Abstract
Background
Obesity and its associated diseases are major health problems characterized by extensive metabolic disturbances. Understanding the causal connections between these phenotypes and variation in metabolite levels can uncover relevant biology and inform novel intervention strategies. Recent studies have combined metabolite profiling with genetic instrumental variable (IV) analysis (Mendelian randomization) to infer the direction of causality between metabolites and obesity, but often omitted a large portion of untargeted profiling data consisting of unknown, unidentified metabolite signals.
Methods
We expanded upon previous research by identifying body mass index (BMI)-associated metabolites in multiple untargeted metabolomics datasets, and then performing bidirectional IV analysis to classify metabolites based on their inferred causal relationships with BMI. Meta-analysis and pathway analysis of both known and unknown metabolites across datasets were enabled by our recently developed bioinformatics suite, PAIRUP-MS.
Results
We identified ten known metabolites that are more likely to be causes (e.g., alpha-hydroxybutyrate) or effects (e.g., valine) of BMI, or may have more complex bidirectional cause-effect relationships with BMI (e.g., glycine). Importantly, we also identified about five times more unknown than known metabolites in each of these three categories. Pathway analysis incorporating both known and unknown metabolites prioritized 40 enriched (p < 0.05) metabolite sets for the cause versus effect groups, providing further support that these two metabolite groups are linked to obesity via distinct biological mechanisms.
Conclusions
These findings demonstrate the potential utility of our approach to uncover causal connections with obesity from untargeted metabolomics datasets. Combining genetically informed causal inference with the ability to map unknown metabolites across datasets provides a path to jointly analyze many untargeted datasets with obesity or other phenotypes. This approach, applied to larger datasets with genotype and untargeted metabolite data, should generate sufficient power for robust discovery and replication of causal biological connections between metabolites and various human diseases.
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Acknowledgements
We thank the Broad Metabolomics Platform and SIGMA T2D Consortium for sharing data resources. This research has been conducted using the UK Biobank Resource under Application Number 11898. This work was supported by the National Heart, Lung, and Blood Institute grant F31HL126581 (YHH), National Institute of Diabetes and Digestive and Kidney Diseases grants T32DK110919 (YHH), K12DK094721 (CMA), and R01DK075787 (JNH), Endocrine Scholars Award (CMA), Doris Duke Charitable Foundation grant 215205 (JNH), Estonian Research Council grants IUT20-60 (AM), PUT1665 (KF), and PUT1660 (TE), European Union through Horizon 2020 grant 692145 (AM), and European Union through the European Regional Development Fund Project No. 2014-2020.4.01.15-0012 (AM). The funding sources had no role in the design, analysis, and interpretation of this research.
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K Fortney and EKM are affiliated with BioAge Labs, Inc.; JNH serves on the Scientific Advisory Board of Camp4 Therapeutics.
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Hsu, YH.H., Astley, C.M., Cole, J.B. et al. Integrating untargeted metabolomics, genetically informed causal inference, and pathway enrichment to define the obesity metabolome. Int J Obes 44, 1596–1606 (2020). https://doi.org/10.1038/s41366-020-0603-x
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DOI: https://doi.org/10.1038/s41366-020-0603-x
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