Abstract 3376: Reprogramming the estrogen signaling network is a potential mechanism for human breast tumorigenesis

Background: The estrogen receptor (ER) is expressed in approximately 70% of sporadic breast cancers and is known to activate genes driving cell proliferation. Much work in the field has characterized the mechanisms of ER signaling behind cancer progression and eventual metastasis, and therapeutic targeting of the ER signaling pathway has proven effective. However, little is known regarding the role of ER in the initial process of transformation. Since ER possesses pro-differentiation roles in normal breast development yet becomes one of the drivers of breast cancer, altered ER function is then likely a crucial early step. We seek to investigate the role of ER from the perspective of the normal breast to allow for mechanistic insights into breast tumorigenesis. Methods: To model the normal breast epithelium, viable tissue specimens from patients undergoing reduction mammoplasty were obtained and enriched for breast epithelial cells via fluorescence-activated cell sorting (FACS). Fresh-frozen ER+ breast tumor samples were then obtained for comparison. RNA-seq, ChIP-seq for ER, and genome-wide CRISPR screening in the presence of estrogen stimulation in both normal breast and breast tumor were performed. These various modalities of genomic data were then integrated and analyzed to identify patterns in breast tumorigenesis. Results: Gene expression profiling via RNA-seq of breast epithelial cells from seven reduction mammoplasties reveals a drastically different and muted normal response to estrogen stimulation. Genes that promote cell cycling and cell proliferation were down-regulated in normal breast but were up-regulated in breast cancer cells. Furthermore, differential ER binding alone via ER ChIP-seq is capable of segregating ten primary breast tumors from seven normal breast tissues with very few overlapping sites. Deeper motif analysis reveals the enrichment of novel transcription cofactor GRHL2 in the binding sites most closely associated to ER+ breast cancer. Further findings via genome-wide CRISPR screens reveal GRHL2 as an essential gene in luminal ER+ breast cancer with predictive ability in cancer risk and prognosis. Conclusions: Cellular reprogramming of the ER signaling network may alter the functioning phenotype of normal mammary epithelial cells and offer insights into the series of events necessary for developing breast cancer. Notably, ER activation does not promote cellular proliferation in normal mammary epithelial cells, and the interaction with transcriptional cofactors such as GRHL2 can be a driving mechanism for breast tumorigenesis. Unraveling the differential ER signaling in normal mammary epithelium and breast cancer will enhance our understanding of breast cancer and aid the development of more effective prevention strategies and targeted therapeutics. Citation Format: David Chi, Hari Singhal, Lewyn Li, Henry W. Long, Judy E. Garber, Myles A. Brown. Reprogramming the estrogen signaling network is a potential mechanism for human breast tumorigenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3376. doi:10.1158/1538-7445.AM2017-3376