Abstract PD1-05: Transgenerational epigenetic silencing of BRCA1 due to a germline variant unmasks a new mechanism for familial breast and ovarian cancer

Abstract

Abstract In families with multiple affected individuals with early-onset breast/ovarian cancer pathogenic variants in BRCA1 or BRCA2 are identified in approximately 20% of the cases. Extensive efforts have been made to identify additional highly penetrant breast cancer genes or alternative mutational mechanisms affecting BRCA1 and BRCA2 to explain the missing heritability. It has been proposed that part of the missing heritability may be explained by gene silencing due to promoter methylation of cancer associated genes, as described in colorectal cancer (MLH1 and MSH2). Here, for the first time, we report two independent families with multiple individuals affected by breast and ovarian cancer with transgenerational promoter methylation of BRCA1. RNA analysis of BRCA1 in the germline of breast/ovarian families, previously found to be BRCA1/2 mutation negative by Sanger sequencing and copy number analysis, identified two families with allelic loss of expression. To investigate the mechanism of transcriptional silencing, a total of 14 affected and unaffected family members from these two families were tested for BRCA1 promoter methylation by pyrosequencing in blood, buccal, and hair follicle cells. Allele specific methylation was determined by clonal bisulphite sequencing. BRCA1 promoter methylation in all three germ layers was present in 11 of 14 family members. Of the 7 women with promoter methylation five were affected with grade 3 breast/high grade serous ovarian cancer. The four males with BRCA1 promoter methylation had no history of cancer. Methylation levels were ˜50%, consistent with the silencing of one allele detected in RNA in these family members. Clonal bisulphite sequencing of an affected family member of each family confirmed that the alternative allele was specifically methylated. Interestingly, in both families the methylation pattern of the BRCA1 promoter segregated with the same novel heterozygous variant in the 5'UTR of BRCA1. These results indicate a novel mechanism for familial breast/ovarian cancer, caused by epigenetic silencing of one allele by transgenerational hypermethylation of the BRCA1 promoter, secondary to a variant in cis of BRCA1. We propose that methylation analyses are indicated in all families affected by early onset breast/ovarian cancer where standard mutation screening of BRCA1/2 has not identified a causative variant. Citation Format: van Veen EM, Smith MJ, Byers HJ, Wallace AJ, Lalloo FI, Newman WG, Evans DGR. Transgenerational epigenetic silencing of BRCA1 due to a germline variant unmasks a new mechanism for familial breast and ovarian cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD1-05.