Exercise intolerance due to cytochrome b mutation

Cytochrome b mutations are rare causes of exercise intolerance. We report an 18‐year‐old man with exercise intolerance since childhood, resting lactic acidosis, cytochrome c oxidase (COX)‐positive ragged‐red fibers, and isolated muscle complex III deficiency due to a heteroplasmic m.14849T>C mutation in cytochrome b. We review previously described patients carrying mutations in the same gene. COX‐positive ragged‐red fibers together with exercise intolerance and lactic acidemia provide a clue for the diagnosis of this rare mitochondrial disorder. Muscle Nerve, 2010     

Ragged Red Fibers in Normal Aging and Inflammatory Myopathy

Ragged red fibers are an important marker for mitochondrial disease. To evaluate the hypothesis that mitochondrial dysfunction may play a role in the pathogensis of aging and inclusion body myositis, we studied the frequency of ragged red fibers in muscle biopsy specimens from 15 young and 13 old normal adults, and from 27 patients with inclusion body myositis, polymyositis, or dermatomyositis. Serial transverse cryostat sections were stained with modified Gomori trichrome, modified succinic dehydrogenase, and cytochrome c oxidase. The frequency of ragged red fibers, determined by measuring the percent number of succinic dehydrogenase–positive ragged red fiber equivalents, was significantly higher in old compared to young normal subjects (0.33 vs. 0.02%, p < 0.0001). With the exception of a single polymyositis biopsy specimen showing a large number of ragged red fibers, the frequency of ragged red fibers in patients with polymyositis or dermatomyositis was similar to that of age-matched normal control subjects. The frequency of ragged red fibers was more than 1% in 7 of 8 patients with inclusion body myositis (maximum, 15%). The modified succinic dehydrogenase stain was more sensitive than the modified Gomori trichrome in detecting accumulation of mitochondria in muscle fibers. Cytochrome c oxidase activity was deficient in most ragged red fibers. We conclude that the number of ragged red fibers increases with normal aging and may reflect an age-related decline in muscle mitochondrial oxidative metabolism. The frequent occurrence of ragged red fibers in inclusion body myositis suggests that mitochondrial function may be impaired in this disease.     

A new mtDNA mutation associated with a progressive encephalopathy and cytochrome c oxidase deficiency

The authors describe a novel pathogenic G5540A transition in the mitochondrial transfer RNA (tRNA)Trp gene of a sporadic encephalomyopathy characterized by spinocerebellar ataxia. Clinical features also included neurosensorial deafness, peripheral neuropathy, and dementia. Biochemistry revealed a severe reduction of cytochrome c oxidase (COX) activity. Single-fiber PCR demonstrated higher levels of mutant genomes in COX-negative ragged red fibers than in normal fibers. These findings confirm that COX is more susceptible than other respiratory chain complexes to mutations in the mitochondrial tRNATrp gene.     

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.     

A novel mutation in the mitochondrial DNA transfer ribonucleic acidAsp gene in a child with myoclonic epilepsy and psychomotor regression.

A novel A7543G mutation was found in the mitochondrial DNA transfer ribonucleic acidAsp gene in an 11-year-old girl with myoclonic seizures, developmental delay, and severe behavioral problems. Muscle histochemistry failed to show any ragged red fibers or cytochrome c oxidase-negative fibers, and muscle biochemistry showed partial cytochrome c oxidase deficiency. The mutation was heteroplasmic in muscle, fibroblasts, and blood from the patient and in blood from other affected family members, and the proportion of mutant mitochondrial DNA correlated with the severity of symptoms.     

A novel mitochondrial tRNAPhe mutation causes MERRF syndrome

A woman with typical features of myoclonic epilepsy with ragged red fibers (MERRF) had a novel heteroplasmic mutation (G611A) in the mitochondrial DNA tRNA phenylalanine gene. The mutation was heteroplasmic (91%) in muscle but undetectable in accessible tissues from the patient and her maternal relatives. Single-fiber PCR analysis showed that the proportion of mutant genomes was higher in cytochrome c oxidase (COX)-negative ragged red fibers (RRFs) than in COX-positive non-RRFs. This report shows that typical MERRF syndrome is not always associated with tRNA lysine mutations.     

Toxic myopathy with multiple deletions in mitochondrial DNA associated with long‐term use of oral anti‐viral drugs for hepatitis B: A case study

Oral nucleoside analogs (NAs) reduce hepatitis B virus (HBV) replication by inhibiting HBV DNA polymerase. However, NAs can also affect human mitochondrial DNA (mtDNA) polymerase, which can lead to mtDNA depletion (quantitative abnormality). Indeed, several mitochondrial myopathy cases have been reported in which a reduced mtDNA copy number was induced by oral NAs for hepatitis B. Herein, we report a case of toxic myopathy with multiple mtDNA deletions (qualitative abnormality) associated with long‐term use of NAs for hepatitis B. A 68‐year‐old woman, who underwent long‐term treatment with lamivudine and adefovir for chronic hepatitis B, developed proximal muscle weakness in the four extremities. Neurological examination showed mild proximal muscle weakness and atrophy in the four extremities. Upon admission to our hospital, her blood lactate/pyruvate ratio during an aerobic exercise test was elevated. Myogenic patterns were observed in lower limb muscles on electromyographic examination. Muscle magnetic resonance imaging revealed diffuse atrophy of proximal muscles in the four extremities with no signal changes. A biopsy from the biceps brachii muscle showed an abnormally large variation in fiber size, scattered muscle fibers with decreased cytochrome c oxidase activity, and ragged‐red fibers. Analysis of mtDNA from skeletal muscle revealed no decrease in copy number but increased incidence of multiple deletions, including a deletion of 4977 base pairs (known as the common deletion) reflecting oxidative stress‐induced mtDNA damage. This case study indicates that long‐term oral antiviral therapy for hepatitis B can induce chronic oxidative damage to mtDNA resulting in qualitative mtDNA abnormalities and toxic myopathy.     

Mitochondrial cytopathy with lactic acidosis, carnitine deficiency and DeToni-Fanconi-Debré syndrome

We reported a 6-year-old girl with mitochondrial cytopathy with lactic acidosis. The patient developed hypotonia, hearing loss, mental retardation, short stature, cataracta, hypoparathyroidism, DeToni-Fanconi-Debré syndrome and carnitine deficiency. Histological examination disclosed ragged red fibers and moderate lipid storage in skeletal muscle tissue and several structural abnormalities of mitochondria both in muscle tissue and proximal renal tubules. Biochemical examination of muscle tissue revealed a partial deficiency of pyruvate dehydrogenase complex and normal activities of cytochrome c oxidase, succinate cytochrome c reductase and NADH cytochrome c reductase. This is the first report of mitochondrial cytopathy representing DeToni-Fanconi-Debré syndrome associated with partial deficiency of pyruvate dehydrogenase complex and normal cytochrome c oxidase activity.     

Residual muscle cytochrome c oxidase activity accounts for submaximal exercise lactate threshold in chronic progressive external ophthalmoplegia

The data from histological, biochemical, and mitochondrial DNA (mtDNA) studies of muscle biopsies from 10 patients affected with chronic progressive external ophthalmoplegia (CPEO) were related to dynamic and metabolic parameters of incremental submaximal exercise. Maximum power output was reduced in all patients as compared to controls. Analysis of the venous lactate curve during exercise revealed a lactate threshold at exercise levels ranging from 40 to 50% of the predicted maximal power output. An earlier significant increase in lactate could be detected by calculating the mean δ lactate. Lactate values were inversely correlated with the cytochrome c oxidase (COX) activity of isolated muscle mitochondria. No relationship was found between lactate values and the number of ragged red fibers, or cytochrome c oxidase-negative fibers or the proportion of deleted mtDNA measured in muscle biopsy specimens. The discussion underscores the value of lactate kinetics in assessing skeletal muscle function, as well as the use of muscle COX levels to predict the effectiveness of wild-type complementation of deleted skeletal muscle mtDNA in in vivo contractile performance of CPEO subjects. © 1996 John Wiley & Sons, Inc.     

A partial deficiency of cytochrome c oxidase in chronic progressive external ophthalmoplegia

A partial deficiency of cytochrome oxidase has been found in 7 patients with chronic progressive external ophthalmoplegia and proximal myopathy or craniosomatic abnormalities. Muscle biopsies from all these patients showed morphological mitochondrial abnormalities (“ragged red” fibres) and cytochemical assay of cytochrome oxidase showed that these fibres contained no demonstrable enzyme activity. The incidence of cytochrome oxidase-negative fibres was greater than that of “ragged-red” fibres suggesting that the enzyme defect preceded the development of morphological mitochondrial changes. Biochemical analysis of skeletal muscle mitochondrial fractions from 3 patients revealed in 1 case a significantly lower concentration of cytochrome aa₃ and a decreased ratio of cytochrome oxidase/succinate-cytochrome c reductase. Fasting blood metabolites were elevated in 2 patients. We suggest that partial cytochrome oxidase deficiency is the underlying defect in mitochondrial myopathy associated with the oculocraniosomatic syndromes.     

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.     

Mitochondrial dysfunction in myofibrillar myopathy

'Myofibrillar myopathy' defines a myopathic condition with focal myofibrillar destruction and accumulation of degraded myofibrillar elements. Despite the fact that a number of mutations in different genes as well as cytotoxic agents lead to the disease, abnormal accumulation of desmin is a typical, common feature. Pathological changes of mitochondrial morphology and function have been observed in animal models with intermediate filament pathology. Therefore, in the present study we tested for mitochondrial pathology in skeletal muscle of five patients with the pathohistological diagnosis of myofibrillar myopathy. Screening for large-scale mtDNA deletions and the frequent MERRF (myoclonic epilepsy; ragged red fibres) and MELAS (mitochondrial encephalomyopathy; lactic acidosis; stroke) point mutations was negative in all patients. Histologically, all muscle biopsies showed nonspecific abnormalities of the oxidative/mitochondrial enzyme stainings (histochemistry for reduced nicotinamide adenine dinucleotide, succinic dehydrogenase, cytochrome c oxidase), only one of them had ragged red fibres and a significant number of cytochrome c oxidase-negative fibres. Upon biochemical investigation, four of our patients showed pathologically low respiratory chain complex I activities. Only one of our patients had a pathologically low complex IV activity, while the measurements of the others were within low normal range. The single patient with pathological values for both complex I and IV was the one with the clear histological hallmarks (ragged red and cytochrome c oxidase-negative fibres) of mitochondrial pathology. She also was the only patient with clinical signs hinting at a mitochondrial disorder. Together with data from observations in desmin- and plectin-deficient mice, our results support the view that desmin intermediate filament pathology in these cases is closely linked to mitochondrial dysfunction in skeletal muscle.     

Unusual clinical presentations in four cases of Leigh disease, cytochrome C oxidase deficiency, and SURF1 gene mutations

Mutations in the SURF1 gene are the most frequent causes of Leigh disease with cytochrome c oxidase deficiency. We describe four children with novel SURF1 mutations and unusual features: three had prominent renal symptoms and one had ragged red fibers in the muscle biopsy. We identified five pathogenic mutations in SURF1: two mutations were novel, an in-frame nonsense mutation (834G-->A) and an out-of-frame duplication (820-824dupTACAT). Although renal manifestations have not been described in association with SURF1 mutations, they can be part of the clinical presentation. Likewise, mitochondrial proliferation in muscle (with ragged red fibers) is most unusual in Leigh disease but might be part of an emerging phenotype.     

Peripheral neuropathy in two children with mitochondrial encephalomyopathy]

Peripheral neuropathy (PN) associated with mitochondrial encephalomyopathy (MEM) has been reported in adult patients, while children with both conditions are rare. Electrophysiological and pathological studies disclosed evidence of PN in a 3-year-old girl and an 8-year-old boy with MEM. In both patients, peripheral nerve conduction velocities were reduced, while amplitudes of evoked potentials were normal. No ragged red fibers were found in the biopsy muscle, while most of the muscle fibers showed poor activity with histochemical staining for cytochrome c oxidase (CCO). Biochemical studies revealed deficiency of CCO in both cases. In the latter patient, CCO activity was also absent in the intramuscular peripheral nerve using CCO staining, and histopathological studies of the sural nerve revealed a marked decrease in the number of large myelinated fibers and an unusual accumulation of the mitochondria in the Schwann cell cytoplasm. These results may support the hypothesis that a common pathogenesis exists in both peripheral nerve and muscle due to mitochondrial dysfunction.     

MELAS without Ragged Red Fibers or Lactic Acidosis Diagnosed by Mitochondrial DNA Testing

Abstract: A case of mitochondrial encephalomyopathy with lactic acidosis, a stroke-like episode (MELAS) without ragged red fiber, diagnosed by mitochondrial DNA testing, is reported. A 37-year-old woman experienced a sudden and recurrent headache with vomiting and stroke-like episodes. Brain CT and MRI showed multiple infarction in the temporal lobes, not corresponding to artery distribution. However, the plasma levels of lactate and pyruvate were normal, and showed no increase after aerobic exercise. Biopsied muscle showed no evidence of ragged red fibers and deficient activity of mitochondrial enzymes in the respiratory chain. The final diagnosis was made by mitochondrial DNA testing. A southern blot analysis after Apa I digestion revealed the A-to-G mutation in the tRNA^{L eu (UUR)}, which is specific to MELAS.     

Ophthalmoplegia,demyelinating neuropathy,leukoencephalopathy, myopathy,and gastrointestinal dysfunction with multiple deletions of mitochondrial DNA: A mitochondrial multisystem disorder in search of a name

This article describes a 37-year-old woman with progressive external ophthalmoplegia, peripheral neuropathy, and chronic intractable diarrhea. Laboratory studies disclosed lactic acidosis, ragged red fibers lacking cytochrome c oxidase, high-normal muscular mitochondrial enzymes, demyelinating neuropathy, leukoencephalopathy and multiple mitochondrial DNA deletions. This is the fourth patient described with this clinical syndrome, which represents a separate entity among multisystemic mitocnondrial disorders. The patient described here is the first with this syndrome to have multiple mitochondrial DNA deletions. © 1994 John Wiley & Sons, Inc.     

Partial cytochrome oxidase deficiency without subsarcolemmal accumulation of mitochondria in chronic progressive external ophthalmoplegia

Chronic progressive external ophthalmoplegia (CPEO) associated with proximal myopathy and/or craniosomatic abnormalities is a rare syndrome in which morphological mitochondrial changes have been found in some fibres (subsarcolemmal accumulation of mitochondria or "ragged red" fibres). We report a 14-year-old boy with CPEO and a mild proximal myopathy without these characteristic "ragged red" fibres. Histochemistry of skeletal muscle showed a mosaic of fibres without detectable cytochrome oxidase activity, while other mitochondrial enzymes were normal. The total cytochrome oxidase activity and cytochrome aa3 concentration in muscle mitochondrial fractions were only 40% of normal. This case is unique in that a biochemical defect was not accompanied by morphological abnormalities and may represent an early stage of CPEO before the development of morphological changes, or alternatively, a new variant of the disease.     

A novel myopathy-associated mitochondrial DNA mutation altering the conserved size of the tRNA(Gln) anticodon loop

We report a novel mitochondrial DNA alteration in a 12-year-old boy with myopathy. We identified a single nucleotide insertion (an adenine) in the mitochondrial tRNA-glutamine gene. This addition of an additional adenine in a polyadenine stretch (at mitochondrial DNA positions 4366-4369), alters the length of the evolutionary conserved anticodon loop from seven to eight bases. The nt-4370 addition was heteroplasmic and was abundant in the patient's muscle. Lower proportions of mutated mitochondrial DNA were observed in skin fibroblasts, but were below detectable levels in white blood cells. A muscle biopsy of the patient showed ragged red fibers and an unusually high percentage of cytochrome c oxidase-deficient fibers (89%). The pathogenicity of the mutation was also evident by the fact that fibers harboring lower levels of the mutation showed normal cytochrome c oxidase activity. The insertion in the anticodon loop of tRNA(Gln) gene identified in our patient may provide a unique tool to study protein synthesis in human mitochondria.     

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.     

Evidence for intramitochondrial complementation between deleted and normal mitochondrial DNA in some patients with mitochondrial myopathy.

Twenty-three patients with mitochondrial myopathies and mitochondrial DNA deletions in muscle were studied by means of deletion mapping and sequencing, histochemistry and polarography. Histochemistry showed significantly less focal cytochrome oxidase deficiency relative to number of ragged red fibres when the deletion did not involve reading frames for cytochrome oxidase subunits. Polarography in such patients showed defects exclusively involving complex I, in contrast to the others with larger deletions who generally had more diffuse respiratory chain defects. Analysis of other published histochemical data showed similar findings to our own. It is concluded that translation of a proportion of deleted mitochondrial DNAs occurs in at least some patients with mitochondrial DNA deletions, implying that deleted and normal mitochondrial genomes share transfer RNAs within mitochondria in such cases.