Evaluation of ABO blood groups as a risk factor for myocardial infarction.

Dear Sir, The immune system and biological roles of blood groups are becoming focuses of interest, while the relevance of ABO blood groups in assessing the risk of many diseases has been investigated for years now. There is a proven association between ABO blood groups and diseases that leads to a shift of the coagulation balance toward thrombus formation. This has been demonstrated in a number of studies of venous thrombosis in non-OO blood group carriers with an increased level of von Willebrand's factor acting as a mediator of enhanced platelet adhesion and aggregation1,2. However, studies on arterial disease and myocardial infarction (MI) have yielded contradictory results, although an association of these processes with the ABO system is physiologically justified because the carbohydrate erythrocyte antigens are also found on platelets and vascular endothelium. Results of a meta-analysis by Clark and Wu, published in March 2011, indicate a very low impact of ABO blood groups on the risk of MI3. The aim of this study was to evaluate the impact of ABO blood group genotypes and the three most common prothrombotic mutations (factor V Leiden, prothrombin G20210A - factor II and MTHFR C677T) on MI development in the Croatian population. This case-control study included 182 patients with acute MI and 236 healthy blood donors with no personal or family history of coronary artery disease or MI. Patients' blood samples were collected at Sestre Milosrdnice University Hospital Centre and control blood samples were collected at the Croatian Institute of Transfusion Medicine. The gender distribution in the group of patients was 63.2% male and 36.8% female (median age 64 years; 24.7% aged <55 years), while that in the control group was 52.5% male and 47.5% female (median age 38 years). All subjects were informed about the study and signed informed consent forms agreeing to participate in it. Following isolation of genomic DNA from EDTA blood samples (QiaAmp DNA Mini kit, Qiagen, Hilden, Germany), ABO genotype was determined in both patients and controls by the method of polymerase chain reaction (PCR) sequence-specific primers. The presence of factor V Leiden, prothrombin G20210A and MTHFR C677T mutations was determined by the real-time PCR method on an AB 7500 real time PCR system using custom-made primers and probes (Applied Biosystems, Branchburg, New Jersey, United States of America). On statistical analysis, logistic regression with odds ratio (OR) and 95% confidence interval (95% CI) and Fisher's exact test were used to assess the impact of ABO blood group genotype combinations and hereditary factors on the development of MI (MedCalc Software, Mariakerke, Belgium). The results of ABO genotyping showed no statistically significant difference in the frequency of OO and non-OO genotype carriers between the group of patients who had had an acute MI and the control group (OR 1.41; 95% CI 0.94–2.11). Comparison of OO genotype and non-OO genotype carriers between MI patients and control subjects yielded a significantly higher OR in the O1A1 genotype carriers vs OO carriers (OR 1.66; 95% CI 1.03–2.68; P <0.05); in O1A2 genotype carriers, the OR was 2.57 (95% CI 1.01–6.55), which did not reach statistical significance. Comparison of OO and non-OO blood group genotype carriers including heterozygous carriers of factor V Leiden and prothrombin G20210A mutations and homozygous carriers of MTHFR 677 TT mutation did not indicate an increased risk of MI. Statistically significant differences were obtained when comparing acute MI patients and the control group according to age and sex, i.e. males had a 1.5-fold greater risk of developing MI (P 55 years had a >20-fold greater risk (OR 21.1; 95% CI 12.64–35.23). In the Croatian population, group A is most common blood group (41%), followed by group O (39%), group B (15%) and then group AB as most infrequent (5%). In the present study, we did not demonstrate that particular non-OO genotypes were associated with different risks of MI compared with the OO genotype. Only the A1 allele had a modest effect on the occurrence of MI as compared with the OO genotype. Similar results indicating a weak correlation of coronary artery disease and MI as its complication in blood group A carriers have already been reported from some earlier studies. At the same time, some studies point to a higher prevalence of both A and B alleles in post-menopausal women and MI patients and some authors concluded that the B allele of the ABO blood group system represents an independent risk factor for MI3. In contrast to these suggested associations with blood group A, B or both alleles, other authors found no statistically significant association between non-OO blood group and arterial forms of cardiovascular disease (MI)4. As far as concerns the widely confirmed increased thrombotic risk in non-OO blood group carriers with venous thrombosis, most studies investigating the association of ABO blood group system and arterial disease have reported quite inconsistent results. Meta-analyses of these studies produced rather low OR (OR 1.25; 95% CI 1.14–1.36) for MI, while A blood group yielded a OR of 1.29 (95% CI 1.16–1.45)1. The majority of retrospective studies found a weak, positive correlation between arterial disease and non-O blood group or A blood group, whereas prospective studies report no risk at all3. The results of our study appear to indicate that the genetic prothrombotic factors are not associated with an increased risk of MI. Many other studies also failed to demonstrate any significant association of MI with factor V Leiden and prothrombin G20210A polymorphisms5. Only a few studies have investigated an association of the MTHFR C677T variant and MI and failed to demonstrate such an association. Some studies report a stronger association of MI with factor V Leiden, and others with prothrombin G20210A, whereas meta-analyses show a modest association of factor V Leiden and FII G20210A mutations with arterial disease and MI. This association is generally stronger in younger patients and more common in women5. The limitations of our study are related to the lack of information on additional and environmental risk factors such as cigarette smoking, hypertension, body mass index, physical inactivity and hyperlipidaemia, considered as modifiable risk factors. Furthermore, the present study was quite small. On the other hand, the subjects included in the study were homogeneous for ethnicity, the patients were selected on the basis of a verified diagnosis, and the results are comparable to those of others studies because the variants of the genetic prothrombotic factors in the Croatian population do not differ significantly from those in other European nations. Considering that MI is a multifactorial disease, the development of the disease is likely to be influenced synergistically by a multitude of factors at lower intensity and further investigation is needed.