Genetic Hearing Loss and Gene Therapy

Expression profiling of cochlear genes uncovers sex-based cellular function in mouse cochleae

Sex is a pivotal biological factor that significantly impacts tissue homeostasis and disease susceptibility. In the auditory system, sex differences have been observed in cochlear physiology and responses to pathological conditions. However, the underlying molecular mechanisms responsible for these differences remain elusive. The current research explores the differences in gene expression profiles in the cochlea between male and female mice, aiming to understand the functional implication of sex-biased gene expression in each sex. Using RNA-sequencing analysis on cochlear tissues obtained from male and female mice, we identified a significant number of genes exhibiting sex-biased expression differences. While some of these differentially expressed genes are located on sex chromosomes, most are found on autosomal chromosomes. Further bioinformatic analysis revealed that these genes are involved in several key cellular functions. In males, these genes are notably linked to oxidative phosphorylation and RNA synthesis and processing, suggesting their involvement in mitochondrial energy production and regulatory control of gene expression. In contrast, sex-biased genes are associated with mechano-transduction and synaptic transmission within female cochleae. Collectively, our study provides valuable insights into the molecular differences between the sexes and emphasizes the need for future research to uncover their functional implications and relevance to auditory health and disease development.

Natural History of KCNQ4 p.G285S Related Hearing Loss, Construction of iPSC and Mouse Model

OBJECTIVE

KCNQ4 is one of the most common disease-causing genes involved in autosomal dominant non-syndromic hearing loss. We previously found that patients with KCNQ4 p.G285S exhibited a much more rapid deterioration in hearing loss than those with other KCNQ4 variants. To determine the rate of hearing loss and assess the disease for further analysis, we performed a long-term follow-up of these patients and generated patient-derived induced pluripotent stem cells (iPSCs), and a mouse model.

METHODS

Patients with KCNQ4 p.G285S from a five-generation family with hearing loss were followed up from 2005 to 2022. iPSCs were generated by stimulating peripheral blood mononuclear cells from the proband, and their pluripotency was determined. The Kcnq4 p.G286S mouse model was generated using CRISPR/Cas9, and its genotype and phenotype were identified.

RESULTS

(1) The annual rates of hearing loss at the frequencies of speech were 0.96 dB for the proband and 0.87 dB for his father during the follow-up period, which were faster than patients with other KCNQ4 variants. (2) The patient-derived iPSC line carrying KCNQ4 p.G285S, possessed the capacity of differentiation and pluripotency capacities. (3) Mutant mice with Kcnq4 p.G286S exhibited hearing loss and outer hair cell loss at 1 month of age.

CONCLUSION

Patients with KCNQ4 p.G285S variant exhibited significantly accelerated progression of hearing loss compared to those with other reported variants. Awareness of the natural history of hearing loss associated with KCNQ4 p.G285S is beneficial for genetic counseling and prognosis. The generation of the iPSCs and mouse model can provide a valuable foundation for further in-depth analyses.

LEVEL OF EVIDENCE

4 Laryngoscope, 134:2356-2363, 2024.

Adeno-Associated Virus-Encapsulated Alginate Microspheres Loaded in Collagen Gel Carriers for Localized Gene Transfer

This work reports localized in vivo gene transfer by biodegradation of the adeno-associated virus-encapsulating alginate microspheres (AAV-AMs) loaded in collagen gel carriers. AAV-AMs are centrifugally synthesized by ejecting a mixed pre-gel solution of alginate and AAV to CaCl2 solution to form an ionically cross-linked hydrogel microsphere immediately. The AAV-AMs are able to preserve the AAV without diffusing out even after spreading them on the cells, and the AAV is released and transfected by the degradation of the alginate microsphere. In addition, AAV-AMs can be stored by cryopreservation until use. By implanting this highly convenient AAV-encapsulated hydrogel, AAV-AMs can be loaded into collagen gel carriers to fix the position of the implanted AAV-AMs and achieve localized gene transfer in vivo. In vivo experiments show that the AAV-AMs loaded in collagen gel carriers are demonstrated to release the encapsulated AAV for gene transfer in the buttocks muscles of mice. While conventional injections caused gene transfer to the entire surrounding tissue, the biodegradation of AAV-AMs shows that gene transfer is achieved locally to the muscles. This means that the proposed AAV-loaded system is shown to be a superior method for selective gene transfer.

The genetic basis and the diagnostic yield of genetic testing related to nonsyndromic hearing loss in Qatar

Hearing loss is the most predominant sensory defect occurring in pediatrics, of which, 66% cases are attributed to genetic factors. The prevalence of hereditary hearing loss increases in consanguineous populations, and the prevalence of hearing loss in Qatar is 5.2%. We aimed to investigate the genetic basis of nonsyndromic hearing loss (NSHL) in Qatar and to evaluate the diagnostic yield of different genetic tests available. A retrospective chart review was conducted for 59 pediatric patients with NSHL referred to the Department of Adult and Pediatric Medical Genetics at Hamad Medical Corporation in Qatar, and who underwent at least one genetic test. Out of the 59 patients, 39 were solved cases due to 19 variants in 11 genes and two copy number variants that explained the NSHL phenotype. Of them 2 cases were initially uncertain and were reclassified using familial segregation. Around 36.8% of the single variants were in GJB2 gene and c.35delG was the most common recurrent variant seen in solved cases. We detected the c.283C > T variant in FGF3 that was seen in a Qatari patient and found to be associated with NSHL for the first time. The overall diagnostic yield was 30.7%, and the diagnostic yield was significantly associated with genetic testing using GJB2 sequencing and using the hearing loss (HL) gene panel. The diagnostic yield for targeted familial testing was 60% (n = 3 patients) and for gene panel was 50% (n = 5). Thus, we recommend using GJB2 gene sequencing as a first-tier genetic test and HL gene panel as a second-tier genetic test for NSHL. Our work provided new insights into the genetic pool of NSHL among Arabs and highlights its unique diversity, this is believed to help further in the diagnostic and management options for NSHL Arab patients.

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.

Postnatal genetic umbilical cord analysis for earliest possible detection of inherited hearing impairment

INTRODUCTION

The most common sensorineural disorder in humans is hearing impairment and approximately 60% of prelingual hearing disorders are genetic. Especially parents with a congenital deaf child want to know as early as possible whether their second born child has the same genetic defect or not. The aim of this study is to demonstrate that postnatal genetic umbilical cord analysis is both the earliest detection possibility and sufficient.

METHODS

We included first born children with severe hearing impairment that underwent cochlear implantation. All included patients were analyzed genetically and exhibited mutations of either DFNB1 loci or SLC26A4 gene. Additionally, the umbilical cord of the sibling underwent genetic analysis to detect hereditary genetic mutations as early as possible.

RESULTS

49 newborn children out of 22 families were included in this study. Genetic analysis revealed clinical relevant mutations in all first born children and in four siblings via umbilical cord analysis. All patients who have been diagnosed with a relevant genetic mutation that caused severe hearing impairment underwent hearing rehabilitation via cochlear implant surgery.

CONCLUSION

This study demonstrates the sufficient and early as possible detection of known genetically hearing disorders via umbilical cord analysis. In case of a known familial genetic hearing disorder, it is advisable to analyze newborn siblings for the corresponding genetic defect as soon as possible, to be able to plan and initiate clinical care for the patient as early as possible. It is also extremely important for the parents to obtain clear information about the auditory status of the newborn.

A review of the auditory-gut-brain axis

Hearing loss places a substantial burden on medical resources across the world and impacts quality of life for those affected. Further, it can occur peripherally and/or centrally. With many possible causes of hearing loss, there is scope for investigating the underlying mechanisms involved. Various signaling pathways connecting gut microbes and the brain (the gut-brain axis) have been identified and well established in a variety of diseases and disorders. However, the role of these pathways in providing links to other parts of the body has not been explored in much depth. Therefore, the aim of this review is to explore potential underlying mechanisms that connect the auditory system to the gut-brain axis. Using select keywords in PubMed, and additional hand-searching in google scholar, relevant studies were identified. In this review we summarize the key players in the auditory-gut-brain axis under four subheadings: anatomical, extracellular, immune and dietary. Firstly, we identify important anatomical structures in the auditory-gut-brain axis, particularly highlighting a direct connection provided by the vagus nerve. Leading on from this we discuss several extracellular signaling pathways which might connect the ear, gut and brain. A link is established between inflammatory responses in the ear and gut microbiome-altering interventions, highlighting a contribution of the immune system. Finally, we discuss the contribution of diet to the auditory-gut-brain axis. Based on the reviewed literature, we propose numerous possible key players connecting the auditory system to the gut-brain axis. In the future, a more thorough investigation of these key players in animal models and human research may provide insight and assist in developing effective interventions for treating hearing loss.

TMEM30A is essential for hair cell polarity maintenance in postnatal mouse cochlea

BACKGROUND

Phosphatidylserine is translocated to the inner leaflet of the phospholipid bilayer membrane by the flippase function of type IV P-tape ATPase (P4-ATPase), which is critical to maintain cellular stability and homeostasis. Transmembrane protein 30A (TMEM30A) is the β-subunit of P4-ATPase. Loss of P4-ATPase function causes sensorineural hearing loss and visual dysfunction in human. However, the function of TMEM30A in the auditory system is unclear.

METHODS

P4-ATPase subtype expression in the cochlea was detected by immunofluorescence staining and quantitative real-time polymerase chain reaction (qRT-PCR) at different developmental stages. Hair cell specific TMEM30A knockout mice and wild-type littermates were used for the following functional and morphological analysis. Auditory function was evaluated by auditory brainstem response. We investigated hair cell and stereocilia morphological changes by immunofluorescence staining. Scanning electron microscopy was applied to observe the stereocilia ultrastructure. Differentially expressed transcriptomes were analyzed based on RNA-sequencing data from knockout and wild-type mouse cochleae. Differentially expressed genes were verified by qRT-PCR.

RESULTS

TMEM30A and subtypes of P4-ATPase are expressed in the mouse cochlea in a temporal-dependent pattern. Deletion of TMEM30A in hair cells impaired hearing onset due to progressive hair cell loss. The disrupted kinocilia placement and irregular distribution of spectrin-α in cuticular plate indicated the hair cell planar polarity disruption in TMEM30A deletion hair cells. Hair cell degeneration begins at P7 and finishes around P14. Transcriptional analysis indicates that the focal adhesion pathway and stereocilium tip-related genes changed dramatically. Without the TMEM30A chaperone, excessive ATP8A2 accumulated in the cytoplasm, leading to overwhelming endoplasmic reticulum stress, which eventually contributed to hair cell death.

CONCLUSIONS

Deletion of TMEM30A led to disrupted planar polarity and stereocilia bundles, and finally led to hair cell loss and auditory dysfunction. TMEM30A is essential for hair cell polarity maintenance and membrane homeostasis. Our study highlights a pivotal role of TMEM30A in the postnatal development of hair cells and reveals the possible mechanisms underlying P4-ATPase-related genetic hearing loss.

Clinical Impact of Genetic Diagnosis of Sensorineural Hearing Loss in Adults

HYPOTHESIS

Adult genetic sensorineural hearing loss (SNHL) may be underestimated.

BACKGROUND

The diagnosis of genetic hearing loss is challenging, given its extreme genetic and phenotypic heterogeneity, particularly in adulthood. This study evaluated the utility of next-generation sequencing (NGS) in the etiological diagnosis of adult-onset SNHL.

MATERIALS AND METHODS

Adults (>16 yr old) with SNHL were recruited at the Otolaryngology Department at Marqués de Valdecilla University Hospital (Spain). Environmental factors, acoustic trauma, endolymphatic hydrops, and age-related hearing loss were excluding criteria. An NGS gene panel was used, including 196 genes (OTOgenics v3) or 229 genes (OTOgenics v4) related to syndromic and nonsyndromic hearing loss.

RESULTS

Sixty-five patients were included in the study (average age at the onset of SNHL, 41 yr). Fifteen pathogenic/likely pathogenic variants considered to be causative were found in 15 patients (23% diagnostic yield) in TECTA (4), KCNQ4 (3), GJB2 (2), ACTG1 (1), COL2A1 (1), COCH (1), COCH/COL2A1 (1), STRC (1), and ABHD12 (1). Three patients had syndromic associations (20% of patients with genetic diagnosis) that had not been previously diagnosed (two Stickler type I and one polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, cataract syndrome). Seven variants of unknown significance were found in COL11A1 (1), GSMDE (2), DNTM1 (1), SOX10 (1), EYA4 (1), and TECTA (1).

CONCLUSION

NGS gene panels can provide diagnostic yields greater than 20% for adult SNHL, with a significant proportion of variant of unknown significance that could potentially contribute to increasing diagnostic output. Identifying a genetic cause enables genetic counseling, provides prognostic information and can reveal unrecognized syndromes contributing to an accurate management of their associated manifestations.

Advancements in prevention and intervention of sensorineural hearing loss

The inner ear is a complex and difficult organ to study, and sensorineural hearing loss (SNHL) is a multifactorial sensorineural disorder with characteristics of impaired speech discrimination, recognition, sound detection, and localization. Till now, SNHL is recognized as an incurable disease because the potential mechanisms underlying SNHL have not been elucidated. The risk of developing SNHL is no longer viewed as primarily due to environmental factors. Instead, SNHL seems to result from a complicated interplay of genetic and environmental factors affecting numerous fundamental cellular processes. The complexity of SNHL is presented as an inability to make an early diagnosis at the earliest stages of the disease and difficulties in the management of symptoms during the process. To date, there are no treatments that slow the neurodegenerative process. In this article, we review the recent advances about SHNL and discuss the complexities and challenges of prevention and intervention of SNHL.

Identification of homozygous missense variant in SIX5 gene underlying recessive nonsyndromic hearing impairment

Hearing impairment (HI) is a heterogeneous condition that affects many individuals globally with different age groups. HI is a genetically and phenotypically heterogeneous disorder. Over the last several years, many genes/loci causing rare autosomal recessive and dominant forms of hearing impairments have been identified, involved in various aspects of ear development. In the current study, two affected individuals of a consanguineous family exhibiting autosomal recessive nonsyndromic hearing impairment (AR-NSHI) were clinically and genetically characterized. The single affected individual (IV-2) of the family was subjected to whole-exome sequencing (WES) accompanied by traditional Sanger sequencing. Clinical examinations using air conduction audiograms of both the affected individuals showed profound hearing loss across all frequencies. WES revealed a homozygous missense variant (c.44G>C) in the SIX5 gene located on chromosome 19q13.32. We report the first case of autosomal recessive NSHI due to a biallelic missense variant in the SIX5 gene. This report further supports the evidence that the SIX5 variant might cause profound HI and supports its vital role in auditory function. Identification of novel candidate genes might help in application of future gene therapy strategies that may be implemented for NSHI, such as gene replacement using cDNA, gene silencing using RNA interference, and gene editing using the CRISPR/Cas9 system.

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.

Newborn Hearing Screening Benefits Children, but Global Disparities Persist

There is substantial evidence that newborn hearing screening (NHS) reduces the negative sequelae of permanent childhood hearing loss (PCHL) if performed in programs that aim to screen all newborns in a region or nation (often referred to as Universal Newborn Hearing Screening or UNHS). The World Health Organization (WHO) has called in two resolutions for the implementation of such programs and for the collection of large-scale data. To assess the global status of NHS programs we surveyed individuals potentially involved with newborn and infant hearing screening (NIHS) in 196 countries/territories (in the following text referred to as countries). Replies were returned from 158 countries. The results indicated that 38% of the world's newborns and infants had no or minimal hearing screening and 33% screened at least 85% of the babies (hereafter referred to as UNHS). Hearing screening programs varied considerably in quality, data acquisition, and accessibility of services for children with PCHL. In this article, we summarize the main results of the survey in the context of several recent WHO publications, particularly the World Report on Hearing, which defined advances in the implementation of NHS programs in the Member States as one of three key indicators of worldwide progress in ear and hearing care (EHC).

In vivo base editing restores sensory transduction and transiently improves auditory function in a mouse model of recessive deafness

Most genetic diseases arise from recessive point mutations that require correction, rather than disruption, of the pathogenic allele to benefit patients. Base editing has the potential to directly repair point mutations and provide therapeutic restoration of gene function. Mutations of transmembrane channel-like 1 gene (TMC1) can cause dominant or recessive deafness. We developed a base editing strategy to treat Baringo mice, which carry a recessive, loss-of-function point mutation (c.A545G; resulting in the substitution p.Y182C) in Tmc1 that causes deafness. Tmc1 encodes a protein that forms mechanosensitive ion channels in sensory hair cells of the inner ear and is required for normal auditory function. We found that sensory hair cells of Baringo mice have a complete loss of auditory sensory transduction. To repair the mutation, we tested several optimized cytosine base editors (CBEmax variants) and guide RNAs in Baringo mouse embryonic fibroblasts. We packaged the most promising CBE, derived from an activation-induced cytidine deaminase (AID), into dual adeno-associated viruses (AAVs) using a split-intein delivery system. The dual AID-CBEmax AAVs were injected into the inner ears of Baringo mice at postnatal day 1. Injected mice showed up to 51% reversion of the Tmc1 c.A545G point mutation to wild-type sequence (c.A545A) in Tmc1 transcripts. Repair of Tmc1 in vivo restored inner hair cell sensory transduction and hair cell morphology and transiently rescued low-frequency hearing 4 weeks after injection. These findings provide a foundation for a potential one-time treatment for recessive hearing loss and support further development of base editing to correct pathogenic point mutations.

Evaluation of DNA damages in congenital hearing loss patients

In the current study, we aimed to compare the level of genetic damages measured as micronucleus (MN), nucleoplasmic bridge (NPB), and nuclear bud formation (NBUD) in congenital hearing loss patients (n = 17) and control group (n = 24). The cytokinesis-blocked micronucleus assay (CBMN) was applied to the blood samples to measure the frequency of the markers in both groups. The frequencies of MN of hearing loss patients were found to be consistently significantly higher than those obtained for the control group (p < 0.0001). Similarly, we found significantly higher frequency of NPB in patients was obtained for the patient group (p < 0.0001). Finally, the frequencies of NBUD in patients is significantly higher than the level measured in the control group (p < 0.0001). Furthermore, the age-adjusted MNL, BNMN, NPB, and NBUD frequencies in the patients were significantly higher than those obtained in the control group. We observed that the frequency of MN in patients was positively correlated with NBUD frequency which may indicate a common mechanism for these biomarkers in the patient group. We found, for the first time, that there were statistically significant higher levels of MN, NPB, and NBUD in sensorineural hearing loss patients. Since the markers we evaluated were linked with crucial diseases, our findings might suggest that sensorineural hearing loss patients are susceptible to several crucial diseases, especially cancer. Furthermore, the results demonstrated the significance of the MN, NPB, and NBUD level and thus provides a potential marker for the diagnosis of congenital hearing loss patients.

Usher Syndrome in the Inner Ear: Etiologies and Advances in Gene Therapy

Hearing loss is the most common sensory disorder with ~466 million people worldwide affected, representing about 5% of the population. A substantial portion of hearing loss is genetic. Hearing loss can either be non-syndromic, if hearing loss is the only clinical manifestation, or syndromic, if the hearing loss is accompanied by a collage of other clinical manifestations. Usher syndrome is a syndromic form of genetic hearing loss that is accompanied by impaired vision associated with retinitis pigmentosa and, in many cases, vestibular dysfunction. It is the most common cause of deaf-blindness. Currently cochlear implantation or hearing aids are the only treatments for Usher-related hearing loss. However, gene therapy has shown promise in treating Usher-related retinitis pigmentosa. Here we review how the etiologies of Usher-related hearing loss make it a good candidate for gene therapy and discuss how various forms of gene therapy could be applied to Usher-related hearing loss.

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.

Novel homozygous variants in the TMC1 and CDH23 genes cause autosomal recessive nonsyndromic hearing loss

Background

Hereditary hearing loss (HL) is a heterogeneous and most common sensory neural disorder. At least, 76 genes have been reported in association with autosomal recessive nonsyndromic HL (ARNSHL). Herein, we subjected two patients with bilateral sensorineural HL in two distinct consanguineous Iranian families to figure out the underlying genetic factors.

Methods

Physical and sensorineural examinations were performed on the patients. Imaging also was applied to unveil any abnormalities in anatomical structures of the middle and inner ear. In order to decipher the possible genetic causes of the verified GJB2‐negative samples, the probands were subjected to whole‐exome sequencing and, subsequently, Sanger sequencing was applied for variant confirmation.

Results

Clinical examinations showed ARNSHL in the patients. After doing whole exome sequencing, two novel variants were identified that were co‐segregating with HL that were absent in 100 ethnically matched controls. In the first family, a novel homozygous variant, NM_138691.2: c.530T>C; p.(lle177Thr), in TMC1 gene co‐segregated with prelingual ARNSHL. In the second family, NM_022124.6: c.2334G>A; p.(Trp778*) was reported as a nonsense variant causing prelingual ARNSHL.

Conclusion

These findings can, in turn, endorse how TMC1 and CDH23 screening is critical to detecting HL in Iranian patients. Identifying TMC1 and CDH23 pathogenic variants doubtlessly help in the detailed genotypic characterization of HL.

Whole-Exome Sequencing in Idiopathic Short Stature: Rare Mutations Affecting Growth

Idiopathic short stature (ISS) is a common diagnosis of exclusion in patients with short stature (SS). In this article, we aimed to identify the genetic causes of SS in patients with ISS and investigate treatment options. Fourteen children with diagnosis of ISS were identified, and whole-exome sequencing (WES) was subsequently conducted on blood-derived DNA. Five patients were correctly diagnosed with ISS and four had rare mutations that have not been previously reported. Four patients had mutations known to cause SS and one had a mutation that was known not to affect height. WES can help identify rare mutations implicated in ISS.

Genetics of pediatric hearing loss: A functional perspective

OBJECTIVES

This article reviews the current role of genetics in pediatric hearing loss (HL).

METHODS

A review of the current literature regarding the genetic basis of HL in children was performed.

RESULTS

To date, 119 nonsyndromic genes have been associated with HL. There are also hundreds of syndromic causes that have HL as part of the clinical phenotype.

CONCLUSIONS

Identifying HL genes coupled with clinical characteristics ("genotype-phenotype") yields a more accurate diagnosis and prognosis. Although the complexity of the auditory apparatus presents challenges, gene therapy is emerging and may be a viable management option in the future.