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

Comparison of vestibular function in hereditary hearing loss patients with GJB2, CDH23, and SLC26A4 variants

To investigate the association between hereditary hearing loss and vestibular function, we compared vestibular function and symptoms among patients with GJB2, SLC26A4, and CDH23 variants. Thirty-nine patients with sensory neural hearing loss (11 males and 28 females) with biallelic pathogenic variants in either GJB2, SLC26A4, or CDH23 were included in this study (13 GJB2, 15 SLC26A4, and 11 CDH23). The patients were examined using caloric testing and cervical and ocular vestibular-evoked myogenic potentials (cVEMP and oVEMP). We also compared vestibular function and symptoms between patients with these gene variants and 78 normal-hearing ears without vestibular symptoms as controls. The frequency of semicircular canal hypofunction in caloric testing was higher in patients with SLC26A4 variants (47%) than in those with GJB2 (0%) and CDH23 variants (27%). According to the cVEMP results, 69% of patients with GJB2 variants had saccular hypofunction, a significantly higher proportion than in those carrying other variants (SLC26A4, 20%; CDH23, 18%). In oVEMP, which reflects utricular function, no difference was observed in the frequency of hypofunction among the three genes (GJB2, 15%; SLC26A4, 40%; and CDH23, 36%). Hence, discernable trends indicate vestibular dysfunction associated with each gene.

D-Galactose and Hypoxia Induce the Early Onset of Age-Related Hearing Loss Deterioration in a Mouse Model

BACKGROUND

We previously showed that aging accelerates after 3 months of exposure to hypoxia and environmental change but not genetic modifications. Here, we aimed to simply induce early-onset age-related hearing loss within a short period based on our previous method.

METHODS

We randomly divided 16 C57BL/6 mice into four groups that were maintained under conditions of normoxia and hypoxia with or without injected D-galactose for 2 months. Deteriorated hearing, the expression of age-related factors, and oxidative stress responses were detected using the click and tone burst auditory brainstem response test, reverse transcription-polymerase chain reaction, and by measuring superoxide dismutase (SOD).

RESULTS

The group maintained under hypoxia combined with D-galactose lost hearing particularly at 24 Hz and 32 Hz at 6 weeks compared with the other groups. Aging-related factors were also significantly decreased in the hypoxia and D-galactose groups. However, SOD levels did not significantly differ among the groups.

CONCLUSION

Age-related hearing loss is an environmental disorder induced by chronic oxidative stress associated with genetic backgrounds. Our findings suggested that D-galactose and hypoxia can induce the phenotypes of age-related hearing loss and aging-associated molecules in a murine model within a short time with environmental stimulation alone.

Phenotypic differences in the inner ears of CBA/CaJ and C57BL/6J mice carrying missense and single base pair deletion mutations in the Cdh23 gene

Different mutations in the cadherin 23 (CDH23) gene in different genetic backgrounds have been linked to either syndromic or nonsyndromic forms of deafness in humans. We previously reported a progressive hearing loss (HL) mouse model, the Cdh23^erl/erl mouse, which carries a 208T > C mutation causing an amino acid substitution at S70P in C57BL/6J mice. To investigate the differences in Cdh23 mutation-related HL in different genetic backgrounds, we used the CRISPR/Cas9 system to generate homozygous mice in the CBA/CaJ background that have the same base pair missense mutation (208T > C) (Cdh23^erl2/erl2 ) as Cdh23^erl/erl mice in the C57BL/6J background or a single base pair deletion (235G) (Cdh23^V2J2/V2J2 ) in the Cdh23 gene at exon 5. The two mutant mice exhibit hearing impairment across a broad range of frequencies. The progression of HL in Cdh23^erl2/erl2 mice is slower than that in Cdh23^erl/erl mice. We also found structural abnormalities in the stereocilia of cochlear hair cells in Cdh23^erl2/erl2 and Cdh23^V2J2/V2J2 mice. Cdh23^V2J2/V2J2 mice show signs of vestibular dysfunction in open field behavior and swimming tests. In addition, we observed hair bundle defects in vestibular hair cells in Cdh23^V2J2/V2J2 mice. Our results suggest an interaction between the erl locus and the C57BL/6J background that exacerbates HL in Cdh23^erl/erl mice. Moreover, our study confirms that the Cdh23 gene is essential for normal hearing and balance. These two novel mutant mouse strains provide excellent models for studying CDH23 mutation-related deafness in humans.

Transcriptomic Analyses of Inner Ear Sensory Epithelia in Zebrafish

Analysis of gene expression has the potential to assist in the understanding of multiple cellular processes including proliferation, cell-fate specification, senesence, and activity in both healthy and disease states. Zebrafish model has been increasingly used to understand the process of hearing and the development of the vertebrate auditory system. Within the zebrafish inner ear, there are three otolith organs, each containing a sensory macula of hair cells. The saccular macula is primarily involved in hearing, the utricular macula is primarily involved in balance and the function of the lagenar macula is not completely understood. The goal of this study is to understand the transcriptional differences in the sensory macula associated with different otolith organs with the intention of understanding the genetic mechanisms responsible for the distinct role each organ plays in sensory perception. The sensory maculae of the saccule, utricle, and lagena were dissected out of adult Et(krt4:GFP)^sqet4 zebrafish expressing green fluorescent protein in hair cells for transcriptional analysis. The total RNAs of the maculae were isolated and analyzed by RNA GeneChip microarray. Several of the differentially expressed genes are known to be involved in deafness, otolith development and balance. Gene expression among these otolith organs was very well conserved with less than 10% of genes showing differential expression. Data from this study will help to elucidate which genes are involved in hearing and balance. Furthermore, the findings of this study will assist in the development of the zebrafish model for human hearing and balance disorders. Anat Rec, 303:527-543, 2020. © 2019 American Association for Anatomy.

Mice heterozygous for the Cdh23/Ahl1 mutation show age-related deficits in auditory temporal processing

A mutation in the Cdh23 gene is implicated in both syndromic and nonsyndromic hearing loss in humans and age-related hearing loss in C57BL/6 mice. It is generally assumed that human patients (as well as mouse models) only have a hearing loss phenotype if the mutation is homozygous. However, a major complaint for patients with a hearing disability is a reduced speech intelligibility that may be related to temporal processing deficits rather than just elevated thresholds. In this study, we used the amplitude modulation following response (AMFR) to test whether mice heterozygous for Cdh23^{735A > G} have an auditory phenotype that includes temporal processing deficits. The hearing of mice heterozygous for the Cdh23^{735A > G} mutation was compared with age-matched mice homozygous for either the mutation or the wild type in 3 cohorts of mice of both sexes at 2-3, 6, and 12 months of age. The AMFR technique was used to generate objective hearing thresholds for all mice across their range of hearing and to test their temporal processing. We found a genotype-dependent hearing loss in mice homozygous for the mutation starting at 5-11 weeks of age, an age when mice on the C57BL/6 background are often presumed to have normal hearing. The heterozygous animals retained normal hearing thresholds up to one year of age. Nevertheless, the heterozygous animals showed a decline in temporal processing abilities at one year of age that was independent of their hearing thresholds. These results suggest that mice heterozygous for the Cdh23 mutation do not have truly normal hearing.

PMCA2 pump mutations and hereditary deafness

Hair cells of the inner ear detect sound stimuli, inertial or gravitational forces by deflection of their apical stereocilia. A small number of stereociliary cation-selective mechanotransduction (MET) channels admit K⁺ and Ca²⁺ ions into the cytoplasm promoting hair cell membrane depolarization and, consequently, neurotransmitter release at the cell basolateral pole. Ca²⁺ influx into the stereocilia compartment is counteracted by the unusual w/a splicing variant of plasma-membrane calcium-pump isoform 2 (PMCA2) which, unlike other PMCA2 variants, increases only marginally its activity in response to a rapid variation of the cytoplasmic free Ca²⁺ concentration ([Ca²⁺]_c). Missense mutations of PMCA2w/a cause deafness and loss of balance in humans. Mouse models in which the pump is genetically ablated or mutated show hearing and balance impairment, which correlates with defects in homeostatic regulation of stereociliary [Ca²⁺]_c, decreased sensitivity of mechanotransduction channels to hair bundle displacement and progressive degeneration of the organ of Corti. These results highlight a critical role played by the PMCA2w/a pump in the control of hair cell function and survival, and provide mechanistic insight into the etiology of deafness and vestibular disorders.

Beyond Cell-Cell Adhesion: Sensational Cadherins for Hearing and Balance

Cadherins form a large family of proteins often involved in calcium-dependent cellular adhesion. Although classical members of the family can provide a physical bond between cells, a subset of special cadherins use their extracellular domains to interlink apical specializations of single epithelial sensory cells. Two of these cadherins, cadherin-23 (CDH23) and protocadherin-15 (PCDH15), form extracellular "tip link" filaments that connect apical bundles of stereocilia on hair cells essential for inner-ear mechanotransduction. As these bundles deflect in response to mechanical stimuli from sound or head movements, tip links gate hair-cell mechanosensitive channels to initiate sensory perception. Here, we review the unusual and diverse structural properties of these tip-link cadherins and the functional significance of their deafness-related missense mutations. Based on the structural features of CDH23 and PCDH15, we discuss the elasticity of tip links and models that bridge the gap between the nanomechanics of cadherins and the micromechanics of hair-cell bundles during inner-ear mechanotransduction.

Zooming in on Cadherin-23: Structural Diversity and Potential Mechanisms of Inherited Deafness

Cadherin-23 (CDH23) is an essential component of hair-cell tip links, fine filaments that mediate inner-ear mechanotransduction. The extracellular domain of CDH23 forms about three-fourths of the tip link with 27 extracellular cadherin (EC) repeats that are structurally similar but not identical to each other. Calcium (Ca²⁺) coordination at the EC linker regions is key for tip-link elasticity and function. There are ∼116 sites in CDH23 affected by deafness-causing mutations, many of which alter conserved Ca²⁺-binding residues. Here we present crystal structures showing 18 CDH23 EC repeats, including the most and least conserved, a fragment carrying disease mutations, and EC repeats with non-canonical Ca²⁺-binding motif sequences and unusual secondary structure. Complementary experiments show deafness mutations' effects on stability and affinity for Ca²⁺. Additionally, a model of nine contiguous CDH23 EC repeats reveals helicity and potential parallel dimerization faces. Overall, our studies provide detailed structural insight into CDH23 function in mechanotransduction.

The tip link protein Cadherin-23: From Hearing Loss to Cancer

Cadherin-23 is an atypical member of the cadherin superfamily, with a distinctly long extracellular domain. It has been known to be a part of the tip links of the inner ear mechanosensory hair cells. Several studies have been carried out to understand the role of Cadherin-23 in the hearing mechanism and defects in the CDH23 have been associated with hearing impairment resulting from defective or absence of tip links. Recent studies have highlighted the role of Cadherin-23 in several pathological conditions, including cancer, suggesting the presence of several unknown functions. Initially, it was proposed that Cadherin-23 represents a yet unspecified subtype of Cadherins; however, no other proteins with similar characteristics have been identified, till date. It has a unique cytoplasmic domain that does not bear a β-catenin binding region, but has been demonstrated to mediate cell-cell adhesions. Several protein interacting partners have been identified for Cadherin-23 and the roles of their interactions in various cellular mechanisms are yet to be explored. This review summarizes the characteristics of Cadherin-23 and its roles in several pathologies including cancer.

Germline Mutations in CDH23, Encoding Cadherin-Related 23, Are Associated with Both Familial and Sporadic Pituitary Adenomas

Pituitary adenoma (PA) is one of the most common intracranial neoplasms. Several genetic predisposing factors for PA have been identified, but they account for a small portion of cases. In this study, we sought to identify the PA genetic risk factors by focusing on causative mutations for PAs. Among the 4 affected and 17 asymptomatic members from one family with familial PA, whole-exome sequencing identified cosegregation of the PA phenotype with the heterozygous missense mutation c.4136G>T (p.Arg1379Leu) in cadherin-related 23 (CDH23). This mutation causes an amino acid substitution in the calcium-binding motif of the extracellular cadherin (EC) domains of CDH23 and is predicted to impair cell-cell adhesion. Genomic screening in a total of 12 families with familial PA (20 individuals), 125 individuals with sporadic PA, and 260 control individuals showed that 33% of the families with familial PA (4/12) and 12% of individuals with sporadic PA (15/125) harbored functional CDH23 variants. In contrast, 0.8% of the healthy control individuals (2/260) carried functional CDH23 variants. Gene-based analysis also revealed a significant association between CDH23 genotype and PA (p = 5.54 × 10^-7). Moreover, PA individuals who did not harbor functional CDH23 variants displayed tumors that were larger in size (p = 0.005) and more invasive (p < 0.001). Therefore, mutations in CDH23 are linked with familial and sporadic PA and could play important roles in the pathogenesis of PA.

Application of Mouse Models to Research in Hearing and Balance

Laboratory mice (Mus musculus) have become the major model species for inner ear research. The major uses of mice include gene discovery, characterization, and confirmation. Every application of mice is founded on assumptions about what mice represent and how the information gained may be generalized. A host of successes support the continued use of mice to understand hearing and balance. Depending on the research question, however, some mouse models and research designs will be more appropriate than others. Here, we recount some of the history and successes of the use of mice in hearing and vestibular studies and offer guidelines to those considering how to apply mouse models.

ER stress inhibitor attenuates hearing loss and hair cell death in Cdh23erl/erl mutant mice

Hearing loss is one of the most common sensory impairments in humans. Mouse mutant models helped us to better understand the mechanisms of hearing loss. Recently, we have discovered that the erlong (erl) mutation of the cadherin23 (Cdh23) gene leads to hearing loss due to hair cell apoptosis. In this study, we aimed to reveal the molecular pathways upstream to apoptosis in hair cells to exploit more effective therapeutics than an anti-apoptosis strategy. Our results suggest that endoplasmic reticulum (ER) stress is the earliest molecular event leading to the apoptosis of hair cells and hearing loss in erl mice. We also report that the ER stress inhibitor, Salubrinal (Sal), could delay the progression of hearing loss and preserve hair cells. Our results provide evidence that therapies targeting signaling pathways in ER stress development prevent hair cell apoptosis at an early stage and lead to better outcomes than those targeting downstream factors, such as tip-link degeneration and apoptosis.

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.

A new Otogelin ENU mouse model for autosomal-recessive nonsyndromic moderate hearing impairment

Approximately 10 % of the population worldwide suffers from hearing loss (HL) and about 60 % of persons with early onset HL have hereditary hearing loss due to genetic mutations. Highly efficient mutagenesis in mice with the chemical mutagen, ethylnitrosourea (ENU), associated with relevant phenotypic tools represents a powerful approach in producing mouse models for hearing impairment. A benefit of this strategy is to generate alleles to form a series revealing the full spectrum of gene function in vivo. It can also mimic the range of human mutations and polymorphisms for HL. In the course of a genome ENU mutagenesis program, we selected a new mouse model for hearing defect based on a dysmorphological screen. We identified by gene mapping the mutation responsible for this phenotype and characterized it at the histological level of the inner ear and evaluated the vestibule by following the recommendations of the standard operating procedures, IMPReSS. We have identified and characterized a new recessive allele of the otogelin gene, Otog^vbd/vbd, due to a homozygous one base pair substitution at the splice donor site of intron 29. This mutation leads to a frame-shift and a premature stop codon. We observed a decrease in the amount of sensory cells in the maculae of Otog^vbd/vbd mice as well as an apparent drastically decreased density to almost absence of the otoconial membrane. Compared to Otog^tm1Prs and twister, the two other existing otogelin alleles, the detailed analysis of Otog^vbd/vbd revealed that these mice share some common behavioural characteristics either with Otog^tm1Prs or twister whereas the fine vestibular phenotype and the hearing defect are different. Our results emphasize the importance of detecting and characterizing a new allele of a gene in order to get comprehensive information about the gene function.

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.

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.

Eeyore: a novel mouse model of hereditary deafness

Animal models that recapitulate human disease are proving to be an invaluable tool in the identification of novel disease-associated genes. These models can improve our understanding of the complex genetic mechanisms involved in disease and provide a basis to guide therapeutic strategies to combat these conditions. We have identified a novel mouse model of non-syndromic sensorineural hearing loss with linkage to a region on chromosome 18. Eeyore mutant mice have early onset progressive hearing impairment and show abnormal structure of the sensory epithelium from as early as 4 weeks of age. Ultrastructural and histological analyses show irregular hair cell structure and degeneration of the sensory hair bundles in the cochlea. The identification of new genes involved in hearing is central to understanding the complex genetic pathways involved in the hearing process and the loci at which these pathways are interrupted in people with a genetic hearing loss. We therefore discuss possible candidate genes within the linkage region identified in eeyore that may underlie the deafness phenotype in these mice. Eeyore provides a new model of hereditary sensorineural deafness and will be an important tool in the search for novel deafness genes.

Cauli: a mouse strain with an Ift140 mutation that results in a skeletal ciliopathy modelling Jeune syndrome

Cilia are architecturally complex organelles that protrude from the cell membrane and have signalling, sensory and motility functions that are central to normal tissue development and homeostasis. There are two broad categories of cilia; motile and non-motile, or primary, cilia. The central role of primary cilia in health and disease has become prominent in the past decade with the recognition of a number of human syndromes that result from defects in the formation or function of primary cilia. This rapidly growing class of conditions, now known as ciliopathies, impact the development of a diverse range of tissues including the neural axis, craniofacial structures, skeleton, kidneys, eyes and lungs. The broad impact of cilia dysfunction on development reflects the pivotal position of the primary cilia within a signalling nexus involving a growing number of growth factor systems including Hedgehog, Pdgf, Fgf, Hippo, Notch and both canonical Wnt and planar cell polarity. We have identified a novel ENU mutant allele of Ift140, which causes a mid-gestation embryonic lethal phenotype in homozygous mutant mice. Mutant embryos exhibit a range of phenotypes including exencephaly and spina bifida, craniofacial dysmorphism, digit anomalies, cardiac anomalies and somite patterning defects. A number of these phenotypes can be attributed to alterations in Hedgehog signalling, although additional signalling systems are also likely to be involved. We also report the identification of a homozygous recessive mutation in IFT140 in a Jeune syndrome patient. This ENU-induced Jeune syndrome model will be useful in delineating the origins of dysmorphology in human ciliopathies.

Skeletal ciliopathies are an emerging field of human disease in which skeletal birth defects arise due to abnormal communication between cells. This failure in communication arises following mutation in components of the primary cilia, a hair-like structure present on every cell. The skeletal ciliopathies are debilitating and in severe cases lead to death in early infancy. However, the mechanisms by which these malformations come about remains unclear. Mouse models are often used to delineate the causes of human birth defects and we have identified a model that mimics one of these conditions known as Jeune syndrome. It is the first mouse model with a mutation in the Ift140 gene, and these mice exhibit phenotypes that are often seen in this set of human syndromes. We have complimented this study with the discovery of a patient that presents with Jeune Syndrome resulting from mutation of human IFT140. This model will allow us to explore the role of IFT140 and the primary cilia in normal human development and provide insight into the field of human skeletal ciliopathies.

Characterization of a novel ENU-generated myosin VI mutant mouse strain with congenital deafness and vestibular dysfunction

Myosin VI (Myo6) is known to play an important role in the mammalian auditory and vestibular systems. We have identified a novel N-ethyl-N-nitrosourea mutagenised mouse strain, charlie, carrying an intronic Myo6 splice site mutation. This mutation (IVS5+5G > A) results in skipping of exon 5, and is predicted to cause a frameshift and premature termination of the protein. We detected essentially no Myo6 transcript in tissue from charlie homozygous mutant mice (Myo6(chl/chl)). Myo6(chl/chl) mice exhibit vestibular dysfunction and profound hearing impairment when first tested at four weeks of age. Analysis of vestibular and cochlear hair cells by scanning electron microscopy and immunohistochemistry revealed highly disorganised hair bundles with irregular orientation and kinocilium position at postnatal stage P2-P3. Within a few weeks, the majority of hair cell stereocilia are missing, or fused and elongated, and degeneration of the sensory epithelium occurs. This novel mouse strain will be an important resource in elucidating the role myosin VI plays in the mammalian auditory system, as well as its non-auditory functions.

Inner ear morphology is perturbed in two novel mouse models of recessive deafness

Human MYO7A mutations can cause a variety of conditions involving the inner ear. These include dominant and recessive non-syndromic hearing loss and syndromic conditions such as Usher syndrome. Mouse models of deafness allow us to investigate functional pathways involved in normal and abnormal hearing processes. We present two novel mouse models with mutations in the Myo7a gene with distinct phenotypes. The mutation in Myo7a^I487N/I487N ewaso is located within the head motor domain of Myo7a. Mice exhibit a profound hearing loss and manifest behaviour associated with a vestibular defect. A mutation located in the linker region between the coiled-coil and the first MyTH4 domains of the protein is responsible in Myo7a^F947I/F947I dumbo. These mice show a less severe hearing loss than in Myo7a^I487N/I487N ewaso; their hearing loss threshold is elevated at 4 weeks old, and progressively worsens with age. These mice show no obvious signs of vestibular dysfunction, although scanning electron microscopy reveals a mild phenotype in vestibular stereocilia bundles. The Myo7a^F947I/F947I dumbo strain is therefore the first reported Myo7a mouse model without an overt vestibular phenotype; a possible model for human DFNB2 deafness. Understanding the molecular basis of these newly identified mutations will provide knowledge into the complex genetic pathways involved in the maintenance of hearing, and will provide insight into recessively inherited sensorineural hearing loss in humans.

Identification of three novel hearing loss mouse strains with mutations in the Tmc1 gene

We report the identification of three new mouse models, baringo, nice, and stitch, with recessively inherited sensorineural deafness due to novel mutations in the transmembrane channel-like gene 1 (Tmc1). These strains were generated by N-ethyl-N-nitrosourea mutagenesis. DNA sequence analysis revealed changes in c.545A>G, c.1345T>C, and c.1661G>T, causing p.Y182C, p.Y449H, and p.W554L amino acid substitutions in baringo, nice, and stitch mutants, respectively. The mutations affect amino acid residues that are evolutionarily conserved across species. Similar to the previously reported Beethoven Tmc1 mutant, both p.Y182C and p.W554L are located outside a predicted transmembrane domain, whereas the p.Y449H mutation resides in the predicted transmembrane domain 4. Homozygous stitch-mutant mice have severe hearing loss at the age of 4 weeks and are deaf by the age of 8 weeks, whereas both baringo and nice mutants are profoundly deaf at the age of 4 weeks. None of the strains displays signs of vestibular dysfunction. Scanning electron microscopy revealed degeneration of outer hair cells in the basal region of baringo, nice, and stitch mutants. Immunolocalization studies revealed expression of TMC1 protein in the hair cells, spiral ganglion neurons, supporting cells, and stria ligament in the inner ear. Reduced levels of TMC1 protein were observed in the spiral ligament of mutants when compared with wild-type animals. These three allelic mutants provide valuable models for studying nonsyndromic recessive sensorineural hearing loss (DFNB7/11) in humans.