Identification of a novel COCH mutation, I109N, highlights the similar clinical features observed in DFNA9 families

AudioGene: refining the natural history of KCNQ4, GSDME, WFS1, and COCH-associated hearing loss

Autosomal dominant non-syndromic hearing loss (ADNSHL) displays gene-specific progression of hearing loss, which is amenable to sequential audioprofiling. We sought to refine the natural history of ADNSHL by examining audiometric data in 5-year increments. 2175 audiograms were included from four genetic causes of ADNSHL-KCNQ4 (DFNA2), GSDME (DFNA5), WFS1 (DFNA6/14/38), and COCH (DFNA9). Annual threshold deterioration (ATD) was calculated for each gene: for the speech-frequency pure tone average, the ATD, respectively, was 0.72 dB/year, 0.94 dB/year, 0.53 dB/year, and 1.41 dB/year, with the largest drops occurring from ages 45-50 (0.89 dB/year; KCNQ4), 5-10 (1.42 dB/year; GSDME), 40-45 (0.83 dB/year; WFS1), and 50-55 (2.09 dB/year; COCH). 5-year interval analysis of audiograms reveals the gene specific natural history of KCNQ4, GSDME, WFS1 and COCH-related progressive hearing loss. Identifying ages at which hearing loss is most rapid informs clinical care and patient expectations. Natural history data are also essential to define outcomes of clinical trials that test novel therapies designed to correct or ameliorate these genetic forms of hearing loss.

Genotype-Phenotype Correlations of Pathogenic COCH Variants in DFNA9: A HuGE Systematic Review and Audiometric Meta-Analysis

Pathogenic missense variants in COCH are associated with DFNA9, an autosomal dominantly inherited type of progressive sensorineural hearing loss with or without vestibular dysfunction. This study is a comprehensive overview of genotype-phenotype correlations using the PRISMA and HuGENet guidelines. Study characteristics, risk of bias, genotyping and data on the self-reported age of onset, symptoms of vestibular dysfunction, normative test results for vestibular function, and results of audiovestibular examinations were extracted for each underlying pathogenic COCH variant. The literature search yielded 48 studies describing the audiovestibular phenotypes of 27 DFNA9-associated variants in COCH. Subsequently, meta-analysis of audiometric data was performed by constructing age-related typical audiograms and by performing non-linear regression analyses on the age of onset and progression of hearing loss. Significant differences were found between the calculated ages of onset and progression of the audiovestibular phenotypes of subjects with pathogenic variants affecting either the LCCL domain of cochlin or the vWFA2 and Ivd1 domains. We conclude that the audiovestibular phenotypes associated with DFNA9 are highly variable. Variants affecting the LCCL domain of cochlin generally lead to more progression of hearing loss when compared to variants affecting the other domains. This review serves as a reference for prospective natural history studies in anticipation of mutation-specific therapeutic interventions.

On the pathophysiology of DFNA9: Effect of pathogenic variants in the COCH gene on inner ear functioning in human and transgenic mice

DeaFNess Autosomal Dominant 9 (DFNA9) is a dominant hereditary non-syndromic form of progressive sensorineural hearing loss often associated with vestibular dysfunction. DFNA9 is caused by pathogenic variants in the COCH gene. This gene encodes for cochlin, a protein that is abundantly expressed in the spiral ligament and spiral limbus of the inner ear but the function of cochlin is still not fully understood. There are 22 known pathogenic variants located in different domains of the COCH gene that can cause DFNA9, all expressing slightly different phenotypes. It is believed that COCH mutations affect the intracellular trafficking of cochlin which could explain the characteristic pathology seen in temporal bones of DFNA9 patients. This pathology involves a widespread accumulation of acellular eosinophilic deposits throughout the labyrinth. To gain a better understanding of the pathology underlying DFNA9, different mouse models were developed. The objective of this review is to describe the different pathogenic variants in the COCH gene and their effect on intracellular trafficking, associated phenotypes and histopathological findings in both patients and mouse models.

Cochlear involvement in patients with systemic autoimmune rheumatic diseases: a clinical and laboratory comparative study

PURPOSE

Inner ear involvement has been reported in systemic rheumatic disease while detection of cochlin-specific antibodies has been reported in patients with idiopatic sensorineural hearing loss, suggesting cochlin's strong link to autoimmune hearing loss. The aim of this cross-sectional study was to calculate the prevalence of sensorineural hearing loss (SNHL) in patients with systemic rheumatic diseases, and to investigate any potential correlation with human antibodies to cochlin.

METHODS

Patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Sjogren's syndrome (SS) and systemic sclerosis (SSc) according to the criteria of American College of Rheumatology were included in the study. All patients underwent a complete ear-nose-throat physical examination and audiological evaluation with pure tone audiometry and impedance audiometry. Pure tone average was calculated, taking as a starting point the hearing loss in dB according to the recommendation 02/1 of "Bureau International d' Audiophonologie" (BIAP) so as an average hearing threshold value. Sera of all patients were tested for the presence of IgG antibodies to human cochline (COCH-IgG). Sex and age-matched healthy subjects were included as controls to each group.

RESULTS

A total of 133 patients were studied; 60 with RA, 41 with SLE, 24 with SS and 8 with SSc. 61.4% of patients reported vertigo, 41% hyperacousis, 39% hearing loss, 38% tinnitus, 37.9% headache and 2.1% sensation of ear pressure with unremarkable otoscopy. The prevalence of SNHL calculated for patients affected by RA, SLE, SS and SSc was 66.6%, 31.71%, 54.17%, and 75% respectively. The calculated average hearing thresholds value in RA was increased in comparison to SLE (p < 0.05). In addition it was also higher in patients with RA and secondary SS, in comparison to RA patients (p > 0.05). There was statistically significant correlation of average hearing threshold with disease activity score 28 (DAS28) in RA, but no correlation observed with disease activity index (SLEDAI) in SLE. COCH-IgG antibodies were detected in only two samples. The results were compared with those of their respective sex and age-matched healthy subjects.

CONCLUSION

Our study revealed increased prevalence of SNHL in patients with systemic autoimmune rheumatic disease but no correlation of hearing loss with COCHIgG antibodies. The mechanism of inner ear damage remains unknown; thus, additional prospective studies will be needed to elucidate its pathogenesis.

Genes important for otoneurological diagnostic purposes - current status and future prospects

SUMMARY

This review focuses on the current knowledge of the genes responsible for non-syndromic hearing loss that can be useful for otoneurological diagnostic purposes. From among a large number of genes that have been associated with non-syndromic hearing impairment, we selected several best-known genes, including the COCH gene, GJB2, GJB6 and SLC26A4, and we describe their role and effects of mutations and prevalence of mutations in various populations. Next, we focus on genes associated with tinnitus. Important areas for further research include assessment of genes potentially involved in pathophysiology of tinnitus and vertigo, which have traditionally been considered as being of otological aetiology, while advances in neuroimaging techniques have increasingly shifted studies toward neurological correlations.

Does Otovestibular Loss in the Autosomal Dominant Disorder DFNA9 Have an Impact of on Cognition? A Systematic Review

Background and Purpose: Cognitive impairment has been observed in patients with bilateral vestibular loss (BVL) and in patients with sensorineural hearing loss (SNHL). DFNA9 is an autosomal dominant disorder that causes a combination of both sensory deficits by the 3rd to 5th decade. We therefore hypothesize a combined detrimental effect on cognition. The aim of this systematic review was to identify studies related to DFNA9 in general and its relationship with cognitive impairment more specifically.

Materials and Methods: Several databases including Medline, Cochrane Database of Systematic Reviews, Cochrane Central Register of Controlled Trials, ISI Web of Knowledge, and Web of Science were searched to accumulate information about DFNA9-mutations, including phenotype, genotype, pathophysiology, quality of life (QOL), and imaging in general and cognitive function more specifically. A qualitative analysis was performed on the 55 articles that qualified.

Results: The clinical features of DFNA9 are different along the 24 COCH mutations, described up to now. Vestibular symptoms generally present themselves a few years after SNHL onset in mutations associated with the vWFA-domain although they can precede SNHL onset in other mutations associated with the LCCL-domain. QoL has not been studied extensively in DFNA9, although scarce work is available on the positive impact of cochlear implantation to rehabilitate hearing. No studies were found evaluating cognition in DFNA9 patients.

Conclusion: Although cognitive impairment has been demonstrated in patients with hearing loss as well as in patients with BVL, no studies have been reported on the combination of both sensory deficits, such as in DFNA9. Further research is warranted to correlate otovestibular status with cognition.

Different Phenotypes of the Two Chinese Probands with the Same c.889G>A (p.C162Y) Mutation in COCH Gene Verify Different Mechanisms Underlying Autosomal Dominant Nonsyndromic Deafness 9

OBJECTIVES

By analyzing the different phenotypes of two Chinese DFNA9 families with the same mutation located in the intervening region between the LCCL and vWFA domains of cochlin and testing the functional changes in the mutant cochlin, we investigated the different pathogeneses for mutations in LCCL and vWFA domains.

METHODS

Targeted next-generation sequencing for deafness-related genes was used to identify the mutation in the proband in family #208. The probands of family #208 and family #32 with the same p.C162Y mutation were followed for more than 3 years to evaluate the progression of hearing loss and vestibular dysfunction using pure-tone audiometry, caloric testing, electrocochleogram, vestibular-evoked myogenic potential, and video head-impulse test. The disruption of normal cleavage to produce secreted LCCL domain fragments and the tendency to form aggregations of mutant cochlins were tested by in vitro cell experiments.

RESULTS

The two families showed different clinical symptoms. Family #32 was identified as having early-onset, progressive sensorineural hearing loss, similar to the symptoms in DFNA9 patients with cochlin mutations in the vWFA domain. The proband of family #208 endured late-onset recurrent paroxysmal vertigo attacks and progressively deteriorating hearing, similar to symptoms in those with cochlin mutations in the LCCL domain. We therefore suggest that the disrupted cleavage of the LCCL domain fragment is likely to cause vestibular dysfunction, and aggregation of mutant cochlin caused by mutations in the vWFA domain is responsible for early-onset hearing loss. The p.C162Y mutation causes either disruption of LCCL domain fragment cleavage or aggregation of mutant cochlin, resulting in the different phenotypes in the two families.

CONCLUSION

This study demonstrates that DFNA9 families with the same genotype may have significantly different phenotypes. The mutation site in cochlin is related to the pathological mechanism underlying the different phenotypes.

Massively Parallel Sequencing of a Chinese Family with DFNA9 Identified a Novel Missense Mutation in the LCCL Domain of COCH

DFNA9 is a late-onset, progressive, autosomal dominantly inherited sensorineural hearing loss with vestibular dysfunction, which is caused by mutations in the COCH (coagulation factor C homology) gene. In this study, we investigated a Chinese family segregating autosomal dominant nonsyndromic sensorineural hearing loss. We identified a missense mutation c.T275A p.V92D in the LCCL domain of COCH cosegregating with the disease and absent in 100 normal hearing controls. This mutation leads to substitution of the hydrophobic valine to an acidic amino acid aspartic acid. Our data enriched the mutation spectrum of DFNA9 and implied the importance for mutation screening of COCH in age related hearing loss with vestibular dysfunctions.

Phenotype Description of a Novel DFNA9/COCH Mutation, I109T

Objectives:

This is a report of the audiological and vestibular characteristics of a Dutch DFNA9 family with a novel mutation, I109T, in the LCCL domain of COCH.

Methods:

From the family with the novel I109T COCH mutation, audiometric data were collected and analyzed longitudinally. Results were compared to those obtained in previously identified P51S, G88E, and G87W COCH mutation carriers. Special attention was also given to a comparison of age-related features such as progressive hearing loss and vestibular impairment.

Results:

A novel mutation (I109T) in COCH segregates with hearing impairment and vestibular dysfunction in the present family. Pure tone thresholds, phoneme recognition scores, and vestibular responses of the I109T mutation carriers were essentially similar to those previously established in P51S, G87W, and G88E mutation carriers. Deterioration of hearing in the I109T mutation carriers started at 43 years of age, and vestibular function deteriorated at least 7 years later.

Conclusions:

The phenotype associated with the novel COCH (I109T) mutation is largely similar to that associated with P51S and G88E mutation carriers. However, subtle differences in terms of onset age and rate of progression seem to exist.

Distinct vestibular phenotypes in DFNA9 families with COCH variants

Mutations of COCH can cause hearing loss and less frequently vestibular symptoms. However, vestibular phenotypes, especially in terms of the location of specific variants are not well documented yet. In this study, a retrospective and prospective cohort survey was performed in two tertiary referral hospitals to demonstrate vestibular phenotypes of DFNA9 subjects with a focus on the relationship with the location of COCH mutations. Two DFNA9 subjects were recruited from the previously collected cohort, each segregating p.G38D and p.C162Y of the COCH gene. Another two DFNA9 families were newly detected by targeted resequencing of known 129 deafness genes (TRS-129). These two families segregated the p.G38D variant of the COCH gene as the causative mutation, making p.G38D the most frequent COCH mutation in our Korean cohorts. Regarding the detailed clinical phenotype of the four DFNA9 families with documented vestibular phenotypes, we were able to classify them into two groups: one (p.C162Y variant) with a Meniere's disease (MD)-like phenotype and the other three (p.G38D variant) with significant bilateral vestibular loss without any definite MD symptoms. Distinct vestibular phenotypes depending on the location of COCH mutations were demonstrated, and this study correlates a genotype of p.G38D in COCH to the phenotype of bilateral total vestibular loss, therefore expanding the vestibular phenotypic spectrum of DFNA9 to range from bilateral vestibular loss without episodic vertigo to MD-like features with devastating episodic vertigo. In addition, the p.G38D variant of the COCH gene is suggested to be a frequent cause of progressive audiovestibular dysfunction in Koreans eventually requiring cochlear implantation.

A novel frameshift variant of COCH supports the hypothesis that haploinsufficiency is not a cause of autosomal dominant nonsyndromic deafness 9

COCH (coagulation factor C homology) encodes cochlin, and certain mutations of COCH cause autosomal dominant nonsyndromic deafness 9 (DFNA9). Hearing loss due to COCH mutation begins in adulthood, and 17 missense mutations and two in-frame mutations have been reported. Studies with animal and cellular models have suggested that the underlying biological mechanism of DFNA9 is the dominant-negative effect of mutated COCH and not haploinsufficiency. However, no human cases of DFNA9 that support this hypothesis have been reported. The proband of the present case was an 18-year-old male with congenital or infantile hearing loss. Targeted next-generation sequencing analysis detected a heterozygous novel frameshift mutation of COCH (c.146dupT, p.C50LfsX8) in the proband, whose hearing loss began earlier than what is typical for DFNA9. His mother also carried the mutation but had normal hearing. Consequently, the mutation was not considered to be the cause of the proband's hearing loss. This family is the first case of a truncating COCH variant and supports the hypothesis that COCH haploinsufficiency is not the cause of hearing loss in humans.

Identification of pathogenic mechanisms of COCH mutations, abolished cochlin secretion, and intracellular aggregate formation: genotype-phenotype correlations in DFNA9 deafness and vestibular disorder

Mutations in COCH (coagulation factor C homology) cause autosomal-dominant nonsyndromic hearing loss with variable degrees of clinical onset and vestibular malfunction. We selected eight uncharacterized mutations and performed immunocytochemical and Western blot analyses to track cochlin through the secretory pathway. We then performed a comprehensive analysis of clinical information from DFNA9 patients with all 21 known COCH mutations in conjunction with cellular and molecular findings to identify genotype-phenotype correlations. Our studies revealed that five mutants were not secreted into the media: two von Willebrand factor A (vWFA) domain mutants, which were not transported from the endoplasmic reticulum to Golgi complex and formed high-molecular-weight aggregates in cell lysates, and three LCCL domain mutants, which were detected as intracellular dimeric cochlins. Mutant cochlins that were not secreted and accumulated in cells result in earlier age of onset of hearing defects. In addition, individuals with LCCL domain mutations show accompanying vestibular dysfunction, whereas those with vWFA domain mutations exhibit predominantly hearing loss. This is the first report showing failure of mutant cochlin transport through the secretory pathway, abolishment of cochlin secretion, and formation and retention of dimers and large multimeric intracellular aggregates, and high correlation with earlier onset and progression of hearing loss in individuals with these DFNA9-causing mutations.

Detailed hearing and vestibular profiles in the patients with COCH mutations

OBJECTIVES

To evaluate the clinical features of Japanese DFNA9 families with mutations of the COCH gene.

METHODS

Mutation screening was performed using targeted next-generation sequencing (NGS) for 63 previously reported deafness genes. The progression of hearing loss and vestibular dysfunction were evaluated by pure-tone audiometry, caloric testing, cVEMP, and computed dynamic posturography.

RESULTS

We detected 1 reported mutation of p.G88E and 2 novel mutations of p.I372T and p.C542R. The patients with the novel mutations of p.I372T and p.C542R within the vWFA2 domain showed early onset progressive hearing loss, and the patients with the p.G88E mutation showed late onset hearing loss and acute hearing deterioration over a short period. Vestibular symptoms were reported in the patients with p.G88E and p.C542R. Vestibular testing was performed for the family with the p.G88E mutation. Severe vestibular dysfunction was observed in the proband, and the proband's son showed unilateral semicircular canal dysfunction with mild hearing loss.

CONCLUSIONS

Targeted exon resequencing of selected genes using NGS successfully identified mutations in the relatively rare deafness gene, COCH, in the Japanese population. The phenotype is compatible with that described in previous reports. Additional supporting evidence concerning progressive hearing loss and deterioration of vestibular function was obtained from our study.

Exome sequencing identifies a novel frameshift mutation of MYO6 as the cause of autosomal dominant nonsyndromic hearing loss in a Chinese family

Autosomal dominant types of nonsyndromic hearing loss (ADNSHL) are typically postlingual in onset and progressive. High genetic heterogeneity, late onset age, and possible confounding due to nongenetic factors hinder the timely molecular diagnoses for most patients. In this study, exome sequencing was applied to investigate a large Chinese family segregating ADNSHL in which we initially failed to find strong evidence of linkage to any locus by whole-genome linkage analysis. Two affected family members were selected for sequencing. We identified two novel mutations disrupting known ADNSHL genes and shared by the sequenced samples: c.328C>A in COCH (DFNA9) resulting in a p.Q110K substitution and a deletion c. 2814_2815delAA in MYO6 (DFNA22) causing a frameshift alteration p.R939Tfs*2. The pathogenicity of novel coding variants in ADNSHL genes was carefully evaluated by analysis of co-segregation with phenotype in the pedigree and in light of established genotype-phenotype correlations. The frameshift deletion in MYO6 was confirmed as the causative variant for this pedigree, whereas the missense mutation in COCH had no clinical significance. The results allowed us to retrospectively identify the phenocopy in one patient that contributed to the negative finding in the linkage scan. Our clinical data also supported the emerging genotype-phenotype correlation for DFNA22.

Clinical characterization of a novel COCH mutation G87V in a Chinese DFNA9 family

OBJECTIVES

To characterize the clinical features of a Chinese DFNA9 family associated with a novel COCH mutation and to confirm the proposed genotype-phenotype correlation of COCH.

METHODS

Mutation screening of 79 deafness genes was performed in the proband by targeted next-generation sequencing. Co-segregation of the disease phenotype and the detected variants was confirmed in all family members by PCR amplification and Sanger sequencing. The progression of hearing impairment in affected family members was followed and the concomitant vestibular dysfunction was verified by the caloric vestibulo-ocular reflex test.

RESULTS

A novel COCH mutation p.G87V was identified in the family segregating with late-onset, progressive sensorineural hearing impairment and consistent vestibular dysfunction.

CONCLUSION

The p.G87V mutation leads to a very similar phenotype as a previously reported p.G87W mutation of COCH. Our study suggested that the G87 residue is critical for function of COCH and further confirms a previously proposed genotype-phenotype correlation for DFNA9.

Phenotype analysis of an Australian DFNA9 family with the 1109N COCH mutation

OBJECTIVES

We studied the clinical characteristics of an Australian family with an autosomal dominant sensorineural hearing impairment (DFNA9) caused by an I109N mutation in COCH.

METHODS

Retrospective analyses of audiometric data from 8 mutation carriers of an Australian DFNA9 family with the I109N COCH mutation were performed. Cross-sectional hearing levels related to age, age-related typical audiograms, and speech recognition scores related to age and to the level of hearing impairment were investigated. Data were compared to those obtained in previously identified DFNA9 families with P51S, V66G, G87W, G88E, I109T, and C542F COCH mutations.

RESULTS

Deterioration of hearing in the I109N mutation carriers started before the age of 40 years. The audiometric characteristics of the I109N mutation carriers are essentially similar to those previously established in I109T mutation carriers and, to a lesser extent, in P51S, G87W, and G88E mutation carriers.

CONCLUSIONS

The phenotype associated with the I109N COCH mutation is largely similar to that associated with the I109T, P51S, G87W, and G88E mutation carriers. However, subtle differences seem to exist in terms of age of onset and rate of progression.

Hearing and vestibular deficits in the Coch(-/-) null mouse model: comparison to the Coch(G88E/G88E) mouse and to DFNA9 hearing and balance disorder

Two mouse models, the Coch(G88E/G88E) or "knock-in" and the Coch(-/-) or "knock-out" (Coch null), have been developed to study the human late-onset, progressive, sensorineural hearing loss and vestibular dysfunction known as DFNA9. This disorder results from missense and in-frame deletion mutations in COCH (coagulation factor C homology), encoding cochlin, the most abundantly detected protein in the inner ear. We have performed hearing and vestibular analyses by auditory brainstem response (ABR) and vestibular evoked potential (VsEP) testing of the Coch(-/-) and Coch(G88E/G88E) mouse models. Both Coch(-/-) and Coch(G88E/G88E) mice show substantially elevated ABRs at 21 months of age, but only at the highest frequency tested for the former and all frequencies for the latter. At 21 months, 9 of 11 Coch(-/-) mice and 4 of 8 Coch(G88E/G88E) mice have absent ABRs. Interestingly Coch(-/+) mice do not show hearing deficits, in contrast to Coch(G88E/+), which demonstrate elevated ABR thresholds similar to homozyotes. These results corroborate the DFNA9 autosomal dominant mode of inheritance, in addition to the observation that haploinsufficiency of Coch does not result in impaired hearing. Vestibular evoked potential (VsEP) thresholds were analyzed using a two factor ANOVA (Age X Genotype). Elevated VsEP thresholds are detected in Coch(-/-) mice at 13 and 21 months, the two ages tested, and as early as seven months in the Coch(G88E/G88E) mice. These results indicate that in both mouse models, vestibular function is compromised before cochlear function. Analysis and comparison of hearing and vestibular function in these two DFNA9 mouse models, where deficits occur at such an advanced age, provide insight into the pathology of DFNA9 and age-related hearing loss and vestibular dysfunction as well as an opportunity to investigate potential interventional therapies.

The genetic bases for non-syndromic hearing loss among Chinese

Deafness is an etiologically heterogeneous trait with many known genetic, environmental causes or a combination thereof. The identification of more than 120 independent genes for deafness has provided profound new insights into the pathophysiology of hearing. However, recent findings indicate that a large proportion of both syndromic and non-syndromic forms of deafness in the Chinese population are caused by defects in a small number of genes. Studies of the genetic epidemiology and molecular genetic features revealed that there is a clear relevance of genes causing deafness in Chinese deaf patients as well as a unique spectrum of common and rare deafness gene mutations in the Chinese population. This review is focused on the genetic aspects of non-syndromic and mitochondrial deafness, in which unique molecular genetic features of hearing impairment have been identified in the Chinese population. The current China population is approximately 1.3 billion. It is estimated that 30,000 infants are born with congenital sensorineural hearing loss each year. Better understanding of the genetic causes of deafness in the Chinese population is important for accurate genetics counseling and early diagnosis for timely intervention and treatment options.

Cochlin expression in anterior segment organ culture models after TGFbeta2 treatment

PURPOSE

To determine the effect of transforming growth factor (TGF)-beta2 treatment on intraocular pressure (IOP), outflow facility, and cochlin expression in vitro in monkey and pig organ-cultured anterior segments (MOCAS and POCAS).

METHODS

MOCAS (rhesus and cynomolgus) or POCAS were infused with media containing 10 ng/mL TGFbeta2 to one segment of each pair and 0.1% BSA (vehicle) to the contralateral segment for up to 14 days at a constant rate. Cochlin expression was determined by immunohistochemical study, ELISA, and Western blot analysis using chicken polyclonal antibodies against different regions of cochlin.

RESULTS

TGFbeta2 infusion produced elevated IOP in MOCAS (usually after 5 days), that was approximately 45% greater than baseline and compared to control segments. Outflow facility (OF) was decreased by approximately 40% compared with pretreatment baseline (n=5). In POCAS (n=7), IOP was increased (approximately 3 days) by approximately 75% compared with baseline and contralateral changes. The IOP elevation subsided thereafter. Cochlin levels increased with duration of TGFbeta2 treatment in the media and in the region of the trabecular meshwork in both species.

CONCLUSIONS

TGFbeta2-induced IOP elevation was associated with an increase in cochlin secretion into the media and expression in the tissue of MOCAS and POCAS. Whether cochlin overexpression contributes to elevated IOP or is a consequence of other changes relevant to IOP elevation remains to be determined.

A targeted Coch missense mutation: a knock-in mouse model for DFNA9 late-onset hearing loss and vestibular dysfunction

Mutations in COCH (coagulation factor C homology) are etiologic for the late-onset, progressive, sensorineural hearing loss and vestibular dysfunction known as DFNA9. We introduced the G88E mutation by gene targeting into the mouse and have created a Coch(G88E/G88E) mouse model for the study of DFNA9 pathogenesis and cochlin function. Vestibular-evoked potential (VsEP) thresholds of Coch(G88E/G88E) mice were elevated at all ages tested compared with wild-type littermates. At the oldest ages, two out of eight Coch(G88E/G88E) mice had no measurable VsEP. Auditory brainstem response (ABR) thresholds of Coch(G88E/G88E) mice were substantially elevated at 21 months but not at younger ages tested. At 21 months, four of eight Coch(G88E/G88E) mice had absent ABRs at all frequencies tested and two of three Coch(G88E)(/+) mice had absent ABRs at three of four frequencies tested. Distortion product otoacoustic emission amplitudes of Coch(G88E/G88E) mice were substantially lower than Coch(+/+) mice and absent in the same Coch(G88E/G88E) mice with absent ABRs. These results suggest that vestibular function is affected beginning as early as 11 months when cochlear function appears to be normal, and dysfunction increases with age. Hearing loss declines substantially at 21 months of age and progresses to profound hearing loss at some to all frequencies tested. This is the only mouse model developed to date where hearing loss begins at such an advanced age, providing an opportunity to study both progressive age-related hearing loss and possible interventional therapies.

Expression studies of osteoglycin/mimecan (OGN) in the cochlea and auditory phenotype of Ogn-deficient mice

Genes involved in the hearing process have been identified through both positional cloning efforts following genetic linkage studies of families with heritable deafness and by candidate gene approaches based on known functional properties or inner ear expression. The latter method of gene discovery may employ a tissue- or organ-specific approach. Through characterization of a human fetal cochlear cDNA library, we have identified transcripts that are preferentially and/or highly expressed in the cochlea. High expression in the cochlea may be suggestive of a fundamental role for a transcript in the auditory system. Herein we report the identification and characterization of a transcript from the cochlear cDNA library with abundant cochlear expression and unknown function that was subsequently determined to represent osteoglycin (OGN). Ogn-deficient mice, when analyzed by auditory brainstem response and distortion product otoacoustic emissions, have normal hearing thresholds.

Cochlin isoforms and their interaction with CTL2 (SLC44A2) in the inner ear

Choline transporter-like protein 2 (CTL2) is a multi-transmembrane protein expressed on inner ear supporting cells that was discovered as a target of antibody-induced hearing loss. Its function is unknown. A 64 kDa band that consistently co-precipitates with CTL2 from inner ear extracts was identified by mass spectroscopy as cochlin. Cochlin is an abundant inner ear protein expressed as multiple isoforms. Its function is also unknown, but it is suspected to be an extracellular matrix component. Cochlin is mutated in individuals with DFNA9 hearing loss. To investigate the CTL2-cochlin interaction, antibodies were raised to a cochlin-specific peptide. The antibodies identify several cochlin polypeptides on western blots and are specific for cochlin. We show that the heterogeneity of the cochlin isoforms is caused, in part, by in vivo post-translational modification by N-glycosylation and, in part, caused by alternative splicing. We verified that antibody to CTL2 co-immunoprecipitates cochlin from the inner ear and antibody to cochlin co-immunoprecipitates CTL2. Using cochlear cross-sections, we show that CTL2 is more widely distributed than previously described, and its prominent expression on cells facing the scala media suggests a possible role in homeostasis. A prominent but previously unreported ribbon-like pattern of cochlin in the basilar membrane was demonstrated, suggesting an important role for cochlin in the structure of the basilar membrane. CTL2 and cochlin are expressed in close proximity in the inner sulcus, the spiral prominence, vessels, limbus, and spiral ligament. The possible functional significance of CTL2-cochlin interactions remains unknown.

Cochlin in the eye: functional implications

Aqueous humor is actively produced in the ciliary epithelium of the anterior chamber and has important functions for the eye. Under normal physiological conditions, the inflow and outflow of the aqueous humor are tightly regulated, but in the pathologic state this balance is lost. Aqueous outflow involves structures of the anterior chamber and experiences most resistance at the level of the trabecular meshwork (TM) that acts as a filter. The modulation of the TM structure regulates the filter and its mechanism remains poorly understood. Proteomic analyses have identified cochlin, a protein of poorly understood function, in the glaucomatous TM but not in healthy control TM from human cadaver eyes. The presence of cochlin has subsequently been confirmed by Western and immunohistochemical analyses. Functionally, cochlin undergoes multimerization induced by shear stress and other changes in the microenvironment. Cochlin along with mucopolysaccharide deposits has been found in the TM of glaucoma patients and in the inner ear of subjects affected by the hearing disorder DNFA9, a late-onset, progressive disease that also involves alterations in fluid shear regimes. In vitro, cochlin induces aggregation of primary TM cells suggesting a role in cell adhesion, possibly in mechanosensation, and in modulation of the TM filter.

The inner ear and the neurologist

Inner ear disorders are common and patients with vestibular failure often present to a neurology clinic because of their dizziness, gait unsteadiness and oscillopsia. Vestibular disorders can be divided into peripheral and central vestibular disorders. Most of the peripheral vestibular disorders have a clinical diagnosis, and a thorough history and examination will often provide a clear direction as to the diagnosis. Correct diagnosis allows treatment for many of the peripheral and central vestibular disorders. As inner ear damage is generally irreversible, early diagnosis allowing prompt treatment is important. The aim of this review is to discuss some audiovestibular conditions that may well appear in a neurology clinic, and to discuss some recent advances within the audiovestibular field that may be of interest to neurologists. Some of the most common audiovestibular conditions will be discussed along side more uncommon conditions.

Clinical Characteristics of a Dutch DFNA9 Family with a Novel COCH Mutation, G87W

The present study aims to report audiological and vestibular characteristics of a Dutch DFNA9 family with a novel mutation, G87W, in the LCCL domain of COCH. From the family with the novel G87W COCH mutation audiometric data were collected and analyzed longitudinally. Results were compared with those obtained in previously identified P51S COCH mutation carriers (n = 74) and with those obtained in G88E mutation carriers. Special attention was also given to a comparison of age-related features, such as progressive hearing loss and vestibular impairment. A novel mutation (G87W) in COCH is indicative of hearing impairment and vestibular dysfunction in the present family. Pure-tone thresholds, phoneme recognition scores, and vestibular responses of the G87W mutation carriers were essentially similar to those previously established in the P51S and G88E mutation carriers. Deterioration of hearing and vestibular function in the G87W mutation carriers started at the age of 43 years. Remarkably, similar to G88E mutation carriers, the proportion of patients over 40 years of age who developed complete vestibular areflexia was significantly lower for the G87W mutation carriers than for the P51S mutation carriers. In conclusion, the phenotype associated with the novel COCH (G87W) mutation is largely similar to that associated with the P51S and G88E mutation carriers. However, subtle differences in terms of onset age and rate of progression seem to exist.

Cochlin immunostaining of inner ear pathologic deposits and proteomic analysis in DFNA9 deafness and vestibular dysfunction

Seven missense mutations and one in-frame deletion mutation have been reported in the coagulation factor C homology (COCH) gene, causing the adult-onset, progressive sensorineural hearing loss and vestibular disorder at the DFNA9 locus. Prevalence of COCH mutations worldwide is unknown, as there is no systematic screening effort for late-onset hearing disorders; however, to date, COCH mutations have been found on four continents and the possibility of COCH playing an important role in presbycusis and disorders of imbalance has been considered. Cochlin (encoded by COCH) has also been shown as a major target antigen for autoimmune sensorineural hearing loss. In this report, we present histopathology, immunohistochemistry and proteomic analyses of inner ear tissues from post-mortem DFNA9 temporal bone samples of an individual from a large Dutch kindred segregating the P51S mutation and adult human unaffected controls, and wild-type (+/+) and Coch null (-/-) knock-out mice. DFNA9 is an inner ear disorder with a unique histopathology showing loss of cellularity and aggregation of abundant homogeneous acellular eosinophilic deposits in the cochlear and vestibular labyrinths, similar to protein aggregation in well-known neurodegenerative disorders. By immunohistochemistry on the DFNA9 temporal bone sections, we have shown cochlin staining of the characteristic cochlear and vestibular deposits, indicating aggregation of cochlin in the same structures in which it is normally expressed. Proteomic analysis identified cochlin as the most abundant protein in mouse and human cochleae. The high-level expression and stability of cochlin in the inner ear, even in the absence and severe atrophy of the fibrocytes that normally express COCH, are shown through these studies and further elucidate the pathobiologic events occurring in DFNA9 leading to hearing loss and vestibular dysfunction.

Cochlin and glaucoma: a mini-review

Primary open angle glaucoma (POAG) is a leading cause of late onset, progressive, irreversible blindness and, although its etiology is poorly understood, elevated intraocular pressure (IOP) often appears to be a contributory factor. Proteomic and Western analyses of trabecular meshwork (TM) from patients with POAG and age-matched controls originally implicated cochlin as possibly contributing to glaucoma pathogenesis. Cochlin deposits were subsequently detected in glaucomatous but not in control TM and older glaucomatous TM was found to contain higher levels of cochlin and significantly lower amounts of collagen type II. More recently, similar results were reported in DBA/2J mice, which at older ages develop elevated IOP, retinal ganglion cell degeneration, and optic nerve damage. Notably, cochlin was absent in TM from C57BL/6J, CD1, and BALBc/ByJ mice, which do not exhibit elevated IOP or glaucoma. Cochlin was found in the TM of very young DBA/2J mice, prior to elevated IOP, suggesting that over time the protein may contribute to the events leading to increased IOP and optic nerve damage. Here we review these findings and describe how future studies in DBA/2J mice can help resolve whether cochlin plays a causal role in mechanisms of POAG and elevated IOP.

Good Speech Recognition and Quality-of-Life Scores after Cochlear Implantation in Patients with DFNA9

OBJECTIVE

To compare audiometric and quality-of-life results in DFNA 9 patients who received a cochlear implant with cochlear implant patients with adult-onset progressive sensorineural hearing loss.

STUDY DESIGN

Prospective comparative design; results were collected cross-sectionally.

SETTING

Tertiary referral center.

PATIENTS

Eleven DFNA 9 patients were included in the study as well as a comparative group of 39 post-lingually deafened cochlear implant subjects with adult-onset progressive sensorineural hearing loss.

INTERVENTIONS

All patients received a cochlear implant. Subjects were implanted with either the Nucleus 24 M/RCS or Med-el Combi 40+ cochlear implant systems implementing the SPEAK, ACE, or CIS+ coding strategies.

MEAN OUTCOME MEASURES

Speech recognition was determined by means of phonetically balanced monosyllabic word lists. The Hearing Handicap Inventory for Adults, the Glasgow Benefit Inventory, and the Scale for the Prediction of Hearing Disability in Sensorineural Hearing Loss were used to quantify the quality of life.

RESULTS

The results show that the speech perception and the quality of life of the DFNA 9 patients do not differ significantly from the control group (p=0.179; p=0.56).

CONCLUSION

In spite of the fact that DFNA 9 is a disease that is known to involve cochlear dendrites, cochlear implantation is a good option for treatment of deafness in DFNA 9.

A novelDFNA9mutation in the vWFA2 domain ofCOCHalters a conserved cysteine residue and intrachain disulfide bond formation resulting in progressive hearing loss and site-specific vestibular and central oculomotor dysfunction

Mutations within the COCH gene (encoding the cochlin protein) lead to auditory and vestibular impairment in the DFNA9 disorder. In this study, we describe the genetic mapping of progressive autosomal dominant sensorineural hearing loss first affecting high-frequency auditory thresholds within a human pedigree to the long arm of chromosome 14 in band q12. A maximal pairwise LOD score of 7.08 was obtained with marker D14S1021. We identified a c.1625G > T mutation in exon 12 of COCH that co-segregates with auditory dysfunction in the pedigree. The mutation results in a predicted p.C542F substitution at an evolutionarily conserved cysteine residue in the C-terminus of cochlin. The c.1625G > T transversion in COCH exon 12 represents the first reported mutation outside of the LCCL domain which is encoded by exons 4 and 5. The 542F mutant cochlin is translated and secreted by transfected mammalian cells. Western blot analysis under non-reducing and reducing conditions suggests that the 542F mutation alters intramolecular cochlin disulfide bond formation. In the vestibular system, a progressive horizontal canal hypofunction and a probable saccular otolith challenge were detected in family members with the c.1625G > T COCH alteration. Abnormal central oculomotor test results in family members with the c.1625G > T COCH alteration imply a possible central nervous system change not previously noted in DFNA9 pedigrees harboring mutations within the LCCL domain.

Targeted disruption of mouse Coch provides functional evidence that DFNA9 hearing loss is not a COCH haploinsufficiency disorder

Dominant progressive hearing loss and vestibular dysfunction DFNA9 is caused by mutations of the human COCH gene. COCH encodes cochlin, a highly abundant secreted protein of unknown function in the inner ear. Cochlin has an N-terminal LCCL domain followed by two vWA domains, and all known DFNA9 mutations are either missense substitutions or an amino acid deletion in the LCCL domain. Here, we have characterized the auditory phenotype associated with a genomic deletion of mouse Coch downstream of the LCCL domain. Homozygous Coch (-/-) mice express no detectable cochlin in the inner ear. Auditory brainstem responses to click and pure-tone stimuli (8, 16, 32 kHz) were indistinguishable among wild type and homozygous Coch (-/-) mice. A Coch-LacZDeltaneo reporter allele detected Coch mRNA expression in nonsensory epithelial and stromal regions of the cochlea and vestibular labyrinth. These data provide functional evidence that DFNA9 is probably not caused by COCH haploinsufficiency, but via a dominant negative or gain-of-function effect, in nonsensory regions of the inner ear.

Cochlin deposits in the trabecular meshwork of the glaucomatous DBA/2J mouse

Cochlin deposits were observed in the trabecular meshwork (TM) of 8-month-old glaucomatous DBA/2J mice, coincident with the reported onset of increased intraocular pressure and optic nerve damage. An age-dependent increase in cochlin was observed up to 10 months of age and was paralleled by a decrease in type II collagen. Similar expression patterns exist in the TM of humans with primary open-angle glaucoma. Cochlin deposits, absent in non-glaucomatous mouse and human TM, may disrupt the TM extracellular matrix and obstruct aqueous humor circulation. Studies of DBA/2J mice offer promise for understanding the role cochlin may play in glaucoma.

Audiometric, Vestibular, and Genetic Aspects of a DFNA9 Family with a G88E COCH Mutation

OBJECTIVES

To perform genetic analysis and to analyze cochleovestibular impairment features in a newly identified Dutch family with nonsyndromic autosomal dominant hearing impairment (DFNA9).

STUDY DESIGN

Genetic analysis was performed using microsatellite markers and single nucleotide polymorphisms. Audiometric data were collected and analyzed longitudinally. Results were compared with those obtained in previously identified P51S COCH mutation carriers (n = 74). Special attention was also given to a comparison of age-related features such as progressive hearing loss and vestibular impairment.

SETTING

Tertiary referral center.

PATIENTS

G88E COCH mutation carriers from a Dutch family.

MAIN OUTCOME MEASURES

The study of clinical features of a DFNA9 family carrying a G88E COCH mutation and to compare this to the symptoms of those carrying a P51S/COCH mutation.

RESULTS

Pure-tone thresholds, phoneme recognition scores, and vestibular responses of the G88E mutation carriers were essentially similar to those previously established in the P51S mutation carriers. Hearing started to deteriorate in G88E mutation carriers from age 46 to 49 years and onward, whereas deterioration of vestibular function started from approximately age 46 years. In the P51S mutation carriers, vestibular impairment started earlier, at approximately age 34 years. However, the difference in age of onset with the G88E mutation carriers was not significant. Remarkably, the proportion of patients who developed complete vestibular areflexia within the age range of 40 to 56 years was significantly lower for the G88E mutation carriers than for the P51S mutation carriers.

CONCLUSION

Apart from a significantly lower frequency of vestibular areflexia between the ages of 40 and 56 years, there are no phenotypic differences between carriers of the G88E and P51S mutations in the COCH gene.

A novel locus for autosomal dominant non-syndromic deafness, DFNA53, maps to chromosome 14q11.2-q12

BACKGROUND

Non-syndromic hearing loss is among the most genetically heterogeneous traits known in humans. To date, at least 50 loci for autosomal dominant non-syndromic sensorineural hearing loss (ADNSSHL) have been identified by linkage analysis.

OBJECTIVE

To report the mapping of a novel autosomal dominant deafness locus on the long arm of chromosome 14 at 14q11.2-q12, DFNA53, in a large multigenerational Chinese family with post-lingual, high frequency hearing loss that progresses to involve all frequencies.

RESULTS

A maximum multipoint LOD score of 5.4 was obtained for marker D14S1280. The analysis of recombinant haplotypes mapped DFNA53 to a 9.6 cM region interval between markers D14S581 and D14S1021. Four deafness loci (DFNA9, DFNA23, DFNB5, and DFNB35) have previously been mapped to the long arm of chromosome 14. The critical region for DFNA53 contains the gene for DFNA9 but does not overlap with the regions for DFNB5, DFNA23, or DFNB35. Screening of the COCH gene (DFNA9), BOCT, EFS, and HSPC156 within the DFNA53 interval did not identify the cause for deafness in this family.

CONCLUSIONS

Identifying the DFNA53 locus is the first step in isolating the gene responsible for hearing loss in this large multigeneration Chinese family.

Proteomics reveal Cochlin deposits associated with glaucomatous trabecular meshwork

The etiology of primary open angle glaucoma, a leading cause of age-related blindness, remains poorly defined, although elevated intraocular pressure (IOP) contributes to the disease progression. To better understand the mechanisms causing elevated IOP from aqueous humor circulation, we pursued proteomic analyses of trabecular meshwork (TM) from glaucoma and age-matched control donors. These analyses demonstrated that Cochlin, a protein associated with deafness disorder DFNA9, is present in glaucomatous but absent in normal TM. Cochlin was also detected in TM from the glaucomatous DBA/2J mouse preceding elevated IOP but found to be absent in three other mouse lines that do not develop elevated IOP. Histochemical analyses revealed co-deposits of Cochlin and mucopolysaccharide in human TM around Schlemm's canal, similar to that observed in the cochlea in DFNA9 deafness. Purified Cochlin was found to aggregate after sheer stress and to induce the aggregation of TM cells in vitro. Age-dependent in vivo increases in Cochlin were observed in glaucomatous TM, concomitant with a decrease in type II collagen, suggesting that Cochlin may disrupt the TM architecture and render components like collagen more susceptible to degradation and collapse. Overall, these observations suggest that Cochlin contributes to elevated IOP in primary open angle glaucoma through altered interactions within the TM extracellular matrix, resulting in cell aggregation, mucopolysaccharide deposition, and significant obstruction of the aqueous humor circulation.

Absence of COCH mutations in patients with Meniere disease

Missense mutations in the coagulation factor C homology (COCH) gene (14q12-q13) cause the autosomal dominant sensorineural hearing loss and vestibular disorder DFNA9 (OMIM 603196), and a high prevalence of symptoms of Meniere disease (MD) has been described in families with a mutation in the COCH gene. In this study, we search for mutations in the COCH gene in peripheral blood from patients with definite MD. DNA was extracted from peripheral blood cells of 30 individuals with MD and 30 controls. Exons 4 and 5 of the COCH gene were amplified by PCR reaction, using primer pairs flanking both exons. Sequences were analysed by a DNA sequencing system and compared with the published COCH cDNA sequence. No differences were found in the nucleotide sequences of exons 4 and 5 in the COCH gene in patients with definite sporadic MD when they were compared with the control group. Patients with definite MD have a low prevalence of mutations in exons 4 and 5 of the COCH gene.

Cochlin, a secreted von Willebrand factor type a domain-containing factor, is regulated by leukemia inhibitory factor in the uterus at the time of embryo implantation

Embryo implantation is a required step in the reproduction of all mammals. In mice, a transient rise in the uterine expression of leukemia inhibitory factor (LIF) occurs on d 4 of pregnancy and is essential for embryo implantation. However, which genes are regulated by LIF in the uterus at implantation has not been determined. We performed a subtractive hybridization assay between luminal epithelial (LE) mRNAs from d 3 and 4 of pregnancy to find genes up-regulated on d 4 and which would be potentially regulated by LIF. One candidate, Coch-5b2, was up-regulated on the day of implantation. Coch mRNA localized to the LE of wild-type mice and was not detected in uteri from Lif-deficient mice. Treatment of LE with LIF, both in vitro and in vivo, resulted in the up-regulation of Coch. Coch is also highly expressed in other tissues, including the spleen and inner ear, but only in the uterus is Coch expression regulated by LIF. Mice were derived in which Coch was either deleted or tagged with a LacZ reporter. In mice carrying the tagged Coch gene, expression of Coch was detected in the LE and also at the site of embryo implantation. However, mice in which the Coch gene was deleted were normal, showing no overt defects in their reproduction. Although loss of Coch expression is not essential to reproduction in mice, it may serve as a useful marker for assessing the state of uterine receptivity in response to LIF at the onset of implantation.

Mutations in the COCH gene are a frequent cause of autosomal dominant progressive cochleo-vestibular dysfunction, but not of Meniere's disease

The COCH gene is the only gene identified in man that causes autosomal dominantly inherited hearing loss associated with vestibular dysfunction. The condition is rare and only five mutations have been reported worldwide. All affected families showed a similar progressive hearing loss and vestibular dysfunction. Since Meniere's disease-like symptoms have also been described in some families, it was suggested that COCH mutations might be present in some patients diagnosed with Meniere's disease. In this study, using a Japanese population, we performed a COCH mutation analysis in 23 patients from independent families with autosomal dominant hearing impairment, four of whom reported vestibular symptoms, and also in 20 Meniere's patients. While a new point mutation, A119 T, was found in a patient with autosomal dominant hearing loss and vestibular symptoms, no mutations were found in the Meniere's patients. Like all other previously identified COCH mutations, the mutation identified here is a missense mutation located in the FCH domain of the protein. The current mutation is located in close spatial proximity to W117, in which a mutation (W117R) had previously been associated with autosomal dominant hearing loss. Model building suggests that, like the W117R mutation, the A119 T mutation does not affect the structural integrity of the FCH domain, but may interfere with the interaction with a yet unknown binding partner. We conclude that mutations in the COCH gene are responsible for a significant fraction of patients with autosomal dominantly inherited hearing loss accompanied by vestibular symptoms, but not for dominant hearing loss without vestibular dysfunction, or sporadic Meniere's disease.

HUMANNONSYNDROMICSENSORINEURALDEAFNESS

Given the unique biological requirements of sound transduction and the selective advantage conferred upon a species capable of sensitive sound detection, it is not surprising that up to 1% of the approximately 30,000 or more human genes are necessary for hearing. There are hundreds of monogenic disorders for which hearing loss is one manifestation of a syndrome or the only disorder and therefore is nonsyndromic. Herein we review the supporting evidence for identifying over 30 genes for dominantly and recessively inherited, nonsyndromic, sensorineural deafness. The state of knowledge concerning their biological roles is discussed in the context of the controversies within an evolving understanding of the intricate molecular machinery of the inner ear.

Subcellular localisation, secretion, and post-translational processing of normal cochlin, and of mutants causing the sensorineural deafness and vestibular disorder, DFNA9

Five missense mutations in the FCH/LCCL domain of the COCH gene, encoding the protein cochlin, are pathogenic for the autosomal dominant hearing loss and vestibular dysfunction disorder, DFNA9. To date, the function of cochlin and the mechanism of pathogenesis of the mutations are unknown. We have used the biological system of transient transfections of the entire protein coding region of COCH into several mammalian cell lines, to investigate various functional properties of cochlin. By western blot analysis of lysates prepared from transfected cells, we show that cochlin is a secreted protein. Immunocytochemistry shows concentrated localisation of cochlin in perinuclear structures consistent with the Golgi apparatus and endoplasmic reticulum, showing intracellular passage through these secretory compartments. We detected that cochlin is proteolytically cleaved between the FCH/LCCL domain and the downstream vWFA domains, resulting in a smaller cochlin isoform of approximately 50 kDa. Interestingly, this isoform lacks the entire mutation bearing FCH/LCCL domain. We have also shown that cochlin is N-glycosylated in its mature secreted form. Previous studies of the FCH/LCCL domain alone, expressed in bacteria, have demonstrated that three of four DFNA9 mutations cause misfolding of this domain. Characteristic eosinophilic deposits in DFNA9 affected inner ear structures could be the result of aberrant folding, secretion, or solubility of mutated cochlins, as in certain other pathological states in which misfolded proteins accumulate and aggregate causing toxicity. To examine the biological consequences of cochlin misfolding, we made separate constructs with three of the DFNA9 mutations and performed parallel studies of the mutated and wild type cochlins. We detected that mutated cochlins are not retained intracellularly, and are able to be secreted adequately by the cells, through the Golgi/ER secretory pathway, and also undergo proteolytic cleavage and glycosylation. These results suggest that DFNA9 mutations may manifest deleterious effects beyond the point of secretion, in the unique environment of the extracellular matrix of the inner ear by disrupting cochlin function or interfering with protein-protein interactions involving the FCH/LCCL domain. It is also possible that the mutations may result in aggregation of cochlin in vivo over a longer time course, as supported by the late onset and progressive nature of this disorder.

Cross-sectional analysis of hearing threshold in relation to age in a large family with cochleovestibular impairment thoroughly genotyped for DFNA9/COCH

Hearing threshold was analyzed for each frequency in relation to age in 88 members of a large Dutch family with cochleovestibular impairment caused by a P51S mutation in the COCH gene within the DFNA9 locus (chromosome 14q12-13). The participants in this study were 34 mutation carriers and 54 relatives without the mutation (control subjects). A sigmoidal dose-response curve with a variable slope was used to fit the mutation carriers' threshold-on-age data. Progression started at about 40 years of age and only lasted for some 20 to 25 years; the associated average progression was 2.9 dB/y for all frequencies. However, some hearing impairment was already present before, predominantly at the high frequencies. The mean thresholds in the young mutation carriers (< 33 years of age) were significantly higher (by 4 to 13 dB) than those in age-matched controls at 2 to 8 kHz. Presumably, mutation carriers have a congenital, stable offset threshold (10 to 29 dB) at these frequencies, and develop progression later in life.

Non-syndromic autosomal-dominant deafness

Non-syndromic deafness is a paradigm of genetic heterogeneity. More than 70 loci have been mapped, and 25 of the nuclear genes responsible for non-syndromic deafness have been identified. Autosomal-dominant genes are responsible for about 20% of the cases of hereditary non-syndromic deafness, with 16 different genes identified to date. In the present article we review these 16 genes, their function and their contribution to deafness in different populations. The complexity is underlined by the fact that several of the genes are involved in both dominant and recessive non-syndromic deafness or in both non-syndromic and syndromic deafness. Mutations in eight of the genes have so far been detected in only single dominant deafness families, and their contribution to deafness on a population base might therefore be limited, or is currently unknown. Identification of all genes involved in hereditary hearing loss will help in the understanding of the basic mechanisms underlying normal hearing, will facilitate early diagnosis and intervention and might offer opportunities for rational therapy.

Molecular genetics of hearing loss

Hereditary isolated hearing loss is genetically highly heterogeneous. Over 100 genes are predicted to cause this disorder in humans. Sixty loci have been reported and 24 genes underlying 28 deafness forms have been identified. The present epistemic stage in the realm consists in a preliminary characterization of the encoded proteins and the associated defective biological processes. Since for several of the deafness forms we still only have fuzzy notions of their pathogenesis, we here adopt a presentation of the various deafness forms based on the site of the primary defect: hair cell defects, nonsensory cell defects, and tectorial membrane anomalies. The various deafness forms so far studied appear as monogenic disorders. They are all rare with the exception of one, caused by mutations in the gene encoding the gap junction protein connexin26, which accounts for between one third to one half of the cases of prelingual inherited deafness in Caucasian populations.

Hereditary deafness and phenotyping in humans

Hereditary deafness has proved to be extremely heterogeneous genetically with more than 40 genes mapped or cloned for non-syndromic dominant deafness and 30 for autosomal recessive non-syndromic deafness. In spite of significant advances in the understanding of the molecular basis of hearing loss, identifying the precise genetic cause in an individual remains difficult. Consequently, it is important to exclude syndromic causes of deafness by clinical and special investigation and to use all available phenotypic clues for diagnosis. A clinical approach to the aetiological investigation of individuals with hearing loss is suggested, which includes ophthalmology review, renal ultrasound scan and neuro-imaging of petrous temporal bone. Molecular screening of the GJB2 (Connexin 26) gene should be undertaken in all cases of non-syndromic deafness where the cause cannot be identified, since it is a common cause of recessive hearing impairment, the screening is straightforward, and the phenotype unremarkable. By the same token, mitochondrial inheritance of hearing loss should be considered in all multigeneration families, particularly if there is a history of exposure to aminoglycoside antibiotics, since genetic testing of specific mitochondrial genes is technically feasible. Most forms of non-syndromic autosomal recessive hearing impairment cause a prelingual hearing loss, which is generally severe to profound and not associated with abnormal radiology. Exceptions to this include DFNB2 (MYO7A), DFNB8/10 (TMPRSS3) and DFNB16 (STRC) where age of onset may sometimes be later on in childhood, DFNB4 (SLC26A4) where there may be dilated vestibular aqueducts and endolymphatic sacs, and DFNB9 (OTOF) where there may also be an associated auditory neuropathy. Unusual phenotypes in autosomal dominant forms of deafness, include low frequency hearing loss in DFNA1 (HDIA1) and DFNA6/14/38 (WFS1), mid-frequency hearing loss in DFNA8/12 (TECTA), DFNA13 (COL11A2) and vestibular symptoms and signs in DFNA9 (COCH) and sometimes in DFNA11 (MYO7A). Continued clinical evaluation of types and course of hearing loss and correlation with genotype is important for the intelligent application of molecular testing in the next few years.