Trastuzumab effects depend on HER2 phosphorylation in HER2-negative breast cancer cell lines

The breast cancer (BC) biomarker HER2 (Human Epidermal Receptor 2) is overexpressed in 25% of BC. Only patients with HER2-positive tumors receive HER2-targeting therapies, like trastuzumab (Herceptin). However, some women with a HER2-negative BC could benefit from trastuzumab. This could be explained by the activation/phosphorylation of HER2 that can be recognized by trastuzumab. The aim of this study is to examine trastuzumab effects on HER2 phosphorylation at tyrosine Y877 (pHER2(Y877)). HER2 and pHER2(Y877) status were evaluated in a cohort of BC patients representative of molecular subtypes distribution (n = 497) and in a series of BC cell lines (n = 7). Immunohistochemistry against pHER2(Y877) was performed on tissue micro arrays. Cellular proliferation assays were performed on BC cell lines presenting different combinations of HER2 and pHER2(Y877) status and treated with increasing doses of trastuzumab (0–150 μg/ml). The prevalence of pHER2(Y877) in this cohort was 6%. Nearly 5% of patients with HER2-negative tumors (n = 406, 82%) overexpressed pHER2(Y877). Among triple negative BC patients (n = 39, 8%), 7.7% expressed pHER2(Y877). Trastuzumab treatment decreased cell proliferation in HER2−/pHER2(Y877)+ BC cell lines, to an extent comparable to what occurs in HER2+ cell lines, but did not affect HER2−/pHER2(Y877)− cell lines. Trastuzumab sensitivity in HER2−/pHER2(Y877)+ cell line is specific to HER2 tyrosine 877 phosphorylation. Hence, with further confirmation in a bigger cohort, trastuzumab treatment could be envisaged as a treatment option to women presenting with HER2−/pHER2+ tumors, representing more than 1000 BC women in Canada in 2019.