Defining at-risk populations for Parkinson's disease: lessons from ongoing studies.

It is currently widely acknowledged that the natural history of PD includes a preclinical phase, and there are increasing efforts to identify markers that would allow the identification of individuals at risk for PD. Here, we discuss the issues related to defining at-risk populations for PD and review findings of current population-based cohorts that have reported potential biomarkers for PD, such as the Honolulu-Asia Aging Study (HAAS) and the PRIPS (Prospective Validation of Risk factors for the development of Parkinson Syndromes) study. We also discuss enriched risk cohorts designed to evaluate specificity and predictive value of markers exemplified by the PARS (Parkinson Associated Risk Study) and the TREND (Tübinger evaluation of Risk factors for the Early detection of NeuroDegeneration) study. Although there is still a long way to go, studies designed according to these concepts might eventually provide sufficient data to form the basis for future screening programs for PD risk to be applied at a population level.