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Parkinson's disease (PD) is the second most common neurodegenerative disease after Alzheimer's disease. It is urgently needed to elucidate the cause of the disease and to establish neuroprotective treatment. We have been working on the etiology and pathogenesis of PD for many years and we found selective loss of mitochondrial complex I and the alpha-ketoglutarate dehydrogenase complex in the nigral neurons of patients with PD. Our observation firmly established mitochondrial defects in PD. Mitochondrial respiratory failure induces oxidative damage in neurons, and we found increase in hydroxynonenal and 8-oxo-deoxyguanine, indices of oxidative damage, in the nigral neurons of PD. These abnormalities can trigger apoptotic cell death. The primary events which induce mitochondrial failure and oxidative damage are not known, however, it has been postulated that the interaction of genetic risk factors and environmental factors would initiate the degenerative process. Based on this assumption, we conducted genetic association studies by the candidate gene methods. We found that polymorphic mutations of superoxide dismutase-2 and 24-kDa subunit of mitochondrial complex I were associated increased risk of developing Parkinson's disease. While we were doing this genetic association study, we found a family, in which parkinsonian phenotype completely segregated with a polymorphic mutation of the superoxide dismutase-2 gene. In this family, 4 out of 6 siblings were affected with early onset parkinsonism and the parents were apparently normal. Thus the mode of inheritance appeared to be autosomal recessive and this type is now called as AR-JP or Park2. We confirmed the linkage of this type of familial Parkinson's disease to the superoxide dismutase loci that is located in the telomeric region of chromosome 6 by the linkage analysis using microsatellite markers in this region. Then we found another family, in which an affected patient showed lack of one of the microsatellite markers (D6S315), which we were using in the linkage analysis. This observation prompted us to initiate the molecular cloning of the disease gene utilizing D6S315 as the initial probe. The molecular cloning was done with the collaboration with Professor Nobuyoshi Shimizu of Keio University. We identified a novel gene and confirmed that mutations of this novel gene were found only in the patients with autosomal recessive Parkinson's disease. The novel gene was named parkin. We conducted mutational analysis on more than 700 families with Parkinson's disease. We also established a method to detect compound heterozygotes of parkin mutations. Mutinous of the parkin gene were found in approximately 50% of autosomal recessive families. Many kinds of exonic deletions and point mutations were found. This type of familial Parkinson's disease had been considered to be unique among Japanese, but since we started mutational analysis of the parkin gene, we confirmed the world wide distribution of parkin gene mutations. Then we analyzed functions of parkin protein with the collaboration with Dr. Keiji Tanaka of Tokyo Metropolitan Institute of Medical Sciences. We found that parkin protein was a ubiquitin-protein ligase of the ubiquitin system. Now we are working on the candidate substrates of parkin protein as a ubiquitin ligase. We found that CDCrel-1, a synaptic vesicle protein, was a candidate substrate of parkin protein. In addition, we found two additional candidate proteins, i.e., alpha-synuclein 22 and PAEL receptor, with the collaboration of Professor Denis Selkoe of Harvard Medical School and Dr. Ryosuke Takahashi of RIKEN, respectively. Accumulation of PAEL receptor in the endoplasmic reticulum causes endoplasmic reticulum stress and apoptotic cell death. We found evidence to indicate accumulation of PAEL receptor and the presence of endoplasmic reticulum stress in a patient with AR-JP (Park2). Thus our studies firmly established that a genetic defect of an enzyme in the ubiquitin-proteasome system induces selective nigral neuronal death. We indicated the important role of the ubiquitin-proteasome system in neurodegeneration in general. In many other neurodegenerative disorders, such as Alzheimer's disease, Huntington's disease, Machado-Joseph disease, dentatorubral-pallidoluysian atrophy, and ALS, ubiquitinated proteins are accumulated in neurons. Thus protein handling in the ubiquitin-proteasome system appears to be affected in these neurodegenerative disorders despite the difference in the primary defects. Our studies also suggest many potential approaches for the discovery of neuroprotective treatment for not only Parkinson's disease but also other neurodegenerative disorders.
Chromosomes, Human, Pair 6↗