Estrogen receptor alpha somatic mutations Y537S and D538G confer breast cancer endocrine resistance by stabilizing the activating function-2 binding conformation

Abstract

Somatic mutations in the estrogen receptor alpha (ERα) gene (ESR1), especially Y537S and D538G, have been linked to acquired resistance to endocrine therapies. Cell-based studies demonstrated that these mutants confer ERα constitutive activity and antiestrogen resistance and suggest that ligand-binding domain dysfunction leads to endocrine therapy resistance. Here, we integrate biophysical and structural biology data to reveal how these mutations lead to a constitutively active and antiestrogen-resistant ERα. We show that these mutant ERs recruit coactivator in the absence of hormone while their affinities for estrogen agonist (estradiol) and antagonist (4-hydroxytamoxifen) are reduced. Further, they confer antiestrogen resistance by altering the conformational dynamics of the loop connecting Helix 11 and Helix 12 in the ligand-binding domain of ERα, which leads to a stabilized agonist state and an altered antagonist state that resists inhibition.

Discovery of Selective Estrogen Receptor Covalent Antagonists for the Treatment of ERαWT and ERαMUT Breast Cancer

“…S7A). These values are in agreement with a recent study [24] but are significantly lower than values published in previous studies [14,25], likely due to differences in experimental design. Based on the precise determination of the affinity for estradiol, we determined that the K i value for non-covalent H3B-9224 was 2-fold higher than 4-OHT for ER WT and ER Y537S ( Supplementary Fig.…”

“…Mechanistically, the Y537 and D538 mutations are located in the AF-2 helix of ER and introduce a stabilizing interaction to shift the dynamic equilibrium towards the agonist conformation even in the absence of ligand [14,15]. We reasoned that for a compound to be more active in the mutant setting, it would necessarily have to overcome the stabilizing effects of the mutation and shift the equilibrium towards the destabilized antagonist conformation.…”

Are We Ready to Use ESR1 Mutations in Clinical Practice

“…This was shown in vitro as well as in vivo using breast cancer cell lines engineered to express the mutations and PDXs derived from human metastatic breast cancer tumors harboring the LBD mutations [10,11,14]. The crystal structure and simulation studies of tamoxifen bound to the D538G and Y537S mutants indicate that the ER LBD mutations lead to decreased affinity to tamoxifen and confer an altered antagonistic conformation facilitating resistance to antagonism by tamoxifen [17]. Fulvestrant also binds to the LBD; however, the structure of the mutant LBD bound to fulvestrant has not been resolved.…”

“…Conversely, antagonist binding to the LBD induces an inactive conformation of H12 that promotes co-repressor binding and inhibition of the receptor activity. The Y537S and D538G ER mutations stabilize H12 in the agonistic conformation, similar to WT-ER bound to E2 [16,17]. In addition, affinity studies showed that the LBD-mutated receptors have a decreased affinity for estradiol (E2).…”

“…These numbers are overall higher compared to the rates seen in the studies of metastatic tissue samples, and this likely due to the ability of plasma samples to capture the heterogeneous genomic landscape of multiple metastatic sites and the high sensitivity of the ddPCR assay. In addition, a number of the ESR1 ctDNA studies have now demonstrated that the presence of ESR1 mutations in metastatic disease is a poor prognostic factor [17,18].…”