Pharmacogenetics of aminoglycoside-related ototoxicity: a systematic review

BACKGROUND

Aminoglycosides (AGs) are important antibiotics in the treatment of Gram-negative sepsis. However, they are associated with the risk of irreversible sensorineural hearing loss (SNHL). Several genetic variants have been implicated in the development of ototoxicity.

OBJECTIVES

To evaluate the pharmacogenetic determinants of AG-related ototoxicity.

METHODS

This study followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses and was registered on Prospero (CRD42022337769). In Dec 2022, PubMed, Cochrane Library, Embase and MEDLINE were searched. Included studies were those reporting original data on the effect of the AG-exposed patient's genome on the development of ototoxicity.

RESULTS

Of 10 202 studies, 31 met the inclusion criteria. Twenty-nine studies focused on the mitochondrial genome, while two studied the nuclear genome. One study of neonates found that 30% of those with the m.1555A > G variant failed hearing screening after AG exposure (level 2 evidence). Seventeen additional studies found the m.1555A > G variant was associated with high penetrance (up to 100%) of SNHL after AG exposure (level 3-4 evidence). Nine studies of m.1494C > T found the penetrance of AG-related SNHL to be up to 40%; however, this variant was also identified in those with SNHL without AG exposure (level 3-4 evidence). The variants m.1005T > C and m.1095T > C may be associated with AG-related SNHL; however, further studies are needed.

CONCLUSIONS

This review found that the m.1555A > G and m.1494C > T variants in the MT-RNR1 gene have the strongest evidence in the development of AG-related SNHL, although study quality was limited (level 2-4). These variants were associated with high penetrance of a SNHL phenotype following AG exposure.

Genetic mutations in patients with nonsyndromic hearing impairment of minority and Han Chinese ethnicities in Qinghai, China

Objective

Mutations in GJB2, SLC26A4, and mitochondrial (mt)DNA 12S rRNA genes are the main cause of nonsyndromic hearing impairment. The present study analyzed these mutations in ethnic minority and Han Chinese patients with nonsyndromic hearing impairment from Qinghai, China.

Methods

The SNPscan assay was used to analyze mutation spectra and frequencies in the two patient groups.

Results

GJB2 mutations were detected in 9.5% (20/210) of minority patients and 20.88% (48/230) of Han Chinese patients. The most common Han Chinese GJB2 variants were c.235delC and c.299_300delAT, whereas c.235delC and c.109G > A were the most prevalent in minority patients. SLC26A4 mutations were detected in 5.71% (12/210) of minority patients and 14.35% (33/230) of Han Chinese patients, and mtDNA 12S rRNA mutations were detected in 4.28% (9/210) of minority patients and 9.13% (21/230) of Han Chinese patients.

Conclusions

These data indicate that the mutation frequencies of three deafness-associated genes were significantly higher in Han Chinese patients than in minority patients. Moreover, the GJB2 mutation spectrum was shown to differ between these two patient groups.

Mitochondrial Disease: Advances in clinical diagnosis, management, therapeutic development, and preventative strategies

PURPOSE OF REVIEW

Primary mitochondrial disease encompasses an impressive range of inherited energy deficiency disorders having highly variable molecular etiologies as well as clinical onset, severity, progression, and response to therapies of multi-system manifestations. Significant progress has been made in primary mitochondrial disease diagnostic approaches, clinical management, therapeutic options, and preventative strategies that are tailored to major mitochondrial disease phenotypes and subclasses.

RECENT FINDINGS

The extensive phenotypic pleiotropy of individual mitochondrial diseases from an organ-based perspective is reviewed. Improved consensus on standards for mitochondrial disease patient care are being complemented by emerging therapies that target specific molecular subtypes of mitochondrial disease. Reproductive counseling options now include preimplantation genetic diagnosis at the time of in vitro fertilization for familial mutations in nuclear genes and some mtDNA disorders. Mitochondrial replacement technologies have promise for some mtDNA disorders, although practical and societal challenges remain to allow their further research analyses and clinical utilization.

SUMMARY

A dramatic increase has occurred in recent years in the recognition, understanding, treatment options, and preventative strategies for primary mitochondrial disease.

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.

Mitochondrial mutations in maternally inherited hearing loss

Background

Although the mitochondrial DNA (mtDNA) mutations m.1555A > G and m.3243A > G are the primary causes of maternally inherited sensorineural hearing loss (SNHL), several other mtDNA mutations are also reported to be associated with SNHL.

Methods

Screening of m.1555A > G and m.3243A > G mutations was performed for 145 probands. Nine probands fulfilled the following criteria: 1) bilateral and symmetric SNHL, 2) ≥ 4 family members with SNHL with a maternal trait of inheritance in ≥ 2 generations, 3) onset of SNHL before the age of 40 years, 4) high-frequency SNHL, and 5) no record of environmental factors related to SNHL. Sequencing of additional mtDNA regions was performed for five subjects meeting the clinical criteria, but the screening results were negative.

Results

Among the nine cases meeting the five clinical criteria detailed above, three had the m.1555A > G mutation in MTRNR1, one had a m.3243A > G mutation in MTTL1, and one case had a m.7511T > C mutation in MTTS1. In the family with the m.7511T > C mutation, penetrance of SNHL among maternally related subjects was 9/17 (53%). The age at onset varied from birth (congenital) to adulthood. Hearing levels varied from normal to moderately impaired, unlike previously reported subjects with this mutation, where some maternal family members presented with profound SNHL. Family members with the m.7511T > C mutation and SNHL did not exhibit any specific clinical characteristics distinct from those of other individuals with SNHL and different mtDNA mutations. Among the 136 probands who did not meet the criteria detailed above, one case had the m.1555A > G mutation, and three cases had the m.3243A > G mutation.

Conclusions

Since five of nine probands with the clinical criteria used in this study had mtDNA mutations, these criteria may be helpful for identification of candidate patients likely to have mtDNA mutations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12881-017-0389-4) contains supplementary material, which is available to authorized users.

Association of nuclear and mitochondrial genes with audiological examinations in Iranian patients with nonaminoglycoside antibiotics-induced hearing loss

Mitochondrial DNA mutations play an important role in causing sensorineural hearing loss. The purpose of this study was to determine the association of the mitochondrial genes RNR1, MT-TL1, and ND1 as well as the nuclear genes GJB2 and GJB6 with audiological examinations in nonfamilial Iranians with cochlear implants, using polymerase chain reaction, DNA sequencing, and RNA secondary structure analysis. We found that there were no novel mutations in the mitochondrial gene 12S rRNA (MT-RNR1) in patients with and without GJB2 mutation (GJB2⁺ and GJB2⁻, respectively), but a total of six polymorphisms were found. No mutations were observed in tRNA^Leu(UUR) (MT-TL1). Furthermore, eight polymorphisms were found in the mitochondrial ND1 gene. Additionally, no mutations were observed in the nuclear GJB6 gene in patients in the GJB2⁻ and GJB2⁺ groups. The speech intelligibility rating and category of auditory perception tests were statistically assessed in patients in the GJB2⁻ and GJB2⁺ groups. The results indicated that there was a significant difference (P<0.05) between the categories of auditory perception score in the GJB2⁻ group compared to that in the GJB2⁺ group. Successful cochlear implantation was observed among individuals with GJB2 mutations (GJB2⁺) and mitochondrial polymorphisms compared to those without GJB2 mutations (GJB2⁻). In conclusion, the outcome of this study suggests that variation in the mitochondrial and nuclear genes may influence the penetrance of deafness. Therefore, further genetic and functional studies are required to help patients in making the best choice for cochlear implants.

Molecular etiology of non-dominant, non-syndromic, mild-to-moderate childhood hearing impairment in Chinese Hans

Childhood hearing impairment (HI) is genetically heterogeneous. Compared with the severe-to-profound HI, the molecular etiology of mild-to-moderate HI in children has been less well characterized, especially for those not inherited in the dominant mode. In this study, we recruited 114 probands with non-dominant, non-syndromic, mild-to-moderate childhood HI. Sequencing of GJB2, SLC26A4, and MTRNR1 identified causative mutations in 30.7% (35/114), 4.4% (5/114), and 4.4% (5/114) of subjects, respectively. A majority (62.9%) of bi-allelic GJB2 mutations have non-truncating mutations in at least one allele. In 10 multiplex probands with no GJB2, SLC26A4, and MTRNR1 mutations identified, targeted next-generation sequencing (NGS) of 79 known deafness genes did not identify any additional causes. Our data showed that the molecular etiology of mild-to-moderate childhood HI is considerably different from what reported for severe-to-profound HI and far from complete for those inherited in non-dominant modes.

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.

Association between idiopathic hearing loss and mitochondrial DNA mutations: a study on 169 hearing-impaired subjects

Mutations in mitochondrial DNA (mtDNA) have been shown to be an important cause of sensorineural hearing loss (SNHL). In this study, we performed a clinical and genetic analysis of 169 hearing-impaired patients and some of their relatives suffering from idiopathic SNHL, both familial and sporadic. The analysis of four fragments of their mtDNA identified several polymorphisms, the well known pathogenic mutation, A1555G, and some novel mutations in different genes, implying changes in the aminoacidic sequence. A novel sporadic mutation in 12S rRNA (MT-RNR1), not previously reported in the literature, was found in a case of possible aminoglycoside-induced progressive deafness.

The Clinical and Audiologic Features of Hearing Loss Due to Mitochondrial Mutations

Objectives

To characterize mitochondrial sequence variants present in a nationwide hereditary deafness DNA repository of samples from deaf subjects and to define the clinical presentation and audiometric characteristics of individuals with a mitochondrial sequence variant.

Study Design

Retrospective review of results for select mitochondrial mutations performed on DNA samples from subjects compiled from 1997 to 2009.

Setting

National hereditary deafness DNA repository.

Subjects and Methods

Available samples from subjects in the repository were screened to identify those with mitochondrial sequence variants. Clinical data on the nature of mutation, type and severity of the hearing loss, and sex, age at diagnosis, family history of hearing loss, and ethnicity were analyzed.

Results

Eighty-six patients were identified with mitochondrial mutations or 3.5% of the subjects studied. Among those with mitochondrial mutations, 21 (24.4%) had the m.7445A>G substitution, 18 (20.9%) had the m.1555A>G substitution, 18 (20.9%) had the m.961T>G substitution, and 29 (33.7%) had a m.961delT+C(n) complex deletion. The majority of patients had bilateral severe to profound hearing loss. Fifty-three (62%) patients were female, and a family history of hearing loss was documented in 66 (76.7%) patients. The deafness was recognized prior to 3 years of age in 26 patients.

Conclusion

Mitochondrial deafness in this sample was associated with a variety of genetic mutations and a wide spectrum of clinical presentations. Because of increased aminoglycoside susceptibility associated with some forms of mitochondrial deafness, matrilineal relatives may be at risk in those cases, highlighting the importance of making an accurate diagnosis prior to exposure.

The mitochondrion: a perpetrator of acquired hearing loss

Age, drugs, and noise are major causes of acquired hearing loss. The involvement of reactive oxygen species (ROS) in hair cell death has long been discussed, but there is considerably less information available as to the mechanisms underlying ROS formation. Most cellular ROS arise in mitochondria and this review will evaluate evidence for mitochondrial pathology in general and dysfunction of the mitochondrial respiratory chain in particular in acquired hearing loss. We will discuss evidence that different pathways can lead to the generation of ROS and that oxidative stress might not necessarily be causal to all three pathologies. Finally, we will detail recent advances in exploiting knowledge of aminoglycoside-mitochondria interactions for the development of non-ototoxic antibacterials. This article is part of a Special Issue entitled "Annual Reviews 2013".

Rational use of aminoglycosides--review and recommendations by the Swedish Reference Group for Antibiotics (SRGA)

The Swedish Reference Group for Antibiotics (SRGA) has carried out a risk-benefit analysis of aminoglycoside treatment based on clinical efficacy, antibacterial spectrum, and synergistic effect with beta-lactam antibiotics, endotoxin release, toxicity, and side effects. In addition, SRGA has considered optimal dosage schedules and advice on serum concentration monitoring, with respect to variability in volume of drug distribution and renal clearance. SRGA recommends that aminoglycoside therapy should be considered in the following situations: (1) progressive severe sepsis and septic shock, in combination with broad-spectrum beta-lactam antibiotics, (2) sepsis without shock, in combination with broad-spectrum beta-lactam antibiotics if the infection is suspected to be caused by multi-resistant Gram-negative pathogens, (3) pyelonephritis, in combination with a beta-lactam or quinolone until culture and susceptibility results are obtained, or as monotherapy if a serious allergy to beta-lactam or quinolone antibiotics exists, (4) serious infections caused by multi-resistant Gram-negative bacteria when other alternatives are lacking, and (5) endocarditis caused by difficult-to-treat pathogens when monotherapy with beta-lactam antibiotics is not sufficient. Amikacin is generally more active against extended-spectrum beta-lactamase (ESBL)-producing and quinolone-resistant Escherichia coli than other aminoglycosides, making it a better option in cases of suspected infection caused by multidrug-resistant Enterobacteriaceae. Based on their resistance data, local drug committees should decide on the choice of first-line aminoglycoside. Unfortunately, aminoglycoside use is rarely followed up with audiometry, and in Sweden we currently have no systematic surveillance of adverse events after aminoglycoside treatment. We recommend routine assessment of adverse effects, including hearing loss and impairment of renal function, if possible at the start and after treatment with aminoglycosides, and that these data should be included in hospital patient safety surveillance and national quality registries.

Novel nucleotide changes in mutational analysis of mitochondrial 12SrRNA gene in patients with nonsyndromic and aminoglycoside-induced hearing loss

Mitochondria have essential role in cellular energy metabolism and defects in their function lead to many metabolic diseases. Mitochondrial DNA (mtDNA) mutations have been associated with number diseases such as nonsyndromic and aminoglycoside-induced hearing loss. Mutational screening of entire 12SrRNA and tRNA (ser (UCN)) genes in 107 unrelated Iranian patients with amino glycoside-induced and nonsyndromic bilateral hearing loss by direct sequencing analysis method were performed. Twenty different homoplasmic sequence variants were identified; including fifteen common polymorphisms, two putatively pathogenic variants: m.921T>C and m.1005T>C, one 12SrRNA sequence variant m.739C>T and two nucleotides substitution; m.1245T>C and m.1545T>C. Deafness-associated mutation, m.1555A>G, was not found. In our patients we found the mutation 1005 was associated with R haplogroup. These finding show that m.1555A>G mutation is not important in our population. Nucleotide change, m.739C>T, previously reported with very low frequency. We suggested the variation of two nucleotides 1245 and 1545 that localized at conserved site of 12SrRNA may be new candidate for amino glycoside-induced and nonsyndromic hearing impairment associated mutations. However, aminoglycoside exposure is a risk factor for clinical phenotype appearance of these mutations.

Analysis of a large-scale screening of mitochondrial DNA m.1555A>G mutation in 2417 deaf-mute students in northwest of China

The ancient Silk Road (also called "Northwest Silk Road") in Northwest China, starting from Xi'an, passes through Gansu, Xinjiang, Central Asia, West Asia, and the land passage connecting the Mediterranean countries. The aim of the present study was to determine the frequency of mitochondrial DNA12SrRNA m.1555A>G mutation in a total of 2417 cases of nonsyndromic deaf-mute patients representative of the general population of Shaanxi, Gansu, Qinghai, Ningxia, and Xinjiang along the Silk Road. Enzyme digestion and direct sequencing were applied to identify sequence variations. The carrier frequency of mitochondrial DNA12S rRNA m.1555A>G mutation was estimated to be 5.21% (126/2417) in the studied population. In detail, the carrier frequency of Uighur and Hui was 1.62% (3/185) and 3.29% (10/304), respectively, compared with 6.09% (113/1856) that of Han. There was a statistically significant difference between Uighur and Han (chi-square test, chi(2) = 6.437, p = 0.011 and p < 0.05), whereas no significant difference in m.1555A>G mutation spectrum or prevalence of mitochondrial DNA12SrRNA was found between Uighur and Hui or Hui and Han. In the 126 m.1555A>G mutation carriers, 52 cases were found to have a clear history of using aminoglycoside antibiotics. Results suggested that the application of aminoglycoside antibiotics in this region is an important reason for higher incidence of m.1555A>G mutation in the deaf-mute population.

Neurodevelopmental disorders in children with severe to profound sensorineural hearing loss: a clinical study

AIM

The effects of sensorineural hearing loss (SNHL) are often complicated by additional disabilities, but the epidemiology of associated disorders is not clearly defined. The aim of this study was to evaluate the frequency and type of additional neurodevelopmental disabilities in a sample of children with SNHL and to investigate the relation between these additional disabilities and the aetiology of deafness.

METHOD

One hundred children with severe/profound SNHL (60 males, 40 females; mean age 5 y 7 mo, SD 3 y 6 mo, range 8 mo-16 y) were investigated using a diagnostic protocol including neurodevelopmental, genetic, neurometabolic, and brain magnetic resonance imaging (MRI) assessment.

RESULTS

Forty-eight per cent of the sample exhibited one or more additional disabilities, with cognitive, behavioural-emotional, and motor disorders being the most frequent. The risk of additional disabilities varied according to the type of aetiology. Thirty-seven out of 80 individuals with available MRIs showed signal abnormalities, in particular brain malformations (46%) and white matter abnormalities (54%). Frequency and type of disability were associated with aetiology (p=0.015) and MRI data (p<0.001).

INTERPRETATION

A multidimensional evaluation, including aetiological, neurodevelopmental, and MRI investigation, is needed for planning therapeutic intervention, such as cochlear implantation in children with severe to profound hearing impairment. The aetiology of deafness is a relevant risk indicator for the presence of an associated disorder.

Hearing loss in a patient with the myopathic form of mitochondrial DNA depletion syndrome and a novel mutation in the TK2 gene

Mitochondrial DNA (mtDNA) depletion syndrome (MDS) is a devastating disorder of infancy caused by a significant reduction of the number of copies of mitochondrial DNA in one or more tissues. We report a Spanish patient with the myopathic form of MDS, harboring two mutations in the thymidine kinase 2 gene (TK2): a previously reported deletion (p.K244del) and a novel nucleotide duplication in the exon 2, generating a frameshift and premature stop codon. Sensorineural hearing loss was a predominant symptom in the patient and a novel feature of MDS due to TK2 mutations. The patient survived up to the age of 8.5 y, which confirms that survival above the age of 5 y is not infrequent in patients with MDS due to TK2 deficiency.

Audiologia em comunidade: relato de experiência

OBJETIVO: caracterizar queixas, sintomas e fatores de risco relacionados à perda auditiva em crianças com suspeita de perda auditiva no Distrito Sanitário I do Recife/Brasil. MÉTODOS: relato de experiência de atuação fonoaudiológica em parceria com Agentes Comunitários de Saúde - ACS que foram capacitados sobre saúde auditiva para identificar crianças com queixas de audição. Foram realizadas visitas aos domicílios indicados e ao posto de saúde e entrevistadas 80 mães obtendo-se informações a respeito de 117 crianças de 0 a 7 anos. Para as análises dos dados foram privilegiadas informações sobre: queixas otológicas, audiológicas e fatores de risco para a audição. RESULTADOS: 35 crianças (29%) apresentaram uma ou mais queixas otológicas e/ou auditivas. Os dois principais sintomas otológicos referidos foram: otalgia (74,2%) e otorréia (34,2%). Os sintomas auditivos mais freqüentes foram: dificuldade de compreender o que os outros falam (25,7%) e diminuição da audição (20%). Com relação aos fatores de risco, verificou-se que 57 (48,7%) crianças possuíam um ou mais fatores, sendo eles: uso materno de álcool (59,6%) durante a gestação, antecedentes hereditários para a surdez (43,8%) e uso materno de drogas ilícitas (39,8%) durante a gestação. CONCLUSÃO: a otalgia foi o principal sintoma referido e a dificuldade de compreensão foi a queixa mais freqüente. Os fatores de risco mais importantes foram o uso materno de álcool e de drogas ilícitas como maconha e craque durante a gestação.

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.

A rapid method for detection of five known mutations associated with aminoglycoside-induced deafness

Background

South Africa has one of the highest incidences of multidrug-resistant tuberculosis (MDR-TB) in the world. Concomitantly, aminoglycosides are commonly used in this country as a treatment against MDR-TB. To date, at least five mutations are known to confer susceptibility to aminoglycoside-induced hearing loss. The aim of the present study was to develop a rapid screening method to determine whether these mutations are present in the South African population.

Methods

A multiplex method using the SNaPshot technique was used to screen for five mutations in the MT-RNR1 gene: A1555G, C1494T, T1095C, 961delT+C(n) and A827G. A total of 204 South African control samples, comprising 98 Mixed ancestry and 106 Black individuals were screened for the presence of the five mutations.

Results

A robust, cost-effective method was developed that detected the presence of all five sequence variants simultaneously. In this pilot study, the A1555G mutation was identified at a frequency of 0.9% in the Black control samples. The 961delT+C(n) variant was present in 6.6% of the Black controls and 2% of the Mixed ancestry controls. The T1095C, C1494T and A827G variants were not identified in any of the study participants.

Conclusion

The frequency of 0.9% for the A1555G mutation in the Black population in South Africa is of concern given the high incidence of MDR-TB in this particular ethnic group. Future larger studies are warranted to determine the true frequencies of the aminoglycoside deafness mutations in the general South African population. The high frequencies of the 961delT+C(n) variant observed in the controls suggest that this change is a common non-pathogenic polymorphism. This genetic method facilitates the identification of individuals at high risk of developing hearing loss prior to the start of aminoglycoside therapy. This is important in a low-resource country like South Africa where, despite their adverse side-effects, aminoglycosides will continue to be used routinely and are accompanied with very limited or no audiological monitoring.

Mutation analysis of mitochondrial DNA 12SrRNA and tRNASer(UCN) genes in non-syndromic hearing loss patients

Specific mitochondrial DNA (mtDNA) mutations in 12SrRNA and tRNASer(UCN) cause non-syndromic hearing loss (NSHL). In this study, we screened 466 hearing loss (HL) patients, negative for GJB2 mutations, for mutations in the two mtDNA genes and flanking regions. In total, 43 different variants were identified, 31 of which were polymorphisms, one was a mutation (m.1555A-->G), two were known variants of controversial pathological nature (m.827A-->G and m.961delTinsC(n)) and nine were newly identified variants. The frequency of m.1555A-->G in this set of HL patients was 0.3%, which was lower than expected. To assess the putative causative nature of controversial or newly identified variants, the frequencies of these variants were determined in 400 Belgian control subjects, and their effect on the secondary structure and their conservation among different species was determined. Our data provide further support for a polymorphic nature of the controversial m.961delTinsC(n) variant. In addition, two of the newly identified variants, m.636A-->G in the 12SrRNA flanking tRNA(Phe) and m.990T-->C in 12SrRNA, may be new candidates for pathogenic HL variants. If the pathogenic nature of m.636A-->G can be confirmed, this would be the first NSHL mutation in tRNA(Phe).