rs1800790, and to a lesser extent by the major allele of IL6 rs2069832, seem to be the driving force between the associations seen, suggesting a restriction of the effect of air pollution to groups of MI survivors of specific genotypes. NO2 was the pollutant with the most con- vincing effects on IL-6 levels with respect to the a priori specified time window of exposure (0–24 hr) when considering the overall popu- lation, regardless of genotype. Modification of air pollution effects by SNPs was, however, clearest for CO, although the other pollutants in general followed a similar pattern. CO has been proposed as a probable culprit to cardio- vascular effects of air pollution (Dales 2004; McGrath 2000) in addition to PM. In this study we did not have sufficient power to more specifically identify the most important air pol- lution component or source, and we acknowl- edge that we studied a mixture of correlated pollutants. Ambient CO levels measured using FGB rs1800790 Major allelic Minor allelic homozygote Heterozygote homozygote (GG) (Ga) (aa) 22 Mhoamjoorzaylgleoltiec Intermediate High High 38 (GG) (n = 212) (n = 125) (n = 11) 906 Heterozygote Low Intermediate High r2s (Ga) (n = 275) (n = 153) (n = 18) IL6 Mhoinmoorzaylgleoltiec Low Low Intermediate (aa) (n = 118) (n = 36) (n = 7) Total number Low Intermediate High in groups n = 429 (45%) n = 372 (39%) n = 154 (16%) ) 15 R Q -6 Ir L I e p10 in n e a c e5 ren llm ffe ra0 i e D vo–5 % ( 0–5 0–5 FGB rs1800790 ) R 15 CO Q -6 Ir iIn L np10 e p = 0.007 p = 0.002 e a c e5 ren llm f ra0 e f i e D vo–5 % ( * * 6–11 12–17 Exposure (hr) In homozygotes for the major allele of IL6 rs2069832 (36% of the study sample), the cor- responding increase was 3.6% (95% CI, 1.0– 6.2%). Differences in the NO2 (Figure 3) and PM2.5 (data not shown) effects across genetic subgroups for different 6-hr exposure windows were similar but weaker. The distributed lag analyses restricted to subjects with at least one minor allele of FGB rs1800790 (GA or AA, 37% of study sample) or one major allele of IL6 2069832 (GA or AA, 83% of study sam- ple) for 6-hr and 24-hr time windows showed associations with increased IL-6 for increases in CO for the 0–5 hr and 6–11 hr preceding sampling [see Supplemental Material, Figure 1 (doi: 10.1289/ehp.0800370.S1)]. The IL-6 increase seen in minor allele holders of FGB rs1800790 for 0–23 hr exposure of CO seemed to be mainly driven by exposure in the 6–11 hr time window. We found a similar but weaker pattern for 0–23 hr exposure in major allele holders of IL6 rs2069832. City-specific modification of IL-6 response to CO and NO2. In a comparison of city-specific analyses for IL6 rs2069832 [see Supplemental Material, Figure 2 (doi: 10.1289/ehp.0800370. S1)], modification of the IL-6 response to pre- ceding 0–24 hr average of increased CO and NO2 showed consistency between the pol- lutants, with the clearest effect modification seen in Helsinki, Barcelona, and Athens. For FGB rs1800790 [see Supplemental Material, Figure 3 (doi: 10.1289/ehp.0800370.S1)], we found the strongest effect modification in Athens and Augsburg, and the effects were con- sistent for both pollutants except in Stockholm. Although we found variation across cities because of reduced power in the city-specific analyses, we found no significant heterogeneity of city-specific effects (p-value > 0.1). Discussion We found evidence of gene–environment interaction where the SNPs IL6 rs2069832 and FGB rs1800790 modified the effects of air pollution on IL-6 levels in MI survivors. All pollutants showed similar patterns of effect, but results were most apparent for CO. The influence of CO on IL-6 levels was strongest in subjects with the major allele of IL6 rs2069832 and the minor allele of FGB rs1800790, and the combination of these two genetic variants, present in 16% of subjects, resulted in a sig- nificant gene–gene–environment interaction, potentially indicating a higher risk for health effects from air pollution in these patients with ischemic heart disease. We found similar magnitudes of effect modification with respect to air pollution for IL6 rs2069840 and IL6 rs2069845, but we found no effect modifica- tion for FGA rs2070011. We found small effects of air pollution on IL-6 levels in the overall population. The groups defined by the minor allele of FGB Environmental Health Perspectives • volume 117 | number 9 | September 2009 1377 Figure 2. (A) Construction of gene–gene interaction variable based on relative change in plasma IL-6 in univari- ate gene–environment interaction analysis of polymorphisms IL6 rs2069832 and FGB rs1800790. (B) Modification by gene–gene interaction of IL6 rs2069832 and FGB rs1800790 of IL-6 response to increased preceding ambient 24-hr air pollutant averages. Error bars indicate 95% CIs, and p-values are for significance of the interaction term. Interquartile ranges: CO, 0.34 mg/m3; NO2, 15.9 µg/m3; PNC, 11,852/cm3; PM2.5, 11.0 µg/m3; PM10, 17.4 µg/m3. *Heterogeneity of the city-specific effect estimates with a p-value < 0.1. IL6 rs20698322 CO Major allelic homozygote Heterozygote Minor allelic homozygote IL6 rs20698322 NO2 p = 0.23 p = 0.10 p = 0.40 p = 0.50 p = 0.34 p = 0.41 p = 0.29 p = 0.17 6–11 12–17 Exposure (hr) 18–23 0–5 6–11 12–17 Exposure (hr) 18–23 Figure 3. Modification by IL6 rs2069832 and FGB rs1800790 genotypes of IL-6 response to increased ambi- ent CO and NO2 in association with 6-hr exposure windows preceding blood sampling. Error bars indicate 95% CIs, and p-values are for significance of the interaction term. Interquartile ranges for 6-hr means: CO, 0.64 mg/m3; NO2, 11.5 µg/m3. *Heterogeneity of the city-specific effect estimates with a p-value < 0.1. Air pollution, genes, and IL-6 fixed monitors have been shown to correlate better to personal exposure to PM rather than to personal exposure to CO (Sarnat et al. 2005; Schwartz et al. 2007), so although sensi- tivity analyses adjusting for the level of PM2.5 in the models showed support for an indepen- dent effect of CO (data not shown), we can not be sure of a specific CO effect. Secondary analyses showed that the interaction effect was clearest and strongest during the first 24 hr and particularly 6–11 hr preceding blood withdrawal, consistent with the short half-life of IL-6. The early effect was further supported by the distributed lag analyses of the subjects carrying the risk allele for IL6 rs2069832 and FGB rs1800790. Given the strong correlation of 6-hr exposure windows, it is probable that effects seen for exposure window 12–17 hr and possibly also 0–5 hr reflect this correla- tion rather than independent effects of these exposure windows. Combined IL6 rs20698322 and FGB rs1800790 Low Intermediate High * * p = 0.001 CO p = 0.03 NO2 p = 0.24 PNC p = 0.02 PM2.5 p = 0.40 PM10 ) 15 R Q -6L Ire10 I p in n ce ea 5 ren llm f ra 0 e f iD voe (–5 % ) R15 Q -6 Ir L pe10 I in n e a c e5 ren llm f ra0 e f i e D vo–5 % ( 18–23 0–5 6–11 12–17 Exposure (hr) 18–23 p = 0.03 p = 0.30 p = 0.48 p = 0.99 FGB rs1800790 ) R 15 NO2 Q -6 Ir L iIn e np10 p = 0.34 p = 0.04 e a c e5 ren llm f ra0 e f i e D vo–5 % (