Women harboring heterozygous germline mutations of BRCA2 have a 50-80% risk of developing breast cancer, yet the early pathogenesis of these cancers is poorly understood. We sought to reveal early steps in BRCA2-associated carcinogenesis through analysis of sorted cell populations from freshly-isolated, non-cancerous breast tissues among a cohort of BRCA2 mutation carriers and matched controls. Single-cell whole-genome sequencing demonstrates that >25% of BRCA2 carrier (BRCA2mut/+) luminal progenitor (LP) cells exhibit sub-chromosomal copy number variations (CNVs), which are rarely observed in non-carriers. Correspondingly, primary BRCA2mut/+ breast epithelia exhibit spontaneous and replication stress-induced DNA damage together with attenuated replication checkpoint and apoptotic responses, associated with an age-associated expansion of the LP compartment in human carrier tissues. These phenotypes are not associated with loss of wild-type BRCA2. Collectively, these findings provide evidence for BRCA2 haploinsufficiency and associated DNA damage in vivo that precede histologic abnormalities. These results provide unanticipated opportunities for new cancer risk assessment and prevention strategies in high-risk patients.
