WO2023016021A1 - 一种碱基编辑工具及其构建方法 - Google Patents

一种碱基编辑工具及其构建方法 Download PDF

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WO2023016021A1
WO2023016021A1 PCT/CN2022/093505 CN2022093505W WO2023016021A1 WO 2023016021 A1 WO2023016021 A1 WO 2023016021A1 CN 2022093505 W CN2022093505 W CN 2022093505W WO 2023016021 A1 WO2023016021 A1 WO 2023016021A1
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gcn4
editing tool
pegrna
base editing
sequence
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谢红娴
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珠海舒桐医疗科技有限公司
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Definitions

  • the invention belongs to the technical field of gene editing, and in particular relates to a base editing tool and a construction method thereof.
  • Genome editing is a genetic engineering technology that uses artificial nucleases to modify the genome in a targeted manner, and is a research hotspot in the field of life sciences today.
  • Artificial nucleases mainly include Zinc Finger Nuclease (ZFN), TALEN Nuclease (Transcription Activator-Like Effector Nuclease) and CRISPR/Cas9 (Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9).
  • ZFN Zinc Finger Nuclease
  • TALEN Nuclease Transcription Activator-Like Effector Nuclease
  • CRISPR/Cas9 Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9
  • the CRISPR/Cas9 system is an immune defense mechanism that exists in bacteria and archaea to resist the invasion of phages and foreign DNA.
  • the gene editing system developed based on the CRISPR system has been widely used in animals, plants and humans. Gene editing of cells.
  • the core of the CRISPR/Cas9 gene editing system is an RNA-protein complex, which consists of two parts, sgRNA and Cas9 nuclease, which can complementarily bind to the target DNA sequence in the genome. When the complex binds to the target site, it activates the nuclease activity of Cas9, thereby cleaving the target DNA and generating a double-strand break (DSB) DNA damage.
  • sgRNA and Cas9 nuclease which can complementarily bind to the target DNA sequence in the genome.
  • DSB further activates the DNA damage repair mechanism in cells, mainly including error-prone non-homologous end joining (Non-homologous End Joining, NHEJ) and high-fidelity homologous recombination repair (Homologous Recombination, HDR).
  • NHEJ Non-homologous End Joining
  • HDR homologous Recombination
  • the repair method of non-homologous end joining will generate insertions or deletions (InDels) of DNA fragments at the target site, resulting in frameshift mutations, resulting in loss of function of the target gene and becoming a null allele (null allele).
  • an endogenous homologous sequence or an exogenously introduced homologous sequence is required as a repair template, and an exogenous fragment is knocked in or a point mutation is introduced at the target site.
  • the efficiency of homologous recombination is much lower than that of non-homologous end joining, resulting in uncontrollable repair results at the target site and tending to generate insertions and deletions of nucleotides.
  • the base editor single-base editing system is mainly composed of sgRNA and fusion protein.
  • the fusion protein is generally composed of a modified Cas9 protein, cytosine deaminase and uracil glycosylase inhibitor.
  • the fusion protein developed by the Changxing research group of the Shanghai Institute of Nutrition and Health, Chinese Academy of Sciences only contains two parts, Cas9 and cytosine deaminase.
  • sgRNA guides the fusion protein to bind to the target site by complementary pairing with the target site.
  • the cytosine deaminase in the fusion protein can deaminate the corresponding cytosine C in the non-complementary chain into uracil U, and DNA replication further makes U replaced by T, and the original base complementary to C on the complementary chain
  • the base guanine G will become adenine A, and the uracil glycosylase inhibitor can inhibit the excision of U, and finally realize the precise editing of C on the non-complementary strand to T and G on the complementary strand to A .
  • the system can be divided into two types, one type uses dCas9 without endonuclease activity, which will not cut the target DNA when editing; the other type uses dCas9 with single-strand nCas9 with DNA nickase activity will generate a nick on a DNA single strand at the target gene site during editing, and then use the complementary strand as a template for synthetic repair. Since nCas9 and dCas9 still maintain the ability to bind to sgRNA, but neither can cause double-stranded DNA breaks, thereby inhibiting the insertion or deletion of DNA fragments mediated by NHEJ.
  • the single base editing system can only deaminate C near the active site of cytidine deaminase, which is called the active window.
  • the active window of cytosine deaminase has 5 nucleotides, which are the 4th to 8th nucleotides from the farthest end of PAM, so that the non-targeted bases in the active window will also occur Substitution. Later researchers tried to shorten the active window and improve the accuracy of gene editing in three ways, but the effect was still not very satisfactory.
  • a new gene editing method developed by David Liu-Guided Editing (Prime Editor, PE), this method can achieve the target site without introducing double-strand breaks (DSB) and donor DNA templates. Insertion, deletion and all 12 types of point mutations (currently ABE and CBE systems can only realize four mutation types: C ⁇ T, G ⁇ A, A ⁇ G, T ⁇ C), which expands the scope of base editing and improves Improve the efficiency of precise editing.
  • Prime editing is a genome editing method based on "search-and-replace".
  • the search function of guide editing is based on the modified guide RNA (the engineered guide RNA, the pegRNA).
  • the pegRNA contains the well-known single guide RNAs (sgRNAs).
  • the difference is that there is a primer binding sequence at its 3' end ( Primer binding site, PBS) and transcription template sequence (RT template).
  • the prime editor protein is a fusion of Cas9 nickase (Cas9 nickase, which only has the function of cutting single strands, H840A) and reverse transcriptase (M-MLV RT).
  • the Cas9 nickase cuts the DNA single strand, and the PBS (primer binding sequence) at the 3' end of the pegRNA can recognize and pair with the complementary sequence before the cutting breakpoint, reverse transcriptase (M-MLV RT)
  • M-MLV RT reverse transcriptase
  • the researchers further optimized and improved the efficiency of reverse transcriptase, and launched a PE2 version with higher editing efficiency than PE.
  • the DNA double strand edited by PE2 is a heterozygous strand, that is, one is an edited strand and the other is a non-edited strand, the template for mismatch repair of a heterozygous double strand is random.
  • their team further developed the PE3 version of the editing system.
  • This version introduces a new nick at a position 50 bp away from the nick caused by pegRNA on the non-edited strand (to avoid double Strand breaks), so that cells can use the edited strand as a template for DNA repair as much as possible, so as to achieve the purpose of precise editing.
  • the purpose of the present invention is to provide a new base editing tool that can further improve the accuracy of base editing, enhance the accuracy of base editing, and increase the selectivity of delivery methods, so as to provide more options for gene editing.
  • the first aspect of the present invention provides a base editing tool, and the base editing tool includes the SunTag system and the PE2 system.
  • the SunTag system is combined with the PE2 system, and the PE2 system is split into relatively independent expression of nCas9 and reverse transcriptase, which are recruited by the SunTag system to form the SunTag-PE2 system.
  • the SunTag system includes GCN4 polypeptide, and the GCN4 polypeptide can be recognized by single-chain variable fragment antibody.
  • SunTag is a signal amplification system. It is multi-copy and contains a 19-amino acid GCN4 polypeptide recognized by a single-chain antibody (scFv).
  • the small soluble tag GB1 is the binding domain of the G protein of group G Streptococcus, fused to the C-terminus of the scFv for protein aggregation.
  • the SunTag system is a synthetic scaffold that can recruit up to 24 copies of a protein. Initially, it was used to combine with GFP protein to amplify the effect of fluorescent labeling, and was used to track single molecules in living cells without affecting the function of the protein.
  • the SunTag system has been successfully applied to fluorescence imaging and targeted demethylation of specific DNA sites, and it has been reported that the combination of the SunTag system and the BE system further broadens the editing window and improves the editing accuracy compared with BE3.
  • the original BE3 The editing window is 4-8bp upstream of PAM, and the BE-Plus system combined with SunTag system and BE system extends the editing window of 9-16bp upstream of PAM.
  • the current BE3 system has two mutation directions of C>G and A>T, and the PE2 system has raised genome editing to a new level.
  • This method allows the introduction of all mutation types, including insertions, deletions, and 12 bases- Base conversion, and by designing the sequence of the pegRNA, an editing window of 3-29bp upstream of the PAM can be achieved.
  • the key to the PE2 system is to improve the editing efficiency, which is mainly achieved by optimizing different RTs and pegRNA designs, but there is no report on the combination of the SunTag system and the PE2 system, mainly because the combination of the SunTag system and the PE2 system is not simply a combination of The BE system is replaced by the PE2 system.
  • the difficulty of combining the SunTag system and the PE2 system includes the influence of steric hindrance when the two systems are combined.
  • the combination method of the two is closely related to the editing efficiency of the combined system. If the combination method is not selected properly , and even results in poorer editing efficiency than the original PE2 system.
  • the PE2 system includes pegRNA, Cas9 nickase with only single-stranded DNA nickase activity, and reverse transcriptase, and the sequence of the pegRNA includes sgRNA sequence, primer binding sequence, and transcription template sequence.
  • the PE system can efficiently generate mutations such as precise base transitions or transversions, insertions and deletions without the need to introduce double-strand breaks and exogenous DNA templates.
  • the present invention combines the SunTag system with the PE2 system, splits the PE system into relatively independent expression of nCas9 and reverse transcriptase, recruits them through the SunTag system to improve their editing efficiency, and explores the relationship between nCas9 nuclease and reverse transcriptase in PE. Optimize the ratio.
  • the SunTag-PE2 system can further improve the accuracy of base editing, enhance the accuracy of base editing, increase the selectivity of delivery methods, and further improve editing efficiency.
  • multiple GCN4 polypeptides are connected to the C-terminal and/or N-terminal of the Cas9 nickase.
  • the number of GCN4 polypeptides is 1-20.
  • the amino acid sequence of the GCN4 polypeptide is EELLSKNYHLENEVARLKK (SEQ ID NO.14).
  • the C-terminal and N-terminal of the Cas9 nickase are connected with 1-5 GCN4 polypeptides respectively.
  • the linker sequence used to connect the Cas9 nickase and the GCN4 polypeptide is a flexible linker.
  • the amino acid sequence of the flexible linker used to connect the Cas9 nickase and the GCN4 polypeptide is SGGSSGGSSGSETPGTSESATTPESSGGSSGGSS (SEQ ID NO.15).
  • the reverse transcriptase is connected to the single-chain variable fragment to form a fusion protein, and the fusion protein is connected to the pegRNA.
  • the linker sequence used to connect the reverse transcriptase and the single-chain variable fragment is flexible linker or GS linker.
  • the amino acid sequence of the flexible linker is SGGSSGGSSGSETPGTSESATTPESSGGSSGGSS (SEQ ID NO.15).
  • amino acid sequence of the GS linker is (GGGS)nG, where n is an integer greater than or equal to 1.
  • n is an integer between 2-10.
  • the amino acid sequence of the GS linker is GGGSGGGSGGGSGGGSG (SEQ ID NO.16).
  • the base editing tool includes a pU6-pegRNA-scFv-linker-RT plasmid and a pCMV-PE2-n ⁇ GCN4 plasmid, wherein the pU6-pegRNA-scFv-linker-RT plasmid includes a reverse recorder, single-chain variable fragment, and pegRNA; the pCMV-PE2-n ⁇ GCN4 plasmid includes Cas9 protease, and n GCN4 polypeptides connected to the C-terminal and/or N-terminal of the Cas9 protease, wherein, 1 ⁇ n ⁇ 20.
  • the second aspect of the present invention provides a method for constructing the base editing tool, including:
  • the base editing tool or the base editing tool constructed by the construction method can edit the target sequence as shown in SEQ ID NO.1 as shown in SEQ ID NO.2
  • the sequence shown, the pegRNA sequence used is shown in SEQ ID NO.3.
  • the C-terminus and the N-terminus of the Cas9 nickase are respectively linked to one of the GCN4 polypeptides.
  • the linker sequence used to connect the reverse transcriptase and the single-chain variable fragment is GS linker.
  • the Cas9 nickase is H840A.
  • the third aspect of the present invention also provides an application of the base editing tool in life science research, agricultural production and biomedicine.
  • the present invention has the following advantages compared with the prior art:
  • the present invention combines the SunTag system with the PE system, and uses the SunTag system and the PE2 version with higher editing efficiency to construct the SunTag-PE2 system. After optimization, it is compared with the original PE2 system on 293T cells, which can further improve the editing efficiency of the PE2 system. Accuracy, enhance editing accuracy, increase the selectivity of delivery methods, and provide more choices for gene editing. This technology will promote the rapid development of life science research, agricultural production and biomedicine in my country and the world. important application value.
  • Figure 1 is a schematic diagram of the PE system.
  • the search function of guide editing is based on the modified guide RNA (the engineered guide RNA, the pegRNA).
  • the pegRNA contains the well-known single guide RNAs (sgRNAs). The difference is that there is a primer binding sequence at its 3' end ( Primer binding site, PBS) and transcription template sequence (RT template).
  • the prime editor protein is a fusion of Cas9 nickase (Cas9 nickase, which only has the function of cutting single strands, H840A) and reverse transcriptase (M-MLV RT);
  • Fig. 2 is a schematic flow chart of editing target DNA by PE system.
  • the Cas9 nickase cuts the DNA single strand.
  • the PBS (primer binding sequence) at the 3' end of the pegRNA can recognize and pair with the complementary sequence before the cutting breakpoint.
  • the reverse transcriptase converts the pegRNA
  • the artificially designed template sequence after the PBS sequence is used as the template for reverse transcription, and the target sequence is directly polymerized onto the nicked DNA strand;
  • FIG. 3 is a schematic diagram of the SunTag-PE2 system. Combine the SunTag system with the PE2 system, and connect n GCN4s to the C-terminus of nCas9 to form a pCMV-PE2-n ⁇ GCN4 plasmid, where n represents the number of different GCN4s; the reverse transcriptase (M-MLV RT) is connected to the The scFv is connected to form a fusion protein, and the pegRNA sequence is connected to the plasmid containing RT to form a pegRNA-scFv-flexible-linker-RT-HEK3 (+1-CTTins) plasmid, where +1 represents that the position of the target mutation is located in the HEK3 sgRNA-guided
  • CTTins represent the insertion of three bases that are mutated into CTT.
  • nCas9-GCN4 was guided to the binding site by sgRNA, and scFv-RT was recruited around the binding site to induce the insertion of the three bases of CTT;
  • Figure 4 is a schematic diagram of the SunTag-PE2 system and a comparison with the original PE2 system.
  • GCN4 to the C-terminus of nCas9 to construct plasmids: pCMV-PE2-1 ⁇ GCN4, pCMV-PE2-2 ⁇ GCN4, pCMV-PE2-3 ⁇ GCN4, pCMV-PE2-5 ⁇ GCN4, pCMV-PE2-5 ⁇ GCN4, pCMV-PE2-5 ⁇ GCN4, PE2-10 ⁇ GCN4, co-transfected with pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins) plasmid, and compared the effect of the target mutation with the original PE2 system;
  • Figure 5 is the editing effect of the optimization method 1 of the SunTag-PE2 system. Connect different numbers of GCN4 to the N-terminus of nCas9 at the same time, and connect different numbers of GCN4 to the C-terminus respectively for combination to construct plasmids respectively:
  • Figure 6 is a schematic diagram of the optimization method 2 of the SunTag-PE2 system and the editing effect. Replace the flexible linker with GS linker to construct the plasmid pegRNA-scFv-GS-linker-RT-HEK3(+1-CTTins).
  • the primers used in the examples were synthesized by Suzhou Jinweizhi Biotechnology Co., Ltd.; the PCR reagents were from Beijing Quanshijin Biotechnology Co., Ltd. PCR SuperMix (Cat. No. AS111-02).
  • M-MLV RT The reverse transcriptase (M-MLV RT) is linked to the scFv through a flexible linker to form a fusion protein, and the pegRNA sequence is linked to the RT-containing plasmid to construct the pegRNA-scFv-flexible-linker-RT-HEK3 (+1-CTTins) plasmid. After the construction is completed, it is confirmed by conventional sequencing that the sequence of the constructed vector is correct and there are no mutations, and completely correct clones are selected for amplification and plasmid extraction.
  • pCMV-PE2-1 ⁇ GCN4 plasmid sequence the complete sequence is shown in SEQ ID NO.5.
  • pCMV-PE2-2 ⁇ GCN4 plasmid sequence replace 1 ⁇ GCN4 (SEQ ID NO.6) in the pCMV-PE2-1 ⁇ GCN4 plasmid sequence with 2 ⁇ GCN4 (SEQ ID NO.7).
  • pCMV-PE2-3 ⁇ GCN4 plasmid sequence replace 1 ⁇ GCN4 (SEQ ID NO.6) in the pCMV-PE2-1 ⁇ GCN4 plasmid sequence with 3 ⁇ GCN4 (SEQ ID NO.8).
  • pCMV-PE2-5 ⁇ GCN4 plasmid sequence replace 1 ⁇ GCN4 (SEQ ID NO.6) in the pCMV-PE2-1 ⁇ GCN4 plasmid sequence with 5 ⁇ GCN4 (SEQ ID NO.9).
  • pCMV-PE2-10 ⁇ GCN4 plasmid sequence replace 1 ⁇ GCN4 (SEQ ID NO.6) in the pCMV-PE2-1 ⁇ GCN4 plasmid sequence with 10 ⁇ GCN4 (SEQ ID NO.10).
  • Plasmids pCMV-PE2-1 ⁇ GCN4, pCMV-PE2-2 ⁇ GCN4, pCMV-PE2-3 ⁇ GCN4, pCMV-PE2-5 ⁇ GCN4, pCMV-PE2-10 ⁇ GCN4 were combined with pegRNA-scFv-flexible-linker -RT-HEK3 (+1-CTTins) plasmids were co-transfected, and the target mutation efficiency was tested separately.
  • the human embryonic kidney cell line 293T was cultured in complete DMEM medium containing 10% serum in a 37°C, 5% CO 2 incubator. When the cell confluency reached 90%, it was digested with 0.25% trypsin, then digested with complete DMEM medium, seeded into a 12-well plate, and cultured for 24 hours.
  • each well was transfected with 0.5ug of plasmid containing pegRNA and 0.5ug of plasmid containing nCas9, using Roche’s X-tremeGENE HP DNA Transfection Reagent transfection reagent to perform transfection according to the instructions, with an equal amount of empty vector (1 ⁇ g) as negative control.
  • the transfected cells continued to be cultured in a 37°C, 5% CO 2 incubator.
  • digest with 0.25% trypsin routinely, stop the digestion with DMEM complete medium, collect the cells into a centrifuge tube, centrifuge at 300g for 5 minutes, discard the medium, wash once with PBS, centrifuge again at 300g for 5 minutes, discard Remove PBS, obtain cell slag, use cell genome extraction kit (Quanshijin Biotechnology Co., Ltd., article number: EE101-01) to extract cell genome DNA, and measure DNA concentration.
  • cell genome extraction kit Quanshijin Biotechnology Co., Ltd., article number: EE101-01
  • the primer sequence corresponding to the sgRNA of HEK3 in this embodiment is:
  • HEK3-NGS-F ACACTCTTTCCCTACACGACGCTCTTCCGATCTctggcctgggtcaatccttg.
  • HEK3-NGS-R GTGACTGGAGTTCAGACGTGTGCTCTTCCGATCTgctgcacatatagcccctg.
  • I7 primer and I5 primer use commercial illumina sequencing adapter primers: Hieff NGS384 Dual Index Primer Kit for (Cat. No. 12613ES02).
  • GCN4 Different numbers of GCN4 were connected to the N-terminal of nCas9 at the same time, and different numbers of GCN4 were connected to the C-terminal respectively for combination to construct the following pCMV-PE2-n ⁇ GCN4 plasmids:
  • 1 ⁇ GCN4-pCMV-PE2-1 ⁇ GCN4 plasmid sequence the complete sequence is shown in SEQ ID NO.12.
  • 1 ⁇ GCN4-pCMV-PE2-2 ⁇ GCN4 plasmid sequence Replace 1 ⁇ GCN4 (SEQ ID NO.6) with 2 ⁇ GCN4 at the N-terminus of nCas9 of the GCN(1x)-pCMV-PE2-GCN(1x) plasmid (SEQ ID NO.7), all the other 14 pCMV-PE2-n ⁇ GCN4 plasmids were constructed by analogy.
  • Example 2 The 16 pCMV-PE2-n ⁇ GCN4 plasmids in Example 2 were co-transfected with the pegRNA-scFv-flexible-linker-RT-HEK3 (+1-CTTins) plasmid respectively, and the effect of the target mutation was compared with the original PE2 system , the method is the same as in Example 1, the results are shown in Figure 6, the histogram results show that the editing efficiency is further improved after the flexible linker is replaced by the GS linker, and the 1 ⁇ GCN4-pCMV-PE2-1 ⁇ GCN4 plasmid and the pegRNA-scFv-GS- Linker-RT-HEK3 (+1-CTTins) plasmid co-transfection effect is the best, about 18%, obviously better than the original PE2 system effect.

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Abstract

提供了一种碱基编辑工具及其构建方法和应用。该碱基编辑工具包括SunTag系统和PE2系统。该碱基编辑工具能够提高PE2系统编辑的准确性，增加递送方式的选择性。

Description

一种碱基编辑工具及其构建方法 技术领域
本发明属于基因编辑技术领域，具体涉及一种碱基编辑工具及其构建方法。
背景技术
基因组编辑(简称为基因编辑)技术是利用人工核酸酶对基因组进行靶向修饰的遗传工程技术，是当今生命科学领域的研究热点。人工核酸酶主要包含锌指核酸酶(Zinc Finger Nuclease，ZFN)、TALEN核酸酶(Transcription Activator-Like Effector Nuclease)以及CRISPR/Cas9(Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9)。利用这些技术，人们可以根据自身研究的需求对感兴趣的基因进行敲除或过表达等操作，进而研究基因的功能和调控机制。
CRISPR/Cas9系统是存在于细菌和古细菌中的一种免疫防御机制，用来抵抗噬菌体以及外源DNA的入侵，基于CRISPR系统开发而来的基因编辑系统已经被广泛运用于动物、植物和人细胞的基因编辑。CRISPR/Cas9基因编辑系统的核心是一个RNA蛋白复合物，由能与基因组中靶DNA序列互补结合的sgRNA和Cas9核酸酶两部分组成。当该复合物与靶位点结合之后，会激活Cas9的核酸酶活性，从而切割靶DNA，产生双链断裂(DSB)的DNA损伤。DSB进一步激活细胞内的DNA损伤修复机制，主要包括易错的非同源末端连接(Non-homologous End Joining，NHEJ)以及高保真的同源重组修复(Homologous Recombination，HDR)。
非同源末端连接的修复方式会在靶位点处产生DNA片段的插入或缺失(InDels)，导致移码突变，从而造成靶基因的功能丧失，成为无效等位基因(null allele)。当通过同源重组的方式进行修复时，需要内源的同源序列或者外源导入的同源序列作为修复模板，在靶位点处敲入外源片段或引入点突变。但是，在细胞中，同源重组的效率远远低于非同源末端连接，导致靶位点处修复结果不可控，并倾向于产生核苷酸的插入和缺失。
为了提高定点突变的效率，将CRISPR/Cas9和其他酶产物进行结合的单碱基编辑系统被相继报道。例如：哈佛大学生物化学家David Liu组开发了base editor系统。base editor单碱基编辑系统主要由sgRNA和融合蛋白两部分组成，其中融合蛋白一般由改造的Cas9蛋白、胞嘧啶脱氨酶和尿嘧啶糖基化酶抑制子三者构成。而中科院上海营养与健康研究院常兴课题组开发的融合蛋白仅包含Cas9和胞嘧啶脱氨酶两部分。sgRNA通过与靶位点互补配对，引导融 合蛋白结合到靶位点发挥作用。融合蛋白中的胞嘧啶脱氨酶能够使非互补链中相应的胞嘧啶C经脱氨基作用转变为尿嘧啶U，而DNA复制进一步使得U被T代替，而互补链上原来与C互补的碱基鸟嘌呤G将会变成腺嘌呤A，而尿嘧啶糖基化酶抑制子则能够抑制U的切除，最终实现非互补链上的C替换为T和互补链上G替换为A的精确编辑。根据融合蛋白中Cas9蛋白突变体的不同，可以将该系统分为两类，一类选用了无核酸内切酶活性的dCas9，进行编辑时不会切割靶DNA；另一类选用了有单链DNA切口酶活性的nCas9，进行编辑时会在靶基因位点的一条DNA单链产生切口，再以互补链为模板进行合成修复。由于nCas9和dCas9仍保持与sgRNA结合的能力，但均不会引起双链DNA的断裂，从而抑制了由NHEJ介导的DNA片段的插入或缺失。单碱基编辑系统只能将在胞嘧啶核苷脱氨酶活性位点附近的C脱氨基，这个区域称为活性窗口。通常胞嘧啶脱氨酶的活性窗口有5个核苷酸，为距离PAM最远端数起的第4～8位核苷酸，这样就会使活性窗口中非靶向的碱基也会发生替换作用。后来的研究者试图通过三种方式来缩短活性窗口，提高基因编辑的精准性，但是效果仍然不十分理想。
接着，David Liu开发的一种新的基因编辑方法——引导编辑(Prime Editor，PE)，这种方法可以在不引入双链断裂(DSB)和供体DNA模板的前提下，实现靶标位点的插入、缺失和所有12种类型点突变(目前ABE和CBE系统仅能实现C→T，G→A，A→G，T→C四种突变类型)，扩展了碱基编辑的范围，提高了精准编辑的效率。引导编辑(prime editing)是一种基于“搜索和替换“(search-and-replace)的基因组编辑方式。引导编辑的搜索功能是基于改造的向导RNA(the engineered guide RNA，the pegRNA)，pegRNA中包含我们熟知的single guide RNAs(sgRNAs)，不同的是，在其3’端还有一段引物结合序列(Primer binding site，PBS)和转录模板序列(RT template)。prime editor蛋白由Cas9切口酶(Cas9 nickase，仅有切割单链的功能，H840A)和逆转录酶(M-MLV RT)融合而成。这样，Cas9切口酶在pegRNA上的sgRNA序列指引下，切割DNA单链，pegRNA 3’端的PBS(引物结合序列)可以与切割断点前的互补序列识别配对，逆转录酶(M-MLV RT)以pegRNA上PBS序列后的人工设计的模板序列为模板进行逆转录，将目标序列直接聚合到切口的DNA链上。在PAM识别链上进行单链切割后，会在这条单链DNA上形成3’末端(3’flaps)和5’末端(5’flaps)两段序列，而5’末端会被具有5’核酸内切酶和5’核酸外切酶活性FEN1和具有5’核酸外切酶活性的EXO1蛋白切割。而3’末端通过逆转录酶合成的编辑序列，就可以更大概率的保留在修复后的序列中。通过这套系统，可以在无需引入双链断裂和外源DNA模板的情况下，有效地产生精确的碱基转换或颠换、插入和缺失等变异。在PE的基础上，研发人员进一步优化和提高了逆转 录酶的效率，推出了比PE编辑效率更高的PE2版本。由于PE2编辑后的DNA双链为杂合链，即一条为编辑链，一条为非编辑链，因而杂合双链错配修复的模板是随机的。为了解决因错配修复造成的编辑效率降低，他们团队进一步开发出了PE3版本编辑系统，这一版本在非编辑链上距离pegRNA造成的切口处50bp的位置引入了一个新的切口(避免产生双链断裂)，从而让细胞尽量多的以编辑链为模板进行DNA修复，以达到精准编辑的目的。
然而，不同的PE碱基编辑系统对同一位点进行编辑时，编辑效率不尽相同。pegRNA中PBS序列长度，逆转录模板长度以及切口sgRNA的位置都会或多或少影响最终的基因编辑效率，但如何影响并没有明显的规律，进行插入和缺失编辑时，随着插入或缺失片段的增长，其编辑效率是逐渐降低的。因此提高精准编辑的效率十分必要。此外，目前的PE系统(nCas9与逆转录酶M-MLV RT融合蛋白)大于6.3kb，超出了AAV包装能力，为PE基因编辑工具的递送，带来巨大挑战，亟待解决其递送负担问题，进一步推动引导编辑技术的发展及其转化应用。
发明内容
本发明的目的是提供一种能够实现进一步提高碱基编辑的准确性、增强碱基编辑精准度、增加递送方式选择性的新的碱基编辑工具，为基因编辑提供更多的选择。
为达到上述目的，本发明采用的技术方案是：
本发明第一方面提供一种碱基编辑工具，所述的碱基编辑工具包括SunTag系统和PE2系统。
具体地，将所述的SunTag系统和所述的PE2系统相结合，将PE2系统拆分成相对独立的nCas9和逆转录酶各自表达，并通过SunTag系统募集，形成SunTag-PE2系统。
具体地，所述的SunTag系统包括GCN4多肽，所述的GCN4多肽能够被单链可变片段抗体识别。
SunTag是一个信号扩增系统。是多拷贝的、含有一个单链抗体(scFv)识别的19个氨基酸GCN4多肽。小可溶性标签GB1是g群链球菌g蛋白的结合域，融合到scFv的c端以蛋白聚集。SunTag系统是一个合成的支架，最多能招募蛋白质的24个拷贝。最初，是用于与GFP蛋白结合以放大荧光标记效应，用于实现活细胞内单分子的追踪，而不会影响蛋白质的功能。SunTag系统已成功应用于荧光成像和特定DNA位点的靶向去甲基化，并且有报道利用SunTag系统和BE系统结合，和BE3相比，进一步拓宽编辑窗口和提高编辑精准度，原始的BE3的编辑窗口在PAM上游的4-8bp，SunTag系统和BE系统结合的BE-Plus系统扩展PAM上游的9-16bp的编辑窗口。但是目前的BE3系统为C>G、A>T两种突变方向，而PE2系统将基因组编辑提升到了 一个新的水平，这种方法允许引入所有突变类型，包括插入、缺失和12种碱基-碱基转换，并且通过设计pegRNA的序列，可以实现PAM上游3-29bp的编辑窗口。目前PE2系统的关键在于提高编辑效率，主要通过优化不同的RTs和pegRNA设计来实现，却未见将SunTag系统和PE2系统相结合的报道，主要是因为SunTag系统和PE2系统的结合并非简单的将BE系统替换为PE2系统，SunTag系统和PE2系统结合的难点包括两种系统结合时的空间位阻的影响，两者的结合方式与结合后的系统的编辑效率密切相关，如果结合方式没有选择好，甚至会出现编辑效率比原始PE2系统差的结果。
具体地，所述的PE2系统包括pegRNA、仅有单链DNA切口酶活性的Cas9切口酶、以及逆转录酶，所述的pegRNA的序列包括sgRNA序列、引物结合序列、以及转录模板序列。
PE系统可以在无需引入双链断裂和外源DNA模板的情况下，有效地产生精确的碱基转换或颠换、插入和缺失等变异。
本发明利用SunTag系统和PE2系统结合，将PE系统拆分成相对独立的nCas9和逆转录酶各自表达，通过SunTag系统募集以提高其编辑效率，并探索了PE中nCas9核酸酶和逆转录酶的优化配比。经优化后，SunTag-PE2系统可以进一步提高碱基编辑的准确性、增强碱基编辑精准度、增加递送方式选择性、以及进一步提高编辑效率。
优选地，将多个所述的GCN4多肽与所述的Cas9切口酶的C端和/或N端相连。
进一步优选地，所述的GCN4多肽的数量为1～20。
根据一种具体实施方式，GCN4多肽的氨基酸序列为EELLSKNYHLENEVARLKK(SEQ ID NO.14)。
再进一步优选地，所述的Cas9切口酶的C端和N端分别连接1～5个所述的GCN4多肽。
优选地，用于连接所述Cas9切口酶和所述GCN4多肽的接头序列为flexible linker。
根据一种具体实施方式，用于连接所述Cas9切口酶和所述GCN4多肽的flexible linker的氨基酸序列为SGGSSGGSSGSETPGTSESATTPESSGGSSGGSS(SEQ ID NO.15)。
优选地，将所述的逆转录酶和所述的单链可变片段连接形成融合蛋白，将所述的融合蛋白与所述的pegRNA相连。
再进一步优选地，用于连接所述的逆转录酶和所述的单链可变片段的接头序列为flexible linker或GS linker。
根据一种具体实施方式，所述的flexible linker的氨基酸序列为SGGSSGGSSGSETPGTSESATTPESSGGSSGGSS(SEQ ID NO.15)。
具体地，所述的GS linker的氨基酸序列为(GGGS)nG，n为大于等于1的整数。
进一步地，n为2～10之间的整数。
根据一种具体实施方式，所述的GS linker的氨基酸序列为GGGSGGGSGGGSGGGSG(SEQ ID NO.16)。
根据一种具体实施方式，所述碱基编辑工具包括pU6-pegRNA-scFv-linker-RT质粒以及pCMV-PE2-n×GCN4质粒，其中，所述pU6-pegRNA-scFv-linker-RT质粒包括逆转录酶、单链可变片段、以及pegRNA；所述pCMV-PE2-n×GCN4质粒包括Cas9蛋白酶，以及连接在所述Cas9蛋白酶C端和/或N端的n个GCN4多肽，其中，1≤n≤20。
本发明第二方面提供一种所述的碱基编辑工具的构建方法，包括：
将逆转录酶、单链可变片段、以及pegRNA相连并插到pU6上，构建pU6-pegRNA-scFv-linker-RT质粒；
将n个GCN4多肽与Cas9蛋白酶的C端和/或N端相连，然后插入到pCMV上，构建pCMV-PE2-n×GCN4质粒，
其中，1≤n≤20。
根据一种具体地实施方式，所述的碱基编辑工具或所述的构建方法构建的碱基编辑工具，能够将如SEQ ID NO.1所示的目标序列编辑为如SEQ ID NO.2所示序列，采用的pegRNA序列如SEQ ID NO.3所示。
优选地，所述的Cas9切口酶的C端和N端分别连接1个所述的GCN4多肽。
优选地，用于连接所述的逆转录酶和所述的单链可变片段的接头序列为GS linker。
优选地，所述的Cas9切口酶为H840A。
本发明第三方面还提供一种所述的碱基编辑工具在生命科学研究、农业生产和生物医药中的应用。
由于上述技术方案运用，本发明与现有技术相比具有下列优点：
本发明利用SunTag系统和PE系统结合，采用SunTag系统和编辑效率更高的PE2版本，构建SunTag-PE2系统，经优化后，在293T细胞上和原始的PE2系统进行比较，能够进一步提高PE2系统编辑的准确性，增强编辑精准度，增加递送方式的选择性，为基因编辑提供更多的选择，该技术将推动我国及全世界在生命科学研究、农业生产和生物医药等领域的快速发展，具有重要的应用价值。
附图说明
图l为PE系统的示意图。引导编辑的搜索功能是基于改造的向导RNA(the engineered guide RNA，the pegRNA)，pegRNA中包含我们熟知的single guide RNAs(sgRNAs)，不同的是，在其3’端还有一段引物结合序列(Primer binding site，PBS)和转录模板序列(RT template)。prime editor蛋白由Cas9切口酶(Cas9 nickase，仅有切割单链的功能，H840A)和逆转录酶(M-MLV RT)融合而成；
图2为PE系统编辑目标DNA的流程示意图。Cas9切口酶在pegRNA上的sgRNA序列指引下，切割DNA单链，pegRNA 3’端的PBS(引物结合序列)可以与切割断点前的互补序列识别配对，逆转录酶(M-MLV RT)以pegRNA上PBS序列后的人工设计的模板序列为模板进行逆转录，将目标序列直接聚合到切口的DNA链上；
图3为SunTag-PE2系统的示意图。将SunTag系统和PE2系统结合，n个GCN4与nCas9的C端连接，构成pCMV-PE2-n×GCN4质粒，其中n代表不同的GCN4的数量；逆转录酶(M-MLV RT)通过flexible linker与scFv连接形成融合蛋白，pegRNA序列连在含有RT的质粒上，构成pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒，其中+1代表目标突变的位置位于HEK3的sgRNA引导的靶序列被Cas9切割的断点后第一个碱基位置，CTTins代表目标突变为CTT这3个碱基插入。当这两个质粒共转染时，nCas9-GCN4被sgRNA引导到结合位点，scFv-RT是围绕结合位点招募来诱导CTT这3个碱基插入的；
图4为SunTag-PE2系统的示意图以及和原始的PE2系统的效果比较图。在nCas9的C端连接不同数量的GCN4，从而分别构建质粒：pCMV-PE2-1×GCN4，pCMV-PE2-2×GCN4，pCMV-PE2-3×GCN4，pCMV-PE2-5×GCN4，pCMV-PE2-10×GCN4，与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染，和原始的PE2系统比较目标突变的效果；
图5为SunTag-PE2系统的优化方式1的编辑效果。在nCas9的N端同时连接不同数量的GCN4，并且分别和C端同时连接不同数量的GCN4进行组合，从而分别构建质粒：
1×GCN4-pCMV-PE2-1×GCN4，1×GCN4-pCMV-PE2-2×GCN4，
1×GCN4-pCMV-PE2-3×GCN4，1×GCN4-pCMV-PE2-5×GCN4，
2×GCN4-pCMV-PE2-1×GCN4，2×GCN4-pCMV-PE2-2×GCN4，
2×GCN4-pCMV-PE2-3×GCN4，2×GCN4-pCMV-PE2-5×GCN4，
3×GCN4-pCMV-PE2-1×GCN4，3×GCN4-pCMV-PE2-2×GCN4，
3×GCN4-pCMV-PE2-3×GCN4，3×GCN4-pCMV-PE2-5×GCN4，
5×GCN4-pCMV-PE2-1×GCN4，5×GCN4-pCMV-PE2-2×GCN4，
5×GCN4-pCMV-PE2-3×GCN4，5×GCN4-pCMV-PE2-5×GCN4，
与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染，和原始的PE2系统比较目标突变的效果；
图6为SunTag-PE2系统的优化方式2示意图以及编辑效果。将flexible linker替换成GS linker，构建质粒pegRNA-scFv-GS-linker-RT-HEK3(+1-CTTins)。
具体实施方式
为更好地说明本发明的目的、技术方案和优点，下面将结合具体实施例对本发明作进一步说明。
实施例中使用的引物由苏州金唯智生物科技有限公司合成；PCR试剂采用北京全式金生物科技有限公司的
Figure PCTCN2022093505-appb-000001
PCR SuperMix(货号：AS111-02)。
为了详细说明本发明的SunTag-PE2系统，以下实施例针对文献中编辑效果较为稳定的pU6-HEK3_pegRNA_CTTins，本领域技术人员可根据以下实施例设计针对其他基因的目标位点的碱基编辑的SunTag-PE2系统。
以下实施例将目标序列：
Figure PCTCN2022093505-appb-000002
精准编辑为：
catttgtaggcttgatgctttttttctgcttctccagccctggcctgggtcaatccttggggcccagactgagcacgCTTtgatggcagaggaaaggaagccctgcttcctcc(SEQ ID NO.2)(目标位点插入CTT这3个碱基，大写字母表示)。
实施例1
1、构建pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒
采用文献中验证有效的pU6-HEK3_pegRNA_CTTins的序列HEK3_pegRNA_CTTins：
Figure PCTCN2022093505-appb-000003
逆转录酶(M-MLV RT)通过flexible linker与scFv连接形成融合蛋白，pegRNA序列连在含有RT的质粒上，构建pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒。构 建完成后，通过常规测序比对确定构建载体序列正确无突变，挑选出完全正确的克隆进行扩增并提取质粒。
pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒完整序列如SEQ ID NO.4所示。
2、在nCas9的C端连接不同数量的GCN4，从而分别构建质粒：pCMV-PE2-1×GCN4，pCMV-PE2-2×GCN4，pCMV-PE2-3×GCN4，pCMV-PE2-5×GCN4，pCMV-PE2-10×GCN4。构建完成后，通过常规测序比对确定构建载体序列正确无突变，挑选出完全正确的克隆进行扩增并提取质粒。
pCMV-PE2-1×GCN4质粒序列：完整序列如SEQ ID NO.5所示。
pCMV-PE2-2×GCN4质粒序列：将pCMV-PE2-1×GCN4质粒序列中的1×GCN4(SEQ ID NO.6)替换成2×GCN4(SEQ ID NO.7)。
pCMV-PE2-3×GCN4质粒序列：将pCMV-PE2-1×GCN4质粒序列中的1×GCN4(SEQ ID NO.6)替换成3×GCN4(SEQ ID NO.8)。
pCMV-PE2-5×GCN4质粒序列：将pCMV-PE2-1×GCN4质粒序列中的1×GCN4(SEQ ID NO.6)替换成5×GCN4(SEQ ID NO.9)。
pCMV-PE2-10×GCN4质粒序列：将pCMV-PE2-1×GCN4质粒序列中的1×GCN4(SEQ ID NO.6)替换成10×GCN4(SEQ ID NO.10)。
3、比较SunTag-PE2系统和原始的PE2系统的目标编辑效果
将质粒pCMV-PE2-1×GCN4、pCMV-PE2-2×GCN4、pCMV-PE2-3×GCN4、pCMV-PE2-5×GCN4、pCMV-PE2-10×GCN4分别与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染，并分别测试目标突变效率。
具体方法步骤如下：
(1)细胞培养
人胚肾细胞系293T用含有10％血清的DMEM完全培养基在37℃、5％CO 2培养箱里培养。待细胞融合度达到90％时用0.25％的胰酶消化后，用DMEM完全培养基终止消化，接种到12孔板中，继续培养24小时。
(2)质粒转染
24小时后，确认细胞贴壁良好，细胞融合度达到80％，即可进行转染。每孔转染0.5ug含有pegRNA的质粒以及0.5ug的含有nCas9的质粒，使用Roche公司的X-tremeGENE HP DNA Transfection Reagent转染试剂按照说明书要求进行转染，以等量的空载体(1μg)作为阴性对照。转染后的细胞继续在37℃、5％CO 2培养箱中培养。
(3)基因组DNA的提取
转染48小时后，常规0.25％的胰酶消化，用DMEM完全培养基终止消化，收集细胞到离心管中，300g离心5分钟,弃除培养基，PBS洗涤一次，再次300g离心5分钟，弃除PBS，获得细胞渣，使用细胞基因组提取试剂盒(全式金生物技术有限公司，货号：EE101-01)提取细胞基因组DNA，测量DNA浓度。
(4)扩增子建库引物的设计
分别根据HEK3的sgRNA靶向的基因序列(SEQ ID NO.11)设计引物，引物两端横跨靶点(产物长度优选150～200bp，并且靶点距离引物两段的距离应当差别不大)，以扩增出目的片段。本实施例中与HEK3的sgRNA相对应的引物序列为：
HEK3-NGS-F：ACACTCTTTCCCTACACGACGCTCTTCCGATCTctggcctgggtcaatccttg。
HEK3-NGS-R：GTGACTGGAGTTCAGACGTGTGCTCTTCCGATCTgctgcacatactagcccctg。
(5)扩增子建库的PCR反应：
以上述提取的基因组DNA为模版，用上述引物进行PCR反应。本实验使用的高保真kapa聚合酶为KAPA HiFi HotStart ReadyMix(货号：KK2602)。
第一轮PCR:
名称 体积
KAPA HiFi HotStart ReadyMix 12.5ul
DNA 200ng
HEK3-NGS-F(10μM) 0.5
HEK3-NGS-R(10μM) 0.5
补无菌去离子水 To 25μl
总共 25μl
PCR反应体系：
98℃for 3min；25 cycles of(98℃for 20s,65℃for 15s,72℃for 15s),72℃ for 1min,4℃for∞。
第二轮PCR:
名称 体积
KAPA HiFi HotStart ReadyMix 12.5μl
第一轮的PCR产物 1μl
I7primer(10μM) 2
I5primer(10μM) 2
补无菌去离子水 To 25μl
总共 25μl
I7 primer和I5 primer采用商业的illumina测序接头引物：Hieff NGS384 Dual Index Primer Kit for
Figure PCTCN2022093505-appb-000004
(货号12613ES02)。
PCR反应体系：
98℃for 3min；11 cycles of(98℃for 20s,65℃for 15s,72℃for 15s),72℃ for 1min,4℃for∞。
PCR反应完成后，取PCR产物进行琼脂糖凝胶电泳，将目标片段大小257bp的扩增子文库进行高通量测序分析目标突变效率，结果见图4，pCMV-PE2-1×GCN4及pCMV-PE2-3×GCN4与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染的效果最好，约为8％，和原始的PE2系统效果接近。
实施例2
对实施例1的SunTag-PE2系统的优化方式1：
在nCas9的N端同时连接不同数量的GCN4，并且分别和C端同时连接不同数量的GCN4进行组合，从而分别构建以下pCMV-PE2-n×GCN4质粒：
1×GCN4-pCMV-PE2-1×GCN4，1×GCN4-pCMV-PE2-2×GCN4，
1×GCN4-pCMV-PE2-3×GCN4，1×GCN4-pCMV-PE2-5×GCN4，
2×GCN4-pCMV-PE2-1×GCN4，2×GCN4-pCMV-PE2-2×GCN4，
2×GCN4-pCMV-PE2-3×GCN4，2×GCN4-pCMV-PE2-5×GCN4，
3×GCN4-pCMV-PE2-1×GCN4，3×GCN4-pCMV-PE2-2×GCN4，
3×GCN4-pCMV-PE2-3×GCN4，3×GCN4-pCMV-PE2-5×GCN4，
5×GCN4-pCMV-PE2-1×GCN4，5×GCN4-pCMV-PE2-2×GCN4，
5×GCN4-pCMV-PE2-3×GCN4，5×GCN4-pCMV-PE2-5×GCN4，
1×GCN4-pCMV-PE2-1×GCN4质粒序列：完整序列如SEQ ID NO.12所示。
1×GCN4-pCMV-PE2-2×GCN4质粒序列：在GCN(1x)-pCMV-PE2-GCN(1x)质粒的nCas9的N端将1×GCN4(SEQ ID NO.6)替换成2×GCN4(SEQ ID NO.7)，其余14个pCMV-PE2-n×GCN4质粒以此类推，构建而得。
将上述16个pCMV-PE2-n×GCN4质粒分别与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染，和原始的PE2系统比较目标突变的效果，方法同实施例1，结果见图5，柱状图结果显示1×GCN4-pCMV-PE2-1×GCN4与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染的效果最好，约为16％，明显优于原始的PE2系统效果。
实施例3
对实施例2的SunTag-PE2系统的优化方式2：
将pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒中的flexible linker替换成GS linker，构建pegRNA-scFv-GS-linker-RT-HEK(+1-CTTins)质粒：逆转录酶(M-MLV RT)通过GS linker与scFv连接形成融合蛋白，pegRNA序列连在含有RT的质粒上，构成pegRNA-scFv-GS-linker-RT-HEK3(+1-CTTins)质粒。构建完成后，通过常规测序比对确定构建载体序列正确无突变，挑选出完全正确的克隆进行扩增并提取质粒。
pegRNA-scFv-GS-linker-RT-HEK3(+1-CTTins)质粒完整序列如SEQ ID NO.13所示。
将实施例2中的16个pCMV-PE2-n×GCN4质粒分别与pegRNA-scFv-flexible-linker-RT-HEK3(+1-CTTins)质粒共转染，和原始的PE2系统比较目标突变的效果，方法同实施例1，结果见图6，柱状图结果显示，将flexible linker替换成GS linker后编辑效率进一步提升，并且1×GCN4-pCMV-PE2-1×GCN4质粒与pegRNA-scFv-GS-linker-RT-HEK3(+1-CTTins)质粒共转染的效果最好，约为18％，明显优于原始的PE2系统效果。
上述实施例只为说明本发明的技术构思及特点，其目的在于让熟悉此项技术的人士能够了解本发明的内容并据以实施，并不能以此限制本发明的保护范围。凡根据本发明精神实质所作的等效变化或修饰，都应涵盖在本发明的保护范围之内。

Claims (14)

  1. 一种碱基编辑工具，其特征在于：所述的碱基编辑工具包括SunTag系统和PE2系统。
  2. 根据权利要求1所述的碱基编辑工具，其特征在于：将所述的SunTag系统和所述的PE2系统相结合形成SunTag-PE2系统，所述的SunTag系统包括GCN4多肽，所述的GCN4多肽能够被单链可变片段抗体识别，所述的PE2系统包括pegRNA、仅有单链DNA切口酶活性的Cas9切口酶、以及逆转录酶，所述的pegRNA的序列包括sgRNA序列、引物结合序列、以及转录模板序列；将多个所述的GCN4多肽与所述的Cas9切口酶的C端和/或N端相连。
  3. 根据权利要求1所述的碱基编辑工具，其特征在于：所述的SunTag系统包括一个或多个GCN4多肽，所述的GCN4多肽能够被单链可变片段抗体识别，所述的PE2系统包括pegRNA、仅有单链DNA切口酶活性的Cas9切口酶、以及逆转录酶，所述的pegRNA的序列包括sgRNA序列、引物结合序列、以及转录模板序列；所述GCN4多肽连接于所述的Cas9切口酶的C端和/或N端。
  4. 根据权利要求2或3所述的碱基编辑工具，其特征在于：所述的GCN4多肽的数量为1～20。
  5. 根据权利要求4所述的碱基编辑工具，其特征在于：所述的Cas9切口酶的C端和N端分别连接有1～5个所述的GCN4多肽。
  6. 根据权利要求2或3所述的碱基编辑工具，其特征在于：所述Cas9切口酶和所述GCN4多肽通过flexible linker连接。
  7. 根据权利要求2或3所述的碱基编辑工具，其特征在于：所述的逆转录酶和所述的单链可变片段连接形成融合蛋白，所述的融合蛋白与所述的pegRNA相连。
  8. 根据权利要求7所述的碱基编辑工具，其特征在于：所述的逆转录酶和所述的单链可变片段通过flexible linker或GS linker连接。
  9. 根据权利要求1所述的碱基编辑工具，其特征在于：所述碱基编辑工具包括pU6-pegRNA-scFv-linker-RT质粒以及pCMV-PE2-n×GCN4质粒，其中，所述pU6-pegRNA-scFv-linker-RT质粒包括逆转录酶、单链可变片段、以及pegRNA；所述pCMV-PE2-n×GCN4质粒包括Cas9蛋白酶，以及连接在所述Cas9蛋白酶C端和/或N端的n个GCN4多肽，其中，1≤n≤20。
  10. 根据权利要求1所述的碱基编辑工具，其特征在于：所述的碱基编辑工具能够将如SEQ ID NO.1所示的目标序列编辑为如SEQ ID NO.2所示序列，采用的pegRNA序列如SEQ ID NO.3所示。
  11. 根据权利要求2或3所述的碱基编辑工具，其特征在于：所述的Cas9切口酶的C端和N端分别连接1个所述的GCN4多肽。
  12. 根据权利要求2或3所述的碱基编辑工具，其特征在于：所述的逆转录酶和所述的单链可变片段通过GS linker连接。
  13. 一种如权利要求1至12中任一项所述的碱基编辑工具的构建方法，其特征在于：所述的构建方法包括：
    将逆转录酶、单链可变片段、以及pegRNA相连并插到pU6上，构建pU6-pegRNA-scFv-linker-RT质粒；
    将n个GCN4多肽与Cas9蛋白酶的C端和/或N端相连，然后连接到pCMV上，构建pCMV-PE2-n×GCN4质粒，
    其中，1≤n≤20。
  14. 如权利要求1至12中任一项所述的碱基编辑工具在生命科学研究、农业生产和生物医药中的应用。
PCT/CN2022/093505 2021-08-10 2022-05-18 一种碱基编辑工具及其构建方法 WO2023016021A1 (zh)

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