Mutations in Cdh23 cause nonsyndromic hearing loss in waltzer mice

A novel copy number variant in the murine Cdh23 gene gives rise to profound deafness and vestibular dysfunction

Hearing loss is the most common congenital sensory deficit worldwide and exhibits high genetic heterogeneity, making molecular diagnoses elusive for most individuals. Detecting novel mutations that contribute to hearing loss is crucial to providing accurate personalized diagnoses, tailored interventions, and improving prognosis. Copy number variants (CNVs) are structural mutations that are understudied, potential contributors to hearing loss. Here, we present the Abnormal Wobbly Gait (AWG) mouse, the first documented mutant exhibiting waltzer-like locomotor dysfunction, hyperactivity, circling behaviour, and profound deafness caused by a spontaneous CNV deletion in cadherin 23 (Cdh23). We were unable to identify the causative mutation through a conventional whole-genome sequencing (WGS) and variant detection pipeline, but instead found a linked variant in hexokinase 1 (Hk1) that was insufficient to recapitulate the AWG phenotype when introduced into C57BL/6J mice using CRISPR-Cas9. Investigating nearby deafness-associated genes revealed a pronounced downregulation of Cdh23 mRNA and a complete absence of full-length CDH23 protein, which is critical for the development and maintenance of inner ear hair cells, in whole head extracts from AWG neonates. Manual inspection of WGS read depth plots of the Cdh23 locus revealed a putative 10.4 kb genomic deletion of exons 11 and 12 that was validated by PCR and Sanger sequencing. This study underscores the imperative to refine variant detection strategies to permit identification of pathogenic CNVs easily missed by conventional variant calling to enhance diagnostic precision and ultimately improve clinical outcomes for individuals with genetically heterogenous disorders such as hearing loss.

A CDH23 missense variant in Beauceron dogs with non-syndromic deafness

Congenital coat-colour-related deafness is common among certain canine breeds especially those exhibiting extreme white spotting or merle patterning. We identified a non-syndromic deafness in Beauceron dogs characterised by a bilateral hearing loss in puppies that is not linked to coat colour. Pedigree analysis suggested an autosomal recessive transmission. By combining homozygosity mapping with whole genome sequencing and variant filtering in affected dogs we identified a CDH23:c.700C>T variant. The variant, located in the CHD23 (cadherin related 23) gene, was predicted to induce a CDH23:p.(Pro234Ser) change in the protein. Proline-234 of CDH23 protein is highly conserved across different vertebrate species. In silico tools predicted the CDH23:p.(Pro234Ser) change to be deleterious. CDH23 encodes a calcium-dependent transmembrane glycoprotein localised near the tips of hair-cell stereocilia in the mammalian inner ear. Intact function of these cilia is mandatory for the transformation of the acoustical wave into a neurological signal, leading to sensorineural deafness when impaired. By genotyping a cohort of 90 control Beauceron dogs sampled in France, we found a 3.3% carrier frequency. The CDH23:c.[700C>T] allele is easily detectable with a genetic test to avoid at-risk matings.

The Role of Genetic Variants in the Susceptibility of Noise-Induced Hearing Loss

Noised-induced hearing loss (NIHL) is an acquired, progressive neurological damage caused by exposure to intense noise in various environments including industrial, military and entertaining settings. The prevalence of NIHL is much higher than other occupational injuries in industrialized countries. Recent studies have revealed that genetic factors, together with environmental conditions, also contribute to NIHL. A group of genes which are linked to the susceptibility of NIHL had been uncovered, involving the progression of oxidative stress, potassium ion cycling, cilia structure, heat shock protein 70 (HSP70), DNA damage repair, apoptosis, and some other genes. In this review, we briefly summarized the studies primary in population and some animal researches concerning the susceptible genes of NIHL, intending to give insights into the further exploration of NIHL prevention and individual treatment.

Congenital Deafness and Recent Advances Towards Restoring Hearing Loss

Congenital hearing loss is the most common birth defect, estimated to affect 2-3 in every 1000 births. Currently there is no cure for hearing loss. Treatment options are limited to hearing aids for mild and moderate cases, and cochlear implants for severe and profound hearing loss. Here we provide a literature overview of the environmental and genetic causes of congenital hearing loss, common animal models and methods used for hearing research, as well as recent advances towards developing therapies to treat congenital deafness. © 2021 The Authors.

Identifying genetic risk variants associated with noise-induced hearing loss based on a novel strategy for evaluating individual susceptibility

BACKGROUND

The overall genetic profile for noise-induced hearing loss (NIHL) remains elusive. Herein we proposed a novel machine learning (ML) based strategy to evaluate individual susceptibility to NIHL and identify the underlying genetic risk variants based on a subsample of participants with extreme phenotypes.

METHODS

Five features (age, sex, cumulative noise exposure [CNE], smoking, and alcohol drinking status) of 5,539 shipbuilding workers from large cross-sectional surveys were included in four ML classification models to predict their hearing levels. The area under the curve (AUC) and prediction accuracy were exploited to evaluate the performance of the models. Based on the prediction error of the ML models, the NIHL-susceptible group (n=150) and NIHL-resistant group (n=150) with a paradoxical relationship between hearing levels and features were separately screened, to identify the underlying variants associated with NIHL risk using whole-exome sequencing (WES). Subsequently, candidate risk variants were validated in an additional replication cohort (n=2108), followed by a meta-analysis.

RESULTS

With 10-fold cross-validation, the performances of the four ML models were robust and similar, with average AUCs and accuracies ranging from 0.783 to 0.798 and 73.7% to 73.8%, respectively. The phenotypes of the NIHL-susceptible and NIHL-resistant groups were significantly different (all p<0.001). After WES analysis and filtering, 12 risk variants contributing to NIHL susceptibility were identified and replicated. The meta-analyses showed that the A allele of CDH23 rs41281334 (odds ratio [OR]=1.506, 95% confidence interval [CI]=1.106-2.051) and the C allele of WHRN rs12339210 (OR=3.06, 95% CI=1.398-6.700) were significantly associated with increased risk of NIHL after adjustment for confounding factors.

CONCLUSIONS

This study revealed two genetic variants in CDH23 rs41281334 and WHRN rs12339210 that associated with NIHL risk, based on a promising approach for evaluating individual susceptibility using ML models.

Identification of Novel CDH23 Variants Causing Moderate to Profound Progressive Nonsyndromic Hearing Loss

Mutant alleles of CDH23, a gene that encodes a putative calcium-dependent cell-adhesion glycoprotein with multiple cadherin-like domains, are responsible for both recessive DFNB12 nonsyndromic hearing loss (NSHL) and Usher syndrome 1D (USH1D). The encoded protein cadherin 23 (CDH23) plays a vital role in maintaining normal cochlear and retinal function. The present study’s objective was to elucidate the role of DFNB12 allelic variants of CDH23 in Saudi Arabian patients. Four affected offspring of a consanguineous family with autosomal recessive moderate to profound NSHL without any vestibular or retinal dysfunction were investigated for molecular exploration of genes implicated in hearing impairment. Parallel to this study, we illustrate some possible pitfalls that resulted from unexpected allelic heterogeneity during homozygosity mapping due to identifying a shared homozygous region unrelated to the disease locus. Compound heterozygous missense variants (p.(Asp918Asn); p.(Val1670Asp)) in CDH23 were identified in affected patients by exome sequencing. Both the identified missense variants resulted in a substitution of the conserved residues and evaluation by multiple in silico tools predicted their pathogenicity and variable disruption of CDH23 domains. Three-dimensional structure analysis of human CDH23 confirmed that the residue Asp918 is located at a highly conserved DXD peptide motif and is directly involved in “Ca²⁺” ion contact. In conclusion, our study identifies pathogenic CDH23 variants responsible for isolated moderate to profound NSHL in Saudi patients and further highlights the associated phenotypic variability with a genotypic hierarchy of CDH23 mutations. The current investigation also supports the application of molecular testing in the clinical diagnosis and genetic counseling of hearing loss.

Usher Syndrome: Genetics and Molecular Links of Hearing Loss and Directions for Therapy

Usher syndrome (USH) is an autosomal recessive (AR) disorder that permanently and severely affects the senses of hearing, vision, and balance. Three clinically distinct types of USH have been identified, decreasing in severity from Type 1 to 3, with symptoms of sensorineural hearing loss (SNHL), retinitis pigmentosa (RP), and vestibular dysfunction. There are currently nine confirmed and two suspected USH-causative genes, and a further three candidate loci have been mapped. The proteins encoded by these genes form complexes that play critical roles in the development and maintenance of cellular structures within the inner ear and retina, which have minimal capacity for repair or regeneration. In the cochlea, stereocilia are located on the apical surface of inner ear hair cells (HC) and are responsible for transducing mechanical stimuli from sound pressure waves into chemical signals. These signals are then detected by the auditory nerve fibers, transmitted to the brain and interpreted as sound. Disease-causing mutations in USH genes can destabilize the tip links that bind the stereocilia to each other, and cause defects in protein trafficking and stereocilia bundle morphology, thereby inhibiting mechanosensory transduction. This review summarizes the current knowledge on Usher syndrome with a particular emphasis on mutations in USH genes, USH protein structures, and functional analyses in animal models. Currently, there is no cure for USH. However, the genetic therapies that are rapidly developing will benefit from this compilation of detailed genetic information to identify the most effective strategies for restoring functional USH proteins.

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

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

Spontaneous mutations of the Zpld1 gene in mice cause semicircular canal dysfunction but do not impair gravity receptor or hearing functions

The cupula is a gelatinous membrane overlying the crista ampullaris of the semicircular canal, important for sensing rotation of the head and critical for normal balance. Recently the zona pellucida like domain containing 1 protein (ZPLD1, also known as cupulin) was identified in the cupula of fish. Here, we describe two new spontaneous mutations in the mouse Zpld1 gene, which were discovered by the circling behavior of mutant mice, an indicator of balance dysfunction. The Zpld1 mutant mice exhibited normal hearing function as assessed by auditory brainstem response (ABR) measurements, and their otolithic organs appeared normal. In the inner ear, Zpld1 mRNA expression was detected only in the hair cells and supporting cells of the crista ampullaris. Normal vestibular sensory evoked potential (VsEP) responses and abnormal vestibulo-ocular reflex (VOR) responses demonstrated that the vestibular dysfunction of the Zpld1 mutant mice is caused by loss of sensory input for rotary head movements (detected by cristae ampullaris) and not by loss of input for linear head translations (detected by maculae of the utricle and saccule). Taken together, these results are consistent with ZPLD1 being an important functional component of the cupula, but not tectorial or otoconial membranes.

An update of common autosomal recessive non-syndromic hearing loss genes in Iranian population

Autosomal-recessive genes are responsible for about 80% of the hereditary non-syndromic hearing loss (NSHL) cases. In Iran, due to consanguineous marriages, NSHL is the second most frequent disability after intellectual disability, occurring one in 16 individuals. Enormous heterogeneity in the genetic pathology of hearing loss causes a major challenge in identification of responsible genes. In Iran, GJB2 is responsible for the most cases of pre-lingual and non-syndromic hearing loss (with frequency of 16.7%) which followed by other genes with lower frequency. Although several studies have indicated that a large proportion of both syndromic and non-syndromic hearing loss in Iranian populations are caused by defects in just a few genes, new detection strategies such as NGS (Next-generation sequencing) have increased the spectrum of responsible mutations. However, by applying this technique in Iran patients screening, the role of lots of novel related genes have been reported. In this review, we aim to describe function of these genes and their contribution to non-syndromic genetic hearing loss in Iranian population and we classify the genes by their functions.

Gfi1Cre mice have early onset progressive hearing loss and induce recombination in numerous inner ear non-hair cells

Studies of developmental and functional biology largely rely on conditional expression of genes in a cell type-specific manner. Therefore, the importance of specificity and lack of inherent phenotypes for Cre-driver animals cannot be overemphasized. The Gfi1^Cre mouse is commonly used for conditional hair cell-specific gene deletion/reporter gene activation in the inner ear. Here, using immunofluorescence and flow cytometry, we show that the Gfi1^Cre mice produce a pattern of recombination that is not strictly limited to hair cells within the inner ear. We observe a broad expression of Cre recombinase in the Gfi1^Cre mouse neonatal inner ear, primarily in inner ear resident macrophages, which outnumber the hair cells. We further show that heterozygous Gfi1^Cre mice exhibit an early onset progressive hearing loss as compared with their wild-type littermates. Importantly, vestibular function remains intact in heterozygotes up to 10 months, the latest time point tested. Finally, we detect minor, but statistically significant, changes in expression of hair cell-enriched transcripts in the Gfi1^Cre heterozygous mice cochleae compared with their wild-type littermate controls. Given the broad use of the Gfi1^Cre mice, both for gene deletion and reporter gene activation, these data are significant and necessary for proper planning and interpretation of experiments.

Differential effects of Cdh23(753A) on auditory and vestibular functional aging in C57BL/6J mice

The C57BL/6J (B6) mouse strain carries a cadherin 23 mutation (Cdh23(753A), also known as Ahl), which affects inner ear structures and results in age-related hearing loss. The B6.CAST strain harbors the wild type Cdh23 gene, and hence, the influence of Ahl is absent. The purpose of the present study was to characterize the effect of age and gender on gravity receptor function in B6 and B6.CAST strains and to compare functional aging between auditory and vestibular modalities. Auditory sensitivity declined at significantly faster rates than gravity receptor sensitivity for both strains. Indeed, vestibular functional aging was minimal for both strains. The comparatively smaller loss of macular versus cochlear sensitivity in both the B6 and B6.CAST strains suggests that the contribution of Ahl to the aging of the vestibular system is minimal, and thus very different than its influence on aging of the auditory system. Alternatively, there exist unidentified genes or gene modifiers that serve to slow the degeneration of gravity receptor structures and maintain gravity receptor sensitivity into advanced age.

Tauroursodeoxycholic acid prevents hearing loss and hair cell death in Cdh23(erl/erl) mice

Sensorineural hearing loss has long been the subject of experimental and clinical research for many years. The recently identified novel mutation of the Cadherin23 (Cdh23) gene, Cdh23(erl/erl), was proven to be a mouse model of human autosomal recessive nonsyndromic deafness (DFNB12). Tauroursodeoxycholic acid (TUDCA), a taurine-conjugated bile acid, has been used in experimental research and clinical applications related to liver disease, diabetes, neurodegenerative diseases, and other diseases associated with apoptosis. Because hair cell apoptosis was implied to be the cellular mechanism leading to hearing loss in Cdh23(erl/erl) mice (erl mice), this study investigated TUDCA's otoprotective effects in erl mice: preventing hearing impairment and protecting against hair cell death. Our results showed that systemic treatment with TUDCA significantly alleviated hearing loss and suppressed hair cell death in erl mice. Additionally, TUDCA inhibited apoptotic genes and caspase-3 activation in erl mouse cochleae. The data suggest that TUDCA could be a potential therapeutic agent for human DFNB12.

The Genetics of Deafness in Domestic Animals

Although deafness can be acquired throughout an animal's life from a variety of causes, hereditary deafness, especially congenital hereditary deafness, is a significant problem in several species. Extensive reviews exist of the genetics of deafness in humans and mice, but not for deafness in domestic animals. Hereditary deafness in many species and breeds is associated with loci for white pigmentation, where the cochlear pathology is cochleo-saccular. In other cases, there is no pigmentation association and the cochlear pathology is neuroepithelial. Late onset hereditary deafness has recently been identified in dogs and may be present but not yet recognized in other species. Few genes responsible for deafness have been identified in animals, but progress has been made for identifying genes responsible for the associated pigmentation phenotypes. Across species, the genes identified with deafness or white pigmentation patterns include MITF, PMEL, KIT, EDNRB, CDH23, TYR, and TRPM1 in dog, cat, horse, cow, pig, sheep, ferret, mink, camelid, and rabbit. Multiple causative genes are present in some species. Significant work remains in many cases to identify specific chromosomal deafness genes so that DNA testing can be used to identify carriers of the mutated genes and thereby reduce deafness prevalence.

Gene expression profiles of the cochlea and vestibular endorgans: localization and function of genes causing deafness

OBJECTIVES

We sought to elucidate the gene expression profiles of the causative genes as well as the localization of the encoded proteins involved in hereditary hearing loss.

METHODS

Relevant articles (as of September 2014) were searched in PubMed databases, and the gene symbols of the genes reported to be associated with deafness were located on the Hereditary Hearing Loss Homepage using localization, expression, and distribution as keywords.

RESULTS

Our review of the literature allowed us to systematize the gene expression profiles for genetic deafness in the inner ear, clarifying the unique functions and specific expression patterns of these genes in the cochlea and vestibular endorgans.

CONCLUSIONS

The coordinated actions of various encoded molecules are essential for the normal development and maintenance of auditory and vestibular function.

Fgf10 is required for specification of non-sensory regions of the cochlear epithelium

The vertebrate inner ear is a morphologically complex sensory organ comprised of two compartments, the dorsal vestibular apparatus and the ventral cochlear duct, required for motion and sound detection, respectively. Fgf10, in addition to Fgf3, is necessary for the earliest stage of otic placode induction, but continued expression of Fgf10 in the developing otic epithelium, including the prosensory domain and later in Kolliker׳s organ, suggests additional roles for this gene during morphogenesis of the labyrinth. While loss of Fgf10 was implicated previously in semicircular canal agenesis, we show that Fgf10(-/+) embryos also exhibit a reduction or absence of the posterior semicircular canal, revealing a dosage-sensitive requirement for FGF10 in vestibular development. In addition, we show that Fgf10(-/-) embryos have previously unappreciated defects of cochlear morphogenesis, including a somewhat shortened duct, and, surprisingly, a substantially narrower duct. The mutant cochlear epithelium lacks Reissner׳s membrane and a large portion of the outer sulcus-two non-contiguous, non-sensory domains. Marker gene analyses revealed effects on Reissner׳s membrane as early as E12.5-E13.5 and on the outer sulcus by E15.5, stages when Fgf10 is expressed in close proximity to Fgfr2b, but these effects were not accompanied by changes in epithelial cell proliferation or death. These data indicate a dual role for Fgf10 in cochlear development: to regulate outgrowth of the duct and subsequently as a bidirectional signal that sequentially specifies Reissner׳s membrane and outer sulcus non-sensory domains. These findings may help to explain the hearing loss sometimes observed in LADD syndrome subjects with FGF10 mutations.

Genetics of auditory mechano-electrical transduction

The hair bundles of cochlear hair cells play a central role in the auditory mechano-electrical transduction (MET) process. The identification of MET components and of associated molecular complexes by biochemical approaches is impeded by the very small number of hair cells within the cochlea. In contrast, human and mouse genetics have proven to be particularly powerful. The study of inherited forms of deafness led to the discovery of several essential proteins of the MET machinery, which are currently used as entry points to decipher the associated molecular networks. Notably, MET relies not only on the MET machinery but also on several elements ensuring the proper sound-induced oscillation of the hair bundle or the ionic environment necessary to drive the MET current. Here, we review the most significant advances in the molecular bases of the MET process that emerged from the genetics of hearing.

Identification of CDH23 mutations in Korean families with hearing loss by whole-exome sequencing

Background

Patient genetic heterogeneity renders it difficult to discover disease-cause genes. Whole-exome sequencing is a powerful new strategy that can be used to this end. The purpose of the present study was to identify a hitherto unknown mutation causing autosomal recessive nonsyndromic hearing loss (ARNSHL) in Korean families.

Methods

We performed whole-exome sequencing in 16 individuals from 13 unrelated small families with ARNSHL. After filtering out population-specific polymorphisms, we focused on known deafness genes. Pathogenic effects of the detected mutations on protein structure or function were predicted via in silico analysis.

Results

We identified compound heterozygous CDH23 mutations in hearing-loss genes of two families. These include two previously reported pathological mutations, p.Pro240Leu and p.Glu1595Lys, as well as one novel mutation, p.Asn342Ser. The p.Pro240Leu mutation was found in both families. We also identified 26 non-synonymous variants in CDH23 coding exons from 16 hearing-loss patients and 30 Korean exomes.

Conclusion

The present study is the first to show that CDH23 mutations cause hearing loss in Koreans. Although the precise contribution made by such mutations needs to be determined using a larger patient cohort, our data indicate that mutations in the CDH23 gene are one of the most important causes of non-syndromic hearing loss in East Asians. Further exome sequencing will identify common mutations or polymorphisms and contribute to the molecular diagnosis of, and development of new therapies for, hereditary hearing loss.

Compound heterozygosity of the functionally null Cdh23(v-ngt) and hypomorphic Cdh23(ahl) alleles leads to early-onset progressive hearing loss in mice

The waltzer (v) mouse mutant harbors a mutation in Cadherin 23 (Cdh23) and is a model for Usher syndrome type 1D, which is characterized by congenital deafness, vestibular dysfunction, and prepubertal onset of progressive retinitis pigmentosa. In mice, functionally null Cdh23 mutations affect stereociliary morphogenesis and the polarity of both cochlear and vestibular hair cells. In contrast, the murine Cdh23^ahl allele, which harbors a hypomorphic mutation, causes an increase in susceptibility to age-related hearing loss in many inbred strains. We produced congenic mice by crossing mice carrying the v niigata (Cdh23^v-ngt) null allele with mice carrying the hypomorphic Cdh23^ahl allele on the C57BL/6J background, and we then analyzed the animals’ balance and hearing phenotypes. Although the Cdh23^v-ngt/ahl compound heterozygous mice exhibited normal vestibular function, their hearing ability was abnormal: the mice exhibited higher thresholds of auditory brainstem response (ABR) and rapid age-dependent elevation of ABR thresholds compared with Cdh23^ahl/ahl homozygous mice. We found that the stereocilia developed normally but were progressively disrupted in Cdh23^v-ngt/ahl mice. In hair cells, CDH23 localizes to the tip links of stereocilia, which are thought to gate the mechanoelectrical transduction channels in hair cells. We hypothesize that the reduction of Cdh23 gene dosage in Cdh23^v-ngt/ahl mice leads to the degeneration of stereocilia, which consequently reduces tip link tension. These findings indicate that CDH23 plays an important role in the maintenance of tip links during the aging process.

Myosin light chain kinase regulates hearing in mice by influencing the F-actin cytoskeleton of outer hair cells and cochleae

Myosin light chain kinase (MLCK) phosphorylates myosin regulatory light chains to facilitate its interaction with actin filaments and produce contractile activity. The outer hair cells (OHCs) in the ear contain large amounts of actin and a variety myosins. The stereociliary and somatic motility of OHCs are closely related to hearing. It appears likely that MLCK may play an important role in acoustic trans-duction. In this study, we analyzed, both in vivo and in vitro, the OHCs of mice bearing a specific deletion of the MLCK gene and the OHCs of control mice. The phenotype was assessed by auditory function [acoustic brainstem responses (ABRs) and distortion product otoacoustic emissions (DPOAEs)], inner ear morphology and histology. MLCK-deficient mice aged 6-7 months showed impaired hearing, a 5- to 10-dB sound pressure level (SPL) increase in the ABR thresholds, when responding to clicks and tones of different frequencies (8 and 16 kHz) (P<0.05). The DPOAE amplitudes of 3-month-old MLCK-deficient mice decreased significantly (>10 dB SPL) at low frequencies (4, 5 and 6 kHz). The OHCs in the MLCK-deficient mice increased with abnormal stereocilia. The staining of F-actin and the phosphorylation of the regulatory light chain in MLCK-deficient OHCs was weak. Our results indicate that MLCK may regulate the structure and the motility of stereocilia through F-actin polymerization.

TMHS is an integral component of the mechanotransduction machinery of cochlear hair cells

Hair cells are mechanosensors for the perception of sound, acceleration, and fluid motion. Mechanotransduction channels in hair cells are gated by tip links, which connect the stereocilia of a hair cell in the direction of their mechanical sensitivity. The molecular constituents of the mechanotransduction channels of hair cells are not known. Here, we show that mechanotransduction is impaired in mice lacking the tetraspan TMHS. TMHS binds to the tip-link component PCDH15 and regulates tip-link assembly, a process that is disrupted by deafness-causing Tmhs mutations. TMHS also regulates transducer channel conductance and is required for fast channel adaptation. TMHS therefore resembles other ion channel regulatory subunits such as the transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor regulatory proteins (TARPs) of AMPA receptors that facilitate channel transport and regulate the properties of pore-forming channel subunits. We conclude that TMHS is an integral component of the hair cell's mechanotransduction machinery that functionally couples PCDH15 to the transduction channel.

Cadherins in brain morphogenesis and wiring

Cadherins are Ca(2+)-dependent cell-cell adhesion molecules that play critical roles in animal morphogenesis. Various cadherin-related molecules have also been identified, which show diverse functions, not only for the regulation of cell adhesion but also for that of cell proliferation and planar cell polarity. During the past decade, understanding of the roles of these molecules in the nervous system has significantly progressed. They are important not only for the development of the nervous system but also for its functions and, in turn, for neural disorders. In this review, we discuss the roles of cadherins and related molecules in neural development and function in the vertebrate brain.

Structures of usher syndrome 1 proteins and their complexes

Usher syndrome 1 (USH1) is the most common and severe form of hereditary loss of hearing and vision. Genetic, physiological, and cell biological studies, together with recent structural investigations, have not only uncovered the physiological functions of the five USH1 proteins but also provided mechanistic explanations for the hearing and visual deficiencies in humans caused by USH1 mutations. This review focuses on the structural basis of the USH1 protein complex organization.

Myosin light-chain kinase is necessary for membrane homeostasis in cochlear inner hair cells

The structural homeostasis of the cochlear hair cell membrane is critical for all aspects of sensory transduction, but the regulation of its maintenance is not well understood. In this report, we analyzed the cochlear hair cells of mice with specific deletion of myosin light chain kinase (MLCK) in inner hair cells. MLCK-deficient mice showed impaired hearing, with a 5- to 14-dB rise in the auditory brainstem response (ABR) thresholds to clicks and tones of different frequencies and a significant decrease in the amplitude of the ABR waves. The mutant inner hair cells produced several ball-like structures around the hair bundles in vivo, indicating impaired membrane stability. Inner hair cells isolated from the knockout mice consistently displayed less resistance to hypoosmotic solution and less membrane F-actin. Myosin light-chain phosphorylation was also reduced in the mutated inner hair cells. Our results suggest that MLCK is necessary for maintaining the membrane stability of inner hair cells.

Sensing sound: molecules that orchestrate mechanotransduction by hair cells

Animals use acoustic signals to communicate and to obtain information about their environment. The processing of acoustic signals is initiated at auditory sense organs, where mechanosensory hair cells convert sound-induced vibrations into electrical signals. Although the biophysical principles underlying the mechanotransduction process in hair cells have been characterized in much detail over the past 30 years, the molecular building-blocks of the mechanotransduction machinery have proved to be difficult to determine. We review here recent studies that have both identified some of these molecules and established the mechanisms by which they regulate the activity of the still-elusive mechanotransduction channel.

The sound of silence: mouse models for hearing loss

Sensorineural hearing loss is one of the most common disabilities in humans. It is estimated that about 278 million people worldwide have slight to extreme hearing loss in both ears, which results in an economic loss for the country and personal loss for the individual. It is thus critical to have a deeper understanding of the causes for hearing loss to better manage and treat the affected individuals. The mouse serves as an excellent model to study and recapitulate some of these phenotypes, identify new genes which cause deafness, and to study their roles in vivo and in detail. Mutant mice have been instrumental in elucidating the function and mechanisms of the inner ear. The development and morphogenesis of the inner ear from an ectodermal layer into distinct auditory and vestibular components depends on well-coordinated gene expression and well-orchestrated signaling cascades within the otic vesicle and interactions with surrounding layers of tissues. Any disruption in these pathways can lead to hearing impairment. This review takes a look at some of the genes and their corresponding mice mutants that have shed light on the mechanism governing hearing impairment (HI) in humans.

Auditory and vestibular hair cell stereocilia: relationship between functionality and inner ear disease

The stereocilia of the inner ear are unique cellular structures which correlate anatomically with distinct cochlear functions, including mechanoelectrical transduction, cochlear amplification, adaptation, frequency selectivity and tuning. Their function is impaired by inner ear stressors, by various types of hereditary deafness, syndromic hearing loss and inner ear disease (e.g. Ménière's disease). The anatomical and physiological characteristics of stereocilia are discussed in relation to inner ear malfunctions.

An ENU-induced mutation of Cdh23 causes congenital hearing loss, but no vestibular dysfunction, in mice

Mutations in the human cadherin 23 (CDH23) gene cause deafness, neurosensory, autosomal recessive 12 (DFNB12) nonsyndromic hearing loss or Usher syndrome, type 1D (characterized by hearing impairment, vestibular dysfunction, and visual impairment). Reported waltzer mouse strains each harbor a Cdh23-null mutation and present with hearing loss and vestibular dysfunction. Two additional Cdh23 mouse mutants, salsa and erlong, each carry a homozygous Cdh23 missense mutation and have progressive hearing loss. We report the identification of a novel mouse strain, jera, with inherited hearing loss caused by an N-ethyl-N-nitrosourea-induced c.7079T>A mutation in the Cdh23 gene. The mutation generates a missense change, p.V2360E, in Cdh23. Affected mice have profound sensorineural deafness, with no vestibular dysfunction. The p.V2360E mutation is semidominant because heterozygous mice have milder and more progressive hearing loss in advanced age. The mutation affects a highly conserved Ca(2+)-binding motif in extracellular domain 22, thought to be important for Cdh23 structure and dimerization. Molecular modeling suggests that the Cdh23(V2360E/V2360E) mutation alters the structural conformation of the protein and affects Ca(2+)-binding properties. Similar to salsa mice, but in contrast to waltzer mice, hair bundle development is normal in jera and hearing loss appears to be due to the loss of tip links. Thus, jera is a novel mouse model for DFNB12.

Regulation of PCDH15 function in mechanosensory hair cells by alternative splicing of the cytoplasmic domain

Protocadherin 15 (PCDH15) is expressed in hair cells of the inner ear and in photoreceptors of the retina. Mutations in PCDH15 cause Usher Syndrome (deaf-blindness) and recessive deafness. In developing hair cells, PCDH15 localizes to extracellular linkages that connect the stereocilia and kinocilium into a bundle and regulate its morphogenesis. In mature hair cells, PCDH15 is a component of tip links, which gate mechanotransduction channels. PCDH15 is expressed in several isoforms differing in their cytoplasmic domains, suggesting that alternative splicing regulates PCDH15 function in hair cells. To test this model, we generated three mouse lines, each of which lacks one out of three prominent PCDH15 isoforms (CD1, CD2 and CD3). Surprisingly, mice lacking PCDH15-CD1 and PCDH15-CD3 form normal hair bundles and tip links and maintain hearing function. Tip links are also present in mice lacking PCDH15-CD2. However, PCDH15-CD2-deficient mice are deaf, lack kinociliary links and have abnormally polarized hair bundles. Planar cell polarity (PCP) proteins are distributed normally in the sensory epithelia of the mutants, suggesting that PCDH15-CD2 acts downstream of PCP components to control polarity. Despite the absence of kinociliary links, vestibular function is surprisingly intact in the PCDH15-CD2 mutants. Our findings reveal an essential role for PCDH15-CD2 in the formation of kinociliary links and hair bundle polarization, and show that several PCDH15 isoforms can function redundantly at tip links.

Stereocilin connects outer hair cell stereocilia to one another and to the tectorial membrane

Stereocilin is defective in a recessive form of deafness, DFNB16. We studied the distribution of stereocilin in the developing and mature mouse inner ear and analyzed the consequences of its absence in stereocilin-null (Strc(-/-)) mice that suffer hearing loss starting at postnatal day 15 (P15) and progressing until P60. Using immunofluorescence and immunogold electron microscopy, stereocilin was detected in association with two cell surface specializations specific to outer hair cells (OHCs) in the mature cochlea: the horizontal top connectors that join the apical regions of adjacent stereocilia within the hair bundle, and the attachment links that attach the tallest stereocilia to the overlying tectorial membrane. Stereocilin was also detected around the kinocilium of vestibular hair cells and immature OHCs. In Strc(-/-) mice the OHC hair bundle was structurally and functionally normal until P9. Top connectors, however, did not form and the cohesiveness of the OHC hair bundle progressively deteriorated from P10. The stereocilia were still interconnected by tip links at P14, but these progressively disappeared from P15. By P60 the stereocilia, still arranged in a V-shaped bundle, were fully disconnected from each other. Stereocilia imprints on the lower surface of the tectorial membrane were also not observed in Strc(-/-) mice, thus indicating that the tips of the tallest stereocilia failed to be embedded in this gel. We conclude that stereocilin is essential to the formation of horizontal top connectors. We propose that these links, which maintain the cohesiveness of the mature OHC hair bundle, are required for tip-link turnover.

Hearing loss: a common disorder caused by many rare alleles

Perception of sound is a fundamental role of the auditory system. Traveling with the force of their mechanical energy, sound waves are captured by the ear and activate the sensory pathway of this complex organ. The hair cells, specialized sensory cells within the inner ear, transmit the mechanical energy into electrical nerve stimuli that reach the brain. A large number of proteins are responsible for the overarching tasks required to maintain the complex mechanism of sound sensation. Many hearing disorders are due to single gene defects inherited in a Mendelian fashion, thus enabling clinical diagnostics. However, at the same time, hearing impairment is genetically heterogeneous, with both common and rare forms occurring due to mutations in over 100 genes. The crosstalk between human and mouse genetics has enabled comprehensive studies on gene identification and protein function, taking advantage of the tools animal models have to offer. The aim of the following review is to provide background and examples of human deafness genes and the discovery of their function in the auditory system.

Analysis of subcellular localization of Myo7a, Pcdh15 and Sans in Ush1c knockout mice

Usher syndrome (USH) is the most frequent cause of combined deaf-blindness in man. An important finding from mouse models and molecular studies is that the USH proteins are integrated into a protein network that regulates inner ear morphogenesis. To understand further the function of harmonin in the pathogenesis of USH1, we have generated a targeted null mutation Ush1c mouse model. Here, we examine the effects of null mutation of the Ush1c gene on subcellular localization of Myo7a, Pcdh15 and Sans in the inner ear. Morphology and proteins distributions were analysed in cochlear sections and whole mount preparations from Ush1c(-/-) and Ush1c(-/+) controls mice. We observed the same distribution of Myo7a throughout the cytoplasm in knockout and control mice. However, we detected Pcdh15 at the base of stereocilia and in the cuticular plate in cochlear hair cells from Ush1c(+/-) controls, whereas in the knockout Ush1c(-/-) mice, Pcdh15 staining was concentrated in the apical region of the outer hair cells and no defined staining was detected at the base of stereocilia nor in the cuticular plate. We showed localization of Sans in the stereocilia of controls mouse cochlear hair cells. However, in cochleae from Ush1c(-/-) mice, strong Sans signals were detected towards the base of stereocilia close to their insertion point into the cuticular plate. Our data indicate that the disassembly of the USH1 network caused by absence of harmonin may have led to the mis-localization of the Protocadherin 15 and Sans proteins in the cochlear hair cells of Ush1c(-/-) knockout mice.

A preformed scleral search coil for measuring mouse eye movements

Mice are excellent subjects for use of genetic-manipulation techniques to study the basis of pathological and normal physiology and behavior; however behavioral analyses of associated phenotypes is often limited. To improve the accuracy and specificity of repeated measurements of vestibular function, we developed a miniaturized, contact-lens scleral search coil to measure mouse eye movements. We describe the physical attributes and document its functionality by measuring vestibuloocular responses in normal mice. This coil should greatly improve the sensitivity and documentation of vestibular dysfunction in mouse models of pathology and dysfunction while allowing screening of significant numbers of subjects.

Review series: The cell biology of hearing

Mammals have an astonishing ability to sense and discriminate sounds of different frequencies and intensities. Fundamental for this process are mechanosensory hair cells in the inner ear that convert sound-induced vibrations into electrical signals. The study of genes that are linked to deafness has provided insights into the cell biological mechanisms that control hair cell development and their function as mechanosensors.

Temporary reduction of distortion product otoacoustic emissions (DPOAEs) immediately following auditory brainstem response (ABR)

The hearing status of an experimental animal is typically assessed in the laboratory setting by the combined use of auditory brainstem response (ABR) and distortion product otoacoustic emissions (DPOAEs), carried out in succession, with the former assay preceding the latter. This study reports a cautionary finding that the use of this accepted regimen yields a reduced DPOAE response. When the DPOAEs were performed after ABR testing, transient reduction of the DPOAE amplitudes was observed at all frequencies in both the inbred, C57/B6 and FVB/N, and the outbred, SW mouse strains. DPOAEs were reduced post-ABR in multiple mouse strains which suggests that this finding is not strain-specific but a general consequence of the preceding ABR analysis. The reduction in DPOAE was temporary: when re-tested at one hour, DPOAE amplitudes recovered to pre-ABR levels. In contrast to the ABR's impact on DPOAE response, ABR thresholds were not altered or reduced when preceded immediately by DPOAE measurements. The molecular alterations underlying the ABR-induced transient reduction of DPOAE remain to be determined. To investigate the potential role of reactive oxygen species in post-ABR DPOAE reduction, transgenic mice over-expressing SOD1, the cytoplasmic enzyme critical for removal of superoxide radicals were subjected to the same auditory testing regimen. Similar to their wild type littermates, the SOD1 transgenic mice also demonstrated post-ABR DPOAE reduction, and thus do not support a role for superoxide radicals in transient reduction of DPOAE. While toxic noise exposure is known to negatively impact OAE, transient decrease in DPOAE levels following standard ABR assay has not been previously described. A practical outcome from this study is a recommendation for reversal of the traditional order for carrying out auditory tests, with the OAE measurements preceding ABR assessment, thus ensuring that the DPOAE response is unaffected.

Separate and combined effects of Sod1 and Cdh23 mutations on age-related hearing loss and cochlear pathology in C57BL/6J mice

Both the ahl allele of Cdh23 and the null mutation of Sod1 have been shown to contribute to age-related hearing loss (AHL) in mice, but mixed strain backgrounds have confounded analyses of their individual and combined effects. To test for the effects of Sod1 deficiency independently from those of Cdh23(ahl), we produced mice with four digenic genotypes: Sod1(+/+)Cdh23(ahl)(/ahl), Sod1(+/+)Cdh23(+/+), Sod1(-/-)Cdh23(ahl)(/ahl), and Sod1(-/-)Cdh23(+/+), all on a uniform C57BL(/)6J strain background. We assessed hearing loss by ABR threshold measurements and evaluated cochlear pathologies in age-matched mice of each digenic combination. ABR analysis showed that Sod1(+/+)Cdh23(+/+) mice retain normal hearing up to 15 months of age and that hearing loss of Sod1(+/+)Cdh23(ahl)(/ahl) mice is more age and frequency dependent than that of Sod1(-/-)Cdh23(+/+) mice. ABR results also showed that mice with both gene mutations (Sod1(-/-)Cdh23(ahl)(/ahl)) exhibit the earliest onset and most severe hearing loss, greater than predicted for strictly additive effects. Histological analysis of cochleas showed that hair cell lesions are most severe in Sod1(-)(/-)Cdh23(ahl)(/ahl) mice followed closely by Sod1(+)(/+)Cdh23(ahl)(/ahl) mice and much smaller in Sod1(-)(/-)Cdh23(+)(/+) and Sod1(+)(/+)Cdh23(+)(/+) mice. Despite extensive damage to cochlear hair cells, vestibular hair cells appeared remarkably normal in all strains. Although both Sod1(-/-) and Cdh23(ahl)(/ahl) genotypes had strong effects on hearing loss, the Cdh23(ahl/ahl) genotype was primarily responsible for the increase in hair cell loss, suggesting that the two mutations have different underlying mechanisms of pathology.

Deafness and retinal degeneration in a novel USH1C knock-in mouse model

Usher syndrome is the leading cause of combined deaf-blindness, but the molecular mechanisms underlying the auditory and visual impairment are poorly understood. Usher I is characterized by profound congenital hearing loss, vestibular dysfunction, and progressive retinitis pigmentosa beginning in early adolescence. Using the c.216G>A cryptic splice site mutation in Exon 3 of the USH1C gene found in Acadian Usher I patients in Louisiana, we constructed the first mouse model that develops both deafness and retinal degeneration. The same truncated mRNA transcript found in Usher 1C patients is found in the cochleae and retinas of these knock-in mice. Absent auditory-evoked brainstem responses indicated that the mutant mice are deaf at 1 month of age. Cochlear histology showed disorganized hair cell rows, abnormal bundles, and loss of both inner and outer hair cells in the middle turns and at the base. Retinal dysfunction as evident by an abnormal electroretinogram was seen as early as 1 month of age, with progressive loss of rod photoreceptors between 6 and 12 months of age. This knock-in mouse reproduces the dual sensory loss of human Usher I, providing a novel resource to study the disease mechanism and the development of therapies.

Genetics and pathological mechanisms of Usher syndrome

Usher syndrome (USH) comprises a group of autosomal recessively inherited disorders characterized by a dual sensory impairment of the audiovestibular and visual systems. Three major clinical subtypes (USH type I, USH type II and USH type III) are distinguished on the basis of the severity of the hearing loss, the presence or absence of vestibular dysfunction and the age of onset of retinitis pigmentosa (RP). Since the cloning of the first USH gene (MYO7A) in 1995, there have been remarkable advances in elucidating the genetic basis for this disorder, as evidence for 11 distinct loci have been obtained and genes for 9 of them have been identified. The USH genes encode proteins of different classes and families, including motor proteins, scaffold proteins, cell adhesion molecules and transmembrane receptor proteins. Extensive information has emerged from mouse models and molecular studies regarding pathogenesis of this disorder and the wide phenotypic variation in both audiovestibular and/or visual function. A unifying hypothesis is that the USH proteins are integrated into a protein network that regulates hair bundle morphogenesis in the inner ear. This review addresses genetics and pathological mechanisms of USH. Understanding the molecular basis of phenotypic variation and pathogenesis of USH is important toward discovery of new molecular targets for diagnosis, prevention and treatment of this debilitating disorder.

Ush1c gene expression levels in the ear and eye suggest different roles for Ush1c in neurosensory organs in a new Ush1c knockout mouse

Usher syndrome (USH) is the most common form of deaf-blindness in humans. Molecular characterization revealed that the USH gene products form a macromolecular protein network in hair cells of the inner ear and in photoreceptor cells of the retina via binding to PDZ domains in the scaffold protein harmonin encoded by the Ush1c gene in mice and humans. Although several mouse mutants for the Ush1c gene have been described, we generated a targeted null mutation Ush1c mouse model in which the first four exons of the Ush1c gene were replaced with a reporter gene. Here, we assessed the expression pattern of the reporter gene under control of Ush1c regulatory elements and characterized the phenotype of mice defective for Ush1c. These Ush1 knockout mice are deaf but do not recapitulate vision defects before 10 months of age. Our data show LacZ expression in multiple layers of the retina but in neither outer nor inner segments of the photoreceptor layers in mice bearing the knockout construct at 1-5 months of age. The fact that Ush1c expression is much higher in the ear than in the eye suggests a different role for Ush1c in ear function than in the eye and may explain why Ush1c mutant mice do not recapitulate vision defects.

Adhesive and signaling functions of cadherins and catenins in vertebrate development

Properly regulated intercellular adhesion is critical for normal development of all metazoan organisms. Adherens junctions play an especially prominent role in development because they link the adhesive function of cadherin-catenin protein complexes to the dynamic forces of the actin cytoskeleton, which helps to orchestrate a spatially confined and very dynamic assembly of intercellular connections. Intriguingly, in addition to maintaining intercellular adhesion, cadherin-catenin proteins are linked to several major developmental signaling pathways crucial for normal morphogenesis. In this article we will highlight the key genetic studies that uncovered the role of cadherin-catenin proteins in vertebrate development and discuss the potential role of these proteins as molecular biosensors of external cellular microenvironment that may spatially confine signaling molecules and polarity cues to orchestrate cellular behavior throughout the complex process of normal morphogenesis.

Harmonin mutations cause mechanotransduction defects in cochlear hair cells

In hair cells, mechanotransduction channels are gated by tip links, the extracellular filaments that consist of cadherin 23 (CDH23) and protocadherin 15 (PCDH15) and connect the stereocilia of each hair cell. However, which molecules mediate cadherin function at tip links is not known. Here we show that the PDZ-domain protein harmonin is a component of the upper tip-link density (UTLD), where CDH23 inserts into the stereociliary membrane. Harmonin domains that mediate interactions with CDH23 and F-actin control harmonin localization in stereocilia and are necessary for normal hearing. In mice expressing a mutant harmonin protein that prevents UTLD formation, the sensitivity of hair bundles to mechanical stimulation is reduced. We conclude that harmonin is a UTLD component and contributes to establishing the sensitivity of mechanotransduction channels to displacement.

EHD4 and CDH23 are interacting partners in cochlear hair cells

Cadherin 23 (CDH23), a transmembrane protein localized near the tips of hair cell stereocilia in the mammalian inner ear, is important for delivering mechanical signals to the mechano-electric transducer channels. To identify CDH23-interacting proteins, a membrane-based yeast two-hybrid screen of an outer hair cell (OHC) cDNA library was performed. EHD4, a member of the C-terminal EH domain containing a protein family involved in endocytic recycling, was identified as a potential interactor. To confirm the interaction, we first demonstrated the EHD4 mRNA expression in hair cells using in situ hybridization. Next, we showed that EHD4 co-localizes and co-immunoprecipitates with CDH23 in mammalian cells. Interestingly, the co-immunoprecipitation was found to be calcium-sensitive. To investigate the role of EHD4 in hearing, compound action potentials were measured in EHD4 knock-out (KO) mice. Although EHD4 KO mice have normal hearing sensitivity, analysis of mouse cochlear lysates revealed a 2-fold increase in EHD1, but no increase in EHD2 or EHD3, in EHD4 KO cochleae compared with wild type, suggesting that a compensatory increase in EHD1 levels may account for the absence of a hearing defect in EHD4 KO mice. Taken together, these data indicate that EHD4 is a novel CDH23-interacting protein that could regulate CDH23 trafficking/localization in a calcium-sensitive manner.

Characterization of the Drosophila ortholog of the human Usher Syndrome type 1G protein sans

BACKGROUND

The Usher syndrome (USH) is the most frequent deaf-blindness hereditary disease in humans. Deafness is attributed to the disorganization of stereocilia in the inner ear. USH1, the most severe subtype, is associated with mutations in genes encoding myosin VIIa, harmonin, cadherin 23, protocadherin 15, and sans. Myosin VIIa, harmonin, cadherin 23, and protocadherin 15 physically interact in vitro and localize to stereocilia tips in vivo, indicating that they form functional complexes. Sans, in contrast, localizes to vesicle-like structures beneath the apical membrane of stereocilia-displaying hair cells. How mutations in sans result in deafness and blindness is not well understood. Orthologs of myosin VIIa and protocadherin 15 have been identified in Drosophila melanogaster and their genetic analysis has identified essential roles in auditory perception and microvilli morphogenesis, respectively.

PRINCIPAL FINDINGS

Here, we have identified and characterized the Drosophila ortholog of human sans. Drosophila Sans is expressed in tubular organs of the embryo, in lens-secreting cone cells of the adult eye, and in microvilli-displaying follicle cells during oogenesis. Sans mutants are viable, fertile, and mutant follicle cells appear to form microvilli, indicating that Sans is dispensable for fly development and microvilli morphogenesis in the follicle epithelium. In follicle cells, Sans protein localizes, similar to its vertebrate ortholog, to intracellular punctate structures, which we have identified as early endosomes associated with the syntaxin Avalanche.

CONCLUSIONS

Our work is consistent with an evolutionary conserved function of Sans in vesicle trafficking. Furthermore it provides a significant basis for further understanding of the role of this Usher syndrome ortholog in development and disease.

A mouse model for nonsyndromic deafness (DFNB12) links hearing loss to defects in tip links of mechanosensory hair cells

Deafness is the most common form of sensory impairment in humans and is frequently caused by single gene mutations. Interestingly, different mutations in a gene can cause syndromic and nonsyndromic forms of deafness, as well as progressive and age-related hearing loss. We provide here an explanation for the phenotypic variability associated with mutations in the cadherin 23 gene (CDH23). CDH23 null alleles cause deaf-blindness (Usher syndrome type 1D; USH1D), whereas missense mutations cause nonsyndromic deafness (DFNB12). In a forward genetic screen, we have identified salsa mice, which suffer from hearing loss due to a Cdh23 missense mutation modeling DFNB12. In contrast to waltzer mice, which carry a CDH23 null allele mimicking USH1D, hair cell development is unaffected in salsa mice. Instead, tip links, which are thought to gate mechanotransduction channels in hair cells, are progressively lost. Our findings suggest that DFNB12 belongs to a new class of disorder that is caused by defects in tip links. We propose that mutations in other genes that cause USH1 and nonsyndromic deafness may also have distinct effects on hair cell development and function.

Cadherins and mechanotransduction by hair cells

Mechanotransduction, the conversion of a mechanical stimulus into an electrical signal is crucial for our ability to hear and to maintain balance. Recent findings indicate that two members of the cadherin superfamily are components of the mechanotransduction machinery in sensory hair cells of the vertebrate inner ear. These studies show that cadherin 23 (CDH23) and protocadherin 15 (PCDH15) form several of the extracellular filaments that connect the stereocilia and kinocilium of a hair cell into a bundle. One of these filaments is the tip link that has been proposed to gate the mechanotransduction channel in hair cells. The extracellular domains of CDH23 and PCDH15 differ in their structure from classical cadherins and their cytoplasmic domains bind to distinct effectors, suggesting that evolutionary pressures have shaped the two cadherins for their function in mechanotransduction.

A core cochlear phenotype in USH1 mouse mutants implicates fibrous links of the hair bundle in its cohesion, orientation and differential growth

The planar polarity and staircase-like pattern of the hair bundle are essential to the mechanoelectrical transduction function of inner ear sensory cells. Mutations in genes encoding myosin VIIa, harmonin, cadherin 23, protocadherin 15 or sans cause Usher syndrome type I (USH1, characterized by congenital deafness, vestibular dysfunction and retinitis pigmentosa leading to blindness) in humans and hair bundle disorganization in mice. Whether the USH1 proteins are involved in common hair bundle morphogenetic processes is unknown. Here, we show that mouse models for the five USH1 genetic forms share hair bundle morphological defects. Hair bundle fragmentation and misorientation (25-52 degrees mean kinociliary deviation, depending on the mutant) were detected as early as embryonic day 17. Abnormal differential elongation of stereocilia rows occurred in the first postnatal days. In the emerging hair bundles, myosin VIIa, the actin-binding submembrane protein harmonin-b, and the interstereocilia-kinocilium lateral link components cadherin 23 and protocadherin 15, all concentrated at stereocilia tips, in accordance with their known in vitro interactions. Soon after birth, harmonin-b switched from the tip of the stereocilia to the upper end of the tip link, which also comprises cadherin 23 and protocadherin 15. This positional change did not occur in mice deficient for cadherin 23 or protocadherin 15. We suggest that tension forces applied to the early lateral links and to the tip link, both of which can be anchored to actin filaments via harmonin-b, play a key role in hair bundle cohesion and proper orientation for the former, and in stereociliary elongation for the latter.

Distribution and frequencies of CDH23 mutations in Japanese patients with non-syndromic hearing loss

Mutations in the CDH23 gene are known to be responsible for both Usher syndrome type ID (USH1D) and non-syndromic hearing loss (DFNB12), and the molecular confirmation of the CDH23 gene has become important in the diagnosis of these conditions. The present study was performed to find whether the CDH23 mutations are also responsible for non-syndromic hearing loss in patients in the Japanese population. A total of 51 sequence variants were found in 64 Japanese probands with non-syndromic sensorineural hearing impairment from autosomal recessive families. Among them, at least four missense mutations in six patients from five families were confirmed to be responsible for deafness by segregation study. All mutations detected were missense mutations, corroborating the previous reports regarding DFNB12. The present data confirmed that CDH23 mutations are frequently found and significantly responsible in Japanese. Interestingly, the CDH23 mutation spectrum in Japanese is very different from that found in Caucasians. This Japanese spectrum may be representative of those in Eastern Asian populations and its elucidation is expected to facilitate the molecular diagnosis of DFNB12 and USH1D.