Of the 89 genetic variants tested, we found strong statistical evidence of an association for only one, the IRS1 Gly972Arg polymorphism, although it was based upon a small number of deaths in individuals who would already have been considered at high risk for mortality. Thus, we can not unequivocally implicate any of the genetic variants included in this study as a prognostic factor following ACS, including those in the 9p21 region. The possible explanations for our essentially null findings include inadequate power for survival analysis, given that mortality following ACS occurred in only 11.1% of our case sample, or that the genetic variants included in our study have no impact on post-MI prognosis.
With respect to statistical power, we have presented evidence that at least 73 of the gene variants seemed identical to random variables, and therefore, our data provide no support for the hypothesis that testing these particular variants in larger samples would likely yield positive results. It is unlikely that mortality in direct relation to any of these gene variants is a common occurrence following ACS. Furthermore, as shown in Table , conventional cardiac risk factors such as advanced age and diabetes appear to be dominant factors in determining prognosis, and therefore, it may be challenging to demonstrate an independent genetic effect on mortality without sample sizes far in excess of 800 patients, particularly if genetic or pharmacogenetic interactions are explored.
However, given the limitations of our sample size, we were unable to exclude modest effects in association with the remaining 16 gene variants, 6 of which had nominally statistically significant associations with post-ACS mortality. Of these, only 2 remained significant after adjustment for traditional cardiac risk factors, with ACE1, APOA1, F7, and HFE having no independent association. As might have been expected, known confounding variables such as age, sex, and comorbidity proved to be important predictors of mortality in the Cox regression models. In addition, APOA1 was related to a small number of deaths, and the association of the I/I genotype of ACE1 would seem paradoxical, given that the D/D genotype has been postulated as a cardiac risk factor in most studies.
As with ACE1, the association of the F7 glutamine allele would also have to have been considered paradoxical, a priori, given that this allele was originally reported to be protective against the occurrence of myocardial infarction, and it correlates with lower F7 activity and hence lesser coagulability. However, it was the relatively rare A/A genotype of F7 that conferred the greatest risk of mortality in our population of ACS patients, which is consistent with the hypothesis that rare variants may have relatively large genetic effects. Interestingly, all 5 deceased individuals with the A/A genotype had NSTEMI, which is improbable (P = 0.005) given that only 35% of the cases had this manifestation of ACS, and 4 of 5 had either prior MI (N = 3) or a history of revascularization (N = 1). Thus, further study of the F7 allele is warranted, especially in NSTEMI patients.
In contrast, it is possible to postulate a plausible cardiovascular risk mechanism for the hemochromatosis-associated A allele. Heterozygotes for the A allele, on average, have relatively increased total body iron stores.[19
] In addition, some data indicate that iron excess may lead to free radical formation and thereby confer risk of myocardial infarction.[21
] However, a limitation of our study is a lack of phenotypic data on serum iron stores (e.g., ferritin or serum iron levels). In addition, the existing literature on HFE
does not show a consistent association with cardiac disease, and the allele was not even marginally associated with the occurrence of ACS in our study population.[5
In judging the likely validity of the remaining 2 genetic associations that remained statistically significant despite extensive adjustment for comorbidity and treatment variables, one must consider the magnitude of the risk, the numbers of deaths observed, as well as the relationship of the genotypic risk pattern in the context of the functional biology of each particular gene.[22
Lys469Glu association was related to relatively increased mortality (15.9%) among individuals with the A/A genotype (lysine homozygotes). This is higher than the overall 11.1% mortality rate in the cohort, and there were 43 deaths among individuals with this frequent (34.0%) genotypic class. However, it is the 469Glu allele that is postulated to have deleterious pro-inflammatory effects,[23
] and therefore, no biological mechanism for this association is immediately obvious.
The strongest genetic association was detected for the rare A allele (coding for arginine) of the IRS1
Gly972Arg polymorphism. Even a Bonferroni-corrected P value for this association had borderline (P < 0.1) study-wide significance. IRS1 is activated as an intracellular signal transducer, in adipocytes and skeletal muscle cells, when its tyrosine residues are phosphorylated by the insulin-bound insulin receptor, the function of which is inhibited in vitro
by the Arg972 residue, leading to decreased glucose uptake.[24
] Baroni et al
(1999) reported the A allele as a risk factor for coronary artery disease (18.9% vs. 6.8%),[25
] and there have been inconsistent reports of this polymorphism being more prevalent among diabetics,[26
] perhaps causing defective phosphatidylinositol 3-kinase interaction, peripheral insulin resistance, and impaired insulin secretion. In addition, Harrap et al
reported a subthreshold linkage peak containing IRS1
in a genome-wide scan of 61 sibling pairs with acute coronary syndrome, adding to interest in the IRS1
locus as a cardiac candidate gene.[28
] However, IRS1 was not associated with the primary occurrence of ACS in our previous case-control analysis.[5
] Moreover, due to the rarity of arginine homozygous genotype, the association in our study population was largely based on the deaths of 2 individuals among the 3 with the A/A genotype. Accordingly, further prognostic study of this rare genotype is warranted in larger populations of ACS patients, particularly those with diabetes.
In 3 of the 6 nominally significant bivariate associations described above (ACE
), the allele hypothesized to be protective against ACS occurrence was associated with early death following ACS, creating an apparent paradox. These may be false positive associations. However, such apparently paradoxical, but well-validated, associations have been observed in prognostic studies (e.g., the association between active smoking and lower mortality following ACS).[29
] One theoretical explanation could be survivorship bias, meaning that most individuals with the genetic risk factor never survive the initial cardiac event, and that the remaining individuals are those with the best prognosis. Although potentially interesting, this hypothesis is difficult to investigate except prospectively, with ascertainment prior to the initial cardiac event.