Connexin mutations associated with palmoplantar keratoderma and profound deafness in a single family

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

A Connexin Gene (GJB3) Mutation in a Chinese Family With Erythrokeratodermia Variabilis, Ichthyosis and Nonsyndromic Hearing Loss: Case Report and Mutations Update

Background: Gap junctions formed by connexins are channels on cytoplasm functioning in ion recycling and homeostasis. Some members of connexin family including connexin 31 are significant components in human skin and cochlea. In clinic, mutations of connexin 31 have been revealed as the cause of a rare hereditary skin disease called erythrokeratodermia variabilis (EKV) and non-syndromic hearing loss (NSHL).

Objective: To determine the underlying genetic cause of EKV, ichthyosis and NSHL in three members of a Chinese pedigree and skin histologic characteristics of the EKV patient.

Methods: By performing whole exome sequencing (WES), Sanger sequencing and skin biopsy, we demonstrate a Chinese pedigree carrying a mutation of GJB3 with three patients separately diagnosed with EKV, ichthyosis and NSHL.

Results: The proband, a 6-year-old Chinese girl, presented with demarcated annular red-brown plaques and hyperkeratotic scaly patches on her trunk and limbs. Her mother has ichthyosis with hyperkeratosis and geographic tongue while her younger brother had NSHL since birth. Mutation analysis revealed all of them carried a heterozygous missense mutation c.293G>A of GJB3. Skin biopsy showed many grain cells with dyskeratosis in the granular layer. Acanthosis, papillomatosis, and a mild superficial perivascular lymphocytic infiltrate were observed.

Conclusion: A mutation of GJB3 associated with EKV, ichthyosis and NSHL is reported in this case. The daughter with EKV and the son with NSHL in this Chinese family inherited the mutation from their mother with ichthyosis. The variation of clinical features may involve with genetic, epigenetic and environmental factors.

GJB3/GJB6 screening in GJB2 carriers with idiopathic hearing loss: Is it necessary?

BACKGROUND

Genetic analysis detected excessive mono-allelic recessive GJB2 mutations in individuals with idiopathic deafness; the remaining alleles in trans/cis are underdetermined. The aim of this study was to assess the contributions of variants in GJB3 or GJB6 to non-syndromic sensorineural hearing impairment (NSHI) in Chinese patients with mono-allelic GJB2 mutations.

METHODS

The entire coding sequences of GJB3/GJB6, as well as deletions in GJB6, in a cohort of NSHI patients (n = 100) carrying likely pathogenic heterozygous GJB2 mutations, were tested. Targeted next generation sequencing was further performed in a multiplex family GDHY with moderate to profound NSHI.

RESULTS

Putatively causative GJB3 variant underlied 1% (1/100) in this cohort. In family GDHY, we identified a rare GJB3 c.250G>A mutation, as double heterozygotes with GJB2 c.109G>A and/or a novel GJB2 mutation c.638T>C predicted to be damaging in a digenic inheritance after precluding other attributable mutations from 127 deafness genes. No GJB6 mutation was found.

CONCLUSIONS

GJB3/GJB6 variants account for a low proportion in autosomal recessive GJB2 mutation carriers in our cohort. Environmental causes, or other NSHI relevant genes, revealed by targeted next generation sequencing or whole exome sequencing, may play major roles in triggering deafness in these patients.

The syndromic deafness mutation G12R impairs fast and slow gating in Cx26 hemichannels

Mutations in connexin 26 hemichannels that cause syndromic deafness have a gain-of-function phenotype that is poorly understood. García et al. show that one such mutation impairs fast and slow gating in these hemichannels because of an interaction between the N terminus and intracellular loop.

Mutations in connexin 26 (Cx26) hemichannels can lead to syndromic deafness that affects the cochlea and skin. These mutations lead to gain-of-function hemichannel phenotypes by unknown molecular mechanisms. In this study, we investigate the biophysical properties of the syndromic mutant Cx26G12R (G12R). Unlike wild-type Cx26, G12R macroscopic hemichannel currents do not saturate upon depolarization, and deactivation is faster during hyperpolarization, suggesting that these channels have impaired fast and slow gating. Single G12R hemichannels show a large increase in open probability, and transitions to the subconductance state are rare and short-lived, demonstrating an inoperative fast gating mechanism. Molecular dynamics simulations indicate that G12R causes a displacement of the N terminus toward the cytoplasm, favoring an interaction between R12 in the N terminus and R99 in the intracellular loop. Disruption of this interaction recovers the fast and slow voltage-dependent gating mechanisms. These results suggest that the mechanisms of fast and slow gating in connexin hemichannels are coupled and provide a molecular mechanism for the gain-of-function phenotype displayed by the syndromic G12R mutation.

GJB2 mutations: Genotypic and phenotypic correlation in a cohort of 690 hearing-impaired patients, toward a new mutation?

OBJECTIVES

To analyze the clinical features of hearing impairment and to search for correlations with the genotype in patients with GJB2 mutations.

DESIGN

Case series.

SETTING

Collaborative study in referral centers, institutional practice.

PATIENTS

A total of 690 hearing-impaired patients were genotypically and phenotypically described. The mutations of GJB2 and GJB6 were studied. Heterozygous patients were searched for another mutation by microsatellite approach.

MAIN OUTCOME MEASURES

Prevalence of GJB2 mutations, microsatellite approach, hearing-impairment.

RESULTS

In 498 patients (72,17% of the cohort), no mutation was found. Homozygotous patients were 59 (8,55%), with 51 for c.35delG, 6 for p.M34T and 2 for GJB6. Compound heterozygous were 64 (9,28%) with 56 c.35delG-others mutations. Genotypes with biallelic non sense mutations had a high risk of severe to profound hearing impairment. It was frequently milder in compound heterozygotes than in c.35delG homozygotes. Heterozygous patients were 69 (10%) with 21 c.35delG, 20 p.M34T and 28 others mutations. We selected patients with a complete historical medical file (clinical and audiometric data). Then, we performed a microsatellite approach (multiplex PCR of short DNA fragments) to localize a new pathologic allele. Seventeen heterozygous patients were studied. Six patients (35%) showed the same haplotype. They were compound heterozygous bearing a new pathologic allele.

CONCLUSION

Genotype may affect deafness severity, but environmental and other genetic factors may also modulate the severity and evolution of GJB2-GJB6 deafness. A new haplotype for GJB2 is described but the exact mutation remains unknown.

Prevalence of 35delG and Met34Thr GJB2 variants in Portuguese samples

OBJECTIVE

To estimate the prevalence of 35delG and Met34Thr variants in a Portuguese children's community sample and to compare these frequencies with nonsyndromic hearing-loss patients.

METHODS

502 children were randomly selected among the 8647 participants of the Portuguese birth cohort Generation XXI, and screened for Met34Thr and 35delG variants in the GJB2 gene. These variants were also studied on 89 index-cases, observed in the Clinic of "Hereditary Hearing-loss" in Saint John's Hospital Center, presenting a mild to profound nonsyndromic hearing-loss.

RESULTS

Among the 502 children from Generation XXI, 10 were heterozygous for the 35delG variant (95% Confidence Interval 1.03-3.68) and 1 homozygous (95% Confidence Interval 0.01-1.24). Other 10 children presented heterozygosity for the Met34Thr variant (95% Confidence Interval 1.03-3.68). No homozygous for the Met34Thr or compound heterozygotes (35delG/Met34Thr) were found. In the total of 89 nonsyndromic hearing-loss patients, 5 (95% Confidence Interval 2.11-12.8) were heterozygous and 7 (95% Confidence Interval 3.61-15.6) were homozygous for the 35delG variant. The Met34Thr variant was found in 4 patients, 2 heterozygous (95% Confidence Interval 0.13-8.31) and 2 homozygous (95% Confidence Interval 0.13-8.31).

CONCLUSION

The carrier frequency of 35delG and Met34Thr variants in a Portuguese sample was 1 in 50. Our data suggests that the 35delG mutation has an association with deafness. For the Met34Thr variant, no association was observed. However, Met34Thr seemed to conform to an additive model in hearing-loss.

Improving hearing loss gene testing: a systematic review of gene evidence toward more efficient next-generation sequencing-based diagnostic testing and interpretation

PURPOSE

With next generation sequencing technology improvement and cost reductions, it has become technically feasible to sequence a large number of genes in one diagnostic test. This is especially relevant for diseases with large genetic and/or phenotypic heterogeneity, such as hearing loss. However, variant interpretation remains the major bottleneck. This is further exacerbated by the lack in the clinical genetics community of consensus criteria for defining the evidence necessary to include genes on targeted disease panels or in genomic reports, and the consequent risk of reporting variants in genes with no relevance to disease.

METHODS

We describe a systematic evidence-based approach for assessing gene-disease associations and for curating relevant genes for different disease aspects, including mode of inheritance, phenotypic severity, and mutation spectrum.

RESULTS

By applying this approach to clinically available hearing loss gene panels with a total of 163 genes, we show that a significant number (45%) of genes lack sufficient evidence of association with disease and thus are expected to increase uncertainty and patient anxiety, in addition to intensifying the interpretation burden. Information about all curated genes is summarized. Our retrospective analysis of 539 hearing loss cases tested by our previous OtoGenomeV2 panel demonstrates the impact of including genes with weak disease association in laboratory wet-bench and interpretation processes.

CONCLUSION

Our study is, to our knowledge, the first to highlight the urgent need for defining the clinical validity of gene-disease relationships for more efficient and accurate clinical testing and reporting.Genet Med 18 6, 545-553.

Keratitis-ichthyosis-deafness syndrome-associated Cx26 mutants produce nonfunctional gap junctions but hyperactive hemichannels when co-expressed with wild type Cx43

Mutations in Cx26 gene are found in most cases of human genetic deafness. Some mutations produce syndromic deafness associated with skin disorders, like the Keratitis-Ichthyosis-Deafness syndrome (KID). Because in the human skin connexin 26 (Cx26) is co-expressed with other connexins, like Cx43 and Cx30, and as the KID syndrome is inherited as autosomal dominant condition, it is possible that KID mutations change the way Cx26 interacts with other co-expressed connexins. Indeed, some Cx26 syndromic mutations showed gap junction dominant negative effect when co-expressed with wild-type connexins, including Cx26 and Cx43. The nature of these interactions and the consequences on hemichannels and gap junction channel (GJC) functions remain unknown. In this study, we demonstrate that syndromic mutations, at the N terminus segment of Cx26, change connexin oligomerization compatibility, allowing aberrant interactions with Cx43. Strikingly, heteromeric oligomer formed by Cx43/Cx26 (syndromic mutants) shows exacerbated hemichannel activity but nonfunctional GJCs; this also occurs for those Cx26 KID mutants that do not show functional homomeric hemichannels. Heterologous expression of these hyperactive heteromeric hemichannels increases cell membrane permeability, favoring ATP release and Ca(2+) overload. The functional paradox produced by oligomerization of Cx43 and Cx26 KID mutants could underlie the severe syndromic phenotype in human skin.

The association between GJB2 gene polymorphism and psoriasis: a verification study

Psoriasis is a chronic inflammatory skin disease with multifactorial etiology. Connexin 26 (Cx26), an important gap junction protein, has been found highly expressed in plaques of psoriasis. Recently, genome wide association studies (GWAS) identified one new single nucleotide polymorphism (SNP) in GJB2 gene coding for Cx26 protein associated with psoriasis in Chinese Han population. In this paper, we verified the GWAS data in Chinese Han population. Here we genotyped the polymorphism of GJB2 rs3751385:C>T in 371 psoriasis patients and 330 healthy controls in Chinese Han population using polymerase chain reaction restriction fragment length polymorphism assay (PCR-RFLP). Our case-control assay indicated decreased frequency of the GJB2 rs3751385 C allele in psoriasis patients compared with that in the healthy controls [p = 6.02 × 10(-5), Odds ratio (OR) = 0.793, 95 % confidence interval (CI) 0.706-0.889]. The result suggested that GJB2 gene polymorphism rs3751385:C>T was associated with psoriasis susceptibility of Chinese Han population.

Pathological hemichannels associated with human Cx26 mutations causing Keratitis-Ichthyosis-Deafness syndrome

Connexin (Cx) proteins form intercellular gap junction channels by first assembling into single membrane hemichannels that then dock to connect the cytoplasm of two adjacent cells. Gap junctions are highly specialized structures that allow the direct passage of small molecules between cells to maintain tissue homeostasis. Functional activity of nonjunctional hemichannels has now been shown in several experimental systems. Hemichannels may constitute an important diffusional exchange pathway with the extracellular space, but the extent of their normal physiological role is currently unknown. Aberrant hemichannel activity has been linked to mutations of connexin proteins involved in genetic diseases. Here, we review a proposed role for hemichannels in the pathogenesis of Keratitis-Ichthyosis-Deafness (KID) syndrome associated with connexin26 (Cx26) mutations. Continued functional evaluation of mutated hemichannels linked to human hereditary disorders may provide additional insights into the mechanisms governing their regulation in normal physiology and dysregulation in disease. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Key functions for gap junctions in skin and hearing

Cx (connexin) proteins are components of gap junctions which are aqueous pores that allow intercellular exchange of ions and small molecules. Mutations in Cx genes are linked to a range of human disorders. In the present review we discuss mutations in β-Cx genes encoding Cx26, Cx30, Cx30.3 and Cx31 which lead to skin disease and deafness. Functional studies with Cx proteins have given insights into disease-associated mechanisms and non-gap junctional roles for Cx proteins.

Vohwinkel Syndrome secondary to missense mutation D66H in GJB2 gene (connexin 26) can include epileptic manifestations

Vohwinkel Syndrome (VS) is a type of diffuse hereditary palmoplantar keratodermas (DHPPK) accompanied by skeletal dimorphisms and sensorineural deafness. The most frequently reported genetic substrate in VS is a point mutation of GJB2 gene, responsible for encoding connexin 26, a gap-junction protein with a crucial role in neuronal migration in rats. We report the case of a 21-year-old male who is a second-generation member of a family with VS and developed cryptogenic focal epilepsy. Genetic study showed a nucleotide change (c.196G>C) in exon 1 of GJB2 gene, producing a missense mutation, D66H. It is plausible that a functional alteration of connexin 26, such as that resulting of the mutation of our case, can produce an alteration in cortical development with epileptogenic potential. The present case and experimental evidence that connexin 26 is related to animal epileptogenesis suggest that the phenotypic spectrum of VS could be expanded to include epileptic manifestations.

Mutation analysis of the Cx26, Cx30, and Cx31 genes in autosomal recessive nonsyndromic hearing impairment

CONCLUSION

Biallelic Cx26 mutations are the most common cause of autosomal recessive nonsyndromic hearing impairment (ARNHI) in Switzerland. Mutations in Cx30 and 31, digenic mutations as well as large deletions/duplications, are unlikely to be a major cause of hearing loss in Swiss patients with ARNHI. Multiplex ligation-dependent probe amplification (MLPA) is a highly accurate screening method for detection of c.del(GJB6-D13S1830).

OBJECTIVES

The intent of this study was to investigate the prevalence of the point and digenic mutations including large deletions and duplications in the Cx26, 30, and 31 genes in a Swiss patient cohort with ARNHI and cochlear implant.

PATIENTS AND METHODS

The coding regions of Cx26, 30, and 31 were sequenced in 32 patients. Large deletions/duplications were assessed by MLPA.

RESULTS

In one patient digenic heterozygous mutations involving Cx26 (c.35delG) and Cx30 (c.del(GJB6-D13S1830)) were identified. Biallelic Cx26 mutations were detected in 31%. One putative mutation (c.94C>T) was found in Cx31. MLPA analysis did not reveal any additional deletion or duplication in all three Cx genes, except for the heterozygous c.del(GJB6-D13S1830) deletion.

DNA sequence analysis of GJB2, encoding connexin 26: observations from a population of hearing impaired cases and variable carrier rates, complex genotypes, and ethnic stratification of alleles among controls

Mutations in GJB2 are associated with hereditary hearing loss. DNA sequencing of GJB2 in a cohort of hearing impaired patients and a multi-ethnic control group is reported. Among 610 hearing impaired cases, 43 DNA sequence variations were identified in the coding region of GJB2 including 24 mutations, 8 polymorphisms, 3 unclassified variants (G4D, R127C, M163V), 1 controversial variant (V37I), and 7 novel variants (G12C, N14D, V63A, T86M, L132V, D159, 592_600delinsCAGTGTTCATGACATTC). Sixteen non-coding sequence variations were also identified among cases including the IVS1+1A>G mutation, 2 polymorphisms, and 13 novel variants. A diagnosis of GJB2-associated hearing loss was confirmed for 63 cases (10.3%). Heterozygous mutations were found in 39 cases (6.4%). Eleven cases carrying novel or unclassified variants (1.8 %) and 18 cases carrying the controversial V37I variant were identified (3%). In addition, 294 control subjects from 4 ethnic groups were sequenced for GJB2. Thirteen sequence variations in the coding region of GJB2 were identified among controls including 2 mutations, 6 polymorphisms, 2 unclassified variants (G4D, T123N), 1 controversial variant (V37I), and 2 novel variants (R127L, V207L). Nine sequence variations were identified among controls in the non-coding regions in and around GJB2 exon 2. Of particular interest among controls were the variability in carrier rates and ethnic stratification of alleles, and the complex genotypes among Asians, 47% of whom carried two to four sequence variations in the coding region of GJB2. These data provide new information about carrier rates for GJB2-based hearing loss in various ethnic groups and contribute to evaluation of the pathogenicity of the controversial V37I variant.

Neurotological and neuroanatomical changes in the connexin-26-related HID/KID syndrome

The phenotype of the HID (hystrix-like ichthyosis, deafness)/KID (keratitis, ichthyosis, deafness) syndrome is primarily characterized by skin changes. However, the connexin 26 (Cx 26) autosomal dominant mutation underlying this syndrome is of special neurotological interest. In the present paper, the clinical pattern, audiovestibular and neuroimaging findings and the detailed genetic analysis of 4 patients with identical HID/KID-associated mutation D50N of Cx 26 are reported. The audiological test results demonstrated profound sensorineural hearing loss in all of the patients. Neurotological testing revealed inconsistent abnormalities in dynamic posturography (sensory organization test), but the vestibular ocular reflex upon caloric irrigation was normal in all patients. Vestibular-evoked myogenic potential testing for otolith function (saccule) showed a regular response in 1 patient and pathologic responses in 3 patients, while subjective haptic vertical (utricular function) testing was normal in all of the patients. CCT showed an extended (in length), but very thin (in diameter) bony lining between the basal portion of the internal auditory canal and the vestibule in the 3 scanned patients. Our study provides evidence for functionally intact semicircular canals and normal utricular function in subjects with the autosomal dominant D50N mutation of Cx 26, in contrast to saccular function which was generally compromised and hearing loss which was profound.

Functional analysis of R75Q mutation in the gene coding for Connexin 26 identified in a family with nonsyndromic hearing loss

Mutations in the gene (GJB2) coding for Connexin 26 (Cx26) are responsible for genetic forms of sensorineural hearing loss. This article describes a family characterized by congenital profound hearing loss, inherited in an autosomal dominant fashion and associated to a R75Q substitution in Cx26. Cell transfection and fluorescence imaging, dye transfer experiments and dual patch clamp recording showed that the mutant completely prevents the formation of functional channels despite assembling into junctional plaques, in communication incompetent HeLa cells. The disease is not associated with palmar and plantar keratosis in any of the family members, suggesting that R75Q substitution is not sufficient for the development of the complete syndromic phenotype. The association of palmar and plantar keratosis with profound hearing loss may be dependent on genetic background, requiring a functional interaction between the mutated Cx26 and other epidermally expressed connexins.

Differential induction of connexins 26 and 30 in skin tumors and their adjacent epidermis

Gap junctions (GJs) have been shown to play a role in tumor progression including a variety of keratinocyte-derived and non-keratinocyte-derived skin tumors. Here we show that the synthesis of the GJ proteins connexin 26 and connexin 30 (Cx26 and Cx30) is induced in keratinocyte-derived epithelial skin tumors whereas there is either no change or a downregulation of Cx43. Cx26, Cx30, and Cx43 are absent in non-epithelial skin tumors. Further, Cx26 and Cx30 are induced in the epidermis adjacent to malignant melanoma but absent in the epidermis adjacent to benign non-epithelial skin lesions (melanocytic nevi and angioma). The keratinocyte-derived skin tumors are very heterogeneous regarding the Cx26/Cx30 pattern in the epidermis at the periphery of the tumors. We did not observe any difference in the localization of the very similar proteins Cx26 and Cx30 but a variation in intensity of immunoreactivity. As the staining patterns of Cx26 and Cx30 antibodies are not identical to those of CK6, a marker for hyperproliferation, and CK17, a marker for trauma, we discuss that the induction of these gap junctional proteins exceeds a reflection of reactive hyperproliferative or traumatized epidermis. We further discuss the putative roles of these gap junctional proteins in tumor progression.

A7445G mtDNA mutation present in a Portuguese family exhibiting hereditary deafness and palmoplantar keratoderma

Mitochondrial DNA (mtDNA) A7445G point mutation has been shown to be responsible for familial nonepidermolytic palmoplantar keratoderma (NEPPK) associated with deafness without any additional features. To date, only a few cases have been described. We report a Portuguese pedigree presenting an inherited combination of NEPPK and sensorineural deafness compatible with maternal transmission. Clinical expression and age of onset of NEPPK and deafness were variable. Normal expression patterns of epidermal keratins and filaggrin, intercellular junction proteins including connexin 26, loricrin and cornified envelope proteins, were observed. Molecular analysis revealed that all the affected members, previously screened for Cx26 mutations with negative results, presented the mtDNA A7445G point mutation in the homoplasmic form. To our knowledge, this is the fifth family in whom inherited NEPPK and hearing loss are related to this mitochondrial mutation.

GJB2 mutations in Turkish patients with ARNSHL: prevalence and two novel mutations

Mutations in the connexin 26 gene (GJB2) cause a significant proportion of prelingual non-syndromic autosomal recessive deafness in all populations studied so far. To determine the percentage of hearing loss attributed to GJB2 in northeast Turkey, 93 unrelated patients with autosomal recessive non-syndromic hearing loss (ARNSHL) were screened. Seven different mutations were found in 29 of the patients with severe to profound hearing loss. The 35delG mutation was the most common mutation, accounting for 76% of all mutant GJB2 alleles. Four already described mutations, W24X, 310del14, delE120 and R184P and two novel mutations, Q80K and P173S, were identified. The allelic Delta(GJB6-D13S1830), which can cause hearing loss in combination with GJB2 mutations, was not present in our patients. Our results are comparable to those reported in other regions in Turkey and indicate that GJB2 mutations account for about 30% of Turkish patients with ARNSHL. Besides 35delG, W24X and delE120 occur more than once in the Turkish ARNSHL population with a frequency of about 5%.

Connexin disorders of the skin

Over the past decade, the molecular basis of most disorders of cornification has been unveiled. Among these, a distinct group has emerged because of primary defects in cell-cell communication due to faulty gap junction proteins also known as connexins. This review aims to delineate the cutaneous connexin disorders and to highlight intriguing genotype-phenotype correlations and emanating clinical implications.

Expanding The Phenotypic Spectrum of Cx26 Disorders: Bart–Pumphrey Syndrome is Caused by a Novel Missense Mutation in GJB2

Bart-Pumphrey syndrome (BPS) is an autosomal dominant disorder characterized by sensorineural hearing loss, palmoplantar keratoderma, knuckle pads, and leukonychia, which show considerable phenotypic variability. The clinical features partially overlap with Vohwinkel syndrome and Keratitis-Ichthyosis-Deafness syndrome, both disorders caused by dominant mutations in the GJB2 gene encoding the gap junction protein connexin-26, suggesting an etiological relationship. We report here a novel GJB2 mutation N54K segregating in a family with BPS, which was not detected in 110 control individuals of Northern European ancestry. This non-conservative missense mutation lies within a cluster of pathogenic GJB2 mutations affecting the evolutionary conserved first extracellular loop of Cx26 important for docking of connexin hemichannels and voltage gating. Immunostaining of Cx26 in lesional palmar and knuckle skin was weak or absent, although its adnexal expression appeared normal and the punctate membrane staining of Cx26 and other epidermal connexins was not altered. Nevertheless, the widespread immunostaining of Cx30 throughout the spinous cell layers suggested a compensatory overexpression. Our results emphasize that pleiotropic GJB2 mutations are responsible for at least 5 overlapping dermatological disorders associated with syndromic hearing loss and cover a wide range of severity and organ involvement.

Connexin 26 mutations in nonsyndromic autosomal recessive hearing loss: speech and hearing rehabilitation

OBJECTIVE

Hearing loss is the most common form of sensory impairment, with approximately one infant/1000 born with profound congenital deafness. A pre-lingual bilateral sensorineural hearing impairment poses a substantial problem as it negatively impacts on the subject's ability to conduct a normal social life. The aim of the study was to observe, in a group of children affected by pre-lingual non-syndromic autosomal recessive hearing impairment: (1) the role of the possible mutation of connexin 26 in the pathogenesis of the hearing loss; (2) the audiological and clinical aspects of the hearing impairment; (3) therapy to be adopted for the different patients.

METHODS

The study was carried out on 39 patients, 16 males and 23 females, aged between six and 17 years (mean 12 years), affected by non syndromic congenital deafness, presumably hereditary, referred to the out-patients audiology clinic for children of the Department of Otolaryngology of the Federico II University of Naples.

RESULTS

Our study conducted on 39 children with pre-lingual bilateral sensorineural autosomal recessive deafness showed as follows: (I) from a molecular perspective: an incidence of 41% in the cases studied of mutations in the encoding of the connexin 26 gene; a prevalence in our case study of the 35delG mutation (69%). (II) The characteristics of the hearing impairments in the children studied were homogeneous, regardless of the presence or absence of a connexin 26 mutation: the hearing impairment was pre-lingual bilateral sensorineural, the impairment often involved mainly the high frequencies, but, especially in the severe forms an involvement of all the frequencies was not rare; the hearing impairments were symmetrical and non progressive in time. (III) The results of the application of prosthesis and thereafter rehabilitative language therapy are generally satisfactory but correlated of course to the severity of the hearing loss.

CONCLUSION

In conclusion, we hope that further developments in the research on genetic hearing impairments will promptly result in advances in clinical practice.

Lack of association between Connexin 31 (GJB3) alterations and sensorineural deafness in Austria

Mutations in the gap junction protein beta 3 (GJB3) gene encoding Connexin 31 (Cx31) are known to cause autosomal inherited sensorineural deafness, erythrokeratodermia and neuropathy. The role of Cx31 mutations has not been described in familial cases of non-syndromic hearing impairment (NSHI) in central European populations. To identify mutations in the Austrian population, highly selected familial (n=24) and sporadic (n=21) cases of isolated NSHI were screened by analysis of the complete coding sequence of Cx31, after exclusion of a common Cx26 causing deafness. Three different variations occurring in a total of 37% of all cases were identified. A C94T (R32W) missense mutation was seen in 4.4% of cases and two silent alterations C357T and C798T were detected in 8.9% and 24.4% of cases exclusively in a heterozygous pattern. No correlation between Cx31 alterations and deafness was found. To investigate the role of heterozygous Cx31 variations for a possibly combination allelic disease inheritance with Cx26 mutations as shown for Connexin 30 and Connexin 26, patients with Cx26 variations were tested. Our data suggest that Cx31 alterations are common but have no or a low genetic relevance in the Austrian hearing impaired population with or without Cx26 alterations.

Divergent effects of two sequence variants of GJB3 (G12D and R32W) on the function of connexin 31 in vitro

Recently, we identified several missense mutations of the connexin gene GJB3 encoding connexin 31 (Cx31) in erythrokeratodermia variabilis (EKV), an autosomal dominant skin disorder. These mutations include G12D, which replaces a conserved glycine residue in the amino-terminus of Cx31 and is associated with a severe EKV phenotype. In contrast, the biologic relevance of the GJB3 sequence variant R32W located in the first transmembrane domain of Cx31 is disputed. To examine the effects of these sequence variants on Cx31 biogenesis and gap junction activity we expressed wild type and mutant Cx31-Flag constructs in HeLa cells. Using immunostaining, all expression variants were detected in the cytoplasm and in a punctate pattern at the cell surface, indicating that G12D and R32W did not interfere with either protein synthesis or transport to the cell membrane. Similarly, oligomerization into hemichannels appeared not impaired when expressing either Cx31 mutant as assessed by size exclusion chromatography, immunoblotting and immunostaining. However, dye transfer experiments and monitoring of intracellular calcium levels in response to serum stimulation revealed that G12D-Cx31 did not form functional gap junction channels, probably due to incorrect assembly or altered properties of Cx31 channels. In contrast, intercellular coupling between cells expressing R32W-Cx31 was comparable to that of wtCx31, suggesting that R32W is a functionally inconsequential polymorphism of Cx31.

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.

[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 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.

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.

HID and KID syndromes are associated with the same connexin 26 mutation

BACKGROUND

Keratitis-ichthyosis-deafness (KID) syndrome is a debilitating ectodermal dysplasia that predisposes patients to develop squamous cell carcinomas in addition to leading to profound sensory deafness and erythrokeratoderma. We recently demonstrated that KID can be caused by a specific missense mutation in connexin 26 (GJB2). Another syndrome, called hystrix-like ichthyosis-deafnesss (HID) syndrome, strongly resembles the KID syndrome. These disorders are distinguished mainly on the basis of electron microscopic findings. We hypothesized that KID and HID syndromes may be genetically related.

OBJECTIVE

To demonstrate by mutation analysis that HID and KID syndromes are genetically indistinguishable.

METHODS

DNA was extracted from paraffin-embedded tissue samples of the first HID syndrome patient described in the literature. Since the KID syndrome mutation abolishes an AspI restriction site, we were able to screen the patient's DNA by polymerase chain reaction and subsequent restriction enzyme analysis.

RESULTS

Restriction analysis of the connexin 26 gene in HID syndrome demonstrated the presence of the KID syndrome mutation that we previously described. This result was confirmed by direct DNA sequencing.

CONCLUSIONS

We show that KID and HID syndromes are identical at the molecular level and confirm the clinical impression that these syndromes are one and the same. That previous clinical reports made a distinction may be a consequence of sampling artefacts; alternatively, genetic background effects such as the presence of concurrent mutations in other skin-expressed genes may modify the phenotype.

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.

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.

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.

Genetic analysis of the connexin-26 M34T variant: identification of genotype M34T/M34T segregating with mild-moderate non-syndromic sensorineural hearing loss

Mutations in the human gap junction beta-2 gene (GJB2) that encodes connexin-26 have been shown to cause non-syndromic sensorineural hearing loss (NSSNHL) at the DFNB1 locus on 13q11. Functional and genetic data regarding the disease causing potential of one particular GJB2 sequence variant, 101 T-->C (M34T), have proven contradictory. In this study, we found the prevalence of the M34T allele in a cohort of white sib pairs and sporadic cases with NSSNHL from the United Kingdom and Ireland to be 3.179% of chromosomes screened. Significantly, we identified the first M34T/M34T genotype cosegregating in a single family with mid to high frequency NSSNHL. Screening a control population of 630 subjects we identified 25 M34T heterozygotes; however, no M34T homozygotes were detected. Surprisingly, the majority of M34T alleles (88%) were in cis with a 10 bp deletion in the 5' non-coding sequence. This non-coding deletion was also homozygous in the homozygous M34T subjects. Microsatellite analysis of flanking loci in M34T heterozygotes and controls does not define an extensive ancestral haplotype but preliminary data suggest two common alleles in subjects with the M34T allele. In summary, we provide data that support M34T acting as a recessive GJB2 allele associated with mild-moderate prelingual hearing impairment.

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.