Lack of overlap in genetic risks for Alzheimer's disease and Parkinson's disease.

We explored the question of genetic overlap between Alzheimer's disease (AD) and Parkinson's disease (PD) because evidence suggests clinical, pathologic, and epidemiologic overlap between the two disorders. We compared the frequency of AD and PD between the first-degree relatives of probands with AD and PD and first-degree relatives of spouse control subjects. Using life-table methods, we found increased risk of AD in first-degree relatives of patients with AD and an increased risk of PD in first-degree relatives of patients with PD. The risk of PD in first-degree relatives of patients with AD was not increased, nor was the risk of AD in first-degree relatives of patients with PD increased. These data do not support the hypothesis that important genetic overlap exists between AD and PD.