Compared to estrogen receptor-negative (ER-) breast cancer (BC), ER+ BC often manifests with a latent disease that recurs decades after initial diagnosis. The mechanisms governing dormancy and distant recurrence of ER+ tumors remain elusive due to the lack of preclinical models. Here, we compared the tumor progression of ER+ and triple negative (TN; ER-, progesterone receptor and human epidermal growth factor receptor 2-negative) BCs by grafting cell lines- and patient-derived tumor cells into the milk ducts of immunocompromised mice. In the intraductal model, both ER+ and TN BC cells disseminate already during the in situ stage. TN disseminated tumor cells (DTCs) proliferate and form macro-metastases, while DTCs from ER+ BC xenografts have low proliferative indices, are arrested in the G0/G1 phase of the cell cycle, and express the dormancy marker p27. Dormant DTCs display mesenchymal and niche-specific characteristics, as revealed by three-dimensional single cell resolution imaging, and show molecular features of Epithelial-Mesenchymal Transition (EMT) with decreased CDH1, and in-creased ZEB1, ZEB2, and VIM expression levels. Surprisingly, EMT is neither detected in primary tumor cells, nor required for their invasion or dissemination, but is acquired upon their establishment in distant sites. In ex vivo cultures, dormant lung and brain DTCs, revert back to a proliferative state within 3 weeks through a Mesenchymal-Epithelial Transition (MET).
In vivo, CDH1 restoration in dormant DTCs overcomes dormancy and increases the growth and proliferation of lung metastases, while decreasing the expression of EMT-transcription factors (MET). We conclude that the intraductal model provides exciting new experimental opportunities to study ER+ BC metastasis dormancy, and reveals that EMT could be exploited therapeutically to promote dormancy and prevent distant recurrence.