Mitochondrial myopathy: correlation between oxidative defect and mitochondrial DNA deletions at single fiber level

Mitochondria, oxidative DNA damage, and aging

Protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage is well known to be necessary to longevity. The relevance of mitochondrial DNA (mtDNA) to aging is suggested by the fact that the two most commonly measured forms of mtDNA damage, deletions and the oxidatively induced lesion 8-oxo-dG, increase with age. The rate of increase is species-specific and correlates with maximum lifespan. It is less clear that failure or inadequacies in the protection from reactive oxygen species (ROS) and from mitochondrial oxidative damage are sufficient to explain senescence. DNA containing 8-oxo-dG is repaired by mitochondria, and the high ratio of mitochondrial to nuclear levels of 8-oxo-dG previously reported are now suspected to be due to methodological difficulties. Furthermore, MnSOD -/+ mice incur higher than wild type levels of oxidative damage, but do not display an aging phenotype. Together, these findings suggest that oxidative damage to mitochondria is lower than previously thought, and that higher levels can be tolerated without physiological consequence. A great deal of work remains before it will be known whether mitochondrial oxidative damage is a "clock" which controls the rate of aging. The increased level of 8-oxo-dG seen with age in isolated mitochondria needs explanation. It could be that a subset of cells lose the ability to protect or repair mitochondria, resulting in their incurring disproportionate levels of damage. Such an uneven distribution could exceed the reserve capacity of these cells and have serious physiological consequences. Measurements of damage need to focus more on distribution, both within tissues and within cells. In addition, study must be given to the incidence and repair of other DNA lesions, and to the possibility that repair varies from species to species, tissue to tissue, and young to old.

Apoptosis in mitochondrial myopathies is linked to mitochondrial proliferation

Increased susceptibility to apoptosis has been shown in many models of mitochondrial defects but its relevance to human diseases is still discussed. We addressed the presence of apoptosis in muscle from patients with mitochondrial DNA (mtDNA) disorders. Taking advantage of the mosaic pattern of muscle morphological anomalies associated with heteroplasmic mtDNA alterations, we have used an in situ approach to address the relationship between apoptosis and respiratory defect, mitochondrial proliferation and mutation load. Different patterns of mitochondrial morphological alterations were provided by the analysis of muscles with large mtDNA deletion (16 cases) or with the MELAS mutation (4 cases). The patient's age at biopsy ranged from 0.4 to 66 years and the muscle mutant mtDNA proportion from 32 to 82%. Apoptotic muscle fibres were observed in a small proportion of muscle fibres of 16 out of the 20 biopsies by three different detection methods for different steps of apoptosis: caspase 3 activation, fragmentation of nuclear DNA [terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling (TUNEL) assay] or overexpression of the pro-apoptotic factor Bax. Analysis of apoptotic features in parallel to cytochrome c oxidase (COX) and succinate dehydrogenase activity of more than 34,000 individual muscle fibres showed that apoptosis occurred only in muscle fibres with mitochondrial proliferation (ragged red fibres, RRF) irrespective of their COX activity. Molecular analyses of single muscle fibres evidenced that, as expected, the presence of COX defect was associated with higher proportion of mutant mtDNA and lower amount of normal mtDNA. Within COX-defective fibres, the presence of mitochondrial proliferation was associated with increase of the mtDNA content but without change in the ratio between normal and mutant mtDNA molecules, thus showing that mitochondrial proliferation was accompanied by similar amplification of normal and mutant mtDNA molecules. Within RRF, apoptosis was associated with higher mutation proportion, suggesting that it was provoked by severe respiratory defect in the same time as increased mitochondrial mass. In conclusion, apoptosis most probably contributes to mitochondrial pathology. It is tightly linked to mitochondrial proliferation and high mutation load. When considering training therapeutics, one will have to take into account the possibility to induce apoptosis in parallel to mitochondrial proliferation.

New mutations in TK2 gene associated with mitochondrial DNA depletion

Mitochondrial deoxyribonucleic acid depletion syndromes are autosomal recessive disorders characterized by a reduction of the amount of mitochondrial deoxyribonucleic acid, which impairs the synthesis of respiratory chain complexes. Mutations in the deoxyguanosine kinase and polymerase gamma genes have been identified in hepatocerebral forms, whereas thymidine kinase 2 gene mutations have been found in patients with isolated myopathy, encephalomyopathy, or spinal muscular atrophy. Mutations in the gene encoding the beta subunit of the adenosine diphosphate-forming succinyl-coenzyme A synthetase have also been reported in a family. In this report, the clinical, molecular, morphologic, and biochemical features of five children from two independent families with an infantile encephalomyopathy are characterized. The affected children manifested muscle mitochondrial deoxyribonucleic acid depletion and three novel thymidine kinase 2 gene mutations. They consist of a homozygous substitution resulting in Ala to Val change at the highly conserved position 181 of thymidine kinase in the first family, and two heterozygous substitutions in the second family: a Cys to Trp change at residue 108 and a Leu to Pro change at residue 257 of the enzyme. Common clinical features associated with these TK2 mutations are a normal early developmental phase followed by psychomotor regression, encephalopathy often with epileptic seizures, and myopathy with features of a progressive dystrophic process.

Progressive external ophthalmoplegia characterized by multiple deletions of mitochondrial DNA: unraveling the pathogenesis of human mitochondrial DNA instability and the initiation of a genetic classification

Over the last decade, many sporadic and familial cases have been reported with multiple deletions of mitochondrial DNA (mtDNA) in postmitotic tissues. Most patients suffer from progressive external ophthalmoplegia (PEO) and may have a nuclear gene defect that predisposes to the accumulation of mtDNA deletions. Recently, positional cloning has led to the discovery of mutations in four such nuclear genes. Some mutations are dominant and others recessive. In all autosomal mutations, defective mtDNA replication and/or repair are probably responsible for the generation of secondary mtDNA deletions. There are also data suggestive of a prominent pathogenic role for disturbed nucleotide metabolism. We here present a tentative genotype-phenotype correlation. Since clinical presentations are heterogeneous and overlap with different previously described clinical syndromes, we advocate the use of a genetic, instead of a clinical, classification of disorders with multiple mtDNA deletions.

Antibody-based approaches to diagnosis and characterization of oxidative phosphorylation diseases

Mitochondrial disorders caused by defects in oxidative phosphorylation function are difficult to diagnose. Here we review the emerging use of antibody-based approaches for this diagnosis. Novel methods involving immunohistochemistry and immunocapture of defective enzymes for characterization are described that add to the arsenal of approaches available.

Cellular phenotypes of age-associated skeletal muscle mitochondrial abnormalities in rhesus monkeys

Rhesus monkey vastus lateralis muscle was examined histologically for age-associated electron transport system (ETS) abnormalities: fibers lacking cytochrome c oxidase activity (COX(-)) and/or exhibiting succinate dehydrogenase hyperreactivity (SDH(++)). Two hundred serial cross-sections (spanning 1600 microm) were obtained and analyzed for ETS abnormalities at regular intervals. The abundance and length of ETS abnormal regions increased with age. Extrapolating the data to the entire length of the fiber, up to 60% of the fibers were estimated to display ETS abnormalities in the oldest animal studied (34 years) compared to 4% in a young adult animal (11 years). ETS abnormal phenotypes varied with age and fiber type. Middle-aged animals primarily exhibited the COX(-) phenotype, while COX(-)/SDH(++) abnormalities were more common in old animals. Transition region phenotype was affected by fiber type with type 2 fibers first displaying COX(-) and then COX(-)/SDH(++) while type 1 fibers progressed from normal to SDH(++) and then to COX(-)/SDH(++). In situ hybridizations studies revealed an association of ETS abnormalities with deletions of the mitochondrial genome. By measuring cross-sectional area along the length of ETS abnormal fibers, we demonstrated that some of these fibers exhibit atrophy. Our data suggest mitochondrial (mtDNA) deletions and associated ETS abnormalities are contributors to age-associated fiber atrophy.

Method for in situ investigation of mitochondrial DNA deletions

A number of mitochondrial DNA (mtDNA) deletions have been recently identified in the tissues of patients with mitochondrial diseases and in elderly individuals. To investigate the distribution of mutant mitochondrial genomes within any particular tissue, we have developed a sensitive method based on indirect in situ PCR. Our experiments have shown that the new method had the advantage of selectively amplifying only mtDNA bearing the 4,977 bp deletion. We show that this method is more sensitive than in situ hybridization for detecting the 4977 bp mtDNA deletion while using only a low number of PCR cycles that minimize damage to tissue architecture. By using this method, we have demonstrated that the mutation does not occur uniformly among the cells of a given tissue/organ. This technique will be useful studying the distribution/localization of mtDNA mutations in individual cells of tissues and when combined with enzyme histochemical procedures in adjacent sections will enable the correlation between mtDNA mutations and bioenergy defects in single cells.

Kearns-Sayre syndrome with features of Pearson's marrow-pancreas syndrome and a novel 2905-base pair mitochondrial DNA deletion

Kearns-Sayre syndrome (KSS) and Pearson's marrow-pancreas syndrome (PMPS) are rare disorders caused by the same molecular defect, one of several deletion mutations in mitochondrial DNA (mtDNA). KSS is an encephalomyopathy with ophthalmoplegia, retinal degeneration, ataxia, and endocrine abnormalities. PMPS is a disorder of childhood characterized by refractory anemia, vacuolization of bone marrow cells, and exocrine pancreas dysfunction. Children with PMPS that have a mild phenotype, or are supported through bone marrow failure, often develop the encephalomyopathic features of KSS. The subject of numerous reports in the neuromuscular, genetic, and pediatric literature in recent years, very few cases of either disorder have ever been studied at autopsy. We report the results of our studies of a patient with clinically documented KSS who presented with renal dysfunction and was found to have a novel mtDNA deletion and degenerative changes in the central nervous system, retina, skeletal muscle, and pancreas.

Influences of caloric restriction on age-associated skeletal muscle fiber characteristics and mitochondrial changes in rats and mice

The effect of caloric restriction (CR) initiated in adult rats (17 months of age) on the abundance of deleted mitochondrial genomes, mitochondrial enzymatic abnormalities, and fiber number was examined in rat skeletal muscle. Vastus lateralis muscle from young (3-4 months) ad libitum-fed, old (30-32 months) restricted (35% and 50% CR, designated CR35 and CR50, respectively), and old ad libitum-fed rats (29 months) was studied. CR preserved fiber number and fiber-type composition in the CR50 rats. In the old rats from all groups, individual fibers were found with either no detectable cytochrome-c oxidase activity (COX-), hyperactive for succinate dehydrogenase activity (SDH++), or both COX- and SDH++. Muscle from the CR50 rats contained significantly fewer COX- and SDH++ fibers than did the muscle from the CR35 rats. CR50 rats also had significantly lower numbers of mtDNA deletion products in two (adductor longus and soleus) of the four muscles examined compared to CR35 rats. These data indicate that CR begun in late middle age can retard age-associated fiber loss and fiber-type changes as well as lower the number of skeletal muscle fibers exhibiting mitochondrial enzyme abnormalities. CR can also decrease the accumulation of deleted mitochondrial genomes.

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 delta 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 of 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.

Localization and amount of myoglobin and myoglobin mRNA in ragged-red fiber of patients with mitochondrial encephalomyopathy

The localization and amounts of myoglobin (Mb) and Mb mRNA in ragged-red fibers (RRF) in skeletal muscle of 6 patients with mitochondrial encephalomyopathy were examined immunohistochemically and by in situ hybridization. The amounts of Mb and Mb mRNA were expressed in terms of optical densities (ODs) of Mb immunostaining and Mb mRNA signals. In nonatrophic RRF, Mb was predominantly seen in the ragged-red region and Mb mRNA signals were increased throughout the sarcoplasm. The amounts of Mb and Mb mRNA in nonatrophic RRF were greater than those in nonatrophic non-RRF. In contrast, the localization and amount in atrophic RRF were similar to those in atrophic non-RRF. Thus, Mb synthesis in nonatrophic RRF may increase to compensate for mitochondrial dysfunction and to supply sufficient oxygen to mitochondria, but this compensatory function may be impaired in atrophic RRF.

High levels of mitochondrial DNA deletions in skeletal muscle of old rhesus monkeys

Mitochondrial DNA (mtDNA) deletions increase in abundance with age in many tissues, however, their calculated low levels (usually < 0.1%) in samples from tissue homogenates containing thousands of cells argue against physiologic significance. Through the analysis of defined numbers of cells (skeletal muscle fibers) from rhesus monkeys, we report that the calculated abundance of specific mtDNA deletions is dependent upon the number of fibers analyzed: as the number of fibers decreases, the calculated deletion abundance increases. Also, most mtDNA deletions appear to occur in a mosaic pattern, varying from cell to cell in size, number and abundance. These data support the hypothesis that mtDNA deletions can focally accumulate to high levels contributing to declines in mass and function of aging skeletal muscle.