A de novo mutation in the keratin 9 gene in a family with epidermolytic palmoplantar keratoderma from northern Sweden.

Sir, Palmoplantar keratodermas (PPKs) constitute a heterogeneous group of skin disorders with the distinctive trait of hyperkeratosis of palmoplantar skin. The disorders are classified clinically by the morphology and distribution of the hyperkeratosis, the presence of associated cutaneous and non-cutaneous features and by the mode of transmission (1, 2). Familial diffuse epidermolytic PPK (EPPK) is the most studied keratoderma and is characterized by granular and vacuolar degeneration of the cells of the spinous and granular layer. All mutations reported to date, with one exception, are located in the keratin 9 gene (KRT9) on chromosome 17 (1, 3). The majority of KRT9 mutations reported are missense mutations in exon 1 of the KRT9 gene, but there are reports of a stop codon mutation in exon 1 (4) and of a 3 base pair insertion in exon 6 (5). The position most frequently reported to be mutated in KRT9 is the arginine codon at position 162 in exon 1. In addition to KRT9 mutations, there is a recent study revealing a splice site mutation in the KRT1 gene as the cause of mild EPPK (6). The KRT9 gene appears to be the only keratin gene whose expression is restricted to palmoplantar epidermis (7, 8). Consequently, individuals that carry a mutation in the keratin 9 gene only display the effect of the mutation in the palmoplantar skin. Here we report the first observation of a Swedish family with EPPK and the attribution of the disorder to a de novo mutation in KRT9.