Porokeratosis 5, Disseminated Superficial Actinic Type

Description

Porokeratosis is a rare skin disorder characterized by one or more annular plaques with a surrounding raised horny border that spreads centrifugally. Variants of porokeratosis have been described that differ in morphologic shapes, distribution, and clinical course (Schamroth et al., 1997). However, as noted by Sybert (2010), several families with expression of more than one variant of porokeratosis among members, and individuals expressing more than one variant, have been reported, suggesting that the distinctions among these variants may be artificial.

Disseminated superficial actinic porokeratosis (DSAP) is the most common subtype of porokeratosis. It is characterized by multiple small, annular, anhidrotic, keratotic lesions that are located predominantly on sun-exposed areas of the skin, such as the face, neck, and distal limbs. The lesions typically begin to develop in adolescence and reach near-complete penetrance by the third or fourth decade of life (summary by Wu et al., 2004 and Zhang et al., 2012).

For a discussion of genetic heterogeneity of porokeratosis, see 175800.

Clinical Features

Liu et al. (2008) reported a large 5-generation Chinese family in which 16 individuals had DSAP inherited in an autosomal dominant pattern. All affected individuals had lesions on exposed areas of the skin, including the face, nose, neck, legs, and forearms. The lesions were exacerbated during the summer due to sun exposure, but no obvious pruritus or pain were reported. The proband was a 27-year-old man with normal skin at birth. He started to have features of DSAP on his nose at age 13 years. The lesions expanded over the following 14 years to involve the face, neck, arms, back, abdomen, buttocks, and thighs. Skin biopsy showed a typical cornoid lamella in the epidermis, under which the granular layer was absent or decreased, and a mild lymphocytic infiltration around the blood vessels in the papillary dermis.

Mapping

By genomewide linkage analysis of a Chinese family with DSAP (POROK5), Liu et al. (2008) identified a locus, previously termed DSAP3, on chromosome 1p31.3-p31.1 (maximum 2-point lod score of 5.09 with marker D1S2897). Fine mapping showed that the disease gene was located within an 8.2-cM (11.9-Mb) region between markers D1S438 and D1S464. Eight candidate genes within this region were sequenced, but no mutations were identified.