Pachyonychia Congenita 4
A number sign (#) is used with this entry because pachyonychia congenita-4 (PC4) is caused by heterozygous mutation in the KRT6B gene (148042) on chromosome 12q13.Description
Pachyonychia congenita (PC) is an autosomal dominant genodermatosis with the main clinical features of hypertrophic nail dystrophy, painful and highly debilitating plantar keratoderma, oral leukokeratosis, and a variety of epidermal cysts. Although the condition had previously been subdivided clinically into Jadassohn-Lewandowsky PC type 1 and Jackson-Lawler PC type 2, patients with PC were later found to have a mixed constellation of both types, leading to a classification of PC based on genotype (summary by Sybert, 2010; Eliason et al., 2012; McLean et al., 2011).
For a discussion of genetic heterogeneity of pachyonychia congenita, see 167200.
Historical Classification of Pachyonychia Congenita
Gorlin et al. (1976) suggested that 2 distinct syndromes are subsumed under the designation pachyonychia congenita. PC type 1, the Jadassohn-Lewandowsky type, shows oral leukokeratosis. PC type 2, the Jackson-Lawler type, has natal teeth and epidermoid cysts (cylindromas), but no oral leukoplakia. Corneal dystrophy may be a feature exclusively of the Jackson-Lawler type.
Smith et al. (1998) stated that PC type 2, in contrast to PC type 1, has minimal oral involvement and milder keratoderma, and multiple steatocystomas (184500) is a major clinical feature. Steatocystoma, also known as eruptive vellus cyst, is a cystic hamartoma lined by sebaceous ductal epithelium.
On the basis of a study of 13 patients with PC type 1 or type 2, Terrinoni et al. (2001) concluded that the presence of pilosebaceous cysts following puberty is the best indicator of PC type 2; prepubescent patients are more difficult to classify due to the lack of cysts. Natal teeth are indicative of PC type 2, although their absence does not preclude the PC type 2 diagnosis.Nomenclature
The form of PC caused by mutation in the KRT6B gene, here designated PC4, has also been designated PC-6b (Eliason et al., 2012) and PC-K6b (Shah et al., 2014).Clinical Features
Smith et al. (1998) reported a 5-generation Dutch family segregating autosomal dominant pachyonychia congenita, which was described as the Jackson-Lawler type. Hypertrophic nail dystrophy affected all fingernails and toenails, and focal palmoplantar keratoderma occurred on the pressure points of the feet. Steatcystomas were widespread in postpubescent affected individuals. Mild lingual hyperkeratosis was present on the margins of the tongue.Molecular Genetics
In affected members of a family segregating pachyonychia congenita of the Jackson-Lawler type, Smith et al. (1998) identified heterozygosity for a missense mutation in the KRT6B gene (E472K; 148042.0001).
In 2 families (families 29 and 30) segregating pachyonychia congenita, Smith et al. (2005) identified heterozygosity for the same E472K mutation in the KRT6B gene that had been identified in a Dutch family by Smith et al. (1998). Haplotype analysis showed that family 29 had an independent occurrence of the mutation; haplotype analysis was not possible in family 30 because of lack of samples, but the family was not of Dutch ancestry.