Evidence for genetic causality between iron homeostasis and Parkinson's disease: A two-sample Mendelian randomization study

Background: Parkinson's disease (PD) is a degenerative disease of the central nervous system, and its specific etiology is still unclear. At present, it is believed that the main pathological basis is the reduction of dopamine concentration in the brain striatum. Although many previous studies have believed that iron as an important nutrient element participates in the occurrence and development of PD, whether there is a causal correlation between total iron binding capacity(TIBC), transferring saturation(TSAT), ferritin and serum iron in iron homeostasis indicators and PD, there has been a lack of effective genetic evidence. Methods: We used Mendelian randomization (MR) as an analytical method to effectively evaluate the genetic association between exposure and outcome, based on the largest genome-wide association study (GWAS) data to date. By using randomly assigned genetic instrumental variables (SNPs, Single Nucleotide Polymorphisms) that are not affected by any causal relationship, we effectively evaluated the causal relationship between iron homeostasis indicators and PD while controlling for confounding factors. Results: By coordinated analysis of 86 SNPs associated with iron homeostasis markers and 12,858,066 SNPs associated with PD, a total of 56 SNPs were finally screened for genome-wide significance of iron homeostasis associated with PD. The results of inverse variance weighting(IVW) analysis suggested that iron( beta = - 0.524; 95%cl=-0.046 to -0.002; P=0.032) was considered to have a genetic causal relationship with PD. Cochran's Q, Egger intercept and MR-PRESSO global tests did not detect the existence of heterogeneity and pleiotropy (P>0.05). Mr Steiger directionality test further confirmed our estimation of the potential causal direction of iron and PD (P=0.001). In addition, TIBC (beta=-0.142; 95%Cl=-0.197-0.481; P=0.414), TSAT (beta=-0.316; 95%Cl=0.861-0.229; P=0.255), and ferritin (beta=-0.387; 95%Cl=-1.179-0.405; P=0.338) did not have genetic causal relationships with PD, and the results were not heterogeneous and pleiotropic (P>0.05). In addition, TIBC (beta=-0.142; 95%Cl=-0.197-0.481; P=0.414), TSAT (beta=-0.316; 95%Cl=-0.861-0.229; P=0.255), and ferritin (beta=-0.101; 95%Cl=-0.987 to -0.405; P=0.823) did not have genetic causal relationships with PD, and the results were not heterogeneous and pleiotropic (P>0.05). TIBC (P=0.008), TSAT (P=0.000) and ferritin (P=0.013) were all consistent with the estimation of MR Steiger directivity test. Conclusion: Our study found that among the four iron homeostasis markers, there was a genetic causal association between serum iron and PD, and the serum iron level was negatively correlated with the risk of PD. In addition, TIBC, TSAT, ferritin had no genetic causal relationship with PD.