Keratin 14 gene point mutation in the Köbner and Dowling-Meara types of epidermolysis bullosa simplex as detected by the PASA method

Epidermolysis Bullosa Simplex in Scotland Caused by a Spectrum of Keratin Mutations

Epidermolysis bullosa simplex (EBS) is an inherited skin disorder caused by mutations in keratins K5 (keratin 5) and K14 (keratin 14), with fragility of basal keratinocytes leading to epidermal cytolysis and blistering. Patients present with widely varying severity and are classified in three main subtypes: EBS Weber-Cockayne (EBS-WC), EBS Köbner (EBS-K), and EBS Dowling-Meara (EBS-DM), based on distribution and pattern of blisters. We could identify K5/K14 mutations in 20 out of the 43 families registered as affected by dominant EBS in Scotland; with previous studies this covers 70% of all Scottish EBS patients, making this the most comprehensively analyzed EBS population. Nine mutations are novel. All mutations lie within five previously identified rod domain hotspots and the severest blistering was associated with mutations in the helix boundary motifs. In some cases, the same mutation caused symptoms of EBS-WC and/or EBS-K, both within and between families, suggesting a contribution of additional factors to the phenotype. In some patients, no mutations were found in K5, K14, or K15, suggesting involvement of other genes. The results confirm that EBS is best considered as a single disorder with a spectrum of phenotypic variations, from severe EBS-DM at one extreme to mild EBS-WC at the other.

Mutation Report. Novel keratin 14 gene mutations in patients from Hungary with epidermolysis bullosa simplex

Mutations in genes keratin 5 (KRT5) and 14 (KRT14) encoding the basal type keratin intermediate filaments have been identified in epidermolysis bullosa simplex (EBS) families and are likely to cause skin fragility. Three novel keratin 14 mutations in cases from the Hungarian Epidermolysis Bullosa Centre are reported. In a 7-year-old boy with Dowling-Meara type EBS (DM-EBS), who had severe skin symptoms with extended herpetiform blisters, a novel amino acid substitution N123K in keratin 14 had been detected. A 26-year-old woman with mild DM-EBS with prominent palmoplantar hyperkeratosis and without active blister formation had a novel R125G mutation in keratin 14. In a 6-year-old girl, with Weber-Cockayne type EBS (WC-EBS) with palmoplantar blisters and moderate mental retardation, a novel V133L substitution was detected. Her pedigree showed autosomal dominant mode of inheritance; in the two other families, only the index patients were affected. The N123K and R125G mutations causing DM-EBS phenotypes are located within the helix initiation motif of the rod domain, whereas the very close V133L mutation underlying the WC-EBS phenotype is outside of this region. These novel amino acid substitutions provide further information for genotype-phenotype correlation in KRT14 mutations, and demonstrate the first molecular genetic data in EBS patients from Hungary.

A novel nonsense mutation at E106 of the 2B rod domain of keratin 14 causes dominant epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is classified into three main types and is caused, in most cases, by missense mutations in the genes encoding keratin (K) 5 and K14. In this study, we clinically, ultrastructurally, immunohistochemically, and molecularly studied a patient with a dominant EBS, Köbner type. Using sequence analysis of genomic DNA, a novel K14 nonsense mutation was identified. A heterozygous mutation G1231T of KRT14 was found to be associated with the disease in the patient. The mutation created a premature stop codon (amino acid codon 411, residue 106 of the 2B helix) in the K14 molecule. This residue lies in a highly conserved region and was recently found to be absolutely required for molecular stability and intermediate filament assembly in K5 and K14. The E411X (E106X) heterozygous ablation, missing the last 16 amino acid residues of the 2B and the entire tail domain of K14, led to disease but did not result in clumping of keratin filaments. It is the first premature stop codon mutation of K14 found in dominant EBS.

Epidermolysis bullosa simplex Dowling-Meara due to an arginine to cysteine substitution in exon 1 of keratin 14

Epidermolysis bullosa simplex (EBS) is a blistering disorder affecting the basal layer of the epidermis usually inherited in an autosomal dominant fashion. Most cases are caused by mutations in the genes encoding keratin 5 (K5) and keratin 14 (K14) and are characterized by cytolysis within the basal layer of the epidermis. We report a patient manifesting the Dowling-Meara variant of EBS in whom we characterized a cytosine to thymine transition at codon 125 (R125C) in K14. This missense mutation is located at the amino terminus of the helical rod domain of the keratin 14 molecule, resulting in defective pairing with K5, thereby disrupting keratin tonofibril integrity.

Exempting homologous pseudogene sequences from polymerase chain reaction amplification allows genomic keratin 14 hotspot mutation analysis

In patients with the major forms of epidermolysis bullosa simplex, either of the keratin genes KRT5 or KRT14 is mutated. This causes a disturbance of the filament network resulting in skin fragility and blistering. For KRT5, a genomic mutation detection system has been described previously. Mutation detection of KRT14 on a DNA level is, however, hampered by the presence of a highly homologous but nontranscribed KRT14 pseudogene. Consequently, mutation detection in epidermolysis bullosa simplex has mostly been carried out on cDNA synthesized from KRT5 and KRT14 transcripts in mRNA isolated from skin biopsies. Here we present a genomic mutation detection system for exons 1, 4, and 6 of KRT14 that encode the 1A, L1-2, and 2B domains of the keratin 14 protein containing the mutation hotspots. After cutting the KRT14 pseudogene genomic sequences with restriction enzymes while leaving the homologous genomic sequences of the functional gene intact, only the mutation hotspot-containing exons of the functional KRT14 gene are amplified. This is followed by direct sequencing of the polymerase chain reaction products. In this way, three novel mutations could be identified, Y415H, L419Q, and E422K, all located in the helix termination motif of the keratin 14 rod domain 2B, resulting in moderate, severe, and mild epidermolysis bullosa simplex phenotype, respectively. By obviating the need of KRT14 cDNA synthesis from RNA isolated from skin biopsies, this approach substantially facilitates the detection of KRT14 hotspot mutations.

The clinical spectrum of epidermolysis bullosa simplex

As part of the U.K. National Epidermolysis Bullosa Register, we have systematically recorded clinical information on 130 (77%) of the 168 known Scottish epidermolysis bullosa simplex (EBS) sufferers. Three subtypes of EBS were recognized: Dowling-Meara (EBS-DM), Weber-Cockayne (EBS-WC) and Köbner (EBS-Kb), seen in 5%, 42% and 53% of patients, respectively. As there is considerable overlap between EBS-WC and EBS-Kb, with both phenotypes frequently seen within the same pedigree, EBS-WC is best regarded as a milder variant of EBS-Kb rather than a separate disorder. Improvement with age is common in all variants of EBS, but is not invariable. Pain due to acral blistering in EBS-Kb/EBS-WC has a more marked impact on life-style than the blisters of EBS-DM. Oral blistering, nail involvement and aplasia cutis congenita occur in all EBS subtypes and laryngeal involvement is a feature of EBS-DM. Seasonal variation is not seen in EBS-DM but is common in EBS-Kb/EBS-WC.

Molecular genetics of Meesmann's corneal dystrophy: ancestral and novel mutations in keratin 12 (K12) and complete sequence of the human KRT12 gene

Recently, we identified the first mutations in corneal keratins K3 and K12 in families with Meesmann's corneal dystrophy (MCD). Here, we sequenced all regions of the human K12 gene, to enable mutation detection for all exons using genomic DNA as a template. The human K12 genomic sequence spans 5919 bp and consists of eight exons. A microsatellite dinucleotide repeat was identified within intron 3, which was highly polymorphic and which we developed for use in genotype analysis. In addition, two mutations in the helix initiation motif of K12 were found in families with MCD. A novel mutation was detected in an American kindred, 410T-->C, which predicts the amino acid substitution M129T. In a German family, mutation 428G-->C was identified, predicting amino acid change R135T. The latter mutation was identical to that which we identified in the original kindred described by Meesmann. Using the intragenic microsatellite polymorphism in K12 and additional flanking markers, we were able to show that this family shares a common haplotype with the original Meesmann kindred. These results strongly imply that R135T represents an ancestral mutation in the German population. Both mutations occur in the highly conserved helix initiation motif of the K12 polypeptide. A total of eight mutations have now been reported in the K12 gene.

Immunohistopathologic diagnosis of epidermolysis bullosa

Routine histologic study usually is insufficient to subclassify epidermolysis bullosa (EB); currently, electron microscopic evaluation has been the gold standard. A major advance recently has been made in elucidating the molecular basis of several major forms of EB. Concomitantly, immunoreagents have been developed to map antigens in the basement membrane zone. Some of these reagents facilitate the classification of EB into types and subtypes and can be used as an adjunct informative screening procedure to direct mutation identification efforts using DNA technologies. The current review provides an overview of these recent developments and a more detailed account of the immunohistopathologic diagnosis of EB.

The eye in epidermolysis bullosa

AIMS

To describe the ophthalmic findings in a large cohort of epidermolysis bullosa (EB) patients managed in one large specialist centre.

METHODS

A case note review of consecutive patients seen at Great Ormond Street Children's Hospital. Data on the dermatological disease, ophthalmic history, and examination were collected and coded onto a data sheet.

RESULTS

181 patients: 50 (28%) simplex EB; 15 (8%) junctional EB; 28 (15%) autosomal dominant dystrophic EB; 72 (40%) autosomal recessive dystrophic EB; nine patients (5%) with dystrophic EB whose inheritance could not be ascertained; and seven cases (4%) of EB that could not be classified. Ocular problems were found in 12% (n = 6) of simplex patients and 40% (n = 6) of those with junctional disease. One patient (of 28) in the autosomal dominant dystrophic group had ocular involvement and 51% (37/72) of patients in the autosomal recessive dystrophic group had ophthalmic complications: corneal (25/72), lid ectropions (3/72), lid blisters (5/72), and symblepharon (3/72).

CONCLUSION

Ophthalmic complications are common in EB overall but the incidence varies widely with subtype. Ophthalmic complications are the most severe in the dystrophic recessive and junctional subtypes where there is a need for extra vigilance. The major treatment modality was use of ocular lubricants.

Primers for exon-specific amplification of the KRT5 gene: identification of novel and recurrent mutations in epidermolysis bullosa simplex patients

The KRT5 and KRT14 genes encode the proteins keratin 5 and 14, respectively, which are the primary structural components of the 10-nm intermediate filaments of the mitotic epidermal basal cells. A single mutation in either gene can disrupt the keratin intermediate filament cytoskeleton, resulting in the skin fragility and blistering that is characteristic of the group of inherited disorders known as epidermolysis bullosa simplex. We have established a mutation detection system that facilitates KRT5 gene analysis from leukocyte genomic DNA, obviating the need for a skin sample or keratinocyte culture for cDNA synthesis. KRT5 intronic regions that flanked each exon were sequenced and sets of facing intronic primers were designed for specific amplification of each of the nine KRT5 exons. Direct sequencing of KRT5-amplified exons identified three novel missense mutations. One mutation recurred in two unrelated patients with sporadic EBS. This glutamate to lysine substitution (E477K), located in the highly conserved KLLEGE motif at the end of the central rod domain, is the third recurrent mutation identified in dominant epidermolysis bullosa simplex disease. The corresponding glutamate in keratin 2e was previously reported to be frequently mutated in ichthyosis bullosa of Siemens, suggesting that this highly conserved residue may be a potential mutational hot spot in other type II keratins or nonkeratin intermediate filament proteins.

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.