RNA Interference Prevents Autosomal-Dominant Hearing Loss

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.

Hearing Loss: Genetic Testing, Current Advances and the Situation in Latin America

Congenital hearing loss is the most common birth defect, estimated to affect 2-3 in every 1000 births, with ~50-60% of those related to genetic causes. Technological advances enabled the identification of hundreds of genes related to hearing loss (HL), with important implications for patients, their families, and the community. Despite these advances, in Latin America, the population with hearing loss remains underdiagnosed, with most studies focusing on a single locus encompassing the GJB2/GJB6 genes. Here we discuss how current and emerging genetic knowledge has the potential to alter the approach to diagnosis and management of hearing loss, which is the current situation in Latin America, and the barriers that still need to be overcome.

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.

Usher syndrome: Genetic diagnosis and current therapeutic approaches

Usher Syndrome (USH) is the most common deaf-blind syndrome, affecting approximately 1 in 6000 people in the deaf population. This genetic condition is characterized by a combination of hearing loss (HL), retinitis pigmentosa, and, in some cases, vestibular areflexia. Among the subtypes of USH, USH type 1 is considered the most severe form, presenting profound bilateral congenital deafness, vestibular areflexia, and early onset RP. USH type 2 is the most common form, exhibiting congenital moderate to severe HL for low frequencies and severe to profound HL for high frequencies. Conversely, type 3 is the rarest, initially manifesting mild symptoms during childhood that become more prominent in the first decades of life. The dual impact of USH on both visual and auditory senses significantly impairs patients’ quality of life, restricting their daily activities and interactions with society. To date, 9 genes have been confirmed so far for USH: MYO7A, USH1C, CDH23, PCDH15, USH1G, USH2A, ADGRV1, WHRN and CLRN1. These genes are inherited in an autosomal recessive manner and encode proteins expressed in the inner ear and retina, leading to functional loss. Although non-genetic methods can assist in patient triage and disease extension evaluation, genetic and molecular tests play a pivotal role in providing genetic counseling, enabling appropriate gene therapy, and facilitating timely cochlear implantation (CI). The CRISPR/Cas9 system and viral-based gene replacement therapy have recently emerged as highly promising techniques for treating USH. Regarding drug therapy, PTC-124 and Nb54 have been identified as promising drug interventions for genetic HL in USH. Simultaneously, CI has proven to be critical in the restoration of hearing. This review aims to summarize the genetic and molecular diagnosis of USH and highlight the importance of early diagnosis in guiding appropriate treatment strategies and improving patient prognosis.

Mechanisms and otoprotective strategies of programmed cell death on aminoglycoside-induced ototoxicity

Aminoglycosides are commonly used for the treatment of life-threatening bacterial infections, however, aminoglycosides may cause irreversible hearing loss with a long-term clinical therapy. The mechanism and prevention of the ototoxicity of aminoglycosides are still limited although amounts of studies explored widely. Specifically, advancements in programmed cell death (PCD) provide more new perspectives. This review summarizes the general signal pathways in programmed cell death, including apoptosis, autophagy, and ferroptosis, as well as the mechanisms of aminoglycoside-induced ototoxicity. Additionally, novel interventions, especially gene therapy strategies, are also investigated for the prevention or treatment of aminoglycoside-induced hearing loss with prospective clinical applications.

Genetic correction of induced pluripotent stem cells from a DFNA36 patient results in morphologic and functional recovery of derived hair cell-like cells

BACKGROUND

TMC1 is one of the most common deafness genes causing DFNA36. Patient-derived human induced pluripotent stem cells (iPSCs) provide an opportunity to modelling diseases. TMC1 p.M418K mutation in human is orthologous to Beethoven mice. Here, we investigated the differentiation, morphology and electrophysiological properties of hair cell-like cells (HC-like cells) derived from DFNA36 patient.

METHODS

Inner ear HC-like cells were induced from iPSCs derived from DFNA36 (TMC1 p.M418K) patient (M+/-), normal control (M+/+) and genetic corrected iPSCs (M+/C). Immunofluorescence, scanning electron microscopy and whole-cell patch-clamp were used to study the mechanism and influence of TMC1 p.M418K mutation.

RESULTS

In this study we successfully generated HC-like cells from iPSCs with three different genotypes. HC-like cells from M+/- showed defected morphology of microvilli and physiological properties compared to M+/+. HC-like cells from M+/C showed recovery in morphology of microvilli and physiological properties.

CONCLUSIONS

Our results indicate that TMC1 p.M418K mutation didn't influence inner ear hair cell differentiation but the morphology of microvilli and electrophysiological properties and gene correction induced recovery. CRISPR/Cas9 gene therapy is feasible in human patient with TMC1 p.M418K mutation.

Microbubble-assisted ultrasound for inner ear drug delivery

Treating pathologies of the inner ear is a major challenge. To date, a wide range of procedures exists for administering therapeutic agents to the inner ear, with varying degrees of success. The key is to deliver therapeutics in a way that is minimally invasive, effective, long-lasting, and without adverse effects on vestibular and cochlear function. Microbubble-assisted ultrasound ("sonoporation") is a promising new modality that can be adapted to the inner ear. Combining ultrasound technology with microbubbles in the middle ear can increase the permeability of the round window, enabling therapeutic agents to be delivered safely and effectively to the inner ear in a targeted manner. As such, sonoporation is a promising new approach to treat hearing loss and vertigo. This review summarizes all studies on the delivery of therapeutic molecules to the inner ear using sonoporation.

Recent Therapeutic Progress and Future Perspectives for the Treatment of Hearing Loss

Up to 1.5 billion people worldwide suffer from various forms of hearing loss, with an additional 1.1 billion people at risk from various insults such as increased consumption of recreational noise-emitting devices and ageing. The most common type of hearing impairment is sensorineural hearing loss caused by the degeneration or malfunction of cochlear hair cells or spiral ganglion nerves in the inner ear. There is currently no cure for hearing loss. However, emerging frontier technologies such as gene, drug or cell-based therapies offer hope for an effective cure. In this review, we discuss the current therapeutic progress for the treatment of hearing loss. We describe and evaluate the major therapeutic approaches being applied to hearing loss and summarize the key trials and studies.

Novel autosomal dominant TMC1 variants linked to hearing loss: insight into protein-lipid interactions

BACKGROUND

TMC1, which encodes transmembrane channel-like protein 1, forms the mechanoelectrical transduction (MET) channel in auditory hair cells, necessary for auditory function. TMC1 variants are known to cause autosomal dominant (DFNA36) and autosomal recessive (DFNB7/11) non-syndromic hearing loss, but only a handful of TMC1 variants underlying DFNA36 have been reported, hampering analysis of genotype-phenotype correlations.

METHODS

In this study, we retrospectively reviewed 338 probands in an in-house database of genetic hearing loss, evaluating the clinical phenotypes and genotypes of novel TMC1 variants associated with DFNA36. To analyze the structural impact of these variants, we generated two structural models of human TMC1, utilizing the Cryo-EM structure of C. elegans TMC1 as a template and AlphaFold protein structure database. Specifically, the lipid bilayer-embedded protein database was used to construct membrane-embedded models of TMC1. We then examined the effect of TMC1 variants on intramolecular interactions and predicted their potential pathogenicity.

RESULTS

We identified two novel TMC1 variants related to DFNA36 (c.1256T > C:p.Phe419Ser and c.1444T > C:p.Trp482Arg). The affected subjects had bilateral, moderate, late-onset, progressive sensorineural hearing loss with a down-sloping configuration. The Phe419 residue located in the transmembrane domain 4 of TMC1 faces outward towards the channel pore and is in close proximity to the hydrophobic tail of the lipid bilayer. The non-polar-to-polar variant (p.Phe419Ser) alters the hydrophobicity in the membrane, compromising protein-lipid interactions. On the other hand, the Trp482 residue located in the extracellular linker region between transmembrane domains 5 and 6 is anchored to the membrane interfaces via its aromatic rings, mediating several molecular interactions that stabilize the structure of TMC1. This type of aromatic ring-based anchoring is also observed in homologous transmembrane proteins such as OSCA1.2. Conversely, the substitution of Trp with Arg (Trp482Arg) disrupts the cation-π interaction with phospholipids located in the outer leaflet of the phospholipid bilayer, destabilizing protein-lipid interactions. Additionally, Trp482Arg collapses the CH-π interaction between Trp482 and Pro511, possibly reducing the overall stability of the protein. In parallel with the molecular modeling, the two mutants degraded significantly faster compared to the wild-type protein, compromising protein stability.

CONCLUSIONS

This results expand the genetic spectrum of disease-causing TMC1 variants related to DFNA36 and provide insight into TMC1 transmembrane protein-lipid interactions.

Gene Therapy for Inherited Hearing Loss: Updates and Remaining Challenges

Hearing loss stands as the most prevalent sensory deficit among humans, posing a significant global health challenge. Projections indicate that by 2050, approximately 10% of the world's population will grapple with disabling hearing impairment. While approximately half of congenital hearing loss cases have a genetic etiology, traditional interventions such as hearing aids and cochlear implants do not completely restore normal hearing. The absence of biological treatment has prompted significant efforts in recent years, with a strong focus on gene therapy to address hereditary hearing loss. Although several studies have exhibited promising recovery from common forms of genetic deafness in mouse models, existing challenges must be overcome to make gene therapy applicable in the near future. Herein, we summarize the primary gene therapy strategies employed over past years, provide an overview of the recent achievements in preclinical studies for genetic hearing loss, and outline the current key obstacles to cochlear gene therapy.

Treatment following Triple-AAV Delivery in Mature Murine Model of Human CDH23-Associated Hearing Loss

This study aimed to investigate the transduction efficiency of triple adeno-associated virus (AAV) vectors in the cochleae of adult mice, focusing on large-gene-associated hearing loss (HL). Additionally, we sought to evaluate the feasibility of cochlear gene therapy in a mouse model of human CDH23-mediated HL using the triple AAV approach. To create a reporter protein, we fused EGFP to mCherry, which was then divided into three parts, each packaged in a separate AAV2/2 vector. Four weeks after co-injecting the triple AAV vectors into 4-5-week-old mice, we assessed transduction efficiency. We found that up to 5.9% of inner hair cells were positive for both EGFP and mCherry. Subsequently, we developed triple Cdh23 AAV vectors for therapeutic purposes. After administering these vectors to 4- to 5-week-old C57/BL6 mice, we conducted auditory tests and immunohistochemistry studies over a period of 60 weeks. Co-injecting triple Cdh23-AAVs did not alter auditory function or lead to hair cell degeneration. In conclusion, this study confirms the feasibility of the triple-AAV approach for cochlear gene delivery. While this strategy did not produce any treatment effects, our findings suggest that large deafness genes could be potential future targets for cochlear gene therapy.

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.

Rescue of auditory function by a single administration of AAV-TMPRSS3 gene therapy in aged mice of human recessive deafness DFNB8

Patients with mutations in the TMPRSS3 gene suffer from recessive deafness DFNB8/DFNB10. For these patients, cochlear implantation is the only treatment option. Poor cochlear implantation outcomes are seen in some patients. To develop biological treatment for TMPRSS3 patients, we generated a knockin mouse model with a frequent human DFNB8 TMPRSS3 mutation. The Tmprss3A306T/A306T homozygous mice display delayed onset progressive hearing loss similar to human DFNB8 patients. Using AAV2 as a vector to carry a human TMPRSS3 gene, AAV2-hTMPRSS3 injection in the adult knockin mouse inner ear results in TMPRSS3 expression in the hair cells and the spiral ganglion neurons. A single AAV2-hTMPRSS3 injection in Tmprss3A306T/A306T mice of an average age of 18.5 months leads to sustained rescue of the auditory function to a level similar to wild-type mice. AAV2-hTMPRSS3 delivery rescues the hair cells and the spiral ganglions neurons. This study demonstrates successful gene therapy in an aged mouse model of human genetic deafness. It lays the foundation to develop AAV2-hTMPRSS3 gene therapy to treat DFNB8 patients, as a standalone therapy or in combination with cochlear implantation.

Treatment of monogenic and digenic dominant genetic hearing loss by CRISPR-Cas9 ribonucleoprotein delivery in vivo

Mutations in Atp2b2, an outer hair cell gene, cause dominant hearing loss in humans. Using a mouse model Atp2b2Obl/+, with a dominant hearing loss mutation (Oblivion), we show that liposome-mediated in vivo delivery of CRISPR-Cas9 ribonucleoprotein complexes leads to specific editing of the Obl allele. Large deletions encompassing the Obl locus and indels were identified as the result of editing. In vivo genome editing promotes outer hair cell survival and restores their function, leading to hearing recovery. We further show that in a double-dominant mutant mouse model, in which the Tmc1 Beethoven mutation and the Atp2b2 Oblivion mutation cause digenic genetic hearing loss, Cas9/sgRNA delivery targeting both mutations leads to partial hearing recovery. These findings suggest that liposome-RNP delivery can be used as a strategy to recover hearing with dominant mutations in OHC genes and with digenic mutations in the auditory hair cells, potentially expanding therapeutics of gene editing to treat hearing loss.

Insights into the pathophysiology of DFNA44 hearing loss associated with CCDC50 frameshift variants

Non-syndromic sensorineural hearing loss (SNHL) is the most common sensory disorder, and it presents a high genetic heterogeneity. As part of our clinical genetic studies, we ascertained a previously unreported mutation in CCDC50 [c.828_858del, p.(Asp276Glufs*40)] segregating with hearing impairment in a Spanish family with SNHL associated with the autosomal dominant deafness locus DFNA44, which is predicted to disrupt protein function. To gain insight into the mechanism behind DFNA44 mutations, we analysed two Ccdc50 presumed loss-of-function mouse mutants, which showed normal hearing thresholds up to 6 months of age, indicating that haploinsufficiency is unlikely to be the pathogenic mechanism. We then carried out in vitro studies on a set of artificial mutants and on the p.(Asp276Glufs*40) and p.(Phe292Hisfs*37) human mutations, and determined that only the mutants containing the six-amino-acid sequence CLENGL as part of their aberrant protein tail showed an abnormal distribution consisting of perinuclear aggregates of the CCDC50 protein (also known as Ymer). Therefore, we conclude that the CLENGL sequence is necessary to form these aggregates. Taken together, the in vivo and in vitro results obtained in this study suggest that the two identified mutations in CCDC50 exert their effect through a dominant-negative or gain-of-function mechanism rather than by haploinsufficiency.

Advances in gene therapy hold promise for treating hereditary hearing loss

Gene therapy focuses on genetic modification to produce therapeutic effects or treat diseases by repairing or reconstructing genetic material, thus being expected to be the most promising therapeutic strategy for genetic disorders. Due to the growing attention to hearing impairment, an increasing amount of research is attempting to utilize gene therapy for hereditary hearing loss (HHL), an important monogenic disease and the most common type of congenital deafness. Several gene therapy clinical trials for HHL have recently been approved, and, additionally, CRISPR-Cas tools have been attempted for HHL treatment. Therefore, in order to further advance the development of inner ear gene therapy and promote its broad application in other forms of genetic disease, it is imperative to review the progress of gene therapy for HHL. Herein, we address three main gene therapy strategies (gene replacement, gene suppression, and gene editing), summarizing the strategy that is most appropriate for particular monogenic diseases based on different pathogenic mechanisms, and then focusing on their successful applications for HHL in preclinical trials. Finally, we elaborate on the challenges and outlooks of gene therapy for HHL.

Mutation-agnostic RNA interference with engineered replacement rescues Tmc1-related hearing loss

Hearing loss is the most common sensory deficit, of which genetic etiologies are a frequent cause. Dominant and recessive mutations in TMC1, a gene encoding the pore-forming subunit of the hair cell mechanotransduction channel, cause DFNA36 and DFNB7/11, respectively, accounting for ∼2% of genetic hearing loss. Previous work has established the efficacy of mutation-targeted RNAi in treatment of murine models of autosomal dominant non-syndromic deafness. However, application of such approaches is limited by the infeasibility of development and validation of novel constructs for each variant. We developed an allele-non-specific approach consisting of mutation-agnostic RNAi suppression of both mutant and WT alleles, co-delivered with a knockdown-resistant engineered WT allele with or without the use of woodchuck hepatitis virus post-transcriptional regulatory element (WPRE) to augment transgene expression. This therapeutic construct was delivered into the mature murine model of DFNA36 with an AAV vector and achieved robust hair cell and auditory brainstem response preservation. However, WPRE-enhanced Tmc1 expression resulted in inferior outcomes, suggesting a role for gene dosage optimization in future TMC1 gene therapy development.

Rescue of Auditory Function by a Single Administration of AAV- TMPRSS3 Gene Therapy in Aged Mice of Human Recessive Deafness DFNB8

Patients with mutations in the TMPRSS3 gene suffer from recessive deafness DFNB8/DFNB10 for whom cochlear implantation is the only treatment option. Poor cochlear implantation outcomes are seen in some patients. To develop biological treatment for TMPRSS3 patients, we generated a knock-in mouse model with a frequent human DFNB8 TMPRSS3 mutation. The Tmprss3 A306T/A306T homozygous mice display delayed onset progressive hearing loss similar to human DFNB8 patients. Using AAV2 as a vector to carry a human TMPRSS3 gene, AAV2-h TMPRSS3 injection in the adult knock-in mouse inner ears results in TMPRSS3 expression in the hair cells and the spiral ganglion neurons. A single AAV2-h TMPRSS3 injection in aged Tmprss3 A306T/A306T mice leads to sustained rescue of the auditory function, to a level similar to the wildtype mice. AAV2-h TMPRSS3 delivery rescues the hair cells and the spiral ganglions. This is the first study to demonstrate successful gene therapy in an aged mouse model of human genetic deafness. This study lays the foundation to develop AAV2-h TMPRSS3 gene therapy to treat DFNB8 patients, as a standalone therapy or in combination with cochlear implantation.

Altering gene expression using antisense oligonucleotide therapy for hearing loss

Hearing loss affects more than 430 million people, worldwide, and is the third most common chronic physical condition in the United States and Europe (GBD Hearing Loss Collaborators, 2021; NIOSH, 2021; WHO, 2021). The loss of hearing significantly impacts motor and cognitive development, communication, education, employment, and overall quality of life. The inner ear houses the sensory organs for both hearing and balance and provides an accessible target for therapeutic delivery. Antisense oligonucleotides (ASOs) use various mechanisms to manipulate gene expression and can be tailor-made to treat disorders with defined genetic targets. In this review, we discuss the preclinical advancements within the field of the highly promising ASO-based therapies for hereditary hearing loss disorders. Particular focus is on ASO mechanisms of action, preclinical studies on ASO treatments of hearing loss, timing of therapeutic intervention, and delivery routes to the inner ear.

Molecular mechanisms governing development of the hindbrain choroid plexus and auditory projection: A validation of the seminal observations of Wilhelm His

Studies by His from 1868 to 1904 delineated the critical role of the dorsal roof plate in the development of the hindbrain choroid plexus, and of the rhombic lips in the development of hindbrain auditory centers. Modern molecular studies have confirmed these observations and placed them in a mechanistic context. Expression of the transcription factor Lmx1a/b is crucial to the development of the hindbrain choroid plexus, and also regulates the expression of Atoh1, a transcription factor that is essential for the formation of the cochlear hair cells and auditory nuclei. By contrast, development of the vestibular hair cells, vestibular ganglion and vestibular nuclei does not depend on Lmx1a/b. These findings demonstrate a common dependence on a specific gene for the hindbrain choroid plexus and the primary auditory projection from hair cells to sensory neurons to hindbrain nuclei. Thus, His' conclusions regarding the origins of specific hindbrain structures are borne out by molecular genetic experiments conducted more than a hundred years later.

Current Advances in Adeno-Associated Virus-Mediated Gene Therapy to Prevent Acquired Hearing Loss

Adeno-associated viruses (AAVs) are viral vectors that offer an excellent platform for gene therapy due to their safety profile, persistent gene expression in non-dividing cells, target cell specificity, lack of pathogenicity, and low immunogenicity. Recently, gene therapy for genetic hearing loss with AAV transduction has shown promise in animal models. However, AAV transduction for gene silencing or expression to prevent or manage acquired hearing loss is limited. This review provides an overview of AAV as a leading gene delivery vector for treating genetic hearing loss in animal models. We highlight the advantages and shortcomings of AAV for investigating the mechanisms and preventing acquired hearing loss. We predict that AAV-mediated gene manipulation will be able to prevent acquired hearing loss.

Neurosensory development of the four brainstem-projecting sensory systems and their integration in the telencephalon

Somatosensory, taste, vestibular, and auditory information is first processed in the brainstem. From the brainstem, the respective information is relayed to specific regions within the cortex, where these inputs are further processed and integrated with other sensory systems to provide a comprehensive sensory experience. We provide the organization, genetics, and various neuronal connections of four sensory systems: trigeminal, taste, vestibular, and auditory systems. The development of trigeminal fibers is comparable to many sensory systems, for they project mostly contralaterally from the brainstem or spinal cord to the telencephalon. Taste bud information is primarily projected ipsilaterally through the thalamus to reach the insula. The vestibular fibers develop bilateral connections that eventually reach multiple areas of the cortex to provide a complex map. The auditory fibers project in a tonotopic contour to the auditory cortex. The spatial and tonotopic organization of trigeminal and auditory neuron projections are distinct from the taste and vestibular systems. The individual sensory projections within the cortex provide multi-sensory integration in the telencephalon that depends on context-dependent tertiary connections to integrate other cortical sensory systems across the four modalities.

Genetic insights, disease mechanisms, and biological therapeutics for Waardenburg syndrome

Waardenburg syndrome (WS), also known as auditory-pigmentary syndrome, is the most common cause of syndromic hearing loss (HL), which accounts for approximately 2-5% of all patients with congenital hearing loss. WS is classified into four subtypes depending on the clinical phenotypes. Currently, pathogenic mutations of PAX3, MITF, SOX10, EDN3, EDNRB or SNAI2 are associated with different subtypes of WS. Although supportive techniques like hearing aids, cochlear implants, or other assistive listening devices can alleviate the HL symptom, there is no cure for WS to date. Recently major progress has been achieved in preclinical studies of genetic HL in animal models, including gene delivery and stem cell replacement therapies. This review focuses on the current understandings of pathogenic mechanisms and potential biological therapeutic approaches for HL in WS, providing strategies and directions for implementing WS biological therapies, as well as possible problems to be faced, in the future.

Inner Ear Drug Delivery for Sensorineural Hearing Loss: Current Challenges and Opportunities

Most therapies for treating sensorineural hearing loss are challenged by the delivery across multiple tissue barriers to the hard-to-access anatomical location of the inner ear. In this review, we will provide a recent update on various pharmacotherapy, gene therapy, and cell therapy approaches used in clinical and preclinical studies for the treatment of sensorineural hearing loss and approaches taken to overcome the drug delivery barriers in the ear. Small-molecule drugs for pharmacotherapy can be delivered via systemic or local delivery, where the blood-labyrinth barrier hinders the former and tissue barriers including the tympanic membrane, the round window membrane, and/or the oval window hinder the latter. Meanwhile, gene and cell therapies often require targeted delivery to the cochlea, which is currently achieved via intra-cochlear or intra-labyrinthine injection. To improve the stability of the biomacromolecules during treatment, e.g., RNAs, DNAs, proteins, additional packing vehicles are often required. To address the diverse range of biological barriers involved in inner ear drug delivery, each class of therapy and the intended therapeutic cargoes will be discussed in this review, in the context of delivery routes commonly used, delivery vehicles if required (e.g., viral and non-viral nanocarriers), and other strategies to improve drug permeation and sustained release (e.g., hydrogel, nanocarriers, permeation enhancers, and microfluidic systems). Overall, this review aims to capture the important advancements and key steps in the development of inner ear therapies and delivery strategies over the past two decades for the treatment and prophylaxis of sensorineural hearing loss.

Conversations in Cochlear Implantation: The Inner Ear Therapy of Today

As biomolecular approaches for hearing restoration in profound sensorineural hearing loss evolve, they will be applied in conjunction with or instead of cochlear implants. An understanding of the current state-of-the-art of this technology, including its advantages, disadvantages, and its potential for delivering and interacting with biomolecular hearing restoration approaches, is helpful for designing modern hearing-restoration strategies. Cochlear implants (CI) have evolved over the last four decades to restore hearing more effectively, in more people, with diverse indications. This evolution has been driven by advances in technology, surgery, and healthcare delivery. Here, we offer a practical treatise on the state of cochlear implantation directed towards developing the next generation of inner ear therapeutics. We aim to capture and distill conversations ongoing in CI research, development, and clinical management. In this review, we discuss successes and physiological constraints of hearing with an implant, common surgical approaches and electrode arrays, new indications and outcome measures for implantation, and barriers to CI utilization. Additionally, we compare cochlear implantation with biomolecular and pharmacological approaches, consider strategies to combine these approaches, and identify unmet medical needs with cochlear implants. The strengths and weaknesses of modern implantation highlighted here can mark opportunities for continued progress or improvement in the design and delivery of the next generation of inner ear therapeutics.

Preventing autosomal-dominant hearing loss in Bth mice with CRISPR/CasRx-based RNA editing

CRISPR/RfxCas13d (CasRx) editing system can specifically and precisely cleave single-strand RNAs, which is a promising treatment for various disorders by downregulation of related gene expression. Here, we tested this RNA-editing approach on Beethoven (Bth) mice, an animal model for human DFNA36 due to a point mutation in Tmc1. We first screened 30 sgRNAs in cell cultures and found that CasRx with sgRNA3 reduced the Tmc1Bth transcript by 90.8%, and the Tmc1 wild type transcript (Tmc1+) by 44.3%. We then injected a newly developed AAV vector (AAV-PHP.eB) based CasRx into the inner ears of neonatal Bth mice, and we found that Tmc1Bth was reduced by 70.2% in 2 weeks with few off-target effects in the whole transcriptome. Consistently, we found improved hair cell survival, rescued hair bundle degeneration, and reduced mechanoelectrical transduction current. Importantly, the hearing performance, measured in both ABR and DPOAE thresholds, was improved significantly in all ages over 8 weeks. We, therefore, have validated the CRISPR/CasRx-based RNA editing strategy in treating autosomal-dominant hearing loss, paving way for its further application in many other hereditary diseases in hearing and beyond.

Computational Genomics in the Era of Precision Medicine: Applications to Variant Analysis and Gene Therapy

Rapid methodological advances in statistical and computational genomics have enabled researchers to better identify and interpret both rare and common variants responsible for complex human diseases. As we continue to see an expansion of these advances in the field, it is now imperative for researchers to understand the resources and methodologies available for various data types and study designs. In this review, we provide an overview of recent methods for identifying rare and common variants and understanding their roles in disease etiology. Additionally, we discuss the strategy, challenge, and promise of gene therapy. As computational and statistical approaches continue to improve, we will have an opportunity to translate human genetic findings into personalized health care.

Gene editing in a Myo6 semi-dominant mouse model rescues auditory function

Myosin VI（MYO6） is an unconventional myosin that is vital for auditory and vestibular function. Pathogenic variants in the human MYO6 gene cause autosomal-dominant or -recessive forms of hearing loss. Effective treatments for Myo6 mutation causing hearing loss are limited. We studied whether adeno-associated virus (AAV)-PHP.eB vector-mediated in vivo delivery of Staphylococcus aureus Cas9 (SaCas9-KKH)-single-guide RNA (sgRNA) complexes could ameliorate hearing loss in a Myo6WT/C442Y mouse model that recapitulated the phenotypes of human patients. The in vivo editing efficiency of the AAV-SaCas9-KKH-Myo6-g2 system on Myo6C442Y is 4.05% on average in Myo6WT/C442Y mice, which was ∼17-fold greater than editing efficiency of Myo6WT alleles. Rescue of auditory function was observed up to 5 months post AAV-SaCas9-KKH-Myo6-g2 injection in Myo6WT/C442Y mice. Meanwhile, shorter latencies of auditory brainstem response (ABR) wave I, lower distortion product otoacoustic emission (DPOAE) thresholds, increased cell survival rates, more regular hair bundle morphology, and recovery of inward calcium levels were also observed in the AAV-SaCas9-KKH-Myo6-g2-treated ears compared to untreated ears. These findings provide further reference for in vivo genome editing as a therapeutic treatment for various semi-dominant forms of hearing loss and other semi-dominant diseases.

Capsid-Engineering for Central Nervous System-Directed Gene Therapy with Adeno-Associated Virus Vectors

Closing the gap in knowledge on the cause of neurodegenerative disorders is paving the way toward innovative treatment strategies, among which gene therapy has emerged as a top candidate. Both conventional gene therapy and genome editing approaches are being developed, and a great number of human clinical trials are ongoing. Already 2 years ago, the first gene therapy for a neurodegenerative disease, spinal muscular atrophy type 1 (SMA1), obtained market approval. To realize such innovative strategies, gene therapy delivery tools are key assets. Here, we focus on recombinant adeno-associated virus (AAV) vectors and report on strategies to improve first-generation vectors. Current efforts focus on the viral capsid to modify the host-vector interaction aiming at increasing the efficacy of target cell transduction, at simplifying vector administration, and at reducing the risk of vector dose-related side effects.

Milestones toward cochlear gene therapy for patients with hereditary hearing loss

A number of genes are reportedly responsible for hereditary hearing loss, which accounts for over 50% of all congenital hearing loss cases. Recent advances in genetic testing have enabled the identification of pathogenic variants in many cases, and systems have been developed to provide personalized treatment based on etiology. Gene therapy is expected to become an unprecedented curative treatment. Several reports have demonstrated the successful use of cochlear gene therapy to restore auditory function in mouse models of genetic deafness; however, many hurdles remain to its clinical application in humans. Herein, we focus on the frequency of deafness genes in patients with congenital and late-onset progressive hearing loss and discuss the following points regarding which genes need to be targeted to efficiently proceed with clinical application: (a) which cells' genes are expressed within the cochlea, (b) whether gene transfer to the targeted cells is possible using vectors such as adeno-associated virus, (c) what phenotype of hearing loss in patients is exhibited, and (d) whether mouse models exist to verify the effectiveness of treatment. Moreover, at the start of clinical application, gene therapy in combination with cochlear implantation may be useful for cases of progressive hearing loss.

Prevalence and clinical features of autosomal dominant and recessive TMC1-associated hearing loss

TMC1 is a causative gene for both autosomal dominant non-syndromic hearing loss (DFNA36) and autosomal recessive non-syndromic hearing loss (DFNB7/11). To date, 125 pathogenic variants in TMC1 have been reported. Most of the TMC1 variants are responsible for autosomal recessive hearing loss, with only 8 variants reported as causative for DFNA36. Here, we reported the prevalence of TMC1-associated hearing loss in a large non-syndromic hearing loss cohort of about 12,000 subjects. As a result, we identified 26 probands with TMC1-associated hearing loss, with the estimated prevalence of TMC1-associated hearing loss in the Japanese hearing loss cohort being 0.17% among all patients. Among the 26 probands with TMC1-associated hearing loss, 15 cases were identified from autosomal dominant hearing loss families. Based on the audiometric data from the probands, family members and previously reported cases, we evaluated hearing deterioration for DFNA36 patients. In addition, we performed haplotype analysis for 11 unrelated autosomal dominant hearing loss families carrying the same variant TMC1: NM_138691:c.1627G > A:p.Asp543Asn. The results clearly indicated that the same haplotype was present despite the families being unrelated, supporting the contention that this variant occurred by founder mutation.

An Integrated Perspective of Evolution and Development: From Genes to Function to Ear, Lateral Line and Electroreception

Four sensory systems (vestibular, lateral line, electroreception, auditory) are unique and project exclusively to the brainstem of vertebrates. All sensory neurons depend on a common set of genes (Eya1, Sox2, Neurog1, Neurod1) that project to a dorsal nucleus and an intermediate nucleus, which differentiate into the vestibular ear, lateral line and electroreception in vertebrates. In tetrapods, a loss of two sensory systems (lateral line, electroreception) leads to the development of a unique ear and auditory system in amniotes. Lmx1a/b, Gdf7, Wnt1/3a, BMP4/7 and Atoh1 define the lateral line, electroreception and auditory nuclei. In contrast, vestibular nuclei depend on Neurog1/2, Ascl1, Ptf1a and Olig3, among others, to develop an independent origin of the vestibular nuclei. A common origin of hair cells depends on Eya1, Sox2 and Atoh1, which generate the mechanosensory cells. Several proteins define the polarity of hair cells in the ear and lateral line. A unique connection of stereocilia requires CDH23 and PCDH15 for connections and TMC1/2 proteins to perceive mechanosensory input. Electroreception has no polarity, and a different system is used to drive electroreceptors. All hair cells function by excitation via ribbons to activate neurons that innervate the distinct target areas. An integrated perspective is presented to understand the gain and loss of different sensory systems.

Tsukushi is essential for the formation of the posterior semicircular canal that detects gait performance

Tsukushi is a small, leucine-rich repeat proteoglycan that interacts with and regulates essential cellular signaling cascades in the chick retina and murine subventricular zone, hippocampus, dermal hair follicles, and the cochlea. However, its function in the vestibules of the inner ear remains unknown. Here, we investigated the function of Tsukushi in the vestibules and found that Tsukushi deficiency in mice resulted in defects in posterior semicircular canal formation in the vestibules, but did not lead to vestibular hair cell loss. Furthermore, Tsukushi accumulated in the non-prosensory and prosensory regions during the embryonic and postnatal developmental stages. The downregulation of Tsukushi altered the expression of key genes driving vestibule differentiation in the non-prosensory regions. Our results indicate that Tsukushi interacts with Wnt2b, bone morphogenetic protein 4, fibroblast growth factor 10, and netrin 1, thereby controlling semicircular canal formation. Therefore, Tsukushi may be an essential component of the molecular pathways regulating vestibular development.

Stem Cells and Gene Therapy in Progressive Hearing Loss: the State of the Art

Progressive non-syndromic sensorineural hearing loss (PNSHL) is the most common cause of sensory impairment, affecting more than a third of individuals over the age of 65. PNSHL includes noise-induced hearing loss (NIHL) and inherited forms of deafness, among which is delayed-onset autosomal dominant hearing loss (AD PNSHL). PNSHL is a prime candidate for genetic therapies due to the fact that PNSHL has been studied extensively, and there is a potentially wide window between identification of the disorder and the onset of hearing loss. Several gene therapy strategies exist that show potential for targeting PNSHL, including viral and non-viral approaches, and gene editing versus gene-modulating approaches. To fully explore the potential of these therapy strategies, a faithful in vitro model of the human inner ear is needed. Such models may come from induced pluripotent stem cells (iPSCs). The development of new treatment modalities by combining iPSC modeling with novel and innovative gene therapy approaches will pave the way for future applications leading to improved quality of life for many affected individuals and their families.

Advances and challenges in adeno-associated viral inner-ear gene therapy for sensorineural hearing loss

There is growing attention and effort focused on treating the root cause of sensorineural hearing loss rather than managing associated secondary characteristic features. With recent substantial advances in understanding sensorineural hearing-loss mechanisms, gene delivery has emerged as a promising strategy for the biological treatment of hearing loss associated with genetic dysfunction. There are several successful and promising proof-of-principle examples of transgene deliveries in animal models; however, there remains substantial further progress to be made in these avenues before realizing their clinical application in humans. Herein, we review different aspects of development, ongoing preclinical studies, and challenges to the clinical transition of transgene delivery of the inner ear toward the restoration of lost auditory and vestibular function.

Single and Dual Vector Gene Therapy with AAV9-PHP.B Rescues Hearing in Tmc1 Mutant Mice

AAV-mediated gene therapy is a promising approach for treating genetic hearing loss. Replacement or editing of the Tmc1 gene, encoding hair cell mechanosensory ion channels, is effective for hearing restoration in mice with some limitations. Efficient rescue of outer hair cell function and lack of hearing recovery with later-stage treatment remain issues to be solved. Exogenous genes delivered with the adeno-associated virus (AAV)9-PHP.B capsid via the utricle transduce both inner and outer hair cells of the mouse cochlea with high efficacy. Here, we demonstrate that AAV9-PHP.B gene therapy can promote hair cell survival and successfully rescues hearing in three distinct mouse models of hearing loss. Tmc1 replacement with AAV9-PHP.B in a Tmc1 knockout mouse rescues hearing and promotes hair cell survival with equal efficacy in inner and outer hair cells. The same treatment in a recessive Tmc1 hearing-loss model, Baringo, partially recovers hearing even with later-stage treatment. Finally, dual delivery of Streptococcus pyogenes Cas9 (SpCas9) and guide RNA (gRNA) in separate AAV9-PHP.B vectors selectively disrupts a dominant Tmc1 allele and preserves hearing in Beethoven mice, a model of dominant, progressive hearing loss. Tmc1-targeted gene therapies using single or dual AAV9-PHP.B vectors offer potent and versatile approaches for treating dominant and recessive deafness.

Advances in genome editing for genetic hearing loss

According to the World Health Organization, hearing loss affects over 466 million people worldwide and is the most common human sensory impairment. It is estimated that genetic factors contribute to the causation of approximately 50% of congenital hearing loss. Yet, curative approaches to reversing or preventing genetic hearing impairment are still limited. The clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) systems enable programmable and targeted gene editing in highly versatile manners and offer new gene therapy strategies for genetic hearing loss. Here, we summarize the most common deafness-associated genes, illustrate recent strategies undertaken by using CRISPR-Cas9 systems for targeted gene editing and further compare the CRISPR strategies to non-CRISPR gene therapies. We also examine the merits of different vehicles and delivery forms of genome editing agents. Lastly, we describe the development of animal models that could facilitate the eventual clinical applications of the CRISPR technology to the treatment of genetic hearing diseases.

Gene therapy and gene correction: targets, progress, and challenges for treating human diseases

The field of gene therapy has made significant strides over the last several decades toward the treatment of previously untreatable genetic disease. Gene therapy techniques have been aimed at mitigating disease features of recessive and dominant disorders, as well as several cancers and other diseases. While there have been numerous disease targets of gene therapy trials, only four therapies have reached FDA and/or EMA approval for clinical use. Gene correction using CRISPR-Cas9 is an extension of gene therapy that has received considerable attention in recent years and boasts many possible uses beyond classical gene therapy approaches. While there is significant therapeutic potential using gene therapy and gene correction strategies, a number of hurdles remain to be overcome before they become more common in clinical use, particularly with regards to safety and efficacy. As research progresses in this exciting field, it is likely that these therapies will become first-line treatments and will have tremendous positive impacts on the lives of patients with genetic disorders.

Adeno-associated virus gene replacement for recessive inner ear dysfunction: Progress and challenges

Approximately 3 in 1000 children in the US under 4 years of age are affected by hearing loss. Currently, cochlear implants represent the only line of treatment for patients with severe to profound hearing loss, and there are no targeted drug or biological based therapies available. Gene replacement is a promising therapeutic approach for hereditary hearing loss, where viral vectors are used to deliver functional cDNA to "replace" defective genes in dysfunctional cells in the inner ear. Proof-of-concept studies have successfully used this approach to improve auditory function in mouse models of hereditary hearing loss, and human clinical trials are on the immediate horizon. The success of this method is ultimately determined by the underlying biology of the defective gene and design of the treatment strategy, relying on intervention before degeneration of the sensory structures occurs. A challenge will be the delivery of a corrective gene to the proper target within the therapeutic window of opportunity, which may be unique for each specific defective gene. Although rescue of pre-lingual forms of recessive deafness have been explored in animal models thus far, future identification of genes with post-lingual onset that are amenable to gene replacement holds even greater promise for treatment, since the therapeutic window is likely open for a much longer period of time. This review summarizes the current state of adeno-associated virus (AAV) gene replacement therapy for recessive hereditary hearing loss and discusses potential challenges and opportunities for translating inner ear gene replacement therapy for patients with hereditary hearing loss.

Genome and base editing for genetic hearing loss

Genome editing opens up a new frontier in developing personalized therapeutic solutions. With the unprecedented advance in the discovery and engineering of gene editing nucleases, it has now become potentially feasible to therapeutically influence up to 90% of all human genetic mutations. Hearing loss is one of the most well studied fields from the genetics perspective, with more than one hundred identified deafness genes. Novel viral and non-viral vectors have been established as safe and efficient modalities to deliver transgenes into cells of the cochlea and to the vestibular system in animal models. Recent studies demonstrated proof-of-concept for therapeutic genome and base editing in the mammalian inner ear and preclinical development is ongoing. This review summarizes important advances and future challenges for this transformative therapeutic modality for genetic and non-genetic hearing loss.

Inner Ear Gene Therapies Take Off: Current Promises and Future Challenges

Hearing impairment is the most frequent sensory deficit in humans of all age groups, from children (1/500) to the elderly (more than 50% of the over-75 s). Over 50% of congenital deafness are hereditary in nature. The other major causes of deafness, which also may have genetic predisposition, are aging, acoustic trauma, ototoxic drugs such as aminoglycosides, and noise exposure. Over the last two decades, the study of inherited deafness forms and related animal models has been instrumental in deciphering the molecular, cellular, and physiological mechanisms of disease. However, there is still no curative treatment for sensorineural deafness. Hearing loss is currently palliated by rehabilitation methods: conventional hearing aids, and for more severe forms, cochlear implants. Efforts are continuing to improve these devices to help users to understand speech in noisy environments and to appreciate music. However, neither approach can mediate a full recovery of hearing sensitivity and/or restoration of the native inner ear sensory epithelia. New therapeutic approaches based on gene transfer and gene editing tools are being developed in animal models. In this review, we focus on the successful restoration of auditory and vestibular functions in certain inner ear conditions, paving the way for future clinical applications.

Gene therapy development in hearing research in China

Sensorineural hearing loss, the most common form of hearing impairment, is mainly attributable to genetic mutations or acquired factors, such as aging, noise exposure, and ototoxic drugs. In the field of gene therapy, advances in genetic and physiological studies and profound increases in knowledge regarding the underlying mechanisms have yielded great progress in terms of restoring the auditory function in animal models of deafness. Nonetheless, many challenges associated with the translation from basic research to clinical therapies remain to be overcome before a total restoration of auditory function can be expected. In recent years, Chinese research teams have promoted various developmental efforts in this field, including gene sequencing to identify additional potential loci that cause deafness, studies to elucidate the underlying molecular mechanisms, and research to optimize vectors and delivery routes. In this review, we summarize the state of the field and focus mainly on the progress of gene therapy in animal model studies and the optimization of therapeutic strategies in China.

Diagnostic and therapeutic applications of genomic medicine in progressive, late-onset, nonsyndromic sensorineural hearing loss

The progressive, late-onset, nonsyndromic, sensorineural hearing loss (PNSHL) is the most common cause of sensory impairment globally, with presbycusis affecting greater than a third of individuals over the age of 65. The etiology underlying PNSHL include presbycusis, noise-induced hearing loss, drug ototoxicity, and delayed-onset autosomal dominant hearing loss (AD PNSHL). The objective of this article is to discuss the potential diagnostic and therapeutic applications of genomic medicine in PNSHL. Genomic factors contribute greatly to PNSHL. The heritability of presbycusis ranges from 25 to 75%. Current therapies for PNSHL range from sound amplification to cochlear implantation (CI). PNSHL is an excellent candidate for genomic medicine approaches as it is common, has well-described pathophysiology, has a wide time window for treatment, and is amenable to local gene therapy by currently utilized procedural approaches. AD PNSHL is especially suited to genomic medicine approaches that can disrupt the expression of an aberrant protein product. Gene therapy is emerging as a potential therapeutic strategy for the treatment of PNSHL. Viral gene delivery approaches have demonstrated promising results in human clinical trials for two inherited causes of blindness and are being used for PNSHL in animal models and a human trial. Non-viral gene therapy approaches are useful in situations where a transient biologic effect is needed or for delivery of genome editing reagents (such as CRISPR/Cas9) into the inner ear. Many gene therapy modalities that have proven efficacious in animal trials have potential to delay or prevent PNSHL in humans. The development of new treatment modalities for PNSHL will lead to improved quality of life of many affected individuals and their families.

A Review of Gene, Drug and Cell-Based Therapies for Usher Syndrome

Usher syndrome is a genetic disorder causing neurosensory hearing loss and blindness from retinitis pigmentosa (RP). Adaptive techniques such as braille, digital and optical magnifiers, mobility training, cochlear implants, or other assistive listening devices are indispensable for reducing disability. However, there is currently no treatment to reduce or arrest sensory cell degeneration. There are several classes of treatments for Usher syndrome being investigated. The present article reviews the progress this research has made towards delivering commercial options for patients with Usher syndrome.

Efficient in Utero Gene Transfer to the Mammalian Inner Ears by the Synthetic Adeno-Associated Viral Vector Anc80L65

Sensorineural hearing loss is one of the most common sensory disorders worldwide. Recent advances in vector design have paved the way for investigations into the use of adeno-associated vectors (AAVs) for hearing disorder gene therapy. Numerous AAV serotypes have been discovered to be applicable to inner ears, constituting a key advance for gene therapy for sensorineural hearing loss, where transduction efficiency of AAV in inner ear cells is critical for success. One such viral vector, AAV2/Anc80L65, has been shown to yield high expression in the inner ears of mice treated as neonates or adults. Here, to evaluate the feasibility of prenatal gene therapy for deafness, we assessed the transduction efficiency of AAV2/Anc80L65-eGFP (enhanced green fluorescent protein) after microinjection into otocysts in utero. This embryonic delivery method achieved high transduction efficiency in both inner and outer hair cells of the cochlea. Additionally, the transduction efficiency was high in the hair cells of the vestibules and semicircular canals and in spiral ganglion neurons. Our results support the potential of Anc80L65 as a gene therapy vehicle for prenatal inner ear disorders.

Fetal antisense oligonucleotide therapy for congenital deafness and vestibular dysfunction

Disabling hearing loss impacts ∼466 million individuals worldwide with 34 million children affected. Gene and pharmacotherapeutic strategies to rescue auditory function in mouse models of human deafness are most effective when administered before hearing onset, after which therapeutic efficacy is significantly diminished or lost. We hypothesize that preemptive correction of a mutation in the fetal inner ear prior to maturation of the sensory epithelium will optimally restore sensory function. We previously demonstrated that transuterine microinjection of a splice-switching antisense oligonucleotide (ASO) into the amniotic cavity immediately surrounding the embryo on embryonic day 13-13.5 (E13-13.5) corrected pre-mRNA splicing in the juvenile Usher syndrome type 1c (Ush1c) mouse mutant. Here, we show that this strategy only marginally rescues hearing and partially rescues vestibular function. To improve therapeutic outcomes, we microinjected ASO directly into the E12.5 inner ear. A single intra-otic dose of ASO corrects harmonin RNA splicing, restores harmonin protein expression in sensory hair cell bundles, prevents hair cell loss, improves hearing sensitivity, and ameliorates vestibular dysfunction. Improvements in auditory and vestibular function were sustained well into adulthood. Our results demonstrate that an ASO pharmacotherapeutic administered to a developing organ system in utero preemptively corrects pre-mRNA splicing to abrogate the disease phenotype.

Recent development of AAV-based gene therapies for inner ear disorders

Gene therapy for auditory diseases is gradually maturing. Recent progress in gene therapy treatments for genetic and acquired hearing loss has demonstrated the feasibility in animal models. However, a number of hurdles, such as lack of safe viral vector with high efficiency and specificity, robust deafness large animal models, translating animal studies to clinic etc., still remain to be solved. It is necessary to overcome these challenges in order to effectively recover auditory function in human patients. Here, we review the progress made in our group, especially our efforts to make more effective and cell type-specific viral vectors for targeting cochlea cells.