Introduction

Cardiovascular diseases, including coronary artery disease (CAD) and myocardial infarction (MI), remain important mortality and morbidity factors worldwide. Coronary artery disease is one with a complicated background, resulting from traditional and genetic risk factors and their interaction [1–4]. Endothelial dysfunction is the most important factor in the pathogenesis of CAD. Leukocyte adhesion to the wall of the activated blood vessel endothelium has proved to be one of the initial features of arteriosclerosis. Proinflammatory adhesion molecules such as selectins, integrins, immunoglobulins and chemokines modulate this process [5]. Leukocytes are captured from the bloodstream and then rolled along the endothelial cell surface. E-selectin is expressed on the activated endothelium and plays the main role in monocyte migration as a rolling mediating factor. It is the first stage for endothelial monocyte and leukocyte adhesion and transmigration [6, 7].

The E-selectin gene (SELE) is located on chromosome 1q22-q25. Genetic polymorphisms at the SELE locus may regulate the gene expression levels and affect the biological function of the protein [8, 9]. The SELE gene was determined with many single nucleotide polymorphisms and several polymorphisms of the E-selectin gene have been mentioned to contribute to CAD, hypertension and ischemic cerebrovascular disease [10–12]. Most studies have extensively tested two polymorphisms in particular, A561C (rs5361) and G98T (rs1805193), which have been implicated in susceptibility to CAD [13, 14]. These functional variants seem to play an important role in modifying the secondary structure of E-selectin by exchanging amino acids and regulating cell-cell interactions [15, 16]. Other variations of SELE may also be associated with atherosclerosis or hypertension [17, 18]. We previously showed that the low frequency haplotypes of the E-selectin gene may reduce susceptibility to coronary artery diseases in the Polish population [19]. Although there are many reports confirming a correlation between genetic alterations of SELE and inter-individual variation in the soluble E-selectin (sE-selectin), the relationship between the two still remains controversial [20, 21].

Moreover, it has been established that SELE gene polymorphism, which predisposes to chronic inflammation development/progression, can affect the conditions with underlying inflammatory disorders [22]. According to in vivo studies, E-selectin expression can be elevated continuously as a result of chronic localized inflammation [23], whereas without coexisting inflammation, expression of E-selectin remains at negligible concentrations or it is indeterminable [24, 25]. The pivotal role in the inflammatory process and endothelial function indicate that the SELE gene is a plausible candidate in CAD development. It must be noted, however, that variability across the E-selectin gene and its association with coronary artery disease remain poorly investigated and the previous studies have mostly concentrated on single gene variants.

Therefore, the purpose of this study was to investigate the association between SELE haplotype tagging polymorphisms and coronary artery disease with or without a history of MI, as well as severity of CAD based on the number of coronary vessels involved in a Polish angiographically characterized population.

Material and methods

The study was performed in a cohort of Polish subjects (n = 379) recruited randomly from the inpatient Cardiology Department, Pomeranian Medical University, Szczecin, Poland, including 261 patients with angiographically documented coronary artery disease (CAD) defined as ≥ 50% stenosis of at least one major coronary artery. There were 59 patients with a history of myocardial infarction (CAD/MI(+) group), diagnosed according to recommendations of the Joint European Society of Cardiology/American College of Cardiology Committee. The CAD patients without a history of myocardial infarction (CAD/MI(–), n = 202) were further divided into multi-vessel CAD (MCAD, n = 123) and single-vessel CAD (SCAD, n = 79) disease. The control group included 118 participants without CAD confirmed by coronary angiography (non-CAD group). Patients with clinical diagnosis of cardiomyopathy, coagulopathy, collagenosis and chronic inflammatory disease were excluded from the study. The protocol of the study was approved by the Ethics Committee of the Pomeranian Medical University, with formal informed consent signed by all the participants.

Demographic data were collected during the clinical trial. Hypertension, diabetes and smoking addiction were defined. Routine biochemical blood analyses were done including total cholesterol (TCH), triacylglycerol (TG), HDL cholesterol (HDL-C) and LDL cholesterol (LDL-C). Coronary angiography was performed according to standard procedures using Philips INTEGRIS HM 3000 (Philips, Netherlands) and Philips ALURA (Philips, Netherlands) devices. Significant stenosis was defined based on American Heart Association criteria as ≥ 50% stenosis of the coronary artery lumen.

The genotype data dump file for the CEU (Utah residents with northern and western European ancestry) population (chromosome 1: 166,423,439 – 166,434,836, HapMap release 21, NCBI Human Genome Build 35) was downloaded from the HapMap Genome Browser. The selection of tagging single nucleotide polymorphisms (SNPs) was conducted on HapMap data spanning a region of 11 kb using Haploview’s Tagger in the pairwise mode, r² threshold of 0.8 and the minimum allele frequency of 5%. Eight markers were selected out of 30 SNPs by the tagging algorithm: C1901T (rs3917454, NM_000450.2:c.529+123C>T), A2252G (rs3917412, NM_000450.2:c.529+427A>G), A2692G (rs3917417, NM_000450.2:c.530-304G>A), C2935T (rs3917419, NM_000450.2:c.530-61C>T), A3929C (rs1534904, NM_000450.2:c.716-11A>C), G5310A (rs1076637, NM_000450.2:c.1091-57G>A), T7836C (rs5356, NM_000450.2:c.1800T>C) and C8304T (rs3917438, NM_000450.2:c.*16-200C>T). Genomic DNA was extracted from peripheral blood leukocytes using the QIAamp DNA Mini Kit (QIAGEN, Germany). Seven tagging SNPs were genotyped using a polymerase chain reaction – restriction fragment length polymorphism (PCR-RFLP) method (Table I). The products of digestion were separated in 3% agarose gels. The rs3917417 was genotyped using PCR-DHPLC (with primers 5′-AgACAgTgCAgCATTAgggTTTTA and 5′-TTCTACCCCTTTTCTTTTATTCAA). DHPLC analysis was performed using a Hewlett-Packard 1050/1100 chromatographic system (Agilent, Waldbronn, Germany).

Results

The clinical and biochemical characteristics of the CAD (n = 261) and non-CAD patients (n = 118) are summarized in Table II. Two tag SNPs, c.530-304G>A and c.*16-200C>T, deviated significantly from HWE expectations in the non-CAD group and were excluded from further analyses (Table III). The results from the association analysis between the 6-marker SELE haplotypes (NM_000450.2:c.[529+123C>T; 529+474A>G; 530-61C>T; 716-11A>C; 1091-57G>A; 1800T>C]) and the CAD-related traits are presented in Tables IV–VIII. There were no differences in the SELE haplotype frequencies in the CAD group and in the CAD/MI(–) group as compared with non-CAD group (Tables VI and VII). Also, there was no association of the SELE haplotypes with severity of CAD in the CAD/MI(–) group expressed as the number of vessels involved (Table VIII).

We found an association of the SELE haplotypes with a history of myocardial infarction among CAD patients (CAD/MI(+) vs. non-CAD, CAD/MI(+) vs. CAD/MI(–) (Tables IV and V). Thirty-five haplotypes were reconstructed in 59 CAD/MI(+) and 118 non-CAD patients. Of these, 6 haplotypes had frequencies greater than 5% (accounting for 80.1%), whereas 10 inferred haplotypes had frequencies greater than 1% (87.6%) (Table IV). Among those 10 haplotypes with frequencies greater than 1%, three haplotypes ([C;G;T;C;G;T], [C;G;C;C;A;C], [C;A;C;A;G;T]) were associated (based on adjusted p-values) with a history of myocardial infarction under additive and dominant models and one haplotype [C;G;T;C;G;T] under a recessive model (Table IV). The most common SELE haplotype in this analysis ([C;G;T;C;G;T], 31.2%) was negatively associated with myocardial infarction under the assumption of additive, dominant and recessive effects. Its frequency in the patients with a history of myocardial infarction was 3.0 times lower than in non-CAD individuals (12.7% vs. 37.5%). Two other haplotypes ([C;G;C;C;A;C], [C;A;C;A;G;T]) showed a negative association effect under the assumption of additive and dominant models. The haplotype [C;G;C;C;A;C] was detected only in the non-CAD patients with a frequency of 8.81%.

Thirty-seven SELE haplotypes were reconstructed in 59 CAD/MI(+) and 202 CAD/MI(–) patients. Of these, 5 haplotypes with frequencies greater than 5% accounted for 77.4% of all haplotypes, whereas 12 haplotypes with frequencies above 1% accounted for 92.9% (Table V). Similarly to a previous analysis, we identified three negatively associated haplotypes [C;G;T;C;G;T], [C;A;C;A;G;T], [C;G;C;C;A;C] that overlapped with haplotypes inferred for CAD/MI(+) and non-CAD patients (Table IV). The most common negatively associated haplotype [C;G;T;C;G;T] was 2.9 times less frequent in the CAD/MI(+) group compared with CAD/MI(–) patients (12.7% vs. 36.3%), while [C;G;C;C;A;C] occurred only in CAD/MI(–) patients (0% vs. 6.41%). In addition, we found two haplotypes associated with a history of myocardial infarction ([T;G;T;C;G;T], [C;G;C;C;A;T]) acting in both additive and dominant manner (Table V).

Discussion

In the current study we conducted a haplotype-based analysis of the SELE gene in 261 patients with coronary artery disease (including 59 with myocardial infarction) and 118 control individuals with no significant coronary stenosis. To the best of our knowledge this is the first study investigating the association of the SELE gene with the risk of coronary artery disease and related phenotypes that uses a tagging SNPs approach for the selection of genetic variants.

Our main findings are as follows:

So far, mainly two presumably functional SELE polymorphisms, A561C (rs5361) and G98T (rs1805193), have been investigated in different ethnic groups with respect to CAD [13, 14, 26]. However, the haplotype-based analysis may offer greater power in association studies than the analysis of one SNP at a time, especially when none of the investigated SNPs is a causative marker [27]. Considering the lack of association with increased risk of CAD among Caucasians for G98T in a recent meta-analysis [13], our decision to use tag SNPs in an attempt to capture a high degree of the known common variability, instead of the standard approach to testing individual candidate SNPs, seems well justified.

Our results suggest that E-selectin gene variability may, independently of conventional risk factors, modify the risk of myocardial infarction. However, there was no clear evidence for an association in patients with CAD in whom MI did not occur. Increased levels of E-selectin were reported in both coronary artery disease [28] and myocardial infarction [29]. Atalar et al. [30] found increased levels of selectins, including E-selectin, in patients with unstable angina compared with those with stable angina or without angiographically visible occlusions. Interestingly, sE-selectin levels were higher in patients with acute myocardial infarction preceded by unstable angina compared with sudden onset of infarction [31]. Since at least part of the sE-selectin level in recent years [32], despite variation, can be attributed to SELE gene polymorphism (a borderline association of the 7-marker haplotype with soluble E-selectin [33]), it seems reasonable to propose that the risk-decreasing and risk-increasing SELE haplotype effects for MI possibly reflect extreme areas of the soluble E-selectin level continuum.

Nonetheless, it is unclear why there was no association between SELE haplotypes and the risk of coronary artery disease. None of the SELE haplotypes, except for [C;G;T;C;G;T] under the recessive model (Table VI), were associated with CAD with or without a history of MI (the association of the [C;G;T;C;G;T] haplotype is likely generated by MI patients as it disappears in the no-MI CAD patients, Table VII). It must be noted that traditional risk factors of atherosclerosis are deeply rooted in the Polish population although it should be emphasized that there has been a slight improvement which may lessen the impact of genetic factors. Indeed, the frequency of classic risk factors such as arterial hypertension, smoking, diabetes and BMI differed significantly between CAD and control patients (Table II). The other possibility is the presence of an interaction with other systems that may modify the impact of SELE gene haplotypes on CAD susceptibility. These mechanisms, however, have not been fully explained yet. For example, Wu et al. [33] reported that genotypes/haplotypes of the SELE gene in Taiwanese individuals are independently associated with E-selectin and matrix metalloproteinase 9 (MMP9) levels. Metalloproteinase 9 plays an important role in the destabilization of atherosclerotic plaque [34], but a direct mechanism linking E-selectin and the MMP9 gene is not clear, and further studies are needed to clarify this relationship. It is also possible that the lack of association with susceptibility to CAD may be related to ethnic differences in the social, environmental and genetic conditions contributing towards the development of CAD [35].

However, our study has several limitations. It is important to emphasize that this is the first report demonstrating that the haplotypes capturing the majority of the E-selectin gene variation are correlated with occurrence of MI , but there is still a lack of studies showing this association in other countries and nations. Additionally it is a relatively small sample size, so the observation need to be followed up with a much larger sample size to make a causal inference. The strength of our study lies in the clinically well characterized and homogeneous cohort of patients who had undergone coronary angiography. Homogeneity is a crucial aspect in genetic association studies as population stratification may lead to spurious findings. The geographic area from which the subjects came coincides with the West Pomerania province in Poland and its genetic structure appears to be extremely homogeneous [36]. However, we did not measure the level of E-selectin, and this is a major limitation of our study. Thus, it is not clear whether the MI risk-modifying effect of the SELE haplotypes is mediated through an effect on the serum E-selectin level.

In conclusion, using a tagging SNPs approach we identified both negatively associated and myocardial infarction risk-enhancing common SELE haplotypes in a homogeneous Polish angiographic cohort. However, there was no evidence of an association between the SELE gene and susceptibility to coronary artery disease without a history of MI. Our results suggest that SELE is one of the independent genetic factors modulating the risk of developing myocardial infarction.