Abstract PO3-29-03: An Emerging Class of ER mutations Enhances ER Dimerization and Promotes ER Activity

Abstract Physiological activation of estrogen receptor alpha (ERα) requires the binding of estradiol (E2) to the ligand binding domain (LBD) of the receptor. This interaction triggers a repositioning of helix 12 (H12), facilitating the recruitment of coactivator proteins to the unoccupied coactivator binding groove. However, in the context of breast cancer, point mutations of ERα at H12 lead to its repositioning and self-activation in the absence of E2. Through MSK clinical sequencing breast cancer cohort (MSK-IMPACT), we assayed 8302 samples of ER+ breast cancer patients and identified 649 mutations within the LBD of ERα. 471 of these mutations (73%) were located at H11-H12 loops (D538, Y537, L536), while the remaining 27% were located near the dimer interface of ERα (V422, G442, F461, S463P, L469). Through in vitro biochemical and cell assays, we identified that these mutations promote dimer formation and hormone-independent transcriptional activity. Tumors expressing these mutations exhibited significantly accelerated growth compared to their wild-type controls. Unlike Y537S, cells harboring mutations at the dimer interface maintain their sensitivity to Selective Estrogen Receptor Degraders (SERDs), including Fulvestrant, recently FDA-approved Elacestrant, and Camizestrant, as well as Selective Estrogen Receptor Modulators (SERMs) including Tamoxifen and Raloxifene. By utilizing machine learning and computational structural analyses, we uncovered that these mutants, unlike Y537 repositions H12, act distinctly through conformational change across the ERα dimer interface. Further cell assays proved that the dimerization leads to increased stability of the receptor and facilitates its nuclear localization, which is required for ERα activation. Collectively, our finding unveiled a new class of ER mutations that enforce receptor dimerization and activation of the ER signaling pathway. The discovery opens up a new therapeutic interventional possibility, suggesting that targeting dimerization could emerge as a new strategy to combat these malignancies. Citation Format: Seema Irani, Wuwei Tan, Qing Li, Weiyi Toy, Catherine Jones, Mayur Gadiya, Antonio Marra, John Katzenellenbogen, Kathryn Carlson, Benita S. Katzenellenbogen, Yang Shen, Sarat Chandarlapaty. An Emerging Class of ER mutations Enhances ER Dimerization and Promotes ER Activity abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-29-03.