Clinical and genetic features in two families with MELAS and the T3271C mutation in mitochondrial DNA

The majority of patients with MELAS (mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes) have the A3243G point mutation. The much rarer T3271C mutation has been reported predominantly in Japanese subjects. Our objective was to better define the clinical phenotype and mutation load in patients with MELAS and the T3271C mutation in mitochondrial DNA. We present clinical and molecular genetic data in two pedigrees with the T3271C mutation. The age at onset was 8 years in one proband and 14 years in the other. Both patients had migrainelike headache, seizures, and strokelike episodes. Mutation loads were quantified in multiple tissues from the patients and from family members by polymerase chain reaction-restriction fragment length polymorphism analysis. The symptoms in both probands were typical of MELAS, and, contrary to previous reports, onset was early. Hearing loss was less common than in typical MELAS, and ragged red fibers were absent. The proportion of mutant genomes was consistently and markedly greater in DNA from urinary sediment than from blood. In the mother of one proband, mutant genomes were detected only in DNA from hair follicles and cheek mucosa The phenotype of patients with the T3271C mutation might not be as distinct as that of the A3243G mutation, as previously described. Our data also suggest that urine is a better source of DNA than blood for diagnosis and that multiple tissues should be studied in maternal relatives, especially when the mutation cannot be detected in blood.     

应用实时荧光定量聚合酶链反应结合突变阻滞形成系统进行线粒体脱氧核糖核酸A3243G突变负荷检测

目的 应用实时荧光定量PCR结合突变阻滞形成系统(RT ARMS-qPCR)技术检测线粒体DNA A3243G突变负荷,并探讨线粒体脑肌病伴高乳酸血症和脑卒中样发作(MELAS)综合征患者不同组织中A3243G突变负荷的分布情况.方法 分别构建含线粒体DNA3243位点的野生型(A)和突变型(G)序列的质粒,将这2种质粒按一定比例混合构建成13个含有不同A3243G突变负荷的标准对照,应用PCR-限制性片段多态性技术(RFLP)和RT ARMS-qPCR技术检测13个标准对照及存在A3243G突变的7例MELAS患者、1名携带者以及53名健康对照的外周血、肌肉、毛发及尿沉渣等组织的A3243G突变负荷.结果 标准对照的突变负荷经PCR-RFLP和RT ARMS-qPCR检测的结果均与预期值呈直线相关,决定系数分别为R21=0.885、R22=0.991,RT ARMS-qPCR检测的结果更接近预期值;7例MELAS患者及1名携带者不同组织的突变负荷经RT ARMS-qPCR检测的结果比经PCR-RFLP检测出的高,且RT ARMS-qPCR比PCR-RFLP更容易检测出低负荷的A3243G突变,并发现MELAS患者的肌肉、尿沉渣、头发的A3243G突变负荷均高于外周血.结论 应用RT ARMS-qPCR可以快速、灵敏、准确地检测不同组织的A3243G突变及其突变负荷.     

Quantitation of heteroplasmy of mitochondrial trnaLeu(UUR) gene using PCR-SSCP

We have devised a novel method for quantitative analysis of the MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) tRNA^Leu(UUR) mutation of mitochondrial DNA using a PCR-SSCP (polymerase chain reaction–single-strand conformation polymorphism) method, and compared the results obtained using the PCR-SSCP method with those obtained using other methods including Southern blotting, last one cycle hot PCR, and conventional PCR-RFLP (restriction fragment length polymorphism). The standard curve obtained using the PCR-SSCP method is linear, with a correlation coefficient of 0.999; it was determined that this method is more accurate than other methods for quantitative analysis. The PCR-SSCP method does not require restriction digestions, thereby avoiding potential problems of partial digestions or heteroduplex formation during PCR. The method is quite simple and should have a broad range of application for quantitation of mutant mtDNAs in various mitochondrial encephalo-myopathies. We applied the method for quantitation of mutant mitochondrial DNA carrying a single base substitution in the tRNA^Leu(UUR) gene in two autopsied cases of MELAS. In both cases, the mutant mtDNA is abundantly present (82–95%) with little variation among tissues. © 1995 John Wiley & Sons, Inc.     

A mitochondrial DNA mutation in a patient with an extensive family history of Duchenne muscular dystrophy

One challenge in the molecular diagnosis of mitochondrial DNA (mtDNA) disorders is detection of a low percentage of mutant heteroplasmy. We report a patient who had a delayed molecular diagnosis of mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome due to the complication of an extensive family history of another neuromuscular disease, Duchenne muscular dystrophy, and the failure to detect a low proportion of mutant A3243G mtDNA with a polymerase chain reaction (PCR)/restriction fragment length polymorphism (RFLP)/ethidium bromide detection method. Using an improved, more sensitive allele-specific oligonucleotide (ASO) radioactive dot-blot hybridization method, a low degree of A3243G heteroplasmy was detected in several tissues from this patient. This case underscores the importance of a sensitive mutation detection method and the need for a search for mtDNA mutations if the patient's clinical symptoms suggest a mitochondrial disorder despite the family background of another neuromuscular disease.     

"MELAS" (A3243G) mutation of mitochondrial DNA: a study of the relationships between the clinical phenotype in 19 patients and morphological and molecular data

Nineteen patients were found to harbor the mitochondrial DNA A3243G mutation associated with MELAS syndrome (Mitochondrial myopathy, Encephalopathy, Lactic Acidosis and Stroke-like episodes). Eight of them had presented with stroke-like episodes and therefore had a clinical diagnosis of MELAS syndrome. The other 11 patients had no strokes and presented with generally less severe multisystemic disease. In the two groups, we compared muscle morphology, biochemical activities of muscle respiratory chain, and genetic characteristics: proportion and tissue distribution of the mutation, sequence of the 22 transfer RNA genes of the mitochondrial DNA. The proportion of mutant mtDNA in muscle was always greater than in blood. The number of patients in the two groups was too low to reach significant values. However, the patients with a MELAS syndrome presented with more severe respiratory chain abnormalities and with a proportion of the A3243G mutation that was both higher and more uniformly distributed among tissues. For symptoms others than stroke-like episodes, we did not observe any correlation with the level of mutant mtDNA in muscle. The analysis of the 22 tRNA sequences did not show differences between the two groups, and no co-inherited modifying tRNA genes could explain the variability of severity in our patients.     

Clinical,physiological, and histological features in a kindred with the T3271C MELAS mutation

The majority of patients with MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes) have an A→G mutation at nucleotide 3243 in mitochondrial transfer (t)RNA. To date there have only been 10 reported cases of MELAS syndrome in patients with a T→C mutation at position 3271 of mitochondrial tRNA. Although many of the clinical features are similar between patients with these different mutations, it appears that the age at onset is later for the 3271 mutation. This report provides information from a North American kindred with the 3271 mutation (n = 6 proven; n = 2 probable; n = 3 possible) that adds clinical, physiological, histological, and molecular information to the pool of information on this rare disorder. Many of these features were similar to previous reports of both 3243 and 3271 patients. We conclude that the phenotypic expression of these different mutations are similar, but the age of onset for 3271 patients is later than for 3243 patients. © 1998 John Wiley & Sons, Inc. Muscle Nerve, 21: 25–33, 1998.     

线粒体脑肌病伴高乳酸血症和脑卒中样发作综合征一家系临床特征及线粒体基因A3243G位点点突变异质性水平

目的 调查1个疑似患有母系遗传性线粒体脑肌病伴高乳酸血症和脑卒中样发作(MELAS)综合征家系的临床表现、生物化学检测数据和影像学资料,并探索其与血细胞线粒体基因突变异质性水平的关联性.方法 收集先证者和11位其母系家系成员的一般情况、抽搐及脑卒中样发作等病史,检测家系成员的血常规和运动前后血浆乳酸水平等生化指标,并做头颅磁共振检查.用聚合酶链反应(PCR)-限制性内切酶片段长度多态和DNA测序法检测其成员是否存在线粒体基因组A3243G点突变,并用荧光实时定量PCR定量该突变的水平.结果 该家系部分成员存在抽搐、脑卒中样发作和高乳酸血症等MELAS综合征典型症状,以及身材矮小、运动不耐受和发热、偏头痛等非典型症状.发作期头颅磁共振成像符合MELAS综合征的典型特点,且普遍存在小脑萎缩.母系亲属均存在线粒体基因的A3243G位点点突变,突变异质性水平越高,症状越典型且严重.结论 该调查家系确诊母系遗传性MELAS综合征,其致病基因为线粒体A3243G点突变.外周血血细胞线粒体基因突变异质性水平与亲缘关系、抽搐早现性和血乳酸值等临床表型存在相关性.     

A patient of MELAS with 3271 mutation with fatal outcome after alcohol intake]

A patient of MELAS is reported. A 28-year-old woman was admitted to Shimada Municipal Hospital because of nausea, vomiting, and right homonymous hemianopsia. She had past history of dizziness and convulsion. A brain magnetic resonance imaging showed an ischemic lesion in the left occipital lobe, which disappeared in the follow-up study. Laboratory examination indicated elevated lactate and pyruvate levels in both blood and cerebrospinal fluid. The muscle biopsy demonstrated ragged-red fibers and strongly SDH-reactive blood vessels. PCR-RFLP analysis of DNA extracted from her muscle and blood as well as her mother's blood revealed a T to C mutation at nucleophile position of 3271 in mitochondrial DNA. She was diagnosed as having MELAS and discharged. One year after the first admission, she re-visited our hospital because of three days' duration of fatigability and generalized muscle pain after alcohol intake. She had severe lactic acidosis, rhabdomyolysis and acute renal failure. Despite a continuous hemodialysis and other intensive efforts, the patient died 20 hours later. Alcohol intake has been reported to induce rhabdomyolysis in myopathy with mitochondrial DNA deletions. The course of this patient suggests that alcohol intake can be an aggravating factor also in MELAS.     

Mitochondrial dementia: a sporadic case of progressive cognitive and behavioral decline with hearing loss due to the rare m.3291T>C MELAS mutation

We report the case of a 23-year-old Italian female harboring the rare m.3291T>C mutation in the MT-TL1 gene, that encodes the mitochondrial transfer RNA for leucine 1 (UUA/G). MT-TL1 mutations usually cause the MELAS (mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes) syndrome. Our patient, however, suffered from a non-syndromic mitochondrial disorder (MID), clinically characterized by progressive cognitive and behavioral decline, and hearing loss; brain MRI disclosed diffuse supratentorial and infratentorial atrophy; EKG revealed a Wolff-Parkinson-White syndrome; combined neuroleptic and antidepressant treatment markedly improved her behavioral symptoms. This case expands the clinical spectrum of non-syndromic MIDs, and further confirms that no obvious genotype-phenotype correlation exists for the m.3291T>C DNA mutation; indeed, this mutation has been previously reported in a Japanese child, who suffered from MELAS, and in an Italian child, who presented an apparently isolated mild myopathy. Moreover, it supports the hypothesis that at least in MT-TL1-related MIDs, dementia may be caused by a progressive neurodegenerative process, rather than by injury accumulation due to stroke-like episodes. Finally, our case suggests that common neuroleptic and antidepressant drugs may be clinically efficacious in the management of psychiatric symptoms associated with MIDs.     

Content of mutant mitochondrial DNA and organ dysfunction in a patient with a MELAS subgroup of mitochondrial encephalomyopathies

A point mutation of mitochondrial tRNA^Leu(UUR) gene is responsible for a MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes) subgroup of mitochondrial encephalomyopathies. In most cases, the mutant mitochondrial DNA (mtDNA) coexists with normal mtDNA in a heteroplasmic manner. In order to quantify the content of mutant mtDNA, we developed a quantitative method of PCR. Using this method, the distribution of the mutant mtDNA was examined in 32 different tissues among 18 autopsied organs from a patient with MELAS, who had shown hypophyseal dysfunction. The percentage of the mutant mtDNA at nucleotide number 3243 in each tissue was ranged between 22% and 95%. The content of the mutant mtDNA was at the highest (95%) in the hypophysis and higher in the cerebral cortex than in the white matter. This study shows a possible correlation of tissue dysfunction with accumulation of the mutant mtDNA within the brain.     

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes correlates with heteroplasmic mutations of mitochondrial DNA 3243 A single-case genealogy analysis

BACKGROUND: Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) is a common syndrome of mitochondrial diseases caused primarily by a mutation from adenine to guanine at mitochondrial DNA 3243. However, the correlation between heteroplasmic mutations and clinical characteristics of hereditary MELAS syndrome is unclear. OBJECTIVE: To survey the clinical behaviors, biochemical outcomes, and imaging data in a patient with suspected MELAS syndrome by maternal inheritance, and to investigate the correlation with heteroplasmic mutations of hemocyte mitochondrial DNA. DESIGN, TIME AND SETTING: A case analysis based on hereditary family surgery was performed in the Enliang Hospital of Anshan, Taian County, and biochemical tests and gene diagnosis were erformed at the Department of Laboratory and Institute of Neurology, the First Affiliated Hospital of China Medical University, between March and September 2009. ARTICIPANTS: A 22-year-old female patient with MELAS syndrome was diagnosed in the First Affiliated Hospital of China Medical University in January, 2009. She had five males and seven females in her maternal family. METHODS: We obtained stroke and convulsion history in the patient and her family, as well as erforming routine blood tests, plasma lactic acid levels before and after movement, and magnetic resonance of the head. A mutation at m.3243A > G was verified using polymerase chain reaction-restriction fragment length polymorphism and DNA sequencing, and quantitated using real-time polymerase chain reaction. MAIN OUTCOME MEASURES: Correlation of clinical behaviors and biochemical outcomes, as well as imaging data with heteroplasmic mutations in family members with typical and atypical MELAS syndrome. RESULTS: Some family members had typical symptoms of convulsion, stroke, and MELAS syndrome, as well as atypical symptoms of microsomia, movement intolerance, febrile, and migraine. Magnetic resonance of the head was consistent with typical imaging data of MELAS syndrome during attacks, and family members showed cerebellar atrophy. A heteroplasmic mutation of mitochondrial DNA 3243 occurred in all family members, although higher levels caused severe typical symptoms. The age of first-onset convulsion was negatively correlated with level of heteroplasmic mutation (r= -0.852, P< 0.05), but lactic acid was positively correlated with mutation levels (before movement, r= 0.945, P< 0.001; after movement, r= 0.945, P< 0.001). CONCLUSION: MELAS syndrome was diagnosed in this family by maternal inheritance, and the etiological factor was a mutation of mitochondrial A3243G. The level of heteroplasmic mutation correlated with anticipated convulsion and lactic acid levels.     

线粒体基因G13513A突变导致的线粒体脑肌病六例临床表型分析

目的 报告6例mtDNA G13513A点突变引起的线粒体脑肌病患者的临床、影像学特点,总结mtDNA G13513A突变所致的线粒体病的临床表型.方法 对35例mtDNA常见突变(包括大片段缺失及A3243G、T3271C、A8344G、T8993G/C点突变)检查为阴性的线粒体脑肌病患者,用线粒体DNA全长测序和(或)聚合酶链反应-限制性片段长度多态法检测mtDNA G13513A点突变,分析阳性患者的临床特点,复习文献报道的mtDNA G13513A所致线粒体病的病例.结果 35例患者中有6例存在mtDNA G13513A突变.该6例患者均出现偏盲、轻偏瘫或偏身感觉障碍等卒中样发作表现,其中3例成人发病者以卒中样发作为主要症状,伴随癫痫、头痛、身材矮小、神经性耳聋等,头颅MRI显示以顶-枕-颢叶受累为主的大片病灶,符合成人型线粒体脑肌病伴高乳酸血症和卒中样发作(MELAS)的临床和影像学特点;3例青少年发病者除卒中样发作外,还有构音障碍、共济失调、眼外肌瘫痪等脑干受累的症状,MRI检查可见枕-颞叶大脑皮质非对称性病灶,以及双侧基底节和脑干的对称性病灶,符合青少年型MELAS-Leigh叠加综合征的临床和影像学特点.肌肉病理检查在5例患者发现不整红边纤维.经复习文献,发现mtDNA G13513A突变患者还存在婴幼儿型Leigh或Leigh样综合征表型.结论 mtDNA G13513A点突变是线粒体脑肌病较常见的致病性突变,主要导致Leigh综合征、MELAS-Leigh叠加综合征或MELAS综合征,其临床表型具有年龄依赖性.

Abstract：

Objective To report 6 Chinese patients with mitochondrial encephalomyopathy caused by mitochondrial DNA(mtDNA)G13513A mutation and discuss the mitochondrial phenotype associated with this mutation based on the data of our patient series as well as the reports by others.Methods Direct sequencing of polymerase chain reaction(PCR)products or PCR-RFLP analysis Was performed to screen mtDNA G13513A mutation in 35 cases with mitoehondrial encephalomyopathy.who carried no mtDNA common mutations(1arge 8eale deletion,A3243G,T3271 C,A8344G,or T8993G/C).The clinical features,MRI changes were retrospectively collected and analyzed.Published studies of all patients with mtDNA G13513A mutation were also reviewed.Results Six patients were identified carrying mtDNA G13513A mutation.All patients presented stroke-like episodes with hemianopsia.hemiparesis or hemiparesthesia.Three adult patients presented clinical and radiological features of adult-onset mitochondrial myopathy,encephalopathy,lactic acidosis,and stroke-like episodes(MELAS),including stroke-like episodes,epilepsy,headache,short stature,sensorineural deafness,multifocal lesions on parietal,occipital and temporal lobes on cranial MRI scans.Three iuvenile.onset patients presented the clinical and brain MRI features of MELAS-Leigh syndrome(LS)overlap syndrome.In addition to the stroke-like episodes,they also showed brain stem lesions with dysarthria,ataxia,and ophthalmopJegia. Brain MRI revealed asymmetrical lesions in the cortex of the oecipital and temporal lobes,as well as symmetrical lesions in the bilateral basal ganglia and brainstem.Muslce biopsy showed ragged redfibem in 5 patients.The infant-onset LS or Leigh-like syndrome with mtDNA G135 13A was described in the English literature.Conclusions mtDNA G13513A mutation is a common pathogenic mutmion for mitochondrial encephalomyopathy,which can result in Leigh syndrome,MELAS-LS overlap syndrome and adult MELAS.The onset of various phenotypes is relatively age-dependent.     

MELAS syndrome with mitochondrial tRNA(Leu(UUR)) gene mutation in a Chinese family.

The clinical features of a patient in a Chinese family with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS syndrome) are reported. The study revealed that hearing and visual impairments and miscarriages may be early clinical presentations in MELAS. A heteroplasmic A to G transition in the tRNA(Leu(UUR)) gene was noted at the nucleotide pair 3243 in the mitochondrial DNA of muscle, blood, and hair follicles of the proband and his maternal relatives. Quantitative analysis of the mutated mitochondrial DNA revealed variable proportions in different tissues and subjects of maternal lineage in the family. Muscle tissue contained a higher proportion of the mutant mitochondria than other tissues examined. The function of the reproductive system of the proband seems to be impaired. In one clinically healthy sibling, the 3243rd point mutation was found in sperm mitochondrial DNA, although sperm motility was not affected. It seems that biochemical defects in mitochondrial respiration and oxidative phosphorylation are tissue specific expressions of the 3243rd point mutation in the mitochondrial DNA of the affected target tissues.     

线粒体脑肌病伴有高乳酸血症和卒中样发作综合征的临床及基因突变分析

目的 探讨线粒体脑肌病伴有高乳酸血症和卒中样发作综合征(MELAS)的临床及基因突变特征. 方法 对1例MELAS患者的临床表现、影像学、肌肉病理特点进行分析,并用PCR-RFLP结合基因测序方法进行线粒体基因突变分析. 结果 患者主要临床表现为发作性头痛和呕吐、反复卒中样发作、癫痫、运动不耐受、身材矮小、神经性耳聋、乳酸水平升高等.脑CT见双侧基底节多个钙化灶,MRI见枕叶异常信号,1H-MRS见T2WI异常信号区域有明显的乳酸峰,在T2正常信号区域也有小的乳酸峰.光镜及电镜肌肉病理检查未见明显的线粒体异常,基因检测显示mtDNA A3243G杂合突变. 结论 MELAS的诊断必须结合临床表现、影像学、病理学和基因突变检测等结果进行综合分析,病理学检查阴性不能否定MELAS的诊断,诊断MELAS应常规进行mtDNA突变分析.     

Single-fiber analysis of mitochondrial A3243G mutation in four different phenotypes

Five unrelated patients harboring the A3243G mutation in the mitochondrial DNA (mtDNA) but presenting with different clinical phenotype were studied for their percentage of mutation at the single muscle fiber levels. One patient had a clinically and pathologically defined Leigh syndrome (LS), two showed mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS), another showed progressive external ophthalmoplegia (PEO), and the other showed mitochondrial diabetes mellitus (MDM). The mutation load was greater in the muscle from the patient with LS (92%), who showed more than 80% even in the non-ragged red fibers (RRF) and also presented the highest proportion of RRF. The patients with MELAS had lower mutation levels as well as a lower proportion of RRF, and these two parameters were even lower in the PEO and MDM patients. These results were consistent with the concept that differences in the mutation load and in the somatic distribution of the mutation among different cells and tissues are responsible for the differences in phenotypical expression of the disease. Received: 8 April 1999 / Revised, accepted: 28 June 1999     

MELAS: clinical features, biochemistry, and molecular genetics.

We studied 23 patients with clinically defined mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), 25 oligosymptomatic or asymptomatic maternal relatives, and 50 mitochondrial disease control subjects for the presence of a previously reported heteroplasmic point mutation at nt 3,243 in the transfer RNA(Leu(UUR)) gene of mitochondrial DNA. We found a high concordance between clinical diagnosis of MELAS and transfer RNA(Leu(UUR)) mutation, which was present in 21 of the 23 patients with MELAS, all 11 oligosymptomatic and 12 of 14 asymptomatic relatives, but in only five of 50 patients without MELAS. The proportion of mutant genomes in muscle ranged from 56 to 95% and was significantly higher in the patients with MELAS than in their oligosymptomatic or asymptomatic relatives. In subjects in whom both muscle and blood were studied, the percentage of mutations was significantly lower in blood and was not detected in three of 12 asymptomatic relatives. The activities of complexes I + III, II + III, and IV were decreased in muscle biopsies harboring the mutation, but there was no clear correlation between percentage of mutant mitochondrial DNAs and severity of the biochemical defect.     

The mitochondrial DNA G13513A transition in ND5 is associated with a LHON/MELAS overlap syndrome and may be a frequent cause of MELAS

We report on 4 male patients with clinical, radiological, and muscle biopsy findings typical of the mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) phenotype. Skeletal muscle mitochondrial DNA (mtDNA) analysis showed that all patients harbored a heteroplasmic G13513A mutation in the ND5 subunit gene. One of these cases (Patient 1) presented with symptoms characteristic of Leber's hereditary optic neuropathy (LHON) 2 years before the first stroke-like episode. Quantitative analysis in several postmortem tissue sections showed that the relative proportions of mutant mtDNA were generally lower than those reported with other pathogenic mtDNA mutations. Single-fiber polymerase chain reaction studies demonstrated significantly higher amounts of mutant mtDNA in ragged red fibers (RRFs) compared with non-RRFs. This study indicates that the G13513A transition is likely to be pathogenic, that it can cause an LHON/MELAS overlap syndrome, and that it may be a more frequent cause of MELAS than previously recognized.     

The mutations m.5628T>C and m.8348A>G in single muscle fibers of a patient with chronic progressive external ophthalmoplegia

We identified a double mutation in a patient with chronic progressive external ophthalmoplegia, located in the tRNA(Ala) (m.5628T>C) and tRNA(Lys) (m.8348A>G) genes. Both mutations were previously described separately and considered pathogenic, however the same mutations were also reported as polymorphisms or phenotype modulator. We analyzed the proportion of each mutation in isolated muscle fibers by single fiber-polymerase chain reaction to investigate the contribution of each mutation to mitochondrial deficiency. Our findings demonstrated that the mutations were heteroplasmic in skeletal muscle and both mutations were present in all single muscle fibers. The proportions of the m.5628T>C mutation were not significantly different between normal and cytochrome-c-oxidase (COX) deficient fibers. However, a significant higher proportion of the m.8348A>G mutation was observed in COX deficient fibers. Homoplasmic m.8348A>G was only observed in COX negative fibers. In conclusion, we provide a piece of evidence toward the pathogenicity of the m.8348A>G mutation and suggest that m.5628T>C is probably a neutral polymorphism.