Human keratin diseases: hereditary fragility of specific epithelial tissues

Keratins are heteropolymeric proteins which form the intermediate filament cytoskeleton in epithelial cells. Since 1991, mutations in several keratin genes have been found to cause a variety of human diseases affecting the epidermis and other epithelial structures. Epidermolysis bullosa simplex (EBS) was the first mechanobullous disease for which the underlying genetic lesion was found, with mutations in both the K5 and K14 genes rendering basal epidermal keratinocytes less resilient to trauma, resulting in skin fragility. The site of mutation in the keratin protein correlates with phenotypic severity in this disorder. Since mutations were identified in the basal cell keratins, the total number of keratin genes associated with diseases has risen to eleven. The rod domains of suprabasal keratins K1 and K10 are mutated in bullous congenital ichthyosiform erythroderma (BCIE; also called epidermolytic hyperkeratosis, EH) and mosaicism for K1/K10 mutations results in a nevoid distribution of EH. An unusual mutation in the VI domain of K1 has also been found to cause diffuse non-epidermolytic palmoplantar keratoderma (DNEPPK). Mutations in palmoplantar specific keratin K9 cause epidermolytic palmoplantar keratoderma (EPPK) and mutations in the late differentiation suprabasal keratin K2e cause ichthyosis bullosa of Siemens (IBS). In the last year or so, mutations were discovered in differentiation specific keratins K6a and K16 causing pachyonychia congenita type 1 and K17 mutations occur in pachyonychia congenita type 2. K16 and K17 mutations have also been reported to produce phenotypes with little or no nail changes: K16 mutations can present as focal non-epidermolytic palmoplantar keratoderma (NEPPK) and K17 mutations can result in a phenotype resembling steatocystoma multiplex. Recently, mutation of mucosal keratin pair K4 and K13 has been shown to underlie white sponge nevus (WSN). This year, the first mutations in a keratin-associated protein, plectin, were shown to cause a variant of epidermolysis bullosa associated with late-onset muscular dystrophy (MD-EBS). An unusual mutation has been identified in K5 which is responsible for EBS with mottled pigmentation and genetic linkage analysis suggests that the hair disorder monilethrix is likely to be due to a mutation in a hair keratin. The study of keratin diseases has led to a better understanding of the importance of the intermediate filament cytoskeleton and associated connector molecules in maintaining the structural integrity of the epidermis and other high stress epithelial tissues, as well as allowing diagnosis at the molecular level thus facilitating prenatal testing for this heterogeneous group of genodermatoses.