OTOF mutations revealed by genetic analysis of hearing loss families including a potential temperature sensitive auditory neuropathy allele

AAV-Mediated Gene Therapy Restores Hearing in Patients with DFNB9 Deafness

Mutations in OTOFERLIN (OTOF) lead to the autosomal recessive deafness 9 (DFNB9). The efficacy of adeno-associated virus (AAV)-mediated OTOF gene replacement therapy is extensively validated in Otof-deficient mice. However, the clinical safety and efficacy of AAV-OTOF is not reported. Here, AAV-OTOF is generated using good manufacturing practice and validated its efficacy and safety in mouse and non-human primates in order to determine the optimal injection dose, volume, and administration route for clinical trials. Subsequently, AAV-OTOF is delivered into one cochlea of a 5-year-old deaf patient and into the bilateral cochleae of an 8-year-old deaf patient with OTOF mutations. Obvious hearing improvement is detected by the auditory brainstem response (ABR) and the pure-tone audiometry (PTA) in these two patients. Hearing in the injected ear of the 5-year-old patient can be restored to the normal range at 1 month after AAV-OTOF injection, while the 8-year-old patient can hear the conversational sounds. Most importantly, the 5-year-old patient can hear and recognize speech only through the AAV-OTOF-injected ear. This study is the first to demonstrate the safety and efficacy of AAV-OTOF in patients, expands and optimizes current OTOF-related gene therapy and provides valuable information for further application of gene therapies for deafness.

Slc20a1 and Slc20a2 regulate neuronal plasticity and cognition independently of their phosphate transport ability

In recent years, primary familial brain calcification (PFBC), a rare neurological disease characterized by a wide spectrum of cognitive disorders, has been associated to mutations in the sodium (Na)-Phosphate (Pi) co-transporter SLC20A2. However, the functional roles of the Na-Pi co-transporters in the brain remain still largely elusive. Here we show that Slc20a1 (PiT-1) and Slc20a2 (PiT-2) are the most abundant Na-Pi co-transporters expressed in the brain and are involved in the control of hippocampal-dependent learning and memory. We reveal that Slc20a1 and Slc20a2 are differentially distributed in the hippocampus and associated with independent gene clusters, suggesting that they influence cognition by different mechanisms. Accordingly, using a combination of molecular, electrophysiological and behavioral analyses, we show that while PiT-2 favors hippocampal neuronal branching and survival, PiT-1 promotes synaptic plasticity. The latter relies on a likely Otoferlin-dependent regulation of synaptic vesicle trafficking, which impacts the GABAergic system. These results provide the first demonstration that Na-Pi co-transporters play key albeit distinct roles in the hippocampus pertaining to the control of neuronal plasticity and cognition. These findings could provide the foundation for the development of novel effective therapies for PFBC and cognitive disorders.

Preservation of Distortion Product Otoacoustic Emissions in OTOF -Related Hearing Impairment

OBJECTIVES

Attenuation of otoacoustic emissions over time has been reported for many patients with hearing impairment harboring mutations in the OTOF gene. In this study, the time course of changes of distortion product otoacoustic emissions (DPOAEs) has been analyzed in a cohort of patients in the light of tympanometry results.

DESIGN

The changes of DPOAEs in 16 patients with OTOF -related hearing impairment were retrospectively analyzed.

RESULTS

All but one subject showed DPOAEs bilaterally at the time of diagnosis. Three patients diagnosed as adults still had DPOAEs at ages of 27, 31, and 47 years, respectively. Follow-up was available for 7 children diagnosed at the age of 1 to 3 years, who still showed preservation of DPOAEs at ages of 5 to 16 years. The responses were absent or attenuated in amplitude at some follow-up appointments in association with type B or C tympanograms.

CONCLUSIONS

DPOAEs are preserved much longer than expected in a cohort of patients with OTOF -related hearing impairment. The previously reported loss of DPOAEs may have been caused in some children by increased middle ear impedance due to otitis media.

The natural history, clinical outcomes, and genotype-phenotype relationship of otoferlin-related hearing loss: a systematic, quantitative literature review

Congenital hearing loss affects one in 500 newborns. Sequence variations in OTOF, which encodes the calcium-binding protein otoferlin, are responsible for 1-8% of congenital, nonsyndromic hearing loss and are the leading cause of auditory neuropathy spectrum disorders. The natural history of otoferlin-related hearing loss, the relationship between OTOF genotype and hearing loss phenotype, and the outcomes of clinical practices in patients with this genetic disorder are incompletely understood because most analyses have reported on small numbers of cases with homogeneous OTOF genotypes. Here, we present the first systematic, quantitative literature review of otoferlin-related hearing loss, which analyzes patient-specific data from 422 individuals across 61 publications. While most patients display a typical phenotype of severe-to-profound hearing loss with prelingual onset, 10-15% of patients display atypical phenotypes, including mild-to-moderate, progressive, and temperature-sensitive hearing loss. Patients' phenotypic presentations appear to depend on their specific genotypes. For example, non-truncating variants located in and immediately downstream of the C2E calcium-binding domain are more likely to produce atypical phenotypes. Additionally, the prevalence of certain sequence variants and their associated phenotypes varies between populations due to evolutionary founder effects. Our analyses also suggest otoacoustic emissions are less common in older patients and those with two truncating OTOF variants. Critically, our review has implications for the application and limitations of clinical practices, including newborn hearing screenings, hearing aid trials, cochlear implants, and upcoming gene therapy clinical trials. We conclude by discussing the limitations of available research and recommendations for future studies on this genetic cause of hearing loss.

A 15-year Review of 260 Children With Auditory Neuropathy Spectrum Disorder: II. Management and Outcomes

OBJECTIVES

Following a review of the demographic and clinical characteristics of all pediatric patients diagnosed with auditory neuropathy spectrum disorder (ANSD) by a pediatric health care system from 2005 to 2020, the present report highlights the type and timing of intervention and outcomes in the same 260 patients with ANSD.

DESIGN

This was a retrospective study reviewing the demographic data, medical history, imaging studies, audiological and speech language data, type of audiological intervention (hearing aids or cochlear implants), and mode of communication in 260 pediatric patients diagnosed with ANSD over a 15-year period.

RESULTS

A significant decrease over time in the age at hearing aid fitting was observed. While a similar reduction in the age at implantation occurred over time, cochlear implantation is still rarely performed by 12 months of age in most ANSD patients. Among bilateral ANSD patients fitted with hearing aids, the majority (89.2%) did not benefit from conventional amplification and most received cochlear implants. Some hearing aid benefit for speech and language development was observed in 5.8%, though communication difficulties were persistent and most used a combination of oral and sign language for communication. Only six patients (5%) received significant benefit from their hearing aids for speech and language development.

CONCLUSIONS

This review of ANSD management over a 15-year period reveals that hearing aids are not a viable option to develop speech and language for most infants and children with ANSD. This finding confirms previous reports and suggest that while hearing aid trials are warranted, children must be tracked closely so as to avoid delays in decision making. Cochlear implantation constitutes the major (if not only) rehabilitative intervention that allows for speech perception in patients who do not benefit from conventional amplification.

Otoferlin as a multirole Ca2+ signaling protein: from inner ear synapses to cancer pathways

Humans have six members of the ferlin protein family: dysferlin, myoferlin, otoferlin, fer1L4, fer1L5, and fer1L6. These proteins share common features such as multiple Ca2+-binding C2 domains, FerA domains, and membrane anchoring through their single C-terminal transmembrane domain, and are believed to play a key role in calcium-triggered membrane fusion and vesicle trafficking. Otoferlin plays a crucial role in hearing and vestibular function. In this review, we will discuss how we see otoferlin working as a Ca2+-dependent mechanical sensor regulating synaptic vesicle fusion at the hair cell ribbon synapses. Although otoferlin is also present in the central nervous system, particularly in the cortex and amygdala, its role in brain tissues remains unknown. Mutations in the OTOF gene cause one of the most frequent genetic forms of congenital deafness, DFNB9. These mutations produce severe to profound hearing loss due to a defect in synaptic excitatory glutamatergic transmission between the inner hair cells and the nerve fibers of the auditory nerve. Gene therapy protocols that allow normal rescue expression of otoferlin in hair cells have just started and are currently in pre-clinical phase. In parallel, studies have linked ferlins to cancer through their effect on cell signaling and development, allowing tumors to form and cancer cells to adapt to a hostile environment. Modulation by mechanical forces and Ca2+ signaling are key determinants of the metastatic process. Although ferlins importance in cancer has not been extensively studied, data show that otoferlin expression is significantly associated with survival in specific cancer types, including clear cell and papillary cell renal carcinoma, and urothelial bladder cancer. These findings indicate a role for otoferlin in the carcinogenesis of these tumors, which requires further investigation to confirm and understand its exact role, particularly as it varies by tumor site. Targeting this protein may lead to new cancer therapies.

Temperature-Sensitive Auditory Neuropathy: Report of a Novel Variant of OTOF Gene and Review of Current Literature

Background and objectives: Otoferlin is a multi-C2 domain protein implicated in neurotransmitter-containing vesicle release and replenishment of the cochlear inner hair cell (IHC) synapses. Mutations in the OTOF gene have been associated with two different clinical phenotypes: a prelingual severe-to-profound sensorineural hearing loss (ANSD-DFNB9); and the peculiar temperature-sensitive auditory neuropathy (TS-ANSD), characterized by a baseline mild-to-moderate hearing threshold that worsens to severe-to-profound when the body temperature rises that returns to a baseline a few hours after the temperature has fallen again. The latter clinical phenotype has been described only with a few OTOF variants with an autosomal recessive biallelic pattern of inheritance. Case report: A 7-year-old boy presented a picture compatible with TS-ANSD exacerbated by febrile states or physical exercise with mild-to-moderate hearing loss at low and medium frequencies and a decrease in speech discrimination that worsened with an unfavorable speech-to-noise ratio. Otoacoustic emissions (OAEs) were present whereas auditory brainstem responses (ABRs) evoked by a click or tone-burst were generally absent. No inner ear malformations were described from the CT scan or MRI. Next-generation sequencing (NGS) of the known deafness genes and multi-phasic bioinformatic analyses of the data detected in OTOF a c.2521G>A missense variant and the deletion of 7.4 Kb, which was confirmed by array-comparative genomic hybridization (array-CGH). The proband's parents, who were asymptomatic, were tested by Sanger sequencing and the father presented the c.2521G>A missense variant. Conclusions: The picture presented by the patient was compatible with OTOF-induced TS-ANSD. OTOF has been generally associated with an autosomal recessive biallelic pattern of inheritance; in this clinical report, two pathogenic variants never previously associated with TS-ANSD were described.

Hearing of Otof-deficient mice restored by trans-splicing of N- and C-terminal otoferlin

Mutations to the OTOF gene are among the most common reasons for auditory neuropathy. Although cochlear implants are often effective in restoring sound transduction, there are currently no biological treatments for individuals with variants of OTOF. Previous studies have reported the rescue of hearing in DFNB9 mice using OTOF gene replacement although the efficacy needs improvement. Here, we developed a novel dual-AAV-mediated gene therapy system based on the principles of protein trans-splicing, and we show that this system can reverse bilateral deafness in Otof ^-/- mice after a single unilateral injection. The system effectively expressed exogenous mouse or human otoferlin after injection on postnatal day 0-2. Human otoferlin restored hearing to near wild-type levels for at least 6 months and restored the release of synaptic vesicles in inner hair cells. Our study not only provides a preferential clinical strategy for the treatment of OTOF-related auditory neuropathies, but also describes a route of development for other large-gene therapies and protein engineering techniques.

Current Advances in Gene Therapies of Genetic Auditory Neuropathy Spectrum Disorder

Auditory neuropathy spectrum disorder (ANSD) refers to a range of hearing impairments characterized by an impaired transmission of sound from the cochlea to the brain. This defect can be due to a lesion or defect in the inner hair cell (IHC), IHC ribbon synapse (e.g., pre-synaptic release of glutamate), postsynaptic terminals of the spiral ganglion neurons, or demyelination and axonal loss within the auditory nerve. To date, the only clinical treatment options for ANSD are hearing aids and cochlear implantation. However, despite the advances in hearing-aid and cochlear-implant technologies, the quality of perceived sound still cannot match that of the normal ear. Recent advanced genetic diagnostics and clinical audiology made it possible to identify the precise site of a lesion and to characterize the specific disease mechanisms of ANSD, thus bringing renewed hope to the treatment or prevention of auditory neurodegeneration. Moreover, genetic routes involving the replacement or corrective editing of mutant sequences or defected genes to repair damaged cells for the future restoration of hearing in deaf people are showing promise. In this review, we provide an update on recent discoveries in the molecular pathophysiology of genetic lesions, auditory synaptopathy and neuropathy, and gene-therapy research towards hearing restoration in rodent models and in clinical trials.

Effect of Sensory Modality on Reaction Time in Individuals with Auditory Neuropathy Spectrum Disorder

PURPOSE

To investigate and compare the reaction time of individuals with auditory neuropathy in three modalities, auditory, visual, and audio-visual. The reaction time of individuals with auditory neuropathy was also compared with those with normal hearing. The relationship between reaction time across modalities and the duration of hearing loss in auditory neuropathy was also investigated.

METHODS AND MATERIALS

The reaction time of adults with auditory neuropathy and those with normal hearing was measured in the three modalities using the Choice reaction time task.

RESULTS

The auditory neuropathy group significantly had a longer reaction time than the normal hearing in all modalities. The trend of the mean reaction time differed across groups. Further, a significant difference in reaction time of the auditory neuropathy group was noted between auditory and visual mode, auditory and audio-visual mode. However, no significant difference between visual and audio-visual modalities was noted in reaction time.

CONCLUSION

Significantly longer reaction time in auditory neuropathy is presumed to have resulted from neural conduction delay and impaired processing. The auditory neuropathy group can utilize visual cues for faster processing, and the study recommends an audio-visual mode for their management. In addition, the duration of hearing loss in auditory neuropathy had no relationship with reaction time across all modalities.

Redefining the architecture of ferlin proteins: Insights into multi-domain protein structure and function

Ferlins are complex, multi-domain proteins, involved in membrane trafficking, membrane repair, and exocytosis. The large size of ferlin proteins and the lack of consensus regarding domain boundaries have slowed progress in understanding molecular-level details of ferlin protein structure and function. However, in silico protein folding techniques have significantly enhanced our understanding of the complex ferlin family domain structure. We used RoseTTAFold to assemble full-length models for the six human ferlin proteins (dysferlin, myoferlin, otoferlin, Fer1L4, Fer1L5, and Fer1L6). Our full-length ferlin models were used to obtain objective domain boundaries, and these boundaries were supported by AlphaFold2 predictions. Despite the differences in amino acid sequence between the ferlin proteins, the domain ranges and distinct subdomains in the ferlin domains are remarkably consistent. Further, the RoseTTAFold/AlphaFold2 in silico boundary predictions allowed us to describe and characterize a previously unknown C2 domain, ubiquitous in all human ferlins, which we refer to as C2-FerA. At present, the ferlin domain-domain interactions implied by the full-length in silico models are predicted to have a low accuracy; however, the use of RoseTTAFold and AlphaFold2 as a domain finder has proven to be a powerful research tool for understanding ferlin structure.

Proteomics for comprehensive characterization of extracellular vesicles in neurodegenerative disease

Extracellular vesicles (EVs) are small lipid bilayer particles ubiquitously released by almost every cell type. A specific and selective constituents of EVs loaded with variety of proteins, lipids, small noncoding RNAs, and long non-coding RNAs are reflective of cellular events, type, and physiologic/pathophysiologic status of the cell of origin. Moreover, these molecular contents carry information from the cell of origin to recipient cells, modulating intercellular communication. Recent studies demonstrated that EVs not only play a neuroprotective role by mediating the removal of toxic proteins, but also emerge as an important player in various neurodegenerative disease onset and progression through facilitating of misfolded proteins propagation. For this reason, neurodegenerative disease-associated differences in EV proteome relative to normal EVs can be used to fulfil diagnostic, prognostic, and therapeutic purposes. Nonetheless, characterizing EV proteome obtained from biological samples (brain tissue and body fluids, including urea, blood, saliva, and CSF) is a challenging task. Herein, we review the status of EV proteome profiling and the updated discovery of potential biomarkers for the diagnosis of neurodegenerative disease with an emphasis on the integration of high-throughput advanced mass spectrometry (MS) technologies for both qualitative and quantitative analysis of EVs in different clinical tissue/body fluid samples in past five years.

Prevalence, risk factors, and audiological characteristics of auditory neuropathy

OBJECTIVE

The objective of this study was to determine the prevalence, risk factors, and audiological characteristics of auditory neuropathy spectrum disorder (ANSD) in the pediatric population.

DESIGN

A retrospective review of medical charts was conducted for children visiting two hospitals in Saudi Arabia.

STUDY SAMPLE

Medical records of 1025 patients with sensorineural hearing loss (SNHL) were reviewed. We analyzed the databases for results of audiological examinations, risk factors, and outcomes of intervention including hearing aid (HA) and cochlear implantation (CI).

RESULTS

Out of 1025 children with SNHL, 101 patients (9.85%) were identified to have ANSD. Audiological characteristics of the ANSD group revealed a severe-to-profound degree of hearing loss, all showed type A tympanogram and absent reflexes, absent auditory brainstem response (ABR) findings with present cochlear microphonic while otoacoustic emissions were absent in 54.5% of patients. The most prevalent risk factors for ANSD in this group were family history of hearing loss, consanguinity, hyperbilirubinemia, and low birth weight. Pure tone and speech detection thresholds improved significantly with CI compared to HA use in this sample of patients with ANSD.

CONCLUSION

This study shows that ANSD is not extremely rare among Saudi children with severe to profound hearing loss, with a prevalence of 9.85%.

Genetic etiology of non-syndromic hearing loss in Latin America

Latin America comprises all countries from South and Central America, in addition to Mexico. It is characterized by a complex mosaic of regions with heterogeneous genetic profiles regarding the geographical origin of the ancestors and proportions of admixture between the Native American, European and African components. In the first years following the findings of the role of the GJB2/GJB6 genes in the etiology of hearing loss, most scientific investigations about the genetics of hearing loss in Latin America focused on assessing the frequencies of pathogenic variants in these genes. More recently, modern techniques allowed researchers in Latin America to make exciting contributions to the finding of new candidate genes, novel mechanisms of inheritance in previously known genes, and characterize a wide diversity of variants, many of them unique to Latin America. This review aimed to provide a general landscape of the genetic studies about non-syndromic hearing loss in Latin America and their main scientific contributions. It allows the conclusion that, although there are similar contributions of some genes, such as GJB2/GJB6, when compared to European and North American countries, Latin American populations revealed some peculiarities that indicate the need for tailored strategies of screening and diagnosis to specific geographic regions.

Familial Temperature-Sensitive Auditory Neuropathy: Distinctive Clinical Courses Caused by Variants of the OTOF Gene

Objective: To investigate the clinical course and genetic etiology of familial temperature-sensitive auditory neuropathy (TSAN), which is a very rare subtype of auditory neuropathy (AN) that involves an elevation of hearing thresholds due to an increase in the core body temperature, and to evaluate the genotype-phenotype correlations in a family with TSAN. Methods: Six members of a non-consanguineous Chinese family, including four siblings complaining of communication difficulties when febrile, were enrolled in this study. The clinical and audiological profiles of the four siblings were fully evaluated during both febrile and afebrile episodes, and the genetic etiology of hearing loss (HL) was explored using next-generation sequencing (NGS) technology. Their parents, who had no complaints of fluctuating HL due to body temperature variation, were enrolled for the genetics portion only. Results: Audiological tests during the patients' febrile episodes met the classical diagnostic criteria for AN, including mild HL, poor speech discrimination, preserved cochlear microphonics (CMs), and absent auditory brainstem responses (ABRs). Importantly, unlike the pattern observed in previously reported cases of TSAN, the ABRs and electrocochleography (ECochG) signals of our patients improved to normal during afebrile periods. Genetic analysis identified a compound heterozygous variant of the OTOF gene (which encodes the otoferlin protein), including one previously reported pathogenic variant, c.5098G > C (p.Glu1700Gln), and one novel variant, c.4882C > A (p.Pro1628Thr). Neither of the identified variants affected the C2 domains related to the main function of otoferlin. Both variants faithfully cosegregated with TSAN within the pedigree, suggesting that OTOF is the causative gene of the autosomal recessive trait segregation in this family. Conclusion: The presence of CMs with absent (or markedly abnormal) ABRs is a reliable criterion for diagnosing AN. The severity of the phenotype caused by dysfunctional neurotransmitter release in TSAN may reflect variants that alter the C2 domains of otoferlin. The observations from this study enrich the current understanding of the phenotype and genotype of TSAN and may lay a foundation for further research on its pathogenesis.

Detailed clinical features and genotype-phenotype correlation in an OTOF-related hearing loss cohort in Japan

Mutations in the OTOF gene are a common cause of hereditary hearing loss and the main cause of auditory neuropathy spectrum disorder (ANSD). Although it is reported that most of the patients with OTOF mutations have stable, congenital or prelingual onset severe-to-profound hearing loss, some patients show atypical clinical phenotypes, and the genotype–phenotype correlation in patients with OTOF mutations is not yet fully understood. In this study, we aimed to reveal detailed clinical characteristics of OTOF-related hearing loss patients and the genotype–phenotype correlation. Detailed clinical information was available for 64 patients in our database who were diagnosed with OTOF-related hearing loss. As reported previously, most of the patients (90.6%) showed a “typical” phenotype; prelingual and severe-to-profound hearing loss. Forty-seven patients (73.4%) underwent cochlear implantation surgery and showed successful outcomes; approximately 85–90% of the patients showed a hearing level of 20–39 dB with cochlear implant and a Categories of Auditory Performance (CAP) scale level 6 or better. Although truncating mutations and p.Arg1939Gln were clearly related to severe phenotype, almost half of the patients with one or more non-truncating mutations showed mild-to-moderate hearing loss. Notably, patients with p.His513Arg, p.Ile1573Thr and p.Glu1910Lys showed “true” auditory neuropathy-like clinical characteristics. In this study, we have clarified genotype–phenotype correlation and efficacy of cochlear implantation for OTOF-related hearing loss patients in the biggest cohort studied to date. We believe that the clinical characteristics and genotype–phenotype correlation found in this study will support preoperative counseling and appropriate intervention for OTOF-related hearing loss patients.

The natural history of OTOF-related auditory neuropathy spectrum disorders: a multicenter study

Pathogenic variations in the OTOF gene are a common cause of hearing loss. To refine the natural history and genotype-phenotype correlations of OTOF-related auditory neuropathy spectrum disorders (ANSD), audiograms and distortion product otoacoustic emissions (DPOAEs) were collected from a diverse cohort of individuals diagnosed with OTOF-related ANSD by comprehensive genetic testing and also reported in the literature. Comparative analysis was undertaken to define genotype-phenotype relationships using a Monte Carlo algorithm. 67 audiograms and 25 DPOAEs from 49 unique individuals positive for OTOF-related ANSD were collected. 51 unique OTOF pathogenic variants were identified of which 21 were missense and 30 were loss of function (LoF; nonsense, splice-site, copy number variants, and indels). There was a statistically significant difference in low, middle, and high frequency hearing thresholds between missense/missense and LoF/missense genotypes as compared to LoF/LoF genotypes (average hearing threshold for low, middle and high frequencies 70.9, 76.0, and 73.4 dB vs 88.5, 95.6, and 94.7 dB) via Tukey's test with age as a co-variate (P = 0.0180, 0.0327, and 0.0347, respectively). Hearing declined during adolescence with missense/missense and LoF/missense genotypes, with an annual mid-frequency threshold deterioration of 0.87 dB/year and 1.87 dB/year, respectively. 8.5% of frequencies measured via DPOAE were lost per year in individuals with serial tests. Audioprofiling of OTOF-related ANSD suggests significantly worse hearing with LoF/LoF genotypes. The unique pattern of variably progressive OTOF-related autosomal recessive ANSD may be amenable to gene therapy in selected clinical scenarios.

Molecular genetic landscape of hereditary hearing loss in Pakistan

Approximately 14.5 million Pakistani individuals have a hearing loss and half of these cases may be due to genetic causes. Though significant progress has been made in uncovering genetic variants for recessively inherited nonsyndromic deafness, Pendred syndrome, and Usher syndromes, the same is not true for dominantly inherited hearing loss, most syndromic cases and deafness with complex inheritance patterns. Variants of 57 genes have been reported to cause nonsyndromic recessive deafness in Pakistan, though most are rare. Variants of just five genes GJB2, HGF, MYO7A, SLC26A4, and TMC1 together explain 57% of profound deafness while those of GJB2, MYO15A, OTOF, SLC26A4, TMC1, and TMPRSS3 account for 47% of moderate to severe hearing loss. In contrast, although variants of at least 39 genes have been implicated in different deafness syndromes, their prevalence in the population and the spectrum of mutations have not been explored. Furthermore, research on genetics of deafness has mostly focused on individuals from the Punjab province and needs to be extended to other regions of Pakistan. Identifying the genes and their variants causing deafness in all ethnic groups is important as it will pinpoint rare as well as recurrent mutations. This information may ultimately help in offering genetic counseling and future treatments.

Cochlear Synaptopathy due to Mutations in OTOF Gene May Result in Stable Mild Hearing Loss and Severe Impairment of Speech Perception

OBJECTIVES

Congenital profound hearing loss with preserved cochlear outer hair cell activity (otoacoustic emissions and cochlear microphonic) is the most common phenotype associated with mutations in the OTOF gene. The aim of this study was to investigate the pathophysiological mechanisms behind the auditory dysfunction in five patients (2 adults and 3 children) carrying biallelic mutations in OTOF, who showed an uncommon phenotype of mild hearing impairment associated with severe difficulties in speech perception and delay of language development.

DESIGN

Patients underwent audiometric assessment with pure-tone and speech perception evaluation, and otoacoustic emissions and auditory brainstem response recording. Cochlear potentials were recorded in all subjects through transtympanic electrocochleography in response to clicks delivered in the free field from 120 to 60 dB peak equivalent SPL and were compared to recordings obtained from 20 normally hearing controls and from eight children with profound deafness due to mutations in the OTOF gene. Three patients out of five underwent unilateral cochlear implantation. Speech perception measures and electrically evoked auditory nerve potentials were obtained within 1 year of cochlear implant use.

RESULTS

Pathogenic mutations in the two alleles of OTOF were found in all five patients, and five novel mutations were identified. Hearing thresholds indicated mild hearing loss in four patients and moderate hearing loss in one. Distortion product otoacoustic emissions were recorded in all subjects, whereas auditory brainstem responses were absent in all but two patients, who showed a delayed wave V in one ear. In electrocochleography recordings, cochlear microphonics and summating potentials showed normal latency and peak amplitude, consistently with preservation of both outer and inner hair cell activity. In contrast, the neural compound action potential recorded in normally hearing controls was replaced by a prolonged, low-amplitude negative response. No differences in cochlear potentials were found between OTOF subjects showing mild or profound hearing loss. Electrical stimulation through the cochlear implant improved speech perception and restored synchronized auditory nerve responses in all cochlear implant recipients.

CONCLUSIONS

These findings indicate that disordered synchrony in auditory fiber activity underlies the impairment of speech perception in patients carrying biallelic mutations in OTOF gene who show a stable phenotype of mild hearing loss. Abnormal nerve synchrony with preservation of hearing sensitivity is consistent with selective impairment of vesicle replenishment at the ribbon synapses with relative preservation of synaptic exocytosis. Cochlear implants are effective in restoring speech perception and synchronous activation of the auditory pathway by directly stimulating auditory fibers.

Ultrastructural maturation of the endbulb of Held active zones comparing wild-type and otoferlin-deficient mice

Endbulbs of Held are located in the anteroventral cochlear nucleus and present the first central synapses of the auditory pathway. During development, endbulbs mature functionally to enable rapid and powerful synaptic transmission with high temporal precision. This process is accompanied by morphological changes of endbulb terminals. Loss of the hair cell-specific protein otoferlin (Otof) abolishes neurotransmission in the cochlea and results in the smaller endbulb of Held terminals. Thus, peripheral hearing impairment likely also leads to alterations in the morphological synaptic vesicle (SV) pool size at individual endbulb of Held active zones (AZs). Here, we investigated endbulb AZs in pre-hearing, young, and adult wild-type and Otof^−/− mice. During maturation, SV numbers at endbulb AZs increased in wild-type mice but were found to be reduced in Otof^−/− mice. The SV population at a distance of 0–15 nm was most strongly affected. Finally, overall SV diameters decreased in Otof^−/− animals during maturation.

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Maturation of wt endbulb of Held active zones leads to more synaptic vesicles

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At endbulbs of otoferlin knockout mice, synaptic vesicles decline with age

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Mainly two distinct synaptic vesicle populations are affected

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Synaptic vesicles sizes are reduced in six-month-old otoferlin knockout animals

Novel pathogenic mutations and further evidence for clinical relevance of genes and variants causing hearing impairment in Tunisian population

Introduction

Hearing impairment (HI) is characterized by complex genetic heterogeneity. The evolution of next generation sequencing, including targeted enrichment panels, has revolutionized HI diagnosis.

Objectives

In this study, we investigated genetic causes in 22 individuals with non-GJB2 HI.

Methods

We customized a Haloplex^HS kit to include 30 genes known to be associated with autosomal recessive nonsyndromic HI (ARNSHI) and Usher syndrome in North Africa.

Results

In accordance with the ACMG/AMP guidelines, we report 11 pathogenic variants; as follows; five novel variants including three missense (ESRRB-Tyr295Cys, MYO15A-Phe2089Leu and MYO7A-Tyr560Cys) and two nonsense (USH1C-Gln122Ter and CIB2-Arg104Ter) mutations; two previously reported mutations (OTOF-Glu57Ter and PNPT1-Glu475Gly), but first time identified among Tunisian families; and four other identified mutations namely WHRN-Gly808AspfsX11, SLC22A4-Cys113Tyr and two MYO7A compound heterozygous splice site variants that were previously described in Tunisia. Pathogenic variants in WHRN and CIB2 genes, in patients with convincing phenotype ruling out retinitis pigmentosa, provide strong evidence supporting their association with ARNSHI. Moreover, we shed lights on the pathogenic implication of mutations in PNPT1 gene in auditory function providing new evidence for its association with ARNSHI. Lack of segregation of a previously identified causal mutation OTOA-Val603Phe further supports its classification as variant of unknown significance. Our study reports absence of otoacoustic emission in subjects using bilateral hearing aids for several years indicating the importance of screening genetic alteration in OTOF gene for proper management of those patients.

Conclusion

In conclusion, our findings do not only expand the spectrum of HI mutations in Tunisian patients, but also improve our knowledge about clinical relevance of HI causing genes and variants.

Overloaded Adeno-Associated Virus as a Novel Gene Therapeutic Tool for Otoferlin-Related Deafness

Hearing impairment is the most common sensory disorder in humans. So far, rehabilitation of profoundly deaf subjects relies on direct stimulation of the auditory nerve through cochlear implants. However, in some forms of genetic hearing impairment, the organ of Corti is structurally intact and therapeutic replacement of the mutated gene could potentially restore near natural hearing. In the case of defects of the otoferlin gene (OTOF), such gene therapy is hindered by the size of the coding sequence (~6 kb) exceeding the cargo capacity (<5 kb) of the preferred viral vector, adeno-associated virus (AAV). Recently, a dual-AAV approach was used to partially restore hearing in deaf otoferlin knock-out (Otof-KO) mice. Here, we employed in vitro and in vivo approaches to assess the gene-therapeutic potential of naturally-occurring and newly-developed synthetic AAVs overloaded with the full-length Otof coding sequence. Upon early postnatal injection into the cochlea of Otof-KO mice, overloaded AAVs drove specific expression of otoferlin in ~30% of all IHCs, as demonstrated by immunofluorescence labeling and polymerase chain reaction. Recordings of auditory brainstem responses and a behavioral assay demonstrated partial restoration of hearing. Together, our results suggest that viral gene therapy of DFNB9—using a single overloaded AAV vector—is indeed feasible, reducing the complexity of gene transfer compared to dual-AAV approaches.

The Many Faces of DFNB9: Relating OTOF Variants to Hearing Impairment

The OTOF gene encodes otoferlin, a critical protein at the synapse of auditory sensory cells, the inner hair cells (IHCs). In the absence of otoferlin, signal transmission of IHCs fails due to impaired release of synaptic vesicles at the IHC synapse. Biallelic pathogenic and likely pathogenic variants in OTOF predominantly cause autosomal recessive profound prelingual deafness, DFNB9. Due to the isolated defect of synaptic transmission and initially preserved otoacoustic emissions (OAEs), the clinical characteristics have been termed "auditory synaptopathy". We review the broad phenotypic spectrum reported in patients with variants in OTOF that includes milder hearing loss, as well as progressive and temperature-sensitive hearing loss. We highlight several challenges that must be addressed for rapid clinical and genetic diagnosis. Importantly, we call for changes in newborn hearing screening protocols, since OAE tests fail to diagnose deafness in this case. Continued research appears to be needed to complete otoferlin isoform expression characterization to enhance genetic diagnostics. This timely review is meant to sensitize the field to clinical characteristics of DFNB9 and current limitations in preparation for clinical trials for OTOF gene therapies that are projected to start in 2021.

FGF2 Affects Parkinson's Disease-Associated Molecular Networks Through Exosomal Rab8b/Rab31

Ras-associated binding (Rab) proteins are small GTPases that regulate the trafficking of membrane components during endocytosis and exocytosis including the release of extracellular vesicles (EVs). Parkinson’s disease (PD) is one of the most prevalent neurodegenerative disorder in the elderly population, where pathological proteins such as alpha-synuclein (α-Syn) are transmitted in EVs from one neuron to another neuron and ultimately across brain regions, thereby facilitating the spreading of pathology. We recently demonstrated fibroblast growth factor-2 (FGF2) to enhance the release of EVs and delineated the proteomic signature of FGF2-triggered EVs in cultured primary hippocampal neurons. Out of 235 significantly upregulated proteins, we found that FGF2 specifically enriched EVs for the two Rab family members Rab8b and Rab31. Consequently, we investigated the interactions of Rab8b and Rab31 using a network analysis approach in order to estimate the global influence of their enrichment in EVs. To achieve this, we have demarcated a protein–protein interaction network (PPiN) for these Rabs and identified the proteins associated with PD in various cellular components of the central nervous system (CNS), in different brain regions, and in the enteric nervous system (ENS). A total of 126 direct or indirect interactions were reported for two Rab candidates, out of which 114 are Rab8b interactions and 54 are Rab31 interactions, ultimately resulting in an individual interaction score (IS) of 90.48 and 42.86%, respectively. Conclusively, these results for the first time demonstrate the relevance of FGF2-induced Rab-enrichment in EVs and its potential to regulate PD pathophysiology.

[OTOF-related auditory neuropathy spectrum disorder]

Otoferlin (OTOF) gene mutations are the most common cause of hereditary ANSD according to investigations in several countries.

THE AIM

Of this study was to estimate the prevalence of OTOF mutations in Russian children with ANSD and evaluate audiological and clinical features of OTOF-related ANSD.

PATIENTS AND METHODS

28 children with bilateral ANSDwere enrolled in the investigation. Two step genetic testing was performed: first step - GJB2 gene testing to exclude GJB2-related hearing loss; second step - NGS-based sequencing to explore another 35 hearing loss genes (including OTOF).

RESULTS

OTOF mutations, including 6 new variants, were found in 5 children with ANSD (18%). All 5 children had no risk factors for hearing loss and passed hearing screening. OAE and cochlear microphonics were present till the last testing at the age of 4-5 years. ABR were not detectable. The ASSR were measurable bilaterally at all frequencies in all cases, but they did not correlate with behavioral thresholds that revealed severe hearing loss. Hearing thresholds were stable during follow up period. 3 children underwent cochlear implantation. After cochlear implantation auditory nerve action potentials to electric stimulation were detected within normal range.

CONCLUSION

Genetic testing of children with ANSD and first of all OTOF testing enables to reveal hearing loss etiology and provide the optimal rehabilitation approach, including cochlear implantation, as early as possible.

An integrative approach for pediatric auditory neuropathy spectrum disorders: revisiting etiologies and exploring the prognostic utility of auditory steady-state response

Auditory neuropathy is an important entity in childhood sensorineural hearing loss. Due to diverse etiologies and clinical features, the management is often challenging. This study used an integrative patient-history, audiologic, genetic, and imaging-based approach to investigate the etiologies and audiologic features of 101 children with auditory neuropathy. Etiologically, 48 (47.5%), 16 (15.8%), 11 (10.9%), and 26 (25.7%) children were categorized as having acquired, genetic, cochlear nerve deficiency-related, and indefinite auditory neuropathy, respectively. The most common causes of acquired and genetic auditory neuropathy were prematurity and OTOF mutations, respectively. Patients with acquired auditory neuropathy presented hearing loss earlier (odds ratio, 10.2; 95% confidence interval, 2.2–47.4), whereas patients with genetic auditory neuropathy had higher presence rate of distortion product otoacoustic emissions (odds ratio, 10.7; 95% confidence interval, 1.3–85.4). In patients with different etiologies or pathological sites, moderate to strong correlations (Pearson’s r = 0.51–0.83) were observed between behavioral thresholds and auditory steady-state response thresholds. In conclusion, comprehensive assessments can provide etiological clues in ~75% of the children with auditory neuropathy. Different etiologies are associated with different audiologic features, and auditory steady-state responses might serve as an objective measure for estimating behavioral thresholds.

Positive Selection and Inactivation in the Vision and Hearing Genes of Cetaceans

The transition to an aquatic lifestyle in cetaceans (whales and dolphins) resulted in a radical transformation in their sensory systems. Toothed whales acquired specialized high-frequency hearing tied to the evolution of echolocation, whereas baleen whales evolved low-frequency hearing. More generally, all cetaceans show adaptations for hearing and seeing underwater. To determine the extent to which these phenotypic changes have been driven by molecular adaptation, we performed large-scale targeted sequence capture of 179 sensory genes across the Cetacea, incorporating up to 54 cetacean species from all major clades as well as their closest relatives, the hippopotamuses. We screened for positive selection in 167 loci related to vision and hearing and found that the diversification of cetaceans has been accompanied by pervasive molecular adaptations in both sets of genes, including several loci implicated in nonsyndromic hearing loss. Despite these findings, however, we found no direct evidence of positive selection at the base of odontocetes coinciding with the origin of echolocation, as found in studies examining fewer taxa. By using contingency tables incorporating taxon- and gene-based controls, we show that, although numbers of positively selected hearing and nonsyndromic hearing loss genes are disproportionately high in cetaceans, counts of vision genes do not differ significantly from expected values. Alongside these adaptive changes, we find increased evidence of pseudogenization of genes involved in cone-mediated vision in mysticetes and deep-diving odontocetes.

Auditory Neuropathy Spectrum Disorders: From Diagnosis to Treatment: Literature Review and Case Reports

Auditory neuropathy spectrum disorder (ANSD) refers to a range of hearing impairments characterized by deteriorated speech perception, despite relatively preserved pure-tone detection thresholds. Affected individuals usually present with abnormal auditory brainstem responses (ABRs), but normal otoacoustic emissions (OAEs). These electrophysiological characteristics have led to the hypothesis that ANSD may be caused by various dysfunctions at the cochlear inner hair cell (IHC) and spiral ganglion neuron (SGN) levels, while the activity of outer hair cells (OHCs) is preserved, resulting in discrepancies between pure-tone and speech comprehension thresholds. The exact prevalence of ANSD remains unknown; clinical findings show a large variability among subjects with hearing impairment ranging from mild to profound hearing loss. A wide range of prenatal and postnatal etiologies have been proposed. The study of genetics and of the implicated sites of lesion correlated with clinical findings have also led to a better understanding of the molecular mechanisms underlying the various forms of ANSD, and may guide clinicians in better screening, assessment and treatment of ANSD patients. Besides OAEs and ABRs, audiological assessment includes stapedial reflex measurements, supraliminal psychoacoustic tests, electrocochleography (ECochG), auditory steady-state responses (ASSRs) and cortical auditory evoked potentials (CAEPs). Hearing aids are indicated in the treatment of ANSD with mild to moderate hearing loss, whereas cochlear implantation is the first choice of treatment in case of profound hearing loss, especially in case of IHC presynaptic disorders, or in case of poor auditory outcomes with conventional hearing aids.

Novel OTOF pathogenic variant segregating with non-syndromic hearing loss in a consanguineous family from tribal Rajouri in Jammu and Kashmir

BACKGROUND

Hereditary hearing loss is characterized by a very high genetic heterogeneity. The OTOF (Locus: DFNB9), encoding otoferlin, is reported to be one of the major causes of non-syndromic hearing loss, and is also reported to be the most common cause of non-syndromic recessive auditory neuropathy spectrum disorder.

METHODS

In this study, whole exome sequencing was employed for detection of novel pathogenic variant that segregates with autosomal recessive nonsyndromic hearing loss in a tribal family from Rajouri, Jammu and Kashmir. Proband was a 9-year-old male born to first-cousin parents and presented with sensorineural hearing loss since birth. Family resides in an area with high consanguinity and lack of basic health care facilities including genetic counselling services.

RESULTS

We report a novel OTOF pathogenic variant NM_194248.2:c.4249_4250insG (p.Ser1417CysfsTer4) co-segregating with hearing loss in this family and not present in any public databases.

CONCLUSIONS

Our findings not only extend the geographical and mutational spectrum of autosomal recessive nonsyndromic hearing loss but also support the need for introducing genetic counselling services to rural and tribal areas in India with high consanguinity.

Functions of Vertebrate Ferlins

Ferlins are multiple-C2-domain proteins involved in Ca²⁺-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

Hair Cell Afferent Synapses: Function and Dysfunction

To provide a meaningful representation of the auditory landscape, mammalian cochlear hair cells are optimized to detect sounds over an incredibly broad range of frequencies and intensities with unparalleled accuracy. This ability is largely conferred by specialized ribbon synapses that continuously transmit acoustic information with high fidelity and sub-millisecond precision to the afferent dendrites of the spiral ganglion neurons. To achieve this extraordinary task, ribbon synapses employ a unique combination of molecules and mechanisms that are tailored to sounds of different frequencies. Here we review the current understanding of how the hair cell's presynaptic machinery and its postsynaptic afferent connections are formed, how they mature, and how their function is adapted for an accurate perception of sound.

Expert specification of the ACMG/AMP variant interpretation guidelines for genetic hearing loss

Due to the high genetic heterogeneity of hearing loss (HL), current clinical testing includes sequencing large numbers of genes, which often yields a significant number of novel variants. Therefore, the standardization of variant interpretation is crucial to provide consistent and accurate diagnoses. The Hearing Loss Variant Curation Expert Panel was created within the Clinical Genome Resource to provide expert guidance for standardized genomic interpretation in the context of HL. As one of its major tasks, our Expert Panel has adapted the American College of Medical Genetics and Genomics/Association for Molecular Pathology (ACMG/AMP) guidelines for the interpretation of sequence variants in HL genes. Here, we provide a comprehensive illustration of the newly specified ACMG/AMP HL rules. Three rules remained unchanged, four rules were removed, and the remaining 21 rules were specified. These rules were further validated and refined using a pilot set of 51 variants assessed by curators and disease experts. Of the 51 variants evaluated in the pilot, 37% (19/51) changed category based upon application of the expert panel specified rules and/or aggregation of evidence across laboratories. These HL-specific ACMG/AMP rules will help standardize variant interpretation, ultimately leading to better care for individuals with HL.

A dual-AAV approach restores fast exocytosis and partially rescues auditory function in deaf otoferlin knock-out mice

Normal hearing and synaptic transmission at afferent auditory inner hair cell (IHC) synapses require otoferlin. Deafness DFNB9, caused by mutations in the OTOF gene encoding otoferlin, might be treated by transferring wild‐type otoferlin cDNA into IHCs, which is difficult due to the large size of this transgene. In this study, we generated two adeno‐associated viruses (AAVs), each containing half of the otoferlin cDNA. Co‐injecting these dual‐AAV2/6 half‐vectors into the cochleae of 6‐ to 7‐day‐old otoferlin knock‐out (Otof^−/−) mice led to the expression of full‐length otoferlin in up to 50% of IHCs. In the cochlea, otoferlin was selectively expressed in auditory hair cells. Dual‐AAV transduction of Otof^−/−IHCs fully restored fast exocytosis, while otoferlin‐dependent vesicle replenishment reached 35–50% of wild‐type levels. The loss of 40% of synaptic ribbons in these IHCs could not be prevented, indicating a role of otoferlin in early synapse maturation. Acoustic clicks evoked auditory brainstem responses with thresholds of 40–60 dB. Therefore, we propose that gene delivery mediated by dual‐AAV vectors might be suitable to treat deafness forms caused by mutations in large genes such as OTOF.

Dual AAV-mediated gene therapy restores hearing in a DFNB9 mouse model

Autosomal recessive genetic forms (DFNB) account for most cases of profound congenital deafness. Adeno-associated virus (AAV)-based gene therapy is a promising therapeutic option, but is limited by a potentially short therapeutic window and the constrained packaging capacity of the vector. We focus here on the otoferlin gene underlying DFNB9, one of the most frequent genetic forms of congenital deafness. We adopted a dual AAV approach using two different recombinant vectors, one containing the 5' and the other the 3' portions of otoferlin cDNA, which exceed the packaging capacity of the AAV when combined. A single delivery of the vector pair into the mature cochlea of Otof ^-/- mutant mice reconstituted the otoferlin cDNA coding sequence through recombination of the 5' and 3' cDNAs, leading to the durable restoration of otoferlin expression in transduced cells and a reversal of the deafness phenotype, raising hopes for future gene therapy trials in DFNB9 patients.

Mutational and phenotypic spectrum of OTOF-related auditory neuropathy in Koreans: eliciting reciprocal interaction between bench and clinics

BACKGROUND

While auditory neuropathy spectrum disorder (ANSD) is a heterogeneous disorder and its management quite varies depending upon the etiology, even including self-resolution, OTOF is an important molecular etiology of prelingual ANSD and has emerged as an attractive target for implementation of precision medicine in terms of timing and prognosis prediction of auditory rehabilitation. However, to date, the literature is lacking in the genotype-phenotype relationship of this gene as well as efficient molecular testing strategy in the clinic in many populations and to make things more complicated in Koreans, the most prevalent variant p.Arg1939Gln among Korean ANSD children frequently evaded detection by next generation sequencing (NGS), resulting in delayed genetic diagnosis and late cochlear implantation (CI). The aims of this study are to document the mutational and phenotypic spectrum of OTOF-related ANSD (DFNB9) in the Korean population, further establishing genotype-phenotype correlation and proposing a set of the most commonly found OTOF variants to be screened first.

METHODS

Genetic diagnosis through the NGS-based sequencing was made on patients with ANSD in two tertiary hospitals. Genotype and phenotypes of eleven DFNB9 patients were reviewed. For data analysis, Mann-Whitney test and Fisher's exact test were applied.

RESULTS

This study disclosed four prevalent variants in Koreans: p.Arg1939Gln with an allele frequency of 40.9%, p.Glu841Lys (13.6%), p.Leu1011Pro and p.Arg1856Trp (9.1%). Three novel variants (c.4227 + 5G > C, p.Gly1845Glu, and p.Pro1931Thr) were identified. Interestingly, a significant association of p.Arg1939Gln with worse ASSR thresholds was observed despite consistently no ABR response. Ten of 11 DFNB9 patients received CI for auditory rehabilitation, showing favorable outcomes with more rapid improvement on early-CI group (age at CI ≤ 18 mo.) than late-CI group.

CONCLUSIONS

This study included the largest Korean DFNB9 cohort to date and proposed a set of the most frequent four OTOF variants, allowing the potential prioritization of exons during Sanger sequencing. Further, a significant association of p.Arg1939Gln homozygotes with poor residual hearing was observed. We may have to suspect p.Arg1939Gln homozygosity in cases of poor auditory thresholds in ANSD children with putative negative OTOF variants solely screened by NGS. Reciprocal feedback between bench and clinics regarding DFNB9 would complement each other.

Balancing presynaptic release and endocytic membrane retrieval at hair cell ribbon synapses

The timely and reliable processing of auditory and vestibular information within the inner ear requires highly sophisticated sensory transduction pathways. On a cellular level, these demands are met by hair cells, which respond to sound waves - or alterations in body positioning - by releasing glutamate-filled synaptic vesicles (SVs) from their presynaptic active zones with unprecedented speed and exquisite temporal fidelity, thereby initiating the auditory and vestibular pathways. In order to achieve this, hair cells have developed anatomical and molecular specializations, such as the characteristic and name-giving 'synaptic ribbons' - presynaptically anchored dense bodies that tether SVs prior to release - as well as other unique or unconventional synaptic proteins. The tightly orchestrated interplay between these molecular components enables not only ultrafast exocytosis, but similarly rapid and efficient compensatory endocytosis. So far, the knowledge of how endocytosis operates at hair cell ribbon synapses is limited. In this Review, we summarize recent advances in our understanding of the SV cycle and molecular anatomy of hair cell ribbon synapses, with a focus on cochlear inner hair cells.

Genetic basis of hearing loss in Spanish, Hispanic and Latino populations

Hearing loss (HL) is the most common neurosensory disorder affecting humans. The screening, prevention and treatment of HL require a better understanding of the underlying molecular mechanisms. Genetic predisposition is one of the most common factors that leads to HL. Most HL studies include few Spanish, Hispanic and Latino participants, leaving a critical gap in our understanding about the prevalence, impact, unmet health care needs, and genetic factors associated with hearing impairment among Spanish, Hispanic and Latino populations. The few studies which have been performed show that the gene variants commonly associated with HL in non-Spanish and non-Hispanic populations are infrequently responsible for hearing impairment in Spanish as well as Hispanic and Latino populations (hereafter referred to as Hispanic). To design effective screening tools to detect HL in Spanish and Hispanic populations, studies must be conducted to determine the gene variants that are most commonly associated with hearing impairment in this racial/ethnic group. In this review article, we summarize gene variants and loci associated with HL in Spanish and Hispanic populations. Identifying new genetic variants associated with HL in Spanish and Hispanic populations will pave the way to develop effective screening tools and therapeutic strategies for HL.

Mutations in OTOF, CLDN14 & SLC26A4 genes as major causes of hearing impairment in Dhadkai village, Jammu & Kashmir, India

Background & objectives:

A high incidence of hearing impairment is reported from the village of Dhadkai in the State of Jammu and Kashmir, India. Prevalence of endogamy in this community suggested a common genetic basis for the disorder. A genetic study was undertaken to ascertain the basis for the high incidence of hearing impairment in this region.

Methods:

In a two-step approach to identify the causative mutation/s, a whole-genome-based linkage analysis of an extended family of 45 members was carried out, which included 23 affected and 22 unaffected members. Mutational analysis for the candidate deafness genes helped reveal causative mutations in the family. In addition, seven deafness-causing genes, Cx26, SLC26A4, CLDN14, TMPRSS3, TMC1, TMIE and USH1C, were analyzed in smaller families with hearing impairment.

Results:

In the 45-member extended family, the critical chromosomal region mapped to 2p24-p22. The c.2122C>T (p.R708X) mutation in OTOF in 2p24-p22was identified as being the causal change. Linkage to 2p24-p22 locus was not observed in a particular branch of this extended family. Analysis of seven known deafness-causing genes in this branch revealed a mutation, c.254T>A (p.V85D), in CLDN14. Among seven small families unrelated to the 45-member extended family, hearing loss was attributable to p.R708X in OTOF in three families and to p.V85D in CLDN14 in one family; a new mutation c.1668T>A (p.Y556X) SLC26A4 was identified in two families and the causative change could not be identified in one family.

Interpretation & conclusions:

This study suggested considerable genetic heterogeneity in the causation of hearing loss in Dhadkai. Recessive mutations were observed in at least three genes causing hearing loss: OTOF (p.R708X), SLC26A4 (p.Y556X) and CLDN14 (p.V85D). Mutation p.R708X appeared to be the major cause of hearing impairment in Dhadkai.

AUNA2: A Novel Type of Non-Syndromic Slowly Progressive Auditory Synaptopathy/Auditory Neuropathy with Autosomal-Dominant Inheritance

BACKGROUND

Auditory synaptopathy/neuropathy (AS/AN) is a heterogeneous disorder, which may be caused by environmental factors like postnatal hyperbilirubinemia or by genetic factors. The genetic forms are subdivided into syndromic and non-syndromic types, and show different inheritance patterns with a strong preponderance of autosomal-recessive forms. To date, only a single locus for non-syndromic autosomal-dominant AS/AN (AUNA1) has been reported in a single family, in which a non-coding DIAPH3 mutation was subsequently described as causative.

MATERIALS AND METHODS

Here, we report detailed clinical data on a large German AS/AN family with slowly progressive postlingual hearing loss. Affected family members developed their first symptoms in their second decade. Moderate hearing loss in the fourth decade then progressed to profound hearing impairment in older family members. Comprehensive audiological and neurological tests were performed in the affected family members. Genetic testing comprised linkage analyses with polymorphic markers and a genome-wide linkage analysis using the Affymetrix GeneChip® Human Mapping 250K.

RESULTS AND CONCLUSION

We identified a large family with autosomal-dominant AS/AN. By means of linkage analyses, the AUNA1 locus was excluded, and putatively linked regions on chromosomal bands 12q24 and 13q34 were identified as likely carrying the second locus for autosomal-dominant AS/AN (AUNA2). AUNA2 is associated with a slowly progressive postlingual hearing loss without any evidence for additional symptoms in other organ systems.

Novel compound heterozygous mutations in the OTOF Gene identified by whole-exome sequencing in auditory neuropathy spectrum disorder

Background

Many hearing-loss diseases are demonstrated to have Mendelian inheritance caused by mutations in single gene. However, many deaf individuals have diseases that remain genetically unexplained. Auditory neuropathy is a sensorineural deafness in which sounds are able to be transferred into the inner ear normally but the transmission of the signals from inner ear to auditory nerve and brain is injured, also known as auditory neuropathy spectrum disorder (ANSD). The pathogenic mutations of the genes responsible for the Chinese ANSD population remain poorly understood.

Methods

A total of 127 patients with non-syndromic hearing loss (NSHL) were enrolled in Guangxi Zhuang Autonomous Region. A hereditary deafness gene mutation screening was performed to identify the mutation sites in four deafness-related genes (GJB2, GJB3, 12S rRNA, and SLC26A4). In addition, whole-exome sequencing (WES) was applied to explore unappreciated mutation sites in the cases with the singularity of its phenotype.

Results

Well-characterized mutations were found in only 8.7% (11/127) of the patients. Interestingly, two mutations in the OTOF gene were identified in two affected siblings with ANSD from a Chinese family, including one nonsense mutation c.1273C > T (p.R425X) and one missense mutation c.4994 T > C (p.L1665P). Furthermore, we employed Sanger sequencing to confirm the mutations in each subject. Two compound heterozygous mutations in the OTOF gene were observed in the two affected siblings, whereas the two parents and unaffected sister were heterozygous carriers of c.1273C > T (father and sister) and c.4994 T > C (mother). The nonsense mutation p.R425X, contributes to a premature stop codon, may result in a truncated polypeptide, which strongly suggests its pathogenicity for ANSD. The missense mutation p.L1665P results in a single amino acid substitution in a highly conserved region.

Conclusions

Two mutations in the OTOF gene in the Chinese deaf population were recognized for the first time. These findings not only extend the OTOF gene mutation spectrum for ANSD but also indicate that whole-exome sequencing is an effective approach to clarify the genetic characteristics in non-syndromic ANSD patients.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-017-0400-0) contains supplementary material, which is available to authorized users.

Hair cell synaptic dysfunction, auditory fatigue and thermal sensitivity in otoferlin Ile515Thr mutants

The multi-C₂ domain protein otoferlin is required for hearing and mutated in human deafness. Some OTOF mutations cause a mild elevation of auditory thresholds but strong impairment of speech perception. At elevated body temperature, hearing is lost. Mice homozygous for one of these mutations, Otof^I515T/I515T, exhibit a moderate hearing impairment involving enhanced adaptation to continuous or repetitive sound stimulation. In Otof^I515T/I515T inner hair cells (IHCs), otoferlin levels are diminished by 65%, and synaptic vesicles are enlarged. Exocytosis during prolonged stimulation is strongly reduced. This indicates that otoferlin is critical for the reformation of properly sized and fusion-competent synaptic vesicles. Moreover, we found sustained exocytosis and sound encoding to scale with the amount of otoferlin at the plasma membrane. We identified a 20 amino acid motif including an RXR motif, presumably present in human but not in mouse otoferlin, which reduces the plasma membrane abundance of Ile515Thr-otoferlin. Together, this likely explains the auditory synaptopathy at normal temperature and the temperature-sensitive deafness in humans carrying the Ile515Thr mutation.

Tryptophan-rich basic protein (WRB) mediates insertion of the tail-anchored protein otoferlin and is required for hair cell exocytosis and hearing

The transmembrane recognition complex (TRC40) pathway mediates the insertion of tail-anchored (TA) proteins into membranes. Here, we demonstrate that otoferlin, a TA protein essential for hair cell exocytosis, is inserted into the endoplasmic reticulum (ER) via the TRC40 pathway. We mutated the TRC40 receptor tryptophan-rich basic protein (Wrb) in hair cells of zebrafish and mice and studied the impact of defective TA protein insertion. Wrb disruption reduced otoferlin levels in hair cells and impaired hearing, which could be restored in zebrafish by transgenic Wrb rescue and otoferlin overexpression. Wrb-deficient mouse inner hair cells (IHCs) displayed normal numbers of afferent synapses, Ca²⁺ channels, and membrane-proximal vesicles, but contained fewer ribbon-associated vesicles. Patch-clamp of IHCs revealed impaired synaptic vesicle replenishment. In vivo recordings from postsynaptic spiral ganglion neurons showed a use-dependent reduction in sound-evoked spiking, corroborating the notion of impaired IHC vesicle replenishment. A human mutation affecting the transmembrane domain of otoferlin impaired its ER targeting and caused an auditory synaptopathy. We conclude that the TRC40 pathway is critical for hearing and propose that otoferlin is an essential substrate of this pathway in hair cells.

Heat pulse excitability of vestibular hair cells and afferent neurons

In the present study we combined electrophysiology with optical heat pulse stimuli to examine thermodynamics of membrane electrical excitability in mammalian vestibular hair cells and afferent neurons. We recorded whole cell currents in mammalian type II vestibular hair cells using an excised preparation (mouse) and action potentials (APs) in afferent neurons in vivo (chinchilla) in response to optical heat pulses applied to the crista (ΔT ≈ 0.25°C per pulse). Afferent spike trains evoked by heat pulse stimuli were diverse and included asynchronous inhibition, asynchronous excitation, and/or phase-locked APs synchronized to each infrared heat pulse. Thermal responses of membrane currents responsible for APs in ganglion neurons were strictly excitatory, with Q10 ≈ 2. In contrast, hair cells responded with a mix of excitatory and inhibitory currents. Excitatory hair cell membrane currents included a thermoelectric capacitive current proportional to the rate of temperature rise (dT/dt) and an inward conduction current driven by ΔT An iberiotoxin-sensitive inhibitory conduction current was also evoked by ΔT, rising in <3 ms and decaying with a time constant of ∼24 ms. The inhibitory component dominated whole cell currents in 50% of hair cells at -68 mV and in 67% of hair cells at -60 mV. Responses were quantified and described on the basis of first principles of thermodynamics. Results identify key molecular targets underlying heat pulse excitability in vestibular sensory organs and provide quantitative methods for rational application of optical heat pulses to examine protein biophysics and manipulate cellular excitability.

Molecular study of patients with auditory neuropathy

Auditory neuropathy is a type of hearing loss that constitutes a change in the conduct of the auditory stimulus by the involvement of inner hair cells or auditory nerve synapses. It is characterized by the absence or alteration of waves in the examination of brainstem auditory evoked potentials, with otoacoustic and/or cochlear microphonic issues. At present, four loci associated with non‑syndromic auditory neuropathy have been mapped: Autosomal recessive deafness‑9 [DFNB9; the otoferlin (OTOF) gene] and autosomal recessive deafness‑59 [DFNB59; the pejvakin (PJVK) gene], associated with autosomal recessive inheritance; the autosomal dominant auditory neuropathy gene [AUNA1; the diaphanous‑3 (DIAPH3) gene]; and AUNX1, linked to chromosome X. Furthermore, mutations of connexin 26 [the gap junction β2 (GJB2) gene] have also been associated with the disease. OTOF gene mutations exert a significant role in auditory neuropathy. In excess of 80 pathogenic mutations have been identified in individuals with non‑syndromic deafness in populations of different origins, with an emphasis on the p.Q829X mutation, which was found in ~3% of cases of deafness in the Spanish population. The identification of genetic alterations responsible for auditory neuropathy is one of the challenges contributing to understand the molecular bases of the different phenotypes of hearing loss. Thus, the present study aimed to investigate molecular changes in the OTOF gene in patients with auditory neuropathy, and to develop a DNA chip for the molecular diagnosis of auditory neuropathy using mass spectrometry for genotyping. Genetic alterations were investigated in 47 patients with hearing loss and clinical diagnosis of auditory neuropathy, and the c.35delG mutation in the GJB2 gene was identified in three homozygous patients, and the heterozygous parents of one of these cases. Additionally, OTOF gene mutations were tracked by complete sequencing of 48 exons, although these results are still preliminary. Studying the genetic basis of auditory neuropathy is of utmost importance for obtaining a differential diagnosis, developing more specific treatments and more accurate genetic counseling.

High frequency of OTOF mutations in Chinese infants with congenital auditory neuropathy spectrum disorder

Auditory neuropathy spectrum disorder (ANSD) is one of the most common diseases leading to hearing and speech communication barriers in infants and young children. The OTOF gene is the first gene identified for autosomal recessive non-syndromic ANSD, and patients with OTOF mutations have shown marked improvement of auditory functions from the cochlear implantation, but the true involvement of OTOF mutations in Chinese ANSD patients is still unknown which precludes the effective management of this disease. Here, we investigated the contribution of OTOF mutations to congenital ANSD patients in China. In all, 37 infants and young Children with ANSD were screened for all the exons of OTOF gene, of them 34 patients had no neonatal risk factors who were considered as congenital ANSD. The clinical manifestation and audiometric features were also investigated and compared in patients with and without OTOF mutations. In all, 14 of these subjects were shown to carry two or three mutant alleles of OTOF with the high frequency of 41.2% in congenital ANSD patients. In total, 15 novel pathogenic mutations and 10 reported mutations were identified. Our results confirmed that mutations in OTOF gene were a major cause of congenital ANSD in China. Identification of OTOF mutations can facilitate diagnosis, clinical intervention and counseling for congenital ANSD.

Auditory neuropathy--neural and synaptic mechanisms

Sensorineural hearing impairment is the most common form of hearing loss, and encompasses pathologies of the cochlea and the auditory nerve. Hearing impairment caused by abnormal neural encoding of sound stimuli despite preservation of sensory transduction and amplification by outer hair cells is known as 'auditory neuropathy'. This term was originally coined for a specific type of hearing impairment affecting speech comprehension beyond changes in audibility: patients with this condition report that they "can hear but cannot understand". This type of hearing impairment can be caused by damage to the sensory inner hair cells (IHCs), IHC ribbon synapses or spiral ganglion neurons. Human genetic and physiological studies, as well as research on animal models, have recently shown that disrupted IHC ribbon synapse function--resulting from genetic alterations that affect presynaptic glutamate loading of synaptic vesicles, Ca(2+) influx, or synaptic vesicle exocytosis--leads to hearing impairment termed 'auditory synaptopathy'. Moreover, animal studies have demonstrated that sound overexposure causes excitotoxic loss of IHC ribbon synapses. This mechanism probably contributes to hearing disorders caused by noise exposure or age-related hearing loss. This Review provides an update on recently elucidated sensory, synaptic and neural mechanisms of hearing impairment, their corresponding clinical findings, and discusses current rehabilitation strategies as well as future therapies.

Features of autosomal recessive non-syndromic hearing impairment: a review to serve as a reference

OBJECTIVE

Non-syndromic sensorineural hearing impairment is inherited in an autosomal recessive fashion in 75-85% of cases. To date, 61 genes with this type of inheritance have been identified as related to hearing impairment, and the genetic heterogeneity is accompanied by a large variety of clinical characteristics. Adequate counselling on a patient's hearing prognosis and rehabilitation is part of the diagnosis on the genetic cause of hearing impairment and, in addition, is important for the psychological well-being of the patient.

TYPE OF REVIEW

Traditional literature review.

DATA SOURCE

All articles describing clinical characteristics of the audiovestibular phenotypes of identified genes and related loci have been reviewed.

CONCLUSION

This review aims to serve as a summary and a reference for counselling purposes when a causative gene has been identified in a patient with a non-syndromic autosomal recessively inherited sensorineural hearing impairment.

Temperature sensitive auditory neuropathy

Temperature sensitive auditory neuropathy is a very rare and puzzling disorder. In the present study, we reported three unrelated 2 to 6 year-old children who were diagnosed as auditory neuropathy patients who complained of severe hearing loss when they had fever. Their hearing thresholds varied from the morning to the afternoon. Two of these patients' hearing improved with age, and one patient received positive results from cochlear implant. Genetic analysis revealed that these three patients had otoferlin (OTOF) homozygous or compound heterozygous mutations with the genotypes c.2975_2978delAG/c.4819C>T, c.4819C>T/c.4819C>T, or c.2382_2383delC/c.1621G>A, respectively. Our study suggests that these gene mutations may be the cause of temperature sensitive auditory neuropathy. The long term follow up results suggest that the hearing loss in this type of auditory neuropathy may recover with age.