Abstract P3-05-07: Determining the role of somatic ERα mutations in acquired hormone (or SERM) resistance
Authors: Fanning Sean W , Mayne Christopher , Toy Weiyi , Shen Yang , Sharma Abhishek , Panchamukhi Srinivas , Nowak Jason , Nettles Kendall W , Chandarlapaty Sarat , Katzenellenbogen John A , Greene Geoffrey L 
Published: May 2015

Abstract
The estrogen receptor alpha (ERα) is a member of the nuclear hormone receptor (NHR) family and is critical for the etiology and treatment of breast cancer. Approximately 70% of breast cancers express ERα and many of these are sensitive to anti-estrogen therapies. Selective estrogen receptor modulators (SERMs), such as tamoxifen and raloxifene, are approved to treat or reduce the risk of ER-dependent breast cancers. SERMs act by competitively binding to the ERα ligand-binding domain (LBD). Unfortunately, metastatic breast tumors recur in approximately half of patients and become SERM resistant while remaining ER-positive in many cases. Recently, conserved somatic mutations in the ERα LBD were identified in patients who received SERM/aromatase inhibitor (AI)/selective estrogen receptor disruptor (SERD) therapy for an average of five years. Because these mutations were observed in approximately 25% of tumors and the most frequent mutations (Y537S or D538G) were located in or just prior to helix 12 (H12), the molecular switch that controls AF-2 activity, they represent a possible mechanism for acquired SERM insensitivity for a significant population of patients. Further studies revealed that these mutations conferred hormone-independent ERα activity and that the inhibitory efficacy of currently approved SERMs was reduced. Our goal is to understand the effects of these mutations on the structure and function of ERα in these tumors to guide the generation of novel compounds which bypass the effects of these somatic mutations. Here, we employ x-ray crystallography, molecular dynamics simulations, biochemical assays in addition to breast cancer cell proliferation assays to dissect the role of somatic mutation in acquired hormone/SERM resistance. X-ray crystal structures of the ERα LBD D538G mutant in the unliganded (apo), agonist and SERM-bound states, combined with molecular dynamics simulations, reveal a stabilized loop between H11 and H12 that allows the receptor to preferentially adopt an agonist conformation versus an antagonist conformation. The biochemical and breast cancer cell proliferation assays reveal structural insights that may explain mutant ERα function within the tumor. Further, we use these methods to explore the utility of next generation SERMs and SERDs to inhibit these mutant ERs as well as to guide the synthesis of additional novel compounds. Importantly, our work is expected to yield more potent and effective SERMs/SERDs that overcome the impact of acquired activating mutations and result in improved patient survival.
Citation Format: Sean W Fanning, Christopher Mayne, Weiyi Toy, Yang Shen, Abhishek Sharma, Srinivas Panchamukhi, Jason Nowak, Kendall W Nettles, Sarat Chandarlapaty, John A Katzenellenbogen, Geoffrey L Greene. Determining the role of somatic ERα mutations in acquired hormone (or SERM) resistance [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-07.