Please use this identifier to cite or link to this item: http://hdl.handle.net/2381/32799
Title: Using multiple genetic variants as instrumental variables for modifiable risk factors.
Authors: Palmer, T. M.
Lawlor, D. A.
Harbord, R. M.
Sheehan, Nuala A.
Tobias, J. H.
Timpson, N. J.
Davey Smith, G.
Sterne, J. A.
First Published: 7-Jan-2011
Publisher: SAGE Publications (UK and US)
Citation: Statistical Methods in Medical Research, 2012, 21 (3), pp. 223-242
Abstract: Mendelian randomisation analyses use genetic variants as instrumental variables (IVs) to estimate causal effects of modifiable risk factors on disease outcomes. Genetic variants typically explain a small proportion of the variability in risk factors; hence Mendelian randomisation analyses can require large sample sizes. However, an increasing number of genetic variants have been found to be robustly associated with disease-related outcomes in genome-wide association studies. Use of multiple instruments can improve the precision of IV estimates, and also permit examination of underlying IV assumptions. We discuss the use of multiple genetic variants in Mendelian randomisation analyses with continuous outcome variables where all relationships are assumed to be linear. We describe possible violations of IV assumptions, and how multiple instrument analyses can be used to identify them. We present an example using four adiposity-associated genetic variants as IVs for the causal effect of fat mass on bone density, using data on 5509 children enrolled in the ALSPAC birth cohort study. We also use simulation studies to examine the effect of different sets of IVs on precision and bias. When each instrument independently explains variability in the risk factor, use of multiple instruments increases the precision of IV estimates. However, inclusion of weak instruments could increase finite sample bias. Missing data on multiple genetic variants can diminish the available sample size, compared with single instrument analyses. In simulations with additive genotype-risk factor effects, IV estimates using a weighted allele score had similar properties to estimates using multiple instruments. Under the correct conditions, multiple instrument analyses are a promising approach for Mendelian randomisation studies. Further research is required into multiple imputation methods to address missing data issues in IV estimation.
DOI Link: 10.1177/0962280210394459
ISSN: 0962-2802
eISSN: 1477-0334
Links: http://smm.sagepub.com/content/21/3/223
http://hdl.handle.net/2381/32799
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Creative Commons Attribution (CC BY) http://creativecommons.org/licenses/by/3.0/
Appears in Collections:Published Articles, Dept. of Health Sciences

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