Interaction between fibrinogen and IL-6 genetic variants and associations with cardiovascular disease risk in the Cardiovascular Health Study

Abstract

The inflammatory cytokine interleukin-6 (IL-6) is a main regulator of fibrinogen synthesis, though its interaction with fibrinogen genes (FGA, FGB, FGG) and subsequent impact on cardiovascular disease (CVD) risk is not well-studied. We investigated joint associations of fibrinogen and IL6 tagSNPs with fibrinogen concentrations, carotid intima-media thickness, and myocardial infarction or ischemic stroke in 3900 European-American Cardiovascular Health Study participants. To identify combinations of genetic main effects and interactions associated with outcomes, we used logic regression. We also evaluated whether the relationship between fibrinogen SNPs and fibrinogen level varied by IL-6 level using linear regression models with multiplicative interaction terms. Combinations of fibrinogen and IL6 SNPs were significantly associated with fibrinogen level (p < 0.005), but not with other outcomes. Fibrinogen levels were higher in individuals having FGB1437 (rs1800790) and lacking FGA6534 (rs6050) minor alleles; these SNPs interacted with IL6 rs1800796 to influence fibrinogen level. Marginally significant (p= 0.03) interactions between IL-6 level and FGA and FGG promoter SNPs associated with fibrinogen levels were detected. We identified potential gene-gene interactions influencing fibrinogen levels. Although IL-6 responsive binding sites are present in fibrinogen gene promoter regions, we did not find strong evidence of interaction between fibrinogen SNPs and IL6 SNPs or levels influencing CVD.

Figure 1. Logic regression cross-validation testing results for models of fibrinogen and IL6 SNPs associated with fibrinogen level

Model size (x-axis scale) indicates the number of leaves; shaded boxes indicate the number of trees in the models. The y-axis scale, ‘test score’ reflects the residual variance scores of each model. Lower test scores are indicative of better fitting models. In this figure, both 1 tree and 2 tree models with 2 leaves have the best scores (< 62.75) in 10-fold cross-validation testing.

Figure 2. Interaction between fibrinogen and IL6 SNPs in relation to fibrinogen level: best models identified by logic regression

A. 1 tree with 2 leaves: score is 62.12 E(fibrinogen) = 243.35 + 18.50 ((no FGA6534 minor alleles) and ≥1 minor allele at FGB1437) + 1.22(site A) +17.67(site B) + 0.82(site C) + 0.91(age) − 4.25(male) As shown below, this equation includes one gene-gene interaction term and no main effects. It suggests that holding site, age and sex constant, individuals with no FGA6534 minor alleles and at least 1 minor allele of FGB1437 have 18.5 mg/dL higher fibrinogen level on average than individuals who do not have that combination of genotypes. B. 2 trees with 2 leaves: score is 62.03 E(fibrinogen) = 252.72 − 13.29(no FGB1437 minor alleles) + 10.21(no FGA6534 minor alleles) + 1.16(site A) +17.31(site B) + 0.47(site C) + 0.88(age) − 4.07(male) As shown below, this equation includes a main effect for the same FGA and FGB SNPs, but no interaction term. It suggests that after adjustment for site, age, sex and FGA6534 genotype, individuals with no FGB1437 minor alleles have 13.3 mg/dL lower levels of fibrinogen than individuals with at least one FGB1437 minor allele. After adjustment for site, age, sex and FGB1437 genotype, individuals with no FGA6534 minor alleles have 10.2 mg/dL higher fibrinogen level on average than individuals who have at least one minor allele.

Figure 2. Interaction between fibrinogen and IL6 SNPs in relation to fibrinogen level: best models identified by logic regression

A. 1 tree with 2 leaves: score is 62.12 E(fibrinogen) = 243.35 + 18.50 ((no FGA6534 minor alleles) and ≥1 minor allele at FGB1437) + 1.22(site A) +17.67(site B) + 0.82(site C) + 0.91(age) − 4.25(male) As shown below, this equation includes one gene-gene interaction term and no main effects. It suggests that holding site, age and sex constant, individuals with no FGA6534 minor alleles and at least 1 minor allele of FGB1437 have 18.5 mg/dL higher fibrinogen level on average than individuals who do not have that combination of genotypes. B. 2 trees with 2 leaves: score is 62.03 E(fibrinogen) = 252.72 − 13.29(no FGB1437 minor alleles) + 10.21(no FGA6534 minor alleles) + 1.16(site A) +17.31(site B) + 0.47(site C) + 0.88(age) − 4.07(male) As shown below, this equation includes a main effect for the same FGA and FGB SNPs, but no interaction term. It suggests that after adjustment for site, age, sex and FGA6534 genotype, individuals with no FGB1437 minor alleles have 13.3 mg/dL lower levels of fibrinogen than individuals with at least one FGB1437 minor allele. After adjustment for site, age, sex and FGB1437 genotype, individuals with no FGA6534 minor alleles have 10.2 mg/dL higher fibrinogen level on average than individuals who have at least one minor allele.

Figure 3. Association between fibrinogen level and ln(IL-6), by FGG902 (rs1800792) genotype

Figure 4. Association between fibrinogen level and ln(IL-6), by FGA251 (rs2070006) genotype