Mutation of a type II keratin gene (K6a) in pachyonychia congenita

Epidermolysis bullosa simplex (EBS) is characterized by skin blistering due to basal keratinocyte fragility. In one family studied, inheritance of EBS is linked to the gene encoding keratin 14, and a thymine to cytosine mutation in exon 6 of keratin 14 has introduced a proline in the middle of an alpha-helical region. In a second family, inheritance of EBS is linked to loci that map near the keratin 5 gene. These data indicate that abnormalities of either of the components of the keratin intermediate filament heterodipolymer can impair the mechanical stability of these epithelial cells.

We have isolated the gene for human type I keratin 9 (KRT9) and localised it to chromosome 17q21. Patients with epidermolytic palmoplantar keratoderma (EPPK), an autosomal dominant skin disease, were investigated. Three KRT9 mutations, N160K, R162Q, and R162W, were identified. All the mutations are in the highly conserved coil 1A of the rod domain, thought to be important for heterodimerisation. R162W was detected in five unrelated families and affects the corresponding residue in the keratin 14 and keratin 10 genes that is also altered in cases of epidermolysis bullosa simplex and generalised epidermolytic hyperkeratosis, respectively. These findings provide further evidence that mutations in keratin genes may cause epidermolysis and hyperkeratosis and that hyperkeratosis of palms and soles may be caused by different mutations in the KRT9 gene.