SUN-010 BRCA1 Mutation Influences Progesterone Response in Human Benign Mammospheres

Abstract

Women, who carry a germline mutation in the breast cancer-associated gene 1 (BRCA1) have an increased risk (60-85%) of developing breast cancer during their lifetime. While BRCA1 mutated patients usually develop triple-negative, basal like, aggressive breast tumors, hormone signaling is important in the genesis of BRCA1 mutant breast cancers. Estrogen and progesterone promote carcinogenesis in the breast. Progesterone promotes proliferation of both benign epithelial cells and cancer cells and can expand the mammary stem cell pool. It is unclear how PR functions in the background of BRCA1 mutation in normal non-cancerous mammary epithelial cells before breast cancer develops. In order to determine whether BRCA1 mutation affects hormone response in benign mammary cells, human mammospheres from patients with wild type (WT; n=6) or BRCA1 mutation (n=6) were treated in vitro with a stepwise menstrual cycle hormone regimen of estrogen (E2) and progesterone (P4) over the course of 28 days. During the last 14 days of treatment, the SPRM, telapristal acetate (TPA) was added to the mammospheres. The mammospheres grown from benign breast tissues retained the expression of hormone receptors (ER and PR) even after the 28 day hormone treatment in vitro. Human mammary epithelial cells expressed aldehyde dehydrogenese-1 ALDH-1 with increasing levels observed in the BRCA1 mutated mammospheres compared to the WT mammospheres. RNA sequencing (RNA-seq) analysis revealed distinct differentially expressed genes between the WT and BRCA1 groups. Specifically, genes associated with extracellular matrix (ECM) organization were differentially regulated by TPA in the BRCA1 mutation group but not in the WT group. In fact, these ECM genes were differentially expressed in response to E2+P4 in WT versus BRCA1 mammospheres. In addition, cell cycle genes were responsive to TPA but in the WT group only. Taken together, these data show that BRCA1 mutation influences the hormone response and in particular to TPA compared to WT cells. Our physiologically relevant model systems have provided important insights into the role of progesterone in BRCA1 mutated breast cells. Further analysis of the molecular mechanism of BRCA1 and PR crosstalk is warranted using this model system.