Mendelian randomisation The RCT concept Everything except the intervention is (hoped to be) the same in the two groups Defined study sample Intervention group Control group Measure outcome Measure outcome Randomisation to two groups Randomisation •… is allocation of the units of analysis to the different experimental groups or conditions according to chance, such that each unit has an equal probability of selection into each group • •Most powerful way of ensuring characteristics not systematically allocated to a particular group • •Can randomise in groups (clusters) • The aim of randomisation is to… •create groups that are comparable with respect to known or unknown confounding factors • •There are two steps in the process 1.Generating an unpredictable allocation sequence e.g. tossing a coin, using a computer random number generator 2.Concealing the allocation sequence from the investigators Mendelian randomisation studies Mendelian randomisation studies •Observational design with (almost) RCT strengths •Based on Mendel’s second law: alleles of different genes assort independently of one another during gamete formation •Inheritance of one trait should be independent of inheritance of other traits •Genetic variant used as proxy for exposure is unrelated to conventional vascular risk factors and other disease marker Glynn RJ, Clinical Chemistry 56:3 388–390 (2010) MR studies of biological or behavioural risk factors •Identify genetic marker (often a SNP) •Associated with the risk factor •Not associated with the disease via other pathways (i.e. not associated with other risk factors) •Estimate association between: •Genetic markers and RF •Observational RF and disease •Genetic marker and diseases •Compare observational and MR associations •Observational associations can be biased, confounded, MR should be unbiased Single-nucleotide polymorphism - ISOGG Wiki Estimates of association of each single nucleotide polymorphism with ln concentrations of C reactive protein C Reactive Protein Coronary Heart Disease Genetics Collaboration (CCGC) BMJ 2011;342:bmj.d548 Fig 2 Estimates of association of each single nucleotide polymorphism with ln concentrations of C reactive protein and risk of coronary heart disease (CHD). *Frequency of allele for increased concentrations of circulating ln C reactive protein (that is, risk allele). Associations presented per additional copy of risk allele. †For associations between single nucleotide polymorphism and coronary heart disease, studies with <10 cases or <50 participants were excluded from analyses. Study specific estimates stratified, where appropriate, by sex, ethnicity, and trial arm and combined with random effects models. Maximum available data on genetic variants, circulating C reactive protein, and coronary heart disease used for analyses; sensitivity analyses restricted to participants with data on C reactive protein single nucleotide polymorphisms, circulating C reactive protein, and coronary heart disease did not differ from current analyses. Fig C in appendix 3 on bmj.com shows study specific associations between single nucleotide polymorphism and C reactive protein and coronary heart disease for each single nucleotide polymorphism Association of (1) circulating concentrations and (2) genetically raised concentrations of C reactive protein (CRP) with risk of coronary heart disease (CHD) C Reactive Protein Coronary Heart Disease Genetics Collaboration (CCGC) BMJ 2011;342:bmj.d548 *Corrected for regression dilution in C reactive protein and potential confounding factors Fig 4 Estimates of association of circulating concentrations and genetically raised concentrations of C reactive protein (CRP) with risk of coronary heart disease (CHD). *Corrected for regression dilution in C reactive protein and potential confounding factors. Association of (1) circulating concentrations and (2) genetically raised concentrations of C reactive protein (CRP) with risk of coronary heart disease (CHD) C Reactive Protein Coronary Heart Disease Genetics Collaboration (CCGC) BMJ 2011;342:bmj.d548 *Corrected for regression dilution in C reactive protein and potential confounding factors Fig 4 Estimates of association of circulating concentrations and genetically raised concentrations of C reactive protein (CRP) with risk of coronary heart disease (CHD). *Corrected for regression dilution in C reactive protein and potential confounding factors. Assumptions / limitations •A gene influences disease solely through B. This is unverifiable, as a single gene can influence disease risk through multiple pathways other than B (pleiotropy); •Other alleles, G’, may be correlated with G (linkage disequilibrium) and influence D through other pathways, thereby inducing confounding; •Other characteristics of individuals at birth, C’, that independently predict the development of D can be correlated with G (population stratification) or influence the expression of G (epigenetics), •Both other alleles and patient characteristics can modify the effect of G on B, the effect of G on D, or both. Types of comparisons in different types of studies Study design Type of comparison Ecological studies Comparing disease frequency between populations Cross-sectional studies Comparing disease frequency between persons with and without characteristic of interest Cohort studies Comparing disease incidence between exposed and unexposed persons Case-control studies Comparing frequency of (past) exposure between cases and healthy controls Interventional studies Comparing incidence of events in persons exposed to the intervention of interest and in control group Mendelian randomisation Comparing frequency of events in persons with and without genotype associated with exposure hierarchy of major study designs systematic review of RCTs RCT cohort case control interventional observational validity ecological Mendelian randomisation studies