Nonsyndromic autosomal dominant progressive sensorineural hearing loss: audiologic analysis of a pedigree linked to DFNA2

Impaired surface expression and conductance of the KCNQ4 channel lead to sensorineural hearing loss

KCNQ4, a voltage-gated potassium channel, plays an important role in maintaining cochlear ion homoeostasis and regulating hair cell membrane potential, both essential for normal auditory function. Mutations in the KCNQ4 gene lead to DFNA2, a subtype of autosomal dominant non-syndromic deafness that is characterized by progressive sensorineural hearing loss across all frequencies. Despite recent advances in the identification of pathogenic KCNQ4 mutations, the molecular aetiology of DFNA2 remains unknown. We report here that decreased cell surface expression and impaired conductance of the KCNQ4 channel are two mechanisms underlying hearing loss in DFNA2. In HEK293T cells, a dramatic decrease in cell surface expression was detected by immunofluorescent microscopy and confirmed by Western blot for the pathogenic KCNQ4 mutants L274H, W276S, L281S, G285C, G285S, G296S and G321S, while their overall cellular levels remained normal. In addition, none of these mutations affected tetrameric assembly of KCNQ4 channels. Consistent with these results, all mutants showed strong dominant-negative effects on the wild-type (WT) channel function. Most importantly, overexpression of HSP90β, a key component of the molecular chaperone network that controls the KCNQ4 biogenesis, significantly increased cell surface expression of the KCNQ4 mutants L281S, G296S and G321S. KCNQ4 surface expression was restored or considerably improved in HEK293T cells mimicking the heterozygous condition of these mutations in DFNA2 patients. Finally, our electrophysiological studies demonstrated that these mutations directly compromise the conductance of the KCNQ4 channel, since no significant change in KCNQ4 current was observed after KCNQ4 surface expression was restored or improved.

Genetics of hearing loss: focus on DFNA2

The purpose of this review is to assess the current literature on deafness nonsyndromic autosomal dominant 2 (DFNA2) hearing loss and the mutations linked to this disorder. Hearing impairment, particularly nonsyndromic hearing loss, affects multiple families across the world. After the identification of the DFNA2 locus on chromosome 1p34, multiple pathogenic mutations in two genes (GJB3 and KCNQ4) have been reported. The overwhelming majority of pathogenic mutations linked to this form of nonsyndromic hearing loss have been identified in the KCNQ4 gene encoding a voltage-gated potassium channel. It is believed that KCNQ4 channels are present in outer hair cells and possibly inner hair cells and the central auditory pathway. This form of hearing loss is both phenotypically and genetically heterogeneous and there are still DFNA2 pedigrees that have not been associated with changes in either GJB3 or KCNQ4, suggesting that a possible third gene exists at this locus. Further studies of the DFNA2 locus will lead to a better understanding of progressive hearing loss and provide a better means of early detection and treatment.

Audioprofile-directed successful mutation analysis in a DFNA2/KCNQ4 (p.Leu274His) family

OBJECTIVES

We undertook to show that in a family with nonsyndromic autosomal dominant sensorineural hearing loss, genetic analysis can be successful when there is a match with a specific DFNA audioprofile. We also provide an update of relevant DFNA2/KCNQ4 audioprofiles and report the results of automatic audioprofile analysis using the Internet program AudioGene.

METHODS

Audiometric data and blood samples were obtained from the family W08-0384. Based on the audiograms of the affected participants, mutation analysis of KCNQ4 was started. Original audiometric threshold data were collected for all identified KCNQ4-related DFNA2 families. The Internet computer program AudioGene, recently developed for automatic audioprofile analysis, was accessed.

RESULTS

The family's audioprofile and the program AudioGene predicted the DFNA2/KCNQ4 locus. Mutation analysis of KCNQ4 revealed a c.821T>A (p.Leu274His) mutation of the KCNQ4 gene. This mutation has been previously identified in a Dutch family. Genetic analysis revealed a common haplotype in these two families over a region including the KCNQ4 gene.

CONCLUSIONS

Familiarity with the audioprofiles of DFNA traits may lead to successful mutation analysis of the gene involved, even in a small family in which genetic linkage analysis is not an option. Alternatively, the specially developed program AudioGene can be accessed on the Internet to perform automatic audioprofile analysis of a family's (audiological) phenotype.

Developmental expression of Kcnq4 in vestibular neurons and neurosensory epithelia

Sensory signal transduction of the inner ear afferent neurons and hair cells (HCs) requires numerous ionic conductances. The KCNQ4 voltage-gated M-type potassium channel is thought to set the resting membrane potential in cochlear HCs. Here we describe the spatiotemporal expression patterns of Kcnq4 and the associated alternative splice forms in the HCs of vestibular labyrinth. Whole mount immunodetection, qualitative and quantitative RT-PCR were performed to characterize the expression patterns of Kcnq4 transcripts and proteins. A topographical expression and upregulation of Kcnq4 during development was observed and indicated that Kcnq4 is not restricted to either a specific vestibular structure or cell type, but is present in afferent calyxes, vestibular ganglion neurons, and both type I and type II HCs. Of the four alternative splice variants, Kcnq4_v1 transcripts were the predominant form in the HCs, while Kcnq4_v3 was the major variant in the vestibular neurons. Differential quantitative expression of Kcnq4_v1 and Kcnq4_v3 were respectively detected in the striolar and extra-striolar regions of the utricle and saccule. Analysis of gerbils and rats yielded results similar to those obtained in mice, suggesting that the spatiotemporal expression pattern of Kcnq4 in the vestibular system is conserved among rodents. Analyses of vestibular HCs of Bdnf conditional mutant mice, which are devoid of any innervation, demonstrate that regulation of Kcnq4 expression in vestibular HCs is independent of innervation.

Phenotype determination guides swift genotyping of a DFNA2/KCNQ4 family with a hot spot mutation (W276S)

OBJECTIVE

Genotype a family trait with autosomal dominant nonsyndromic sensorineural hearing impairment guided only by the phenotype.

STUDY DESIGN

Family study.

SETTING

Tertiary referral center.

PATIENTS

Fifteen family members.

METHODS

In the first phase, sequence analysis was performed on DNA isolated from buccal swabs of the proband and her daughter, guided by the phenotype based on audiometric data that were already available. After detection of the W276S missense mutation in the KCNQ4 gene in both patients, this finding was confirmed in the other affected family members. All participants completed a questionnaire, were clinically examined, and underwent standard pure-tone audiometry. The results were analyzed to refine the phenotypic features of the family trait.

RESULTS

All clinically affected participants were carriers of the W276S hotspot mutation in exon 5 of the KCNQ4 gene on chromosome 1p34. Refined phenotypic features confirmed previously described phenotypes of DFNA2 families.

CONCLUSIONS

Phenotype determination can be cost saving and very effective in detecting the genotype of autosomal dominant nonsyndromic hearing impairment, especially when phenotype analyses can be performed on data that are already available or easily collected.

A mutational hot spot in the KCNQ4 gene responsible for autosomal dominant hearing impairment

Several different mutations in the KCNQ4 K+ channel gene are responsible for autosomal dominant nonsyndromic hearing impairment (DFNA2). Here we describe two additional families originating from Europe and Japan with a KCNQ4 missense mutation (W276S) that was previously found in one European family. We compared the disease-associated haplotype of the three W276S-bearing families using closely linked microsatellite markers and intragenic single nucleotide polymorphisms. Differences between the haplotypes were found, excluding a single founder mutation for the families. Therefore, the W276S mutation has occurred three times independently, and most likely represents a hot spot for mutation in the KCNQ4 gene.

Longitudinal and cross-sectional phenotype analysis in a new, large Dutch DFNA2/KCNQ4 family

We analyzed hearing thresholds, speech recognition scores, and vestibular responses in 32 affected persons in a large family with DFNA2/KCNQ4-related hearing impairment caused by a W276S missense mutation. Linear regression analysis of individual longitudinal data revealed significant threshold progression (1 dB/y) and offset (at age zero). The mean offset thresholds were 5, 21, 40, 39, 31, and 51 dB hearing level (HL) at 0.25, 0.5, 1, 2, 4, and 8 kHz, respectively. Cross-sectional analysis of last-visit thresholds against age produced less-steep slopes and higher offset thresholds. Nonlinear regression analysis of last-visit phoneme recognition scores against age in 25 cases showed that speech recognition did not deteriorate before the third decade. A hyperactive vestibuloocular reflex was found in 3 of 11 cases: 2 persons were especially susceptible to motion sickness. Persons with this KCNQ4 mutation showed congenital, progressive high-frequency impairment without substantial loss of speech recognition during the first decades of life.

Phenotype of DFNA11: a nonsyndromic hearing loss caused by a myosin VIIA mutation

OBJECTIVES/HYPOTHESIS

To characterize the audiovestibular phenotype of DFNA11, an autosomal dominant nonsyndromic hearing impairment caused by a mutation in the myosin VIIA gene (MYO7A), including whether DFNA11-affected subjects have retinal degeneration as is characteristic of Usher syndrome type 1B, caused by different MYO7A mutations.

STUDY DESIGN

Retrospective study of audiovestibular and ophthalmological data in a Japanese family linked to DFNA11.

METHODS

Otoscopic examination and pure-tone audiometry were performed in all participants in the family. Selected subjects underwent additional examinations including speech discrimination scoring, acoustic reflex measurements, Békésy audiometry, evoked and distortion-product otoacoustic emissions, auditory brainstem responses, and bithermal caloric testing; visual acuity, ocular tonometry, slit-lamp examination, ophthalmoscopy, and electroretinography; and computed tomography of the temporal bone.

RESULTS

Most affected individuals had moderate cochlear hearing loss beginning in the second decade and progressing at all frequencies. Variable degrees of asymptomatic vestibular dysfunction were present. Computed tomography showed normal inner and middle ear structures. No evidence suggested retinitis pigmentosa.

CONCLUSIONS

The phenotype of DFNA11 is postlingual, nonsyndromic sensorineural hearing loss with gradual progression. Showing moderate hearing loss with asymptomatic variable vestibular dysfunction and no retinal degeneration, the DFNA11 phenotype is mildest among phenotypes caused by MYO7A mutations.

Pediatric fluctuating sensorineural hearing loss: problems in medical management

OBJECTIVES

To discuss the diagnosis and management of children with fluctuating sensorineural hearing loss, especially focusing on those problems dealing with autoimmune inner ear disease.

STUDY DESIGN

A retrospective chart review of a large pediatric otolaryngology practice. A series of 40 children with progressive hearing losses was identified. Of that group, 22 children, aged 1.5 to 12.2 years at first audiogram, were considered to have fluctuating sensorineural hearing loss (FSNHL). Criteria for inclusion in the FSNHL group were threshold variations of 15 dB or more in at least one ear at two or more of the standard audiometric frequencies on at least 2 testing days.

METHODS

Charts were reviewed for age, sex, otologic history, laboratory evaluations, medical or surgical treatments, significant medical history, and family medical history.

RESULTS

Twenty-two children met the criteria for fluctuating sensorineural hearing loss. Of those with fluctuating hearing loss, 15 were idiopathic, 3 had positive lymphocyte transformation tests (LTT) suggestive of autoimmune inner ear disease (AIED), and 4 had fistula on middle ear exploration. Average fluctuation for all groups was 29.1 dB. Average duration of fluctuations was 4.95 years.

CONCLUSIONS

The majority of pediatric FSNHL cases (15 of 22) were idiopathic in nature. Of those FSNHL children with positive LTTs, only one was treated with steroid therapy. In the other patients with positive LTTs, parents or other physicians were often reluctant to treat, or the patient was lost to follow-up. Mean fluctuations varied substantially across all standard audiometric frequencies for all groups.

KCNQ4, a K+ channel mutated in a form of dominant deafness, is expressed in the inner ear and the central auditory pathway

Mutations in the potassium channel gene KCNQ4 underlie DFNA2, an autosomal dominant form of progressive hearing loss in humans. In the mouse cochlea, the transcript has been found exclusively in the outer hair cells. By using specific antibodies, we now show that KCNQ4 is situated at the basal membrane of these sensory cells. In the vestibular organs, KCNQ4 is restricted to the type I hair cells and the afferent calyx-like nerve endings ensheathing these sensory cells. Several lines of evidence suggest that KCNQ4 underlies the I(K,n) and g(K,L) currents that have been described in the outer and type I hair cells, respectively, and that are already open at resting potentials. KCNQ4 is also expressed in neurons of many, but not all, nuclei of the central auditory pathway, and is absent from most other brain regions. It is present, e.g., in the cochlear nuclei, the nuclei of the lateral lemniscus, and the inferior colliculus. This is the first ion channel shown to be specifically expressed in a sensory pathway. Moreover, the expression pattern of KCNQ4 in the mouse auditory system raises the possibility of a central component in the DFNA2 hearing loss.

Non-syndromal autosomal dominant hearing impairment: ongoing phenotypical characterization of genotypes

This review is concerned with the present state of phenotypical characterization of known genotypes of non-syndromal autosomal dominant hearing impairment. A brief outline of history and context of phenotyping and genotyping of hearing impairment is given with particular reference to the most recent developments in this field, followed by descriptions of DFNA1, DFNA2, DFNA5, DFNA6/14, DFNA8/12, DFNA9, DFNA 13, DFNA17 and DFNA21. Phenotyping those known genotypes may support the ongoing search for mutations in the corresponding gene and enhance genetic counselling. It is recommended that sufficient attention is given to a detailed description of the phenotype in each (newly) described hereditary hearing impairment disorder.

The DFNA2 locus for hearing impairment: two genes regulating K+ ion recycling in the inner ear

DFNA2 is a locus for autosomal dominant non-syndromal hearing impairment (ADNSHI) located on chromosome 1p34 and six linked families have been identified. An audiometric study of these families showed that despite small differences in the phenotype all families suffer from progressive hearing impairment starting in the high frequencies. A detailed genetic analysis revealed that this deafness locus contains more than one gene responsible for hearing impairment. Thus far, two genes on chromosome 1p34 have been implicated in ADNSHI. The first, connexin 31 (GJB3), is a member of the connexin gene family. Connexins form gap junctions. These are connections between neighbouring cells that allow transport of small molecules. GJB3 mutations were found in two small Chinese families with ADNSHI. The second is KCNQ4, a voltage-gated K+ channel. Mutations in KCNQ4 were first found in a small French family, later in five of the six linked DFNA2 families. No GJB3 or KCNQ4 mutations were detected in patients of an extended Indonesian DFNA2 family. Two pathways have been proposed for the recycling of K+ from the hair cells back to the endolymph. These pathways involve the use of gap junctions, K+ pumps and K+ channels. The expression of GJB3 and KCNQ4 in the inner ear and their functions suggest that both DFNA2 genes may play a role in K+ homeostasis.

A gene for fluctuating, progressive autosomal dominant nonsyndromic hearing loss, DFNA16, maps to chromosome 2q23-24.3

The sixteenth gene to cause autosomal dominant nonsyndromic hearing loss (ADNSHL), DFNA16, maps to chromosome 2q23-24.3 and is tightly linked to markers in the D2S2380-D2S335 interval. DFNA16 is unique in that it results in the only form of ADNSHL in which the phenotype includes rapidly progressing and fluctuating hearing loss that appears to respond to steroid therapy. This observation suggests that it may be possible to stabilize hearing through medical intervention, once the biophysiology of deafness due to DFNA16 is clarified. Especially intriguing is the localization of several voltage-gated sodium-channel genes to the DFNA16 interval. These cationic channels are excellent positional and functional DFNA16 candidate genes.

Autosomal dominant non-syndromal low-frequency sensorineural hearing impairment linked to chromosome 4p16 (DFNA14): statistical analysis of hearing threshold in relation to age and evaluation of vestibulo-ocular functions

A Dutch kindred was studied with low-frequency sensorineural hearing impairment linked to a new locus on chromosome 4p16 (DFNA14). Of the affected individuals, 21 (aged 11-75 years) were examined and the most recent audiogram was used for cross-sectional analysis of hearing threshold in relation to age. Suitable serial audiograms were available for a longitudinal analysis in nine cases: they had been obtained from the age of six years onwards and covered a follow-up period from 14 to 36 years. The presumably congenital (offset) component of SNHI was extrapolated or estimated from average values and offset thresholds were found of about 45 dB at 0.25-1 kHz, 25 dB at 2 kHz and 10 dB at 4-8 kHz. Significant progression in hearing impairment occurred at all frequencies, but could be attributed to presbyacusis. The combination of congenital, stationary low-frequency SNHI and presbyacusis resulted in an up-sloping audiogram in the first five decades of life, which evolved into a flat-type audiogram in the sixth or seventh decade and a down-sloping audiogram at a more advanced age. With few exceptions, vestibular function was intact.