Keratins are heteropolymeric proteins which fortn the intermediate filament eytoskeleton in epithelial cells, Sinee 1991, mutations in several keratin genes have been found to cause a variety of human diseases affeeting 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 keratinoeytes less resilient to trauma, resulting in skin fragility. The site of mutation in the keratin protein correlates with phenotypie severity in this disorder. Since tnutations were identified in the basal eell keratins, the total nutrtber of keratin genes associated with diseases has risen to eleven. The rod domains of suprabasal keratins Kl and K 10 are mutated in buUous congenital ichthyosiform erythroderma (BCIE; also ealled epidermolytic hyperkeratosis, EH) and mosaieism for Kl/KlO mutations results in a nevoid distribution of EH, An utiusual mutation in the VI domain of Kl has also been found to cause diffuse non-epidermolytie pahnoplantar keratodertna (DNEPPK), Mutations in pahnoplantar speeifie keratin K9 cause epidertnolytic pahnoplantar keratoderma (EPPK) and mutations in the late differentiation suprabasal keratin l<.2e cause iehthyosis tiullosa of Siemens (IBS). In the last year or so, mutations were discovered in differentiation speeifie keratins K6a and K16 eausing pachyonychia eongenita type 1 and KI 7 mutations occur in pachyonychia eongenita type 2, K16 and K17 mutations have also been reported to produee phenotypes with little or no nail changes: K16 tnutations can present as focal non-epidermolytic pahnoplantar keratodertna (NEPPK) and K17 mutations can result in a phenotype resembling steatoeystoma multiplex, Reeently, mutation of mueosal keratin pair K4 and K13 has been shown to underlie white sponge nevus (WSN), This year, the ftrst mutations in a keratin-associated protein, plectin, were shown to cause a variant of epidertnolysis 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 tnutation in a hair keratin. The study of keratin diseases has led to a better understanditig of the itnportanee of the intermediate filament cytoskeletoti and associated cotinector tnolecules iti tnaintaining 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 genodertnatoses.

The intermediate filament cytoskeleton of corneal epithelial cells is composed of cornea-specific keratins K3 and K12 (refs 1,2). Meesmann's corneal dystrophy (MCD) is an autosomal dominant disorder causing fragility of the anterior corneal epithelium, where K3 and K12 are specifically expressed. We postulated that dominant-negative mutations in these keratins might be the cause of MCD. K3 was mapped to the type-II keratin gene cluster on 12q; and K12 to the type-I keratin cluster on 17q using radiation hybrids. We obtained linkage to the K12 locus in Meesmann's original German kindred (Zmax = 7.53; theta = 0) and we also showed that the phenotype segregated with either the K12 or the K3 locus in two Northern Irish pedigrees. Heterozygous missense mutations in K3 (E509K) and in K12 (V143L; R135T) completely co-segregated with MCD in the families and were not found in 100 normal unrelated chromosomes. All mutations occur in the highly conserved keratin helix boundary motifs, where dominant mutations in other keratins have been found to severely compromise cytoskeletal function, leading to keratinocyte fragility phenotypes. Our results demonstrate for the first time the molecular basis of Meesmann's corneal dystrophy.

We studied a kindred with recessive epidermolysis bullosa simplex in which the affected members lacked expression of the basal cell keratin 14. The patients had severe generalized skin blistering that improved slightly with age. The basal cells of the patients did not express keratin 14 and contained no keratin intermediate filaments. The expression of keratin 5, the obligate copolymer of keratin 14, was slightly reduced. The expression of keratin 15, the alternative basal cell keratin, was increased, suggesting upregulation or stabilization to compensate for the lack of keratin 14. The expression of keratin 16, keratin 17, and keratin 19 in the patient's skin was not different from controls. Immunoelectron microscopy showed a loose network of keratin 5/keratin 15 protofilaments in the basal cells. Keratin 15 filaments did not aggregate into higher order bundles. Sequence analysis of genomic DNA revealed a homozygous mutation in the 3'-acceptor splice site of intron 1 (1840 A-->C) in the affected individuals. This mutation led to the skipping of exon 2 in 24% of the KRT14 transcripts and to the use of a cryptic splice site in 76% of the transcripts. Premature termination codons were generated in all transcripts (codons 175+1 or 175+29), leading to a truncated keratin 14 protein within the helical 1B rod domain. The disorder was associated with circumscribed hyperkeratotic lesions with the histology of epidermolytic hyperkeratosis. The prognosis of keratin 14 ablation is much better in the human than in the mouse.