Dyschromatosis symmetrica hereditaria and reticulate acropigmentation of Kitamura: An update.

Dyschromatosis symmetrica hereditaria (DSH) and reticulate acropigmentation of Kitamura (RAK) are rare, inherited pigmentary diseases. DSH shows a mixture of pigmented and depigmented macules on the extremities. RAK shows reticulated, slightly depressed pigmented macules on the extremities. The causative gene of DSH was clarified as ADAR1 by positional cloning including linkage analysis and haplotype analysis in 2003. Ten years later, the causative gene of RAK was identified as ADAM10 by whole-exome sequencing, in 2013. ADAR1 is an RNA-editing enzyme which catalyzes the deamination of adenosine to inosine (A-to-I) in double-stranded RNA substrates during post-transcription processing. Inosine acts as guanine during translation, resulting in codon alterations or alternative splice sites that lead to functional changes in proteins when they occur in coding regions. In 2012, it was clarified that ADAR1 mutations cause Aicardi-Goutières syndrome 6, which is a severe genetic inflammatory disease that affects the brain and the skin. A zinc metalloprotease, a disintegrin and metalloprotease domain-containing protein 10 (ADAM10), is involved in the ectodomain shedding of various membrane proteins and shows various functions in vivo. ADAM10 is known to be involved in the ectodomain shedding of Notch proteins as substrates in the skin. We speculate that the pathogenesis of RAK and Dowling-Degos disease (DDD, a pigmentary disease similar to RAK) is associated with the Notch signaling pathway. In addition, ADAM10 mutations proved to be associated with late-onset Alzheimer disease. This review comprehensively discusses the updated pathophysiology of those genetic pigmentary disorders.