A novel mitochondrial ATPase 6 point mutation in familial bilateral striatal necrosis

A T-to-C transition at nucleotide (nt) 9176 in the mitochondrial adenosine triphosphatase 6 (ATPase 6) gene was detected in 2 brothers with a neurological disorder resembling Leigh syndrome. The mutation was also present in the 2 other siblings and in the mother, who were asymptomatic. In the more severely affected boy (the proband), the mutation was homoplasmic in muscle, leucocytes, and fibroblasts. In leucocytes from his affected brother, 98% of mtDNA was mutant. Heteroplasmy of varying degrees was seen in leucocytes from the mother and the 2 unaffected siblings. The mutation changes a highly conserved leucine residue near the carboxyl terminus of the mitochondrial ATPase 6 subunit to proline. It could not be detected in 168 control subjects. Studies of ATP synthesis and hydrolysis in fibroblasts from the proband were normal.     

Mitochondrial myopathy and respiratory failure associated with a new mutation in the mitochondrial transfer ribonucleic acid glutamic acid gene

We report a novel T14687C mutation in the mitochondrial transfer ribonucleic acid glutamic acid gene in a 16-year-old boy with myopathy and lactic acidosis, retinopathy, and progressive respiratory failure leading to death. A muscle biopsy showed cytochrome c oxidase-negative ragged-red fibers, and biochemical analysis of the respiratory chain enzymes in muscle homogenate revealed complex I and complex IV deficiencies. The mutation, which affects the trinucleotide (TpsiC) loop, was nearly homoplasmic in the muscle DNA of the proband, but it was absent in his blood and in the blood from the asymptomatic mother, suggesting that it may have been a spontaneous somatic mutation in muscle.     

Rapidly progressive neurodegeneration in a case with the 7472insC mutation and the A7472C polymorphism in the mtDNA tRNA ser(UCN) gene

The authors report the clinical, neuroimaging, muscle biopsy and mtDNA findings in a patient affected by bilateral hearing loss and mental retardation since infancy, presenting at age 31 years with a rapid deterioration of mental status and ataxia leading to vegetative condition and death at the age of 32 years. Clinical and genetic studies have been also performed in the mother, affected by neurosensorial hearing loss. Muscle biopsy showed severe mitochondrial alterations in the propositus and evidence of mitochondrial alterations in his mother. Direct mtDNA sequencing in all family members revealed the known 7472insC mutation and the recently described A7472C sequence variation in the tRNA(Ser(UCN))gene. RFLP-PCR confirmed the heteroplasmic nature of the two mutations and failed to find the second transversion in 200 controls. The percentage of mutant genomes harbouring 7472insC ranged from 3 to 7% in asymptomatic family members to 70% in the proband and his mother, whereas the percentage of A7472C mutant genomes was about 90% in all maternal relatives except the proband (56%) and his sister (5%). In conclusion, this is the first report of a rapidly progressive encephalopathy in association with the 7472insC mutation in mtDNA, combined with an A>C variation at the same nucleotide with a possible suppression effect on the pathogenic mutation.     

Complex neurologic syndrome associated with the G1606A mutation of mitochondrial DNA

OBJECTIVES: To confirm the pathogenicity of the G-to-A substitution at nucleotide 1606 (G1606A) mutation in the mitochondrial DNA (mtDNA) tRNA(Val) gene, and to characterize genotype-phenotype correlation. PATIENT AND METHODS: A 37-year-old man since childhood developed a complex clinical picture characterized by hearing loss, migraine, ataxia, seizures, cataracts, retinitis pigmentosa, mental deterioration, and hypothyroidism. Magnetic resonance imaging revealed diffuse calcification of the basal ganglia and cerebral cortical atrophy. Morphologic and biochemical studies of respiratory chain complexes were performed in skeletal muscle. All 22 mitochondrial tRNA genes were screened for mutations by direct sequencing. RESULTS: Biochemical analysis showed normal activities of respiratory chain enzymes and citrate synthase; morphologic examination showed scattered ragged-red fibers and poor or absent cytochrome c oxidase staining in 10% of the fibers. A heteroplasmic G1606A transition in the mtDNA tRNA(Val) gene was found. Mutant DNA was 70% of the total in the proband's muscle. The mutation was absent in blood samples and urinary sediment from his healthy brother and mother. CONCLUSION: This second patient with the G1606A mutation confirms both the pathogenicity of the mutation and its association with a characteristic complex neurologic phenotype.     

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.     

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).     

Leber's hereditary optic neuropathy and complex I deficiency in muscle.

We investigated a family with Leber's hereditary optic neuropathy in which affected individuals were homoplasmic for the point mutation of the NADH-dehydrogenase 4 gene of mitochondrial DNA, described by Wallace and colleagues in 1988. The proband had bilateral optic atrophy, tremor, dystonia, and sharply defined lesions in the putamen on magnetic resonance images. Optic atrophy was found in another 3 of 13 investigated relatives on the maternal side. Additional neurological signs were found but only in patients with optic neuropathy. The morphological appearance and the respiratory chain function of muscle tissue were investigated in the proband, his mother, and 3 siblings. Polarographic measurements revealed complex I deficiency in the 5 investigated subjects. Morphological changes of mitochondria were found in 4 of these subjects. There was no decrease in complex I activity measured as NADH ferricyanide reductase or rotenone-sensitive NADH cytochrome c reductase activities. In other cases with complex I deficiency, good agreement between polarographic and spectrophotometric measurements was found. This study showed that there is decreased activity of complex I of the respiratory chain in muscle and that cerebral striatal lesions occur in Leber's hereditary optic neuropathy with the NADH-dehydrogenase 4 gene point mutation.     

A novel heteroplasmic tRNA(Ser(UCN)) mtDNA point mutation associated with progressive external ophthalmoplegia and hearing loss

We sequenced all mitochondrial tRNA genes from a patient with sporadic external ophthalmoplegia (PEO) and 5% COX-negative fibers in muscle biopsy, who had no detectable large mtDNA deletions. Direct sequencing showed a heteroplasmic mutation at nucleotide 7506 in the dihydrouridine stem of the tRNA(Ser(UCN)) gene. RFLP analysis confirmed that 30% of muscle and 20% of urinary epithelium mtDNA harbored the mutation, which was absent in other tissues of the proband as well as in mtDNA of his mother and 100 patients with various encephalomyopathies. Several point mutations on mitochondrial tRNA genes have been reported in PEO patients without large-scale rearrangements of mtDNA but no point mutations have hitherto been found in the gene coding for tRNA(Ser(UCN)).     

Clinical heterogeneity in two pedigrees with the 3243 bp tRNALeu(UUR) mutation of mitochondrial DNA

We studied two pedigrees with a mutation at the nucleotide 3243 of mitochondrial DNA (mtDNA). The proband from the first pedigree had clinically defined MELAS plus maternally transmitted insulin-dependent diabetes mellitus (IDDM). The propositus of the other pedigree had exercise intolerance, lactic acidosis and ragged-red fibers (RRF). In the first pedigree, both the mother and the sister's proband harbored the point mutation in their muscle. The mother had 40% of mutant mitochondrial genomes and the sister 70%. In the second pedigree, the mutation was present in both muscle and blood from the proband as well as in blood from all other members studied. Proportion of mutant mtDNA was 90% in muscle and ranged from 40% to 90% in blood.     

Complete mitochondrial DNA sequence analysis in a family with early-onset dystonia and optic atrophy.

The combination of optic atrophy and dystonia has been etiologically associated with mitochondrial DNA (mtDNA) mutations. We report here on the complete mtDNA sequence from the proband of a consanguineous family exhibiting "mitochondrial-like" optic atrophy and dystonia. A candidate tRNA(Gly) mutation was identified that was unique to the family. However, the mutation was homoplasmic in both affected and unaffected family members and we were unable to demonstrate a biochemical defect in patient mitochondria. Hence, it is unlikely that a mtDNA mutation accounts for the phenotype in this family.     

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.     

Maternally inherited encephalopathy associated with a single-base insertion in the mitochondrial tRNATrp gene

We identified a single thymidine inserton at nucleotide postion 5537 (T⁵⁵³⁷ⁱ) in the mitochondrial DNA transfer RNA gene for tryptophan in a family in which the proband had a progressive neurological disorder and his brother died in infancy of Leigh syndrome. Muscle biopsy from the proband showed subsarcolemmal proliferation of mitochondria and ecreased activities of oxidative metabolism enzymes, in particular complex IV. Complex IV was also severely reduced in autopsy tissues, including heart and brain tissues, from the Leigh syndrome infant. The novel T⁵⁵³⁷ⁱ mutaion was very abundant in tissues from the proband and the infant (>92%) and less abundant (range, 42–89%) in blood, hair follicles, and skin fibroblasts from 4 maternal relatives, 3 of whom showed a neuropsychiatric disturbance. The mutation was not found in more than 100 control subjects. The degree of heteroplasmy in blood correlated well with the severity of the clinical presentation, suggesting specific segregation with the disease.     

Novel heteroplasmic mutation in the anticodon stem of mitochondrial tRNALys associated with dystonia and stroke‐like episodes

Gal A, Pentelenyi K, Remenyi V, Pal Z, Csanyi B, Tomory G, Rasko I, Molnar MJ. Novel heteroplasmic mutation in the anticodon stem of mitochondrial tRNA^Lys associated with dystonia and stroke‐like episodes.
Acta Neurol Scand: 2010: 122: 252–256.
© 2009 The Authors Journal compilation © 2009 Blackwell Munksgaard. Objectives – We report a novel heteroplasmic mitochondrial tRNA^Lys mutation associated with dystonia, stroke‐like episodes, sensorineural hearing loss and epilepsy in a Hungarian family. Material and methods – A 16‐year‐old boy, his brother and mother were investigated. Thorough clinical investigation as well as electrophysiological, neuroradiological and myopathological examinations were performed. Molecular studies included the analysis of the DYT1, DDP1/TIMM8A (deafness‐dystonia peptid‐1) genes and mitochondrial DNA (mtDNA). Results –  The mtDNA analysis of the proband revealed a heteroplasmic A8332G substitution in the anticodon stem of the tRNA^Lys gene. The mutation segregated in all affected family members. Besides this mutation 16 further mtDNA polymorphisms were detected. Complex I activity of the patient’s fibroblast cultures showed decreased activity confirming mitochondrial dysfunction. Conclusion –  The novel A8332G heteroplasmic mutation is most likely a new cause of dystonia and stroke‐like episodes due to mitochondrial encephalopathy. The synergistic effect of the G8697A, A11812G and T10463C single nucleotide polymorphisms may modify the phenotype.     

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.     

A novel mutation (8342G-->A) in the mitochondrial tRNA(Lys) gene associated with progressive external ophthalmoplegia and myoclonus

We describe a patient who suffered from impaired ocular motility from age 10 years and at 16 years developed ptosis, proximal weakness and progressive fatigability. At 35 years she developed massive myoclonic jerks, and head and distal tremor. A muscle biopsy showed a high percentage of cytochrome c oxidase negative fibers but no ragged-red fibers. A novel heteroplasmic mutation (8342G-->A) was found in the mitochondrial transfer RNA(Lys) gene by single-strand conformation polymorphism screening, followed by sequence and restriction fragment length polymorphism analysis. Approximately 80% of muscle mitochondrial DNA (mtDNA) harbored the mutation, while the mutation was absent in lymphocyte DNA of the proband, as well as of her mother, daughter and a maternal aunt. However, the pathogenicity of the mutation was confirmed by restriction fragment length polymorphism analysis of single muscle fibers, which revealed a significantly greater level of mutant mtDNA in cytochrome c oxidase negative over cytochrome c oxidase positive fibers.     

A novel mutation in the mitochondrial tRNAVal gene associated with a complex neurological presentation

We describe a patient who presented with progressive ataxia, sezures, mental deterioration, mild myopathy, and hearing loss. A novel heteroplasmic G-to-A transition was found, affecting the acceptor stem of the mitochondrial (mt) tRNA^Val gene. Mutant mtDNA was 67% of total mtDNA in the muscle of the proband and was also present at low levels in the muscle of his healthy mother. It was absent in all of the numerous control DNA samples that were tested. Analysis of single muscle fibers revealed a significantly greater level of mutant mtDNA in cytochrome c oxidase-negative fibers. Mutations of mtDNA may be responsible of neurological syndromes that, like the case reported here, are clinically puzzling, and lack typical “mitochondrial” clues, such as elevated levels of blood lactate, overt defects of the respiratory complexes, and clinically documented maternal inheritance.     

Mitochondrial DNA deletion in a child with mitochondrial encephalomyopathy, growth hormone deficiency, and hypoparathyroidism

We report an 11-year-old boy with short stature, bilateral ptosis, sensorineural hearing loss, muscle weakness, and endocrine abnormalities. Brain magnetic resonance imaging (MRI) showed a bilateral abnormal signal in the globus pallidus and in the midbrain tegment. Muscle biopsy specimens showed ragged red and cytochrome c oxidase negative fibers, and biochemical analysis of muscle homogenate showed a partial defect of complex I and IV activities of the respiratory chain enzymes. Analysis of mitochondrial DNA by a polymerase chain reaction screening procedure and Southern blot revealed a novel heteroplasmic single mitochondrial DNA deletion of 7.8 kb in different tissues. This deletion was absent in the blood DNA of his mother and brother. This case further expands and confirms the wide clinical spectrum of mitochondrial disorders associated with single large-scale mitochondrial DNA deletions.     

Phenotypes and mitochondrial DNA substitutions in families with A3243G mutation

OBJECTIVE: To clarify the relationship between mitochondrial DNA (mtDNA) sequence variations and phenotypes in patients with A3243G mutation. MATERIALS AND METHODS: We studied whole mtDNA sequences in two families with A3243G mutation and characteristic clinical features. Two brothers in Family 1 had shown thiamine deficiency and mitochondrial myopathy without central nervous system involvement. In Family 2, a 16-year-old woman showed the symptoms of mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes (MELAS). Her mother had had diabetes mellitus and died at the age of 42. The proportion of A3243G mtDNA in blood was 87 and 89% in the patients of Family 1, and 25% in the patient and less than 5% in the mother of Family 2. RESULTS: The mtDNA analysis revealed the following homoplasmic substitutions: T1520C and C12153T found only in Family 1, and A15954G found only in Family 2. These substitutions were not detected in seven other MELAS patients or in 50 controls. CONCLUSION: These substitutions might be specific to these families and could be one of the factors that modulate their clinical features together with the A3243G mutation.     

Mitochondrial myopathy with exercise intolerance and retinal dystrophy in a sporadic patient with a G583A mutation in the mt tRNA(phe) gene

We describe a second patient with the 583G>A mutation in the tRNA(phe) gene of mitochondrial DNA (mtDNA). This 17-year-old girl had a mitochondrial myopathy with exercise intolerance and an asymptomatic retinopathy. Muscle investigations showed occasional ragged red fibers, 30% cytochrome c oxidase (COX)-negative fibers, and reduced activities of complex I+IV in the respiratory chain. The mutation was heteroplasmic (79%) in muscle but undetectable in other tissues. Analysis of single muscle fibers revealed a significantly higher level of mutated mtDNA in COX-negative fibers. Our study indicates that the 583G>A mutation is pathogenic and expands the clinical spectrum of this mutation.     

Very low levels of the mtDNA A3243G mutation associated with mitochondrial dysfunction in vivo

We studied mitochondrial function in vivo in 2 brothers harboring the mitochondrial DNA A3243G mutation by using magnetic resonance spectroscopy. One brother presented with recurrent strokes and had a mitochondrial respiratory chain complex I defect, with 85% A3243G mutation in his quadriceps. The maximum rate of mitochondrial ATP production in his calf, measured in vivo, was reduced to 21% of the normal mean value. The second brother had mild exercise intolerance, normal muscle histochemistry, and normal respiratory chain activity in vitro. Despite a level of the A3243G mutation of only 5.95% (SD, 4.45; range, 0.7-16.1%) within single muscle fibers from the gastrocnemius muscle, the maximum rate of mitochondrial ATP production in his calf, measured in vivo, was reduced to 35% of the normal mean value. These findings suggest that there may not be a clear genetic threshold level for the expression of the A3243G mutation in skeletal muscle in vivo.