Deficiency of subunits in heart mitochondrial NADH-ubiquinone oxidoreductase of a patient with mitochondrial encephalomyopathy and cardiomyopathy

Mitochondrial DNA repair and aging

The mitochondrial electron transport chain plays an important role in energy production in aerobic organisms and is also a significant source of reactive oxygen species that damage DNA, RNA and proteins in the cell. Oxidative damage to the mitochondrial DNA is implicated in various degenerative diseases, cancer and aging. The importance of mitochondrial ROS in age-related degenerative diseases is further strengthened by studies using animal models, Caenorhabditis elegans, Drosophila and yeast. Research in the last several years shows that mitochondrial DNA is more susceptible to various carcinogens and ROS when compared to nuclear DNA. DNA damage in mammalian mitochondria is repaired by base excision repair (BER). Studies have shown that mitochondria contain all the enzymes required for BER. Mitochondrial DNA damage, if not repaired, leads to disruption of electron transport chain and production of more ROS. This vicious cycle of ROS production and mtDNA damage ultimately leads to energy depletion in the cell and apoptosis.

The mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episode syndrome-associated human mitochondrial tRNALeu(UUR) mutation causes aminoacylation deficiency and concomitant reduced association of mRNA with ribosomes

The pathogenetic mechanism of the mitochondrial tRNA(Leu(UUR)) A3243G transition associated with the mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) syndrome has been investigated in transmitochondrial cell lines constructed by transfer of mutant mitochondrial DNA (mtDNA)-carrying mitochondria from three genetically unrelated MELAS patients or of isogenic wild-type mtDNA-carrying organelles into human mtDNA-less cells. An in vivo footprinting analysis of the mtDNA segment within the tRNA(Leu(UUR)) gene that binds the transcription termination factor failed to reveal any difference in occupancy of sites or qualitative interaction with the protein between mutant and wild-type mtDNAs. Cell lines nearly homoplasmic for the mutation exhibited a strong (70-75%) reduction in the level of aminoacylated tRNA(Leu(UUR)) and a decrease in mitochondrial protein synthesis rate. The latter, however, did not show any significant correlation between synthesis defect of the individual polypeptides and number or proportion of UUR codons in their mRNAs, suggesting that another step, other than elongation, may be affected. Sedimentation analysis in sucrose gradient showed a reduction in size of the mitochondrial polysomes, while the distribution of the two rRNA components and of the mRNAs revealed decreased association of mRNA with ribosomes and, in the most affected cell line, pronounced degradation of the mRNA associated with slowly sedimenting structures. Therefore, several lines of evidence indicate that the protein synthesis defect in A3243G MELAS mutation-carrying cells is mainly due to a reduced association of mRNA with ribosomes, possibly as a consequence of the tRNA(Leu(UUR)) aminoacylation defect.

Multiple respiratory chain abnormalities associated with hypertrophic cardiomyopathy and 3-methylglutaconic aciduria

In a 4.5-month-old boy presenting with marked muscular hypotonia in the neonatal period, hepatomegaly, cardiac hypertrophy, recurrent hypoglycemia, metabolic acidosis, and secondary carnitine deficiency, there was a considerable urinary excretion of 3-methylglutaconic and 3-methylglutaric acid. Estimation of 3-methylglutaconyl-CoA hydratase, 3-hydroxy-3-methylglutaryl-CoA lyase and initial enzymatic steps of cholesterol biosynthesis in cultured fibroblasts and in different tissues postmortem revealed no enzyme deficiency. Analyses of the respiratory chain in postmortem tissues demonstrated severe impairment of complex I (NADH ubiquinone oxidoreductase) and complex IV (cytochrome c oxidase) activities in skeletal muscle and reduced complex IV activity in heart.

Phenotypic expression of mtDNA heteroplasmy in the skeletal muscle of patients with oculomyopathy: defect in mitochondrial protein synthesis

The biochemical consequences of mtDNA heteroplasmy, observed in patients with a range of diseases associated with the mitochondrial respiratory enzymes deficiency is of particular interest, as they might provide information with regard to the regulatory interactions which govern the expression of the human mitochondrial genome. Three patients with chronic progressive external ophthalmoplegia (CPEO) were investigated to study the consequences of mtDNA heteroplasmy on mitochondrial protein synthesis. All 3 patients exhibited partially deleted mtDNA species (varying in size from 10.5 to 14 kb) in their skeletal muscle, which co-existed with the normal 16.5 kb mtDNA. The examination of mitochondrial translation products following the incorporation of [35S]methionine by isolated mitochondria, showed grossly abnormal patterns of mitochondrial translation products, suggesting a major disturbance in the regulation of mitochondrial protein synthesis.

Chronic progressive external ophthalmoplegia in patients with large heteroplasmic mitochondrial DNA deletions: an immunocytochemical study

Immunocytochemical studies with a holocomplex antibody battery in patients with chronic progressive external ophthalmoplegia, with and without large mitochondrial DNA deletions, revealed positive (and often increased) immunoreactivity for all complexes studied in histochemically cytochrome oxidase (COX)-negative areas, suggesting a compensatory up-regulation of these components. Similar findings were observed with subunit-specific probes directed against both nuclear- and mitochondrially encoded gene products. Comparison of staining intensities between the different complexes revealed significantly more variability in COX-negative than COX-positive fibres, suggesting disordered stoichiometric control during up-regulation. These differences were confirmed using statistical models. This data challenges the view that COX-negative fibre segments have little or no mitochondrially coded protein translation.

Fatal infantile mitochondrial cardiomyopathy and myopathy with heterogeneous tissue expression of combined respiratory chain deficiencies

A 5-month-old boy died of progressive heart failure that started at the age of 3 months. Autopsy revealed a mitochondrial cardiomyopathy and a mitochondrial myopathy of the limb muscle and diaphragm. Cytochemically random defects of cytochrome c oxidase were visualized by light and electron microscopy in the diaphragm and especially the heart muscle, the limb muscle showing a diffuse attenuation whereas the liver and kidneys reacted normally. The activities of NADH-dehydrogenase (complex I) and cytochrome c oxidase (complex IV) were severely diminished (20% residual activity of controls) in the skeletal and heart muscle. In the heart, succinate cytochrome c reductase (complex II/III) was additionally decreased to the same degree. Loss of cytochrome c oxidase activity was based on a reduction of both mitochondrial and nuclear derived subunits in the heart and diaphragm as revealed by immunohistochemical analysis, whereas the limb muscle showed a normal immunoreactive protein content. The results illustrate heterogeneous tissue expression of respiratory chain enzyme defects and demonstrate that a cardiomyopathy may be the leading presentation of a mitochondrial disorder in early infancy.

Mitochondrial DNA mutations in mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS)

The total sequences of mitochondrial DNA were determined in two patients with juvenile-onset mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) due to Complex I deficiency. Patients 1 and 2 had three and two unique point mutations, respectively, causing replacement of phylogenically conserved amino acids. A transition from G to A was found at nucleotide position 5601 in the alanine tRNA gene of Patient 2, and a transition from A to G was found at 3243 in the leucine (UUR) tRNA gene of both patients. The latter mutation located at the phylogenically conserved 5' end of the dihydrouridine loop of the tRNA molecule, and was present in two patients with adult-onset MELAS and absent in controls. These results indicate that a mass of mtDNA mutations including the A-to-G transition in the tRNA(Leu) gene is a genetic cause of MELAS.

Isolated and combined deficiencies of NADH dehydrogenase (complex I) in muscle tissue of children with mitochondrial myopathies

We describe eight children with complex I deficiency, four of them with an isolated, the other four with an additional deficiency of complex IV. Clinical, chemical and morphological findings were compared from patients with isolated and combined deficiency. In both groups, the age of onset of symptoms was between the 1st day and the 4th month of life. Clinical and biochemical heterogeneity were observed. We found no correlation between residual activity of complex I in muscle, blood lactate level, and severity of clinical symptoms. Newborns presenting with severe lactic acidosis and children with later onset myopathy were seen in both groups. The group with combined complex I deficiency showed a more severe clinical course. By light microscopy ragged red fibres were only found in two patients with combined deficiency. However, by electron microscopy structural alterations of the mitochondria were observed in six out of seven muscle specimens.

Multiple mitochondrial DNA deletions exist in cardiomyocytes of patients with hypertrophic or dilated cardiomyopathy

Genetic impairment was revealed in idiopathic cardiomyopathy and the responsible DNA locus was estimated. Mitochondrial DNA were amplified from autopsied cardiac specimens from three patients who died from hypertrophic or dilated cardiomyopathy by using polymerase chain reaction (PCR). By using two novel methods for PCR gene amplification, the pleioplasmic existence of multiple populations of differently deleted mitochondrial DNA in all specimens from the patients was confirmed. Mitochondrial DNA with a 7,436 bp deletion which commonly existed among the specimens was sequenced and the direct repeat at each edge of deletion was identified as (CATCAACAACCG) which was located in ATPase 6 gene and in the D-loop region. From our results mitochondrial DNA mutations could also be an important contributory factor to cardiomyopathy.

Progressive cytochrome c oxidase deficiency in a case of Leigh's encephalomyelopathy

We report the morphological, biochemical, immunological, and genetic findings in a patient with the clinical characteristics of Leigh's disease due to multisystemic cytochrome c oxidase (CCO) deficiency. Muscle biopsy at 2 years and 5 months of age showed markedly decreased CCO and cytochrome a + a3, moderately decreased NADH-cytochrome c reductase to 46.3%, and generalized loss of immunologically detectable CCO subunits, but other respiratory chain enzyme proteins were normal. All the tissues examined at autopsy showed decreased activity of all respiratory chain enzymes except complex II. The decrease in cytochromes b and a + a3 were in harmony with decreased enzyme activities in complex III and IV (CCO), respectively. All immunologically detectable subunits of CCO in immunoprecipitation were uniformly decreased in the cardiac and skeletal muscles, but subunits 1 and 4 were selectively decreased in other organs except liver. No large deletion could be detected in the cardiac muscle mtDNA after digestion with restriction enzymes. These results suggest that the respiratory chain enzymes are variable in their activity and the amount of enzyme proteins decreases as the disease progresses.

Mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes with special reference to the mechanism of cerebral manifestations

A 29-year-old man with mitochondrial encephalomyopathy caused by partial deficiency of mitochondrial NADH-ubiquinone oxidoreductase (Complex I) is described. Clinical manifestations were characterized by generalized convulsion, dementia and stroke-like episodes consisting of hemianopsia, Gerstmann's syndrome and visual hallucination. Blood and cerebrospinal fluid lactate and pyruvate levels were elevated. Biochemical studies on a muscle biopsy specimen revealed partial deficiency of Complex I activity, and decreases in the 75-kDa and the 20-kDa subunits of Complex I by immunoblotting analysis. Serial brain CT scans revealed multiple low-density areas with fluctuating densities. Single photon emission tomographic study revealed preservation of blood circulation where CT scans showed diminished density in acute stage, suggesting the presence of abnormal cellular metabolism rather than vascular occlusion as the basic mechanism of his stroke-like episodes. Pathogenesis of neurological manifestations in MELAS is discussed with reference to the possible involvement of free radicals in inducing brain damage.

Pleiotropic molecular defects in energy-transducing complexes in mitochondrial encephalomyopathy (MELAS)

The extent of molecular defects in the mitochondrial energy-transducing system was examined in autopsied tissues of a 14-year-old male with mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS) in order to elucidate the underlying molecular and genetic abnormalities. The patient also had other multiorganic disorders: hypertrophic cardiomyopathy, nephrotic syndrome, and pseudohypoparathyroidism. Enzymic activities of complex I and IV were severely decreased, and those of complex III and V were mildly decreased in the mitochondria isolated from various tissues, but the severity of the deficiencies varied from tissue to tissue. In contrast, complex II and citrate synthase activities were normal or were decreased to a lesser extent than the enzymic activities of other complexes in all the tissues examined. These results suggest that the energy-transducing complexes, namely complexes, I, III, IV, and V, that contain mitochondrially synthesized subunits, were selectively affected. Immunoblot analysis demonstrated that the decreased enzymic activities were based on decreased contents of subunits in these complexes. The multiorganic manifestation of the disorder may result from wide and uneven distribution of abnormal mitochondria that have pleiotropic molecular defects in the energy-transducing complexes among the organs of the patient.

Fatal mitochondrial myopathy with cytochrome-c-oxidase deficiency and subunit-restricted reduction of enzyme protein in two siblings: an autopsy-immunocytochemical study

Lack of cytochrome-c oxidase activity and of cytochromes aa3 + b has been reported previously in the skeletal muscle of one of two siblings (Müller-Höcker et al, 1983). The present study reports a deficiency of immunoreactive enzyme protein in the skeletal muscle of both siblings, who had an identical fatal clinical course. In all specimens the defect did not involve the whole enzyme protein, but was selectively expressed in the mitochondrially derived subunits II/III and nuclear coded subunits VIIbc. Neither the specific fibers of the muscle spindles nor the mitochondria of the heart, liver, kidneys, vessel walls and/or gastrointestinal tract were affected. These results are most consistent with a primary nuclear defect being responsible for the organ specific and subunit selective expression of the enzyme defect.

Familial mitochondrial myopathy associated with peripheral neuropathy: partial deficiencies of complex I and complex IV

Two brothers, 46 and 48 years old, presented with optic atrophy and blepharoptosis since childhood, and later developed muscle wasting and weakness of the extremities, and glove and stocking type sensory impairment. Biopsies of muscles and sural nerves clearly showed mitochondrial myopathy with many ragged-red fibers and peripheral neuropathy with onion-bulb formation. Biochemical studies of muscles disclosed partial deficiencies of complexes I and IV of the mitochondrial respiratory chain in both cases. Since the parents were first cousins, this mitochondrial disorder seemed to be transmitted as an autosomal recessive trait.

Molecular defects of NADH-ubiquinone oxidoreductase (complex I) in mitochondrial diseases

Defects in Complex I of the mitochondrial respiratory chain have been identified in 38 patients. The clinical and laboratory features are reviewed and the results of recently devised strategies aimed at characterizing the primary molecular and genetic abnormalities are presented. Although not exhaustive, these studies have provided a molecular basis for the contention that defects in Complex I may have their origin in nuclear or in mitochondrial genes.

The biochemical basis of mitochondrial diseases

Dysfunctioning of human mitochondria is found in a rapidly increasing number of patients. The mitochondrial system for energy transduction is very vulnerable to damage by genetic and environmental factors. A primary mitochondrial disease is caused by a genetic defect in a mitochondrial enzyme or translocator. More than 60 mitochondrial enzyme deficiencies have been reported. Secondary mitochondrial defects are caused by lack of compounds to enable a proper mitochondrial function or by inhibition of that function. This may result from malnutrition, circulatory or hormonal disturbances, viral infection, poisoning, or an extramitochondrial error of metabolism. Once mitochondrial ATP synthesis decreases, secondary mitochondrial lesions may be generated further, due to changes in synthesis and degradation of mitochondrial phospholipids and proteins, to mitochondrial antibody formation following massive degradation, to accumulation of toxic products as excess acyl-CoA, to the depletion of Krebs cycle intermediates, and to the increase of free radical formation and lipid peroxidation.

Deficiency of subunits of Complex I and mitochondrial encephalomyopathy

Enzymic activities of the respiratory chain and content of immunochemically detectable subunits in NADH-ubiquinone oxidoreductase (Complex I) were measured in mitochondria from the skeletal muscles of 4 patients with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). The rotenone-sensitive NADH-cytochrome c reductase activity was extremely decreased, ranging from 0% to 27% of the control value. In all patients, the content of subunits of Complex I was also reduced in parallel with the rotenone-sensitive NADH-cytochrome c reductase activity. It is suggested that the variation in the degree of deficiency of Complex I subunits could explain the clinical heterogeneity of patients with MELAS.

An animal model of mitochondrial myopathy: a biochemical and physiological investigation of rats treated in vivo with the NADH-CoQ reductase inhibitor, diphenyleneiodonium

Chronic administration of the NADH-CoQ reductase inhibitor, diphenyleneiodonium to rats at two dose levels, 1.0 and 1.5 mg/kg per day, caused a 40% and 60% reduction, respectively, in the in vitro rate of NAD-linked respiration by skeletal muscle mitochondria. At the highest dose, muscle fatigue, lactic acidosis and an over-utilization of phosphocreatine was observed in the gastrocnemius muscle during mild stimulation of 1 Hz frequency. The resynthesis of phosphocreatine following muscle stimulation was about 2 fold slower in the treated animal group. At the low dose, no significant biochemical changes were observed during muscle stimulation at 4 Hz. The results are discussed in terms of skeletal muscle "oxidative reserve", twitch tension maintenance and the relevance to the human diseased state of mitochondrial myopathy.

Mitochondrial myopathy involving ubiquinol-cytochrome c oxidoreductase (complex III) identified by immunoelectron microscopy

The distribution of respiratory chain complexes in bovine heart and human muscle mitochondria has been explored by immunoelectron microscopy with antibodies made against bovine heart mitochondrial proteins in conjunction with protein A-colloidal gold (12-nm particles). The antibodies used were made against NADH-coenzyme Q reductase (complex I), ubiquinol cytochrome c oxidoreductase (complex III), cytochrome c oxidase, core proteins isolated from complex III and the non-heme iron protein of complex III. Labeling of bovine heart tissue with any of these antibodies gave gold particles randomly distributed along the mitochondrial inner membrane. The labeling of muscle tissue from a patient with a mitochondrial myopathy localized by biochemical analysis to complex III was quantitated and compared with the labeling of human control muscle tissue. Complex I and cytochrome c oxidase antibodies reacted to the same level in myopathic and normal muscle samples. Antibodies to complex III or its components reacted very poorly to the patient's tissue but strongly to control muscle samples. Immunoelectron microscopy using respiratory chain antibodies appears to be a promising approach to the diagnosis and characterization of mitochondrial myopathies when only limited amounts of tissue are available for study.

Isolation of a cDNA clone for human cytochrome c1 from a lambda gt11 expression library

Antiserum directed against a purified preparation of beef heart cytochrome bc1 complex has been used to screen a human liver cDNA expression library in lambda gt11. The inserts of two recombinants, which gave strong signals, were found to represent cytochrome c1 by epitope selection using nitrocellulose filters containing the expressed proteins. The amino acid sequence deduced from the nucleotide sequence of an insert DNA revealed a high degree of homology with the sequence of bovine cytochrome c1.

A mitochondrial encephalomyopathy with cardiomyopathy. A case revealing a defect of complex I in the respiratory chain

We describe a 16-year-old Japanese girl with a mitochondrial encephalomyopathy who presented with progressive dementia, limb weakness and atrophy, episodic vomiting, generalized convulsions, myoclonic seizures, and hypertrophic cardiomyopathy. CT scan revealed transient focal low density areas in her occipital and parietal lobes, and cerebellar atrophy. The clinical features were consistent with mitochondrial myopathy, encephalopathy, lactic acidosis, and strokelike episodes (MELAS). Microscopically, most of muscle fibers in the skeletal muscles and heart were occupied by markedly increased mitochondria. Polarographic studies on mitochondria isolated from postmortem heart muscle showed severe impairment of oxidation of NADH-linked substrates in contrast to normal succinate oxidation. The rotenone-sensitive NADH-coenzyme Q reductase activity was markedly decreased in heart, skeletal muscle and liver mitochondria. The biochemical investigations have led to the identification of a defect of complex I in the respiratory chain. Reported cases of a defect of complex I have revealed pure myopathy, encephalopathy or encephalomyopathy. The reason for a varied clinical expression of a single defect remains to be clarified.

Structure and function of mitochondria: their organization and disorders

In this lecture, recent advances in studies on the structure and function of mitochondria were reviewed. In particular, in order to understand the etiology of mitochondrial myopathies, the mechanism of the biogenesis of the mitochondrial structure with proteins synthesized in mitochondria and in the cytoplasm was discussed; namely, how proteins encoded by mitochondrial DNA are biosynthesized, and how nuclealy encoded proteins are targeted into the appropriate compartments inside the mitochondria. Recent advances in mitochondriology have made it possible to isolate and purify the enzyme complexes and their subunits, which are involved in mitochondrial oxidative phosphorylation. Immunochemical analyses using a specific antibody against each complex or subunit enabled us to detect defects in individual subunits in mitochondria isolated from a small amount of biopsied material. Several examples of molecular defects revealed by these methods in patients with mitochondrial myopathies were presented, and the principles of their therapy are discussed on the basis of the pattern of the defect. Specific antibodies are also a powerful tool for the cloning of the human cDNAs for the subunits in the mitochondrial energy-transducing machinery. This approach will hopefully facilitate elucidation of the genetic defects underlying these disorders.