How genetics research in Parkinson's disease is enhancing understanding of the common idiopathic forms of the disease

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Abstract
Rapid progress in genetics has meant that there are now five genes identified for 'Parkinson's disease'. The detailed phenotypes vary, but generally the dominant genes cause a Lewy body disease spectrum whereas recessive genes cause a milder parkinsonism with variable inclusion body pathology. The subject of this review is to highlight these discoveries and to discuss their relationships to idiopathic Parkinson's disease. In January 2004, mutations in PINK1, coding for a mitochondrial kinase, were found to be causal for recessive parkinsonism. Subsequently, several studies have found additional mutations associated with early onset parkinsonism. Some cases have been described with a phenotype much closer to idiopathic Parkinson's disease, but it does not appear that PINK1 is a major risk factor for the sporadic disease. Later in the same year, the LRRK2 gene was shown to cause a dominant disease with a broader phenotype. The protein product was named dardarin and contains GTPase and kinase domains. Lewy bodies have been reported in LRRK2 cases, potentially linking this gene with sporadic Parkinson's disease. One mutation, G2019S, is found in a significant percentage of cases, including sporadic Parkinson's disease. Mutations in these two genes, along with previously described Mendelian variants, are beginning to yield important information about loss of specific neuronal groups or to protein inclusion pathology. How this relates to sporadic Parkinson's disease, however, is not yet fully defined. There are clear phenotypic overlaps with genetic and sporadic Parkinson's disease, especially for the dominant genes, suggesting that common facets of pathogenesis may exist.