New Aspects of Cutaneous Mosaicism

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Abstract
The concept of cutaneous mosaicism has today been proven at the cellular level in at least fifteen different skin disorders. We can distinguish five different patterns of mosaicism, including the phylloid pattern and the lateralization pattern. Etiologically, cutaneous mosaics can be divided into two large categories, epigenetic mosaicism and genomic mosaicism. All forms of epigenetic mosaicism known so far, including the various patterns of X‐inactivation, appear to be caused by the action of retrotransposons. A new concept is functional autosomal mosaicism transmittable through the action of retrotransposons, which has been described in mice and dogs and may explain, for example, the familial occurrence of pigmentary mosaicism along the Blaschko lines in human skin. Among the examples of mosaicism of autosomal lethal mutations, phylloid hypomelanosis is a recently recognized neurocutaneous entity caused by mosaic trisomy 13. Possible examples of a type 2 segmental manifestation now include at least fifteen different autosomally dominant skin disorders. This phenomenon is most frequently found in glomangiomatosis, cutaneous leiomyomatosis, and disseminated superficial actinic porokeratosis. Recently proposed examples of didymosis (twin spotting) include cutis tricolor, paired patches of excessive or absent involvement in Darier disease, and didymosis aplasticosebacea characterized by coexistent aplasia cutis congenita and nevus sebaceus. To the list of possible examples of paradominant inheritance, cutis marmorata telangiectatica congenita and speckled lentiginous nevus syndrome have now been added. Revertant mosaicism giving rise to unaffected skin areas in autosomally recessive cutaneous traits will certainly likewise be recognized more often when clinicians are bearing this concept in mind. Such cases can be taken as examples of “natural gene therapy”.