Abstract 5871: Characterizing hormone therapy-resistance phenotypes in metastatic breast cancer conferred by estrogen receptor (ER) mutations
Authors: Jia Shanhang , Hessenius Ryan , Ngo Marilyn , Wang Peilu , Bahreini Amir , Chen Ning , Ding Zhijie , Shun Tong ying , Taylor Lansing , Puhalla Shannon , Lee Adrian , Oesterreich Steffi , Stern Andrew M. , Miedel Mark T. 
Published: July 2017

Abstract
232,000 new cases of invasive breast cancer will be diagnosed in 2016. 40,300 patients will die primarily from metastatic disease. Mortality results from the ability of cancer to evolve and evade therapy. Estrogen receptor (ER+) breast cancer accounts for 70% of invasive breast cancer. The mainstay treatment of ER+ breast cancer involves estrogen deprivation therapy using aromatase inhibitors as well as estrogen receptor antagonists and degraders. We and others have shown that patients treated with aromatase inhibitors often (14-54%) acquire ESR1 mutations in their metastases in contrast to only a 0.5% ESR1 mutation frequency detectable in their primary tumors. We hypothesize that ESR1 mutations are selected during estrogen deprivation therapy as a result of Darwinian forces of evolution and represent targetable dependencies for ER+ metastatic disease. We reasoned that characterization of the phenotype engendered by ESR1 mutations under physiologically relevant conditions will help us understand the mechanism of ER(+)metastatic cancer. To achieve this objective we have stably expressed the two most common ESR1 mutations observed in the clinic (i.e., D538G and Y537S) in a parental human breast cancer cell line (T47D) using both lentiviral transfection and CRISPR/Cas9 gene editing. We used an estrogen response element (ERE) transactivation luciferase-based reporter assay to determine the estrogen response of each ESR1 mutant expressing cell line and their respective sensitivity to approved and investigational ER antagonist drugs. Expression of each mutant confers estrogen-independent (constitutive) ERE transactivation in contrast to the parental and wild type control cells. Furthermore, partial and potentially clinically relevant resistance of these ESR1 mutant-expressing cells to ER antagonists such as fulvestrant and 4-hydroxytamoxifen was evident. In addition, using this reporter assay, mutant ESR1-expressing cell lines show similar resistance in the absence of estrogen. We are using a human microphysiological model of liver metastasis as a complementary approach to patient-derived xenograft models to investigate metastatic associated phenotypes conferred by these mutations. Since our previous studies indicated the existence of polyclonal mutations within individual patients (P.Wang, A. Bahreini et al., CCR 2016), we are testing the hypothesis that the persistence of this heterogeneity results from cooperation among these mutant-expressing clones. These studies form the basis for our continuing efforts to understand ER+ metastatic disease and use this knowledge to identify more effective therapies.
Note: This abstract was not presented at the meeting.
Citation Format: Shanhang Jia, Ryan Hessenius, Marilyn Ngo, Peilu Wang, Amir Bahreini, Ning Chen, Zhijie Ding, Tong ying Shun, Lansing Taylor, Shannon Puhalla, Adrian Lee, Steffi Oesterreich, Andrew M. Stern, Mark T. Miedel. Characterizing hormone therapy-resistance phenotypes in metastatic breast cancer conferred by estrogen receptor (ER) mutations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5871. doi:10.1158/1538-7445.AM2017-5871