Comparative biochemical studies in fibroblasts from patients with different forms of Leigh syndrome

Primary mitochondrial diseases: The intertwined pathophysiology of bioenergetic dysregulation, oxidative stress and neuroinflammation

OBJECTIVES AND SCOPE

Primary mitochondrial diseases (PMDs) are rare genetic disorders resulting from mutations in genes crucial for effective oxidative phosphorylation (OXPHOS) that can affect mitochondrial function. In this review, we examine the bioenergetic alterations and oxidative stress observed in cellular models of primary mitochondrial diseases (PMDs), shedding light on the intricate complexity between mitochondrial dysfunction and cellular pathology. We explore the diverse cellular models utilized to study PMDs, including patient-derived fibroblasts, induced pluripotent stem cells (iPSCs) and cybrids. Moreover, we also emphasize the connection between oxidative stress and neuroinflammation.

INSIGHTS

The central nervous system (CNS) is particularly vulnerable to mitochondrial dysfunction due to its dependence on aerobic metabolism and the correct functioning of OXPHOS. Similar to other neurodegenerative diseases affecting the CNS, individuals with PMDs exhibit several neuroinflammatory hallmarks alongside neurodegeneration, a pattern also extensively observed in mouse models of mitochondrial diseases. Based on histopathological analysis of postmortem human brain tissue and findings in mouse models of PMDs, we posit that neuroinflammation is not merely a consequence of neurodegeneration but a potential pathogenic mechanism for disease progression that deserves further investigation. This recognition may pave the way for novel therapeutic strategies for this group of devastating diseases that currently lack effective treatments.

SUMMARY

In summary, this review provides a comprehensive overview of bioenergetic alterations and redox imbalance in cellular models of PMDs while underscoring the significance of neuroinflammation as a potential driver in disease progression.

Mitochondrial DNA and Diseases of the Nervous System: The Spectrum

The past 9 years have witnessed the development of a new chapter in human pathology related to mutations in the "other genome" or the "25th chromosome," namely mitochondrial DNA (mtDNA). An astounding array of multisystemic disorders, almost always involving muscle and brain (mitochondrial encephalomyopathies) have been attributed to over 50 point mutations and a multitude of rearrangements in mtDNA. Here, we review the still expanding spectrum of proven or putative mtDNA-related disorders, and we try to explain some peculiarities of these diseases according to the new rules of "mitochondrial genetics." NEURO SCIENTIST 4:53-63, 1998

Impaired Mitochondrial Energy Production Causes Light-Induced Photoreceptor Degeneration Independent of Oxidative Stress

Two insults often underlie a variety of eye diseases including glaucoma, optic atrophy, and retinal degeneration—defects in mitochondrial function and aberrant Rhodopsin trafficking. Although mitochondrial defects are often associated with oxidative stress, they have not been linked to Rhodopsin trafficking. In an unbiased forward genetic screen designed to isolate mutations that cause photoreceptor degeneration, we identified mutations in a nuclear-encoded mitochondrial gene, ppr, a homolog of human LRPPRC. We found that ppr is required for protection against light-induced degeneration. Its function is essential to maintain membrane depolarization of the photoreceptors upon repetitive light exposure, and an impaired phototransduction cascade in ppr mutants results in excessive Rhodopsin1 endocytosis. Moreover, loss of ppr results in a reduction in mitochondrial RNAs, reduced electron transport chain activity, and reduced ATP levels. Oxidative stress, however, is not induced. We propose that the reduced ATP level in ppr mutants underlies the phototransduction defect, leading to increased Rhodopsin1 endocytosis during light exposure, causing photoreceptor degeneration independent of oxidative stress. This hypothesis is bolstered by characterization of two other genes isolated in the screen, pyruvate dehydrogenase and citrate synthase. Their loss also causes a light-induced degeneration, excessive Rhodopsin1 endocytosis and reduced ATP without concurrent oxidative stress, unlike many other mutations in mitochondrial genes that are associated with elevated oxidative stress and light-independent photoreceptor demise.

Some mitochondrial disorders cause blindness through increased oxidative stress. This study shows that in other such disorders, light-activated photoreceptors degenerate because the shortfall in mitochondrial energy production impairs rhodopsin trafficking and induces toxicity.

Mitochondrial dysfunction is associated with a number of metabolic and neurological diseases such as Leigh syndrome and progressive blindness. Increased oxidative stress, which is often associated with mitochondrial dysfunction, is thought to be a common cause of disease progression. Here, we identified nuclear genes that encode mitochondrial proteins, whose loss causes the demise of photoreceptor neurons. Contrary to the common idea that this degeneration is triggered by elevated levels of oxidative stress, we find no change in the levels of oxidative stress. We show that activating photoreceptor neurons with light significantly increases energy production, and that this process is required to sustain their activity. Mitochondrial dysfunction impairs this capacity and leads to a premature termination of the light response. This in turn impairs the cycling of the light-sensitive receptor Rhodopsin in photoreceptors, and Rhodopsin accumulates in the cell inducing toxicity. This distinct mechanism of degeneration suggests that different mitochondrial diseases may follow different paths of disease progression and would hence respond differently to treatments.

Visualization of ATP with luciferin-luciferase reaction in mouse skeletal muscles using an "in vivo cryotechnique"

Adenosine triphosphate (ATP) is a well-known energy source for muscle contraction. In this study, to visualize localization of ATP, a luciferin-luciferase reaction (LLR) was performed in mouse skeletal muscle with an "in vivo cryotechnique" (IVCT). First, to confirm if ATP molecules could be trapped and detected after glutaraldehyde (GA) treatment, ATP was directly attached to glass slides with GA, and LLR was performed. The LLR was clearly detected as an intentional design of the ATP attachment. The intensity of the light unit by LLR was correlated with the concentration of the GA-treated ATP in vitro. Next, LLR was evaluated in mouse skeletal muscles with IVCT followed by freeze-substitution fixation (FS) in acetone-containing GA. In such tissue sections the histological structure was well maintained, and the intensity of LLR in areas between muscle fibers and connective tissues was different. Moreover, differences in LLR among muscle fibers were also detected. For the IVCT-FS tissue sections, diaminobenzidine (DAB) reactions were clearly detected in type I muscle fibers and erythrocytes in capillaries, which demonstrated flow shape. Thus, it became possible to perform microscopic evaluation of the numbers of ATP molecules in the mouse skeletal muscles with IVCT, which mostly reflect living states.

Mitochondrial gene therapy improves respiration, biogenesis, and transcription in G11778A Leber's hereditary optic neuropathy and T8993G Leigh's syndrome cells

Many incurable mitochondrial disorders result from mutant mitochondrial DNA (mtDNA) and impaired respiration. Leigh's syndrome (LS) is a fatal neurodegenerative disorder of infants, and Leber's hereditary optic neuropathy (LHON) causes blindness in young adults. Treatment of LHON and LS cells harboring G11778A and T8993G mutant mtDNA, respectively, by >90%, with healthy donor mtDNA complexed with recombinant human mitochondrial transcription factor A (rhTFAM), improved mitochondrial respiration by ∼1.2-fold in LHON cells and restored >50% ATP synthase function in LS cells. Mitochondrial replication, transcription, and translation of key respiratory genes and proteins were increased in the short term. Increased NRF1, TFAMB1, and TFAMA expression alluded to the activation of mitochondrial biogenesis as a mechanism for improving mitochondrial respiration. These results represent the development of a therapeutic approach for LHON and LS patients in the near future.

Biochemical consequences in yeast of the human mitochondrial DNA 8993T>C mutation in the ATPase6 gene found in NARP/MILS patients

We have created and analyzed the properties of a yeast model of the human mitochondrial DNA T8993C mutation that has been associated with maternally-inherited Leigh syndrome and/or with neurogenic muscle weakness, ataxia and retinitis pigmentosa. This mutation changes a highly conserved leucine to proline in the Atp6p subunit of the ATP synthase, at position 156 in the human protein, position 183 in yeast. In vitro the yeast T8993C mitochondria showed a 40-50% decrease in the rate of ATP synthesis. The ATP-driven translocation of protons across the inner mitochondrial membrane was normal in the mutant and fully sensitive to oligomycin, an inhibitor of the ATP synthase proton channel. However under conditions of maximal ATP hydrolytic activity, using non-osmotically protected mitochondria, the mutant ATPase activity was poorly inhibited by oligomycin (by 40% versus 85% in wild type cells). These anomalies were attributed by BN-PAGE and mitochondrial protein synthesis analyses to a less efficient incorporation of Atp6p within the ATP synthase. Interestingly, the cytochrome c oxidase content was selectively decreased by 40-50% in T8993C yeast, apparently due to a reduced synthesis of its mitochondrially encoded Cox1p subunit. This observation further supports the existence of a control of cytochrome c oxidase expression by the ATP synthase in yeast mitochondria. Despite the ATPase deficiency, growth of the atp6-L183P mutant on respiratory substrates and the efficiency of oxidative phosphorylation were similar to that of wild type, indicating that the mutation did not affect the proton permeability of the mitochondrial inner membrane.

Pathogenesis and treatment of mitochondrial disorders

In the past 50 years, our understanding of the biochemical and molecular causes of mitochondrial diseases, defined restrictively as disorders due to defects of the mitochondrial respiratory chain (RC), has made great strides. Mitochondrial diseases can be due to mutations in mitochondrial DNA (mtDNA) or in nuclear DNA (nDNA) and each group can be subdivided into more specific classes. Thus, mtDNA-related disorders can result from mutations in genes affecting protein synthesis in toto or mutations in protein-coding genes. Mendelian mitochondrial disorders can be attributed to mutations in genes that (i) encode subunits of the RC ("direct hits"); (ii) encode assembly proteins or RC complexes ("indirect hits"); (iii) encode factors needed for mtDNA maintenance, replication, or translation (intergenomic signaling); (iv) encode components of the mitochondrial protein import machinery; (v) control the synthesis and composition of mitochondrial membrane phospholipids; and (vi) encode proteins involved in mitochondrial dynamics.In contrast to this wealth of knowledge about etiology, our understanding of pathogenic mechanism is very limited. We discuss pathogenic factors that can influence clinical expression, especially ATP shortage and reactive oxygen radicals (ROS) excess. Therapeutic options are limited and fall into three modalities: (i) symptomatic interventions, which are palliative but crucial for day-to-day management; (ii) radical approaches aimed at correcting the biochemical or molecular error, which are interesting but still largely experimental; and (iii) pharmacological means of interfering with the pathogenic cascade of events (e.g. boosting ATP production or scavenging ROS), which are inconsistently and incompletely effective, but can be safe and helpful.

Mitochondrial ATP synthase disorders: molecular mechanisms and the quest for curative therapeutic approaches

In mammals, the majority of cellular ATP is produced by the mitochondrial F1F(O)-ATP synthase through an elaborate catalytic mechanism. While most subunits of this enzymatic complex are encoded by the nuclear genome, a few essential components are encoded in the mitochondrial genome. The biogenesis of this multi-subunit enzyme is a sophisticated multi-step process that is regulated on levels of transcription, translation and assembly. Defects that result in diminished abundance or functional impairment of the F1F(O)-ATP synthase can cause a variety of severe neuromuscular disorders. Underlying mutations have been identified in both the nuclear and the mitochondrial DNA. The pathogenic mechanisms are only partially understood. Currently, the therapeutic options are extremely limited. Alternative methods of treatment have however been proposed, but still encounter several technical difficulties. The application of novel scientific approaches promises to deepen our understanding of the molecular mechanisms of the ATP synthase, unravel novel therapeutic pathways and improve the unfortunate situation of the patients suffering from such diseases.

Biochemical and genetic analysis of Leigh syndrome patients in Korea

Sixteen Korean patients with Leigh syndrome were identified at the Seoul National University Children's Hospital in 2001-2006. Biochemical or molecular defects were identified in 14 patients (87.5%). Thirteen patients had respiratory chain enzyme defects; 9 had complex I deficiency, and 4 had combined defects of complex I+III+IV. Based on the biochemical defects, targeted genetic studies in 4 patients with complex I deficiency revealed two heteroplasmic mitochondrial DNA mutations in ND genes. One patient had the mitochondrial DNA T8993G point mutation. No mitochondrial DNA defects were identified in 11 (68.7%) of our LS patients, who probably have mutations in nuclear DNA. Although a limited study based in a single tertiary medical center, our findings suggest that isolated complex I deficiency may be the most common cause of Leigh syndrome in Korea.

A yeast model of the neurogenic ataxia retinitis pigmentosa (NARP) T8993G mutation in the mitochondrial ATP synthase-6 gene

NARP (neuropathy, ataxia, and retinitis pigmentosa) and MILS (maternally inherited Leigh syndrome) are mitochondrial disorders associated with point mutations of the mitochondrial DNA (mtDNA) in the gene encoding the Atp6p subunit of the ATP synthase. The most common and studied of these mutations is T8993G converting the highly conserved leucine 156 into arginine. We have introduced this mutation at the corresponding position (183) of yeast Saccharomyces cerevisiae mitochondrially encoded Atp6p. The "yeast NARP mutant" grew very slowly on respiratory substrates, possibly because mitochondrial ATP synthesis was only 10% of the wild type level. The mutated ATP synthase was found to be correctly assembled and present at nearly normal levels (80% of the wild type). Contrary to what has been reported for human NARP cells, the reverse functioning of the ATP synthase, i.e. ATP hydrolysis in the F(1) coupled to F(0)-mediated proton translocation out of the mitochondrial matrix, was significantly compromised in the yeast NARP mutant. Interestingly, the oxygen consumption rate in the yeast NARP mutant was decreased by about 80% compared with the wild type, due to a selective lowering in cytochrome c oxidase (complex IV) content. This finding suggests a possible regulatory mechanism between ATP synthase activity and complex IV expression in yeast mitochondria. The availability of a yeast NARP model could ease the search for rescuing mechanisms against this mitochondrial disease.

The role of mitochondria in inherited neurodegenerative diseases

In the past decade, the genetic causes underlying familial forms of many neurodegenerative disorders, such as Huntington's disease, Parkinson's disease, Alzheimer's disease, amyotrophic lateral sclerosis, Friedreich ataxia, hereditary spastic paraplegia, dominant optic atrophy, Charcot-Marie-Tooth type 2A, neuropathy ataxia and retinitis pigmentosa, and Leber's hereditary optic atrophy have been elucidated. However, the common pathogenic mechanisms of neuronal death are still largely unknown. Recently, mitochondrial dysfunction has emerged as a potential 'lowest common denominator' linking these disorders. In this review, we discuss the body of evidence supporting the role of mitochondria in the pathogenesis of hereditary neurodegenerative diseases. We summarize the principal features of genetic diseases caused by abnormalities of mitochondrial proteins encoded by the mitochondrial or the nuclear genomes. We then address genetic diseases where mutant proteins are localized in multiple cell compartments, including mitochondria and where mitochondrial defects are likely to be directly caused by the mutant proteins. Finally, we describe examples of neurodegenerative disorders where mitochondrial dysfunction may be 'secondary' and probably concomitant with degenerative events in other cell organelles, but may still play an important role in the neuronal decay. Understanding the contribution of mitochondrial dysfunction to neurodegeneration and its pathophysiological basis will significantly impact our ability to develop more effective therapies for neurodegenerative diseases.

Metallothionein isoform 2A expression is inducible and protects against ROS-mediated cell death in rotenone-treated HeLa cells

The role of MT (metallothionein) gene expression was investigated in rotenone-treated HeLa cells to induce a deficiency of NADH:ubiquinone oxidoreductase (complex I). Complex I deficiency leads to a diversity of cellular consequences, including production of ROS (reactive oxygen species) and apoptosis. HeLa cells were titrated with rotenone, resulting in dose-dependent decrease in complex I activity and elevated ROS production at activities lower than 33%. Expression of MT2A (MT isoform 2A), but not MT1A or MT1B RNA, was significantly inducible by rotenone (up to 7-fold), t-BHP (t-butyl hydroperoxide; 5-fold) and CdCl2 (50-fold), but not ZnCl2. Myxothiazol treatment did not elevate either ROS or MT2A levels, which supports a ROS-related mechanism for rotenone-induced MT2A expression. To evaluate the role of MT2A expression, MT2A and MT1B were overexpressed in HeLa cells and treated with rotenone. Compared with control and MT1B-overexpressing cells, ROS production was significantly lower and cell viability higher in MT2A-overexpressing HeLa cells when ROS production was enhanced by treatment with t-BHP. Mitochondrial membrane potential was noticeably less reduced in both MT-overexpressing cell lines. MT2A overexpression in rotenone-treated cells also significantly reduced or delayed apoptosis induction, as measured by caspase 3/7 activity and cytosolic nucleosome enrichment. We conclude that MT2A offers significant protection against the main death-causing consequences of rotenone-induced complex I deficiency in HeLa cells. Our results are in support of the protective role against oxidative stress ascribed to MTs and provide evidence that MT2A expression may be a beneficial downstream adaptive response in complex I-deficient cells.

The role of mitochondria in the pathogenesis of neurodegenerative diseases

A growing body of evidence indicates that mitochondrial dysfunction may play an important role in the pathogenesis of many neurodegenerative disorders. Because mitochondrial metabolism is not only the principal source of high energy intermediates, but also of free radicals, it has been suggested that inherited or acquired mitochondrial defects could be the cause of neuronal degeneration as a consequence of energy defects and oxidative damage. Mitochondrial respiratory chain dysfunction has been reported in association with primary mitochondrial DNA abnormalities, and also as a consequence of mutations in nuclear genes directly involved in mitochondrial functions, such as SURF1, frataxin, and paraplegin. Defects of oxidative phosphorylation and increased free radical production have also been observed in diseases that are not due to primary mitochondrial abnormalities. In these cases, the mitochondrial dysfunction is likely to be an epiphenomenon, which, nevertheless, could be of importance in precipitating a cascade of events leading to cell death. In either case, understanding the role of mitochondria in the pathogenesis of neurodegenerative diseases could be important for the development of therapeutic strategies in these disorders.

Ageing and subcellular distribution of mitochondria: role of mitochondrial DNA deletions and energy production

The rapid growing population of elderly illustrates the importance of understanding the mechanisms responsible for ageing and the detrimental effects on health associated with increasing age. One of the primary mechanisms may be because of the accumulation of mtDNA damage and oxidative damage with age. Previous studies have examined this correlation in post-mitotic tissues such as skeletal muscle, heart and brain with decreased mitochondrial function, such as enzymatic activities of the electron transport chain and ATP production. However, regional differences in the subcellular location of mitochondria exist and most studies have failed to differentiate the effects of these two autonomous fractions, the subsarcolemmal and intermyofibrillar populations. Hence, while future research attempts to explain the mechanisms responsible for ageing in the mitochondrion, it should also take into account the independent pathways of these two distinctly different populations.

Inhibition of mitochondrial Na+-Ca2+ exchange restores agonist-induced ATP production and Ca2+ handling in human complex I deficiency

Human mitochondrial complex I (NADH:ubiquinone oxidoreductase) of the oxidative phosphorylation system is a multiprotein assembly comprising both nuclear and mitochondrially encoded subunits. Deficiency of this complex is associated with numerous clinical syndromes ranging from highly progressive, often early lethal encephalopathies, of which Leigh disease is the most frequent, to neurodegenerative disorders in adult life, including Leber's hereditary optic neuropathy and Parkinson disease. We show here that the cytosolic Ca2+ signal in response to hormonal stimulation with bradykinin was impaired in skin fibroblasts from children between the ages of 0 and 5 years with an isolated complex I deficiency caused by mutations in nuclear encoded structural subunits of the complex. Inhibition of mitochondrial Na+-Ca2+ exchange by the benzothiazepine CGP37157 completely restored the aberrant cytosolic Ca2+ signal. This effect of the inhibitor was paralleled by complete restoration of the bradykinin-induced increases in mitochondrial Ca2+ concentration and ensuing ATP production. Thus, impaired mitochondrial Ca2+ accumulation during agonist stimulation is a major consequence of human complex I deficiency, a finding that may provide the basis for the development of new therapeutic approaches to this disorder.

A mitochondrial ATPase 6 mutation is associated with Leigh syndrome in a family and affects proton flow and adenosine triphosphate output when modeled in Escherichia coli

A multidisciplinary strategy was used to identify the molecular defect in a family with Leigh syndrome (LS). The propositus presented severe developmental delay, an ataxic-spastic gait and seizures. She died at 3.5 y of age from cardiorespiratory arrest. Postmortem examination disclosed pathological features typical of LS. A 12-y-old sister is affected with the same disease. Respiratory chain enzyme complex activities in skeletal muscle biopsy were normal. Adenosine triphosphate (ATP) synthesis during oxidative phosphorylation in skin fibroblasts mitochondria showed a severely hampered ATP production. Mitochondrial DNA sequencing revealed a new mutation in the ATPase 6 gene (T9176G). Site-directed mutagenesis in Escherichia coli strains was used to measure H+ pumping and ATP synthesis. Results were comparable to findings obtained in human cells. These data corroborate the use of E. coli strains as a feasible "animal" model for functional studies in pathogenic mutations of the ATPase 6 gene.

A zebrafish model for pyruvate dehydrogenase deficiency: rescue of neurological dysfunction and embryonic lethality using a ketogenic diet

Defects in the pyruvate dehydrogenase (PDH) complex result in severe neurological dysfunction, congenital lactic acidosis, growth retardation, and early death. Current treatments for PDH deficiency are administered postnatally and are generally unsuccessful. Because many patients with this disease are born with irreversible defects, a model system for the development of effective pre- and postnatal therapies would be of great value. In a behavioral genetic screen aimed to identify zebrafish with visual function defects, we previously isolated two alleles of the recessive lethal mutant no optokinetic response a (noa). Here we report that noa is deficient for dihydrolipoamide S-acetyltransferase (Dlat), the PDH E2 subunit, and exhibits phenotypes similar to human patients with PDH deficiency. To rescue the deficiency, we added ketogenic substrates to the water in which the embryos develop. This treatment successfully restored vision, promoted feeding behavior, reduced lactic acidosis, and increased survival. Our study demonstrates an approach for establishing effective therapies for PDH deficiency and other congenital diseases that affect early embryonic development.

An algal nucleus-encoded subunit of mitochondrial ATP synthase rescues a defect in the analogous human mitochondrial-encoded subunit

Unlike most organisms, the mitochondrial DNA (mtDNA) of Chlamydomonas reinhardtii, a green alga, does not encode subunit 6 of F(0)F(1)-ATP synthase. We hypothesized that C. reinhardtii ATPase 6 is nucleus encoded and identified cDNAs and a single-copy nuclear gene specifying this subunit (CrATP6, with eight exons, four of which encode a mitochondrial targeting signal). Although the algal and human ATP6 genes are in different subcellular compartments and the encoded polypeptides are highly diverged, their secondary structures are remarkably similar. When CrATP6 was expressed in human cells, a significant amount of the precursor polypeptide was targeted to mitochondria, the mitochondrial targeting signal was cleaved within the organelle, and the mature polypeptide was assembled into human ATP synthase. In spite of the evolutionary distance between algae and mammals, C. reinhardtii ATPase 6 functioned in human cells, because deficiencies in both cell viability and ATP synthesis in transmitochondrial cell lines harboring a pathogenic mutation in the human mtDNA-encoded ATP6 gene were overcome by expression of CrATP6. The ability to express a nucleus-encoded version of a mammalian mtDNA-encoded protein may provide a way to import other highly hydrophobic proteins into mitochondria and could serve as the basis for a gene therapy approach to treat human mitochondrial diseases.

Rescue of a deficiency in ATP synthesis by transfer of MTATP6, a mitochondrial DNA-encoded gene, to the nucleus

A T-->G transversion at nt 8993 in mitochondrial DNA of MTATP6 (encoding ATPase 6 of complex V of the respiratory chain) causes impaired mitochondrial ATP synthesis in two related mitochondrial disorders: neuropathy, ataxia and retinitis pigmentosa and maternally inherited Leigh syndrome. To overcome the biochemical defect, we expressed wildtype ATPase 6 protein allotopically from nucleus-transfected constructs encoding an amino-terminal mitochondrial targeting signal appended to a recoded ATPase 6 gene (made compatible with the universal genetic code) that also contained a carboxy-terminal FLAG epitope tag. After transfection of human cells, the precursor polypeptide was expressed, imported into and processed within mitochondria, and incorporated into complex V. Allotopic expression of stably transfected constructs in cytoplasmic hybrids (cybrids) homoplasmic with respect to the 8993T-->G mutation showed a significantly improved recovery after growth in selective medium as well as a significant increase in ATP synthesis. This is the first successful demonstration of allotopic expression of an mtDNA-encoded polypeptide in mammalian cells and could form the basis of a genetic approach to treat a number of human mitochondrial disorders.

Pathogenesis of primary defects in mitochondrial ATP synthesis

Maternally inherited mutations in the mtDNA-encoded ATPase 6 subunit of complex V (ATP synthase) of the respiratory chain/oxidative phosphorylation system are responsible for a subgroup of severe and often-fatal disorders characterized predominantly by lesions in the brain, particularly in the striatum. These include NARP (neuropathy, ataxia, and retinitis pigmentosa), MILS (maternally inherited Leigh syndrome), and FBSN (familial bilateral striatal necrosis). Of the five known pathogenic mutations causing these disorders, four are located at two codons (156 and 217), each of which can suffer mutations converting a conserved leucine to either an arginine or a proline. Based on the accumulating data on both the structure of ATP synthase and the mechanism by which rotary catalysis couples proton flow to ATP synthesis, we propose a model that may help explain why mutations at codons 156 and 217 are pathogenic.

Mitochondrial dysfunction and neuromuscular disease

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

Structure, functioning, and assembly of the ATP synthase in cells from patients with the T8993G mitochondrial DNA mutation. Comparison with the enzyme in Rho(0) cells completely lacking mtdna

The structure and functioning of the ATP synthase of human fibroblast cell lines with 91 and 100%, respectively, of the T8993G mutation have been studied, with MRC5 human fibroblasts and Rho(0) cells derived from this cell line as controls. ATP hydrolysis was normal but ATP synthesis was reduced by 60% in the 100% mutants. Both activities were highly oligomycin-sensitive. The levels of F(1)F(0) were close to normal, and the enzyme was stable. It is concluded that the loss of ATP synthesis is because of disruption of the proton translocation step within the F(0) part. This is supported by membrane potential measurements using the dye JC-1. Cells with a 91% mutation load grew well and showed only a 25% loss in ATP synthesis. This much reduced effect for only a 9% difference in mutation load mirrors the reduced pathogenicity in patients. F(1)F(0) has been purified for the first time from human cell lines. A partial complex was obtained from Rho(0) cells containing the F(1) subunits associated with several stalk, as well as F(0) subunits, including oligomycin sensitivity conferring protein, b, and c subunits. This partial complex no longer binds inhibitor protein.

Rearrangements of human mitochondrial DNA (mtDNA): new insights into the regulation of mtDNA copy number and gene expression

Mitochondria from patients with Kearns-Sayre syndrome harboring large-scale rearrangements of human mitochondrial DNA (mtDNA; both partial deletions and a partial duplication) were introduced into human cells lacking endogenous mtDNA. Cytoplasmic hybrids containing 100% wild-type mtDNA, 100% mtDNA with partial duplications, and 100% mtDNA with partial deletions were isolated and characterized. The cell lines with 100% deleted mtDNAs exhibited a complete impairment of respiratory chain function and oxidative phosphorylation. In contrast, there were no detectable respiratory chain or protein synthesis defects in the cell lines with 100% duplicated mtDNAs. Unexpectedly, the mass of mtDNA was identical in all cell lines, despite the fact that different lines contained mtDNAs of vastly different sizes and with different numbers of replication origins, suggesting that mtDNA copy number may be regulated by tightly controlled mitochondrial dNTP pools. In addition, quantitation of mtDNA-encoded RNAs and polypeptides in these lines provided evidence that mtDNA gene copy number affects gene expression, which, in turn, is regulated at both the post-transcriptional and translational levels.

Catalytic activities of mitochondrial ATP synthase in patients with mitochondrial DNA T8993G mutation in the ATPase 6 gene encoding subunit a

We investigated the biochemical phenotype of the mtDNA T8993G point mutation in the ATPase 6 gene, associated with neurogenic muscle weakness, ataxia, and retinitis pigmentosa (NARP), in three patients from two unrelated families. All three carried >80% mutant genome in platelets and were manifesting clinically various degrees of the NARP phenotype. Coupled submitochondrial particles prepared from platelets capable of succinate-sustained ATP synthesis were studied using very sensitive and rapid luminometric and fluorescence methods. A sharp decrease (>95%) in the succinate-sustained ATP synthesis rate of the particles was found, but both the ATP hydrolysis rate and ATP-driven proton translocation (when the protons flow from the matrix to the cytosol) were minimally affected. The T8993G mutation changes the highly conserved residue Leu(156) to Arg in the ATPase 6 subunit (subunit a). This subunit, together with subunit c, is thought to cooperatively catalyze proton translocation and rotate, one with respect to the other, during the catalytic cycle of the F(1)F(0) complex. Our results suggest that the T8993G mutation induces a structural defect in human F(1)F(0)-ATPase that causes a severe impairment of ATP synthesis. This is possibly due to a defect in either the vectorial proton transport from the cytosol to the mitochondrial matrix or the coupling of proton flow through F(0) to ATP synthesis in F(1). Whatever mechanism is involved, this leads to impaired ATP synthesis. On the other hand, ATP hydrolysis that involves proton flow from the matrix to the cytosol is essentially unaffected.

Clinical and molecular studies in three Portuguese mtDNA T8993G families

The T8993G mutation in the mitochondrial DNA adenosine triphosphatase 6 gene represents an important cause of maternally inherited Leigh's syndrome. Reported are the clinical findings and mutational loads in three Portuguese T8993G pedigrees. Polymerase chain reaction-restriction fragment length polymorphism analyses demonstrated the T8993G mutation in a high percentage of tissues from all patients (97% +/- 2.3%), but it was less abundant in the blood from 14 maternal relatives. The disease progressed severely in the probands but did not have the fatal course reported by others. To test whether this prolonged course was related to the presence of a specific, disease-associated haplogroup the origin of the mutational event in Portugal was traced. Haplotype investigation revealed an independent occurrence of the mutation in the three probands. These analyses represent the first molecular characterization of Portuguese patients with Leigh's syndrome.

Mitochondrial DNA mutations at nucleotide 8993 show a lack of tissue- or age-related variation

Two pathogenic mitochondrial DNA mutations, a T-to-G substitution (8993T > G) and a T-to-C substitution (8993T > C), at nucleotide 8993 have been reported. We describe 13 pedigrees with mitochondrial DNA mutations at nucleotide 8993; 10 pedigrees with the 8993T > G mutation and three with the 8993T > C mutation. Prenatal diagnosis of the nucleotide 8993 mutations is technically possible. However, there are three major concerns: (i) that there is variation in mutant loads among tissues; (ii) that the mutant load in a tissue may change over time; and (iii) that the genotype-phenotype correlation is not clearly understood. We have used the 13 pedigrees to determine specifically the extent of tissue- and age-related variation of the two mutations at nucleotide 8993 in the mitochondrial DNA. The tissue variation was investigated by analysing two or more different tissues from a total of 18 individuals. The age-related variation of the mutation was investigated by comparing the amount of both mutations in blood taken at birth and at a later age. No substantial tissue variation was found, nor was there any substantial change in the proportion of either mutation over periods of 8-23 years in the four individuals studied. In addition, we noted that two features were remarkably common in families with nucleotide 8993 mutations, namely (i) unexplained infant death (8 cases in 13 pedigrees); and (ii) de novo mutations (5 of the 10 8993T > G pedigrees).

A calcium signaling defect in the pathogenesis of a mitochondrial DNA inherited oxidative phosphorylation deficiency

In recent years, genetic defects of the mitochondrial genome (mtDNA) were shown to be associated with a heterogeneous group of disorders, known as mitochondrial diseases, but the cellular events deriving from the molecular lesions and the mechanistic basis of the specificity of the syndromes are still incompletely understood. Mitochondrial calcium (Ca2+) homeostasis depends on close contacts with the endoplasmic reticulum and is essential in modulating organelle function. Given the strong dependence of mitochondrial Ca2+ uptake on the membrane potential and the intracellular distribution of the organelle, both of which may be altered in mitochondrial diseases, we investigated the occurrence of defects in mitochondrial Ca2+ handling in living cells with either the tRNALys mutation of MERRF (myoclonic epilepsy with ragged-red fibers) or the ATPase mutation of NARP (neurogenic muscle weakness, ataxia and retinitis pigmentosa). There was a derangement of mitochondrial Ca2+ homeostasis in MERRF, but not in NARP cells, whereas cytosolic Ca2+ responses were normal in both cell types. Treatment of MERRF cells with drugs affecting organellar Ca2+ transport mostly restored both the agonist-dependent mitochondrial Ca2+ uptake and the ensuing stimulation of ATP production. These results emphasize the differences in the cellular pathogenesis of the various mtDNA defects and indicate specific pharmacological approaches to the treatment of some mitochondrial diseases.

Genetic counseling and prenatal diagnosis for the mitochondrial DNA mutations at nucleotide 8993

Mitochondrial genetics is complicated by heteroplasmy, or mutant load, which may be from 1%-99%, and thus may produce a gene dosage-type effect. Limited data are available for genotype/phenotype correlations in disorders caused by mtDNA mutations; therefore, prenatal diagnosis for mtDNA mutations has been hindered by an inability to predict accurately the clinical severity expected from a mutant load measured in fetal tissue. After reviewing 44 published and 12 unpublished pedigrees, we considered the possibility of prenatal diagnosis for two common mtDNA mutations at nucleotide 8993. We related the severity of symptoms to the mutant load and predicted the clinical outcome of a given mutant load. We also used the available data to generate empirical recurrence risks for genetic counseling, which may be used in conjunction with prenatal diagnosis.

Personality profiles of mothers of children with mitochondrial disorders

Mothers of children with mitochondrial disorders, inherited neurodegenerative diseases, are faced with a frightening diagnosis and numerous demands associated with caring for these children. The psychological profile of mothers whose children carry a mitochondrial disorder is unknown. Forty-two mothers of children with mitochondrial disorders were interviewed and administered the Minnesota Multiphasic Personality Inventory--Second Edition (MMPI-2). Fifty-six per cent of the mothers had scores in the pathological range on three or more scales, most notably on Hypochondriasis, Hysteria and Paranoia scales. The MMPI-2 profile is associated with situational anxiety and stress or may be associated with carrier status characteristics. Whatever the cause, future studies need to determine whether supportive services can reduce the level of anxiety and stress in mothers of children with these disorders.

Functional consequences of the 3460-bp mitochondrial DNA mutation associated with Leber's hereditary optic neuropathy

Complex I is the largest of the mitochondrial respiratory chain proteins, and contains subunits encoded by both mitochondrial and nuclear genomes. Leber's hereditary optic neuropathy has been clearly linked to mutations of mitochondrial DNA complex I genes, and variable complex I functional defects have been reported. We have confirmed an approximate 60% defect in mitochondrial NADH CoQ1 reductase activity in cultured fibroblasts bearing the 3460-bp G to A mutation within the ND1 gene. However complex I-linked ATP synthesis was found to be normal in these fibroblasts. A 60% rotenone-induced decrease in complex I activity was shown to reduce ATP synthesis in normal fibroblasts, indicating that this level of complex I activity was below the threshold required to affect ATP synthesis. Although 3460 LHON mitochondria were less sensitive to rotenone inhibition, this did not explain the decreased complex I activity as the rotenone insensitive activity was not increased, nor did the inhibitor diphenyleneiodonium inhibit the NADH CoQ1 reductase activity to a greater extent. Decreased NADH cytochrome c reductase activity in cybrids homoplasmic for the 3460 LHON mtDNA mutation confirmed that the decrease in complex I activity was not specific to the assay used and was not caused by inhibitory effects of ubiquinone analogues used in the NADH CoQ1 reductase assay. These findings have important implications for our understanding of complex I dysfunction in the pathogenesis of 3460 Leber's hereditary optic neuropathy.

Oligomycin induces a decrease in the cellular content of a pathogenic mutation in the human mitochondrial ATPase 6 gene

A T --> G mutation at position 8993 in human mitochondrial DNA is associated with the syndrome neuropathy, ataxia, and retinitis pigmentosa and with a maternally inherited form of Leigh's syndrome. The mutation substitutes an arginine for a leucine at amino acid position 156 in ATPase 6, a component of the F0 portion of the mitochondrial ATP synthase complex. Fibroblasts harboring high levels of the T8993G mutation have decreased ATP synthesis activity, but do not display any growth defect under standard culture conditions. Combining the notions that cells with respiratory chain defects grow poorly in medium containing galactose as the major carbon source, and that resistance to oligomycin, a mitochondrial inhibitor, is associated with mutations in the ATPase 6 gene in the same transmembrane domain where the T8993G amino acid substitution is located, we created selective culture conditions using galactose and oligomycin that elicited a pathological phenotype in T8993G cells and that allowed for the rapid selection of wild-type over T8993G mutant cells. We then generated cytoplasmic hybrid clones containing heteroplasmic levels of the T8993G mutation, and showed that selection in galactose-oligomycin caused a significant increase in the fraction of wild-type molecules (from 16 to 28%) in these cells.

Comparative biochemical studies of ATPases in cells from patients with the T8993G or T8993C mitochondrial DNA mutations

We performed comparative biochemical studies in cultured fibroblast mitochondria from patients with the T8993G or the T8993C point mutations in the ATPase 6 gene of mitochondrial DNA. We found that ATP production was much more severely decreased in cells from patients with the T8993G mutation than in those from patients with the T8993C mutation. Kinetic studies suggest that both mutations affect only the F0 sector of the mitochondrial ATPase complex. We conclude that these two mutations, which result in the substitution of different amino acids at the same site of the ATPase, result in an enzyme with different biochemical characteristics.

Mitochondria in neuromuscular disorders

This review considers primary mitochondrial diseases affecting the respiratory chain. As diseases due to mitochondrial DNA defects defy traditional anatomical classifications, we have not limited our discussion to neuromuscular disorders, but have extended it to include mitochondrial encephalomyopathies. Primary mitochondrial diseases can be due to mutations in either the nuclear or the mitochondrial genome. Nuclear mutations can affect (i) genes encoding enzymatic or structural mitochondrial proteins; (ii) translocases; (iii) mitochondrial protein importation; and (iv) intergenomic signaling. We review briefly recent molecular data and outstanding questions regarding these mendelian disorders, with special emphasis on cytochrome c oxidase deficiency and coenzyme Q10 deficiency. Mitochondrial DNA mutations fall into three main categories: (i) sporadic rearrangements (deletions/duplications); (ii) maternally inherited rearrangements (duplications); and (iii) maternally inherited point mutations. We summarize the most common clinical presentations and discuss pathogenic mechanisms, which remain largely elusive. Uncertainties about pathogenesis extend to the process of cell death, although excitotoxicity in neurons and apoptosis in muscle seem to have important roles.

Fulminant Leigh syndrome and sudden unexpected death in a family with the T9176C mutation of the mitochondrial ATPase 6 gene

We report an Italian family in which the T-to-C point mutation at nucleotide 9176 of the mitochondrial adenosine triphosphate synthetase (mtATPase) 6 gene is associated with an early-onset fulminant form of Leigh syndrome and with sudden unexpected death in two siblings, respectively. Polymerase chain reaction single-strand conformation polymorphism (PCR-SSCP) analysis and direct sequencing revealed that the mutation was homoplasmic in mitochondrial DNA of the proband. The T9176C mutation changes a highly conserved leucine to a proline in subunit 6 of the mtATPase gene and is maternally inherited, but the maternal relatives are asymptomatic. This point mutation was initially described in two brothers with bilateral striatal necrosis, a milder variant of Leigh syndrome.

Mitochondrial DNA mutations and pathogenesis

Approximately there years ago, this journal published a review on the clinical and molecular analysis of mitochondrial encephalomyopathies, with emphasis on defects in mitochondrial DNA (mtDNA). At the time, approximately 30 point mutations associated with a variety of maternally-inherited (or rarely, sporadic) disorders had been described. Since that time, almost twenty new pathogenic mtDNA point mutations have been described, and the pace of discovery of such mutations shows no signs of abating. This accumulating body of data has begun to reveal some patterns that may be relevant to pathogenesis.