Keratosis palmoplantaris cum degeneratione granulosa Q82.8

The name and description of "keratosis palmoplantaris cum degeneratione granulosa" is only used here for historical reasons. In fact, this autosomal-dominantly inherited flat palmoplantar keratosis is referred to as keratosis palmoplantaris diffusa with mutation in KRT9 . Its clinical variability is explained by 26 mutations in the KRT9 gene (17q12-q21) that have been identified to date. KRT9 is possibly a heterodimer partner for keratin 6C (KRT6C) (Li P et al. 2023). Mutations in KRT6C are associated with a mild form of pachyonychia congenita.

Most likely, epidermolytic palmoplantar keratosis type Vörner (EPPK) and palmoplantar keratosis type Unna-Thost are distinct variants of the same disease. However, it is possible that they are one and the same disease(PPK type Unna-Thost-Vörner).

In contrast, keratosis palmoplantaris diffusa circumscripta type Norrbotten(synonyms: non-epidermolytic palmoplantar keratoderma; autosomal dominant diffuse palmoplantar keratoderma, Norrbotten type; diffuse palmoplantar keratoderma, Bothnian type) as diffuse palmoplantar keratosis with non-epidermolytic hyperkeratoses and with an underlying mutation in the aquaporin 5 gene (AQP5, 12q13.12), it occupies a genetically and phenotypically clearly defined special position.

Defective structure of keratin due to point mutations in the gene for keratin 9 (KRT9), which is mapped on chromosome 17q12-q21. Mutations in the KRT1 gene are rarer.

Keratin 9 is expressed in the suprabasal layers of the epidermis and is specific for the skin on the palms of the hands and soles of the feet. Mutations in the keratin 9 gene lead to the breakdown of the keratin filament network and cytolysis of corneocytes.

Diffuse, non-transgenic cornification with sharp edges, red rim and ankle-cushion-like hyperkeratosis on the extensor sides of hands and feet, also watch-glass nails. Occasionally hyperhidrosis palmaris or plantaris.

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3. Li P et al. (2023) Proteomic profiling reveals KRT6C as a probable hereterodimer partner for KRT9: New insights into re-classifying epidermolytic palmoplantar keratoderma (EPPK) and a milder form of pachyonychia congenita (PC-K6c) as a group of genetic cutaneous disorders. J Proteomics 287:104971.

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