Co-Occurring Gain-of-Function Mutations in HER2 and HER3 Modulate HER2/HER3 Activation, Breast Cancer Progression, and HER2 Inhibitor Sensitivity

Activating mutations in HER2 (ERBB2) drive the growth of a subset of breast and other cancers and tend to co-occur with HER3 (ERBB3) missense mutations. To characterize the role of HER3 mutations in HER2-mutant tumors, we integrated computational structural modeling with biochemical and cell biological analyses. Computational modeling predicted that the frequent HER3E928G kinase domain mutation enhances the affinity of HER2/HER3, stabilizes the HER2 active conformation, and reduces binding of HER2 to its inhibitor neratinib, potentially explaining the lower clinical response of HER2/HER3 co-mutant tumors. Co-expression of mutant HER2 and HER3 enhanced HER2/HER3 co-immunoprecipitation and ligand-independent activation of HER2/HER3 and PI3K/AKT, resulting in enhanced growth, invasiveness, and resistance to HER2-targeted therapies, which was reversed by treatment with PI3Kα inhibitors.SignificanceThe HER2 TKI neratinib has shown clinical activity against cancers harboring HER2 activating mutations. However, not all HER2-mutant tumors respond, suggesting that co-occurring mutations may limit sensitivity. We found that co-occurring HER3 mutations resulted in reduced sensitivity to HER2 TKIs and HER2 antibodies by disrupting neratinib binding and preventing the dissociation of HER2/HER3, respectively. Cells expressing HER2/HER3 mutations were sensitive to the combination of a HER2 TKI and a PI3Kα inhibitor, suggesting a potential therapeutic strategy against these refractory tumors. Our results provide a mechanistic rationale for the evolutionary selection of co-occurring HER2/HER3 mutations and the recent clinical observations that HER3 mutations are associated with a lesser response to neratinib in HER2-mutant cancers.