Connexin mutations in skin disease and hearing loss

The role of Connexin26 regulated by MiR-2114-3p in the pathogenesis of ovarian cancer

OBJECTIVES

The purpose of our research was to determine the expression of Cx26 and miR-2114-3p, and their effects on proliferation, migration, and invasion in ovarian cancer and their mechanisms.

MATERIALS AND METHODS

Transcriptome sequencing was performed and differentially expressed Cx26 was screened. The mRNA and protein levels of Cx26 in EOC and normal ovarian tissues were verified. The relationship between Cx26 levels and prognostics was analyzed. Cx26 Lentiviral vectors were constructed to detect its effect on ovarian cancer. WB verified that PI3K/AKT pathway was the possible signal pathway regulated by Cx26. The interaction between miR-2114-3p and Cx26 was detected by double luciferase reporter assay and qrt-PCR. CCK8, clone formation, transwell, and flow cytometry assays were conducted in cells transfected miR-2114-3p plasmids. The vivo experiment investigated the effects of Cx26 on subcutaneous tumor growth, PI3K expression, proliferation proteins Ki67 and PCNA.

RESULTS

Cx26 was up-regulated in EOC tissue and cell lines, and was associated with poor prognosis of ovarian cancer, while miR-2114-3p was down-regulated in EOC cell lines. Cx26 was a direct target of miR-2114-3p. Cx26 overexpression and miR-2114-3p inhibition promoted the growth, motility, invasiveness, and S phase arrest of EOC cells. Additionally, Cx26 could activated PI3K pathway whatever in vivo and in vitro.

CONCLUSIONS

Dysregulation of Cx26 is critical in EOC patients. Manipulation of this mechanism may influence the survival of EOC patients. MiR-2114-3p regulates the tumor-promoting activity of Cx26 in EOC. By inhibiting the PI3K pathway or knocking down Cx26 effectively inhibits tumor growth in EOC cells and Nude mouse model.

GJB4 and GJC3 variants in non-syndromic hearing impairment in Ghana

IMPACT STATEMENT

Although connexins are known to be the major genetic factors associated with HI, only a few studies have investigated GJB4 and GJC3 variants among hearing-impaired patients. This study is the first to report GJB4 and GJC3 variants from an African HI cohort. We have demonstrated that GJB4 and GJC3 genes may not contribute significantly to HI in Ghana, hence these genes should not be considered for routine clinical screening in Ghana. However, it is important to study a larger population to determine the association of GJB4 and GJC3 variants with HI.

Calcium interactions with Cx26 hemmichannel: Spatial association between MD simulations biding sites and variant pathogenicity

Connexinophaties are a collective of diseases related to connexin channels and hemichannels. In particular many Cx26 alterations are strongly associated to human deafness. Calcium plays an important role on this structures regulation. Here, using calcium as a probe, extensive atomistic Molecular Dynamics simulations were performed on the Cx26 hemichannel embedded in a lipid bilayer. Exploring different initial conditions and calcium concentration, simulation reached ∼4 μs. Several analysis were carried out in order to reveal the calcium distribution and localization, such as electron density profiles, density maps and distance time evolution, which is directly associated to the interaction energy. Specific amino acid interactions with calcium and their stability were capture within this context. Few of these sites such as, GLU42, GLU47, GLY45 and ASP50, were already suggested in the literature. Besides, we identified novel calcium biding sites: ASP2, ASP117, ASP159, GLU114, GLU119, GLU120 and VAL226. To the best of our knowledge, this is the first time that these sites are reported within this context. Furthermore, since various pathologies involving the Cx26 hemichannel are associated with pathogenic variants in the corresponding CJB2 gene, using ClinVar, we were able to spatially associate the 3D positions of the identified calcium binding sites within the framework of this work with reported pathogenic variants in the CJB2 gene. This study presents a first step on finding associations between molecular features and pathological variants of the Cx26 hemichannel.

Genetic Aetiology of Nonsyndromic Hearing Loss in Moravia-Silesia

BACKGROUND AND OBJECTIVE

Hearing loss is the most common sensory deficit in humans. The aim of this study was to clarify the genetic aetiology of nonsyndromic hearing loss in the Moravian-Silesian population of the Czech Republic.

PATIENTS AND METHODS

This study included 200 patients (93 males, 107 females, mean age 16.9 years, ranging from 4 months to 62 years) with nonsyndromic sensorineural hearing loss. We screened all patients for mutations in GJB2 and the large deletion del(GJB6-D13S1830). We performed further screening for additional genes (SERPINB6, TMIE, COCH, ESPN, ACTG1, KCNQ4, and GJB3) with Sanger sequencing on a subset of patients that were negative for GJB2 mutations.

RESULTS

We detected biallelic GJB2 mutations in 44 patients (22%). Among these patients, 63.6%, 9.1% and 2.3% exhibited homozygous c.35delG, p.Trp24*, and p.Met34Thr mutations, respectively. The remaining 25% of these patients exhibited compound heterozygous c.35delG, c.-23+1G>A, p.Trp24*, p.Val37Ile, p.Met34Thr, p.Leu90Pro, c.235delC, c.313_326del14, p.Ser139Asn, and p.Gly147Leu mutations. We found a monoallelic GJB2 mutation in 12 patients (6.6%). We found no pathogenic mutations in the other tested genes. Conclusions: One fifth of our cohort had deafness related to GJB2 mutations. The del(GJB6-D13S1830), SERPINB6, TMIE, COCH, ESPN, ACTG1, GJB3, and KCNQ4 mutations were infrequently associated with deafness in the Moravian-Silesian population. Therefore, we suggest that del(GJB6-D13S1830) testing should be performed only when patients with deafness carry the monoallelic GJB2 mutation.

p63 is a key regulator of iRHOM2 signalling in the keratinocyte stress response

Hyperproliferative keratinocytes induced by trauma, hyperkeratosis and/or inflammation display molecular signatures similar to those of palmoplantar epidermis. Inherited gain-of-function mutations in RHBDF2 (encoding iRHOM2) are associated with a hyperproliferative palmoplantar keratoderma and squamous oesophageal cancer syndrome (termed TOC). In contrast, genetic ablation of rhbdf2 in mice leads to a thinning of the mammalian footpad, and reduces keratinocyte hyperproliferation and migration. Here, we report that iRHOM2 is a novel target gene of p63 and that both p63 and iRHOM2 differentially regulate cellular stress-associated signalling pathways in normal and hyperproliferative keratinocytes. We demonstrate that p63-iRHOM2 regulates cell survival and response to oxidative stress via modulation of SURVIVIN and Cytoglobin, respectively. Furthermore, the antioxidant compound Sulforaphane downregulates p63-iRHOM2 expression, leading to reduced proliferation, inflammation, survival and ROS production. These findings elucidate a novel p63-associated pathway that identifies iRHOM2 modulation as a potential therapeutic target to treat hyperproliferative skin disease and neoplasia.

Prevalence, Risk Factors, and Comorbidities of Hidradenitis Suppurativa

It is challenging to estimate a true prevalence of hidradenitis suppurativa (HS) because it is underdiagnosed and misdiagnosed. Prevalences have been reported from 0.00033% to 4.1%. The incidence seems to be rising. In addition to dermatologic symptoms, HS is associated with metabolic syndrome, and increased cardiovascular risk. The majority of HS patients are smokers. Additional somatic comorbidities complicating HS include autoimmune conditions, follicular syndromes, rheumatologic conditions, and malignancies. HS patients are troubled by psychological comorbidities. When treating HS patients it is imperative not only to treat the skin symptoms, but also address the screening and treatment of possible comorbidities.

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.

Government-funded universal newborn hearing screening and genetic analyses of deafness predisposing genes in Taiwan

OBJECTIVE

To investigate the association of eight connexin genes (GJB2, GJB4, GJA1P1, GJB6, GJB3, GJA1, GJB1, and GJC3) and the SLC26A4 gene with congenital hearing impairment among infants in a universal newborn hearing screening program.

METHOD

From September 2009 to October 2013, the consecutive neonates born in all six branches of Taipei City Hospital were enrolled. Infants who failed the newborn hearing screening and were diagnosed with hearing impairment underwent the genetic analyses.

RESULT

15,404 neonates were born at Taipei City Hospital, and 15,345 neonates underwent newborn hearing screening. Among them, 32 infants were diagnosed with unilateral or bilateral hearing impairment. 26 of them underwent analyses of the connexin genes and the SLC26A4 gene. Of the connexin genes, two infants carried a GJB3 mutation (heterozygous c.580G>A and heterozygous c.520G>A, respectively). Only one infant carried a GJB2 mutation (homozygous c.235delC). One infant carried a GJA1P1 mutation (heterozygous c.929delC) and another carried a GJB4 mutation (heterozygous c.302G>A). Additionally, one infant carried a GJA1P1 novel variant (heterozygous c.1081C>T). Another infant carried a GJA1 novel variant (heterozygous c.1-33C>G). Of the SLC26A4 gene, one infant carried heterozygous c.919-2A>G mutation and a novel variant (heterozygous c.164+1G>C), and high-resolution computed tomography (HRCT) of the temporal bone revealed bilateral enlarged vestibular aqueducts. One infant carried heterozygous c.919-2A>G mutation and no inner ear anomalies were demonstrated by HRCT of the temporal bone. Another infant carried a novel variant (heterozygous c.818C>T).

CONCLUSION

These results provide a genetic profile of the connexin genes and SLC26A4 gene among infants with hearing impairment detected by a universal newborn hearing screening program in Taiwan. Further studies and long-term follow up of this cohort are warranted to determine the pathogenicity of each variants and the long-term hearing consequence.

Characterization of spectrum, de novo rate and genotype-phenotype correlation of dominant GJB2 mutations in Chinese hans

Dominant mutations in GJB2 may lead to various degrees of sensorineural hearing impairment and/or hyperproliferative epidermal disorders. So far studies of dominant GJB2 mutations were mostly limited to case reports of individual patients and families. In this study, we identified 7 families, 11 subjects with dominant GJB2 mutations by sequencing of GJB2 in 2168 Chinese Han probands with sensorineural hearing impairment and characterized the associated spectrum, de novo rate and genotype-phenotype correlation. We identified p.R75Q, p.R75W and p.R184Q as the most frequent dominant GJB2 mutations among Chinese Hans, which had a very high de novo rate (71% of probands). A majority (10/11) of subjects carrying dominant GJB2 mutations exhibited palmoplantar keratoderma in addition to hearing impairment. In two families segregated with additional c.235delC or p.V37I mutations of GJB2, family members with the compound heterozygous mutations exhibited more severe phenotype than those with single dominant GJB2 mutation. Our study suggested that the high de novo mutation rate gives rise to a significant portion of dominant GJB2 mutations. The severity of the hearing and epidermal phenotypes associated with dominant GJB2 mutations may be modified by additional recessive mutations of GJB2.

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

An unusual mucocutaneous syndrome with sensorineural deafness due to connexin 26 mutations

Mutations of the GJB2 gene, which encodes connexin 26, are related to a range of conditions associated with sensorineural deafness and keratinization disorders. We present the case of a newborn girl with sensorineural deafness, erythematous hyperkeratotic plaques on intertriginous areas, and parakeratosis on the oral and esophageal mucosa. She had an F142L mutation in exon 1 of the GJB2 gene, which was described previously in a patient with a similar phenotype.

Unfolded protein response in keratinocytes: impact on normal and abnormal keratinization

The unfolded protein response (UPR) is a signaling pathway from the endoplasmic reticulum (ER) to the nucleus that protects cells from stress caused by misfolded or unfolded proteins. As such, ER stress is an ongoing challenge for all cells, given the central biologic importance of secretion as part of normal physiologic functions. Mild UPR is activated by mild ER stress, which occurs under normal conditions. Abnormal UPR is activated by severe ER stress, which occurs under pathological conditions. Abnormal UPR activation is associated with a number of diseases, including diabetes mellitus and Alzheimer's disease. Within skin tissues, keratinocytes in the epidermis are especially dependent upon a mild UPR for normal differentiation in the course of their differentiation into secretory cells in the uppermost granular layers. Association between abnormal UPR activation and hereditary keratoses, including Darier's disease, keratosis linearis with ichthyosis congenita and keratoderma syndrome, erythrokeratoderma variabilis, and ichthyosis follicularis with atrichia and photophobia syndrome, have been elucidated recently. This review describes the UPR in normal and abnormal keratinization and discusses the regulation of abnormal UPR activation by chemical chaperones as a potential treatment for one of the hereditary keratoses.

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.

Biological role of connexin intercellular channels and hemichannels

Gap junctions (GJ) and hemichannels (HC) formed from the protein subunits called connexins are transmembrane conduits for the exchange of small molecules and ions. Connexins and another group of HC-forming proteins, pannexins comprise the two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than one connexin or pannexin. While connexin expression and channel activity may vary as a function of physiological and pathological states of the cell and tissue, only a few studies suggest the involvement of pannexin HC in acquired pathological conditions. Importantly, genetic mutations in connexin appear to interfere with GJ and HC function which results in several diseases. Thus connexins could serve as potential drug target for therapeutic intervention. Growing evidence suggests that diseases resulting from HC dysfunction might open a new direction for development of specific HC reagents. This review provides a comprehensive overview of the current studies of GJ and HC formed by connexins and pannexins in various tissue and organ systems including heart, central nervous system, kidney, mammary glands, ovary, testis, lens, retina, inner ear, bone, cartilage, lung and liver. In addition, present knowledge of the role of GJ and HC in cell cycle progression, carcinogenesis and stem cell development is also discussed.

Posttranslational modifications in connexins and pannexins

Posttranslational modification is a common cellular process that is used by cells to ensure a particular protein function. This can happen in a variety of ways, e.g., from the addition of phosphates or sugar residues to a particular amino acid, ensuring proper protein life cycle and function. In this review, we assess the evidence for ubiquitination, glycosylation, phosphorylation, S-nitrosylation as well as other modifications in connexins and pannexin proteins. Based on the literature, we find that posttranslational modifications are an important component of connexin and pannexin regulation.

Modulation of gap junction channels and hemichannels by growth factors

Gap junction hemichannels and cell-cell channels have roles in coordinating numerous cellular processes, due to their permeability to extra and intracellular signaling molecules. Another mechanism of cellular coordination is provided by a vast array of growth factors that interact with relatively selective cell membrane receptors. These receptors can affect cellular transduction pathways, including alteration of intracellular concentration of free Ca(2+) and free radicals and activation of protein kinases or phosphatases. Connexin and pannexin based channels constitute recently described targets of growth factor signal transduction pathways, but little is known regarding the effects of growth factor signaling on pannexin based channels. The effects of growth factors on these two channel types seem to depend on the cell type, cell stage and connexin and pannexin isoform expressed. The functional state of hemichannels and gap junction channels are affected in opposite directions by FGF-1 via protein kinase-dependent mechanisms. These changes are largely explained by channels insertion in or withdrawal from the cell membrane, but changes in open probability might also occur due to changes in phosphorylation and redox state of channel subunits. The functional consequence of variation in cell-cell communication via these membrane channels is implicated in disease as well as normal cellular responses.

Cell-cell connectivity: desmosomes and disease

Cell-cell connectivity is an absolute requirement for the correct functioning of cells, tissues and entire organisms. At the level of the individual cell, direct cell-cell adherence and communication is mediated by the intercellular junction complexes: desmosomes, adherens, tight and gap junctions. A broad spectrum of inherited, infectious and auto-immune diseases can affect the proper function of intercellular junctions and result in either diseases affecting specific individual tissues or widespread syndromic conditions. A particularly diverse group of diseases result from direct or indirect disruption of desmosomes--a consequence of their importance in tissue integrity, their extensive distribution, complex structure, and the wide variety of functions their components accomplish. As a consequence, disruption of desmosomal assembly, structure or integrity disrupts not only their intercellular adhesive function but also their functions in cell communication and regulation, leading to such diverse pathologies as cardiomyopathy, epidermal and mucosal blistering, palmoplantar keratoderma, woolly hair, keratosis, epidermolysis bullosa, ectodermal dysplasia and alopecia. Here, as well as describing the importance of the other intercellular junctions, we focus primarily on the desmosome, its structure and its role in disease. We will examine the various pathologies that result from impairment of desmosome function and thereby demonstrate the importance of desmosomes to tissues and to the organism as a whole.

Detection of the GJB2 Mutation in Iranian Children with Hearing Loss Treated with Cochlear Implantation

The 35delG mutation in the gap junction protein, β2, 26kDa (GJB2) gene is the most common mutation that has been found in children with non syndromic hearing loss. Testing for the GJB2 gene mutation is simple and can directly answer the concerns of the parents about cause of the disorder and prognosis for their children. Cochlear implantation (CI) is one of the methods of hearing rehabilitation in patients with complete hearing loss. The present study was designed for genetic assessment of children who were referred for CI. Connexin 26 (Cx26) gene analyses were performed on 42 children with non syndromic hearing loss who were referred to the Baqiyatallah Hospital, Tehran, Iran for genetic consultation and CI. Clinical history was obtained and an examination conducted on each individual. Genomic DNA was extracted from peripheral blood and mutation identification of the Cx26 gene was performed by polymerase chain reaction (PCR) amplification and direct sequencing of the coding sequence of the gene. Cochlear implantation was performed for all patients and treatment response was assessed for all of them based on speech intelligibility rating (SIR) before and after CI. We found six patients (14.3%) with the 35delG mutation on the Cx26 gene, two homozygotes and four heterozygotes. No other mutation was detected. Treatment response in children with the homozygous 35delG mutation was better than in heterozygous patients, and treatment response in children with the mutation was better than in children with no mutation. Mutation screening for finding deafness causing mutations in the GJB2 gene is a useful predictor of post-implantation speech perception. We suggest microarray or other advanced mutation detection methods for assessment of other genes that might be responsible for non syndromic deafness.

Phenotypes of Two Dutch DFNA3 Families with Mutations in GJB2

Objectives:

We describe the phenotype of 2 Dutch DFNA3 families with mutations in the GJB2 gene.

Methods:

Two patients from family 1 and one isolated patient from family 2 were studied. The audiometric examination consisted of pure tone and speech audiometry. Two patients underwent vestibular testing and high-resolution computed tomographic scanning of the temporal bone. Mutation analysis of GJB2 and GJB6 was performed.

Results:

All 3 patients had severe to profound sensorineural hearing impairment. Cochlear implantation was performed in 2 patients, and their phoneme recognition scores were good. Mutation analyses revealed a p. Arg184Gln mutation in GJB2 in family 1 and a p. Arg75Trp mutation in GJB2 in family 2. No mutations in GJB6 were identified. Vestibular function tests and computed tomographic scans yielded normal findings in the examined subjects.

Conclusions:

Severe to profound sensorineural hearing impairment was found in these DFNA3 patients, and was well rehabilitated with cochlear implantation. A thorough genotype-phenotype correlation is difficult because of the small number of affected patients and the limited clinical data of these patients. More clinical data on DFNA3 families need to be published in order to create a reliable and precise phenotype characterization.

Development of gap junctional intercellular communication within the lateral wall of the rat cochlea

Auditory function depends on gap junctional intercellular communication (GJIC) between fibrocytes within the cochlear spiral ligament, and basal cells and intermediate cells within stria vascularis. This communication within the lateral wall is hypothesized to support recirculation of K+ from perilymph to the intra-strial space, and thus is essential for the high [K+] measured within endolymph, and the generation of the endocochlear potential. In rats, the [K+] within endolymph reaches adult levels by postnatal day 7 (P7), several days before hearing onset, suggesting that GJIC matures before auditory responses are detectable. In this study we have mapped the postnatal development of GJIC within the cochlear lateral wall, to determine the stage at which direct communication first exists between the spiral ligament and stria vascularis. Connexin 30 immunofluorescence revealed a progressive increase of gap junction plaque numbers from P0 onwards, initially in the condensing mesenchyme behind strial marginal cells, and spreading throughout the lateral wall by P7-P8. Whole-cell patch clamp experiments revealed compartmentalized intercellular dye-coupling in the lateral wall between P2 and P5. There was extensive dye-coupling throughout the fibrocyte syncytium by P7. Also, by P7 dye introduced to fibrocytes could also be detected within strial basal cells and intermediate cells. These data suggest that lateral wall function matures several days in advance of hearing onset, and provide anatomical evidence of the existence of a putative K+ recirculation pathway within the cochlear lateral wall.

Do cell junction protein mutations cause an airway phenotype in mice or humans?

Cell junction proteins connect epithelial cells to each other and to the basement membrane. Genetic mutations of these proteins can cause alterations in some epithelia leading to varied phenotypes such as deafness, renal disease, skin disorders, and cancer. This review examines if genetic mutations in these proteins affect the function of lung airway epithelia. We review cell junction proteins with examples of disease mutation phenotypes in humans and in mouse knockout models. We also review which of these genes are expressed in airway epithelium by microarray expression profiling and immunocytochemistry. Last, we present a comprehensive literature review to find the lung phenotype when cell junction and adhesion genes are mutated or subject to targeted deletion. We found that in murine models, targeted deletion of cell junction and adhesion genes rarely result in a lung phenotype. Moreover, mutations in these genes in humans have no obvious lung phenotype. Our research suggests that simply because a cell junction or adhesion protein is expressed in an organ does not imply that it will exhibit a drastic phenotype when mutated. One explanation is that because a functioning lung is critical to survival, redundancy in the system is expected. Therefore mutations in a single gene might be compensated by a related function of a similar gene product. Further studies in human and animal models will help us understand the overlap in the function of cell junction gene products. Finally, it is possible that the human lung phenotype is subtle and has not yet been described.

Trafficking abnormality and ER stress underlie functional deficiency of hearing impairment-associated connexin-31 mutants

Hearing impairment (HI) affects 1/1000 children and over 2% of the aged population. We have previously reported that mutations in the gene encoding gap junction protein connexin-31 (C×31) are associated with HI. The pathological mechanism of the disease mutations remains unknown. Here, we show that expression of C×31 in the mouse inner ear is developmentally regulated with a high level in adult inner hair cells and spiral ganglion neurons that are critical for the hearing process. In transfected cells, wild type C×31 protein (C×31wt) forms functional gap junction at cell-cell-contacts. In contrast, two HI-associated C×31 mutants, C×31R180X and C×31E183K resided primarily in the ER and Golgi-like intracellular punctate structures, respectively, and failed to mediate lucifer yellow transfer. Expression of C×31 mutants but not C×31wt leads to upregulation of and increased association with the ER chaperone BiP indicating ER stress induction. Together, the HI-associated C×31 mutants are impaired in trafficking, promote ER stress, and hence lose the ability to assemble functional gap junctions. The study reveals a potential pathological mechanism of HI-associated C×31 mutations.

Hypothesizing an ancient Greek origin of the GJB2 35delG mutation: can science meet history?

One specific mutation of the GJB2 gene that encodes the connexin 26 protein, the 35delG mutation, has become a major interest among scientists who focus on the genetics of nonsyndromic hearing loss. The mutation accounts for the majority of GJB2 mutations detected in Caucasian populations and represents one of the most frequent disease mutations identified so far. The debate was so far between the arguments whether or not the 35delG mutation constitutes a mutational hot-spot or a founder effect; however, it was recently clarified that the latter seems the most likely. In an attempt to explore the origin and propagation of the 35delG mutation, several groups have reported the prevalence of the mutation and the carrier rates in different populations worldwide. It is now certain that the theory of a common founder prevails and that the highest carrier frequencies of the 35delG mutation are observed in southern European populations, giving rise to a discussion regarding the origin of the 35delG mutation. In this study, we discuss data previously published by our and other groups and also compare the haplotype distribution of the mutation in southern Europe, trying to understand the pathways of science and history and the conflict between them.

Easy, rapid, and cost-effective methods for identifying carriers of recurrent GJB2 mutations causing nonsyndromic hearing impairment in the Greek population

A variety of techniques have been developed for screening the GJB2 gene for known and unknown mutations, especially the most common mutation in the Caucasian population, the c.35delG. Other mutations that have been so far characterized in the GJB2 gene seem to have different geographical distributions, and therefore there is an interest in identifying recurrent mutations specific for each population and developing easy and rapid screening techniques. Here we present easy screening protocols for already identified recurrent mutations in the Greek population. Developing easy, rapid, and cost-effective screening methods will facilitate the detection of GJB2 recurrent mutation carriers, at large, in the Greek population.

A novel dominant and a de novo mutation in the GJB2 gene (connexin-26) cause keratitis-ichthyosis-deafness syndrome: implication for cochlear implantation

OBJECTIVE

Keratitis-ichthyosis-deafness (KID) syndrome is a rare congenital disorder, characterized by hyperkeratosis and erythrokeratoderma associated with profound sensorineural hearing loss. Additional concomitant phenomena of the KID syndrome are dystrophic nails, dental abnormalities, scarring alopecia, and vascularizing keratitis. The disorder is caused by mutation in the GJB2 gene (connexin-26), a gap junction protein. The aim of this study was to explore the feasibility and procedure of cochlear implantation in patients with KID syndrome and to assess the genetic causes.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center. Cochlear implant program.

PATIENTS

We report on 2 cases of KID syndrome with congenital profound hearing loss. A 50-year-old woman with skin necrosis and implant extrusion 5 years after cochlear implantation and a 10-month-old infant girl with bilateral deafness, alopecia, bright light sensitivity, and congenital dermatosis.

INTERVENTION

Genetic analysis. Cochlear implantation.

MAIN OUTCOME MEASURES

Mutation analysis, surgical suitability, and hearing rehabilitation.

RESULTS

We detected a novel heterozygous missense mutation (Ile30Asn) in Patient 1 and a de novo mutation (Asp50Asn) in the GJB2 gene (connexin-26) in Patient 2. To decrease the risk of skin flap necrosis, we describe alternative surgical cochlear implantation techniques with a novel very thin receiver/stimulator (Nucleus CI 513; Cochlear Corp.). The postoperative course of both patients has been without any problems until now.

CONCLUSION

The combination of the cutaneous lesions with visual and auditory impairment demands to diagnose impaired hearing as early as possible. It would be helpful to search for KID syndrome in dealing with patients with deafness, skin lesions of unknown cause, and wound healing problems to choose the right method of surgical treatment and subsequent aftercare.

Dominant connexin26 mutants associated with human hearing loss have trans-dominant effects on connexin30

Dominant mutations in GJB2, the gene encoding the human gap junction protein connexin26 (Cx26), cause hearing loss. We investigated whether dominant Cx26 mutants interact directly with Cx30. HeLa cells stably expressing nine dominant Cx26 mutants, six associated with non-syndromic hearing loss (W44C, W44S, R143Q, D179N, R184Q and C202F) and three associated with hearing loss and palmoplantar keratoderma (G59A, R75Q and R75W), individually or together with Cx30, were analyzed by immunocytochemistry, co-immunoprecipitation, and functional assays (scrape-loading and/or fluorescence recovery after photobleaching). When expressed alone, all mutants formed gap junction plaques, but with impaired intercellular dye transfer. When expressed with Cx30, all mutants co-localized and co-immunoprecipitated with Cx30, indicating they likely co-assembled into heteromers. Furthermore, 8/9 Cx26 mutants inhibited the transfer of neurobiotin or calcein, indicating that these Cx26 mutants have trans-dominant effects on Cx30, an effect that may contribute to the pathogenesis of hearing loss.

EKV mutant connexin 31 associated cell death is mediated by ER stress

The epidermis expresses a number of connexin (Cx) proteins that are implicated in gap junction-mediated cell communication. Distinct dominantly inherited mutations in Cx31 cause the skin disease erythrokeratoderma variabilis (EKV) and hearing loss with or without neuropathy. Functional studies reveal tissue-specific effects of these Cx31 disease-associated mutations. The Cx31 mutants (R42P)Cx31, (C86S)Cx31 and (G12D)Cx31 are associated with EKV and the mutant (66delD)Cx31 with peripheral neuropathy and hearing loss, however the mechanisms of pathogenesis remain to be elucidated. Expression of (R42P)Cx31, (C86S)Cx31 and (G12D)Cx31 in vitro, but not (WT)Cx31 or (66delD)Cx31, cause elevated levels of cell-type specific cell death. Previous studies suggest that Cx-associated cell death may be related to abnormal ‘leaky’ hemichannels but we produced direct evidence against that being the major mechanism. Additionally, our immunocytochemistry showed upregulation of components of the unfolded protein response (UPR) in cells expressing the EKV-associated Cx31 mutants but not (WT)Cx31 or (66delD)Cx31. We conclude that the endoplasmic reticulum (ER) stress leading to the UPR is the main mechanism of mutant Cx31-associated cell death. These results indicate that, in vivo, ER stress may lead to abnormal keratinocyte differentiation and hyperproliferation in EKV patient skin.

New evidence for the correlation of the p.G130V mutation in the GJB2 gene and syndromic hearing loss with palmoplantar keratoderma

The GJB2 gene located on chromosome 13q12 and encoding the connexin 26 (Cx26) protein, a transmembrane protein involved in cell-cell attachment of almost all tissues, including the skin, causes autosomal recessive and sometimes dominant nonsyndromic sensorineural hearing loss. GJB2 mutations have also been identified in syndromic disorders exhibiting hearing loss associated with skin problems. Recently, a new mutation, p.G130V in the GJB2 gene has been reported as causative for Vohwinkel syndrome. In this case the p.G130V mutation was found in two patients (son and father) with palmoplantar keratoderma. The father also showed also skin constrictions of the 2nd and 3rd toes of the right foot. Here, we report on another family with palmoplantar keratoderma associated with a dominant form of hearing loss confirming the genotype-phenotype correlation between the mutation p.G130V and the skin abnormalities observed in syndromic disorders with hearing loss as described by [Snoeckx et al. (2005) Hum Mutat 26:60-65].

Gap-junction channels dysfunction in deafness and hearing loss

Gap-junction channels connect the cytoplasm of adjacent cells, allowing the diffusion of ions and small metabolites. They are formed at the appositional plasma membranes by a family of related proteins named connexins. Mutations in connexins 26, 31, 30, 32, and 43 have been associated with nonsyndromic or syndromic deafness. The majority of these mutations are inherited in an autosomal recessive manner, but a few of them have been associated with dominantly inherited hearing loss. Mutations in the connexin26 gene (GJB2) are the most common cause of genetic deafness. This review summarizes the most relevant and recent information about different mutations in connexin genes found in human patients, with emphasis on GJB2. The possible effects of the mutations on channel expression and function are discussed, in addition to their possible physiologic consequences for inner ear physiology. Finally, we propose that connexin channels (gap junctions and hemichannels) may be targets for age-related hearing loss induced by oxidative damage.

Sudden hearing loss in a family with GJB2 related progressive deafness

Mutations of GJB2, the gene encoding connexin 26, have been associated with prelingual, sensorineural hearing loss of mild to profound severity. One specific mutation, the 35delG, has accounted for the majority of mutations detected in the GJB2 gene in Caucasian populations. Recent studies have described progression of hearing loss in a proportion of cases with GJB2 deafness. We report an unusual family with four 35delG homozygous members, in which the parents were deaf-mute whilst both children had a postlingual progressive hearing loss. Furthermore, the son suffered from sudden hearing loss.

Differential potency of dominant negative connexin43 mutants in oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a congenital autosomal dominant disorder with phenotypic variability, which has been associated with mutations in the GJA1 gene encoding connexin43 (Cx43). Given that Cx43 mutants are thought to be equally co-expressed with wild-type Cx43 in ODDD patients, it is imperative to examine the consequence of these mutants in model systems that reflect this molar ratio. To that end, we used differential fluorescent protein tagging of mutant and wild-type Cx43 to quantitatively monitor the ratio of mutant/wild-type within the same putative gap junction plaques and co-immunoprecipitation to determine if the mutants interact with wild-type Cx43. Together the fluorescence-based assay was combined with patch clamp analysis to assess the dominant negative potency of Cx43 mutants. Our results revealed that the ODDD-linked Cx43 mutants, G21R and G138R, as well as amino terminus green fluorescent protein-tagged Cx43, were able to co-localize with wild-type Cx43 at the gap junction plaque-like structures and to co-immunoprecipitate with wild-type Cx43. All Cx43 mutants demonstrated dominant negative action on gap junctional conductance of wild-type Cx43 but not that of Cx32. More interestingly, these Cx43 mutants demonstrated different potencies in inhibiting the function of wild-type Cx43 with the G21R mutant being two times more potent than the G138R mutant. The potency difference in the dominant negative properties of ODDD-linked Cx43 mutants may have clinical implications for the various symptoms and disease severity observed in ODDD patients.

Three-dimensional structure of a human connexin26 gap junction channel reveals a plug in the vestibule

Connexin molecules form intercellular membrane channels facilitating electronic coupling and the passage of small molecules between adjoining cells. Connexin26 (Cx26) is the second smallest member of the gap junction protein family, and mutations in Cx26 cause certain hereditary human diseases such as skin disorders and hearing loss. Here, we report the electron crystallographic structure of a human Cx26 mutant (M34A). Although crystallization trials used hemichannel preparations, the density map revealed that two hemichannels redocked at their extracellular surfaces into full intercellular channels. These orthorhombic crystals contained two sets of symmetry-related intercellular channels within three lipid bilayers. The 3D map shows a prominent density in the pore of each hemichannel. This density contacts the innermost helices of the surrounding connexin subunits at the bottom of the vestibule. The density map suggests that physical blocking may play an important role that underlies gap junction channel regulation. Our structure allows us to suggest that the two docked hemichannels can be independent and may regulate their activity autonomously with a plug in the vestibule.

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.

Species specificity of mammalian connexin-26 to form open voltage-gated hemichannels

Mutations of connexin-26 (Cx26) cause nonsyndromic hearing loss and other syndromes affecting ectoderm-derived tissues. While the exact mechanisms underlying these diseases remain elusive, Cx's are generally considered to mediate cell-to-cell communication by forming gap junction channels. We show here that unlike rat Cx26, human and sheep Cx26 form voltage-gated hemichannels when expressed in oocytes and Neuro2A cells. A single evolutionary amino acidic change at position 159 of the rodent protein, the replacement of aspartic acid with asparagine in the human and sheep proteins, accounts for this species specificity. At the resting potential and in normal millimolar extracellular calcium, open human Cx26 hemichannels can be detected both electrophysiologically and by dye uptake, although they did not affect cell viability. These hemichannels opened at approximately -50 mV and their activation increased by depolarization until they inactivate at positive membrane potentials. Single-channel analysis revealed that activation and inactivation involved two distinct voltage gating mechanisms and that the fully open hemichannel displays a conductance twice that of the intercellular channel. The existence of a hemichannel that opens under physiological control of the membrane potential may have important implications for the normal and pathological activity of Cx26 in humans, particularly with respect to hearing and the epidermis.

Functional Characterization of aGJA1Frameshift Mutation Causing Oculodentodigital Dysplasia and Palmoplantar Keratoderma

A frameshift mutation generated from a dinucleotide deletion (780-781del) in the GJA1 gene encoding Cx43 results in a frameshift yielding 46 aberrant amino acids after residue 259 and a shortened protein of 305 residues compared with the 382 in wild-type Cx43. This frameshift mutant (fs260) causes oculodentodigital dysplasia (ODDD) that includes the added condition of palmoplantar keratoderma. When expressed in a variety of cell lines, the fs260 mutant was typically localized to the endoplasmic reticulum and other intracellular compartments. The fs260 mutant, but not the G138R ODDD-linked Cx43 mutant or a Cx43 mutant truncated at residue 259 (T259), reduced the number of apparent gap junction plaques formed from endogenous Cx43 in normal rat kidney cells or keratinocytes. Interestingly, mutation of a putative FF endoplasmic reticulum retention motif encoded within the 46 aberrant amino acid domain failed to restore efficient assembly of the fs260 mutant into gap junctions. Dual whole cell patch-clamp recording revealed that fs260-expressing N2A cells exerted severely reduced electrical coupling in comparison to wild-type Cx43 or the T259 mutant, whereas single patch capacitance recordings showed that fs260 could also dominantly inhibit the function of wild-type Cx43. Co-expression studies further revealed that the dominant negative effect of fs260 on wild-type Cx43 was dose-dependent, and at a predicted 1:1 expression ratio the fs260 mutant reduced wild-type Cx43-mediated gap junctional conductance by over 60%. These results suggest that the 46 aberrant amino acid residues associated with the frameshift mutant are, at least in part, responsible for the manifestation of palmoplantar keratoderma symptoms.