Notch2 pathway mediates breast cancer cellular dormancy and mobilisation in bone and contributes to haematopoietic stem cell mimicry

BACKGROUND: Recurrence after >5-year disease-free survival affects one-fifth of breast cancer patients and is the clinical manifestation of cancer cell reactivation after persistent dormancy. METHODS: We investigated cellular dormancy in vitro and in vivo using breast cancer cell lines and cell and molecular biology techniques. RESULTS: We demonstrated cellular dormancy in breast cancer bone metastasis, associated with haematopoietic stem cell (HSC) mimicry, in vivo competition for HSC engraftment and non-random distribution of dormant cells at the endosteal niche. Notch2 signal implication was demonstrated by immunophenotyping the endosteal niche-associated cancer cells and upon co-culture with sorted endosteal niche cells, which inhibited breast cancer cell proliferation in a Notch2-dependent manner. Blocking this signal by in vivo acute administration of the γ-secretase inhibitor, dibenzazepine, induced dormant cell mobilisation from the endosteal niche and colonisation of visceral organs. Sorted Notch2(HIGH) breast cancer cells exhibited a unique stem phenotype similar to HSCs and in vitro tumour-initiating ability in mammosphere assay. Human samples confirmed the existence of a small Notch2(HIGH) cell population in primary and bone metastatic breast cancers, with a survival advantage for Notch2(HIGH) vs Notch2(LOW) patients. CONCLUSIONS: Notch2 represents a key determinant of breast cancer cellular dormancy and mobilisation in the bone microenvironment.