Differential Genomic Interactions Drive Progesterone Receptor Isoform Specific Functions in Breast Cancer

Progesterone is critical for normal breast development and function, and has been shown to stimulate proliferation of normal breast epithelial cells by increasing stem and progenitor cell numbers. Breast cancer incidence is increased in women exposed to progesterone analogues in combined estrogen plus progestin hormone replacement therapy, but not in women taking estrogen alone. Classical progesterone signaling is mediated through the nuclear progesterone receptor (PR), which occurs as two related but functionally different isoforms, PRA and PRB. PRA and PRB are co-expressed equally in normal breast tissue but become dysregulated in breast cancer where PRA often becomes predominant. PRA predominance in breast cancer is associated with poorer outcome and higher risk of distant metastasis in tamoxifen treated patients. We show using integrated analysis of ChIP-seq, ATAC-seq and transcriptomic profiling in a breast cancer cell line model of acquired PRA predominance that: 1) PRA and PRB have different requirements with regard to chromatin accessibility; 2) PRA predominance reshapes the PR cistrome and the associated transcriptome to affect genes not normally regulated by PR when PRA and PRB are equivalently expressed, possibly through assisted loading with multiple other transcription factors; 3) Genes regulated by PR only when PRA is predominant are associated with poorer breast cancer outcome and involved in multiple cancer-associated pathways including those that regulate cell proliferation and adhesion. Our data suggest a mechanism for the poorer disease outcome seen in breast cancers with a predominance of PRA.