A spectrum of mutations in keratins K6a, K16 and K17 causing pachyonychia congenita.

BACKGROUND: Pachyonychia congenita (PC) is a rare autosomal dominant keratin disorder, subdivided into two major variants, PC-1 and PC-2. Predominant characteristics include hypertrophic nail dystrophy, focal palmoplantar keratoderma and oral leukokeratosis. Multiple steatocystomas that develop during puberty are a useful feature distinguishing PC-2 from PC-1. At the molecular level it has been shown that mutations in keratin K6a or K16 cause PC-1 whereas those in K6b or K17 lead to PC-2. OBJECTIVE: To identify mutations in 22 families presenting with clinical symptoms of either PC-1/focal non-epidermolytic palmoplantar keratoderma (FNEPPK) or PC-2. METHODS: Mutation analysis was performed on genomic DNA from PC patients by direct sequencing. RESULTS: Here, we report four new missense and five known mutations in K6a; one new deletion and three previously identified missense mutations in K16; plus one known mutation in K17. CONCLUSION: With one exception, all these heterozygous mutations are within the highly conserved helix boundary motif regions at either end of the keratin rod domain. In one sporadic case, a unique mutation in K16 resulting in deletion of 24bp was found within the central rod domain, in a child with a phenotype predominantly consisting of focal plantar keratoderma. The identification of mutations in cases of PC is prerequisite for future development of gene-specific and/or mutation-specific therapies.

Development of therapeutic siRNAs for pachyonychia congenita.

Pachyonychia congenita (PC) is an autosomal-dominant keratin disorder where the most painful, debilitating aspect is plantar keratoderma. PC is caused by mutations in one of four keratin genes; however, most patients carry K6a mutations. Knockout mouse studies suggest that ablation of one of the several K6 genes can be tolerated owing to compensatory expression of the others. Here, we have developed potent RNA interference against K6a as a paradigm for treating a localized dominant skin disorder. Four small interfering RNAs (siRNAs) were designed against unique sequences in the K6a 3'-untranslated region. We demonstrated near-complete ablation of endogenous K6a protein expression in two keratinocyte cell lines, HaCaT and NEB-1, by transient transfection of each of the four K6a siRNAs. The siRNAs were effective at very low, picomolar concentrations. One potent lead K6a inhibitor, which was highly specific for K6a, was tested in a mouse model where reporter gene constructs were injected intradermally into mouse paw and luciferase activity was used as an in vivo readout. Imaging in live mice using the Xenogen IVIS system demonstrated that the K6a-specific siRNA strongly inhibited bicistronic K6a-luciferase gene expression in vivo. These data suggest that siRNAs can specifically and very potently target mutated genes in the skin and support development of these inhibitors as potential therapeutics.