抑癌基因BRCA2对于细胞应对DNA双链断裂损伤不可或缺。其蛋白通过招募修复因子、激活同源DNA配对支持损伤位点同源重组修复(HR)。BRCA2突变可导致基因组不稳定和家族遗传性卵巢/乳腺癌高发。本项目前期应用全外显子测序在癌症高危家族中发现了一个新的BRCA2异亮氨酸>甲硫氨酸胚系突变。根据前期研究，提出科学假说：BRCA2特异位点的异亮氨酸侧链甲基是一个固有的甲基“信号”，它的缺失导致修复相关蛋白招募受损，ssDNA检索同源dsDNA、D-环形成过程受到抑制，造成HR受阻。本课题利用点突变、CRISPR/Cas9、DNA单链交换分析、CoIP等方法在细胞系、原代细胞和小鼠模型进行此突变抑制HR过程的细胞学评价和体内外机制研究。从临床样本、细胞功能、生化分子机制和小鼠模型四个层次阐明此BRCA2突变的甲基效应，为BRCA2蛋白的固有甲基“信号”理论提供实验证据以及癌症的诊治提供新靶点。

The tumor suppressor gene BRCA2 is indispensable for cells to cope with DNA double-strand breaks. BRCA2 participates in key HR processes by recruiting repair proteins, activating homologous DNA sequence pairing and exchanging. Mutations in BRCA2 can result in impaired genomic stability and a high incidence of familial ovarian/breast cancer.Using the whole-exome sequencing, we found one novel BRCA2 germline mutation (I>M) in the high-risk family of ovarian/breast cancer, which is significantly associated with the high incidence of cancer in this family. According to the previous study, the scientific hypothesis is proposed: the branched methyl group of isoleucine at the key site of BRCA2 is an intrinsic methyl "signal", and its deletion leads to the damaged recruitment of repair-related proteins, and/or inhibition of single-strand DNA retrieval of genetic information from homologous double-strand DNA, and the HR critical steps blocked. In this study, we use site mutation, CRISPR/Cas9, DNA single-strand exchange analysis, co-immunoprecipitation to evaluate the inhibition of HR, cell function and the molecular mechanism in cell lines, primary cells and mouse models. We elucidate the methyl effect and mechanism of BRCA2 point mutation from four diractions: clinical samples, cell function,biochemical molecular mechanism and mouse models. We provide experimental evidence for the inherent methyl "signal" theory of BRCA2 protein and provide new targets for cancer diagnosis and treatment.