Genetically decreased vitamin D and risk of Alzheimer disease

Objective: To test whether genetically decreased vitamin D levels are associated with Alzheimer disease (AD) using mendelian randomization(MR), a method that minimizes bias due to confounding or reverse causation. Methods: We selected single nucleotide polymorphisms ( SNPs) that are strongly associated with 25-hydroxyvitamin D (25OHD) levels ( p −8 ) from the Study of Underlying Genetic Determinants of Vitamin D and Highly Related Traits (SUNLIGHT) Consortium (N = 33,996) to act as instrumental variables for the MR study. We measured the effect of each of these SNPson 25OHD levels in the Canadian Multicentre Osteoporosis Study (CaMos; N = 2,347) and obtained the corresponding effect estimates for each SNPon AD risk from the International Genomics of Alzheimer9s Project (N = 17,008 AD cases and 37,154 controls). To produce MR estimates, we weighted the effect of each SNPon AD by its effect on 25OHD and meta-analyzed these estimates using a fixed-effects modelto provide a summary effect estimate. Results: The SUNLIGHT Consortium identified 4 SNPsto be genome-wide significant for 25OHD, which described 2.44% of the variance in 25OHD in CaMos. All 4 SNPsmap to genes within the vitamin D metabolic pathway. MR analyses demonstrated that a 1-SD decrease in natural log–transformed 25OHD increased AD risk by 25% (odds ratio 1.25, 95% confidence interval 1.03–1.51, p = 0.021). After sensitivity analysis in which we removed SNPspossibly influenced by pleiotropyand population stratification, the results were largely unchanged. Conclusions: Our results provide evidence supporting 25OHD as a causal risk factor for AD. These findings provide further rationale to understand the effect of vitamin D supplementation on cognition and AD risk in randomized controlled trials.