[How frequently does genetic mosaicism occur in the skin?].

BACKGROUND: Until recently, cutaneous mosaicism was considered a rare phenomenon. Its practical significance was considered minimal.
OBJECTIVES: The following questions will be considered: How often are mosaic skin disorders seen in dermatological practice? In which ways can special dermatological competence contribute to assure an appropriate genetic counseling?
METHODS: This review is based on the analysis of recent research articles and on the author's book "Mosaicism in Human Skin" (Berlin, Springer 2014).
RESULTS: The following categories can be distinguished: punctual versus disseminated mosaicism; segmental manifestation of lethal autosomal mutations; type 1 versus type 2 segmental involvement in autosomal dominant skin disorders; isolated versus superimposed manifestation of polygenic skin disorders; twin spotting; epigenetic mosaicism; revertant mosaicism.
CONCLUSIONS: Cutaneous mosaicism occurs so frequently that dermatologists can note it every day in their practice, usually in the form of punctual mosaicism. In the group of autosomal dominant genodermatoses, the type 1 segmental manifestation implies a slightly increased risk that the disorder will affect the patient's offspring in a diffuse form, whereas in cases of type 2 segmental involvement this risk is 50%. In the group of common skin disorders with a polygenic background, cellular analysis of a superimposed segmental manifestation may contribute to elucidate the genetic basis of such diseases. In the group of epigenetically controlled functional mosaics of the skin, we discriminate between X-linked and autosomal forms that are always inheritable. From the concept of revertant mosaicism, a new approach to treat severe genodermatoses can perhaps be developed.