A clinical-genetic study of Parkinson's disease in a genetically isolated community.

The role of genetic factors in idiopathic, late-onset Parkinson's disease (PD) remains unclear, in spite of the recent advances in the genetics of early-onset forms of familial parkinsonism. There is increasing interest in using genetically isolated populations to unravel the genetics of complex diseases such as late-onset PD. We have studied genetic and clinical features of 109 patients with parkinsonism from an area comprising a genetically isolated population in the South-West of the Netherlands. Of the 109 patients with ascertained parkinsonism, 41 patients were diagnosed with PD and could be linked to a common founder 14 generations ago. The distribution of ages at onset of PD in the genetically isolated population was significantly bimodal, showing two peaks (one with a mean at age 67 years and another with a mean at 44 years, the former peak being significantly larger than that in a population-based study, the Rotterdam Study). In other clinical features, the only statistically significant difference between early-onset and late-onset PD was a decreased motor and cognitive function in patients with late-onset PD. Involvement of other PD genes including DJ-1, a gene implicated in a kindred with early-onset parkinsonism from the same genetic isolate, was excluded in other PD patients in the population. The finding of a common ancestor in 41 idiopathic-PD patients along with the exclusion of known PD genes and loci suggests the presence of at least one other, yet unknown, susceptibility gene involved in PD in this population.