The impact of tumor epithelial and microenvironmental heterogeneity on treatment responses in HER2-positive breast cancer.

Despite the availability of multiple HER2-targeted treatments, therapeutic resistance in HER2+ breast cancer remains a clinical challenge. Intratumor heterogeneity for HER2 and resistance-conferring mutations (e.g., PIK3CA) have been investigated in response and resistance to HER2-targeting agents, while the role of divergent cellular phenotypes and tumor epithelial-stromal cell interactions is less well understood. Here, we assessed the effect of intratumor cellular genetic heterogeneity for ERBB2 copy number and PIK3CA mutation on different types of neoadjuvant HER2-targeting therapies and clinical outcome in HER2+ breast cancer. We found that the frequency of cells lacking HER2 was a better predictor of response to HER2-targeted treatment than intratumor heterogeneity. We also compared the efficacy of different therapies in the same tumor using patient-derived xenograft models of heterogeneous HER2+ breast cancer and single cell approaches. Stromal determinants were better predictors of response than tumor epithelial cells, and we identified alveolar epithelial and fibroblastic reticular cells as well as Lyve1+ macrophages as putative drivers of therapeutic resistance. Our results demonstrate that both pre-existing and acquired resistance to HER2-targeting agents involves multiple mechanisms including the tumor microenvironment. Furthermore, our data also suggest that intratumor heterogeneity for HER2 should be incorporated into treatment design.