Mouse otocyst transuterine gene transfer restores hearing in mice with connexin 30 deletion-associated hearing loss

Hereditary Hearing Loss: A Systematic Review of Potential Treatments and Interventions

PURPOSE

The purpose of this study was to systematically review the research literature with regards to treatments and intervention methods for hereditary hearing loss. Our goal was to provide reference guidelines for the rational use of medication and gene-targeted therapy for patients with hereditary hearing loss and discuss the future development of research in this area.

METHOD

We searched two core databases, PubMed and Web of Science, for relevant literature relating to potential treatments and interventional methods for hereditary hearing loss. Then, we used Microsoft Excel to perform basic statistical analysis of the data, the R language to perform bibliometric analyses, and VOSviewer and CiteSpace to visualize data. In addition, we clustered and descriptively analyzed the data and identified the relative importance of each approach with regard to precise patient outcomes.

RESULTS

In this study, we followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standardized screening process and identified a total of 103 research articles. The average annual growth rate of publications in this area was 12.73%. The country with the highest number of publications and citations was the United States; 80 of these publications (associated with 76.92% of funding) were supported by grants from 16 countries. Potential treatments and interventions were clustered according to the stage of research and showed that 8.74% remain in the research design stage, 59.22% are in the clinical validation stage, and 32.04% are being applied in the clinic. The main research focus in this field is cochlear implants and gene therapy.

CONCLUSIONS

Hereditary hearing loss is in a critical period of transition from preventive to therapeutic research. Gene-targeted interventions represent one of the most promising and effective treatments.

SUPPLEMENTAL MATERIAL

https://doi.org/10.23641/asha.24309193.

Differential regulation of hair cell actin cytoskeleton mediated by SRF and MRTFB

The MRTF-SRF pathway has been extensively studied for its crucial role in driving the expression of a large number of genes involved in actin cytoskeleton of various cell types. However, the specific contribution of MRTF-SRF in hair cells remains unknown. In this study, we showed that hair cell-specific deletion of Srf or Mrtfb, but not Mrtfa, leads to similar defects in the development of stereocilia dimensions and the maintenance of cuticular plate integrity. We used fluorescence-activated cell sorting-based hair cell RNA-Seq analysis to investigate the mechanistic underpinnings of the changes observed in Srf and Mrtfb mutants, respectively. Interestingly, the transcriptome analysis revealed distinct profiles of genes regulated by Srf and Mrtfb, suggesting different transcriptional regulation mechanisms of actin cytoskeleton activities mediated by Srf and Mrtfb. Exogenous delivery of calponin 2 using Adeno-associated virus transduction in Srf mutants partially rescued the impairments of stereocilia dimensions and the F-actin intensity of cuticular plate, suggesting the involvement of Cnn2, as an Srf downstream target, in regulating the hair bundle morphology and cuticular plate actin cytoskeleton organization. Our study uncovers, for the first time, the unexpected differential transcriptional regulation of actin cytoskeleton mediated by Srf and Mrtfb in hair cells, and also demonstrates the critical role of SRF-CNN2 in modulating actin dynamics of the stereocilia and cuticular plate, providing new insights into the molecular mechanism underlying hair cell development and maintenance.

Mitochondrial alterations in the cochlea of Cdk5rap1-knockout mice with age-related hearing loss

Previous studies have revealed that age-related hearing loss (AHL) in Cdk5 regulatory subunit-associated protein 1 (Cdk5rap1)-knockout mice is associated with pathology in the cochlea. Here, we aimed to identify mitochondrial alterations in the cochlea of Cdk5rap1-knockout mice with AHL. Mitochondria in the spiral ganglion neurons (SGNs) and hair cells (HCs) were normal despite senescence; however, the mitochondria of types I, II, and IV spiral ligament fibrocytes were ballooned, damaged, and ballooned, respectively, in the stria vascularis. Our results suggest that the accumulation of dysfunctional mitochondria in the lateral wall, rather than the loss of HCs and SGNs, leads to the onset of AHL. Our results provide valuable information regarding the underlying mechanisms of AHL and the relationship between aberrant tRNA modification-induced hearing loss and mitochondrial dysfunction.

Arginine-Rich Cell-Penetrating Peptide-Mediated Transduction of Mouse Nasal Cells with FOXP3 Protein Alleviates Allergic Rhinitis

Intranasal corticosteroids are effective medications against allergic rhinitis (AR). However, mucociliary clearance promptly eliminates these drugs from the nasal cavity and delays their onset of action. Therefore, a faster, longer-lasting therapeutic effect on the nasal mucosa is required to enhance the efficacy of AR management. Our previous study showed that polyarginine, a cell-penetrating peptide, can deliver cargo to nasal cells; moreover, polyarginine-mediated cell-nonspecific protein transduction into the nasal epithelium exhibited high transfection efficiency with minimal cytotoxicity. In this study, poly-arginine-fused forkhead box P3 (FOXP3) protein, the "master transcriptional regulator" of regulatory T cells (Tregs), was administered into the bilateral nasal cavities of the ovalbumin (OVA)-immunoglobulin E mouse model of AR. The effects of these proteins on AR following OVA administration were investigated using histopathological, nasal symptom, flow cytometry, and cytokine dot blot analyses. Polyarginine-mediated FOXP3 protein transduction induced Treg-like cell generation in the nasal epithelium and allergen tolerance. Overall, this study proposes FOXP3 activation-mediated Treg induction as a novel and potential therapeutic strategy for AR, providing a potential alternative to conventional intranasal drug application for nasal drug delivery.

Recent advances and future challenges in gene therapy for hearing loss

Hearing loss is the most common sensory deficit experienced by humans and represents one of the largest chronic health conditions worldwide. It is expected that around 10% of the world's population will be affected by disabling hearing impairment by 2050. Hereditary hearing loss accounts for most of the known forms of congenital deafness, and over 25% of adult-onset or progressive hearing loss. Despite the identification of well over 130 genes associated with deafness, there is currently no curative treatment for inherited deafness. Recently, several pre-clinical studies in mice that exhibit key features of human deafness have shown promising hearing recovery through gene therapy involving the replacement of the defective gene with a functional one. Although the potential application of this therapeutic approach to humans is closer than ever, substantial further challenges need to be overcome, including testing the safety and longevity of the treatment, identifying critical therapeutic time windows and improving the efficiency of the treatment. Herein, we provide an overview of the recent advances in gene therapy and highlight the current hurdles that the scientific community need to overcome to ensure a safe and secure implementation of this therapeutic approach in clinical trials.

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.

Towards the Clinical Application of Gene Therapy for Genetic Inner Ear Diseases

Hearing loss, the most common human sensory defect worldwide, is a major public health problem. About 70% of congenital forms and 25% of adult-onset forms of deafness are of genetic origin. In total, 136 deafness genes have already been identified and there are thought to be several hundred more awaiting identification. However, there is currently no cure for sensorineural deafness. In recent years, translational research studies have shown gene therapy to be effective against inherited inner ear diseases, and the application of this technology to humans is now within reach. We provide here a comprehensive and practical overview of current advances in gene therapy for inherited deafness, with and without an associated vestibular defect. We focus on the different gene therapy approaches, considering their prospects, including the viral vector used, and the delivery route. We also discuss the clinical application of the various strategies, their strengths, weaknesses, and the challenges to be overcome.

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.

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.

Virally Mediated Connexin 26 Expression in Postnatal Scala Media Significantly and Transiently Preserves Hearing in Connexin 30 Null Mice

Non-sensory cells in the sensory epithelium of the cochlea are connected extensively by gap junctions. Functionally null mutations in GJB6 (encoding Cx30) cause hearing loss in humans. In this study, we injected AAV1-CB7-Gjb2 into the scala media between P0-2 in the cochlea of Gjb6^−/− mice. The injection increased Cx26 expression and significantly preserved auditory functions. However, the hearing preservation gradually declined and essentially disappeared 3 months after the injections. In contrast, the morphological preservation was still significant at 3 months post-injection. We found that the expression of Cx26, at both the mRNA and protein levels, showed substantial decreases during the 3-month period. Curiously, treatments by injecting AAV1-CB7-Gjb6 with the identical approach failed to yield any hearing preservation. Our results demonstrated the first successful cochlear gene therapy treatment in mouse models by virally expressing a companion gene of Gjb6.

Dynamic Spatiotemporal Expression Changes in Connexins of the Developing Primate's Cochlea

Connexins are gap junction components that are essential for acquiring normal hearing ability. Up to 50% of congenital, autosomal-recessive, non-syndromic deafness can be attributed to variants in GJB2, the gene that encodes connexin 26. Gene therapies modifying the expression of connexins are a feasible treatment option for some patients with genetic hearing losses. However, the expression patterns of these proteins in the human fetus are not fully understood due to ethical concerns. Recently, the common marmoset was used as a primate animal model for the human fetus. In this study, we examined the expression patterns of connexin 26 and connexin 30 in the developing cochlea of this primate. Primate-specific spatiotemporal expression changes were revealed, which suggest the existence of primate-specific control of connexin expression patterns and specific functions of these gap junction proteins. Moreover, our results indicate that treatments for connexin-related hearing loss established in rodent models may not be appropriate for human patients, underscoring the importance of testing these treatments in primate models before applying them in human clinical trials.

Cdk5 regulatory subunit-associated protein 1 knockout mice show hearing loss phenotypically similar to age-related hearing loss

Mitochondrial dysfunction is associated with aging and age-related hearing loss (AHL). However, the precise mechanisms underlying the pathophysiology of hearing loss remain unclear. Cdk5 regulatory subunit-associated protein 1 (CDK5RAP1) enables efficient intramitochondrial translation by catalyzing the deposition of 2-methylthio modifications on mitochondrial tRNAs. Here we investigated the effect of defective mitochondrial protein translation on hearing and AHL in a Cdk5rap1 deficiency C57BL/6 mouse model. Compared to control C57BL/6 mice, Cdk5rap1-knockout female mice displayed hearing loss phenotypically similar to AHL from an early age. The premature hearing loss in Cdk5rap1-knockout mice was associated with the degeneration of the spiral ligament and reduction of endocochlear potentials following the loss of auditory sensory cells. Furthermore, cultured primary mouse embryonic fibroblasts displayed early onset of cellular senescence associated with high oxidative stress and cell death. These results indicate that the CDK5RAP1 deficiency-induced defective mitochondrial translation might cause early hearing loss through the induction of cellular senescence and cochlear dysfunction in the inner ear. Our results suggest that the accumulation of dysfunctional mitochondria might promote AHL progression. Furthermore, our findings suggest that mitochondrial dysfunction and dysregulated mitochondrial tRNA modifications mechanistically cause AHL. Understanding the mechanisms underlying AHL will guide future clinical investigations and interventions in the attempt to mitigate the consequences of AHL.

Gene Therapy to the Retina and the Cochlea

Vision and hearing disorders comprise the most common sensory disorders found in people. Many forms of vision and hearing loss are inherited and current treatments only provide patients with temporary or partial relief. As a result, developing genetic therapies for any of the several hundred known causative genes underlying inherited retinal and cochlear disorders has been of great interest. Recent exciting advances in gene therapy have shown promise for the clinical treatment of inherited retinal diseases, and while clinical gene therapies for cochlear disease are not yet available, research in the last several years has resulted in significant advancement in preclinical development for gene delivery to the cochlea. Furthermore, the development of somatic targeted genome editing using CRISPR/Cas9 has brought new possibilities for the treatment of dominant or gain-of-function disease. Here we discuss the current state of gene therapy for inherited diseases of the retina and cochlea with an eye toward areas that still need additional development.

Gene therapy for genetic mutations affecting non-sensory cells in the cochlea

Congenital hearing loss (HL) affects about 1 in every 500 infants. Among those affected more than half are caused by genetic mutations. According to the cellular sites affected by mutations in the cochlea, deafness genes could be classified into three major groups: those affecting the function of hair cells and synapses, cochlear supporting cells, and cells in the stria vascularis (SV) as well as in the lateral wall. The second and third groups account for more than half of all sensorineural hearing loss (SNHL) cases caused by genetic mutations. Current major treatment options for SNHL patients are hearing aids and cochlear implants (CIs). Hearing aids can only help patients with moderate to severe HL. Resolution of CIs is still improving and these devices are quite expensive especially when lifetime rehabilitation and maintenance costs are included. Tremendous efforts have been made to find novel treatments that are expected to restore hearing with higher-resolution and more natural quality, and to have a significantly lower cost over the lifetime of uses. Gene therapy studies have made impressive progresses in preclinical trials. This review focuses on deafness genes that affect supporting cells and cells in the SV of the cochlea. We will discuss recent progresses and remaining challenges for gene therapies targeting mutations in deafness genes belonging to this category.

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.

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.

Fetal gene therapy and pharmacotherapy to treat congenital hearing loss and vestibular dysfunction

Disabling hearing loss is expected to affect over 900 million people worldwide by 2050. The World Health Organization estimates that the annual economic impact of hearing loss globally is US$ 750 billion. The inability to hear may complicate effective interpersonal communication and negatively impact personal and professional relationships. Recent advances in the genetic diagnosis of inner ear disease have keenly focused attention on strategies to restore hearing and balance in individuals with defined gene mutations. Mouse models of human hearing loss serve as the primary approach to test gene therapies and pharmacotherapies. The goal of this review is to articulate the rationale for fetal gene therapy and pharmacotherapy to treat congenital hearing loss and vestibular dysfunction. The differential onset of hearing in mice and humans suggests that a prenatal window of therapeutic efficacy in humans may be optimal to restore sensory function. Mouse studies demonstrating the utility of early fetal intervention in the inner ear show promise. We focus on the modulation of gene expression through two strategies that have successfully treated deafness in animal models and have had clinical success for other conditions in humans: gene replacement and antisense oligonucleotide-mediated modulation of gene expression. The recent establishment of effective therapies targeting the juvenile and adult mouse provide informative counterexamples where intervention in the maturing and fully functional mouse inner ear may be effective. Distillation of the current literature leads to the conclusion that novel therapeutic strategies to treat genetic deafness and imbalance will soon translate to clinical trials.

Tsukushi is essential for the development of the inner ear

Tsukushi (TSK)—a small, secreted, leucine-rich-repeat proteoglycan—interacts with and regulates essential cellular signaling cascades. However, its functions in the mouse inner ear are unknown. In this study, measurement of auditory brainstem responses, fluorescence microscopy, and scanning electron microscopy revealed that TSK deficiency in mice resulted in the formation of abnormal stereocilia in the inner hair cells and hearing loss but not in the loss of these cells. TSK accumulated in nonprosensory regions during early embryonic stages and in both nonprosensory and prosensory regions in late embryonic stages. In adult mice, TSK was localized in the organ of Corti, spiral ganglion cells, and the stria vascularis. Moreover, loss of TSK caused dynamic changes in the expression of key genes that drive the differentiation of the inner hair cells in prosensory regions. Finally, our results revealed that TSK interacted with Sox2 and BMP4 to control stereocilia formation in the inner hair cells. Hence, TSK appears to be an essential component of the molecular pathways that regulate inner ear development.

Prenatal electroporation-mediated gene transfer restores Slc26a4 knock-out mouse hearing and vestibular function

The otocyst, an anlage of the inner ear, presents an attractive target to study treatment strategies for genetic hearing loss and inner ear development. We have previously reported that electroporation-mediated transuterine gene transfer of Connexin30, utilizing a monophasic pulse into Connexin30^−/− mouse otocysts at embryonic day 11.5, is able to prevent putative hearing deterioration. However, it is not clear whether supplementary gene transfer can rescue significant morphological changes, caused by genetic deficits. In addition, with the transuterine gene transfer technique utilized in our previous report, the survival rate of embryos and their mothers after treatment was low, which became a serious obstacle for effective in vivo experiments. Here, we set out to elucidate the feasibility of supplementation therapy in Slc26a4 deficient mice, utilizing biphasic pulses, optimized by modifying pulse conditions. Modification of the biphasic pulse conditions during electroporation increased the survival rate. In addition, supplementation of the target gene cDNA into the otocysts of homozygous Slc24a4 knockout mice significantly prevented enlargement of the endolymphatic space in the inner ear areas; moreover, it rescued hearing and vestibular function of mice in vivo.

Structure and Function of Cochlear Gap Junctions and Implications for the Translation of Cochlear Gene Therapies

Connexins (Cxs) are ubiquitous membrane proteins that are found throughout vertebrate organs, acting as building blocks of the gap junctions (GJs) known to play vital roles in the normal function of many organs. Mutations in Cx genes (particularly GJB2, which encodes Cx26) cause approximately half of all cases of congenital hearing loss in newborns. Great progress has been made in understanding GJ function and the molecular mechanisms for the role of Cxs in the cochlea. Data reveal that multiple types of Cxs work together to ensure normal development and function of the cochlea. These findings include many aspects not proposed in the classic K+ recycling theory, such as the formation of normal cochlear morphology (e.g., the opening of the tunnel of Corti), the fine-tuning of the innervation of nerve fibers to the hair cells (HCs), the maturation of the ribbon synapses, and the initiation of the endocochlear potential (EP). New data, especially those collected from targeted modification of major Cx genes in the mouse cochlea, have demonstrated that Cx26 plays an essential role in the postnatal maturation of the cochlea. Studies also show that Cx26 and Cx30 assume very different roles in the EP generation, given that only Cx26 is required for normal hearing. This article will review our current understanding of the molecular structure, cellular distribution, and major functions of cochlear GJs. Potential implications of the knowledge of cochlear GJs on the design and implementation of translational studies of cochlear gene therapies for Cx mutations are also discussed.

Genetic Therapies for Hearing Loss: Accomplishments and Remaining Challenges

More than 15 years have passed since the official completion of the Human Genome Project. Predominantly due to this project, over one hundred genes have now been linked to hearing loss. Although major advancements have been made in the understanding of underlying pathologies in deafness as a consequence of these gene discoveries, biological treatments for these conditions are still not available and current treatments rely on amplification or prosthetics. A promising approach for developing treatments for genetic hearing loss is the most simplistic one, that of gene therapy. Gene therapy would intuitively be ideal for these conditions since it is directed at the very source of the problem. Recent achievements in this field in laboratory models spike hope and optimism among scientists, patients, and industry, and suggest that this approach can mature into clinical trials in the coming years. Here we review the existing literature and discuss the different aspects of developing gene therapy for genetic hearing loss.

Role of Dach1 revealed using a novel inner ear-specific Dach1-knockdown mouse model

The Dach1 gene is expressed in the inner ear of normal mouse embryos in the area that differentiates into the cochlear stria vascularis (SV). We hypothesised that Dach1 downregulation in the inner ear would lead to SV dysplasia. However, because Dach1 knockout is embryonic lethal in mice, the role of Dach1 in the inner ear is unclear. Here, we established inner ear-specific Dach1-knockdown mice and showed that Dach1 downregulation resulted in hearing loss, reduced endocochlear potential and secondary outer hair cell loss. There were no abnormalities in marginal cells and basal cells in the SV or spiral ligament in inner ear-specific Dach1-knockdown mature mice. However, intermediate cell dysplasia and thinning of the SV were observed. Moreover, dynamic changes in the expression of key genes related to the epithelial-mesenchymal transition were observed in the lateral wall of the cochlear epithelium, which differentiated into the SV in inner ear-specific Dach1-knockdown mice at embryonic stages. In summary, suppression of Dach1 expression in the inner ear caused the epithelial-mesenchymal transition in the lateral wall of cochlear epithelium, resulting in loss of intermediate cells in the SV and SV dysplasia.

This article has an associated First Person interview with the first author of the paper.

Summary: Inner ear-specific downregulation of Dach1 reveals that the epithelial–mesenchymal transition is crucial for the generation of the stria vascularis.

First person – Toru Miwa

First Person is a series of interviews with the first authors of a selection of papers published in Biology Open, helping early-career researchers promote themselves alongside their papers. Toru Miwa is first author on ‘Role of Dach1 revealed using a novel inner ear-specific Dach1-knockdown mouse model’, published in BiO. Toru conducted the research described in this article while a post-doc fellow of ENT in University of Southern California, in Takahiro Ohyama's lab at the Keck School of Medicine, USA. He is now a head physician specializing in ENT at JCHO Kumamoto General Hospital, Japan, investigating the inner ear.

Engraftment of Human Stem Cell-Derived Otic Progenitors in the Damaged Cochlea

Most cases of sensorineural deafness are caused by degeneration of hair cells. Although stem/progenitor cell therapy is becoming a promising treatment strategy in a variety of organ systems, cell engraftment in the adult mammalian cochlea has not yet been demonstrated. In this study, we generated human otic progenitor cells (hOPCs) from induced pluripotent stem cells (iPSCs) in vitro and identified these cells by the expression of known otic markers. We showed successful cell transplantation of iPSC-derived-hOPCs in an in vivo adult guinea pig model of ototoxicity. The delivered hOPCs migrated throughout the cochlea, engrafted in non-sensory regions, and survived up to 4 weeks post-transplantation. Some of the engrafted hOPCs responded to environmental cues within the cochlear sensory epithelium and displayed molecular features of early sensory differentiation. We confirmed these results with hair cell progenitors derived from Atoh1-GFP mice as donor cells. These mouse otic progenitors transplanted using the same in vivo delivery system migrated into damaged cochlear sensory epithelium and adopted a partial sensory cell fate. This is the first report of the survival and differentiation of hOPCs in ototoxic-injured mature cochlear epithelium, and it should stimulate further research into cell-based therapies for treatment of deafness.

Gene Therapy in Mouse Models of Deafness and Balance Dysfunction

Therapeutic strategies to restore hearing and balance in mouse models of inner ear disease aim to rescue sensory function by gene replacement, augmentation, knock down or knock out. Modalities to achieve therapeutic effects have utilized virus-mediated transfer of wild type genes and small interfering ribonucleic acids; systemic and focal administration of antisense oligonucleotides (ASO) and designer small molecules; and lipid-mediated transfer of Cas 9 ribonucleoprotein (RNP) complexes. This work has established that gene or drug administration to the structurally and functionally immature, early neonatal mouse inner ear prior to hearing onset is a prerequisite for the most robust therapeutic responses. These observations may have significant implications for translating mouse inner ear gene therapies to patients. The human fetus hears by gestational week 19, suggesting that a corollary window of therapeutic efficacy closes early in the second trimester of pregnancy. We hypothesize that fetal therapeutics deployed prior to hearing onset may be the most effective approach to preemptively manage genetic mutations that cause deafness and vestibular dysfunction. We assert that gene therapy studies in higher vertebrate model systems with fetal hearing onset and a comparable acoustic range and sensitivity to that of humans are an essential step to safely and effectively translate murine gene therapies to the clinic.

Differentiation and transplantation of human induced pluripotent stem cell-derived otic epithelial progenitors in mouse cochlea

Background

Inner ear hair cells as mechanoreceptors are extremely important for hearing. Defects in hair cells are a major cause of deafness. Induced pluripotent stem cells (iPSCs) are promising for regenerating inner ear hair cells and treating hearing loss. Here, we investigated migration, differentiation, and synaptic connections of transplanted otic epithelial progenitors (OEPs) derived from human iPSCs in mouse cochlea.

Methods

Human urinary cells isolated from a healthy donor were reprogramed to form iPSCs that were induced to differentiate into OEPs and hair cell-like cells. Immunocytochemistry, electrophysiological examination, and scanning electron microscopy were used to examine characteristics of induced hair cell-like cells. OEP-derived hair cell-like cells were cocultured with spiral ganglion neurons (SGNs), and the markers of synaptic connections were detected using immunocytochemistry and transmission electron microscope. In vivo, OEPs derived from iPSCs were transplanted into the cochlea of mice by injection through the round window. Migration, differentiation, and synaptic connections of transplanted cells were also examined by thin cochlear sectioning and immunohistochemistry.

Results

The induced hair cell-like cells displayed typical morphological characteristics and electrophysiological properties specific to inner hair cells. In vitro, OEP-derived hair cell-like cells formed synaptic connections with SGNs in coculture. In vivo, some of the transplanted cells migrated to the site of the resident hair cells in the organ of Corti, differentiated into hair cell-like cells, and formed synaptic connections with native SGNs.

Conclusions

We conclude that the transplantation of OEPs is feasible for the regeneration of hair cells. These results present a substantial reference for a cell-based therapy for the loss of hair cells.

Cochlear Gene Therapy for Sensorineural Hearing Loss: Current Status and Major Remaining Hurdles for Translational Success

Sensorineural hearing loss (SNHL) affects millions of people. Genetic mutations play a large and direct role in both congenital and late-onset cases of SNHL (e.g., age-dependent hearing loss, ADHL). Although hearing aids can help moderate to severe hearing loss the only effective treatment for deaf patients is the cochlear implant (CI). Gene- and cell-based therapies potentially may preserve or restore hearing with more natural sound perception, since their theoretical frequency resolution power is much higher than that of cochlear implants. These biologically-based interventions also carry the potential to re-establish hearing without the need for implanting any prosthetic device; the convenience and lower financial burden afforded by such biologically-based interventions could potentially benefit far more SNHL patients. Recently major progress has been achieved in preclinical studies of cochlear gene therapy. This review critically evaluates recent advances in the preclinical trials of gene therapies for SNHL and the major remaining challenges for the development and eventual clinical translation of this novel therapy. The cochlea bears many similarities to the eye for translational studies of gene therapies. Experience gained in ocular gene therapy trials, many of which have advanced to clinical phase III, may provide valuable guidance in improving the chance of success for cochlear gene therapy in human trials. A discussion on potential implications of translational knowledge gleaned from large numbers of advanced clinical trials of ocular gene therapy is therefore included.

Outlook and future of inner ear therapy

Drug delivery to the inner ear is an ideal method to treat a wide variety of otologic conditions. A broad range of potential applications is just beginning to be explored. New approaches combine principles of inner ear pharmacokinetics with emerging technologies of drug delivery including novel delivery systems, drug-device combinations, and new categories of drugs. Strategies include cell-specific targeting, manipulation of gene expression, local activation following systemic delivery, and use of stem cells, viral vectors, and gene editing systems. Translation of these therapies to the clinic remains challenging given the potential risks of intracochlear and intralabyrinthine trauma, our limited understanding of the etiologies of particular inner ear disorders, and paucity of accurate diagnostic tools at the cellular level. This review provides an overview of future methods, delivery systems, disease targets, and clinical considerations required for translation to clinical medicine.

Cochleovestibular gene transfer in neonatal mice by canalostomy

Impairments of the inner ear result in sensorineural hearing loss and vestibular dysfunction in humans. A large proportion of these disorders are congenital, and involve both auditory and vestibular systems. Therefore, genetic interventions to correct deficits must be administered during early developmental stages. In this study, we evaluated inner ear gene transfer in neonatal mice by canalostomy using an adeno-associated virus serotype 8 (AAV8) vector. AAV8 with the green fluorescence protein (GFP) gene was inoculated into the inner ear of the neonatal mice through the posterior semicircular canal (canalostomy). At 30 days following surgery, animals were subjected to swim tests and auditory brainstem response measurements. Then, the animals were euthanized and temporal bones were harvested for whole-mount preparation. GFP expression and morphological changes in the inner ear were assessed by immunohistochemistry. After surgery, no signs of vestibular dysfunction were found, and there were no significant differences in the auditory brainstem response threshold between AAV8-inoculated ears and nonsurgery ears. In the surgery ears, extensive GFP expression and no morphological lesions were detected in the cochlear and vestibular end organs. Robust GFP expression was found in inner hair cells, marginal cells, vestibular hair cells, and vestibular supporting cells. In conclusion, AAV8 inoculation through canalostomy into the inner ears of neonatal mice led to extensive overexpression of exogenous genes in the inner ear without affecting hearing or vestibular function. It serves as a promising approach for gene therapy of congenital cochleovestibular diseases.

Engraftment of Human Pluripotent Stem Cell-derived Progenitors in the Inner Ear of Prenatal Mice

There is, at present, no curative treatment for genetic hearing loss. We have previously reported that transuterine gene transfer of wild type CONNEXIN30 (CX30) genes into otocysts in CX30-deleted mice could restore hearing. Cell transplantation therapy might be another therapeutic option, although it is still unknown whether stem cell-derived progenitor cells could migrate into mouse otocysts. Here, we show successful cell transplantation of progenitors of outer sulcus cell-like cells derived from human-derived induced pluripotent stem cells into mouse otocysts on embryonic day 11.5. The delivered cells engrafted more frequently in the non-sensory region in the inner ear of CX30-deleted mice than in wild type mice and survived for up to 1 week after transplantation. Some of the engrafted cells expressed CX30 proteins in the non-sensory region. This is the first report that demonstrates successful engraftment of exogenous cells in prenatal developing otocysts in mice. Future studies using this mouse otocystic injection model in vivo will provide further clues for developing treatment modalities for congenital hearing loss in humans.

Age-dependent gene expression in the inner ear of big brown bats (Eptesicus fuscus)

For echolocating bats, hearing is essential for survival. Specializations for detecting and processing high frequency sounds are apparent throughout their auditory systems. Recent studies on echolocating mammals have reported evidence of parallel evolution in some hearing-related genes in which distantly related groups of echolocating animals (bats and toothed whales), cluster together in gene trees due to apparent amino acid convergence. However, molecular adaptations can occur not only in coding sequences, but also in the regulation of gene expression. The aim of this study was to examine the expression of hearing-related genes in the inner ear of developing big brown bats, Eptesicus fuscus, during the period in which echolocation vocalizations increase dramatically in frequency. We found that seven genes were significantly upregulated in juveniles relative to adults, and that the expression of four genes through development correlated with estimated age. Compared to available data for mice, it appears that expression of some hearing genes is extended in juvenile bats. These results are consistent with a prolonged growth period required to develop larger cochlea relative to body size, a later maturation of high frequency hearing, and a greater dependence on high frequency hearing in echolocating bats.

Molecular composition and distribution of gap junctions in the sensory epithelium of the human cochlea-a super-resolution structured illumination microscopy (SR-SIM) study

BACKGROUND

Mutations in the GJB2 gene, which encodes the Connexin26 (Cx26) protein, are the most common cause of childhood hearing loss in American and European populations. The cochlea contains a gap junction (GJ) network in the sensory epithelium and two connective tissue networks in the lateral wall and spiral limbus. The syncytia contain the GJ proteins beta 2 (GJB2/Cx26) and beta 6 (GJB6/Cx30). Our knowledge of their expression in humans is insufficient due to the limited availability of tissue. Here, we sought to establish the molecular arrangement of GJs in the epithelial network of the human cochlea using surgically obtained samples.

METHODS

We analyzed Cx26 and Cx30 expression in GJ networks in well-preserved adult human auditory sensory epithelium using confocal, electron, and super-resolution structured illumination microscopy (SR-SIM).

RESULTS

Cx30 plaques (<5 μm) dominated, while Cx26 plaques were subtle and appeared as 'mini-junctions' (2-300 nm). 3-D volume rendering of Z-stacks and orthogonal projections from single optical sections suggested that the GJs are homomeric/homotypic and consist of assemblies of identical GJs composed of either Cx26 or Cx30. Occasionally, the two protein types were co-expressed, suggesting functional cooperation.

CONCLUSIONS

Establishing the molecular composition and distribution of the GJ networks in the human cochlea may increase our understanding of the pathophysiology of Cx-related hearing loss. This information may also assist in developing future strategies to treat genetic hearing loss.

Transplanting mouse induced pluripotent stem cells into mouse otocysts in vivo

The otocyst is an attractive target for studying treatment strategies for genetic hearing loss and for understanding inner ear development. We have previously reported that trans-uterine supplemental gene therapy in vivo into the otocysts of mice, which had a loss of function mutation in a causative gene of deafness, was able to prevent putative hearing loss. We herein set out to clarify the feasibility of allogenic cell transplantation into the mouse otocysts in vivo. We transplanted naive mouse-derived induced pluripotent stem cells (miPSCs) into the otocysts of wild type mice or connexin (Cx) 30 deficient mice, at embryonic day 11.5 (E11.5). The transplanted m-iPSCs survived in the lumens of the inner ears at E13.5 and E15.5 in wild type mice. In the Cx30 deficient mouse, the transplanted cells survived similarly, with some of the transplanted cells migrating into the lining cells of the lumens of the inner ears at E13.5 and showing tumorigenic cell proliferation at E15.5. In addition, engrafted cells appear to be able to differentiate after the cell transplantation. Our results suggest that otocyst transplanted cells survived and differentiated. A Cx30 deficiency may facilitate cell migration. These findings may offer some hope for cell transplantation therapy for profound genetic hearing loss caused by a Cxs deficiency.

Cochlear hair cell regeneration after noise-induced hearing loss: Does regeneration follow development?

Noise-induced hearing loss (NIHL) affects a large number of military personnel and civilians. Regenerating inner-ear cochlear hair cells (HCs) is a promising strategy to restore hearing after NIHL. In this review, we first summarize recent transcriptome profile analysis of zebrafish lateral lines and chick utricles where spontaneous HC regeneration occurs after HC damage. We then discuss recent studies in other mammalian regenerative systems such as pancreas, heart and central nervous system. Both spontaneous and forced HC regeneration occurs in mammalian cochleae in vivo involving proliferation and direct lineage conversion. However, both processes are inefficient and incomplete, and decline with age. For direct lineage conversion in vivo in cochleae and in other systems, further improvement requires multiple factors, including transcription, epigenetic and trophic factors, with appropriate stoichiometry in appropriate architectural niche. Increasing evidence from other systems indicates that the molecular paths of direct lineage conversion may be different from those of normal developmental lineages. We therefore hypothesize that HC regeneration does not have to follow HC development and that epigenetic memory of supporting cells influences the HC regeneration, which may be a key to successful cochlear HC regeneration. Finally, we discuss recent efforts in viral gene therapy and drug discovery for HC regeneration. We hope that combination therapy targeting multiple factors and epigenetic signaling pathways will provide promising avenues for HC regeneration in humans with NIHL and other types of hearing loss.

Virally mediated Kcnq1 gene replacement therapy in the immature scala media restores hearing in a mouse model of human Jervell and Lange-Nielsen deafness syndrome

Mutations in the potassium channel subunit KCNQ1 cause the human severe congenital deafness Jervell and Lange-Nielsen (JLN) syndrome. We applied a gene therapy approach in a mouse model of JLN syndrome (Kcnq1(-/-) mice) to prevent the development of deafness in the adult stage. A modified adeno-associated virus construct carrying a Kcnq1 expression cassette was injected postnatally (P0-P2) into the endolymph, which resulted in Kcnq1 expression in most cochlear marginal cells where native Kcnq1 is exclusively expressed. We also found that extensive ectopic virally mediated Kcnq1 transgene expression did not affect normal cochlear functions. Examination of cochlear morphology showed that the collapse of the Reissner's membrane and degeneration of hair cells (HCs) and cells in the spiral ganglia were corrected in Kcnq1(-/-) mice. Electrophysiological tests showed normal endocochlear potential in treated ears. In addition, auditory brainstem responses showed significant hearing preservation in the injected ears, ranging from 20 dB improvement to complete correction of the deafness phenotype. Our results demonstrate the first successful gene therapy treatment for gene defects specifically affecting the function of the stria vascularis, which is a major site affected by genetic mutations in inherited hearing loss.

Methionine Sulfoxide Reductase B3-Targeted In Utero Gene Therapy Rescues Hearing Function in a Mouse Model of Congenital Sensorineural Hearing Loss

AIMS

Methionine sulfoxide reductase B3 (MsrB3), which stereospecifically repairs methionine-R-sulfoxide, is an important Msr protein that is associated with auditory function in mammals. MsrB3 deficiency leads to profound congenital hearing loss due to the degeneration of stereociliary bundles and the apoptotic death of cochlear hair cells. In this study, we investigated a fundamental treatment strategy in an MsrB3 deficiency mouse model and confirmed the biological significance of MsrB3 in the inner ear using MsrB3 knockout (MsrB3(-/-)) mice.

RESULTS

We delivered a recombinant adeno-associated virus encoding the MsrB3 gene directly into the otocyst at embryonic day 12.5 using a transuterine approach. We observed hearing recovery in the treated ears of MsrB3(-/-) mice at postnatal day 28, and we confirmed MsrB3 mRNA and protein expression in cochlear extracts. Additionally, we demonstrated that the morphology of the stereociliary bundles in the rescued ears of MsrB3(-/-) mice was similar to those in MsrB3(+/+) mice.

INNOVATION

To our knowledge, this is the first study to demonstrate functional and morphological rescue of the hair cells of the inner ear in the MsrB3 deficiency mouse model of congenital genetic sensorineural hearing loss using an in utero, virus-mediated gene therapy approach.

CONCLUSION

Our results provide insight into the role of MsrB3 in hearing function and bring us one step closer to hearing restoration as a fundamental therapy.

Perinatal Gjb2 gene transfer rescues hearing in a mouse model of hereditary deafness

Hearing loss is the most widespread sensory disorder, with an incidence of congenital genetic deafness of 1 in 1600 children. For many ethnic populations, the most prevalent form of genetic deafness is caused by recessive mutations in the gene gap junction protein, beta 2, 26 kDa (GJB2), which is also known as connexin 26 (Cx26). Despite this knowledge, existing treatment strategies do not completely recover speech perception. Here we used a gene delivery system to rescue hearing in a mouse model of Gjb2 deletion. Mice lacking Cx26 are characterized by profound deafness from birth and improper development of cochlear cells. Cochlear delivery of Gjb2 using an adeno-associated virus significantly improved the auditory responses and development of the cochlear structure. Using gene replacement to restore hearing in a new mouse model of Gjb2-related deafness may lead to the development of therapies for human hereditary deafness.

New treatment options for hearing loss

Hearing loss is the most common form of sensory impairment in humans and affects more than 40 million people in the United States alone. No drug-based therapy has been approved by the Food and Drug Administration, and treatment mostly relies on devices such as hearing aids and cochlear implants. Over recent years, more than 100 genetic loci have been linked to hearing loss and many of the affected genes have been identified. This understanding of the genetic pathways that regulate auditory function has revealed new targets for pharmacological treatment of the disease. Moreover, approaches that are based on stem cells and gene therapy, which may have the potential to restore or maintain auditory function, are beginning to emerge.

Potential treatments for genetic hearing loss in humans: current conundrums

Genetic defects are a major cause of hearing loss in newborns. Consequently, hearing loss has a profound negative impact on human daily living. Numerous causative genes for genetic hearing loss have been identified. However, presently, there are no truly curative treatments for this condition. There have been several recent reports on successful treatments in mice using embryonic gene therapy, neonatal gene therapy and neonatal antisense oligonucleotide therapy. Herein, we describe state-of-the-art research on genetic hearing loss treatment through gene therapy and discuss the obstacles to overcome in curative treatments of genetic hearing loss in humans.

Strategies for developing novel therapeutics for sensorineural hearing loss

Sensorineural hearing loss (SNHL) is a common disability in the world; however, at present, options for the pharmacological treatment of SNHL are very limited. Previous studies involving human temporal bone analyses have revealed that the degeneration of the cochlea is a common mechanism of SNHL. A major problem for the development of novel pharmacotherapy for SNHL has been the limited regeneration capacity in mammalian cochlear cells. However, recent progress in basic studies has led to several effective strategies for the induction of regeneration in the mammalian cochlea, in accordance with the stage of degeneration. In addition, recent advances in the identification of human deafness genes and their characterization in mouse models have elucidated cellular and/or molecular mechanisms of SNHL, which will contribute to clarify molecular targets of pharmacotherapy for treatment of SNHL.

Mutations in Cx30 that are linked to skin disease and non-syndromic hearing loss exhibit several distinct cellular pathologies

Connexin 30 (Cx30), a member of the large gap-junction protein family, plays a role in the homeostasis of the epidermis and inner ear through gap junctional intercellular communication (GJIC). Here, we investigate the underlying mechanisms of four autosomal dominant Cx30 gene mutations that are linked to hearing loss and/or various skin diseases. First, the T5M mutant linked to non-syndromic hearing loss formed functional gap junction channels and hemichannels, similar to wild-type Cx30. The loss-of-function V37E mutant associated with Clouston syndrome or keratitis-ichthyosis-deafness syndrome was retained in the endoplasmic reticulum and significantly induced apoptosis. The G59R mutant linked to the Vohwinkel and Bart-Pumphrey syndromes was retained primarily in the Golgi apparatus and exhibited loss of gap junction channel and hemichannel function but did not cause cell death. Lastly, the A88V mutant, which is linked to the development of Clouston syndrome, also significantly induced apoptosis but through an endoplasmic-reticulum-independent mechanism. Collectively, we discovered that four unique Cx30 mutants might cause disease through different mechanisms that also likely include their selective trans-dominant effects on coexpressed connexins, highlighting the overall complexity of connexin-linked diseases and the importance of GJIC in disease prevention.

Virally expressed connexin26 restores gap junction function in the cochlea of conditional Gjb2 knockout mice

Mutations in GJB2, which codes for the gap junction protein connexin26, are the most common causes of human nonsyndromic hereditary deafness. We inoculated modified adeno-associated viral vectors into the scala media of early postnatal conditional Gjb2 knockout mice to drive exogenous connexin26 expression. We found extensive virally-expressed connexin26 in cells lining the scala media, and intercellular gap junction network was re-established in the organ of Corti of mutant mouse cochlea. Widespread ectopic connexin26 expression neither formed ectopic gap junctions nor affected normal hearing thresholds in wild type mice, suggesting that autonomous cellular mechanisms regulate proper membrane trafficking of exogenously-expressed connexin26 and govern the functional manifestation of them. Functional recovery of gap-junction-mediated coupling among the supporting cells was observed. We found that both cell death in the organ of Corti and degeneration of spiral ganglion neurons in the cochlea of mutant mice were substantially reduced, although auditory brainstem responses did not show significant hearing improvement. This is the first report demonstrating that virally-mediated gene therapy restored extensive gap junction intercellular network among cochlear non-sensory cells in vivo. Such a treatment performed at early postnatal stages resulted in a partial rescue of disease phenotypes in the cochlea of the mutant mice.