Abstract P3-04-08: The role of HER2 mutations in resistance to endocrine therapy in ER+ breast cancer

Resistance to endocrine therapies in estrogen receptor positive (ER+) metastatic breast cancer is widespread, and understanding the mechanisms whereby these tumors acquire resistance is a critical need. Through whole-exome sequencing of metastatic tumor biopsies from patients with endocrine resistant ER+ metastatic breast cancer, we identified 13 different HER2 mutations, including five in the kinase domain, four in the signaling domain, three in the extracellular domain, and one in the transmembrane region of the protein. Two of the kinase domain mutations (L755S and V777L) have been previously described and shown to be activating and resistant to reversible anti-HER2 targeted therapies; the remaining mutations have not been reported. In several of these patients, whole exome sequencing of a pre-treatment primary tumor did not identify the HER2 mutations seen in the corresponding metastatic tumor, suggesting that they were acquired during therapy. To examine the role of HER2 mutations in endocrine resistance, we generated ER+ breast cancer cell lines (MCF7 and T47D) stably expressing the HER2 mutants observed in our clinical data. Several mutants promoted enhanced growth in charcoal dextran-stripped media, which lacks estradiol and mimics treatment with aromatase inhibitor. In addition, several mutants conferred varying degrees of resistance to fulvestrant and tamoxifen. Taken together, these results suggest that HER2 mutations are associated with acquired resistance to endocrine therapies in patients with ER+ breast cancer. The ability of irreversible anti-HER2 agents as well as other agents that target the HER2 pathway to overcome this resistance is being tested for individual HER2 mutations in vitro . The results from these studies may provide a clinical rationale for therapeutic combination strategies in patients with refractory tumors that have acquired endocrine resistance through HER2 mutations. Citation Format: Nayar U, Cohen O, Oh C, Wagle N. The role of HER2 mutations in resistance to endocrine therapy in ER+ breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-08.