Patent Publication Number: US-2021171935-A1

Title: Method for modifying genome sequence to introduce specific mutation to targeted dna sequence by base-removal reaction, and molecular complex used therein

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is a continuation of copending U.S. patent application Ser. No. 15/523,939, filed on May 2, 2017, which is the U.S. national phase of International Patent Application No. PCT/JP2015/080958, filed on Nov. 2, 2015, which claims the benefit of Japanese Patent Application No. 2014-224745, filed on Nov. 4, 2014, which are incorporated by reference in their entireties herein. 
    
    
     INCORPORATION-BY-REFERENCE OF MATERIAL ELECTRONICALLY SUBMITTED 
     Incorporated by reference in its entirety herein is a computer-readable nucleotide/amino acid sequence listing submitted concurrently herewith and identified as follows: 49,254 bytes ASCII (Text) file named “752116SequenceListing.txt,” created Feb. 11, 2021. 
     TECHNICAL FIELD 
     The present invention relates to a modification method of a genome sequence, which enables modification of a nucleic acid base in a particular region of a genome, without cleaving double stranded DNA (no cleavage or single strand cleavage), or inserting a foreign DNA fragment, but utilizing a base excision reaction, and a complex of a nucleic acid sequence-recognizing module and DNA glycosylase to be used therefor. 
     BACKGROUND ART 
     In recent years, genome editing is attracting attention as a technique for modifying the object gene and genome region in various species. Conventionally, as a method of genome editing, a method utilizing an artificial nuclease comprising a molecule having a sequence-independent DNA cleavage ability and a molecule having a sequence recognition ability in combination has been proposed (non-patent document 1). 
     For example, a method of performing recombination at a target gene locus in DNA in a plant cell or insect cell as a host, by using a zinc finger nuclease (ZFN) wherein a zinc finger DNA binding domain and a non-specific DNA cleavage domain are linked (patent document 1), a method of cleaving or modifying a target gene in a particular nucleotide sequence or a site adjacent thereto by using TALEN wherein a transcription activator-like (TAL) effector which is a DNA binding module that the plant pathogenic bacteria  Xanthomonas  has, and a DNA endonuclease are linked (patent document 2), a method utilizing CRISPR-Cas9 system wherein DNA sequence CRISPR (Clustered Regularly interspaced short palindromic repeats) that functions in an acquired immune system possessed by eubacterium and archaebacterium, and nuclease Cas (CRISPR-associated) protein family having an important function along with CRISPR are combined (patent document 3) and the like have been reported. Furthermore, a method of cleaving a target gene in the vicinity of a particular sequence, by using artificial nuclease wherein a PPR protein constituted to recognize a particular nucleotide sequence by a continuation of PPR motifs each consisting of 35 amino acids and recognizing one nucleic acid base, and nuclease are linked (patent document 4) has also been reported. 
     These genome editing techniques basically presuppose double stranded DNA breaks (DSB). However, since they include unexpected genome modifications, side effects such as strong cytotoxicity, chromosomal rearrangement and the like occur, and they have common problems of impaired reliability in gene therapy, extremely small number of surviving cells by nucleotide modification, and difficulty in genetic modification itself in primate ovum and unicellular microorganisms. 
     On the other hand, as a method of performing nucleotide modification without causing DSB, utilization of DNA glycosylase has been proposed. For example, patent document 5 describes that mutant DNA glycosylase having an activity to eliminate a thymine or cytosine base from a single stranded or double stranded DNA (TDG activity or CDG activity) was obtained by introducing mutation into human uracil-DNA glycosylase (UDG) and, using the enzyme, the efficiency of mutation induction in  Escherichia coli  was improved. 
     Furthermore, patent document 6 and non-patent document 2 describe that targeted nucleotide modification into a genome region having a Tet operator (TetO) DNA sequence specifically recognized by TetR is possible by using a fusion protein of yeast 3-methyladenine-DNA glycosylase (MAG1) and Tet repressor protein (TetR). However, MAG1 essentially removes a special DNA base that was injured by alkylation and the like, which is mainly 3-methyladenine, and the ability to remove a base from normal bases, particularly base pairs without a mismatch, is extremely low. Therefore, it is difficult to observe the effect of mutation introduction by MAG1 in normal cells, and a detectable mutation rate has been obtained only by overexpressing MAG1 in cells with disrupted genes of base excision repair system. In addition, when the DNA-repair system is weakened, the mutation rate of the entire genome also increases, which makes practicalization a far goal. However, when a mutant UDG having CDG activity to act on a normal cytosine base was used for this system instead of MAG1, the efficiency of mutation induction did not increase (patent document 6, non-patent document 2). 
     DOCUMENT LIST 
     Patent Documents 
     
         
         patent document 1: JP-B-4968498 
         patent document 2: National Publication of International Patent Application No. 2013-513389 
         patent document 3: National Publication of International Patent Application No. 2010-519929 
         patent document 4: JP-A-2013-128413 
         patent document 5: WO 97/25416 
         patent document 6: WO 2014/127287 
       
    
     Non-Patent Documents 
     
         
         non-patent document 1: Kelvin M Esvelt, Harris H Wang (2013) Genome-scale engineering for systems and synthetic biology, Molecular Systems Biology 9: 641 
         non-patent document 2: Shwan P. Finney-Manchester, Narendra Maheshri, (2013) Harnessing mutagenic homologous recombination for targeted mutagenesis in vivo by TaGTEAM, Nucleic Acids Research 41(9): e99 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     An object of the present invention is to provide a novel method of genome editing for modifying a nucleic acid base of a particular sequence of a gene without DSB or insertion of foreign DNA fragment, i.e., by non-cleavage of a double stranded DNA or single strand cleavage, and a complex of a nucleic acid sequence-recognizing module and a mutation introducing enzyme therefor. 
     Means of Solving the Problems 
     The present inventors have already reported that they have successfully modified, without DSB, a genome sequence by nucleic acid base conversion in a region containing a particular DNA sequence, by using deaminase that catalyzes a deamination reaction and linking the enzyme and a molecule having a DNA sequence recognition ability (WO 2015/133554). In expectation of affording a mutation introduction tendency different from that of deaminase and the like, the present invention is based on an idea of causing a base excision reaction by hydrolysis of N-glycosidic bond of DNA, and then inducing mutation introduction in a repair process of cells. Thus, attempts have been made to use an enzyme having CDG activity or TDG activity, which is a mutant of yeast mitochondrial uracil-DNA glycosylase (UNG 1), as an enzyme that performs such base excision reaction. 
     The present inventors assumed that one of the reasons for a failure to improve efficiency of mutation induction by conventional methods (e.g., patent document 6 and non-patent document 2) even by using mutant UDG having CDG activity is that the enzyme causes base excision everywhere in the double stranded DNA, which in turn causes high cytotoxicity and makes it difficult to express the enzyme protein itself. Under such hypothesis, the present inventors modified a genome sequence by nucleic acid base conversion in a region containing a particular DNA sequence, by introducing a mutation that lowers an action on a DNA having an unrelaxed double helix structure (unrelaxed (double-helical) DNA) into a mutant UDG having CDG activity or TDG activity, and linking the mutant enzyme and a molecule having a DNA sequence recognition ability. 
     To be specific, CRISPR-Cas system (CRISPR-mutant Cas) was used. That is, a DNA encoding an RNA molecule wherein genome specific CRISPR-RNA:crRNA (gRNA) containing a sequence complementary to a target sequence of a gene to be modified is linked to an RNA (trans-activating crRNA: tracrRNA) for recruiting Cas protein was produced, a DNA wherein a DNA encoding a mutant Cas protein (dCas) wherein cleavage ability of both strands of a double stranded DNA is inactivated and a mutant CDG or TDG gene are linked was produced, and these DNAs were introduced into a host yeast cell containing a gene to be modified. As a result, mutation could be introduced randomly within the range of several hundred nucleotides of the object gene including the target sequence. Furthermore, mutation could be introduced extremely efficiently by targeting multiple regions in the object gene. That is, a host cell introduced with DNA was seeded in a nonselective medium, and the sequence of the object gene was examined in randomly selected colonies. As a result, introduction of mutation was confirmed in almost all colonies. The efficiency of mutation induction was further improved by coexpressing mutant AP endonuclease lacking its function as an enzyme of a base excision repair system, thereby increasing the frequency of repair errors in an abasic site (AP site). 
     It was also confirmed that a system using a heterogenous mutant UDG also functions in yeast. Unexpectedly, UDG derived from vaccinia virus (vvUDG) does not cause cytotoxicity even in the absence of introduction of mutation that lowers the action on an unrelaxed double helix structure of DNA, and mutant UDG introduced solely with mutations that change substrate specificity rather showed higher efficiency of mutation induction. That is, vvUDG was suggested to be an enzyme having a natively low action on DNA having an unrelaxed double-helix structure. This was also supported by the fact that the mutated sites by vvUDG are concentrated in a particular base within the target sequence (i.e., single strand part). In addition, the activity of vvUDG could be increased by co-expressing A20, which is known as a cofactor. 
     The present inventors obtained an idea of utilization of split Cas9 system as a means to decrease non-specific mutation by UDG, in addition to the introduction of mutation into the enzyme. Split Cas9 is a variant of Cas9, which is designed to function only when N-terminal fragment and C-terminal fragment split from Cas9 protein are associated with gRNA (Nat Biotechnol. 33(2): 139-142 (2015); PNAS 112(10): 2984-2989 (2015)). The present inventors constructed a system expressing split dCas9-mutant UNG1 as one fusion protein consisting of N-terminal fragment of dCas9—N-terminal fragment of mutant UNG1—C-terminal fragment of dCas9—C-terminal fragment of mutant UNG1, and a system separately expressing N-terminal fragment of dCas9—C-terminal fragment of mutant UNG1, and N-terminal fragment of mutant UNG1—C-terminal fragment of dCas9, and then examined the frequency of mutation and non-specific mutation at the target site by using a sequence in the Can1 gene as a target, as well as cell proliferation. As a result, in any system, the cell survival rate on a nonselective medium is hardly different between mutant UNG1 introduced only with mutation (N222D) that imparts CDG activity, and mutant UNG1 further introduced with mutation (R308C) that decreases the action on an unrelaxed double helix structure of DNA, and the cytotoxicity was markedly reduced by utilization of the split enzyme. When split enzyme was used, the frequency of non-specific mutation using thialysine resistance as an index was also reduced without depending on the presence or absence of R308 mutation. The efficiency of mutation induction at the original target site rather decreased, since addition of the R308C mutation also decreases the CDG activity. It was suggested, therefore, a mutation that reduces the action on an unrelaxed double helix structure of DNA is preferably not added to UDG when a split enzyme is used. 
     The present inventor have conducted further studies based on these findings and completed the present invention. 
     Therefore, the present invention is as described below. 
     [1] A method of modifying a targeted site of a double stranded DNA in a cell, comprising a step of contacting a complex wherein a nucleic acid sequence-recognizing module that specifically binds to a target nucleotide sequence in a given double stranded DNA and DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure (unrelaxed DNA) are bonded, with said double stranded DNA, to convert one or more nucleotides in the targeted site to other one or more nucleotides or delete one or more nucleotides, or insert one or more nucleotides into said targeted site, without cleaving at least one strand of said double stranded DNA in the targeted site.
 
[2] The method of the above-mentioned [1], wherein the aforementioned nucleic acid sequence-recognizing module is selected from the group consisting of a CRISPR-Cas system wherein at least one DNA cleavage ability of Cas is inactivated, a zinc finger motif, a TAL effector and a PPR motif.
 
[3] The method of the above-mentioned [1], wherein the aforementioned nucleic acid sequence-recognizing module is a CRISPR-Cas system wherein at least one DNA cleavage ability of Cas is inactivated.
 
[4] The method of the above-mentioned [1] or [2], wherein the double stranded DNA is further contacted with a factor that changes a DNA double stranded structure.
 
[5] The method of any of the above-mentioned [1]-[4], which uses two or more kinds of nucleic acid sequence-recognizing modules each specifically binding to a different target nucleotide sequence.
 
[6] The method of the above-mentioned [5], wherein the aforementioned different target nucleotide sequences are present in different genes.
 
[7] The method of any of the above-mentioned [1]-[6], wherein the aforementioned DNA glycosylase has cytosine-DNA glycosylase (CDG) activity or thymine-DNA glycosylase (TDG) activity.
 
[8] The method of the above-mentioned [7], wherein the aforementioned DNA glycosylase having CDG activity or TDG activity is a mutant of uracil-DNA glycosylase (UDG).
 
[9] The method of any of the above-mentioned [1]-[8], further comprising contacting the double stranded DNA with an AP endonuclease having binding capacity to an abasic site but lacking nuclease activity.
 
[10] The method of any of the above-mentioned [1]-[9], wherein the aforementioned DNA glycosylase has natively low reactivity with a DNA having an unrelaxed double helix structure.
 
[11] The method of the above-mentioned [10], wherein the aforementioned DNA glycosylase is a mutant of uracil-DNA glycosylase (UDG) derived from a virus belonging to Poxviridae and having CDG activity or TDG activity.
 
[12] The method of the above-mentioned [11], wherein the double stranded DNA is further contacted with A20 protein.
 
[13] The method of any of the above-mentioned [1]-[9], wherein the aforementioned DNA glycosylase is a mutant having reduced reactivity with a DNA having an unrelaxed double helix structure (unrelaxed DNA) as compared to a wild-type one.
 
[14] The method of any of the above-mentioned [1]-[9], wherein the aforementioned DNA glycosylase, and an element of the aforementioned nucleic acid sequence-recognizing module which is directly bonded to the DNA glycosylase are respectively split into two fragments, the fragments of either of the DNA glycosylase and the element are respectively linked to the fragments of the other to provide two partial complexes, and when the partial complexes are refolded with each other, the nucleic acid sequence-recognizing module is capable of specifically binding to the target nucleotide sequence and the specific bond enables the DNA glycosylase to exhibit enzyme activity.
 
[15] The method of the above-mentioned [14], wherein the element of the nucleic acid sequence-recognizing module which is directly bonded to the aforementioned DNA glycosylase is a Cas protein wherein at least one of the DNA cleavage abilities is inactivated.
 
[16] The method of the above-mentioned [14] or [15], wherein the aforementioned two partial complexes are provided as separate molecule complexes, and are refolded by association thereof in the cell.
 
[17] The method of any of the above-mentioned [1]-[16], wherein the double stranded DNA is contacted with the complex by introducing a nucleic acid encoding the complex into a cell having the double stranded DNA.
 
[18] The method of the above-mentioned [17], wherein the aforementioned cell is a prokaryotic cell.
 
[19] The method of the above-mentioned [17], wherein the aforementioned cell is a eukaryotic cell.
 
[20] The method of the above-mentioned [17], wherein the aforementioned cell is a microbial cell.
 
[21] The method of the above-mentioned [17], wherein the aforementioned cell is a plant cell.
 
[22] The method of the above-mentioned [17], wherein the aforementioned cell is an insect cell.
 
[23] The method of the above-mentioned [17], wherein the aforementioned cell is an animal cell.
 
[24] The method of the above-mentioned [17], wherein the aforementioned cell is a vertebrate cell. 25 [25] The method of the above-mentioned [17], wherein the aforementioned cell is a mammalian cell.
 
[26] The method of any of the above-mentioned [18]-[25], wherein the aforementioned cell is a polyploid cell, and all of the targeted sites in alleles on a homologous chromosome are modified.
 
[27] A nucleic acid-modifying enzyme complex wherein a nucleic acid sequence-recognizing module that specifically binds to a target nucleotide sequence in a given double stranded DNA and DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure (unrelaxed DNA) are bonded, which converts one or more nucleotides in the targeted site to other one or more nucleotides or deletes one or more nucleotides, or inserts one or more nucleotides into said targeted site, without cleaving at least one strand of said double stranded DNA in the targeted site.
 
[28] A nucleic acid encoding the nucleic acid-modifying enzyme complex of the above-mentioned [27].
 
     Effect of the Invention 
     According to the genome editing technique of the present invention, since it does not accompany insertion of a foreign DNA or double stranded DNA breaks, the technique is superior in safety, and has no small possibility of affording a solution to cases causing biological or legal disputes on conventional methods as relating to gene recombination. It is also possible to set an appropriate range of mutation introduction up to the range of several hundred bases including the target sequence, and the technique can also be applied to topical evolution induction by introduction of random mutation into a particular restricted region, which has been almost impossible heretofore. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic showing of the mechanism of DNA sequence specific targeting of uracil-DNA glycosylase mutant. 
         FIG. 2  is a schematic showing of a mechanism of the genetic modification method of the present invention using DNA glycosylase and the CRISPR-Cas system. 
         FIG. 3  shows the results of verification, by using a budding yeast, of the effect of the genetic modification method of the present invention comprising a CRISPR-Cas system and mutant uracil-DNA glycosylase derived from budding yeast (having CDG activity and decreased reactivity with DNA having unrelaxed double helix structure) in combination. 
         FIG. 4  shows the results of verification, by using a budding yeast, of the effect of the genetic modification method of the present invention comprising a CRISPR-Cas system and budding yeast-derived mutant uracil-DNA glycosylase (having CDG activity or TDG activity and decreased reactivity with DNA having unrelaxed double helix structure) in combination. 
         FIG. 5  shows the analysis results of mutated sites in the canavanine-resistant colony obtained in  FIG. 4 . 
         FIG. 6  shows the results when an expression construct constructed such that budding yeast-derived mutant uracil-DNA glycosylase and dCas9 are bound via SH3 domain and a binding ligand thereof is introduced into a budding yeast together with two kinds of gRNA. 
         FIG. 7  shows that the efficiency of mutation induction by mutant UNG1 (“Ung” in Figure means UNG1 derived from yeast, hereinafter the same) is improved by coexpression of AP endonuclease (Ape1) mutants (E96Q, D210N). 
         FIG. 8  shows that cytotoxicity in a host yeast introduced with mutant UNG1 having CDG activity (N222D) or TDG activity (Y164A) can be avoided by introducing a mutation (L304A) that decreases reactivity with a DNA having an unrelaxed double helix structure. 
         FIG. 9  shows survival rate and efficiency of mutation induction in a yeast introduced with a heterogenous mutant uracil-DNA glycosylase derived from  Escherichia coli  or vaccinia virus (EcUDG or vvUDG). vvUDG did not cause cytotoxicity in a host yeast even without introduction of a mutation (R187C) that decreases reactivity with a DNA having an unrelaxed double helix structure. The efficiency of mutation induction by vvUDG was remarkably improved by coexpression of A20 protein. 
         FIG. 10  shows comparison of mutation introduction sites between mutant UNG1 derived from yeast and mutant UDG derived from vaccinia virus (vvUDG) in the target nucleotide sequences and in the vicinity thereof. 
         FIG. 11  shows that non-specific mutation by mutant UNG1 can be reduced by utilizing a split enzyme. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The present invention provides a method of modifying a nucleotide of a targeted site of a double stranded DNA by utilizing an error in base excision repair (BER) of cells, by excising a base from a nucleotide in a single strand region or in a region in which the double helix structure is relaxed in the target nucleotide sequence of the double stranded DNA and in the vicinity thereof, without cleaving at least one strand of the double stranded DNA to be modified. The method is characterized in that it contains a step of contacting a complex wherein the nucleic acid sequence-recognizing module that specifically binds to the target nucleotide sequence in the double stranded DNA, and DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure are bonded with the double stranded DNA to convert the targeted site, i.e., the target nucleotide sequence and nucleotides in the vicinity thereof, to other nucleotides or delete same, or insert one or more nucleotides into the targeted site. 
     In the present invention, the “modification” of a double stranded DNA means that a nucleotide (e.g., dC) on a DNA strand is converted to other nucleotide (e.g., dT, dA or dG), or deleted, or a nucleotide or a nucleotide sequence is inserted between certain nucleotides on a DNA strand. While the double stranded DNA to be modified is not particularly limited, it is preferably a genomic DNA. The “targeted site” of a double stranded DNA means the whole or partial “target nucleotide sequence”, which a nucleic acid sequence-recognizing module specifically recognizes and binds to, or the vicinity of the target nucleotide sequence (one or both of 5′ upstream and 3′ downstream), and the range thereof can be appropriately adjusted between 1 base and several hundred bases according to the object. 
     In the present invention, the “nucleic acid sequence-recognizing module” means a molecule or molecule complex having an ability to specifically recognize and bind to a particular nucleotide sequence (i.e., target nucleotide sequence) on a DNA strand. Binding of the nucleic acid sequence-recognizing module to a target nucleotide sequence enables a DNA glycosylase linked to the module to specifically act on a targeted site of a double stranded DNA. 
     In the present invention “DNA glycosylase” means an enzyme that hydrolyzes N-glycosidic bond of DNA. DNA glycosylase originally plays a role of eliminating injured bases from DNA in BER. In the present invention, DNA glycosylase capable of acting on normal bases (i.e., dC, dT, dA and dG, or those after epigenetic modification) in DNA is preferable. A mutant DNA glycosylase which natively does not react with a normal base or has low reactivity but which acquired reactivity or has improved reactivity with a normal base by mutation is encompassed in the DNA glycosylase in the present invention and can be preferably used. An abasic site (apurinic/apyrimidic (AP) site) resulting from the base excision reaction by the enzyme is treated with an enzyme at the downstream of the BER pathway, such as AP endonuclease, DNA polymerase, DNA ligase and the like. 
     With “sufficiently low reactivity with a DNA having an unrelaxed double helix structure” means that a base excision reaction in a region where a DNA having an unrelaxed double helix structure is formed is performed only at a frequency sufficient to suppress cytotoxicity to a level uninfluential on the survival of the cells. As used herein, a “DNA having an unrelaxed double helix structure” refers to having formed a firm double helix structure (namely, unrelaxed double-helical DNA (or simply unrelaxed DNA)), and does not include the state of single strand DNA from completely dissociated pairing bases, and the state of double strands in an unwound and relaxed double helix structure forming base pairs (relaxed double-stranded DNA). Examples of the DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure include DNA glycosylase with natively sufficiently low reactivity with a DNA having an unrelaxed double helix structure, mutant DNA glycosylase introduced with a mutation to decrease reactivity with a DNA having an unrelaxed double helix structure as compared to wild-type and the like. Furthermore, DNA glycosylase which is a split enzyme designed to be split into two fragments, wherein respective fragments are bonded to either one of the nucleic acid sequence-recognizing module split into two to form two complexes, when the both complexes are refolded, the nucleic acid sequence-recognizing module can be specifically bonded to the target nucleotide sequence, and, due to the specific bond, the DNA glycosylase can catalize the base excision reaction is also encompassed in the “DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure” in the present invention. 
     In the present invention, the “nucleic acid-modifying enzyme complex” means a molecule complex comprising a complex wherein the above-mentioned nucleic acid sequence-recognizing module and a DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure are linked, which molecule complex is imparted with a particular nucleotide sequence recognition ability, and an activity to catalyze a base excision reaction of nucleic acid. The “complex” here encompasses not only one constituted of multiple molecules, but also one having a nucleic acid sequence-recognizing module and DNA glycosylase in a single molecule, like a fusion protein. In addition, the “nucleic acid-modifying enzyme complex” in the present invention also encompasses a molecule complex in which nucleic acid sequence-recognizing module split into two fragments and either fragments of DNA glycosylase split into two fragments are linked to each other to form two “partial complexes”, and the molecule complex acquires a nucleotide sequence recognition ability and a base excision reaction catalyst activity when the partial complexes are refolded with each other. 
     While DNA glycosylase to be used in the present invention is not particularly limited as long as it can hydrolyze N-glycosidic bond of DNA to catalyze a reaction for releasing a base, DNA glycosylase capable of acting on normal bases (i.e., dC, dT, dA and dG, or those after epigenetic modification, for example, 5-methylcytosine etc.) in DNA is preferable to increase versatility as a genome editing technique. Examples of such enzyme include an enzyme having CDG activity to catalyze a reaction for releasing cytosine, an enzyme having TDG activity to catalyze a reaction for releasing thymine, an enzyme having activity to catalyze a reaction for releasing 5-methylcytosine (5-mCDG activity) and the like. While DNA glycosylase natively having CDG activity has not been reported to date, in almost all species, a mutant UDG having CDG activity or TDG activity that recognizes cytidine or thymidine as a substrate can be obtained by introducing a mutation into a particular amino acid residue of UNG known as the major UDG responsible for the removal of uracil incorporated in DNA (in the case of eucaryote, it has two splice variants of mitochondrial localization UNG1 and nuclear localization UNG2, which have a common amino acid sequence except N-terminal sequence containing each oraganelle localization signal) (see  FIG. 1 , upper panel). More specifically, for example, in the case of UNG1 derived from yeast (SEQ ID NO: 2; GenBank accession No. NP_013691), CDG activity can be imparted by substituting the 222nd asparagine from the N-terminal with aspartic acid (the mutant is also referred to as N222D), and TDG activity can be imparted by substituting the 164th tyrosine with alanine (the mutant is also referred to as Y164A) (Kavli B. et al., EMBO J. (1996) 15(13): 3442-7). The present inventors have newly found that TDG activity higher than that of Y164A mutant can be imparted by substituting the 164th tyrosine with glycine (the mutant is also referred to as Y164G). Furthermore, since cytosine is obtained by substituting the carbonyl group at the 4-position of uracil with an amino group and thymine is equivalent to uracil in which the 5-position is methylated, DNA glycosylase having an activity to release 5-methylcytosine, derived from methylation of the 5-position of cytosine, from DNA (5-mCDG activity) can be obtained by introducing double mutation into the 222nd asparagine residue and the 164th tyrosine residue (e.g., N222D/Y164A or N222D/Y164G). Since base excision of 5-methylcytosine can change the methylation state of the genomic DNA, the genome editing technique of the present invention also enables epigenome editing to change epigenetic modification. 
     When UNG2 is used, mutant UNG having CDG activity, TDG activity or 5-mCDG activity can be obtained by introducing similar mutation into the amino acid residue corresponding to the above-mentioned mutated site. 
     In the mutant UNG, the above-mentioned site may be substituted with an amino acid other than the above-mentioned amino acid, or the mutation may be introduced into a site other than the above-mentioned site, as long as it can act on a normal base. Whether the mutant UNG can act on a normal base can be confirmed by the method described in, for example, Kavli B. et al., EMBO J. (1996) 15(13): 3442-7. 
     While the derivation of UNG is not particularly limited, for example, ung derived  Escherichia coli  (Varshney, U. et al. (1988) J. Biol. Chem., 263, 7776-7784), UNG1 or UNG2 derived from yeast, mammal (e.g., human, mouse, swine, bovine, horse, monkey etc.) and the like, or UDG derived from virus (e.g., Poxviridae (vaccinia virus etc.), Herpesviridae and the like) can be used. For example, UniprotKB Nos. P13051-2 and P13051-1 can be referred to for the amino acid sequences of human UNG1 and UNG2, respectively. In addition, UniprotKB No. P12295 can be referred to for the amino acid sequence of  Escherichia coli  (K-12 strain) ung, and UniprotKB No. P20536 can be referred to for the amino acid sequence of vaccinia virus (Copenhagen Strain) UDG. The amino acid sequence of UNG is highly preserved among species, and the corresponding site for mutation can be identified by aligning the amino acid sequence of UNG1 or UNG2 derived from a desired organism with that of the above-mentioned yeast UNG1. For example, in the case of human UNG1, the amino acid corresponding to N222 of yeast UNG1 is the 204th asparagine (N204), and the amino acid corresponding to Y164 is the 147th tyrosine (Y147). In the case of  Escherichia coli  ung, the amino acid corresponding to N222 of yeast UNG1 is the 123rd asparagine (N123), and the amino acid corresponding to Y164 is the 66th tyrosine (Y66). 
     In the case of UDG derived from Poxviridae virus such as vaccinia virus, smallpoxvirus, monkeypoxvirus, fowlpox virus, swinepox virus, rabbit fibroma virus, the amino acid corresponding to N222 of yeast UNG1 is the 120th asparagine (N120), and the amino acid corresponding to Y164 is the 70th tyrosine (Y70). 
     While UNG can remove uracil from single stranded DNA and double stranded DNA, it has higher affinity for single stranded DNA. This tendency is also found in the above-mentioned mutant UNG conferred with CDG activity and TDG activity. However, since cytosine and thymine exist everywhere in the genomic DNA, mutant UNG having CDG activity or TDG activity acts, unlike wild-type UNG that exclusively uses, as a substrate, uracil which is rarely introduced into genomic DNA by an error on replication or deamination of cytosine, on any site in the nucleotide sequence targeted by a nucleic acid sequence-recognizing module and double stranded DNA in the vicinity thereof to remove cytosine or thymine, thus causing strong cytotoxicity. Therefore, DNA glycosylase to be used in the present invention is required to show sufficiently low reactivity with a DNA having an unrelaxed double helix structure. When mutant UNG is used as DNA glycosylase, the cytotoxicity by the enzyme can be avoided by further introducing a mutation that decreases reactivity with a DNA having an unrelaxed double helix structure to make a base excision reaction by mutant UNG having CDG activity or TDG activity more selective to the region of a relaxed double-stranded or single stranded DNA (see  FIG. 1 , middle panel). Specifically, for example, in the case of UNG, a mutation that decreases the reactivity with U-A 25 mer double stranded DNA to not more than 1/20, more preferably not more than 1/50, further preferably not more than 1/100, in vitro, that of the wild-type can be mentioned. However, as long as the reactivity with a DNA having an unrelaxed double helix structure is decreased to a level free from lethal cytotoxicity in vivo, it is not limited by the reactivity in vitro. Whether the DNA glycosylase to be used has “sufficiently low reactivity with a DNA having an unrelaxed double helix structure” can be confirmed, for example, by inserting the DNA glycosylase into the construct described in  FIG. 2 , introducing the construct together with the guide RNA into the object cell by the method described in Example 1, culturing the obtained transformant, and verifying the survivability thereof. Alternatively, a candidate of DNA glycosylase with “sufficiently low reactivity with a DNA having an unrelaxed double helix structure” can also be screened for by evaluating, as one of the criteria, the reactivity with a double stranded DNA oligomer in vitro by the method described in Chen, C. Y. et al. DNA Repair (Amst) (2005) 4(7): 793-805. 
     As a DNA glycosylase fulfilling the above-mentioned conditions in the case of, for example, UNG1 derived from yeast (SEQ ID NO: 2), a mutant in which the 304th leucine from the N-terminal is substituted with alanine can be mentioned (the mutant is also referred to as L304A) (Slupphaug, G. et al. Nature (1996) 384(6604): 87-92). Alternatively, a mutant in which the 308th arginine is substituted with glutamic acid or cysteine (the mutants are also referred to as R308E, R308C, respectively) also shows remarkably decreased reactivity with a DNA having an unrelaxed double helix structure (Chen, C. Y. et al. DNA Repair (Amst) (2005) 4(7): 793-805). When a different species derived from UNG is used, the corresponding site for mutation can be identified by aligning the amino acid sequence of UNG1 or UNG2 derived from a desired organism with that of the above-mentioned yeast UNG1. For example, in the case of human UNG1, the amino acid corresponding to L304 of yeast UNG1 is the 272nd leucine (L272), and the amino acid corresponding to R308 is the 276th arginine (R276). In the case of  Escherichia coli  ung, the amino acid corresponding to L304 of yeast UNG1 is the 191st leucine (L191), and the amino acid corresponding to R308 is the 195th arginine (R195). In the case of vaccinia virus UDG, the amino acid corresponding to R308 of yeast UNG1 is the 187th arginine (R187). 
     The substrate specificity of UNG1(ung) to yeast ( Saccharomyces cerevisiae ),  Escherichia coli , human and Poxviridae virus, and the site of mutation that changes the reactivity with a DNA having an unrelaxed double helix structure are shown in Table 1. Mutant amino acid is indicated with one letter, and the number shows the position of mutant amino acid when N-terminal amino acid is 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 
                   Escherichia 
                 
                   
                 pox 
                 change of mutant form and 
               
               
                 yeast 
                 
                   coli 
                 
                 human 
                 virus 
                 phenotype 
               
               
                   
               
             
            
               
                 Y164 
                 Y66 
                 Y147 
                 Y70 
                 recognizes thymine by 
               
               
                   
                   
                   
                   
                 mutation to A, G 
               
               
                 N222 
                 N123 
                 N204 
                 N120 
                 recognizes cytosine by 
               
               
                   
                   
                   
                   
                 mutation to D 
               
               
                 L304 
                 L191 
                 L272 
                 — 
                 reactivity with DNA having 
               
               
                   
                   
                   
                   
                 unrelaxed double helix 
               
               
                   
                   
                   
                   
                 structure decreases by 
               
               
                   
                   
                   
                   
                 mutation to A 
               
               
                 R308 
                 R195 
                 R276 
                 R187 a)   
                 reactivity with DNA having 
               
               
                   
                   
                   
                   
                 unrelaxed double helix 
               
               
                   
                   
                   
                   
                 structure decreases by 
               
               
                   
                   
                   
                   
                 mutation to E, C 
               
               
                   
               
               
                   a) In vaccinia virus UDG etc., wild-type itself is considered to have natively low reactivity with DNA having unrelaxed double helix structure. 
               
            
           
         
       
     
     From the above, preferable examples of the “DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure” to be used in the present invention include N222D/L304A double mutant, N222D/R308E double mutant, N222D/R308C double mutant, Y164A/L304A double mutant, Y164A/R308E double mutant, Y164A/R308C double mutant, Y164G/L304A double mutant, Y164G/R308E double mutant, Y164G/R308C double mutant, N222D/Y164A/L304A triple mutant, N222D/Y164A/R308E triple mutant, N222D/Y164A/R308C triple mutant, N222D/Y164G/L304A triple mutant, N222D/Y164G/R308E triple mutant, N222D/Y164G/R308C triple mutant and the like of yeast UNG1. When different UNG is used instead of yeast UNG1, a mutant having an amino acid corresponding to each of the above-mentioned mutants, which is introduced with a similar mutation, may be used. 
     Alternatively, as a “DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure”, DNA glycosylase with natively low reactivity with a DNA having an unrelaxed double helix structure, and high selectivity to relaxed double stranded or single stranded DNA can also be used. Examples of such DNA glycosylase include SMUG1 having UDG activity (Single strand-selective Monofunctional Uracil-DNA Glycosylase). While a mutation conferring CDG activity or TDG activity to SMUG1 is not known, it has been reported that the SMUG1 is important for removal of uracil resulting from deamination of cytosine (Nilsen, H. et al. EMBO J. (2001) 20: 4278-4286). The present inventors have already developed a method for specifically introducing a mutation into the targeted nucleotide sequence and the vicinity thereof, by combining cytidine deaminase capable of converting cytosine to uracil, and a nucleic acid sequence-recognizing module similar to that in the present invention (WO 2015/133554). By combining with the technique, cytosine can be artificially converted to uracil in the targeted site in genomic DNA, after which SMUG1 can be further reacted to release the uracil from DNA. While the derivation of SMUG1 is not particularly limited, for example, SMUG1 derived from  Escherichia coli , yeast, mammal (e.g., human, mouse, swine, bovine, horse, monkey etc.) and the like can be used. For example, UniprotKB No. Q53HC7-1 and Q53HV7-2 can be referred to for the two amino acid sequences of the two isoforms of human SMUG1. In addition, while the derivation of cytidine deaminase is not particularly limited, for example,  Petromyzon marinus -derived PmCDA1 ( Petromyzon marinus  cytosine deaminase 1), or AID (Activation-induced cytidine deaminase; AICDA) derived from mammal (e.g., human, swine, bovine, horse, monkey etc.) can be used. For example, GenBank accession No. EF094822 and ABO15149 can be referred to for the base sequence and the amino acid sequence of PmCDA1 cDNA, and GenBank accession No. NM_020661 and NP_065712 can be referred to for the base sequence and the amino acid sequence of human AID cDNA. 
     In another preferable embodiment, as the DNA glycosylase with natively low reactivity with a DNA having an unrelaxed double helix structure, UDG derived from virus belonging to Poxviridae such as vaccinia virus and the like can be mentioned. As shown in the below-mentioned Examples, vaccinia virus-derived UDG (vvUDG) is considered to be sufficiently low in the reactivity with a DNA having an unrelaxed double helix structure to a level free from toxicity in a host cell, because it shows growth equivalent to that in yeast UNG1 and  Escherichia coli  ung, introduced with a mutation that decreases reactivity with a DNA having an unrelaxed double helix structure (e.g., R187C), on a nonselective medium, even when such mutation is not introduced. When UDG derived from Poxviridae virus such as vvUDG and the like is used as a DNA glycosylase, a mutation (e.g., R187C) corresponding to the mutation that decreases reactivity with a DNA having an unrelaxed double helix structure in yeast UNG1 can be further introduced. However, when such mutation decreases the CDG activity (N120D) and TDG activity (Y70A, Y70G) of UDG, it is desirably avoided. From the above, preferable examples of UDG derived from Poxviridae virus such as vvUDG to be used in the present invention include N120D mutant, Y70G or Y70A mutant, N120D/Y70G double mutant or N120D/Y70A double mutant and the like. 
     When UDG derived from Poxviridae virus such as mutant vvUDG and the like is used as the DNA glycosylase, it is preferable to contact A20 protein, which interacts with UDG to form a heterodimer that functions as a processivity factor of viral DNA polymerase, with double stranded DNA together with UDG. Since combined use with A20 protein increases CDG activity or TDG activity of mutant UDG, for example, the efficiency of mutation induction into thymine, of mutant vvUDG having low TDG activity (Y70G) as compared to CDG activity (N120D) can be improved by the combined use with A20 protein. While the derivation of A20 is not particularly limited, for example, A20 derived from virus belonging to Poxviridae such as vaccinia virus, smallpoxvirus, monkeypoxvirus, fowlpox virus, swinepox virus, rabbit fibroma virus can be used. For example, UniprotKB No. P20995 can be referred for the amino acid sequence of vaccinia virus (Copenhagen Strain) A20. 
     In yet another preferable embodiment, a split enzyme designed such that nucleic acid sequence-recognizing module and DNA glycosylase are each split into two fragments, either fragments are linked to each other to form two partial complexes, these complexes are associated to reconstitute a functional nucleic acid sequence-recognizing module, and the module is bonded to the target nucleotide sequence to reconstitute a functional DNA glycosylase can be used as a DNA glycosylase having low reactivity with a DNA having an unrelaxed double helix structure. In the split enzyme, since the enzyme activity is exhibited only when it is bonded to the target nucleotide sequence, even when the DNA glycosylase itself to be reconstituted does not have a reduced reactivity with a DNA having an unrelaxed double helix structure, it consequently acts selectively on the single stranded DNA or relaxed double stranded DNA part in the target nucleotide sequence and the vicinity thereof. For example, nucleic acid sequence-recognizing module and DNA glycosylase are each split into N-terminal side fragments and C-terminal side fragments, for example, N-terminal side fragments are linked to each other to give a partial complex, C-terminal side fragments are linked to each other to give a partial complex (or N-terminal side fragments of nucleic acid sequence-recognizing module and C-terminal side fragments of DNA glycosylase are linked to give a partial complex, and N-terminal side fragments of DNA glycosylase and C-terminal side fragments of nucleic acid sequence-recognizing module are linked to give a partial complex), and they are associated, whereby functional nucleic acid sequence-recognizing module and functional DNA glycosylase can be reconstituted. The combination of the fragments to be linked is not particularly limited as long as a complex of the functional nucleic acid sequence-recognizing module and the functional DNA glycosylase is reconstituted when the two partial complexes are associated. The two partial complexes may be provided as separate molecules, or may be provided as a single fusion protein by linking them directly or via a suitable linker. The split site in DNA glycosylase is not particularly limited as long as two split fragments can be reconstituted as functional DNA glycosylase, and DNA glycosylase may be split at one site to provide N-terminal side fragment and C-terminal side fragment, or not less than 3 fragments obtained by splitting at two or more sites may be appropriately linked to give two fragments. The three-dimensional structures of various UDG proteins are known, and those of ordinary skill in the art can appropriately select the split sites based on such information. For example, yeast UNG1 (SEQ ID NO: 2) can be split between the 258th amino acid and 259th amino acid from the N-terminal to give N-terminal side fragment (1-258) and C-terminal side fragment (259-359). 
     As mentioned above, in the base excision repair (BER) mechanism, when a base is excised by DNA glycosylase, AP endonuclease puts a nick in the abasic site (AP site), and exonuclease completely excises the AP site. When the AP site is excised, DNA polymerase produces a new base by using the base of the opposing strand as a template, and DNA ligase finally seals the nick to complete the repair. Mutant AP endonuclease that has lost the enzyme activity but maintains the binding capacity to the AP site is known to competitively inhibit BER. Therefore, when the mutant AP endonuclease is contacted with double stranded DNA together with DNA glycosylase, the repair of the AP site by endogenous BER mechanism in the host cell is inhibited, and the frequency of repair errors, namely, efficiency of mutation induction, is improved. For example, the efficiency of mutation induction into thymine in mutant yeast UNG1 having lower TDG activity (Y164G) as compared to CDG activity (N222D) can be improved by using mutant AP endonuclease in combination. While the derivation of AP endonuclease is not particularly limited, for example, AP endonuclease derived from  Escherichia coli , yeast, mammal (e.g., human, mouse, swine, bovine, horse, monkey etc.) and the like can be used. For example, UniprotKB No. P27695 can be referred to for the amino acid sequence of human Ape1. Examples of the mutant AP endonuclease that has lost the enzyme activity but maintains the binding capacity to the AP site include proteins having mutated activity site and mutated Mg (cofactor)-binding site. For example, E96Q, Y171A, Y171F, Y171H, D210N, D210A, N212A and the like can be mentioned for human Ape1. 
     A target nucleotide sequence in a double stranded DNA to be recognized by the nucleic acid sequence-recognizing module in the nucleic acid-modifying enzyme complex of the present invention is not particularly limited as long as the module specifically binds to, and may be any sequence in the double stranded DNA. The length of the target nucleotide sequence only needs to be sufficient for specific binding of the nucleic acid sequence-recognizing module. For example, when mutation is introduced into a particular site in the genomic DNA of a mammal, it is not less than 12 nucleotides, preferably not less than 15 nucleotides, more preferably not less than 17 nucleotides, according to the genome size thereof. While the upper limit of the length is not particularly limited, it is preferably not more than 25 nucleotides, more preferably not more than 22 nucleotides. 
     As the nucleic acid sequence-recognizing module in the nucleic acid-modifying enzyme complex of the present invention, CRISPR-Cas system wherein at least one DNA cleavage ability of Cas is inactivated (CRISPR-mutant Cas), zinc finger motif, TAL effector and PPR motif and the like, as well as a fragment containing a DNA binding domain of a protein that specifically binds to DNA, such as restriction enzyme, transcription factor, RNA polymerase and the like, and free of a DNA double strand cleavage ability and the like can be used, but the module is not limited thereto. Preferably, CRISPR-mutant Cas, zinc finger motif, TAL effector, PPR motif and the like can be mentioned. 
     A zinc finger motif is constituted by linkage of 3-6 different Cys2His2 type zinc finger units (1 finger recognizes about 3 bases), and can recognize a target nucleotide sequence of 9-18 bases. A zinc finger motif can be produced by a known method such as Modular assembly method (Nat Biotechnol (2002) 20: 135-141), OPEN method (Mol Cell (2008) 31: 294-301), CoDA method (Nat Methods (2011) 8: 67-69),  Escherichia coli  one-hybrid method (Nat Biotechnol (2008) 26:695-701) and the like. The above-mentioned patent document 1 can be referred to as for the detail of the zinc finger motif production. 
     A TAL effector has a module repeat structure with about 34 amino acids as a unit, and the 12th and 13th amino acid residues (called RVD) of one module determine the binding stability and base specificity. Since each module is highly independent, TAL effector specific to a target nucleotide sequence can be produced by simply connecting the module. For TAL effector, a production method utilizing an open resource (REAL method (Curr Protoc Mol Biol (2012) Chapter 12: Unit 12.15), FLASH method (Nat Biotechnol (2012) 30: 460-465), and Golden Gate method (Nucleic Acids Res (2011) 39: e82) etc.) have been established, and a TAL effector for a target nucleotide sequence can be designed comparatively conveniently. The above-mentioned patent document 2 can be referred to as for the detail of the production of TAL effector. 
     PPR motif is constituted such that a particular nucleotide sequence is recognized by a continuation of PPR motifs each consisting of 35 amino acids and recognizing one nucleic acid base, and recognizes a target base only by 1, 4 and ii(-2) amino acids of each motif. Motif constitution has no dependency, and is free of interference of motifs on both sides. Therefore, like TAL effector, a PPR protein specific to the target nucleotide sequence can be produced by simply connecting PPR motifs. The above-mentioned patent document 4 can be referred to as for the detail of the production of PPR motif. 
     When a fragment of restriction enzyme, transcription factor, RNA polymerase and the like is used, since the DNA binding domains of these proteins are well known, a fragment containing the domain and free of a DNA double strand cleavage ability can be easily designed and constructed. 
     As mentioned above, DNA glycosylase used for the nucleic acid-modifying enzyme complex of the present invention is preferably mutant UDG conferred with CDG activity or TDG activity, more preferably UNG, and it needs to be sufficiently low in the reactivity with a DNA having an unrelaxed double helix structure so that CDG activity or TDG activity will not act on anywhere in the targeted site (see  FIG. 1 , middle panel) Therefore, the targeted site is desirably in the state of single stranded DNA, or relaxed DNA structure resulting from unwinding of at least firm double helix structure, which enables the DNA glycosylase to act efficiently when brought into contact with DNA glycosylase. When CRISPR-Cas system is used as the nucleic acid sequence-recognizing module, guide RNA complementary to the target nucleotide sequence recognizes the sequence of the object double stranded DNA and specifically forms a hybrid with the target nucleotide sequence, whereby the targeted site becomes the state of single strand or unwound double helix structure (relaxed double stranded) state. Therefore, DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure selectively acts on cytosine or thymine in the targeted site and can excise the base (see  FIG. 1 , lower panel). On the other hand, when zinc finger motif, TAL effector, PPR motif and the like are used as the nucleic acid sequence-recognizing module, since the module itself does not have a function to change the structure of double stranded DNA (cause distortion of double helix structure), it is desirable to contact the nucleic acid-modifying enzyme complex of the present invention in combination with a factor (e.g., gyrase, topoisomerase, helicase etc.), which changes the structure of the object double stranded DNA, with the double stranded DNA. 
     Any of the above-mentioned nucleic acid sequence-recognizing module can be provided as a fusion protein with the above-mentioned DNA glycosylase, or a protein binding domain such as SH3 domain, PDZ domain, GK domain, GB domain and the like and a binding partner thereof may be fused with a nucleic acid sequence-recognizing module and a DNA glycosylase, respectively, and provided as a protein complex via an interaction of the domain and a binding partner thereof. Alternatively, a nucleic acid sequence-recognizing module and a DNA glycosylase may be each fused with intein, and they can be linked by ligation after protein synthesis. 
     The nucleic acid-modifying enzyme complex of the present invention containing a complex (including fusion protein) wherein a nucleic acid sequence-recognizing module and DNA glycosylase are bonded is contacted with a double stranded DNA (e.g., genomic DNA) by introducing the complex or a nucleic acid encoding the complex into a cell having the object double stranded DNA. In consideration of the introduction and expression efficiency, it is desirable to introduce the nucleic acid-modifying enzyme complex into the cell in the form of a nucleic acid encoding the complex, rather than the complex itself, and allow for expression of the complex in the cell. Also when mutant AP endonuclease, A20 protein and the like are used in combination, it is desirable to introduce them into the cell in the form of a nucleic acid encoding them, and allow for expression of the complex in the cell. 
     Therefore, the nucleic acid sequence-recognizing module and the DNA glycosylase are preferably prepared as a nucleic acid encoding a fusion protein thereof, or in a form capable of forming a complex in a host cell after translation into a protein by utilizing a binding domain, intein and the like, or as a nucleic acid encoding each of them. The nucleic acid here may be a DNA or an RNA. When it is a DNA, it is preferably a double stranded DNA, and provided in the form of an expression vector disposed under regulation of a functional promoter in a host cell. When it is an RNA, it is preferably a single stranded RNA. 
     When nucleic acid sequence-recognizing module and DNA glycosylase are each split into two fragments, the fragments of either of them are respectively linked to the fragments of the other to provide two partial complexes, for example, a DNA encoding the N-terminal side fragment and a DNA encoding the C-terminal side fragment of the nucleic acid sequence-recognizing module are respectively prepared by the PCR method using suitable primers; and a DNA encoding the N-terminal side fragment and a DNA encoding the C-terminal side fragment of DNA glycosylase are prepared in the same manner and, for example, the DNAs encoding the N-terminal side fragments, and the DNAs encoding the C-terminal side fragments are linked to each other by a conventional method, whereby a DNA encoding the two partial complexes can be produced. Alternatively, a DNA encoding the N-terminal side fragment of the nucleic acid sequence-recognizing module and a DNA encoding the C-terminal side fragment of the DNA glycosylase are linked; and a DNA encoding the N-terminal side fragment of the DNA glycosylase and a DNA encoding the C-terminal side fragment of the nucleic acid sequence-recognizing module are linked, whereby a DNA encoding the two partial complexes can also be produced. The combination of the fragments to be linked is not particularly limited as long as a complex of the functional nucleic acid sequence-recognizing module and the functional DNA glycosylase is reconstituted when the two partial complexes are associated. The two partial complexes are not only expressed as separate molecules, but may also be expressed as a single fusion protein by linking nucleic acids encoding them directly or via a suitable linker, which protein forms a complex of the functional nucleic acid sequence-recognizing module and the functional DNA glycosylase by intramolecular association. 
     Since the complex of the present invention wherein a nucleic acid sequence-recognizing module and a DNA glycosylase are bonded does not accompany double stranded DNA breaks (DSB), genome editing with low toxicity is possible, and the genetic modification method of the present invention can be applied to a wide range of biological materials. Therefore, the cells to be introduced with nucleic acid encoding nucleic acid sequence-recognizing module and/or DNA glycosylase can encompass cells of any species, from bacterium of  Escherichia coli  and the like which are prokaryotes, cells of microorganism such as yeast and the like which are lower eucaryotes, to cells of vertebrata including mammals such as human and the like, and cells of higher eukaryote such as insect, plant and the like. 
     A DNA encoding a nucleic acid sequence-recognizing module such as zinc finger motif, TAL effector, PPR motif and the like can be obtained by any method mentioned above for each module. A DNA encoding a sequence-recognizing module of restriction enzyme, transcription factor, RNA polymerase and the like can be cloned by, for example, synthesizing an oligoDNA primer covering a region encoding a desired part of the protein (part containing DNA binding domain) based on the cDNA sequence information thereof, and amplifying by the RT-PCR method using, as a template, the total RNA or mRNA fraction prepared from the protein-producing cells. 
     A DNA encoding DNA glycosylase can also be cloned similarly by synthesizing an oligoDNA primer based on the cDNA sequence information thereof, and amplifying by the RT-PCR method using, as a template, the total RNA or mRNA fraction prepared from the enzyme-producing cells. For example, a DNA encoding UNG1 of yeast can be cloned by designing suitable primers for the upstream and downstream of CDS based on the cDNA sequence (accession No. NM_001182379) registered in the NCBI database, and cloning from yeast-derived mRNA by the RT-PCR method. 
     A nucleic acid encoding DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure can be obtained by a site specific mutagenesis method known per se by using the obtained cDNA as a template, and introducing a mutation imparting CDG activity, TDG activity or 5-mCDG activity, and a mutation that decreases reactivity with a DNA having an unrelaxed double helix structure. When DNA glycosylase with natively sufficiently low reactivity with a DNA having an unrelaxed double helix structure, such as vvUDG and the like, only a mutation imparting CDG activity, TDG activity or 5-mCDG activity can be introduced. 
     The cloned DNA may be directly, or after digestion with a restriction enzyme when desired, or after addition of a suitable linker (e.g., GS linker, GGGAR linker etc.), spacer (e.g., FLAG sequence etc.) and/or a nuclear localization signal (NLS) (each organelle localization signal when the object double stranded DNA is mitochondria or chloroplast DNA), ligated with a DNA encoding a nucleic acid sequence-recognizing module to prepare a DNA encoding a fusion protein. Since UNG1 and UNG2 each have a mitochondria localization signal and a nuclear localization signal on the N-terminal, they may also be utilized as they are. Alternatively, for example, when UNG1 is used for nucleotide modification targeting nuclear genomic DNA, it is possible to remove the mitochondria localization signal and separately link a nuclear localization signal. 
     Alternatively, a DNA encoding a nucleic acid sequence-recognizing module, and a DNA encoding a DNA glycosylase may be each fused with a DNA encoding a binding domain or a binding partner thereof, or both DNAs may be fused with a DNA encoding a separation intein, whereby the nucleic acid sequence-recognizing conversion module and the DNA glycosylase are translated in a host cell to form a complex. In these cases, a linker and/or a nuclear localization signal can be linked to a suitable position of one of or both DNAs when desired. 
     A DNA encoding a nucleic acid sequence-recognizing module and a DNA encoding a DNA glycosylase can be obtained by chemically synthesizing the DNA strand, or by connecting synthesized partly overlapping oligoDNA short strands by utilizing the PCR method and the Gibson Assembly method to construct a DNA encoding the full length thereof. The advantage of constructing a full-length DNA by chemical synthesis or a combination of PCR method or Gibson Assembly method is that the codon to be used can be designed in CDS full-length according to the host into which the DNA is introduced. In the expression of a heterologous DNA, the protein expression level is expected to increase by converting the DNA sequence thereof to a codon highly frequently used in the host organism. As the data of codon use frequency in host to be used, for example, the genetic code use frequency database (www.kazusa.or.jp/codon/index.html) disclosed in the home page of Kazusa DNA Research Institute can be used, or documents showing the codon use frequency in each host may be referred to. By reference to the obtained data and the DNA sequence to be introduced, codons showing low use frequency in the host from among those used for the DNA sequence may be converted to a codon coding the same amino acid and showing high use frequency. 
     An expression vector containing a DNA encoding a nucleic acid sequence-recognizing module and/or a DNA glycosylase can be produced, for example, by linking the DNA to the downstream of a promoter in a suitable expression vector. 
     As the expression vector,  Escherichia coli -derived plasmids (e.g., pBR322, pBR325, pUC12, pUC13);  Bacillus subtilis -derived plasmids (e.g., pUB110, pTP5, pC194); yeast-derived plasmids (e.g., pSH19, pSH15); insect cell expression plasmids (e.g., pFast-Bac); animal cell expression plasmids (e.g., pA1-11, pXT1, pRc/CMV, pRc/RSV, pcDNAI/Neo); bacteriophages such as Aphage and the like; insect virus vectors such as baculovirus and the like (e.g., BmNPV, AcNPV); animal virus vectors such as retrovirus, vaccinia virus, adenovirus and the like, and the like are used. 
     As the promoter, any promoter appropriate for a host to be used for gene expression can be used. In a conventional method using DSB, since the survival rate of the host cell sometimes decreases markedly due to the toxicity, it is desirable to increase the number of cells by the start of the induction by using an inductive promoter. However, since sufficient cell proliferation can also be afforded by expressing the nucleic acid-modifying enzyme complex of the present invention, a constitution promoter can also be used without limitation. 
     For example, when the host is an animal cell, SRα promoter, SV40 promoter, LTR promoter, CMV (cytomegalovirus) promoter, RSV (Rous sarcoma virus) promoter, MoMuLV (Moloney mouse leukemia virus) LTR, HSV-TK (simple herpes virus thymidine kinase) promoter and the like are used. Of these, CMV promoter, SRα promoter and the like are preferable. 
     When the host is  Escherichia coli , trp promoter, lac promoter, recA promoter, λP L  promoter, lpp promoter, T7 promoter and the like are preferable. 
     When the host is genus  Bacillus , SPO1 promoter, SPO2 promoter, penP promoter and the like are preferable. 
     When the host is a yeast, Gal1/10 promoter, PHO5 promoter, PGK promoter, GAP promoter, ADH promoter and the like are preferable. 
     When the host is an insect cell, polyhedrin promoter, P10 promoter and the like are preferable. 
     When the host is a plant cell, CaMV35S promoter, CaMV19S promoter, NOS promoter and the like are preferable. 
     As the expression vector, besides those mentioned above, one containing enhancer, splicing signal, terminator, polyA addition signal, a selection marker such as drug resistance gene, auxotrophic complementary gene and the like, replication origin and the like on demand can be used. 
     An RNA encoding a nucleic acid sequence-recognizing module and/or a DNA glycosylase can be prepared by, for example, transcription to mRNA in a vitro transcription system known per se by using a vector encoding DNA encoding the above-mentioned nucleic acid sequence-recognizing module and/or a DNA glycosylase as a template. 
     A complex of a nucleic acid sequence-recognizing module and a DNA glycosylase can be intracellularly expressed by introducing an expression vector containing a DNA encoding a nucleic acid sequence-recognizing module and/or a DNA glycosylase into a host cell, and culturing the host cell. 
     As the host, genus  Escherichia , genus  Bacillus , yeast, insect cell, insect, animal cell and the like are used. 
     As the genus  Escherichia, Escherichia coli  K12⋅DH1 [Proc. Natl. Acad. Sci. USA, 60, 160 (1968)],  Escherichia coli  JM103 [Nucleic Acids Research, 9, 309 (1981)],  Escherichia coli  JA221 [Journal of Molecular Biology, 120, 517 (1978)],  Escherichia coli  HB101 [Journal of Molecular Biology, 41, 459 (1969)],  Escherichia coli  C600 [Genetics, 39, 440 (1954)] and the like are used. 
     As the genus  Bacillus, Bacillus subtilis  MI114 [Gene, 24, 255 (1983)],  Bacillus subtilis  207-21 [Journal of Biochemistry, 95, 87 (1984)] and the like are used. 
     As the yeast,  Saccharomyces cerevisiae  AH22, AH22R − , NA87-11A, DKD-5D, 20B-12,  Schizosaccharomyces pombe  NCYC1913, NCYC2036,  Pichia pastoris  KM71 and the like are used. 
     As the insect cell when the virus is AcNPV, cells of cabbage armyworm larva-derived established line ( Spodoptera frugiperda  cell; Sf cell), MG1 cells derived from the mid-intestine of  Trichoplusia ni , High Five™ cells derived from an egg of  Trichoplusia ni, Mamestra brassicae -derived cells,  Estigmena acrea -derived cells and the like are used. When the virus is BmNPV, cells of  Bombyx mori -derived established line ( Bombyx mori  N cell; BmN cell) and the like are used as insect cells. As the Sf cell, for example, Sf9 cell (ATCC CRL1711), Sf21 cell [all above, In Vivo, 13, 213-217 (1977)] and the like are used. 
     As the insect, for example, larva of  Bombyx mori, Drosophila , cricket and the like are used [Nature, 315, 592 (1985)]. 
     As the animal cell, cell lines such as monkey COS-7 cell, monkey Vero cell, Chinese hamster ovary (CHO) cell, dhfr gene-deficient CHO cell, mouse L cell, mouse AtT-20 cell, mouse myeloma cell, rat GH3 cell, human FL cell and the like, pluripotent stem cells such as iPS cell, ES cell and the like of human and other mammals, and primary cultured cells prepared from various tissues are used. Furthermore, zebrafish embryo,  Xenopus  oocyte and the like can also be used. 
     As the plant cell, suspend cultured cells, callus, protoplast, leaf segment, root segment and the like prepared from various plants (e.g., grain such as rice, wheat, corn and the like, product crops such as tomato, cucumber, egg plant and the like, garden plants such as carnation,  Eustoma russellianum  and the like, experiment plants such as tobacco,  Arabidopsis thaliana  and the like, and the like) are used. 
     All the above-mentioned host cells may be haploid (monoploid), or polyploid (e.g., diploid, triploid, tetraploid and the like). In the conventional mutation introduction methods, mutation is, in principle, introduced into only one homologous chromosome to produce a hetero gene type. Therefore, desired phenotype is not expressed unless dominant mutation occurs, and homozygousness inconveniently requires labor and time. In contrast, according to the present invention, since mutation can be introduced into any allele on the homologous chromosome in the genome, desired phenotype can be expressed in a single generation even in the case of recessive mutation, which is extremely useful since the problem of the conventional method can be solved. 
     An expression vector can be introduced by a known method (e.g., lysozyme method, competent method, PEG method, CaCl 2  coprecipitation method, electroporation method, the microinjection method, the particle gun method, lipofection method,  Agrobacterium  method and the like) according to the kind of the host. 
       Escherichia coli  can be transformed according to the methods described in, for example, Proc. Natl. Acad. Sci. USA, 69, 2110 (1972), Gene, 17, 107 (1982) and the like. 
     The genus  Bacillus  can be introduced into a vector according to the methods described in, for example, Molecular &amp; General Genetics, 168, 111 (1979) and the like. 
     A yeast can be introduced into a vector according to the methods described in, for example, Methods in Enzymology, 194, 182-187 (1991), Proc. Natl. Acad. Sci. USA, 75, 1929 (1978) and the like. 
     An insect cell and an insect can be introduced into a vector according to the methods described in, for example, Bio/Technology, 6, 47-55 (1988) and the like. 
     An animal cell can be introduced into a vector according to the methods described in, for example, Cell Engineering additional volume 8, New Cell Engineering Experiment Protocol, 263-267 (1995) (published by Shujunsha), and Virology, 52, 456 (1973). 
     A cell introduced with a vector can be cultured according to a known method according to the kind of the host. 
     For example, when  Escherichia coli  or genus  Bacillus  is cultured, a liquid medium is preferable as a medium to be used for the culture. The medium preferably contains a carbon source, nitrogen source, inorganic substance and the like necessary for the growth of the transformant. Examples of the carbon source include glucose, dextrin, soluble starch, sucrose and the like; examples of the nitrogen source include inorganic or organic substances such as ammonium salts, nitrate salts, corn steep liquor, peptone, casein, meat extract, soybean cake, potato extract and the like; and examples of the inorganic substance include calcium chloride, sodium dihydrogen phosphate, magnesium chloride and the like. The medium may contain yeast extract, vitamins, growth promoting factor and the like. The pH of the medium is preferably about 5-about 8. 
     As a medium for culturing  Escherichia coli , for example, M9 medium containing glucose, casamino acid [Journal of Experiments in Molecular Genetics, 431-433, Cold Spring Harbor Laboratory, New York 1972] is preferable. Where necessary, for example, agents such as 3β-indolylacrylic acid may be added to the medium to ensure an efficient function of a promoter.  Escherichia coli  is cultured at generally about 15-about 43° C. Where necessary, aeration and stirring may be performed. 
     The genus  Bacillus  is cultured at generally about 30-about 40° C. Where necessary, aeration and stirring may be performed. 
     Examples of the medium for culturing yeast include Burkholder minimum medium [Proc. Natl. Acad. Sci. USA, 77, 4505 (1980)], SD medium containing 0.5% casamino acid [Proc. Natl. Acad. Sci. USA, 81, 5330 (1984)] and the like. The pH of the medium is preferably about 5-about 8. The culture is performed at generally about 20° C.-about 35° C. Where necessary, aeration and stirring may be performed. 
     As a medium for culturing an insect cell or insect, for example, Grace&#39;s Insect Medium [Nature, 195, 788 (1962)] containing an additive such as inactivated 10% bovine serum and the like as appropriate and the like are used. The pH of the medium is preferably about 6.2-about 6.4. The culture is performed at generally about 27° C. Where necessary, aeration and stirring may be performed. 
     As a medium for culturing an animal cell, for example, minimum essential medium (MEM) containing about 5-about 20% of fetal bovine serum [Science, 122, 501 (1952)], Dulbecco&#39;s modified Eagle medium (DMEM) [Virology, 8, 396 (1959)], RPMI 1640 medium [The Journal of the American Medical Association, 199, 519 (1967)], 199 medium [Proceeding of the Society for the Biological Medicine, 73, 1 (1950)] and the like are used. The pH of the medium is preferably about 6-about 8. The culture is performed at generally about 30° C.-about 40° C. Where necessary, aeration and stirring may be performed. 
     As a medium for culturing a plant cell, for example, MS medium, LS medium, B5 medium and the like are used. The pH of the medium is preferably about 5-about 8. The culture is performed at generally about 20° C.-about 30° C. Where necessary, aeration and stirring may be performed. 
     As mentioned above, a complex of a nucleic acid sequence-recognizing module and a DNA glycosylase, i.e., nucleic acid-modifying enzyme complex, can be expressed intracellularly. 
     An RNA encoding a nucleic acid sequence-recognizing module and/or a DNA glycosylase can be introduced into a host cell by microinjection method, lipofection method and the like. RNA introduction can be performed once or repeated multiple times (e.g., 2-5 times) at suitable intervals. 
     When a complex of a nucleic acid sequence-recognizing module and a DNA glycosylase is expressed by an expression vector or RNA molecule introduced into the cell, the nucleic acid sequence-recognizing module specifically recognizes and binds to a target nucleotide sequence in the double stranded DNA (e.g., genomic DNA) of interest and, due to the action of the DNA glycosylase linked to the nucleic acid sequence-recognizing module, base excision reaction occurs in the sense strand or antisense strand of the targeted site (whole or partial target nucleotide sequence or appropriately adjusted within several hundred bases including the vicinity thereof) and an abasic site (AP site) is produced in one of the strands of the double stranded DNA. Then, the base excision repair (BER) system in the cell operates, AP endonuclease first recognizes the AP site and cleaves the phosphoric acid bond in one of DNA strand, and exonuclease removes nucleotide subjected to base excision. Then, DNA polymerase inserts a new nucleotide by using the opposing strand DNA as a template and finally DNA ligase repairs the joint. Various mutations are introduced by a repair miss occurring at any stage of this BER. As mentioned above, the BER mechanism in the cell is inhibited, and the frequency of repair miss, and thus, efficiency of mutation induction can be improved by using a mutant AP endonuclease which lost enzyme activity but retains binding capacity to AP site in combination. 
     As for zinc finger motif, production of many actually functionable zinc finger motifs is not easy, since production efficiency of a zinc finger that specifically binds to a target nucleotide sequence is not high and selection of a zinc finger having high binding specificity is complicated. While TAL effector and PPR motif have a high degree of freedom of target nucleic acid sequence recognition as compared to zinc finger motif, a problem remains in the efficiency since a large protein needs to be designed and constructed every time according to the target nucleotide sequence. Furthermore, since these nucleic acid sequence-recognizing modules do not have a function to change the structure of double stranded DNA (causing strain in the double helix structure), for a DNA glycosylase with sufficiently low reactivity with a DNA having an unrelaxed double helix structure to efficiently act on the targeted site, it is necessary to separately contact a factor that changes the structure of the double stranded DNA with the object double stranded DNA, thus making the operation complicated. 
     In contrast, since the CRISPR-Cas system recognizes the object double stranded DNA sequence by a guide RNA complementary to the target nucleotide sequence, any sequence can be targeted by simply synthesizing an oligoDNA capable of specifically forming a hybrid with the target nucleotide sequence. Moreover, at the targeted site, since the double stranded DNA is unwound to generate a region having a single stranded structure, and a region adjacent thereto which has a structure of relaxed double stranded DNA, DNA glycosylase can be made to act efficiently in a targeted site-specific manner, without combining factors that change the structure of double stranded DNA. 
     Therefore, in a more preferable embodiment of the present invention, a CRISPR-Cas system wherein at least one DNA cleavage ability of Cas is inactivated (CRISPR-mutant Cas), is used as a nucleic acid sequence-recognizing module. 
       FIG. 2  is a schematic showing of the double stranded DNA modification method of the present invention using CRISPR-mutant Cas as a nucleic acid sequence-recognizing module. 
     The nucleic acid sequence-recognizing module of the present invention using CRISPR-mutant Cas is provided as a complex of an RNA molecule consisting of a guide RNA complementary to the target nucleotide sequence and tracrRNA necessary for recruiting mutant Cas protein, and a mutant Cas protein. 
     The Cas protein to be used in the present invention is not particularly limited as long as it belongs to the CRISPR system, and preferred is Cas9. Examples of Cas9 include, but are not limited to,  Streptococcus pyogenes -derived Cas9 (SpCas9),  Streptococcus thermophilus -derived Cas9 (StCas9) and the like. Preferred is SpCas9. AS a mutant Cas to be used in the present invention, any of Cas wherein the cleavage ability of the both strands of the double stranded DNA is inactivated and one having nickase activity wherein at least one cleavage ability of one strand alone is inactivated can be used. For example, in the case of SpCas9, a D10A mutant in which the 10th Asp residue is converted to an Ala residue and lacking cleavage ability of a strand opposite to the strand forming a complementary strand with a guide RNA, or H840A mutant in which the 840th His residue is converted to an Ala residue and lacking cleavage ability of strand complementary to guide RNA, or a double mutant thereof can be used, and other mutant Cas can be used similarly. 
     DNA glycosylase is provided as a complex with mutant Cas by a method similar to the coupling scheme with the above-mentioned zinc finger and the like. Alternatively, a DNA glycosylase and mutant Cas can also be bound by utilizing RNA aptamers MS2F6, PP7 and the like and RNA scaffold by binding proteins thereto. Guide RNA forms a complementary strand with the target nucleotide sequence, mutant Cas is recruited by the tracrRNA attached and mutant Cas recognizes DNA cleavage site recognition sequence PAM (protospacer adjacent motif) (when SpCas9 is used, PAM is 3 bases of NGG (N is any base), and, theoretically, can target any position on the genome). One or both DNAs cannot be cleaved, and, due to the action of the DNA glycosylase linked to the mutant Cas, base excision occurs in the targeted site (appropriately adjusted within several hundred bases including whole or partial target nucleotide sequence) and a AP site occurs in the double stranded DNA. Various mutations are introduced due to the errors made by the BER system of the cell to be repaired (see, for example,  FIG. 5 ). 
     Even when CRISPR-mutant Cas is used as a nucleic acid sequence-recognizing module, a nucleic acid sequence-recognizing module and a DNA glycosylase are desirably introduced, in the form of a nucleic acid encoding same, into a cell having a double stranded DNA of interest, similar to when zinc finger and the like are used as a nucleic acid sequence-recognizing module. 
     A DNA encoding Cas can be cloned by a method similar to the above-mentioned method for a DNA encoding a DNA glycosylase, from a cell producing the enzyme. A mutant Cas can be obtained by introducing a mutation to convert an amino acid residue of the part important for the DNA cleavage activity (e.g., 10th Asp residue and 840th His residue for Cas9, though not limited thereto) to other amino acid, into a DNA encoding cloned Cas, by a site specific mutation induction method known per se. 
     Alternatively, a DNA encoding mutant Cas can also be constructed as a DNA showing codon usage suitable for expression in a host cell to be used, by a method similar to those mentioned above for a DNA encoding a nucleic acid sequence-recognizing module and a DNA encoding a DNA glycosylase, and by a combination of chemical synthesis or PCR method or Gibson Assembly method. For example, CDS sequence and amino acid sequence optimized for the expression of SpCas9 in eukaryotic cells are shown in SEQ ID NOs: 3 and 4. In the sequence shown in SEQ ID NO: 3, when “A” is converted to “C” in base No. 29, a DNA encoding a D10A mutant can be obtained, and when “CA” is converted to “GC” in base No. 2518-2519, a DNA encoding an H840A mutant can be obtained. 
     A DNA encoding a mutant Cas and a DNA encoding a DNA glycosylase may be linked to allow for expression as a fusion protein, or designed to be separately expressed using a binding domain, intein and the like, and form a complex in a host cell via protein-protein interaction and protein ligation. Alternatively, a design may be employed in which a DNA encoding mutant Cas and a DNA encoding DNA glycosylase are each split into two fragments at suitable split site, either fragments are linked to each other directly or via a suitable linker to express a nucleic acid-modifying enzyme complex as two partial complexes, which are associated and refolded in the cell to reconstitute functional mutant Cas having a particular nucleic acid sequence recognition ability, and a functional DNA glycosylase having a base excision reaction catalyst activity is reconstituted when the mutant Cas is bonded to the target nucleotide sequence. For example, a DNA encoding the N-terminal side fragment and a DNA encoding the C-terminal side fragment of mutant Cas are respectively prepared by the PCR method by using suitable primers; a DNA encoding the N-terminal side fragment and a DNA encoding the C-terminal side fragment of DNA glycosylase are prepared in the same manner; for example, the DNAs encoding the N-terminal side fragments are linked to each other, and the DNAs encoding the C-terminal side fragments are linked to each other by a conventional method, whereby a DNA encoding two partial complexes can be produced. Alternatively, a DNA encoding the N-terminal side fragment of mutant Cas and a DNA encoding the C-terminal side fragment of DNA glycosylase are linked; and a DNA encoding the N-terminal side fragment of DNA glycosylase and a DNA encoding the C-terminal side fragment of mutant Cas are linked, whereby a DNA encoding two partial complexes can also be produced. Respective partial complexes may be linked to allow for expression as a fusion protein, or designed to be separately expressed using a binding domain, intein and the like, and form a complex in a host cell via protein-protein interaction and protein ligation. Two partial complexes may be linked to be expressed as a fusion protein. The split site of the mutant Cas is not particularly limited as long as the two split fragments can be reconstituted such that they recognize and bind to the target nucleotide sequence, and it may be split at one site to provide N-terminal side fragment and C-terminal side fragment, or not less than 3 fragments obtained by splitting at two or more sites may be appropriately linked to give two fragments. The three-dimensional structures of various Cas proteins are known, and those of ordinary skill in the art can appropriately select the split site based on such information. For example, since the region consisting of the 94th to the 718th amino acids from the N terminus of SpCas9 is a domain (REC) involved in the recognition of the target nucleotide sequence and guide RNA, and the region consisting of the 1099th amino acid to the C-terminal amino acid is the domain (PI) involved in the interaction with PAM, the N-terminal side fragment and the C-terminal side fragment can be split at any site in REC domain or PI domain, preferably in a region free of a structure (e.g., between 204th and 205th amino step from the N-terminal (204 . . . 205), between 535th and 536th amino acids from the N-terminal (535 . . . 536) and the like) (see, for example, Nat Biotechnol. 33(2): 139-142 (2015)). 
     The obtained DNA encoding a mutant Cas and/or a DNA glycosylase can be inserted into the downstream of a promoter of an expression vector similar to the one mentioned above, according to the host. 
     On the other hand, a DNA encoding guide RNA and tracrRNA can be obtained by designing an oligoDNA sequence linking guide RNA sequence complementary to the target nucleotide sequence and known tracrRNA sequence (e.g., gttttagagctagaaatagcaagttaaaataaggctagtccgttatcaacttgaaaaagtggc accgagtcggtggtgctttt; SEQ ID NO: 5) and chemically synthesizing using a DNA/RNA synthesizer. While a DNA encoding guide RNA and tracrRNA can also be inserted into an expression vector similar to the one mentioned above, according to the host. As the promoter, pol III system promoter (e.g., SNR6, SNR52, SCR1, RPR1, U6, H1 promoter etc.) and terminator (e.g., T 6  sequence) are preferably used. 
     An RNA encoding mutant Cas and/or a DNA glycosylase can be prepared by, for example, transcription to mRNA in a vitro transcription system known per se by using a vector encoding the above-mentioned mutant Cas and/or DNA encoding a DNA glycosylase as a template. 
     Guide RNA-tracrRNA can be obtained by designing an oligoDNA sequence linking a sequence complementary to the target nucleotide sequence and known tracrRNA sequence and chemically synthesizing using a DNA/RNA synthesizer. 
     A DNA or RNA encoding mutant Cas and/or a DNA glycosylase, guide RNA-tracrRNA or a DNA encoding same can be introduced into a host cell by a method similar to the above, according to the host. 
     Since conventional artificial nuclease accompanies double stranded DNA breaks (DSB), inhibition of growth and cell death assumedly caused by disordered cleavage of chromosome (off-target cleavage) occur by targeting a sequence in the genome. The effect thereof is particularly fatal for many microorganisms and prokaryotes, and prevents applicability. In the present invention, mutation is introduced not by DNA cleavage but by a base excision reaction on the DNA base, and therefore, drastic reduction of toxicity can be realized. 
     The modification of the double stranded DNA in the present invention does not prevent occurrence of cleavage of the double stranded DNA in a site other than the targeted site (appropriately adjusted within several hundred bases including whole or partial target nucleotide sequence). However, one of the greatest advantages of the present invention is avoidance of toxicity by off-target cleavage, which is generally applicable to any species. In preferable one embodiment, therefore, the modification of the double stranded DNA in the present invention does not accompany cleavage of DNA strand not only in a targeted site of a given double stranded DNA but in a site other than same. 
     As shown in the below-mentioned Examples, when sequence-recognizing modules are produced corresponding to the adjacent multiple target nucleotide sequences, and simultaneously used, the mutation introduction can be efficiency increased than by using a single nucleotide sequence as a target. As the effect thereof, similarly mutation induction is realized even when both target nucleotide sequences partly overlap or when the both are apart by about 600 bp. It can occur both when the target nucleotide sequences are in the same direction (target nucleotide sequences are present on the same strand), and when they are opposed (target nucleotide sequence is present on each strand of double stranded DNA). 
     The genome sequence modification method of the present invention can introduce mutation into almost all cells in which the nucleic acid-modifying enzyme complex of the present invention has been expressed, by selecting a suitable target nucleotide sequence. Thus, insertion and selection of a selection marker gene, which are essential in the conventional genome editing, are not necessary. This dramatically facilitates and simplifies gene manipulation and enlarges the applicability to crop breeding and the like since a recombinant organism with foreign DNA is not produced. 
     Since the genome sequence modification method of the present invention shows extremely high efficiency of mutation induction, and does not require selection by markers, modification of multiple DNA regions at completely different positions as targets can be performed. Therefore, in one preferable embodiment of the present invention, two or more kinds of nucleic acid sequence-recognizing modules that specifically bind to different target nucleotide sequences (which may be present in one object gene, or two or more different object genes, which object genes may be present on the same chromosome or different chromosomes) can be used. In this case, each one of these nucleic acid sequence-recognizing modules and DNA glycosylase form a nucleic acid-modifying enzyme complex. Here, a common DNA glycosylase can be used. For example, when CRISPR-Cas system is used as a nucleic acid sequence-recognizing module, a common complex of a Cas protein and a DNA glycosylase (including fusion protein) is used, and two or more kinds of chimeric RNAs of tracrRNA and each of two or more guide RNAs that respectively form a complementary strand with a different target nucleotide sequence are produced and used as guide RNA-tracrRNAs. On the other hand, when zinc finger motif, TAL effector and the like are used as nucleic acid sequence-recognizing modules, for example, a DNA glycosylase can be fused with a nucleic acid sequence-recognizing module that specifically binds to a different target nucleotide. 
     To express the nucleic acid-modifying enzyme complex of the present invention in a host cell, as mentioned above, an expression vector containing a DNA encoding the nucleic acid-modifying enzyme complex, or an RNA encoding the nucleic acid-modifying enzyme complex is introduced into a host cell. For efficient introduction of mutation, it is desirable to maintain an expression of nucleic acid-modifying enzyme complex of a given level or above for not less than a given period. From such aspect, it is ensuring to introduce an expression vector (plasmid etc.) autonomously replicatable in a host cell. However, since the plasmid etc. are foreign DNAs, they are preferably removed rapidly after successful introduction of mutation. When mutant AP endonuclease is used in combination, since the mutant enzyme inhibits the BER mechanism in the host cell, it may induce undesirable spontaneous mutations outside the target region. Thus, it is preferable to also remove a plasmid containing a DNA encoding the mutant enzyme promptly after introduction of the desired mutation. Therefore, though subject to change depending on the kind of host cell and the like, for example, the introduced plasmid is desirably removed from the host cell after a lapse of for 6 hr-2 days from the introduction of an expression vector by using various plasmid removal methods well known in the art. 
     Alternatively, as long as expression of a nucleic acid-modifying enzyme complex, which is sufficient for the introduction of mutation, is obtained, it is preferable to introduce mutation into the object double stranded DNA by transient expression by using an expression vector without autonomous replicatability in a host cell (e.g., vector etc. lacking replication origin that functions in host cell and/or gene encoding protein necessary for replication) or RNA. 
     The present invention is explained in the following by referring to Examples, which are not to be construed as limitative. 
     EXAMPLES 
     In the below-mentioned Examples, experiments were performed as follows. 
     &lt;Cell Line, Culture, Transformation, and Expression Induction&gt; 
     Budding yeast  Saccharomyces cerevisiae  BY4741 strain (requiring leucine and uracil) was cultured in a standard YPDA medium or SD medium with a Dropout composition meeting the auxotrophicity. The culture performed was stand culture in an agar plate or shaking culture in a liquid medium between 25° C. and 30° C. Transformation was performed by an acetic acid lithium method, and selection was made in SD medium meeting appropriate auxotrophicity. For expression induction by galactose, after preculture overnight in an appropriate SD medium, culture in SR medium with carbon source changed from 2% glucose to 2% raffinose overnight, and further culture in SGal medium with carbon source changed to 0.2% galactose for 3 hr to about two nights were conducted for expression induction. 
     For the measurement of the number of surviving cells and Can1 mutation rate, a cell suspension was appropriately diluted, applied on SD plate medium or SD-Arg+60 mg/l Canavanine plate medium or SD+300 mg/l Canavanine plate medium, applied, and the number of colonies that emerge 3 days later was counted as the number of surviving cells. Using number of surviving colonies in SD plate as the total number of cells, and the number of surviving colonies in Canavanine plate as resistant mutant strain number, the mutation rate was calculated and evaluated. The site of mutation introduction was identified by amplifying DNA fragments containing the target gene region of each strain by a colony PCR method, performing DNA sequencing, and performing an alignment analysis based on the sequence of  Saccharomyces  Genome Database (www.yeastgenome.org/). 
     &lt;Nucleic Acid Operation&gt; 
     DNA was processed or constructed by any of PCR method, restriction enzyme treatment, ligation, Gibson Assembly method, and artificial chemical synthesis. For plasmid, as a yeast  Escherichia coli  shuttle vector, pRS315 for leucine selection and pRS426 for uracil selection were used as the backbone. Plasmid was amplified by  Escherichia coli  line XL-10 gold or DH5α, and introduced into yeast by the acetic acid lithium method. 
     &lt;Construct&gt; 
     For inducible expression, budding yeast pGal1/10 (SEQ ID NO: 6) which is a bidirectional promoter induced by galactose was used. At the downstream of the promoter, a nuclear localization signal (ccc aag aag aag agg aag gtg; SEQ ID NO: 7 (PKKKRV; encoding SEQ ID NO: 8)) was added to  Streptococcus pyogenes -derived Cas9 gene ORF having a codon optimized for eucaryon expression (SEQ ID NO: 3) and the sequence of ORF (ORF of wild-type gene is shown in SEQ ID NO: 1, Y164A mutation is substitution of base number 490-491 ta with gc (ta490gc); Y164G mutation is substitution of base number 490-491 ta with gg (ta490gg); N222D mutation is substitution of base number 664 a with g (a664g); L304A mutation is substitution of base number 910-911 tt with gc (tt910gc); R308E mutation is substitution of base number 922-923 ag with ga (ag922ga); R308C mutation is substitution of base number 922-924 aga with tgt (aga922tgt)) of wild-type or various mutant uracil-DNA glycosylase genes (UNG1 derived from yeast  Saccharomyces cerevisiae ), excluding a region (base number 1-60) encoding the mitochondria localization signal, was ligated via a linker sequence and expressed as a fusion protein. For comparison, UNG1 gene instead of deaminase gene (PmCDA1 derived from  Petromyzon marinus Petromyzon marinus ) was ligated and expressed as a fusion protein. As a linker sequence, 2×GS linker (two repeats of ggt gga gga ggt tct; SEQ ID NO: 9 (GGGGS; encoding SEQ ID NO: 10)) was used. As a terminator, budding yeast-derived ADH1 terminator (SEQ ID NO: 11) and Top2 terminator (SEQ ID NO: 12) were ligated. In the domain integration method, Cas9 gene ORF was ligated to SH3 domain (SEQ ID NOs: 13 and 14) via 2×GS linker to give one protein, mutant yeast UNG1 added with SH3 ligand sequence (SEQ ID NOs: 15 and 16) as another protein and they were ligated to Gal1/10 promoter on both directions and simultaneously expressed. These were incorporated into pRS315 plasmid. 
     In Cas9, mutation to convert the 10th aspartic acid to alanine (D10A, corresponding DNA sequence mutation a29c) and mutation to convert the 840th histidine to alanine (H840A, corresponding DNA sequence mutation ca2518gc) were introduced to remove cleavage ability of each side of DNA strand. 
     gRNA as a chimeric structure with tracrRNA (derived from  Streptococcus pyogenes ; SEQ ID NO: 5) was disposed between SNR52 promoter (SEQ ID NO: 17) and Sup4 terminator (SEQ ID NO: 18), and incorporated into pRS426 plasmid. As gRNA target base sequence, 793-812 (aacccaggtgcctggggtcc; SEQ ID NO: 19) and 767-786 complementary strand sequence (ataacggaatccaactgggc; SEQ ID NO: 20) of CAN1 gene ORF were used. For simultaneous expression of multiple targets, a sequence from a promoter to a terminator as one set and a plurality thereof were incorporated into the same plasmid. They were introduced into cells along with Cas9-UNG1 expression plasmid, intracellularly expressed, and a complex of gRNA-tracrRNA and Cas9-UNG1 was formed. 
     Example 1: Modification of Genome Sequence by Linking DNA Sequence Recognition Ability of CRISPR-Cas to Mutant Uracil-DNA Glycosylase (1) 
     To test the effect of genome sequence modification technique of the present invention by utilizing mutant uracil—DNA glycosylase and CRISPR-Cas nucleic acid sequence recognition ability, introduction of mutation into CAN1 gene encoding canavanine transporter that acquire canavanine-resistance due to gene deficiency was tried. As gRNA, a sequence complementary to 793-812 of CAN1 gene ORF and a sequence complementary to 767-786 complementary strand sequence were used, a chimeric RNA expression vector obtained by linking thereto  Streptococcus pyogenes -derived tracrRNA, and a vector expressing a protein obtained by fusing dCas9 with impaired nuclease activity by introducing mutations (D10A and H840A) into  Streptococcus pyogenes -derived Cas9 (SpCas9), and wild-type yeast-derived UNG1 or yeast-derived UNG1 introduced with various mutations (N222D single mutation and double mutation of N222D and L304A, R308E or R308C mutation) were constructed, introduced into the budding yeast by the acetic acid lithium method, and coexpressed. The results are shown in  FIG. 3 . When cultured on a canavanine-containing SD plate, only the cells subjected to introduction and expression of gRNA-tracrRNA and dCas9-mutant UNG1 (double mutant of N222D mutation imparting CDG activity and L304A, R308E or R308C mutation that decreases reactivity with DNA having an unrelaxed double helix structure) formed canavanine-resistant colonies. With N222D single mutation, the cytotoxicity was strong, and cell culture and evaluation were difficult, and therefore, the results are not shown. From the above, it was shown that target specific mutation introduction becomes possible by decreasing the reactivity of DNA glycosylase with a DNA having an unrelaxed double helix structure. 
     Example 2: Modification of Genome Sequence by Linking DNA Sequence Recognition Ability of CRISPR-Cas to Mutant Uracil-DNA Glycosylase (2) 
     Using yeast UNG1 introduced with double mutation of R308C mutation that decreases reactivity with a DNA having an unrelaxed double helix structure, and N222D mutation imparting CDG activity, or Y164A or Y164G mutation imparting TDG activity, and by a method similar to that in Example 1, introduction of mutation into CAN1 gene was tried. The results are shown in  FIG. 4 . It was shown that R308C N222D can achieve efficiency of mutation induction comparable to that of deaminase PmCDA1, and mutant strain can be obtained even without selection. It was shown that thymine base could be edited, since canavanine-resistant colony was also obtained in R308C Y164A. Y164G mutation improved the efficiency of mutation induction. 
     Then, each canavanine-resistant clone was subjected to the sequence analysis of the Can1 gene region. The results are shown in  FIG. 5 . Mutations were somewhat randomly centered around two adjacent target sites (767-786, 793-812). This is different from the pinpoint introduction of mutation by deaminase (WO 2015-133554), and suggests that the genome editing technique of the present invention is suitable for random introduction of mutation into the target nucleotide sequence and in the vicinity thereof. As assumed, point mutation from C or G was mainly found in N222D, and point mutation from T or A was mainly found in Y164A and Y164G. 
     Example 3: Use of Different Coupling Scheme 
     Whether mutation can be introduced into a targeted gene even when Cas9 and DNA glycosylase are not used as a fusion protein but when a nucleic acid-modifying enzyme complex is formed via a binding domain and a ligand thereof was examined. As Cas9, dCas9 used in Example 1 was used, yeast UNG1 mutant (double mutant of N222D or Y164A mutation, and R308E or R308C mutant) was used as DNA glycosylase, SH3 domain was fused with the former, and a binding ligand thereof was fused with the latter to produce various constructs shown in  FIG. 6 . In the same manner as in Example 1, sequences in the CAN1 gene were used as gRNA targets, and these constructs were introduced into a budding yeast. As a result, even when dCas9 and DNA glycosylase were bound via the binding domain, mutation was efficiently introduced into the targeted site of the CAN1 gene ( FIG. 6 ). 
     Example 4 Improvement of Efficiency of Mutation Induction by Coexpression of Mutant AP Endonuclease 
     Using yeast UNG1 introduced with double mutation of R308C mutation that decreases reactivity with a DNA having an unrelaxed double helix structure, and N222D mutation imparting CDG activity or Y164G mutation imparting TDG activity, and mutant human APE1 (E96Q, D210N) which lost enzyme activity but retained binding capacity to AP site, and by a method similar to that in Example 1, introduction of mutation into CAN1 gene was tried. The results are shown in  FIG. 7 . When mutant APE1 was coexpressed, the number of canavanine-resistant colonies increased even in Y164G, R308C that showed low efficiency when used alone, and efficiency of mutation introduction targeting thymine was remarkably improved. 
     Example 5 Reduction of Cytotoxicity by Introduction of Mutation that Decreases Reactivity with DNA Having an Unrelaxed Double Helix Structure 
     An influence of the presence or absence of mutation (L304A) that decreases reactivity with a DNA having an unrelaxed double helix structure in UNG1 on the survival rate of the host yeast was examined. The results are shown in  FIG. 8 . The host yeast introduced with mutant UNG1 having only the mutation imparting CDG activity (N222D) or TDG activity (Y164A) showed a marked decrease in the survival rate as compared to the yeast introduced with wild-type UNG1. This is assumed to be because wild-type UNG1 removes uracil which is an aberrant base that appears rarely in DNA, whereas mutant UNG1 having CDG activity or TDG activity removes cytosine or thymine anywhere on the genomic DNA and produces mutations undesirable for the survival of the cell. On the other hand, when the reactivity with a DNA having an unrelaxed double helix structure is decreased by introducing L304 mutation, the survival rate of the host yeast recovered remarkably and cytotoxicity could be avoided. 
     Example 6 Utilization of Heterogenous Uracil-DNA Glycosylase 
     Whether introduction of targeted mutation into the host yeast is possible even when heterogenous mutant UNG1 is used instead of mutant UNG1 derived from yeast was examined. Two kinds of  Escherichia coli -derived mutant ungs (EcUDG) (N123D/L191A double mutant, Y66G/L191A double mutant) and four kinds of vaccinia virus-derived mutant UDGs (vvUDG) (N120D/R187C double mutant, Y70G/R187C double mutant, N120D mutant, Y70G mutant) were used. The results are shown in  FIG. 9 . While both EcUDG, vvUDG were functional in yeast, the efficiency of mutation induction was low as compared to yeast UNG1, and it was shown that the use of allogeneic DNA glycosylase was advantageous. Surprisingly, it was clarified that cytotoxicity was absent in vvUDG, regardless of the presence or absence of R187C mutation corresponding to R308C mutation of yeast UNG1. As a result of sequence analysis, since mutation by vvUDG was concentrated in a specific base in the target nucleotide sequence regardless of the presence or absence of R187C mutation (see  FIG. 10 ), vvUDG was suggested to be a DNA glycosylase with natively sufficiently low reactivity with a DNA having an unrelaxed double helix structure. The efficiency of mutation induction by vvUDG was remarkably increased in virus DNA polymerase by coexpressing A20, which interacts with vvUDG and acts as a processivity factor ( FIG. 9 ). 
     Example 7 Reduction of Non-Specific Mutation by Utilization of Split Enzyme 
     In addition to the utilization of mutation that decreases reactivity with a DNA having an unrelaxed double helix structure, and DNA glycosylase with natively low reactivity with a DNA having a double helix structure such as vvUDG, utilization of split enzyme technique was tried as a different means for reducing non-specific mutation by DNA glycosylase. The plasmids shown in  FIG. 11  containing DNA encoding various split enzymes were introduced into the host yeast together with a plasmid containing a DNA encoding guide RNA-tracrRNA by a method similar to that in Example 1, and the cell number, the number of canavanine-resistant (mutation at targeted site) colonies, the number of thialysine-resistant (non-specific mutation) colonies on a nonselective medium were examined. The survival rate of the host yeast on a nonselective medium was equivalent to that when mutant UNG1 introduced with mutation (R308C) that decreases reactivity with a DNA having an unrelaxed double helix structure was introduced even when any split enzyme was used. Thus, it was shown that cytotoxicity can be sufficiently decreased by the utilization of a split enzyme, even without introducing a mutation that decreases reactivity with a DNA having an unrelaxed double helix structure, whereby it was suggested that non-specific mutation can be suppressed ( FIG. 11 ). In fact, the frequency of non-specific mutation by using thialysine-resistance as an index was decreased by the utilization of a split enzyme ( FIG. 11 ). 
     INDUSTRIAL APPLICABILITY 
     The present invention makes it possible to safely introduce site specific mutation into any species without accompanying insertion of a foreign DNA or double-stranded DNA breaks. It is also possible to set a wide range of mutation introduction to target nucleotide sequence and several hundred bases in the vicinity thereof, and the technique can also be applied to topical evolution induction by introduction of random mutation into a particular restricted region, which has been almost impossible heretofore, and is extremely useful. Furthermore, when mutation imparting CDG activity and mutation imparting TDG activity are imparted to UNG, base excision using 5-methylcytidine as a substrate becomes possible. According to the present invention, therefore, the epigenome information can be rewritten into, for example, region-specific release of methylation state to change the gene expression pattern and the like. Therefore, artificial cell differentiation, cancer cell inhibition, modification of gene function without rewriting genome sequence, and the like become possible. 
     This application is based on a patent application No. 2014-224745 filed in Japan (filing date: Nov. 4, 2014), the contents of which are incorporated in full herein.