An overview of hereditary hearing loss

Autosomal recessive non-syndromic hearing loss genes in Pakistan during the previous three decades

Hearing loss is a clinically and genetically heterogeneous disorder, with over 148 genes and 170 loci associated with its pathogenesis. The spectrum and frequency of causal variants vary across different genetic ancestries and are more prevalent in populations that practice consanguineous marriages. Pakistan has a rich history of autosomal recessive gene discovery related to non-syndromic hearing loss. Since the first linkage analysis with a Pakistani family that led to the mapping of the DFNB1 locus on chromosome 13, 51 genes associated with this disorder have been identified in this population. Among these, 13 of the most prevalent genes, namely CDH23, CIB2, CLDN14, GJB2, HGF, MARVELD2, MYO7A, MYO15A, MSRB3, OTOF, SLC26A4, TMC1 and TMPRSS3, account for more than half of all cases of profound hearing loss, while the prevalence of other genes is less than 2% individually. In this review, we discuss the most common autosomal recessive non-syndromic hearing loss genes in Pakistani individuals as well as the genetic mapping and sequencing approaches used to discover them. Furthermore, we identified enriched gene ontology terms and common pathways involved in these 51 autosomal recessive non-syndromic hearing loss genes to gain a better understanding of the underlying mechanisms. Establishing a molecular understanding of the disorder may aid in reducing its future prevalence by enabling timely diagnostics and genetic counselling, leading to more effective clinical management and treatments of hearing loss.

Development of Chinese herbal medicine for sensorineural hearing loss

According to the World Health Organization's world report on hearing, nearly 2.5 billion people worldwide will suffer from hearing loss by 2050, which may contribute to a severe impact on individual life quality and national economies. Sensorineural hearing loss (SNHL) occurs commonly as a result of noise exposure, aging, and ototoxic drugs, and is pathologically characterized by the impairment of mechanosensory hair cells of the inner ear, which is mainly triggered by reactive oxygen species accumulation, inflammation, and mitochondrial dysfunction. Though recent advances have been made in understanding the ability of cochlear repair and regeneration, there are still no effective therapeutic drugs for SNHL. Chinese herbal medicine which is widely distributed and easily accessible in China has demonstrated a unique curative effect against SNHL with higher safety and lower cost compared with Western medicine. Herein we present trends in research for Chinese herbal medicine for the treatment of SNHL, and elucidate their molecular mechanisms of action, to pave the way for further research and development of novel effective drugs in this field.

Improving the accuracy and internal consistency of regression-based clustering of high-dimensional datasets

Component-wise Sparse Mixture Regression (CSMR) is a recently proposed regression-based clustering method that shows promise in detecting heterogeneous relationships between molecular markers and a continuous phenotype of interest. However, CSMR can yield inconsistent results when applied to high-dimensional molecular data, which we hypothesize is in part due to inherent limitations associated with the feature selection method used in the CSMR algorithm. To assess this hypothesis, we explored whether substituting different regularized regression methods (i.e. Lasso, Elastic Net, Smoothly Clipped Absolute Deviation (SCAD), Minmax Convex Penalty (MCP), and Adaptive-Lasso) within the CSMR framework can improve the clustering accuracy and internal consistency (IC) of CSMR in high-dimensional settings. We calculated the true positive rate (TPR), true negative rate (TNR), IC and clustering accuracy of our proposed modifications, benchmarked against the existing CSMR algorithm, using an extensive set of simulation studies and real biological datasets. Our results demonstrated that substituting Adaptive-Lasso within the existing feature selection method used in CSMR led to significantly improved IC and clustering accuracy, with strong performance even in high-dimensional scenarios. In conclusion, our modifications of the CSMR method resulted in improved clustering performance and may thus serve as viable alternatives for the regression-based clustering of high-dimensional datasets.

Aminoglycoside treatment alters hearing-related genes and depicts behavioral defects in Drosophila

The hearing organ of Drosophila is present within the second segment of antennae. The hearing organ of Drosophila (Johnston's organ [JO]) shares much structural, developmental, and functional similarity with the vertebrate hearing organ (Organ of Corti). JO is evolving as a potential model system to examine the hearing-associated defects in vertebrates. In the vertebrates, aminoglycosides like gentamicin, kanamycin, and neomycin have been known to cause defects in the hearing organ. However, a complete mechanism of toxicity is not known. Taking the evolutionary conservation into account the current study aims to test various concentrations of aminoglycoside on the model organism, Drosophila melanogaster. The current study uses the oral route to check the toxicity of various aminoglycosides at different concentrations (50, 100, 150, 200, and 250 μg ml^{- 1} ). In Drosophila, many foreign particles enter the body through the gut via food. The aminoglycoside treated third instar larvae show defective crawling and sound avoidance behavior. The adult flies release lower amounts of acetylcholine esterase and higher amounts of reactive oxygen species than control untreated animals, accompanied by defective climbing and aggressive behavior. All these behavioral defects are further confirmed by the altered expression level of hearing genes such as nompC, inactive, nanchung, pyrexia. All the behavioral and genetic defects are reported as a readout of aminoglycoside toxicity.

Relationship between Nutrient Intake and Hearing Loss According to the Income Level of Working-Aged Adults: A Korean National Health and Nutrition Survey

The relationship between hearing impairment and nutrition has been extensively investigated; however, few studies have focused on this topic in working-age adults by income level. Herein, we aimed to determine the differences in hearing impairment among working-age adults by income level and identify the nutritional factors that affect hearing loss in various socioeconomic groups. Seven-hundred-and-twenty participants had hearing impairment, while 10,130 had normal hearing. After adjustment for propensity score matching, income and smoking status were identified as significant variables. By assessing the relationship between hearing impairment and nutrient intake by income level using multiple regression analyses, significant nutrients differed for each income category. Carbohydrate and vitamin C levels were significant in the low-income group; protein, fat, and vitamin B1 levels were significant in the middle-income group; and carbohydrates were significant in the high-income group. Income was significantly associated with hearing impairment in working-age adults. The proportion of individuals with hearing impairment increased as income decreased. The association between hearing impairment and nutritional intake also differed by income level. Our findings may enable the establishment of health policies for preventing hearing impairment in working-age adults by income level.

Hearing loss in Africa: current genetic profile

Hearing impairment (HI) is highly heterogeneous with over 123 associated genes reported to date, mostly from studies among Europeans and Asians. Here, we performed a systematic review of literature on the genetic profile of HI in Africa. The study protocol was registered on PROSPERO, International Prospective Register of Systematic Reviews with the registration number “CRD42021240852”. Literature search was conducted on PubMed, Scopus, Africa-Wide Information, and Web of Science databases. A total of 89 full-text records was selected and retrieved for data extraction and analyses. We found reports from only 17/54 (31.5%) African countries. The majority (61/89; 68.5%) of articles were from North Africa, with few reports found from sub-Saharan Africa. The most common method used in these publications was targeted gene sequencing (n = 66/111; 59.5%), and only 13.5% (n = 15/111) used whole-exome sequencing. More than half of the studies were performed in families segregating HI (n = 51/89). GJB2 was the most investigated gene, with GJB2: p.(R143W) founder variant only reported in Ghana, while GJB2: c.35delG was common in North African countries. Variants in MYO15A were the second frequently reported in both North and Central Africa, followed by ATP6V1B1 only reported from North Africa. Usher syndrome was the main syndromic HI molecularly investigated, with variants in five genes reported: USH2A, USH1G, USH1C, MYO7A, and PCDH15. MYO7A: p.(P1780S) founder variant was reported as the common Usher syndrome variant among Black South Africans. This review provides the most comprehensive data on HI gene variants in the largely under-investigated African populations. Future exomes studies particularly in multiplex families will likely provide opportunities for the discovery of the next sets of novel HI genes, and well as unreported variants in known genes to further our understanding of HI pathobiology, globally.

Identification of Hearing Loss-Associated Variants of PTPRQ, MYO15A, and SERPINB6 in Pakistani Families

The inner ear is an essential part of a well-developed and well-coordinated hearing system. However, hearing loss can make communication and interaction more difficult. Inherited hearing loss (HL) can occur from pathogenic genetic variants that negatively alter the intricate inner ear sensory mechanism. Recessively inherited forms of HL are highly heterogeneous and account for a majority of prelingual deafness. The current study is designed to investigate genetic causes of HL in three consanguineous Pakistani families. After IRB approval, the clinical history and pure tone audiometric data was obtained for the clinical diagnosis of HL segregating in these three Pakistani families. We performed whole exome sequencing (WES) followed by Sanger sequencing in order to identify and validate the HL-associated pathogenic variants, respectively. The 3-D molecular modeling and the Ramachandran analysis of the identified missense variants were compiled to evaluate the impact of the variants on the encoded proteins. Clinical evaluation revealed prelingual severe to profound sensorineural HL segregating among the affected individuals in all three families. Genetic analysis revealed segregation of several novel variants associated with HL, including a canonical splice-site variant (c.55-2A>G) of PTPRQ in family GCFHL-01, a missense variant [c.1079G>A; p.(Arg360Gln)] of SERPINB6 in family LUHL-01, and an insertion variant (c.10208-10211insCCACCAGGCCCGTGCCTC) within MYO15A in family LUHL-011. All the identified variants had very low frequencies in the control databases. The molecular modeling of p.Arg360Gln missense variant also predicted impaired folding of SERPINB6 protein. This study reports the identification of novel disease-causing variants in three known deafness genes and further highlights the genetic heterogeneity of HL in Pakistani population.

Genetic etiology study of four Chinese families with two nonsyndromic deaf children in succession by targeted next-generation sequencing

BACKGROUND

Genetic components contribute significantly to the cause of hearing loss. Nonsyndromic hearing loss has been shown to have high genetic heterogeneity. For families who had given birth to two nonsyndromic deaf children in succession, it seems that their deafness was highly related to genetics.

OBJECTIVES

This study aimed to disclose the genetic causes of the subjects from the four Chinese families with two nonsyndromic deaf children in succession who failed to find the genetic etiology of the hearing loss by common deafness genetic screening (GJB2, GJB3, SLC26A4, and MT-RNR1, including 20 hot variants in 4 genes).

METHODS

Targeted next-generation sequencing (NGS) of 127 known deafness genes was performed in probands of four families, followed by a series of comprehensive analyses of all family members combined with a literature review of related genes.

RESULTS

We identified pathogenic variants in three families including c.919-2A>G/c.1985G>A in SLC26A4; c.109G>A (p.V37I) in GJB2; and m.7505T>C in MT-TS1. Sanger sequencing confirmed that these variants segregated with the hearing impairment of each family. We also identified c.331C>T/c.625-5C>T/c.5717G>A in CDH23; c.138T>C in POU3F4 in two families, in which the pathogenicity in clinical was likely pathogenic or unknown.

CONCLUSIONS

Using the NGS detection technology, we found the genetic etiology of the HL in part of deaf families. Our study provided a useful piece of information for the variant spectrum of hearing loss in Chinese families with two deaf children in succession.

Etiologies of Childhood Hearing Impairment in Schools for the Deaf in Mali

Objectives: To identify the etiologies of hearing impairment (HI) in schools for students who are deaf and to use a systematic review to summarize reports on the etiologies and clinical and genetic features of HI in Mali. Methods: We included individuals with HI that started before the age of 15 years old. Patients were carefully evaluated under standard practices, and pure-tone audiometry was performed where possible. We then searched for articles published on HI in the Malian population from the databases' inception to March 30, 2020. Results: A total of 117 individuals from two schools for the deaf were included, and a male predominance (sex ratio 1.3; 65/52) was noted. HI was pre-lingual in 82.2% (n = 117), and the median age at diagnosis was 12 years old. The etiologies were environmental in 59.4% (70/117), with meningitis being the leading cause (40%, 20/70), followed by cases with genetic suspicion (29.3%, 21/117). In 11.3% (8/117) of patients, no etiology was identified. Among cases with genetic suspicion, three were syndromic, including two cases of Waardenburg syndrome, while 15 individuals had non-syndromic HI. An autosomal recessive inheritance pattern was observed in 83.3% of families (15/18), and consanguinity was reported in 55.5% (10/18) of putative genetic cases. Conclusion: This study concludes that environmental factors are the leading causes of HI in Mali. However, genetic causes should be investigated, particularly in the context of a population with a high consanguinity rate.

Connexin Genes Variants Associated with Non-Syndromic Hearing Impairment: A Systematic Review of the Global Burden

Mutations in connexins are the most common causes of hearing impairment (HI) in many populations. Our aim was to review the global burden of pathogenic and likely pathogenic (PLP) variants in connexin genes associated with HI. We conducted a systematic review of the literature based on targeted inclusion/exclusion criteria of publications from 1997 to 2020. The databases used were PubMed, Scopus, Africa-Wide Information, and Web of Science. The protocol was registered on PROSPERO, the International Prospective Register of Systematic Reviews, with the registration number “CRD42020169697”. The data extracted were analyzed using Microsoft Excel and SPSS version 25 (IBM, Armonk, New York, United States). A total of 571 independent studies were retrieved and considered for data extraction with the majority of studies (47.8% (n = 289)) done in Asia. Targeted sequencing was found to be the most common technique used in investigating connexin gene mutations. We identified seven connexin genes that were associated with HI, and GJB2 (520/571 publications) was the most studied among the seven. Excluding PLP in GJB2, GJB6, and GJA1 the other connexin gene variants (thus GJB3, GJB4, GJC3, and GJC1 variants) had conflicting association with HI. Biallelic GJB2 PLP variants were the most common and widespread variants associated with non-syndromic hearing impairment (NSHI) in different global populations but absent in most African populations. The most common GJB2 alleles found to be predominant in specific populations include; p.Gly12ValfsTer2 in Europeans, North Africans, Brazilians, and Americans; p.V37I and p.L79Cfs in Asians; p.W24X in Indians; p.L56Rfs in Americans; and the founder mutation p.R143W in Africans from Ghana, or with putative Ghanaian ancestry. The present review suggests that only GJB2 and GJB3 are recognized and validated HI genes. The findings call for an extensive investigation of the other connexin genes in many populations to elucidate their contributions to HI, in order to improve gene-disease pair curations, globally.

A Rare Mutation in the MARVELD2 Gene Can Cause Nonsyndromic Hearing Loss

The MARVELD2 gene which is located on the 5q13.2 may cause nonsyndromic hearing loss (NSHL) with autosomal recessive inherited pattern. So far c.1331+1G>A (IVS4+1G>A); NM_001038603.3, variant in deafness, has only reported previously in one Pakistani family in 2008 and it is reported for the first time in Iran and second time in the world. The case is a 21-year-old Iranian woman who has NSHL referred for genetic consultation, and her parents had a consanguineous marriage. To study the responsible genes for the mentioned disorder, whole exome sequencing (WES) was performed for the case. The result of WES analysis revealed a transition at the splice donor variant site of the MARVELD2 gene. The NGS result was confirmed by Sanger sequencing.

Hearing Impairment Overview in Africa: the Case of Cameroon

The incidence of hearing impairment (HI) is higher in low- and middle-income countries when compared to high-income countries. There is therefore a necessity to estimate the burden of this condition in developing world. The aim of our study was to use a systematic approach to provide summarized data on the prevalence, etiologies, clinical patterns and genetics of HI in Cameroon. We searched PubMed, Scopus, African Journals Online, AFROLIB and African Index Medicus to identify relevant studies on HI in Cameroon, published from inception to 31 October, 2019, with no language restrictions. Reference lists of included studies were also scrutinized, and data were summarized narratively. This study is registered with PROSPERO, number CRD42019142788. We screened 333 records, of which 17 studies were finally included in the review. The prevalence of HI in Cameroon ranges from 0.9% to 3.6% in population-based studies and increases with age. Environmental factors contribute to 52.6% to 62.2% of HI cases, with meningitis, impacted wax and age-related disorder being the most common ones. Hereditary HI comprises 0.8% to 14.8% of all cases. In 32.6% to 37% of HI cases, the origin remains unknown. Non-syndromic hearing impairment (NSHI) is the most frequent clinical entity and accounts for 86.1% to 92.5% of cases of HI of genetic origin. Waardenburg and Usher syndromes account for 50% to 57.14% and 8.9% to 42.9% of genetic syndromic cases, respectively. No pathogenic mutation was described in GJB6 gene, and the prevalence of pathogenic mutations in GJB2 gene ranged from 0% to 0.5%. The prevalence of pathogenic mutations in other known NSHI genes was <10% in Cameroonian probands. Environmental factors are the leading etiology of HI in Cameroon, and mutations in most important HI genes are infrequent in Cameroon. Whole genome sequencing therefore appears as the most effective way to identify variants associated with HI in Cameroon and sub-Saharan Africa in general.

Fokouo JV, Oluwole OG, Nguefack S, Chimusa ER, Wonkam A. Hearing Impairment Overview in Africa: the Case of Cameroon

The incidence of hearing impairment (HI) is higher in low- and middle-income countries when compared to high-income countries. There is therefore a necessity to estimate the burden of this condition in developing world. The aim of our study was to use a systematic approach to provide summarized data on the prevalence, etiologies, clinical patterns and genetics of HI in Cameroon. We searched PubMed, Scopus, African Journals Online, AFROLIB and African Index Medicus to identify relevant studies on HI in Cameroon, published from inception to 31 October, 2019, with no language restrictions. Reference lists of included studies were also scrutinized, and data were summarized narratively. This study is registered with PROSPERO, number CRD42019142788. We screened 333 records, of which 17 studies were finally included in the review. The prevalence of HI in Cameroon ranges from 0.9% to 3.6% in population-based studies and increases with age. Environmental factors contribute to 52.6% to 62.2% of HI cases, with meningitis, impacted wax and age-related disorder being the most common ones. Hereditary HI comprises 0.8% to 14.8% of all cases. In 32.6% to 37% of HI cases, the origin remains unknown. Non-syndromic hearing impairment (NSHI) is the most frequent clinical entity and accounts for 86.1% to 92.5% of cases of HI of genetic origin. Waardenburg and Usher syndromes account for 50% to 57.14% and 8.9% to 42.9% of genetic syndromic cases, respectively. No pathogenic mutation was described in GJB6 gene, and the prevalence of pathogenic mutations in GJB2 gene ranged from 0% to 0.5%. The prevalence of pathogenic mutations in other known NSHI genes was <10% in Cameroonian probands. Environmental factors are the leading etiology of HI in Cameroon, and mutations in most important HI genes are infrequent in Cameroon. Whole genome sequencing therefore appears as the most effective way to identify variants associated with HI in Cameroon and sub-Saharan Africa in general.

Alterations of structural and functional connectivity in profound sensorineural hearing loss infants within an early sensitive period: A combined DTI and fMRI study

Due to heightened level of neuroplasticity, there is a sensitive period (2-4 years after birth) that exists for optimal central auditory development. Using diffusion tensor imaging combined with resting-state functional connectivity (rsFC) analysis, this study directly investigates the structural connectivity alterations of the whole brain white matter (WM) and the functional reorganization of the auditory network in infants with sensorineural hearing loss (SNHL) during the early sensitive period. 46 bilateral profound SNHL infants prior to cochlear implantation (mean age, 17.59 months) and 33 healthy controls (mean age, 18.55 months) were included in the analysis. Compared with controls, SNHL infants showed widespread WM alterations, including bilateral superior longitudinal fasciculus, inferior fronto-occipital fasciculus, inferior longitudinal fasciculus, right corticospinal tract, posterior thalamic radiation and left uncinate fasciculus. Moreover, SNHL infants demonstrated increased rsFC between left/right primary auditory cortex seeds and right insula and superior temporal gyrus. In conclusion, this study suggests that SNHL in the early sensitive period is associated with diffuse WM alterations that mainly affect the auditory and language pathways. Furthermore, increased rsFC in areas mainly associated with auditory and language networks may potentially reflect reorganization and compensatory activation in response to auditory deprivation during the early sensitive period.

Post-Translational Modifications of the Mini-Chromosome Maintenance Proteins in DNA Replication

The eukaryotic mini-chromosome maintenance (MCM) complex, composed of MCM proteins 2-7, is the core component of the replisome that acts as the DNA replicative helicase to unwind duplex DNA and initiate DNA replication. MCM10 tightly binds the cell division control protein 45 homolog (CDC45)/MCM2-7/ DNA replication complex Go-Ichi-Ni-San (GINS) (CMG) complex that stimulates CMG helicase activity. The MCM8-MCM9 complex may have a non-essential role in activating the pre-replicative complex in the gap 1 (G1) phase by recruiting cell division cycle 6 (CDC6) to the origin recognition complex (ORC). Each MCM subunit has a distinct function achieved by differential post-translational modifications (PTMs) in both DNA replication process and response to replication stress. Such PTMs include phosphorylation, ubiquitination, small ubiquitin-like modifier (SUMO)ylation, O-N-acetyl-D-glucosamine (GlcNAc)ylation, and acetylation. These PTMs have an important role in controlling replication progress and genome stability. Because MCM proteins are associated with various human diseases, they are regarded as potential targets for therapeutic development. In this review, we summarize the different PTMs of the MCM proteins, their involvement in DNA replication and disease development, and the potential therapeutic implications.

Genetic screening involving 101 hot spots for neonates not passing newborn hearing screening and those random recruited in Dongguan

In order to investigate essential molecular causes for hearing loss and mutation frequency of deafness-related genes, 1315 newborns who did not pass the Newborn Hearing Screening (NHS) (audio-no-pass) and 1000 random-selected infants were subjected to detection for 101 hotspot mutations in 18 common deafness-related genes. Totally, 23 alleles of 7 deafness genes were detected out. Significant difference (χ² = 25.320, p = 0.000) existed in causative mutation frequency between audio-no-pass group (81/1315, 6.160%) and random-selected cohort (18/1000, 1.80%). Of the genes detected out, GJB2 gene mutation was with significant difference (χ² = 75.132, p = 0.000) between audio-no-pass group (417/1315, 31.711%) and random-selected cohort (159/1000, 15.900%); c.109G > A was the most common allele, as well as the only one with significantly different allele frequency (χ² = 79.327, p = 0.000) between audio-no-pass group (392/1315, 16.84%) and random-selected cohort (140/1000, 7.55%), which suggested c.109G > A mutation was critical for newborns' hearing loss. This study performed detection for such a large scale of deafness-associated genes and for the first time compared mutations between audio-no-pass and random-recruited neonates, which not only provided more reliable DNA diagnosis result for medical practioners and enhanced clinical care for the newborns, but gave more accurate estimation for mutation frequency.

Fluvastatin protects cochleae from damage by high-level noise

Exposure to noise and ototoxic drugs are responsible for much of the debilitating hearing loss experienced by about 350 million people worldwide. Beyond hearing aids and cochlear implants, there have been no other FDA approved drug interventions established in the clinic that would either protect or reverse the effects of hearing loss. Using Auditory Brainstem Responses (ABR) in a guinea pig model, we demonstrate that fluvastatin, an inhibitor of HMG-CoA reductase, the rate-limiting enzyme of the mevalonate pathway, protects against loss of cochlear function initiated by high intensity noise. A novel synchrotron radiation based X-ray tomographic method that imaged soft tissues at micrometer resolution in unsectioned cochleae, allowed an efficient, qualitative evaluation of the three-dimensional internal structure of the intact organ. For quantitative measures, plastic embedded cochleae were sectioned followed by hair cell counting. Protection in noise-exposed cochleae is associated with retention of inner and outer hair cells. This study demonstrates the potential of HMG-CoA reductase inhibitors, already vetted in human medicine for other purposes, to protect against noise induced hearing loss.

Discovery of MYH14 as an important and unique deafness gene causing prelingually severe autosomal dominant nonsyndromic hearing loss

BACKGROUND

Pathogenic variants of MYH14 are known to be associated (in either a syndromic or nonsyndromic manner) with hearing loss. Interestingly, all reported cases to date of MYH14-related nonsyndromic hearing loss with detailed phenotypes have demonstrated mild-to-moderate progressive hearing loss with postlingual onset.

METHODS

In the present study, targeted resequencing (TRS) of known deafness genes was performed to identify the causative variant in two multiplex families segregating autosomal dominant (AD) inherited hearing loss.

RESULTS

TRS uncovered two novel variants of MYH14 (c.A572G: p.Asp191Gly in the myosin head domain and c.C73T:p.Gln25* in exon 2) from two multiplex deafness Korean families. Notably, both probands showed phenotypes of congenital or prelingual severe hearing loss. It is remarkably uncommon to encounter such a severe-to-profound, prelingual, AD hearing loss. Given that the first variant, p. Asp191Gly, was the first documented missense allele discovered in the myosin head domain of this gene related to either congenital or prelingual severe nonsyndromic hearing loss, and also that the second variant, p. Gln25*, lead to a null allele, more severe phenotypes from our probands may have been the result of either genotype-phenotype correlation or genetic backgrounds, or both.

CONCLUSIONS

In the present study, we report that MYH14 can manifest as nonsyndromic prelingual severe sensorineural hearing loss in an AD fashion in Koreans. The results of the present study suggest that further genetic studies of similar patients should consider MYH14 as a causative gene, and cochlear implantation during infant or early childhood should be indicated for those patients with certain MYH14 pathogenic variants.

GPRASP2, a novel causative gene mutated in an X-linked recessive syndromic hearing loss

Background

A substantial amount of nuclear genes have been identified to be implicated in genetic hearing loss, while X-linked hearing loss is genetically heterogeneous and relatively infrequent.

Objective

To identify the causative gene mutation in a five-generation Chinese family with an X-linked recessive syndromic hearing loss (SHL).

Methods

Targeted X-chromosome exome sequencing was conducted, and cosegregation analysis was performed in the members of the affected family. The in silico and expression studies were also performed.

Results

A 2-bp missense mutation (c.1717_1718GC>AA, p.A573N) in the G protein-coupled receptor associated sorting protein 2 (GPRASP2) gene was identified in four hemizygous male patients and two heterozygous female carriers, which was cosegregated with the clinical phenotypes in this family. In silico analysis supported that this gene mutation is functionally deleterious, and it was detected that homologous Gprasp2 was highly expressed in multiple structures of the mouse cochlea, which suggested that GPRASP2 might be the genetic cause for the described disease phenotypes.

Conclusions

This study presented a novel X-linked SHL combined with unique and unrecognised clinical features, and a missense variation of GPRASP2 was first identified to be implicated in X-linked SHL.

Application of a New Genetic Deafness Microarray for Detecting Mutations in the Deaf in China

Objective

The aim of this study was to evaluate the GoldenGate microarray as a diagnostic tool and to elucidate the contribution of the genes on this array to the development of both nonsyndromic and syndromic sensorineural hearing loss in China.

Methods

We developed a microarray to detect 240 mutations underlying syndromic and nonsyndromic sensorineural hearing loss. The microarray was then used for analysis of 382 patients with nonsyndromic sensorineural hearing loss (including 15 patients with enlarged vestibular aqueduct syndrome), 21 patients with Waardenburg syndrome, and 60 unrelated controls. Subsequently, we analyzed the sensitivity, specificity, and reproducibility of this new approach after Sanger sequencing-based verification, and also determined the contribution of the genes on this array to the development of distinct hearing disorders.

Results

The sensitivity and specificity of the microarray chip were 98.73% and 98.34%, respectively. Genetic defects were identified in 61.26% of the patients with nonsyndromic sensorineural hearing loss, and 9 causative genes were identified. The molecular etiology was confirmed in 19.05% and 46.67% of the patients with Waardenburg syndrome and enlarged vestibular aqueduct syndrome, respectively.

Conclusion

Our new mutation-based microarray comprises an accurate and comprehensive genetic tool for the detection of sensorineural hearing loss. This microarray-based detection method could serve as a first-pass screening (before next-generation-sequencing screening) for deafness-causing mutations in China.

Prevalence and audiological profiles of GJB2 mutations in a large collective of hearing impaired patients

Mutations in the GJB2 gene are known to represent the commonest cause of hereditary and congenital hearing loss. In this study, a complete sequencing of the GJB2 gene in a cohort of 506 patients from a single, large cochlear implant program in Europe was performed. Audiological testing for those patients who could actively participate was performed using pure tone audiometry (PTA). Those unable to undergo PTA were measured using click-auditory brainstem response (ABR). Data analysis was performed to determine genotype-phenotype correlations of the mutational status vs. audiological profiles and vs. age at the time of presentation. An overall prevalence of biallelic mutations of 13.4% was found for the total collective. When subsets of younger patients were examined, the prevalence increased to 27% of those up to age 18 and 35% of those up to age 5 at the time of testing, respectively. This increase was found to be highly significant (p < 0.001). Analysis of the mean PTA thresholds revealed a strong correlation between allele combination status and mean PTA (p = 0.021). The prevalence of simple heterozygotes was found to be approximately 10.1%, which is around 3.3 times the value expected in the general population. As GJB2 follows a recessive pattern of inheritance, the question arises as to why such a large fraction of simple heterozygotes was observed among the hearing impaired patients included in this study.

Targeted Exome Sequencing of Deafness Genes After Failure of Auditory Phenotype-Driven Candidate Gene Screening

OBJECTIVE

To demonstrate the efficacy and advantages of targeted exome sequencing (TES) of known deafness genes in cases with failed or misleading auditory phenotype-driven candidate gene screening.

STUDY DESIGN

Prospective cohort survey.

SETTING

Otolaryngology department of a tertiary referral hospital.

PATIENTS

Six hearing-impaired probands with seemingly non-syndromic features from six deaf families were enrolled in this study after failure of genetic diagnosis using auditory phenotype-driven candidate gene screening.

INTERVENTION

TES of known deafness genes was performed in the six probands, and a final causative variant was pursued using subsequent filtering steps.

MAIN OUTCOME MEASURE

Potential causative variants determined using TES were confirmed by previously introduced filtering steps.

RESULTS

We detected causative variants in three (50%) of six families, and these variants were in the COCH, PAX3, and GJB2 genes. Additionally, we also recapitulated the recent finding from other report arguing for the non-pathogenic potential of MYO1A variant.

CONCLUSIONS

TES of a deafness panel provides a comprehensive genetic screening tool that can be implemented without being misled by the audiogram configuration information and can complement incomplete clinical physical examinations. In addition, the secondary incidental finding obtained by TES contributes useful information regarding the deafness field.

Genetic Effects on Sensorineural Hearing Loss and Evidence-based Treatment for Sensorineural Hearing Loss

In this article, the mechanism of inheritance behind inherited hearing loss and genetic susceptibility in noise-induced hearing loss are reviewed. Conventional treatments for sensorineural hearing loss (SNHL), i.e. hearing aid and cochlear implant, are effective for some cases, but not without limitations. For example, they provide little benefit for patients of profound SNHL or neural hearing loss, especially when the hearing loss is in poor dynamic range and with low frequency resolution. We emphasize the most recent evidence-based treatment in this field, which includes gene therapy and allotransplantation of stem cells. Their promising results have shown that they might be options of treatment for profound SNHL and neural hearing loss. Although some treatments are still at the experimental stage, it is helpful to be aware of the novel therapies and endeavour to explore the feasibility of their clinical application.

The 100 Most Cited Turkish Papers in the Otorhinolaryngology Journals of Web of Science

Objective

The aim of the study was to analyze the 100 most cited publications with Turkish origin in the Web of Science Otorhinolaryngology (ORL) journals.

Methods

The Web of Science database was searched in terms of citations for publications originating from Turkey in ORL journals since 1983. After the identification of the 100 most cited articles, analysis was performed for the first author, institution, city, publication type, subject related to subspecialty, and journals having the most cited articles. Moreover, the number of ORL publications and citations of countries was determined in descending order using the same database.

Results

A total of 3948 ORL articles with Turkish origin was identified. The number of citations was 181 for the first and 28 for the last in the 100 most cited articles. As there was more than one article with 28 citations, 101 articles were analyzed. The number of the articles was 76, 22, and 3 for the university, education/research, and state hospitals, respectively. Hacettepe University, Ankara Numune Hospital, and Gazi University were the three leading institutions having the most cited articles, and Ankara was the first city. While 98 of 101 articles were original research, the number of case reports and review articles were 2 and 1, respectively. Thirty-five articles were related to otology, 23 to pediatric ORL, 20 to rhinology and head and neck surgery, and 3 to facial plastic surgery. Laryngoscope, Otolaryngology-Head and Neck Surgery, and International Journal of Pediatric Otorhinolaryngology were the leading 3 journals with the most cited articles coming from Turkey. The evaluation of countries revealed that Turkey was among the first 10 countries in terms of number of ORL articles but fell behind for the number of citations.

Conclusion

This bibliometric study is the first one regarding the contribution of Turkish authors and institutions to ORL literature. Similar studies might be periodically repeated to determine national development in the field of ORL and place of Turkey in the world.

Whole Exome Sequencing Identified MCM2 as a Novel Causative Gene for Autosomal Dominant Nonsyndromic Deafness in a Chinese Family

We report the genetic analysis of autosomal dominant, nonsyndromic, progressive sensorineural hearing loss in a Chinese family. Using whole exome sequencing, we identified a missense variant (c.130C>T, p.R44C) in the MCM2 gene, which has a pro-apoptosis effect and is involved in the initiation of eukaryotic genome replication. This missense variant is very likely to be the disease causing variant. It segregated with hearing loss in this pedigree, and was not found in the dbSNP database or databases of genomes and SNP in the Chinese population, in 76 patients with sporadic hearing loss, or in 145 normal individuals. We performed western blot and immunofluorescence to test the MCM2 protein expression in the cochlea of rats and guinea pigs, demonstrating that MCM2 was widely expressed in the cochlea and was also surprisingly expressed in the cytoplasm of terminally differentiated hair cells. We then transiently expressed the variant MCM2 cDNA in HEK293 cells, and found that these cells displayed a slight increase in apoptosis without any changes in proliferation or cell cycle, supporting the view that this variant is pathogenic. In summary, we have identified MCM2 as a novel gene responsible for nonsyndromic hearing loss of autosomal dominant inheritance in a Chinese family.

The validity of family history as a risk factor in pediatric hearing loss

OBJECTIVES

A family history of permanent childhood hearing loss is considered a risk factor for pediatric hearing loss, although its validity has been sparsely examined. This study aimed to: (1) investigate the prevalence and yield of this risk factor for congenital and postnatal hearing losses, (2) define the audiometric characteristics of hearing loss in children with positive family histories, and (3) assess the nature of the familial relationships.

METHOD

A retrospective cohort study including all children born in Queensland, Australia between September 2004 and December 2011 who had completed Healthy Hearing's newborn hearing screen (n=380,895).

RESULTS

(1) Prevalence of the risk factor was 1.09% (4138/380,895). Prevalence of the risk factor in congenital cases was 7.29% and in postnatal cases was 36.84%. A low yield was identified for both congenital and postnatal groups (1.43% and 1.7%, respectively). (2) The degree of loss in congenital cases was highly varied, whereas the predominant degree in postnatal cases was mild. The most frequent type of loss for congenital cases was sensorineural, whereas for postnatal cases it was conductive. (3) Maternal or sibling relationships were most commonly reported for congenital losses, and maternal or paternal relationships for postnatal losses.

CONCLUSIONS

Children with a family history of pediatric hearing loss should have their hearing screened at birth and be monitored throughout early childhood. However, more efficient surveillance methods should be considered in view of the high prevalence with low yield.

Genetics of hearing loss in Africans: use of next generation sequencing is the best way forward

Hearing loss is the most common communication disorder affecting about 1-7/1000 births worldwide. The most affected areas are developing countries due toextensively poor health care systems. Environmental causes contribute to 50-70% of cases, specifically meningitis in sub-Saharan Africa. The other 30-50% is attributed to genetic factors. Nonsyndromic hearing loss is the most common form of hearing loss accounting for up to 70% of cases. The most common mode of inheritance is autosomal recessive. The most prevalent mutations associated with autosomal recessive nonsyndromic hearing loss (ARNSHL) are found within connexin genes such as GJB2, mostly in people of European and Asian origin. For example, the c.35delG mutation ofGJB2 is found in 70% of ARNSHL patients of European descentand is rare in populations of otherethnicities. Other GJB2 mutations have been reported in various populations. The second most common mutations are found in theconnexin gene, GJB6, also with a high prevalencein patients of European descent. To date more than 60 genes have been associated with ARNSHL. We previously showed that mutations in GJB2, GJB6 and GJA1 are not significant causes of ARNSHL inpatients from African descents, i.e. Cameroonians and South AfricansIn order to resolve ARNSHL amongst sub-Saharan African patients, additional genes would need to be explored. Currently at least 60 genes are thought to play a role in ARNSHL thus the current approach using Sanger sequencing would not be appropriate as it would be expensive and time consuming. Next Generation sequencing (NGS) provides the best alternative approach. In this review, we reported on the success of using NGSas observed in various populations and advocate for the use of NGS to resolve cases of ARNSHL in sub-Saharan African populations.

Autosomal dominant hearing loss resulting from p.R75Q mutation in the GJB2 gene: nonsyndromic presentation in a South Indian family

Mutations in the GJB2 gene encoding the gap junction protein Connexin 26 have been associated with autosomal recessive as well as dominant nonsyndromic hearing loss. Owing to the involvement of connexins in skin homeostasis, GJB2 mutations have also been associated with syndromic forms of hearing loss showing various skin manifestations. We report an assortatively mating hearing impaired family of south Indian origin with three affected members spread over two generations, having p.R75Q mutation in the GJB2 gene in the heterozygous condition. The inheritance pattern was autosomal dominant with mother and son being affected. Dermatological and histopathologic examinations showed absence of palmoplantar keratoderma. To the best of our knowledge, this is the first report from India on p.R75Q mutation in the GJB2 gene with nonsyndromic hearing loss.

Hearing impairment in Estonia: an algorithm to investigate genetic causes in pediatric patients

PURPOSE

The present study was initiated to establish the etiological causes of early onset hearing loss (HL) among Estonian children between 2000-2009.

METHODS

The study group consisted of 233 probands who were first tested with an arrayed primer extension assay, which covers 199 mutations in 7 genes (GJB2, GJB6, GJB3, SLC26A4, SLC26A5 genes, and two mitochondrial genes - 12S rRNA, tRNASer(UCN)). From probands whose etiology of HL remained unknown, DNA analysis of congenital cytomegalovirus (CMV) infection and G-banded karyotype and/or chromosomal microarray analysis (CMA) were performed.

RESULTS

In 110 (47%) cases, the etiology of HL was genetic and in 5 (2%) congenital CMV infection was diagnosed. We found mutations with clinical significance in GJB2 (100 children, 43%) and in 2 mitochondrial genes (2 patients, 1%). A single mutation in SLC26A4 gene was detected in 5 probands (2.2%) and was considered diagnostic. In 4 probands a heterozygous IVS2-2A>G change in the SLC26A5 gene was found. We did not find any instances of homozygosity for this splice variant in the probands. CMA identified in 4 probands chromosomal regions with the loss of one allele. In 2 of them we were able to conclude that the found abnormalities are definitely pathogenic (12q13.3-q14.2 and 17q22-23.2 microdeletion), but the pathogenity of 2 other findings (3p26.2 and 1p33 microdeletion) remained unknown.

CONCLUSION

This practical diagnostic algorithm confirmed the etiology of early onset HL for 115 Estonian patients (49%). This algorithm may be generalized to other populations for clinical application.

Genetic spectrum of autosomal recessive non-syndromic hearing loss in Pakistani families

The frequency of inherited bilateral autosomal recessive non-syndromic hearing loss (ARNSHL) in Pakistan is 1.6/1000 individuals. More than 50% of the families carry mutations in GJB2 while mutations in MYO15A account for about 5% of recessive deafness. In the present study a cohort of 30 ARNSHL families was initially screened for mutations in GJB2 and MYO15A. Homozygosity mapping was performed by employing whole genome single nucleotide polymorphism (SNP) genotyping in the families that did not carry mutations in GJB2 or MYO15A. Mutation analysis was performed for the known ARNSHL genes present in the homozygous regions to determine the causative mutations. This allowed the identification of a causative mutation in all the 30 families including 9 novel mutations, which were identified in 9 different families (GJB2 (c.598G>A, p.Gly200Arg); MYO15A (c.9948G>A, p.Gln3316Gln; c.3866+1G>A; c.8767C>T, p.Arg2923* and c.8222T>C, p.Phe2741Ser), TMC1 (c.362+18A>G), BSND (c.97G>C, p.Val33Leu), TMPRSS3 (c.726C>G, p.Cys242Trp) and MSRB3 (c.20T>G, p.Leu7Arg)). Furthermore, 12 recurrent mutations were detected in 21 other families. The 21 identified mutations included 10 (48%) missense changes, 4 (19%) nonsense mutations, 3 (14%) intronic mutations, 2 (9%) splice site mutations and 2 (9%) frameshift mutations. GJB2 accounted for 53% of the families, while mutations in MYO15A were the second most frequent (13%) cause of ARNSHL in these 30 families. The identification of novel as well as recurrent mutations in the present study increases the spectrum of mutations in known deafness genes which could lead to the identification of novel founder mutations and population specific mutated deafness genes causative of ARNSHL. These results provide detailed genetic information that has potential diagnostic implication in the establishment of cost-efficient allele-specific analysis of frequently occurring variants in combination with other reported mutations in Pakistani populations.

Common molecular etiology of patients with nonsyndromic hearing loss in Tibetan, Tu nationality, and Mongolian patients in the northwest of China

CONCLUSION

In the northwest of China, the prevalence of mutations of the three prominent deafness-related genes, GJB2, SLC26A4, and mitochondrial DNA (mtDNA) 12S rRNA, among Tibetan, Tu nationality, and Mongolian subjects is high, at 19%, 28.57%, and 21.05%, respectively. Molecular genetic screening for these mutations and genetic counseling are effective methods to prevent the occurrence of hereditary hearing loss.

OBJECTIVE

To analyze the prevalence of the three common deafness genes GJB2, mtDNA, and SLC26A4 gene mutations in Tibetan, Tu nationality, and Mongolian patients with nonsyndromic hearing impairment in the Northwest region of China.

METHODS

Genomic DNA was extracted from a total of 189 Tibetan, Tu nationality, and Mongolian probands from the northwest of China. PCR and direct sequencing were used to analyze the coding region of GJB2, mtDNA, and SLC26A4 genes.

RESULTS

The mutant allele rate of GJB2 gene was 6.2% in Tibetan and 11.22% in Tu nationality patients, c.235delC was the most prevalent mutation, accounting for 75% of the mutant GJB2 alleles. Mutant allele frequency of SLC26A4 in Tibetan, Tu nationality, and Mongolian subjects was 4.54%, 6.12%, and 15.79% respectively; p.IVS7-2A>G was the most common form. Mongolian cases were significantly higher than Tibetan cases (χ² = 7.281, p = 0.007 and p < 0.05). mtDNA A1555G mutation was detected in six Tibetan, five Tu nationality, and one Mongolian subject; one Tibetan patient carried the C1494T mutation.

Novel form of X-linked nonsyndromic hearing loss with cochlear malformation caused by a mutation in the type IV collagen gene COL4A6

Hereditary hearing loss is the most common human sensorineural disorder. Genetic causes are highly heterogeneous, with mutations detected in >40 genes associated with nonsyndromic hearing loss, to date. Whereas autosomal recessive and autosomal dominant inheritance is prevalent, X-linked forms of nonsyndromic hearing impairment are extremely rare. Here, we present a Hungarian three-generation family with X-linked nonsyndromic congenital hearing loss and the underlying genetic defect. Next-generation sequencing and subsequent segregation analysis detected a missense mutation (c.1771G>A, p.Gly591Ser) in the type IV collagen gene COL4A6 in all affected family members. Bioinformatic analysis and expression studies support this substitution as being causative. COL4A6 encodes the alpha-6 chain of type IV collagen of basal membranes, which forms a heterotrimer with two alpha-5 chains encoded by COL4A5. Whereas mutations in COL4A5 and contiguous X-chromosomal deletions involving COL4A5 and COL4A6 are associated with X-linked Alport syndrome, a nephropathy associated with deafness and cataract, mutations in COL4A6 alone have not been related to any hereditary disease so far. Moreover, our index patient and other affected family members show normal renal and ocular function, which is not consistent with Alport syndrome, but with a nonsyndromic type of hearing loss. In situ hybridization and immunostaining demonstrated expression of the COL4A6 homologs in the otic vesicle of the zebrafish and in the murine inner ear, supporting its role in normal ear development and function. In conclusion, our results suggest COL4A6 as being the fourth gene associated with X-linked nonsyndromic hearing loss.

Combination of retinitis pigmentosa and hearing loss caused by a novel mutation in PRPH2 and a known mutation in GJB2: importance for differential diagnosis of Usher syndrome

Purpose of this study was to molecularly characterize a family in which two brothers (46 and 36 years) presented with a combination of retinitis pigmentosa (RP) and severe sensorineural hearing loss while father and sister (71 and 41 years) presented with isolated RP. Retinal phenotype was compared with phenotype of 17 patients with Usher syndrome type 1. Ophthalmological examination included assessment of Snellen visual acuity, color vision with Ishihara tables, Goldmann perimetry (targets II/1-4) and microperimetry. Fundus autofluorescence imaging and optical coherence tomography were performed. Direct sequencing of all coding exons and flanking intronic sequences of GJB2 (gap junction protein, beta 2) and PRPH2 (peripherin 2) genes was performed in younger brother. Other family members were analyzed with sequencing (GJB2), high resolution melt analysis (GJB2) or restriction enzymes (PRPH2). Brothers with hearing loss were found to carry a homozygous c.35 delG mutation in GJB2, the most common mutation associated with recessive hearing loss. All patients were found to carry a novel heterozygous mutation c.389T>C (p.Leu130Pro) on PRPH2. Age of onset was higher in PRPH2 than USH1 patients, however with some overlap. Differentiation from retinal phenotype of USH1 could only be made in the oldest patient, who retained good central visual function after more than three decades of disease.

Otolaryngologic manifestations of craniofacial syndromes

This review describes important aspects of the most commonly encountered craniofacial syndromes. The goal is to provide otolaryngologists and other health care providers with critical information necessary to manage these patients appropriately. The algorithm provided in this article should be helpful in guiding the treatment of craniofacial patients based on their unique otolaryngologic characteristics. The principles highlighted in the algorithm can be applied to other craniofacial syndromes not addressed here, including Pierre Robin sequence and Down syndrome.

The contribution of the mitochondrial COI/tRNA(Ser(UCN)) gene mutations to non-syndromic and aminoglycoside-induced hearing loss in Polish patients

Mutations in mitochondrial DNA have been implicated in both, non-syndromic and aminoglycoside-induced hearing loss. In the present study, we have performed the systematic mutation screening of the COI/tRNA(Ser(UCN)) genes in 250 unrelated Polish subjects with hearing impairment. Three different homoplasmic sequence variants were identified, including one common polymorphism m.7476 C>T in tRNA(Ser(UCN)) and two mutations, m.7444 G>A and m.7445 A>G localized in the COI/precursor of tRNA(Ser(UCN)). The incidence of m.7444 G>A substitution was estimated at 1.6% (4/250), however variable penetrance of hearing loss, age of onset and hearing thresholds among m.7444 G>A carriers was observed. Two subjects had the positive history of aminoglycoside exposure and one of them harbored both m.7444 G>A and 12S rRNA m.1555 A>G mutations. Those suggest that m.7444 G>A itself is not sufficient to produce a clinical phenotype and additional modifier factors are required for pathogenic manifestation of m.7444 G>A substitution. Moreover, we have described the first Polish family with non-syndromic hearing loss, harboring m.7445 A>G mutation. The penetrance of hearing loss in this pedigree was 58% when aminoglycoside-induced hearing impairment was included, and 8% when ototoxic effect was excluded. This finding strongly suggests the possible role of m.7445 A>G in susceptibility to aminoglycoside induced-hearing loss.

Alteration of rod and cone function in children with Usher syndrome

PURPOSE

To evaluate the retinal function, with emphasis on phenotype and rate of progression, in infants and children with different genotypes of Usher syndrome.

METHODS

Fourteen children (2-10 years of age) with retinitis pigmentosa and hearing impairment were examined with full-field electroretinography (ERG) during general anesthesia, ophthalmologic examination, and genetic analysis. Five children were repeatedly examined (follow-up 5-10 years) with full-field ERG under local anesthesia and in 2 children multifocal ERG and optical coherence tomography (OCT) were performed. These results were compared to full-field ERG data from 58 children without retinal eye disorder.

RESULTS

Six children were genotyped as Usher 1B, 2A, and 3A. Full-field ERG demonstrated early alterations corresponding to a rod-cone dystrophy in all children. A remaining rod function could be verified in the majority of the children up to 4 years of age. After 4 years of age, there was a further deterioration of the rod function; the progress was severe in Usher types 1 and 2 and moderate in Usher type 3. In all children, the cone function was moderately reduced, in a few cases almost normal. The results from the 58 children without retinal disorder confirm that full-field ERG during general anesthesia is reliable. Multifocal ERG confirmed a preserved central cone function and in OCT there were discrete structural alterations.

CONCLUSIONS

Full-field ERG during general anesthesia in children with Usher syndrome demonstrates variable phenotypes and different degrees in rate of progression during childhood.

Mutation analysis of mitochondrial 12S rRNA gene in Polish patients with non-syndromic and aminoglycoside-induced hearing loss

Mutations in mitochondrial DNA have been reported as associated with non-syndromic and aminoglycoside-induced hearing loss. In the present study, we have performed mutational screening of entire 12S rRNA gene in 250 unrelated patients with non-syndromic and aminoglycoside-induced hearing loss. Twenty-one different homoplasmic sequence variants were identified, including eight common polymorphisms, one deafness-associated mutation m.1555 A>G and three putatively pathogenic variants: m.669 T>C, m.827 A>G, m.961 delT+C(n)ins. The incidence of m.1555 A>G was estimated for 3.6% (9/250); however, where aminoglycoside exposure was taken as a risk factor, the frequency was 5.5% (7/128). Substitution m.669 T>C was identified only in patients with hearing impairment and episode of aminoglycoside exposure, which may suggest that such additional risk factors must appear to induce clinical phenotype. Moreover, two 12S rRNA sequence variants: m.988 G>A and m.1453 A>G, localized at conserved sites and affected RNA secondary structure, may be new candidates for non-syndromic and aminoglycoside-induced hearing loss associated mutations.

Screening of the general Polish population for deafness-associated mutations in mitochondrial 12S rRNA and tRNA Ser(UCN) genes

Mutations in mitochondrial DNA are associated potentially with nonsyndromic and aminoglycoside-induced hearing loss. Several nucleotide changes associated with hearing impairment were described; however, a variable frequency of deafness-associated mutations in different populations has been observed. The aim of the present study was to determine the frequency of pathological mutations in mitochondrial 12S rRNA and tRNA(Ser(UCN)) genes in a group of 500 individuals representative of the general population of Poland. Mutational screening of 12S rRNA revealed the presence of three deafness-associated mutations, A827G, T961C, and A1555G, and one potentially pathogenic substitution, T669C. The carrier frequency of pathological mutations was estimated to be 1.2% (6/500) in the general Polish population. A deafness-associated G7444A mutation in the precursor of tRNA(Ser(UCN)) gene was identified in 8/500 (1.6%) unrelated blood donors. Seven nucleotide changes identified in 12S rRNA (G709A, G750A, G930A, T1243C, T1420C, and G1438A) and tRNA(Ser(UCN)) (C7476T), based on a frequency exceeding 1.0%, were considered as polymorphisms of 12S rRNA and tRNA(Ser(UCN)) in the studied population. Mitochondrial 12S rRNA gene seems to be the hot spot for deafness-associated mutations in the Polish population. The relatively high carrier frequency of tRNA(Ser(UCN)) G7444A (1/62) suggests that this substitution might be a nonpathogenic polymorphism in the Polish population.