In this issue of JAMA Dermatology, Atzmony and colleagues1 present a superb study breaking new ground in the field of porokeratosis. The authors detail molecular associations in 3 patients between type 2 segmental mosaicism and early-onset and linearly arranged lesions in varying clinical types of porokeratosis.
Simply put, the term mosaicism refers to the occurrence of a genetically distinct population of cells arising within a genetically homogeneous zygote. Type 1 segmental mosaicism results from an early postzygotic mutation in an otherwise healthy embryo and reflects heterozygosity. By contrast, the type 2 segmental mosaicism originates in a heterozygous embryo and reflects early postzygotic loss of the corresponding wild-type allele. Type 1 manifests at an age when the nonsegmental phenotype would appear, whereas type 2 manifests much earlier and may even be present at birth. In type 1, the degree of involvement corresponds to that of the nonsegmental phenotype, whereas in type 2 the segmental lesions are far more pronounced and superimposed on the ordinary trait.2
Happle R. Molecular Corroboration of Type 2 Segmental Mosaicism in Various Types of Porokeratosis. JAMA Dermatol. 2019;155(5):531–532. doi:10.1001/jamadermatol.2019.0134
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