Decreased activities of mitochondrial respiratory chain complexes in non-mitochondrial respiratory chain diseases

The aim of this study was to illustrate the difficulties in establishing a diagnosis of mitochondrial respiratory chain (MRC) disorders based on clinical grounds in combination with intermediate activities of the MRC enzyme complexes. We reviewed retrospectively all medical and laboratory records of patients initially considered likely to have MRC disorders on clinical grounds, and subsequently diagnosed with other disorders (n = 20; 11 males, 9 females). Data were retrieved from hospital records, referral letters, and results of enzymatic analysis at a reference laboratory. Clinical symptoms included developmental delay, epilepsy, hypotonia, movement disorder, spastic quadriplegia, tetany, microcephaly, visual problems, carpopedal spasms, dysmorphism, hearing loss, muscle weakness and rhabdomyolysis, and fulminant hepatitis. Blood and cerebrospinal fluid lactate levels were elevated in 13/20 and 9/20 respectively. One or more MRC complex activities (expressed as ratios relative to citrate synthase and/or complex II activity) were less than 50% of control mean activity in 11/20 patients (including patients with deficiencies of pyruvate dehydrogenase complex, pantothenate kinase, holocarboxylase synthetase, long-chain hydroxy acyl-CoA dehydrogenase, molybdenum co-factor, and neonatal haemochromatosis). One patient had a pattern suggestive of mitochondrial proliferation. We conclude that intermediate results of MRC enzymes should be interpreted with caution and clinicians should be actively looking for other underlying diagnoses.     

Normal muscle respiratory chain enzymes can complicate mitochondrial disease diagnosis

This report presents a case of mitochondrial respiratory chain deficiency in a neonate with elevated plasma lactate, hypotonia, developmental delay, and dysmorphic features. The initial biochemical analyses of muscle tissue for mitochondrial function were normal. Additional testing on skin fibroblasts performed owing to a high clinical suspicion of a possible mitochondrial disorder indicated a deficiency of mitochondrial complex I. Western blotting of samples obtained both from muscle and fibroblast tissues also revealed an extensive defect in mitochondrial respiratory chain complex I, confirming the diagnosis. These observations underscore the fact that both enzymatic and immunological assays should be undertaken in alternate tissues when muscle biopsies are inconclusive in highly suspected cases.     

Exercise intolerance and the mitochondrial respiratory chain

The syndrome of exercise intolerance, cramps, and myoglobinuria is a common presentation of metabolic myopathies and has been associated with several specific inborn errors of glycogen or lipid metabolism. As disorders in fuel utilization presumably impair muscle energy production, it was more than a little surprising that exercise intolerance and myoglobinuria had not been associated with defects in the mitochondrial respiratory chain, the terminal energy-yielding pathway. Recently, however, specific defects in complex I, complex III, and complex IV have been identified in patients with severe exercise intolerance with or without myoglobinuria. All patients were sporadic cases and all harbored mutations in protein-coding genes of muscle mtDNA, suggesting that these were somatic mutations not affecting the germ-line. Another respiratory chain defect, primary coenzyme Q10 (CoQ10) deficiency, also causes exercise intolerance and recurrent myoglobinuria, usually in conjunction with brain symptoms, such as seizures or cerebellar ataxia. Primary CoQ10 deficiency is probably due to mutations in nuclear gene(s) encoding enzymes involved in CoQ10 biosynthesis.     

Epilepsia partialis continua and defects in the mitochondrial respiratory chain

Epilepsia partialis continua (EPC) is characterized by continuous myoclonic or clonic jerks repeated at short intervals followed by a slowly progressive neurological disorder. We report three patients with EPC and a defect in the mitochondrial respiratory chain. METHODS: Clinical, neuroradiological, and biochemical data were reported. RESULTS: The patients presented continuous myoclonic jerks at age of 8 months, 11 months and 6 years, respectively. Two of the three patients had a previous developmental delay. Neurological examination at first admission revealed extrapyramidal symptoms in all patients. Initial biological investigations suggested mitochondrial dysfunction. Initial EEG showed a continuous discharge of periodic spikes (0.5-1Hz). MRI studies were initially normal then progressed to cerebral hemiatrophia. EEG revealed both correlation and absence of correlation between spikes or sharp waves and myoclonic jerks. The activity of one or several complexes of the mitochondrial respiratory chain was reduced in the muscle samples of the three patients. No mutation of mtDNA was found. CONCLUSION: Our report suggests that EPC can be due to mitochondrial respiratory chain disorders. Some clinical findings and initial investigations were indicative of a disorder of mitochondrial metabolism. Previous developmental delay, extrapyramidal symptoms and other organ involvement should suggest a possible mitochondrial etiology of EPC. In case of infant presenting EPC, mitochondrial respiratory chain disorder should be considered first.     

Variability in the activity of respiratory chain enzymes in mitochondrial myopathies

Summary Four patients with mitochondrial abnormality had multiple muscle biopsies at several year intervals during which respiratory chain enzyme activities were shown to be quite variable. In three patients, progression of the disease paralleled the decrease in respiratory chain enzyme activity. In one patient, the clinical and pathological findings improved with age as is seen in the benign infantile form of cytochromec oxidase (CCO) deficiency. The variability in these mitochondrial disorders may result from the varied proportions of normal and abnormal mitochondria in the muscle cells in which the mitochondria are said to be randomly replicated from numerous mitochondrial DNA copies.This work was partially supported by a Grant-in-Aid for Scientific Research (No. 61570397) from the Ministry of Education, Science and Culture of Japan     

Epidemiology of pediatric mitochondrial respiratory chain disorders in northwest Spain

Our knowledge of mitochondrial respiratory chain diseases has increased dramatically in recent years, but relatively little information is available about their prevalence and incidence, either in pediatric or adult patients. This study reports incidence and prevalence estimates, and summarizes the clinical, biochemical, histologic, and genetic characteristics of 51 patients age 0-16 years. The overall annual incidence of all mitochondrial respiratory chain diseases was estimated to be 1.43 cases per 10(5) in the population as a whole, and 2.85 cases per 10(5) in the under-6 population. The overall prevalence of all mitochondrial respiratory chain diseases was estimated as 7.5 cases per 10(5) in the under-19 population, and 8.7 cases per 10(5) in the under-16 population. These incidence and prevalence estimates are higher than in most previous studies of pediatric populations. Estimated prevalences of specific mitochondrial respiratory chain diseases were 2.05 cases per 10(5) for Leigh syndrome, 0.68 per 10(5) for mitochondrial deoxyribonucleic acid (mtDNA) deletions and deletions-duplications, 1.59 per 10(5) for mtDNA depletions, and 0.45 per 10(5) for mtDNA point mutations. Leigh syndrome was the most frequent clinical syndrome. The estimates of the prevalences of mtDNA deletions, deletions-duplications, and point mutations set forth here are lower than in similar previous studies, whereas the estimate of the prevalence of mtDNA depletions is rather higher. Sixteen of these patients manifested phenotypic syndromes that have not been previously reported in association with mitochondrial respiratory chain diseases.     

Flow cytometric evaluation of defects of the mitochondrial respiratory chain.

Cultured human skin fibroblasts from 12 patients with a variety of mitochondrial respiratory chain defects were examined for their capacity to oxidize dihydrorhodamine-123 to the fluorescent molecule rhodamine-123 using a flow cytometer. We found that cells from patients with functional defects in respiratory chain enzymes were less able to oxidize dihydrorhodamine-123 than those of healthy controls. Ten of the cell strains had reduced activity in at least one of the respiratory chain complexes and also showed significantly reduced fluorescence when compared to the mean of eight normal control cell strains. One patient had mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (with the A3243G mutation) and reduced respiratory chain activities in muscle and liver. Molecular analysis did not show the mutation in cultured skin fibroblasts, and had correspondingly normal fluorescence. The 12th cell strain showed reduced fluorescence but did not reach statistical significance. This strategy could be of use in helping direct further investigations in patients, and in studying the biochemical pathogenesis of mitochondrial DNA mutations in cybrid studies.     

Epilepsy and respiratory chain defects in children with mitochondrial encephalopathies

OBJECTIVE: The purpose of this study was to determine the relationship between epilepsy and respiratory chain defects in children with mitochondrial encephalopathies (ME). STUDY DESIGN: We conducted a retrospective review of the medical records of children referred for evaluation of an ME. Only patients assigned a definite diagnosis of ME using modified Walker criteria and with a respiratory chain defect were included. Clinical data pertaining to the ME and epilepsy type were collected. Mitochondria were isolated by subcellular fractionation from a vastus lateralis muscle biopsy and studies were performed using polarographic and spectroscopic techniques for the quantitative determination of NADH and cytochrome components of the respiratory chain. RESULTS: A total of 38 children with ME were identified. Seizures were present in 61%. Sixteen of 23 children with epilepsy (70%) had refractory epilepsy associated with a progressive encephalopathy. Children with epilepsy had a significantly higher incidence of complex I defects than children without epilepsy (p<0.01). Complex III and IV defects were significantly higher in patients without epilepsy (p<0.01 and p<0.05, respectively) than in those with epilepsy. CONCLUSIONS: Epilepsy is an important component of ME. The higher incidence of complex I defects in patients with epilepsy suggests a possible relationship between mitochondrial oxidative stress dysfunction and epileptogenic process.     

Chronic levodopa administration alters cerebral mitochondrial respiratory chain activity

Parkinson's disease (PD) is characterized mainly by a loss of nigrostriatal dopamine neurons. Thus far, the actual physiopathology of PD remains uncertain, although recent studies have found decreased activity of complex I, one of the enzymatic units of the mitochondrial respiratory chain, in various tissues of PD patients. Because most, if not all, of PD patients are treated chronically with levodopa, the precursor of dopamine, and because we have shown previously that catecholamines may alter mitochondrial respiration, we assessed the effects of chronic administration of levodopa on complex I activity in rat brain. We found that chronic administration of levodopa, at a dose used in PD patients, caused a significant reduction in complex I activity while it did not affect the activities of complex II, complex IV, and citrate synthase. Reduction in complex I activity correlated well with catecholamine innervation as the reduction was observed mainly in the striatum and substantia nigra and to a lesser extent in the frontal cortex but not in the cerebellum. Moreover, the levodopa-induced decrease of complex I activity was reversible since activities at 1, 3, and 7 days after the last injection showed a progressive return to control values. Incubation of whole brain mitochondria in vitro showed that both levodopa and dopamine inhibit complex I activity in a dose- and time- dependent manner. In contrast, other compounds such as homovanillic acid, 3,4-dihydroxyphenylacetic acid, and 3-O-methyl-dopa were minimally effective. Reduced glutathione, ascorbate, superoxide dismutase, and catalase prevented the effect of levodopa and dopamine on complex I. Various inhibitors of monoamine oxidase also prevented the effect of dopamine. In agreement with a critical role of monoamine oxidase in this effect in vitro, we observed that noncatecholamine substrates of this enzyme such as serotonin and β-phenylethylamine were potent inhibitors of complex I. However, autoxidation may also be involved in this process because the effect of levodopa, 6-hydroxydopa, and 6-hydroxydopamine on complex I activity was only partially suppressed by monoamine oxidase inhibitors. These observations demonstrate that the chronic administration of levodopa can cause alterations in mitochondrial respiratory chain activity in rats that are most likely related to an oxidative stress provoked by the increase in dopamine turnover. We suggest that this mechanism may exaggerate a mitochondrial defect already present in the brains of PD patients and thus may play a role in the progression of PD.     

Liver-specific mitochondrial respiratory chain complex I deficiency in fatal influenza encephalopathy

We report on a 4-year-old boy who died from influenza encephalopathy. The clinical course and microscopic findings of the autopsied liver were compatible with Reye's syndrome. We examined the mitochondrial respiratory chain function by blue native polyacrylamide gel electrophoresis (BN-PAGE), western blotting, and respiratory chain enzyme activity assays. The activity of liver respiratory chain complex (CO) I was markedly decreased (7.2% of the respective control activity); whereas, the other respiratory chain complex activities were substantially normal (CO II, 57.9%; CO III, 122.3%; CO IV, 161.0%). The activities of CO I-IV in fibroblasts were normal (CO I, 82.0%; CO II, 83.1%; CO III, 72.9%; CO IV, 97.3%). The patient was diagnosed with liver-specific complex I deficiency. This inborn disorder may have contributed to the fatal outcome. We propose that relying only on fibroblast respiratory chain complex activities may lead to the misdiagnosis of liver-specific complex I deficiency.     

Kearns-Sayre syndrome: mitochondrial encephalomyopathy caused by deficiency of the respiratory chain

We report the cases of a 46 year old woman and of a 18 year-old boy who met the criteria for Kearns-Sayre syndrome. Additional atypic features were present in one case: family history, psychosis and acute respiratory failure. In both cases histoenzymatic analysis of the muscle biopsy and biochemical studies of mitochondria isolated from the muscle sample demonstrated mitochondrial myopathy associated with combined partial deficiency of complexes I and IV of the electron transfer chain. Although there is no correlation between clinical and biological data in the mitochondrial myopathies our cases confirm that such defects are involved in Kearns-Sayre syndrome. Improvement with coenzyme Q10 therapy in these patients is reported.     

Chronic administration of methylphenidate activates mitochondrial respiratory chain in brain of young rats

Methylphenidate is frequently prescribed for the treatment of attention deficit/hyperactivity disorder. Psychostimulants can cause long-lasting neurochemical and behavioral adaptations. The exact mechanisms underlying its therapeutic and adverse effects are still not well understood. In this context, it was previously demonstrated that methylphenidate altered brain metabolic activity, evaluated by glucose consumption. Most cell energy is obtained through oxidative phosphorylation, in the mitochondrial respiratory chain. Tissues with high energy demands, such as the brain, contain a large number of mitochondria. In this work, our aim was to measure the activities of mitochondrial respiratory chain complexes II and IV and succinate dehydrogenase in cerebellum, prefrontal cortex, hippocampus, striatum, and cerebral cortex of young rats (starting on 25th post-natal day and finishing on 53rd post-natal day) chronically treated with methylphenidate. Our results showed that mitochondrial respiratory chain enzymes activities were increased by chronic administration of this drug. Succinate dehydrogenase was activated in cerebellum, prefrontal cortex and striatum, but did not change in hippocampus and brain cortex. Complex II activity was increased in cerebellum and prefrontal cortex and was not affected in hippocampus, striatum and brain cortex. Finally, complex IV activity was increased in cerebellum, hippocampus, striatum and brain cortex, and was not affected in prefrontal cortex. These findings suggest that chronic exposure to methylphenidate in young rats increases mitochondrial enzymes involved in brain metabolism. Further research is being carried out in order to better understand the effects of this drug on developing nervous system and the potential consequences in adulthood resulting from early-life drug exposure.     

Deficiency of complex II of the mitochondrial respiratory chain in late-onset optic atrophy and ataxia

Defects of the mitochondrial respiratory chain are increasingly being recognized as an important cause of neurological disease in humans. In many of these patients, the biochemical defect results from an abnormality of the mitochondrial genome. Respiratory chain defects involving complex II, which is entirely encoded by the nuclear genome, are comparatively rare. We report the clinical and biochemical findings in 2 elderly sisters who presented with late-onset neurodegenerative disease. In both patients, a partial deficiency of complex II (approximately 50% of control values) was shown to be present in mitochondria from muscle and platelets. The enzyme defect was not expressed in cultured skin fibroblasts or immortalized lymphocytes. There was an overexpression of the 70-kd flavoprotein subunit in muscle mitochondria from both patients, although we showed that this subunit is present in normal amounts in mitochondrial membranes. Our studies highlight the diversity of the clinical presentation of respiratory chain disease and that complex II deficiency should enter the differential diagnosis of certain patients with late-onset neurodegenerative disease.     

Delayed manifestation of mitochondrial myopathy--complex I and IV deficiency of the mitochondrial respiratory chain with progressive paresis]

We describe a 53-year-old patient with a progressive mitochondrial myopathy of late-onset, restricted to skeletal muscle only without external ophthalmoplegia. The myopathy developed at the age of 46 years initially with exercise intolerance and subsequently progressive permanent muscle weakness. Muscle biopsy revealed severe myopathic changes, ragged red fibers, and a marked multifocal cytochrome-c-oxidase deficiency. Biochemical analysis showed a deficiency of complexes I and IV of the mitochondrial respiratory chain. Genetic analysis of mitochondrial DNA revealed no deletions. Mitochondrial myopathies restricted to skeletal muscle have to be considered in the differential diagnosis of late-onset progressive myopathies.     

Lafora disease: diagnosis by liver biopsy

We have studied four patients who had a clinical course compatible with Lafora disease. The diagnosis was confirmed in one by the presence of Lafora bodies in central nervous system neurons at autopsy and was supported in another by findings in the cerebral biopsy of a sibling. Our patients had no clinically apparent liver disease, but liver specimens in each instance showed a distinctive histological abnormality, with hepatocytes containing inclusions having a ground-glass appearance. The liver biopsy findings appear to be relatively specific for this disorder and can easily be differentiated from those in other liver diseases.