Segmental forms of autosomal dominant skin disorders: the puzzle of mosaicism

Autosomal dominant inherited disorders of the skin sometimes present as a segmental phenotype. In recent years molecular studies have demonstrated that genetic mosaicism leads to such a clinical manifestation. In general the skin outside the segmental disorder is normal. This rather common variant of segmental manifestation has been termed type 1. Recently, Happle delineated a second type of segmental manifestation of autosomal dominant genodermatosis. This variant is characterized by a more diffuse clinical presentation of the disease, and a very marked linear pattern can be recognized. An explanation of this phenotype is a germline mutation of the gene manifests after a postzygotic mutation leading to double inactivation of the gene. The severe linear manifestation then reflects a doubling of the genetic burden. We present a number of clinical cases to demonstrate this phenomenon, and we present a case of the segmental Naegeli-Franceschetti-Jadassohn syndrome born to a mother with the diffuse manifestation of the disorder.