Association between C reactive protein and coronary heart disease: mendelian randomisation analysis based on individual participant data

Abstract

Objective: To use genetic variants as unconfounded proxies of C reactive protein concentration to study its causal role in coronary heart disease.

Design: Mendelian randomisation meta-analysis of individual participant data from 47 epidemiological studies in 15 countries.

Participants: 194 418 participants, including 46 557 patients with prevalent or incident coronary heart disease. Information was available on four CRP gene tagging single nucleotide polymorphisms (rs3093077, rs1205, rs1130864, rs1800947), concentration of C reactive protein, and levels of other risk factors.

Main outcome measures: Risk ratios for coronary heart disease associated with genetically raised C reactive protein versus risk ratios with equivalent differences in C reactive protein concentration itself, adjusted for conventional risk factors and variability in risk factor levels within individuals.

Results: CRP variants were each associated with up to 30% per allele difference in concentration of C reactive protein (P<10(-34)) and were unrelated to other risk factors. Risk ratios for coronary heart disease per additional copy of an allele associated with raised C reactive protein were 0.93 (95% confidence interval 0.87 to 1.00) for rs3093077; 1.00 (0.98 to 1.02) for rs1205; 0.98 (0.96 to 1.00) for rs1130864; and 0.99 (0.94 to 1.03) for rs1800947. In a combined analysis, the risk ratio for coronary heart disease was 1.00 (0.90 to 1.13) per 1 SD higher genetically raised natural log (ln) concentration of C reactive protein. The genetic findings were discordant with the risk ratio observed for coronary heart disease of 1.33 (1.23 to 1.43) per 1 SD higher circulating ln concentration of C reactive protein in prospective studies (P=0.001 for difference).

Conclusion: Human genetic data indicate that C reactive protein concentration itself is unlikely to be even a modest causal factor in coronary heart disease.

Fig 1 Associations of four principal single nucleotide polymorphisms (SNP) related to C reactive protein with various characteristics in individuals free from known coronary heart disease at time of measurement. Estimates presented are based on random effects meta-analysis of study specific associations of each SNP with panel of risk factors, adjusted, where appropriate, for ethnicity. Per allele model corresponds to association per addition of risk allele for each SNP

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

Fig 3 Estimates of association of each haplotype with ln concentrations of CRP and risk of coronary heart disease. *Based on seven haplotypes and therefore do not add up to 1. Haplotypes 6 and 7 excluded because they represent individuals for whom it was not possible to assign to primary haplotypes because of missing data in rs3093077 and rs1800947. See table A in appendix 1, fig F in appendix 3, and appendix 5 for details of these haplotypes and explanation of methods. Additive haplotype model was used to estimate effect of each haplotype relative to two copies of haplotype 1. See appendix 5 for details of this model. Data limited to populations of European descent, for which it was possible to assign haplotypes based on linkage disequilibrium information between single nucleotide polymorphisms (see appendix 5). Studies with <10 cases or <50 participants excluded. Study specific estimates were stratified, where appropriate, by sex, ethnicity, and trial arm and combined with multivariate random effects meta-analysis. Data available on up to 25 960 cases and up to 139 251 participants of European descent from 33 studies. Fig G in appendix 3 shows study specific associations between haplotype and C reactive protein and coronary heart disease for each haplotype

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.