Gap Junction Beta Research Articles

Molecular and hereditary mechanisms of sensorineural hearing loss with focus on selected endocrinopathies

Hearing loss is one of the most widespread sensory disorders. The incidence of deafness in general population is 1:1000 newborns. About one half of the cases of the congenital sensorineural hearing loss (SNHL) is inherited. Recessive mutations in the gap junction beta 2 (GJB2) gene are the most common genetic causes of the nonsyndromic SNHL. The GJB2 encodes a protein connexin 26 which forms a subunit of gap junction essential for the correct function of the inner ear. The syndromic SNHL is associated with a wide range of other symptoms, which encompass also dysfunctions of endocrine organs. The Pendred syndrome associated with the hearing impairment is characterized by a prelingual, bilateral sever to profound SNHL, goiter, and iodine organification defect. It is an autosomal recessive disorder, which develops due to mutations in pendrin, an anion channel encoded by SLC26A4 gene. Another important type of syndromic hearing loss is the Maternally Inherited Diabetes and Deafness syndrome, which is caused by several mitochondrial DNA mutations. These mutations are clinically manifested by a hearing impairment with development of the diabetes in the adult age. Hearing impairment occurs during puberty when sensation of high frequency tones is affected following with further progress to profound bilateral sensorineural hearing impairment in the whole frequency range. This review deals with the molecular mechanisms of common genetic causes of the hereditary SNHL along with the selected endocrinopathies emphasizing that the DNA analyses along with the functional studies significantly contribute to the early SNHL diagnosis followed by personalized therapy and genetic counseling.

Identification of a p.R143Q dominant mutation in the gap junction beta-2 gene in three Chinese patients with different hearing phenotypes

Conclusions: p.R143Q mutation in GJB2 can cause mild to profound bilateral sensorineural hearing impairment. Although not common in Chinese patients with hearing loss, it is important to identify the specific phenotype and genotype correlations of the dominant mutation in GJB2. Objective: Mutations in the GJB2 gene are the most common cause of nonsyndromic autosomal recessive sensorineural hearing loss. A few mutations in GJB2 have also been reported to cause dominant nonsyndromic or syndromic hearing loss. However, dominant mutation in GJB2 is not common in Chinese populations. Methods: Three patients with hearing impairment from Chinese families are reported here. Temporal CT scan, complete physical (including skin and hair) and otoscopic examinations, and an audiological study, including tympanometry, auditory brainstem response (ABR), auditory steady-state response (ASSR), and 40 Hz auditory event-related potential (40 Hz-AERP), were carried out. The two exons of GJB2, the coding exons of SLC26A4, and mitochondrial 12S rRNA were sequenced. Results: Sequencing of GJB2 in two patients showed a heterozygous c.428G>A (p.R143Q) mutation, and the third patient was identified with c.299delAT and c.428G>A compound heterozygous mutation. Sequence analysis of the coding exons of SLC26A4 and mitochondrial 12S rRNA was performed but no sequence aberration or deletion was found.

Peripheral lymphangiogenesis in mice depends on ectodermal connexin-26 (Gjb2)

In order to study the specific function of connexin-26 (Cx26, also known as gap junction beta-2 protein; Gjb2), we generated knockin mice that expressed either a floxed lacZ reporter or, after Cre-mediated deletion, connexin-32 (Cx32)-coding DNA, both driven by the endogenous Cx26 promoter. Heterozygous Cx26knock-inCx32 (Cx26KICx32) embryos developed normally until embryonic day 14.5 but died before birth with severe lymphedemas. Although the jugular lymph sacs were normally developed, these embryos had a strongly reduced dermal lymphatic capillary network. By analyses of β-galactosidase reporter protein expression and lymphatic or blood endothelial-specific marker proteins, we demonstrated that Cx26 expression is temporally closely linked to lymphangiogenesis. No obvious phenotypic abnormalities were observed in Cx26KICx32 mice when Cre-mediated recombination was directed to mesenchyme or blood endothelium using the Prx1-Cre or Tie2-Cre mouse strains, respectively. By contrast, keratin-5-Cre-mediated replacement of Cx26 with Cx32 or deletion of both Cx26 alleles revealed severe lymphedemas similar to the general Cx26KICx32 phenotype. Thus, conditional ablation of Cx26 (loss of function) in ectoderm leads to partial disruption of lymphatic capillaries and embryonic death. We conclude that appropriate development of dermal lymphatic vessels in mice is dependent on the expression of Cx26 in the ectoderm.

Peripheral lymphangiogenesis in mice depends on ectodermal connexin-26 (Gjb2)

In order to study the specific function of connexin-26 (Cx26, also known as gap junction beta-2 protein; Gjb2), we generated knockin mice that expressed either a floxed lacZ reporter or, after Cre-mediated deletion, connexin-32 (Cx32)-coding DNA, both driven by the endogenous Cx26 promoter. Heterozygous Cx26knock-inCx32 (Cx26KICx32) embryos developed normally until embryonic day 14.5 but died before birth with severe lymphedemas. Although the jugular lymph sacs were normally developed, these embryos had a strongly reduced dermal lymphatic capillary network. By analyses of β-galactosidase reporter protein expression and lymphatic or blood endothelial-specific marker proteins, we demonstrated that Cx26 expression is temporally closely linked to lymphangiogenesis. No obvious phenotypic abnormalities were observed in Cx26KICx32 mice when Cre-mediated recombination was directed to mesenchyme or blood endothelium using the Prx1-Cre or Tie2-Cre mouse strains, respectively. By contrast, keratin-5-Cre-mediated replacement of Cx26 with Cx32 or deletion of both Cx26 alleles revealed severe lymphedemas similar to the general Cx26KICx32 phenotype. Thus, conditional ablation of Cx26 (loss of function) in ectoderm leads to partial disruption of lymphatic capillaries and embryonic death. We conclude that appropriate development of dermal lymphatic vessels in mice is dependent on the expression of Cx26 in the ectoderm.

Phenotype expression in women with CMT1X

Charcot-Marie-Tooth disease type 1X (CMT1X) is the second most common inherited peripheral neuropathy. Women with CMT1X typically have a less severe phenotype than men, perhaps because of X-inactivation patterns. Our objective was to determine the phenotype of women with CMT1X and whether X-inactivation patterns in white blood cells (WBCs) differ between females with CMT1X and controls. Thirty-one women with CMT1X were evaluated using the CMT neuropathy score (CMTNS) and the CMT symptom score in cross-sectional and longitudinal analyses. Lower scores correspond to less disability. WBCs were analyzed for X-inactivation pattern by androgen receptor X-inactivation assay in 14 patients and 23 controls. The 31 women's mean CMTNS was 8.35. Two-thirds of the cohort had a mild CMTNS (mean 4.85) and one-third had a moderate CMTNS (mean 14.73). Three patients had a CMTNS of 0. The pattern of X-inactivation did not differ between the affected and control groups. Women with CMT1X presented with variable impairment independent of age, type of mutation, or location of mutation. No evidence supported the presence of a gap junction beta-1 (GJB1) mutation affecting the pattern of X-inactivation in blood. Further studies are planned to determine whether X-inactivation is the mechanism for CMT1X females' variable phenotypes.

C042 Auditory brainstem implantation outcomes: Longitudinal study results

To assess the molecular etiology of nonsyndromic sensorineural hearing loss (NSHL) in members of an affected Chinese family.Common hearing-related genes including gap junction beta 2 (GJB2), SLC26A4, mitochondrial DNA 12S rRNA, GJB3 and GJB6 were examined in a family consisting of a normal hearing father, an NSHL-affected mother, one normal-hearing child and three NSHL-affected children. Specific primers were used in polymerase chain reactions to amplify the coding regions of the above genes from the peripheral blood DNA from each family member, and the genes were analyzed by direct sequencing. The subjects were evaluated for phenotypic characterization using audiometric testing and radiological examination of the inner ear.Pathogenic mutations in the GJB2 gene were identified. The affected mother showed a heterozygous G → A transition at nucleotide 232, resulting in an alanine to threonine substitution at codon 78 (p.A78T), and the normal hearing father had a c.35insG insertion mutation. The three affected children displayed heterozygosity for the GJB2 mutations, showing a previously unreported combination of c.35insG and c.232G>A.The GJB2 mutations account for a significant proportion of NSHL in affected individuals worldwide. Genetic and audiological data analysis of a Chinese family with NSHL revealed a novel c.35insG/c.232G>A compound heterozygous state. Our results highlight the complexity of the GJB2 genotypes and phenotypes.

Evidence for the absence of mutations at GJB3, GJB4 and LOR in progressive symmetrical erythrokeratodermia

Progressive symmetrical erythrokeratodermia (PSEK) is a rare inherited cornification disorder characterized by symmetrical erythematous hyperkeratotic plaques. The genetic basis for PSEK is not clear. PSEK shares many clinical features with erythrokeratodermia variabilis (EKV), which is associated with mutations in genes coding for gap junction beta (GJB) 3 and 4. A mutation in the loricrin gene (LOR) was found in patients with PSEK, who were members of a family with Vohwinkel syndrome. It would therefore be of interest to determine if PSEK is also caused by mutations in these genes. To examine the mutation status of GJB3, GJB4 and LOR in patients with PSEK and in control subjects. Genomic DNA samples from 25 patients with PSEK and 56 healthy controls were examined by sequencing analysis of the coding sequences of GJB3, GJB4 and LOR. There were no mutations found in any of these three genes. PSEK is a disorder distinct from EKV, and the true pathogenesis of PSEK remains unknown.

In Silico Study of Human Gap Junction Beta-2 Protein by Homology Modeling

Abstract Asp66his, Asp54Lys, and Asp50Asn are mutations in connexin 26 that are observed in the clinic and give rise to autosomal dominant syndromes. They are the result of point mutations in the human gap junction β-2 gene. In order to investigate the structural mechanism of Bart-Pumphrey Syndrome, Keratitis-Ichthyosis-Deafness Syndrome, and Vohwinkel Syndrome, homology model-ing was carried out. Asp66 has direct contact with Asn62 by two hydrogen bonds in the wild-type protein, and in Asp66His, the biggest change observed is a tre-mendous energy increase caused by hydrogen bond breakage to Asn62. Shifts in the side chain and new hy-drogen bond formation are observed for Lys54 com-pared to the wild-type protein (Asn54) and result in clos-er contact to Val84. Asp50Asn causes a significant de-crease in bond energy, and residual charge reversal re-pels the ion and metabolites and, hence, inhibits their transportation. Such perturbations are likely to be a fac-tor contributing to abnormal functioning of ion channels, resulting cell death and disease.Keywords: Bart-Pumphrey syndrome, connexin 26, gap junction, β-2 protein, hearing impairment, Keratitis-Ich-thyosis-Deafness syndrome, knuckle pads, leukonychia, vohwinkel syndrome

Six New Gap Junction Beta 1 Gene Mutations and Their Phenotypic Expression in Czech Patients with Charcot-Marie-Tooth Disease

X-linked Charcot-Marie-Tooth (CMTX) disease is a hereditary motor and sensory neuropathy caused by mutations in the gap junction beta 1 gene (GJB1 codes for connexin 32). In this study we report six novel mutations p.Met1Arg, p.Leu9Phe, p.Ser17Tyr, p.Val63Phe, p.Val170Ile, and p.Leu212Phe in GJB1 and their phenotypic expression. These mutations affect both intracellular and extracellular parts of the GJB1 protein. The screened patients had previously excluded the duplication/deletion on 17p11.2 and the male-to-male transfer in the pedigree. Except p.Val170Ile, all reported mutations segregated with the CMT phenotype in the families and caused CMTX1 neuropathy. Mutations were not found in 200 control DNA samples. Additionally, we performed in silico analysis of the novel mutations with the program PANTHER. The PANTHER scored five mutations, all but p.Val170Ile, as likely deleterious and supported the pathogenicity of the found mutations. These results provided evidence that these five mutations are causative for CMTX1.

A novel 355–357delGAG mutation and frequency of connexin-26 (GJB2) mutations in Iranian patients

The common form of autosomal recessive non-syndromic deafness is caused by the mutation in gap junction beta 2 (GJB2) gene (GenBank M86849, OMIM# 121011) which is located at the DFNB1 locus at 13q11. GJB2 is a small gene about 5500-bp length with two exons, of which only one contains the coding region (Kelley et al. 2000). The sequence of the coding region consists of 681 bp, encoding a gap-junction protein with 226 amino acids (Schrijver 2004). The genetics of hearing loss is highly heterogeneous and more than 100 mutations in connexin 26 (GJB2) genes are reported to be responsible for 30%–40% of hereditary hearing loss in deaf subjects (Ballana et al. 2001; Schrijver 2004). The most frequent mutation 35delG has been detected in different populations; especially in European countries where it is established to be due to founder effect (Van Laer et al. 2001; Rothrock et al. 2003). In this study, we performed mutation screening in 33 families who met clinical criteria of non-syndromic hereditary hearing loss (NSHHL) to evaluate the type and frequency of GJB2 mutations in Iranian population.

GJB1 gene mutations in suspected inflammatory demyelinating neuropathies not responding to treatment

It is generally accepted that while inflammatory demyelinating neuropathies often cause patchy demyelination resulting in conduction block, temporal dispersion and variation in conduction velocities, demyelinating hereditary neuropathies such as Charcot–Marie–Tooth...

Genetic evaluation of American minority pediatric cochlear implant recipients

To review the results of genetic evaluation of American minority pediatric cochlear implant recipients over a 5-year period. Case series review of pediatric cochlear implant recipients of Caribbean Hispanic and African American admixture descent with severe to profound sensorineural hearing loss at a tertiary care children's hospital. Out of 28 patients receiving cochlear implants, 14 were of Caribbean Hispanic or African American admixture ancestry. Six (43%) had environmental risk factors for sensorineural hearing loss. Eight (57%) patients had presumed genetic sensorineural hearing loss; two of whom were syndromic and six non-syndromic. Patients with no clear etiology for hearing loss were tested for Gap Junction Beta 2 (GJB2) mutations. Within this admixture group, we found no biallelic mutations in GJB2, while two patients, both with environmental risk factors for sensorineural hearing loss, had monoallelic GJB2 variants. One patient of mixed ethnicity (Caribbean Hispanic, Turkish, Macedonian), not included as part of the 14, had the common Caucasian founder mutation, 35delG, along with a heterozygous polymorphism in the GJB2 gene. This extends previous data showing a paucity of GJB2 mutations in these admixture populations. We found no biallelic GJB2 mutations in our admixture cochlear implant population, and two sequence variants of the gene, only one of which was disease causing. This suggests that the incidence of GJB2 mutations in these admixture populations is low. Hence, there may be low cost-benefit of GJB2 mutation analysis in these admixture populations with severe to profound non-syndromic sensorineural hearing loss.

Experimental Therapeutics in Hereditary Neuropathies: The Past, the Present, and the Future

Hereditary neuropathies represent approximately 40% of undiagnosed neuropathies in a tertiary clinic setting. The Charcot-Marie-Tooth neuropathies (CMT) are the most common. Mutations in more than 40 genes have been identified to date in CMT. Approximately 50% of CMT cases are accounted for by CMT type 1A, due to a duplication within the peripheral myelin protein 22 gene (PMP22). Mutations in the gap junction beta 1 gene (GJB1), the myelin protein zero gene (MPZ), and the mitofusin 2 gene (MFN2) account for a substantial proportion of other genetically definable CMT. Some 15% of demyelinating CMT and 70% of axonal CMT await genetic clarification. Other hereditary neuropathies include the hereditary sensory and autonomic neuropathies, the familial amyloid polyneuropathies, and multisystem disorders (e.g., lipid storage diseases and inherited ataxias) that have peripheral neuropathy as a major or minor component. This review surveys the challenges of developing effective therapies for hereditary neuropathies in terms of past, present, and future experimental therapeutics in CMT.

Spectrum of GJB2 mutations causing deafness in the British Bangladeshi population

Mutations in Gap Junction Beta 2 (GJB2) (the gene encoding the protein Connexin 26) have been found to be a major cause of non-syndromic sensorineural recessive deafness. The mutations in GJB2 causing hearing impairment vary in different populations. The aim of this study was to determine the prevalence and spectrum of GJB2 mutations in prelingual deafness in a population of Bangladeshi origin in the UK. Cross-sectional survey. Community based audiology clinic and tertiary level genetics department. Fifty-three families (67 patients) with sensorineural hearing loss of unknown cause were included in the study. Detailed history and examination excluded syndromic and environmental causes of hearing loss in the subjects as far as possible. Genetic analysis was performed, specifically looking for mutations in the GJB2 gene. Of the 53 families, 14 were confirmed to have biallelic pathogenic mutations in GJB2 (26%). The most common mutations of GJB2 in this population were W24X, IVS1+1, M1V, W77X and Q124X, W24X being the most common mutation seen in 57% of patients. Mutations in GJB2 are responsible for over one quarter of non-syndromic sensorineural deafness in the British Bangladeshi population. It is recommended that all Bangladeshi patients with non-syndromic hearing loss should be first tested for GJB2 mutations before requesting other aetiological investigations.

DYSBINDIN GENE AND EARLY ONSET SCHIZOPHRENIA: EVIDENCE FROM A FAMILY-BASED ASSOCIATION STUDY

The gap junction beta-2 (GJB2) gene is the most common genetic cause of hereditary deafness worldwide. Especially, the 235delC mutation in GJB2 is most prevalent in East Asia. In this study, we generated two iPSC lines from PBMCs of siblings carrying homozygous 235delC mutation which exhibits an audiometric phenotype of profound hearing loss. These iPSC lines had normal karyotype, showed expression of pluripotency markers, and could differentiate into three germ layers. These disease specific iPSC lines may be useful for the construction of the disease models and for the elucidation of pathogenesis in GJB2-related deafness.

Gap junction beta 1 (GJB1) gene mutations in Italian patients with X-linked Charcot-Marie-Tooth disease

X-linked Charcot-Marie-Tooth disease (CMT1X) is a peripheral neuropathy transmitted in a dominant manner and caused by mutations in the Connexin 32 (Cx32) gene (GJB1, gap junction beta 1). Here we report the mutation analysis of the GJB1 gene in 76 subjects with possible CMT1 and absence of 17p11.2 duplication, and in 38 CMT2 patients without mutations in CMT2-associated-genes, selected from a cohort of 684 patients with peripheral sensory-motor neuropathy. The analysis was performed by direct sequencing of the coding sequence and exon/intron boundaries of the GJB1 gene. The mutation screening identified 22 mutations in GJB1, eight of which have not been previously published: six point mutations (c.50C > G, c.107T > A, c.545C > T, c.545C > G, c.548G > C, c.791G > T) and two deletions (c.84delC, c.573_581delCGTCTTCAT). The GJB1 mutation frequency (19.3%) and the clinical heterogeneity of our patients suggest searching for GJB1 mutations in all CMT cases without the 17p11.2 duplication, regardless of the gender of the proband, as well as in CMT2 patients with possible X-linked inheritance.

Clinical and Electrophysiological Features in Charcot-Marie-Tooth Disease With Mutations in the NEFL Gene

To date, 13 different neurofilament light-chain polypeptide gene (NEFL) mutations have been identified in 55 patients with Charcot-Marie-Tooth disease (CMT) from 16 families. NEFL mutations were found to be associated with axonal and demyelinating variants of CMT. To describe the clinical features of 11 patients with CMT and NEFL mutations and to explore possible genotype-phenotype correlations. Standardized neuromuscular and nerve conduction studies were performed, and the coding regions of the peripheral myelin protein 22 (PMP22), myelin protein zero (MPZ), gap junction beta-1 protein (GJB1), and NEFL genes were analyzed by direct DNA sequencing. Two university hospitals in Austria (referral centers for neuromuscular disorders). Patients Eleven patients with CMT and NEFL mutations. Main Outcome Measure We genotyped NEFL in all of the patients and healthy relatives and correlated the genotype with the phenotype. A novel NEFL mutation (p.L93P) was detected in 1 family with 4 affected individuals exhibiting a severe CMT phenotype. Nerve conduction velocities were intermediately slowed to a range of 35 to 39 m/s. In a second family and in a sporadic patient, a p.P8R mutation was identified with intermediate and severe nerve conduction slowing. The results argue against an obvious genotype-phenotype correlation regarding disease onset, degree of muscle weakness, and nerve conduction slowing caused by NEFL mutations.

Additional clinical manifestations in children with sensorineural hearing loss and biallelicGJB2mutations: Who should be offeredGJB2testing?

Sensorineural hearing loss (SNHL), the most common sensory impairment noted at birth, occurs in 3 out of every 1,000 births live births. At least half of congenital SNHL is genetic in origin, with nonsyndromic, or isolated hearing loss, accounting for approximately 70% of the total genetic causes. Syndromic hearing loss (hearing loss associated with other clinical findings) makes up the remaining 30%. Worldwide, mutations in the gap junction beta 2 (GJB2) gene, encoding the connexin 26 (Cx26) protein, are responsible for approximately 30% of all cases of childhood SNHL. GJB2 mutations have been primarily associated with nonsyndromic forms of bilateral SNHL although rare syndromic forms involving dermatologic manifestations have also been reported. In general, unless skin findings are present, children with bilateral SNHL and other structural or developmental abnormalities are not generally thought of as candidates for GJB2 testing. We evaluated 163 individuals with biallelic GJB2 mutations and SNHL for the presence of other clinical findings. Twenty-nine of the 163 (18%) were found to have structural and/or developmental abnormalities in addition to the SNHL and four subjects had diagnoses that were felt to account for their hearing loss prior to being screened for GJB2 mutations. Although the GJB2 mutations are likely not responsible for these additional clinical manifestations, this study underscores the importance of considering GJB2 mutational analysis in individuals with more than just isolated SNHL given the high prevalence of GJB2-related hearing loss.

CMT1X phenotypes represent loss of GJB1 gene function

To investigate possible genotype-phenotype correlations and to evaluate the natural history of patients with Charcot-Marie-Tooth disease type 1X (CMT1X). CMT1X is caused by over 260 distinct mutations in the gap junction beta 1 (GJB1) gene, located on the X chromosome, which encodes the gap junction protein connexin 32 (Cx32). The natural history of CMT1X is poorly understood, and it remains unknown whether particular mutations cause more severe neuropathies through abnormal gain-of-function mechanisms. We evaluated 73 male patients with CMT1X, who each have 1 of 28 different GJB1 mutations predicted to affect nearly all domains of Cx32. Disability was evaluated quantitatively by the CMT Neuropathy Score (CMTNS) as well as by the CMT Symptom Score (CMTSS) and the CMT Examination Score (CMTES), which are both based on the CMTNS. Patients were also evaluated by neurophysiology. In all patients, disability increased with age, and the degree of disability was comparable with that observed in patients with a documented GJB1 deletion. Disability correlated with a loss of motor units as assessed by motor unit number estimates. Taken together, these data suggest that most GJB1 mutations cause neuropathy by a loss of normal connexin 32 function. Therefore, treatment of male patients with Charcot-Marie-Tooth disease type 1X may prove amenable to gene replacement strategies.

V37I connexin 26 allele in patients with sensorineural hearing loss: Evidence of its pathogenicity

Sensorineural hearing loss (SNHL) is the most common inherited sensory disorder, reported in 1-3 of every 1,000 births. It has been estimated that 50% of all cases of prelingual SNHL are genetically determined. There is tremendous genetic heterogeneity, with multiple dominant and recessive loci. Mutations of the gap junction beta-2 gene (GJB2) emerge as a leading cause of autosomal recessive non-syndromic SNHL. Over 90 sequence alterations have been reported, the pathogenicity of some of them being unknown or unclear. The status of the V37I allele of connexin 26 (GJB2 amino acid product) with regards to its association with SNHL has been controversial. This study examines the pathogenicity of V37I by comparing the frequency of this allele in 40 patients with SNHL of Chinese and Caucasian descent with the frequency of the allele in 100 anonymized, ethnically matched controls. The V37I allele was identified in 43.75 and 11.5% of the patient and control alleles of Chinese ethnicity, respectively, but was not found in either Caucasian cohort. We also compiled the audiograms of 15 individuals with SNHL homozygous for the V37I allele, and showed that these individuals present with a mild to moderate SNHL. These results indicate that (1) the V37I allele is common in individuals of Chinese descent but rarely present in individuals of Caucasian decent; and (2) the V37I allele is pathogenic, but produces milder hearing loss compared to nonsense mutations of connexin 26 such as the 35delG mutation.