A missense mutation in connexin26, D66H, causes mutilating keratoderma with sensorineural deafness (Vohwinkel's syndrome) in three unrelated families

The role of connexins in human disease

Connexins are the building blocks of gap junctions. In forming a gap junction, six connexins oligomerize to form a hexameric torus called a connexon. The number of gap junctions in a cell ranges from a few to over 105 and imparts to interconnected cells a uniform phenotype. The crucial role that gap junctions play in normal physiology is reflected by the diverse spectrum of human diseases in which allele variants of different gap junction genes are implicated. In particular, mutations in GJB2 are a major cause of autosomal recessive non-syndromic deafness. This discovery has impacted medical practice and makes it incumbent on clinicians to familiarize themselves with the genetic advances that are rapidly occurring in our field.

Frequencies of gap- and tight-junction mutations in Turkish families with autosomal-recessive non-syndromic hearing loss

Mutations in genes encoding gap- and tight-junction proteins have been shown to cause distinct forms of hearing loss. We have now determined the GJB2[connexin 26 (Cx26)] mutation spectrum in 60 index patients from mostly large Turkish families with autosomal-recessive inherited non-syndromic sensorineural hearing loss (NSSHL). GJB2 mutations were found in 31.7% of the families, and the GJB2-35delG mutation accounted for 73.6% of all GJB2 mutations. The carrier frequency of GJB2-35delG in the normal Turkish population was found to be 1.17% (five in 429). In addition to the described W24X, 233delC, 120delE and R127H mutations, we also identified a novel mutation, Q80R, in the GJB2 gene. Interestingly, the Q80R allele was inherited on the same haplotype as V27I and E114G polymorphisms. As little is known about the mutation frequencies of most other recently identified gap- and tight-junction genes as a cause for hearing loss, we further screened our patients for mutations in GJB3 (Cx31), GJA1 (Cx43), DeltaGJB6-D13S1830 (Cx30) and the gene encoding the tight-junction protein, claudin 14 (CLDN14). Several novel polymorphisms, but no disease-associated mutations, were identified in the CLND14 and GJA1 genes, and we were unable to detect the DeltaGJB6-D13S1830 deletion. A novel putative mutation, P223T, was found in the GJB3 gene in heterozygous form in a family with two affected children. Our data shows that the frequency of GJB2 mutations in Turkish patients with autosomal-recessive NSSHL and the carrier rate of the GJB2-35delG mutation in the Turkish population, is much lower than described for other Mediterranean countries. Furthermore, mutations in other gap- and tight-junction proteins are not a frequent cause of hearing loss in Turkey.

Ceratodermia mutilante de Vohwinkel: relato de três casos em uma família

A ceratodermia hereditária mutilante ou síndrome de Vohwinkel é afecção dermatológica rara caracterizada pelo espessamento cutâneo das palmas, plantas e dorso de mãos e pés, e por constrições em faixa dos dedos (pseudoainhum). São relatados três casos em uma família, envolvendo duas gerações. Em todos havia, adicionalmente, perda auditiva neurossensorial. Discutem-se o diagnóstico diferencial com outras ceratodermias palmoplantares e o tratamento com retinóides. Trata-se da primeira descrição dessa entidade no Brasil.

A novel dominant missense mutation--D179N--in the GJB2 gene (Connexin 26) associated with non-syndromic hearing loss

Mutations of the GJB2 gene, encoding Connexin 26, are the most common cause of hereditary congenital hearing loss in many countries, and account for up to 50% of cases of autosomal-recessive non-syndromic deafness. By contrast, only a few GJB2 mutations have been reported to cause an autosomal-dominant form of non-syndromic deafness. We report on a family from southern Italy in whom dominant, non-syndromic, post-lingual hearing loss is associated with a novel missense mutation in the GJB2 gene. Direct sequencing of the gene showed a heterozygous G-->A transition at nucleotide 535, resulting in an aspartic acid to asparagine amino acid substitution at codon 179 (D179N). This mutation occurred in the second extracellular domain (EC2), which would seem to be very important for connexon-connexon interaction.

Mutation spectrum of the connexin 26 (GJB2) gene in Taiwanese patients with prelingual deafness

PURPOSE

To determine the mutation spectrum of the connexin 26 gene among 324 Taiwanese patients with prelingual deafness and the carrier rate of gene mutation in another 432 unrelated control subjects.

METHODS

The coding region of the connexin 26 gene was sequenced in both directions to detect mutation in all 756 samples.

RESULTS

Among the 756 samples tested, 21 connexin 26 variants were detected, including 7 novel ones. The 235delC mutation was the most common, accounting for 57.6% of the mutant alleles. Among patients, 48 (14.8%) had connexin 26 gene mutations. In the control group, the carrier rate of connexin 26 mutation was estimated at 2.8%.

CONCLUSION

The mutation spectrum of the connexin 26 gene is wide, with more than half of the patients having only one mutation detected. Thus, further efforts are needed to look for possible existence of a second mutant allele.

Novel mutations in GJB2 encoding connexin-26 in Japanese patients with keratitis-ichthyosis-deafness syndrome

BACKGROUND

Germline missense mutations in the GJB2 gene that encodes connexin-26 (Cx26) have recently been found to be the cause of the keratitis-ichthyosis-deafness (KID) syndrome.

OBJECTIVES

To define the GJB2 mutations in three Japanese patients with KID syndrome.

METHODS

Genomic DNA was extracted from peripheral blood and used to amplify the GJB2 gene. Direct sequencing and endonuclease digestion were used for mutation analysis and DNA-based diagnosis.

RESULTS

We identified two heterozygous mis-sense mutations (D50Y, D50N) in the GJB2 gene in three Japanese patients with KID syndrome. All mutations were located on the first extracellular domain of Cx26.

CONCLUSIONS

These data expand the GJB2 mutation database and show that a dominant mutation of Cx26 can cause KID syndrome in Japanese patients.

Cx26 affects the in vitro reconstruction of human epidermis

To study the function of connexins in human keratinocytes, we have used a three-dimensional culture system, in which a tissue is reconstructed using cells from the outer root sheet of hair follicles. This tissue reproduces in vitro the histological organisation of human epidermis in situ and the normal distribution of several keratinocyte markers. Furthermore, it shows characteristics of a differentiating epidermis, including the expression of connexin26. Connexin26 protein expression is increased under physiological and pathological conditions resulting in increased keratinocyte turnover. Loss of this protein in keratinocytes, obtained from patients carrying a stop mutation, resulted in a reduced stratification of the in vitro reconstructed tissue, probably due to a lower proliferation and migration capacity of the keratinocytes, although dye coupling and persistence of other gap junctions is maintained. No changes were seen in tissues reconstructed with keratinocytes from patients carrying a non stop mutation of connexin30. The data indicate that, at least in vitro, connexin26 affects the function of human keratinocytes, independently of obvious changes in coupling.

Connexin 26 expression and mutation analysis in epidermal disease

Gap junctional communication has a key role in the co-ordination of keratinocyte differentiation. Multiple connexins are expressed in the epidermis and mutations in four of these connexins are associated with disorders of keratinisation. Specific autosomal dominant Cx26 mutations have been associated with syndromes of skin disease and hearing loss. Here we describe the characterization of a new Cx26 polyclonal antibody raised against the cytoplasmic region of the protein. It has been used to investigate Cx26 protein localization in epidermal disease and in the study of mutant Cx26 proteins.

Functional analysis of a dominant mutation of human connexin26 associated with nonsyndromic deafness

Cx26 has been implicated in dominant (DFNA3) and recessive (DFNB1) forms of nonsyndromic sensorineural deafness. While most homozygous DFNB1 Cx26 mutations result in a simple loss of channel activity, it is less clear how heterozygous mutations in Cx26 linked to DFNA3 cause hearing loss. We have tested the ability of one dominant mutation (W44C) to interfere with wild-type human Cx26 (HCx26wt). HCx26wt induced robust electrical conductance between paired oocytes, and facilitated dye transfer between transfected HeLa cells. In contrast, oocyte pairs injected with only W44C were not electrically coupled above background levels, and W44C failed to dye couple transfected HeLa cells. Moreover, W44C dramatically inhibited intercellular conductance of HCx26wt when co-expressed in an equal ratio, and the low levels of residual conductance displayed altered gating properties. A nonfunctional recessive mutation (W77R) did not inhibit the ability of HCx26wt to form functional channels when co-injected in the same oocyte pairs, nor did it alter HCx26wt gating. These results provide evidence for a functional dominant negative effect of the W44C mutant on HCx26wt and explain how heterozygous Cx26 mutations could contribute to autosomal dominant deafness, by resulting in a net loss, and/or alteration, of Cx26 function.

Acrokeratosis verruciformis of Hopf is caused by mutation in ATP2A2: evidence that it is allelic to Darier's disease

Acrokeratosis verruciformis of Hopf is a localized disorder of keratinization affecting the distal extremities. Onset is early in life and the disease is inherited in an autosomal dominant fashion. Although histology of acrokeratosis verruciformis lesions shows no evidence of dyskeratosis, a possible relationship with Darier's disease has long been postulated on the basis of clinical similarity. ATP2A2 encoding the sarco(endo)plasmic reticulum Ca2+ ATPase2 pump has been identified as the defective gene in Darier's disease. In this report, we studied a family affected with acrokeratosis verruciformis in six generations and identified a heterozygous P602L mutation in ATP2A2. This mutation predicts a nonconservative amino acid substitution in the ATP binding domain of the molecule. The mutation segregates with the disease phenotype in the family and was not found in 50 controls. Moreover, functional analysis of the P602L mutant showed that it has lost its ability to transport Ca2+. This result demonstrates loss of function of the sarco(endo)plasmic reticulum Ca2+ ATPase2 mutant in acrokeratosis verruciformis, thus providing evidence that acrokeratosis verruciformis and Darier's disease are allelic disorders.

Connexin 43 (GJA1) mutations cause the pleiotropic phenotype of oculodentodigital dysplasia

Gap junctions are assemblies of intercellular channels that regulate a variety of physiologic and developmental processes through the exchange of small ions and signaling molecules. These channels consist of connexin family proteins that allow for diversity of channel composition and conductance properties. The human connexin 43 gene, or GJA1, is located at human chromosome 6q22-q23 within the candidate region for the oculodentodigital dysplasia locus. This autosomal dominant syndrome presents with craniofacial (ocular, nasal, and dental) and limb dysmorphisms, spastic paraplegia, and neurodegeneration. Syndactyly type III and conductive deafness can occur in some cases, and cardiac abnormalities are observed in rare instances. We found mutations in the GJA1 gene in all 17 families with oculodentodigital dysplasia that we screened. Sixteen different missense mutations and one codon duplication were detected. These mutations may cause misassembly of channels or alter channel conduction properties. Expression patterns and phenotypic features of gja1 animal mutants, reported elsewhere, are compatible with the pleiotropic clinical presentation of oculodentodigital dysplasia.

Ultraviolet A radiation transiently disrupts gap junctional communication in human keratinocytes

Ultraviolet A (UVA) (320-400 nm) radiation is known to cause cutaneous aging and skin cancer. We studied the effect of UVA (365 nm) radiation on the human epidermis by focusing on keratinocyte gap junction-mediated intercellular communication (GJIC). We observed a dose-dependent 10-fold decrease in GJIC induced by UVA in normal human keratinocytes. This decrease in GJIC was associated with time-dependent internalization of connexin43 (Cx43). UVA radiation also damaged the actin cytoskeleton, as shown by microfilament disappearance. Importantly, the decrease in GJIC was transient when keratinocytes were irradiated with 10 J/cm(2) UVA, with a return to baseline values after 8 h. Concomitantly, Cx43 was relocalized and the actin cytoskeleton was restored. UVA irradiation and 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment activated protein kinase C and reduced GJIC. However, Cx43 localization and phosphorylation were differently regulated by the two treatments. This suggests that at least two different pathways may mediate the observed fall in GJIC. These findings identify keratinocyte GJIC as a new UVA target that might sensitize human skin to photoaging and cancer formation.

Genetic disorders of palm skin and nail

The outer part of the skin, the epidermis, is specialized to protect the human body from its environment. Because of the high levels of physical stress experienced by the human hand in everyday use, the epidermis of the hand is especially toughened. In particular, the epidermis of the palm is highly specialized to resist mechanical trauma. Like the epidermis, the nails are composed of specialized epithelial cells and are especially strong. In recent years it has become apparent that the physical strength of epithelial cells comes from the keratin cytoskeleton--a dense meshwork of filaments extending throughout the cytoplasm. Keratins are a large family of intermediate filament proteins encoded by more than 50 distinct genes in humans. These different keratin genes are expressed in well-defined combinations in specific epithelial tissues. Several keratin genes are expressed in palmoplantar epidermis and in the stratified epithelia of the nail bed. Genetic mutations in these genes lead to fragility of these tissues and result in a range of genetic disorders characterized by blistering and thickening of palm and sole skin and/or nails. Study of these diseases has shed new light on the vital structural role of keratins in maintaining the integrity of epithelial cells.

Progress in molecular genetics of heritable skin diseases: the paradigms of epidermolysis bullosa and pseudoxanthoma elasticum

The 42nd Annual Symposium on the Biology of the Skin, entitled "The Genetics of Skin Disease", was held in Snowmass Village, Colorado, in July 1993. That meeting presented the opportunity to discuss how modern approaches to molecular genetics and molecular biology could be applied to understanding the mechanisms of skin diseases. The published proceedings of this meeting stated that "It is an opportune time to examine the genetics of skin disease" (Norris et al, 1994). Indeed, this meeting just caught the wave of early pioneering studies that have helped us to understand the molecular basis of a large number of genodermatoses. This overview presented in the 50th Annual Symposium on the biology of the skin, highlights the progress made in the molecular genetics of heritable skin diseases over the past decade.

[Incidence of A1555G mutations in the mitochondrial DNA and 35delG in the GJB2 gene (connexin-26) in families with late onset non-syndromic sensorineural hearing loss from Cantabria]

INTRODUCTION

Sensorineural deafness is a very common disorder in humans, which affects approximately 10% of the population. Genetic causes are suggested to be responsible for more than half of the cases. The A1555G mutation in the mitochondrial 12S rRNA gene and the 35delG mutation in the GJB2 gene are the most common mutations for sensorineural deafness in the Spanish population.

METHODS

A genetic study was carried out in order to determine the frequency of the mutations A1555G in the mitochondrial DNA and 35delG in the connexin-26 gene in 21 patients from 21 non-consanguineous unrelated families affected by late-onset bilateral non-syndromic sensorineural hearing loss from Cantabria.

RESULTS

The A1555G mutation was found in 6 patients. Five of these 6 patients had been treated with aminoglycosides. In all of them the auditory impairment affected mainly the high frequencies. The 35delG mutation was not found in any of the patients.

CONCLUSIONS

The A1555G mutation in the mitochondrial DNA has been found to be the most common amongst the Cantabrian population. The A1555G mutation should be suspected in those members of families affected by sensorineural hearing impairment with a maternal inheritance pattern and ototoxicity from treatment with aminoglycoside antibiotics. The 35delG mutation in the GJB2 gene does not seem to be a major cause of deafness in families with late-onset non-syndromic sensorineural hearing loss in our area.

The novel R75Q mutation in the GJB2 gene causes autosomal dominant hearing loss and palmoplantar keratoderma in a Turkish family

Dominant mutations in the GJB2 gene encoding connexin 26 (Cx26) can cause non-syndromic hearing impairment alone or in association with palmoplantar keratoderma (PPK). We have identified the novel G224A (R75Q) mutation in the GJB2 gene in a four-generation family from Turkey with autosomal dominant inherited hearing impairment and PPK. The age of onset and progression of hearing loss were found to be variable among affected family members, but all of them had more severe impairment at higher hearing frequencies. Interestingly, the novel R75Q mutation affects the same amino acid residue as described recently in a small family (R75W) with profound prelingual hearing loss and PPK. However, the R75W mutation was also observed in a control individual without PPK and unknown hearing status. Therefore, the nature of the R75W mutation remains ambiguous. Our molecular findings provide further evidence for the importance of the conserved R75 in Cx26 for the physiological function of the inner ear and the epidermal cells of the skin.

The molecular basis of hereditary palmoplantar keratodermas

In recent years, the gene defects causing many types of hereditary palmoplantar keratoderma have been discovered. These genes encode a variety of proteins involved in the terminal differentiation of keratinocytes and the formation of the cornified cell envelope. In this article, we review the molecular defects underlying various palmoplantar keratodermas with particular attention to the role of these molecules in the terminal differentiation of palmoplantar epidermis. Of the proteins involved in keratodermas, loricrin, keratins, and desmosomal proteins provide the protein structure of the cornified cell envelope. Connexins form intercellular gap junctions, which regulate ionic calcium signals necessary for the expression of the proteins that form the cornified cell envelope. Cathepsins likely mediate enzymatic processes necessary for the formation and dissolution of the cornified cell envelope. The clinical phenotypes produced by various mutations affecting these proteins are discussed vis-à-vis data from genetic, cellular, and molecular experiments.

Non-syndromic autosomal-dominant deafness

Non-syndromic deafness is a paradigm of genetic heterogeneity. More than 70 loci have been mapped, and 25 of the nuclear genes responsible for non-syndromic deafness have been identified. Autosomal-dominant genes are responsible for about 20% of the cases of hereditary non-syndromic deafness, with 16 different genes identified to date. In the present article we review these 16 genes, their function and their contribution to deafness in different populations. The complexity is underlined by the fact that several of the genes are involved in both dominant and recessive non-syndromic deafness or in both non-syndromic and syndromic deafness. Mutations in eight of the genes have so far been detected in only single dominant deafness families, and their contribution to deafness on a population base might therefore be limited, or is currently unknown. Identification of all genes involved in hereditary hearing loss will help in the understanding of the basic mechanisms underlying normal hearing, will facilitate early diagnosis and intervention and might offer opportunities for rational therapy.

Effectiveness of sequencing connexin 26 (GJB2) in cases of familial or sporadic childhood deafness referred for molecular diagnostic testing

PURPOSE

Hearing loss is a common congenital disorder that is frequently associated with mutations in the GJB2 gene encoding the connexin 26 protein (Cx26). We sought to evaluate the effectiveness of direct DNA sequencing for detection of Cx26 mutations as a clinical diagnostic test.

METHODS

We designed a clinical assay using a three-step polymerase chain reaction (PCR)-based DNA sequencing strategy to detect all possible mutations in the open reading frame and flanking sequences of Cx26. The results of the first 324 cases of childhood deafness referred for diagnostic testing were analyzed.

RESULTS

A total of 127 of the 324 (39.2%) cases had at least one mutant Cx26 allele (36.1% of sporadic cases, 70% of familial cases). Of these 127 case, 57 (44.8%) were homozygotes or compound heterozygotes. Thirty-four different mutations were identified, including 10 novel mutations, 6 of which (T8M, K15T, R32L, M93I, N206S, and 511-512insAACG) may be pathogenic. We also provide new evidence on the pathogenicity or nonpathogenicity of 12 previously reported mutations, and clarify the confusing nomenclature of the 313-326del14 mutation.

CONCLUSION

A simple and rigorous method for efficient PCR-based sequence analysis of Cx26 is a sensitive clinical assay for evaluating deaf children. Its widespread use is likely to identify additional pathogenic mutations and lead to a better understanding of the clinical significance of previously identified mutations.

GJB2 (connexin 26) variants and nonsyndromic sensorineural hearing loss: a HuGE review

Despite the enormous heterogeneity of genetic hearing loss, variants in one locus, Gap Junction Beta 2 or GJB2 (connexin 26), account for up to 50% of cases of nonsyndromic sensorineural hearing loss in some populations. This article reviews genetic epidemiology studies of the alleles of GJB2, prevalence rates, genotype-phenotype relations, contribution to the incidence of hearing loss, and other issues related to the clinical validity of genetic testing for GJB2. This review focuses primarily on three alleles: 167 Delta T, 35 Delta G, and 235 Delta C. These alleles are recessive for nonsyndromic prelingual sensorineural hearing loss, and the evidence suggests complete penetrance but variable expressivity.

Connexin mutations in hearing loss, dermatological and neurological disorders

Gap junctions are important structures in cell-to-cell communication. Connexins, the protein units of gap junctions, are involved in several human disorders. Mutations in beta-connexin genes cause hearing, dermatological and peripheral nerve disorders. Recessive mutations in the gene encoding connexin 26 (GJB2) are the most common cause of childhood-onset deafness. The combination of mutations in the GJB2 and GJB6 (Cx30) genes also cause childhood hearing impairment. Although both recessive and dominant connexin mutants are functionally impaired, dominant mutations might have in addition a dominant-negative effect on wild-type connexins. Some dominant mutations in beta-connexin genes have a pleiotropic effect at the level of the skin, the auditory system and the peripheral nerves. Understanding the genotype-phenotype correlations in diseases caused by mutations in connexin genes might provide important insight into the mechanisms that lead to these disorders.

A recurrent mutation in the loricrin gene underlies the ichthyotic variant of Vohwinkel syndrome

Vohwinkel syndrome (VS) is a family of genodermatoses which exhibits extensive clinical and genetic heterogeneity. Here, we studied a pedigree originating from the UK with typical features of the ichthyotic variant of VS and identified a recurrent insertion mutation in the loricrin gene resulting in a mutant polypeptide with an unusual C terminus. Functional studies in transgenic mice have shown that the accumulation of mutant loricrin in the nucleus appears to interfere with the later stages of epidermal differentiation, thereby explaining the clinical manifestations of ichthyosis, keratoderma and pseudoainhum. Our findings extend the body of evidence implicating mutations in the loricrin gene as the underlying cause of VS.

Missense mutations in GJB2 encoding connexin-26 cause the ectodermal dysplasia keratitis-ichthyosis-deafness syndrome

Keratitis-ichthyosis-deafness syndrome (KID) is a rare ectodermal dysplasia characterized by vascularizing keratitis, profound sensorineural hearing loss (SNHL), and progressive erythrokeratoderma, a clinical triad that indicates a failure in development and differentiation of multiple stratifying epithelia. Here, we provide compelling evidence that KID is caused by heterozygous missense mutations in the connexin-26 gene, GJB2. In each of 10 patients with KID, we identified a point mutation leading to substitution of conserved residues in the cytoplasmic amino terminus or first extracellular domain of Cx26. One of these mutations was detected in six unrelated sporadic case subjects and also segregated in one family with vertical transmission of KID. These results indicate the presence of a common, recurrent mutation and establish its autosomal dominant nature. Cx26 and the closely related Cx30 showed differential expression in epidermal, adnexal, and corneal epithelia but were not significantly altered in lesional skin. However, mutant Cx26 was incapable of inducing intercellular coupling in vitro, which indicates its functional impairment. Our data reveal striking genotype-phenotype correlations and demonstrate that dominant GJB2 mutations can disturb the gap junction system of one or several ectodermal epithelia, thereby producing multiple phenotypes: nonsyndromic SNHL, syndromic SNHL with palmoplantar keratoderma, and KID. Decreased host defense and increased carcinogenic potential in KID illustrate that gap junction communication plays not only a crucial role in epithelial homeostasis and differentiation but also in immune response and epidermal carcinogenesis.

Structural and functional diversity of connexin genes in the mouse and human genome

Gap junctions are clustered channels between contacting cells through which direct intercellular communication via diffusion of ions and metabolites can occur. Two hemichannels, each built up of six connexin protein subunits in the plasma membrane of adjacent cells, can dock to each other to form conduits between cells. We have recently screened mouse and human genomic data bases and have found 19 connexin (Cx) genes in the mouse genome and 20 connexin genes in the human genome. One mouse connexin gene and two human connexin genes do not appear to have orthologs in the other genome. With three exceptions, the characterized connexin genes comprise two exons whereby the complete reading frame is located on the second exon. Targeted ablation of eleven mouse connexin genes revealed basic insights into the functional diversity of the connexin gene family. In addition, the phenotypes of human genetic disorders caused by mutated connexin genes further complement our understanding of connexin functions in the human organism. In this review we compare currently identified connexin genes in both the mouse and human genome and discuss the functions of gap junctions deduced from targeted mouse mutants and human genetic disorders.

Auditory manifestations of Keratitis-Ichthyosis-Deafness (KID) syndrome

OBJECTIVE

Evaluation of the auditory manifestations of Keratitis-Ichthyosis-Deafness (KID) syndrome, a rare genodermatosis characterized by follicular hyperkeratosis, vascularizing keratitis, and congenital hearing loss.

STUDY DESIGN

Five individuals with sporadic KID syndrome were evaluated in the outpatient audiology clinic at the Warren Grant Magnuson Clinical Center of the National Institutes of Health.

METHODS

Audiologic examinations included pure-tone audiometry, speech audiometry, and middle ear immittance testing. Auditory brainstem responses and otoacoustic emissions were analyzed in 2 subjects.

RESULTS

Four subjects had prelingual, bilateral, profound sensorineural hearing loss, whereas the fifth subject had significant residual hearing that exhibited no progression on serial audiograms. All 5 subjects had a history of non-erosive keratosis obturans and cutaneous cysts in the external ear canals that prevented continuous use of ear molds.

CONCLUSIONS

The sensorineural hearing loss in KID syndrome is generally prelingual and profound. This combination of auditory and cutaneous phenotypes is similar to those previously reported for KID syndrome. KID syndrome presents a difficult challenge for communication rehabilitation because keratitis may impair the perception of sign and spoken language, and the cutaneous manifestations routinely curtail use of external amplification devices.

Molecular genetics of hearing loss

Hereditary isolated hearing loss is genetically highly heterogeneous. Over 100 genes are predicted to cause this disorder in humans. Sixty loci have been reported and 24 genes underlying 28 deafness forms have been identified. The present epistemic stage in the realm consists in a preliminary characterization of the encoded proteins and the associated defective biological processes. Since for several of the deafness forms we still only have fuzzy notions of their pathogenesis, we here adopt a presentation of the various deafness forms based on the site of the primary defect: hair cell defects, nonsensory cell defects, and tectorial membrane anomalies. The various deafness forms so far studied appear as monogenic disorders. They are all rare with the exception of one, caused by mutations in the gene encoding the gap junction protein connexin26, which accounts for between one third to one half of the cases of prelingual inherited deafness in Caucasian populations.

Hereditary deafness and phenotyping in humans

Hereditary deafness has proved to be extremely heterogeneous genetically with more than 40 genes mapped or cloned for non-syndromic dominant deafness and 30 for autosomal recessive non-syndromic deafness. In spite of significant advances in the understanding of the molecular basis of hearing loss, identifying the precise genetic cause in an individual remains difficult. Consequently, it is important to exclude syndromic causes of deafness by clinical and special investigation and to use all available phenotypic clues for diagnosis. A clinical approach to the aetiological investigation of individuals with hearing loss is suggested, which includes ophthalmology review, renal ultrasound scan and neuro-imaging of petrous temporal bone. Molecular screening of the GJB2 (Connexin 26) gene should be undertaken in all cases of non-syndromic deafness where the cause cannot be identified, since it is a common cause of recessive hearing impairment, the screening is straightforward, and the phenotype unremarkable. By the same token, mitochondrial inheritance of hearing loss should be considered in all multigeneration families, particularly if there is a history of exposure to aminoglycoside antibiotics, since genetic testing of specific mitochondrial genes is technically feasible. Most forms of non-syndromic autosomal recessive hearing impairment cause a prelingual hearing loss, which is generally severe to profound and not associated with abnormal radiology. Exceptions to this include DFNB2 (MYO7A), DFNB8/10 (TMPRSS3) and DFNB16 (STRC) where age of onset may sometimes be later on in childhood, DFNB4 (SLC26A4) where there may be dilated vestibular aqueducts and endolymphatic sacs, and DFNB9 (OTOF) where there may also be an associated auditory neuropathy. Unusual phenotypes in autosomal dominant forms of deafness, include low frequency hearing loss in DFNA1 (HDIA1) and DFNA6/14/38 (WFS1), mid-frequency hearing loss in DFNA8/12 (TECTA), DFNA13 (COL11A2) and vestibular symptoms and signs in DFNA9 (COCH) and sometimes in DFNA11 (MYO7A). Continued clinical evaluation of types and course of hearing loss and correlation with genotype is important for the intelligent application of molecular testing in the next few years.

A large deletion including most of GJB6 in recessive non syndromic deafness: a digenic effect?

Congenital profound deafness has a known genetic origin in more than 50% of all cases. The majority of the non syndromic hearing loss (NSHL) show an autosomal recessive inheritance. Mutations in the GJB2 gene (connexin 26) account for more than 50% of the recessive non syndromic deafness (DFNB1) among 30 loci. Other connexin genes have been more rarely involved and attention was given here to the GJB6 gene (connexin 30). We show that homozygous deletion of a minimal 150 kb region encompassing this gene causes NSHL. More strikingly, association of this deletion in trans of the GJB2 gene 35delG or E47X mutations is also associated with NSHL.

Loricrin keratoderma: a cause of congenital ichthyosiform erythroderma and collodion baby

A group of hereditary palmoplantar keratodermas due to heterozygous mutation in the loricrin gene has recently been identified. Of five reported pedigrees, four presented as mutilating keratoderma with ichthyosis (variant Vohwinkel syndrome), and one as progressive symmetric erythrokeratoderma. We report a new Japanese pedigree of loricrin keratoderma. A 14-year-old male and his 11-year-old female sibling had both been born as collodion babies and were initially diagnosed as having non-bullous congenital ichthyosiform erythroderma, but later developed palmoplantar keratoderma with pseudoainhum. Their father was similarly affected. Direct sequencing of genomic DNA revealed a G residue insertion at codon 230-231 of the loricrin gene. Antibody studies confirmed the presence of mutant loricrin in the retained nuclei. We conclude that loricrin gene mutation may present as congenital ichthyosiform erythroderma, and should be included in the differential diagnosis of collodion baby.

An Indian case of keratoderma hereditarium mutilans (Vohwinkel's syndrome) associated with ichthyosiform dermatosis

Keratoderma hereditarium mutilans (KHM) or Vohwinkel's syndrome (VS) is usually inherited as an autosomal dominant disease, but a recessive type has occasionally been described. We report a 19-year-old male born to consanguineous parents, who presented with KHM associated with generalized ichthyosiform dermatosis. On examination, his generalized ichthyosiform dermatosis was accompanied by severe 'honey comb' like palmoplantar keratoderma and marked hyperhidrosis. Bilateral fifth toes showed complete constricting bands just distal to the metatarsophalangeal joints leading to pseudoainhum. The histopathologic examination of a biopsy from the sole showed hyperkeratosis, marked parakeratosis, hypergranulosis, acanthosis, and a few mononuclear cells in the papillary dermis, consistent with the diagnosis of KHM. Several hereditary disorders may show varying degrees of hyperkeratosis and constriction of digits; their differentiating features are discussed. This case is reported for its rarity and interesting associated findings.

Multiple epidermal connexins are expressed in different keratinocyte subpopulations including connexin 31

Recent genetic studies have demonstrated the importance of epidermal gap junctions with mutations in four beta-connexins associated with autosomal dominant epidermal disease. One of these disorders, erythrokeratoderma variabilis, is associated with germline mutations in the genes encoding connexins (Cx) Cx31 and Cx30.3. Towards understanding the functional mechanism of Cx31 mutations in epidermal disease, we have developed and characterized a polyclonal antibody raised against human Cx31. Using this antibody to immunostain normal epidermis, Cx31 protein was found to be expressed predominately in the stratum granulosum with a punctate pattern of staining at the plasma membrane. In addition, we used reverse transcriptase polymerase chain reaction and, where reagents were available, immunocytochemistry to investigate which other connexins are expressed in the epidermis. Surprisingly, this analysis revealed that there are at least 10 connexins expressed with an overlapping distribution and localization to distinct keratinocyte subpopulations. These data provide additional evidence for multiple gap junction channel types in the human epidermis. Elucidation of this complexity of channel types with respect to specific permeabilities and function of each wildtype and mutant channel type in epidermal biology will require further investigations.

Ectodermal dysplasias: a new clinical-genetic classification

The ectodermal dysplasias (EDs) are a large and complex nosological group of diseases, first described by Thurnam in 1848. In the last 10 years more than 170 different pathological clinical conditions have been recognised and defined as EDs, all sharing in common anomalies of the hair, teeth, nails, and sweat glands. Many are associated with anomalies in other organs and systems and, in some conditions, with mental retardation.The anomalies affecting the epidermis and epidermal appendages are extremely variable and clinical overlap is present among the majority of EDs. Most EDs are defined by particular clinical signs (for example, eyelid adhesion in AEC syndrome, ectrodactyly in EEC). To date, few causative genes have been identified for these diseases. We recently reviewed genes known to be responsible for EDs in light of their molecular and biological function and proposed a new approach to EDs, integrating both molecular-genetic data and corresponding clinical findings. Based on our previous report, we now propose a clinical-genetic classification of EDs, expand it to other entities in which no causative genes have been identified based on the phenotype, and speculate on possible candidate genes suggested by associated "non-ectodermal" features.

trans-dominant inhibition of connexin-43 by mutant connexin-26: implications for dominant connexin disorders affecting epidermal differentiation

Dominant mutations of GJB2-encoding connexin-26 (Cx26) have pleiotropic effects, causing either hearing impairment (HI) alone or in association with palmoplantar keratoderma (PPK/HI). We examined a British family with the latter phenotype and identified a new dominant GJB2 mutation predicted to eliminate the amino acid residue E42 (ΔE42) in Cx26. To dissect the pathomechanisms that result in diverse phenotypes of dominant GJB2 mutations, we studied the effect of three Cx26 mutants (ΔE42, D66H and R75W) identified in individuals with PPK/HI, and another (W44C) present in individuals with non-syndromic HI on gap junctional intercellular communication. We expressed mutant Cx26 alone and together with the epidermal connexins Cx26, Cx37 and Cx43 in paired Xenopus oocytes, and measured the intercellular coupling by dual voltage clamping. Homotypic expression of each connexin as well as co-expression of wild-type (wt) Cx26/wtCx43 and wtCx26/wtCx37 yielded variable, yet robust, levels of channel activity. However, all four Cx26 mutants were functionally impaired and failed to induce intercellular coupling. When co-expressed with wtCx26, all four mutants suppressed the wtCx26 channel activity consistent with a dominant inhibitory effect. However, only those Cx26 mutants associated with a skin phenotype also significantly (P<0.05) inhibited intercellular conductance of co-expressed wtCx43, indicating a direct interaction of mutant Cx26 units with wtCx43. These results demonstrate, for the first time, a trans-dominant negative effect of Cx26 mutants in vitro. Furthermore, they support a novel concept suggesting that the principal mechanism for manifestation of dominant GJB2 mutations in the skin is their dominant interference with the function of wtCx43. This assumption is further corroborated by our finding that Cx26 and Cx43 focally colocalize at gap junctional plaques in affected skin tissue of two carriers of ΔE42.

W44C mutation in the connexin 26 gene associated with dominant non-syndromic deafness

Although more than 50% of recessive non-syndromic deafness is attributed to mutations in the connexin 26 (Cx26) gene, only a few reported families have shown dominant transmission of the trait. The W44C mutation was originally reported in two families from the same geographic region of France, which exhibited dominant non-syndromic hearing loss. In this report, we describe a third family with early-onset severe-to-profound non-syndromic hearing loss segregating with the W44C mutation. Our observation places W44C among recurrent mutations in the Cx26 gene and emphasizes the importance of screening for this as well as other Cx26 mutations in autosomal dominant families.

Epithelial structural proteins of the skin and oral cavity: function in health and disease

Epithelial tissues function to protect the organism from physical, chemical, and microbial damage and are essential for survival. To perform this role, epithelial keratinocytes undergo a well-defined differentiation program that results in the expression of structural proteins which maintain the integrity of epithelial tissues and function as a protective barrier. This review focuses on structural proteins of the epidermis and oral mucosa. Keratin proteins comprise the predominant cytoskeletal component of these epithelia. Keratin filaments are attached to the plasma membrane via desmosomes, and together these structural components form a three-dimensional array within the cytoplasm of epithelial cells and tissues. Desmosomes contain two types of transmembrane proteins, the desmogleins and desmocollins, that are members of the cadherin family. The desmosomal cadherins are linked to the keratin cytoskeleton via several cytoplasmic plaque proteins, including desmoplakin and plakoglobin (gamma-catenin). Epidermal and oral keratinocytes express additional differentiation markers, including filaggrin and trichohyalin, that associate with the keratin cytoskeleton during terminal differentiation, and proteins such as loricrin, small proline-rich proteins, and involucrin, that are cross-linked into the cornified envelope by transglutaminase enzymes. The importance of these cellular structures is highlighted by the large numbers of genetic and acquired (autoimmune) human disorders that involve mutations in, or autoantibodies to, keratins and desmosomal and cornified envelope proteins. While much progress has been made in the identification of the structural proteins and enzymes involved in epithelial differentiation, regulation of this process is less clear. Both calcium and retinoids influence epithelial differentiation by altering the transcription of target genes and by regulating activity of enzymes critical in epithelial differentiation, such as transglutaminases, proteinases, and protein kinases. These studies have furthered our understanding of how epithelial tissue and cell integrity is maintained and provide a basis for the future treatment of skin and oral disorders by gene therapy and other novel therapeutics.

Recent advances in the molecular basis of inherited skin diseases

Over the last few years the molecular basis of several inherited skin diseases has been delineated. Some discoveries have stemmed from a candidate gene approach using clinical, biochemical, immunohistochemical, and ultrastructural clues, while others have arisen from genetic linkage and positional cloning analyses. Notable advances have included elucidation of specific gene pathology in the major forms of inherited skin fragility, ichthyosis, and keratoderma. These findings have led to a better understanding of the significance of individual structural proteins and regulatory enzymes in keratinocyte adhesion and differentiation. From a clinical perspective, the advances have led to better genetic counseling in many disorders, the development of DNA-based prenatal diagnosis, and a foundation for planning newer forms of treatment, including somatic gene therapy, in selected conditions.

Vohwinkel's syndrome in three generations

Vohwinkel's syndrome or keratoderma hereditaria mutilans is a diffuse, honeycombed, palmar, and plantar keratosis usually accompanied by pseudoainhum near the distal interphalangeal creases. The mutilating keratoderma associated with sensorineural hearing loss is thought to have an etiologic basis, resting on a mutation of the GJB2 gene, which encodes the gap junction protein connexin26 (Cx26). This specific mutation results in impaired epidermal differentiation as well as inner ear function. We describe a patient with Vohwinkel's syndrome accompanied by high-frequency sensorineural hearing loss whose mother and son were similarly affected.

A case of erythrokeratoderma variabilis without mutations in connexin 31

Erythrokeratoderma (EK) variabilis is a heterogeneous group of diseases characterized by migratory erythematous patches and hyperkeratotic plaques. Mutations in connexin 31 have recently been found to underlie several cases of EK variabilis. We describe a Japanese girl with extensive lesions that appeared to be a form of EK variabilis, clinically resembling genodermatose en cocardes (Degos). Our patient had characteristic migratory rosette or target-like erythematous keratotic plaques with peripheral scaling in addition to relatively fixed keratotic plaques. Sequencing of the connexin 31 gene did not detect mutations. Skin biopsy showed parakeratotic hyperkeratosis with hypergranulosis. Immunohistochemically, suprabasal keratins, involucrin and profilaggrin were unequivocally expressed, while loricrin expression was greatly diminished and deiminated K1 was undetectable. Our results confirm aetiological heterogeneity in EK. The histological features suggest disruption of keratinocyte terminal differentiation at a very late stage.

Genetic correction of inherited epidermal disorders

Genetic correction of monogenic human skin disorders represents a potentially effective molecular therapy for severe diseases in which current therapy is only palliative. The stratified epithelium of the epidermis represents the tissue location with the largest number of genetic skin diseases yet characterized. Specific requirements of successful gene delivery in this setting include correct targeting within tissue, durability, and a lack of immunogenecity. Progress toward this goal has advanced from identification of disease genes to reintroduction of wild-type genes to patient cell lines and primary cells in vitro. This initial work has been extended to gene-based correction of diseased tissue regenerated in vivo in the form of human patient skin xenografts on immune-deficient mice. Efforts in this human tissue model have laid the foundation for future efforts to extend this progress toward ex vivo cutaneous gene therapy trials in humans.

The genodermatoses: candidate diseases for gene therapy

Tremendous progress has been made in understanding the genetic basis of different forms of genodermatoses, a group of heritable diseases displaying a spectrum of phenotypic manifestations and clinical severity. The information about the underlying mutations in the candidate gene/protein systems has provided the basis for initial development of cutaneous gene therapy, and these heritable conditions appear to serve as appropriate candidate diseases for such efforts. Because of its accessibility and the fact that resident skin cells, such as epidermal keratinocytes and dermal fibroblasts, can be readily propagated in culture, skin serves as an appropriate target tissue for gene therapy. Various strategic considerations, including the use of in vivo or ex vivo approaches, gene replacement versus gene repair, utilization of different delivery systems, etc., require careful prioritization depending on the type of mutations and their pathogenetic consequences at the mRNA and protein levels.

Transgenic mice expressing a mutant form of loricrin reveal the molecular basis of the skin diseases, Vohwinkel syndrome and progressive symmetric erythrokeratoderma

Mutations in the cornified cell envelope protein loricrin have been reported recently in some patients with Vohwinkel syndrome (VS) and progressive symmetric erythrokeratoderma (PSEK). To establish a causative relationship between loricrin mutations and these diseases, we have generated transgenic mice expressing a COOH-terminal truncated form of loricrin that is similar to the protein expressed in VS and PSEK patients. At birth, transgenic mice (ML.VS) exhibited erythrokeratoderma with an epidermal barrier dysfunction. 4 d after birth, high-expressing transgenic animals showed a generalized scaling of the skin, as well as a constricting band encircling the tail and, by day 7, a thickening of the footpads. Histologically, ML. VS transgenic mice also showed retention of nuclei in the stratum corneum, a characteristic feature of VS and PSEK. Immunofluorescence and immunoelectron microscopy showed the mutant loricrin protein in the nucleus and cytoplasm of epidermal keratinocytes, but did not detect the protein in the cornified cell envelope. Transfection experiments indicated that the COOH-terminal domain of the mutant loricrin contains a nuclear localization signal. To determine whether the ML.VS phenotype resulted from dominant-negative interference of the transgene with endogenous loricrin, we mated the ML.VS transgenics with loricrin knockout mice. A severe phenotype was observed in mice that lacked expression of wild-type loricrin. Since loricrin knockout mice are largely asymptomatic (Koch, P.K., P. A. de Viragh, E. Scharer, D. Bundman, M.A. Longley, J. Bickenbach, Y. Kawachi, Y. Suga, Z. Zhou, M. Huber, et al., J. Cell Biol. 151:389-400, this issue), this phenotype may be attributed to expression of the mutant form of loricrin. Thus, deposition of the mutant protein in the nucleus appears to interfere with late stages of epidermal differentiation, resulting in a VS-like phenotype.

Inherited palmoplantar keratoderma and sensorineural deafness associated with A7445G point mutation in the mitochondrial genome

We report a French pedigree with members having an inherited combination of non-epidermolytic palmoplantar keratoderma (NEPPK) and sensorineural deafness. The penetrance of both features was incomplete. Additional ectodermal defects were absent. The expression of numerous epidermal proteins (keratins, fillagrin, cornified envelope proteins, intercellular junction proteins including connexin 26, and loricrin) defined with immunolabelling was normal in the proband. The combination was shown to be associated with the A7445G point mutation in the mitochondrial genome (mtDNA). This mutation is responsible for a subtype of NEPPK which is so far the only mtDNA mutation-associated keratoderma.