Three keratin gene mutations account for the majority of dominant simplex epidermolysis bullosa cases within the population of Ireland

Descriptive Study of the Clinical and Molecular Features of Epidermolysis Bullosa Patients in a Romanian European Reference Network-Skin Affiliated Reference Center

BACKGROUND

During the last 10 years, in Romania, progress has been made for the welfare of patients suffering from epidermolysis bullosa (EB). In five university hospitals, affiliated with the National Program for the Treatment of Rare Diseases, highly trained specialists diagnose and treat patients with this rare condition. Regarding diagnosis, limitations still exist as immunofluorescence mapping and molecular genetic analysis are not accessible, and generally not reimbursed. Our objective is to present the experience in diagnosing EB patients at Colentina Clinical Hospital, highlighting genotype-phenotype correlations observed in our cohort of patients.

METHODS

The records of the patients enrolled between 2012 and 2024 were analyzed considering clinical aspects, and, when available, immunofluorescence mapping, transmission electron microscopy, and genetic molecular analysis.

RESULTS

Fifty-six patients were identified, of whom 31 cases were of dystrophic EB, three were of junctional EB, and 11 were of simplex EB. For 11 cases, the EB type could not be determined. Regarding EB simplex, two patients with KRT5 mutations and three patients with KRT14 mutations with various clinical expressions, from mild phenotype to severe forms, were identified. Three severe junctional EB patients were registered in our database and for one of the patients, two previously unreported mutations in the LAMA3 gene were identified. Regarding dystrophic EB, 31 cases were identified, of which 25 were recessive dystrophic cases and six were dominant dystrophic cases. Molecular genetic testing was performed for 15 patients, and the most common variant was c.425A>G, identified in six cases.

DISCUSSIONS

Two previously unreported mutations were identified, namely, COL7A1 c.5416G>C, a heterozygous missense variant in a patient with a mild phenotype, mainly with nail involvement, and COL7A1 c.5960del, a variant that generates a frameshift in exon 72 resulting in a premature stop codon; this variant was identified in two siblings with a severe recessive dystrophic.

CONCLUSION

Important steps have been made in identifying the correct and complete diagnosis, as well as the characterization of EB patients addressing our reference center. The findings underscore the pivotal role of molecular genetic testing in confirming diagnoses and elucidating inheritance patterns, especially in cases with atypical presentations or de novo mutations.

A Novel Mutation p.L461P in KRT5 Causing Localized Epidermolysis Bullosa Simplex

Background

Epidermolysis bullosa (EB) is a rare genetic disease with widely different clinical manifestations, but the relationship between genotype and phenotype is not fully understood. In the present study, we recruited a Chinese family in which two members had been diagnosed with localized EB simplex (EBS), with clinical manifestation, including blisters and erosions on the soles of the feet since infancy.

Objective

To identify and confirm the genetic variation in a Chinese family diagnosed as localized EBS.

Methods

Our study included two patients, other healthy members of the family, and 100 normal controls. Genomic DNA samples were isolated from each participant, and then polymerase chain reaction (PCR) direct sequencing was performed.

Results

The results of PCR direct sequencing revealed a novel heterozygous missense mutation in codon 461 of exon 7 of KRT5 (c.1382T>C), which led to an amino acid change (p.L461P) in the patients with EBS but was absent in unaffected family members and 100 unrelated control samples.

Conclusion

The present study broadens the mutational spectrum of EBS, and this knowledge could be harnessed for prenatal screening, gene diagnosis, and gene therapy for localized EBS.

Two novel recessive mutations in KRT14 identified in a cohort of 21 Spanish families with epidermolysis bullosa simplex

BACKGROUND

Basal epidermolysis bullosa simplex (EBS) is a group of blistering genodermatoses mostly caused by mutations in the keratin genes, KRT5 and KRT14. Recessive mutations represent about 5% of all EBS mutations, being common and specific in populations with high consanguinity, where affected patients show severe phenotypes.

OBJECTIVES

To accomplish the first mutational analysis in patients of Spanish origin with EBS and to delineate a comprehensive genotype-phenotype correlation.

METHODS

Twenty-one EBS families were analysed. Immunofluorescence mapping at the dermoepidermal junction level was performed on skin biopsies from patients. Mutation screening of the entire coding sequences of KRT5 and KRT14 in genomic DNA was assessed by polymerase chain reaction and direct sequencing.

RESULTS

KRT5 or KRT14 causative mutations were identified in 18 of the 21 EBS families. A total of 14 different mutations were disclosed, of which 12 were dominant missense mutations and two truncating recessive mutations. Five of the 14 mutations were novel including three dominant in KRT5 (p.V186E, p.T321P and p.A428T) and two recessive in KRT14 (p.K116X and p.K250RfsX8). The two patients with EBS carrying homozygous recessive mutations were affected by severe phenotypes and belonged to consanguineous families. All five families with the EBS Dowling-Meara subtype carried recurrent mutations affecting the highly conserved ends of the α-helical rod domain of K5 and K14. The seven mutations associated with the localized EBS subtype were widely distributed along the KRT5 and KRT14 genes. Two families with mottled pigmentation carried the P25L mutation in KRT5, commonly associated with this subtype.

CONCLUSIONS

This study further confirms the genotype-phenotype correlation established for EBS in other ethnic groups, and is the first in a Mediterranean country (excluding Israel). This study adds two novel recessive mutations to the worldwide record to date, which includes a total of 14 mutations. As in previous reports, the recessive mutations resulted in a lack of keratin K14, giving rise to a generalized and severe presentation.

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.

A mutation in the hair matrix and cuticle keratin KRTHB5 gene causes ectodermal dysplasia of hair and nail type

BACKGROUND

Ectodermal dysplasias are developmental disorders affecting tissues of ectodermal origin. To date, four different types of ectodermal dysplasia involving only hair and nails have been described. In an effort to understand the molecular bases of this form of ectodermal dysplasia, large Pakistani consanguineous kindred with multiple affected individuals has been ascertained from a remote region in Pakistan.

OBJECTIVE

To identify the gene underlying the phenotype.

METHODS

Microsatellite markers were genotyped in candidate regions and two point and multipoint parametric linkage analysis carried out.

RESULTS

The disease locus was mapped to a 16.6 centimorgan region on chromosome 12q12-q14.1 (Zmax = 8.2), which harbours six type II hair keratin genes. DNA sequence analysis revealed a homozygous missense mutation in the hair matrix and cuticle keratin KRTHB5, leading to histidine substitution of a conserved arginine residue (R78H) located in the head domain.

CONCLUSIONS

This report provides the first direct evidence relating to the molecular pathogenesis of pure hair-nail ectodermal dysplasias.

Modeling effects of mutations in coiled-coil structures: case study using epidermolysis bullosa simplex mutations in segment 1a of K5/K14 intermediate filaments

The sequence of a protein chain determines both its conformation and its function in vivo. An attempt is made to gain an understanding of the classes of deformations that can arise in an important structural motif, the alpha-helical coiled coil, as a consequence of mutations occurring in its underlying heptad substructure. In order to do so we consider the model structure of segment 1A in intermediate filaments and then investigate the structures arising from each of the 22 mutations observed in cytokeratin K5/K14 molecules that lead to variants of epidermolysis bullosa simplex. These are refined separately using a molecular dynamics protocol. The mutations often result in a significant distortion of the backbone over a turn or so of the alpha helix in either the chain itself or its constituent partner, leading to the likelihood of impaired chain aggregation and hence molecular assembly. One mutant (K14-L143P; 1A-28) gave rise to structural distortion along almost the entire length of segment 1A. The remaining structures showed less deformation, and normal-looking intermediate filaments are likely in vivo. In addition, an identical mutation in the same position in each of the chains in the heterodimer did not necessarily give equivalent structural distortions. Although proline mutations frequently lead to the most severe structural deformations, a non-proline substitution (K14-R125S; 1A-10) gave rise to the largest local structural disruption that was observed. Unexpectedly, mutations in positions a and d were not always of the greatest structural significance, although three in position a were shown by AGADIR to result in a significant increase in alpha-helix stability.

A Novel Mutation in the Second Half of the Keratin 17 1A Domain in a Large Pedigree with Delayed-Onset Pachyonychia Congenita Type 2

Pachyonychia congenita type 2 (PC-2), also known as Jackson-Lawler type PC, is an autosomal dominant disorder characterized by hypertrophic nail dystrophy associated with focal keratoderma and multiple pilosebaceous cysts. We report a large Chinese pedigree of typical delayed-onset PC-2 that includes 19 affected members. Direct sequencing of PCR products revealed a novel heterozygous 325A-->G mutation in the affected members. This mutation predicts the substitution of asparagine by aspartic acid in codon 109 (N109D) located in the second half of the keratin 17 1A domain, where similar mutation in keratin 5 is associated with the mild Weber-Cockayne form of epidermolysis bullosa simplex.

Functional testing of keratin 14 mutant proteins associated with the three major subtypes of epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is a group of autosomal dominantly inherited skin disorders characterized by the development of intra-epidermal skin blisters on mild mechanical trauma. The three major clinical subtypes (Weber-Cockayne, Koebner and Dowling-Meara) are all caused by mutations in either the keratin 5 (KRT5) or keratin 14 (KRT14) gene. Previously, we identified three novel KRT14 missense mutations in Danish EBS patients associated with the three different forms of EBS (1). The identified KRT14 mutations represent the full spectrum of the classical EBS subtypes. In the present study we investigated these mutations in a cellular expression system in order to analyse their effects on the keratin cytoskeleton. KRT14 expression vectors were constructed by fusing the nucleotide sequence encoding the FLAG reporter peptide to the 3' end of the KRT14 cDNA sequences. The expression vectors were transiently transfected into normal human primary keratinocytes (NHK), HaCaT or HeLa cells in order to analyze the ability of the mutant K14 proteins to integrate into the existing endogenous keratin filament network (KFN). No effect on the keratin cytoskeleton was observed upon transfection of NHK with the various K14 constructs neither with nor without a subsequently induced heat-stress. In contrast, all constructs, including wild-type K14, caused collapse of the endogenous KFN in a small fraction of the transfected HeLa and HaCaT cells. However, overexpression of the mutation associated with the most severe form of the disease, EBS Dowling-Meara, resulted in a higher number of transfected HaCaT cells with KFN collapse (P < 0.001). Thus, although a background KFN perturbance was observed upon transfection with the wild-type K14 construct, the mutant protein associated with the most severe form of EBS worsened the KFN perturbation significantly compared with the mutant proteins associated with the milder forms of the disease and the normal K14 protein. This shows that the clinical severity of disease-associated mutations identified in patients can be tested using this expression system, although it can not at present be used to discriminate between the milder forms. Assessment of the endogenous K14 protein expression in NHK and HaCaT cells indicated that the higher level of endogenous keratin expression in NHK might make these cells more resistant to perturbation of the keratin cytoskeleton by overexpressed K14 protein than HaCaT cells.

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.

A novel keratin 5 mutation (K5V186L) in a family with EBS-K: a conservative substitution can lead to development of different disease phenotypes

Epidermolysis bullosa simplex is a hereditary skin blistering disorder caused by mutations in the KRT5 or KRT14 genes. More than 50 different mutations have been described so far. These, and reports of other keratin gene mutations, have highlighted the existence of mutation "hotspots" in keratin proteins at which sequence changes are most likely to be detrimental to protein function. Pathogenic mutations that occur outside these hotspots are usually associated with less severe disease phenotypes. We describe a novel K5 mutation (V186L) that produces a conservative amino acid change (valine to leucine) at position 18 of the 1A helix. The phenotype of this case is unexpectedly severe for the location of the mutation, which lies outside the consensus helix initiation motif mutation hotspot, and other mutations at this position have been associated in Weber--Cockayne (mild) epidermolysis bullosa simplex only. The mutation was confirmed by mismatch-allele-specific polymerase chain reaction and the entire KRT5 coding region was sequenced, but no other changes were identified. De novo K5/K14 (mutant and wild-type) filament assembly in cultured cells was studied to determine the effect of this mutation on filament polymerization and stability. A computer model of the 1A region of the K5/K14 coiled-coil was generated to visualize the structural impact of this mutation and to compare it with an analogous mutation causing mild disease. The results show a high level of concordance between genetic, cell culture and molecular modeling data, suggesting that even a conservative substitution can cause severe dysfunction in a structural protein, depending on the size and structure of the amino acid involved.

A novel mutation within the rhodopsin gene (Thr-94-Ile) causing autosomal dominant congenital stationary night blindness

More than 100 mutations within the rhodopsin gene have been found to be responsible for some forms of retinitis pigmentosa, a progressive retinal degeneration characterized by night blindness and subsequent disturbance of day vision that may eventually result in total blindness. Congenital stationary night blindness (CSNB) is an uncommon inherited retinal dysfunction in which patients complain of night vision difficulties of a nonprogressive nature only and in which generally there is no involvement of day vision. We report the results of molecular genetic analysis of an Irish family segregating an autosomal dominant form of CSNB in which a previously unreported threonine-to-isoleucine substitution at codon 94 in the rhodopsin gene was found to segregate with the disease. Computer modeling suggests that constitutive activation of transducin by the altered rhodopsin protein may be a mechanism for disease causation in this family. Only two mutations within the rhodopsin gene have been previously reported in patients with congenital stationary night blindness, constitutive activation also having been proposed as a possible disease mechanism.

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.

Using transgenic models to study the pathogenesis of keratin-based inherited skin diseases

In the past decade, the production of transgenic animals whose genome is modified to contain DNA transgenes of interest has significantly contributed to expand our understanding of the molecular etiology and pathobiology of several inherited skin diseases. This technology has led to the discovery that mutations affecting specific keratin genes are responsible for a wide spectrum of inherited bullous diseases, which are collectively characterized by blistering after minor trauma. Type I and type II keratin proteins are restricted to, and very abundant in, epithelial cells, where they occur as a pancytoplasmic network of cytoskeletal filaments. Although it had long been suspected that a primary function of keratin filaments may be to contribute to the physical strength of epithelial sheets, a formal demonstration came from studies of transgenic mouse models and patients suffering from keratin-based blistering diseases. Here we review the basic characteristics of keratin gene and their proteins and relate them to the molecular pathogenesis of relevant inherited skin blistering diseases. A particular emphasis is placed on the role of transgenic mouse models in the past, current, and future studies of these genodermatoses.

Identification of novel and known mutations in the genes for keratin 5 and 14 in Danish patients with epidermolysis bullosa simplex: correlation between genotype and phenotype

Epidermolysis bullosa simplex (EBS) is a group of autosomal dominant inherited skin diseases caused by mutations in either the keratin 5 (K5) or the keratin 14 (K14) genes and characterized by development of intraepidermal skin blisters. The three major subtypes of EBS are Weber-Cockayne, Koebner, and Dowling-Meara, of which the Dowling-Meara form is the most severe. We have investigated five large Danish families with EBS and two sporadic patients with the Dowling-Meara form of EBS. In the sporadic Dowling-Meara EBS patients, a novel K14 mutation (N123S) and a previously published K5 mutation (N176S) were identified, respectively. A novel K14 mutation (K116N) was found in three seemingly unrelated families, whereas another family harbored a different novel K14 mutation (L143P). The last family harbored a novel K5 mutation (L325P). The identified mutations were not present in more than 100 normal chromosomes. Six polymorphisms were identified in the K14 gene and their frequencies were determined in normal controls. These polymorphisms were used to show that the K14 K116N mutation was located in chromosomes with the same haplotype in all three families, suggesting a common ancestor. We observed a strict genotype-phenotype correlation in the investigated patients as the same mutation always resulted in a similar phenotype in all individuals with the mutation, but our results also show that it is not possible to predict the EBS phenotype merely by the location (i.e., head, rod, or linker domains) of a mutation. The nature of the amino acid substitution must also be taken into account.