A novel mutation and large size polymorphism affecting the V2 domain of keratin 1 in an African-American family with severe, diffuse palmoplantar keratoderma of the ichthyosis hystrix Curth-Macklin type

Keratin gene mutations affecting nonhelical head and tail domains are not usually associated with prominent skin blistering and keratin filament clumping. Instead, they have been associated with several distinct clinical phenotypes, such as epidermolysis bullosa simplex with mottled pigmentation (mutation P25L in the V1 domain of keratin 5), epidermolysis bullosa simplex with migratory circinate erythema (frameshift mutation c1649delG in the V2 domain of keratin 5), striate palmoplantar keratoderma (PPK), and ichthyosis hystrix Curth-Macklin (different frameshift mutations in the V2 domain of keratin 1 (K1)). We have studied a family with severe, diffuse, nonepidermolytic PPK and verrucous hyperkeratotic plaques over the joints and in flexures and identified a new KRT1 gene mutation that is predicted to completely alter the K1 tail domain. In addition, a new K1 size polymorphism has been detected, which is especially prevalent among the African-American population. These results further emphasize the functional importance of the nonhelical tail domain in keratin molecules despite the obvious variability in the number of glycine loop motifs and underscore the broad phenotypic spectrum of disorders due to dominant keratin tail mutations.     

Two cases of primarily palmoplantar keratoderma associated with novel mutations in keratin 1

Mutations in keratin 1 were initially described in the classical form of bullous congenital ichthyosiform erythroderma (also known as epidermolytic hyperkeratosis). More recently the range of phenotypes associated with mutations in this gene has been extended to include annular ichthyosiform erythroderma and mild epidermolytic palmoplantar keratoderma. Here we present two novel mutations in the keratin 1 gene (KRT1): a 5' donor splice site mutation in exon 1 (591 + 2T > A) that predicts a 22 amino acid in-frame deletion in the keratin 1 1A domain; and an in-frame deletion in exon 7 (1376del24) that predicts a foreshortened 2B coiled-coil domain of keratin 1. In each case these mutations are associated with palmoplantar keratoderma and mild ichthyosis, largely limited to the flexural areas. These mutations appear to have a less damaging effect than previously reported mis-sense mutations sited in the helix boundary motifs. This report extends the range of phenotypes associated with mutations in KRT1.     

Splice site and deletion mutations in keratin (KRT1 and KRT10) genes: unusual phenotypic alterations in Scandinavian patients with epidermolytic hyperkeratosis

Epidermolytic hyperkeratosis is a rare autosomal dominant inherited skin disorder caused by keratin 1 or keratin 10 mutations. Keratins are major structural proteins of the epidermis, and in keratinocytes committed to terminal differentiation the intermediate filaments are composed of keratin 1 and keratin 10 heterodimers. The majority of reported mutations (86.6%) are heterozygous single point mutations and most of these are located in the 1A and 2B regions of the highly conserved keratin alpha-helical rod domain. We have studied eight Scandinavian families with epidermolytic hyperkeratosis and identified three point mutations, two codon deletions, two splice site mutations, and a complex deletion/insertion. Two of the point mutations were in the KRT1 gene (F191C and K177N) and the other was in KRT10 (L453P). All three patients had associated palmoplantar keratoderma. The splice site mutations in KRT1 both caused a large deletion removing 22 codons (delta176-197) from the 1A helical domain. Codon deletions were found in KRT1 (delta170-173) and in KRT10 (delta161-162) in two patients with a severe phenotype. A final patient had a more complex mutation with a large deletion (442 bp) together with a large insertion (214 bp) of unknown origin that caused deletion of exon 6 in KRT1. In conclusion, we have found eight novel keratin mutations that cause epidermolytic hyperkeratosis with differing phenotypes. Even when a large part of keratin 1 (46 amino acids) is deleted, surprisingly mild phenotypes can result, suggesting that genotype-phenotype relationships in epidermolytic hyperkeratosis are complex and do not solely depend on the type of mutation but also depend on interactions between the behavior of the mutant protein and the cellular environment.     

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.     

Epidermolytic palmoplantar keratoderma due to a novel type of keratin mutation, a 3-bp insertion in the keratin 9 helix termination motif

Epidermolytic palmoplantar keratoderma (EPPK) is an autosomal dominant genodermatosis characterized by diffuse keratoderma, typically with an erythematous border. Histologically, palmoplantar epidermis shows suprabasal cytolysis and ultrastructurally, tonofilament aggregation with overlying epidermolytic hyperkeratosis. Mutations in the KRT9 gene, encoding keratin 9 (K9), a cytoskeletal protein expressed exclusively in suprabasal keratinocytes of palmoplantar epidermis, have been reported to cause EPPK. To date, all KRT9 defects reported in EPPK have been missense mutations in exon 1, which encodes the start of the alpha-helical rod domain. However, based on studies of other keratin disorders, it was postulated that mutations at the other end of the rod domain might also produce the EPPK phenotype. Here, we report the first mutation in the 2B domain of KRT9, 1362ins3, leading to an insertion of histidine in the helix termination motif of the K9 polypeptide. Insertional mutations have not been previously described in keratins. The phenotype of this case is similar to EPPK caused by 1A domain mutations, demonstrating that mutations in either of the helix boundary motif sequences of K9 are detrimental to keratin function and keratinocyte structure.     

R156C mutation of keratin 10 causes mild form of epidermolytic hyperkeratosis

A 37-year-old Japanese male presented to us with persistent asteatotic skin with mild erythema on the trunk and extremities. Skin biopsy from the left knee showed marked epidermal acanthosis and hyperkeratosis, and milder granular degeneration. Ultrastructural analysis revealed clumping of the keratin filaments within suprabasal keratinocytes of the epidermis. Following direct sequencing, we found a single nucleotide substitution in one allele at the residue position 466 of the 1A rod domain segment (CGC to TGC, arginine to cysteine; R156C) in keratin 10. Clinical manifestations and molecular analysis indicated that R156C mutation in keratin 10 gene (KRT10) causes a mild form of epidermolytic hyperkeratosis (EHK) in the presented case.     

A premature stop codon mutation in the 2B helix termination peptide of keratin 5 in a German epidermolysis bullosa simplex Dowling-Meara case.

Epidermolysis bullosa simplex (EBS) is caused by defective assembly of keratin intermediate filaments in basal keratinocytes and recent studies indicated causal mutations in the keratin KRT5 and KRT14 genes. In this study, we describe a novel KRT5 mutation in a German sporadic case of EBS Dowling-Meara. Transition of G to T (nucleotide position 2334) leads to a premature stop codon (E477stop, residue 93 of the 2B helix) in the last residue of the highly conserved helix-termination peptide K/LLEGE of the 2B rod domain of keratin K5. This represents the first premature stop codon mutation identified within the K/LLEGE motif of any disorder reported so far that is caused by keratin mutations.     

Evidence for novel functions of the keratin tail emerging from a mutation causing ichthyosis hystrix

Unraveling the molecular basis of inherited disorders of epithelial fragility has led to understanding of the complex structure and function of keratin intermediate filaments. Keratins are organized as a central alpha-helical rod domain flanked by nonhelical, variable end domains. Pathogenic mutations in 19 different keratin genes have been identified in sequences corresponding to conserved regions at the beginning and end of the rod. These areas have been recognized as zones of overlap between aligned keratin proteins and are thought to be crucial for proper assembly of keratin intermediate filaments. Consequently, all keratin disorders of skin, hair, nail, and mucous membranes caused by mutations in rod domain sequences are characterized by perinuclear clumping of fragmented keratin intermediate filaments, thus compromising mechanical strength and cell integrity. We report here the first mutation in a keratin gene (KRT1) that affects the variable tail domain (V2) and results in a profoundly different abnormality of the cytoskeletal architecture leading to a severe form of epidermal hyperkeratosis known as ichthyosis hystrix Curth-Macklin. Structural analyses disclosed a failure in keratin intermediate filament bundling, retraction of the cytoskeleton from the nucleus, and failed translocation of loricrin to the desmosomal plaques. These data provide the first in vivo evidence for the crucial role of a keratin tail domain in supramolecular keratin intermediate filament organization and barrier formation.     

Expression of a truncated keratin 5 may contribute to severe palmar--plantar hyperkeratosis in epidermolysis bullosa simplex patients.

Epidermolysis bullosa simplex are dominant disorders of skin fragility characterized by intraepidermal blistering upon mild mechanical trauma. Skin fragility is caused by expression of either an abnormal keratin 5 or an abnormal keratin 14 protein, which compromises the structure and function of the keratin cytoskeleton of basal cells. We report an epidermolysis bullosa simplex patient with a novel single base substitution (A-->T1414) that changes the lysine residue at amino acid 472 to a non-sense codon (K472X). This change predicts the synthesis of a truncated keratin 5, missing 119 amino acids, including the entire tail domain and the highly conserved KLLEGE motif at the carboxy terminus of the 2B domain of the central rod. Expression of an altered keratin 5, of predicted mass and pI for the product of the K472X allele, was documented by one- and two-dimensional western blots of protein extracts from patient skin. Ultrastructural analysis of the patient's nonhyperkeratotic skin was remarkable for basal keratinocytes with dense and irregular keratin filaments proximal to the basement membrane. Keratinocytes, transfected with a cDNA carrying the A-->T1414 non-sense mutation, overexpressed a truncated keratin 5, and showed a disorganized and collapsed keratin filament cytoskeleton. This is the second epidermolysis bullosa simplex patient reported with a premature termination mutation in the KLLEGE motif. The remarkable occurrence of severe palmar--plantar hyperkeratosis in both patients suggests that the keratin 5 tail domain may have unrecognized, but important, normal functions in palmar-plantar tissues.     

A novel asparagine-->aspartic acid mutation in the rod 1A domain in keratin 2e in a Japanese family with ichthyosis bullosa of Siemens

Ichthyosis bullosa of Siemens is a unique type of congenital ichthyosis characterized by mild hyperkeratosis over the flexural areas and blister formation after mechanical trauma and superficial denuded areas in the hyperkeratotic skin. Recently, mutations in the helix initiation or termination motifs of keratin 2e (KRT2E) have been described in ichthyosis bullosa of Siemens patients. The majority of the mutations reported to date lie in the 2B region. We report a novel amino acid substitution mutation (asparagine-->aspartic acid) in codon 192 at the conserved 1A helix initiation site of the rod domain of KRT2E in a Japanese family with ichthyosis bullosa of Siemens. Our data indicate aspartic acid substitution in codon 192 in the 1A helix initiation site is deleterious to keratin filament network integrity and leads to ichthyosis bullosa of Siemens phenotype.     

Human keratin diseases:

Abstract 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 K 10 are mutated in bullous congenital ichthyosiform erythroderma (BC1E; also called epidermolytic hyperkeratosis, EH) and mosaicism for K 1/K 10 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 iehthyosis 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 K1 7 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 K.5 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.     

Arginine in the beginning of the 1A rod domain of the keratin 10 gene is the hot spot for the mutation in epidermolytic hyperkeratosis

Keratin intermediate filaments are expressed in specific type I/type II pairs in the stage of differentiation of keratinocytes. The mutations in the keratin genes expressed in the epidermis are etiologically responsible for several epidermal genetic skin diseases, such as epidermolysis bullosa simplex, epidermolytic hyperkeratosis (EHK), ichthyosis bullosa of Siemens, palmoplantar keratoderma, pachyonchia congenita and white sponge nevus. The mutations of keratins 1/10 which are expressed in spinous and granular layers are confirmed to cause EHK. There are several trials to correlate between the clinical phenotypes and sites of mutations of the keratin genes. One of these is that EHK is divided into two groups: the palms and soles involvement (PS) group and the non-palms and soles (NPS) group. So far the PS group had the mutations in the keratin 1 and the NPS group in keratin 10. Most of the mutations of the NPS group were reported in the beginning of the 1A rod domain and over 2/3 of the mutations in the 1A rod domain were the base pair substitution of arginine. Here we find two different mutations in two unrelated Korean kindreds classified as NPS group-R156C and R156H-in the 1A rod domain of keratin 10. Our results are compatible with the above classification and suggest that the arginine in the beginning of the 1A rod domain is the hot spot for the mutation of the keratin 10 gene.     

伴移行性环形红斑的单纯性大疱性表皮松解症一家系报道及基因检测

目的:单纯性大疱性表皮松解症伴有移行性环状红斑(EBS-Migr)是EBS的一种罕见亚型,是一种常染色体显性遗传病,本文报道一家系,并进行基因检测。方法:收集临床资料,提取患儿及其父母外周血DNA,对患儿及其父母进行全基因组外显子测序,经生物学分析,获得致病变异;再采用Sanger测序法进行验证。结果:患儿及其父亲KRT5外显子9存在杂合突变(移码缺失c.1653＿1654delCT),其母亲未发现该突变。结论:该患儿诊断为伴移行性环状红斑的单纯性大疱性表皮松解症,致病机制为KRT5外显子9发生杂合突变(移码缺失c.1653＿1654delCT),既往未见报道。     

A novel mutation in the 2B domain of keratin 2e causing ichthyosis bullosa of Siemens

Ichthyosis bullosa of Siemens (IBS; MIM: 146800) is an autosomal dominant disorder of keratinization characterized by epidermolytic hyperkeratosis without erythroderma. The clinical features are less marked than those of bullous congenital ichthyosiform erythroderma with relatively mild hyperkeratosis usually limited to the skin flexures. Mutations in the epithelial cytokeratin 2e (K2e), which is expressed in a differentiation-specific fashion in the upper spinous and granular layers of the epidermis, have been shown to cause IBS. We detected a novel mutation in a three generation kindred with IBS (1448T-->A) within exon 7 of the KRT2E gene. This is predictive of an I483N substitution in the 2B domain of K2e. This extends the range of mutations reported to date and illustrates the usefulness of molecular genetics in the diagnosis of this disorder.     

Mutations in the keratin 9 gene in Pakistani families with epidermolytic palmoplantar keratoderma

Background. Keratins are heteropolymeric proteins that form the intermediate filament cytoskeleton in epithelial cells. The common basic structure of all keratins is organized in a central α‐helical rod domain flanked by nonhelical, variable head and tail regions. Most mutations in keratins are found in the central α‐helical rod domain. Keratin 9 (K9) is expressed only in the suprabasal layers of palmoplantar epidermis. Mutations in the keratin 9 gene (KRT9) have been shown to cause epidermolytic palmoplantar keratoderma (EPPK; OMIM 144200), an autosomal dominant genodermatosis characterized clinically by diffuse hyperkeratosis limited to the palms and soles, and histologically by epidermolysis in suprabasal layers of the epidermis. Aim. To elucidate the genetic basis of EPPK in five Pakistani families. Methods. Using microsatellite markers localized to the areas around the type I keratin gene cluster on chromosome 17q21, genotyping of these families was performed, followed by sequencing of the KRT9 gene. Results. The analysis resulted in the identification of two novel (p.M157K and p.Y454H) and two recurrent (p.M157T and p.R163Q) mutations in the KRT9 of all five families. All mutations occurred within the highly conserved helix initiation or termination motif of K9. Conclusions. The affected members of all five families possess mutations in the KRT9 gene that severely affect heterodimer formation with the type II keratin partner. The results of our study further underscore the crucial role of K9 protein in the palmoplantar epidermis.     

A usual frameshift and delayed termination codon mutation in keratin 5 causes a novel type of epidermolysis bullosa simplex with migratory circinate erythema

We report here two unrelated families in Japan and Korea having patients with a unique type of epidermolysis bullosa simplex and a novel mutation in the keratin gene KRT5, i.e., a frameshift and delayed stop codon inconsistent with any subtype described before. The patients showed migratory circinate erythema and multiple vesicles on the circular belt-like areas affected by erythema. Electron microscopy of skin biopsies showed a reduction in the number of keratin intermediate filaments in the basal cells without tonofilament clumping. We identified a novel heterozygous deletion mutation (1649delG of KRT5) in both cases. This deletion is predicted to produce a mutant keratin 5 protein with a frameshift of its terminal 41 amino acids and 35 amino acids longer than the wild-type keratin 5 protein due to a delayed termination codon. As the same abnormal elongated mutant KRT5 gene was found in the independent families, the predicted abnormal elongated keratin protein is likely to lead to an atypical clinical phenotype that has never been reported, possibly by interfering with the functional interaction between keratin and its associated proteins.     

Linkage of the epidermolytic hyperkeratosis phenotype and the region of the type II keratin gene cluster on chromosome 12.

Bullous congenital ichthyosiform erythroderma (epidermolytic hyperkeratosis) is a severe, generalized, lifelong disease of the skin. As in epidermolysis bullosa simplex, intraepidermal blisters and clumping of keratin intermediate filaments are characteristic. We report here linkage of the inheritance of this disease to the region of chromosome 12q containing the genes encoding type II keratins. This suggests that keratin gene mutations may underlie this complex hyperproliferative and hyperkeratotic phenotype.     

A novel mutation of keratin 5 in epidermolysis bullosa simplex with migratory circinate erythema

Epidermolysis bullosa simplex migratory circinate erythema (EBS-Migr) is an uncommon subtype of EBS. We report a case of EBS-MIGR with a novel heterozygous pathogenic mutation in exon 9 (frameshift deletion c.1650delC) and likely benign heterozygous mutation in exon 2 (missense c.591C > A) of keratin 5. This novel pathogenic mutation in KRT5 expands the molecular spectrum of this rare subtype of EBS.