7324 Regulation of Super Enhancers by Phosphorylated Progesterone Receptors Drives Breast Cancer Stem Cell Expansion

Abstract Disclosure: N. Gillis: None. C.A. Lange: None. Current treatments for estrogen receptor (ER) positive breast cancer largely target rapidly dividing tumor cells. However, aggressive breast cancers often contain long-lived and weakly proliferative cancer stem cells (CSCs) that readily escape treatments aimed at cycling cells. We previously reported that progesterone receptors (PR) and their target genes aid the growth of these CSCs and help them evade anti-estrogen therapy. As proliferation and stemness are directed by separate signaling pathways that lead to the expression of distinct gene programs, it's crucial to understand how PR facilitates this cellular transformation at the genomic level. Establishing a gene expression program that may limit proliferation but encourages stemness requires coordination between transcription factors and their coregulators to alter the nuclear microenvironment and chromatin landscape.To reveal these mechanisms, we profiled PR and ER genomic binding using CUT&RUN in the T47D luminal breast cancer cell line grown in ultra-low attachment (3D) mammosphere conditions. Our analysis of genomic binding profiles displayed a pattern of PR/ER co-occupancy at distant sites more than 10 kilobases away from the nearest transcriptional start site. We used Rank Ordering of Super Enhancer (ROSE) analysis on these PR/ER binding sites to predict which are likely super enhancer elements capable of coordinating the regulation of multi-gene subsets. We employed a genetic approach to directly test the function of novel enhancer sites linked to PR-target genes by disrupting the PR/ER binding with a dCas9-KRAB construct directed to the predicted enhancer elements. We propose that abnormal growth factor signaling in breast cancer cells permits PR/ER complex formation which promotes expression of genes that induce endocrine resistance and CSC expansion through regulation of super enhancer elements. Further studies to elucidate these mechanisms are ongoing. Understanding the intricacies of PR/ER crosstalk is the crucial next step in developing new therapies that target PR-driven processes in ER+ breast cancers. Presentation: 6/3/2024