Acquired HER2 mutations in ER+ metastatic breast cancer confer resistance to ER-directed therapies

Estrogen receptor positive (ER+) breast cancers that develop resistance to therapies that target the ER are the most common cause of breast cancer death. Beyond mutations in ER, which occur in 25-30% of patients treated with aromatase inhibitors (AIs), our understanding of clinical mechanisms of resistance to ER-directed therapies remains incomplete. We identified activating HER2 mutations in metastatic biopsies from eight patients with ER+ metastatic breast cancer who had developed resistance to ER-directed agents, including AIs, tamoxifen, and fulvestrant. Examination of treatment-naïve primary tumors in five patients revealed no evidence of pre-existing mutations in four of five patients, suggesting that these mutations were acquired under the selective pressure of ER-directed therapy. These mutations were mutually exclusive with ER mutations, suggesting a distinct mechanism of acquired resistance to ER-directed therapies. In vitro analysis confirmed that these mutations conferred estrogen independence. In addition, and in contrast to ER mutations, these mutations resulted in resistance to tamoxifen, fulvestrant, and the CDK4/6 inhibitor palbociclib. Resistance was overcome by combining ER-directed therapy with the irreversible HER2 kinase inhibitor neratinib, highlighting an effective treatment strategy in these patients. Examination of the transcriptional output (mRNA) of the HER2 activating mutations compared with controls under various drugs. Specifically, we expressed the activating mutations S653C, L755S, V777L, and L869R in ER+/HER2- breast cancer cell line (T47D), and controls (GFP, wild-type HER2, kinase-dead HER2, and ESR1 Y537S). Cell were then treated with DMSO, 1μM fulvestrant, 1μM neratinib, 10μM palbociclib, 1μM fulvestrant + 1μM neratinib, or 1μM fulvestrant + 10μM palbociclib for 24 hours. All experimental conditions were done in 6 replicates, sequenced in 3 replicates