Systematic review of Mendelian randomization studies on Parkinson's disease

Background: Parkinson's disease (PD) is known to be associated with non-genetic factors. To infer causality, Mendelian randomization (MR) studies are increasingly used. Here, genetic variants are used as instrumental variables for the risk factor but have no direct effect on PD themselves. Methods: We performed a systematic literature review on MR studies for PD. Studies were identified searching the PubMed database. Upon data extraction, we evaluated the methodological quality and summarized the evidence. Results: Twelve articles were included. Most studies showed "good"methodological quality, but most did not report proper power estimations. Twelve analyses yielded nominally significant effects. Conclusions: Our systematic review shows that most MR studies were well performed and allow to identify causal exposures, which may inform further studies on the prevention and early intervention of PD.