The heteroplasmic m.14709T>C mutation in the tRNA(Glu) gene in two Tunisian families with mitochondrial diabetes

Heteroplasmy and Individual Mitogene Pools: Characteristics and Potential Roles in Ecological Studies

The mitochondrial genome (mitogenome or mtDNA), the extrachromosomal genome, is a multicopy circular DNA with high mutation rates due to replication and repair errors. A mitochondrion, cell, tissue, organ, or an individual body may hold multiple variants, both inherited and developed over a lifetime, which make up individual mitogene pools. This phenomenon is also called mtDNA heteroplasmy. MtDNA variants influence cellular and tissular functions and are consequently subjected to selection. Although it has long been recognized that only inheritable germline heteroplasmies have evolutionary significance, non-inheritable somatic heteroplasmies have been overlooked since they directly affect individual fitness and thus indirectly affect the fate of heritable germline variants. This review focuses on the characteristics, dynamics, and functions of mtDNA heteroplasmy and proposes the concept of individual mitogene pools to discuss individual genetic diversity from multiple angles. We provide a unique perspective on the relationship between individual genetic diversity and heritable genetic diversity and guide how the individual mitogene pool with novel genetic markers can be applied to ecological research.

Insulin Resistance in Mitochondrial Diabetes

Mitochondrial diabetes (MD) is generally classified as a genetic defect of β-cells. The main pathophysiology is insulin secretion failure in pancreatic β-cells due to impaired mitochondrial ATP production. However, several reports have mentioned the presence of insulin resistance (IR) as a clinical feature of MD. As mitochondrial dysfunction is one of the important factors causing IR, we need to focus on IR as another pathophysiology of MD. In this special issue, we first briefly summarized the insulin signaling and molecular mechanisms of IR. Second, we overviewed currently confirmed pathogenic mitochondrial DNA (mtDNA) mutations from the MITOMAP database. The variants causing diabetes were mostly point mutations in the transfer RNA (tRNA) of the mitochondrial genome. Third, we focused on these variants leading to the recently described "tRNA modopathies" and reviewed the clinical features of patients with diabetes. Finally, we discussed the pathophysiology of MD caused by mtDNA mutations and explored the possible mechanism underlying the development of IR. This review should be beneficial to all clinicians involved in diagnostics and therapeutics related to diabetes and mitochondrial diseases.

Ageing, Metabolic Dysfunction, and the Therapeutic Role of Antioxidants

The gradual ageing of the world population has been accompanied by a dramatic increase in the prevalence of obesity and metabolic diseases, especially type 2 diabetes. The adipose tissue dysfunction associated with ageing and obesity shares many common physiological features, including increased oxidative stress and inflammation. Understanding the mechanisms responsible for adipose tissue dysfunction in obesity may help elucidate the processes that contribute to the metabolic disturbances that occur with ageing. This, in turn, may help identify therapeutic targets for the treatment of obesity and age-related metabolic disorders. Because oxidative stress plays a critical role in these pathological processes, antioxidant dietary interventions could be of therapeutic value for the prevention and/or treatment of age-related diseases and obesity and their complications. In this chapter, we review the molecular and cellular mechanisms by which obesity predisposes individuals to accelerated ageing. Additionally, we critically review the potential of antioxidant dietary interventions to counteract obesity and ageing.

Mutational Analysis of Mitochondrial tRNA Genes in 200 Patients with Type 2 Diabetes Mellitus

Objective

Previous studies showed that variants in mitochondrial DNA (mtDNA) are associated with type 2 diabetes mellitus (T2DM). However, the relationships between mitochondrial tRNA (mt-tRNA) variants and T2DM remain poorly understood.

Methods

In this study, we performed a mutational screening of 22 mt-tRNA genes in a cohort of 200 Han Chinese subjects with T2DM and 200 control subjects through PCR–Sanger sequencing. The identified mt-tRNA variants were assessed for their pathogenicity via the phylogenetic approach, structural and functional analysis. Furthermore, two Han Chinese pedigrees with maternally inherited diabetes and deafness (MIDD) were reported by clinical and genetic assessments.

Results

A total of 49 genetic variants in mt-tRNA genes were identified; among them, 31 variants (17 pathogenic/likely pathogenic) were absent in controls, located at extremely conserved nucleotides, may have potential structural and functional significance, thereby considered to be T2DM-associated variants. In addition, sequence analysis of entire mitochondrial genomes of the matrilineal relatives from two MIDD pedigrees revealed the occurrence of tRNA^Leu(UUR) A3243G and T3290C mutations, as well as sets of polymorphisms belonging to mitochondrial haplogroups F2 and D4. However, the lack of any functional variants in connexin 26 gene (GJB2) and tRNA 5-methylaminomethyl-2-thiouridylate (TRMU) suggested that nuclear genes may not play active roles in clinical expression of MIDD in these pedigrees.

Conclusion

Our data indicated that mt-tRNA variants were associated with T2DM, screening for mt-tRNA pathogenic mutations was recommended for early detection and prevention of mitochondrial diabetes.

Obesity associated with a novel mitochondrial tRNACys 5802A>G mutation in a Chinese family

A Chinese family with matrilineally inherited obesity was assessed and its clinical, genetic, and molecular profiling was conducted. Obesity was observed in matrilineal relatives (3 out of 7) of a single generation (of 3 alive generations) in this family. On pedigree analysis and sequencing of their mitochondrial DNA (mtDNA), a novel homoplasmic mutation of the mitochondrial tRNA^Cys gene (5802A>G) was identified in these individuals. This mutation correlated with a destabilized conserved base pair in this tRNA anticodon stem. Position 30 is known to be crucial for carrying out effective codon recognition and stability of tRNA. In accordance with the importance of this conserved site, we observed that the predicted structure of tRNA^Cys with the mutation was noticeably remodeled in a molecular dynamics simulation when compared with the isoform of the wild-type. All other 46 mutations observed in the individual’s mtDNA were known variants belonging to haplogroup D4. Thus, this is the first report that provides evidence of the association between a mutation in tRNA and an enhanced risk of maternally transmissible obesity, offering more insights into obesity and its underlying nature.

Overview of Atypical Diabetes

Although type 1 diabetes mellitus and, to a lesser extent, type 2 diabetes mellitus, are the prevailing forms of diabetes in youth, atypical forms of diabetes are not uncommon and may require etiology-specific therapies. By some estimates, up to 6.5% of children with diabetes have monogenic forms. Mitochondrial diabetes and cystic fibrosis related diabetes are less common but often noted in the underlying disease. Atypical diabetes should be considered in patients with a known disorder associated with diabetes, aged less than 25 years with nonautoimmune diabetes and without typical characteristics of type 2 diabetes mellitus, and/or with comorbidities associated with atypical diabetes.

Genetic cause of heterogeneous inherited myopathies in a cohort of Greek patients

Inherited muscle disorders are caused by pathogenic changes in numerous genes. Herein, we aimed to investigate the etiology of muscle disease in 24 consecutive Greek patients with myopathy suspected to be genetic in origin, based on clinical presentation and laboratory and electrophysiological findings and absence of known acquired causes of myopathy. Of these, 16 patients (8 females, median 24 years-old, range 7 to 67 years-old) were diagnosed by Whole Exome Sequencing as suffering from a specific type of inherited muscle disorder. Specifically, we have identified causative variants in 6 limb-girdle muscular dystrophy genes (6 patients; ANO5, CAPN3, DYSF, ISPD, LAMA2, SGCA), 3 metabolic myopathy genes (4 patients; CPT2, ETFDH, GAA), 1 congenital myotonia gene (1 patient; CLCN1), 1 mitochondrial myopathy gene (1 patient; MT-TE) and 3 other myopathy-associated genes (4 patients; CAV3, LMNA, MYOT). In 6 additional family members affected by myopathy, we reached genetic diagnosis following identification of a causative variant in an index patient. In our patients, genetic diagnosis ended a lengthy diagnostic process and, in the case of Multiple acyl-CoA dehydrogenase deficiency and Pompe's disease, it enabled specific treatment to be initiated. These results further expand the genotypic and phenotypic spectrum of inherited myopathies.

In silico model of mtDNA mutations effect on secondary and 3D structure of mitochondrial rRNA and tRNA in Leber's hereditary optic neuropathy

The Leber's hereditary optic neuropathy (LHON) is a rare disease caused by mitochondrial DNA (mtDNA) mutations. Beside primary mutations, the effect of secondary mtDNA mutations in still unclear. We examined the effect of secondary mtDNA mutations on secondary structure of different mitochondrial RNAs. Whole mitochondrial genome sequence of LHON patients has been obtained from in six non related pedigrees by Sanger sequencing method. The effect of mutations located in mitochondrial RNA genes was examined by creating in silico models of RNA secondary and regional 3D structure, accompanied by sequence conservation analysis. All three primary LHON mutations (m.3460G>A, m.11778G>A and m.14484 T>C) were revealed in study families. Four mutations in MT-RNR1 gene (m.750A>G, m.956delC, m.1438A>G and m.1555A>G) were identified and only an m.1555A>G causes significant changes of secondary structure of mitochondrial 12S ribosomal RNA (rRNA), while it is the only mutation which does not alter its 3D structure. Five mutations (m.1811A>G, m.2706A>G, m.2831G>A, m.3010G>A and m.3197T>C) were discovered in MT-RNR2 gene and all of them induced substantial alterations of mitochondrial 16S rRNA secondary structure. Significant changes of mitochondrial 16S rRNA 3D structure are caused by m.1811A>G, m.2706A>G, m.3010G>A and m.3197T>C. A single insertion variant (m.15986insG) has been found in the MT-TP gene which encodes mitochondrial transfer RNA for Proline (tRNA Pro). This mutation does not cause substantial changes of tRNA for Proline secondary structure, while the 3D geometry remains without major changes. Most of the mutation loci exhibited high level of sequence conservation. Presence of multiple mutations in a single family appears to cause more extensive changes in mitochondrial 12S and 16S rRNA, then their individual influence. The effect of discovered mutations on in silico modelled RNA structure is in a significant correlation with the present knowledge about the potential of these mutation to participate in the pathophysiology of LHON and other human diseases. The presence of certain multiple mitochondrial RNA mutations could be a possible explanation of LHON clinical presentation in some families. All revealed mutations have been evaluated for the first time in terms of in silico structural modelling. The application of bioinformatics tools such as secondary and 3D RNA structure prediction can have a great advantage in better understanding of the molecular standpoint of the LHON pathophysiology and clinical phenotype.

The first concurrent detection of mitochondrial DNA m.3243A>G mutation, deletion, and depletion in a family with mitochondrial diabetes

BACKGROUND

Mitochondrial diabetes (MD) is a rare monogenic form of diabetes and divided into type l and type 2. It is characterized by a strong familial clustering of diabetes with the presence of maternal transmission in conjunction with bilateral hearing impairment in most of the carriers. The most common form of MD is associated with the m.3243A>G mutation in the mitochondrial MT-TL1, but there are also association with a range of other point mutations, deletion, and depletion in mtDNA.

METHODS

The mitochondrial genome anomalies were investigated in a family with clinical features of MD, which includes a proband presenting severe MD conditions including cardiomyopathy, retinopathy, and psychomotor retardation.

RESULTS

By investigating the patient's blood leukocytes and skeletal muscle, we identified the m.3243A>G mutation in heteroplasmic state. This mutation was absent in the rest of the family members. In addition, our analysis revealed in the proband a large mtDNA heteroplasmic deletion (~1 kb) and a reduction in mtDNA copy number.

CONCLUSION

Our study points out, for the first time, a severe phenotypic expression of the m.3243A>G point mutation in association with mtDNA deletion and depletion in MD.

Clinical phenotype of mitochondrial diabetes due to rare mitochondrial DNA mutations

OBJECTIVE

While the most frequent mutation responsible for mitochondrial diabetes is the point mutation m.3243 A>G of mitochondrial DNA (mtDNA), few data are available about the role of rare mtDNA mutations in the pathophysiology of diabetes. The main objective of our study was to describe the phenotypic characteristics of patients suffering from diabetes linked to rare mtDNA mutations.

RESEARCH DESIGN AND METHODS

We performed a post-hoc analysis of a prospective multicenter cohort of 743 patients with mitochondrial disorder (previously published by the French Network of Mitochondrial Diseases), associated to a literature review of the PubMed database from 1992 to May 2016. We extracted all reported patients with diabetes and identified rare mtDNA mutations and described their clinical and metabolic phenotypes.

RESULTS

The 50 identified patients (10 from the princeps study; 40 from the review of the literature) showed a heterogeneous metabolic phenotype in terms of age, symptoms prior to diagnosis, treatments, and associated clinical and biological signs. However, neurological symptoms were more frequent in case of rare mtDNA mutations compared to the classical m.3243 A>G mutation (P=0.024). In contrast, deafness (65% vs. 95%, P=3.7E-5), macular pattern dystrophy (20% vs. 86%, P=1.6E-10) and nephropathy (8% vs. 28%, P=0.018) were significantly less frequent than in case of the classical m.3243 A>G mutation.

CONCLUSION

Although no specific metabolic phenotype could be identified suggesting or eliminating implication of rare mtDNA mutations in diabetes, clinical phenotypes featured more frequent neurological signs.

A Novel Mutation of Mitochondrial T14709C Causes Myoclonic Epilepsy with Ragged Red Fibers Syndrome in a Chinese Patient

Background:

Myoclonic epilepsy with ragged red fibers (MERRF) syndrome is characterized by myoclonus, generalized epilepsy, cerebellar ataxia, and ragged red fibers (RRFs) in the muscle. T-to-C transition at nucleotide position 14709 in the mitochondrial tRNA glutamic acid (tRNA^Glu) gene has previously been associated with maternally inherited diabetes and deafness. However, the association between MERRF and mitochondrial T14709C mutation (m.T14709C) has never been reported before.

Methods:

Clinical information of a 17-year-old patient was collected; muscle biopsy and next-generation sequencing (NGS) of whole mitochondrial and neuromuscular disease panel were then conducted. Finally, sanger sequencing was carried out to confirm the mutations.

Results:

The patient presented a typical MERRF phenotype with muscle weakness, epileptic seizure, clonic episodes, cerebellar ataxia, and spinal scoliosis. Muscle biopsy showed RRFs which indicated abnormal mitochondrial functions. NGS of whole mitochondrial gene revealed m.T14709C mutation, confirmed by Sanger sequencing.

Conclusion:

We present a sporadic patient with typical MERRF presentation carrying the mutation of m.T14709C, which expanded the spectrum of m.T14709C.

Association study of apoptosis gene polymorphisms in mitochondrial diabetes: A potential role in the pathogenicity of MD

Mitochondrial diabetes (MD) is a heterogeneous disorder characterized by a chronic hyperglycemia and is maternally transmitted. Syndromic MD is a subgroup of MD including diabetic microangiopathy and macroangiopathy, in addition to extrapancreatic disorder. MD is caused by genetic mutations and deletions affecting mitochondrial DNA. This mitochondrial damage initiates apoptosis. In this study, we hypothesized that functional polymorphisms in genes involved in apoptotic pathway could be associated with the development of apoptosis in MD disease and increased its risk. Detection of apoptosis was confirmed on muscle biopsies taken from MD patients using the TUNEL method and the Cytochrome c protein expression level. We genotyped then 11 published SNPs from intrinsic and extrinsic apoptotic pathway and assessed the signification of these polymorphisms in 43 MD patients and 100 healthy controls. We found 10 selected polymorphisms (p53 (rs1042522 and rs17878362), BCL2 (rs2279115), BAX (rs1805419), BAK1 (rs210132 and rs2227925), CASP3 (rs1405937), CASP7 (rs2227310), CASP8 (rs1045485) and CASP10 (rs13006529)) with a potential apoptosis effect in MD patients compared to control population. Specifically, SNPs involved in the intrinsic pathway (p53, BCL2, BAK1 and CASP3) presented the highest risk of apoptosis. Our result proved that apoptosis initiated by mtDNA mutations, can be emphasized by a functional apoptotic polymorphisms associated with high expression of cytochrome c protein and more myofibers with apoptosis in syndromic MD subgroup compared with non-syndromic MD subgroup.

Whole mitochondrial genome screening of a family with maternally inherited diabetes and deafness (MIDD) associated with retinopathy: A putative haplotype associated to MIDD and a novel MT-CO2 m.8241T>G mutation

Mitochondrial diseases are a clinically heterogeneous group of disorders that arise as a result of dysfunction of the mitochondrial respiratory chain. They can be caused by mutations in both nuclear and mitochondrial DNA. In fact, mitochondrial DNA (mtDNA) defects are known to be associated with a large spectrum of human diseases and patients might present wide range of clinical features with various combinations. Our study reported a Tunisian family with clinical features of maternally inherited diabetes and deafness (MIDD). Accordingly, we performed a whole mitochondrial genome mutational analysis, results revealed a haplotype composed by "A750G, A1438G, G8860A, T12705, T14766C and T16519C", in homoplasmic state, in the mother and transmitted to her daughter and her son. The patient with MIDD2 and retinopathy presented, in addition to this haplotype associated to the MIDD, two de novo variations including a novel one m.8241T>G (p. F219C) in MT-CO2 gene and a known one m.13276G>A (p. M314V) in MT-ND5 gene. The coexistence of these two mutations could explain the retinopathy observed in this patient.

A Deafness- and Diabetes-associated tRNA Mutation Causes Deficient Pseudouridinylation at Position 55 in tRNAGlu and Mitochondrial Dysfunction

Several mitochondrial tRNA mutations have been associated with maternally inherited diabetes and deafness. However, the pathophysiology of these tRNA mutations remains poorly understood. In this report, we identified the novel homoplasmic 14692A→G mutation in the mitochondrial tRNA^Glu gene among three Han Chinese families with maternally inherited diabetes and deafness. The m.14692A→G mutation affected a highly conserved uridine at position 55 of the TΨC loop of tRNA^Glu The uridine is modified to pseudouridine (Ψ55), which plays an important role in the structure and function of this tRNA. Using lymphoblastoid cell lines derived from a Chinese family, we demonstrated that the m.14692A→G mutation caused loss of Ψ55 modification and increased angiogenin-mediated endonucleolytic cleavage in mutant tRNA^Glu The destabilization of base-pairing (18A-Ψ55) caused by the m.14692A→G mutation perturbed the conformation and stability of tRNA^Glu An approximately 65% decrease in the steady-state level of tRNA^Glu was observed in mutant cells compared with control cells. A failure in tRNA^Glu metabolism impaired mitochondrial translation, especially for polypeptides with a high proportion of glutamic acid codons such as ND1, ND6, and CO2 in mutant cells. An impairment of mitochondrial translation caused defective respiratory capacity, especially reducing the activities of complexes I and IV. Furthermore, marked decreases in the levels of mitochondrial ATP and membrane potential were observed in mutant cells. These mitochondrial dysfunctions caused an increasing production of reactive oxygen species in the mutant cells. Our findings may provide new insights into the pathophysiology of maternally inherited diabetes and deafness, which is primarily manifested by the deficient nucleotide modification of mitochondrial tRNA^Glu.

Mitochondrial complex I and V gene polymorphisms in type II diabetes mellitus among high risk Mizo-Mongoloid population, Northeast India

Introduction

The study was carried out to identify the polymorphisms in mitochondrial genes (ATPase and ND1) in type 2 Diabetes Mellitus (T2DM) from Mizo population and to correlate the involvement of demographic factors.

Findings

In the present study, 58 patients and 50 healthy volunteers were considered. The mutations observed were mostly base substitutions and were similar as reported for other populations. Three mutations are unreported and were found to be novel polymorphisms for diabetic disease. One heteroplasmic variation (MT3970 C > T) was found in 36.36 % of samples. Subjects with excessive smoked meat consumption and customary habit of smoking (ORs: 4.92; 95 % CI: 0.96–25.21) were found to be more prone to T2DM. Mitochondrial genes sequence analysis revealed the genetic variability between the healthy and diabetic samples.

Conclusion

Mitochondrial ATPase and ND1 gene polymorphisms may be involved in triggering the risk for T2DM.

Electronic supplementary material

The online version of this article (doi:10.1186/s41021-016-0034-z) contains supplementary material, which is available to authorized users.

Whole mitochondrial genome analysis in two families with dilated mitochondrial cardiomyopathy: detection of mutations in MT-ND2 and MT-TL1 genes

Pathogenic mitochondrial DNA (mtDNA) mutations leading to mitochondrial dysfunction can cause cardiomyopathy and heart failure. These mutations were described in the mt-tRNA genes and in the mitochondrial protein-coding genes. The aim of this study was to identify the genetic defect in two patients belonging to two families with cardiac dysfunction associated to a wide spectrum of clinical phenotypes. The sequencing analysis of the whole mitochondrial DNA in the two patients and their parents revealed the presence of known polymorphisms associated to cardiomyopathy and two pathogenic mutations in DNA extracted from blood leucocytes: the heteroplasmic m.3243A > G mutation in the MT-TL1 gene in patient A; and the homoplasmic m.5182C > T mutation in the ND2 gene in patient B. Secondary structure analysis of the ND2 protein further supported the deleterious role of the m.5182C > T mutation, as it was found to be involved an extended imbalance in its hydrophobicity and affect its function. In addition, the mitochondrial variants identified in patients A and B classify both of them in the same haplogroup H2a2a1.

A novel mutation MT-COIII m.9267G>C and MT-COI m.5913G>A mutation in mitochondrial genes in a Tunisian family with maternally inherited diabetes and deafness (MIDD) associated with severe nephropathy

Mitochondrial diabetes (MD) is a heterogeneous disorder characterized by a chronic hyperglycemia, maternal transmission and its association with a bilateral hearing impairment. Several studies reported mutations in mitochondrial genes as potentially pathogenic for diabetes, since mitochondrial oxidative phosphorylation plays an important role in glucose-stimulated insulin secretion from beta cells. In the present report, we studied a Tunisian family with mitochondrial diabetes (MD) and deafness associated with nephropathy. The mutational analysis screening revealed the presence of a novel heteroplasmic mutation m.9276G>C in the mitochondrial COIII gene, detected in mtDNA extracted from leukocytes of a mother and her two daughters indicating that this mutation is maternally transmitted and suggest its implication in the observed phenotype. Bioinformatic tools showed that m.9267G>C mutation (p.A21P) is « deleterious » and it can modify the function and the stability of the MT-COIII protein by affecting the assembly of mitochondrial COX subunits and the translocation of protons then reducing the activity of the respective OXPHOS complexes of ATP synthesis. The nonsynonymous mutation (p.A21P) has not been reported before, it is the first mutation described in the COXIII gene which is related to insulin dependent mitochondrial diabetes and deafness and could be specific to the Tunisian population. The m.9267G>C mutation was present with a nonsynonymous inherited mitochondrial homoplasmic variation MT-COI m.5913 G>A (D4N) responsible of high blood pressure, a clinical feature detected in all explored patients.

Maternally inherited diabetes is associated with a homoplasmic T10003C mutation in the mitochondrial tRNA(Gly) gene

In this report, we investigate molecular pathogenic mechanism of a diabetes-associated homoplasmic mitochondrial tRNA mutation in a Han Chinese family with maternally transmitted diabetes mellitus. Of 10 adult matrilineal relatives, 5 individuals suffered from diabetes (4 subjects with only diabetes, one subject with both diabetes and hearing impairment), while other five matrilineal relatives (one with hearing loss) had glucose intolerance. The average age at onset of diabetes in matrilineal relatives was 50 years. Molecular analysis of their mitochondrial genomes identified the novel homoplasmic T10003C mutation in the tRNA(Gly) gene belonging to haplogroup M11b. The T10003C mutation is expected to form a base-pairing (13C-22G) at the highly conserved D-stem of tRNA(Gly), thereby affecting secondary structure and function of this tRNA. A tRNA Northern analysis revealed that the T10003C mutation caused ~70% reduction in the steady-state level of tRNA(Gly). An in vivo translation analysis showed ~33% reduction in the rate of mitochondrial translation in mutant cells. Oxygen consumption analysis showed the defects in overall respiratory capacity or the ATP-linked, proton leak, and maximal respiration in mutant cells. As a result, the cellular energy deficiency contributes to the development of diabetes in subjects carrying the T10003C mutation. These data provide the first direct evidence that the tRNA(Gly) mutation might be associated with diabetes. Thus, our findings may provide new insights into the understanding of pathophysiology of maternally inherited diabetes.

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

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