中国汉族乳腺癌BRCA1/2始祖突变的鉴定及功能研究

BRCA1/2 genetic testing plays a crucial important role in the prevention and early-detection of breast cancer. Due to the complexity and expensiveness of BRCA1/2 genetic testing, currently, only women with familial breast cancer may receive the entire-coding region sequencing of BRCA1/2 genes. In some ethnic populations, founder mutations are identified and these founder mutations account for the majority of mutations in the ethnic groups; genetic testing for BRCA1/2 founder mutations is simple and inexpensive. Whether high frequency founder mutations are existed in Chinese women is currently unknown. We previously found 6 recurrent mutations in 409 Chinese women with familial breast cancer patients. In the current study, we extended our sample size to find more recurrent mutations and to further identify whether these recurrent mutations are founder mutations through haplotype analysis, we also investigate whether the founder mutations affect the function of BRCA1/2 through homologous recombination assay. Our study may identify the specific founder mutations in Chinese women, therefore, we are able to screen the founder mutations in the population level and to find high-risk women, it would be very useful for prevention and early-detection of breast cancer.

BRCA1/2基因检测对于乳腺癌的预防及早诊有重要意义。目前BRCA1/2全编码序列测序成本较高，仅对家族性乳腺癌患者进行基因突变检测。BRCA1/2始祖突变是存在于特定种群中的高频率突变，始祖突变的定点检测经济快速，但在我国尚未发现高频率的始祖突变。前期，我们在409例中国家族性乳腺癌中发现了6种BRCA1/2基因的致病性重复突变，但尚不清楚它们是否是中国人群的始祖突变。本课题将在此基础上扩大样本发现更多重复突变，并使用单体型分析的方法进行始祖突变的鉴定。对鉴定出的始祖突变，通过同源重组修复实验等方法探讨对BRCA1/2功能的影响。本研究有可能发现中国人群特异性始祖突变，这有利于在大样本人群中进行始祖突变的筛查、及早发现乳腺癌高风险人群，对乳腺癌的预防、早诊具有重要意义。

BRCA1和BRCA2是重要的乳腺癌易感基因，其突变频率和突变谱因受检者的民族和地区不同而不同。我们一共在9928例非筛选性乳腺癌患者检测出181例BRCA1突变和312例BRCA2突变。其中有9种BRCA1突变和17种BRCA2突变发生次数超过4次，频率最高的突变为BRCA1 c.5470_5477del，共31例（0.3%）患者携带此突变。这些重复突变有的已在世界上其他地区有所报道，有的为我国人民所特有。为了鉴定这些突变是否为始祖突变，该研究纳入了166例携带此26种BRCA1/2突变的乳腺癌患者及50例健康人群，我们检测了其BRCA1基因内部及侧翼的9个微卫星灶标记和BRCA2基因侧翼的7个微卫星灶标记等位基因型，并进一步构建单倍体型。单倍体分析结果显示31例BRCA1 c.5470_5477del 突变携带者有共同的单倍体型，其他突变患者不携带此单倍体型，仅一例健康人携带此单倍体类型，统计分析认为该31例患者携带的共同突变来自同一祖先，该突变为始祖突变。进一步研究发现，携带BRCA1 c.5470_5477del突变的患者与不携带BRCA1/2突变的患者相比，其患病年龄更年轻，更可能有乳腺癌家族史，更可能患三阴性乳腺癌、双侧乳腺癌、病理分级高的乳腺癌。在预后方面，该突变携带者的疾病特异性生存（DSS）显著差于非携带者。此外，单倍体型分析显示所有BRCA1 c.3359_3363del（6例）, c.3916_3917del（4例）, c.5521delA（5例）突变携带者各自共享一种单倍体型，这表明这几种突变也是始祖突变。而携带其他BRCA1/2重复突变的患者，仅部分患者拥有相同的单倍体型，每种突变均源自多次、独立的突变事件。综上所述，我国乳腺癌患者BRCA1/2重复突变有的表现出始祖效应，有的源自独立突变事件，但有证据表明BRCA1始祖突变的存在。始祖突变的鉴定有可能使BRCA1/2检测更为有效、简化、廉价。