The effects of a connexin 26 mutation--35delG--on oto-acoustic emissions and brainstem evoked potentials: homozygotes and carriers

Population-scale analysis of common and rare genetic variation associated with hearing loss in adults

To better understand the genetics of hearing loss, we performed a genome-wide association meta-analysis with 125,749 cases and 469,497 controls across five cohorts. We identified 53/c loci affecting hearing loss risk, including common coding variants in COL9A3 and TMPRSS3. Through exome sequencing of 108,415 cases and 329,581 controls, we observed rare coding associations with 11 Mendelian hearing loss genes, including additive effects in known hearing loss genes GJB2 (Gly12fs; odds ratio [OR] = 1.21, P = 4.2 × 10^-11) and SLC26A5 (gene burden; OR = 1.96, P = 2.8 × 10^-17). We also identified hearing loss associations with rare coding variants in FSCN2 (OR = 1.14, P = 1.9 × 10^-15) and KLHDC7B (OR = 2.14, P = 5.2 × 10^-30). Our results suggest a shared etiology between Mendelian and common hearing loss in adults. This work illustrates the potential of large-scale exome sequencing to elucidate the genetic architecture of common disorders where both common and rare variation contribute to risk.

Cx26 partial loss causes accelerated presbycusis by redox imbalance and dysregulation of Nfr2 pathway

Mutations in GJB2, the gene that encodes connexin 26 (Cx26), are the most common cause of sensorineural hearing impairment. The truncating variant 35delG, which determines a complete loss of Cx26 protein function, is the prevalent GJB2 mutation in several populations. Here, we generated and analyzed Gjb2+/- mice as a model of heterozygous human carriers of 35delG. Compared to control mice, auditory brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs) worsened over time more rapidly in Gjb2+/- mice, indicating they were affected by accelerated age-related hearing loss (ARHL), or presbycusis. We linked causally the auditory phenotype of Gjb2+/- mice to apoptosis and oxidative damage in the cochlear duct, reduced release of glutathione from connexin hemichannels, decreased nutrient delivery to the sensory epithelium via cochlear gap junctions and deregulated expression of genes that are under transcriptional control of the nuclear factor erythroid 2-related factor 2 (Nrf2), a pivotal regulator of tolerance to redox stress. Moreover, a statistically significant genome-wide association with two genes (PRKCE and TGFB1) related to the Nrf2 pathway (p-value < 4 × 10-2) was detected in a very large cohort of 4091 individuals, originating from Europe, Caucasus and Central Asia, with hearing phenotype (including 1076 presbycusis patients and 1290 healthy matched controls). We conclude that (i) elements of the Nrf2 pathway are essential for hearing maintenance and (ii) their dysfunction may play an important role in the etiopathogenesis of human presbycusis.

Rescue of Outer Hair Cells with Antisense Oligonucleotides in Usher Mice Is Dependent on Age of Treatment

The absence of functional outer hair cells is a component of several forms of hereditary hearing impairment, including Usher syndrome, the most common cause of concurrent hearing and vision loss. Antisense oligonucleotide (ASO) treatment of mice with the human Usher mutation, Ush1c c.216G>A, corrects gene expression and significantly improves hearing, as measured by auditory-evoked brainstem responses (ABRs), as well as inner and outer hair cell (IHC and OHC) bundle morphology. However, it is not clear whether the improvement in hearing achieved by ASO treatment involves the functional rescue of outer hair cells. Here, we show that Ush1c c.216AA mice lack OHC function as evidenced by the absence of distortion product otoacoustic emissions (DPOAEs) in response to low-, mid-, and high-frequency tone pairs. This OHC deficit is rescued by treatment with an ASO that corrects expression of Ush1c c.216G>A. Interestingly, although rescue of inner hairs cells, as measured by ABR, is achieved by ASO treatment as late as 7 days after birth, rescue of outer hair cells, measured by DPOAE, requires treatment before post-natal day 5. These results suggest that ASO-mediated rescue of both IHC and OHC function is age dependent and that the treatment window is different for the different cell types. The timing of treatment for congenital hearing disorders is of critical importance for the development of drugs such ASO-29 for hearing rescue.

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.

Prevalence of 35delG and Met34Thr GJB2 variants in Portuguese samples

OBJECTIVE

To estimate the prevalence of 35delG and Met34Thr variants in a Portuguese children's community sample and to compare these frequencies with nonsyndromic hearing-loss patients.

METHODS

502 children were randomly selected among the 8647 participants of the Portuguese birth cohort Generation XXI, and screened for Met34Thr and 35delG variants in the GJB2 gene. These variants were also studied on 89 index-cases, observed in the Clinic of "Hereditary Hearing-loss" in Saint John's Hospital Center, presenting a mild to profound nonsyndromic hearing-loss.

RESULTS

Among the 502 children from Generation XXI, 10 were heterozygous for the 35delG variant (95% Confidence Interval 1.03-3.68) and 1 homozygous (95% Confidence Interval 0.01-1.24). Other 10 children presented heterozygosity for the Met34Thr variant (95% Confidence Interval 1.03-3.68). No homozygous for the Met34Thr or compound heterozygotes (35delG/Met34Thr) were found. In the total of 89 nonsyndromic hearing-loss patients, 5 (95% Confidence Interval 2.11-12.8) were heterozygous and 7 (95% Confidence Interval 3.61-15.6) were homozygous for the 35delG variant. The Met34Thr variant was found in 4 patients, 2 heterozygous (95% Confidence Interval 0.13-8.31) and 2 homozygous (95% Confidence Interval 0.13-8.31).

CONCLUSION

The carrier frequency of 35delG and Met34Thr variants in a Portuguese sample was 1 in 50. Our data suggests that the 35delG mutation has an association with deafness. For the Met34Thr variant, no association was observed. However, Met34Thr seemed to conform to an additive model in hearing-loss.

Cellular and Deafness Mechanisms Underlying Connexin Mutation-Induced Hearing Loss - A Common Hereditary Deafness

Hearing loss due to mutations in the connexin gene family, which encodes gap junctional proteins, is a common form of hereditary deafness. In particular, connexin 26 (Cx26, GJB2) mutations are responsible for ~50% of non-syndromic hearing loss, which is the highest incidence of genetic disease. In the clinic, Cx26 mutations cause various auditory phenotypes ranging from profound congenital deafness at birth to mild, progressive hearing loss in late childhood. Recent experiments demonstrate that congenital deafness mainly results from cochlear developmental disorders rather than hair cell degeneration and endocochlear potential reduction, while late-onset hearing loss results from reduction of active cochlear amplification, even though cochlear hair cells have no connexin expression. However, there is no apparent, demonstrable relationship between specific changes in connexin (channel) functions and the phenotypes of mutation-induced hearing loss. Moreover, new experiments further demonstrate that the hypothesized K⁺-recycling disruption is not a principal deafness mechanism for connexin deficiency induced hearing loss. Cx30 (GJB6), Cx29 (GJC3), Cx31 (GJB3), and Cx43 (GJA1) mutations can also cause hearing loss with distinct pathological changes in the cochlea. These new studies provide invaluable information about deafness mechanisms underlying connexin mutation-induced hearing loss and also provide important information for developing new protective and therapeutic strategies for this common deafness. However, the detailed cellular mechanisms underlying these pathological changes remain unclear. Also, little is known about specific mutation-induced pathological changes in vivo and little information is available for humans. Such further studies are urgently required.

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.

Age-Related Hearing Impairment (ARHI) associated with GJB2 single mutation IVS1+1G>A in the Yakut population isolate in Eastern Siberia

Age-Related Hearing Impairment (ARHI) is one of the frequent sensory disorders registered in 50% of individuals over 80 years. ARHI is a multifactorial disorder due to environmental and poor-known genetic components. In this study, we present the data on age-related hearing impairment of 48 heterozygous carriers of mutation IVS1+1G>A (GJB2 gene) and 97 subjects with GJB2 genotype wt/wt in the Republic of Sakha/Yakutia (Eastern Siberia, Russia). This subarctic territory was found as the region with the most extensive accumulation of mutation IVS1+1G>A in the world as a result of founder effect in the unique Yakut population isolate. The GJB2 gene resequencing and detailed audiological analysis in the frequency range 0.25, 0.5, 1.0, 2.0, 4.0, 8.0 kHz were performed in all examined subjects that allowed to investigate genotype-phenotype correlations between the presence of single mutation IVS1+1G>A and hearing of subjects from examined groups. We revealed the linear correlation between increase of average hearing thresholds at speech frequencies (PTA_{0.5,1.0,2.0,4.0 kHz}) and age of individuals with GJB2 genotype IVS1+1G>A/wt (r_s = 0.499, p = 0.006860 for males and r_s = 0.427, p = 0.000277 for females). Moreover, the average hearing thresholds on high frequency (8.0 kHz) in individuals with genotype IVS1+1G>A/wt (both sexes) were significantly worse than in individuals with genotype wt/wt (p<0.05). Age of hearing loss manifestation in individuals with genotype IVS1+1G>A/wt was estimated to be ∼40 years (r_s = 0.504, p = 0.003). These findings demonstrate that the single IVS1+1G>A mutation (GJB2) is associated with age-related hearing impairment (ARHI) of the IVS1+1G>A carriers in the Yakuts.

Distortion-product otoacoustic emissions at ultra-high frequencies in parents of individuals with autosomal recessive hearing loss

Purpose: To evaluate the cochlear function of parents of individuals with autosomal recessive gene Gap Junction Protein Beta-2 hearing loss by ultra-high frequencies distortion-product otoacoustic emissions (DPOAEs), compared with responses of a control group matched for age and gender. Methods: We studied 56 subjects aged from 20 to 58 years, divided into two groups. The study group comprised 28 parents of hearing-impaired patients due to autosomal recessive inheritance, 14 females aged 20.0-55.0 years (mean 32.8 years) and 14 males aged 20.0-58.0 years (mean 35.2 years). Control group was composed of normal hearing individuals, 14 males and 14 females age-matched to the study group. The subjects underwent tests for audiometry, tympanometry, and DPOAE in the frequency range of 9.000-16.000 Hz. Results: We found 64.3% of normal results of DPOAE in the study group compared to 91.1% in the control. There were significant differences between groups in the ears and DPOAE responses, and the mean level of response was in 10 dBNPS in study group and 14 dBNPS in the control. The Pearson's correlation between age and DPOAE in ultra-high frequencies showed no statistical significance. Conclusion: DPOAE at ultra-high frequencies were able to identify individuals from both groups, suggesting that heterozygous individuals for the Gap Junction Protein Beta-2 gene mutation may have damage to the cochlear function before clinical manifestation in audiometry.

Active cochlear amplification is dependent on supporting cell gap junctions

Mammalian hearing relies upon active cochlear mechanics, which arises from outer hair cell (OHC) electromotility and hair bundle movement, to amplify acoustic stimulations increasing hearing sensitivity and frequency selectivity. Here we describe the novel finding that gap junctions between cochlear supporting cells also have a critical role in active cochlear amplification in vivo. We find that targeted-deletion of connexin26 (Cx26) in Deiters cells (DCs) and outer pillar cells (OPCs), which constrain OHCs standing on the basilar membrane, causes a leftward shift in OHC electromotility towards hyperpolarization, and reduces active cochlear amplification with hearing loss. Coincident with large reduction in distortion product otoacoustic emission (DPOAE) and severe hearing loss at high frequencies, the shift is larger in shorter OHCs. Our study demonstrates that active cochlear amplification in vivo is dependent on supporting cell gap junctions. These new findings also show that Cx26 deficiency can reduce active cochlear amplification to induce hearing loss.

Hearing function in heterozygous carriers of a pathogenic GJB2 gene mutation

The most frequent hereditary hearing loss is caused by mutations in the GJB2 gene coding for the gap junction beta 2 protein Connexin 26 (Cx26). In contrast to many studies performed in patients with bi-allelic mutations, audiometric studies on heterozygotes are sparse and often contradictory. To evaluate hearing function in heterozygous carriers of the GJB2 c.35delG mutation, audiometry over the extended frequency range and the recording of otoacoustic emissions (OAEs), i.e., transient-evoked OAEs (TEOAEs) and distortion product OAEs (DPOAEs), were performed in a group of parents and grandparents of deaf children homozygous for the GJB2 c.35delG mutation. The comparison of audiograms between control and heterozygous subjects was enabled using audiogram normalization for age and sex. Hearing loss, estimated with this procedure, was found to be significantly larger in GJB2 c.35delG heterozygous females in comparison with controls for the frequencies of 8-16 kHz; the deterioration of hearing in heterozygous men in comparison with controls was not statistically significant. A comparison of TEOAE responses and DPOAE levels between GJB2 c.35delG heterozygotes and controls did not reveal any significant differences. The results prove the importance of using audiometry over the extended frequency range and audiogram normalization for age and sex to detect minor hearing impairments, even in a relatively small group of subjects of different ages.

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.

Auditory measurements in parents of individuals with autosomal recessive hearing loss

BACKGROUND

Audiological evaluation of parents of individuals with autosomal recessive hearing loss.

AIM

To study the audiological profile of parents of individuals with autosomal recessive hearing loss, inferred by family history or by molecular tests that detected heterozygous mutations in the GJB2 gene. This gene codes Connexin 26.

METHOD

Participants were 36 subjects, ranging between 30 and 60 years, who were divided into two groups: a control group composed by individuals without auditory complaints and without family history of hearing loss, and a research group composed by heterozygous parents of individuals with autosomal recessive hearing loss or heterozygous for connexin 26 mutations. All subjects underwent pure tone audiometry (0.25 to 8 kHz), high frequencies audiometry (9 to 20 kHz) and distortion product otoacoustic emissions (DPOAE).

RESULTS

There were significant differences between the groups when considering the amplitude of DPOAE in the frequencies of 1001 and 1501 Hz. Amplitude was higher in the control group. There was no significant difference between the groups for pure tone thresholds from 0.25 to 20 KHz.

CONCLUSION

The DPOAE were more effective, in comparison to the pure tone audiometry, to detect auditory differences between the groups. More studies of this type are necessary to confirm the observed results.

The search of a genetic basis for noise-induced hearing loss (NIHL)

BACKGROUND AND AIM

Knowledge about the genetic factors responsible for noise-induced hearing loss (NIHL) is still limited. This study investigated whether genetic factors are associated or not to susceptibility to NIHL.

SUBJECTS AND METHODS

The family history and genotypes were studied for candidate genes in 107 individuals with NIHL, 44 with other causes of hearing impairment and 104 controls. Mutations frequently found among deaf individuals were investigated (35delG, 167delT in GJB2, Δ(GJB6- D13S1830), Δ(GJB6- D13S1854) in GJB6 and A1555G in MT-RNR1 genes); allelic and genotypic frequencies were also determined at the SNP rs877098 in DFNB1, of deletions of GSTM1 and GSTT1 and sequence variants in both MTRNR1 and MTTS1 genes, as well as mitochondrial haplogroups.

RESULTS

When those with NIHL were compared with the control group, a significant increase was detected in the number of relatives affected by hearing impairment, of the genotype corresponding to the presence of both GSTM1 and GSTT1 enzymes and of cases with mitochondrial haplogroup L1.

CONCLUSION

The findings suggest effects of familial history of hearing loss, of GSTT1 and GSTM1 enzymes and of mitochondrial haplogroup L1 on the risk of NIHL. This study also described novel sequence variants of MTRNR1 and MTTS1 genes.

Reduced electromotility of outer hair cells associated with connexin-related forms of deafness: an in silico study of a cochlear network mechanism

Mutations in the GJB2 gene encoding for the connexin 26 (Cx26) protein are the most common source of nonsyndromic forms of deafness. Cx26 is a building block of gap junctions (GJs) which establish electrical connectivity in distinct cochlear compartments by allowing intercellular ionic (and metabolic) exchange. Animal models of the Cx26 deficiency in the organ of Corti seem to suggest that the hearing loss and the degeneration of outer hair cells (OHCs) and inner hair cells is due to failed K(+) and metabolite homeostasis. However, OHCs can develop normally in some mutants, suggesting that the hair cells death is not the universal mechanism. In search for alternatives, we have developed an in silico large scale three-dimensional model of electrical current flow in the cochlea in the small signal, linearised, regime. The effect of mutations was analysed by varying the magnitude of resistive components representing the GJ network in the organ of Corti. The simulations indeed show that reduced GJ conductivity increases the attenuation of the OHC transmembrane potential at frequencies above 5 kHz from 6.1 dB/decade in the wild-type to 14.2 dB/decade. As a consequence of increased GJ electrical filtering, the OHC transmembrane potential is reduced by up to 35 dB at frequencies >10 kHz. OHC electromotility, driven by this potential, is crucial for sound amplification, cochlear sensitivity and frequency selectivity. Therefore, we conclude that reduced OHC electromotility may represent an additional mechanism underlying deafness in the presence of Cx26 mutations and may explain lowered OHC functionality in particular reported Cx26 mutants.

The role of potassium recirculation in cochlear amplification

PURPOSE OF REVIEW

Normal cochlear function depends on maintaining the correct ionic environment for the sensory hair cells. Here we review recent literature on the cellular distribution of potassium transport-related molecules in the cochlea.

RECENT FINDINGS

Transgenic animal models have identified novel molecules essential for normal hearing and support the idea that potassium is recycled in the cochlea. The findings indicate that extracellular potassium released by outer hair cells into the space of Nuel is taken up by supporting cells, that the gap junction system in the organ of Corti is involved in potassium handling in the cochlea, that the gap junction system in stria vascularis is essential for the generation of the endocochlear potential, and that computational models can assist in the interpretation of the systems biology of hearing and integrate the molecular, electrical, and mechanical networks of the cochlear partition. Such models suggest that outer hair cell electromotility can amplify over a much broader frequency range than expected from isolated cell studies.

SUMMARY

These new findings clarify the role of endolymphatic potassium in normal cochlear function. They also help current understanding of the mechanisms of certain forms of hereditary hearing loss.

Comprehensive molecular etiology analysis of nonsyndromic hearing impairment from typical areas in China

Background

Every year, 30,000 babies are born with congenital hearing impairment in China. The molecular etiology of hearing impairment in the Chinese population has not been investigated thoroughly. To provide appropriate genetic testing and counseling to families, we performed a comprehensive investigation of the molecular etiology of nonsyndromic deafness in two typical areas from northern and southern China.

Methods

A total of 284 unrelated school children with hearing loss who attended special education schools in China were enrolled in this study, 134 from Chifeng City in Inner Mongolia and the remaining 150 from Nangtong City in JiangSu Province. Screening was performed for GJB2, GJB3, GJB6, SLC26A4, 12S rRNA, and tRNA^ser(UCN) genes in this population. All patients with SLC26A4 mutations or variants were subjected to high-resolution temporal bone CT scan to verify the enlarged vestibular aqueduct.

Results

Mutations in the GJB2 gene accounted for 18.31% of the patients with nonsyndromic hearing loss, 1555A>G mutation in mitochondrial DNA accounted for 1.76%, and SLC26A4 mutations accounted for 13.73%. Almost 50% of the patients with nonsyndromic hearing loss in these typical Chinese areas carried GJB2 or SLC26A4 mutations. No significant differences in mutation spectrum or prevalence of GJB2 and SLC26A4 were found between the two areas.

Conclusion

In this Chinese population, 54.93% of cases with hearing loss were related to genetic factors. The GJB2 gene accounted for the etiology in about 18.31% of the patients with hearing loss, SLC26A4 accounted for about 13.73%, and mtDNA 1555A>G mutation accounted for 1.76%. Mutations in GJB3, GJB6, and mtDNA tRNA^ser(UCN) were not common in this Chinese cohort. Conventionally, screening is performed for GJB2, SLC26A4, and mitochondrial 12S rRNA in the Chinese deaf population.

Correlation between audiometric data and the 35delG mutation in ten patients

Mutations in the connexin 26 gene seem to be extremely common in non-syndromic hereditary deafness genesis, especially the 35delG, but there are still only a few studies that describe the audiometric characteristics of patients with these mutations.

Aim

to analyze the audiometric characteristics of patients with mutations in the connexin 26 gene in order to outline genotype-phenotype correlation.

Materials and Methods

Tonal audiometries of 33 index cases of non-syndromic sensorineural hearing loss were evaluated and eight affected relatives. Specific molecular tests were carried out to analyze mutations in the connexin 26 gene. Experiment Design: Retrospective, cross-sectional study.

Results

A 27.3% prevalence of mutation 35delG was found in the index cases and 12.5% among the relatives affected. In relation to hearing loss degree, 41.5% of the patients were found with profound hearing loss, 39% with severe HL and 19.5% with moderate HL with homozygote and heterozygote patients for the 35delG predominating in the severe-moderate hearing losses.

Conclusion

Our results suggest that the audiometric data associated with the molecular diagnose of hearing loss helped us to outline a genotype-phenotype correlation in ten patients with 35delG mutation. However, it is still necessary to run multicentric studies to verify the real phenotypic expression in the Brazilian population, as far as the 35delG mutation is concerned.

Distinct and gradient distributions of connexin26 and connexin30 in the cochlear sensory epithelium of guinea pigs

Connexin26 (Cx26) and Cx30 are predominant isoforms of gap junction channels in the cochlea and play a critical role in hearing. In this study, the cellular distributions of Cx26 and Cx30 in the cochlear sensory epithelium of guinea pigs were examined by immunofluorescent staining and confocal microscopy in whole mounts of the cochlear sensory epithelium and dissociated cell preparations. The expression of Cx26 and Cx30 demonstrated a longitudinal gradient distribution in the epithelium and was reduced threefold from the cochlear apex to base. The reduction was more pronounced in the Deiters cells and pillar cells than in the Hensen cells. Cx26 was expressed in all types of supporting cells, but little Cx30 labeling was seen in the Hensen cells. Cx26 expression in the Hensen cells was concentrated mainly in the second and third rows, forming a distinct band along the sensory epithelium at its outer region. In the dissociated Deiters cells and pillar cells, Cx30 showed dense labeling at the cell bodies and processes in the reticular lamina. Cx26 labeling largely overlapped that of Cx30 in these regions. Cx26 and Cx30 were also coexpressed in the gap junctional plaques between Claudius cells. Neither Cx26 nor Cx30 labeling was seen in the hair cells and spiral ganglion neurons. These observations demonstrate that Cx26 and Cx30 have a longitudinal gradient distribution and distinct cellular expression in the auditory sensory epithelium. This further supports our previous reports that Cx26 and Cx30 can solely and concertedly perform different functions in the cochlea.

Connexin 26 variants and auditory neuropathy/dys-synchrony among children in schools for the deaf

Genetic and auditory studies of 731 children with severe-to-profound hearing loss in US schools for the deaf and 46 additional children receiving clinical services for hearing loss ranging from moderate to profound demonstrated that mutations in the connexin 26 (GJB2) and connexin 30 (GJB6) genes explain at least 12% of those with nonsyndromic sensorineural deafness. Otoacoustic emissions (OAEs) testing to detect functional outer hair cells indicated that 76 of the children had emissions and therefore may have (as yet unconfirmed) auditory neuropathy/dys-synchrony (AN/AD). Five of these children with OAEs were GJB2 homozygotes or compound heterozygotes with the genotypes 35delG/35delG, W77X/W77X, 35delG/360delGAG, 35delG/V95M, and V84M/M34T. In particular, unilateral AN/AD was confirmed in a child with moderate hearing loss and the 35delG/V95M genotype. Detecting OAEs in individuals with GJB2 mutations suggests that lack of functional gap junctions as a result of GJB2 mutations does not necessarily destroy all outer hair cell function.

Audiometric evaluation of carriers of the connexin 26 mutation 35delG

Mutation in a gap junction protein gene (GJB2 also named connexin 26) is a major cause of autosomal recessive congenital deafness, which is responsible for about 80% of the cases in Mediterranean families, but actually little is known about the influence of GJB2 mutations on the hearing of obligate carriers. We examined GJB2 35delG mutation carrier individuals to test the possible presence and incidence of audiometric abnormalities among carriers of 35delG mutations. Tonal audiometric analysis was performed on a 35delG mutation carrier group (H) and on a non-carrier control group (N). Audiometric evaluations in the control group showed the presence of thresholds within normal limits at all frequencies, while carriers of 35delG mutations presented a decrease of hearing principally at 6,000 and 8,000 Hz. The difference at 6,000 and 8,000 Hz between groups H and N is statistically significant.

Prevalence of the GJB2 mutations and the del(GJB6-D13S1830) mutation in Brazilian patients with deafness

Mutations in the GJB2 gene are the most common cause of sensorineural non-syndromic deafness in different populations. One specific mutation, 35delG, has accounted for the majority of the mutations detected in the GJB2 gene in many countries. The aim of this study was to determine the prevalence of GJB2 mutations and the del(GJB6-D13S1830) mutation in non-syndromic deaf Brazilians. The 33 unrelated probands were examined by clinical evaluation to exclude syndromic forms of deafness. Mutation analysis in the GJB2 gene and the testing for the del(GJB6-D13S1830) were performed in both the patients and their family members. The 35delG mutation was found in nine of the probands or in 14 of the mutated alleles. The V37I mutation and the del(GJB6-D13S1830) mutation were also found in two patients, both are compound heterozygote with 35delG mutation. These findings strengthen the importance of genetic diagnosis, providing early treatment, and genetic counseling of deaf patients.

Cochlear function in young and adult Fischer 344 rats

Fischer 344 (F344) rats are often used as an animal model for investigation of the mechanisms underlying age-related hearing loss. The aim of this study was to assess cochlear function in young (1-month-old) and adult (6-month-old) F344 rats using recording of otoacoustic emissions and auditory brainstem responses (ABRs). The results were compared with control groups of Long Evans (LE) rats of the same ages. The results demonstrate a significant increase in the hearing threshold in F344 rats in comparison with LE rats, expressed mainly at low frequencies (1-2 kHz). In F344 rats, transient evoked otoacoustic emissions were not measurable and distortion product otoacoustic emissions could be detected within a frequency range of 2.4-6.3 kHz. Tympanometric measurements did not reveal any differences in middle ear parameters between F344 and LE rats. The amplitudes of click-evoked ABRs were significantly lower in 6-month-old F344 rats than in LE rats, but other parameters of the ABRs were almost identical in both rat strains. The results demonstrate a significant deficit in low-frequency hearing and altered otoacoustic emissions in both young and adult F344 rats, suggesting a defect of the inner ear sensory epithelium at the apical part of the cochlea.

Progress in understanding GJB2-linked deafness

Mutations in the GJB2 gene (encoding for Connexin 26 protein) represent a leading cause of genetic hearing impairment. Extensive epidemiological and molecular studies have been reported, describing GJB2 mutations type, frequency and distribution. Moreover, several aspects of GJB2 mutations pathogenic effects have been elucidated taking advantage of in vitro and in vivo experimental approaches. Progress through reported studies is reviewed, highlighting recent major achievements in this field. Attention is focused on different unresolved questions regarding GJB2 deafness pathogenesis and genotype-phenotype relationships. Clarification of these important clues will significantly increase our understanding of the molecular basis of hearing loss and will improve the effectiveness of diagnosis and counselling of this frequent disease.

Otoacoustic emissions

PURPOSE OF REVIEW

Otoacoustic emissions offer the practitioner a number of beneficial features as a noninvasive and objective measure of the ear's ability to process acoustic stimuli.

RECENT FINDINGS

Since their discovery, a number of clinical applications of otoacoustic emissions have been established, including their utility in the differential diagnosis of sensorineural hearing loss, in the screening of cochlear function in infants and other difficult-to-test patients, and in the monitoring of outer hair cell healthiness in patients who are exposed to potentially damaging agents or who have progressive hearing ailments.

SUMMARY

Clinical applications of OAEs have developed very rapidly and OAEs have become the standard of care, at least, for pediatric patients.

Mutation spectrum of the connexin 26 (GJB2) gene in Taiwanese patients with prelingual deafness

PURPOSE

To determine the mutation spectrum of the connexin 26 gene among 324 Taiwanese patients with prelingual deafness and the carrier rate of gene mutation in another 432 unrelated control subjects.

METHODS

The coding region of the connexin 26 gene was sequenced in both directions to detect mutation in all 756 samples.

RESULTS

Among the 756 samples tested, 21 connexin 26 variants were detected, including 7 novel ones. The 235delC mutation was the most common, accounting for 57.6% of the mutant alleles. Among patients, 48 (14.8%) had connexin 26 gene mutations. In the control group, the carrier rate of connexin 26 mutation was estimated at 2.8%.

CONCLUSION

The mutation spectrum of the connexin 26 gene is wide, with more than half of the patients having only one mutation detected. Thus, further efforts are needed to look for possible existence of a second mutant allele.

Genetic screening for deafness

Genetic testing for deafness has become a reality. It has changed the paradigm for evaluating deaf and hard-of-hearing persons and will be used by physicians for diagnostic purposes and as a basis for treatment and management options. Although mutation screening is currently available for only a limited number of genes, in these specific instances, diagnosis, carrier detection, and reproductive risk counseling can be provided. In the coming years there will be an expansion of the role of genetic testing and counseling will not be limited to reproductive issues. Treatment and management decisions will be made based on specific genetic diagnoses. Although genetic testing may be a confusing service for the practicing otolaryngologist, it is an important part of medical care. New discoveries and technologies will expand and increase the complexity of genetic testing options and it will become the responsibility of otolaryngologists to familiarize themselves with current discoveries and accepted protocols for genetic testing.

Otoacoustic emissions and brainstem evoked potentials in compound carriers of connexin 26 mutations

This study compares the effects of mutations in the gap junction protein connexin 26 (Cx26), on outer hair cells (OHCs), inner hair cells (IHCs) and auditory nerve/brainstem among carriers of these mutations. One hundred and twenty eight individuals, from a village with widespread consanguinity and congenital deafness, due to three Cx26 mutations, were selected among relatives of deaf persons, and divided into non-carriers, carriers of one mutation, homozygous to one mutation, or compound heterozygous carriers of two different mutations. Distortion product otoacoustic emissions (DPOAEs), auditory brainstem responses (ABRs) and audiometric evaluation were compared in these genetic groups. Hearing loss among homozygotes and compound heterozygotes was comparable and ranged from mild to profound. Most ABRs from these groups showed no responses or partial responses (peaks III, V) with prolonged latencies, but some individuals had all peaks at normal latencies. DPOAEs were absent, except sporadic responses. Carriers of one mutation had significantly smaller DPOAEs compared to non-carriers, although normal pure tone audiograms and ABRs were found in these groups. In conclusion, based on DPOAEs, Cx26 mutations may impact OHC function among carriers of one or two Cx26 mutations. IHC/nerve impairment among homozygotes and compound heterozygotes is variable. OHCs may be more susceptible to Cx26 mutations compared to IHCs and the auditory nerve and brainstem pathway activated by them.

GJB2 (connexin 26) variants and nonsyndromic sensorineural hearing loss: a HuGE review

Despite the enormous heterogeneity of genetic hearing loss, variants in one locus, Gap Junction Beta 2 or GJB2 (connexin 26), account for up to 50% of cases of nonsyndromic sensorineural hearing loss in some populations. This article reviews genetic epidemiology studies of the alleles of GJB2, prevalence rates, genotype-phenotype relations, contribution to the incidence of hearing loss, and other issues related to the clinical validity of genetic testing for GJB2. This review focuses primarily on three alleles: 167 Delta T, 35 Delta G, and 235 Delta C. These alleles are recessive for nonsyndromic prelingual sensorineural hearing loss, and the evidence suggests complete penetrance but variable expressivity.