One or more genetic variants located on chromosome 9p21 and tagged by SNP rs2383206 are major risk factors for coronary artery disease among individuals with type 2 diabetes. In our population of diabetic subjects, this effect is stronger than that reported in the general population due to a positive interaction between the genetic variant(s) and hyperglycemia. As was found in the general population, this association is not mediated by an effect of these genetic variants on other cardiovascular risk factors, since it is not attenuated by adjustment for these variables (10
). This synergism between 9p21 locus and hyperglycemia on the risk of coronary artery disease translates into a similar interaction with respect to cardiovascular mortality among individuals with type 2 diabetes.
Other genes, including ADIPOQ
, and TNFAIP3
, have been reported to host polymorphisms influencing cardiovascular risk in type 2 diabetes (19
). However, the present finding stands out from the previous ones in two respects. First, it concerns a genetic effect that was identified through a genome-wide approach and has been extensively replicated in the general population (10
). Second, it is the first to demonstrate synergism with poor glycemic control.
The interaction between 9p21 allele and glycemic control may help explain the discrepancy between the potent proatherogenic effects of glucose observed in vitro
and the evidence from large clinical trials of limited benefit of good glycemic control on cardiovascular outcomes in diabetic subjects (2
). Poor glycemic control has an especially strong impact on cardiovascular risk in individuals who are homozygous for allele G at rs2383206, about 30% of individuals with type 2 diabetes. The other 70% are not as sensitive to the atherogenic effects of hyperglycemia. This heterogeneity could explain the past difficulties in demonstrating an association between glycemic control and cardiovascular outcomes.
Our findings are at variance with those by Broadbent et al., who found that the strength of the association between 9p21 variant and CAD was similar among diabetic and non-diabetic subjects (29
). That study, however, included individuals with both type 1 and type 2 diabetes. Furthermore, the estimate of the association between 9p21 variants and CAD in the diabetic stratum was based on a small number of controls (n=156) in whom asymptomatic CAD had not been excluded as had been in ours. Most importantly, that study did not present data on history of glycemic control. Thus, a fair comparison with our study is not possible.
SNP rs2383206 is placed in a 190 Kb region of high linkage disequilibrium containing two known genes (CDKN2A
), which code for three proteins (p16INK4a
, ARF, and p15INK4b
) that are expressed at high levels in a wide range of cell types, including endothelial and inflammatory cells. All three proteins are inhibitors of cyclin-dependent kinases controlling cell proliferation, cell aging, and apoptosis – functions that are all potentially relevant to the atherosclerotic process (30
). Excess glucose is believed to foster atherosclerosis through multiple pathways involving the build-up of advance glycation end-products or AGE, activation of protein kinase C, increased production of polyols and hexosamine, and increased oxidative stress (18
). At what level the cellular pathways controlled by the 9p21 polymorphism(s) and those induced by high glucose intersect remain to be determined. However, one should also consider the possibility that metabolic alterations that are associated with poor glycemic control, rather than hyperglycemia per se
, are the actual factors responsible for the synergism with the 9p21 locus. The finding that the interaction was unaffected by adjustment for some of these metabolic traits such as blood pressure and cholesterol levels at examination is against his hypothesis, but further analyses using more precise indicators based on repeated measures are certainly warranted. Another aspect that remains to be fully clarified is whether the association with type 2 diabetes reported in a region immediately centromeric to that associated with CAD (33
), plays any role in the observed interaction, On the other hand, a recent multicentric study has shown that the SNP associated with type 2 diabetes (rs10811661) is not associated with increased risk of CAD or other arterial disorders (36
). Similarly, in an exploratory study of our cases and controls, this SNP was not associated with CAD nor was there any evidence that this SNP mediated the interaction between 9p21 variant and HbA1c levels by determining worse glycemic control (Doria et al., unpublished data).
Whether knowledge of modest genetic effects can improve disease prediction and treatment of common disorders remains uncertain (37
). However, the magnitude of the joint effect of poor glycemic control and 9p21 locus favors some clinical benefit. If the probability of clinically significant CAD is about 30% for unselected type 2 diabetic subjects, one can estimate from our data that this probability goes up to 60% for diabetic individuals who have Hba1c values in the upper tertile of the distribution and carry the high risk genotype. On the other hand, the availability of a test improving prediction does not necessarily imply that such test should be adopted in clinical practice. This decision should be based on an investigation of the cost-effectiveness of prevention strategies targeted at high-risk individuals rather than to the entire population of diabetic subjects. This analysis must weigh the costs of the genetic test and available prevention strategies and the effectiveness of these strategies specifically in these high-risk patients.
Our study has two unique strengths, namely the contrast achieved by comparing angiographically-confirmed CAD cases with controls for whom CAD was ruled out by an exercise stress test, and the accurate assessment of long-term glycemic control through multiple HbA1c measurements spanning many years before study entry, in contrast to the cross-sectional measurements available to most studies of CAD in type 2 diabetes. However, some limitations should be acknowledged. One is that the analysis of the interaction between genotype and glycemic control was based on small effective sample sizes. This translated into relatively high p values for interaction (0.004–0.05), raising the concern of a false-positive result. The fact that we found a similar interaction between glycemic control and the 9p21 locus for a related outcome (cardiovascular mortality) in an independent study based on a different design makes this possibility less likely. However, additional replication studies are necessary to establish the interaction with statistical confidence. Another limitation concerns generalizability. Cases and controls were recruited at the Joslin Diabetes Center and Beth Israel Deaconess Medical Center. It is possible that the strength of the association in our study may have been overestimated due to selective referral of especially severe CAD cases to these specialized centers. However, this does not seem to be the case since only half of the cases had three stenotic vessels and less than half had a previous myocardial infarction. Generalizability is also affected by the fact that the additional risk to patients with the risk genotype was assessed against a select group of controls rather than the general population of patients with type 2 diabetes. Finally, a potential limitation concerns the effectiveness of the exercise treadmill test to exclude CAD in controls. Large meta-analyses evaluating the accuracy of the exercise treadmill test for the detection of CAD in the setting of normal ECG have determined the sensitivity of this test to be 72% with a specificity of 77% (15
). Although the prevalence of asymptomatic CAD in diabetic patients has not been thoroughly investigated, a single, large prospective study found myocardial perfusion defects in 16% of asymptomatic diabetic patients (40
). Assuming this prevalence of CAD in the population eligible for entry into the control group, the negative predictive value of our combined selection criteria is 93.5%. Therefore, only a small proportion of the individuals in the control group might have had asymptomatic obstructive CAD. Furthermore, such misclassification, if present, would have biased the results towards the null hypothesis, making our findings of association even more notable.
In conclusion, 9p21 locus and poor glycemic control interact in determining the risk of CAD in type 2 diabetes. This finding may have implications for our understanding of atherogenesis in diabetes and for the design of more effective prevention strategies. More broadly, it illustrates the complex etiology of multifactorial disorders and highlights the importance of accounting for gene-environment and gene-gene interactions in the quest for genetic factors contributing to these conditions.