An MR framework that aims to quantify whether a risk factor-outcome association (that is either established or has supportive evidence from MR analyses) is mediated by another risk factor. In many published examples, initial MR will assess evidence for a causal effect of the risk factor on an outcome and then go on to undertake two-step MR. First, genetic IVs associated with the risk factor are used to determine the causal effect of the risk factor on the potential mediator (step one). Secondly, genetic IVs associated with the potential mediator and independent of those used for step one are used to determine the effect of the potential mediator on the outcome of interest (step two). Where evidence is found for a causal effect of a risk factor on an outcome, a risk factor on a mediator and a mediator on an outcome, methods such as multivariable MR or product-of-coefficients can be used to explore the extent of this mediation.

The IV assumptions must be met for the two steps of the MR analyses of the (i) risk factor on mediator and (ii) mediator on outcome. This methodology can be extended to networks of relationships (i.e., network MR).

## References

- Burgess S, Daniel RM, Butterworth AS, Thompson SG, Consortium EP-I. Network Mendelian randomization: using genetic variants as instrumental variables to investigate mediation in causal pathways. Int J Epidemiol 2015;44:484-95.
- Richmond RC, Hemani G, Tilling K, Davey Smith G, Relton CL. Challenges and novel approaches for investigating molecular mediation. Hum Mol Genet 2016;25:R149-R156.
- Relton CL, Davey Smith G. Two-step epigenetic Mendelian randomization: a strategy for establishing the causal role of epigenetic processes in pathways to disease. Int J Epidemiol 2012;41:161-76.
- Xu L, Borges MC, Hemani G, Lawlor DA. The role of glycaemic and lipid risk factors in mediating the effect of BMI on coronary heart disease: a two-step, two-sample Mendelian randomisation study. Diabetologia 2017;60:2210-2220.

## Other terms in 'Definition of MR and study designs':

- Bidirectional MR
- Binary exposure MR
- Factorial MR
- Instrumental variable (IV)
- Mendelian randomization (MR)
- Multivariable MR
- One-sample MR
- Two-sample MR
- Two-sample MR with individual participant data (IPD)
- Two-sample MR with summary (aggregate) data