MERRF/MELAS overlap syndrome associated with 3243 tRNALeu(UUR) mutation of mitochondrial DNA

We report a case of myoclonus epilepsy associated with ragged-red fibers (MERRF)/mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episode (MELAS) overlap syndrome with hearing loss, external ophthalmoplegia, and myoclonus epilepsy in addition to stroke-like episode and diabetes mellitus. Pathologically, there was degeneration in the dentate nuclei, substantia nigra, red nucleus, and subthalamic nucleus which has been reported as characteristic of MERRF, as well as necrotic lesions of various stages in the cerebral cortex, characteristic of MELAS. The gene study disclosed 3243 mutation in the tRNA^Leu(UUR) gene of mitochondrial DNA. This case is the first neuropathological report of MERRF/MELAS overlap syndrome verified by gene analyses.     

Sporadic MERRF/MELAS overlap syndrome associated with the 3243 tRNALeu(UUR) mutation of mitochondrial DNA

We studied a patient with a mitochondrial encephalomyopathy characterized by the presence of all the cardinal features of both myoclonic epilepsy and ragged-red fibers (MERRF) and mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndromes. Muscle biopsy showed ragged-red fibers (RRF). Some RRF were cytochrome c oxidase (COX)-negative, while some others stained positive for COX. Muscle biochemistry revealed defects of complexes I and IV of the respiratory chain. Both muscle and blood mitochondrial DNA from the patient showed the presence of the mutation at nucleotide position 3243 in the tRNA^Leu(UUR) gene and the absence of point mutations related to MERRF syndrome. The proportions of mutant mtDNA were 70% in muscle and 30% in blood. The mutation was absent in blood from all maternal relatives, in hair follicles from the mother, and in muscle from one sister of the proband. Therefore, there was no evidence of maternal inheritance. © 1996 John Wiley & Sons, Inc.     

Novel cluster of tRNALeu(UUR) mutations in a sporadic case of infantile myopathy restricted to muscle tissue

In a previous study we reported on a case with severe infantile, mitochondrial myopathy caused by somatic mutation [12]. In the present study we give evidence for asymmetric tissue distribution of the mutations. Mitochondrial DNA (mtDNA) analysis showed a cluster of nearly homoplasmic point mutations in the tRNA gene for leucine (UUR) (A3259 G, A3261 G, A3266 G, A3268 G). The mutation is abundant in muscle, but is not found in blood cells. This cluster of mutations is sporadic, because the search for mutant molecules in the blood of the healthy mother and maternal grandmother did not show these alterations.     

Genetic, pathogenetic, and phenotypic implications of the mitochondrial A3243G tRNALeu(UUR) mutation

Mitochondrial disorders are frequently caused by mutations in mitochondrial genes and usually present as multisystem disease. One of the most frequent mitochondrial mutations is the A3,243G transition in the tRNALeu(UUR) gene. The phenotypic expression of the mutation is variable and comprises syndromic or non-syndromic mitochondrial disorders. Among the syndromic manifestations the mitochondrial encephalopathy, lactacidosis, and stroke-like episode (MELAS) syndrome is the most frequent. In single cases the A3,243G mutation may be associated with maternally inherited diabetes and deafness syndrome, myoclonic epilepsy and ragged-red fibers (MERRF) syndrome, MELAS/MERRF overlap syndrome, maternally inherited Leigh syndrome, chronic external ophthalmoplegia, or Kearns-Sayre syndrome. The wide phenotypic variability of the mutation is explained by the peculiarities of the mitochondrial DNA, such as heteroplasmy and mitotic segregation, resulting in different mutation loads in different tissues and family members. Moreover, there is some evidence that additional mtDNA sequence variations (polymorphisms, haplotypes) influence the phenotype of the A3,243G mutation. This review aims to give an overview on the actual knowledge about the genetic, pathogenetic, and phenotypic implications of the A3,243G mtDNA mutation.     

A novel mutation in the mitochondrial DNA tRNA Leu (UUR) gene associated with late-onset ocular myopathy

We identified a novel G3283A transition in the mitochondrial DNA tRNA(Leu (UUR)) gene in a patient with ptosis, ophthalmoparesis and hyporeflexia. Muscle biopsy showed cytochrome oxidase positive ragged-red fibers, and defects of complexes I, III and IV of the mitochondrial respiratory chain. The mutation was heteroplasmic in muscle of the proband, being absent in her blood. Ragged-red fibers harbored greater levels of mutant genomes than normal fibers. The G3283A mutation affects a strictly conserved base pair in the TPsiC stem of the gene and was not found in controls, thus satisfying the accepted criteria for pathogenicity.     

Increased risk for cardiorespiratory failure associated with the A3302G mutation in the mitochondrial DNA encoded tRNALeu(UUR) gene

Screening the mitochondrial DNA of a 64-year-old woman with mitochondrial myopathy revealed 76% of the tRNA(Leu(UUR)) A3302G mutation in muscle. Muscle of her affected son carried 96% mutated mitochondrial DNA. Both patients were biopsied twice, showing isolated complex I deficiency in the son's first biopsy, additional increased (within normal range) complex II + III activities in his second biopsy, combined complex I, II + III deficiency in mothers first biopsy and additional complex IV deficiency in her second biopsy. After a stay in the mountains, the son died of cardiac arrhythmia. The A3302G mutation has been reported before and is associated with mitochondrial myopathy and cardiorespiratory failure. Pathogenesis is explained by abnormal mtRNA processing, which was also reported for the adjacent C3303T mutation associated with cardiomyopathy and/or skeletal myopathy. Our findings suggest that a high mutation load of the A3302G mutation can lead to fatal cardiorespiratory failure, likely triggered by low environmental oxygen pressure and exercise.     

The A to G transition at nt 3243 of the mitochondrial tRNALeu(UUR) may cause an MERRF syndrome.

OBJECTIVE--To verify the phenotype to genotype correlations of mitochondrial DNA (mtDNA) related disorders in an atypical maternally inherited encephalomyopathy. METHODS--Neuroradiological, morphological, biochemical, and molecular genetic analyses were performed on the affected members of a pedigree harbouring the heteroplasmic A to G transition at nucleotide 3243 of the mitochondrial tRNALeu(UUR), which is usually associated with the syndrome of mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS). RESULTS--The proband was affected by a fullblown syndrome of myoclonic epilepsy with ragged red fibres (MERRF), severe brain atrophy, and basal ganglia calcifications, without the MRI T2 hyperintense focal lesions which are pathognomonic of MELAS. Oligosymptomatic relatives were variably affected by lipomas, goitre, brain atrophy, and basal ganglia calcifications. Muscle biopsies in the proband and his mother showed a MELAS-like pattern with cytochrome c oxidase hyperreactive ragged red fibres and strongly succinate dehydrogenase reactive vessels. Quantification of the A3243G mutation disclosed 78% and 70% of mutated mtDNA in the muscle of the severely affected proband and of his oligosymptomatic mother respectively. Nucleotide sequencing of the mitochondrial tRNALeu(UUR) and tRNALys in the proband's muscle failed to show any additional nucleotide change which could account for the clinical oddity of this pedigree by modulating the expression of the primary pathogenic mutation. CONCLUSION--So far, MERRF has been associated with mutations of the mitochondrial tRNALys, and MELAS with mutations of the mitochondrial tRNALeu(UUR). Now MERRF may also be considered among the clinical syndromes associated with the A to G transition at nt 3243 of the tRNALeu(UUR).     

Multimodal evoked potentials in progressive external ophthalmoplegia with mitochondrial myopathy

Multimodal evoked potentials were studied in 13 patients affected by progressive external ophthalmoplegia with histologically proven mitochondrial myopathy. Progressive external ophthalmoplegia occurred with craniosomatic spreading in all the patients and with a varying degree of nervous and/or other system involvement in most of them. In all but one of the subjects, at least one evoked potential modality was abnormal; 11 of them demonstrated an abnormal visual evoked potential, but this finding might have been influenced by concurrent retinal dysfunction. Abnormalities in brainstem auditory evoked potentials and/or somatosensory evoked potentials, revealing an impairment of central sensory pathways, were detected in 7 subjects, 5 of whom lacked clinical evidence of central nervous system involvement. Thus, evoked potentials represent an useful tool for the detection of subclinical central nervous system involvement in patients affected by progressive external ophthalmoplegia with mitochondrial myopathy.     

Early mitochondrial changes in chronic progressive ocular myopathy

Two sisters with chronic progressive external ophthalmoplegia (CPEO) and their in all 7 healthy children were investigated. Both ophthalmoplegic patients had histopathological changes typical of mitochondrial myopathy. The same type of muscular pathology was also found among the healthy children. The most common muscular changes were subsarcolemmal accumulation of pathological mitochondria, including vacuoles, abnormal cristae and sometimes also inclusion bodies. Biochemical studies showed partial complex III deficiency, with low succinate-cytochrome c reductase activity in 1 of the ophthalmoplegic patients. These findings suggest that CPEO is a slowly progressive muscle disease, starting early in life. The widespread occurrence among the children may indicate maternal inheritance.     

A new mitochondrial DNA mutation (A3288G) in the tRNA(Leu(UUR)) gene associated with familial myopathy.

We describe a family with a maternally inherited mitochondrial myopathy and an A3288G mutation in the tRNA(Leu(UUR)) gene. The proband had muscle cramping and mild weakness while her brother had long-standing limb and respiratory muscle weakness and her daughter had elevated serum CK. The mutation, which was nearly homoplasmic in muscle and heteroplasmic in blood, affects the TpsiC loop at a conserved site and was not found in 107 controls. This report confirms the frequent association of tRNA(Leu(UUR)) mutations with respiratory muscle involvement and bolsters the concept that tRNA(Leu(UUR)) is a hotspot for mtDNA mutations.     

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.     

慢性进行性眼外肌瘫痪型线粒体肌病(附3个家系报告)

目的 探讨家族性慢性进行性眼外肌瘫痪（CPEO）型线粒体肌病的临床、遗传和病理特点。方法 回顾性分析CPEO型线粒体肌病3个家系21例患者的临床表现、家系调查及5例肌活检病理学资料。结果 患者临床均表现为眼睑下垂和眼球运动障碍,伴或不伴有肌无力。1家系符合常染色体显性遗传规律,另2个家系符合母系遗传规律。病理改变：光镜下为破碎红纤维（RRFs）和细胞色素C氧化酶（COX）缺失纤维;电镜为肌膜下、肌原纤维间线粒体数量增多,嵴内可见电子致密颗粒或晶格样包涵体。结论 3个家系及其亲子代问临床与病理表现相似,提示不同遗传方式所致CPEO型线粒体肌病临床表现是相同的。     

Mitochondrial myopathy of childhood associated with depletion of mitochondrial DNA.

We have studied five children with mitochondrial myopathy manifesting within or soon after the first year of life. Muscle biopsies showed ragged-red fibers and decreased respiratory chain activity. All five patients had a severe decrease (2 to 34% of normal) in the amount of muscle mitochondrial DNA (mtDNA). The depletion of mtDNA correlated with absence of mtDNA-encoded translation products and with loss of cytochrome c oxidase enzyme activity in individual muscle fibers. This mitochondrial myopathy of childhood illustrates one phenotypic expression of a novel pathogenetic mechanism in mitochondrial diseases, the specific depletion of mtDNA in affected tissues.     

Mitochondrial myopathy of cerebro-hepato-renal (Zellweger) syndrome

The muscles of four infants with cerebro-hepato-renal (Zellweger) syndrome were studied during life and/or at necropsy. A mitochondrial myopathy was demonstrated, similar to mitochondrial alterations demonstrated in liver and brain in this disease. Muscle fibers with red-staining subsarcolemmal aggregates were identified with Gomori trichrome stain in two cases. Subsarcolemmal and intermyofibrillar zones of increased concentrations of NADH-TR, SDH, and cytochrome-c-oxidase activity were demonstrated histochemically in all four cases. Degenerative and cytoarchitectural changes in muscle fibers were not found. Ultrastructural studies showed large aggregates of mitochondria and increased lipid in the subsarcolemmal and intermyofibrillar spaces. Degenerative changes in mitochondria and lipid also were demonstrated, but paracrystalline inclusions were not seen. The distribution of these changes was not uniform between patients or between different muscles in the same patient. The diaphragm was affected more severely than proximal or distal muscles of the extremities. Direct involvement of muscle mitochondria in this disease may interfere with energy metabolism and contribute to the clinical findings of hypotonia, weakness, and respiratory insufficiency. The muscle biopsy with histochemistry and electron microscopy may be used as a diagnostic adjunct in suspected cases, but the variation encountered dictates dictates caution in the interpretation of negative findings.     

A novel mutation in the mitochondrial tRNA(Phe) gene associated with mitochondrial myopathy

We report a novel heteroplasmic T-->C mutation at nt position 582 within the mitochondrial tRNA(Phe) gene of a 70-year-old woman with mitochondrial myopathy. No other family members were affected, suggesting that our patient was a sporadic case. The muscle showed frequent ragged red fibers and 43% cytochrome c oxidase deficient fibers. The mutation alters a conserved base pairing in the aminoacyl acceptor stem. The mutation load was 70% in muscle homogenate and varied from 0 to 95% in individual muscle fiber segments. Cytochrome c oxidase-negative fibers showed significantly higher levels of mutated mtDNA (>75%) than Cytochrome c oxidase-positive fibers (<55%). This mutation adds to the previously described four pathogenic mutations in the tRNA(Phe) gene.