Abstract

Patient-derived circulating tumour cells are used to characterize the dynamics and underlying plasticity of HER2 expression in non-HER2-amplified breast tumours. These authors characterize circulating tumour cells from patients with advanced estrogen-receptor-positive/HER2-negative breast cancer and find that they can interconvert between two different states. One subpopulation acquires HER2 expression, displays activation of multiple RTK signalling pathways and is highly proliferative. A second population lacks HER2 expression but has elevated Notch1 levels. The HER2-negative population is less proliferative; it is highly resistant to cytotoxic agents but sensitive to Notch1 inhibitors. The rapid interconversion between proliferative and drug-resistant circulating tumour cell subpopulations raises the possibility that simultaneous combination therapy may be of clinical value. Circulating tumour cells in women with advanced oestrogen-receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative breast cancer acquire a HER2-positive subpopulation after multiple courses of therapy1,2. In contrast to HER2-amplified primary breast cancer, which is highly sensitive to HER2-targeted therapy, the clinical significance of acquired HER2 heterogeneity during the evolution of metastatic breast cancer is unknown. Here we analyse circulating tumour cells from 19 women with ER+/HER2− primary tumours, 84% of whom had acquired circulating tumour cells expressing HER2. Cultured circulating tumour cells maintain discrete HER2+ and HER2− subpopulations: HER2+ circulating tumour cells are more proliferative but not addicted to HER2, consistent with activation of multiple signalling pathways; HER2− circulating tumour cells show activation of Notch and DNA damage pathways, exhibiting resistance to cytotoxic chemotherapy, but sensitivity to Notch inhibition. HER2+ and HER2− circulating tumour cells interconvert spontaneously, with cells of one phenotype producing daughters of the opposite within four cell doublings. Although HER2+ and HER2− circulating tumour cells have comparable tumour initiating potential, differential proliferation favours the HER2+ state, while oxidative stress or cytotoxic chemotherapy enhances transition to the HER2− phenotype. Simultaneous treatment with paclitaxel and Notch inhibitors achieves sustained suppression of tumorigenesis in orthotopic circulating tumour cell-derived tumour models. Together, these results point to distinct yet interconverting phenotypes within patient-derived circulating tumour cells, contributing to progression of breast cancer and acquisition of drug resistance.