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

Engineering of the AAV-Compatible Hair Cell-Specific Small-Size Myo15 Promoter for Gene Therapy in the Inner Ear

Adeno-associated virus (AAV)-mediated gene therapy is widely applied to treat numerous hereditary diseases in animal models and humans. The specific expression of AAV-delivered transgenes driven by cell type-specific promoters should further increase the safety of gene therapy. However, current methods for screening cell type-specific promoters are labor-intensive and time-consuming. Herein, we designed a "multiple vectors in one AAV" strategy for promoter construction in vivo. Through this strategy, we truncated a native promoter for Myo15 expression in hair cells (HCs) in the inner ear, from 1,611 bp down to 1,157 bp, and further down to 956 bp. Under the control of these 2 promoters, green fluorescent protein packaged in AAV-PHP.eB was exclusively expressed in the HCs. The transcription initiation ability of the 2 promoters was further verified by intein-mediated otoferlin recombination in a dual-AAV therapeutic system. Driven by these 2 promoters, human otoferlin was selectively expressed in HCs, resulting in the restoration of hearing in treated Otof -/- mice for at least 52 weeks. In summary, we developed an efficient screening strategy for cell type-specific promoter engineering and created 2 truncated Myo15 promoters that not only restored hereditary deafness in animal models but also show great potential for treating human patients in future.

Slow kinesin-dependent microtubular transport facilitates ribbon synapse assembly in developing cochlear inner hair cells

Sensory synapses are characterized by electron-dense presynaptic specializations, so-called synaptic ribbons. In cochlear inner hair cells (IHCs), ribbons play an essential role as core active zone (AZ) organizers, where they tether synaptic vesicles, cluster calcium channels and facilitate the temporally-precise release of primed vesicles. While a multitude of studies aimed to elucidate the molecular composition and function of IHC ribbon synapses, the developmental formation of these signalling complexes remains largely elusive to date. To address this shortcoming, we performed long-term live-cell imaging of fluorescently-labelled ribbon precursors in young postnatal IHCs to track ribbon precursor motion. We show that ribbon precursors utilize the apico-basal microtubular (MT) cytoskeleton for targeted trafficking to the presynapse, in a process reminiscent of slow axonal transport in neurons. During translocation, precursor volume regulation is achieved by highly dynamic structural plasticity - characterized by regularly-occurring fusion and fission events. Pharmacological MT destabilization negatively impacted on precursor translocation and attenuated structural plasticity, whereas genetic disruption of the anterograde molecular motor Kif1a impaired ribbon volume accumulation during developmental maturation. Combined, our data thus indicate an essential role of the MT cytoskeleton and Kif1a in adequate ribbon synapse formation and structural maintenance.

A cure for deafness?

Congenital deafness critically challenges the ability of affected children to acquire spoken language and normal social interactions. The clinical study by Lv et al. recently published in The Lancet1 presents a major breakthrough as the first successful causative treatment aiming to restore natural hearing in deaf children.

Gene therapy for deafness: are we there now?

This Commentary discusses the successes of the recent first in human trials for gene therapy of otoferlin-deficient hearing impairment.

Clinical and genetic architecture of a large cohort with auditory neuropathy

Auditory neuropathy (AN) is a unique type of language developmental disorder, with no precise rate of genetic contribution that has been deciphered in a large cohort. In a retrospective cohort of 311 patients with AN, pathogenic and likely pathogenic variants of 23 genes were identified in 98 patients (31.5% in 311 patients), and 14 genes were mutated in two or more patients. Among subgroups of patients with AN, the prevalence of pathogenic and likely pathogenic variants was 54.4% and 56.2% in trios and families, while 22.9% in the cases with proband-only; 45.7% and 25.6% in the infant and non-infant group; and 33.7% and 0% in the bilateral and unilateral AN cases. Most of the OTOF gene (96.6%, 28/29) could only be identified in the infant group, while the AIFM1 gene could only be identified in the non-infant group; other genes such as ATP1A3 and OPA1 were identified in both infant and non-infant groups. In conclusion, genes distribution of AN, with the most common genes being OTOF and AIFM1, is totally different from other sensorineural hearing loss. The subgroups with different onset ages showed different genetic spectrums, so did bilateral and unilateral groups and sporadic and familial or trio groups.

Autosomal Recessive Non-Syndromic Deafness: Is AAV Gene Therapy a Real Chance?

The etiology of sensorineural hearing loss is heavily influenced by genetic mutations, with approximately 80% of cases attributed to genetic causes and only 20% to environmental factors. Over 100 non-syndromic deafness genes have been identified in humans thus far. In non-syndromic sensorineural hearing impairment, around 75-85% of cases follow an autosomal recessive inheritance pattern. In recent years, groundbreaking advancements in molecular gene therapy for inner-ear disorders have shown promising results. Experimental studies have demonstrated improvements in hearing following a single local injection of adeno-associated virus-derived vectors carrying an additional normal gene or using ribozymes to modify the genome. These pioneering approaches have opened new possibilities for potential therapeutic interventions. Following the PRISMA criteria, we summarized the AAV gene therapy experiments showing hearing improvement in the preclinical phases of development in different animal models of DFNB deafness and the AAV gene therapy programs currently in clinical phases targeting autosomal recessive non syndromic hearing loss. A total of 17 preclinical studies and 3 clinical studies were found and listed. Despite the hurdles, there have been significant breakthroughs in the path of HL gene therapy, holding great potential for providing patients with novel and effective treatment.

Preparing for Otoferlin gene therapy trials: A survey of NHS Paediatric Audiology and Cochlear Implant services on diagnosis and management of Auditory Neuropathy Spectrum Disorder

OBJECTIVES

Gene therapy for monogenic hearing loss is on the horizon. The first trials in patients with Auditory Neuropathy Spectrum Disorder (ANSD) due to pathogenic variants in the Otoferlin (OTOF) gene will open this year. In the UK, the new NHS Genomic Medicine Service (GMS) offers genetic testing in each child diagnosed with congenital or early onset sensorineural hearing loss. This survey study aims to map preexisting clinical pathways for the diagnosis and management of children with ANSD and identify opportunities for improvement in early identification of OTOF- related ANSD.

METHODS

A Google form with 24 questions in English covering the ANSD clinical pathway was developed with clinicians involved in the diagnosis and management ANSD. The survey was disseminated via email to all Lead clinicians of NHS Tertiary Paediatric Audiology and Cochlear Implant Services within the UK.

RESULTS

Data was received from 27 (34 %) NHS Tertiary Paediatric Audiology Services and 8 (n = 57 %) Paediatric Cochlear Implant Services. Services follow existing national guidance and provide multidisciplinary care with structured patient pathways for referral, diagnosis, and management of children with ANSD and multidisciplinary input throughout. Clinicians are aware of the genetic causes of ANSD and new processes for genetic testing, but do not uniformly refer children with ANSD for testing for OTOF pathogenic variants. As such, they had difficulty estimating numbers of children with OTOF pathogenic variants under their care.

CONCLUSION

Those results highlight the urgency of implementing hearing gene panel sequencing for all children with ANSD to provide opportunities for early diagnosis and candidacy for OTOF gene therapy trials.

Advancements and future prospects of adeno-associated virus-mediated gene therapy for sensorineural hearing loss

Sensorineural hearing loss (SNHL), a highly prevalent sensory impairment, results from a multifaceted interaction of genetic and environmental factors. As we continually gain insights into the molecular basis of auditory development and the growing compendium of deafness genes identified, research on gene therapy for SNHL has significantly deepened. Adeno-associated virus (AAV), considered a relatively secure vector for gene therapy in clinical trials, can deliver various transgenes based on gene therapy strategies such as gene replacement, gene silencing, gene editing, or gene addition to alleviate diverse types of SNHL. This review delved into the preclinical advances in AAV-based gene therapy for SNHL, spanning hereditary and acquired types. Particular focus is placed on the dual-AAV construction method and its application, the vector delivery route of mouse inner ear models (local, systemic, fetal, and cerebrospinal fluid administration), and the significant considerations in transforming from AAV-based animal model inner ear gene therapy to clinical implementation.

Probing the role of the C2F domain of otoferlin

Afferent synapses of cochlear inner hair cells (IHCs) employ a unique molecular machinery. Otoferlin is a key player in this machinery, and its genetic defects cause human auditory synaptopathy. We employed site-directed mutagenesis in mice to investigate the role of Ca2+ binding to the C2F domain of otoferlin. Substituting two aspartate residues of the C2F top loops, which are thought to coordinate Ca2+-ions, by alanines (OtofD1841/1842A) abolished Ca2+-influx-triggered IHC exocytosis and synchronous signaling in the auditory pathway despite substantial expression (~60%) of the mutant otoferlin in the basolateral IHC pole. Ca2+ influx of IHCs and their resting membrane capacitance, reflecting IHC size, as well as the number of IHC synapses were maintained. The mutant otoferlin showed a strong apex-to-base abundance gradient in IHCs, suggesting impaired protein targeting. Our results indicate a role of the C2F domain in otoferlin targeting and of Ca2+ binding by the C2F domain for IHC exocytosis and hearing.

Advances in cochlear gene therapies

PURPOSE OF REVIEW

Hearing loss is the most common sensory deficit and in young children sensorineural hearing loss is most frequently genetic in etiology. Hearing aids and cochlear implant do not restore normal hearing. There is significant research and commercial interest in directly addressing the root cause of hearing loss through gene therapies. This article provides an overview of major barriers to cochlear gene therapy and recent advances in preclinical development of precision treatments of genetic deafness.

RECENT FINDINGS

Several investigators have recently described successful gene therapies in many common forms of genetic hearing loss in animal models. Elegant strategies that do not target a specific pathogenic variant, such as mini gene replacement and mutation-agnostic RNA interference (RNAi) with engineered replacement, facilitate translation of these findings to development of human therapeutics. Clinical trials for human gene therapies are in active recruitment.

SUMMARY

Gene therapies for hearing loss are expected to enter clinical trials in the immediate future. To provide referral for appropriate trials and counseling regarding benefits of genetic hearing loss evaluation, specialists serving children with hearing loss such as pediatricians, geneticists, genetic counselors, and otolaryngologists should be acquainted with ongoing developments in precision therapies.

Diversity matters - extending sound intensity coding by inner hair cells via heterogeneous synapses

Our sense of hearing enables the processing of stimuli that differ in sound pressure by more than six orders of magnitude. How to process a wide range of stimulus intensities with temporal precision is an enigmatic phenomenon of the auditory system. Downstream of dynamic range compression by active cochlear micromechanics, the inner hair cells (IHCs) cover the full intensity range of sound input. Yet, the firing rate in each of their postsynaptic spiral ganglion neurons (SGNs) encodes only a fraction of it. As a population, spiral ganglion neurons with their respective individual coding fractions cover the entire audible range. How such "dynamic range fractionation" arises is a topic of current research and the focus of this review. Here, we discuss mechanisms for generating the diverse functional properties of SGNs and formulate testable hypotheses. We postulate that an interplay of synaptic heterogeneity, molecularly distinct subtypes of SGNs, and efferent modulation serves the neural decomposition of sound information and thus contributes to a population code for sound intensity.

En route to sound coding strategies for optical cochlear implants

Hearing loss is the most common human sensory deficit. Severe-to-complete sensorineural hearing loss is often treated by electrical cochlear implants (eCIs) bypassing dysfunctional or lost hair cells by direct stimulation of the auditory nerve. The wide current spread from each intracochlear electrode array contact activates large sets of tonotopically organized neurons limiting spectral selectivity of sound coding. Despite many efforts, an increase in the number of independent eCI stimulation channels seems impossible to achieve. Light, which can be better confined in space than electric current may help optical cochlear implants (oCIs) to overcome eCI shortcomings. In this review, we present the current state of the optogenetic sound encoding. We highlight optical sound coding strategy development capitalizing on the optical stimulation that requires fine-grained, fast, and power-efficient real-time sound processing controlling dozens of microscale optical emitters as an emerging research area.

Deafness: from genetic architecture to gene therapy

Progress in deciphering the genetic architecture of human sensorineural hearing impairment (SNHI) or loss, and multidisciplinary studies of mouse models, have led to the elucidation of the molecular mechanisms underlying auditory system function, primarily in the cochlea, the mammalian hearing organ. These studies have provided unparalleled insights into the pathophysiological processes involved in SNHI, paving the way for the development of inner-ear gene therapy based on gene replacement, gene augmentation or gene editing. The application of these approaches in preclinical studies over the past decade has highlighted key translational opportunities and challenges for achieving effective, safe and sustained inner-ear gene therapy to prevent or cure monogenic forms of SNHI and associated balance disorders.

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.

Comparison of three AUC techniques for the determination of the loading status and capsid titer of AAVs

Due to the rise of adeno-associated viruses (AAVs) as gene therapy delivery vectors, boundary sedimentation velocity analytical ultracentrifugation (boundary SV-AUC) has been developed into a widely used quality control assay even for release analytics. It can be considered as the "gold standard" for the determination of the loading status of empty, partially filled, and full capsids especially when conducted in multiwavelength (MWL) mode. It can be considered to provide the most accurate determination of the loading status, and it also provides information on the capsid titer, aggregates, and potential contaminants such as free DNA. MWL boundary SV-AUC can be regarded as a multi-attribute (MAM) method for the characterization of AAVs. One major drawback of the method is the high sample consumption both in terms of concentration and volume. Here, we compare two alternative AUC techniques, band SV-AUC and analytical CsCl density gradient sedimentation equilibrium AUC (CsCl SE-AUC) with the boundary SV-AUC and the MWL-SV-AUC experiment. Our data show a high consistency of the determined full/empty ratios between these techniques if the appropriate wavelengths and extinction coefficients are used.

Mini-PCDH15 gene therapy rescues hearing in a mouse model of Usher syndrome type 1F

Usher syndrome type 1 F (USH1F), caused by mutations in the protocadherin-15 gene (PCDH15), is characterized by congenital deafness, lack of balance, and progressive blindness. In hair cells, the receptor cells of the inner ear, PCDH15 is a component of tip links, fine filaments which pull open mechanosensory transduction channels. A simple gene addition therapy for USH1F is challenging because the PCDH15 coding sequence is too large for adeno-associated virus (AAV) vectors. We use rational, structure-based design to engineer mini-PCDH15s in which 3-5 of the 11 extracellular cadherin repeats are deleted, but which still bind a partner protein. Some mini-PCDH15s can fit in an AAV. An AAV encoding one of these, injected into the inner ears of mouse models of USH1F, produces a mini-PCDH15 which properly forms tip links, prevents the degeneration of hair cell bundles, and rescues hearing. Mini-PCDH15s may be a useful therapy for the deafness of USH1F.

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.

The Genomics of Auditory Function and Disease

Current estimates suggest that nearly half a billion people worldwide are affected by hearing loss. Because of the major psychological, social, economic, and health ramifications, considerable efforts have been invested in identifying the genes and molecular pathways involved in hearing loss, whether genetic or environmental, to promote prevention, improve rehabilitation, and develop therapeutics. Genomic sequencing technologies have led to the discovery of genes associated with hearing loss. Studies of the transcriptome and epigenome of the inner ear have characterized key regulators and pathways involved in the development of the inner ear and have paved the way for their use in regenerative medicine. In parallel, the immense preclinical success of using viral vectors for gene delivery in animal models of hearing loss has motivated the industry to work on translating such approaches into the clinic. Here, we review the recent advances in the genomics of auditory function and dysfunction, from patient diagnostics to epigenetics and gene therapy.

Is there an unmet medical need for improved hearing restoration?

Hearing impairment, the most prevalent sensory deficit, affects more than 466 million people worldwide (WHO). We presently lack causative treatment for the most common form, sensorineural hearing impairment; hearing aids and cochlear implants (CI) remain the only means of hearing restoration. We engaged with CI users to learn about their expectations and their willingness to collaborate with health care professionals on establishing novel therapies. We summarize upcoming CI innovations, gene therapies, and regenerative approaches and evaluate the chances for clinical translation of these novel strategies. We conclude that there remains an unmet medical need for improving hearing restoration and that we are likely to witness the clinical translation of gene therapy and major CI innovations within this decade.

Analyzing efficacy, stability, and safety of AAV-mediated optogenetic hearing restoration in mice

This longitudinal study revealed stable expression of the fast-gating channelrhodopsin f-Chrimson in cochlear neurons of mice over at least 2 yr upon a single postnatal AAV dosing of the cochlea.

AAV-mediated optogenetic neural stimulation has become a clinical approach for restoring function in sensory disorders and feasibility for hearing restoration has been indicated in rodents. Nonetheless, long-term stability and safety of AAV-mediated channelrhodopsin (ChR) expression in spiral ganglion neurons (SGNs) remained to be addressed. Here, we used longitudinal studies on mice subjected to early postnatal administration of AAV2/6 carrying fast gating ChR f-Chrimson under the control of the human synapsin promoter unilaterally to the cochlea. f-Chrimson expression in SGNs in both ears and the brain was probed in animals aged 1 mo to 2 yr. f-Chrimson was observed in SGNs at all ages indicating longevity of ChR-expression. SGN numbers in the AAV-injected cochleae declined with age faster than in controls. Investigations were extended to the brain in which viral transduction was observed across the organ at varying degrees irrespective of age without observing viral spread-related pathologies. No viral DNA or virus-related histopathological findings in visceral organs were encountered. In summary, our study demonstrates life-long (24 mo in mice) expression of f-Chrimson in SGNs upon single AAV-dosing of the cochlea.

Choice of vector and surgical approach enables efficient cochlear gene transfer in nonhuman primate

Inner ear gene therapy using adeno-associated viral vectors (AAV) promises to alleviate hearing and balance disorders. We previously established the benefits of Anc80L65 in targeting inner and outer hair cells in newborn mice. To accelerate translation to humans, we now report the feasibility and efficiency of the surgical approach and vector delivery in a nonhuman primate model. Five rhesus macaques were injected with AAV1 or Anc80L65 expressing eGFP using a transmastoid posterior tympanotomy approach to access the round window membrane after making a small fenestra in the oval window. The procedure was well tolerated. All but one animal showed cochlear eGFP expression 7–14 days following injection. Anc80L65 in 2 animals transduced up to 90% of apical inner hair cells; AAV1 was markedly less efficient at equal dose. Transduction for both vectors declined from apex to base. These data motivate future translational studies to evaluate gene therapy for human hearing disorders.

Gene therapy using Adeno-associated viral vectors (AAV) rescues hearing and balance deficits in mouse models of human disorders. Here, the authors show that AAVAnc80L65 allows efficient cochlear gene transfer in nonhuman primates, and motivate future studies to evaluate gene therapy for hearing and balance disorders.

Electrocochleography in Auditory Neuropathy Related to Mutations in the OTOF or OPA1 Gene

Auditory Neuropathy (AN) is characterized by disruption of temporal coding of acoustic signals in auditory nerve fibers resulting in alterations of auditory perceptions. Mutations in several genes have been associated to the most forms of AN. Underlying mechanisms include both pre-synaptic and post-synaptic damage involving inner hair cell (IHC) depolarization, neurotransmitter release, spike initiation in auditory nerve terminals, loss of auditory fibers and impaired conduction. In contrast, outer hair cell (OHC) activities (otoacoustic emissions [OAEs] and cochlear microphonic [CM]) are normal. Disordered synchrony of auditory nerve activity has been suggested as the basis of both the alterations of auditory brainstem responses (ABRs) and reduction of speech perception. We will review how electrocochleography (ECochG) recordings provide detailed information to help objectively define the sites of auditory neural dysfunction and their effect on receptor summating potential (SP) and neural compound action potential (CAP), the latter reflecting disorders of ribbon synapses and auditory nerve fibers.

Cabp2-Gene Therapy Restores Inner Hair Cell Calcium Currents and Improves Hearing in a DFNB93 Mouse Model

Clinical management of auditory synaptopathies like other genetic hearing disorders is currently limited to the use of hearing aids or cochlear implants. However, future gene therapy promises restoration of hearing in selected forms of monogenic hearing impairment, in which cochlear morphology is preserved over a time window that enables intervention. This includes non-syndromic autosomal recessive hearing impairment DFNB93, caused by defects in the CABP2 gene. Calcium-binding protein 2 (CaBP2) is a potent modulator of inner hair cell (IHC) voltage-gated calcium channels Ca_V1.3. Based on disease modeling in Cabp2^–/– mice, DFNB93 hearing impairment has been ascribed to enhanced steady-state inactivation of IHC Ca_V1.3 channels, effectively limiting their availability to trigger synaptic transmission. This, however, does not seem to interfere with cochlear development and does not cause early degeneration of hair cells or their synapses. Here, we studied the potential of a gene therapeutic approach for the treatment of DFNB93. We used AAV2/1 and AAV-PHP.eB viral vectors to deliver the Cabp2 coding sequence into IHCs of early postnatal Cabp2^–/– mice and assessed the level of restoration of hair cell function and hearing. Combining in vitro and in vivo approaches, we observed high transduction efficiency, and restoration of IHC Ca_V1.3 function resulting in improved hearing of Cabp2^–/– mice. These preclinical results prove the feasibility of DFNB93 gene therapy.

Otoferlin Is Required for Proper Synapse Maturation and for Maintenance of Inner and Outer Hair Cells in Mouse Models for DFNB9

Deficiency of otoferlin causes profound prelingual deafness in humans and animal models. Here, we closely analyzed developmental deficits and degenerative mechanisms in Otof knock-out (Otof^–/–) mice over the course of 48 weeks. We found otoferlin to be required for proper synapse development in the immature rodent cochlea: In absence of otoferlin, synaptic pruning was delayed, and postsynaptic boutons appeared enlarged at 2 weeks of age. At postnatal day 14 (P14), we found on average ∼15 synapses per inner hair cell (IHC) in Otof^–/– cochleae as well as in wild-type controls. Further on, the number of synapses in Otof^–/– IHCs was reduced to ∼7 at 8 weeks of age and to ∼6 at 48 weeks of age. In the same period, the number of spiral ganglion neurons (SGNs) declined in Otof^–/– animals. Importantly, we found an age-progressive loss of IHCs to an overall number of 75% of wildtype IHCs. The IHC loss more prominently but not exclusively affected the basal aspects of the cochlea. For outer hair cells (OHCs), we observed slightly accelerated age-dependent degeneration from base to apex. This was associated with a progressive decay in DPOAE amplitudes for high frequency stimuli, which could first be observed at the age of 24 weeks in Otof^–/– mice. Our data will help to plan and predict the outcome of a gene therapy applied at various ages of DFNB9 patients.

Truncation of the otoferlin transmembrane domain alters the development of hair cells and reduces membrane docking

Release of neurotransmitter from sensory hair cells is regulated by otoferlin. Despite the importance of otoferlin in the auditory and vestibular pathways, the functional contributions of the domains of the protein have not been fully characterized. Using a zebrafish model, we investigated a mutant otoferlin with a stop codon at the start of the transmembrane domain. We found that both the phenotype severity and the expression level of mutant otoferlin changed with the age of the zebrafish. At the early developmental time point of 72 h post fertilization, low expression of the otoferlin mutant coincided with synaptic ribbon deficiencies, reduced endocytosis, and abnormal transcription of several hair cell genes. As development proceeded, expression of the mutant otoferlin increased, and both synaptic ribbons and hair cell transcript levels resembled wild type. However, hair cell endocytosis deficits and abnormalities in the expression of GABA receptors persisted even after up-regulation of mutant otoferlin. Analysis of membrane-reconstituted otoferlin measurements suggests a function for the transmembrane domain in liposome docking. We conclude that deletion of the transmembrane domain reduces membrane docking, attenuates endocytosis, and results in developmental delay of the hair cell.