No Clear Support for a Role for Vitamin D in Parkinson's Disease: A Mendelian Randomization Study

Background: Observational studies have found that relative to healthy controls, patients with Parkinson's disease have lower circulating concentrations of 25-hydroxyvitamin D, a clinical biomarker of vitamin D status. However, the causality of this association is uncertain. We undertook a Mendelian randomization study to investigate whether genetically decreased 25-hydroxyvitamin D concentrations are associated with PD to minimize confounding and prevent bias because of reverse causation. Methods: As instrumental variables for the Mendelian randomization analysis, we used 4 single-nucleotide polymorphisms that affect 25-hydroxyvitamin D concentrations (rs2282679 in GC, rs12785878 near DHCR7, rs10741657 near CYP2R1, and rs6013897 near CYP24A1). Summary effect size estimates of the 4 single-nucleotide polymorphisms on PD were obtained from the International Parkinson's Disease Genomics Consortium (including 5333 PD cases and 12,019 controls). The estimates of the 4 single-nucleotide polymorphisms were combined using an inverse-variance weighted meta-analysis. Results: Of the 4 single-nucleotide polymorphisms associated with 25-hydroxyvitamin D concentrations, one (rs6013897 in CYP24A1) was associated with PD (odds ratio per 25-hydroxyvitamin D-decreasing allele, 1.09; 95% confidence interval, 1.02-1.16; P = 0.008), whereas no association was observed with the other 3 single-nucleotide polymorphisms (P > 0.23). The odds ratio of PD per genetically predicted 10% lower 25-hydroxyvitamin D concentration, based on the 4 single-nucleotide polymorphisms, was 0.98 (95% confidence interval, 0.93-1.04; P = 0.56). Conclusions: This Mendelian randomization study provides no clear support that lowered 25-hydroxyvitamin D concentration is causally associated with risk of PD. (C) 2017 International Parkinson and Movement Disorder Society