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Porokeratosis at a Glance
  • A chronic progressive disorder of keratinization, characterized clinically by hyperkeratotic papules or plaques surrounded by a thread-like elevated border that expands centrifugally.
  • At least six clinical variants of porokeratosis have been described.
  • The classic form, porokeratosis of Mibelli, presents in infancy or childhood as asymptomatic small brown to skin-colored annular papules with a characteristic raised border.
  • Disseminated superficial actinic porokeratosis is the most common type, with multiple papules distributed symmetrically on sun-exposed areas.
  • Linear porokeratosis presents at birth or in childhood with lesions distributed along Blaschko's lines.
  • Punctate porokeratosis appears during or after adolescence as 1- to 2-mm papules on the palms or soles.
  • In all variants, a thin column of parakeratotic cells (cornoid lamella) corresponds to the hyperkeratotic border and extends throughout the stratum corneum in histologic sections.
  • A genetically heterogeneous disorder; the majority of forms may be inherited as autosomal dominant traits.
  • Malignant epithelial neoplasms are reported in all subtypes except the punctate variety.

Porokeratosis is a morphologically distinct disorder of keratinization, characterized clinically by hyperkeratotic papules or plaques surrounded by a thread-like elevated border that expands centrifugally. Histologically, a thin column of parakeratotic cells extends throughout the stratum corneum and is seen in all variants. This distinctive histopathologic feature, known as the cornoid lamella, corresponds to the raised hyperkeratotic border evident clinically.

At least six clinical variants of porokeratosis are recognized; however, the clinical distinction between these morphological variants may not be justified (Box 52-1). Reports of one type of porokeratosis coexisting with other forms and different types developing in multiple members of an affected family suggest more similarities than disparities, particularly in the disseminated forms.13

Box 52-1 Clinical Variants of Porokeratosis

Porokeratosis is a genetically heterogeneous disorder with multiple loci identified to date; however, the pathogenetic mechanisms remain elusive. Loci at chromosome bands 12q23.2–24.1 and 15q25 (DSAP1 and DSAP2) have been reported in familial disseminated superficial actinic porokeratoses; a further locus has been identified for disseminated superficial porokeratosis (DSP) at 18p11.3.4,5 The locus at DSAP1 corresponds to a candidate gene, SART3 (squamous cell antigen recognized by T cells 3); this encodes a tumor rejection antigen thought to be involved in the regulation of messenger RNA splicing. Fine mapping of the locus at DSAP1 has also revealed mutations in another potential candidate gene, SSH1 (slingshot 1), and a variation in the promoter region of ARPC3, which play a key role in actin dynamics....

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