Brca1 haploinsufficiency promotes early tumor onset and epigenetic alterations in a mouse model of hereditary breast cancer (ATAC-Seq)

Carriers of germline BRCA1 mutations face a high risk of developing breast cancer; however, the mechanisms underlying this tumor predisposition are not completely understood. Here, using a newly developed genetically engineered mouse model for germline Brca1 heterozygosity that allows spatiotemporal control of full Brca1 inactivation, we demonstrate that the early tumor onset in a Brca1-heterozygous background cannot be fully explained by the conventional “two-hit” hypothesis. Specifically, when both Brca1-heterozygous and wild-type conditional mice are induced to Brca1 nullizygosity simultaneously, heterozygous mice develop tumors sooner, suggesting the existence of inherent tumor-promoting alterations in the Brca1-heterozygous state prior to tumor formation. Single-cell RNA-seq and ATAC-seq analyses uncover a unique set of differentially accessible chromatin regions in ostensibly normal Brca1-heterozygous mammary epithelial cells, distinct from wild-type cells and partially mimicking the chromatin and RNA-level changes in tumor cells, thereby pointing to an epigenetic priming mechanism for promoting early tumor onset in the Brca1-heterozygous background. Transcription factor analyses of the altered chromatin regions identify loss of ELF5 and gain of AP-1 binding sites in association with these epigenetically primed chromatin regions, and in vivo experiment reveals that Brca1-heterozygous mice are reliant on the AP-1 pathway for accelerated tumorigenesis. Finally, analysis of downstream target genes identifies Wnt10a as a candidate driver of early tumor onset, and functional experiments demonstrate that it activates the WNT pathway and is sufficient to substantially accelerate tumor formation. Taken together, these findings provide the first in vivo demonstration of a role of BRCA1 haploinsufficiency in accelerating tumor formation and implicate AP-1 and WNT10A as promoters of tumor formation in BRCA1-related breast cancer, advancing our understanding of the disease and informing potential strategies for therapeutic intervention.