Circulating Breast Cancer Cells Interconvert between Distinct States

In patients with advanced estrogen receptor (ER)⁺/HER2⁻ breast cancer, a subpopulation of HER2⁺ circulating tumor cells (CTC) has been observed to emerge after multiple courses of therapy, but the role of acquired HER2 heterogeneity in disease progression is unclear. Jordan and colleagues found that 16 of 19 patients with ER⁺/HER2⁻ recurrent metastatic breast cancer acquired HER2⁺ CTCs after multiple courses of therapy. Single-cell RNA sequencing revealed a bimodal distribution of HER2 expression in CTCs indicative of distinct HER2⁺ and HER2⁻ subpopulations. HER2⁺ CTCs had an enhanced proliferation rate and generated larger tumors and more frequent lung metastases in mice. However, despite the enhanced proliferation rate of HER2⁺ cells, both populations coexisted, suggesting the possibility of interconversion between subpopulations. Indeed, spontaneous interconversion between subpopulations was observed in vitro and in vivo, with HER2⁺ and HER2⁻ cells producing cells of the opposite type within four doublings. The subpopulations exhibited distinct expression profiles with HER2⁺ CTCs having increased expression of pro-growth genes and phosphorylation of receptor tyrosine kinases (RTK), and HER2⁻ CTCs having increased expression of proteins involved in Notch signaling and DNA damage. A drug screen revealed that HER2⁺ CTCs were not dependent on HER2 but sensitive to combined inhibition of multiple RTKs, whereas HER2⁻ cells were specifically sensitive to inhibitors of Notch activity and resistant to chemotherapeutic agents. Consistent with these findings, suppression of HER2 induced a rapid conversion from a HER2⁺ to a HER2⁻ state, as did chemotherapeutic agents or oxidative stress, indicating that the switch to a less proliferative state promotes a more drug-resistant phenotype. Moreover, in orthotopic mammary xenografts generated from a mixture of HER2⁺ and HER2⁻ CTCs, combined chemotherapy and Notch inhibition delayed tumor recurrence compared with chemotherapy alone. The finding that HER2⁺ and HER2⁻ cells can spontaneously interconvert to switch between proliferative and drug-resistant states suggests that combination therapy targeting both subpopulations may be required to eliminate these metastatic tumors.

Jordan NV, Bardia A, Wittner BS, Benes C, Ligorio M, Zheng Y, et al. HER2 expression identifies dynamic functional states within circulating breast cancer cells. Nature 2016 Aug 24 [Epub ahead of print].

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