ESR1 Mutation Status as a Biomarker for Radiation Response in ER-Positive Breast Cancer

Abstract

Acquired mutations in oncogenic drivers in response to systemic therapies is a common mechanism of treatment resistance. How such mutations alter response to radiation therapy is poorly understood. Acquired mutations in the gene encoding for Estrogen Receptor (ESR1) are one of the most common mechanisms of endocrine therapy resistance in breast cancer. The goal of this study is to define the role of ESR1 mutations in altering response to therapeutic radiation in breast cancer and to develop therapeutic approaches to overcome such resistance. To test if ESR1 mutations influence response to radiation therapy, we employed ER-positive MCF7 and T47D cell lines which have been genome edited to knock in two of the most common ESR1 mutations found in breast cancer patients (Y537S and D538G). Clonogenic survival assays were carried out to characterize differences in radiation sensitivity of these cells. γ-H2AX and Comet assays were employed to study the differences in residual unrepaired DNA damage following radiation in these cell lines. MCF7 cell line-derived xenografts harboring the wild type or mutant ESR1 were employed to study differences in radiation resistance of these tumors in vivo. ER-positive breast cancer cell lines harboring ESR1 mutations exhibit resistance to radiation compared to the corresponding isogenic cell lines with the wild type ESR1. Following exposure to radiation, the ESR1 mutant cells have significantly less residual, unrepaired DNA damage. We also show an endogenous interaction between mutant ER proteins and BRD4, a bromodomain and extraterminal domain (BET) family protein. Recent studies have established a key role for BRD4 in repairing DNA double strand breaks via non-homologous end joining pathway. Pharmacologic BET inhibition with OTX015 overcomes ESR1 mutation-induced resistance to therapeutic radiation in breast cancer, both in vitro and in vivo. ESR1 mutations, which confer resistance to endocrine therapies, also confer resistance to radiation therapy. Recent studies show that ESR1 mutations are also enriched in non-metastatic, ER-positive breast cancer patients receiving neoadjuvant endocrine therapy. Our findings suggest that patients who develop ESR1 mutations under selective pressure of neoadjuvant endocrine therapy may respond poorly to subsequent radiation therapy and may experience increased risk of locoregional failure. In fact, a recent study reported enrichment of ESR1 mutations in locoregionally recurrent breast cancers. Our findings support prospective clinical studies for personalization of radiation treatments in breast cancer based on ESR1 mutation status.