A novel GJB2 mutation p.Asn54His in a patient with palmoplantar keratoderma, sensorineural hearing loss and knuckle pads

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

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

A novel autosomal recessive GJB2-associated disorder: Ichthyosis follicularis, bilateral severe sensorineural hearing loss, and punctate palmoplantar keratoderma

Ichthyosis follicularis, a distinct cutaneous entity reported in combination with atrichia, and photophobia has been associated with mutations in MBTPS2. We sought the genetic cause of a novel syndrome of ichthyosis follicularis, bilateral severe sensorineural hearing loss and punctate palmoplantar keratoderma in two families. We performed whole exome sequencing on three patients from two families. The pathogenicity and consequences of mutations were studied in the Xenopus oocyte expression system and by molecular modeling analysis. Compound heterozygous mutations in the GJB2 gene were discovered: a pathogenic c.526A>G; p.Asn176Asp, and a common frameshift mutation, c.35delG; p.Gly12Valfs*2. The p.Asn176Asp missense mutation was demonstrated to significantly reduce the cell-cell gap junction channel activity and increase the nonjunctional hemichannel activity in the Xenopus oocyte expression system. Molecular modeling analyses of the mutant Cx26 protein revealed significant changes in the structural characteristics and electrostatic potential of the Cx26, either in hemichannel or gap junction conformation. Thus, association of a new syndrome of an autosomal recessive disorder of ichthyosis follicularis, bilateral severe sensorineural hearing loss and punctate palmoplantar keratoderma with mutations in GJB2, expands the phenotypic spectrum of the GJB2-associated disorders. The findings attest to the complexity of the clinical consequences of different mutations in GJB2.

Crucial motifs and residues in the extracellular loops influence the formation and specificity of connexin docking

Most of the early studies on gap junction (GJ) channel function and docking compatibility were on rodent connexins, while recent research on GJ channels gradually shifted from rodent to human connexins largely due to the fact that mutations in many human connexin genes are found to associate with inherited human diseases. The studies on human connexins have revealed some key differences from those found in rodents, calling for a comprehensive characterization of human GJ channels. Functional studies revealed that docking and formation of functional GJ channels between two hemichannels are possible only between docking-compatible connexins. Two groups of docking-compatible rodent connexins have been identified. Compatibility is believed to be due to their amino acid residue differences at the extracellular loop domains (E1 and E2). Sequence alignment of the E1 and E2 domains of all connexins known to make GJs revealed that they are highly conserved and show high sequence identity with human Cx26, which is the only connexin with near atomic resolution GJ structure. We hypothesize that different connexins have a similar structure as that of Cx26 at the E1 and E2 domains and use the corresponding residues in their E1 and E2 domains for docking. Based on the Cx26 GJ structure and sequence analysis of well-studied connexins, we propose that the E1-E1 docking interactions are staggered with each E1 interacting with two E1s on the docked connexon. The putative E1 docking residues are conserved in both docking-compatible and -incompatible connexins, indicating that E1 does not likely serve a role in docking compatibility. However, in the case of E2-E2 docking interactions, the putative docking residues are only conserved within the docking-compatible connexins, suggesting the E2 is likely to serve the function of docking compatibility. Docking compatibility studies on human connexins have attracted a lot of attention due to the fact that putative docking residues are mutational hotspots for several connexin-linked human diseases. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Functional Analysis of a Novel Connexin30 Mutation in a Large Family with Hearing Loss, Pesplanus, Ichthyosis, Cutaneous Nodules, and Keratoderma

Mutations in the gap-junction gene Cx30 (Connexin30, GJB6) are a known cause of hearing loss. Here, we report our findings on a large multigeneration family in which severe to profound sensorineural hearing impairment is associated with a variety of skin-related anomalies. Genome-wide analysis of the family showed that the locus maps to chromosome region 13ptel-q12.1 and that a novel mutation, p.N54K, in Cx30, cosegregates with the phenotype. Unlike wild-type Cx30, p.N54K Cx30 is predominantly localized in the cytoplasm and does not permit transfer of neurobiotin, suggesting improper cellular localization and abolishment of gap-junction activity.

Connexins and skin disease: insights into the role of beta connexins in skin homeostasis

Cell-to-cell communication triggered by connexin channels plays a central role in maintaining epidermal homeostasis. Here, we discuss the role of the beta connexin subgroup, where site-specific mutations in at least 4 of these proteins lead to distinctive non-inflammatory and inflammatory hyperproliferative epidermal disorders. Recent advances in the molecular pathways evoked and correlation with clinical outcome are discussed. The latest data provide increasing evidence that connexins in the epidermis are sensors to environmental stress and that targeting aberrant hemichannel activity holds significant therapeutic potential for inflammatory skin disorders.

R75Q de novo dominant mutation of GJB2 in a Chinese family with hearing loss and palmoplantar keratoderma

OBJECTIVES

Mutations in the GJB2 gene encoding connexin 26 (Cx26) are major causes of hereditary deafness. This study aimed to characterize the mutation profiles of the GJB2 gene in a Chinese family with sensorineural hearing loss.

METHODS

A Chinese family that included three individuals with sensorineural hearing loss and palmoplantar keratoderma underwent complete physical examinations, audiological examinations including pure tone audiometry and auditory brainstem response, skin pathological examination, and temporal CT scans. The entire coding region of GJB2, GJB3, GJB6, and the coding exons (exon7+8 and 19) of SLC26A4, mitochondrial 12SrRNA, and tRNA Ser (UCN) were sequenced. Structural analysis was performed to detect the effects of mutation on the tertiary structure of Cx26.

RESULTS

A dominant GJB2 mutation, c.224G>A (p.Arg75Gln, p.R75Q), was detected in the family. No other mutation was identified in GJB2, GJB3, GJB6, or the coding exons (exon7+8 and 19) of SLC26A4, mitochondrial 12SrRNA, and tRNA Ser (UCN). Structural analysis revealed that the p.R75Q mutation likely affects the structural stability and permeation properties of the Cx26 gap junction channel.

CONCLUSION

Our findings provide further evidence of a correlation between the p.R75Q mutation in Cx26 and a syndromic hearing impairment with palmoplantar keratoderma.

Extracellular domains play different roles in gap junction formation and docking compatibility

GJ (gap junction) channels mediate direct intercellular communication and play an important role in many physiological processes. Six connexins oligomerize to form a hemichannel and two hemichannels dock together end-to-end to form a GJ channel. Connexin extracellular domains (E1 and E2) have been shown to be important for the docking, but the molecular mechanisms behind the docking and formation of GJ channels are not clear. Recent developments in atomic GJ structure and functional studies on a series of connexin mutants revealed that E1 and E2 are likely to play different roles in the docking. Non-covalent interactions at the docking interface, including hydrogen bonds, are predicted to form between interdocked extracellular domains. Protein sequence alignment analysis on the docking compatible/incompatible connexins indicate that the E1 domain is important for the formation of the GJ channel and the E2 domain is important in the docking compatibility in heterotypic channels. Interestingly, the hydrogen-bond forming or equivalent residues in both E1 and E2 domains are mutational hot spots for connexin-linked human diseases. Understanding the molecular mechanisms of GJ docking can assist us to develop novel strategies in rescuing the disease-linked connexin mutants.

GJB2 Gene Mutations in Syndromic Skin Diseases with Sensorineural Hearing Loss

The GJB2 gene is located on chromosome 13q12 and it encodes the connexin 26, a transmembrane protein involved in cell-cell attachment of almost all tissues. GJB2 mutations cause autosomal recessive (DFNB1) and sometimes dominant (DFNA3) non-syndromic sensorineural hearing loss. Moreover, it has been demonstrated that connexins are involved in regulation of growth and differentiation of epidermis and, in fact, GJB2 mutations have also been identified in syndromic disorders with hearing loss associated with various skin disease phenotypes. GJB2 mutations associated with skin disease are, in general, transmitted with a dominant inheritance pattern. Nonsyndromic deafness is caused prevalently by a loss-of-function, while literature evidences suggest for syndromic deafness a mechanism based on gain-of-function. The spectrum of skin manifestations associated with some mutations seems to have a very high phenotypic variability. Why some mutations can lead to widely varying cutaneous manifestations is poorly understood and in particular, the reason why the skin disease-deafness phenotypes differ from each other thus remains unclear. This review provides an overview of recent findings concerning pathogenesis of syndromic deafness imputable to GJB2 mutations with an emphasis on relevant clinical genotype-phenotype correlations. After describing connexin 26 fundamental characteristics, the most relevant and recent information about its known mutations involved in the syndromic forms causing hearing loss and skin problems are summarized. The possible effects of the mutations on channel expression and function are discussed.

The role of connexins in ear and skin physiology - functional insights from disease-associated mutations

Defects in several different connexins have been associated with several different diseases. The most common of these is deafness, where a few mutations in connexin (Cx) 26 have been found to contribute to over 50% of the incidence of non-syndromic deafness in different human populations. Other mutations in Cx26 or Cx30 have also been associated with various skin phenotypes linked to deafness (palmoplanta keratoderma, Bart-Pumphrey syndrome, Vohwinkel syndrome, keratitis-ichthyosis-deafness syndrome, etc.). The large array of disease mutants offers unique opportunities to gain insights into the underlying function of gap junction proteins and their channels in the normal and pathogenic physiologies of the cochlea and epidermis. This review focuses on those mutants where the impact on channel function has been assessed, and correlated with the disease phenotype, or organ function in knock-out mouse models. These approaches have provided evidence supporting a role of gap junctions and hemichannels in K(+) removal and recycling in the ear, as well as possible roles for nutrient passage, in the cochlea. In contrast, increases in hemichannel opening leading to increased cell death, were associated with several keratitis-ichthyosis-deafness syndrome skin disease/hearing mutants. In addition to providing clues for therapeutic strategies, these findings allow us to better understand the specific functions of connexin channels that are important for normal tissue function. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Two Iranian families with a novel mutation in GJB2 causing autosomal dominant nonsyndromic hearing loss

Mutations in GJB2, encoding connexin 26 (Cx26), cause both autosomal dominant and autosomal recessive nonsyndromic hearing loss (ARNSHL) at the DFNA3 and DFNB1 loci, respectively. Most of the over 100 described GJB2 mutations cause ARNSHL. Only a minority has been associated with autosomal dominant hearing loss. In this study, we present two families with autosomal dominant nonsyndromic hearing loss caused by a novel mutation in GJB2 (p.Asp46Asn). Both families were ascertained from the same village in northern Iran consistent with a founder effect. This finding implicates the D46N missense mutation in Cx26 as a common cause of deafness in this part of Iran mandating mutation screening of GJB2 for D46N in all persons with hearing loss who originate from this geographic region.

Hereditary palmoplantar keratoderma and deafness resulting from genetic mutation of Connexin 26

Gap junctions, which mediate rapid intercellular communication, consist of connexins, small transmembrane proteins that belong to a large family of proteins found throughout the species. Mutations in the GJB2 gene, encoding Connexin 26, can cause nonsyndromic autosomal recessive or dominant hearing loss with or without skin manifestations. A 3-yr-old Korean female and her mother presented to our clinic with diffuse hyperkeratosis of the palms and soles (May 3, 2007). Skin biopsies from the soles of both patients demonstrated histopathological evidence of palmoplantar keratoderma. The patient and a number of her maternal family members also had congenital hearing loss. The combination of congenital hearing loss and palmoplantar keratoderma, inherited as an autosomal dominant trait, led us to test for a mutation in the GJB2 gene in both patients. The results showed the R75W mutation of the GJB2 gene in both. In conclusion, the simultaneous occurrence of a GJB2 mutation in a mother and daughter suggests that R75W mutation cause autosomal dominant hearing loss presenting with palmoplantar keratoderma. To the best of our knowledge, this is the first report of a GJB2 mutation associated with syndromic autosomal dominant hearing loss and palmoplantar keratoderma in a Korean family.

A novel mutation in the connexin 26 gene (GJB2) in a child with clinical and histological features of keratitis-ichthyosis-deafness (KID) syndrome

BACKGROUND

Keratitis-ichthyosis-deafness (KID) syndrome is a rare congenital ectodermal disorder, caused by heterozygous missense mutation in GJB2, encoding the gap junction protein connexin 26. The commonest mutation is the p.Asp50Asn mutation, and only a few other mutations have been described to date.

AIM

To report the fatal clinical course and characterize the genetic background of a premature male neonate with the clinical and histological features of KID syndrome.

METHODS

Genomic DNA was extracted from peripheral blood and used for PCR amplification of the GJB2 gene. Direct sequencing was used for mutation analysis.

RESULTS

The clinical features included hearing impairment, ichthyosiform erythroderma with hyperkeratotic plaques, palmoplantar keratoderma, alopecia of the scalp and eyelashes, and a thick vernix caseosa-like covering of the scalp. On histological analysis, features characteristic of KID syndrome, such as acanthosis and papillomatosis of the epidermis with basket-weave hyperkeratosis, were seen. The skin symptoms were treated successfully with acitretin 0.5 mg/kg. The boy developed intraventricular and intracerebral haemorrhage, leading to hydrocephalus. His condition was further complicated by septicaemia and meningitis caused by infection with extended-spectrum beta-lactamase-producing Klebsiella pneumoniae. Severe respiratory failure followed, and the child died at 46 weeks of gestational age (13 weeks postnatally). Sequencing of the GJB2 gene showed that the child was heterozygous for a novel nucleotide change, c.263C>T, in exon 2, leading to a substitution of alanine for valine at position 88 (p.Ala88Val).

CONCLUSIONS

This study has identified a new heterozygous de novo mutation in the Cx26 gene (c.263C>T; p.Ala88Val) leading to KID syndrome.

Connexin-26 mutations in deafness and skin disease

Gap junctions allow the exchange of ions and small molecules between adjacent cells through intercellular channels formed by connexin proteins, which can also form functional hemichannels in nonjunctional membranes. Mutations in connexin genes cause a variety of human diseases. For example, mutations in GJB2, the gene encoding connexin-26 (Cx26), are not only a major cause of nonsyndromic deafness, but also cause syndromic deafness associated with skin disorders such as palmoplantar keratoderma, keratitis-ichthyosis deafness syndrome, Vohwinkel syndrome, hystrix-ichthyosis deafness syndrome and Bart-Pumphrey syndrome. The most common mutation in the Cx26 gene linked to nonsyndromic deafness is 35DeltaG, a frameshift mutation leading to an early stop codon. The large number of deaf individuals homozygous for 35DeltaG do not develop skin disease. Similarly, there is abundant experimental evidence to suggest that other Cx26 loss-of-function mutations cause deafness, but not skin disease. By contrast, Cx26 mutations that cause both skin diseases and deafness are all single amino acid changes. Since nonsyndromic deafness is predominantly a loss-of-function disorder, it follows that the syndromic mutants must show an alteration, or gain, of function to cause skin disease. Here, we summarise the functional consequences and clinical phenotypes resulting from Cx26 mutations that cause deafness and skin disease.