Shin-ichiro Ikebe

Increase of deleted mitochondrial DNA in the striatum in Parkinson's disease and senescence

A mutant mitochondrial DNA (mtDNA) with a 4,977-bp deletion was detected in the parkinsonian brain by using the polymerase chain reaction. Although the deleted mtDNA was detectable even in the brain of aged controls, the proportion of deleted mtDNA to normal mtDNA in the striatum was higher in the parkinsonian patients than in the controls. In both the parkinsonian patients and the aged controls, the proportion was higher in the striatum than in the cerebral cortex. These results indicate that age-related accumulation of deleted mtDNA is accelerated in the parkinsonian striatum and suggest that the deletion contributes to pathophysiological processes underlying Parkinsons disease.

Quantitative determination of deleted mitochondrial DNA relative to normal DNA in parkinsonian striatum by a kinetic PCR analysis

Deleted mitochondrial DNA (mtDNA) was accumulated in the parkinsonian striatum, but the same deleted mtDNA was also detectable in the control striatum when cycles of polymerase chain reaction were increased. To discriminate between these pathological and physiological conditions, we quantitatively analyzed the proportion of deleted mtDNA to normal mtDNA by measuring the incorporation of alpha-[32P]deoxycytosine triphosphate into mtDNA fragments by using a laser image analyzer. To estimate the molar ratio of the deleted mtDNA to normal mtDNA, the radioactivity was normalized by each fragment size. By plotting logarithms of normalized radioactivities against PCR amplification cycles, straight lines were obtained with different slopes. By extrapolation of the line to the zero amplification, the proportion of mutant mtDNA to normal mtDNA in the original sample from the parkinsonian striatum was estimated to be ca. 5%, which was at least ten times higher than the proportion of ca. 0.3% in the control striatum. These results indicate that phenotype of the mutant mtDNA as Parkinsons disease is expressed when the proportion of deleted mtDNA to normal mtDNA exceeds a threshold of ten times higher value than in the normal subject.

Point mutations of mitochondrial genome in Parkinson's disease

Oxidative stress and subsequent energy crisis have been proposed as the cause of nigral neuronal cell death in Parkinsons disease. We have reported defects in the mitochondrial respiratory chain and increased amount of deleted mitochondrial genome in the nigrostriatal system of patients with Parkinsons disease. Deletion in mitochondrial DNA could be ascribed to somatically acquired premature aging leading to cell death. To elucidate the contribution of maternally transmitted point mutations in mitochondrial DNA to the premature DNA damages, we employed a direct sequencing system and analyzed the total nucleotide sequences of mitochondrial DNA in the brains of five patients with idiopathic Parkinsons disease. There were no predominant point mutations among the patients in contrast to some neuromuscular diseases. However, each patient had several point mutations that would result in a significant change in the gene products. Some of these mutations may be involved either in the increased production of oxygen radicals from the mitochondrial respiratory chain or in the increased susceptibility of the respiratory chain components to oxidative damage. We propose that some of these mutations can be regarded as one of the risk factors accelerating degeneration of nigrostriatal pathway in Parkinsons disease.

Mitochondrial leucine tRNA mutation in a mitochondrial encephalomyopathy.

We have identified another A-to-G transition in the dihydrouridine loop of the leucine tRNA (UUR) gene in mtDNa of two independent patients with mitochondrial myopathy, encephalopathy, lacticacidosis, and stroke-like syndrome (MELAS)

Mitochondrial DNA Mutations as an Etiology of Human Degenerative Diseases

Because mitochondrial DNA (mtDNA) is exclusively maternally transmitted, mutations of mtDNA are implicated to be the cause of maternally inherited diseases. Recent extensive studies have clarified three types of mtDNA mutations in several human diseases.