Abstract PD8-09: Androgen and ESR1 mutant receptors mechanistically collaborate for overexpression of genes associated with poor outcomes in ER-positive metastatic breast cancer

Abstract Background: Recurrences are delayed for years with endocrine therapy (ET), but resistance ultimately evolves with the development of ER+ metastatic breast cancer (MBC). ESR1 mutations are acquired in MBC undergoing ET in about 20-40% of patients. We have previously demonstrated that AR overexpression confers resistance to both tamoxifen and aromatase inhibitor treatments. We discovered that AR protein is up-regulated in tumors expressing ESR1 mutations, and herein explore the functional consequences of AR-ESR1 mutant co-expression on metastatic progression of breast cancer. Objective: We hypothesize that ET resistance mechanisms driven by AR overexpression and acquisition of ESR1 mutations co-evolve under the selective pressure of therapy to drive distant metastasis. We investigated whether AR engages in novel genomic binding interactions with mutant ER to promote tumor progression. Methods: Expression profiling of CRISPR-Cas9 engineered cell lines was integrated with chromatin immunoprecipitation-sequencing (ChIP-Seq) to define the genomic distribution of ER and AR binding sites. Genomic distribution of AR-ER co-occupied binding sites from MBC patient tumors, and a gene expression signature associated with patient outcomes was generated using using Cox Regression analysis to calculate Hazard Ratios for recurrence free survival from gene expression in the KMPlotter Breast Cancer Dataset and signature validation by Kaplan Meier analysis in the ER+ METABRIC patient cohort. Results: AR protein was post-transcriptionally elevated in ESR1 mutant cells, including distant metastatic tumors from ESR1 mutant xengraft models. Transcriptome data showed significant elevation of Cancer Hallmark pathways, including epithelial-mesenchymal transition (EMT) and estrogen response, but down-regulation of androgen response genes. However, both ER and AR functions were constitutively activated, independent of their respective ligands in ESR1 mutant-expressing models. AR was disproportionally re-distributed to selected enhancer genomic regions, but also heterochromatin in ESR1 mutant compared to cells with WT ER in the presence of estrogen. In addition, AR-ER co-occupied sites were increased in enhancer regions in ESR1 mutant cells, and these sites best discriminated distal ER enhancer regions in non-responder patients and MBC. . The expression of genes co-bound by AR and ER were significantly associated with shorter recurrence-free and overall survival in ER+ breast cancer. Treatment with AR agonists blocked distant metastasis of ESR1 mutant PDX models in vivo, in part due to reduction in ER protein and ER binding to select genomic sites. Conclusions: Our results demonstrate that unliganded AR collaborates with ESR1 mutations to regulate genes associated with the metastasis of ER+ breast cancer. We hypothesize that in an ESR1 mutant genomic background, AR acquires ER co-activator functions that are blocked with AR agonists, suggesting a unique hormonal therapeutic vulnerability in tumors with ESR1 mutations. Citation Format: Suzanne Fuqua, Thomas L Gonzalez, Guowei Gu, Sarah Herzog, Yassine Rechoum, Amanda Beyer, David Edwards, Hangqing Lin, BAsil Paul, Ralf Kittler, Nancy Weigel, Sandra Grimm, Cristian Coarfa, Tasneem Bawa-Khalfe. Androgen and ESR1 mutant receptors mechanistically collaborate for overexpression of genes associated with poor outcomes in ER-positive metastatic breast cancer [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PD8-09.