Cochleosaccular dysplasia associated with a connexin 26 mutation in keratitis-ichthyosis-deafness syndrome

Research progress in delineating the pathological mechanisms of GJB2-related hearing loss

Hearing loss is the most common congenital sensory impairment. Mutations or deficiencies of the GJB2 gene are the most common genetic cause of congenital non-syndromic deafness. Pathological changes such as decreased potential in the cochlea, active cochlear amplification disorders, cochlear developmental disorders and macrophage activation have been observed in various GJB2 transgenic mouse models. In the past, researchers generally believed that the pathological mechanisms underlying GJB2-related hearing loss comprised a K⁺ circulation defect and abnormal ATP-Ca²⁺ signals. However, recent studies have shown that K⁺ circulation is rarely associated with the pathological process of GJB2-related hearing loss, while cochlear developmental disorders and oxidative stress play an important, even critical, role in the occurrence of GJB2-related hearing loss. Nevertheless, these research has not been systematically summarized. In this review, we summarize the pathological mechanisms of GJB2-related hearing loss, including aspects of K⁺ circulation, developmental disorders of the organ of Corti, nutrition delivery, oxidative stress and ATP-Ca²⁺ signals. Clarifying the pathological mechanism of GJB2-related hearing loss can help develop new prevention and treatment strategies.

Antibody gene transfer treatment drastically improves epidermal pathology in a keratitis ichthyosis deafness syndrome model using male mice

BACKGROUND

Keratitis ichthyosis deafness (KID) syndrome is a rare disorder caused by hemichannel (HC) activating gain-of-function mutations in the GJB2 gene encoding connexin (Cx) 26, for which there is no cure, or current treatments based upon the mechanism of disease causation.

METHODS

We applied Adeno Associated Virus (AAV) mediated mAb gene transfer (AAVmAb) to treat the epidermal features of KID syndrome with a well-characterized HC blocking antibody using male mice of a murine model that replicates the skin pathology of the human disease.

FINDINGS

We demonstrate that in vivo AAVmAb treatment significantly reduced the size and thickness of KID lesions, in addition to blocking activity of mutant HCs in the epidermis in vivo. We also show that AAVmAb treatment eliminated abnormal keratinocyte proliferation and enlarged cell size, decreased apoptosis, and restored the normal distribution of keratin expression.

INTERPRETATION

Our findings reinforce the critical role played by increased HC activity in the skin pathology associated with KID syndrome. They also underscore the clinical potential of anti-HC mAbs coupled with genetic based delivery systems for treating the underlying mechanistic basis of this disorder. Inhibition of HC activity is an ideal therapeutic target in KID syndrome, and the genetic delivery of mAbs targeted against mutant HCs could form the basis of new therapeutic interventions to treat this incurable disease.

FUNDING

Fondazione Telethon grant GGP19148 and University of Padova grant Prot. BIRD187130 to FM; Foundation for Ichthyosis and Related Skin Types (FIRST) and National Institutes of Health grant EY 026911 to TWW.

KI04 an Aminoglycosides-Derived Molecule Acts as an Inhibitor of Human Connexin46 Hemichannels Expressed in HeLa Cells

BACKGROUND

Connexins (Cxs) are proteins that help cells to communicate with the extracellular media and with the cytoplasm of neighboring cells. Despite their importance in several human physiological and pathological conditions, their pharmacology is very poor. In the last decade, some molecules derived from aminoglycosides have been developed as inhibitors of Cxs hemichannels. However, these studies have been performed in E. coli, which is a very simple model. Therefore, our main goal is to test whether these molecules have similar effects in mammalian cells.

METHODS

We transfected HeLa cells with the human Cx46tGFP and characterized the effect of a kanamycin-derived molecule (KI04) on Cx46 hemichannel activity by time-lapse recordings, changes in phosphorylation by Western blot, localization by epifluorescence, and possible binding sites by molecular dynamics (MD).

RESULTS

We observed that kanamycin and KI04 were the most potent inhibitors of Cx46 hemichannels among several aminoglycosides, presenting an IC50 close to 10 μM. The inhibitory effect was not associated with changes in Cx46 electrophoretic mobility or its intracellular localization. Interestingly, 5 mM DTT did not reverse KI04 inhibition, but the KI04 effect completely disappeared after washing out KI04 from the recording media. MD analysis revealed two putative binding sites of KI04 in the Cx46 hemichannel.

RESULTS

These results demonstrate that KI04 could be used as a Cx46 inhibitor and could help to develop future selective Cx46 inhibitors.

Keratitis-ichthyosis-deafness syndrome: A comprehensive review of cutaneous and systemic manifestations

Keratitis-ichthyosis-deafness syndrome is a rare genetic disease presenting with cutaneous, ocular, and otic defects. This comprehensive review provides insight into the clinical presentations, highlighting the cutaneous manifestations including histopathology and treatment options.

F-Actin Dysplasia Involved in Organ of Corti Deformity in Gjb2 Knockdown Mouse Model

Mutations in the GJB2 gene encoding connexin26 (Cx26) protein are one of the most common causes of hereditary deafness. Previous studies have found that different Cx26-null mouse models have severe hearing loss and deformity of the organ of Corti (OC) as well as a reduction in microtubules in pillar cells (PCs). To explore the underlying mechanism of OC deformity caused by Cx26 downregulation further, we established Cx26 knockdown (KD) mouse models at postnatal days (P)0 and P8. The actin filaments contained in the pillar cells of mice in the P0 KD group were reduced by 54.85% and vinculin was increased by 22%, while the outer hair cells (OHCs) showed normal F-actin content. In the P8 KD group, PCs and OHCs of mice also showed almost normal F-actin content. The G-actin/F-actin ratio increased by 38% in the P0 KD group. No significant change was found in the mRNA or protein expression level of G-actin or the cadherin–catenin core complex in the P0 KD group at P6. Moreover, immunofluorescence showed that the intensity of LRRK2 was reduced by 97% in the P0 KD group at P6. Our results indicate that Cx26 is involved in the maturation of the cytoskeleton during the development of the OC at the early postnatal stage. The polymerization of G-actin into F-actin is prevented in Cx26 KD mice.

Cochlear histopathology in human genetic hearing loss: State of the science and future prospects

Sensorineural hearing loss (SNHL) is an extraordinarily common disability, affecting 466 million people across the globe. Half of these incidents are attributed to genetic mutations that disrupt the structure and function of the cochlea. The human cochlea's interior cannot be imaged or biopsied without damaging hearing; thus, everything known about the morphologic correlates of hereditary human deafness comes from histopathologic studies conducted in either cadaveric human temporal bone specimens or animal models of genetic deafness. The purpose of the present review is to a) summarize the findings from all published histopathologic studies conducted in human temporal bones with known SNHL-causing genetic mutations, and b) compare the reported phenotypes of human vs. mouse SNHL caused by the same genetic mutation. The fact that human temporal bone histopathologic analysis has been reported for only 22 of the nearly 200 identified deafness-causing genes suggests a great need for alternative and improved techniques for studying human hereditary deafness; in light of this, the present review concludes with a summary of promising future directions, specifically in the fields of high resolution cochlear imaging, intracochlear fluid biopsy, and gene therapy.

Connexins and Disease

Inherited or acquired alterations in the structure and function of connexin proteins have long been associated with disease. In the present work, we review current knowledge on the role of connexins in diseases associated with the heart, nervous system, cochlea, and skin, as well as cancer and pleiotropic syndromes such as oculodentodigital dysplasia (ODDD). Although incomplete by virtue of space and the extent of the topic, this review emphasizes the fact that connexin function is not only associated with gap junction channel formation. As such, both canonical and noncanonical functions of connexins are fundamental components in the pathophysiology of multiple connexin related disorders, many of them highly debilitating and life threatening. Improved understanding of connexin biology has the potential to advance our understanding of mechanisms, diagnosis, and treatment of disease.

Developmental abnormalities in supporting cell phalangeal processes and cytoskeleton in the Gjb2 knockdown mouse model

Mutations in the GJB2 gene [which encodes connexin 26 (Cx26)] are the most common causes of hereditary hearing loss in humans, and previous studies showed postnatal development arrest of the organ of Corti in different Cx26-null mouse models. To explore the pathological changes and the mechanism behind the cochlear abnormalities in these mice further, we established transgenic mouse models by conditional knockdown of cochlear Cx26 at postnatal day (P) 0 and P8. Auditory brainstem responses were recorded and the morphological features in the organ of Corti were analyzed 18 days after Cx26 knockdown. Mice in the P0 knockdown group displayed severe hearing loss at all frequencies, whereas mice in the P8 knockdown group showed nearly normal hearing. In the P8 knockdown group, the organ of Corti displayed normal architecture, and no ultrastructural changes were observed. In the P0 knockdown group, the phalangeal processes of Deiter's cells did not develop into finger-like structures, and the formation of microtubules in the pillar cells was significantly reduced; moreover, the amount of acetylated α-tubulin was reduced in pillar cells. Our results indicate that Gjb2 participates in postnatal development of the cytoskeleton in pillar cells during structural maturation of the organ of Corti. In P0 knockdown mice, the reduction in microtubules in pillar cells might be responsible for the failure of the tunnel of Corti to open, and the malformed phalangeal processes might negatively affect the supporting framework of the organ of Corti, which would be a new mechanism of Gjb2-related hearing loss.

Summary: A reduction in connexin 26 before opening of the tunnel of Corti impedes microtubule formation in supporting cells, and this may lead to cochlear developmental abnormalities and deafness in the Gjb2 knockdown mouse model.

Comparison of the Motor Performance and Vestibular Function in Infants with a Congenital Cytomegalovirus Infection or a Connexin 26 Mutation: A Preliminary Study

OBJECTIVES

Hearing-impaired children are at risk for vestibular damage and delayed motor development. Two major causes of congenital hearing loss are cytomegalovirus (CMV) infection and connexin (Cx) 26 mutations. Comparison of the motor performance and vestibular function between these specific groups is still underexplored. The objective of this study was to investigate the impact of congenital (c)CMV and Cx26 on the motor performance and vestibular function in 6 months old infants.

DESIGN

Forty children (mean age 6.7 months; range 4.8 to 8.9 months) participated in this cross-sectional design and were recruited from the Flemish CMV registry. They were divided into five age-matched groups: normal-hearing control, asymptomatic cCMV, normal-hearing symptomatic cCMV, hearing-impaired symptomatic cCMV, and hearing-impaired Cx26. Children were examined with the Peabody Developmental Motor Scales-2 and cervical vestibular-evoked myogenic potential (cVEMP) test.

RESULTS

Symptomatic hearing-impaired cCMV children demonstrated a significantly lower gross motor performance compared with the control group (p = 0.005), the asymptomatic cCMV group (p = 0.034), and the Cx26 group (0.016). In this symptomatic hearing-impaired cCMV group, 4 out of 8 children had absent cVEMP responses that were related to the weakest gross motor performance. The Cx26 children showed no significant delay in motor development compared with the control children and none of these children had absent cVEMP responses.

CONCLUSIONS

The weakest gross motor performance was found in symptomatic hearing-impaired cCMV-infected children with absent cVEMP responses. These results suggest that abnormal saccular responses are a major factor for this delayed motor development, although more work is needed including comprehensive vestibular function testing to verify this.

Connexin channel and its role in diabetic retinopathy

Diabetic retinopathy is the leading cause of blindness in the working age population. Unfortunately, there is no cure for this devastating ocular complication. The early stage of diabetic retinopathy is characterized by the loss of various cell types in the retina, namely endothelial cells and pericytes. As the disease progresses, vascular leakage, a clinical hallmark of diabetic retinopathy, becomes evident and may eventually lead to diabetic macular edema, the most common cause of vision loss in diabetic retinopathy. Substantial evidence indicates that the disruption of connexin-mediated cellular communication plays a critical role in the pathogenesis of diabetic retinopathy. Yet, it is unclear how altered communication via connexin channel mediated cell-to-cell and cell-to-extracellular microenvironment is linked to the development of diabetic retinopathy. Recent observations suggest the possibility that connexin hemichannels may play a role in the pathogenesis of diabetic retinopathy by allowing communication between cells and the microenvironment. Interestingly, recent studies suggest that connexin channels may be involved in regulating retinal vascular permeability. These cellular events are coordinated at least in part via connexin-mediated intercellular communication and the maintenance of retinal vascular homeostasis. This review highlights the effect of high glucose and diabetic condition on connexin channels and their impact on the development of diabetic retinopathy.

Comparative functional characterization of novel non-syndromic GJB2 gene variant p.Gly45Arg and lethal syndromic variant p.Gly45Glu

We characterized a novel GJB2 missense variant, c.133G>A, p.Gly45Arg, and compared it with the only other variant at the same amino acid position of the connexin 26 protein (Cx26) reported to date: c.134G>A, p.Gly45Glu. Whereas both variants are associated with hearing loss and are dominantly inherited, p.Gly45Glu has been implicated in the rare fatal keratitis-ichthyosis-deafness (KID) syndrome, which results in cutaneous infections and septicemia with premature demise in the first year of life. In contrast, p.Gly45Arg appears to be non-syndromic. Subcellular localization experiments in transiently co-transfected HeLa cells demonstrated that Cx26-WT (wild-type) and p.Gly45Arg form gap junctions, whereas Cx26-WT with p.Gly45Glu protein does not. The substitution of a nonpolar amino acid glycine in wildtype Cx26 at position 45 with a negatively charged glutamic acid (acidic) has previously been shown to interfere with Ca²⁺ regulation of hemichannel gating and to inhibit the formation of gap junctions, resulting in cell death. The novel variant p.Gly45Arg, however, changes this glycine to a positively charged arginine (basic), resulting in the formation of dysfunctional gap junctions that selectively affect the permeation of negatively charged inositol 1,4,5-trisphosphate (IP₃) and contribute to hearing loss. Cx26 p.Gly45Arg transfected cells, unlike cells transfected with p.Gly45Glu, thrived at physiologic Ca²⁺ concentrations, suggesting that Ca²⁺ regulation of hemichannel gating is unaffected in Cx26 p.Gly45Arg transfected cells. Thus, the two oppositely charged amino acids that replace the highly conserved uncharged glycine in p.Gly45Glu and p.Gly45Arg, respectively, produce strikingly different effects on the structure and function of the Cx26 protein.

Connexin26 Mutations Causing Palmoplantar Keratoderma and Deafness Interact with Connexin43, Modifying Gap Junction and Hemichannel Properties

Mutations in GJB2 (Cx26) cause either deafness, or deafness associated with skin diseases. That different disorders can be caused by distinct mutations within the same gene suggests that unique channel activities are influenced by each class of mutation. We have examined the functional characteristics of two human mutations, Cx26-H73R and Cx26-S183F, causing palmoplantar keratoderma (PPK) and deafness. Both failed to form gap junction channels or hemichannels when expressed alone. Co-expression of the mutants with wild-type Cx43 showed a trans-dominant inhibition of Cx43 gap junction channels, without reductions in Cx43 protein synthesis. In addition, the presence of mutant Cx26 shifted Cx43 channel gating and kinetics towards a more Cx26-like behavior. Co-immunoprecipitation showed Cx43 being pulled down more efficiently with mutant Cx26, than wild-type, confirming the enhanced formation of heteromeric connexons. Finally, the formation of heteromeric connexons resulted in significantly increased Cx43 hemichannel activity in the presence of Cx26 mutants. These findings suggest a common mechanism whereby Cx26 mutations causing PPK and deafness trans-dominantly influence multiple functions of wild-type Cx43. They also implicate a role for aberrant hemichannel activity in the pathogenesis of PPK, and further highlight an emerging role for Cx43 in genetic skin diseases.

Phenotype in a patient with p.D50N mutation in GJB2 gene resemble both KID and Clouston syndromes

Keratitis-ichthyosis-deafness (KID) syndrome (OMIM 148210) is a rare ectodermal dysplasia syndrome characterized by vascularizing keratitis, congenital profound sensorineural hearing loss, and progressive erythrokeratoderma. We have found a 148G-A transition in the GJB2 gene, resulting in an asp50-to-asn (D50N) substitution in a girl with congenital deafness. This finding allowed us to diagnose а KID syndrome. But clinical features were uncommon because of a mild skin manifestation, lack of keratitis and unusual appearance resembling Clouston syndrome. Molecular genetic tests showed that it was de novo mutation because parents have normal genotype. Several autosomal dominant mutations in the GJB2 gene (сonnexin 26) now established to underlie many of the affected cases, with the majority of patients harboring the p.D50N mutation. Skin disease-associated mutation of connexin proteins can cause functional disturbances in gap junction intercellular conductance. It is likely that multiple disease mechanisms are involved across the wide spectrum of hereditary diseases relating to connexin proteins. The clinical data may provide additional insights into the dysregulation mechanisms of mutations result in the disease.

From Hyperactive Connexin26 Hemichannels to Impairments in Epidermal Calcium Gradient and Permeability Barrier in the Keratitis-Ichthyosis-Deafness Syndrome

The keratitis-ichthyosis-deafness (KID) syndrome is characterized by corneal, skin, and hearing abnormalities. KID has been linked to heterozygous dominant missense mutations in the GJB2 and GJB6 genes, encoding connexin26 and 30, respectively. In vitro evidence indicates that KID mutations lead to hyperactive (open) hemichannels, which in some cases is accompanied by abnormal function of gap junction channels. Transgenic mouse models expressing connexin26 KID mutations reproduce human phenotypes and present impaired epidermal calcium homeostasis and abnormal lipid composition of the stratum corneum affecting the water barrier. Here we have compiled relevant data regarding the KID syndrome and propose a mechanism for the epidermal aspects of the disease.

Diseases associated with leaky hemichannels

Hemichannels (HCs) and gap junction channels (GJCs) formed by protein subunits called connexins (Cxs) are major pathways for intercellular communication. While HCs connect the intracellular compartment with the extracellular milieu, GJCs allow the interchange of molecules between cytoplasm of two contacting cells. Under physiological conditions, HCs are mostly closed, but they can open under certain stimuli allowing the release of autocrine and paracrine molecules. Moreover, some pathological conditions, like ischemia or other inflammation conditions, significantly increase HCs activity. In addition, some mutations in Cx genes associated with human diseases, such as deafness or cataracts, lead to the formation of more active HCs or “leaky HCs.” In this article we will revise cellular and molecular mechanisms underlying the appearance of leaky HCs, and the consequences of their expression in different cellular systems and animal models, in seeking a common pattern or pathological mechanism of disease.

Aberrant Cx26 hemichannels and keratitis-ichthyosis-deafness syndrome: insights into syndromic hearing loss

Mutation of the GJB2 gene, which encodes the connexin 26 (Cx26) gap junction (GJ) protein, is the most common cause of hereditary, sensorineural hearing loss. Cx26 is not expressed in hair cells, but is widely expressed throughout the non-sensory epithelial cells of the cochlea. Most GJB2 mutations produce non-syndromic deafness, but a subset produces syndromic deafness in which profound hearing loss is accompanied by a diverse array of infectious and neoplastic cutaneous disorders that can be fatal. Although GJ channels, which are assembled by the docking of two, so-called hemichannels (HCs), have been the main focus of deafness-associated disease models, it is now evident that the HCs themselves can function in the absence of docking and contribute to signaling across the cell membrane as a novel class of ion channel. A notable feature of syndromic deafness mutants is that the HCs exhibit aberrant behaviors providing a plausible basis for disease that is associated with excessive or altered contributions of Cx26 HCs that, in turn, lead to compromised cell integrity. Here we discuss some of the aberrant Cx26 HC properties that have been described for mutants associated with keratitis-ichthyosis-deafness (KID) syndrome, a particularly severe Cx26-associated syndrome, which shed light on genotype-phenotype relationships and causes underlying cochlear dysfunction.

Charge at the 46th residue of connexin 50 is crucial for the gap-junctional unitary conductance and transjunctional voltage-dependent gating

Gap-junction (GJ) channels are twice the length of most membrane channels, yet they often have large unitary channel conductance (γj). What factors make this possibly the longest channel so efficient in passing ions are not fully clear. Here we studied the lens connexin (Cx) 50 GJs, which display one of the largest γj and the most sensitive transjunctional voltage-dependent gating (Vj gating) among all GJ channels. Introduction of charged residues into a putative pore-lining domain (the first transmembrane and the first extracellular loop border) drastically altered the apparent γj. Specifically, G46D and G46E increased the Cx50 γj from 201 to 256 and 293 pS, respectively and the G46K channel showed an apparent γj of only 20 pS. G46K also drastically altered Vj gating properties in homotypic G46K and heterotypic Cx50/G46K channels, causing an apparent loss of fast Vj-dependent gating transitions and leaving only loop gating transitions at the single channel current records. Both macroscopic and single channel currents of heterotypic Cx50/G46K channels showed a prominent rectification. Our homology structural models indicate that the pore surface electrostatic potentials are a dictating factor in determining the γj. Our data demonstrate, at the whole GJ channel level, a crucial role of the surface charge properties in the first transmembrane/first extracellular border domain in determining the efficiency of ion permeation and the Vj gating of Cx50 and possibly other GJ channels.

Timed conditional null of connexin26 in mice reveals temporary requirements of connexin26 in key cochlear developmental events before the onset of hearing

Mutations in the Gjb2 gene, which encodes a gap junction protein connexin26 (Cx26), are the most prevalent form of hereditary deafness in humans and represent about half of non-syndromic congenital deafness cases in many ethnic populations. Cochlear potassium (K+) recycling in mature cochlea is required for normal hearing. It is thought that gap junctions are essential for K+ recycling and that Gjb2 mutations cause Gjb2-associated deafness by disrupting K+ recycling in mature cochlea. Here we present evidence showing that Gjb2 is required for normal development of the neonatal organ of Corti prior to the onset of the hearing in mice. In the conditional Gjb2 null (cCx26 null) mice, ribbon synapses in inner hair cells remained poorly developed, the afferent type I fibers failed to finish the refinement process to form convergent innervation to individual inner hair cells. The spontaneous depolarizing activities in the supporting cells, which normally diminish in the wild type cochleae after postnatal day 8 (P8), remained strong in the cochlea after P8 in the mutant mice. Furthermore, the deafness phenotype was readily generated only if the Cx26 expression in the organ of Corti was significantly reduced before P6. Similar amount of Cx26 reduction in more mature cochleae had a much weaker effect in damaging the hearing sensitivity. Our findings indicated that Cx26 plays essential roles in the maturation process of the organ of Corti prior to the establishment of high K+ in the endolymph and the onset of hearing. These results suggest that successful treatment of Cx26 deafness requires early intervention before the cochlea fully matures.

Aberrant connexin26 hemichannels underlying keratitis-ichthyosis-deafness syndrome are potently inhibited by mefloquine

Keratitis-ichthyosis-deafness (KID) syndrome is an ectodermal dysplasia caused by dominant mutations of connexin26 (Cx26). Loss of Cx26 function causes non-syndromic sensorineural deafness, without consequence in the epidermis. Functional analyses have revealed that a majority of KID-causing mutations confer a novel expansion of hemichannel activity, mediated by connexin channels in a non-junctional configuration. Inappropriate Cx26 hemichannel opening is hypothesized to compromise keratinocyte integrity and epidermal homeostasis. Pharmacological modulators of Cx26 are needed to assess the pathomechanistic involvement of hemichannels in the development of hyperkeratosis in KID syndrome. We have used electrophysiological assays to evaluate small molecule analogs of quinine for suppressive effects on aberrant hemichannel currents elicited by KID mutations. Here, we show that mefloquine inhibits several mutant hemichannel forms implicated in KID syndrome when expressed in Xenopus laevis oocytes (IC₅₀≈16µM), using an extracellular divalent cation, zinc (Zn⁺⁺), as a non-specific positive control for comparison (IC₅₀≈3µM). Furthermore, we used freshly isolated transgenic keratinocytes to show that micromolar concentrations of mefloquine attenuated increased macroscopic membrane currents in primary mouse keratinocytes expressing human Cx26-G45E, a mutation causing a lethal form of KID syndrome.

Revertant mutation releases confined lethal mutation, opening Pandora's box: a novel genetic pathogenesis

When two mutations, one dominant pathogenic and the other “confining” nonsense, coexist in the same allele, theoretically, reversion of the latter may elicit a disease, like the opening of Pandora's box. However, cases of this hypothetical pathogenic mechanism have never been reported. We describe a lethal form of keratitis-ichthyosis-deafness (KID) syndrome caused by the reversion of the GJB2 nonsense mutation p.Tyr136X that would otherwise have confined the effect of another dominant lethal mutation, p.Gly45Glu, in the same allele. The patient's mother had the identical misssense mutation which was confined by the nonsense mutation. The biological relationship between the parents and the child was confirmed by genotyping of 15 short tandem repeat loci. Haplotype analysis using 40 SNPs spanning the >39 kbp region surrounding the GJB2 gene and an extended SNP microarray analysis spanning 83,483 SNPs throughout chromosome 13 in the family showed that an allelic recombination event involving the maternal allele carrying the mutations generated the pathogenic allele unique to the patient, although the possibility of coincidental accumulation of spontaneous point mutations cannot be completely excluded. Previous reports and our mutation screening support that p.Gly45Glu is in complete linkage disequilibrium with p.Tyr136X in the Japanese population. Estimated from statisitics in the literature, there may be approximately 11,000 p.Gly45Glu carriers in the Japanese population who have this second-site confining mutation, which acts as natural genetic protection from the lethal disease. The reversion-triggered onset of the disesase shown in this study is a previously unreported genetic pathogenesis based on Mendelian inheritance.

Loss of gene functions due to nonsense mutations is a typical pathogenic mechanism of hereditary diseases. They may, however, in certain genetic contexts, confine the effects of other dominant pathogenic mutations and suppress disease manifestations. We report the first instance in the literature where the reversion of a “confining” nonsense mutation in GJB2 gene released the dominant pathogenic effect of a coexsisting gain-of-function mutation, eliciting the lethal form of keratitis-ichthyosis-deafness syndrome (KID). We describe this form of KID syndrome caused by the reversion of the GJB2 nonsense mutation p.Tyr136X that would otherwise have confined the effect of another dominant lethal mutation, p.Gly45Glu, in the same allele. The patient's mother had the identical misssense mutation which was confined by the nonsense mutation. An epidemiologic estimation demonstrates that approximately 11,000 individuals in the Japanese population may have the same lethal GJB2 mutation, nonetheless protected from the manifestation of the syndrome because they also inherit the common “confining” nonsense mutation. The reversion-triggered onset of the disease shown in this study is a previously unreported genetic pathogenesis based on Mendelian inheritance.

The D50N mutation and syndromic deafness: altered Cx26 hemichannel properties caused by effects on the pore and intersubunit interactions

Mutations in the GJB2 gene, which encodes Cx26, are the most common cause of sensorineural deafness. In syndromic cases, such as keratitis-ichthyosis-deafness (KID) syndrome, in which deafness is accompanied by corneal inflammation and hyperkeratotic skin, aberrant hemichannel function has emerged as the leading contributing factor. We found that D50N, the most frequent mutation associated with KID syndrome, produces multiple aberrant hemichannel properties, including loss of inhibition by extracellular Ca(2+), decreased unitary conductance, increased open hemichannel current rectification and voltage-shifted activation. We demonstrate that D50 is a pore-lining residue and that negative charge at this position strongly influences open hemichannel properties. Examination of two putative intersubunit interactions involving D50 suggested by the Cx26 crystal structure, K61-D50 and Q48-D50, showed no evidence of a K61-D50 interaction in hemichannels. However, our data suggest that Q48 and D50 interact and disruption of this interaction shifts hemichannel activation positive along the voltage axis. Additional shifts in activation by extracellular Ca(2+) remained in the absence of a D50-Q48 interaction but required an Asp or Glu at position 50, suggesting a separate electrostatic mechanism that critically involves this position. In gap junction (GJ) channels, D50 substitutions produced loss of function, whereas K61 substitutions functioned as GJ channels but not as hemichannels. These data demonstrate that D50 exerts effects on Cx26 hemichannel and GJ channel function as a result of its dual role as a pore residue and a component of an intersubunit complex in the extracellular region of the hemichannel. Differences in the effects of substitutions in GJ channels and hemichannels suggest that perturbations in structure occur upon hemichannel docking that significantly impact function. Collectively, these data provide insight into Cx26 structure-function and the underlying bases for the phenotypes associated with KID syndrome patients carrying the D50N mutation.

Conserved glycine at position 45 of major cochlear connexins constitutes a vital component of the Ca²⁺ sensor for gating of gap junction hemichannels

Mutations in gap junction (GJ) family of proteins, especially in the connexin (Cx) 26, are responsible for causing severe congenital hearing loss in a significant portion of patients (30-50% in various ethnic groups). Substitution of glycine at the position 45 of Cx26 to glutamic acid (p.G45E mutation) causes the Keratitis-ichthyosis-deafness (KID) syndrome. Previous studies have suggested that this point mutation caused a gain-of-function defect. However, the molecular mechanism of KID syndrome remains unclear. Since glycine at this position is conserved in many Cxs expressed in the cochlea, we tested the hypothesis that glycine at position 45 is an important component of the sensor regulating the Ca(2+) gating of GJ hemichannels. Using reconstituted Cx30, 32 and 43 expressed in the HEK 293 cells, we compared the functions of wild type and p.G45E mutant Cxs. We found that G45E in Cx30 resulted in similar deleterious cellular effects as Cx26 did. Cell death occurred within 24h of transfection, which was rescued by increasing extracellular Ca(2+) concentration ([Ca(2+)]o). Dye loading assay showed that Cx30 G45E, similar to Cx26 G45E, had leaky hemichannels at physiological [Ca(2+)]o (1.2 mM). Higher [Ca(2+)]o reduced the dye loading in a dose-dependent manner. Whole cell membrane current recordings also indicated that G45E caused increased hemichannel activities. p.G45E mutations of Cx32 and 43 also resulted in leaky hemichannels compared to their respective wild types in lower [Ca(2+)]o. Our data in this study provided further support for the hypothesis that glycine at position 45 is a conserved Ca(2+) sensor for the gating of GJ hemichannels among multiple Cx subtypes expressed in the cochlea.

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.

The Cx26-G45E mutation displays increased hemichannel activity in a mouse model of the lethal form of keratitis-ichthyosis-deafness syndrome

Mutations in the GJB2 gene (Cx26) cause deafness in humans. Most are loss-of-function mutations and cause nonsyndromic deafness. Some mutations produce a gain of function and cause syndromic deafness associated with skin disorders, such as keratitis-ichthyosis-deafness syndrome (KIDS). Cx26-G45E is a lethal mutation linked to KIDS that forms constitutively active connexin hemichannels. The pathomechanism(s) by which mutant Cx26 hemichannels perturb normal epidermal cornification are poorly understood. We created an animal model for KIDS by generating an inducible transgenic mouse expressing Cx26-G45E in keratinocytes. Cx26-G45E mice displayed reduced viability, hyperkeratosis, scaling, skin folds, and hair loss. Histopathology included hyperplasia, acanthosis, papillomatosis, increased cell size, and osteal plugging. These abnormalities correlated with human KIDS pathology and were associated with increased hemichannel currents in transgenic keratinocytes. These results confirm the pathogenic nature of the G45E mutation and provide a new model for studying the role of aberrant connexin hemichannels in epidermal differentiation and inherited connexin disorders.

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

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

Differentially altered Ca2+ regulation and Ca2+ permeability in Cx26 hemichannels formed by the A40V and G45E mutations that cause keratitis ichthyosis deafness syndrome

Mutations in GJB2, which encodes Cx26, are one of the most common causes of inherited deafness in humans. More than 100 mutations have been identified scattered throughout the Cx26 protein, most of which cause nonsyndromic sensorineural deafness. In a subset of mutations, deafness is accompanied by hyperkeratotic skin disorders, which are typically severe and sometimes fatal. Many of these syndromic deafness mutations localize to the amino-terminal and first extracellular loop (E1) domains. Here, we examined two such mutations, A40V and G45E, which are positioned near the TM1/E1 boundary and are associated with keratitis ichthyosis deafness (KID) syndrome. Both of these mutants have been reported to form hemichannels that open aberrantly, leading to “leaky” cell membranes. Here, we quantified the Ca²⁺ sensitivities and examined the biophysical properties of these mutants at macroscopic and single-channel levels. We find that A40V hemichannels show significantly impaired regulation by extracellular Ca²⁺, increasing the likelihood of aberrant hemichannel opening as previously suggested. However, G45E hemichannels show only modest impairment in regulation by Ca²⁺ and instead exhibit a substantial increase in permeability to Ca²⁺. Using cysteine substitution and examination of accessibility to thiol-modifying reagents, we demonstrate that G45, but not A40, is a pore-lining residue. Both mutants function as cell–cell channels. The data suggest that G45E and A40V are hemichannel gain-of-function mutants that produce similar phenotypes, but by different underlying mechanisms. A40V produces leaky hemichannels, whereas G45E provides a route for excessive entry of Ca²⁺. These aberrant properties, alone or in combination, can severely compromise cell integrity and lead to increased cell death.

Temporal bone abnormalities in children with GJB2 mutations

OBJECTIVES

To determine the incidence of temporal bone abnormalities in children with sensorineural hearing loss (SNHL) and pathogenic biallelic GJB2 mutations.

STUDY DESIGN

Retrospective analysis of a large cohort of pediatric patients with biallelic GJB2 mutations and SNHL (observational case series).

METHODS

Blinded review of all available temporal bone computed tomographic (CT) and magnetic resonance imaging (MRI) studies in this cohort.

RESULTS

Out of 158 patients with biallelic GJB2 mutations, 113 had CT and/or MRI studies available for review. Definite, although generally subtle, inner ear abnormalities were present in 12/113. There were malformations of the semicircular canals (SCC) in 4/12, of the internal auditory canal in 2/12, of the cochlear nerve canal (CNC) in 6, and unilateral cochlear malformation in 1/12. MRI in 1/5 showed mildly hypoplastic cochlear nerve. There was no correlation between SNHL severity and presence/absence/type of malformations or genotype.

CONCLUSIONS

Our study of 113 biallelic GJB2 patients with SNHL and temporal bone imaging is the largest study to date. We found only 10% had any abnormalities, most subtle, and none had EVA. Additionally, there was no correlation between SNHL severity and presence/absence/type of malformations or genotype. Disparities between our group and previous reports may be due to differences in degree of hearing loss, types of mutations, populations studied, and radiologic factors for both image acquisition and interpretation.

Vestibular function of patients with profound deafness related to GJB2 mutation

CONCLUSION

GJB2 mutations are responsible not only for deafness but also for the occurrence of vestibular dysfunction. However, vestibular dysfunction tends to be unilateral and less severe in comparison with that of bilateral deafness.

OBJECTIVES

The correlation between the cochlear and vestibular end-organs suggests that some children with congenital deafness may have vestibular impairments. On the other hand, GJB2 gene mutations are the most common cause of nonsyndromic deafness. The vestibular function of patients with congenital deafness (CD), which is related to GJB2 gene mutation, remains to be elucidated. The purpose of this study was to analyze the relationship between GJB2 gene mutation and vestibular dysfunction in adults with CD.

METHODS

A total of 31 subjects, including 10 healthy volunteers and 21 patients with CD, were enrolled in the study. A hearing test and genetic analysis were performed. The vestibular evoked myogenic potentials (VEMPs) were measured and a caloric test was performed to assess the vestibular function. The percentage of vestibular dysfunction was then statistically analyzed.

RESULTS

The hearing level of all CD patients demonstrated a severe to profound impairment. In seven CD patients, their hearing impairment was related to GJB2 mutation. Five of the seven patients with CD related to GJB2 mutation demonstrated abnormalities in one or both of the two tests. The percentage of vestibular dysfunction of the patients with CD related to GJB2 mutation was statistically higher than in patients with CD unrelated to GJB2 mutation and in healthy controls.

Germline mosaicism in keratitis-ichthyosis-deafness syndrome: pre-natal diagnosis in a familial lethal form

Keratitis-ichthyosis-deafness (KID) syndrome is an autosomal dominant congenital ectodermal defect characterized by the association of skin lesions, hearing loss and keratitis. Most of the cases appear to be sporadic. KID syndrome is mostly related to mutations of GJB2 gene encoding connexin-26. Recently, a lethal form of the disease during the first year of life has been reported in two unrelated Caucasian patients. This rare lethal form is caused by the G45E mutation of GJB2 gene. We here report the first pre-natal molecular genetic diagnosis of the lethal form of KID syndrome relating to a G45E mutation. In the same family, the occurrence of this condition in three other siblings born to African non-consanguineous healthy parents lead to perform pre-natal diagnosis for this last pregnancy. Molecular analysis confirms the diagnosis of the lethal form of KID for the fetus. These results establish the role of germline mosaicism in KID syndrome and warrant careful genetic counseling. Furthermore, analysis of our cases and the literature allowed us to define a characteristic severe neonatal phenotype including facial dysmorphy, severe cornification with massive focal hyperkeratosis of the skin with erythroderma, dystrophic nails, complete atrichia and absence of foreskin.

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.

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.

Diverse deafness mechanisms of connexin mutations revealed by studies using in vitro approaches and mouse models

Mutations in connexins (Cxs), the constitutive protein subunits of gap junction (GJ) intercellular channels, are one of the most common human genetic defects that cause severe prelingual non-syndromic hearing impairments. Many subtypes of Cxs (e.g., Cxs 26, 29, 30, 31, 43) and pannexins (Panxs) are expressed in the cochlea where they contribute to the formation of a GJ-based intercellular communication network. Cx26 and Cx30 are the predominant cochlear Cxs and they co-assemble in most GJ plaques to form hybrid GJs. The cellular localization of specific Cx subtypes provides a basis for understanding the molecular structure of GJs and hemichannels in the cochlea. Information about the interactions among the various co-assembled Cx partners is critical to appreciate the functional consequences of various types of genetic mutations. In vitro studies of reconstituted GJs in cell lines have yielded surprisingly heterogeneous mechanisms of dysfunction caused by various Cx mutations. Availability of multiple lines of Cx-mutant mouse models has provided some insight into the pathogenesis processes in the cochlea of deaf mice. Here we summarize recent advances in understanding the structure and function of cochlear GJs and give a critical review of current findings obtained from both in vitro studies and mouse models on the mechanisms of Cx mutations that lead to cell death in the cochlea and hearing loss.

Spatiotemporal loss of K+ transport proteins in the developing cochlear lateral wall of guinea pigs with hereditary deafness

Genetic deafness is one of the most common human genetic birth defects. To understand the molecular mechanisms underlying human hereditary deafness, deaf animal strains have proved to be invaluable models. The German waltzing guinea pig is a new strain of animals with unidentified gene mutation(s), displaying recessively inherited cochleovestibular impairment. Histological investigations of the homozygous animals (gw/gw) revealed a collapse of the endolymphatic compartment and malformation of stria vascularis. RT-PCR showed a significant reduction in expression of the strial intermediate cell-specific gene Dct and the tight-junction gene Cldn11 in the embryonic day (E)40 and adult gw/gw cochlear lateral wall. Immunohistochemical analysis of the gw/gw cochlea showed loss of the tight junction protein CLDN11 in strial basal cells from E40, loss of the potassium channel subunit KCNJ10 in strial intermediate cells from E50, and loss of the Na-K-Cl cotransporter SLC12A2 in strial marginal cells from E50. In addition, a temporary loss of the gap junction protein GJB2 (connexin 26) between fibrocytes in the spiral ligament of the E50 gw/gw cochlea was observed. The barrier composed of tight junctions between strial basal cells was disrupted in the gw/gw cochlea as indicated by a biotin tracer permeability assay. In conclusion, spatiotemporal loss of K+ transport proteins in the cochlear lateral wall is caused by malformation of the stria vascularis in the developing German waltzing guinea pig inner ear. This new animal strain may serve as a good model for studying human genetic deafness due to disruption of inner ear ion homeostasis.

A familial case of Keratitis-Ichthyosis-Deafness (KID) syndrome with the GJB2 mutation G45E

Keratitis-Ichthyosis-Deafness (KID) syndrome (OMIM 148210) is a congenital ectodermal defect. KID consists of an atypical ichthyosiform erythroderma associated with congenital sensorineural deafness. A rare form of the KID syndrome is a fatal course in the first year of life due to severe skin lesion infections and septicaemia. KID appears to be genetically heterogeneous and may be caused by mutations in connexin 26 or connexin 30 genes. GJB2 mutations in the connexin 26 gene are the main cause of the disease. Most of the cases caused by GJB2 mutations are sporadic, but dominant transmission has also been described. To date, the rare lethal form of the disease has been only observed in two Caucasian sporadic patients with the GJB2 mutation, with the p.Gly45Glu (G45E) arising de novo. We have reported an African family with dizygotic twins suffering from a lethal form of KID. The dizygosity of the twins was confirmed by microsatellite markers. The two patients were heterozygous for the G45E mutation of GJB2, whereas the mutation was not detected in the two parents. The unusual transmission of the disease observed in this family could be explained by the occurrence of a somatic or more probably a germinal mosaic in one of the parents.

Aberrant hemichannel properties of Cx26 mutations causing skin disease and deafness

Mutations in the human GJB2 gene, which encodes connexin26 (Cx26), underlie various forms of hereditary deafness and skin disease. While it has proven difficult to discern the exact pathological mechanisms that cause these disorders, studies have shown that the loss or abnormal function of Cx26 protein has a profound effect on tissue homeostasis. Here, we used the Xenopus oocyte expression system to examine the functional characteristics of a Cx26 mutation (G45E) that results in keratitis-ichthyosis-deafness syndrome (KIDS) with a fatal outcome. Our data showed that oocytes were able to express both wild-type Cx26 and its G45E variant, each of which formed hemichannels and gap junction channels. However, Cx26-G45E hemichannels displayed significantly greater whole cell currents than wild-type Cx26, leading to cell lysis and death. This severe phenotype could be rescued in the presence of elevated Ca(2+) levels in the extracellular milieu. Cx26-G45E could also form intercellular channels with a similar efficiency as wild-type Cx26, however, with increased voltage sensitive gating. We also compared Cx26-G45E with a previously described Cx26 mutant, A40V, which has an overlapping human phenotype. We found that both dominant Cx26 mutants elicited similar functional consequences and that cells coexpressing mutant and wild-type connexins predominantly displayed mutant-like behavior. These data suggest that mutant hemichannels may act on cellular homeostasis in a manner that can be detrimental to the tissues in which they are expressed.