Promoter hypermethylation and BRCA1 inactivation in sporadic breast and ovarian tumors

Background: Inherited mutations in the BRCA1 gene may be responsible for al most half of inherited breast carcinomas. However, somatic (acquired) mutat ions in BRCA1 have not been reported, despite frequent loss of heterozygosi ty (LOH or loss of one copy of the gene) at the BRCA1 locus and loss of BRC A1 protein in tumors. To address whether BRCA1 may be inactivated by pathwa ys other than mutations in sporadic tumors, we analyzed the role of hyperme thylation of the gene's promoter region. Methods: Methylation patterns in t he BRCA1 promoter were assessed in breast cancer cell lines, xenografts, an d 215 primary breast and ovarian carcinomas by methylation-specific polymer ase chain reaction (PCR), BRCA1 RTA expression was determined in cell lines and seven xenografts by reverse transcription-PCR, P values are two-sided. Results: The BRCA1 promoter was found to be unmethylated in all normal tis sues and cancer cell lines tested, However, BRCA1 promoter hypermethylation nas present in tao breast cancer xenografts, both of which had loss of the BRCA1 transcript. BRCA1 promoter hypermethylation aas present in 11 (13%) of 84 unselected primary breast carcinomas. BRCA1 methylation was strikingl y associated with the medullary (67% methylated; P = .0002 versus ductal) a nd mucinous (55% methylated; P = .0033 versus ductal) subtypes, which are o verrepresented in BRCA1 families. In a second series of 66 ductal breast tu mors informative for LOH nine (20%) of 45 tumors with LOH had BRCA1 hyperme thylation, while one (5%) of 21 without LOB ass methylated (P = .15). In ov arian neoplasms, BRCA1 methylation was found only in tumors with LOH, four (31%) of 13 versus none of 18 without LOH (P = .02). The BRCA1 promoter was unmethylated in other tumor types. Conclusion: Silencing of the BRCA1 gene by promoter hypermethylation occurs in primary breast and ovarian carcinom as, especially in the presence of LOH and in specific histopathologic subgr oups. These findings support a role for this tumor suppressor gene in spora dic breast and ovarian tumorigenesis.