Cancer stem cells and epithelial-mesenchymal transition: revisiting minimal residual disease.

The cancer stem cell (CSC) hypothesis provides an attractive model of tumour development and progression, holding that solid tumours are hierarchically organized and sustained by a minority of the tumour cell population with stem cell properties, such as self-renewal, tumorigenicity and multilineage differentiation capacity. Therapeutic resistance, underlying tumour recurrence and the lack of curative treatments in metastatic disease, raise the question if conventional anticancer therapies target the right cells. Indeed, these treatments might miss CSCs, which represent a more chemoresistant and radioresistant subpopulation within cancer. Recently, a direct link between the epithelial-mesenchymal transition process and the gain of stem cell competence were demonstrated in cultured breast cells. In particular, it was shown that the induction of EMT program not only allow cancer cells to disseminate from the primary tumor, but also promotes their self-renewal capability. Furthermore, the expression of stemness and EMT markers in CTCs were associated with resistance to conventional anti-cancer therapies and treatment failure, highlighting the urgency of improving tools for detecting and eliminating minimal residual disease.