Epigenetic inactivation of BRCA1 through promoter hypermethylation and its clinical importance in triple-negative breast cancer

Background Triple-negative breast cancer (TNBC) has many similarities with basal-like breast cancer. Additionally, TNBCs are associated with Breast cancer susceptibility gene I (BRCA1) functional loss, which leads to impaired homologous recombination-mediated DNA repair. Although somatic mutations in BRCA1 rarely occur in sporadic breast cancer, lower than normal rates of expression of BRCA1 is reported to be an important factor that contributes to tumorigenesis in sporadic tumors. The epigenetic inactivation of BRCA1 expression might thus play an important role in sporadic breast cancer cases. Patients and Methods Breast cancer specimens were obtained from 69 TNBC and 161 non-TNBC patients who underwent surgery without neoadjuvant systemic therapy. BRCA1 promoter methylation status was investigated using combined bisulfite and restriction analysis. BRCA1 mRNA expression was evaluated using quantitative reverse transcriptase polymerase chain reaction and BRCA1 protein expression was assessed using immunohistochemistry. Results BRCA1 promoter methylation was found in 11 tumors and all of these were in TNBC cases (P <.0001). BRCA1 promoter methylation was significantly associated with lymphovessel invasion (P =.02), high nuclear grade (P =.05), low BRCA1 mRNA expression (P <.0001), and loss of BRCA1 protein expression (P =.0015). BRCA1 promoter methylation was significantly associated with shorter overall survival (P =.038). Conclusion BRCA1 promotor methylation was found only in TNBC cases and the methylated cases account for 16% of TNBC. BRCA1 promoter methylation was significantly associated with reduced BRCA1 expression, aggressive phenotype, and poor prognosis. BRCA1 promoter methylation is an important mechanism that leads to functional loss of BRCA1.