Associations between Potentially Modifiable Risk Factors and Alzheimer Disease: A Mendelian Randomization Study

作者：Ostergaard Soren D^*; Mukherjee Shubhabrata; Sharp Stephen J; Proitsi Petroula; Lotta Luca A; Day Felix; Perry John R B; Boehme Kevin L; Walter Stefan; Kauwe John S; Gibbons Laura E; Larson Eric B; Powell John F; Langenberg Claudia; Crane Paul K; Wareham Nicholas J; Scott Robert A

来源：PLoS Medicine, 2015, 12(6): e1001841.

DOI：10.1371/journal.pmed.1001841

Background
Potentially modifiable risk factors including obesity, diabetes, hypertension, and smoking are associated with Alzheimer disease (AD) and represent promising targets for intervention. However, the causality of these associations is unclear. We sought to assess the causal nature of these associations using Mendelian randomization (MR).
Methods and Findings
We used SNPs associated with each risk factor as instrumental variables in MR analyses. We considered type 2 diabetes (T2D, N-SNPs = 49), fasting glucose (N-SNPs = 36), insulin resistance (N-SNPs = 10), body mass index (BMI, N-SNPs = 32), total cholesterol (N-SNPs = 73), HDL-cholesterol (N-SNPs = 71), LDL-cholesterol (N-SNPs = 57), triglycerides (N-SNPs = 39), systolic blood pressure (SBP, N-SNPs = 24), smoking initiation (N-SNPs = 1), smoking quantity (N-SNPs = 3), university completion (N-SNPs = 2), and years of education (N-SNPs = 1). We calculated MR estimates of associations between each exposure and AD risk using an inverse-variance weighted approach, with summary statistics of SNP-AD associations from the International Genomics of Alzheimer's Project, comprising a total of 17,008 individuals with AD and 37,154 cognitively normal elderly controls. We found that genetically predicted higher SBP was associated with lower AD risk (odds ratio [OR] per standard deviation [15.4 mm Hg] of SBP [95% CI]: 0.75 [0.62-0.91]; p = 3.4 x 10(-3)). Genetically predicted higher SBP was also associated with a higher probability of taking antihypertensive medication (p = 6.7 x 10(-8)). Genetically predicted smoking quantity was associated with lower AD risk (OR per ten cigarettes per day [95% CI]: 0.67 [0.51-0.89]; p = 6.5 x 10(-3)), although we were unable to stratify by smoking history; genetically predicted smoking initiation was not associated with AD risk (OR = 0.70 [0.37, 1.33]; p = 0.28). We saw no evidence of causal associations between glycemic traits, T2D, BMI, or educational attainment and risk of AD (all p > 0.1). Potential limitations of this study include the small proportion of intermediate trait variance explained by genetic variants and other implicit limitations of MR analyses.
Conclusions
Inherited lifetime exposure to higher SBP is associated with lower AD risk. These findings suggest that higher blood pressure-or some environmental exposure associated with higher blood pressure, such as use of antihypertensive medications-may reduce AD risk.

• 出版日期2015-6