The A1555G mitochondrial DNA mutation in Greek patients with non-syndromic, sensorineural hearing loss

Increased risk of hearing loss associated with MT-RNR1 gene mutations: a real-world investigation among Han Taiwanese Population

BACKGROUND

Previous studies have implicated inherited mutations in mitochondrial DNA (mtDNA) in sensorineural hearing loss (SNHL). However, the definitive association between mitochondrial 12S rRNA (MT-RNR1) variants and hearing loss in the population has not been well established, particularly in Asia. The objective of this retrospective cohort study was to assess the association between MT-RNR1 variants and the risk of SNHL in patients in Taiwan.

METHODS

The cohort included 306,068 participants from Taiwan between January 2003 and December 2020. Participants were classified based on genetic variants, particularly mitochondrial mutations (rs267606618, rs267606619, rs267606617). MT-RNR1 variant cases were matched 1:10 with non-mutant patients by age, gender, and visit year, excluding those with pre-existing hearing loss. The primary endpoint was SNHL, identified using specific ICD-TM codes with a 90% positive predictive value. Medication exposure history was determined via self-report or electronic medical records in the hospital. Cox proportional hazard regression models were used to assess the association between MT-RNR1 variants and hearing loss, adjusting for various covariates. Kaplan-Meier survival curves and log-rank tests compared hearing loss incidence between groups.

RESULTS

The mean age of the mtDNA variants group is 32.4 years, with a standard deviation of 19.2 years. The incidence density of hearing loss for the mutation group was 36.42 per 10,000 person-years (95% Confidence Interval [CI], 27.21-47.73), which was higher than the 23.77per 10,000 person-years (95% CI, 21.32-26.42) in the wild-type group (p = 0.0036). Additionally, diabetes mellitus was associated with an increased risk of developing SNHL in individuals with MT-RNR1 variants (adjusted hazard ratio = 1.76 [95% CI, 1.00-3.09], p < 0.05).

CONCLUSION

This study highlights the increased risk of hearing loss in patients carrying MT-RNR1 variants, particularly those with diabetes mellitus. Future research that integrates genetic and clinical data is crucial for developing more precise interventions to monitor and treat hearing loss in this vulnerable population.

Consensus on intratympanic drug delivery for Menière's disease

Purpose

Intratympanic (IT) drug delivery receives attention due to its effectivity in treatment for Menière’s disease (MD). Due to the release of the consensuses and new evidence on IT drug delivery for MD have been published, the review with a view to supplementing the details of IT treatment of MD is indispensable.

Methods

The literatures on IT injection for MD treatment over the last two decades are retrieved, International consensus (ICON) on treatment of Menière’s disease (2018), Clinical Practice Guideline (2020) and European Position statement on Diagnosis and Treatment of Meniere’s Disease (2018) are taken into account for reference, and follow advice from experts from Europe, USA and China.

Results

Experts agree on the following: (1) The effectiveness of IT methylprednisolone (ITM) on vertigo control seems to be somewhat better than that of IT dexamethasone (ITD), and ITM can restore hearing in some cases. (2) Due to the ototoxicity of aminoglycosides, the application of intratympanic gentamicin (ITG) in MD patients with good hearing is conservative. However, some studies suggest that ITG with low doses has no significant effect on hearing, which needs to be further proved by clinical studies with high levels of evidence. (3) Currently, generally accepted treatment endpoint of ITG is no vertigo attack in a 12-month period or a vestibular loss in objective tests in the affected ear.

Conclusion

More studies with high level of evidence are needed to evaluate the drug type, efficacy, and therapeutic endpoint of IT therapy for MD.

Genetic etiology of non-syndromic hearing loss in Europe

Hearing impairment not etiologically associated with clinical signs in other organs (non-syndromic) is genetically heterogeneous, so that over 120 genes are currently known to be involved. The frequency of mutations in each gene and the most frequent mutations vary throughout populations. Here we review the genetic etiology of non-syndromic hearing impairment (NSHI) in Europe. Over the years, epidemiological data were scarce because of the large number of involved genes, whose screening was not cost-effective until implementation of massively parallel DNA sequencing. In Europe, the most common form of autosomal recessive NSHI is DFNB1, which accounts for 11-57% of the cases. Mutations in STRC account for 16% of the recessive cases, and only a few more (MYO15A, MYO7A, LOXHD1, USH2A, TMPRSS3, CDH23, TMC1, OTOF, OTOA, SLC26A4, ADGRV1 and TECTA) have contributions higher than 2%. As regards autosomal-dominant NSHI, DFNA22 (MYO6) and DFNA8/12 (TECTA) represent the most common forms, accounting for 21% and 18% of elucidated cases, respectively. The contribution of ACTG1 and WFS1 drops to 9% in both cases, followed by POU4F3 (6.5%), MYO7A (5%), MYH14 and COL11A2 (4% each). Four additional genes contribute 2.5% each one (MITF, KCNQ4, EYA4, SOX10) and the remaining are residually represented. X-linked hearing loss and maternally-inherited NSHI have minor contributions in most countries. Further knowledge on the genetic epidemiology of NSHI in Europe needs a standardization of the experimental approaches and a stratification of the results according to clinical features, familial history and patterns of inheritance, to facilitate comparison between studies.

Prevalence of the mitochondrial 1555 A>G and 1494 C>T mutations in a community-dwelling population in Japan

Single nucleotide polymorphisms in mitochondrial DNA, such as mitochondrial 1555 A>G (m.1555 A>G) and mitochondrial 1494 C>T (m.1494 C>T), are known to be causative mutations of nonsyndromic hearing loss following exposure to aminoglycoside antibiotics. The prevalence of the m.1555 A>G and m.1494 C>T mutations has not been reported for the general population in Japan. The purpose of this study was to investigate the prevalence of m.1555 A>G and m.1494 C>T mutations in a community-dwelling population in Japan in order to prevent aminoglycoside-induced hearing loss. We recruited participants older than 20 years of age to the Iwaki Health Promotion Project in 2014, 2015, and 2016, resulting in the recruitment of 1,683 participants. For each participant, we performed a hearing test and a genetic test for the m.1555 A>G and m.1494 C>T mutations using the TaqMan genotyping method. The m.1555 A>G mutation was detected in only 1 of the 1,683 participants (0.06%). This carrier of the m.1555 A>G mutation was a 69-year-old male with bilateral, symmetric, and high-frequency hearing loss. We provided genetic counseling and distributed a drug card advising him to avoid the administration of aminoglycoside antibiotics. In contrast, the m.1494 C>T mutation was not detected in this study population.

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.

Mutations in the two ribosomal RNA genes in mitochondrial DNA among Finnish children with hearing impairment

Background

Mutations in the two MT-RNR genes in mitochondrial DNA can cause hearing impairment that presents with variable severity and age of onset. In order to study the prevalence of mutations in MT-RNR1 and MT-RNR2 genes among Finnish children, we studied a ten-year cohort of hearing impaired children born in Northern Finland.

Methods

We studied children, who had been born in Northern Finland in 1993–2002 and who had been ascertained to have hearing impairment by 31 December 2007. Samples from 103 children were sequenced in order to find mutations in the MT-RNR1 and MT-RNR2 genes.

Results

One child harboured the pathogenic m.1555A > G mutation in MT-RNR1 suggesting a frequency of 4.4/100,000 in the Finnish paediatric population. In addition, eight rare variants and 13 polymorphisms were found in MT-RNR1 and MT-RNR2 genes. Five of the rare variants were deemed to be haplogroup-specific polymorphisms rather than putative pathogenic mutations, while the remaining three variants have been reported in various haplogroups. Among them m.990 T > C occurs at a conserved site.

Conclusions

The presence of m.990 T > C variant in various haplogroups and the rather high degree of conservation at this site suggest that this transition is a pathogenic rather than homoplasic neutral variant. Identification of further patients with m.990 T > C and segregation analysis in their families should help in determining the pathogenic potential of this variant.

Novel mutations affecting the secondary structure of MT-RNR1 gene: a causal relationship with profound nonsyndromic hearing impairment

Mutations in mitochondrial DNA (mtDNA) are one of the most important causes of sensorineural hearing loss, especially in the MT-RNR1 gene. In the present study we have performed mutational screening for m.1555A>G and a region of the MT-RNR1 gene in 303 unrelated patients (including family members of 25 probands) with nonsyndromic hearing loss and 200 controls. Three homoplasmic variants, namely, m.1453A>G, 1462G>A, and 1508C>T, were identified in addition to the known deafness-associated m.1555A>G mutation in the MT-RNR1 gene. All the variants were detected only in the patients and not in the controls. m.1555A>G was detected in three probands amounting to 1.0%. Prediction of RNA secondary structure showed changes in all the three variants, the most severe being in m.1453A>G that was inherited in a typical maternal pattern in two families. Screening of GJB2 and GJB6 genes in all these probands revealed cosegregation of the p.W24X mutation (GJB2) in one family with m.1453A>G. Only the proband carrying the p.W24X mutation in a homozygous state expressed the condition while heterozygous and normal homozygous relatives had normal hearing in spite of having the mutation in MT-RNR1. The conservation index (CI) of m.1453A>G was found to be 82%, suggesting it to be a possibly deleterious mutation. Functional studies using cell lines derived from muscle tissue of these patients may reveal the pathogenic mechanism of deafness in them.

Large-scale screening of mitochondrial DNA mutations among Iranian patients with prelingual nonsyndromic hearing impairment

Hereditary hearing impairment (HI) is a genetically heterogeneous disorder caused by mutations either in nuclear DNA (nDNA) or in mitochondrial DNA (mtDNA). The nDNA mutations account for the majority of prelingual nonsyndromic HI (NSHI). The present survey was conducted to screen for known pathogenic mtDNA mutations including A1555G, A3243G, C1494T, and A7445G to provide an accurate estimate of their prevalence in prelingual NSHI for the first time in the Iranian subpopulations. One thousand unrelated probands with NSHI (including both GJB2-negative and GJB2 heterozygote cases) and 1000 healthy matched controls were investigated using the PCR/RFLP method followed by DNA sequencing to confirm the observed mtDNA mutations. Two of the studied mutations, namely A3243G and A7445G, were each found in a single family (a frequency of 0.1% for each). Mutation screening for A3243G followed by DNA sequencing led to the identification of G3316A substitution, with no prior link to HI. Surprisingly, screening for A3243G in the studied population identified 6 cases (0.6%) in probands and 10 (1%) in normal subjects. A1555G, the most common mtDNA mutation associated with deafness in other populations, was not found in the studied samples. To conclude, our findings indicate G3316A as a nonpathogenic variant in the prelingual NSHI subpopulations of Iran and suggest that mtDNA mutations do not play a major role in the etiology of NSHI in Iran.

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.