Cancer risk assessment of premalignant breast tissues from patients with BRCA mutations by genome profiling.

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.