携带 BRCA1/2 基因种系致病性变异的患者特别容易患乳腺癌。目前还没有开发出临床测试来准确、定量地评估他们患乳腺癌的风险。我们假设异常细胞克隆扩增可能在正常乳腺组织中开始，而没有表现出病理变化。为了评估正常乳房中克隆扩张的发生率，我们收集了 24 名接受手术切除的乳腺癌患者和 5 名接受预防性乳房切除术的致病性 BRCA1/2 变异携带者的正常乳腺组织。对 14 名个体的 97 份样本进行了全外显子组测序 (WES)，对 26 名个体的 321 份样本进行了 TOP panel（针对 464 个基因的基因 panel），其中包括 8 名具有 BRCA1/2 基因种系致病性变异的个体。在正常乳腺组织中以相当大的变异等位基因频率 (VAF) 鉴定出 PIK3CA、ARHGAP35、HRAS 和 NF1 内的复发性致癌突变，表明克隆扩张。此外，使用针对 25 个基因的乳腺癌面板 (BCP) 对 937 个正常乳腺组织进行了评估，以确定克隆扩增的确切患病率和分布。为了评估克隆扩增，我们开发了克隆性评分，它是从给定乳房获得的样本的克隆细胞分数的平均值。宏观正常乳腺组织的平均克隆性评分为0.95（0-2.46），有和没有2期或以上晚期乳腺癌病史的病例之间存在显着差异（p = 0.01）。对42个克隆大小相对较大（VAF > 3%）的样本进行额外的WES证实，这些细胞克隆含有多个突变（10.7个突变/样本），并且现有突变的数量与克隆大小一致（R = 0.50）。结果表明，即使在病理学和/或放射学检测到癌症之前，乳腺癌高风险女性的正常乳腺组织中也发生了克隆变化，克隆性评分显示出有可能成为评估癌症风险克隆扩增的有效方法为乳腺癌高危患者提供适当预防选择的评估。© 2024。作者。

Patients with germline pathogenic variants of BRCA1/2 genes have a particular predisposition to develop breast cancer. No clinical test has been developed to accurately and quantitatively evaluate their risk of developing breast cancer. We hypothesized that aberrant cell clonal expansion may be initiated in normal breast tissues without manifesting pathologic changes. To assess the prevalence of clonal expansion in the normal breast, we collected normal breast tissue from 24 breast cancer patients who had undergone surgical resection and 5 carriers of pathogenic BRCA1/2 variant who had undergone prophylactic mastectomy. Whole-exome sequencing (WES) was conducted in 97 specimens from 14 individuals, and TOP panel, a gene panel targeting 464 genes, was conducted in 321 specimens from 26 individuals, including 8 individuals with germline pathogenic variants of BRCA1/2 genes. Recurrent oncogenic mutations within PIK3CA, ARHGAP35, HRAS, and NF1 were identified in normal breast tissue at considerable variant allelic frequencies (VAF), suggesting clonal expansion. In addition, 937 normal breast tissues were evaluated using the Breast Cancer Panel (BCP) targeting 25 genes to determine the exact prevalence and distribution of clonal expansion. To assess the clonal expansion, we developed the clonality score, which is the mean value of clonal cell fractions for samples obtained from a given breast. The average clonality score in macroscopically normal breast tissue was 0.95 (0-2.46), with a significant difference between cases with and without a history of breast cancer of stage 2 or more advanced stage (p = 0.01). Additional WES on 42 samples with relatively large clone size (VAF > 3%) confirmed that these cell clones harbored multiple mutations (10.7 mutations/sample), and the number of existing mutations was consistent with the clone size (R = 0.50). The results suggest that clonal changes occur in normal breast tissue of women at high risk for breast cancer even before cancer is detected pathologically and/or radiologically, and the clonality score shows the potential to be a valid method of evaluating clonal expansion for cancer-risk assessment that provides appropriate preventive options for patients at high risk for breast cancer.© 2024. The Author(s).