Antioxidant treatment improves in vivo cardiac and skeletal muscle bioenergetics in patients with Friedreich's ataxia

Friedreich's ataxia (FA) is the most common form of autosomal recessive spinocerebellar ataxia and is often associated with a cardiomyopathy. The disease is caused by an expanded intronic GAA repeat, which results in deficiency of a mitochondrial protein called frataxin. In the yeast YFH1 knockout model of the disease there is evidence that frataxin deficiency leads to a severe defect of mitochondrial respiration, intramitochondrial iron accumulation, and associated production of oxygen free radicals. Recently, the analysis of FA cardiac and skeletal muscle samples and in vivo phosphorus magnetic resonance spectroscopy (31P-MRS) has confirmed the deficits of respiratory chain complexes in these tissues. The role of oxidative stress in FA is further supported by the accumulation of iron and decreased aconitase activities in cardiac muscle. We used 31P-MRS to evaluate the effect of 6 months of antioxidant treatment (Coenzyme Q10 400 mg/day, vitamin E 2,100 IU/day) on cardiac and calf muscle energy metabolism in 10 FA patients. After only 3 months of treatment, the cardiac phosphocreatine to ATP ratio showed a mean relative increase to 178% (p = 0.03) and the maximum rate of skeletal muscle mitochondrial ATP production increased to 139% (p = 0.01) of their respective baseline values in the FA patients. These improvements, greater in prehypertrophic hearts and in the muscle of patients with longer GAA repeats, were sustained after 6 months of therapy. The neurological and echocardiographic evaluations did not show any consistent benefits of the therapy after 6 months. This study demonstrates partial reversal of a surrogate biochemical marker in FA with antioxidant therapy and supports the evaluation of such therapy as a disease-modifying strategy in this neurodegenerative disorder.

Mitochondrial dysfunction in friedreich's ataxia

Friedreich ataxia (FRDA) is an autosomal recessive degenerative disorder caused in the vast majority of cases by a GAA triplet expansion in the FRDA gene on chromosome 9q13. The FRDA gene product, frataxin, is a widely expressed mitochondrial protein which is severely reduced in FRDA patients. Loss of the homologue of frataxin in yeast is associated with mitochondrial iron overload, increased sensitivity to oxidative stress and profound deficit of oxidative phosphorylation. The demonstration that the human pathology of FRDA is also characterised by mitochondrial iron accumulation, deficit of respiratory chain complex activities and in vivo deficit of tissue energy metabolism establishes FRDA as a 'new' nuclear encoded mitochondrial disease.

The place of COMT inhibitors in the armamentarium of drugs for the treatment of Parkinson's disease

Catechol-O-methyl transferase (COMT) inhibitors block the peripheral metabolism of levodopa, increase its plasma half-life, and enhance its brain availability. Two COMT inhibitors, tolcapone and entacapone, have recently been made available as adjunctive agents to levodopa. In PD patients with motor fluctuations, they have been shown to increase "on" time and reduce "off" time. In patients with more advanced disease, they provide similar benefits, but patients tend to experience less overall benefit and a greater likelihood of developing dopaminergic adverse events. Accordingly, closer monitoring is required. In stable patients who have not yet developed motor complications, there are preliminary data suggesting that they experience improvements in motor function and in activities of daily living. Finally, there are theoretical reasons to consider administering a COMT inhibitor to patients from the onset of levodopa therapy in order to reduce the likelihood that motor complications will develop. COMT inhibitors are easy to administer, do not require titration, and are generally well tolerated particularly in patients with relatively mild disease. Adverse events are primarily dopaminergic and can usually be controlled by levodopa dose adjustments. COMT inhibitors have thus proven to be a useful addition to the therapeutic armamentarium of PD.

Expression of mutant alpha-synuclein causes increased susceptibility to dopamine toxicity

Mutations of the alpha-synuclein gene have been identified in autosomal dominant Parkinson's disease (PD). Transgenic mice overexpressing wild-type human alpha-synuclein develop motor impairments, intraneuronal inclusions and loss of dopaminergic terminals in the striatum. To study the mechanism of action through which mutant alpha-synuclein toxicity is mediated, we have generated stable, inducible cell models expressing wild-type or PD-associated mutant (G209A) alpha-synuclein in human-derived HEK293 cells. Increased expression of either wild-type or mutant alpha-synuclein resulted in the formation of cytoplasmic aggregates which were associated with the vesicular (including monoaminergic) compartment. Expression of mutant alpha-synuclein induced a significant increase in sensitivity to dopamine toxicity compared with the wild-type protein expression. These results provide an explanation for the preferential dopaminergic neuronal degeneration seen in both the PD G209A mutant alpha-synuclein families and suggest that similar mechanisms may underlie or contribute to cell death in sporadic PD.

Continuous dopamine-receptor stimulation in early Parkinson's disease

Chronic L-dopa therapy is associated with the development of motor complications in the majority of Parkinson's disease (PD) patients. Although the precise mechanism responsible for these events is not known, increasing laboratory and clinical evidence points to a sequence of events that is initiated by abnormal pulsatile stimulation of dopamine receptors by the intermittent administration of agents with short half-lives such as L-dopa. Initiating therapy with a long-acting dopamine agonist has been shown to delay the onset and reduce the severity of motor complications in MPTP monkeys and PD patients. Administering L-dopa with a catechol-O-methyltransferase (COMT) inhibitor to block its peripheral metabolism increases its plasma half-life and might have a similar effect. Thus, a rational strategy for treating PD would be to initiate therapy with a long-acting dopamine-receptor agonist and supplement at the appropriate time with L-dopa combined with a COMT inhibitor.

Mitochondrial disorders

The rate of advance of our understanding of mitochondrial pathology continues to accelerate. Trends in genotype-phenotype correlations in mitochondrial DNA mutations continue to be developed; the latest of these is the association of exercise intolerance with cytochrome b mutations and onset in infancy of multisystem disorders associated with cytochrome oxidase assembly defects. New models for mitochondrial disease are being developed. Drugs, toxins and deficiency of nuclear encoded proteins that are targeted at mitochondria are now recognized as important causes of secondary mitochondrial respiratory chain deficiency.

Secondary abnormalities of mitochondrial DNA associated with neurodegeneration

The central nervous system has a particularly high energy requirement, thus making it very susceptible to defects in mitochondrial function. A number of neurodegenerative diseases, in particular Parkinson's disease (PD), Huntington's disease (HD) and Friedreich's ataxia (FRDA), are associated with mitochondrial dysfunction. The identification of a mitochondrial complex-I defect in PD provides a link between toxin models of the disease, and clues to the pathogenesis of idiopathic PD. We have undertaken genomic transplantation studies involving the transfer of mitochondrial DNA (mtDNA) from PD patients with a complex-I defect to a novel nuclear background. Histochemical, immunohistochemical and functional analysis of the resulting cybrids all showed a pattern in the PD clones indicative of a mtDNA mutation. There is good evidence for the involvement of defective energy metabolism and excitotoxicity in the aetiology of HD. We, and others, have shown a severe deficiency of complex II/III confined to the striatum that mimics the toxin-induced animal models of HD. There is also a milder defect in complex IV in the caudate. The tricarboxylic acid cycle enzyme aconitase is particularly sensitive to inhibition by peroxynitrite and superoxide radicals. We have found this enzyme to be severely decreased in HD caudate, putamen and cortex in a pattern that parallels the severity of neuronal loss seen. We propose a scheme for the role of nitric oxide, free radicals and excitotoxicity in the pathogenesis of HD. FRDA is caused by an expanded GAA repeat in intron 1 of the X25 gene encoding a protein called frataxin. Frataxin is widely expressed and is a mitochondrial protein, although its function is unknown. We have found abnormal magnetic resonance spectroscopy in the skeletal muscle of FRDA patients, which parallels our biochemical findings of reduced complexes I-III in patients' heart and skeletal muscle. There is also reduced aconitase activity in these areas. Increased iron deposition was seen in patients' tissues in a pattern consistent with a mitochondrial location. The mitochondrial iron accumulation, defective respiratory chain activity and aconitase dysfunction suggest that frataxin may be involved in mitochondrial iron regulation. There is also evidence that oxidative stress contributes to cellular toxicity.

Oxidative-phosphorylation defects in liver of patients with Wilson's disease

BACKGROUND

Wilson's disease (WD) is caused by mutations in a P-type ATPase and is associated with copper deposition in liver and brain. The WD protein is present in the trans-Golgi network and may also be imported into mitochondria. The WD protein functions as a P-type copper transporting ATPase in the Golgi but any action in mitochondria is at present unknown.

METHODS

We studied mitochondrial function and aconitase activity in WD liver tissue and compared the results with those in a series of healthy controls and patients without WD.

FINDINGS

There was evidence of severe mitochondrial dysfunction in the livers of patients with WD. Enzyme activities were decreased as follows: complex I by 62%, complex II+III by 52%, complex IV by 33%, and aconitase by 71%. These defects did not seem to be secondary to penicillamine use, cholestasis, or poor hepatocellular synthetic function.

INTERPRETATION

The results show that there is a defect of energy metabolism in WD. The pattern of enzyme defects suggests that free-radical formation and oxidative damage, probably mediated via mitochondrial copper accumulation, are important in WD pathogenesis. These results provide a rationale for a study of the use of antioxidants in WD.

Abnormal in vivo skeletal muscle energy metabolism in Huntington's disease and dentatorubropallidoluysian atrophy

We studied in vivo muscle energy metabolism in patients with Huntington's disease (HD) and dentatorubropallidoluysian atrophy (DRPLA) using 31P magnetic resonance spectroscopy (MRS). Twelve gene-positive HP patients (4 presymptomatic patients) and 2 gene-positive DRPLA patients (1 presymptomatic patient) were studied. 31P-MRS at rest showed a reduced phosphocreatine-to-inorganic phosphate ratio in the symptomatic HD patients and DRPLA patient. Muscle adenosine triphosphate/(phosphocreatine + inorganic phosphate) at rest was significantly reduced in both groups of symptomatic and presymptomatic HD subjects and was below the normal range in the 2 DRPLA subjects. During recovery from exercise, the maximum rate of mitochondrial adenosine triphosphate production was reduced by 44% in symptomatic HD patients and by 35% in presymptomatic HD carriers. The maximum rate of mitochondrial adenosine triphosphate production in muscle was also reduced by around 46% in the 2 DRPLA subjects. Our findings show that HD and DRPLA share a deficit of in vivo mitochondrial oxidative metabolism, supporting a role for mitochondrial dysfunction as a factor involved in the pathogenesis of these polyglutamine repeat-mediated neurodegenerative disorders. The identification of 31P-MRS abnormalities may offer a surrogate biochemical marker by which to study disease progression and the effects of treatment in HD and DRPLA.

A new family with paroxysmal exercise induced dystonia and migraine: a clinical and genetic study

OBJECTIVE

To characterise the phenotype of a family with paroxysmal exercise induced dystonia (PED) and migraine and establish whether it is linked to the paroxysmal non-kinesigenic dyskinesia (PNKD) locus on chromosome 2q33-35, the familial hemiplegic migraine (FHM) locus on chromosome 19p, or the familial infantile convulsions and paroxysmal choreoathetosis (ICCA syndrome) locus on chromosome 16.

METHODS

A family, comprising 30 members, was investigated. Fourteen family members in two generations including three spouses were examined. Haplotypes were reconstructed for all the available family members by typing several microsatellite markers spanning the PNKD, FHM, and ICCA loci. Additionally, the four exons containing the known FHM mutations were sequenced.

RESULTS

Of 14 members examined four were definitely affected and one member was affected by history. The transmission pattern in this family was autosomal dominant with reduced penetrance. Mean age of onset in affected members was 12 (range 9-15 years). Male to female ratio was 3:1. Attacks of PED in affected members were predominantly dystonic and lasted between 15 and 30 minutes. They were consistently precipitated by walking but could also occur after other exercise. Generalisation did not occur. Three of the affected members in the family also had migraine without aura. Linkage of the disease to the PNKD, FHM, or ICCA loci was excluded as no common haplotype was shared by all the affected members for each locus. In addition, direct DNA sequential analysis of the FHM gene (CACNL1A4) ruled out all known FHM point mutations.

CONCLUSIONS

This family presented with the classic phenotype of PED and is not linked to the PNKD, FHM, or ICCA loci. A new gene, possibly coding for an ion channel, is likely to be the underlying cause of the disease.

Sleep attacks (sleep episodes) with pergolide

Increased sedation, somnolence, and sleep episodes seem to occur with several, if not all, dopamine agonists and dopaminergic treatment. Patients at risk of sleep episodes can be identified by well-chosen questions, and could be managed by appropriate measures including dose reduction.

Cytochrome oxidase immunohistochemistry: clues for genetic mechanisms

Cytochrome c oxidase (COX) is encoded by three mitochondrial and nine nuclear genes. COX deficiency is genetically heterogeneous but current diagnostic methods cannot easily distinguish between mitochondrial and nuclear defects. We hypothesized that there may be differential expression of COX subunits depending on the underlying mutation. COX subunit expression was investigated in five patients with known mtDNA mutations. Severe and selective reduction of mtDNA-encoded COX subunits I and II was consistently observed in all these patients and was restricted to COX-deficient fibres. Immunostaining of nuclear-encoded subunits COX IV and Va was normal, whilst subunit VIc, also nuclear-encoded, was decreased. Twelve of 36 additional patients with histochemically defined COX deficiency also had this pattern of staining, suggesting that they had mtDNA defects. Clinical features in this group were heterogeneous, including infantile encephalopathy, multisystem disease, cardiomyopathy and childhood-onset isolated myopathy. The remaining patients did not have the same pattern of immunostaining. Fourteen had reduced staining of all subunits, whilst 10 had normal staining of all subunits despite reduced enzyme activity. Patients with COX deficiency secondary to mtDNA mutations have a specific pattern of subunit loss, but the majority of children with COX deficiency do not have this pattern of subunit loss and are likely to have nuclear gene defects.

Mitochondrial respiratory chain disorders II: neurodegenerative disorders and nuclear gene defects

The first part of this review (Lancet 2000; 355: 299) covered primary disorders of mitochondrial DNA (mtDNA). This section will cover nuclear-encoded defects of the oxidative phosphorylation (OXPHOS) system, including mtDNA mutations that are secondary to nuclear gene mutations and nuclear gene defects responsible for secondary OXPHOS deficiency (panel). The latter group of diseases are predominantly neurodegenerative. The mitochondrion's role in apoptosis and its contribution to the pathogenesis of neurodegenerative diseases are also covered.

Mitochondrial respiratory chain disorders I: mitochondrial DNA defects

Mitochondria have a pivotal role in cell metabolism, being the major site of ATP production via oxidative phosphorylation (OXPHOS); they have a critical role in apoptotic cell death; and they also contribute to human genetics since mitochondria have a functional genome separate from that of nuclear DNA. Defects of mitochondrial metabolism are associated with a wide spectrum of disease. An Important part of this spectrum is caused by mutations of mitochondrial DNA (mtDNA). These class I OXPHOS diseases are covered in part I of this two-part review. Dysfunction of mitochondrial OXPHOS has also emerged as an important component of a range of predominantly neurodegenerative diseases in which the mitochondrial abnormality is most probably secondary. These class II OXPHOS diseases are due to mutations of genes not encoding OXPHOS subunits or are caused by exogenous or endogenous OXPHOS toxins. Class II mitochondrial diseases and the mitochondrion's role in apoptosis are covered in part II (Lancet 2000; 355: 389-94).

Clinical, biochemical and molecular genetic correlations in Friedreich's ataxia

Friedreich's ataxia (FRDA) is an autosomal recessive disorder with a frequency of 1 in 50 000 live births. In 97% of patients it is caused by the abnormal expansion of a GAA repeat in intron 1 of the FRDA gene on chromosome 9, which encodes a 210 amino acid protein called frataxin. Frataxin is widely expressed and has been localized to mitochondria although its function is unknown. We have investigated mitochondrial function, mitochondrial DNA levels, aconitase activity and iron content in tissues from FRDA patients. There were significant reductions in the activities of complex I, complex II/III and aconitase in FRDA heart. Respiratory chain and aconitase activities were decreased although not significantly in skeletal muscle, but were normal in FRDA cerebellum and dorsal root ganglia, although there was a mild decrease in aconitase activity in the latter. Mitochondrial DNA levels were reduced in FRDA heart and skeletal muscle, although in skeletal muscle this was paralleled by a decline in citrate synthase activity. Increased iron deposition was seen in FRDA heart, liver and spleen in a pattern consistent with a mitochondrial location. The iron accumulation, mitochondrial respiratory chain and aconitase dysfunction and mitochondrial DNA depletion in FRDA heart samples largely paralleled those in the yeast YFH1 knockout model, suggesting that frataxin may be involved in mitochondrial iron regulation or iron sulphur centre synthesis. However, the severe deficiency in aconitase activity also suggests that oxidant stress may induce a self-amplifying cycle of oxidative damage and mitochondrial dysfunction, which may contribute to cellular toxicity.

Mitochondrial dysfunction and free radical damage in the Huntington R6/2 transgenic mouse

Huntington's disease is a progressive neurodegenerative disease caused by an abnormally expanded (>36) CAG repeat within the ITI5 gene encoding a widely expressed 349-kd protein, huntingtin. The medium spiny neurons of the caudate preferentially degenerate in Huntington's disease, with the presence of neuronal intranuclear inclusions. Excitotoxicity is thought to be important in the pathogenesis of Huntington's disease; the recently described mitochondrial respiratory chain and aconitase defects in Huntington's disease brain are consistent with this hypothesis. A transgenic mouse model (R6/2) of Huntington's disease develops a movement disorder, muscle wasting, and premature death at about 14 to 16 weeks. Selective neuronal death in these mice is not seen until 14 weeks. Biochemical analysis of R6/2 mouse brain at 12 weeks demonstrated a significant reduction in aconitase and mitochondrial complex IV activities in the striatum and a decrease in complex IV activity in the cerebral cortex. Increased immunostaining for inducible nitric oxide synthase and nitrotyrosine was seen in the transgenic mouse model but not control mouse brains. These results extend the parallels between Huntington's disease and the transgenic mouse model to biochemical events and suggest complex IV deficiency and elevated nitric oxide and superoxide radical generation precede neuronal death in the R6/2 mouse and contribute to pathogenesis.

MitBASE : a comprehensive and integrated mitochondrial DNA database. The present status

MitBASE is an integrated and comprehensive database of mitochondrial DNA data which collects, under a single interface, databases for Plant, Vertebrate, Invertebrate, Human, Protist and Fungal mtDNA and a Pilot database on nuclear genes involved in mitochondrial biogenesis in Saccharomyces cerevisiae. MitBASE reports all available information from different organisms and from intraspecies variants and mutants. Data have been drawn from the primary databases and from the literature; value adding information has been structured, e.g., editing information on protist mtDNA genomes, pathological information for human mtDNA variants, etc. The different databases, some of which are structured using commercial packages (Microsoft Access, File Maker Pro) while others use a flat-file format, have been integrated under ORACLE. Ad hoc retrieval systems have been devised for some of the above listed databases keeping into account their peculiarities. The database is resident at the EBI and is available at the following site: http://www3.ebi.ac.uk/Research/Mitbase/mitbas e.pl. The impact of this project is intended for both basic and applied research. The study of mitochondrial genetic diseases and mitochondrial DNA intraspecies diversity are key topics in several biotechnological fields. The database has been funded within the EU Biotechnology programme.

A missense mutation of cytochrome oxidase subunit II causes defective assembly and myopathy

We report the first missense mutation in the mtDNA gene for subunit II of cytochrome c oxidase (COX). The mutation was identified in a 14-year-old boy with a proximal myopathy and lactic acidosis. Muscle histochemistry and mitochondrial respiratory-chain enzymology demonstrated a marked reduction in COX activity. Immunohistochemistry and immunoblot analyses with COX subunit-specific monoclonal antibodies showed a pattern suggestive of a primary mtDNA defect, most likely involving CO II, for COX subunit II (COX II). mtDNA-sequence analysis demonstrated a novel heteroplasmic T-->A transversion at nucleotide position 7,671 in CO II. This mutation changes a methionine to a lysine residue in the middle of the first N-terminal membrane-spanning region of COX II. The immunoblot studies demonstrated a severe reduction in cross-reactivity, not only for COX II but also for the mtDNA-encoded subunit COX III and for nuclear-encoded subunits Vb, VIa, VIb, and VIc. Steady-state levels of the mtDNA-encoded subunit COX I showed a mild reduction, but spectrophotometric analysis revealed a dramatic decrease in COX I-associated heme a3 levels. These observations suggest that, in the COX protein, a structural association of COX II with COX I is necessary to stabilize the binding of heme a3 to COX I.

Mitochondrial myopathies and encephalomyopathies

Defects of mitochondrial metabolism result in a wide variety of human disorders, which can present at any time from infancy to late adulthood and involve virtually any tissue either alone or in combination. Abnormalities of the electron transport and oxidative phosphorylation (OXPHOS) system are probably the most common cause of mitochondrial diseases. Thirteen of the protein subunits of OXPHOS are encoded by mitochondrial DNA (mtDNA) and mutations of this genome are important causes of OXPHOS deficiency. The link between genotype and phenotype with respect to mtDNA mutations is not clear: the same mutation may result in a variety of phenotypes, and the same phenotype may be seen with a variety of different mtDNA mutations. The pathogenesis of mtDNA mutations is unclear although OXPHOS and ATP deficiency, and free radical generation, are thought to contribute to tissue dysfunction. There is now strong evidence for mitochondrial dysfunction in neurodegenerative disorders. In some cases, e.g. Friedreich's ataxia, hereditary spastic paraplegia, this is a result of a mutation of a nuclear gene encoding a mitochondrial protein, whilst in others, e.g. Huntington's disease, amyotrophic lateral sclerosis, the OXPHOS defect is secondary to events induced by a mutation in a nuclear gene encoding a non-mitochondrial protein. In yet a third group, e.g. Parkinson's disease, Alzheimer's disease, the relationship of the mitochondrial defect to aetiology and pathogenesis is unclear.

Deficit of in vivo mitochondrial ATP production in patients with Friedreich ataxia

Friedreich ataxia (FRDA), the most common of the inherited ataxias, is an autosomal recessive degenerative disorder, characterized clinically by onset before the age of 25 of progressive gait and limb ataxia, absence of deep tendon reflexes, extensor plantar responses, and loss of position and vibration sense in the lower limbs. FRDA is caused by a GAA triplet expansion in the first intron of the FRDA gene on chromosome 9q13 in 97% of patients. The FRDA gene encodes a widely expressed 210-aa protein, frataxin, which is located in mitochondria and is severely reduced in FRDA patients. Frataxin function is still unknown but the knockout of the yeast frataxin homologue gene (YFH1) showed a severe defect of mitochondrial respiration and loss of mtDNA associated with elevated intramitochondrial iron. Here we report in vivo evidence of impaired mitochondrial respiration in skeletal muscle of FRDA patients. Using phosphorus magnetic resonance spectroscopy we demonstrated a maximum rate of muscle mitochondrial ATP production (V(max)) below the normal range in all 12 FRDA patients and a strong negative correlation between mitochondrial V(max) and the number of GAA repeats in the smaller allele. Our results show that FRDA is a nuclear-encoded mitochondrial disorder affecting oxidative phosphorylation and give a rationale for treatments aimed to improve mitochondrial function in this condition.