Audiologic phenotype and progression in GJB2 (Connexin 26) hearing loss

DFNB1 Non-syndromic Hearing Impairment: Diversity of Mutations and Associated Phenotypes

The inner ear is a very complex sensory organ whose development and function depend on finely balanced interactions among diverse cell types. The many different kinds of inner ear supporting cells play the essential roles of providing physical and physiological support to sensory hair cells and of maintaining cochlear homeostasis. Appropriately enough, the gene most commonly mutated among subjects with hereditary hearing impairment (HI), GJB2, encodes the connexin-26 (Cx26) gap-junction channel protein that underlies both intercellular communication among supporting cells and homeostasis of the cochlear fluids, endolymph and perilymph. GJB2 lies at the DFNB1 locus on 13q12. The specific kind of HI associated with this locus is caused by recessively-inherited mutations that inactivate the two alleles of the GJB2 gene, either in homozygous or compound heterozygous states. We describe the many diverse classes of genetic alterations that result in DFNB1 HI, such as large deletions that either destroy the GJB2 gene or remove a regulatory element essential for GJB2 expression, point mutations that interfere with promoter function or splicing, and small insertions or deletions and nucleotide substitutions that target the GJB2 coding sequence. We focus on how these alterations disrupt GJB2 and Cx26 functions and on their different effects on cochlear development and physiology. We finally discuss the diversity of clinical features of DFNB1 HI as regards severity, age of onset, inner ear malformations and vestibular dysfunction, highlighting the areas where future research should be concentrated.

ACEMg Diet Supplement Modifies Progression of Hereditary Deafness

Dietary supplements consisting of beta-carotene (precursor to vitamin A), vitamins C and E and the mineral magnesium (ACEMg) can be beneficial for reducing hearing loss due to aminoglycosides and overstimulation. This regimen also slowed progression of deafness for a boy with GJB2 (CONNEXIN 26) mutations. To assess the potential for treating GJB2 and other forms of hereditary hearing loss with ACEMg, we tested the influence of ACEMg on the cochlea and hearing of mouse models for two human mutations: GJB2, the leading cause of childhood deafness, and DIAPH3, a cause of auditory neuropathy. One group of mice modeling GJB2 (Gjb2-CKO) received ACEMg diet starting shortly after they were weaned (4 weeks) until 16 weeks of age. Another group of Gjb2-CKO mice received ACEMg in utero and after weaning. The ACEMg diet was given to mice modeling DIAPH3 (Diap3-Tg) after weaning (4 weeks) until 12 weeks of age. Control groups received food pellets without the ACEMg supplement. Hearing thresholds measured by auditory brainstem response were significantly better for Gjb2-CKO mice fed ACEMg than for the control diet group. In contrast, Diap3-Tg mice displayed worse thresholds than controls. These results indicate that ACEMg supplementation can influence the progression of genetic hearing loss.

The pathological effects of connexin 26 variants related to hearing loss by in silico and in vitro analysis

Gap junctions (GJs) are intercellular channels associated with cell-cell communication. Connexin 26 (Cx26) encoded by the GJB2 gene forms GJs of the inner ear, and mutations of GJB2 cause congenital hearing loss that can be syndromic or non-syndromic. It is difficult to predict pathogenic effects using only genetic analysis. Using ionic and biochemical coupling tests, we evaluated the pathogenic effects of Cx26 variants using computational analyses to predict structural abnormalities. For seven out of ten variants, we predicted the variation would result in a loss of GJ function, whereas the others would completely fail to form GJs. Functional studies demonstrated that, although all variants were able to function normally as hetero-oligomeric GJ channels, six variants (p.E47K, p.E47Q, p.H100L, p.H100Y, p.R127L, and p.M195L) did not function normally as homo-oligomeric GJ channels. Interestingly, GJs composed of the Cx26 variant p.R127H were able to function normally, even as homo-oligomeric GJ channels. This study demonstrates the particular location and property of an amino acid are more important mainly than the domain where they belong in the formation and function of GJ, and will provide information that is useful for the accurate diagnosis of hearing loss.

Acquired Hearing Loss in Children

Hearing loss is the most common congenital sensory impairment. According to National Health and Nutrition Examination Survey data from 2001 to 2008, 20.3% of subjects aged greater than or equal to 12 had unilateral or bilateral hearing loss. The World Health Organization notes that, worldwide, there are 360 million people with disabling hearing loss, with 50% preventable. Although many hearing losses are acquired, many others are manifestations of preexisting conditions. The purpose of a pediatric hearing evaluation is to identify the degree and type of hearing loss and etiology and to outline a comprehensive strategy that supports language and social development and communication.

TMPRSS3 mutations in autosomal recessive nonsyndromic hearing loss

Nonsyndromic genetic deafness is highly heterogeneous in its clinical presentation, pattern of inheritance and underlying genetic causes. Mutations in TMPRSS3 gene encoding transmembrane serine protease account for <1 % of autosomal recessive nonsyndromic hearing loss (ARNSHL) in Caucasians. Targeted next generation sequencing in the index family with profound deaf parents and a son, and Sanger sequencing of selected TMPRSS3 gene regions in a cohort of thirty-five patients with suspected ARNSHL was adopted. A son and his mother in the index family were homozygous for TMPRSS3 c.208delC (p.His70Thrfs*19) variant. Father was digenic compound heterozygote for the same variant and common GJB2 c.35delG variant. Three additional patients from the ARNSHL cohort were homozygous for TMPRSS3 c.208delC. TMPRSS3 defects seem to be an important cause of ARNSHL in Slovenia resulting in uniform phenotype with profound congenital hearing loss, and satisfactory hearing and speech recognition outcome after cochlear implantation. Consequently, TMPRSS3 gene analysis should be included in the first tier of genetic investigations of ARNSHL along with GJB2 and GJB6 genes.

Connexin26 (GJB2) deficiency reduces active cochlear amplification leading to late-onset hearing loss

Connexin26 (Cx26, GJB2) mutations account for >50% of nonsyndromic hearing loss. The deafness is not always congenital. A large group of these patients (∼30%) demonstrate a late-onset hearing loss, starting in childhood. They have normal hearing early in life and are therefore good candidates for applying protective and therapeutic interventions. However, the underlying deafness mechanism is unclear. In this study, we used a time-controlled, inducible gene knockout technique to knockout Cx26 expression in the cochlea after birth. We found that deletion of Cx26 after postnatal day 5 (P5) in mice could lead to late-onset hearing loss. Similar to clinical observations, the mice demonstrated progressive, mild to moderate hearing loss. The hearing loss initiated at high frequencies and then extended to the middle- and low-frequency range. The cochlea showed normal development and had no apparent hair cell loss. However, distortion product otoacoustic emission (DPOAE) was reduced. The reduction was also progressive and large at high-frequencies. Consistent with DPOAE reduction, we found that outer hair cell electromotility-associated nonlinear capacitance was shifted to the right and the slope of voltage dependence was reduced. The endocochlear potential was reduced in Cx26 conditional knockout (cKO) mice but the reduction was not associated with progressive hearing loss. These data suggest that Cx26 deficiency may impair active cochlear amplification leading to late-onset hearing loss. Our study also helps develop newer protective and therapeutic interventions to this common nonsyndromic hearing loss.

American College of Medical Genetics and Genomics guideline for the clinical evaluation and etiologic diagnosis of hearing loss

Hearing loss is a common and complex condition that can occur at any age, can be inherited or acquired, and is associated with a remarkably wide array of etiologies. The diverse causes of hearing loss, combined with the highly variable and often overlapping presentations of different forms of hearing loss, challenge the ability of traditional clinical evaluations to arrive at an etiologic diagnosis for many deaf and hard-of-hearing individuals. However, identifying the etiology of a hearing loss may affect clinical management, improve prognostic accuracy, and refine genetic counseling and assessment of the likelihood of recurrence for relatives of deaf and hard-of-hearing individuals. Linguistic and cultural identities associated with being deaf or hard of hearing can complicate access to and the effectiveness of clinical care. These concerns can be minimized when genetic and other health-care services are provided in a linguistically and culturally sensitive manner. This guideline offers information about the frequency, causes, and presentations of hearing loss and suggests approaches to the clinical evaluation of deaf and hard-of-hearing individuals aimed at identifying an etiologic diagnosis and providing informative and effective patient education and genetic counseling.

ACEMg supplementation ameliorates progressive Connexin 26 hearing loss in a child

Mutations in the gene encoding Connexin 26 are the most common cause of genetic hearing loss. The hearing loss is typically stable but may be progressive. The reason for progression is unknown. Antioxidants have been associated with attenuation of hearing loss from other insults. One antioxidant regimen consists of beta-carotene (metabolized to vitamin A), vitamin C, vitamin E, and magnesium (ACEMg). We present a child with Connexin 26 related hearing loss who experienced progressive hearing loss over 7 years of observation. He was given ACEMg daily for 3 years, during which time his progressive hearing loss was ameliorated.

Identification of four novel connexin 26 mutations in non-syndromic deaf patients: genotype-phenotype analysis in moderate cases

This paper presents a mutation as well as a genotype-phenotype analysis of the GJB2 and GJB6 genes in 476 samples from non-syndromic unrelated Argentinean deaf patients (104 familial and 372 sporadic cases). Most of them were of prelingual onset (82 %) and 27 % were cochlear implanted. Variation of sequences was detected in 171 of the 474 patients (36 %). Overall, 43 different sequence variations were identified in GJB2 and GJB6. Four of them are reported for the first time in GJB2: c.233dupG, p.Ala78Ser, p.Val190Asp and p.Cys211Tyr. Mutations in GJB6 were detected in 3 % of patients [nine del(GJB6-D13S1830) and three del(GJB6-D13S1854)]. Of the 43 different variations identified in GJB2, 6 were polymorphisms and of the others, 10 (27 %) were truncating and 27 (73 %) were nontruncating. Patients with two truncating mutations had significantly worse hearing impairment than all other groups. Moderate phenotypes were observed in a group of patients carrying biallelic mutations (23 %). This work shows the high prevalence of GJB2 mutations in the Argentinean population and presents an analysis of moderate phenotypes in our cohort.

Etiology and audiological outcomes at 3 years for 364 children in Australia

Hearing loss is an etiologically heterogeneous trait with differences in the age of onset, severity and site of lesion. It is caused by a combination of genetic and/or environmental factors. A longitudinal study to examine the efficacy of early intervention for improving child outcomes is ongoing in Australia. To determine the cause of hearing loss in these children we undertook molecular testing of perinatal “Guthrie” blood spots of children whose hearing loss was either detected via newborn hearing screening or detected later in infancy. We analyzed the GJB2 and SLC26A4 genes for the presence of mutations, screened for the mitochondrial DNA (mtDNA) A1555G mutation, and screened for congenital CMV infection in DNA isolated from dried newborn blood spots. Results were obtained from 364 children. We established etiology for 60% of children. One or two known GJB2 mutations were present in 82 children. Twenty-four children had one or two known SLC26A4 mutations. GJB2 or SLC26A4 changes with unknown consequences on hearing were found in 32 children. The A1555G mutation was found in one child, and CMV infection was detected in 28 children. Auditory neuropathy spectrum disorder was confirmed in 26 children whose DNA evaluations were negative. A secondary objective was to investigate the relationship between etiology and audiological outcomes over the first 3 years of life. Regression analysis was used to investigate the relationship between hearing levels and etiology. Data analysis does not support the existence of differential effects of etiology on degree of hearing loss or on progressiveness of hearing loss.

"Accelerated ear-age": a new measure of chemotherapy-induced ototoxicity

Currently, there are several different scales that grade chemotherapy-induced ototoxicity. This report highlights how the implications of the conclusions drawn from each scale differ and compare these prior scales to a more functionally based scale developed at Children's Hospital Boston. Additionally, this report introduces the concept of "ear-age," akin to the age at which one would expect the observed decrease in hearing as a consequence of normative aging (but documented in a child or young adult following chemotherapy).

Clinical trial of a farnesyltransferase inhibitor in children with Hutchinson-Gilford progeria syndrome

Hutchinson-Gilford progeria syndrome (HGPS) is an extremely rare, fatal, segmental premature aging syndrome caused by a mutation in LMNA that produces the farnesylated aberrant lamin A protein, progerin. This multisystem disorder causes failure to thrive and accelerated atherosclerosis leading to early death. Farnesyltransferase inhibitors have ameliorated disease phenotypes in preclinical studies. Twenty-five patients with HGPS received the farnesyltransferase inhibitor lonafarnib for a minimum of 2 y. Primary outcome success was predefined as a 50% increase over pretherapy in estimated annual rate of weight gain, or change from pretherapy weight loss to statistically significant on-study weight gain. Nine patients experienced a ≥50% increase, six experienced a ≥50% decrease, and 10 remained stable with respect to rate of weight gain. Secondary outcomes included decreases in arterial pulse wave velocity and carotid artery echodensity and increases in skeletal rigidity and sensorineural hearing within patient subgroups. All patients improved in one or more of these outcomes. Results from this clinical treatment trial for children with HGPS provide preliminary evidence that lonafarnib may improve vascular stiffness, bone structure, and audiological status.

Novel connexin 30 and connexin 26 mutational spectrum in patients with progressive sensorineural hearing loss

Objective:

Mutations in the gap junction protein beta-2 gene (‘GJB2’) are known to be responsible for mild to profound congenital and late-onset hearing loss. This study aimed to investigate the molecular basis of progressive hearing loss compared with non-progressive hearing loss.

Methods:

Following clinical otorhinolaryngological evaluation, a genetic analysis was performed in a cohort of 72 patients with progressive sensorineural hearing loss.

Results:

Pathological genotypes were established in 16 patients (22.2 per cent). Six different gap junction protein beta-2 gene mutations were detected in 15 patients, with the c.35delG mutation responsible for 56 per cent of the mutated alleles. A novel gap junction protein beta-6 gene (‘GJB6’) mutation (p.Met203Val) was observed in one patient with mild progressive hearing loss.

Conclusion:

Analyses of gap junction protein beta-2 and -6 genes revealed that similar pathological genotypes, occurring with similar frequencies, were responsible for progressive hearing loss, compared with reported genotypes for non-progressive hearing loss patients. Thus, genotype cannot be used to differentiate non-progressive from progressive hearing loss cases; in this study, patients both with and without an established pathological genotype had a similar clinical course.

Homozygous M34T mutation of the GJB2 gene associates with an autosomal recessive nonsyndromic sensorineural hearing impairment in Finnish families

CONCLUSION

The genetic and audiological data support the hypothesis that the p.M34T is a pathogenic mutation in the Finnish population. The p.M34T mutation displays an autosomal recessive pattern of inheritance and is associated with mild to moderate nonsyndromic sensorineural hearing impairment (SNHI) in the homozygous state. The audiograms often display a hearing impairment notch at 2-4 kHz in young patients, which may aid in the early diagnosis.

OBJECTIVES

The aim of the study was to assess whether the p.M34T mutation in the GJB2 gene may associate with nonsyndromic SNHI.

METHODS

We systematically reviewed the families with children diagnosed with nonsyndromic SNHI caused by a homozygous p.M34T mutation at the Kuopio and Oulu University Hospital Clinics. The children were re-examined and audiological and genetic data were obtained from their parents and healthy siblings to study genotype-phenotype correlation.

RESULTS

We describe 11 patients from 6 families including 5 sibling pairs from 6 to 23 years of age with homozygous p.M34T genotype all having mild nonsyndromic SNHI. In addition, we found three patients with compound p.M34T mutation also exhibiting mild to moderate SNHI.

Prevalence and audiological features in carriers of GJB2 mutations, c.35delG and c.101T>C (p.M34T), in a UK population study

OBJECTIVES

To determine the carrier rate of the GJB2 mutation c.35delG and c.101T>C in a UK population study; to determine whether carriers of the mutation had worse hearing or otoacoustic emissions compared to non-carriers.

DESIGN

Prospective cohort study.

SETTING

University of Bristol, UK.

PARTICIPANTS

Children in the Avon Longitudinal Study of Parents and Children. 9202 were successfully genotyped for the c.35delG mutation and c.101>T and classified as either carriers or non-carriers.

OUTCOME MEASURES

Hearing thresholds at age 7, 9 and 11 years and otoacoustic emissions at age 9 and 11.

RESULTS

The carrier frequency of the c.35delG mutation was 1.36% (95% CI 1.13 to 1.62) and c.101T>C was 2.69% (95% CI 2.37 to 3.05). Carriers of c.35delG and c.101T>C had worse hearing than non-carriers at the extra-high frequency of 16 kHz. The mean difference in hearing at age 7 for the c.35delG mutation was 8.53 dB (95% CI 2.99, 14.07) and 12.57 dB at age 9 (95% CI 8.10, 17.04). The mean difference for c.101T>C at age 7 was 3.25 dB (95% CI -0.25 to 6.75) and 7.61 dB (95% CI 4.26 to 10.96) at age 9. Otoacoustic emissions were smaller in the c.35delG mutation carrier group: at 4 kHz the mean difference was -4.95 dB (95% CI -6.70 to -3.21) at age 9 and -3.94 dB (95% CI -5.78 to -2.10) at age 11. There was weak evidence for differences in otoacoustic emissions amplitude for c.101T>C carriers.

CONCLUSION

Carriers of the c.35delG mutation and c.101T>C have worse extra-high-frequency hearing than non-carriers. This may be a predictor for changes in lower-frequency hearing in adulthood. The milder effects observed in carriers of c.101T>C are in keeping with its classification as a mutation causing mild/moderate hearing loss in homozygosity or compound heterozygosity.

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.