Dominant and recessive compound heterozygous mutations in epidermolysis bullosa simplex demonstrate the role of the stutter region in keratin intermediate filament assembly

Damaged Keratin Filament Network Caused by KRT5 Mutations in Localized Recessive Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a blistering dermatosis that is mostly caused by dominant mutations in KRT5 and KRT14. In this study, we investigated one patient with localized recessive EBS caused by novel homozygous c.1474T > C mutations in KRT5. Biochemical experiments showed a mutation-induced alteration in the keratin 5 structure, intraepidermal blisters, and collapsed keratin intermediate filaments, but no quantitative change at the protein levels and interaction between keratin 5 and keratin 14. Moreover, we found that MAPK signaling was inhibited, while desmosomal protein desmoglein 1 (DSG1) was upregulated upon KRT5 mutation. Inhibition of EGFR phosphorylation upregulated DSG1 levels in an in vitro model. Collectively, our findings suggest that this mutation leads to localized recessive EBS and that keratin 5 is involved in maintaining DSG1 via activating MAPK signaling.

Keratins as an Inflammation Trigger Point in Epidermolysis Bullosa Simplex

Epidermolysis bullosa simplex (EBS) is a group of inherited keratinopathies that, in most cases, arise due to mutations in keratins and lead to intraepidermal ruptures. The cellular pathology of most EBS subtypes is associated with the fragility of the intermediate filament network, cytolysis of the basal layer of the epidermis, or attenuation of hemidesmosomal/desmosomal components. Mutations in keratins 5/14 or in other genes that encode associated proteins induce structural disarrangements of different strengths depending on their locations in the genes. Keratin aggregates display impaired dynamics of assembly and diminished solubility and appear to be the trigger for endoplasmic reticulum (ER) stress upon being phosphorylated by MAPKs. Global changes in cellular signaling mainly occur in cases of severe dominant EBS mutations. The spectrum of changes initiated by phosphorylation includes the inhibition of proteasome degradation, TNF-α signaling activation, deregulated proliferation, abnormal cell migration, and impaired adherence of keratinocytes. ER stress also leads to the release of proinflammatory danger-associated molecular pattern (DAMP) molecules, which enhance avalanche-like inflammation. Many instances of positive feedback in the course of cellular stress and the development of sterile inflammation led to systemic chronic inflammation in EBS. This highlights the role of keratin in the maintenance of epidermal and immune homeostasis.

Grainyhead-like transcription factors: guardians of the skin barrier

There has been selective pressure to maintain a skin barrier since terrestrial animals evolved 360 million years ago. These animals acquired an unique integumentary system with a keratinized, stratified, squamous epithelium surface barrier. The barrier protects against dehydration and entry of microbes and toxins. The skin barrier centres on the stratum corneum layer of the epidermis and consists of cornified envelopes cemented by the intercorneocyte lipid matrix. Multiple components of the barrier undergo cross-linking by transglutaminase (TGM) enzymes, while keratins provide additional mechanical strength. Cellular tight junctions also are crucial for barrier integrity. The grainyhead-like (GRHL) transcription factors regulate the formation and maintenance of the integument in diverse species. GRHL3 is essential for formation of the skin barrier during embryonic development, whereas GRHL1 maintains the skin barrier postnatally. This is achieved by transactivation of Tgm1 and Tgm5, respectively. In addition to its barrier function, GRHL3 plays key roles in wound repair and as an epidermal tumour suppressor. In its former role, GRHL3 activates the planar cell polarity signalling pathway to mediate wound healing by providing directional migration cues. In squamous epithelium, GRHL3 regulates the balance between proliferation and differentiation, and its loss induces squamous cell carcinoma (SCC). In the skin, this is mediated through increased expression of MIR21, which reduces the expression levels of GRHL3 and its direct target, PTEN, leading to activation of the PI3K-AKT signalling pathway. These data position the GRHL family as master regulators of epidermal homeostasis across a vast gulf of evolutionary history.

Structure-Function Analyses of a Keratin Heterotypic Complex Identify Specific Keratin Regions Involved in Intermediate Filament Assembly

Intermediate filaments (IFs) provide vital mechanical support in a broad array of cell types. Interference with this role causes cell fragility and accounts for a large number of human diseases. Gaining an understanding of the structure of IFs is paramount to understanding their function and designing therapeutic agents for relevant diseases. Here, we report the 2.6-Å resolution crystal structure of a complex of interacting 2B domains of keratin 5 (K5) and K14. K5 and K14 form a long-range, left-handed coiled coil, with participating α helices aligned in parallel and in register. Follow-up mutagenesis revealed that specific contacts between interacting 2B domains play a crucial role during 10-nm IF assembly, likely at the step of octamer-octamer association. The resulting structural model represents an atomic-resolution visualization of 2B-2B interactions important to filament assembly and provides insight into the defects introduced by mutations in IF genes associated with human skin diseases.

Biallelic KRT5 mutations in autosomal recessive epidermolysis bullosa simplex, including a complete human keratin 5 "knock-out"

Epidermolysis bullosa simplex (EBS) is usually inherited as an autosomal dominant disease due to monoallelic gain-of-function mutations in KRT5 or KRT14. Although autosomal recessive forms of EBS have been associated with mutations in at least 10 genes, recessive EBS due to homozygous biallelic KRT5 mutations has not been reported previously; it has been hypothesized that it would result in prenatal lethality. We sought the genetic causes of EB in a cohort of 512 distinct EB families by performing whole exome sequencing (WES) and using an EB-targeting next-generation sequencing (NGS) panel of 21 genes. The pathogenicity and consequences of the mutations were determined by expression profiling and at tissue and ultrastructural levels. Two pathogenic, homozygous missense variants of KRT5 in two patients with generalized EBS and a homozygous null mutation in a patient who died as a neonate from complications of EB were found. The two missense mutations disrupted keratin 5 expression on immunofluorescence microscopy, and the human "knock-out" of KRT5 showed no RNA and protein expression. Collectively, these findings identify biallelic KRT5 mutations with a phenotypic spectrum varying from mild, localized and generalized to perinatal lethal, expanding the genotypic profile of autosomal recessive EBS.

Keratins and epidermolysis bullosa simplex

Keratin intermediate filaments play an important role in maintaining the integrity of the skin structure. Understanding the importance of this subject is possible with the investigation of keratin defects in epidermolysis bullosa simplex (EBS). Nowadays, in addition to clinical criteria, new molecular diagnostic methods, such as next generation sequencing, can help to distinguish the subgroups of EBS more precisely. Because the most important and most commonly occurring molecular defects in these patients are the defects of keratins 5 and14 (KRT5 and KRT14), comprehending the nature structure of these proteins and their involved processes can be very effective in understanding the pathophysiology of this disease and providing new and effective therapeutic platforms to treat it. Here, we summarized the various aspects of the presence of KRT5 and KRT14 in the epidermis, their relation to the incidence and severity of EBS phenotypes, and the processes with which these proteins can affect them.

"Nails Only" Phenotype and Partial Dominance of p.Glu170Lys Mutation in a Family with Epidermolysis Bullosa Simplex

Epidermolysis bullosa (EB) is a heterogeneous group of rare, chronic, inherited skin disorders characterized by marked mechanical fragility of epithelial tissues, with blistering and erosions after minor trauma. We present the first report of a nails-only phenotype in two patients with epidermolysis bullosa simplex (EBS) and a heterozygous pGlu170Lys mutation and the second reported case of EBS associated with a homozygous p.Glu170Lys mutation in the KRT5 gene. Our findings may be relevant for genetic counseling and for understanding the inheritance pattern of EBS.

Mapping genotype-phenotype associations of nsSNPs in coiled-coil oligomerization domains of the human proteome

We assessed the impact of disease mutations (DMs) versus polymorphisms (PYs) in coiled-coil (CC) domains in UniProt by modeling the structural and functional impact of variants in silico with the CC prediction program Multicoil. The structural impact of variants was evaluated with respect to three main metrics: the oligomerization score-to determine whether the variant is stabilizing or destabilizing-the oligomerization state, and the register-specific score. The functional impact was queried indirectly in several ways. First, we examined marginally stable CCs that were either stabilized or destabilized by the variant. Second, we looked for variants that altered the register of the wild-type CC near wild-type irregularities of likely functional importance, such as skips and stammers. Third, we searched for variants that altered the oligomerization state of the CC. DMs tended to be more destabilizing than PYs; but interestingly, PYs were more frequently associated with predicted changes in the oligomerization state. The functional impact was also queried by testing the association of CC variants with multiple phenotypes, that is, pleiotropy. Mutations in CC regions of proteins cause 155 different phenotypes and are more frequently associated with pleiotropy than proteins in general. Importantly, the CC region itself often encodes the pleiotropy.

Novel sporadic and recurrent mutations in KRT5 and KRT14 genes in Polish epidermolysis bullosa simplex patients: further insights into epidemiology and genotype-phenotype correlation

Epidermolysis bullosa simplex (EBS) is a hereditary genodermatosis characterised by trauma-induced intraepidermal blistering of the skin. EBS is mostly caused by mutations in the KRT5 and KRT14 genes. Disease severity partially depends on the affected keratin type and may be modulated by mutation type and location. The aim of our study was to identify the molecular defects in KRT5 and KRT14 in a cohort of 46 Polish and one Belarusian probands with clinical suspicion of EBS and to determine the genotype-phenotype correlation. The group of 47 patients with clinical recognition of EBS was enrolled in the study. We analysed all coding exons of KRT5 and KRT14 using Sanger sequencing. The pathogenic status of novel variants was evaluated using bioinformatical tools, control group analysis (DNA from 100 healthy population-matched subjects) and probands' parents testing. We identified mutations in 80 % of patients and found 29 different mutations, 11 of which were novel and six were found in more than one family. All novel mutations were ascertained as pathogenic. In the majority of cases, the most severe genotype was associated with mutations in highly conserved regions. In some cases, different inheritance mode and clinical significance, than previously reported by others, was observed. We report 11 novel variants and show novel genotype-phenotype correlations. Our data give further insight into the natural history of EBS molecular pathology, epidemiology and mutation origin.

Exome sequencing reveals a novel mutation, p.L325H, in the KRT5 gene associated with autosomal dominant Epidermolysis Bullosa Simplex Koebner type in a large family from western India

We report a large, non-consanguineous family comprising five generations of individuals residing in Gujarat, India affected with localized Epidermolysis Bullosa Simplex (EBS) Koebner type. We analyzed 14 individuals including 9 affected individuals from this family. Exome sequencing in two cases suggested a novel non-synonymous variation, p.L325H, in the KRT5 gene. The present analysis also reports the first causative mutation of EBS Koebner type from India.

Structural basis for heteromeric assembly and perinuclear organization of keratin filaments

There is as yet no high-resolution data regarding the structure and organization of keratin intermediate filaments, which are obligate heteropolymers providing vital mechanical support in epithelia. We report the crystal structure of interacting 2B regions from the central coiled-coil domains of keratins 5 and 14 (K5 and K14), expressed in progenitor keratinocytes of epidermis. The interface of the K5-K14 coiled-coil heterodimer has asymmetric salt bridges, hydrogen bonds and hydrophobic contacts, and its surface exhibits a notable charge polarization. A trans-dimer homotypic disulfide bond involving Cys367 in K14's stutter region occurs in the crystal and in skin keratinocytes, where it is concentrated in a keratin filament cage enveloping the nucleus. We show that K14-Cys367 impacts nuclear shape in cultured keratinocytes and that mouse epidermal keratinocytes lacking K14 show aberrations in nuclear structure, highlighting a new function for keratin filaments.

Hereditary leukonychia, or porcelain nails, resulting from mutations in PLCD1

Hereditary leukonychia (porcelain nails or white nails) is a rare nail disorder with an unknown genetic basis. To identify variants in a gene underlying this phenotype, we identified four families of Pakistani origin showing features of hereditary leukonychia. All 20 nails of each affected individual were chalky and white in appearance, consistent with total leukonychia, with no other cutaneous, appendageal, or systemic findings. By using Affymetrix 10K chip, we established linkage to chromosome 3p21.3-p22 with a LOD score (Z) of 5.1. We identified pathogenic mutations in PLCD1 in all four families, which encodes phosphoinositide-specific phospholipase C delta 1 subunit, a key enzyme in phosphoinositide metabolism. We then identified localization of PLCD1 in the nail matrix. It was recently shown that PLCD1 is a component of the human nail plate by proteomic analysis and is localized in the matrix of human nails. Furthermore, mutations detected in PLCD1 resulted in reduced enzymatic activity in vitro. Our data show that mutations in PLCD1 underlie hereditary leukonychia, revealing a gene involved in molecular control of nail growth.

Consequences of two different amino-acid substitutions at the same codon in KRT14 indicate definitive roles of structural distortion in epidermolysis bullosa simplex pathogenesis

Numerous inherited diseases develop due to missense mutations, leading to an amino-acid substitution. Whether an amino-acid change is pathogenic depends on the level of deleterious effects caused by the amino-acid alteration. We show an example of different structural and phenotypic consequences caused by two individual amino-acid changes at the same position. Epidermolysis bullosa simplex (EBS) is a genodermatosis resulting from KRT5 or KRT14 mutations. Mutation analysis of an EBS family revealed that affected individuals were heterozygous for a, to our knowledge, previously unreported mutation of c.1237G>C (p.Ala413Pro) in KRT14. Interestingly, 2 of 100 unrelated normal controls were heterozygous, and 1 of the 100 was homozygous for a different mutation in this position, c.1237G>A (p.Ala413Thr). In silico modeling of the protein demonstrated deleterious structural effects from proline substitution but not from threonine substitution. In vitro transfection studies revealed a significantly larger number of keratin-clumped cells in HaCaT cells transfected with mutant KRT14 complementary DNA (cDNA) harboring p.Ala413Pro than those transfected with wild-type KRT14 cDNA or mutant KRT14 cDNA harboring p.Ala413Thr. These results show that changes in two distinct amino acids at a locus are destined to elicit different phenotypes due to the degree of structural distortion resulting from the amino-acid alterations.

Keratin gene mutations in disorders of human skin and its appendages

Keratins, the major structural protein of all epithelia are a diverse group of cytoskeletal scaffolding proteins that form intermediate filament networks, providing structural support to keratinocytes that maintain the integrity of the skin. Expression of keratin genes is usually regulated by differentiation of the epidermal cells within the stratifying squamous epithelium. Amongst the 54 known functional keratin genes in humans, about 22 different genes including, the cornea, hair and hair follicle-specific keratins have been implicated in a wide range of hereditary diseases. The exact phenotype of each disease usually reflects the spatial expression level and the types of mutated keratin genes, the location of the mutations and their consequences at sub-cellular levels as well as other epigenetic and/or environmental factors. The identification of specific pathogenic mutations in keratin disorders formed the basis of our understanding that led to re-classification, improved diagnosis with prognostic implications, prenatal testing and genetic counseling in severe keratin genodermatoses. Molecular defects in cutaneous keratin genes encoding for keratin intermediate filaments (KIFs) causes keratinocytes and tissue-specific fragility, accounting for a large number of genetic disorders in human skin and its appendages. These diseases are characterized by keratinocytes fragility (cytolysis), intra-epidermal blistering, hyperkeratosis, and keratin filament aggregation in severely affected tissues. Examples include epidermolysis bullosa simplex (EBS; K5, K14), keratinopathic ichthyosis (KPI; K1, K2, K10) i.e. epidermolytic ichthyosis (EI; K1, K10) and ichthyosis bullosa of Siemens (IBS; K2), pachyonychia congenita (PC; K6a, K6b, K16, K17), epidermolytic palmo-plantar keratoderma (EPPK; K9, (K1)), monilethrix (K81, K83, K86), ectodermal dysplasia (ED; K85) and steatocystoma multiplex. These keratins also have been identified to have roles in apoptosis, cell proliferation, wound healing, tissue polarity and remodeling. This review summarizes and discusses the clinical, ultrastructural, molecular genetics and biochemical characteristics of a broad spectrum of keratin-related genodermatoses, with special clinical emphasis on EBS, EI and PC. We also highlight current and emerging model tools for prognostic future therapies. Hopefully, disease modeling and in-depth understanding of the molecular pathogenesis of the diseases may lead to the development of novel therapies for several hereditary cutaneous diseases.

Compound heterozygosity for dominant and recessive GJB2 mutations: effect on phenotype and review of the literature

Mutations in GJB2 (which encodes the gap-junction protein connexin 26) are the most common cause of genetic deafness in many populations. To date, more than 100 deafness-causing mutations have been described in this gene. The majority of these mutations are inherited in an autosomal recessive manner, but approximately 19 GJB2 mutations have been associated with dominantly inherited hearing loss. One, W44C, was first identified in two families from France. We subsequently described a family in the United States with the same mutation. In these families, W44C segregates with a dominantly inherited, early-onset, progressive, sensorineural deafness that is worse in the high frequencies. Since that report, we have tested additional family members and identified two siblings who are compound heterozygous for the W44C and K15T mutations. Their father, the original proband, is heterozygous for the dominant W44C mutation, and their mother is compound heterozygous for two recessively inherited mutations, K15T and 35delG. Both children have a profound, sensorineural deafness and use manual communication, in contrast to their parents and other relatives whose hearing losses are less severe and who can communicate orally. The difference in phenotype may be a result of the disruption of different functions of the gap-junction protein by the two mutations, which have an additive effect.

A spectrum of mutations in keratins K6a, K16 and K17 causing pachyonychia congenita

BACKGROUND

Pachyonychia congenita (PC) is a rare autosomal dominant keratin disorder, subdivided into two major variants, PC-1 and PC-2. Predominant characteristics include hypertrophic nail dystrophy, focal palmoplantar keratoderma and oral leukokeratosis. Multiple steatocystomas that develop during puberty are a useful feature distinguishing PC-2 from PC-1. At the molecular level it has been shown that mutations in keratin K6a or K16 cause PC-1 whereas those in K6b or K17 lead to PC-2.

OBJECTIVE

To identify mutations in 22 families presenting with clinical symptoms of either PC-1/focal non-epidermolytic palmoplantar keratoderma (FNEPPK) or PC-2.

METHODS

Mutation analysis was performed on genomic DNA from PC patients by direct sequencing.

RESULTS

Here, we report four new missense and five known mutations in K6a; one new deletion and three previously identified missense mutations in K16; plus one known mutation in K17.

CONCLUSION

With one exception, all these heterozygous mutations are within the highly conserved helix boundary motif regions at either end of the keratin rod domain. In one sporadic case, a unique mutation in K16 resulting in deletion of 24bp was found within the central rod domain, in a child with a phenotype predominantly consisting of focal plantar keratoderma. The identification of mutations in cases of PC is prerequisite for future development of gene-specific and/or mutation-specific therapies.

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.

Filaggrin mutations are genetic modifying factors exacerbating X-linked ichthyosis

Mutations inactivating the STS gene cause X-linked ichthyosis (XLI), whereas null mutations in the FLG gene cause ichthyosis vulgaris. Two brothers presented with XLI. One had a typical fine scaling, and the other was much more severely affected. Both patients carried STS missense mutation T165I. Furthermore, the more severely affected patient also carried heterozygous FLG mutation R501X, which was absent from his mildly affected brother. These data suggest that disrupting epidermal differentiation via different pathways can increase phenotypic severity. Owing to the high population frequency of FLG mutations, filaggrin is a possible genetic modifier in other genodermatoses.

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.

Insights into genotype-phenotype correlation in pachyonychia congenita from the human intermediate filament mutation database

Keratins are the intermediate filament proteins specifically expressed by epithelial cells. The Human Genome Project has uncovered a total of 54 functional keratin genes that are differentially expressed in specific epithelial structures of the body, many of which involve the epidermis and its appendages. Pachyonychia congenita (PC) is a group of autosomal dominant genodermatoses affecting the nails, thick skin and other ectodermal structures, according to specific sub-type. The major clinical variants of the disorder (PC-1 and PC-2) are known to be caused by dominant-negative mutations in one of four differentiation-specific keratins: K6a, K6b, K16, and K17. A total of 20 human keratin genes are currently linked to single-gene disorders or are predisposing factors in complex traits. In addition, a further six intermediate filament genes have been linked to other non-epithelial genetic disorders. We have established a comprehensive mutation database that catalogs all published independent occurrences of intermediate filament mutations (http://www.interfil.org), with details of phenotypes, published papers, patient support groups and other information. Here, we review the genotype-phenotype trends emerging from the spectrum of mutations in these genes and apply these correlations to make predictions about PC phenotypes based on the site of mutation and keratin pair involved.

Preservation of phenotype in an organotypic cell culture model of a recessive keratinization defect of Norfolk terrier dogs

The purpose of this study is to reproduce in vitro a recessive keratinization defect of Norfolk terrier dogs characterized by a lack of keratin 10 (K10) production. Keratinocytes from skin biopsy samples of four normal dogs and two affected dogs were cultured organotypically with growth factor-supplemented media in order to stimulate cornification. The cultured epidermis from the normal dogs closely resembled the normal epidermis in vivo and cornified. The cultured epidermis from the affected dogs displayed many phenotypic alterations identified in skin biopsies from dogs with this heritable defect. Immunohistochemistry and immunoblotting showed a marked decrease in K10 from the cultures of the affected keratinocytes, compared to that in K10 from the cultures of the normal keratinocytes. Real-time reverse transcription polymerase chain reaction quantitation showed a 31-fold decrease in K10, a 1.75-fold increase in K1 and a 136-fold increase in K2e between the affected and the normal epidermis. Organotypic keratinocytes showed a 241-fold decrease in K10, a 31-fold decrease in K1 and a 1467-fold decrease in K2e between the affected and normal cultures. Although in vitro keratin expression did not precisely simulate in vivo, the morphology of the normal and the affected epidermis was largely preserved; thus, this culture system may provide an alternative to in vivo investigations for cutaneous research involving cornification.

Mild recessive epidermolytic hyperkeratosis associated with a novel keratin 10 donor splice-site mutation in a family of Norfolk terrier dogs

BACKGROUND

Epidermolytic hyperkeratosis in humans is caused by dominant-negative mutations in suprabasal epidermal keratins 1 and 10. However, spontaneous keratin mutations have not been confirmed in a species other than human.

OBJECTIVES

To describe an autosomal recessive, mild, nonpalmar/plantar epidermolytic ichthyosis segregating in an extended pedigree of Norfolk terrier dogs due to a splice-site mutation in the gene encoding keratin 10 (KRT10).

METHODS

Dogs were evaluated clinically, and skin samples were examined by light and electron microscopy. Genomic DNA samples and cDNA from skin RNA were sequenced and defined a mutation in KRT10. Consequences of the mutation were evaluated by assessing protein expression with immunohistochemistry and Western blotting and gene expression with real-time RT-PCR (reverse transcriptase-polymerase chain reaction).

RESULTS

Adult dogs with the disease had generalized, pigmented hyperkeratosis with epidermal fragility. Light microscopic examination defined epidermolysis with hyperkeratosis; ultrastructural changes included a decrease in tonofilaments and abnormal filament aggregation in upper spinous and granular layer keratinocytes. Affected dogs were homozygous for a single base GT-->TT change in the consensus donor splice site of intron 5 in KRT10. Keratin 10 protein was not detected with immunoblotting in affected dogs. Heterozygous dogs were normal based on clinical and histological appearance and keratin 10 protein expression. The mutation caused activation of at least three cryptic or alternative splice sites. Use of the cryptic sites resulted in transcripts containing premature termination codons. One transcript could result in shortening of the proximal portion of the 2B domain before the stutter region. Quantitative real-time PCR indicated a significant decrease in KRT10 mRNA levels in affected dogs compared with wild-type dogs.

CONCLUSIONS

This disease is the first confirmed spontaneous keratin mutation in a nonhuman species and is the first reported recessive form of epidermolytic hyperkeratosis.

Inherited defects in keratins

In the years following the initial reports of keratin gene mutations in epidermolysis bullosa simplex, great strides have been made in understanding the basic biology of human keratins and in understanding the etiology and pathogenesis of a number of specific human single gene disorders. A total of 19 human keratin genes is now linked to specific diseases. This article summarizes current knowledge in relation to basic keratin biology, known disease associations, and genotype correlation in this diverse and complex group of conditions.

The keratins and their disorders

Diseases caused by mutations in gene encoding keratin intermediate filaments (IF) are characterized by a loss of structural integrity in the cells expressing those keratins in vivo. This is manifested as cell fragility, compensatory epidermal hyperkeratosis, and keratin filament aggregation in some affected tissues. Keratin disorders are a novel molecular category including quite different phenotypes such as epidermolysis bullosa simplex (EBS), bullous congenital ichthyosiform erthroderma (BCIE), pachyonychia congenital (PC), steatocystoma multiplex, ichthyosis bullosa of Siemens (IBS), and white sponge nevus (WSN) of the orogenital mucosa.

Keratins and skin disorders

The association of keratin mutations with genetic skin fragility disorders is now one of the best-established examples of cytoskeleton disorders. It has served as a paradigm for many other diseases and has been highly informative for the study of intermediate filaments and their associated components, in helping to understand the functions of this large family of structural proteins. The keratin diseases have shown unequivocally that, at least in the case of the epidermal keratins, a major function of intermediate filaments is to provide physical resilience for epithelial cells. This review article reflects on the variety of phenotypes arising from mutations in keratins and the reasons for this variation.

Atypical epidermolytic palmoplantar keratoderma presentation associated with a mutation in the keratin 1 gene

BACKGROUND

Epidermolytic palmoplantar keratoderma (EPPK) is an autosomal dominant genodermatosis characterized by epidermolytic hyperkeratosis strictly confined to the palms and soles, and usually associated with mutations in the keratin K9 gene (KRT9). Mutations in the keratin K1 gene (KRT1) have been shown to underlie a variety of phenotypes typically involving generalized epidermolytic hyperkeratosis, but in some cases the phenotype can be more regionally restricted.

OBJECTIVES

To identify the genetic defect in two unrelated families initially presenting with EPPK but where careful examination revealed hyperkeratosis extending on to the proximal wrist flexure. Methods Linkage analysis and DNA sequencing.

RESULTS

We found that this phenotype is caused by a heterozygous missense mutation in the K1 gene, designated I479T. This mutation lies in the highly conserved helix termination motif of K1, previously shown to be important for keratin assembly and filament formation. In general, mutations in this region of keratins are associated with more severe disease phenotypes. However, K1 mutations in this region and the I479T mutation in particular have previously been associated with both severe and mild bullous congenital ichthyosiform erythroderma phenotypes. When further clinical enquiries were made, several affected individuals in the families studied here were found to have had transient flexural peeling and hyperkeratosis in the neonatal period.

CONCLUSIONS

K1 mutations may underlie a phenotype closely resembling EPPK. A history of transient flexural peeling and hyperkeratosis in childhood and palmoplantar keratoderma which extends beyond the boundary of the palmoplantar margins may indicate a K1 mutation rather than a K9 defect. As K1 mutations are also associated with severe widespread phenotypes, with important implications for prognostic and genetic counselling, whole body examination is recommended for patients presenting with EPPK.

A Heritable Keratinization Defect of the Superficial Epidermis in Norfolk Terriers

Although well-characterized in man, abnormal cornification secondary to heritable superficial keratin defects is rarely reported in animals. This report describes a mild cornification defect in seven related Norfolk terrier dogs. Lesions were present at birth and pedigree analysis suggested an autosomal recessive mode of inheritance. The affected dogs had hyperpigmented skin with scaling following mild trauma. The lesions were generalized but most prominent in the glabrous skin of the axillary and inguinal regions-areas where the epidermis is not protected by hair and is subject to frequent trauma. The most striking histological change was vacuolation in the upper epidermis, which often resulted in epidermolysis and blister formation. All of the affected dogs showed similar gross and histological changes. Ultrastructural changes included abnormal keratin filament clumping, prominent clear spaces in the cytoplasm of suprabasal keratinocytes, and abnormal keratohyaline granules. Immunohistochemical labelling for keratin 10 demonstrated a lack of expression in the superficial epidermis of affected dogs. All of the morphological changes noted in the Norfolk terriers were consistent with a mild form of a heritable defect in superficial keratin synthesis.

Citoqueratinas

As citoqueratinas (CQ) são constituintes do citoesqueleto das células epiteliais, pertencendo aos filamentos intermediários; sua distribuição é específica para cada subtipo de epitélio, permitindo que sejam utilizadas como importantes marcadores de sua diferenciação. Anticorpos monoclonais permitem sua localização nos tecidos e são utilizados no diagnóstico de tumores. Na última década inúmeras mutações foram descritas em seus genes, levando a alteração em sua estrutura molecular, esclarecendo várias enfermidades cutâneas, como epidermólise bolhosa simples (CQ 5 ou 14), hiperqueratose epidermolítica (CQ 1 ou 10), hiperqueratose palmoplantar epidermolítica (CQ 9) e paquioníquia congênita (CQ 6, 16 ou 17).

Mutational analysis of the ATP2A2 gene in two Darier disease families with intrafamilial variability

BACKGROUND

Darier disease (DD), an autosomal dominant genodermatosis characterized by warty papules and plaques over seborrhoeic areas, is caused by mutations in the ATP2A2 gene, which encodes the sarco/endoplasmic reticulum Ca(2+) ATPase type 2 isoform (SERCA2). While markedly different clinical severity within DD-affected family members is known, the pathomechanism has not been elucidated.

OBJECTIVES

Based on the hypothesis that multiple ATP2A2 mutations might contribute to the pathomechanism, we have analysed two DD families in which the clinical severity differs markedly within a single pedigree, and, as controls, eight DD families without differing clinical severity.

METHODS

All the exons and intron-exon borders of ATP2A2 were directly sequenced from the genomic DNA extracted from all the subjects.

RESULTS

We identified the heterozygous mutations, G233R in pedigree 1 and C318R in pedigree 2, respectively, whereas no other ATP2A2 mutations in any of severely affected individuals were found. In eight DD pedigrees as control, we have found M1V, N39D, L180R, A838P and 2170 insertion G in each of five pedigrees, but no mutation was found in three DD pedigrees.

CONCLUSIONS

Our results together with previous data indicate that the distribution of mutations is scattered over the entire ATP2A2 without any, as yet, discernible 'hotspots'. The mutations in pedigrees 1 and 2 with intrafamiliar clinical differences occurred around the Ca(2+)-binding sites on SERCA2, which might be associated with differences in clinical severity. These variations in ATP2A2 mutations alone cannot account for the clinical heterogeneity within DD pedigrees.

Phenotypes, genotypes and their contribution to understanding keratin function

A large number of mutations in keratin genes underlie inherited tissue fragility disorders of epithelia. The genotype-phenotype correlations emerging from these studies provide a rich source of information about the function of keratins that would have taken decades to achieve by a purely transgenic approach. Human disease studies are being supplemented by engineered mouse mutant studies, which give access to the effects of genetic alterations unlikely to occur naturally. Evidence is emerging that the great diversity of keratins might be required to enable cells to adapt their structure in response to different signalling pathways.

Light and death: photons and apoptosis

Phototherapies like photodynamic therapy (PDT), UVA1, UVB, and PUVA treat skin diseases. These phototherapies work because they alter cytokine profiles, change immune cytotoxicity in the skin, and directly kill diseased cells by apoptosis. Apoptosis is a term that only describes the morphologic changes a cell undergoes during this mode of cell death. The terms "immediate", "intermediate", and "delayed" apoptosis segregate the different apoptotic mechanisms into three kinetic categories, whereas the terms preprogrammed cell death (pre-PCD) and programmed cell death (PCD) describe the underlying mechanisms. Immediate apoptosis (T< or =0.5 h post-exposure) is triggered by singlet-oxygen damage that opens the mitochondrial megachannel, which can be mediated by PDT or UVA1 radiation. It is a pre-PCD mechanism of apoptosis, i.e., protein synthesis is not required post-insult, because all the necessary components are constitutively synthesized and only need to be activated. Intermediate apoptosis (T< or =4 h>0.5 h) is initiated by receptor cross-linking on the plasma membrane, which can be achieved using high doses of UVB or UVC radiation. It is also a pre-PCD mechanism. Delayed apoptosis (T>4 h) is induced by DNA damage that can be caused by X-rays, PUVA, UVC, UVB, UVA, and PDT. It is a PCD mechanism of apoptosis, i.e., protein synthesis is required post-insult. These three apoptotic mechanisms each access one of two "points-of-no-return" located on the mitochondrial membrane, which activate different, but not mutually exclusive, final pathways of apoptosis. This review discusses the latest findings on these apoptotic mechanisms and their implications in phototherapies.