Abstract
There is increasing evidence on inflammation as a determinant in the pathogenesis of Parkinson's disease. But, its role in parkinsonian neurodegeneration remains elusive: it´s not clear if inflammatory cascades are causes or consequences of dopamine neurons death. In the present study, we aim at performing an in-depth statistical investigation of the causal relationship between inflammation and Parkinson's disease using a two-sample Mendelian randomization design. Genetic instruments were selected using summary-level data from the largest to date genome-wide association studies (sample size ranging from 13,955 to 204,402 individuals) conducted on European population for the following inflammation biomarkers: C-reactive protein, interleukin-6, interleukin 1 receptor antagonist, and tumor necrosis factor α. Genetic association data on Parkinson's disease (56,306 cases and 1,417,791 controls) and age at onset of Parkinson's disease (28,568 cases) were obtained from the International Parkinson's Disease Genomics Consortium. On primary analysis, causal associations were estimated on sets of strong (P-value < 5 × 10-8; F-statistic > 10) and independent (linkage disequilibrium r2<0.001) genetic instruments using the inverse-variance weighted method. In sensitivity analysis, we estimated causal effects using robust Mendelian randomization methods and after removing pleiotropic genetic variants. Reverse causation was also explored. We repeated the analysis on different data sources for inflammatory biomarkers to check findings' consistency. In all the three data sources selected for interleukin-6, we found statistical evidence for earlier age at onset of Parkinson's disease associated with increased interleukin-6 concentration (years difference per 1 log-unit increase = -2.364, 95% CI = -4.789 to 0.060; years difference per 1 log-unit increase = -2.011, 95% CI = -3.706 to -0.317; years difference per 1 log-unit increase = -1.569, 95% CI = -2.891 to -0.247; ). We did not observe any statistical evidence for causal effects of C-reactive protein, interleukin 1 receptor antagonist, and tumor necrosis factor α on both Parkinson's disease and its age at onset. Results after excluding possible pleiotropic genetic variants were consistent with findings from primary analyses. When investigating reverse causation, we did not find evidence for a causal effect of Parkinson's disease or age at onset on any biomarkers of inflammation. We found evidence for a causal association between the onset of Parkinson's disease and interleukin-6. The findings of this study suggest that the pro-inflammatory activity of the interleukin-6 cytokine could be a determinant of prodromal Parkinson's disease.