Happle R

Department of Dermatology, Philipp University of Marburg, Germany

Objective: The various patterns of cutaneous mosaicism (lines of Blaschko, checkerboard pattern, phylloid pattern) will be considered and categorized according to the underlying genetic mechanisms.

Methods and Results: Two major categories are genomic mosaicism and functional X-chromosome mosaicism. Genomic mosaicism: Lethal autosomal mutations include those of McCune-Albright syndrome, Schimmelpenning syndrome and Proteus syndrome. A new phenotype to be distinguished from hypomelanosis of Ito is phylloid hypomelanosis reflecting mosaic trisomy 13. Non-lethal autosomal mutations may become manifest in segmental neurofibromatosis and the epidermal nevus of the epidermolytic type. Functional X-chromosome mosaicism is visualized in incontinentia pigmenti, MIDAS syndrome, X-linked dominant chondrodysplasia punctata and CHILD syndrome, as well as in non-lethal X-linked phenotypes such as X-linked hypohidrotic ectodermal dysplasia.

Examples of twin spotting are vascular twin nevi, phacomatosis pigmentovascularis, and phacomatosis pigmentokeratotica.

Two different types of segmental manifestation of autosomal dominant skin disorders can be distinguished. A degree of severity similar to that observed in the non-segmental phenotype reflects heterozygosity for the underlying postzygotic mutation. A far more severe degree of segmental manifestation reflects loss of heterozygosity for the same allele, as seen in neurofibromatosis 1, glomangiomatosis, cutaneous leiomyomatosis, epidermolytic hyperkeratosis of Brocq, Hailey-Hailey disease and disseminated actinic superficial porokeratosis.

Conclusion: The skin is particularly suitable to study various states of mosaicism. Hence, pediatricians should look on nevoid skin disorders with a "prepared mind".