Constitutional BRCA1 methylation and risk of incident triple-negative breast cancer and high-grade serous ovarian cancer.

Abstract

10509 Background: About 25% of all triple-negative breast cancer (TNBC) and 10–20% of high-grade serous ovarian cancers (HGSOC) harbor BRCA1 promoter methylation. While constitutional BRCA1 promoter methylation has been observed in normal tissues of some individuals, the potential role of normal tissue methylation as a risk factor for incident TNBC or HGSOC risk is unknown. Methods: The objective of this study was to assess potential correlation between white blood cell (WBC) BRCA1 promoter methylation and subsequent risk of incident TNBC and HGSOC. To do so, we analyzed samples from women participating in the Women’s Health Initiative (WHI) study who had not been diagnosed with either breast or ovarian cancer prior to study entrance. A total of n = 636 women developing incident TNBC and 509 women developing incident HGSOC were matched with cancer-free controls (n = 1838 and 2979) in a nested case-control design. Cancers were confirmed after central medical record review. Blood samples, collected at entry, were analyzed for BRCA1 promoter methylation by massive parallel sequencing. Associations between BRCA1 methylation and incident TNBC and incident HGSOC were analyzed by unconditional logistic regression. Results: Age at entry was 62 (median; range 50 to 79) years, with median interval to diagnosis of 9 (TNBC) and 10 (HGSOC) years. The presence of methylated BRCA1 alleles was significantly associated with higher risk of incident TNBC (OR 2.44, 95% CI 1.79–3.33; P <.001) and incident HGSOC (OR 1.87, 95% CI 1.32–2.61; P <.001). Restricting analyses to individuals with > 5 years between sampling and cancer diagnosis yielded similar results (> 5 years; TNBC: OR 2.53, 95% CI 1.81–3.54; P <.001; HGSOC: OR 1.81, 95% CI 1.21–2.63; P =.003). Across individuals, methylation was not haplotype-specific, arguing against an underlying cis-acting factor. Within individuals, BRCA1 methylation was observed on the same allele, indicating clonal expansion from a single methylation event. Conclusions: Constitutional normal tissue BRCA1 promoter methylation is significantly associated with risk of incident TNBC and HGSOC with implications for prediction of these cancers not associated with germline mutations. These findings warrant further research to determine if constitutional methylation of tumor suppressor genes are pan-cancer risk factors.