Recessive palmoplantar keratodermas: a tale of wings, hands, hair and cancer.

The palmoplantar keratodermas (PPKs) are a heterogeneous group of disorders of cornification affecting the palms and soles. Of late, a number of rare, recessive PPKs such as odonto-onycho-dermal dysplasia have been elucidated. Surprisingly, these results indicate that correct palmoplantar keratinization depends on intact Wingless (WNT) signalling. WNT was originally discovered in the fruit fly where it is required for wing morphogenesis. This ancient signalling pathway is now emerging as a master regulator of differentiation in a variety of tissues, including the intestine and the hair follicle. It is also becoming increasingly clear that deregulation of WNT signalling is involved in neoplasia. Thus, a single pathway unites several seemingly disparate processes and disorders. The keratodermas are emerging as model systems in which to study WNT signalling. Moreover, as agents that are in the dermatological arsenal can modulate WNT signalling, some insight into its workings is of importance to the practicing dermatologist. In this review, I outline how WNT signalling is involved in epidermal differentiation and skin cancer and what these new insights mean for everyday dermatology.