HER2 Heterogeneity Affects Trastuzumab Responses and Survival in Patients With HER2-Positive Metastatic Breast Cancer

Objectives: Heterogeneity of HER2 gene amplification is found in a subset of breast cancers. We investigated the impact of HER2 heterogeneity on trastuzumab responses and clinical outcomes in 112 patients with HER2-positive metastatic breast cancer. Methods: Regional and genetic heterogeneity of HER2 gene amplification was determined in three different areas of each tumor by immunohistochemistry and silver in situ hybridization. We also assessed the overall levels of HER2 amplification and the proportion of tumor cells with a HER2 /CEP17 ratio of more than 2.2 or strong and complete membranous (3+) expression of HER2 protein. Results: HER2 regional and genetic heterogeneity based on the HER2 /CEP17 ratio was confirmed in 8.7% and 2.7% of cases, respectively. Poor response to trastuzumab was associated with overall low-level or equivocal amplification, HER2 regional heterogeneity by the HER2 /CEP17 ratio, the HER2 /CEP17 ratio of more than 2.2 in less than 80% of tumor cells, and HER2 immunohistochemical expression of 3+ in less than 75% of tumor cells. In survival analyses, low-level or equivocal HER2 amplification, HER2 regional heterogeneity based on the HER2 /CEP17 ratio, and the HER2 /CEP17 ratio of more than 2.2 in less than 80% of tumor cells were associated with shorter time to progression and lower overall survival in univariate and multivariate analyses. Conclusions: These results suggest that accurate assessment of HER2 status, including HER2 heterogeneity, is important in predicting trastuzumab responses and outcomes in patients with HER2-positive metastatic breast cancer.