Mendelian Randomization Suggests No Causal Association Between C-reactive Protein and Carotid Intima-media Thickness in the Young Finns Study
To the Editor:
It is unclear whether C-reactive protein (CRP), a nonspecific marker of acute phase inflammatory response, is causally related to arterial intima-media thickness (IMT), a risk factor for coronary heart disease (CHD). Previous evidence from conventional observational studies is inconsistent and suggests that the association may be biased or confounded.1 According to the Mendelian randomization approach, the genetic variants in the CRP gene (CRP) may represent good instruments for CRP levels that are largely free from reverse causation bias and confounding.1 If the association between CRP and IMT is causal, then genetic variants in CRP should be related to IMT to the extent predicted by the magnitude of their association with average CRP levels.
We examined the causality between CRP and carotid IMT by determining haplotypes from genetic variants in CRP among 1609 individuals (768 men and 841 women) participating in the Cardiovascular Risk in Young Finns study.2 We genotyped 5 single nucleotide polymorphisms (SNPs) in the CRP gene: CRP-717A>G (rs 2794521); CRP-286C>T>A (rs3091244); CRP +1059G>C (rs1800947); CRP +1444T>C (rs1130864); and CRP +1846G>A (rs1205). The SNPs were in Hardy-Weinberg equilibrium and strongly linked D′ values ranging between 0.98 to 0.99. After exclusion of rare haplotypes (frequency <1%), 5 haplotypes remained for analysis. We assessed serum high-sensitive CRP in 1980 (at age 3 to 18) and 2001 (at age 24 to 39), and carotid IMT in 2001 to 2002. Potential confounding factors measured included adult biological risk factors (body mass index, systolic and diastolic blood pressure, total, HDL, and LDL cholesterol, triglycerides), smoking, alcohol consumption, and occupational status.
Details of the methods and results are provided in the online supplement (available at http://atvb.ahajournals.org). Of the 55 associations between haplotypes and potential confounders, there was no strong evidence of any robust association between haplotypes and potential confounding factors (P<0.05 only for 2 associations). This finding supports the assumption that the haplotypes represent a nonconfounded estimate of CRP levels. In contrast, circulating CRP was associated with all biological risk factors and smoking.
We used instrumental variable methods to obtain estimates of the causal (unbiased and nonconfounded) association between circulating CRP and IMT.3 Although the ordinary least squares regression analysis suggested a positive association of CRP levels measured in adulthood and mean life course CRP levels (higher CRP levels associated with greater IMT), the instrumental variables analysis suggested inverse associations of all three of childhood, adulthood, and life course CRP (higher CRP levels associated with lower IMT), though these were imprecisely estimated (Table).
Thus, our Mendelian randomization analysis with a null association between CRP haplotypes and IMT provides no support for the hypothesis that circulating CRP would causally influence IMT in a healthy population of young adults. The few previous Mendelian randomisation studies used CRP genotypes rather than haplotypes, they genotyped a smaller number of SNPs than was done in this study, their assessment of circulating CRP was limited to adulthood, and instead of IMT, they were focused on risk factors, such as blood pressure4 or components of the metabolic syndrome,5 or CHD events.6 However, similar to our results, those studies found no evidence to causally link CRP to coronary outcomes. In a cohort of adults aged 65 or older, CRP genotypes were associated with cardiovascular events, but not with carotid IMT.7
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Davey Smith G, Lawlor DA, Harbord R, Timpson N, Rumley A, Lowe GD, Day IN, Ebrahim S. Association of C-reactive protein with blood pressure and hypertension: life course confounding and mendelian randomization tests of causality. Arterioscler Thromb Vasc Biol. 2005; 25: 1051–1056.
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Lange LA, Carlson CS, Hindorff LA, Lange EM, Walston J, Durda JP, Cushman M, Bis JC, Zeng D, Lin D, Kuller LH, Nickerson DA, Psaty BM, Tracy RP. Reiner AP. Association of polymorphisms in the CRP gene with circulating C-reactive protein levels and cardiovascular events. J Am Med Assoc. 2006; 296: 2703–2711.