Novel keratin 16 mutations and protein expression studies in pachyonychia congenita type 1 and focal palmoplantar keratoderma

Abstract

Pachyonychia congenita type 1 (PC-1) is an autosomal dominant ectodermal dysplasia characterized by nail dystrophy, focal non-epidermolytic palmoplantar keratoderma (FNEPPK) and oral lesions. We have previously shown that mutations in keratin 16 (K16) cause fragility of specific epithelia resulting in phenotypes of PC-1 or FNEPPK alone. Here, we report 2 novel mutations in K16 causing distinct phenotypes. A heterozygous missense mutation (L124R) was detected in a kindred with PC-1. In a family where mild FNEPPK was the only phenotype, a 23 bp deletion and a separate 1 bp deletion downstream were found in exon 6: [1244-1266del; 1270delG]. At the protein level, these mutations remove 8 residues and substitute 2 residues in the helix termination motif (HTM) of the K16 polypeptide. The HTM sequence is conserved in all known intermediate filament proteins and for convenience, this complex mutation was designated deltaHTM. Transient expression of K16 cDNAs carrying either the L124R or the deltaHTM mutation in epithelial cell line PtK2 produced aggregation of the keratin cytoskeleton. However, the aggregates observed with the deltaHTM mutation were morphologically different and appeared to be less disruptive to the endogenous cytoskeleton. Therefore, loss of the HTM sequence may render this mutant K16 less capable of contributing to filament assembly and decrease its dominant-negative effect, resulting in the milder FNEPPK phenotype.

Clinical and Pathological Features of Pachyonychia Congenita

“…Thus, at least some cases of steatocystoma multiplex are allelic variants of PC-2. Similarly, some families carrying mutations in K16 present with focal non-epidermolytic palmoplantar keratoderma with very trivial or no nail changes (Shamsher et al, 1995;Smith et al, 2000), showing that this condition is allelic with PC-1 (see also Smith et al, this issue). Even within a single family, considerable clinical variability may be observed, suggesting that environmental or co-inherited genetic modifiers play an important role in disease manifestation.…”

Insights into Genotype–Phenotype Correlation in Pachyonychia Congenita from the Human Intermediate Filament Mutation Database

“…It may be that these larger deletions, particularly those removing substantial portions of the boundary motif sequences, render the mutant polypeptide less able to assemble and are therefore less disruptive. This seems to be the case with a similar deletion in K16 leading to a mild PC-related phenotype, discussed below (Smith et al, 2000).…”

The molecular basis of human keratin disorders

“…In the 2B domain, only one missense mutation, p.Lys354Asn, has been reported in a girl with a delayed onset of the clinical signs of PC-1 (Connors et al 2001). Certain K16 mutations have been reported to cause FNEPPK focal keratoderma without nail changes or other features of PC-1 including missense mutations in the HIM (p.Asn125Ser, p.Arg127Cys) and a complex 14 bp deletion in the HTM (Shamsher et al 1995;Smith et al 2000).…”