Paucity of deleted mitochondrial DNAs in brain regions of Huntington's disease patients

Age-related mitochondrial alterations in brain and skeletal muscle of the YAC128 model of Huntington disease

Mitochondrial dysfunction and bioenergetics failure are common pathological hallmarks in Huntington's disease (HD) and aging. In the present study, we used the YAC128 murine model of HD to examine the effects of mutant huntingtin on mitochondrial parameters related to aging in brain and skeletal muscle. We have conducted a cross-sectional natural history study of mitochondrial DNA changes in the YAC128 mouse. Here, we first show that the mitochondrial volume fraction appears to increase in the axons and dendrite regions adjacent to the striatal neuron cell bodies in old mice. Mitochondrial DNA copy number (mtDNAcn) was used as a proxy measure for mitochondrial biogenesis and function. We observed that the mtDNAcn changes significantly with age and genotype in a tissue-specific manner. We found a positive correlation between aging and the mtDNAcn in striatum and skeletal muscle but not in cortex. Notably, the YAC128 mice had lower mtDNAcn in cortex and skeletal muscle. We further show that mtDNA deletions are present in striatal and skeletal muscle tissue in both young and aged YAC128 and WT mice. Tracking gene expression levels cross-sectionally in mice allowed us to identify contributions of age and genotype to transcriptional variance in mitochondria-related genes. These findings provide insights into the role of mitochondrial dynamics in HD pathogenesis in both brain and skeletal muscle, and suggest that mtDNAcn in skeletal muscle tissue may be a potential biomarker that should be investigated further in human HD.

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

Mitochondrial DNA 4977 bp deletion and OH8dG levels correlate in the brain of aged subjects but not Alzheimer's disease patients

The levels of mitochondrial DNA 4977 bp deletion (mtDNA4977) and mitochondrial DNA 8'-hydroxy-2'-deoxyguanosine (OH8dG) were determined in the same samples from two brain areas of healthy subjects and Alzheimer's disease (AD) patients. A positive correlation between the age-related increases of mtDNA4977 and of OH8dG levels was found in the brain of healthy individuals. On the contrary, in both brain areas of AD patients, mtDNA4977 levels were very low in the presence of high OH8dG amounts. These results might be explained assuming that the increase of OH8dG above a threshold level, as in AD patients, implies consequences for mtDNA replication and neuronal cell survival.

Kearns-Sayre syndrome: unusual pattern of expression of subunits of the respiratory chain in the cerebellar system

Kearns-Sayre syndrome (KSS) is a sporadic multisystem disorder of oxidative phosphorylation associated with clonally expanded rearrangements of mitochondrial DNA (mtDNA). Mitochondrial dysfunction in the central nervous system of patients with KSS accounts for the neurological manifestations of the disease. To gain further insight into the pathogenesis of neuronal dysfunction in KSS, we used antibodies against mtDNA-encoded and nuclear DNA-encoded subunits of the mitochondrial respiratory chain to study the expression of these proteins in the cerebellar cortex, dentate nucleus, and inferior olivary nucleus from 2 autoptic cases of KSS. Neuropathological examination showed a moderate loss of Purkinje cells and spongiform degeneration of the cerebellar white matter. By using immunohistochemistry, we found a decreased expression of mtDNA-encoded proteins only in neurons of the dentate nucleus. We suggest that mitochondrial abnormalities in the dentate nucleus in conjunction with loss of Purkinje cells and spongiform degeneration of the cerebellar white matter may be important factors in the genesis of the cerebellar dysfunction in KSS.

Mitochondrial DNA mutations and age

Apopotic cell death is reported to be prominent in the stable tissues of the failing heart, in cardiomyopathies (CM), in the sinus node of complete heart block, in B cells of diabetes mellitus, and in neurodegenerative diseases. Recently, mitochondrial (mt) control of nuclear apoptosis was demonstrated in the cell-free system. The mt bioenergetic crisis induced by exogenously added factors such as respiratory inhibitors leads to the collapse of mt transmembrane potential, to the opening of the inner membrane pore, to the release of the apoptotic protease activating factors into cytosol, and subsequently to nuclear DNA fragmentation. However, the endogenous factor for the mt bioenegertic crisis in naturally occurring cell death under the physiological conditions without vascular involvement has remained unknown. Recently devised, the total detection system for deletion demonstrates the extreme fragmentation of mtDNA in the cardiac myocytes of senescence, and mt CM harboring maternally inherited point mutations in mtDNA and on the cultured cell line with or without mtDNA disclosed that mtDNA is unexpectedly fragile to hydroxyl radial damage and hence to oxygen stress. The great majority of wild-type mtDNA fragmented into over two hundreds types of deleted mtDNA related to oxidative damage, resulting in pleioplasmic defects in the mt energy transducing system. The mtDNA fragmentation to this level is demonstrated in cardiac myocytes of normal subjects over age 80, of an mtCM patient who died at age 20 and one who died at age 19, of a recipient of heart transplantation at age 7 with severe mtCM, and in mtDNA of a cultured cell line under hyperbaric oxygen stress for two days, leading a majority of cells to apoptotic death on the third day. The extreme fragility of mtDNA could be the missing link in the apoptosis cascade that is the physiological basis of aging and geriatrics of such stable tissues as nerve and muscle.

Recent developments in the molecular genetics of mitochondrial disorders

Rapid progress has been made in the identification of mitochondrial DNA mutations which are typically associated with diseases of the nervous system and muscle. The well established mitochondrial disorders are maternally inherited and males and females are equally affected. An exception is Leber's hereditary optic atrophy (LHON) which is observed much more frequently in males than in females. There are three common point mutations in LHON which can be homoplasmic or heteroplasmic. In mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) most mutations are single base changes and lie within the tRNA-Leu gene. Point mutations in myoclonic epilepsy with ragged red fibres (MERRF) usually occur within the tRNA-Lys gene but mutations of the tRNA-Leu gene are also observed. MELAS and MERRF mutations are heteroplasmic and there is considerable clinical overlap between these diseases. Point mutations within the ATPase6 gene result in either neuropathy, ataxia and retinitis pigmentosa (NARP) or in Leigh's syndrome. The latter occurs if the mutation is present in the majority of mitochondria (extreme heteroplasmy). Finally, mitochondrial DNA deletions are the cause underlying Kearns-Sayre syndrome (KSS). Apart from the well-established mitochondrial diseases, there is increasing evidence that mitochondrial mutations may also play a role in the neurodegenerative disorders Parkinson, Alzheimer and Huntington disease. The complex I defect found in Parkinson disease is especially interesting in this respect. However, no causative mitochondrial mutation has as yet been established in any of these three common disorders.

The 'common deletion' is not increased in parkinsonian substantia nigra as shown by competitive polymerase chain reaction

Previous studies have estimated levels of mitochondrial DNA (mtDNA) carrying the 4,977-base-pair 'common deletion' in tissues from patients with Parkinson's disease (PD) by using semiquantitative techniques. The role of this deleted mtDNA species in the pathogenesis of PD has remained controversial. We have applied competitive polymerase chain reaction to achieve exact quantitation of deleted mtDNA in the substantia nigra and additional brain regions of cases with neuropathologically confirmed Lewy-body parkinsonism. In addition, genotyping was carried out for CYP2D6G1,934A and CYP2D6C2,938T alleles and the mitochondrial ND2 (nucleotide 5,460) and transfer RNA for glutamine (nucleotide 4,336) sequence variants. Parkinsonian brains showed 1-3% deleted mtDNA in the substantia nigra, that is, deletion levels were not higher than in age-matched controls. Our findings suggest that the defect in complex I of the respiratory chain observed in PD is not primarily due to the 'common deletion.'

Mitochondrial encephalomyopathies: what next?

In few areas of medicine has progress been more spectacular than in the field of mitochondrial diseases, especially those related to mtDNA mutations. Much remains to be done, however, and this brief review discusses the following areas of research where progress has been more limited or data are still controversial: (1) the molecular basis of respiratory-chain defects due to nuclear DNA mutations; (2) defects of mitochondrial protein importation; (3) defects of intergenomic signalling; (4) pathophysiology of mtDNA-related disorders; (5) ageing and age-related neurodegenerative diseases; (6) therapy; and (7) genetic counselling.