Current Advances in Gene Therapies of Genetic Auditory Neuropathy Spectrum Disorder

Outcomes of Cochlear Implantation in Adolescents With Auditory Neuropathy Spectrum Disorder: Scoping Review and Case Report

OBJECTIVE

To assess outcomes of CI in adolescent patients with ANSD, a population which has not yet been comprehensively reviewed through a scoping review.

METHODS

A scoping review in accordance with Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A comprehensive search of MEDLINE, EMBASE, Cochrane DSR, Cochrane CENTRAL, CINAHL, and Web of Science was performed. Cohort and case studies evaluating outcomes of CI in adolescents with ANSD were selected. A case report of an adolescent ANSD patient who underwent CI from our tertiary care academic hospital setting is also reported.

RESULTS

Our search identified a total of 17 articles after screening 488 articles. Among the 24 patients isolated from the selected articles, the vast majority showed some level of improvement in their hearing ability (n = 21, 87.5%). Additionally, of the 20 individuals whose hearing outcomes had a comparison cohort of subjects under 10 years old, nearly half showed better or similar levels of hearing improvement (n = 9, 45%).

CONCLUSIONS

Adolescents with ANSD receiving CI have notable improvements in hearing outcomes, but to a lesser degree than younger cohorts with ANSD. As such, CI should be considered as a valid treatment option for adolescents with ANSD. However, the benefit of such intervention has a wide variability, presumably based on the different pathologies that can cause their hearing loss and not necessarily the age at implantation.

Identification of genetic mechanisms of non-isolated auditory neuropathy with various phenotypes in Chinese families

BACKGROUND

Non-isolated auditory neuropathy (AN), or syndromic AN, is marked by AN along with additional systemic manifestations. The diagnostic process is challenging due to its varied symptoms and overlap with other syndromes. This study focuses on two mitochondrial function-related genes which result in non-isolated AN, FDXR and TWNK, providing a summary and enrichment analysis of genes associated with non-isolated AN to elucidate the genotype-phenotype correlation and underlying mechanisms.

METHODS

Seven independent Chinese Han patients with mutations in FDXR and TWNK underwent comprehensive clinical evaluations, genetic testing, and bioinformatics analyses. Diagnostic assessments included auditory brainstem response and distortion product otoacoustic emissions, supplemented by other examinations. Whole exome sequencing and Sanger sequencing validated genetic findings. Pathogenicity was assessed following American College of Medical Genetics and Genomics guidelines. Genes associated with non-isolated AN were summarized from prior reports, and functional enrichment analysis was conducted using Gene Ontology databases.

RESULTS

A total of 11 variants linked to non-isolated AN were identified in this study, eight of which were novel. Patients' age of hearing loss onset ranged from 2 to 25 years, averaging 11 years. Hearing loss varied from mild to profound, with 57.1%(4/7) of patients having risk factors and 71.4%(5/7) exhibiting additional systemic symptoms such as muscle weakness, ataxia, and high arches. Functional enrichment analysis revealed that genes associated with non-isolated AN predominantly involve mitochondrial processes, affecting the central and peripheral nervous, musculoskeletal, and visual systems.

CONCLUSION

This study identifies novel mutations in FDXR and TWNK that contribute to non-isolated AN through mitochondrial dysfunction. The findings highlight the role of mitochondrial processes in non-isolated AN, suggesting potential relevance as biomarkers for neurodegenerative diseases. Further research is required to explore these mechanisms and potential therapies.

Modeling of auditory neuropathy spectrum disorders associated with the TEME43 variant reveals impaired gap junction function of iPSC-derived glia-like support cells

Auditory neuropathy spectrum disorder (ANSD) is an auditory dysfunction disorder characterized by impaired speech comprehension. Its etiology is complex and can be broadly categorized into genetic and non-genetic factors. TMEM43 mutation is identified as a causative factor in ANSD. While some studies have been conducted using animal models, its pathogenic mechanisms in humans remain unclear. TMEM43 is predominantly expressed in cochlear glia-like support cells (GLSs) and plays a vital role in gap junction intercellular communication. In this work, we utilized induced pluripotent stem cells from an ANSD patient carrying the TMEM43 gene mutation c.1114C>T (p.Arg372Ter) and directed their differentiation toward GLSs to investigate the effect of TMEM43 mutation on the function of gap junctions in cochlear GLSs in vitro. Reduced expression of genes associated with GLSs characteristics and reduced gap junction intercellular communication in TMEM43 mutant cell lines were observed compared to controls. Transcriptome analysis revealed that differentially expressed genes were significantly enriched in pathways related to cell proliferation, differentiation, extracellular space and adhesion. Furthermore, significant alterations were noted in the PI3K-Akt signaling pathway and the calcium signaling pathway, which could potentially influence gap junction function and contribute to hearing loss. In summary, our study based on patient-derived iPSCs sheds new light on the molecular mechanisms by which TMEM43 mutations may lead to ANSD. These mutations could result in developmental defects in GLSs and a diminished capacity for gap junction function, which may be implicated in the auditory deficits observed in ANSD patients. Our study explored the pathological effects of the TMEM43 mutation and its causal relationship with ANSD using a patient-derived iPSC-based GLSs model, providing a foundation for future mechanistic studies and potential drug screening efforts.

Brazilian Society of Otology task force - cochlear implant ‒ recommendations based on strength of evidence

OBJECTIVE

To make evidence-based recommendations for the indications and complications of Cochlear Implant (CI) surgery in adults and children.

METHODS

Task force members were educated on knowledge synthesis methods, including electronic database search, review and selection of relevant citations, and critical appraisal of selected studies. Articles written in English or Portuguese on cochlear implantation were eligible for inclusion. The American College of Physicians' guideline grading system and the American Thyroid Association's guideline criteria were used for critical appraisal of evidence and recommendations for therapeutic interventions.

RESULTS

The topics were divided into 2 parts: (1) Evaluation of candidate patients and indications for CI surgery; (2) CI surgery - techniques and complications.

CONCLUSIONS

CI is a safe device for auditory rehabilitation of patients with severe-to-profound hearing loss. In recent years, indications for unilateral hearing loss and vestibular schwannoma have been expanded, with encouraging results. However, for a successful surgery, commitment of family members and patients in the hearing rehabilitation process is essential.

A novel therapeutic pathway to the human cochlear nerve

Traditional approaches to the human cochlear nerve have been impeded by its bony encasement deep inside the skull base. We present an innovative, minimally invasive, therapeutic pathway for direct access to the nerve to deliver novel regenerative therapies. Neuroanatomical studies on 10 cadaveric human temporal bones were undertaken to identify a potentially safe therapeutic pathway to the cochlear nerve. Simulations based on three-dimensional delineation of anatomical structures obtained from synchrotron phase-contrast imaging were analyzed. This enabled the identification of an approach to the nerve in the fundus of the internal auditory meatus by trephining the medial modiolar wall of the cochlea via the round window for a median depth of 1.48 mm (range 1.21-1.91 mm). The anatomical access was validated on 9 additional human temporal bones using radio-opaque markers and contrast injection with micro-computed tomography surveillance. We thus created an effective conduit for the delivery of therapeutic agents to the cochlear nerve.

Advances in the Study of Etiology and Molecular Mechanisms of Sensorineural Hearing Loss

Sensorineural hearing loss (SNHL), a multifactorial progressive disorder, results from a complex interplay of genetic and environmental factors, with its underlying mechanisms remaining unclear. Several pathological factors are believed to contribute to SNHL, including genetic factors, ion homeostasis, cell apoptosis, immune inflammatory responses, oxidative stress, hormones, metabolic syndrome, human cytomegalovirus infection, mitochondrial damage, and impaired autophagy. These factors collectively interact and play significant roles in the onset and progression of SNHL. The present review offers a comprehensive overview of the various factors that contribute to SNHL, emphasizes recent developments in understanding its etiology, and explores relevant preventive and intervention measures.

Tissue engineering strategies for spiral ganglion neuron protection and regeneration

Cochlear implants can directly activate the auditory system's primary sensory neurons, the spiral ganglion neurons (SGNs), via circumvention of defective cochlear hair cells. This bypass restores auditory input to the brainstem. SGN loss etiologies are complex, with limited mammalian regeneration. Protecting and revitalizing SGN is critical. Tissue engineering offers a novel therapeutic strategy, utilizing seed cells, biomolecules, and scaffold materials to create a cellular environment and regulate molecular cues. This review encapsulates the spectrum of both human and animal research, collating the factors contributing to SGN loss, the latest advancements in the utilization of exogenous stem cells for auditory nerve repair and preservation, the taxonomy and mechanism of action of standard biomolecules, and the architectural components of scaffold materials tailored for the inner ear. Furthermore, we delineate the potential and benefits of the biohybrid neural interface, an incipient technology in the realm of implantable devices. Nonetheless, tissue engineering requires refined cell selection and differentiation protocols for consistent SGN quality. In addition, strategies to improve stem cell survival, scaffold biocompatibility, and molecular cue timing are essential for biohybrid neural interface integration.

A novel mutation in the OTOF gene in a Chinese family with auditory neuropathy

Gene therapy for monogenic auditory neuropathy (AN) has successfully improved hearing function in target gene-deficient mice. Accurate genetic diagnosis can not only clarify the etiology but also accurately locate the lesion site, providing a basis for gene therapy and guiding patient intervention and management strategies. In this study, we collected data from a family with a pair of sisters with prelingual deafness. According to their auditory tests, subject Ⅱ-1 was diagnosed with profound sensorineural hearing loss (SNHL), Ⅱ-2 was diagnosed with AN, Ⅰ-1 was diagnosed with high-frequency SNHL, and Ⅰ-2 had normal hearing. Using whole-exome sequencing (WES), one nonsense mutation, c.4030C>T (p.R1344X), and one missense mutation, c.5000C>A (p.A1667D), in the OTOF (NM_001287489.1) gene were identified in the two siblings. Their parents were heterozygous carriers of c.5000C>A (father) and c.4030C>T (mother). We hypothesized that c.5000C>A is a novel pathogenic mutation. Thus, subject Ⅱ-1 should also be diagnosed with AN caused by OTOF mutations. These findings not only expand the OTOF gene mutation spectrum for AN but also indicate that WES is an effective approach for accurately diagnosing AN.

Predictors of cochlear implant outcomes in pediatric auditory neuropathy: A matched case-control study

OBJECTIVES

Current evidence supports the benefits of cochlear implants (CIs) in children with hearing loss, including those with auditory neuropathy spectrum disorder (ANSD). However, there is limited evidence regarding factors that hold predictive value for intervention outcomes.

DESIGN

This retrospective case-control study consisted of 66 children with CIs, including 22 with ANSD and 44 with sensorineural hearing loss (SNHL) matched on sex, age, age at CI activation, and the length of follow-up with CIs (1:2 ratio). The case and control groups were compared in the results of five open-set speech perception tests, and a Forward Linear Regression Model was used to identify factors that can predict the post-CI outcomes.

RESULTS

There was no significant difference in average scores between the two groups across five outcome measures, ranging from 88.40% to 95.65%. The correlation matrix revealed that younger ages at hearing aid fitting and CI activation positively influenced improvements in speech perception test scores. Furthermore, among the variables incorporated in the regression model, the duration of follow-up with CIs, age at CI activation, and the utilization of two CIs demonstrated prognostic significance for improved post-CI speech perception outcomes.

CONCLUSIONS

Children with ANSD can achieve similar open-set speech perception outcomes as children with SNHL. A longer CI follow-up, a lower age at CI activation, and the use of two CIs are predictive for optimal CI outcome.

The human OPA1delTTAG mutation induces adult onset and progressive auditory neuropathy in mice

Dominant optic atrophy (DOA) is one of the most prevalent forms of hereditary optic neuropathies and is mainly caused by heterozygous variants in OPA1, encoding a mitochondrial dynamin-related large GTPase. The clinical spectrum of DOA has been extended to a wide variety of syndromic presentations, called DOAplus, including deafness as the main secondary symptom associated to vision impairment. To date, the pathophysiological mechanisms underlying the deafness in DOA remain unknown. To gain insights into the process leading to hearing impairment, we have analyzed the Opa1delTTAG mouse model that recapitulates the DOAplus syndrome through complementary approaches combining morpho-physiology, biochemistry, and cellular and molecular biology. We found that Opa1delTTAG mutation leads an adult-onset progressive auditory neuropathy in mice, as attested by the auditory brainstem response threshold shift over time. However, the mutant mice harbored larger otoacoustic emissions in comparison to wild-type littermates, whereas the endocochlear potential, which is a proxy for the functional state of the stria vascularis, was comparable between both genotypes. Ultrastructural examination of the mutant mice revealed a selective loss of sensory inner hair cells, together with a progressive degeneration of the axons and myelin sheaths of the afferent terminals of the spiral ganglion neurons, supporting an auditory neuropathy spectrum disorder (ANSD). Molecular assessment of cochlea demonstrated a reduction of Opa1 mRNA level by greater than 40%, supporting haploinsufficiency as the disease mechanism. In addition, we evidenced an early increase in Sirtuin 3 level and in Beclin1 activity, and subsequently an age-related mtDNA depletion, increased oxidative stress, mitophagy as well as an impaired autophagic flux. Together, these results support a novel role for OPA1 in the maintenance of inner hair cells and auditory neural structures, addressing new challenges for the exploration and treatment of OPA1-linked ANSD in patients.

NADH improves AIF dimerization and inhibits apoptosis in iPSCs-derived neurons from patients with auditory neuropathy spectrum disorder

Auditory neuropathy spectrum disorder (ANSD) is a hearing impairment involving disruptions to inner hair cells (IHCs), ribbon synapses, spiral ganglion neurons (SGNs), and/or the auditory nerve itself. The outcomes of cochlear implants (CI) for ANSD are variable and dependent on the location of lesion sites. Discovering a potential therapeutic agent for ANSD remains an urgent requirement. Here, 293T stable transfection cell lines and patient induced pluripotent stem cells (iPSCs)-derived auditory neurons carrying the apoptosis inducing factor (AIF) p.R422Q variant were used to pursue a therapeutic regent for ANSD. Nicotinamide adenine dinucleotide (NADH) is a main electron donor in the electron transport chain (ETC). In 293T stable transfection cells with the p.R422Q variant, NADH treatment improved AIF dimerization, rescued mitochondrial dysfunctions, and decreased cell apoptosis. The effects of NADH were further confirmed in patient iPSCs-derived neurons. The relative level of AIF dimers was increased to 150.7 % (P = 0.026) from 59.2 % in patient-neurons upon NADH treatment. Such increased AIF dimerization promoted the mitochondrial import of coiled-coil-helix-coiled-coil-helix domain-containing protein 4 (CHCHD4), which further restored mitochondrial functions. Similarly, the content of mitochondrial calcium (mCa2+) was downregulated from 136.7 % to 102.3 % (P = 0.0024) in patient-neurons upon NADH treatment. Such decreased mCa2+ levels inhibited calpain activity, ultimately reducing the percentage of apoptotic cells from 30.5 % to 21.1 % (P = 0.021). We also compared the therapeutic effects of gene correction and NADH treatment on hereditary ANSD. NADH treatment had comparable restorative effects on functions of ANSD patient-specific cells to that of gene correction. Our findings offer evidence of the molecular mechanisms of ANSD and introduce NADH as a potential therapeutic agent for ANSD therapy.

Polygenic Risk Score-Based Association Analysis of Speech-in-Noise and Hearing Threshold Measures in Healthy Young Adults with Self-reported Normal Hearing

PURPOSE

Speech-in-noise (SIN) traits exhibit high inter-subject variability, even for healthy young adults reporting normal hearing. Emerging evidence suggests that genetic variability could influence inter-subject variability in SIN traits. Genome-wide association studies (GWAS) have uncovered the polygenic architecture of various adult-onset complex human conditions. Polygenic risk scores (PRS) summarize complex genetic susceptibility to quantify the degree of genetic risk for health conditions. The present study conducted PRS-based association analyses to identify PRS risk factors for SIN and hearing threshold measures in 255 healthy young adults (18-40 years) with self-reported normal hearing.

METHODS

Self-reported SIN perception abilities were assessed by the Speech, Spatial, and Qualities of Hearing Scale (SSQ12). QuickSIN and audiometry (0.25-16 kHz) were performed on 218 participants. Saliva-derived DNA was used for low-pass whole genome sequencing, and 2620 PRS variables for various traits were calculated using the models derived from the polygenic risk score (PGS) catalog. The regression analysis was conducted to identify predictors for SSQ12, QuickSIN, and better ear puretone averages at conventional (PTA0.5-2), high (PTA4-8), and extended-high (PTA12.5-16) frequency ranges.

RESULTS

Participants with a higher genetic predisposition to HDL cholesterol reported better SSQ12. Participants with high PRS to dementia revealed significantly elevated PTA4-8, and those with high PRS to atrial fibrillation and flutter revealed significantly elevated PTA12.5-16.

CONCLUSION

These results indicate that healthy individuals with polygenic risk of certain health conditions could exhibit a subclinical decline in hearing health measures at young ages, decades before clinically meaningful SIN deficits and hearing loss could be observed. PRS could be used to identify high-risk individuals to prevent hearing health conditions by promoting a healthy lifestyle.