Abstract:
The present invention is in the field of genetic editing tools and methods of genetic engineering. It relates to the engineering of rare-cutting endonucleases designed to contract highly repetitive motives in chromosomes, which are at the origin of certain genetic diseases, in particular the so-called “triplet repeat diseases”, such as the Huntington disease. The invention encompasses the method for contracting the repetitive motives, the rare-cutting endonucleases for use to contract repetitive motives in a gene subjected to repeat disorder, the polynucleotides and vectors encoding thereof as well as the resulting pharmaceutical compositions.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is in the field of genetic editing tools and use thereof. It relates to the engineering of rare-cutting endonucleases designed to contract highly repetitive motives in chromosomes, which are at the origin of certain genetic diseases, in particular the so-called “triplet repeat diseases”, such as the Huntington disease. The invention encompasses the method for contracting the repetitive motives, the rare-cutting endonucleases for use to contract repetitive motives in a gene subjected to repeat disorder, the polynucleotides and vectors encoding thereof as well as the resulting pharmaceutical compositions. 
       BACKGROUND OF THE INVENTION 
       [0002]    Since the early 1990s, expansion of unstable nucleotide (microsatellite) repeats, notably trinucleotide repeat was identified as a novel mutational mechanism underlying certain human diseases. Over the years, several additional developmental and neuromuscular disorders were identified to be caused by either an insertion or a duplication of trinucleotide repeats as well as unstable tetra-, penta-, hexanucleotide, and longer repeats (Mirkin 2007). This insertion or duplication of polynucleotide repeats can induce a protein loss of function, a RNA toxic gain of function or a protein toxic gain of function leading to the disorder. Examples of such disorders include Huntington disease, inherited ataxias, fragile X syndrome, myotonic dystrophy a common genetic muscular dystrophy, a group of dominantly inherited ataxias, and most recently an unstable hexanucleotide repeat in the C9ORF72 gene as a frequent cause of frontotemporal dementia/amyotrophic lateral sclerosis (DeJesus-Hernandez, Mackenzie et al. 2011; Renton, Majounie et al. 2011) (see for review (Nelson, Orr et al. 2013)). 
         [0003]    Treatment options for most of repeat expansion disorders are very limited. One of the most attractive therapeutic strategies envisaged for various neurodegenerative diseases is gene therapy. Indeed, several strategies to turn off expression of repeat expanded have been developed. In particular, silencing the mutant gene using RNA interference technology within cell has been realized for preventing the toxic function of the protein or RNA (Wang, Liu et al. 2005; Machida, Okada et al. 2006; DiFiglia, Sena-Esteves et al. 2007). However, basically the design of RNA interference does not allow the distinction between the normal and repeat expansion sequences and induce simultaneous reduction of both the mutant and wild type gene (Caplen, Taylor et al. 2002). However, the huntingtin protein is widely expressed and is required for neuronal function and survival in the brain (Duyao, Auerbach et al. 1995; Dragatsis, Levine et al. 2000). Thus, it is important to reduce specifically expression of the mutant gene, while leaving the expression of the wild type protein unaffected. 
         [0004]    Recently, Zinc Finger proteins were designed to bind poly-trinucleotides repeat of the huntingtin gene, responsible for the Huntington disease. Zinc fingers were concatenated into long chains with appropriate linker to obtain an optimal configuration for repressing preferably the repeat expanded huntingtin gene compared with the shorter repeats. This strategy allows more efficient repression of mutant gene expression compared to wild type gene. However it has not been known whether the repression would be sufficient to reduce protein levels for gene therapy (Garriga-Canut, Agustin-Pavon et al., International application: WO2013/130824). 
         [0005]    A previous study (Richard, Dujon et al. 1999) has suggested that Induction of a cleavage event within the repeat sequence was associated with contraction of trinucleotide repeat arrays, which may be explained by two different mechanisms: (1) the two ends of the break are available to invade the template, but they can invade at any location within the template, since they carry repeated sequences that are homologous to the template; or (2) only one end invades the template and the newly synthesized strand is displaced from its template, but can anneal with the other end containing repeats (Richard, Dujon et al. 1999). However, due to the highly frequency of repeat sequences within the genome, engineered DNA binding nuclease designed to be specific to said repeat sequences, are likely to induce off-site mutagenesis at several positions throughout the human genome. Consequently, the ability to create a cleavage in the repeat sequence only at the desired genomic position would be highly desirable. 
         [0006]    To overcome the above limitations, the present inventors have developed a genetic therapeutic strategy to decrease the number of expanded polynucleotide repeats by using DNA binding nucleases, while maintaining the integrity of the genome and functionality of the corrected gene. This strategy mainly relies on the design of the DNA binding nucleases along with the selection of genome sequences to specifically target the repeat sequence associated with the triplet repeat disorders. 
       SUMMARY OF THE INVENTION 
       [0007]    In a general aspect, the present invention relates to a rare-cutting endonuclease for use to contract polynucleotide repeats, preferably in a specific gene subjected to repeat disorder. In particular, the rare-cutting endonuclease is engineered to specifically cleave repeated sequences, characterized in that said rare-cutting endonuclease recognizes a target sequence comprising a region adjacent to the repeat sequence. The present invention relates to a method of engineering a rare-cutting endonuclease used to induce contraction within highly repetitive motives in a specific region. Preferably, said rare-cutting endonuclease targets a sequence comprising the region adjacent to the repeat sequence, such that the rare-cutting endonuclease specifically binds the selected target sequence and cleaves the repeat sequence. Cleavage of the repeat sequence induces a repairing process conducting to the contraction of the repeat sequence within the specific gene and thus the decrease of the expanded repeat sequence to an approximately wild type configuration. Preferably, said rare-cutting endonuclease is a Cas9-guide RNA complex which specifically cleaves a repeat sequence characterized in that the guide RNA hybridizes a target sequence comprising a region adjacent to the repeat sequence. Preferably, said rare-cutting endonuclease is a modular DNA binding nuclease, which comprises a DNA binding domain such as TALE, MBBBD, Zinc Finger (ZF) domain fused a catalytic domain of an endonuclease. Said DNA binding nuclease can act as a monomer or a dimer. The dimeric DNA binding nuclease comprises a first DNA binding domain capable of binding a sequence adjacent to the repeat sequence fused to a nuclease catalytic domain and a second DNA binding domain capable of binding repeat sequence fused to a nuclease catalytic domain (see  FIG. 1 ). Said nuclease catalytic domain which acts as a dimer is preferably FokI catalytic domain. The rare-cutting endonuclease of the present invention is particularly suitable for treating or preventing repeat disease, such as Huntington disease, by contracting the highly repetitive motives region. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES AND THE TABLES 
         [0008]      FIG. 1 : A schematic representation of a TALE-nuclease engineered to specifically cleave a repeat sequence. 
           [0009]      FIG. 2 : A schematic representation of the use of dimeric TALE-nuclease engineered to specifically cleave a repeat sequence. One TALE-nuclease half-domain is engineered to recognize a target sequence comprising a region adjacent to the repeat sequence and another TALE-nuclease half-domain is engineered to recognize a target sequence within the repeat sequence such as the dimeric TALE-nuclease cleaves the repeat sequence. The cleavage of the repeat sequence induces a repair process, such as single-strand annealing (SSA) process resulting in the contraction of the repeats. 
           [0010]      FIG. 3 : A schematic representation of the use of monomeric TALE-nuclease engineered to specifically cleave a repeat sequence. Monomeric TALE-nuclease engineered specifically to cleave a repeat sequence, recognizes a target sequence comprising a region adjacent to the repeat sequence. The cleavage of the repeat sequence induces a repair process, such as single-strand annealing (SSA) process resulting in the contraction of the repeats. 
           [0011]      FIG. 4 : A schematic representation of the use of Cas9-guide RNA complex engineered to specifically cleave a repeat sequence. The guide RNA is engineered to specifically recognize a target sequence comprising a region adjacent to the repeat sequence such that the Cas9-guide RNA complex cleaves the repeat sequence. The cleavage of the repeat sequence induces a repair process, such as single-strand annealing (SSA) process resulting in the contraction of the repeats. 
       
    
    
       [0012]    Table 1: List of sequences targeted by the two TALEN pairs. The 16 by sequence targeted by the TALEN (position T0 is omitted) flanking the repeated sequence is underlined. 
         [0013]    Table 2: Activity of TALEN in our yeast SSA assay previously described (International PCT Applications WO 2004/067736 and in (Epinat, Arnould et al. 2003; Chames, Epinat et al. 2005; Arnould, Chames et al. 2006; Smith, Grizot et al. 2006) at 37° C. − represent no detectable activity, + indicate a weak activity and ++ represent a high activity. n.a. indicates no available data. 
       DESCRIPTION OF THE INVENTION 
       [0014]    Unless specifically defined herein, all technical and scientific terms used have the same meaning as commonly understood by a skilled artisan in the fields of gene therapy, biochemistry, genetics, and molecular biology. 
         [0015]    The practice of the present invention will employ, unless otherwise indicated, conventional techniques of cell biology, cell culture, molecular biology, transgenic biology, microbiology, recombinant DNA, and immunology, which are within the skill of the art. Such techniques are explained fully in the literature. See, for example, Current Protocols in Molecular Biology (Frederick M. AUSUBEL, 2000, Wiley and son Inc, Library of Congress, USA); Molecular Cloning: A Laboratory Manual, Third Edition, (Sambrook et al, 2001, Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press); Oligonucleotide Synthesis (M. J. Gait ed., 1984); Mullis et al. U.S. Pat. No. 4,683,195; Nucleic Acid Hybridization (B. D. Harries &amp; S. J. Higgins eds. 1984); Transcription And Translation (B. D. Hames &amp; S. J. Higgins eds. 1984); Culture Of Animal Cells (R. I. Freshney, Alan R. Liss, Inc., 1987); Immobilized Cells And Enzymes (IRL Press, 1986); B. Perbal, A Practical Guide To Molecular Cloning (1984); the series, Methods In ENZYMOLOGY (J. Abelson and M. Simon, eds.-in-chief, Academic Press, Inc., New York), specifically, Vols. 154 and 155 (Wu et al. eds.) and Vol. 185, “Gene Expression Technology” (D. Goeddel, ed.); Gene Transfer Vectors For Mammalian Cells (J. H. Miller and M. P. Calos eds., 1987, Cold Spring Harbor Laboratory); Immunochemical Methods In Cell And Molecular Biology (Mayer and Walker, eds., Academic Press, London, 1987); Handbook Of Experimental Immunology, Volumes I-IV (D. M. Weir and C. C. Blackwell, eds., 1986); and Manipulating the Mouse Embryo, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1986). 
       Engineered Rare-Cutting Endonucleases for Use to Contract Polynucleotide Repeats 
       [0016]    The present invention relates to a rare-cutting endonuclease which is capable of specifically recognizing and cleaving a repeat sequence. To avoid off-site targeting, the inventors engineered a rare-cutting endonuclease to specifically cleave repeated sequence, characterized in that said rare-cutting endonuclease recognizes a target sequence comprising a region adjacent to the repeat sequence. The cleavage of the repeat sequence induces a repairing process conducting to the contraction of polynucleotide repeats, preferably present in a gene subjected to repeat disorder. In a particular embodiment, the present invention relates to a method of engineering a rare-cutting endonuclease which specifically cleaves a repeat sequence. In particular, said method comprises the steps of: (a) selecting a target sequence comprising a region adjacent to the repeat sequence; (b) engineering a rare-cutting endonuclease capable of recognizing said target sequence and cleaving the repeat sequence. 
         [0017]    The target sequence according to the present invention can be present in a chromosome, an episome, an organellar genome such as mitochondrial or chloroplast genome or genetic material that can exist independently to the main body of genetic material such as an infecting viral genome, plasmids, episomes, transposons for example. A target nucleic acid sequence can be within the coding sequence of a gene, within transcribed non-coding sequence such as, for example, leader sequences, trailer sequence or introns, or within non-transcribed sequence, either upstream or downstream of the coding sequence. The nucleic acid target sequence is defined by the 5′ to 3′ sequence of one strand of said target. In particular the target sequence comprises a part of the repeat sequence and a sequence adjacent thereto. 
         [0018]    The repeat sequence can be trinucleotide repeats, but also tetra-, penta- or hexa-nucleotides. As non limiting examples, said repeat sequence can be (CGC)n, (GAA)n, (CTG)n, (CCTG)n, (CGG)n, (ATTCT)n, (CAG)n wherein n can be comprised between 1 to 20000, preferably between 10 to 15000, preferably more than 20 (see for review: (Orr and Zoghbi 2007)). Said target sequence comprises a part of a repeat sequence comprising at least 3, preferably at least 4, 5, 6, 7, 8, 9, 10 nucleotides. 
         [0019]    The region adjacent to the repeat sequence needs to be sufficiently long to be specifically recognized by the rare-cutting endonuclease. The region adjacent to the repeat sequence comprises at least 5 nucleotides, preferably at least 6, 7, 8, 9, 10, 11, 12, 15 nucleotides. In a more preferred embodiment said adjacent sequence comprises between 5 and 10 nucleotides. The adjacent sequence can be in the 5′ or the 3′ region to the repeat sequence. Said target sequence is preferably within a genetic sequence in which expansion of unstable repeats can cause neurological disorder. As non limiting example, said genetic sequence can be selected from the group consisting of: 5′untranslated region (UTR) sequence of Fragile X mental retardation 1 gene (FMR1, MIM number: 309550, NG_007529.1) comprising (CGG)n repeat units; 5′ UTR sequence of Fragile X mental retardation 2 gene (FMR2, MIM number 300806, NG_016313.1) comprising (CCG)n repeat units, the first intron of the Friedreich ataxia 1 gene (FRDA, MIM number: 606829, NG_008845.2) comprising (GAA)n repeat unit; 3′UTR sequence of dystrophia myotonica-protein kinase gene (DMPK, MIM number 605377, NG_009784.1) comprising (CTG)n repeat units; the first intron of the Zing finger 9 gene (ZNF9, MIM number: 602668, NG_011902.1) comprising (CCTG)n repeat units; Ataxin 8 (ATXN8, MIM number: 613289, GenBank: DQ641254.1) comprising (CAG)n repeat units; Ataxin 8 opposite strand (ATXN8OS, MIM number: 603680, NR_002717.2) comprising (CTG)n repeat units, intron 9 of the ataxin 10 gene (ATXN10, MIM number: 611150, NG_016212.1) comprising (CAGT)n repeat units; 5′ UTR sequence of protein phosphatase 2 regulatory subunit B beta gene (PPP2R2B, MIM number: 604325, NG_011570.1) comprising (CAG)n repeat units; N-terminus of the huntingtin gene (HTT, MIM number: 613004, NG_009378.1) comprising (CAG)n repeat units; ataxin 1 (ATXN1, MIM number: 601556, NG_011571.1) comprising (CAG)n repeat units; ataxin 2 (ATXN2; MIM number: 601517, NG_011572.1) comprising (CAG)n repeat; ataxin 3 (ATXN3, MIM number: 607047, NG_008198.1) comprising (CAG)n repeat units; the exon 47 of Calcium Channel, voltage-dependent, P/Q type, alpha-1A subunit gene (CACNA1A, MIM number: 601011, NC_000019.9) comprising (CAG)n repeat units; ataxin 7 (ATXN7, MIM number: 607640, NG_008227.1) comprising (CAG)n repeat units; TATA box-binding protein gene (TBP, MIM number: 60075, NG_008165.1) comprising a (CAG)n and/or (CAA)n repeat units; the exon 1 of spinal and Androgen receptor gene (AR, MIM number: 313700, NG_009014.2) comprising (CAG)n repeat units; atrophin 1 gene (ATN1, MIM number: 607462, NG_008047.1) comprising (CAG)n repeat units and homologue thereof. 
         [0020]    In a more preferred embodiment, said target sequence is selected within the sequence encoding huntinting protein (SEQ ID NO: 1), preferably within sequence encoding the N-terminal part of the huntingtin protein (SEQ ID NO: 2), more preferably the target sequence is selected within the sequence SEQ ID NO: 3. 
         [0021]    By “rare-cutting endonuclease”, it is meant any wild type or variant enzyme capable of catalyzing the hydrolysis (cleavage) of bonds between nucleic acids within a DNA or RNA molecule, preferably a DNA molecule. A rare-cutting endonucelase is highly specific, recognizing nucleic acid target sites ranging from 10 to 45 base pairs (bp) in length, usually ranging from 10 to 35 base pairs in length. The endonuclease according to the present invention recognizes and cleaves nucleic acid at specific polynucleotide sequences, further referred to as “target sequence”. The rare-cutting endonuclease can recognize and generate a single- or double-strand break at specific polynucleotides sequences. 
         [0022]    The rare-cutting endonuclease according to the present invention can be a Cas9 endonuclease. Recently, a new genome engineering tool has been developed based on the RNA-guided Cas9 nuclease (Gasiunas, Barrangou et al. 2012; Jinek, Chylinski et al. 2012; Cong, Ran et al. 2013; Mali, Yang et al. 2013) from the type II prokaryotic CRISPR (Clustered Regularly Interspaced Short palindromic Repeats) adaptive immune system (see for review (Sorek, Lawrence et al. 2013)). The CRISPR Associated (Cas) system was first discovered in bacteria and functions as a defense against foreign DNA, either viral or plasmid. CRISPR-mediated genome engineering first proceeds by the selection of target sequence often flanked by a short sequence motif, referred as the proto-spacer adjacent motif (PAM). Following target sequence selection, a specific crRNA, complementary to this target sequence is engineered. Trans-activating crRNA (tracrRNA) required in the CRISPR type II systems paired to the crRNA and bound to the provided Cas9 protein. Cas9 acts as a molecular anchor facilitating the base pairing of tracRNA with cRNA (Deltcheva, Chylinski et al. 2011). In this ternary complex, the dual tracrRNA:crRNA structure acts as guide RNA that directs the endonuclease Cas9 to the cognate target sequence. In the present invention the guide RNA can hybridize the target sequence which comprises a region adjacent to the repeat sequence. Target recognition by the Cas9-tracrRNA:crRNA complex is initiated by scanning the target sequence for homology between the target sequence and the crRNA. In addition to the target sequence-crRNA complementarity, DNA targeting requires the presence of a short motif adjacent to the protospacer (protospacer adjacent motif - PAM). Following pairing between the dual-RNA and the target sequence, Cas9 subsequently introduces a blunt double strand break 3 bases upstream of the PAM motif (Garneau, Dupuis et al. 2010). According to the present invention, following the hybridization of the dual-RNA (guide RNA) and the target sequence which comprises a region adjacent to the repeat sequence, Cas9 cleaves repeated sequence (see  FIG. 4 ). 
         [0023]    Rare-cutting endonuclease can also be a homing endonuclease, also known under the name of meganuclease. Such homing endonucleases are well-known to the art (Stoddard 2005). Homing endonucleases recognize a DNA target sequence and generate a single- or double-strand break. Homing endonucleases are highly specific, recognizing DNA target sites ranging from 12 to 45 base pairs (bp) in length, usually ranging from 14 to 40 by in length. The homing endonuclease according to the invention may for example correspond to a LAGLIDADG endonuclease, to a HNH endonuclease, or to a GIY-YIG endonuclease. Preferred homing endonuclease according to the present invention can be an I-CreI variant. A “variant” endonuclease, i.e. an endonuclease that does not naturally exist in nature and that is obtained by genetic engineering or by random mutagenesis can bind DNA sequences different from that recognized by wild-type endonucleases (see international application WO2006/097854). 
         [0024]    Said rare-cutting endonuclease can be a modular DNA binding nuclease or a chimeric endonuclease. By chimeric endonuclease or modular DNA binding nuclease is meant any fusion proteins comprising at least one catalytic domain of an endonuclease and at least one DNA binding domain or protein specifying a nucleic acid target sequence. 
         [0025]    The DNA binding domain is generally a RNA or DNA-binding domain formed by an independently folded polypeptide protein domain that contains at least one motif that recognizes double- or single-stranded polynucleotides. Said nucleic acid binding domain preferably recognizes a specific nucleic acid sequence named target sequence. wMany such polypeptides have been described in the art having the ability to bind specific nucleic acid sequences. Such binding domains often comprise, as non limiting examples, helix-turn helix domains, leucine zipper domains, winged helix domains, helix-loop-helix domains, HMG-box domains, Immunoglobin domains, B3 domain or engineered zinc finger domain. 
         [0026]    According to a preferred embodiment of the invention, the DNA binding domain is derived from a Transcription Activator like Effector (TALE), wherein sequence specificity is driven by a series of 33-35 amino acids repeats originating from  Xanthomonas  or  Ralstonia  bacterial proteins. These repeats differ essentially by two amino acids positions that specify an interaction with a base pair (Boch, Scholze et al. 2009; Moscou and Bogdanove 2009). Each base pair in the DNA target is contacted by a single repeat, with the specificity resulting from the two variant amino acids of the repeat (the so-called repeat variable dipeptide, RVD). TALE binding domains may further comprise an N-terminal translocation domain responsible for the requirement of a first thymine base (T 0 ) of the targeted sequence and a C-terminal domain that containing a nuclear localization signals (NLS). A TALE nucleic acid binding domain generally corresponds to an engineered core TALE scaffold comprising a plurality of TALE repeat sequences, each repeat comprising a RVD specific to each nucleotides base of a TALE recognition site. In the present invention, each TALE repeat sequence of said core scaffold is made of 30 to 42 amino acids, more preferably 33 or 34 wherein two critical amino acids (the so-called repeat variable dipeptide, RVD) located at positions 12 and 13 mediates the recognition of one nucleotide of said TALE binding site sequence; equivalent two critical amino acids can be located at positions other than 12 and 13 specially in TALE repeat sequence taller than 33 or 34 amino acids long. Preferably, RVDs associated with recognition of the different nucleotides are HD for recognizing C, NG for recognizing T, NI for recognizing A, NN for recognizing G or A. In another embodiment, critical amino acids 12 and 13 can be mutated towards other amino acid residues in order to modulate their specificity towards nucleotides A, T, C and G and in particular to enhance this specificity. By other amino acid residues is intended any of the twenty natural amino acid residues or unnatural amino acids derivatives. 
         [0027]    A TALE nucleic acid binding domain usually comprises between 8 and 30 TALE repeat sequences. More preferably, said core scaffold of the present invention comprises between 8 and 20 TALE repeat sequences; again more preferably 15 TALE repeat sequences. It can also comprise an additional single truncated TALE repeat sequence made of 20 amino acids located at the C-terminus of said set of TALE repeat sequences, i.e. an additional C-terminal half- TALE repeat sequence. The TALE nucleic acid binding domains according to the present invention preferably comprise the nucleic acid sequences selected from the group consisting of SEQ ID NO: 4 and SEQ ID NO: 5. In another embodiment, said engineered TALE binding domain comprises a nucleic acid sequence having at least 80%, more preferably 90%, again more preferably 95% identity with the nucleic acid sequences selected from the group consisting of SEQ ID NO: 4 and SEQ ID NO: 5. 
         [0028]    Other engineered DNA binding domains are modular base-per-base specific nucleic acid binding domains (MBBBD) (PCT/US2013/051783). Said MBBBD can be engineered, for instance, from the newly identified proteins, namely EAV36_BURRH, E5AW43_BURRH, E5AW45_BURRH and 
         [0029]    E5AW46_BURRH proteins from the recently sequenced genome of the endosymbiont fungi  Burkholderia Rhizoxinica  (Lackner, Moebius et al. 2011). MBBBD proteins comprise modules of about 31 to 33 amino acids that are base specific. These modules display less than 40% sequence identity with  Xanthomonas  TALE common repeats, whereas they present more polypeptides sequence variability. When they are assembled together, these modular polypeptides can although target specific nucleic acid sequences in a quite similar fashion as  Xanthomonas  TAL-nucleases. 
         [0030]    According to a preferred embodiment of the present invention, said DNA binding domain is an engineered MBBBD binding domain comprising between 10 and 30 modules, preferably between 16 and 20 modules. The different domains from the above proteins (modules, N and C-terminals) from  Burkholderia  and  Xanthomonas  are useful to engineer new proteins or scaffolds having binding properties to specific nucleic acid sequences. In particular, additional N-terminal and C-terminal domains of engineered MBBBD can be derived from natural TALE like AvrBs3, PthXo1, AvrHah1, PthA, Tal1c as non-limiting examples. 
         [0031]    “TALE-nuclease” or “MBBBD-nuclease” refers to engineered proteins resulting from the fusion of a DNA binding domain typically derived from Transcription Activator like Effector proteins (TALE) or MBBBD binding domain, with an endonuclease catalytic domain. Such catalytic domain is preferably a nuclease domain and more preferably a domain having endonuclease activity, like for instance I-TevI, ColE7, NucA and Fok-I. In a particular embodiment, said nuclease is a monomeric TALE-Nuclease or MBBBD-nuclease. A monomeric Nuclease is a nuclease that does not require dimerization for specific recognition and cleavage, such as the fusions of engineered DNA binding domain with the catalytic domain of I-TevI described in WO2012138927 (see  FIG. 3 ). In another particular embodiment, said rare-cutting endonuclease is a dimeric TALE-nuclease or MBBBD-nuclease, preferably comprising a DNA binding domain fused to FokI (see  FIG. 1 ). Said dimeric nuclease comprises a first DNA binding nuclease capable of binding a target sequence comprising a region adjacent to the repeat sequence and a second DNA binding nuclease capable of binding a target sequence within the repeat sequence, such that the dimeric nuclease induces a cleavage event within the repeat sequence (see  FIG. 2 ). TALE-nuclease have been already described and used to stimulate gene targeting and gene modifications (Boch, Scholze et al. 2009; Moscou and Bogdanove 2009; Christian, Cermak et al. 2010). Such engineered TALE-nucleases are commercially available under the trade name TALEN™ (Cellectis, 8 rue de la Croix Jarry, 75013 Paris, France). 
         [0032]    In another aspect, the present invention also relates to the rare-cutting endonucleases disclosed here, preferably rare-cutting endonucleases obtainable by the method described above. In a preferred embodiment, the present invention relates to the rare-cutting endonuclease which has at least 70%, preferably 80%, 85%, 90%; 95% identity with the amino acid sequence selected from the group consisting of: SEQ ID NO: 8, SEQ ID NO: 10 and SEQ ID NO: 15. 
       Polynucleotides, Vectors: 
       [0033]    The present invention also relates to polynucleotides, vectors encoding the above described rare-cutting endonuclease according to the invention. In a preferred embodiment, the present invention relates to a polynucleotide comprising the nucleic acid sequence selected from the group consisting of: SEQ ID NO: 9, SEQ ID NO: 11 and SEQ ID NO: 16. In a preferred embodiment, the polynucleotide has at least 70%, preferably at least 80%, more preferably at least 90%, 95% 97% or 99% sequence identity with nucleic acid sequence selected from the group consisting of SEQ ID NO: 9, SEQ ID NO: 11 and SEQ ID NO: 16. 
         [0034]    The polynucleotide may consist in an expression cassette or expression vector (e.g. a plasmid for introduction into a bacterial host cell, or a viral vector such as a baculovirus vector for transfection of an insect host cell, or a plasmid or viral vector such as a lentivirus for transfection of a mammalian host cell). 
         [0035]    In a particular embodiment, the different nucleic acid sequences can be included in one polynucleotide or vector which comprises a nucleic acid sequence encoding ribosomal skip sequence such as a sequence encoding a 2A peptide. 2A peptides, which were identified in the Aphthovirus subgroup of picornaviruses, causes a ribosomal “skip” from one codon to the next without the formation of a peptide bond between the two amino acids encoded by the codons (see (Donnelly and Elliott 2001; Donnelly, Luke et al. 2001; Atkins, Wills et al. 2007; Doronina, Wu et al. 2008)). By “codon” is meant three nucleotides on an mRNA (or on the sense strand of a DNA molecule) that are translated by a ribosome into one amino acid residue. Thus, two polypeptides can be synthesized from a single, contiguous open reading frame within an mRNA when the polypeptides are separated by a 2A oligopeptide sequence that is in frame. Such ribosomal skip mechanisms are well known in the art and are known to be used by several vectors for the expression of several proteins encoded by a single messenger RNA. 
         [0036]    Those skilled in the art will recognize that, in view of the degeneracy of the genetic code, considerable sequence variation is possible among these polynucleotide molecules. Preferably, the nucleic acid sequences of the present invention are codon-optimized for expression in mammalian cells, preferably for expression in human cells. Codon-optimization refers to the exchange in a sequence of interest of codons that are generally rare in highly expressed genes of a given species by codons that are generally frequent in highly expressed genes of such species, such codons encoding the amino acids as the codons that are being exchanged. 
       A Method for Contracting a Repeat Sequence Subjected to Repeat Disorder 
       [0037]    In another aspect, the present invention also relates to a method for contracting a repeat sequence within a genetic sequence subjected to repeat disorder into a living cell. This method comprises the steps of: (a) selecting a target sequence comprising a region adjacent to the repeat sequence; (b) providing at least one rare-cutting endonuclease capable of binding said target sequence and cleaving the repeat sequence; (c) introducing said rare-cutting endonuclease into said cell and (d) contacting said rare-cutting endonuclease with the genetic sequence such that said rare-cutting endonuclease cleaves the repeat sequence inducing a repair process conducting to the contraction of said repeat sequence. In a preferred embodiment said repair process is the single strand annealing (SSA). Single strand annealing (SSA) is a process that is initiated when a cleavage is made between two repeated sequences oriented in the same direction. Single stranded regions are created adjacent to the break that extend to the repeated sequences such that the complementary strands can anneal to each other. This annealed intermediate can be processed by digesting away the single stranded tails and filling in the gaps annealing process. In particular embodiment, the method comprises expressing within a cell the rare-cutting endonuclease capable of binding the target sequence according to the present invention. In a more particular embodiment, the method comprises transforming the cell with at least one polyucleotide encoding the rare-cutting endonuclease as described above and expressing said polynucleotide into said cell. 
         [0038]    The method described above involves introducing rare-cutting endonuclease into a cell. As non-limiting example, said rare-cutting endouclease can be introduced as transgenes encoded by one plasmidic vector. Said plasmid vector can also contain a selection marker which provides for identification and/or selection of cells which received said vector. 
         [0039]    Polypeptides may be synthesized in situ in the cell as a result of the introduction of polynucleotides encoding said polypeptides into the cell. Alternatively, said polypeptides could be produced outside the cell and then introduced thereto. Methods for introducing a polynucleotide construct into cells are known in the art and including as non limiting examples stable transformation methods wherein the polynucleotide construct is integrated into the genome of the cell, transient transformation methods wherein the polynucleotide construct is not integrated into the genome of the cell and virus mediated methods. Said polynucleotides may be introduced into a cell by for example, recombinant viral vectors (e.g. retroviruses, adenoviruses), liposome and the like. For example, transient transformation methods include for example microinjection, electroporation or particle bombardment. Said polynucleotides may be included in vectors, more particularly plasmids or virus, in view of being expressed in cells. 
         [0040]    The present invention also relates to isolated cells or cell lines susceptible to be obtained by the method described in the above paragraph. In particular, said isolated cell comprises at least one rare-cutting endonuclease as described above. In another embodiment, said isolated cell comprises a reduced repeat expanded sequence. In a preferred embodiment, said isolated cell is a mammalian cell. 
       Applications 
       [0041]    In another aspect, said rare-cutting endonuclease according to the present invention can be used to treat or prevent disease caused by the expansion of unstable repeats, preferably as non limiting examples: Fragile X syndrome (FRAXA), Fragile XE syndrome (FRAXE), Friedreich Ataxia (FRDA), Myotonic dystrophy (DM1), Fragile X-Associated Tremor Ataxia syndrome (FXTAS), CAG repeat expansion disease such as spinal and bulbar muscular atrophy (SBMA), Huntington disease (HD), spinocerebellar ataxia type 1, dentatorubal-pallidoluysian atrophy, Machado-Joseph disease, spinocerebellar ataxia 2, spinocerebellar ataxia 6 and spinocerebellar ataxia 7. Said rare cutting endonuclease of the present invention is preferably used to treat Huntington disease. Said rare-cutting endonuclease can be administrating directly to subjects (in vivo) using for example viral vector. Said rare-cutting endonuclease can be administrated by systemic administration (e.g. intravenous, intraperitoneal, intramuscular, subdermal or intracranial infusion) or topical application. Alternatively said rare-cutting endonuclease can be used to treat cells in vitro and then the modified cells are administrated to subjects, usually after selection for cells which have incorporated the vector (ex vivo). 
         [0042]    The present invention also relates to a pharmaceutical composition comprising the rare-cutting endonuclease according to the present invention specific to a repeat sequence. The pharmaceutical composition according to the present invention can be used for contracting a specific repeat sequence within a cell. Pharmaceutically acceptable carriers are determined in part by the particular composition being administrated, as well as by the particular method used to administer the composition. Accordingly, there is a wide variety of suitable formulations of pharmaceutical compositions available. 
         [0043]    The methods and compositions of the present invention are also useful for the design and implementation of in vitro and in vivo models, for example, animal models of repeat disorders, which allows for the study of these disorders. Non-limiting examples of suitable in vitro models include cells or cell lines from any organism, including fibroblast. Non limiting-examples of suitable animals for use as animal models include, invertebrates ( C. elegans, drsophilia ), rodents (e.g., rat or mouse), primate (e.g., non-human primates). 
       Definitions 
       [0044]    In the description above, a number of terms are used extensively. The following definitions are provided to facilitate understanding of the present embodiments. 
         [0045]    As used herein, “a” or “an” may mean one or more than one. 
         [0046]    As used herein, the term “about” indicates that a value includes the inherent variation of error for the method being employed to determine a value, or the variation that exists among experiments.
       Amino acid residues in a polypeptide sequence are designated herein according to the one-letter code, in which, for example, Q means Gln or Glutamine residue, R means Arg or Arginine residue and D means Asp or Aspartic acid residue.   Amino acid substitution means the replacement of one amino acid residue with another, for instance the replacement of an Arginine residue with a Glutamine residue in a peptide sequence is an amino acid substitution.   Nucleotides are designated as follows: one-letter code is used for designating the base of a nucleoside: a is adenine, t is thymine, c is cytosine, and g is guanine. For the degenerated nucleotides, r represents g or a (purine nucleotides), k represents g or t, s represents g or c, w represents a or t, m represents a or c, y represents t or c (pyrimidine nucleotides), d represents g, a or t, v represents g, a or c, b represents g, t or c, h represents a, t or c, and n represents g, a, t or c.   As used herein, “nucleic acid” or “nucleic acid molecule” refers to nucleotides and/or polynucleotides, such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), oligonucleotides, fragments generated by the polymerase chain reaction (PCR), and fragments generated by any of ligation, scission, endonuclease action, and exonuclease action. Nucleic acid molecules can be composed of monomers that are naturally-occurring nucleotides (such as DNA and RNA), or analogs of naturally-occurring nucleotides (e.g., enantiomeric forms of naturally-occurring nucleotides), or a combination of both. Modified nucleotides can have alterations in sugar moieties and/or in pyrimidine or purine base moieties. Sugar modifications include, for example, replacement of one or more hydroxyl groups with halogens, alkyl groups, amines, and azido groups, or sugars can be functionalized as ethers or esters. Moreover, the entire sugar moiety can be replaced with sterically and electronically similar structures, such as aza-sugars and carbocyclic sugar analogs. Examples of modifications in a base moiety include alkylated purines and pyrimidines, acylated purines or pyrimidines, or other well-known heterocyclic substitutes. Nucleic acid monomers can be linked by phosphodiester bonds or analogs of such linkages. Nucleic acids can be either single stranded or double stranded.   By “gene” is meant the basic unit of heredity, consisting of a segment of DNA arranged in a linear manner along a chromosome, which codes for a specific protein or segment of protein. A gene typically includes a promoter, a 5′ untranslated region, one or more coding sequences (exons), optionally introns, a 3′ untranslated region. The gene may further comprise a terminator, enhancers and/or silencers.   The term “cleavage” refers to the breakage of the covalent backbone of a polynucleotide. Cleavage can be initiated by a variety of methods including, but not limited to, enzymatic or chemical hydrolysis of a phosphodiester bond. Both single-stranded cleavage and double-stranded cleavage are possible, and double-stranded cleavage can occur as a result of two distinct single-stranded cleavage events. Double stranded DNA, RNA, or DNA/RNA hybrid cleavage can result in the production of either blunt ends or staggered ends.   By “catalytic domain” is intended the protein domain or module of an enzyme containing the active site of said enzyme; by active site is intended the part of said enzyme at which catalysis of the substrate occurs. Enzymes, but also their catalytic domains, are classified and named according to the reaction they catalyze. The Enzyme Commission number (EC number) is a numerical classification 10 scheme for enzymes, based on the chemical reactions they catalyze.   According to the invention, by “homologous” is meant, with respect to a first sequence of amino acids, any amino acid sequence having at least 60% or at least 70%, at least 80% , at least 85%, at least 90%, at least 95%, at least 98%, at least 99% homology with said first amino acid sequence, and having a similar biological activity.       
 
         [0055]    Sequence homology can be identified by any method commonly used in the field by one skilled in the art. Various alignment algorithms and/or programs may be used to calculate the identity between two sequences, including FASTA, or BLAST which are available as a part of the GCG sequence analysis package (University of Wisconsin, Madison, Wis.), and can be used with, e.g., default setting.
       “identity” refers to sequence identity between two nucleic acid molecules or polypeptides. Identity can be determined by comparing a position in each sequence which may be aligned for purposes of comparison. When a position in the compared sequence is occupied by the same base, then the molecules are identical at that position. A degree of similarity or identity between nucleic acid or amino acid sequences is a function of the number of identical or matching nucleotides at positions shared by the nucleic acid sequences. Various alignment algorithms and/or programs may be used to calculate the identity between two sequences, including FASTA, or BLAST which are available as a part of the GCG sequence analysis package (University of Wisconsin, Madison, Wis.), and can be used with, e.g., default setting.   By “hybridization sequence” is meant the sequence part of the oligonucleotide that can hybridize to one of the other oligonucleotides under standard low stringent conditions. Such conditions can be for instance at room temperature for 2 hours by using a buffer containing 25% formamide, 4x SSC, 50 mM NaH2PO4/Na2HPO4 buffer; pH 7.0,5× Denhardt&#39;s, 1 mM EDTA,1 mg/ml DNA +20 to 200 ng/ml probe to be tested (approx. 20-200 ng/ml)). This can be also predicted by standard calculation of hybridization using the number of complementary bases within the sequence and the content in G-C at room temperature as provided in the literature. Preferentially, the hybridization sequences are complementary to each other pursuant to the complementarity between two nucleic acid strands relying on Watson-Crick base pairing between the strands, i.e. the inherent base pairing between adenine and thymine (A-T) nucleotides and guanine and cytosine (G-C) nucleotides. Accurate base pairing equates with Watson-Crick base pairing includes base pairing between standard and modified nucleosides and base pairing between modified nucleosides, where the modified nucleosides are capable of substituting for the appropriate standard nucleosides according to the Watson-Crick pairing. The complementary sequence of the single-strand oligonucleotide can be any length that supports specific and stable hybridization between the two single-strand oligonucleotides under the reaction conditions.   By “ delivery vector” or “ delivery vectors” is intended any delivery vector which can be used in the present invention to put into cell contact (i.e “contacting”) or deliver inside cells or subcellular compartments agents/chemicals and molecules (proteins or nucleic acids) needed in the present invention. It includes, but is not limited to liposomal delivery vectors, viral delivery vectors, drug delivery vectors, chemical carriers, polymeric carriers, lipoplexes, polyplexes, dendrimers, microbubbles (ultrasound contrast agents), nanoparticles, emulsions or other appropriate transfer vectors. These delivery vectors allow delivery of molecules, chemicals, macromolecules (genes, proteins), or other vectors such as plasmids, peptides developed by Diatos. In these cases, delivery vectors are molecule carriers. By “delivery vector” or “delivery vectors” is also intended delivery methods to perform transfection.   The terms “vector” or “vectors” refer to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked. A “vector” in the present invention includes, but is not limited to, a viral vector, a plasmid, a RNA vector or a linear or circular DNA or RNA molecule which may consists of a chromosomal, non chromosomal, semi-synthetic or synthetic nucleic acids.       
 
         [0060]    Preferred vectors are those capable of autonomous replication (episomal vector) and/or expression of nucleic acids to which they are linked (expression vectors). Large numbers of suitable vectors are known to those of skill in the art and commercially available.
       Viral vectors include retrovirus, adenovirus, parvovirus (e. g. adenoassociated viruses), coronavirus, negative strand RNA viruses such as orthomyxovirus (e. g., influenza virus), rhabdovirus (e. g., rabies and vesicular stomatitis virus), paramyxovirus (e. g. measles and Sendai), positive strand RNA viruses such as picornavirus and alphavirus, and double-stranded DNA viruses including adenovirus, herpesvirus (e. g., Herpes Simplex virus types 1 and 2, Epstein-Barr virus, cytomegalovirus), and poxvirus (e. g., vaccinia, fowlpox and canarypox). Other viruses include Norwalk virus, togavirus, flavivirus, reoviruses, papovavirus, hepadnavirus, and hepatitis virus, for example. Examples of retroviruses include: avian leukosis-sarcoma, mammalian C-type, B-type viruses, D type viruses, HTLV-BLV group, lentivirus, spumavirus (Coffin, J. M., Retroviridae: The viruses and their replication, In Fundamental Virology, Third Edition, B. N. Fields, et al., Eds., Lippincott-Raven Publishers, Philadelphia, 1996).   By cell or cells is intended any prokaryotic or eukaryotic living cells, cell lines derived from these organisms for in vitro cultures, primary cells from animal or plant origin.   By “primary cell” or “primary cells” are intended cells taken directly from living tissue (i.e. biopsy material) and established for growth in vitro, that have undergone very few population doublings and are therefore more representative of the main functional components and characteristics of tissues from which they are derived from, in comparison to continuous tumorigenic or artificially immortalized cell lines. These cells thus represent a more valuable model to the in vivo state they refer to.       
 
         [0064]    In the frame of the present invention, “eukaryotic cells” refer to a fungal, plant or animal cell or a cell line derived from the organisms listed below and established for in vitro culture. 
         [0065]    More preferably the animal cell is of the genus  Homo, Rattus, Mus, Sus, Bos, Danio, Canis, Felis, Equus, Salmo, Oncorhynchus, Gallus, Meleagris, Drosophila, Caenorhabditis ; more preferably, the animal cell is of the species  Homo sapiens, Rattus norvegicus, Mus musculus, Sus scrofa, Bos taurus, Danio rerio, Canis lupus, Felis catus, Equus caballus, Salmo salar, Oncorhynchus mykiss, Gallus gallus, Meleagris gallopavo, Drosophila melanogaster, Caenorhabditis elegans.    
         [0066]    In the present invention, the cell can be a mammalian cell, a fish cell, an insect cell or cell lines derived from these organisms for in vitro cultures or primary cells taken directly from living tissue and established for in vitro culture. As non limiting examples cell lines can be selected from the group consisting of CHO-K1 cells; HEK293 cells; Caco2 cells; U2-OS cells; NIH 3T3 cells; NSO cells; SP2 cells; CHO-S cells; DG44 cells; K-562 cells, U-937 cells; MRC5 cells; IMR90 cells; Jurkat cells; HepG2 cells; HeLa cells; HT-1080 cells; HCT-116 cells; Hu-h7 cells; Huvec cells; Molt 4 cells. Are also encompassed in the scope of the present invention stem cells and induced Pluripotent Stem cells (iPS). 
         [0067]    All these cell lines can be modified by the method of the present invention to provide cell line models.
       The term “subject” as used herein includes all members of the animal kingdom including non-human primates and humans.       
 
       EXAMPLES 
     Cloning of the RVD Array Collection in the TALE Backbone 
       [0069]    The two TALE backbones used in these experiment (pCLS9303 and pCLS9312, SEQ ID NO: 4 and 5) contain, between the C-terminal and the N-terminal domains, two BsmBI restriction sites. The individual repeat arrays targeting the region flanking the repeated trinucleotides (SEQ ID NO: 6 to 7) were subcloned in the pCLS9303 using type Ils restriction enzymes BsmBI for the receiving plasmid and BbvI and SfaNI for the inserted RVD array, leading to pCLS9984 and pCLS16715 (SEQ ID NO: 9 encoded SEQ ID NO: 8 and SEQ ID NO: 11 encoded SEQ ID NO: 10). The individual repeat arrays targeting the repeated trinulceotides (SEQ ID NO: 14) was subcloned in the pCLS9312 using type Is restriction enzymes BsmBl for the receiving plasmid and BbvI and SfaNl for the inserted RVD array, leading to pCLS9996 (SEQ ID NO: 16 encoded SEQ ID NO: 15). The monoclonality DNA sequence of each individual clone was assessed by DNA sequencing. 
       TALE-Nuclease Activities in Yeast 
       [0070]    The two yeast target reporter plasmids containing the TALEN™ DNA target sequences were constructed as previously described (International PCT Applications WO 2004/067736 and in (Epinat, Arnould et al. 2003; Chames, Epinat et al. 2005; Arnould, Chames et al. 2006; Smith, Grizot et al. 2006). The TALEN™ pairs (pCLS9984/pCLS9996 and pCLS16715/pCLS9996) were tested at 37° C. and 30° C. in our yeast SSA assay previously described (International PCT Applications WO 2004/067736 and in (Epinat, Arnould et al. 2003; Chames, Epinat et al. 2005; Arnould, Chames et al. 2006; Smith, Grizot et al. 2006) on both targets (SEQ ID NO: 12 to 13, Table 1). TALEN™ cleavage activity levels on their targets in yeast are shown in Table 2. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 List of sequences targeted by the two TALEN pairs. The 16 bp sequence 
               
               
                 targeted by the TALEN ™ (position T0 is omitted) flanking the repeated 
               
               
                 sequence is underlined 
               
             
          
           
               
                 Name 
                 Sequence 
               
               
                   
               
               
                 TiFLAN 
                 TCTCAAGATT TCGCTGCAGCAGCAGC AGCAGCAGCAGCAGCAGCAGCAGCAGCAGCAG 
               
               
                   
                 CAGCAGCAGCA 
               
               
                   
               
               
                 TiFLAN2_T01.1 
                 T GTGATCCCCCCAGCAG CAGCAGCAGCAGCAGCAGCAGCAGCAGCA 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Activity of TALEN ™ in our yeast SSA assay 
               
               
                 previously described (International PCT  
               
               
                 Applications WO 2004/067736 and in (Epinat, 
               
               
                 Arnould et al. 2003; Chames, Epinat et al. 2005; 
               
               
                 Arnould, Chames et al. 2006; Smith, Grizot et al. 
               
               
                 2006; Smith, Grizot et al. 2006) at 37° C. and 
               
               
                 30° C.-represent no detectable activity, + 
               
               
                 indicate a weak activity and ++ represent a 
               
               
                 high activity. n.a. indicates no available data. 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 pCLS9984/ 
                 pCLS16715/ 
               
               
                 At 37° C. 
                 pCLS9996 
                 pCLS9996 
               
               
                   
               
               
                 TiFLAN 
                 +++ 
                 - 
               
               
                   
               
               
                 TiFLAN2_T01.1 
                 + 
                 +++ 
               
               
                   
               
               
                   
                 pCLS9984/ 
                 pCLS16715/ 
               
               
                 At 30° C. 
                 pCLS9996 
                 pCLS9996 
               
               
                   
               
               
                 TiFLAN 
                 +++ 
                 - 
               
               
                   
               
               
                 TiFLAN2_T01.1 
                 - 
                 ++ 
               
               
                   
               
             
          
         
       
     
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