利用单碱基编辑系统定点编辑哈萨克羊MSTN基因的研究

肌肉生长抑制素（myostatin，MSTN）可负调控骨骼肌的发育。本研究利用胞嘧啶碱基编辑器（cytidine base editor，CBE）在哈萨克羊MSTN基因编码区提前引入终止密码子，以期达到敲除MSTN基因的目的。共设计2条靶向哈萨克羊MSTN基因不同外显子的sgRNAs，分别连接至pGL3-U6-sgRNA-PGK-puromycin质粒，并与pCMV-AncBE4 max-P2A-GFP质粒共转染哈萨克羊胎儿成纤维细胞，经Cruiser^TM酶酶切检测、Sanger测序和TA克隆分析。结果显示，成功筛选出可以在哈萨克羊MSTN基因外显子提前引入终止密码子的2条sgRNAs，其中STOPsg-1靶位点编辑效率为26.7%，STOPsg-2靶位点的编辑效率为6.7%。本研究成功运用CBE（AncBE4 max）技术在哈萨克羊胎儿成纤维细胞MSTN基因编码区实现定点编辑，为培育精确编辑MSTN基因的哈萨克羊奠定基础。

Study of Kazakh Sheep MSTN Gene Site-directed Editing Based on the Base Editing System

Myostatin (MSTN) can negatively regulate the growth and development of skeletal muscle. In this study, the MSTN gene exon of Kazakh sheep was edited to create a stop codon in order to knock out the MSTN gene by using cytidine base editor (CBE). Two sgRNAs targeting to loci across exons of MSTN gene of Kazakh sheep were designed in the study. The sgRNAs were ligated to pGL3-U6-sgRNA-PGK-puromycin plasmid and then co-transfected with pCMV-AncBE4 max-P2A-GFP plasmid into Kazakh sheep fetal fibroblasts. The Cruiser^TM assay, Sanger sequencing and TA clone analysis were performed. The results showed that two sgRNAs could create stop codons in the MSTN gene exons of Kazakh sheep. The editing efficiency of the STOPsg-1 target site was 26.7%, and the editing efficiency of the STOPsg-2 target site was 6.7%. In this study, the CBE (AncBE4 max) technique was successfully used to edit the coding region of MSTN gene in fetal fibroblasts of Kazak sheep in the present study, which lay a technical foundation for breeding Kazakh sheep with precise editing of MSTN gene.