Beyond Mendelian randomization: how to interpret evidence of shared genetic predictors.
OBJECTIVE
Mendelian randomization is a popular technique for assessing and estimating the causal effects of risk factors. If genetic variants which are instrumental variables for a risk factor are shown to be additionally associated with a disease outcome, then the risk factor is a cause of the disease. However, in many cases, the instrumental variable assumptions are not plausible, or are in doubt. In this paper, we provide a theoretical classification of scenarios in which a causal conclusion is justified or not justified, and discuss the interpretation of causal effect estimates.


RESULTS
A list of guidelines based on the 'Bradford Hill criteria' for judging the plausibility of a causal finding from an applied Mendelian randomization study is provided. We also give a framework for performing and interpreting investigations performed in the style of Mendelian randomization, but where the choice of genetic variants is statistically, rather than biologically motivated. Such analyses should not be assigned the same evidential weight as a Mendelian randomization investigation.


CONCLUSIONS
We discuss the role of such investigations (in the style of Mendelian randomization), and what they add to our understanding of potential causal mechanisms. If the genetic variants are selected solely according to statistical criteria, and the biological roles of genetic variants are not investigated, this may be little more than what can be learned from a well-designed classical observational study.
Cardiovascular Epidemiology Unit, Department of Public Health and Primary Care, 2 Worts Causeway, Cambridge CB1 8RN, UK; Homerton College, University of Cambridge, Hills Road, Cambridge CB2 8PH, UK. Electronic address: sb452@medschl.cam.ac.uk.
Stephen
S
Burgess
Cardiovascular Epidemiology Unit, Department of Public Health and Primary Care, 2 Worts Causeway, Cambridge CB1 8RN, UK.
Adam S
AS
Butterworth
Department of Health Sciences, Adrian Building, University Road, Leicester, LE1 7RH, UK.
John R
JR
Thompson
D003627:Data Interpretation, Statistical; D014644:Genetic Variation; D057182:Mendelian Randomization Analysis
Aetiology; Causal inference; Genetic predictors; Genetic variants; Instrumental variable; Mendelian randomization; Translational Genetics