Electron transfer complex I defect in idiopathic dystonia

Differential expression of striatal proteins in a mouse model of DOPA-responsive dystonia reveals shared mechanisms among dystonic disorders

Dystonia is characterized by involuntary muscle contractions that cause debilitating twisting movements and postures. Although dysfunction of the basal ganglia, a brain region that mediates movement, is implicated in many forms of dystonia, the underlying mechanisms are unclear. The inherited metabolic disorder DOPA-responsive dystonia is considered a prototype for understanding basal ganglia dysfunction in dystonia because it is caused by mutations in genes necessary for the synthesis of the neurotransmitter dopamine, which mediates the activity of the basal ganglia. Therefore, to reveal abnormal striatal cellular processes and pathways implicated in dystonia, we used an unbiased proteomic approach in a knockin mouse model of DOPA-responsive dystonia, a model in which the striatum is known to play a central role in the expression of dystonia. Fifty-seven of the 1805 proteins identified were differentially regulated in DOPA-responsive dystonia mice compared to control mice. Most differentially regulated proteins were associated with gene ontology terms that implicated either mitochondrial or synaptic dysfunction whereby proteins associated with mitochondrial function were generally over-represented and proteins associated with synaptic function were largely under-represented. Remarkably, nearly 20% of the differentially regulated striatal proteins identified in our screen are associated with pathogenic variants that cause inherited disorders with dystonia as a sign in humans suggesting shared mechanisms across many different forms of dystonia.

[Update on the etiology and pathogenesis of muscle dystonia]

Muscle dystonia is one of the most common extrapyramidal diseases and is the third most common after essential tremor and Parkinson's disease. The introduction of diagnostic methods expanded the understanding of the genetic basis of muscle dystonia and neurophysiological mechanisms of dystonic phenomena. However, the questions of the etiology and pathogenesis of dystonia still remain the subject of close interest of researchers. The review provides up-to-date information about the etiology and pathogenesis of muscle dystonia. Recent changes in the genetic nomenclature of dystonia are described. Modern ideas about the pathogenetic significance of such mechanisms as abnormalities of neural inhibition, disturbances of sensorimotor integration, and abnormalities of neural plasticity are considered. Recent research data support the concept of systemic sensorimotor disintegration, including not only basal ganglia dysfunction, but also motor network disorders involving the cerebellum, cortex, midbrain, thalamus and other areas.

A case of mitochondrial cytopathy with exertion induced dystonia

Paroxysmal dystonias are a group of relatively benign hyperkinetic childhood movement disorders of varied etiology. Mitochondrial diseases are well known to produce persistent dystonias as sequelae, but paroxysmal exertion induced dystonia has been reported in only one case to the best of our knowledge. Two siblings born to consanguineous parents presented with early-onset exertion induced dystonia, which was unresponsive to diphenylhydantoin and carbamazepine. A trial with valproate in one of the siblings turned fatal within 24 h. Based on this clue, the second child was investigated and found to suffer from complex I deficiency with a paternally inherited dominant nuclear DNA mutation, which is responsive to the mitochondrial cocktail. Exertion induced dystonia can be a rare manifestation of complex I deficiency.

The focal dystonias: current views and challenges for future research

The most common forms of dystonia are those that develop in adults and affect a relatively isolated region of the body. Although these adult-onset focal dystonias are most prevalent, knowledge of their etiologies and pathogenesis has lagged behind some of the rarer generalized dystonias, in which the identification of genetic defects has facilitated both basic and clinical research. This summary provides a brief review of the clinical manifestations of the adult-onset focal dystonias, focusing attention on less well understood clinical manifestations that need further study. It also provides a simple conceptual model for the similarities and differences among the different adult-onset focal dystonias as a rationale for lumping them together as a class of disorders while at the same time splitting them into subtypes. The concluding section outlines some of the most important research questions for the future. Answers to these questions are critical for advancing our understanding of this group of disorders and for developing novel therapeutics.

The genetics of dystonias

Dystonia has been defined as a syndrome of involuntary, sustained muscle contractions affecting one or more sites of the body, frequently causing twisting and repetitive movements or abnormal postures. Dystonia is also a clinical sign that can be the presenting or prominent manifestation of many neurodegenerative and neurometabolic disorders. Etiological categories include primary dystonia, secondary dystonia, heredodegenerative diseases with dystonia, and dystonia plus. Primary dystonia includes syndromes in which dystonia is the sole phenotypic manifestation with the exception that tremor can be present as well. Most primary dystonia begins in adults, and approximately 10% of probands report one or more affected family members. Many cases of childhood- and adolescent-onset dystonia are due to mutations in TOR1A and THAP1. Mutations in THAP1 and CIZ1 have been associated with sporadic and familial adult-onset dystonia. Although significant recent progress had been made in defining the genetic basis for most of the dystonia-plus and heredodegenerative diseases with dystonia, a major gap remains in understanding the genetic etiologies for most cases of adult-onset primary dystonia. Common themes in the cellular biology of dystonia include G1/S cell cycle control, monoaminergic neurotransmission, mitochondrial dysfunction, and the neuronal stress response.

Oxidative stress and hypertension

Mammalian cells are capable of generating metabolites of oxygen, referred to as reactive oxygen species (ROS) via the action of several enzymes. In vascular cells, ROS are predominantly produced by the NADPH oxidases, uncoupled nitric oxide synthase, xanthine oxidase and by mitochondrial sources. In hypertension, ROS production by these sources is increased, and this not only contributes to hypertension, but also causes vascular disease and dysfunction. ROS production in other organs, particularly the kidney and the centers within the brain, likely participate in blood pressure regulation. Despite the wealth of data supporting a role of ROS in hypertension and other cardiovascular diseases, treatment with commonly employed antioxidants have failed, and in some cases have proven harmful, prompting a reconsideration of the concept of oxidative stress. Within the cell, ROS are produced locally and have important signaling roles, such that scavenging of these species by exogenous antioxidants is difficult and could produce untoward effects. In this article, we consider these tissues and discuss potential new approaches to treatment of "oxidative stress".

Convergent mechanisms in etiologically-diverse dystonias

INTRODUCTION

Dystonia is a neurological disorder associated with twisting motions and abnormal postures, which compromise normal movements and can be both painful and debilitating. It can affect a single body part (focal), several contiguous regions (segmental), or the entire body (generalized), and can arise as a result of numerous causes, both genetic and acquired. Despite the diversity of causes and manifestations, shared clinical features suggest that common mechanisms of pathogenesis may underlie many dystonias.

AREAS COVERED

Shared themes in etiologically-diverse dystonias exist at several biological levels. At the cellular level, abnormalities in the dopaminergic system, mitochondrial function and calcium regulation are often present. At the anatomical level, the basal ganglia and the cerebellum are frequently implicated. Global CNS dysfunction, specifically aberrant neuronal plasticity, inhibition and sensorimotor integration, are also observed in a number of dystonias. Using clinical data and data from animal models, this article seeks to highlight shared pathways that may be critical in understanding mechanisms and identifying novel therapeutic strategies in dystonia.

EXPERT OPINION

Identifying shared features of pathogenesis can provide insight into the biological processes that underlie etiologically diverse dystonias, and can suggest novel targets for therapeutic intervention that may be effective in a broad group of affected individuals.

Altered nicotinamide adenine dinucleotide (NADH) fluorescence in dt sz mutant hamsters reflects differences in striatal metabolism between severe and mild dystonia

The dt(sz) mutant hamster represents a unique rodent model of idiopathic paroxysmal dystonia. Previous data, collected post-mortem or in anesthetized hamsters under basal conditions, indicated the critical involvement of enhanced striatal neuronal activity. To assess the importance of an enhanced striatal neuronal activity directly during a dystonic episode, continuous monitoring of changes in brain metabolism and therefore neuronal activity indirectly in awake, freely moving animals is necessary. Determination of CNS metabolism by NADH measurement by laser-induced fluorescence spectroscopy in conscious dt(sz) and nondystonic control hamsters revealed reversible decreased NADH fluorescence during dystonic episodes. The degree of change corresponded to the severity of dystonia. This study represents the first application of this innovative method in freely moving animals exhibiting a movement disorder. Our data clearly confirm that the expression of paroxysmal dystonia in dt(sz) mutant hamsters is associated with enhanced striatal neuronal activity and further underscore the versatile application of NADH fluorescence measurements in neuroscience.

Experimental therapeutics for dystonia

Dystonia is a neurological syndrome characterized by excessive involuntary muscle contractions leading to twisting movements and unnatural postures. It has many different clinical manifestations, and many different causes. More than 3 million people worldwide suffer from dystonia, yet there are few broadly effective treatments. In the past decade, progress in research has advanced our understanding of the pathogenesis of dystonia to a point where drug discovery efforts are now feasible. Several strategies can be used to develop novel therapeutics for dystonia. Existing therapies have only modest efficacy, but may be refined and improved to increase benefits while reducing side effects. Identifying rational targets for drug intervention based on the pathogenesis of dystonia is another strategy. The surge in both basic and clinical research discoveries has provided insights at all levels, including etiological, physiological and nosological, to enable such a targeted approach. The empirical approach to drug discovery, whereby compounds are identified using a nonmechanistic strategy, is complementary to the rational approach. With the recent development of multiple animal models of dystonia, it is now possible to develop assays and perform drug screens on vast numbers of compounds. This multifaceted approach to drug discovery in dystonia will likely provide lead compounds that can then be translated for clinical use.

Clinical spectrum, morbidity, and mortality in 113 pediatric patients with mitochondrial disease

OBJECTIVES

The aim of this study was to elucidate the frequency of major clinical manifestations in children with mitochondrial disease and establish their clinical course, prognosis, and rates of survival depending on their clinical features.

METHODS

We performed a retrospective review of the medical records of 400 patients who were referred for evaluation of mitochondrial disease. By use of the modified Walker criteria, only patients who were assigned a definite diagnosis were included in the study.

RESULTS

A total of 113 pediatric patients with mitochondrial disease were identified. A total of 102 (90%) patients underwent a muscle biopsy as part of the diagnostic workup. A significant respiratory chain (RC) defect, according to the diagnostic criteria, was found in 71% of the patients who were evaluated. In this cohort, complex I deficiency (32%) and combined complex I, III, and IV deficiencies (26%) were the most common causes of RC defects, followed by complex IV (19%), complex III (16%), and complex II deficiencies (7%). Pathogenic mitochondrial DNA abnormalities were found in 11.5% of the patients. A substantial fraction (40%) of patients with mitochondrial disorders exhibited cardiac disease, diagnosed by Doppler echocardiography; however, the majority (60%) of patients had predominant neuromuscular manifestations. No correlation between the type of RC defect and the clinical presentation was observed. Overall, the mean age at presentation was 40 months. However, the mean age at presentation was 33 months in the cardiac group and 44 months in the noncardiac group. Twenty-six (58%) patients in the cardiac group exhibited hypertrophic cardiomyopathy, 29% had dilated cardiomyopathy, and the remainder (13%) had left ventricular noncompaction. Patients with cardiomyopathy had an 18% survival rate at 16 years of age. Patients with neuromuscular features but no cardiomyopathy had a 95% survival at the same age.

CONCLUSIONS

This study gives strong support to the view that in patients with RC defects, cardiomyopathy is more common than previously thought and tends to follow a different and more severe clinical course. Although with a greater frequency than previously reported, mitochondrial DNA mutations were found in a minority of patients, emphasizing that most mitochondrial disorders of childhood follow a Mendelian pattern of inheritance.

The rotenone model of Parkinson's disease: genes, environment and mitochondria

Parkinson's disease (PD) is occasionally caused by single gene mutations or by single toxic exposures, but most cases of PD are probably caused by some combination of genetic susceptibility and environmental exposure. Using rotenone as a prototype for an environmental toxicant, we argue here that genetic and environmental causes of PD converge on common pathogenic mechanisms. If so, protective strategies devised for one type of PD may be broadly useful for other forms of the disease.

Bilateral striatal necrosis associated with a novel mutation in the mitochondrial ND6 gene

We report the molecular findings in two independent patients presenting with progressive generalized dystonia and bilateral striatal necrosis in whom we have identified a mutation (T14487C) in the mitochondrial ND6 gene. The mutation is heteroplasmic in all samples analyzed, and it fulfills all accepted criteria of pathogenicity. Transmitochondrial cell lines harboring 100% mutant mitochondrial DNA showed a marked decrease in the activity of complex I of the respiratory chain supporting the pathogenic role of T14487C.

Focal and segmental primary dystonia in north-western Germany--a clinico-genetic study

OBJECTIVES

To determine the frequency of familial focal and segmental dystonias in a large patient cohort with primary dystonia from north-western Germany.

MATERIALS AND METHODS

In this study, 130 patients with focal or segmental dystonia were examined and a family history was obtained. Whenever possible, affected relatives were examined (a total of 789 first-degree relatives). Data on disease duration, age at disease onset and age of the patients were investigated by Student's t-test and a segregation analysis was performed by Weinberg's proband method.

RESULTS

Age at onset of disease was significantly later in the blepharospasm group. Only in the writer's cramp group were women outnumbered by men. A positive family history was found in 15 of the 130 index patients (11.5%). None of 102 index patients tested carried the GAG deletion in the DYT1 gene.

CONCLUSIONS

In accordance with previous series our study provides evidence that primary focal dystonia may have a genetic etiology, most probably caused by an autosomal dominant trait with reduced penetrance.

Therapeutic potential of N-acetylcysteine in age-related mitochondrial neurodegenerative diseases

Increasing lines of evidence suggest a key role for mitochondrial damage in neurodegenerative diseases. Brain aging, Parkinson's disease, Alzheimer's disease, Huntington's disease and Friedreich's ataxia have been associated with several mitochondrial alterations including impaired oxidative phosphorylation. Mitochondrial impairment can decrease cellular bioenergetic capacity, which will then increase the generation of reactive oxygen species resulting in oxidative damage and programmed cell death. This paper reviews the mechanisms of N-acetylcysteine action at the cellular level, and the possible usefulness of this antioxidant for the treatment of age-associated neurodegenerative diseases. First, this thiol can act as a precursor for glutathione synthesis as well as a stimulator of the cytosolic enzymes involved in glutathione regeneration. Second, N-acetylcysteine can act by direct reaction between its reducing thiol group and reactive oxygen species. Third, it has been shown that N-acetylcysteine can prevent programmed cell death in cultured neuronal cells. And finally, N-acetylcysteine also increases mitochondrial complex I and IV specific activities both in vitro and in vivo in synaptic mitochondrial preparations from aged mice. In view of the above, and because of the ease of its administration and lack of toxicity in humans, the potential usefulness of N-acetylcysteine in the treatment of age-associated mitochondrial neurodegenerative diseases deserves investigation.

Metabolic defects caused by treatment with the tetrahydropyridine analog of haloperidol (HPTP), in baboons

Mounting evidence suggests that compromised cellular energy production is a major contributor to idiopathic and drug-induced degenerative processes. Our interest in neurotoxins have prompted us to examine in the baboon the effects of HPTP, the tetrahydropyridine dehydration product of haloperidol, on urinary chemical markers that reflect defects in mitochondrial respiration. Urinary dicarboxylic acid and conjugate profiles, similar to those seen in humans with inborn errors of mitochondrial metabolism and toxin-induced Jamaican vomiting sickness (JVS) were observed in the treated baboons. We interpret these results as evidence that HPTP and/or HPTP metabolites inhibit mitochondrial respiration in the baboon and speculate that analogous effects may occur in haloperidol-treated individuals.

Increased tissue copper and manganese content in the lentiform nucleus in primary adult-onset dystonia

We analyzed trace metals in frozen brain tissue of several subcortical nuclei from 3 patients with primary adult-onset dystonia and 10 control subjects. Copper levels were significantly increased in the globus pallidus and putamen of patients with dystonia A slight increase in manganese content was identified in the putamen and thalamus of patients with dystonia. Our findings show for the first time an accumulation of trace metals in the lentiform nuclei in patients with primary dystonia, which may play a pathogenetic role in primary dystonia and may explain recent ultrasound and magnetic resonance imaging findings.

Dystonia as a presenting feature of the 3243 mitochondrial DNA mutation

A variety of neurologic phenotypes have been described in patients with mitochondrial disorders. We report a 32-year-old man in whom dystonia was the salient and presenting feature of a mitochondrial DNA mutation. He presented at age 23 with writer's cramp and progressed over 5 years to exhibit dystonia in facial muscles and lower limbs. He also has exercise intolerance, mild, bilateral ptosis, proximal muscle weakness, and sensorineural hearing loss. Molecular genetic analysis of blood, urine, and muscle biopsy demonstrated the presence of a heteroplasmic point mutation at nucleotide position 3243. The 3243 mtDNA mutation has pleomorphic manifestations, and dystonia should be added to the list of associated clinical features.

Mitochondrial pathology in human schizophrenic striatum: a postmortem ultrastructural study

Studies using in vivo imaging or microscopic analysis of autopsy specimens indicate abnormalities in the striatum of schizophrenics including lower striatal metabolism, a change which can be normalized by antipsychotic medication. To investigate the possibility that abnormalities in schizophrenia brain may be due, in part, to pathology in mitochondria, organelles which generate energy, postmortem brain tissue from schizophrenic and control cases was obtained from the Maryland Brain Collection. Mitochondria in electron micrographs of striatal neuropil were counted and digitized. The caudate and the putamen of the schizophrenic subjects contained significantly (P < 0.05) fewer (a decrease of approximately 20%) mitochondrial profiles throughout the neuropil than did normal controls. The numbers of mitochondrial profiles per axon terminal appeared lower in the subset of schizophrenics off-drug as compared to either the subset of schizophrenics on-drug or to controls, suggesting that neuroleptic treatment may normalize this measure. The structural integrity of mitochondrial profiles in the schizophrenic striata was not obviously different from that of controls. Fewer mitochondrial profiles suggest decreased energy demands or diminished capacity to respond to energy requirements in the structures that contain them. These data are consistent with other studies showing decreased metabolism in the striatum of schizophrenics and may identify, in part, the anatomical basis of this deficit.

1H magnetic resonance spectroscopy of the lentiform nucleus in primary focal hand dystonia

Several radiologic findings point toward the lentiform nucleus as a possible site of lesion in primary dystonia. Histologic examinations, however, have shown inconsistent results. 1H-magnetic resonance spectroscopy (MRS) has proved helpful to assess neuronal degeneration in a variety of basal ganglia disorders. MRS data of dystonia patients are, however, lacking so far. 1H-MRS centered on the lentiform nuclei was performed in 14 patients with primary focal hand dystonia and in 12 healthy control subjects using a 1.5-T MR imager. No statistically significant differences of N-acetylaspartate/creatine and lactate/creatine ratios were found between patients and control subjects. Based on these data, the authors found no evidence that primary focal dystonia was associated with a conspicuous loss of lentiform nucleus neurons or a marked disturbance of the aerobic metabolism, although minor abnormalities cannot be excluded because of the possibly limited sensitivity of the method.

Mitochondrial DNA in focal dystonia: a cybrid analysis

The cause and pathophysiology of dystonia remain unknown. The recent identification of mitochondrial complex I deficiency in platelets from patients with sporadic focal dystonia suggests that a defect of energy metabolism may be relevant in a proportion of patients. We have addressed the possible contribution of mitochondrial DNA (mtDNA) to the complex I deficiency in dystonia by the use of genome transfer technology. Platelets from patients deficient for complex I were fused with A549 p0 (mtDNA-less) cells to form cybrids comprising the A549 nucleus and dystonia mtDNA. Mixed cybrid cell lines were analyzed for 9 controls and 9 dystonia patients, and clonal cybrid lines were generated for 2 control and 2 dystonia patients. Subsequent biochemical analysis showed that the dystonia complex I defect was complemented in both the mixed and the clonal cybrid lines. These results contrast with similar studies in mitochondrial myopathy and Parkinson's disease patients, in which the mitochondrial defect was maintained in at least a proportion of A549 cybrids, and suggest that the complex I defect in dystonia is not caused by an mtDNA mutation.

Dystonia

Many different disorders have dystonia as the only or primary sign. The list of causes for dystonia increases yearly and now includes three mapped loci for primary torsion dystonia, although other susceptibility genes are suspected. Study of one of these primary torsion dystonia loci (DYT1) has culminated in the cloning of a gene which codes for a novel protein, torsin A. Physiological and positron emission tomography analyses suggest that dystonia results from impaired inhibition at cortical and subcortical levels; these physiological changes may in turn be due to striatal dysfunction and a mismatch or imbalance between the direct and indirect pathways. Future study of normal and mutant torsin A, as well as the identification of other primary torsion dystonia genes, should help elucidate the mechanisms underlying dystonia.

Human complex I defects in neurodegenerative diseases

Complex I deficiency, either specific or associated with other respiratory chain defects, has been identified in myopathies, encephalomyopathies and in three 'neurodegenerative' disorders: Parkinson's disease, dystonia and Leber's hereditary optic neuropathy. The complex I defect is expressed in blood in all these three but, to date, only in LHON have specific mitochondrial DNA mutations been identified. Recent work with rho degrees cybrids indicates that, in a subgroup of patients at least, the complex I deficiency is determined by mtDNA, in contrast to dystonia where a nuclear gene defect or toxic influence appears a more likely cause. The actions of specific toxins, e.g., MPTP continue to play an important role in our understanding of pathogenesis of neurodegeneration, particularly in PD.

Frequency of familial inheritance among 488 index patients with idiopathic focal dystonia and clinical variability in a large family

Idiopathic torsion dystonia is characterized by involuntary twisting movements and postures. One molecularly defined form with generalized dystonia has been shown to be autosomal dominantly inherited with reduced penetrance in chromosome 9q34.1, especially in Ashkenazi Jewish families, while other generalized families from Europe and families with other subtypes of dystonia have been excluded from linkage to this locus. Genealogical studies suggest that the much more frequent focal dystonia follows an autosomal dominant inheritance with reduced penetrance as well. For our study, 488 patients with focal dystonia, without a tendency for generalization, were interviewed for their family history. Evidence for hereditary disposition was found in 88 individuals. In a second step, all available family members of 17 of the 488 index patients (chosen for cooperation) were clinically examined. Objective diagnosis of affected relative was established in 13 families, whereas only 4 of the 17 index patients had previously admitted a positive family history. Furthermore, a large three-generation family with focal dystonia linked to chromosome 18p (linkage data described elsewhere) was identified. The familial pattern of all reported families is compatible with autosomal dominant inheritance with reduced penetrance. Assessment only on patients' report leads to underestimation of the frequency of familial idiopathic focal dystonia.

Evidence for DYT7 being a common cause of cervical dystonia (torticollis) in Central Europe

Adult-onset focal idiopathic torsion dystonias (AFITD), such as torticollis, have a prevalence similar to that of multiple sclerosis and usually seem sporadic. Only recently has one large AFITD pedigree "K" with autosomal dominant inheritance and reduced penetrance from Northwest Germany provided the opportunity to identify a gene locus on chromosome 18p. We have now tested the relevance of this DYT7 gene locus in a collective of 18 nuclear AFITD families from Central Europe who were genotyped with chromosome 18p microsatellites. In three families, the affected relatives did not share a chromosome 18p haplotype, suggesting locus heterogeneity in AFITD. In the remaining 15 families, significant allelic association was observed for marker D18S1098. This result suggests that DYT7 is a common cause for AFITD at least in Central Europe, that many patients are descended from a common ancestor, and that the DYT7 gene is mapped in a 4.4 centimorgan subregion of chromosome 18p.

Complex I function in familial and sporadic dystonia

A significant proportion of patients with inborn errors of the mitochondrial respiratory chain exhibit movement disorders, particularly dystonia. Point mutations of mitochondrial DNA (mtDNA) are usually expressed systemically, and defects of platelet respiratory chain function have been described in patients with mtDNA mutations and Leber's hereditary optic neuropathy (LHON). Recent reports have documented families with dystonia in association with LHON and mtDNA complex I gene mutations. We have examined mitochondrial function in platelet mitochondria from patients with familial generalized dystonia (linked or not linked to 9q34) and sporadic focal dystonia. We confirm a previous report of a specific complex I defect in patients with sporadic focal dystonia but could not find any abnormality in patients with familial generalized dystonia, linked or not to 9q34. These results support the existence of a mitochondrial deficiency in sporadic focal dystonia and provide a biochemical dimension to the clinical and genetic distinction between focal and generalized familial dystonia.

[3H]dihydrorotenone binding to NADH: ubiquinone reductase (complex I) of the electron transport chain: an autoradiographic study

Abnormalities of mitochondrial energy metabolism may play a role in normal aging and certain neurodegenerative disorders. In this regard, complex I of the electron transport chain has received substantial attention, especially in Parkinson's disease. The conventional method for studying complex I has been quantitation of enzyme activity in homogenized tissue samples. To enhance the anatomic precision with which complex I can be examined, we developed an autoradiographic assay for the rotenone site of this enzyme. [3H]dihydrorotenone ([3H]DHR) binding is saturable (KD = 15-55 nM) and specific, and Hill slopes of 1 suggest a single population of binding sites. Nicotinamide adenine dinucleotide (NADH) enhances binding 4- to 80-fold in different brain regions (EC50 = 20-40 microM) by increasing the density of recognition sites (Bmax). Nicotinamide adenine dinucleotide phosphate also increases binding, but NAD+ does not. In skeletal muscle, heart, and kidney, binding was less affected by NADH. [3H]DHR binding is inhibited by rotenone (IC50 = 8-20 nM), meperidine (IC50 = 34-57 microM), amobarbitol (IC50 = 375-425 microM), and MPP+ (IC50 = 4-5 mM), consistent with the potencies of these compounds in inhibiting complex I activity. Binding is heterogeneously distributed in brain with the density in gray matter structures varying more than 10-fold. Lesion studies suggest that a substantial portion of binding is associated with nerve terminals. [3H]DHR autoradiography is the first quantitative method to examine complex I with a high degree of anatomic precision. This technique may help to clarify the potential role of complex I dysfunction in normal aging and disease.

Respiration and growth defects in transmitochondrial cell lines carrying the 11778 mutation associated with Leber's hereditary optic neuropathy

Mitochondrial DNA from two genetically unrelated patients carrying the mutation at position 11778 that causes Leber's hereditary optic neuropathy has been transferred with mitochondria into human mtDNA-less rho0206 cells. As analyzed in several transmitochondrial cell lines thus obtained, the mutation, which is in the gene encoding subunit ND4 of the respiratory chain NADH dehydrogenase (ND), did not affect the synthesis, size, or stability of ND4, nor its incorporation into the enzyme complex. However, NADH dehydrogenase-dependent respiration, as measured in digitonin-permeabilized cells, was specifically decreased by approximately 40% in cells carrying the mutation. This decrease, which was significant at the 99.99% confidence level, was correlated with a significantly reduced ability of the mutant cells to grow in a medium containing galactose instead of glucose, indicating a clear impairment in their oxidative phosphorylation capacity. On the contrary, no decrease in rotenone-sensitive NADH dehydrogenase activity, using a water-soluble ubiquinone analogue as electron acceptor, was detected in disrupted mitochondrial membranes. This is the first cellular model exhibiting in a foreign nuclear background mitochondrial DNA-linked biochemical defects underlying the optic neuropathy phenotype.

Use of transmitochondrial cybrids to assign a complex I defect to the mitochondrial DNA-encoded NADH dehydrogenase subunit 6 gene mutation at nucleotide pair 14459 that causes Leber hereditary optic neuropathy and dystonia

A heteroplasmic G-to-A transition at nucleotide pair (np) 14459 within the mitochondrial DNA (mtDNA)-encoded NADH dehydrogenase subunit 6 (ND6) gene has been identified as the cause of Leber hereditary optic neuropathy (LHON) and/or pediatric-onset dystonia in three unrelated families. This ND6 np 14459 mutation changes a moderately conserved alanine to a valine at amino acid position 72 of the ND6 protein. Enzymologic analysis of mitochondrial NADH dehydrogenase (complex I) with submitochondrial particles isolated from Epstein-Barr virus-transformed lymphoblasts revealed a 60% reduction (P < 0.005) of complex I-specific activity in patient cell lines compared with controls, with no differences in enzymatic activity for complexes II plus III, III and IV. This biochemical defect was assigned to the ND6 np 14459 mutation by using transmitochondrial cybrids in which patient Epstein-Barr virus-transformed lymphoblast cell lines were enucleated and the cytoplasts were fused to a mtDNA-deficient (p 0) lymphoblastoid recipient cell line. Cybrids harboring the np 14459 mutation exhibited a 39% reduction (p < 0.02) in complex I-specific activity relative to wild-type cybrid lines but normal activity for the other complexes. Kinetic analysis of the np 14459 mutant complex I revealed that the Vmax of the enzyme was reduced while the Km remained the same as that of wild type. Furthermore, specific activity was inhibited by increasing concentrations of the reduced coenzyme Q analog decylubiquinol. These observations suggest that the np 14459 mutation may alter the coenzyme Q-binding site of complex I.

Leber's hereditary optic neuropathy plus dystonia is caused by a mitochondrial DNA point mutation

A novel point mutation in the ND6 subunit of complex I at position 14,459 of the mitochondrial DNA (MTND6*LDY T14459A) was identified as a candidate mutation for the highly tissue-specific disease. Leber's hereditary optic neuropathy plus dystonia. Since the MTND6*LDYT14459A mutation was identified in a single family, other pedigrees with the mutation are needed to confirm its association with the disease. Clinical, biochemical, and genetic characterization is reported in two additional pedigrees. Leber's hereditary optic neuropathy developed in two family members in one pedigree. The daughter had clinically silent basal ganglia lesions. In a second pedigree, a single individual presented with childhood-onset generalized dystonia and bilateral basal ganglia lesions. Patient groups that included individuals with Leigh's disease, dystonia plus complex neurodegeneration, and Leber's hereditary optic neuropathy did not harbor the MTND6*LDYT14459A mutation, suggesting that this mutation displays a high degree of tissue specificity, thus producing a narrow phenotypic range. These results confirm the association of the MTND6*LDYT14459A mutation with Leber's hereditary optic neuropathy and/or dystonia. As the first genetic abnormality that has been identified to cause generalized dystonia, this mutation suggests that nuclear DNA or mitochondrial DNA mutations in oxidative phosphorylation genes are important considerations in the pathogenesis of dystonia.

Respiratory chain and mitochondrial deoxyribonucleic acid in blood cells from patients with focal and generalized dystonia

An increasing number of neurodegenerative diseases seem to be associated with or even due to disturbances of cerebral energy metabolism. One generally accepted example is complex I deficiency in substantia nigra from patients with Parkinson's disease. Reports on a complex I defect in platelets from patients with dystonia led us to check for disturbances of the respiratory chain or of the mitochondrial genome in isolated mitochondria from patients with focal or generalized dystonia. We could not confirm the idea of mitochondrial disturbance in platelets from patients with dystonia because we did not find abnormal enzyme activities or any deletions of the mitochondrial genome. Thus, we do not think that blood cells such as platelets can serve as markers for neurodegenerative disorders such as dystonia.

A mitochondrial DNA mutation at nucleotide pair 14459 of the NADH dehydrogenase subunit 6 gene associated with maternally inherited Leber hereditary optic neuropathy and dystonia

A five-generation Hispanic family expressing maternally transmitted Leber hereditary optic neuropathy and/or early-onset dystonia associated with bilateral basal ganglia lesions was studied. Buffy coat mitochondrial DNA (mtDNA) from a severely affected child was amplified by the polymerase chain reaction and greater than 90% sequenced. The mtDNA proved to be a Native American haplogroup D genotype and differed from the standard "Cambridge" sequence at 40 nucleotide positions. One of these variants, a G-to-A transition at nucleotide pair (np) 14459, changed a moderately conserved alanine to a valine at NADH dehydrogenase subunit 6 (ND6) residue 72. The np 14459 variant was not found in any of 38 Native American haplogroup D mtDNAs, nor was it detected in 108 Asian, 103 Caucasian, or 99 African mtDNAs. Six maternal relatives in three generations were tested and were found to harbor the mutation, with one female affected with Leber hereditary optic neuropathy being heteroplasmic. Thus, the np 14459 G-to-A missense mutation is specific to this family, alters a moderately conserved amino acid in a complex I gene, is a unique mtDNA variant in Native American haplogroup D, and is heteroplasmic, suggesting that it is the disease-causing mutation.

Characterization of assembly intermediates of NADH:ubiquinone oxidoreductase (complex I) accumulated in Neurospora mitochondria by gene disruption

NADH:ubiquinone oxidoreductase, the respiratory chain complex I of mitochondria, is an assembly of some 25 nuclear-encoded and 7 mitochondrially encoded subunits. The complex has an overall L-shaped structure formed by a peripheral arm and an elongated membrane arm. The peripheral arm containing one FMN and at least three iron-sulphur clusters constitutes the NADH dehydrogenase segment of the electron pathway. The membrane arm with at least one iron-sulphur cluster constitutes the ubiquinone reducing segment. We are studying the assembly of the complex in Neurospora crassa. By disrupting the gene of a nuclear-encoded subunit of the membrane arm a mutant was generated that cannot form complex I. The mutant rather pre-assembles the peripheral arm with all redox groups and the ability to catalyse NADH oxidation by artificial electron acceptors. The final assembly of the membrane arm is blocked in the mutant leading to accumulation of complementary assembly intermediates. One intermediate is associated with a protein that is not present in the fully assembled complex I. The results demonstrate that the two arms of complex I are assembled independently on separate pathways, and gave a first insight into the assembly pathway of the membrane arm. It is also shown for the first time that the obligate aerobic fungus N. crassa can grow and respire without an intact complex I. Gene replacement in this fungus is therefore a tool for investigation of this complex.

The NADH:ubiquinone oxidoreductase (complex I) of respiratory chains

The inner membranes of mitochondria contain three multi-subunit enzyme complexes that act successively to transfer electrons from NADH to oxygen, which is reduced to water (Fig. I). The first enzyme in the electron transfer chain, NADH:ubiquinone oxidoreductase (or complex I), is the subject of this review. It removes electrons from NADH and passes them via a series of enzyme-bound redox centres (FMN and Fe-S clusters) to the electron acceptor ubiquinone. For each pair of electrons transferred from NADH to ubiquinone it is usually considered that four protons are removed from the matrix (see section 4.1 for further discussion of this point).