Mutation in the mitochondrial 12S rRNA gene in two families from Mongolia with matrilineal aminoglycoside ototoxicity

Establishment and application of a reverse dot blot assay for 13 mutations of hearing-loss genes in primary hospitals in China

Background

Hearing loss is a common sensorineural dysfunction with a high incidence in China. Although genetic factors are important causes of hearing loss, hearing-related gene detection has not been widely adopted in China.

Objective

Establishing a rapid and efficient method to simultaneously detect hotspot hearing loss gene mutations.

Methods

A reverse dot blot assay combined with a flow-through hybridization technique was developed for the simultaneous detection of 13 hotspot mutations of 4 hearing loss-related genes including GJB2, GJB3, SLC26A4, and the mitochondrial gene MT-RNR1. This method involved PCR amplification systems and a hybridization platform.

Results

The technique can detect 13 hotspot mutations of 4 hearing loss-related genes. And a total of 213 blood samples were used to evaluate the availability of this method.

Discussion

Our reverse dot blot assay was a simple, rapid, accurate, and cost-effective method to identify hotspot mutations of 4 hearing loss-related genes in a Chinese population.

Clinical evaluation and etiologic diagnosis of hearing loss: A clinical practice resource of the American College of Medical Genetics and Genomics (ACMG)

Hearing loss is a common and complex condition that can occur at any age, can be inherited or acquired, and is associated with a remarkably wide array of etiologies. The diverse causes of hearing loss, combined with the highly variable and often overlapping presentations of different forms of hearing loss, challenge the ability of traditional clinical evaluations to arrive at an etiologic diagnosis for many deaf and hard-of-hearing individuals. However, identifying the etiology of hearing loss may affect clinical management, improve prognostic accuracy, and refine genetic counseling and assessment of the likelihood of recurrence for relatives of deaf and hard-of-hearing individuals. Linguistic and cultural identities associated with being deaf or hard-of-hearing can complicate access to and the effectiveness of clinical care. These concerns can be minimized when genetic and other health care services are provided in a linguistically and culturally sensitive manner. This clinical practice resource offers information about the frequency, causes, and presentations of hearing loss and suggests approaches to the clinical and genetic evaluation of deaf and hard-of-hearing individuals aimed at identifying an etiologic diagnosis and providing informative and effective patient education and genetic counseling.

Genetic susceptibility to aminoglycoside ototoxicity

INTRODUCTION

Aminoglycosides are a well-known clinically relevant antibiotic family used to treat bacterial infections in humans and animals and can produce toxic side effects. Aminoglycoside-induced hearing loss (HL) has been shown to have a genetic susceptibility. Mitochondrial DNA mutations have been implicated in inherited and acquired hearing impairment.

OBJECTIVE

Literature review of genetic mutations associated with aminoglycoside-induced ototoxicity.

METHODS

PubMed was accessed from 1993 to 2017 using the search terms "aminoglycoside, genetic, ototoxicity, hearing loss". Exclusion criteria consisted of a literature in a language other than English, uncompleted or ongoing studies, literature with non-hearing related diseases, literature on ototoxicity due to cisplatin/carboplatin based chemotherapy, literature on ototoxicity from loop diuretics, animal studies, literature studying oto-protective agents, and literature without documented aminoglycoside exposure.

RESULTS

108 articles were originally identified, and 25 articles were included in our review. Mitochondrial 12S rRNA mutations were identified in all 25 studies in a total of 220 patients. Eight studies identified A1555G mutation as primary genetic factor underlying HL in cases of aminoglycoside-induced ototoxicity. The next most common mutation identified was C1494T.

DISCUSSION

Mitochondrial 12s rRNA mutation A1555G was present in American, Chinese, Arab-Israeli, Spanish and Mongolian ethnicities. All mutations leading to aminoglycoside ototoxicity were mitochondrial mutations.

CONCLUSIONS

Consideration of preexisting genetic defects may be valuable in treatments involving aminoglycosides. In particular populations such as those of Chinese origin, clinicians should continue to consider the increased susceptibility to aminoglycosides.

Investigation of the mtDNA mutations in Syrian families with non-syndromic sensorineural hearing loss

OBJECTIVE

Hearing loss is a common sensory disorder, and at least 50% of cases are due to a genetic etiology. Several mitochondrial DNA mutations (mtDNA) have been reported to be associated with nonsyndromic hearing loss (NSHL) in different population. However, There is no previous available data about the frequency of mtDNA mutations as etiology for deafness in Syrian. The aim of present study is to investigate the incidence of common mt DNA mutations in our families with congenital hearing loss and not related to the ototoxicity or aminoglycosides.

METHODS

A total of 50 deaf families were enrolled in the present study. Direct sequencing and PCR-RFLP methods were employed to detect seven mt DNA mutations, including A1555G, A3243G, C1494T, G3316A, T7510C, A7445G, and 7472insC.

RESULTS

Our results revealed a high prevalence of mt DNA mutation (10%) in deaf families (5/50). In surprising, the unexpected mutations were observed. The G3316A mutation was found in 2 families as homoplasmic genotype. Also, we found the homoplasmic and heteroplasmic genotype for the C1494T mutation in two families. In one family the heteroplasmic genotype for T7510C mutation was observed; this family harbor 35delG mutation in GJB2 gene. None of the common mtDNA mutations (A1555G, A3243G) and other mutations (A7445G, 7472insC) were detected here.

CONCLUSION

Our findings indicate to significant contribution of the mt DNA mutations in our families with NSHL. The presented data is the first report about mt DNA and it will improve the genetic counseling of hearing impaired in Syrian families.

The Mitochondrial COI/tRNASER(UCN) G7444A Mutation may be Associated with Hearing Impairment in a Han Chinese Family

Variations in mitochondrial genome have been found to be associated with hearing loss. Of these, the mitochondrial 12S rRNA and tRNA^Ser(UCN) are the hot-spots for pathogenic variants associated with deafness. To understand the putative role of mitochondrial DNA (mtDNA) variants in hearing loss, we recently screened the variants in mitochondrial genomes in patients with deafness from the Hangzhou area of Zhejiang Province, People’s Republic of China (PRC). In this study, we describe a maternally-inherited Han Chinese family with high penetrance of hearing loss, notably, the penetrance of hearing loss in this family were 80.0 and 40.0%, when the aminoglycoside was included or excluded. Three matrilineal relatives in this pedigree exhibited different levels of hearing loss with different age at onset. In addition, sequence analysis of the complete mitochondrial genome showed the presence of the well-known C1494T pathogenic variant in the 12S rRNA gene and the G7444A pathogenic variant in the COI/ tRNA^Ser(UCN). The C1494T anomaly had been reported to be a pathogenic mutation associated with aminoglycoside-induced and nonsyndromic hearing loss (AINHL), while the G7444A was considered as a secondary mutation associated with deafness. However, the lack of functional variants in GJB2 and TRMU genes suggested that nuclear modified genes may not play important roles in deafness expression. Thus, the combination of G7444A and C1494T pathogenic variants in the mitochondrial genome may account for the high penetrance of hearing loss in this Chinese family.

Adverse effects of a single dose of gentamicin in adults: a systematic review

AIMS

To systematically review the frequency and type of adverse events associated with a single dose of intravenous or intramuscular gentamicin in adults, for any indication, in studies where a comparator was available.

METHODS

A review protocol was developed and registered (PROSPERO: CRD42013003229). Studies were eligible for review if they: recruited participants aged ≥16 years; used gentamicin intramuscularly or intravenously as a single one-off dose; compared gentamicin to another medication or placebo; and monitored adverse events. MEDLINE, EMBASE, Cochrane Library, trial registries, conference proceedings and other relevant databases were searched up to November 2016. Risk of bias was assessed on all included studies.

RESULTS

In total, 15 522 records were identified. After removal of duplicates, screening of title/abstracts for relevance and independent selection of full texts by two reviewers, 36 studies were included. Across all the included studies, 24 107 participants received a single one-off dose of gentamicin (doses ranged from 1 mg kg^-1 to 480 mg per dose). Acute kidney injury was described in 2520 participants receiving gentamicin. The large majority of cases were reversible. There were no cases of ototoxicity reported in patients receiving gentamicin. A meta-analysis was not performed due to study heterogeneity.

CONCLUSIONS

A significant number of patients saw a transient rise in creatinine after a single dose of gentamicin at doses up to 480 mg. Persistent renal impairment and other adverse events were relatively rare.

Mitochondrial COI/tRNASer(UCN) G7444A mutation may be associated with hearing impairment in a Han Chinese family

Mutations in mitochondrial genome have been found to be associated with hearing loss. Of these, the mitochondrial 12S rRNA and tRNA^Ser(UCN) are the hot spots for pathogenic mutations associated with deafness. To understand the putative role of mitochondrial DNA (mtDNA) mutations in hearing loss, we recently initiated a mutational screening for the mtDNA mutations in Hangzhou area from Zhejiang Province. In this study, we described a maternally inherited Han Chinese family with high penetrance of hearing loss, notably, the penetrances of hearing loss in this family were 80% and 40%, when the aminoglycoside was included or excluded. Three matrilineal relatives in this pedigree exhibited different levels of hearing loss with different age at onset. In addition, sequence analysis of the complete mitochondrial genome showed the presence of the well-known C1494T mutation in 12S rRNA gene and the G7444A mutation in the COI/tRNA^Ser(UCN). The C1494T mutation had been reported to be a pathogenic mutation associated with aminoglycoside-induced and non-syndromic hearing loss. While the G7444A mutation was considered as a secondary mutation associated with deafness. However, the lack of functional variants in GJB2 and TRMU genes suggested that nuclear modified genes may not play important roles in deafness expression. Thus, the combination of G7444A and C1494T mutations in mitochondrial genome may account for the high penetrance of hearing loss in this Chinese family.

Genetic Epidemiology of Mitochondrial Pathogenic Variants Causing Nonsyndromic Hearing Loss in a Large Cohort of South Indian Hearing Impaired Individuals

Mitochondria play a critical role in the generation of metabolic energy in the form of ATP. Tissues and organs that are highly dependent on aerobic metabolism are involved in mitochondrial disorders including nonsyndromic hearing loss (NSHL). Seven pathogenic variants leading to NSHL have so far been reported on two mitochondrial genes: MT-RNR1 encoding 12SrRNA and MT-TS1 encoding tRNA for Ser((UCN)) . We screened 729 prelingual NSHL subjects to determine the prevalence of MT-RNR1 variants at position m.961, m.1555A>G and m.1494C>T, and MT-TS1 m.7445A>G, m.7472insC m.7510T>C and m.7511T>C variants. Mitochondrial pathogenic variants were found in eight probands (1.1%). Five of them were found to have the m.1555A>G variant, two others had m.7472insC and one proband had m.7444G>A. The extended relatives of these probands showed variable degrees of hearing loss and age at onset. This study shows that mitochondrial pathogenic alleles contribute to about 1% prelingual hearing loss. This study will henceforth provide the reference for the prevalence of mitochondrial pathogenic alleles in the South Indian population, which to date has not been estimated. The m.1555A>G variant is a primary predisposing genetic factor for the development of hearing loss. Our study strongly suggests that mitochondrial genotyping should be considered for all hearing impaired individuals and particularly in families where transmission is compatible with maternal inheritance, after ruling out the most common variants.

A reverse dot blot assay for the screening of twenty mutations in four genes associated with NSHL in a Chinese population

Background

Congenital deafness is one of the most distressing disorders affecting humanity and exhibits a high incidence worldwide. Most cases of congenital deafness in the Chinese population are caused by defects in a limited number of genes. A convenient and reliable method for detecting common deafness-related gene mutations in the Chinese population is required.

Methods

We developed a PCR-reverse dot blot (RDB) assay for screening 20 hotspot mutations of GJB2, GJB3, SLC26A4, and MT-RNR1, which are common non-syndromic hearing loss (NSHL)–associated genes in the Chinese population. The PCR-RDB assay consists of multiplex PCR amplifications of 10 fragments in the target sequence of the four above-mentioned genes in wild-type and mutant genomic DNA samples followed by hybridization to a test strip containing allele-specific oligonucleotide probes. We applied our method to a set of 225 neonates with deafness gene mutations and 30 normal neonates.

Results

The test was validated through direct sequencing in a blinded study with 100% concordance.

Conclusions

The results demonstrated that our reverse dot blot assay is a reliable and effective genetic screening method for identifying carriers and individuals with NSHL among the Chinese population.

Mitochondrial DNA mutations associated with aminoglycoside induced ototoxicity

Aminoglycosides (AmAn) are widely used for their great efficiency against gram-negative bacterial infections. However, they can also induce ototoxic hearing loss, which has affected millions of people around the world. As previously reported, individuals bearing mitochondrial DNA mutations in the 12S rRNA gene, such as m.1555A>G and m.1494C>T, are more prone to AmAn-induced ototoxicity. These mutations cause human mitochondrial ribosomes to more closely resemble bacterial ribosomes and enable a stronger aminoglycoside interaction. Consequently, exposure to AmAn can induce or worsen hearing loss in these individuals. Furthermore, a wide range of severity and penetrance of hearing loss was observed among families carrying these mutations. Studies have revealed that these mitochondria mutations are the primary molecular mechanism of genetic susceptibility to AmAn ototoxicity, though nuclear modifier genes and mitochondrial haplotypes are known to modulate the phenotypic manifestation.

Etiologic diagnosis of sensorineural hearing loss in adults

OBJECTIVE: To determine the etiology of adult-onset sensorineural hearing loss.

STUDY DESIGN AND SETTING: This is a prospective cohort study of 60 adult subjects with bilateral sensorineural hearing loss of no obvious etiology by medical history and physical examination. These patients were evaluated at an academic medical center and underwent evaluation by high-resolution computed tomography of the temporal bone, autoimmune panel, and DNA testing for mutations of both the GJB2 gene and the mitochondrial DNA (1555A>G and 7445A>G).

RESULTS: An etiologic diagnosis was achieved in 6 patients: cochlear otosclerosis, 1 case; dilated vestibular aqueduct, 1 case; a mitochondrial DNA 7445A>G mutation, 3 cases; and a mitochondrial DNA 1555A>G mutation, 1 case.

CONCLUSION: This result underscores the importance of a search for the etiology of a hearing deficit in adult patients. There are specific interventions now available for the management of hearing-impaired patients with cochlear otosclerosis and mitochondrial DNA mutations.

Two novel compound heterozygous families with a trimutation in the GJB2 gene causing sensorineural hearing loss

BACKGROUND

Sensorineural hearing loss (SNHL) is a genetically heterogeneous disease. GJB2 gene mutations seem to be the most frequent cause of hereditary hearing impairment in several populations. There is variability in the mutations in the GJB2 gene worldwide; this remarks the influence of ethnic background in SNHL.

OBJECTIVE

To describe the presence of two trimutations in the GJB2 gene in two Mexican families with hereditary SNHL.

MATERIALS AND METHODS

Two unrelated Mexican families with prelingual SNHL were included in the study. Analysis of the GJB2 gene through PCR and DNA direct sequencing analysis was performed in all members of the families and in 100 normal controls.

RESULTS

Affected member of the family 1 showed the trimutation p.S19R/p.R32S/p.E47*, whereas affected members of the family 2 showed the trimutation p.F31I/p.W44*/p.V84M. Parents of both families were heterozygous with normal audition.

CONCLUSION

We found a novel mutation in the GJB2 gene and two trimutations with SNHL not previously reported. This remarks the complexity in the pattern of mutations in the GJB2 gene in SNHL and enriches the spectrum of the type of molecular defects in the GJB2 gene.

Successful MDR-TB treatment regimens including amikacin are associated with high rates of hearing loss

Background

Aminoglycosides are a critical component of multidrug-resistant tuberculosis (MDR-TB) treatment but data on their efficacy and adverse effects in Botswana is scarce. We determined the effect of amikacin on treatment outcomes and development of hearing loss in MDR-TB patients.

Methods

Patients started on MDR-TB treatment between 2006 and 2012 were included. Multivariate analysis was used to determine the effect of amikacin on treatment outcomes and development of hearing loss.

Results

437 MDR-TB patients were included, 288 (66%) of whom were HIV co-infected. 270 (62%) developed hearing loss, of whom 147 (54%) had audiometry. Of the 313 (72%) patients who completed treatment, 228 (73%) had a good outcome (cure or treatment completion). Good outcome was associated with longer amikacin treatment (adjusted OR [aOR] 1.13, 95% CI 1.06 - 1.21) and higher dosage (aOR 1.90, 95% CI 1.12 – 2.99). Longer amikacin duration (aOR 1.98, 95% CI 1.86 – 2.12) and higher dosage per weight per month (aOR 1.15, 95% CI 1.04 – 1.28) were associated with development of hearing loss. Amikacin treatment duration modified the effect of the dosage on the risk of hearing loss, increasing this risk as the duration increased.

Conclusions

Amikacin was effective for MDR-TB treatment, but was associated with a high incidence of hearing loss especially in our study population. Total treatment duration and average monthly amikacin dose were associated with improved outcomes; however these were also associated with development of hearing loss.

Complete mitochondrial genome analysis and clinical documentation of a five-generational Indian family with mitochondrial 1555A>G mutation and postlingual hearing loss

Hearing loss is the most common sensory disorder and is genetically heterogeneous. Apart from nuclear gene mutations, a number of inherited mitochondrial mutations have also been implicated. The m.1555A>G mutation in the mitochondrial MT-RNR1 gene is reported as the most common mutation causing nonsyndromic hearing loss in various ethnic populations. We report here for the first time the clinical, genetic and molecular characterisation of a single large five-generational Tamil-speaking South Indian family with maternally inherited nonsyndromic postlingual hearing loss. Molecular analysis led to identification of m.1555A>G in 28 maternal relatives with variable degree of phenotypic expression. The penetrance of hearing loss among the maternal relatives in this family was 55%. Sequence analysis of the complete mitochondrial genome in 36 members of this pedigree identified 25 known variants and one novel variant co-transmitted along with m.1555A>G mutation. The mtDNA haplotype analysis revealed that the maternal relatives carry the R*T2 haplotype similar to Europeans and South Asians. Sequencing of the coding exon of GJB2 nuclear gene did not show any pathogenic mutations. The results suggest that other nuclear or environmental modifying factors could have played a role in the differential expression of mutation m.1555A>G in postlingual hearing loss in this family.

American College of Medical Genetics and Genomics guideline for the clinical evaluation and etiologic diagnosis of hearing loss

Hearing loss is a common and complex condition that can occur at any age, can be inherited or acquired, and is associated with a remarkably wide array of etiologies. The diverse causes of hearing loss, combined with the highly variable and often overlapping presentations of different forms of hearing loss, challenge the ability of traditional clinical evaluations to arrive at an etiologic diagnosis for many deaf and hard-of-hearing individuals. However, identifying the etiology of a hearing loss may affect clinical management, improve prognostic accuracy, and refine genetic counseling and assessment of the likelihood of recurrence for relatives of deaf and hard-of-hearing individuals. Linguistic and cultural identities associated with being deaf or hard of hearing can complicate access to and the effectiveness of clinical care. These concerns can be minimized when genetic and other health-care services are provided in a linguistically and culturally sensitive manner. This guideline offers information about the frequency, causes, and presentations of hearing loss and suggests approaches to the clinical evaluation of deaf and hard-of-hearing individuals aimed at identifying an etiologic diagnosis and providing informative and effective patient education and genetic counseling.

Screening of mitochondrial mutations in Tunisian patients with mitochondrial disorders: an overview study

To investigate the spectrum of common mitochondrial mutations in Tunisia during the years of 2002-2012, 226 patients with mitochondrial disorders were clinically diagnosed with hearing loss, Leigh syndrome (LS), diabetes, cardiomyopathy, Kearns-Sayre syndrome (KSS), Pearson syndrome (PS), myopathy, mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes syndrome (MELAS) and Wolfram syndrome. Restriction fragment length polymorphism (PCR-RFLP), radioactive PCR, single specific primer-PCR (SSP-PCR) analysis and PCR-sequencing methods were used to identify the mutations. Two cases with m.1555A>G mutation and two families with the novel 12S rRNA m.735A>G transition were detected in patients with hearing loss. Three cases with m.8993T>G mutation, two patients with the novel m.5523T>G and m.5559A>G mutations in the tRNA(Trp) gene, and two individuals with the undescribed m.9478T>C mutation in the cytochrome c oxidase subunit III (COXIII) gene were found with LS. In addition, one case with hypertrophic cardiomyopathy and deafness presented the ND1 m.3395A>G mutation and the tRNA(Ile) m.4316A>G variation. Besides, multiple mitochondrial deletions were detected in patients with KSS, PS, and Wolfram syndrome. The m.14709T>C mutation in the tRNA(Glu) was reported in four maternally inherited diabetes and deafness patients and a novel tRNA(Val) m.1640A>G mutation was detected in a MELAS patient.

Etiology of unilateral hearing loss in a national hereditary deafness repository

PURPOSE

The aim of this study was to characterize the genetic, audiologic, and epidemiologic characteristics of unilateral hearing loss (HL) in a national hereditary deafness repository.

MATERIALS AND METHODS

This is a prospective clinical study involving 34 subjects identified in a national hereditary deafness repository. Clinical data and family history of HL were obtained on enrollment. Candidate deafness genes were screened by single-stranded conformation polymorphism, and mutations were confirmed with sequencing.

RESULTS

Thirty-four subjects (19 males, 15 females) with unilateral HL were identified, ranging in age from 2 months to 36 years. The mean age at diagnosis was 7 years, and the left ear was affected in 62% of the cases. The racial distribution of our sample was 62% white, 23% African American, and 15% Hispanic. Imaging results were available in 47%, and most (69%) were considered normal. Nineteen percent had enlarged vestibular aqueducts, 2 had ipsilateral Mondini dysplasia, and 1 had a common cavity deformity. Twenty subjects (59%) had a family history of HL, with 26% specifically reporting familial unilateral HL. Mutational screening revealed sequence variants in the GJB2 (connexin 26), GJB3 (connexin 31), TECTA, and COCH genes. Two novel mutations were detected in COCH and TECTA.

CONCLUSIONS

Sequence variants in known deafness genes were detected in more than one-third of our study population, suggesting that gene/gene or gene/environmental interactions may indeed play a role in the etiology of some cases of unilateral deafness. Further prospective studies including congenital cytomegalovirus screening at birth and molecular screening of deafness genes in children with congenital unilateral HL will be required to establish the etiology of unilateral deafness with certainty.

Mitochondrial DNA mutation screening in an ethnically diverse nonsyndromic deafness cohort

Deafness is a heterogeneous trait with many known genetic and environmental causes. Hereditary hearing loss is an extremely common disorder in the general population. Mutations in mitochondrial DNA (mtDNA) are known to be associated with nonsyndromic deafness (NSD) and syndromic deafness. The objective of this article is to investigate the frequency of common mitochondrial mutations (A1555G, G7444A, and A3243G) in an ethnically diverse cohort of probands with NSD from South Florida. These patients were ascertained at the University of Miami. Polymerase chain reaction-restriction fragment length polymorphism analysis and direct sequencing methods were used for mutation screening in a cohort of 217 patients with NSD. The frequency of common mitochondrial mutations is 1.84% (4/217) in this cohort. A1555G and G7444A accounted for four patients with NSD. Our mutation frequencies are comparable with those previously reported in other populations, indicating that mutations in mtDNA are an important cause of NSD in our patient cohort.

GJB2 and mitochondrial DNA 1555A>G mutations in students with hearing loss in the Hubei Province of China

OBJECTIVES

The GJB2 and MTRNR1 1555A>G mutations are the prevalent causes of hearing loss worldwide. However, the mutation profiles of the two genes are dependent on the ethnic or geographic origins. Therefore, this study was to characterize the forms and frequencies of the two genes in 813 students with hearing loss in Hubei province, Central China.

METHODS

Blood samples from 813 students were obtained with informed consent. Genomic DNA was extracted from peripheral blood leukocytes. The target fragments were amplified by polymerase chain reaction (PCR). Sequencing (or enzyme digestion) was applied to identify sequence variations.

RESULTS

Ten different mutations were identified in GJB2 in 146 of the 813 (17.96%) patients and 11.81% (96/813) patients had homoplasmic mtDNA 1555A>G mutation.

CONCLUSIONS

This study demonstrated the high prevalence of GJB2 and mtDNA 1555A>G mutations in Central Chinese population. Therefore, it will be effective to perform GJB2 and mtDNA 1555A>G mutation analysis for genetic screening for hearing loss in this population.

Absence of GJB2 gene mutations, the GJB6 deletion (GJB6-D13S1830) and four common mitochondrial mutations in nonsyndromic genetic hearing loss in a South African population

OBJECTIVE

The purpose of this study was to determine the prevalence of mutations in the GJB2 gene, the GJB6-D13S1830 deletion and the four common mitochondrial mutations (A1555G, A3243G, A7511C and A7445G) in a South African population.

METHODS

Using single-strand conformation polymorphism and direct sequencing for screening GJB2 mutation; Multiplex PCR Amplification for GJB6-D13S1830 deletion and Restriction Fragment-Length Polymorphism (PCR-RFLP) analysis for the four common mtDNA mutations. We screened 182 hearing impaired students to determine the frequency of these mutations in the population.

RESULTS

None of the reported disease causing mutations in GJB2 nor any novel pathogenic mutations in the coding region were detected, in contrast to the findings among Caucasians. The GJB6-D13S1830 deletion and the mitochondrial mutations were not observed in this group.

CONCLUSION

These results suggest that GJB2 may not be a significant deafness gene among sub-Saharan Africans, pointing to other unidentified genes as responsible for nonsyndromic hearing loss in these populations.

Mitochondrial 12S rRNA mutations associated with aminoglycoside ototoxicity

The mitochondrial 12S rRNA is a hot spot for mutations associated with both aminoglycoside-induced and nonsyndromic hearing loss. Of those, the homoplasmic 1555A>G and 1494C>T mutations at the highly conserved decoding region of the 12S rRNA have been associated with hearing loss worldwide. In particular, these two mutations account for a significant number of cases of aminoglycoside ototoxicity. The 1555A>G or 1494C>T mutation is expected to form a novel 1494C-G1555 or 1494U-A1555 base-pair at the highly conserved A-site of 12S rRNA. These transitions make the human mitochondrial ribosomes more bacteria-like and alter binding sites for aminoglycosides. As a result, the exposure to aminoglycosides can induce or worsen hearing loss in individuals carrying one of these mutations. Biochemical characterization demonstrated an impairment of mitochondrial protein synthesis and subsequent defects in respiration in cells carrying the A1555G or 1494C>T mutation. Furthermore, a wide range of severity, age-at-onset and penetrance of hearing loss was observed within and among families carrying these mutations. Nuclear modifier genes, mitochondrial haplotypes and aminoglycosides should modulate the phenotypic manifestation of the 12S rRNA 1555A>G and 1494C>T mutations. Therefore, these data provide valuable information and technology: (1) to predict which individuals are at risk for ototoxicity; (2) to improve the safety of aminoglycoside antibiotic therapy; and (3) eventually to decrease the incidence of hearing loss.

The search of a genetic basis for noise-induced hearing loss (NIHL)

BACKGROUND AND AIM

Knowledge about the genetic factors responsible for noise-induced hearing loss (NIHL) is still limited. This study investigated whether genetic factors are associated or not to susceptibility to NIHL.

SUBJECTS AND METHODS

The family history and genotypes were studied for candidate genes in 107 individuals with NIHL, 44 with other causes of hearing impairment and 104 controls. Mutations frequently found among deaf individuals were investigated (35delG, 167delT in GJB2, Δ(GJB6- D13S1830), Δ(GJB6- D13S1854) in GJB6 and A1555G in MT-RNR1 genes); allelic and genotypic frequencies were also determined at the SNP rs877098 in DFNB1, of deletions of GSTM1 and GSTT1 and sequence variants in both MTRNR1 and MTTS1 genes, as well as mitochondrial haplogroups.

RESULTS

When those with NIHL were compared with the control group, a significant increase was detected in the number of relatives affected by hearing impairment, of the genotype corresponding to the presence of both GSTM1 and GSTT1 enzymes and of cases with mitochondrial haplogroup L1.

CONCLUSION

The findings suggest effects of familial history of hearing loss, of GSTT1 and GSTM1 enzymes and of mitochondrial haplogroup L1 on the risk of NIHL. This study also described novel sequence variants of MTRNR1 and MTTS1 genes.

Mitochondrial 12S rRNA variants in 1642 Han Chinese pediatric subjects with aminoglycoside-induced and nonsyndromic hearing loss

In this report, we investigated the frequency and spectrum of mitochondrial 12S rRNA variants in a large cohort of 1642 Han Chinese pediatric subjects with aminoglycoside-induced and nonsyndromic hearing loss. Mutational analysis of 12S rRNA gene in these subjects identified 68 (54 known and 14 novel) variants. The frequencies of known 1555A>G and 1494C>T mutations were 3.96% and 0.18%, respectively, in this cohort with nonsyndromic and aminoglycoside-induced hearing loss. Prevalence of other putative deafness-associated mutation at positions 1095 and 961 were 0.61% and 1.7% in this cohort, respectively. Furthermore, the 745A>G, 792C>T, 801A>G, 839A>G, 856A>G, 1027A>G, 1192C>T, 1192C>A, 1310C>T, 1331A>G, 1374A>G and 1452T>C variants conferred increased sensitivity to ototoxic drugs or nonsyndromic deafness as they were absent in 449 Chinese controls and localized at highly conserved nucleotides of this rRNA. However, other variants appeared to be polymorphisms. Moreover, 65 Chinese subjects carrying the 1555A>G mutation exhibited bilateral and sensorineural hearing loss. A wide range of severity, age-of-onset and audiometric configuration was observed among these subjects. In particular, the sloping and flat-shaped patterns were the common audiograms in individuals carrying the 1555A>G mutation. The phenotypic variability in subjects carrying these 12S rRNA mutations indicated the involvement of nuclear modifier genes, mitochondrial haplotypes, epigenetic and environmental factors in the phenotypic manifestation of these mutations. Therefore, our data demonstrated that mitochondrial 12S rRNA is the hot spot for mutations associated with aminoglycoside ototoxicity.

Mitochondrial haplotypes may modulate the phenotypic manifestation of the deafness-associated 12S rRNA 1555A>G mutation

Mitochondrial 12S rRNA 1555A>G mutation is one of the important causes of aminoglycoside-induced and nonsyndromic deafness. Our previous investigations showed that the A1555G mutation was a primary factor underlying the development of deafness but was insufficient to produce deafness phenotype. However, it has been proposed that mitochondrial haplotypes modulate the phenotypic manifestation of the 1555A>G mutation. Here, we performed systematic and extended mutational screening of 12S rRNA gene in a cohort of 1742 hearing-impaired Han Chinese pediatric subjects from Zhejiang Province, China. Among these, 69 subjects with aminoglycoside-induced and nonsyndromic deafness harbored the homoplasmic 1555A>G mutation. These translated to a frequency of approximately 3.96% for the 1555A>G mutation in this hearing-impaired population. Clinical and genetic characterizations of 69 Chinese families carrying the 1555A>G mutation exhibited a wide range of penetrance and expressivity of hearing impairment. The average penetrances of deafness were 29.5% and 17.6%, respectively, when aminoglycoside-induced hearing loss was included or excluded. Furthermore, the average age-of-onset for deafness without aminoglycoside exposure ranged from 5 and 30years old, with the average of 14.5years. Their mitochondrial genomes exhibited distinct sets of polymorphisms belonging to ten Eastern Asian haplogroups A, B, C, D, F, G, M, N, R and Y, respectively. These indicated that the 1555A>G mutation occurred through recurrent origins and founder events. The haplogroup D accounted for 40.6% of the patient's mtDNA samples but only 25.8% of the Chinese control mtDNA samples. Strikingly, these Chinese families carrying mitochondrial haplogroup B exhibited higher penetrance and expressivity of hearing loss. In addition, the mitochondrial haplogroup specific variants: 15927G>A of haplogroup B5b, 12338T>C of haplogroup F2, 7444G>A of haplogroup B4, 5802T>C, 10454T>C, 12224C>T and 11696G>A of D4 haplogroup, 5821G>A of haplogroup C, 14693A>G of haplogroups Y2 and F, and 15908T>C of Y2 may enhance the penetrace of hearing loss in these Chinese families. Moreover, the absence of mutation in nuclear modifier gene TRMU suggested that TRMU may not be a modifier for the phenotypic expression of the 1555A>G mutation in these Chinese families. These observations suggested that mitochondrial haplotypes modulate the variable penetrance and expressivity of deafness among these Chinese families.

Screening of the mitochondrial A1555G mutation in patients with sensorineural hearing loss

The A1555G mitochondrial mutation is the main alteration associated with aminoglycoside-induced deafness.

Aim

to investigate the prevalence of the A1555G mutation in patients sensorineural hearing loss patients with and without aminoglycosides antibiotic use.

Material and Method

a study of 27 cases with deafness as the sample, and 100 neonates with normal hearing as the control group. DNA was extracted from blood leukocyte samples, and specific oligonucleotide primers were designed to amplify the cytochrome b gene and the region which encloses the A1555G mutation of the mitocondrial DNA using the polymerase chain reaction and restriction fragment length polymorphism.

Design

a cross-sectional case study.

Results

a region of the cytochrome b gene was amplified and the presence of the mtDNA was confirmed in all of the 127 cases. The A1555G mutation was not found in any of the 27 patients with hearing loss or the control group with 100 neonates.

Conclusion

the results agree with studies stating that the A1555G mutation is not prevalent in the Americas. There is interest in establishing the real prevalence of this mutation and to investigate other mutations that may cause hearing loss, associated or not with the use of aminoglycosides, in the Brazilian population.

Synergistic ototoxicity due to noise exposure and aminoglycoside antibiotics

Acoustic exposure to high intensity and/or prolonged noise causes temporary or permanent threshold shifts in auditory perception, reflected by reversible or irreversible damage in the cochlea. Aminoglycoside antibiotics, used for treating or preventing life-threatening bacterial infections, also induce cytotoxicity in the cochlea. Combined noise and aminoglycoside exposure, particularly in neonatal intensive care units, can lead to auditory threshold shifts greater than simple summation of the two insults. The synergistic toxicity of acoustic exposure and aminoglycoside antibiotics is not limited to simultaneous exposures. Prior acoustic insult which does not result in permanent threshold shifts potentiates aminoglycoside ototoxicity. In addition, exposure to subdamaging doses of aminoglycosides aggravates noise-induced cochlear damage. The mechanisms by which aminoglycosides cause auditory dysfunction are still being unraveled, but likely include the following: 1) penetration into the endolymphatic fluid of the scala media, 2) permeation of nonselective cation channels on the apical surface of hair cells, and 3) generation of toxic reactive oxygen species and interference with other cellular pathways. Here we discuss the effect of combined noise and aminoglycoside exposure to identify pivotal synergistic events that can potentiate ototoxicity, in addition to a current understanding of aminoglycoside trafficking within the cochlea. Preventing the ototoxic synergy of noise and aminoglycosides is best achieved by using non-ototoxic bactericidal drugs, and by attenuating perceived noise intensity when life-saving aminoglycoside therapy is required.

The genetic bases for non-syndromic hearing loss among Chinese

Deafness is an etiologically heterogeneous trait with many known genetic, environmental causes or a combination thereof. The identification of more than 120 independent genes for deafness has provided profound new insights into the pathophysiology of hearing. However, recent findings indicate that a large proportion of both syndromic and non-syndromic forms of deafness in the Chinese population are caused by defects in a small number of genes. Studies of the genetic epidemiology and molecular genetic features revealed that there is a clear relevance of genes causing deafness in Chinese deaf patients as well as a unique spectrum of common and rare deafness gene mutations in the Chinese population. This review is focused on the genetic aspects of non-syndromic and mitochondrial deafness, in which unique molecular genetic features of hearing impairment have been identified in the Chinese population. The current China population is approximately 1.3 billion. It is estimated that 30,000 infants are born with congenital sensorineural hearing loss each year. Better understanding of the genetic causes of deafness in the Chinese population is important for accurate genetics counseling and early diagnosis for timely intervention and treatment options.

Rastreamento da mutação mitocondrial A1555G em pacientes com deficiência auditiva sensorioneural

A mutação mitocondrial A1555G é a principal alteração associada à surdez ocasionada pelo uso de aminoglicosídeos. OBJETIVO: Investigar a prevalência da mutação A1555G em pacientes com deficiência auditiva sensorioneural com e sem uso de antibióticos aminoglicosídeos. MATERIAL E MÉTODO: Estudo em amostras de 27 pacientes com surdez, como casos, e em 100 neonatos, com audição normal, como grupo controle. O DNA foi extraído de leucócitos de amostras de sangue e "primers" específicos foram utilizados para amplificar o gene do citocromo b e a região que abrange a mutação A1555G do DNA mitocondrial, usando as técnicas da Reação em Cadeia da Polimerase e do Polimorfismo no Comprimento de Fragmentos de Restrição. DESENHO CIENTÍFICO: Estudo de casos em corte transversal. RESULTADOS: A região do gene do citocromo b foi amplificada, sendo confirmada a presença do DNA mitocondrial em todas as 127 amostras do estudo. A mutação A1555G não foi identificada nos 27 pacientes com deficiência auditiva e no grupo controle (100 neonatos). CONCLUSÕES: Os resultados são concordantes com estudos que relatam que a mutação A1555G não é prevalente nas Américas. Há interesse na determinação da real prevalência dessa mutação e na investigação de outras mutações que possam ocasionar deficiência auditiva associada ou não ao uso de aminoglicosídeos na população brasileira.

Co-occurrence of A1555G and G11778A in a Chinese family with high penetrance of Leber's hereditary optic neuropathy

Co-occurrence of double pathogenic mtDNA mutations with different claimed pathological roles in one mtDNA is infrequent. It is tentative to believe that each of these pathogenic mutations would have its own deleterious effect. Here we reported one three-generation Chinese family with a high penetrance of LHON (78.6%). Analysis of the complete mitochondrial genome in the proband revealed the presence of the LHON primary mutation G11778A in the NADH dehydrogenase 4 (ND4) gene and a deafness-associated mutation A1555G in the 12S rRNA gene. The other mtDNA variants in this family suggested a haplogroup status G2b. Although A1555G has long been confirmed to be a primary mutation for aminoglycoside-induced and non-syndromic hearing loss, none of the maternally related members in this family showed hearing impairment. It thus seems that the occurrence of A1555G in this family had no pathological manifestation. However, whether A1555G has a synergistic effect with G11778A and contribute to the high penetrance of LHON remained an open question. To our knowledge, this is the first report that identified the co-existence of a deafness mutation A1555G and a primary LHON mutation G11778A in one family.

Aminoglycoside-Induced Vestibular Injury: Maintaining a Sense of Balance

Objective

To describe the mechanism and risk factors for the development of aminoglycoside-induced vestibular injury and discuss their implications for therapeutic monitoring of aminoglycoside antibiotics.

Data Sources:

A MEDLINE search (1975–January 2008) was performed to identify literature on aminoglycoside-induced vestibular injury and risk factors associated with this outcome and their impact on therapeutic drug monitoring. Additional references were identified through review of bibliographies of identified articles.

Study Selection And Data Extraction:

Data on the mechanisms of vestibular toxicity and its development in association with aminoglycoside exposure were extracted from identified references.

Data Synthesis:

The mechanism leading to the development of irreversible vestibular injury from exposure to aminoglycosides appears to be through the excessive production of oxidative free radicals. This production and subsequent toxicity appears to be a time-dependent process and is unrelated to dose or serum concentration. For similarly designed studies, the pooled incidence of vestibular toxicity is 10.9% for gentamicin, 7.4% for amikacin, 3.5% for tobramycin, and 1.1% for netilmicin. Current evidence suggests that this form of drug toxicity is not restricted to traditionally dosed systemic therapy, since intraperitoneal administration, high-dose once-daily administration, topical inhalation, and eardrop administration have all been associated with the development of this adverse outcome.

Conclusions:

Given the lack of association between serum concentrations and vestibulotoxicity, it is imperative for the pharmacist to interview the patient and not focus solely on maintaining target range drug concentrations. Minimizing the duration of exposure to aminoglycosides is recommended to reduce the risk from this form of drug toxicity.

Audiological and genetic features of the mtDNA mutations

CONCLUSIONS

Significant difference in the incidence of mitochondrial DNA (mtDNA) mutations was found between the Chinese and USA populations. The identification of the mtDNA A1555G mutation in a large proportion of Chinese probands with nonsyndromic sensorineural hearing loss (NSHL) provides a molecular explanation for the high prevalence of aminoglycoside-induced deafness in China.

OBJECTIVE

The aim was to characterize the audiological and genetic features of NSHL due to mutations in mtDNA.

SUBJECTS AND METHODS

The mtDNA and audiogram analyses were performed in 498 NSHL patients (290 from China and 208 from the USA) with and without history of aminoglycoside exposure. A PCR and restriction enzyme digestion protocol was used for mutational screening and the European Workshop on Genetic Hearing Loss criteria were applied for audiological classification.

RESULTS

All Chinese probands (15.5%) with mtDNA mutation were found to carry the homoplasmic mtDNA A1555G mutation, whereas four probands (1.9%) from the USA were found to carry the mtDNA A1555G and two (1%) had mtDNA G7444A. Approximately 63% of the probands with mtDNA mutations had post-lingual hearing loss and 56.8% of them had a medical history of exposure to aminoglycosides. Hearing losses are bilateral, sensorineural, and symmetric. The main audiogram shapes found were sloping.

Mitochondrial ND5 T12338C, tRNA(Cys) T5802C, and tRNA(Thr) G15927A variants may have a modifying role in the phenotypic manifestation of deafness-associated 12S rRNA A1555G mutation in three Han Chinese pedigrees

We report here on the clinical, genetic, and molecular characterization of three Han Chinese pedigrees with aminoglycoside-induced and nonsyndromic hearing loss. Clinical evaluation revealed the variable phenotype of hearing impairment including severity, age-at-onset, audiometric configuration in these subjects. The penetrance of hearing loss in WZD8, WZD9, and WZD10 pedigrees were 46%, 46%, and 50%, respectively, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrance of hearing loss in these pedigrees were 23%, 31%, and 37.5%, respectively. Mutational analysis of the complete mitochondrial genomes showed the homoplasmic A1555G mutation and distinct sets of mitochondrial DNA variants belonging to haplogroups D4b2b, B5b1, and F2, respectively. Of these, the tRNA(Cys) T5802C, tRNA(Thr) A15924C, and ND5 T12338C variants are of special interest as these variants occur at positions which are highly evolutionarily conserved nucleotides of tRNAs or amino acid of polypeptide. These homoplasmic mtDNA variants were absent among 156 unrelated Chinese controls. The T5802C and G15927A variants disrupted a highly conserved A-U or C-G base-pairing at the anticodon-stem of tRNA(Cys) or tRNA(Thr), while the ND5 T12338C mutation resulted in the replacement of the translation-initiating methionine with a threonine, and also located in two nucleotides adjacent to the 3' end of the tRNA(Leu(CUN)). Thus, mitochondrial dysfunctions, caused by the A1555G mutation, would be worsened by these mtDNA variants. Therefore, these mtDNA mutations may have a potential modifier role in increasing the penetrance and expressivity of the deafness-associated 12S rRNA A1555G mutation in those Chinese pedigrees.

New polymorphic mtDNA restriction site in the 12S rRNA gene detected in Tunisian patients with non-syndromic hearing loss

The 12S rRNA gene was shown to be a hot spot for aminoglycoside-induced and non-syndromic hearing loss since several deafness-associated mtDNA mutations were identified in this gene. Among them, we distinguished the A1555G, the C1494T and the T1095C mutations and C-insertion or deletion at position 961. One hundred Tunisian patients with non-syndromic hearing loss and 100 hearing individuals were analysed in this study. A PCR-RFLP analysis with HaeIII restriction enzyme showed the presence of the A1555G mutation in the 12S rRNA gene in only one out of the 100 patients. In addition, PCR-RFLP and radioactive PCR revealed the presence of a new HaeIII polymorphic restriction site in the same gene of 12S rRNA site in 4 patients with non-syndromic hearing loss. UVIDOC-008-XD analyses showed the presence of this new polymorphic restriction site with a variable heteroplasmic rates at position +1517 of the human mitochondrial genome. On the other hand, direct sequencing of the entire mitochondrial 12S rRNA gene in the 100 patients and in 100 hearing individuals revealed the presence of the A750G and A1438G polymorphisms and the absence of the C1494T, T1095C and 961insC mutations in all the tested individuals. Sequencing of the whole mitochondrial genome in the 4 patients showing the new HaeIII polymorphic restriction site revealed only the presence of the A8860G transition in the MT-ATP6 gene and the A4769G polymorphism in the ND2 gene.

Maternally inherited aminoglycoside-induced and nonsyndromic hearing loss is associated with the 12S rRNA C1494T mutation in three Han Chinese pedigrees

We report here the clinical, genetic and molecular characterization of three Han Chinese pedigrees with maternally transmitted aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the wide range of severity, age-at-onset and audiometric configuration of hearing impairment in matrilineal relatives in these families. The penetrances of hearing loss in these pedigrees were 28%, 20%, and 15%, with an average of 21%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees were 21%, 13% and 8%, with an average of 14%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA C1494T mutation, in addition to distinct sets of mtDNA polymorphism belonging to Eastern Asian haplogroups F1a1, F1a1 and D5a2, respectively. This suggested that the C1494T mutation occurred sporadically and multiplied through evolution of the mtDNA. The absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in their mtDNA suggests that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the C1494T mutation in those Chinese families. In addition, the lack of significant mutation in the GJB2 gene ruled out the possible involvement of GJB2 in the phenotypic expression of the C1494T mutation in those affected subjects. However, aminoglycosides and other nuclear modifier genes play a modifying role in the phenotypic manifestation of the C1494T mutation in these Chinese families.

Very low penetrance of hearing loss in seven Han Chinese pedigrees carrying the deafness-associated 12S rRNA A1555G mutation

Mutations in mitochondrial DNA (mtDNA) have been found to be associated with sensorineural hearing loss. We report here the clinical, genetic and molecular characterizations of seven Han Chinese pedigrees with aminoglycoside-induced and nonsyndromic bilateral hearing loss. Clinical evaluation revealed the variable phenotype of hearing impairment including severity, age-at-onset and audiometric configuration in these subjects. The penetrance of hearing loss in these pedigrees ranged from 3% to 29%, with an average of 13.6%, when aminoglycoside-induced deafness was included. When the effect of aminoglycosides was excluded, the penetrances of hearing loss in these seven pedigrees varied from 0% to 17%, with an average of 5.3%. Sequence analysis of the complete mitochondrial genomes in these pedigrees showed the presence of the deafness-associated 12S rRNA A1555G mutation, in addition to distinct sets of mtDNA polymorphism belonging to East Asian haplogroups B4, D4, D5 and F1, respectively. This suggested that the A1555G mutation occurred sporadically and multiplied through evolution of the mtDNA in China. Despite the presence of several evolutionary conservative variants in protein-encoding genes, there was the absence of functionally significant mutations in tRNA and rRNAs or secondary LHON mutations in these seven Chinese families. These suggest that these mtDNA haplogroup-specific variants may not play an important role in the phenotypic expression of the A1555G mutation in those Chinese families with very low penetrance of hearing loss. However, aminoglycosides appear to be a major modifier factor for the phenotypic manifestation of the A1555G mutation in these Chinese families.

The mitochondrial tRNA(Ala) T5628C variant may have a modifying role in the phenotypic manifestation of the 12S rRNA C1494T mutation in a large Chinese family with hearing loss

We report here the clinical, genetic, and molecular characterization of a large Han Chinese family with aminoglycoside-induced and nonsyndromic hearing loss. Two and 13 of 66 matrilineal relatives suffered from aminoglycoside-induced and nonsyndromic hearing loss, respectively. These matrilineal relatives exhibited a wide range of severity of hearing loss, varying from profound to normal hearing. In the absence of aminoglycosides, the age-at-onset of hearing impairment in these matrilineal relatives ranged from 13 to 50years. Furthermore, these affected matrilineal relatives shared some common features: bilateral hearing loss of high frequencies and symmetries. Sequence analysis of mitochondrial DNA (mtDNA) in the pedigree identified the homoplasmic 12S rRNA C1494T mutation and other 34 variants belonging to Eastern Asian haplogroup F1. Of these, the variant T5628C occurs at an extremely conserved nucleotide (A31) of tRNA(Ala). This variant converted a very conservative A-U to a G-U base-pairing at AC-stem of this tRNA. The disruption of this base-pairing in tRNAs by mtDNA mutations has been associated with several clinical abnormalities. The alteration of structure of the tRNA(Ala) by the T5628C mutation may lead to a failure in tRNA metabolism and lead to impairment of mitochondrial translation, thereby worsening mitochondrial dysfunctions, caused by the C1494T mutation. Therefore, this mtDNA mutation may influence the phenotypic manifestation of the 12S rRNA C1494T mutation in this Chinese pedigree.

Carrier frequency of the 35delG and A1555G deafness mutations in the Argentinean population. Impact on the newborn hearing screening

OBJECTIVE

Hearing loss is a complex multifactorial disorder caused by genetic and environmental factors. The 35delG mutation in the GJB2 gene is the most prevalent mutation in Caucasian patients with genetic sensorineural deafness. The A1555G mutation in the mitochondrial 12S rRNA is the main genetic alteration associated with aminoglycoside-induced deafness. The aim of this study was to evaluate the prevalence of both mutations in general population of Argentina.

METHODS

A total of 712 samples of unrelated healthy blood donors and 330 newborn dried blood spots were studied by PCR-RFLP.

RESULTS

The 35delG mutation was detected in 11/ 712 unrelated blood donors. The carrier frequency found in this sample (1/65) proved to be lower than that found in Southern European countries, mainly Spain and Italy, from where Argentina originally received its major immigration waves. When the populations of Southern Europe were considered altogether, this difference reached statistical significance. The A1555G mutation was not found in any of the 1042 samples tested.

CONCLUSIONS

Taking into account the 35delG carrier frequency found in this study, it could be estimated that 130-160 children with congenital deafness due to mutations in the connexin genes would be born per year in Argentina. In contrast, the mitochondrial mutation A1555G appears to be infrequent in general Argentinean population.

Mitochondrial rRNA and tRNA and hearing function

The human ear is a delicate sensory apparatus of hearing for normal communication, and its proper functioning is highly dependent on mitochondrial oxidative phosphorylation. The first mitochondrial point mutation for nonsyndromic and aminoglycoside-induced hearing loss was identified in 1993. Since then a number of inherited mitochondrial mutations have been implicated in hearing loss. Most of the molecular defects responsible for mitochondrial disorder-associated hearing loss are mutations in the 12S rRNA gene and tRNA genes. In this review, after a short description of normal hearing mechanisms and mitochondrial genetics, we outline the recent advances that have been made in the identification of deafness-associated mitochondrial mutations, and discuss how mitochondrial dysfunction contributes to hearing loss.

Variants in mitochondrial tRNAGlu, tRNAArg, and tRNAThr may influence the phenotypic manifestation of deafness-associated 12S rRNA A1555G mutation in three Han Chinese families with hearing loss

We report here on the clinical, genetic, and molecular characterization of three Han Chinese pedigrees with aminoglycoside-induced and nonsyndromic hearing loss. Clinical evaluation revealed the variable phenotype of hearing loss including severity, age-at-onset, audiometric configuration in these subjects. Penetrances of hearing loss in BJ107, BJ108, and BJ109 pedigrees are 35%, 63%, and 67%, respectively. Mutational analysis of the complete mitochondrial genomes in these pedigrees showed the identical homoplasmic A1555G mutation and distinct sets of mitochondrial DNA (mtDNA) variants belonging to haplogroups N, F, and M, respectively. Of these variants, the A14693G mutation in the tRNA(Glu), the T15908C mutation in the tRNA(Thr), and the T10454C mutation in the tRNA(Arg) are of special interest as these mutations occur at positions which are highly evolutionarily conserved nucleotides of corresponding tRNAs. These homoplasmic mtDNA mutations were absent among 156 unrelated Chinese controls. The A14693G and T10454C mutations occur at the highly conserved bases of the TpsiC-loop of tRNA(Glu) and tRNA(Arg), respectively. Furthermore, the T15908C mutation in the tRNA(Thr) disrupts a highly conserved A-U base-pairing at the D-stem of this tRNA. The alteration of structure of these tRNAs by these mtDNA mutations may lead to a failure in tRNA metabolism, thereby causing impairment of mitochondrial translation. Thus, mitochondrial dysfunctions, caused by the A1555G mutation, would be worsened by these mtDNA mutations. Therefore, these mtDNA mutations may have a potential modifier role in increasing the penetrance and expressivity of the deafness-associated 12S rRNA A1555G mutation in those Chinese pedigrees.