ΔNp63/p40 correlates with the location and phenotype of basal/mesenchymal cancer stem-like cells in human ER+ and HER2+ breast cancers

Delta Np63, also known as p40, regulates stemness of normal mammary gland epithelium and provides stem cell characteristics in basal and HER2-driven murine breast cancer models. Whilst Delta Np63/p40 is a characteristic feature of normal basal cells and basal-type triple-negative breast cancer, some receptor-positive breast cancers express Delta Np63/p40 and its overexpression imparts cancer stem cell-like properties in ER+ cell lines. However, the incidence of ER+ and HER2(+) tumours that express Delta Np63/p40 is unclear and the phenotype of Delta Np63/p40(+) cells in these tumours remains uncertain. Using immunohistochemistry with p63 isoform-specific antibodies, we identified a Delta Np63/p40(+) tumour cell subpopulation in 100 of 173 (58%) non-triple negative breast cancers and the presence of this population associated with improved survival in patients with ER-/HER2(+) tumours (p = 0.006). Furthermore, 41% of ER+/PR+ and/or HER2(+) locally metastatic breast cancers expressed Delta Np63/p40, and these cells commonly accounted for <1% of the metastatic tumour cell population that localised to the tumour/stroma interface, exhibited an undifferentiated phenotype and were CD44(+)/ALDH(-). In vitro studies revealed that MCF7 and T47D (ER+) and BT-474 (HER2(+)) breast cancer cell lines similarly contained a small subpopulation of Delta Np63/p40(+) cells that increased in mammospheres. In vivo, MCF7 xenografts contained Delta Np63/p40(+) cells with a similar phenotype to primary ER+ cancers. Consistent with tumour samples, these cells also showed a distinct location at the tumour/stroma interface, suggesting a role for paracrine factors in the induction or maintenance of Delta Np63/p40. Thus, Delta Np63/p40 is commonly present in a small population of tumour cells with a distinct phenotype and location in ER+ and/or HER2(+) human breast cancers.