Epidermal stem cell dynamics.

Wong and Reiter have explored the possibility that hair follicle stem cells can give rise to basal cell carcinoma (BCC). They expressed in mice an inducible human BCC-derived oncogenic allele of Smoothened, SmoM2, under the control of either the cytokeratin 14 (K14) or cytokeratin 15 (K15) promoter. Smoothened encodes a G-protein-coupled receptor protein in the hedgehog pathway, the misregulation of which is implicated in BCC and other human cancers. Chronic injury is thought to be a contributing factor. The authors used K14 as a marker for stem cells in the basal layer of the epidermis and K15 as a marker for epidermal stem cells in the bulge of hair follicles. Upon activation, K14 construct-bearing mice readily formed BCC-like tumours, whereas this was not the case in K15:SmoM2-carrying mice. Upon wounding the epidermis, however, there was widespread BCC-like tumour formation in the skin of K15:SmoM2 mice. The authors conclude that wounding recruited bulge epidermal stem cells to the surface, allowing the cells to escape quiescence in the stem cell niche and to arrive in an environment where the hedgehog pathway becomes activated and therefore tumorigenesis is elicited. While this is a provocative result and the authors' conclusion may well be correct, there are alternative explanations.