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

Proteomic profiling reveals KRT6C as a probable hereterodimer partner for KRT9: New insights into re-classifying epidermolytic palmoplantar keratoderma (EPPK) and a milder form of pachyonychia congenita (PC-K6c) as a group of genetic cutaneous disorders

Epidermolytic palmoplantar keratoderma (EPPK), a highly penetrant autosomal dominant genodermatosis, is characterized by diffuse keratoses on palmplantar epidermis. The keratin 9 gene (KRT9) is responsible for EPPK. To date, phenotypic therapy is the primary treatment for EPPK. Because KRT9 pairs with a type II keratin-binding partner to function in epidermis, identifying the interaction partner is an essential first step in revealing EPPK pathogenesis and its fundamental treatment. In this study, we proved that keratin 6C (KRT6C) is a probable hereterodimer partner for KRT9. In silico model for KRT6C/KRT9 shows a typical coiled-coil structure in their 2B domains. Proteomics analysis shows that KRT6C/KRT9 pair is in a densely connected protein-protein interaction network, where proteins participate jointly in regulating cytoskeleton organization and keratinization. This study shows that co-immunoprecipitation coupled with mass spectroscopy and proteomics analysis provide a sensitive approach, which compensates for inevitable inadequacies of anti-keratin 6C antibody and helps discover the probable hereterodimer partner KRT6C for KRT9. The acknowledgement of KRT6C/KRT9 pairwise relationship may help re-classify EPPK and PC-K6c (a milder form of pachyonychia congenita, caused by KRT6C) as a group of hereditary defects at a molecular-based level, and lay foundation for deciphering the keratin network contributing to EPPK and PC-K6c. SIGNIFICANCE OF THE STUDY: What is already known about this topic? KRT9 and KRT6C are disease-causing factors for epidermolytic palmoplantar keratoderma (EPPK) and a milder form of pachyonychia congenita (PC-K6c), respectively. EPPK and PC-K6c have some symptom similarities. Keratins are the major structural proteins in epithelial cells. Each of the type I keratin is matched by a particular type II keratin to assemble a coiled-coil heterodimer. The hereterodimer partner for KRT9 is unknown. What does this study add? We discovered and proved that KRT6C is a probable hereterodimer partner for KRT9 in palmplantar epidermis in a native endogenous environment by using co-immunoprecipitation coupled with mass spectroscopy and proteomics analysis, etc. The proteomics analysis shows that KRT6C/KRT9 keratin pair is in a densely connected protein-protein interaction network, where proteins participate jointly in regulating intermediate filament-based cytoskeleton organization and keratinization processes. What are the implications of this work? The new understanding of probable KRT6C/KRT9 pairwise correlation may help re-classify the genetic cutaneous disorders EPPK and PC-K6c as a group of hereditary defects at a molecular-based level, and lay foundation for pathogenic mechanism research in EPPK and PC-K6c. The densely related network components derived from the proteomic data using Metascape in the study and pairwise regulation fashion of specific keratin pairs should attract more attention in the further explorations when investigators concern the physiological functions of keratins and the pathogenesis of related skin diseases.

Case Report: A Novel GJB2 Missense Variant Inherited From the Low-Level Mosaic Mother in a Chinese Female With Palmoplantar Keratoderma With Deafness

Dominant variants in the gap junction beta-2 (GJB2) gene may lead to various degrees of syndromic hearing loss (SHL) which is manifest as sensorineural hearing impairment and hyperproliferative epidermal disorders, including palmoplantar keratoderma with deafness (PPKDFN). So far, only a few GJB2 dominant variants causing PPKDFN have been discovered. Through the whole-exome sequencing (WES), a Chinese female patient with severe palmoplantar hyperkeratosis and delayed-onset hearing loss has been identified. She had a novel heterozygous variant, c.224G>C (p.R75P), in the GJB2 gene, which was unreported previously. The proband’s mother who had a mild phenotype was suggested the possibility of mosaicism by WES (∼120×), and the ultra-deep targeted sequencing (∼20,000×) was used for detecting low-level mosaic variants which provided accurate recurrence-risk estimates and genetic counseling. In addition, the analysis of protein structure indicated that the structural stability and permeability of the connexin 26 (Cx26) gap junction channel may be disrupted by the p.R75P variant. Through retrospective analysis, it is detected that the junction of extracellular region-1 (EC1) and transmembrane region-2 (TM2) is a variant hotspot for PPKDFN, such as p.R75. Our report reflects the important and effective diagnostic role of WES in PPKDFN and low-level mosaicism, expands the spectrum of the GJB2 variant, and furthermore provides strong proof about the relevance between the p.R75P variant in GJB2 and PPKDFN.

Hereditary Palmoplantar Keratoderma: A Practical Approach to the Diagnosis

The ridged skin of the palms and soles has several unique features: (i) presence of dermatoglyphics created by alternating ridges and grooves forming a unique pattern, (ii) presence of the highest density of eccrine sweat glands and absence of pilosebaceous units, and (iii) differential expression of keratins compared to the glabrous skin. These features explain the preferential localization of palmoplantar keratoderma (PPK) and several of its characteristic clinical features. PPK develops as a compensatory hyperproliferation of the epidermis and excessive production of stratum corneum in response to altered cornification of the palmoplantar skin due to mutations in the genes encoding several of the proteins involved in it. PPK can manifest as diffuse, focal, striate, or punctate forms per se or as a feature of several dermatological or systemic diseases. There is a wide genetic and phenotypic heterogeneity in hereditary PPK, due to which reaching an accurate diagnosis only on the basis of clinical features may be sometimes challenging for the clinicians in the absence of molecular studies. Nevertheless, recognizing the clinical patterns of keratoderma, extent of involvement, degree of mutilation, and associated appendageal and systemic involvement may help in delineating different forms. Molecular studies, despite high cost, are imperative for accurate classification, recognizing clinical patterns in resource poor settings is important for appropriate diagnosis, genetic counseling, and management. This review intends to develop a practical approach for clinical diagnosis of different types of hereditary PPK with reasonable accuracy.

Novel Splice-Site Mutation of KRT1 Underlies Diffuse Palmoplantar Keratoderma in a Large Chinese Pedigree

AIMS

To identify potential causative gene mutations in a large Han Chinese pedigree with diffuse nonepidermolytic palmoplantar keratoderma (NEPPK).

METHODS

We enrolled 11 patients and 8 healthy individuals from a pedigree with NEPPK and 100 randomly selected healthy controls. Biopsy samples were obtained from the proband. Genomic DNA was extracted from a peripheral blood sample from each participant. Mutation detection via polymerase chain reaction and Sanger sequencing of relevant potential causative genes, including KRT1, KRT6C, KRT10, KRT16, AQP5, and SERPINB7, was performed. Comparisons were made between sequencing outcomes and currently available reference genome databases, including HGMD Pro, Pubmed, 1000 Genomics, and dbSNP.

RESULTS

Histological findings, clinical features, and medical history were in accordance with the diagnosis of diffuse NEPPK. We identified a novel splice-site mutation c.1255-1G > C in intron 6 of KRT1 in all individuals with NEPPK in the pedigree.

CONCLUSIONS

Diffuse NEPPK is a relatively rare subtype of palmoplantar keratoderma. The results of this study expand the spectrum of KRT1 mutations in diffuse NEPPK and provide insights into the understanding of its underlying pathological mechanisms and phenotype-genotype correlations.

Hereditary palmoplantar keratodermas. Part I. Non-syndromic palmoplantar keratodermas: classification, clinical and genetic features

The term palmoplantar keratoderma (PPK) indicates any form of persistent thickening of the epidermis of palms and soles and includes genetic as well as acquired conditions. We review the nosology of hereditary PPKs that comprise an increasing number of entities with different prognoses, and a multitude of associated cutaneous and extracutaneous features. On the basis of the phenotypic consequences of the underlying genetic defect, hereditary PPKs may be divided into the following: (i) non-syndromic, isolated PPKs, which are characterized by a unique or predominant palmoplantar involvement; (ii) non-syndromic PPKs with additional distinctive cutaneous and adnexal manifestations, here named complex PPKs; (iii) syndromic PPKs, in which PPK is associated with specific extracutaneous manifestations. To date, the diagnosis of the different hereditary PPKs is based mainly on clinical history and features combined with histopathological findings. In recent years, the exponentially increasing use of next-generation sequencing technologies has led to the identification of several novel disease genes, and thus substantially contributed to elucidate the molecular basis of such a heterogeneous group of disorders. Here, we focus on hereditary non-syndromic isolated and complex PPKs. Syndromic PPKs are reviewed in the second part of this 2-part article, where other well-defined genetic diseases, which may present PPK among their phenotypic manifestations, are also listed and diagnostic and therapeutic approaches for PPKs are summarized.

Progress towards genetic and pharmacological therapies for keratin genodermatoses: current perspective and future promise

Hereditary keratin disorders of the skin and its appendages comprise a large group of clinically heterogeneous disfiguring blistering and ichthyotic diseases, primarily characterized by the loss of tissue integrity, blistering and hyperkeratosis in severely affected tissues. Pathogenic mutations in keratins cause these afflictions. Typically, these mutations in concert with characteristic features have formed the basis for improved disease diagnosis, prognosis and most recently therapy development. Examples include epidermolysis bullosa simplex, keratinopathic ichthyosis, pachyonychia congenita and several other tissue-specific hereditary keratinopathies. Understanding the molecular and genetic events underlying skin dysfunction has initiated alternative treatment approaches that may provide novel therapeutic opportunities for affected patients. Animal and in vitro disease modelling studies have shed more light on molecular pathogenesis, further defining the role of keratins in disease processes and promoting the translational development of new gene and pharmacological therapeutic strategies. Given that the molecular basis for these monogenic disorders is well established, gene therapy and drug discovery targeting pharmacological compounds with the ability to reinforce the compromised cytoskeleton may lead to promising new therapeutic strategies for treating hereditary keratinopathies. In this review, we will summarize and discuss recent advances in the preclinical and clinical modelling and development of gene, natural product, pharmacological and protein-based therapies for these disorders, highlighting the feasibility of new approaches for translational clinical therapy.

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.

Autosomal dominant prelingual hearing loss with palmoplantar keratoderma syndrome: Variability in clinical expression from mutations of R75W and R75Q in the GJB2 gene

About one to three of a 1,000 neonates are afflicted at birth with a serious hearing impairment, with about half of the cases due to genetic causes. Genetic causes of hearing impairment are very heterogeneous. About half of all cases of genetically caused nonsyndromic hearing loss can be ascribed to mutations in the GJB2 gene (connexin 26) and to deletions in the GJB6 gene(connexin 30). Thus far, about 90 different mutations have been identified in the GJB2 gene, of which the majority are autosomal recessive. Ten mutations are autosomal dominant and are in most cases associated with various skin diseases: the keratitis-ichthyosis-deafness (KID) syndrome, Vohwinkel syndrome and palmoplantar keratoderma with deafness. To date, the following mutations have been identified which lead to the Palmoplantar Keratoderma syndrome with deafness; Gly59Ala, Gly59Arg, His73Arg, Arg75Trp, and Arg75Gln. We are reporting on four patients with severe hearing impairment. They are members of three unrelated families, who are carriers of mutations Arg75Trp or Arg75Gln, but unlike patients of other publications, do not all present with Palmoplantar Keratoderma syndrome. Our investigations document additional evidence for the correlation between the cited mutations in the GJB2 gene and a syndromic hearing impairment with palmoplantar keratoderma.

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.

Mutation L437P in the 2B domain of keratin 1 causes diffuse palmoplantar keratoderma in a Chinese pedigree

Diffuse palmoplantar keratoderma (DPPK) is an autosomal dominant genodermatosis characterized by uniform hyperkeratosis of the palm and sole epidermis. This disorder can be caused by mutations in the genes keratin 1, keratin 9, keratin 16, desmoglein 1 and plakoglobin. Here we present a DPPK Chinese pedigree and identify the aetiology as a novel missense mutation, L437P, located in a highly conserved helix motif in domain 2B of KRT1. Functional analysis shows that overexpression of the L437P mutant in cultured cells leads to abnormal intermediate filament networks and filament aggregation. This gain-of-function mutation highlights the role of domain 2B in mediating filament assembly.

Keratin 1 gene mutation detected in epidermal nevus with epidermolytic hyperkeratosis

Since 1994, four cases of epidermal nevus with epidermolytic hyperkeratosis (EH) caused by keratin 10 gene mutations have been reported, although no keratin 1 (K1) gene mutation has yet been reported. We detected a K1 gene (KRT1) mutation in epidermal nevus with EH in a 10-year-old Japanese male. The patient showed well-demarcated verrucous, hyperkeratotic plaques mainly on the trunk, covering 15% of the entire body surface. No hyperkeratosis was seen on the palms or soles. He had no family history of skin disorders. His lesional skin showed typical granular degeneration and, ultrastructurally, clumped keratin filaments were observed in the upper epidermis. Direct sequence analysis of genomic DNA extracted from lesional skin revealed a heterozygous 5' donor splice site mutation c.591+2T>A in KRT1. This mutation was not detected in genomic DNA samples from the patient's peripheral blood leukocytes or those of other family members. The identical splice mutation was previously reported in a family with palmoplantar keratoderma and mild ichthyosis, and was demonstrated to result in a 22 amino-acid deletion p.Val175_Lys196del in the H1 and 1A domains of K1. To our knowledge, the present patient is the first reported case of epidermal nevus associated with EH caused by a K1 gene mutation in a mosaic pattern.

Hereditary diffuse palmoplantar keratodermas in Slovenia: epidemiologic foci in remote rural areas

BACKGROUND

Previous studies carried out in Slovenia revealed a high frequency of cases of hereditary diffuse palmoplantar keratodermas (DPPK). The relatively small total population of about two million in a small territory and an efficient public health service were favorable preconditions for such a study.

METHODS

Existing hospital and outpatient department records served as starting points. Patients were invited to come for a follow-up examination, and visiting the patients at their homes enabled us to gather further data. Thus efforts were made to include all patients with hereditary DPPK in Slovenia.

RESULTS

Altogether 170 DPPK patients were detected, giving a prevalence of 8.3 per 100,000 inhabitants. The patients originated from remote, mostly mountainous districts, where the local DPPK prevalence highly significantly exceeded the average Slovene prevalence. The segregation ratio showed an autosomal dominant mode of inheritance. The percentage of persons affected was 34.4% (95% confidence interval 29.8-39.4), lower than expected for autosomal dominant inheritance (the difference is highly significant, P < 0.00001; exact binomial test).

CONCLUSION

One autosomal dominant gene alone does not fully explain the transmission of the disorder to siblings. Evidence is produced that additional factors are necessary for the transmission of this genetic condition. The degree of consanguinity and the physical pressure on palms and soles seem to play an important part. It is reasonable to expect that molecular-biology studies linked to the epidemiological data could contribute to the solution of the problem.

Two families with Greither's syndrome caused by a keratin 1 mutation

Transgrediens et progrediens palmoplantar keratoderma, known as Greither's syndrome, was originally described in 1952 and is characterized by diffuse keratoderma of the palms and soles, extending to the back aspects (transgrediens) and involving the skin over the Achilles' tendon. Patchy hyperkeratosis also develops on the shins, knees, elbows, and sometimes on the skin flexures. We describe two unrelated families affected with Greither's syndrome, in which the same dominant missense mutation gave rise to the amino acid change N188S in K1. The previously reported cases of Greither's syndrome showed phenotypic variability suggestive of different underlying gene defects. Our findings suggest that at least some cases of Greither's syndrome are caused by keratin mutations.

Mutation S233L in the 1B domain of keratin 1 causes epidermolytic palmoplantar keratoderma with "tonotubular" keratin

Epidermolytic palmoplantar keratoderma (EPPK) is an autosomal dominant genodermatosis characterized by epidermolytic hyperkeratosis restricted to the palm and sole epidermis. The disorder is normally associated with dominant-negative mutations in the keratin 9 (K9) gene; however, a small number of cases have been reported where causative mutations were identified in the K1 gene. Here, we present two unrelated Dutch EPPK families with striking ultrastructural findings: tubular keratin structures in the cytoplasm of suprabasal cells. Similar structures were reported previously in a German EPPK family and were termed "tonotubular" keratin. After excluding the involvement of the K9 gene by complete sequencing, we identified a novel mutation, S233L, at the beginning of the 1B domain of K1 in both families. Protein expression studies in cultured cells indicated pathogenicity of this mutation. This is the first report of a genetic defect in this domain of K1. The unusual gain-of-function mutation points to a subtle role of the 1B domain in mediating filament-filament interactions with regular periodicity.

A case of bullous congenital ichthyosiform erythroderma (BCIE) caused by a mutation in the 1A helix initiation motif of keratin 1

Bullous congenital ichthyosiform erythroderma (BCIE) is an autosomally dominant inherited disorder characterized by erythematous, erosive, and bullous skin lesions over the entire body at birth and abnormal hyperkeratosis on the palmoplantar sufaces as the patient grows older. BCIE is caused by a mutation in the keratin 1 (K1) and/or keratin 10 (K10) genes, and most pathogenic mutations are found within the helix initiation and termination motifs of the central helical rod domain (K1 and K10) or the upstream H1 homology domain (K10). In addition to inherited cases, sporadic cases due to a new mutation account for approximately half the total cases of BCIE. We report herein a typical sporadic case of BCIE with erythroderma, erosion, and blisters on the entire body surface at birth and palmoplantar and flexuaral areas of hyperkeratosis in the later stage. We found in this case a novel mutation, 559C to T, at amino acid position 187, which resulted in a leucine to phenylalanine substitution within the helix initiation motif of K1.

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.

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.

Palmoplantar keratodermas

The palmoplantar skin is a highly specialized tissue which is able to resist mechanical trauma and other physical stress. In recent years the more descriptive classification of keratodermas has switched to an exact molecular genetic view where gene functions are considered. Palmoplantar keratodermas can be separated in the following functional subgroups: disturbed gene fuctions in structural proteins (keratins), cornified envelope (loricrin, transglutaminase), cohesion (plakophilin, desmoplakin, desmoglein1), cell-to-cell communication (connexins), and transmembrane signal transduction (cathepsin C). This review intends to emphasize the typical clinical aspects and symptom complexes associated with palmoplantar keratodermas which enable the astute dermatologist to make a clinical diagnosis. In addition the molecular genetic knowledge on the topic is given which is necessary to confirm the clinical diagnosis.

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.

Epidermolytic hyperkeratosis type PS-1 caused by aberrant splicing of KRT1

Mutations in the keratin 1 (KRT1) gene underlie epidermolytic hyperkeratosis (EHK). This autosomal dominant disorder is characterized by phenotypic heterogeneity. In the present study, we assessed a 33-year-old individual presenting with severe palmoplantar keratoderma and histopathological findings suggestive of EHK. We analysed genomic DNA extracted from the patient's blood lymphocytes for pathogenic mutations in KRT1. A heterozygous 4-bp deletion was identified in intron 1 of the gene (591+3_+6delGAGT), suggesting the possibility that it may interfere with the normal splicing of intron 1. We detected a 66-bp deletion in KRT1 mRNA extracted from the patient's skin, predicted to result in the translation of a mutant KRT1 lacking 22 amino acids, including the conserved helix initiation motif. The identification of this unusual and novel mutation underscores the diagnostic importance of sequence analysis of keratin gene noncoding regions.

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.

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.