Cancer stem cells

The concept of cancer stem cells (CSCs) emerged from our understanding of the way in which normal tissues are generated from multipotent stem cells. Regenerative tissues exhibit a cellular hierarchy of differentiation, which is maintained by stem cells. Evidence from experimental models has indicated that a similar hierarchy is seen in at least some cancers, where CSCs give rise to disordered and dysfunctional tissues, leading to disease. The CSC model proposes that tumours can be divided into at least two distinct populations. The stem cells are a specialized population of cancer cells with the unique property of long-term self-renewal that maintain the growth of the cancerous clone. These stem cells give rise to the second population of cells, which form the bulk of the tumour, and lack indefinite self-renewal. Recently, our understanding of CSCs has been refined through combining genetic, epigenetic, and functional models of tumorigenesis. Malignant transformation occurs as the result of sequential acquisition of genetic mutations. Capacity for self-renewal is essential for a clone to survive and progress to become cancerous. If an oncogenic mutation occurs in a cell that is incapable of self-renewal, the clone will become exhausted through differentiation. CSCs may survive anticancer chemotherapy and increasing evidence indicates their role in mediating treatment resistance and relapse. Therefore, strategies to eradicate cancers must effectively target the stem cells that maintain their growth. CSC-directed therapeutic strategies are currently being explored in experimental studies and clinical trials but reducing toxicity to normal tissue stem cells represents a significant challenge.