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

Structural analysis of mitochondrial rRNA gene variants identified in patients with deafness

The last few years have witnessed dramatic advances in our understanding of the structure and function of the mammalian mito-ribosome. At the same time, the first attempts to elucidate the effects of mito-ribosomal fidelity (decoding accuracy) in disease have been made. Hence, the time is right to push an important frontier in our understanding of mitochondrial genetics, that is, the elucidation of the phenotypic effects of mtDNA variants affecting the functioning of the mito-ribosome. Here, we have assessed the structural and functional role of 93 mitochondrial (mt-) rRNA variants thought to be associated with deafness, including those located at non-conserved positions. Our analysis has used the structural description of the human mito-ribosome of the highest quality currently available, together with a new understanding of the phenotypic manifestation of mito-ribosomal-associated variants. Basically, any base change capable of inducing a fidelity phenotype may be considered non-silent. Under this light, out of 92 previously reported mt-rRNA variants thought to be associated with deafness, we found that 49 were potentially non-silent. We also dismissed a large number of reportedly pathogenic mtDNA variants, 41, as polymorphisms. These results drastically update our view on the implication of the primary sequence of mt-rRNA in the etiology of deafness and mitochondrial disease in general. Our data sheds much-needed light on the question of how mt-rRNA variants located at non-conserved positions may lead to mitochondrial disease and, most notably, provide evidence of the effect of haplotype context in the manifestation of some mt-rRNA variants.

Mitochondrial DNA Polymorphism in HV1 and HV2 Regions and 12S rDNA in Perimenopausal Hypertensive Women

Estrogens enhance cellular mitochondrial activity. The diminution of female hormones during menopause may have an effect on the mitochondrial genome and the expression of mitochondrial proteins. Hence, oxidative stress and the pro-inflammatory state contribute to the formation of systemic illnesses including arterial hypertension (AH). This study aimed to determine the types and frequency of mutations in the mitochondrial DNA (mtDNA) nucleotide sequence in the hypervariable regions 1 and 2 (HV1 and HV2) and the 12S RNA coding sequence of the D-loop in postmenopausal women with hypertension. In our study, 100 women were investigated, 53 of whom were postmenopausal and 47 of whom were premenopausal (53.9 ± 3.7 years vs. 47.7 ± 4.2 years, respectively). Of those studied, 35 premenopausal and 40 postmenopausal women were diagnosed with AH. A medical checkup with 24 h monitoring of blood pressure (RR) and heart rate was undertaken (HR). The polymorphism of the D-loop and 12S rDNA region of mtDNA was examined. Changes in the nucleotide sequence of mtDNA were observed in 23% of the group of 100 women. The changes were identified in 91.3% of HV1 and HV2 regions, 60.9% of HV1 segments, 47.5% of HV2 regions, and 43.5% of 12S rDNA regions. The frequency of nucleotide sequence alterations in mtDNA was substantially higher in postmenopausal women (34%) than in premenopausal women (10.6%), p = 0.016. A higher frequency of changes in HV1 + HV2 sections in postmenopausal women (30.2%) compared to the premenopausal group (10.6%) was detected, p = 0.011. Only postmenopausal women were found to have modifications to the HV2 segment and the 12S rDNA region. After menopause, polymorphism in the mtDNA region was substantially more frequent in women with arterial hypertension than before menopause (p = 0.030; 37.5% vs. 11.5%). Comparable findings were observed in the HV2 and HV1 regions of the AH group (35% vs. 11.5%), p = 0.015, in the HV1 segment (25% vs. 11.5%), p = 0.529, and in the HV2 segment, 12S rDNA (25% vs. 0%). More than 80% of all changes in nucleotide sequence were homoplasmic. The mtDNA polymorphisms of the nucleotide sequence in the HV1 and HV2 regions, the HV2 region alone, and the 12S RNA coding sequence were associated with estrogen deficiency and a more severe course of arterial hypertension, accompanied by symptoms of adrenergic stimulation.

Insight of the mitochondrial genomes of the Orang Asli and Malays: The heterogeneity and the disease-associated variants

Orang Asli are the oldest inhabitants in Peninsular Malaysia that forms as a national minority while the Malays are the majority. The study aimed to screen the mitochondrial genomes of the Orang Asli and the Malays to discover the disease-associated variants. A total of 99 Orang Asli from six tribes (Bateq, Cheq Wong, Orang Kanaq, Kensiu, Lanoh, and Semai) were recruited. Mitochondrial genome sequencing was conducted using a next-generation sequencing platform. Furthermore, we retrieved mitochondrial DNA sequences from the Malays for comparison. The clinical significance, pathogenicity prediction and frequency of variants were determined using online tools. Variants associated with mitochondrial diseases were detected in the 2 populations. A high frequency of variants associated with mitochondrial diseases, breast cancer, prostate cancer, and cervical cancer were detected in the Orang Asli and modern Malays. As medicine evolves to adopt prediction and prevention of diseases, this study highlights the need for intervention to adopt genomics medicine to strategise better healthcare management as a way forward for Precision Health.

Mitochondrial mutations in non-syndromic hearing loss at UAE

INTRODUCTION

Hearing loss (HL) is a common sensory disorder over the world, and it has been estimated that genetic etiology is involved in more than 50% of the cases in developed countries. Both nuclear and mitochondrial genes were reported as responsible for hereditary HL. Mitochondrial mutations leading to HL have so far been reported in the MT-RNR1 gene, mitochondrially encoded 12S rRNA.

METHODS

To study the molecular contribution of mitochondrial 12S rRNA gene mutations in UAE-HL, a cohort of 74 unrelated UAE patients with no gap junction protein beta 2 (GJB2) mutations were selected for mitochondrial 12S rRNA gene mutational screening using Sanger sequencing and whole-exome sequencing. Detected DNA variants were analyzed by bioinformatics tools to predict their pathogenic effects.

RESULTS

Our analysis revealed the presence of two known deafness mutations; m.669T > C and m.827A > G in two different deaf individuals. Furthermore, whole-exome sequencing was done for these two patients and showed the absence of any nuclear mutations. Our study supports the pathogenic effect of the m.669T > C and m.827A > G mutations and showed that mitochondrial mutations have a contribution of 2.7% in our cohort.

CONCLUSIONS

This is the first report of mtDNA mutations in the UAE which revealed that both variants m.669T > C and m.827A > G should be included in the molecular diagnosis of patients with maternally inherited HL in UAE.

Mitochondrial DNA Mutation Analysis in Breast Cancer: Shifting From Germline Heteroplasmy Toward Homoplasmy in Tumors

Studies have suggested a potential role of somatic mitochondrial mutations in cancer development. To analyze the landscape of somatic mitochondrial mutation in breast cancer and to determine whether mitochondrial DNA (mtDNA) mutational burden is correlated with overall survival (OS), we sequenced whole mtDNA from 92 matched-paired primary breast tumors and peripheral blood. A total of 324 germline variants and 173 somatic mutations were found in the tumors. The most common germline allele was 663G (12S), showing lower heteroplasmy levels in peripheral blood lymphocytes than in their matched tumors, even reaching homoplasmic status in several cases. The heteroplasmy load was higher in tumors than in their paired normal tissues. Somatic mtDNA mutations were found in 73.9% of breast tumors; 59% of these mutations were located in the coding region (66.7% non-synonymous and 33.3% synonymous). Although the CO1 gene presented the highest number of mutations, tRNA genes (T,C, and W), rRNA 12S, and CO1 and ATP6 exhibited the highest mutation rates. No specific mtDNA mutational profile was associated with molecular subtypes of breast cancer, and we found no correlation between mtDNA mutational burden and OS. Future investigations will provide insight into the molecular mechanisms through which mtDNA mutations and heteroplasmy shifting contribute to breast cancer development.

Low penetrance of hearing loss in two Chinese families carrying the mitochondrial tRNASer(UCN) mutations

Mutations in mitochondrial DNA (mtDNA), especially in mitochondrial 12S rRNA and transfer RNA(tRNA)^Ser(UCN) genes, are important causes of non-syndromic hearing loss. However, the molecular mechanism underlying mt-tRNA mutations in clinical hearing impairment are not fully understood. The present study assessed the molecular characterization of two Chinese families with non-syndromic hearing loss, who both exhibited very low penetrance of deafness (9.1 and 12.5% for Family 1 and 2, respectively). Mutational analysis of the complete mtDNA genes identified the presence of cytochrome c oxidase 1/tRNA^Ser(UCN) G7444A and tRNA^Ser(UCN) C7492T mutations, together with polymorphisms belonging to human mitochondrial haplogroup D4 and G2b, respectively. Moreover, the G7444A and C7492T mutations occurred at highly conserved tRNA^Ser(UCN) nucleotides and may cause tRNA metabolism failure, which is involved in mitochondrial translation defects. Therefore, the G7444A and C7492T mutations may lead to the mitochondrial dysfunction that responsible for deafness. However, the absence of any functional variants in Gap junction β-2, Solute Carrier Family 26 Member 4 and TRNA 5-methylaminomethyl-2-thiouridylate methyltransferase suggested that nuclear genes may not play active roles in the occurrence of deafness. In the present study, the observed incomplete penetrance of hearing loss and mild mitochondrial dysfunction indicated that mtDNA G7444A and C7492T mutations are insufficient to produce the deafness phenotype. Therefore, other risk factors such as environmental factors and epigenetic regulation may be involved in the pathogenesis of hearing loss in the families recruited in the present study.

New mtDNA Association Model, MutPred Variant Load, Suggests Individuals With Multiple Mildly Deleterious mtDNA Variants Are More Likely to Suffer From Atherosclerosis

The etiology of common complex diseases is multifactorial, involving both genetic, and environmental factors. A role for mitochondrial dysfunction and mitochondrial DNA (mtDNA) variation has been suggested in the pathogenesis of common complex traits. The aim of this study was to investigate a potential role of mtDNA variants in the development of obesity, diabetes, and atherosclerosis in the Polish population. Whole mtDNA sequences from 415 Polish individuals representing three disease cohorts and a control group were obtained using high-throughput sequencing. Two approaches for the assessment of mtDNA variation were applied, traditional mitochondrial haplogroup association analysis and the mutational or variant load model using the MutPred pathogenicity prediction algorithm for amino acid substitutions in humans. We present a possible association between mildly deleterious mtDNA variant load and atherosclerosis that might be due to having more than one likely mildly deleterious non-synonymous substitution. Moreover, it seems largely dependent upon a few common haplogroup associated variants with MutPred score above 0.5.

The prevalence of mitochondrial mutations associated with aminoglycoside-induced deafness in ethnic Latvian population: the appraisal of the evidence

Aminoglycosides are potent antibiotics which are used to treat severe gram-negative infections, neonatal sepsis, and multidrug-resistant tuberculosis. Ototoxicity is a well-known side effect of aminoglycosides, and a rapid, profound, and irreversible hearing loss can occur in predisposed individuals. MT-RNR1 gene encoding the mitochondrial ribosomal 12S subunit is a hot spot for aminoglycoside-induced hearing loss mutations, however, a variability in the nature and frequency of genetic changes in different populations exists. The objective of this study was to analyze MT-RNR1 gene mutations in a Baltic-speaking Latvian population, and to estimate the prevalence of such genetic changes in the population-specific mitochondrial haplogroups. In the cohort of 191 ethnic non-related Latvians, the presence of two deafness-associated mutations, m.1555A>G and m.827A>G, three potentially pathogenic variations, m.961insC(n), m.961T>G and m.951G>A, and one unknown substitution, m961T>A was detected, and the aggregate frequency of all variants was 7.3%. All genetic changes were detected in samples belonged to the haplogroups H, U, T, and J. The presence of several aminoglycoside ototoxicity-related MT-RNR1 gene mutations in Baltic-speaking Latvian population indicates the necessity to include ototoxicity-related mutation analysis in the future studies in order to determine the feasibility of DNA screening for patients before administration of aminoglycoside therapy.

Near-Infrared Spectroscopy Screening to Allow Detection of Pathogenic Mitochondrial DNA Variants in Individuals with Unexplained Abnormal Fatigue: A Preliminary Study

Unexplained abnormal fatigue is characterized by chronic fatigue persisting for at least six months and not sufficiently explained by any recognized medical condition. In this pilot study, twelve individuals with abnormal fatigue remaining unexplained after thorough screening were investigated using a near-infrared (NIR) spectroscopy handgrip test. Four of them were found to have an abnormal oxygen extraction pattern similar to participants with documented mitochondrial myopathy. In three of the four individuals, diverse mitochondrial abnormalities were documented by spectrophotometric, immunocytological, fluorescent, and morphological analyses performed in skeletal muscle and in cultured skin fibroblasts. Three of the four participants with decreased muscular oxygen extraction were each shown to harbor a different homoplasmic pathogenic mitochondrial DNA point mutation (m.961T > C, m.1555A > G, m.14484T > C). In the fourth participant, the presence of multiple large mitochondrial DNA deletions was suspected in muscle tissue. In contrast, none of the eight abnormally fatigued participants with normal NIR spectroscopy results harbored either a pathogenic mitochondrial DNA point mutation or large deletions ( P < 0.001). This pilot study shows that NIR spectroscopy may serve as a noninvasive screening tool to delineate a subgroup (of participants) with mitochondrial dysfunction among the large group of individuals with unexplained abnormal fatigue.

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.

Analysis of mitochondrial alterations in Brazilian patients with sensorineural hearing loss using MALDI-TOF mass spectrometry

BACKGROUND

Mutations in the mitochondrial DNA (mtDNA) have been associated with aminoglycoside-induced and nonsyndromic deafness in different populations. In the present study, we investigated the contribution of mutations in mitochondrial genes to the etiology of hearing loss in a Brazilian sample.

METHODS

Using mass spectrometry genotyping technology, combined with direct sequencing, 50 alterations previously described in 14 mitochondrial genes were screened in 152 patients with sensorineural hearing loss and in104 normal hearing controls.

RESULTS

Fifteen known mitochondrial alterations were detected (G709A, A735G, A827G, G988A, A1555G, T4363C, T5628C, T5655C, G5821A, C7462T, G8363A, T10454C, G12236A, T1291C, G15927A). Pathogenic mutations in MT-RNR1 and MT-TK genes were detected in 3 % (5/152) of the patients with hearing loss.

CONCLUSIONS

This study contributed to show the spectrum of mitochondrial variants in Brazilian patients with hearing loss. Frequency of A1555G was relatively high (2.6 %), indicating that this mutation is an important cause of hearing loss in our population. This work reports for the first time the investigation and the detection of the tRNA(Lys) G8363A mutation in Brazilian patients with maternally inherited sensorineural hearing loss.

Mitochondrial COI/tRNASer(UCN) G7444A mutation may be associated with aminoglycoside-induced and non-syndromic hearing impairment

Mutations in mitochondrial DNA (mtDNA) have been reported to have important roles in aminoglycoside-induced hearing impairment; however, the underlying molecular mechanisms have remained largely elusive. The current study presented a case of a Chinese patient with maternally inherited aminoglycoside-induced hearing impairment. A profound hearing impairment was identified by clinical evaluation; furthermore, analysis of the mitochondrial genome sequence of the patient revealed the presence of an A1555G mutation in the 12S rRNA as well as a G7444A mutation in the COI/tRNASer(UCN) gene. As the G7444A mutation is highly conserved between various species, it may be a modifying factor with regard to the pathological effects of the A1555G mutation.

A meta-analysis and systematic review of the prevalence of mitochondrially encoded 12S RNA in the general population: Is there a role for screening neonates requiring aminoglycosides?

BACKGROUND

This was a meta-analysis and systematic review to determine the global prevalence of the mitochondrially encoded 12S RNA (MT-RNR1) genetic mutation in order to assess the need for neonatal screening prior to aminoglycoside therapy.

MATERIALS AND METHODS

A comprehensive search of MEDLINE, EMBASE, Ovid, Database of Abstracts of Reviews of Effect, Cochrane Library, Clinical Evidence and Cochrane Central Register of Trials was performed including cross-referencing independently by 2 assessors. Selections were restricted to human studies in English. Meta-analysis was done with MetaXL 2013.

RESULTS

Forty-five papers out of 295 met the criteria. Pooled prevalence in the general population for MT-RNR1 gene mutations (A1555G, C1494T, A7445G) was 2% (1-4%) at 99%.

CONCLUSION

Routine screening for MT-RNR1 mutations in the general population prior to treatment with aminoglycosides appear desirable but poorly supported by the weak level of evidence available in the literature. Routine screening in high-risk (Chinese and Spanish) populations appear justified.

Second stage of Universal Neonatal Hearing Screening - a way for diagnosis and beginning of proper treatment for infants with hearing loss

PURPOSE

To analyze retrospectively the results of hearing testing in infants at the second stage of the Polish Universal Neonatal Hearing Screening Program carried out in the Department of Otolaryngology at the Medical University of Warsaw.

MATERIAL/METHODS

A total of 351 infants referred to our Department for the second stage of UNHS were included in the study. There were 39.60% infants referred due to positive result of hearing screening at the first stage of the Program performed in neonatal units, 55.27% with negative screening but risk factors present, and 5.13% without any tests due to equipment failure in the maternity unit.

RESULTS

Risk factors were identified in 86.61% of the infants. The most frequent ones were hyperbilirubinemia (71.51%), premature birth (63.25%), and ototoxic medication (62.11%). Otoacoustic emission test showed fail results in 17.66% of the infants, and auditory brainstem responses confirmed hearing loss in 16.81%. Correlation between risk factors and confirmed hearing loss was found for hyperbilirubinemia, low birth weight, intensive therapy for at least 7 days, low Apgar scores, and craniofacial abnormalities.

CONCLUSIONS

The early identification of infants with hearing loss is essential for early intervention. Not only infants who fail the initial screening but also the ones with risk factors of hearing impairment should be referred to the centers that are capable of providing the necessary diagnostic services required for the second stage of the UNHSP. Those two steps are needed to both minimize the risk of overlooking a child with hearing loss and properly diagnose hearing impairment.

The risk factor profile of children covered by the Polish universal neonatal hearing screening program and its impact on hearing loss incidence

OBJECTIVE

The high frequency of risk factors detected within the newborn population increases the total number of children that should receive regular follow-ups. However, in some circumstances, this could be beyond the capacity of the health system. Therefore, careful interpretation and selection of risk factors, and in particular of those factors not strictly defined, should be carried out during screening. The aim of the study was to analyse the risk factor profile of children covered by the national universal neonatal hearing screening program and to correlate it with hearing loss incidence.

PATIENTS AND METHODS

The analysis of records in the program database collected from 472 neonatal and well-baby units over a period of 10 years (2002-2012), focused on children with at least one risk factor. The analysis was subdivided into distribution of risk factors as well as to risk factors and hearing loss correlation.

RESULTS

In the studied cohort of n=137,432 children (4% of the total number of screened children) single risk factors were most frequently detected, accounting for 71% of records. The association of two or more risk factors appeared in 659 configurations (29%), with a mean of 3.1 coexisting risk factors and a maximum of 9. Hearing loss was dependent on the number of risk factors in a child, but reached its maximum with the association of 6 factors.

CONCLUSIONS

The detection of postnatal hearing loss should be continued in order to increase our understanding of hearing incidence and the role of environmental factors. To optimize screening, discussions between specialists (mostly related to the issue of risk factors detected and registered in the earliest stage of screening programs) would be beneficial.

Utility of Genetic Testing for the Detection of Late-Onset Hearing Loss in Neonates

Purpose

The purpose of this study was to demonstrate the utility of molecular testing in the detection of potentially important causes of delayed hearing loss missed by current audiometric screening at birth.

Method

We enrolled infants who had received a newborn audiometric hearing screen and a filter paper blood collection for state newborn screening. A central laboratory ran the SoundGene ® panel.

Results

Of 3,681 infants studied, 35 (0.95%) had a positive SoundGene panel, 16 had mitochondrial mutations, 9 had Pendred mutations, 5 were cytomegalovirus (CMV) DNA positive, 2 had connexin mutations, and 3 had a combination of different mutations. Infants with an abnormal SoundGene panel were at increased risk for hearing loss compared to neonates without mutations. Three (8.6%) of the 35 subjects had persistent hearing loss compared to 5 (0.21%) of 2,398 subjects with no report of mutation ( p < .01). Of 3,681 infants studied, 8 (0.22%) had persistent hearing loss: 5 (62.5%) had abnormal newborn audiometric screens, 2 (25%) had an abnormal SoundGene panel (1 was CMV positive, 1 had a mitochondrial mutation), and 1 (12.5%) had no identifiable risk factors.

Conclusion

A positive SoundGene panel identifies infants who are not identified by audiometric testing and may be at risk for hearing loss.

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.

Mitochondrial DNA variations in Madras motor neuron disease

Although the Madras motor neuron disease (MMND) was found three decades ago, its genetic basis has not been elucidated, so far. The symptom at onset was impaired hearing, upper limb weakness and atrophy. Since some clinical features of MMND overlap with mitochondrial disorders, we analyzed the complete mitochondrial genome of 45 MMND patients and found 396 variations, including 13 disease-associated, 2 mt-tRNA and 33 non-synonymous (16 MT-ND, 10 MT-CO, 3 MT-CYB and 4 MT-ATPase). A rare variant (m.8302A>G) in mt-tRNA(Leu) was found in three patients. We predict that these variation(s) may influence the disease pathogenesis along with some unknown factor(s).

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.

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.

Mitochondrial genetic background plays a role in increasing risk to asthma

A number of studies suggest that mitochondrial dysfunction plays a role in the pathogenesis of asthma. To shed light for the first time on the role of the mitochondrial genome in the etiology of asthma we analyzed the mitochondrial tRNA genes and part of their flanking regions in patients with asthma compared with a set of healthy controls. We found a total of 10 mutations in 56 out of 76 asthmatic patients. Four of these mutations were not found in the control group, five were observed at a significantly lower frequency in controls, but none of the combinations of mutations detected in asthma patients was observed in the controls. Furthermore, we observed that 27.6% of the asthma patients (vs. 4% of the controls) belonged to the haplogroup U (Fisher test P = 0.00) and a positive significant correlation was found between the occurrence of the haplogroup U and the severity of the disease (Fisher test P = 0.02). Whereas further studies in larger cohorts are needed to confirm these observations we suggest that the mitochondrial genetic background plays a key role in asthma development.

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

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

The prevalence of mitochondrial mutations associated with aminoglycoside-induced sensorineural hearing loss in an NICU population

Several mitochondrial DNA variants increase risk for developing sensorineural hearing loss following exposure to aminoglycoside antibiotics, a particular concern in the premature infant population, as many of these babies spend time in neonatal intensive care units and are treated with aminoglycosides. To determine the relative prevalence of five mitochondrial DNA variants in the 12S rRNA gene, MT-RNR1, we genotyped 703 neonatal intensive care unit patients and 1473 individuals from the general Iowa population. We found that the aggregate frequency of these variants (∼1.8%) was comparable between populations. Although no hearing loss was detected by newborn hearing screens in the at-risk patients, these neonatal intensive care unit graduates have an increased life-time risk for developing aminoglycoside-induced deafness.

Homoplasmy of the G7444A mtDNA and heterozygosity of the GJB2 c.35delG mutations in a family with hearing loss

OBJECTIVE

Mitochondrial mutations have been shown to be responsible for syndromic as well as non-syndromic hearing loss. The G7444A mitochondrial DNA mutation affects COI/the precursor of tRNA(Ser(UCN)), encoding the first subunit of cytochrome oxidase. Here we report on the first Greek family with the G7444A mitochondrial DNA mutation.

METHODS

Clinical, cytogenetic, and molecular methods were employed in this study.

RESULTS

We describe the high variability of phenotypes among three family members harboring the G7444A mutation and also the frequent GJB2 c.35delG mutation of the nuclear genome in heterozygosity. Their phenotypes ranged from normal hearing to deafness, while the proband presented with several other symptoms.

CONCLUSIONS

The G7444A mitochondrial DNA mutation has been reported in only a few cases worldwide, alone or in cosegregation with other mitochondrial DNA mutations, but to our knowledge, never before in coexistence with the GJB2 c.35delG mutation.

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.

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.