Mendelian Randomization - the Key to Understanding Aspects of Parkinson's Disease Causation?

Parkinson's disease has multiple determinants and is associated with a wide range of exposures that appear to modify risk in traditional observational studies, including numerous lifestyle and environmental factors. Across other fields of medicine, Mendelian randomization has emerged as a powerful method to examine whether associations between exposures and disease outcomes are causal. Here we discuss the concept of Mendelian randomization, its potential relevance to Parkinson's disease, and suggest avenues through which the method could be employed to further understanding of the causal basis of Parkinson's disease.

Noyce AJ ，Nalls MA 《-》

The Parkinson's Disease Mendelian Randomization Research Portal.

Mendelian randomization is a method for exploring observational associations to find evidence of causality. To apply Mendelian randomization between risk factors/phenotypic traits (exposures) and PD in a large, unbiased manner, and to create a public resource for research. We used two-sample Mendelian randomization in which the summary statistics relating to single-nucleotide polymorphisms from 5,839 genome-wide association studies of exposures were used to assess causal relationships with PD. We selected the highest-quality exposure genome-wide association studies for this report (n = 401). For the disease outcome, summary statistics from the largest published PD genome-wide association studies were used. For each exposure, the causal effect on PD was assessed using the inverse variance weighted method, followed by a range of sensitivity analyses. We used a false discovery rate of 5% from the inverse variance weighted analysis to prioritize exposures of interest. We observed evidence for causal associations between 12 exposures and risk of PD. Of these, nine were effects related to increasing adiposity and decreasing risk of PD. The remaining top three exposures that affected PD risk were tea drinking, time spent watching television, and forced vital capacity, but these may have been biased and were less convincing. Other exposures at nominal statistical significance included inverse effects of smoking and alcohol. We present a new platform which offers Mendelian randomization analyses for a total of 5,839 genome-wide association studies versus the largest PD genome-wide association studies available (https://pdgenetics.shinyapps.io/MRportal/). Alongside, we report further evidence to support a causal role for adiposity on lowering the risk of PD. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Noyce AJ ，Bandres-Ciga S ，Kim J ，Heilbron K ，Kia D ，Hemani G ，Xue A ，Lawlor DA ，Smith GD ，Duran R ，Gan-Or Z ，Blauwendraat C ，Gibbs JR ，23andMe Research Team5, International Parkinson's Disease Genomics Consortium (IPDGC) ，Hinds DA ，Yang J ，Visscher P ，Cuzick J ，Morris H ，Hardy J ，Wood NW ，Nalls MA ，Singleton AB ... -  《-》

A Mendelian randomization study investigating the causal role of inflammation on Parkinson's disease.

There is increasing evidence for inflammation as a determinant in the pathogenesis of Parkinson's disease, but its role in parkinsonian neurodegeneration remains elusive. It is not clear whether inflammatory cascades are causes or consequences of dopamine neuron death. In the present study, we aim to perform 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 genome-wide association studies to date (sample size ranging from 13 955 to 204 402 individuals) conducted on a European population for the following inflammation biomarkers: C-reactive protein, interleukin-6, interleukin 1 receptor antagonist and tumour 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 the consistency of the findings. In all the three data sources selected for interleukin-6, we found statistical evidence for an earlier age at onset of Parkinson's disease associated with increased interleukin-6 concentration [years difference per 1 log-unit increase = -2.364, 95% confidence interval (CI) = -4.789-0.060; 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 tumour 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.

Bottigliengo D ，Foco L ，Seibler P ，Klein C ，König IR ，Del Greco M F ... -  《-》

Association between telomere length and Parkinson's disease: a Mendelian randomization study.

In this study, we examined the potential association of telomere length with Parkinson's disease (PD) using the publicly available genome-wide association study summary statistics from the International Parkinson's Disease Genomics Consortium involving up to 37,688 patients with PD and 449,056 controls in Mendelian randomization framework. The Mendelian randomization approach has the potential to investigate a causal relationship between a risk factor and a disease, avoiding confounding and reverse causation that often present in conventional epidemiological studies. We did not find that longer telomeres were associated with higher risks of PD (odds ratio: 1.18, 95% confidence interval: 0.94, 1.48, p = 0.15). Our study, therefore, did not provide evidence to support a potential causal relationship between telomere length and PD.

Chen R ，Zhan Y 《-》

Brain Proteome-Wide and Transcriptome-Wide Asso-ciation Studies, Bayesian Colocalization, and Mendelian Randomization Analyses Reveal Causal Genes of Parkinson's Disease.

How genome-wide associated loci confer risk for Parkinson's disease is unclear. We aim to reveal causal genes through effects on brain proteins to provide new pathogenesis insights for Parkinson's disease. Proteome-wide and transcriptome-wide associations were determined by functional summary-based imputation leveraging data from genome-wide association summary (56 306 Europeans, 1.4 million controls), brain proteomes (528 cases from 2 separate data sets), and transcriptome (452 cases), followed by Mendelian randomization, Bayesian colocalization, cell-type-specific and brain regional expression, and drug-gene interaction analyses. As a result, genetically regulated protein abundances of 11 genes were associated with Parkinson's disease. Five genes (CD38, GPNMB, TMEM175, RAB7L1, and HIP1R) were colocalized. Four genes (GPNMB, SEC23IP, CD38, and DGKQ) demonstrated Mendelian randomized correlations (p < 8.10 × 10-5). Higher GPNMB level (1.47, 1.28-1.68) and lower CD38 level (0.319, 0.24-0.43) were causally associated with higher risk of Parkinson's disease, consistent with transcriptomic evaluations. CD38 and GPNMB were preferentially enriched in astrocytes and oligodendrocyte precursor cells, respectively. And CD38 and GPNMB were suggested to be the targets of many oncological drugs from Drug-Gene Interaction database. In conclusion, utilizing multidimensional data, GPNMB and CD38 were prioritized as the causal genes of Parkinson's disease, crucial for mechanistic and therapeutic investigations.

Zhou S ，Tian Y ，Song X ，Xiong J ，Cheng G ... -  《-》