Unbiased metabolome screen links serum urate to risk of Alzheimer's disease.

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease caused by a combination of genetic and environmental risk factors. The serum metabolome refers to a set of small-molecules which are an important determinant of cellular health. We obtained genome-wide association study (GWAS) summary statistics for serum concentrations of 376 metabolites which were population matched with 2 GWAS studies of AD. For each metabolite we performed 2-sample MR (2SMR) using an inverse variance weighted (IVW) estimate for significance testing. After Bonferroni multiple testing correction one metabolite was causally linked to AD in both GWAS: serum urate. This result was supported by robust 2SMR measures and sensitivity analyses. We applied 2SMR to test for a causal relationship between serum urate and other neurodegenerative diseases: Parkinson disease (PD) and Amyotrophic lateral sclerosis (ALS). In ALS but not PD we identified a nominally significant link between serum urate and disease-risk, although in this case increased serum urate was protective. We conclude that serum urate is a modulator of risk for neurodegeneration. Our work has implications for the design of preventative interventions.

A review of Mendelian randomization in amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis is a relatively common and rapidly progressive neurodegenerative disease that, in the majority of cases, is thought to be determined by a complex gene-environment interaction. Exponential growth in the number of performed genome-wide association studies combined with the advent of Mendelian randomization is opening significant new opportunities to identify environmental exposures that increase or decrease the risk of amyotrophic lateral sclerosis. Each of these discoveries has the potential to shape new therapeutic interventions. However, to do so, rigorous methodological standards must be applied in the performance of Mendelian randomization. We have reviewed Mendelian randomization studies performed in amyotrophic lateral sclerosis to date. We identified 20 Mendelian randomization studies, including evaluation of physical exercise, adiposity, cognitive performance, immune function, blood lipids, sleep behaviours, educational attainment, alcohol consumption, smoking and type 2 diabetes mellitus. We have evaluated each study using gold standard methodology supported by the Mendelian randomization literature and the STROBE-Mendelian randomization checklist. Where discrepancies exist between Mendelian randomization studies, we suggest the underlying reasons. A number of studies conclude that there is a causal link between blood lipids and risk of amyotrophic lateral sclerosis; replication across different datasets and even different populations adds confidence. For other putative risk factors, such as smoking and immune function, Mendelian randomization studies have provided cause for doubt. We highlight the use of positive control analyses in choosing exposure single nucleotide polymorphisms (SNPs) to make up the Mendelian randomization instrument, use of SNP clumping to avoid false positive results due to SNPs in linkage and the importance of multiple testing correction. We discuss the implications of survival bias for study of late age of onset diseases such as amyotrophic lateral sclerosis and make recommendations to mitigate this potentially important confounder. For Mendelian randomization to be useful to the amyotrophic lateral sclerosis field, high methodological standards must be applied to ensure reproducibility. Mendelian randomization is already an impactful tool, but poor-quality studies will lead to incorrect interpretations by a field that includes non-statisticians, wasted resources and missed opportunities.