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.

Liver failure associated with mitochondrial DNA depletion

BACKGROUND/AIMS

Liver failure in infancy can result from several disorders of the mitochondrial respiratory chain. In some patients, levels of mitochondrial DNA are markedly reduced, a phenomenon referred to as mitochondrial DNA depletion. To facilitate diagnosis of this condition, we have reviewed the clinical and pathological features in five patients with mitochondrial DNA depletion.

METHODS

Cases were identified by preparing Southern blots of DNA from muscle and liver, hybridising with appropriate probes and quantifying mitochondrial DNA relative to nuclear DNA.

RESULTS

All our patients with mitochondrial DNA depletion died of liver failure. Other problems included hypotonia, hypoglycaemia, neurological abnormalities (including Leigh syndrome) and cataracts. Liver histology showed geographic areas of fatty change, bile duct proliferation, collapse of liver architecture and fibrosis; some cells showed decreased cytochrome oxidase activity. Muscle from three patients showed mitochondrial proliferation, with loss of cytochrome oxidase activity in some fibres but not in others; in these cases, muscle mitochondrial DNA levels were less than 5% of the median control value. The remaining two patients (from a single pedigree) had normal muscle histology and histochemistry associated with less severe depletion of mitochondrial DNA in muscle.

CONCLUSIONS

Liver failure is common in patients with mitochondrial DNA depletion. Associated clinical features often include neuromuscular disease. Liver and muscle histology can be helpful in making the diagnosis. Mitochondrial DNA levels should be measured whenever liver failure is thought to have resulted from respiratory chain disease.

Indices of oxidative stress in Parkinson's disease, Alzheimer's disease and dementia with Lewy bodies

The cause of neuronal cell death in Parkinson's disease is unknown but there is accumulating evidence suggesting that oxidative stress may be involved in this process. Current evidence shows that in the substantia nigra there is altered iron metabolism, decreased levels of reduced glutathione and an impairment of mitochondrial complex I activity. However, these changes seem to be unique to the substantia nigra and have not been found in other areas of the brain known to be altered in Parkinson's disease, such as substantia innominata. In addition they do not appear to be related to the presence of Lewy bodies, as other areas of the brain containing Lewy bodies do not show evidence of either oxidative stress or mitochondrial dysfunction. Oxidative stress has now been demonstrated in Alzheimer's disease and its presence appears to be correlated with regions of marked pathological changes.

Two pregnant women with vomiting and fits

Two women with hyperemesis gravidarum were first seen with a short history of confusion diplopia, unsteadiness, and fits caused by Wernicke's encephalopathy. The neurologic presentation had been precipitated by a carbohydrate load inadvertently administered without vitamin supplementation. We stress the importance of prescribing thiamine supplements to all women with prolonged vomiting during pregnancy.

Free radical scavengers protect dopaminergic cell lines from apoptosis induced by complex I inhibitors

The cause of dopaminergic neurodegeneration in Parkinson's disease remains unclear, but may involve both oxidative stress and mitochondrial complex I inhibition. We have demonstrated that complex I inhibitors, including rotenone, MPP+, isoquinoline and tetrahydroisoquinoline, induce apoptosis in PC12 and SK-N-MC dopaminergic cell lines which was decreased by pretreatment with N-acetylcysteine, TEMPO, dihydrolipoic acid or pyrrolidine dithiocarbamate. These results indicate that the pathway leading to apoptosis following complex I inhibition involves free radical generation. The free radical generation may result directly from inhibition of the mitochondrial respiratory chain or indirectly during the apoptotic process itself. This has important implications for our understanding of the relationship between complex I deficiency and oxidative stress and neurodegeneration in Parkinson's disease.

Genetic counselling in mitochondrial diseases

Mitochondrial disorders may be caused by mutations either in mitochondrial or in nuclear genes involved in the synthesis or regulation of respiratory chain subunits. The unique nature of the mitochondrial genome calls for a different approach to genetic counselling and risk analysis.

In vivo skeletal muscle mitochondrial function in Leber's hereditary optic neuropathy assessed by 31P magnetic resonance spectroscopy

We used 31P magnetic resonance spectroscopy (31P-MRS) to assess in vivo skeletal muscle mitochondrial function in 10 Leber's hereditary optic neuropathy patients/carriers with a mitochondrial DNA (mtDNA) mutation at one of three nucleotide positions, 11,778, 14,484, and 3,460. We studied one affected patient for each mutation and two unaffected carriers with the 11,778 or 3,460 mutation and three carriers with 14,484. All subjects were homoplasmic except the two 3,460 carriers, who showed 80% and 15% of mutated mtDNA. 31P-MRS at rest disclosed some abnormalities in all subjects. In particular, the phosphorylation potential was below the normal range in all cases. During recovery from exercise, the maximum rate of mitochondrial ATP production (Vmax) was reduced to 27% of normal in the 11,778 mutation and to 53% in the 14,484 mutation patient/carrier groups. Mitochondrial Vmax was within the normal range in all subjects with the 3,460 mutation but correlated inversely with the percentage of mutated mtDNA. This in vivo study shows that the 11,778 mutation causes a mitochondrial impairment more severe than the 14,484 and that the 3,460 mutation results in only a mild depression of muscle mitochondrial function.

Molecular mechanisms in mitochondrial DNA depletion syndrome

Depletion of mitochondrial DNA (mtDNA) appears to be an important cause of mitochondrial dysfunction in neonates and infants. We have identified another child in whom depletion of mtDNA was demonstrated in liver and serial skeletal muscle biopsies. A primary myoblast culture from the patient initially showed normal levels of mtDNA, but there was a progressive loss of mtDNA in later cell passages and clonal myoblast cell cultures, similar to that observed in the skeletal muscle tissue of the patient. Thus, these clonal myoblast cultures provide an in vitro model of the in vivo mtDNA dynamics. The levels of mitochondrial mRNAs for subunits I and II of cytochrome c oxidase declined with declining mtDNA levels, but the fall in mitochondrial transcript levels lagged behind that of the mtDNA levels. Levels of cytochrome c oxidase subunit I and II polypeptides, however, declined ahead of declining mtDNA levels. Immunocytochemistry showed that between individual cells of the clonal myoblast cultures, the expression of the mitochondrially encoded subunit I of cytochrome c oxidase was heterogeneous, suggesting variable levels of mtDNA. Transfer of patient mitochondria with residual mtDNA levels to control cells devoid of mtDNA (rho0 cells) led to restoration of mtDNA levels and, hence, suggests a nuclear involvement in the depletion.

Mitochondrial respiratory chain function in multiple system atrophy

Multiple system atrophy (MSA) is a clinico-pathological entity distinct from idiopathic Parkinson's disease (PD) that is responsible for 5-10% of cases of parkinsonism. Degeneration of nigral neurones is a feature of both diseases. A specific deficiency of mitochondrial complex I activity has been found in PD substantia nigra. We have analysed mitochondrial function in substantia nigra and platelets from MSA patients to identify any respiratory chain defect in this disorder and to determine its tissue specificity. As our MSA patients had been on L-DOPA, we also sought to establish whether this treatment could cause the complex I defect as seen in PD. We found no significant difference in respiratory chain activity corrected for mitochondrial mass between control and MSA patients in either of the tissues studied. These results provide a biochemical dimension to the differences between MSA and idiopathic PD. In addition, the fact that L-DOPA failed to induce a complex I defect in MSA substantia nigra suggests that this treatment is unlikely to cause the complex I deficiency in PD, without additional factors that may operate in PD.

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.

Mitochondrial dysfunction in neurodegeneration

Numerous toxins are known to interfere with mitochondrial respiratory chain functions. Use has been made of these in the development of pesticides and herbicides, and accidental use in man has led to the development of animal models for human disease. The propensity for mitochondrial toxins to induce neuronal cell death may well reflect not only their metabolic pathways but also the sensitivity of neurons to inhibition of oxidative phosphorylation. Thus, the accidental exposure of humans to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and to 3-nitropropionic acid had led to primate models of Parkinson's disease and Huntington's Disease, respectively. These models were made all the more remarkable when identical biochemical deficiencies were identified in relevant areas of human suffering from the respective idiopathic diseases. The place of complex I deficiency in Parkinson's disease remains undetermined, but there is recent evidence to suggest that, in some cases at least, it may play a primary role. The complex II/III deficiency in Huntington's disease is likely to be secondary and induced by other pathogenetic factors. The potential to intervene in the cascade of reactions involving mitochondrial dysfunction and cell death offers prospects for the development of new treatment strategies either for neuroprotection in prophylaxis or rescue.

Pathogenesis of Parkinson's disease

The aetiology and pathogenesis of Parkinson's disease (PD) remain unknown. There is a consensus emerging that there are likely to be multiple aetiologies that may result in the clinical and pathological abnormalities common to the majority of patients with idiopathic PD. Genetic factors have been suggested as important in either the cause of PD or in determining susceptibility. The recent linkage in one large pedigree of a gene for autosomal dominant parkinsonism to chromosome 4q21-23 and the subsequent identification of a mutation in the alpha-synuclein gene of this region are important steps towards identifying a biochemical deficiency capable of causing selective dopaminergic cell death. However, the relevance of such a defect to the majority of patients with apparent sporadic PD remains to be established. Factors that may predispose to substantia nigral cell loss, including mitochondrial dysfunction and oxidative damage, could be common to a number of separate aetiologies. A better understanding of these and their relationship to neuronal loss may provide further clues to aetiology.

Congenital muscular dystrophy with severe retrocollis and mental retardation: a report of two siblings

Two siblings with a congenital muscular dystrophy and severe mental retardation which was not due to dystrophin, merosin, or adhalin deficiency are described. These cases overlap with congenital muscular dystrophy of the Fukuyama-type but are less severe. Atypical features include limited facial involvement, retained ambulation, and severe retrocollis.

Mitochondrial disorders

There have been significant advances in our understanding of the contribution of mitochondria to basic cellular function such as energy supply, calcium homeostasis and, more recently, programmed cell death. Mitochondria now appear to play an important role in the final common pathway leading to apoptosis. Study of inborn errors of the respiratory chain is now focussed on understanding pathogenesis, in particular the role of the cell nucleus in determining the expression of mitochondrial DNA mutations. Respiratory chain deficiencies induced by exogenous or endogenous toxins are important in the aetiology and pathogenesis of certain neurodegenerative diseases such as Parkinson's disease and Huntington's disease. A potential role for inborn mitochondrial defects in these disorders has not yet been defined but is currently attracting interest.

Neuromyelitis optica (Devic's syndrome): no association with the primary mitochondrial DNA mutations found in Leber hereditary optic neuropathy

Devic's neuromyelitis optica is a rare syndrome characterised by the combination of acute or subacute optic neuritis and transverse myelitis, in some cases considered to be a variant of multiple sclerosis. Mutations of mitochondrial DNA (mtDNA) associated with Leber hereditary optic neuropathy (LHON) have been identified in some patients with multiple sclerosis in whom optic neuritis is a prominent early feature. Using restriction enzyme digestion of mtDNA products amplified by the polymerase chain reaction, the primary LHON mtDNA mutations at positions 3460 bp, 11,778 bp, and 14,484 bp have been excluded in four women with Devic's neuromyelitis optica. A mutation at 4160 bp associated in some LHON families with more widespread neurological disease was also not detected. It is concluded that the primary mtDNA mutations currently associated with LHON are not responsible for the prominence of optic nerve disease in Devic's neuromyelitis optica.

Detection of nitrosyl complexes in human substantia nigra, in relation to Parkinson's disease

Idiopathic Parkinson's disease (PD) involves a documentable decline in the activity of mitochondrial complex I in substantia nigra (1-3). We have EPR spectroscopy to investigate complex I in human substantia nigra and globus pallidus. EPR signals characteristic of the iron-sulfur centers of complexes I and II were observed with globus pallidus, with no significant difference between control and PD. These complex 1 signals could not be clearly observed in substantia nigra. Instead, nitric oxide (NO.) radicals in PD nigra were detected at g approximately 2.08, 1.98 due to [haem-NO] formation. Although an EPR signal indicative of haem-NO was observed with control nigra, it lacked the distinctive g approximately 1.98 trough observed with PD nigra. As PD is associated with a reactive gliosis, the difference in the haem-NO EPR signal, between control and PD nigra, may result from cytotoxic NO. generated by microglia in PD substantia nigra.

Oxidative stress and mitochondrial dysfunction in neurodegeneration

Rapid advances are being made in our understanding of the pathogenesis of neurodegenerative diseases, particularly those in which specific DNA mutations have been identified. beta-amyloid has been shown to induce free radical formation both directly and via an effect on endothelial function. There is presuasive evidence for cytochrome oxidase dysfunction with oxidative stress and damage in the brains of patients with Alzheimer's disease. The confirmation of the complex II inhibitor 3-nitropropionic acid as a toxin model for Huntington's disease, together with the demonstration of reduced mitochondrial function in Huntington's disease caudate, supports the proposition that mutant huntingtin may exert its effect through an abnormality of energy metabolism.

Oxidative stress and Parkinson's disease

The underlying mechanism of cell death in substantia nigra of Parkinson's disease patients remains unknown. Biochemical changes occurring in substantia nigra in Parkinson's disease (increased iron levels, inhibition of complex I activity and decreased reduced glutathione levels; GSH) suggest that oxidative stress and free radical species may be involved. In particular, a decrease in GSH levels may be an early component of the process, since this also occurs in incidental Lewy body disease (presymptomatic Parkinson's disease). GSH is lost only from the substantia nigra in Parkinson's disease and this does not occur in other neurodegenerative disorders of the basal ganglia. GSH loss appears to be global throughout the substantia nigra and not localized to either the glia or neuronal elements. The activity of enzymes involved in the glutathione cycle are normal with the exception of gamma-glutamyltranspeptidase, the activity of which is increased. This could result in increased removal and degradation of glutathione from cells. Depletion of GSH in rat using L-buthionine-[S, R]-sulfoxamine (BSO) potentiates 6-hydroxydopamine (6-OHDA) toxicity but does not in itself produce degeneration of the nigrostriatal pathway. Oxidative stress may be a potentially important factor in the degeneration of the substantia nigra in Parkinson's disease and warrants further investigation into its role in this process.

Mitochondrial defect in Huntington's disease caudate nucleus

Although the Huntington's disease (HD) gene defect has been identified, the structure and function of the abnormal gene product and the pathogenetic mechanisms involved in producing death of selective neuronal populations are not understood. Indirect evidence from several sources indicates that a defect of energy metabolism and consequent excitotoxicity are involved in HD. Toxin models of HD may be induced by 3-nitropropionic acid or malonate, both inhibitors of succinate dehydrogenase, complex II of the mitochondrial respiratory chain. We analyzed mitochondrial respiratory chain function in the caudate nucleus (n = 10) and platelets (n = 11) from patients with HD. In the caudate nucleus, severe defects of complexes II and III (53-59%, p < 0.0005) and a 32-38% (p < 0.01) deficiency of complex IV activity were demonstrated. No deficiencies were found in platelet mitochondrial function. The mitochondrial defect identified in HD caudate parallels that induced by HD neurotoxin models and further supports the role of abnormal energy metabolism in HD. The relationship of the mitochondrial defect to the role of huntingtin is not known.

HLA class I genotypes in Leber's hereditary optic neuropathy

There is evidence that mitochondrial DNA (mtDNA) encoded peptides can restrict the immune response in rodents and that these peptides are presented by classical and 'neoclassical' class I major histocompatibility complex (MHC) molecules. We investigated the frequency of HLA-A and two HLA-B genotypes in index cases of 77 families with Leber's hereditary optic neuropathy (LHON), on the basis that there may be an autoimmune component to this disease. There was no association between LHON and any genotype. We conclude that the classical class I MHC loci are not major determinants of the development of blindness in LHON.