Deafness due to A1555G mitochondrial mutation without use of aminoglycoside

High prevalence of m.1555A > G in patients with hearing loss in the Baikal Lake region of Russia as a result of founder effect

Mitochondrial forms account approximately 1-2% of all nonsyndromic cases of hearing loss (HL). One of the most common causative variants of mtDNA is the m.1555A > G variant of the MT-RNR1 gene (OMIM 561000). Currently the detection of the m.1555A > G variant of the MT-RNR1 gene is not included in all research protocols. In this study this variant was screened among 165 patients with HL from the Republic of Buryatia, located in the Baikal Lake region of Russia. In our study, the total contribution of the m.1555A > G variant to the etiology of HL was 12.7% (21/165), while the update global prevalence of this variant is 1.8% (863/47,328). The m.1555A > G variant was notably more prevalent in Buryat (20.2%) than in Russian patients (1.3%). Mitogenome analysis in 14 unrelated Buryat families carrying the m.1555A > G variant revealed a predominant lineage: in 13 families, a cluster affiliated with sub-haplogroup A5b (92.9%) was identified, while one family had the D5a2a1 lineage (7.1%). In a Russian family with the m.1555A > G variant the lineage affiliated with sub-haplogroup F1a1d was found. Considering that more than 90% of Buryat families with the m.1555A > G variant belong to the single maternal lineage cluster we conclude that high prevalence of this variant in patients with HL in the Baikal Lake region can be attributed to a founder effect.

Carrier frequencies, trends, and geographical distribution of hearing loss variants in China: The pooled analysis of 2,161,984 newborns

The aim of this study is to comprehensively investigate the prevalence and distribution patterns of three common genetic variants associated with hearing loss (HL) in Chinese neonatal population. Methods: Prior to June 30, 2023, an extensive search and screening process was conducted across multiple literature databases. R software was utilized for conducting meta-analyses, cartography, and correlation analyses. Results: Firstly, our study identified a total of 99 studies meeting the inclusion criteria. Notably, provinces such as Qinghai, Tibet, Jilin, and Heilongjiang lack large-scale genetic screening data for neonatal deafness. Secondly, in Chinese newborns, the carrier frequencies of GJB2 variants (c.235delC, c.299_300delAT) were 1.63 % (95 %CI 1.52 %-1.76 %) and 0.33 % (95 %CI 0.30 %-0.37 %); While SLC26A4 variants (c.919-2A > G, c.2168A > G) exhibited carrier rates of 0.95 % (95 %CI 0.86 %-1.04 %) and 0.17 % (95 %CI 0.15 %-0.19 %); Additionally, Mt 12S rRNA m.1555 A > G variant was found at a rate of 0.24 % (95 % CI 0.22 %-0.26 %). Thirdly, the mutation rate of GJB2 c.235delC was higher in the east of the Heihe-Tengchong line, whereas the mutation rate of Mt 12S rRNA m.1555 A > G variant exhibited the opposite pattern. Forthly, no significant correlation exhibited the opposite pattern of GJB2 variants, but there was a notable correlation among SLC26A4 variants. Lastly, strong regional distribution correlations were evident between mutation sites from different genes, particularly between SLC26A4 (c.919-2A > G and c.2168A > G) and GJB c.299_300delAT. Conclusions: The most prevalent deafness genes among Chinese neonates were GJB2 c.235delC variant, followed by SLC26A4 c.919-2A > G variant. These gene mutation rates exhibit significant regional distribution characteristics. Consequently, it is imperative to enhance genetic screening efforts to reduce the incidence of deafness in high-risk areas.

Deciphering exome sequencing data: Bringing mitochondrial DNA variants to light

The expanding use of exome sequencing (ES) in diagnosis generates a huge amount of data, including untargeted mitochondrial DNA (mtDNA) sequences. We developed a strategy to deeply study ES data, focusing on the mtDNA genome on a large unspecific cohort to increase diagnostic yield. A targeted bioinformatics pipeline assembled mitochondrial genome from ES data to detect pathogenic mtDNA variants in parallel with the "in-house" nuclear exome pipeline. mtDNA data coming from off-target sequences (indirect sequencing) were extracted from the BAM files in 928 individuals with developmental and/or neurological anomalies. The mtDNA variants were filtered out based on database information, cohort frequencies, haplogroups and protein consequences. Two homoplasmic pathogenic variants (m.9035T>C and m.11778G>A) were identified in 2 out of 928 unrelated individuals (0.2%): the m.9035T>C (MT-ATP6) variant in a female with ataxia and the m.11778G>A (MT-ND4) variant in a male with a complex mosaic disorder and a severe ophthalmological phenotype, uncovering undiagnosed Leber's hereditary optic neuropathy (LHON). Seven secondary findings were also found, predisposing to deafness or LHON, in 7 out of 928 individuals (0.75%). This study demonstrates the usefulness of including a targeted strategy in ES pipeline to detect mtDNA variants, improving results in diagnosis and research, without resampling patients and performing targeted mtDNA strategies.

Carrier frequencies of hearing loss variants in newborns of China: A meta-analysis

OBJECTIVE

The objective of this study was to review the carrier frequencies of hearing loss gene variants, such as GJB2, SLC26A4, and MT-RNR1 in newborns of China.

DESIGN

PubMed, Embase, BioCentral, CNKI, WanFang, and VIP databases were used for searching relevant literature studies published during the period of January 2007 and January 2016. Meta-analysis was performed by using the R software. The estimated rate and its 95% confidence intervals (CI) of the relevant indexes in newborns were collected and calculated using a fixed-effects model or a random-effects model when appropriate.

RESULTS

In total, 35 of 958 published literature studies in Chinese and English were selected. The overall results showed that in newborns of China, the carrier frequencies of GJB2 variants (235 delC, 299 delAT) were 1.64% (95% CI 1.52% to 1.77%) and 0.33% (95% CI 0.19% to 0.51%); SLC26A4 variants (IVS7-2 A > G, 2168 A > G) were 1.02% (95% CI 0.91% to 1.15%) and 0.14% (95% CI 0.06% to 0.25%); MT-RNR1 variants (1555 A > G, 1449 C > T) were 0.20% (95% CI 0.17% to 0.23%) and 0.03% (95% CI 0.02% to 0.05%).

CONCLUSIONS

There are high carrier frequencies of GJB2 variants among newborns in China, followed by SLC26A4 and MT-RNR1 variants. In order to achieve "early detection, early diagnosis and early treatment" and reduce the incidence of hereditary hearing loss in offspring, a comprehensive combination of neonatal hearing screening and deafness gene detection should be recommended and implemented in China.

Contribution of the tRNAIle 4317A→G mutation to the phenotypic manifestation of the deafness-associated mitochondrial 12S rRNA 1555A→G mutation

The 1555A→G mutation in mitochondrial 12S rRNA has been associated with aminoglycoside-induced and non-syndromic deafness in many individuals worldwide. Mitochondrial genetic modifiers are proposed to influence the phenotypic expression of m.1555A→G mutation. Here, we report that a deafness-susceptibility allele (m.4317A→G) in the tRNA^Ile gene modulates the phenotype expression of m.1555A→G mutation. Strikingly, a large Han Chinese pedigree carrying both m.4317A→G and m.1555A→G mutations exhibited much higher penetrance of deafness than those carrying only the m.1555A→G mutation. The m.4317A→G mutation affected a highly conserved adenine at position 59 in the T-loop of tRNA^Ile We therefore hypothesized that the m.4317A→G mutation alters both structure and function of tRNA^Ile Using lymphoblastoid cell lines derived from members of Chinese families (three carrying both m.1555A→G and m.4317A→G mutations, three harboring only m.1555A→G mutation, and three controls lacking these mutations), we found that the cell lines bearing both m.4317A→G and m.1555A→G mutations exhibited more severe mitochondrial dysfunctions than those carrying only the m.1555A→G mutation. We also found that the m.4317A→G mutation perturbed the conformation, stability, and aminoacylation efficiency of tRNA^Ile These m.4317A→G mutation-induced alterations in tRNA^Ile structure and function aggravated the defective mitochondrial translation and respiratory phenotypes associated with the m.1555A→G mutation. Furthermore, mutant cell lines bearing both m.4317A→G and m.1555A→G mutations exhibited greater reductions in the mitochondrial ATP levels and membrane potentials and increasing production of reactive oxygen species than those carrying only the m.1555A→G mutation. Our findings provide new insights into the pathophysiology of maternally inherited deafness arising from the synergy between mitochondrial 12S rRNA and tRNA mutations.

Genetics of Tinnitus: Still in its Infancy

Tinnitus is the perception of a phantom sound that affects between 10 and 15% of the general population. Despite this considerable prevalence, treatments for tinnitus are presently lacking. Tinnitus exhibits a diverse array of recognized risk factors and extreme clinical heterogeneity. Furthermore, it can involve an unknown number of auditory and non-auditory networks and molecular pathways. This complex combination has hampered advancements in the field. The identification of specific genetic factors has been at the forefront of several research investigations in the past decade. Nine studies have examined genes in a case-control association approach. Recently, a genome-wide association study has highlighted several potentially significant pathways that are implicated in tinnitus. Two twin studies have calculated a moderate heritability for tinnitus and disclosed a greater concordance rate in monozygotic twins compared to dizygotic twins. Despite the more recent data alluding to genetic factors in tinnitus, a strong association with any specific genetic locus is lacking and a genetic study with sufficient statistical power has yet to be designed. Future research endeavors must overcome the many inherent limitations in previous study designs. This review summarizes the previously embarked upon tinnitus genetic investigations and summarizes the hurdles that have been encountered. The identification of candidate genes responsible for tinnitus may afford gene based diagnostic approaches, effective therapy development, and personalized therapeutic intervention.

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.

Genetics of non syndromic hearing loss

Non Syndromic Hearing Loss is an important cause for hearing loss. One in 1000 newborns have some hearing impairment. Over 400 genetic syndromes have been described. Non Syndromic Hearing Loss (NSHL) can be inherited in an Autosomal Dominant, Autosomal Recessive or a Sex Linked fashion. There are several reasons why genetic testing should be done in cases of NSHL, the main reasons being for genetic screening and for planning treatment. This review describes the genes involved in NSHL and the genetic mechanisms involved in the pathogenesis of the disease.

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.

MT-CYB mutations in hypertrophic cardiomyopathy

Mitochondrial dysfunction is a characteristic of heart failure. Mutations in mitochondrial DNA, particularly in MT-CYB coding for cytochrome B in complex III (CIII), have been associated with isolated hypertrophic cardiomyopathy (HCM). We hypothesized that MT-CYB mutations might play an important causal or modifying role in HCM. The MT-CYB gene was sequenced from DNA isolated from blood from 91 Danish HCM probands. Nonsynonymous variants were analyzed by bioinformatics, molecular modeling and simulation. Two germline-inherited, putative disease-causing, nonsynonymous variants: m.15024G>A; p.C93Y and m.15482T>C; p.S246P were identified. Modeling showed that the p.C93Y mutation leads to disruption of the tertiary structure of Cytb by helix displacement, interfering with protein–heme interaction. The p.S246P mutation induces a diproline structure, which alters local secondary structure and induces a kink in the protein backbone, interfering with macromolecular interactions. These molecular effects are compatible with a leaky phenotype, that is, limited but progressive mitochondrial dysfunction. In conclusion, we find that rare, putative leaky mtDNA variants in MT-CYB can be identified in a cohort of HCM patients. We propose that further patients with HCM should be examined for mutations in MT-CYB in order to clarify the role of these variants.

Systematic analysis of mitochondrial genes associated with hearing loss in the Japanese population: dHPLC reveals a new candidate mutation

Background

Variants of mitochondrial DNA (mtDNA) have been evaluated for their association with hearing loss. Although ethnic background affects the spectrum of mtDNA variants, systematic mutational analysis of mtDNA in Japanese patients with hearing loss has not been reported.

Methods

Using denaturing high-performance liquid chromatography combined with direct sequencing and cloning-sequencing, Japanese patients with prelingual (N = 54) or postlingual (N = 80) sensorineural hearing loss not having pathogenic mutations of m.1555A > G and m.3243A > G nor GJB2 were subjected to mutational analysis of mtDNA genes (12S rRNA, tRNA^Leu(UUR), tRNA^Ser(UCN), tRNA^Lys, tRNA^His, tRNA^Ser(AGY), and tRNA^Glu).

Results

We discovered 15 variants in 12S rRNA and one homoplasmic m.7501A > G variant in tRNA^Ser(UCN); no variants were detected in the other genes. Two criteria, namely the low frequency in the controls and the high conservation among animals, selected the m.904C > T and the m.1105T > C variants in 12S rRNA as candidate pathogenic mutations. Alterations in the secondary structures of the two variant transcripts as well as that of m.7501A > G in tRNA^Ser(UCN) were predicted.

Conclusions

The m.904C > T variant was found to be a new candidate mutation associated with hearing loss. The m.1105T > C variant is unlikely to be pathogenic. The pathogenicity of the homoplasmic m.7501T > A variant awaits further study.

Value of genetic testing in the otological approach for sensorineural hearing loss

Sensorineural hearing loss (SNHL) is one of the most common disabilities in human, and genetics is an important aspect for SNHL, especially in children. In recent 10 years, our knowledge in genetic causes of SNHL has made a significant advance, and now it is used for diagnosis and other clinical practices. Hereditary hearing loss can be classified into syndromic and nonsyndromic hearing loss. As the nonsyndromic deafness genes, more than 100 loci for deafness genes have been determined, and more than 40 genes were identified. Furthermore, more than 300 forms of syndromic hearing loss have been characterized, and each syndrome may have several causative genes. In childhood hearing loss, early educational intervention is required in addition to medical intervention for normal development of speech and language. In addition, even severe to profound hearing loss may be restored very effectively by hearing aids or cochlear implants. Because of these features of SNHL, genetic testing has exceptionally high value in the medical practice for hereditary hearing loss. Several strategies are used for genetic testing of SNHL for accurate and efficient identification of the genetic causes, and the results were used for explanation of the cause, prediction of auditory features, prevention of deafness, management of associated symptoms, determination of therapy, and genetic counseling. Identification of damaged cells in the inner ear and the underlying mechanism by genetic testing undoubtedly facilitates development and introduction of novel and specific therapies to distinct types of SNHL.

Mutations in MYH9 exons 1, 16, 26, and 30 are infrequently found in Japanese patients with nonsyndromic deafness

Mutations in MYH9 result in the autosomal dominant giant platelet disorders with leukocyte inclusion bodies with varying degrees of Alport manifestations, including nephritis, deafness, and cataracts. A specific MYH9 mutation in exon 16, R705H, causes nonsyndromic deafness DFNA17. We searched for mutations in MYH9 exons 1, 16, 26, and 30 in a total of 157 Japanese patients with nonsyndromic deafness without known cause of hearing loss, but no mutations were found. We conclude that mutations in MYH9 are infrequently found in patients with nonsyndromic deafness and suggest that MYH9 mutations infrequently cause isolated sensorineural hearing loss. Thus, MYH9 may not currently be a good candidate gene for efficient screening of genetic causes in nonsyndromic deafness.

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.

Aminoglycoside ototoxicity in Nicaraguan children: patient risk factors and mitochondrial DNA results

OBJECTIVE

Aminoglycoside ototoxicity remains a major problem in developing countries and accounts for 3 percent to 30 percent of hearing loss. This high prevalence rate may be related to genetic susceptibility from mitochondrial mutations in the 12S rRNA gene, comorbidity factors, or unregulated use of the medication. This study investigates the risk factors and prevalence of mtDNA mutations in serum from rural Nicaragua children with aminoglycoside-related hearing loss.

STUDY DESIGN

Cross sectional study.

SUBJECTS

Deaf children from rural Nicaragua.

METHODS

Mitochondrial DNA isolated from serum collected from 31 deaf children with childhood or in utero exposure to gentamicin was amplified, sequenced, and analyzed for mutations in the 12S rRNA gene.

RESULTS

No known pathologic mutations of the 12S rRNA gene were identified in this subpopulation of deaf children. In addition, patients with gentamicin exposure were often likely to have other comorbidity factors.

CONCLUSION

These results suggest that genetic susceptibility is not a major factor in the high rate of gentamicin ototoxicity in this population sample. The high prevalence of gentamicin ototoxicty in this population is presumed to be due to unrestricted access to the drug.

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.

A maternal hereditary deafness pedigree of the A1555G mitochondrial mutation, causing aminoglycoside ototoxicity predisposition

Objective:

To characterise the hearing loss, and the frequency of the mitochondrial deoxyribonucleic acid 12S ribosomal ribonucleic acid A1555G mutation, in a large pedigree of aminoglycoside-induced deafness.

Design:

Hearing loss was clinically assessed. Blood samples were collected from 27 family members (19 matrilinear and eight non-matrilinear) and leukocyte deoxyribonucleic acid was extracted. Mitochondrial deoxyribonucleic acid fragments, spanning the 1555 location, were amplified by polymerase chain reaction. Polymerase chain reaction products were analysed by restriction fragment length polymorphism and deoxyribonucleic acid sequencing.

Results:

We detected the A1555G mutation in all 19 matrilinear relatives. Of these 19, two exhibited congenital deafness, four had no hearing deficits and the remaining 13 suffered mild to profound hearing loss.

Conclusion:

We confirmed that the A1555G mutation is a ‘hot spot’ associated with non-syndromic, inherited hearing loss. This mutation may play a vital role in the pathogenesis of hearing impairment, and can result in various grades of deafness.

Mitochondrial non-syndromic sensorineural hearing loss: a clinical, audiological and pathological study from Italy, and revision of the literature

Over the last decade, a number of distinct mutations in the mtDNA (mitochondrial DNA) have been found to be associated with both syndromic and non-syndromic forms of hearing impairment. Their real incidence as a cause of deafness is poorly understood and generally underestimated. Among the known mtDNA mutations, the A1555G mutation in the 12S gene has been identified to be one of the most common genetic cause of deafness, and it has been described to be both associated to non-syndromic progressive SNHL (sensorineural hearing loss) and to aminoglycoside-induced SNHL. In the present study, we have investigated the presence of mtDNA alterations in patients affected by idiopathic non-syndromic SNHL, both familiar and sporadic, in order to evaluate the frequency of mtDNA alterations as a cause of deafness and to describe the audiological manifestations of mitochondrial non-syndromic SNHL. In agreement with previous studies, we found the A1555G mutation to be responsible for a relevant percentage (5.4%) of cases affected with isolated idiopathic sensorineural hearing impairment.

Pharmacogenomic testing to prevent aminoglycoside-induced hearing loss in cystic fibrosis patients: potential impact on clinical, patient, and economic outcomes

BACKGROUND

Aminoglycosides are commonly used in cystic fibrosis patients to treat Pseudomonas aeruginosa respiratory infections. Aminoglycoside-induced hearing loss may occur in 1%-15% of patients with cystic fibrosis, ranging from mild to severe. Recently, a genetic test to identify patients with a mitochondrial mutation (A1555G) that may predispose patients to this adverse event has become available. Although the A1555G variant is very rare, it seems to confer a high risk of severe hearing loss in patients exposed to aminoglycosides.

OBJECTIVE

The objective was to evaluate the potential clinical, patient, and economic outcomes associated with the use of A1555G testing in a cystic fibrosis population, and explore data gaps and uncertainty in its clinical implementation.

METHODS

We developed a decision-analytic model to evaluate a hypothetical cohort of patients with cystic fibrosis from a societal perspective. Clinical and economic data were derived primarily from a critical literature review. The incidence of aminoglycoside-induced severe hearing loss, quality-adjusted life-years, and total health care costs were evaluated. Sensitivity analyses were conducted to evaluate uncertainty in our results.

RESULTS

In the base-case analysis, A1555G testing decreased the risk of severe aminoglycoside-induced hearing loss by 0.12% in the cystic fibrosis population. The discounted incremental cost per quality-adjusted life-years gained was $79,300, but varied widely from $33,000 to testing being dominated by the no testing strategy (higher costs and lower quality-adjusted life-years with testing) in sensitivity analyses. If avoidance of aminoglycosides in patients testing positive leads to an absolute increase in the lifetime risk of death from Pseudomonas infection of 0.8% or greater, A1555G testing would lead to a decrease in quality-adjusted life-years.

CONCLUSIONS

The results of our analysis suggest that there are significant data gaps and uncertainty in the outcomes with A1555G testing, but it is not likely cost-effective, and could lead to worse patient outcomes due to avoidance of first-line therapy in the >95% of patients who are false-positives. Additional research is needed before pharmacogenetic testing for the A1555G mitochondrial mutation can be recommended, even in a population with a high likelihood of exposure to aminoglycosides.

Prevalence and Clinical Features of the Mitochondrial m.1555A>G Mutation in Taiwanese Patients with Idiopathic Sensorineural Hearing Loss and Association of Haplogroup F with Low Penetrance in Three Families

OBJECTIVE

The m.1555A>G mutation in the mitochondria 12S rRNA gene has been reported to be an important cause of nonsyndromic hereditary hearing loss. However, remarkable interfamilial and intrafamilial variations in the phenotypes of the mutation preclude precise prognosis during genetic counseling. Hence, this study was performed to explore the factors that might contribute to the differences in the phenotypes, including aminoglycoside exposure, mutation load and mitochondrial DNA (mtDNA) background. Also reported were the prevalence and the clinical features of the m.1555A>G mutation in the hearing-impaired Taiwanese patients.

DESIGN

Mutations in the 12S rRNA gene were screened in a panel of 315 unrelated Taiwanese families with idiopathic sensorineural hearing loss. The clinical features in families with m.1555A>G mutation were analyzed, and the roles of aminoglycoside exposure, mutation load and mtDNA background in disease expression were investigated. Penetrance was then compared among families with different mtDNA backgrounds.

RESULTS

The m.1555A>G mutation was identified in a total of 10 (3.2%) families, and was characterized clinically by progressive, postlingual and bilaterally symmetric sensorineural hearing loss and normal temporal bone radiological results. The m.1555A>G mutation was homoplasmic (i.e., all the mitochondrial DNA carries the mutation) in all the matrilineal relatives in these 10 pedigrees. Among the 44 hearing-impaired relatives of the 10 pedigrees, only two recalled definite episodes of aminoglycoside-induced hearing loss. mtDNA backgrounds in these 10 families could be categorized into 6 main haplogroups (A, B, D, F, M7, N*), including three families belonging to haplogroup F, two belonging to haplogroup A, two belonging to haplogroup M7, and three belonging to haplogroups B, N* and D, respectively. Penetrance differed among various haplogroups, and certain haplogroups appeared to be associated with a lower penetrance, like the three haplogroup F families, in which the penetrance ranged from 13 to 33%. Further analysis confirmed a heterogeneous distribution of hearing-impaired subjects among various haplogroups (Chi-square test, p = 0.018).

CONCLUSIONS

The mitochondrial m.1555A>G mutation accounted for 3.2% of the Taiwanese families (0% of the simplex families and 11% of multiplex families respectively) with sensorineural hearing impairment of unknown etiology. Since it was identified in a variety of mtDNA backgrounds, the mutation appeared to arise from multiple origins in Taiwanese. As subjects with various haplogroups demonstrated different penetrance, mtDNA background might exert effects on the disease expression of the m.1555A>G mutation.

Mitochondrial deafness

Non-syndromic deafness can be caused by mutations in both nuclear and mitochondrial genes. More than 50 nuclear genes have been shown to be involved in non-syndromic hearing loss, but mutations in mitochondrial DNA (mtDNA) might also cause hearing impairment. As mitochondria are responsible for oxidative phosphorylation, the primary energy-producing system in all eukaryotic cells, mitochondrial dysfunction has pleiotropic effects. Many mutations in mtDNA can lead to multisystem disorders, such as Kearns-Sayre syndrome, NARP, MELAS, or MERRF syndromes, the presentation of which may include hearing loss. A more specific association of mitochondrially inherited deafness and diabetes known as MIDD syndrome can be caused by a limited number of specific mitochondrial mutations. In addition, several rare mutations in the mitochondrial MTTS1 and MTRNR1 genes have been found to be responsible for non-syndromic hearing loss. The most frequent form of non-syndromic deafness is presbyacusis, affecting more than 50% of the elderly. This age-related hearing loss is a paradigm for multifactorial inheritance, involving a multitude of inherited and acquired mutations in the nuclear and mitochondrial genomes, each with a low penetrance, in complex interplay with environmental factors, such as ototoxic medication, that accumulate with age. This study reviews the different mitochondrial mutations, leading to syndromic and especially non-syndromic deafness.