the full-length recognition sequence. Genetic selection techniques have been used to identify mutations in I-CreI that alter base preference at a single position in this recognition sequence (Sussman et al. (2004), J. Mol. Biol. 342: 31-41; Chames et al. (2005), Nucleic Acids Res. 33: e178; Seligman et al. (2002), Nucleic Acids Res. 30: 3870-9) or, more recently, at three positions in the recognition sequence (Arnould et al. (2006), J. Mol. Biol. 355: 443-58). The I-CreI protein-DNA interface contains nine amino acids that contact the DNA bases directly and at least an additional five positions that can form potential contacts in modified interfaces. The size of this interface imposes a combinatorial complexity that is unlikely to be sampled adequately in sequence libraries constructed to select for enzymes with drastically altered cleavage sites.

There remains a need for nucleases that will facilitate precise modification of a genome. In addition, there remains a need for techniques for generating nucleases with pre-determined, rationally-designed recognition sequences that will allow manipulation of genetic sequences at specific genetic loci and for techniques utilizing such nucleases to genetically engineer organisms with precise sequence modifications.

SUMMARY

The present invention is based, in part, upon the identification and characterization of specific amino acid residues in the LAGLIDADG family of meganucleases that make contacts with DNA bases and the DNA backbone when the meganucleases associate with a double-stranded DNA recognition sequence, and thereby affect the specificity and activity of the enzymes. This discovery has been used, as described in detail below, to identify amino acid substitutions which can alter the recognition sequence specificity and/or DNA-binding affinity of the meganucleases, and to rationally design and develop meganucleases that can recognize a desired DNA sequence that naturally-occurring meganucleases do not recognize. The invention also provides methods that use such meganucleases to produce recombinant nucleic acids and organisms by utilizing the meganucleases to cause recombination of a desired genetic sequence at a limited number of loci within the genome of the organism, for gene therapy, for treatment of pathogenic infections, and for in vitro applications in diagnostics and research.

Thus, in some embodiments, the invention provides recombinant meganucleases having altered specificity for at least one recognition sequence half-site relative to a wild-type I-CreI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 2-153 of the wild-type I-CreI meganuclease of SEQ ID NO: 1, but in which the recombinant meganuclease has specificity for a recognition sequence half-site which differs by at least one base pair from a half-site within an I-CreI meganuclease recognition sequence selected from SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4 and SEQ ID NO: 5, and in which the recombinant meganuclease includes at least one modification listed in Table 1 which is not an excluded modification found in the prior art.

In other embodiments, the invention provides recombinant meganucleases having altered specificity for at least one recognition sequence half-site relative to a wild-type I-MsoI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 6-160 of the I-MsoI meganuclease of SEQ ID NO: 6, but in which the recombinant meganuclease has specificity for a recognition sequence half-site which differs by at least one base pair from a half-site within an I-MsoI meganuclease recognition sequence selected from SEQ ID NO: 7 and SEQ ID NO: 8, and in which the recombinant meganuclease includes at least one modification listed in Table 2 which is not an excluded modification found in the prior art.

In other embodiments, the invention provides recombinant meganucleases having altered specificity for a recognition sequence relative to a wild-type I-SceI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 3-186 of the I-SceI meganuclease of SEQ ID NO: 9, but in which the recombinant meganuclease has specificity for a recognition sequence which differs by at least one base pair from an I-SceI meganuclease recognition sequence of SEQ ID NO: 10 and SEQ ID NO: 11, and in which the recombinant meganuclease includes at least one modification listed in Table 3 which is not an excluded modification found in the prior art.

In other embodiments, the invention provides recombinant meganucleases having altered specificity for at least one recognition sequence half-site relative to a wild-type I-CeuI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 5-211 of the I-CeuI meganuclease of SEQ ID NO: 12, but in which the recombinant meganuclease has specificity for a recognition sequence half-site which differs by at least one base pair from a half-site within an I-CeuI meganuclease recognition sequence selected from SEQ ID NO: 13 and SEQ ID NO: 14, and in which the recombinant meganuclease includes at least one modification listed in Table 4 which is not an excluded modification found in the prior art.

The meganucleases of the invention can include one, two, three or more of the modifications which have been disclosed herein in order to affect the sequence specificity of the recombinant meganucleases at one, two, three or more positions within the recognition sequence. The meganucleases can include only the novel modifications disclosed herein, or can include the novel modifications disclosed herein in combination with modifications found in the prior art. Specifically excluded, however, are recombinant meganucleases comprising only the modifications of the prior art.

In another aspect, the invention provides for recombinant meganucleases with altered binding affinity for double-stranded DNA which is not sequence-specific. This is accomplished by modifications of the meganuclease residues which make contacts with the backbone of the double-stranded DNA recognition sequence. The modifications can increase or decrease the binding affinity and, consequently, can increase or decrease the overall activity of the enzyme. Moreover, increases/decreases in binding and activity have been found to causes decreases/increases in sequence specificity. Thus, the invention provides a means for altering sequence specificity generally by altering DNA-binding affinity.

Thus, in some embodiments, the invention provides for recombinant meganucleases having altered binding affinity for double-stranded DNA relative to a wild-type I-CreI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 2-153 of the I-CreI meganuclease of SEQ ID NO: 1, and in which the DNA-binding affinity has been either (1) increased by at least one modification corresponding to a substitution selected from (a) substitution of E80, D137, 181, L112, P29, V64 or Y66 with H, N, Q, S, T, K or R, or (b) substitution of T46, T140 or T143 with K or R; or, conversely, (2) decreased by at least one modification corresponding to a substitution selected from (a) substitution of K34, K48, R51, K82, K116 or K139 with H, N, Q, S, T, D or E, or (b) substitution of 181, L112, P29, V64, Y66, T46, T140 or T143 with D or E.

In other embodiments, the invention provides for recombinant meganucleases having altered binding affinity for double-stranded DNA relative to a wild-type I-MsoI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 6-160 of the I-MsoI meganuclease of SEQ ID NO: 6, and in which the DNA-binding affinity has been either (1) increased by at least one modification corresponding to a substitution selected from (a) substitution of E147, 185, G86 or Y118 with H, N, Q, S, T, K or R, or (b) substitution of Q41, N70, S87, T88, H89, Q122, Q139, 5150 or N152 with K or R; or, conversely, (2) decreased by at least one modification corresponding to a substitution selected from (a) substitution of K36, R51, K123, K143 or R144 with H, N, Q, S, T, D or E, or (b) substitution of 185, G86, Y118, Q41, N70, S87, T88, H89, Q122, Q139, 5150 or N152 with D or E.

In other embodiments, the invention provides for recombinant meganucleases having altered binding affinity for double-stranded DNA relative to a wild-type I-SceI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 3-186 of the I-SceI meganuclease of SEQ ID NO: 9, and in which the DNA-binding affinity has been either (1) increased by at least one modification corresponding to a substitution selected from (a) substitution of D201, L19, L80, L92, Y151, Y188, I191, Y199 or Y222 with H, N, Q, S, T, K or R, or (b) substitution of N15, N17, S81, H84, N94, N120, T156, N157, 5159, N163, Q165, 5166, N194 or 5202 with K or R; or, conversely, (2) decreased by at least one modification corresponding to a substitution selected from (a) substitution of K20, K23, K63, K122, K148, K153, K190, K193, K195 or K223 with H, N, Q, S, T, D or E, or (b) substitution of L19, L80, L92, Y151, Y188, I191, Y199, Y222, N15, N17, S81, H84, N94, N120, T156, N157, 5159, N163, Q165, 5166, N194 or 5202 with D or E.

In other embodiments, the invention provides for recombinant meganucleases having altered binding affinity for double-stranded DNA relative to a wild-type I-CeuI meganuclease, in which the meganuclease includes a polypeptide having at least 85% sequence similarity to residues 5-211 of the I-CeuI meganuclease of SEQ ID NO: 12, and in which the DNA-binding affinity has been either (1) increased by at least one modification corresponding to a substitution selected from (a) substitution of D25 or D128 with H, N, Q, S, T, K or R, or (b) substitution of S68, N70, H94, S117, N120, N129 or H172 with K or R; or, conversely, (2) decreased by at least one modification corresponding to a substitution selected from (a) substitution of K21, K28, K31, R112, R114 or R130 with H, N, Q, S, T, D or E, or (b) substitution of S68, N70, H94, 5117, N120, N129 or H172 with D or E.

The meganucleases of the invention can include one, two, three or more of the modifications of backbone contact residues which have been disclosed herein in order to affect DNA-binding affinity. In addition, these modifications affecting DNA-binding affinity can be combined with one or more of the novel modifications of the base contact residues described above which alter the sequence specificity of the recombinant meganucleases at specific positions within the recognition sequence, or with the prior art modifications described above, or with a combination of the novel modifications and prior art modifications. In particular, by combining backbone contact modifications and base contact modifications, recombinant meganucleases can be rationally-designed with desired specificity and activity. For example, increases in DNA-binding affinity can be designed which may offset losses in affinity resulting from designed changes to base contact residues, or decreases in affinity can be designed which may also decrease sequence specificity and broaden the set of recognition sequences for an enzyme.

In another aspect, the invention provides for rationally-designed meganuclease monomers with altered affinity for homo- or heterodimer formation. The affinity for dimer formation can be measured with the same monomer (i.e., homodimer formation) or with a different monomer (i.e., heterodimer formation) such as a reference wild-type meganuclease. These recombinant meganucleases have modifications to the amino acid residues which are present at the protein-protein interface between monomers in a meganuclease dimer. The modifications can be used to promote heterodimer formation and create meganucleases with non-palindromic recognition sequences.

Thus, in some embodiments, the invention provides recombinant meganuclease monomers having altered affinity for dimer formation with a reference meganuclease monomer, in which the recombinant monomer includes a polypeptide having at least 85% sequence similarity to residues 2-153 of the I-CreI meganuclease of SEQ ID NO: 1, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of K7, K57 or K96 with D or E, or (b) substitution of E8 or E61 with K or R. Based upon such recombinant monomers, the invention also provides recombinant meganuclease heterodimers including (1) a first polypeptide having at least 85% sequence similarity to residues 2-153 of the I-CreI meganuclease of SEQ ID NO: 1, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of K7, K57 or K96 with D or E, and (2) a second polypeptide having at least 85% sequence similarity to residues 2-153 of the I-CreI meganuclease of SEQ ID NO: 1, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (b) substitution of E8 or E61 with K or R.

In other embodiments, the invention provides recombinant meganuclease monomers having altered affinity for dimer formation with a reference meganuclease monomer, in which the recombinant monomer includes a polypeptide having at least 85% sequence similarity to residues 6-160 of the I-MsoI meganuclease of SEQ ID NO: 6, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of R302 with D or E, or (b) substitution of D20, E11 or Q64 with K or R. Based upon such recombinant monomers, the invention also provides recombinant meganuclease heterodimers including (1) a first polypeptide having at least 85% sequence similarity to residues 6-160 of the I-MsoI meganuclease of SEQ ID NO: 6, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of R302 with D or E, and (2) a second polypeptide having at least 85% sequence similarity to residues 6-160 of the I-MsoI meganuclease of SEQ ID NO: 6, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (b) substitution of D20, E11 or Q64 with K or R.

In other embodiments, the invention provides recombinant meganuclease monomers having altered affinity for dimer formation with a reference meganuclease monomer, in which the recombinant monomer includes a polypeptide having at least 85% sequence similarity to residues 5-211 of the I-CeuI meganuclease of SEQ ID NO: 12, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of R93 with D or E, or (b) substitution of E152 with K or R. Based upon such recombinant monomers, the invention also provides recombinant meganuclease heterodimers including (1) a first polypeptide having at least 85% sequence similarity to residues 5-211 of the I-CeuI meganuclease of SEQ ID NO: 12, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (a) substitution of R93 with D or E, and (2) a second polypeptide having at least 85% sequence similarity to residues 5-211 of the I-CeuI meganuclease of SEQ ID NO: 12, but in which affinity for dimer formation has been altered by at least one modification corresponding to a substitution selected from (b) substitution of E152 with K or R.

The recombinant meganuclease monomers or heterodimers with altered affinity for dimer formation can also include one, two, three or more of the modifications of base contact residues described above; one, two, three or more of the modifications of backbone contact residues described above; or combinations of both. Thus, for example, the base contacts of a monomer can be modified to alter sequence specificity, the backbone contacts of a monomer can be modified to alter DNA-binding affinity, and the protein-protein interface can be modified to affect dimer formation. Such a recombinant monomer can be combined with a similarly modified monomer to produce a rationally-designed meganuclease heterodimer with desired sequence specificity and activity.

In another aspect, the invention provides for various methods of use for the rationally-designed meganucleases described and enabled herein. These methods include producing genetically-modified cells and organisms, treating diseases by gene therapy, treating pathogen infections, and using the recombinant meganucleases for in vitro applications for diagnostics and research.

Thus, in one aspect, the invention provides methods for producing a genetically-modified eukaryotic cell including an exogenous sequence of interest inserted in a chromosome, by transfecting the cell with (i) a first nucleic acid sequence encoding a meganuclease of the invention, and (ii) a second nucleic acid sequence including said sequence of interest, wherein the meganuclease produces a cleavage site in the chromosome and the sequence of interest is inserted into the chromosome at the cleavage site either by homologous recombination or non-homologous end joining.

Alternatively, in another aspect, the invention provides methods for producing a genetically-modified eukaryotic cell including an exogenous sequence of interest inserted in a chromosome, by introducing a meganuclease protein of the invention into the cell, and transfecting the cell with a nucleic acid including the sequence of interest, wherein the meganuclease produces a cleavage site in the chromosome and the sequence of interest is inserted into the chromosome at the cleavage site either by homologous recombination or non-homologous end joining.

In another aspect, the invention provides methods for producing a genetically-modified eukaryotic cell by disrupting a target sequence in a chromosome, by transfecting the cell with a nucleic acid encoding a meganuclease of the invention, wherein the meganuclease produces a cleavage site in the chromosome and the target sequence is disrupted by non-homologous end joining at the cleavage site.

In another aspect, the invention provides methods of producing a genetically-modified organism by producing a genetically-modified eukaryotic cell according to the methods described above, and growing the genetically-modified eukaryotic cell to produce the genetically-modified organism. In these embodiments, the eukaryotic cell can be selected from a gamete, a zygote, a blastocyst cell, an embryonic stem cell, and a protoplast cell.

In another aspect, the invention provides methods for treating a disease by gene therapy in a eukaryote, by transfecting at least one cell of the eukaryote with one or more nucleic acids including (i) a first nucleic acid sequence encoding a meganuclease of the invention, and (ii) a second nucleic acid sequence including a sequence of interest, wherein the meganuclease produces a cleavage site in the chromosome and the sequence of interest is inserted into the chromosome by homologous recombination or non-homologous end-joining, and insertion of the sequence of interest provides gene therapy for the disease.

Alternatively, in another aspect, the invention provides methods for treating a disease by gene therapy in a eukaryote, by introducing a meganuclease protein of the invention into at least one cell of the eukaryote, and transfecting the cell with a nucleic acid including a sequence of interest, wherein the meganuclease produces a cleavage site in the chromosome and the sequence of interest is inserted into the chromosome at the cleavage site by homologous recombination or non-homologous end-joining, and insertion of the sequence of interest provides gene therapy for the disease.

In another aspect, the invention provides methods for treating a disease by gene therapy in a eukaryote by disrupting a target sequence in a chromosome of the eukaryotic, by transfecting at least one cell of the eukaryote with a nucleic acid encoding a meganuclease of the invention, wherein the meganuclease produces a cleavage site in the chromosome and the target sequence is disrupted by non-homologous end joining at the cleavage site, wherein disruption of the target sequence provides the gene therapy for the disease.

In another aspect, the invention provides methods for treating a viral or prokaryotic pathogen infection in a eukaryotic host by disrupting a target sequence in a genome of the pathogen, by transfecting at least one infected cell of the host with a nucleic acid encoding a meganuclease of the invention, wherein the meganuclease produces a cleavage site in the genome and the target sequence is disrupted by either (1) non-homologous end joining at the cleavage site or (2) by homologous recombination with a second nucleic acid, and wherein disruption of the target sequence provides treatment for the infection.

More generally, in another aspect, the invention provides methods for rationally-designing recombinant meganucleases having altered specificity for at least one base position of a recognition sequence, by (1) determining at least a portion of a three-dimensional structure of a reference meganuclease-DNA complex; (2) identifying amino acid residues forming a base contact surface at the base position; (3) determining a distance between a β-carbon of at least a first residue of the contact surface and at least a first base at the base position; and (4) identifying an amino acid substitution to promote the desired change by either (a) for a first residue which is <6 Å from the first base, selecting a substitution from Group 1 and/or Group 2 which is a member of an appropriate one of Group G, Group C, Group T or Group A; or (b) for a first residue which is >6 Å from said first base, selecting a substitution from Group 2 and/or Group 3 which is a member of an appropriate one of Group G, Group C, Group T or Group A, where each of the Groups is defined herein. This method may be repeated for additional contact residues for the same base, and for contact residues for the other base at the same position, as well as for additional positions.

In addition, in another general aspect, the invention provides methods for rationally-designing a recombinant meganuclease having increased DNA-binding affinity, by (1) determining at least a portion of a three-dimensional structure of a reference meganuclease-DNA complex; (2) identifying amino acid contact residues forming a backbone contact surface; and (3) identifying an amino acid substitution to increase the DNA-binding affinity by (a) for a contact residue having a negatively-charged or hydrophobic side chain, selecting a substitution having an uncharged/polar or positively-charged side chain; or (b) for a contact residue having an uncharged/polar side chain, selecting a substitution having a positively-charged side chain. Conversely, the invention also provides methods for rationally-designing a recombinant meganuclease having decreased DNA-binding affinity, by (1) determining at least a portion of a three-dimensional structure of a reference meganuclease-DNA complex; (2) identifying amino acid contact residues forming a backbone contact surface; (3) identifying an amino acid substitution to decrease the DNA-binding affinity by (a) for a contact residue having a positively-charged side chain, selecting a substitution having an uncharged/polar or negatively-charged side chain; or (b) for a contact residue having an hydrophobic or uncharged/polar side chain, selecting a substitution having a negatively-charged side chain.

These and other aspects and embodiments of the invention will be apparent to one of ordinary skill in the art based upon the following detailed description of the invention.

FIG. 1(A) illustrates the interactions between the I-CreI homodimer and its naturally-occurring double-stranded recognition sequence, based upon crystallographic data. This schematic representation depicts the recognition sequence (SEQ ID NO: 2 and SEQ ID NO: 3), shown as unwound for illustration purposes only, bound by the homodimer, shown as two ovals. The bases of each DNA half-site are numbered −1 through −9, and the amino acid residues of I-CreI which form the recognition surface are indicated by one-letter amino acid designations and numbers indicating residue position. Solid black lines: hydrogen bonds to DNA bases. Dashed lines: amino acid positions that form additional contacts in enzyme designs but do not contact the DNA in the wild-type complex. Arrows: residues that interact with the DNA backbone and influence cleavage activity.

FIG. 1(B) illustrates the wild-type contacts between the A-T base pair at position −4 of the cleavage half-site on the right side of FIG. 1(A). Specifically, the residue Q26 is shown to interact with the A base. Residue 177 is in proximity to the base pair but not specifically interacting.

FIG. 1(C) illustrates the interactions between a rationally-designed variant of the I-CreI meganuclease in which residue 177 has been modified to E77. As a result of this change, a G-C base pair is preferred at position −4. The interaction between Q26 and the G base is mediated by a water molecule, as has been observed crystallographically for the cleavage half-site on the left side of FIG. 1(A).

FIG. 1(D) illustrates the interactions between a rationally-designed variant of the I-CreI meganuclease in which residue Q26 has been modified to E26 and residue 177 has been modified to R77. As a result of this change, a C-G base pair is preferred at position −4.

FIG. 1(E) illustrates the interactions between a rationally-designed variant of the I-CreI meganuclease in which residue Q26 has been modified to A26 and residue 177 has been modified to Q77. As a result of this change, a T-A base pair is preferred at position −4.

FIG. 2(A) shows a comparison of one recognition sequence for each of the wild type I-CreI meganuclease (WT) and 11 rationally-designed meganuclease heterodimers of the invention. Bases that are conserved relative to the WT recognition sequence are shaded. The 9 bp half-sites are bolded. WT: wild-type (SEQ ID NO: 4); CF: ΔF508 allele of the human CFTR gene responsible for most cases of cystic fibrosis (SEQ ID NO: 25); MYD: the human DM kinase gene associated with myotonic dystrophy (SEQ ID NO: 27); CCR: the human CCR5 gene (a major HIV co-receptor) (SEQ ID NO: 26); ACH: the human FGFR3 gene correlated with achondroplasia (SEQ ID NO: 23); TAT: the HIV-1 TAT/REV gene (SEQ ID NO: 15); HSV: the HSV-1 UL36 gene (SEQ ID NO: 28); LAM: the bacteriophage λ p05 gene (SEQ ID NO: 22); POX: the Variola (smallpox) virus gp009 gene (SEQ ID NO: 30); URA: the Saccharomyces cerevisiae URA3 gene (SEQ ID NO: 36); GLA: the Arabidopsis thaliana GL2 gene (SEQ ID NO: 32); BRP: the Arabidopsis thaliana BP-1 gene (SEQ ID NO: 33).

FIG. 2(B) illustrates the results of incubation of each of wild-type I-CreI (WT) and 11 rationally-designed meganuclease heterodimers with plasmids harboring the recognition sites for all 12 enzymes for 6 hours at 37° C. Percent cleavage is indicated in each box.

FIG. 3 illustrates cleavage patterns of wild-type and rationally-designed I-CreI homodimers. (A) wild type I-CreI. (B) I-CreI K116D. (C-L) rationally-designed meganucleases of the invention. Enzymes were incubated with a set of plasmids harboring palindromes of the intended cleavage half-site the 27 corresponding single-base pair variations. Bar graphs show fractional cleavage (F) in 4 hours at 37° C. Black bars: expected cleavage patterns based on Table 1. Gray bars: DNA sites that deviate from expected cleavage patterns. White circles indicate bases in the intended recognition site. Also shown are cleavage time-courses over two hours. The open circle time-course plots in C and L correspond to cleavage by the CCR1 and BRP2 enzymes lacking the E80Q mutation. The cleavage sites correspond to the 5′ (left column) and 3′ (right column) half-sites for the heterodimeric enzymes described in FIG. 2(A).

DETAILED DESCRIPTION

The present invention is based, in part, upon the identification and characterization of specific amino acids in the LAGLIDADG family of meganucleases that make specific contacts with DNA bases and non-specific contacts with the DNA backbone when the meganucleases associate with a double-stranded DNA recognition sequence, and which thereby affect the recognition sequence specificity and DNA-binding affinity of the enzymes. This discovery has been used, as described in detail below, to identify amino acid substitutions in the meganucleases that can alter the specificity and/or affinity of the enzymes, and to rationally design and develop meganucleases that can recognize a desired DNA sequence that naturally-occurring meganucleases do not recognize, and/or that have increased or decreased specificity and/or affinity relative to the naturally-occurring meganucleases. Furthermore, because DNA-binding affinity affects enzyme activity as well as sequence-specificity, the invention provides rationally-designed meganucleases with altered activity relative to naturally-occurring meganucleases. In addition, the invention provides rationally-designed meganucleases in which residues at the interface between the monomers associated to form a dimer have been modified in order to promote heterodimer formation. Finally, the invention provides uses for the rationally-designed meganucleases in the production of recombinant cells and organisms, as well as in gene therapy, anti-pathogen, anti-cancer, and in vitro applications, as disclosed herein.

As a general matter, the invention provides methods for generating rationally-designed LAGLIDADG meganucleases containing altered amino acid residues at sites within the meganuclease that are responsible for (1) sequence-specific binding to individual bases in the double-stranded DNA recognition sequence, or (2) non-sequence-specific binding to the phosphodiester backbone of a double-stranded DNA molecule. Because enzyme activity is correlated to DNA-binding affinity, however, altering the amino acids involved in binding to the DNA recognition sequence can alter not only the specificity of the meganuclease through specific base pair interactions, but also the activity of the meganuclease by increasing or decreasing overall binding affinity for the double-stranded DNA. Similarly, altering the amino acids involved in binding to the DNA backbone can alter not only the activity of the enzyme, but also the degree of specificity or degeneracy of binding to the recognition sequence by increasing or decreasing overall binding affinity for the double-stranded DNA.

As described in detail below, the methods of rationally-designing meganucleases include the identification of the amino acids responsible for DNA recognition/binding, and the application of a series of rules for selecting appropriate amino acid changes. With respect to meganuclease sequence specificity, the rules include both steric considerations relating to the distances in a meganuclease-DNA complex between the amino acid side chains of the meganuclease and the bases in the sense and anti-sense strands of the DNA, and considerations relating to the non-covalent chemical interactions between functional groups of the amino acid side chains and the desired DNA base at the relevant position.

Finally, a majority of natural meganucleases that bind DNA as homodimers recognize pseudo- or completely palindromic recognition sequences. Because lengthy palindromes are expected to be rare, the likelihood of encountering a palindromic sequence at a genomic site of interest is exceedingly low. Consequently, if these enzymes are to be redesigned to recognize genomic sites of interest, it is necessary to design two enzyme monomers recognizing different half-sites that can heterodimerize to cleave the non-palindromic hybrid recognition sequence. Therefore, in some aspects, the invention provides rationally-designed meganucleases in which monomers differing by at least one amino acid position are dimerized to form heterodimers. In some cases, both monomers are rationally-designed to form a heterodimer which recognizes a non-palindromic recognition sequence. A mixture of two different monomers can result in up to three active forms of meganuclease dimer: the two homodimers and the heterodimer. In addition or alternatively, in some cases, amino acid residues are altered at the interfaces at which monomers can interact to form dimers, in order to increase or decrease the likelihood of formation of homodimers or heterodimers.

Thus, in one aspect, the invention provide methods for rationally designing LAGLIDADG meganucleases containing amino acid changes that alter the specificity and/or activity of the enzymes. In another aspect, the invention provides the rationally-designed meganucleases resulting from these methods. In another aspect, the invention provides methods that use such rationally-designed meganucleases to produce recombinant nucleic acids and organisms in which a desired DNA sequence or genetic locus within the genome of an organism is modified by the insertion, deletion, substitution or other manipulation of DNA sequences. In another aspect, the invention provides methods for reducing the survival of pathogens or cancer cells using rationally-designed meganucleases which have pathogen-specific or cancer-specific recognition sequences.

The patent and scientific literature referred to herein establishes knowledge that is available to those of skill in the art. The issued U.S. patents, allowed applications, published foreign applications, and references, including GenBank database sequences, that are cited herein are hereby incorporated by reference to the same extent as if each was specifically and individually indicated to be incorporated by reference.

As used herein, the term “meganuclease” refers to an endonuclease that binds double-stranded DNA at a recognition sequence that is greater than 12 base pairs. Naturally-occurring meganucleases can be monomeric (e.g., I-SceI) or dimeric (e.g., I-CreI). The term meganuclease, as used herein, can be used to refer to monomeric meganucleases, dimeric meganucleases, or to the monomers which associate to form a dimeric meganuclease. The term “homing endonuclease” is synonymous with the term “meganuclease.”

As used herein, the term “LAGLIDADG meganuclease” refers either to meganucleases including a single LAGLIDADG motif, which are naturally dimeric, or to meganucleases including two LAGLIDADG motifs, which are naturally monomeric. The term “mono-LAGLIDADG meganuclease” is used herein to refer to meganucleases including a single LAGLIDADG motif, and the term “di-LAGLIDADG meganuclease” is used herein to refer to meganucleases including two LAGLIDADG motifs, when it is necessary to distinguish between the two. Each of the two structural domains of a di-LAGLIDADG meganuclease which includes a LAGLIDADG motif can be referred to as a LAGLIDADG subunit.

As used herein, the term “rationally-designed” means non-naturally occurring and/or genetically engineered. The rationally-designed meganucleases of the invention differ from wild-type or naturally-occurring meganucleases in their amino acid sequence or primary structure, and may also differ in their secondary, tertiary or quaternary structure. In addition, the rationally-designed meganucleases of the invention also differ from wild-type or naturally-occurring meganucleases in recognition sequence-specificity and/or activity.

As used herein, with respect to a protein, the term “recombinant” means having an altered amino acid sequence as a result of the application of genetic engineering techniques to nucleic acids which encode the protein, and cells or organisms which express the protein. With respect to a nucleic acid, the term “recombinant” means having an altered nucleic acid sequence as a result of the application of genetic engineering techniques. Genetic engineering techniques include, but are not limited to, PCR and DNA cloning technologies; transfection, transformation and other gene transfer technologies; homologous recombination; site-directed mutagenesis; and gene fusion. In accordance with this definition, a protein having an amino acid sequence identical to a naturally-occurring protein, but produced by cloning and expression in a heterologous host, is not considered recombinant.

As used herein with respect to recombinant proteins, the term “modification” means any insertion, deletion or substitution of an amino acid residue in the recombinant sequence relative to a reference sequence (e.g., a wild-type).

As used herein, the term “genetically-modified” refers to a cell or organism in which, or in an ancestor of which, a genomic DNA sequence has been deliberately modified by recombinant technology. As used herein, the term “genetically-modified” encompasses the term “transgenic.”

As used herein, the term “wild-type” refers to any naturally-occurring form of a meganuclease. The term “wild-type” is not intended to mean the most common allelic variant of the enzyme in nature but, rather, any allelic variant found in nature. Wild-type meganucleases are distinguished from recombinant or non-naturally-occurring meganucleases.

As used herein, the term “recognition sequence half-site” or simply “half site” means a nucleic acid sequence in a double-stranded DNA molecule which is recognized by a monomer of a mono-LAGLIDADG meganuclease or by one LAGLIDADG subunit of a di-LAGLIDADG meganuclease.

As used herein, the term “recognition sequence” refers to a pair of half-sites which is bound and cleaved by either a mono-LAGLIDADG meganuclease dimer or a di-LAGLIDADG meganuclease monomer. The two half-sites may or may not be separated by base pairs that are not specifically recognized by the enzyme. In the cases of I-CreI, I-MsoI and I-CeuI, the recognition sequence half-site of each monomer spans 9 base pairs, and the two half-sites are separated by four base pairs which are not recognized specifically but which constitute the actual cleavage site (which has a 4 base pair overhang). Thus, the combined recognition sequences of the I-CreI, I-MsoI and I-CeuI meganuclease dimers normally span 22 base pairs, including two 9 base pair half-sites flanking a 4 base pair cleavage site. The base pairs of each half-site are designated −9 through −1, with the −9 position being most distal from the cleavage site and the −1 position being adjacent to the 4 central base pairs, which are designated N1-N4. The strand of each half-site which is oriented 5′ to 3′ in the direction from −9 to −1 (i.e., towards the cleavage site), is designated the “sense” strand and the opposite strand is designated the “antisense strand”, although neither strand may encode protein. Thus, the “sense” strand of one half-site is the antisense strand of the other half-site. See, for example, FIG. 1(A). In the case of the I-SceI meganuclease, which is a di-LAGLIDADG meganuclease monomer, the recognition sequence is an approximately 18 bp non-palindromic sequence, and there are no central base pairs which are not specifically recognized. By convention, one of the two strands is referred to as the “sense” strand and the other the “antisense” strand, although neither strand may encode protein.

As used herein, the term “specificity” means the ability of a meganuclease to recognize and cleave double-stranded DNA molecules only at a particular sequence of base pairs referred to as the recognition sequence, or only at a particular set of recognition sequences. The set of recognition sequences will share certain conserved positions or sequence motifs, but may be degenerate at one or more positions. A highly-specific meganuclease is capable of cleaving only one or a very few recognition sequences. Specificity can be determined in a cleavage assay as described in Example 1. As used herein, a meganuclease has “altered” specificity if it binds to and cleaves a recognition sequence which is not bound to and cleaved by a reference meganuclease (e.g., a wild-type) or if the rate of cleavage of a recognition sequence is increased or decreased by a statistically significant (p<0.05) amount relative to a reference meganuclease.

As used herein, the term “degeneracy” means the opposite of “specificity.” A highly-degenerate meganuclease is capable of cleaving a large number of divergent recognition sequences. A meganuclease can have sequence degeneracy at a single position within a half-site or at multiple, even all, positions within a half-site. Such sequence degeneracy can result from (i) the inability of any amino acid in the DNA-binding domain of a meganuclease to make a specific contact with any base at one or more positions in the recognition sequence, (ii) the ability of one or more amino acids in the DNA-binding domain of a meganuclease to make specific contacts with more than one base at one or more positions in the recognition sequence, and/or (iii) sufficient non-specific DNA binding affinity for activity. A “completely” degenerate position can be occupied by any of the four bases and can be designated with an “N” in a half-site. A “partially” degenerate position can be occupied by two or three of the four bases (e.g., either purine (Pu), either pyrimidine (Py), or not G).

As used herein with respect to meganucleases, the term “DNA-binding affinity” or “binding affinity” means the tendency of a meganuclease to non-covalently associate with a reference DNA molecule (e.g., a recognition sequence or an arbitrary sequence). Binding affinity is measured by a dissociation constant, KD (e.g., the KD of I-CreI for the WT recognition sequence is approximately 0.1 nM). As used herein, a meganuclease has “altered” binding affinity if the KD of the recombinant meganuclease for a reference recognition sequence is increased or decreased by a statistically significant (p<0.05) amount relative to a reference meganuclease.

As used herein with respect to meganuclease monomers, the term “affinity for dimer formation” means the tendency of a meganuclease monomer to non-covalently associate with a reference meganuclease monomer. The affinity for dimer formation can be measured with the same monomer (i.e., homodimer formation) or with a different monomer (i.e., heterodimer formation) such as a reference wild-type meganuclease. Binding affinity is measured by a dissociation constant, KD. As used herein, a meganuclease has “altered” affinity for dimer formation if the KD of the recombinant meganuclease monomer for a reference meganuclease monomer is increased or decreased by a statistically significant (p<0.05) amount relative to a reference meganuclease monomer.

As used herein, the term “palindromic” refers to a recognition sequence consisting of inverted repeats of identical half-sites. In this case, however, the palindromic sequence need not be palindromic with respect to the four central base pairs, which are not contacted by the enzyme. In the case of dimeric meganucleases, palindromic DNA sequences are recognized by homodimers in which the two monomers make contacts with identical half-sites.

As used herein, the term “pseudo-palindromic” refers to a recognition sequence consisting of inverted repeats of non-identical or imperfectly palindromic half-sites. In this case, the pseudo-palindromic sequence not only need not be palindromic with respect to the four central base pairs, but also can deviate from a palindromic sequence between the two half-sites. Pseudo-palindromic DNA sequences are typical of the natural DNA sites recognized by wild-type homodimeric meganucleases in which two identical enzyme monomers make contacts with different half-sites.

As used herein, the term “non-palindromic” refers to a recognition sequence composed of two unrelated half-sites of a meganuclease. In this case, the non-palindromic sequence need not be palindromic with respect to either the four central base pairs or the two monomer half-sites. Non-palindromic DNA sequences are recognized by either di-LAGLIDADG meganucleases, highly degenerate mono-LAGLIDADG meganucleases (e.g., I-CeuI) or by heterodimers of mono-LAGLIDADG meganuclease monomers that recognize non-identical half-sites.

As used herein, the term “activity” refers to the rate at which a meganuclease of the invention cleaves a particular recognition sequence. Such activity is a measurable enzymatic reaction, involving the hydrolysis of phosphodiester bonds of double-stranded DNA. The activity of a meganuclease acting on a particular DNA substrate is affected by the affinity or avidity of the meganuclease for that particular DNA substrate which is, in turn, affected by both sequence-specific and non-sequence-specific interactions with the DNA.

As used herein, the term “homologous recombination” refers to the natural, cellular process in which a double-stranded DNA-break is repaired using a homologous DNA sequence as the repair template (see, e.g. Cahill et al. (2006), Front. Biosci. 11:1958-1976). The homologous DNA sequence may be an endogenous chromosomal sequence or an exogenous nucleic acid that was delivered to the cell. Thus, in some embodiments, a rationally-designed meganuclease is used to cleave a recognition sequence within a target sequence and an exogenous nucleic acid with homology to or substantial sequence similarity with the target sequence is delivered into the cell and used as a template for repair by homologous recombination. The DNA sequence of the exogenous nucleic acid, which may differ significantly from the target sequence, is thereby incorporated into the chromosomal sequence. The process of homologous recombination occurs primarily in eukaryotic organisms. The term “homology” is used herein as equivalent to “sequence similarity” and is not intended to require identity by descent or phylogenetic relatedness.

As used herein, the term “non-homologous end-joining” refers to the natural, cellular process in which a double-stranded DNA-break is repaired by the direct joining of two non-homologous DNA segments (see, e.g. Cahill et al. (2006), Front. Biosci. 11:1958-1976). DNA repair by non-homologous end joining is error-prone and frequently results in the untemplated addition or deletion of DNA sequences at the site of repair. Thus, in certain embodiments, a rationally-designed meganuclease can be used to produce a double-stranded break at a meganuclease recognition sequence within a target sequence to disrupt a gene (e.g., by introducing base insertions, base deletions, or frameshift mutations) by non-homologous end-joining. In other embodiments, an exogenous nucleic acid lacking homology to or substantial sequence similarity with the target sequence may be captured at the site of a meganuclease-stimulated double-stranded DNA break by non-homologous end joining (see, e.g. Salomon, et al. (1998), EMBO J. 17:6086-6095). The process of non-homologous end joining occurs in both eukaryotes and prokaryotes such as bacteria.

As used herein, the term “sequence of interest” means any nucleic acid sequence, whether it codes for a protein, RNA, or regulatory element (e.g., an enhancer, silencer, or promoter sequence), that can be inserted into a genome or used to replace a genomic DNA sequence using a meganuclease protein. Sequences of interest can have heterologous DNA sequences that allow for tagging a protein or RNA that is expressed from the sequence of interest. For instance, a protein can be tagged with tags including, but not limited to, an epitope (e.g., c-myc, FLAG) or other ligand (e.g., poly-His). Furthermore, a sequence of interest can encode a fusion protein, according to techniques known in the art (see, e.g., Ausubel et al., Current Protocols in Molecular Biology, Wiley 1999). In some embodiments, the sequence of interest is flanked by a DNA sequence that is recognized by the recombinant meganuclease for cleavage. Thus, the flanking sequences are cleaved allowing for proper insertion of the sequence of interest into genomic recognition sequences cleaved by the recombinant meganuclease. In some embodiments, the entire sequence of interest is homologous to or has substantial sequence similarity with the a target sequence in the genome such that homologous recombination effectively replaces the target sequence with the sequence of interest. In other embodiments, the sequence of interest is flanked by DNA sequences with homology to or substantial sequence similarity with the target sequence such that homologous recombination inserts the sequence of interest within the genome at the locus of the target sequence. In some embodiments, the sequence of interest is substantially identical to the target sequence except for mutations or other modifications in the meganuclease recognition sequence such that the meganuclease can not cleave the target sequence after it has been modified by the sequence of interest.

As used herein with respect to both amino acid sequences and nucleic acid sequences, the terms “percentage similarity” and “sequence similarity” refer to a measure of the degree of similarity of two sequences based upon an alignment of the sequences which maximizes similarity between aligned amino acid residues or nucleotides, and which is a function of the number of identical or similar residues or nucleotides, the number of total residues or nucleotides, and the presence and length of gaps in the sequence alignment. A variety of algorithms and computer programs are available for determining sequence similarity using standard parameters. As used herein, sequence similarity is measured using the BLASTp program for amino acid sequences and the BLASTn program for nucleic acid sequences, both of which are available through the National Center for Biotechnology Information (www.ncbi.nlm.nih.gov/), and are described in, for example, Altschul et al. (1990), J. Mol. Biol. 215:403-410; Gish and States (1993), Nature Genet. 3:266-272; Madden et al. (1996), Meth. Enzymol. 266:131-141; Altschul et al. (1997), Nucleic Acids Res. 25:33 89-3402); Zhang et al. (2000), J. Comput. Biol. 7(1-2):203-14. As used herein, percent similarity of two amino acid sequences is the score based upon the following parameters for the BLASTp algorithm: word size=3; gap opening penalty=−11; gap extension penalty=−1; and scoring matrix=BLOSUM62. As used herein, percent similarity of two nucleic acid sequences is the score based upon the following parameters for the BLASTn algorithm: word size=11; gap opening penalty=−5; gap extension penalty=−2; match reward=1; and mismatch penalty=−3.

As used herein with respect to modifications of two proteins or amino acid sequences, the term “corresponding to” is used to indicate that a specified modification in the first protein is a substitution of the same amino acid residue as in the modification in the second protein, and that the amino acid position of the modification in the first proteins corresponds to or aligns with the amino acid position of the modification in the second protein when the two proteins are subjected to standard sequence alignments (e.g., using the BLASTp program). Thus, the modification of residue “X” to amino acid “A” in the first protein will correspond to the modification of residue “Y” to amino acid “A” in the second protein if residues X and Y correspond to each other in a sequence alignment, and despite the fact that X and Y may be different numbers.

As used herein, the recitation of a numerical range for a variable is intended to convey that the invention may be practiced with the variable equal to any of the values within that range. Thus, for a variable which is inherently discrete, the variable can be equal to any integer value within the numerical range, including the end-points of the range. Similarly, for a variable which is inherently continuous, the variable can be equal to any real value within the numerical range, including the end-points of the range. As an example, and without limitation, a variable which is described as having values between 0 and 2 can take the values 0, 1 or 2 if the variable is inherently discrete, and can take the values 0.0, 0.1, 0.01, 0.001, or any other real values ≧0 and ≦2 if the variable is inherently continuous.

As used herein, unless specifically indicated otherwise, the word “or” is used in the inclusive sense of “and/or” and not the exclusive sense of “either/or.”

In one aspect of the invention, methods for rationally designing recombinant LAGLIDADG family meganucleases are provided. In this aspect, recombinant meganucleases are rationally-designed by first predicting amino acid substitutions that can alter base preference at each position in the half-site. These substitutions can be experimentally validated individually or in combinations to produce meganucleases with the desired cleavage specificity.

In accordance with the invention, amino acid substitutions that can cause a desired change in base preference are predicted by determining the amino acid side chains of a reference meganuclease (e.g., a wild-type meganuclease, or a non-naturally-occurring reference meganuclease) that are able to participate in making contacts with the nucleic acid bases of the meganuclease\'s DNA recognition sequence and the DNA phosphodiester backbone, and the spatial and chemical nature of those contacts. These amino acids include but are not limited to side chains involved in contacting the reference DNA half-site. Generally, this determination requires having knowledge of the structure of the complex between the meganuclease and its double-stranded DNA recognition sequence, or knowledge of the structure of a highly similar complex (e.g., between the same meganuclease and an alternative DNA recognition sequence, or between an allelic or phylogenetic variant of the meganuclease and its DNA recognition sequence).

Three-dimensional structures, as described by atomic coordinates data, of a polypeptide or complex of two or more polypeptides can be obtained in several ways. For example, protein structure determinations can be made using techniques including, but not limited to, X-ray crystallography, NMR, and mass spectrometry. Another approach is to analyze databases of existing structural co-ordinates for the meganuclease of interest or a related meganuclease. Such structural data is often available from databases in the form of three-dimensional coordinates. Often this data is accessible through online databases (e.g., the RCSB Protein Data Bank at www.rcsb.org/pdb).

Structural information can be obtained experimentally by analyzing the diffraction patterns of, for example, X-rays or electrons, created by regular two- or three-dimensional arrays (e.g., crystals) of proteins or protein complexes. Computational methods are used to transform the diffraction data into three-dimensional atomic co-ordinates in space. For example, the field of X-ray crystallography has been used to generate three-dimensional structural information on many protein-DNA complexes, including meganucleases (see, e.g., Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774).

Nuclear Magnetic Resonance (NMR) also has been used to determine inter-atomic distances of molecules in solution. Multi-dimensional NMR methods combined with computational methods have succeeded in determining the atomic co-ordinates of polypeptides of increasing size (see, e.g., Tzakos et al. (2006), Annu. Rev. Biophys. Biomol. Struct. 35:19-42.).

Alternatively, computational modeling can be used by applying algorithms based on the known primary structures and, when available, secondary, tertiary and/or quaternary structures of the protein/DNA, as well as the known physiochemical nature of the amino acid side chains, nucleic acid bases, and bond interactions. Such methods can optionally include iterative approaches, or experimentally-derived constraints. An example of such computational software is the CNS program described in Adams et al. (1999), Acta Crystallogr. D. Biol. Crystallogr. 55 (Pt 1): 181-90. A variety of other computational programs have been developed that predict the spatial arrangement of amino acids in a protein structure and predict the interaction of the amino acid side chains of the protein with various target molecules (see, e.g., U.S. Pat. No. 6,988,041).

Thus, in some embodiments of the invention, computational models are used to identify specific amino acid residues that specifically interact with DNA nucleic acid bases and/or facilitate non-specific phosphodiester backbone interactions. For instance, computer models of the totality of the potential meganuclease-DNA interaction can be produced using a suitable software program, including, but not limited to, MOLSCRIPT™ 2.0 (Avatar Software AB, Stockholm, Sweden), the graphical display program 0 (Jones et. al. (1991), Acta Crystallography, A47: 110), the graphical display program GRASP™ (Nicholls et al. (1991), PROTEINS, Structure, Function and Genetics 11(4): 281ff), or the graphical display program INSIGHT™ (TSI, Inc., Shoreview, Minn.). Computer hardware suitable for producing, viewing and manipulating three-dimensional structural representations of protein-DNA complexes are commercially available and well known in the art (e.g., Silicon Graphics Workstation, Silicon Graphics, Inc., Mountainview, Calif.).

Specifically, interactions between a meganuclease and its double-stranded DNA recognition sequences can be resolved using methods known in the art. For example, a representation, or model, of the three dimensional structure of a multi-component complex structure, for which a crystal has been produced, can be determined using techniques which include molecular replacement or SIR/MIR (single/multiple isomorphous replacement) (see, e.g., Brunger (1997), Meth. Enzym. 276: 558-580; Navaza and Saludjian (1997), Meth. Enzym. 276: 581-594; Tong and Rossmann (1997), Meth. Enzym. 276: 594-611; and Bentley (1997), Meth. Enzym. 276: 611-619) and can be performed using a software program, such as AMoRe/Mosflm (Navaza (1994), Acta Cryst. A50: 157-163; CCP4 (1994), Acta Cryst. D50: 760-763) or XPLOR (see, Brünger et al. (1992), X-PLOR Version 3.1. A System for X-ray Crystallography and NMR, Yale University Press, New Haven, Conn.).

The determination of protein structure and potential meganuclease-DNA interaction allows for rational choices concerning the amino acids that can be changed to affect enzyme activity and specificity. Decisions are based on several factors regarding amino acid side chain interactions with a particular base or DNA phosphodiester backbone. Chemical interactions used to determine appropriate amino acid substitutions include, but are not limited to, van der Waals forces, steric hindrance, ionic bonding, hydrogen bonding, and hydrophobic interactions. Amino acid substitutions can be selected which either favor or disfavor specific interactions of the meganuclease with a particular base in a potential recognition sequence half-site in order to increase or decrease specificity for that sequence and, to some degree, overall binding affinity and activity. In addition, amino acid substitutions can be selected which either increase or decrease binding affinity for the phosphodiester backbone of double-stranded DNA in order to increase or decrease overall activity and, to some degree, to decrease or increase specificity.

Thus, in specific embodiments, a three-dimensional structure of a meganuclease-DNA complex is determined and a “contact surface” is defined for each base-pair in a DNA recognition sequence half-site. In some embodiments, the contact surface comprises those amino acids in the enzyme with β-carbons less than 9.0 Å from a major groove hydrogen-bond donor or acceptor on either base in the pair, and with side chains oriented toward the DNA, irrespective of whether the residues make base contacts in the wild-type meganuclease-DNA complex. In other embodiments, residues can be excluded if the residues do not make contact in the wild-type meganuclease-DNA complex, or residues can be included or excluded at the discretion of the designer to alter the number or identity of the residues considered. In one example, as described below, for base positions −2, −7, −8, and −9 of the wild-type I-CreI half-site, the contact surfaces were limited to the amino acid positions that actually interact in the wild-type enzyme-DNA complex. For positions −1, −3, −4, −5, and −6, however, the contact surfaces were defined to contain additional amino acid positions that are not involved in wild-type contacts but which could potentially contact a base if substituted with a different amino acid.

It should be noted that, although a recognition sequence half-site is typically represented with respect to only one strand of DNA, meganucleases bind in the major groove of double-stranded DNA, and make contact with nucleic acid bases on both strands. In addition, the designations of “sense” and “antisense” strands are completely arbitrary with respect to meganuclease binding and recognition. Sequence specificity at a position can be achieved either through interactions with one member of a base pair, or by a combination of interactions with both members of a base-pair. Thus, for example, in order to favor the presence of an A/T base pair at position X, where the A base is on the “sense” strand and the T base is on the “antisense” strand, residues are selected which are sufficiently close to contact the sense strand at position X and which favor the presence of an A, and/or residues are selected which are sufficiently close to contact the antisense strand at position X and which favor the presence of a T. In accordance with the invention, a residue is considered sufficiently close if the β-carbon of the residue is within 9 Å of the closest atom of the relevant base.

Thus, for example, an amino acid with a β-carbon within 9 Å of the DNA sense strand but greater than 9 Å from the antisense strand is considered for potential interactions with only the sense strand. Similarly, an amino acid with a β-carbon within 9 Å of the DNA antisense strand but greater than 9 Å from the sense strand is considered for potential interactions with only the antisense strand. Amino acids with β-carbons that are within 9 Å of both DNA strands are considered for potential interactions with either strand.

For each contact surface, potential amino acid substitutions are selected based on their predicted ability to interact favorably with one or more of the four DNA bases. The selection process is based upon two primary criteria: (i) the size of the amino acid side chains, which will affect their steric interactions with different nucleic acid bases, and (ii) the chemical nature of the amino acid side chains, which will affect their electrostatic and bonding interactions with the different nucleic acid bases.

With respect to the size of side chains, amino acids with shorter and/or smaller side chains can be selected if an amino acid β-carbon in a contact surface is <6 Å from a base, and amino acids with longer and/or larger side chains can be selected if an amino acid β-carbon in a contact surface is >6 Å from a base. Amino acids with side chains that are intermediate in size can be selected if an amino acid β-carbon in a contact surface is 5-8 Å from a base.

The amino acids with relatively shorter and smaller side chains can be assigned to Group 1, including glycine (G), alanine (A), serine (S), threonine (T), cysteine (C), valine (V), leucine (L), isoleucine (I), aspartate (D), asparagine (N) and proline (P). Proline, however, is expected to be used less frequently because of its relative inflexibility. In addition, glycine is expected to be used less frequently because it introduces unwanted flexibility in the peptide backbone and its very small size reduces the likelihood of effective contacts when it replaces a larger residue. On the other hand, glycine can be used in some instances for promoting a degenerate position. The amino acids with side chains of relatively intermediate length and size can be assigned to Group 2, including lysine (K), methionine (M), arginine (R), glutamate (E) and glutamine (Q). The amino acids with relatively longer and/or larger side chains can be assigned to Group 3, including lysine (K), methionine (M), arginine (R), histidine (H), phenylalanine (F), tyrosine (Y), and tryptophan (W). Tryptophan, however, is expected to be used less frequently because of its relative inflexibility. In addition, the side chain flexibility of lysine, arginine, and methionine allow these amino acids to make base contacts from long or intermediate distances, warranting their inclusion in both Groups 2 and 3. These groups are also shown in tabular form below:

Group 1 Group 2 Group 3 glycine (G) glutamine (Q) arginine (R) alanine (A) glutamate (E) histidine (H) serine (S) lysine (K) phenylalanine (F) threonine (T) methionine (M) tyrosine (Y) cysteine (C) arginine (R) tryptophan (W) valine (V) lysine (K) leucine (L) methionine (M) isoleucine (I) aspartate (D) asparagine (N) proline (P)

With respect to the chemical nature of the side chains, the different amino acids are evaluated for their potential interactions with the different nucleic acid bases (e.g., van der Waals forces, ionic bonding, hydrogen bonding, and hydrophobic interactions) and residues are selected which either favor or disfavor specific interactions of the meganuclease with a particular base at a particular position in the double-stranded DNA recognition sequence half-site. In some instances, it may be desired to create a half-site with one or more complete or partial degenerate positions. In such cases, one may choose residues which favor the presence of two or more bases, or residues which disfavor one or more bases. For example, partial degenerate base recognition can be achieved by sterically hindering a pyrimidine at a sense or antisense position.

Recognition of guanine (G) bases is achieved using amino acids with basic side chains that form hydrogen bonds to N7 and 06 of the base. Cytosine (C) specificity is conferred by negatively-charged side chains which interact unfavorably with the major groove electronegative groups present on all bases except C. Thymine (T) recognition is rationally-designed using hydrophobic and van der Waals interactions between hydrophobic side chains and the major groove methyl group on the base. Finally, adenine (A) bases are recognized using the carboxamide side chains Asn and Gln or the hydroxyl side chain of Tyr through a pair of hydrogen bonds to N7 and N6 of the base. Lastly, His can be used to confer specificity for a purine base (A or G) by donating a hydrogen bond to N7. These straightforward rules for DNA recognition can be applied to predict contact surfaces in which one or both of the bases at a particular base-pair position are recognized through a rationally-designed contact.

Thus, based on their binding interactions with the different nucleic acid bases, and the bases which they favor at a position with which they make contact, each amino acid residue can be assigned to one or more different groups corresponding to the different bases they favor (i.e., G, C, T or A). Thus, Group G includes arginine (R), lysine (K) and histidine (H); Group C includes aspartate (D) and glutamate (E); Group T includes alanine (A), valine (V), leucine (L), isoleucine (I), cysteine (C), threonine (T), methionine (M) and phenylalanine (F); and Group A includes asparagine (N), glutamine (N), tyrosine (Y) and histidine (H). Note that histidine appears in both Group G and Group A; that serine (S) is not included in any group but may be used to favor a degenerate position; and that proline, glycine, and tryptophan are not included in any particular group because of predominant steric considerations. These groups are also shown in tabular form below:

Group G Group C Group T Group A arginine (R) aspartate (D) alanine (A) asparagine (N) lysine (K) glutamate (E) valine (V) glutamine (Q) histidine (H)

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