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

Pachyonychia congenita type 1 (PC-1) is an autosomal dominant ectodermal dy splasia characterized by nail dystrophy, focal non-epidermolytic palmoplant ar keratoderma (FNEPPK) and oral lesions. We have previously shown that mut ations 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 we re found in exon 6: [1244-1266del; 1270delG]. At the protein level, these m utations remove 8 residues and substitute 2 residues in the helix terminati on motif (HTM) of the K16 polypeptide. The HTM sequence is conserved in all known intermediate filament proteins and for convenience, this complex mut ation was designated Delta HTM. Transient expression of K16 cDNAs carrying either the L124R or the Delta HTM mutation in epithelial cell line PtK2 pro duced aggregation of the keratin cytoskeleton. However, the aggregates obse rved with the Delta HTM mutation were morphologically different and appeare d 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.