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Mendelian Randomization

  • Sandeep Grover
  • Fabiola Del Greco M.
  • Catherine M. Stein
  • Andreas Ziegler
Protocol
Part of the Methods in Molecular Biology book series (MIMB, volume 1666)

Abstract

Confounding and reverse causality have prevented us from drawing meaningful clinical interpretation even in well-powered observational studies. Confounding may be attributed to our inability to randomize the exposure variable in observational studies. Mendelian randomization (MR) is one approach to overcome confounding. It utilizes one or more genetic polymorphisms as a proxy for the exposure variable of interest. Polymorphisms are randomly distributed in a population, they are static throughout an individual’s lifetime, and may thus help in inferring directionality in exposure–outcome associations. Genome-wide association studies (GWAS) or meta-analyses of GWAS are characterized by large sample sizes and the availability of many single nucleotide polymorphisms (SNPs), making GWAS-based MR an attractive approach. GWAS-based MR comes with specific challenges, including multiple causality. Despite shortcomings, it still remains one of the most powerful techniques for inferring causality.

With MR still an evolving concept with complex statistical challenges, the literature is relatively scarce in terms of providing working examples incorporating real datasets. In this chapter, we provide a step-by-step guide for causal inference based on the principles of MR with a real dataset using both individual and summary data from unrelated individuals. We suggest best possible practices and give recommendations based on the current literature.

Key words

Mendelian randomization Instrumental variable Observational epidemiology Reverse causation Unobserved confounding Genome-wide association study Individual data Summary data Pleiotropy Causal inference 

Notes

Acknowledgment

S.G. greatly acknowledges the organizers of the recent workshop on Mendelian Randomization run through the University of Cambridge’s Department for Public Health and Primary Care. This work was supported by a grant from the German Research Foundation (Research Unit ProtectMove, FOR 2488).

Supplementary material

421123_2_En_29_MOESM1_ESM.csv (59 kb)
Dataset 1 Descriptive table of prioritized SNPs for estimating the causal estimates using individual and summary data (CSV 1 kb)
421123_2_En_29_MOESM2_ESM.csv (1 kb)
Dataset 2 Example dataset with quality-controlled individual data on phenotypic variables and prioritized genetic instruments (CSV 59 kb)
421123_2_En_29_MOESM3_ESM.csv (1 kb)
Dataset 3 Example dataset with summary data on genetic instrument–exposure and genetic instrument–outcome associations (CSV 1019 bytes)

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Copyright information

© Springer Science+Business Media LLC 2017

Authors and Affiliations

  • Sandeep Grover
    • 1
  • Fabiola Del Greco M.
    • 2
  • Catherine M. Stein
    • 3
  • Andreas Ziegler
    • 1
  1. 1.Institut für Medizinische Biometrie und StatistikUniversität zu Lübeck, Universitätsklinikum Schleswig-HolsteinLübeckGermany
  2. 2.Center for BiomedicineEURAC ResearchBolzanoItaly
  3. 3.Case Western Reserve UniversityClevelandUSA

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