[Parkin gene and its function; a key to understand nigral degeneration]

In most patients with Parkinson's disease (PD), the contribution of genetic factors as well as environmental factors remains to be elucidated. But, it has become clear that genetic factors contribute to the pathogenesis of PD after identification of the distinct genetic loci for certain forms of familial PD. We recently identified the novel large gene "parkin" responsible for an autosomal recessive form of familial parkinsonism (AR-JP). AR-JP is a distinct clinical and genetic entity characterized by early onset before 40 years. Pathological changes in this form revealed selective degeneration of the pigmented neurons in the substantia nigra and locus coeruleus, but no Lewy bodies were found. The parkin gene encodes a novel protein of 465 amino acids. The parkin gene is mildly homologous to ubiquitin at the N-terminal portion and has a RING-finger motif at the C-terminal portion. We found variable different homozygous deletions involving exons 3, 4, 5, 3 to 4, 3 to 5, and 3 to 7 in AR-JP families from Japan. In addition to exonic deletions, we identified a one base deletion in exon 5 in two AR-JP families. Although we have identified several mutations in parkin gene, characterization of its gene product, "Parkin protein" has not yet been established. To elucidate the molecular mechanism underlying the disease, we have analyzed the subcellular localization of the Parkin protein by immunohistochemical and immunoblotting studies on patients with AR-JP and sporadic PD using two antibodies. Parkin protein was absent in all regions of the brains of AR-JP patients. Parkin protein was not decreased in brains of sporadic PD patients. Parkin protein was located in both Golgi complex and cytosol. Taken together, the Parkin protein may play a role in vesicular transport system in association with the Golgi complex.