Ten simple rules for conducting a mendelian randomization study

Mendelian randomization (MR) is an epidemiological technique for estimating causal relationships using observational data, which has become very popular in recent years following publication of a seminal article by Smith and Ebrahim in 2003 1. MR is a specific form of “instrumental variables” (IV) analysis (the latter being first invented by Phillip and Sewall Wright in the 1920s 2) that uses genetic variants to proxy a modifiable variable (which we term the “exposure” variable here) in order to estimate the causal relationship between the exposure and an outcome of interest. To understand how this causal inference technique works, it is useful to think of MR as similar to a “natural” randomized controlled trial 3 where individuals are randomly assigned to groups based on the alleles that they inherit from their parents (Fig 1). MR takes advantage of Mendel’s laws of segregation and independent assortment, which state that offspring inherit alleles randomly from their parents and randomly with respect to other genes in the genome (with certain exceptions 1). Therefore, genetic variants that are related to an exposure of interest can be used to proxy the part of the exposure variable that is independent of possible confounding influences from the environment and other traits. Providing several assumptions are satisfied (see below), and the principle of gene–environment equivalence (i.e., perturbing the exposure genetically has the same effect as perturbing the exposure by other means), statistical association between the genetic variant and the outcome is indicative of a causal relationship between the exposure and the outcome and can be used to estimate the magnitude of the causal relationship using IV methods. Although originally developed as a way to estimate causal relationships between modifiable environmental exposures and medically relevant outcomes, in recent years, MR has been utilized in many other situations including studies of molecular biomarkers, in pharmacogenetics, in the social sciences, and in other discplines that use observational frameworks 4,5. Open in a separate window Fig 1 Similarities between the MR study design and a randomized controlled trial. MR, mendelian randomization.