Causal effects of circulating cytokine concentrations on risk of Alzheimer’s disease: A bidirectional two-sample Mendelian randomization study

Background There is considerable interest in the role of neuroinflammation in the pathogenesis of Alzheimer’s disease. Evidence from observational studies suggests an association between cytokine concentrations and Alzheimer’s disease. However, establishing a causal role of cytokine concentrations on risk of Alzheimer’s disease is challenging due to bias from reverse causation and residual confounding.

Methods We used two-sample MR to explore causal effects of circulating cytokine concentrations on Alzheimer’s disease and vice versa, employing genetic variants associated with cytokine concentrations (N=8,293) and Alzheimer’s disease (71,880 cases / 383,378 controls) from the largest non-overlapping genome-wide association studies (GWAS) of European ancestry.

Results There was weak evidence to suggest that 1 standard deviation (SD) increase in levels of CTACK (CCL27) (OR= 1.09 95%CI: 1.01 to 1.19, p=0.03) increased risk of Alzheimer’s disease. There was also weak evidence of a causal effect of 1 SD increase in levels of MIP-1b (CCL4) (OR=1.04 95%CI: 0.99 to 1.09, p=0.08), Eotaxin (OR=1.08 95%CI: 0.99 to 1.17, p =0.10), GROa (CXCL1) (OR=1.04 95%CI: 0.99 to 1.10, p=0.15), MIG (CXCL9) (OR=1.17 95%CI: 0.97 to 1.41, p=0.10), IL-8 (Wald Ratio: OR=1.21 95%CI: 0.97 to 1.51, p=0.09) and IL-2 (Wald Ratio: OR=1.21 95%CI: 0.94 to 1.56, p=0.14) on greater risk of Alzheimer’s disease. There was little evidence of a causal effect of genetic liability to Alzheimer’s disease on circulating cytokine concentrations.

Conclusions Our study provides some evidence supporting a causal role of cytokines in the pathogenesis of Alzheimer’s disease. However, more studies are needed to elucidate the specific mechanistic pathways via which cytokines alter the risk of Alzheimer’s disease.