Diabetes & Metabolic Syndrome: Clinical Research & Reviews
Original articlePrevalence of endothelial nitric oxide synthase E298D polymorphism in Turkish patients with essential hypertension
Introduction
Nitric oxide (NO), produced by the nitric oxide synthase in the endothelial cells, regulates vasomotor tone and blood flow through its activation of the endothelium-derived relaxing factor (EDRF) [1]. Association exists between NOS3 gene single nucleotide polymorphisms (SNPs) and hypertension [2]. Both human and animal studies have shown onset of hypertension due to loss of nitric oxide, which act as endothelium-derived relaxing factor [3]. Nitric oxide is catalyzed by endothelial nitric oxide synthase (eNOS), an enzyme with multiple genetic variants that might confer risk for hypertension [4]. An association between eNOS polymorphism and reduced eNOS expression and activity has been reported. Human eNOS gene is located on chromosome 7q35–36, consists of 26 exons spanning approximately 21 kb with multiple polymorphisms [5]. The common polymorphism G894T in exon 7 of the eNOS gene results in the substitution of glutamic acid (E) at codon 298 by aspartic acid (D) (E298D). This is the only known polymorphism changing the eNOS protein sequence, leading to speculation that genetic variation at this site may alter eNOS activity or regulation and possibly leads to endothelial dysfunction and to pathogenesis of several cardiovascular diseases [6]. The biological significance of the substitution of G to T in the eNOS gene locus is unclear. Fatini et al. have published a LightCycler assay for the analysis of this polymorphism [7]. E298D polymorphism of the eNOS gene has been extensively studied, and reported to be associated with the reduced expression and activity of eNOS [8]. Tesauro et al. [9] suggested that the eNOS E298 is subject to selective proteolytic cleavage in endothelial cells and vascular tissues and therefore individuals with Asp 298 may display low eNOS activity. Further, The codon 298 falls between the critical residues of the heme domain (100–200) and the binding sites for l-arginine and tetrahydrobiopterin (350–450) [10]. It has been speculated that in the absence of tetrahydrobiopterin, the eNOS enzyme becomes dysfunctional leading to decreased generation of NO and increased superoxide synthesis. By some investigators it has been suggested that Glu298Asp substitution may affect the response of vascular endothelium to increased oxidative stress [11], [12]. Whether the E298D mutation has any effect on the binding properties of eNOS and can give rise to a dysfunctional enzyme remains to be explored. Essential hypertension (eHT) is the major risk factor for the coronary artery disease (CAD) in south East Asians [13]. In addition, it is associated very significantly with the pathophysiology of a variety of vascular disorders like renal and cardiac failure, ocular damage and stroke [14]. Obesity and other environmental factors [15] also substantially contribute to the high incidence of hypertension, twin studies suggest that in human beings a significant part of the inter-individual variability of blood pressure is heritable [16]. However, genetic studies of multifactorial disorders such as hypertension have proven difficult due to the multiplicity of genes underlying complex phenotypes and the modest effect of individual polymorphisms or genes (eNOS4) [17].
Various studies have stated eNOS gene E298D polymorphism to be associated with coronary spasm [18], essential hypertension [19] and the risk of acute myocardial infarction [20], [21], [22]. Many studies have reported associations of eNOS gene polymorphism with atherosclerosis and the risk factor hypertension [19], [23], [24], [25], [26]. However, many other studies have failed to show such associations [17], [27], [28], [29], [30], [31], [32], [33]. NO is closely related with lipid metabolism. Recent epidemiological studies have clarified blood lipid levels to be closely related to NO and the function of endothelial cells [34]. Pereira et al. [35] have recently reported that eNOS Glu298Asp polymorphism influences the relationship between serum lipid levels and blood pressure. In this study, the relationship between eNOS E298D genotypes and blood pressure, lipid profiles and glucose was systematically analyzed in order to explore the roles of eNOS E298D polymorphism in the development of essential hypertension.
Section snippets
Subjects
We studied 56 eHT patients (21 men, 35 women), 36 controls (14 men, 22 women), who were angiographically proven to have normal coronaries from Group Florence Nightingale Hospital (Istanbul, Turkey). The criteria for CHD was narrowing of artery 50% or more with angiography, who had stable angina pectoris and who were ischemia positive with noninvasive tests. Other disease such as CHD, dyslipidemia (HDL-cholesterol levels <45 mg/dl, triglyceride levels >150 mg/dl and LDL-cholesterol levels >130
Analytical methods
The plasma glucose concentration was measured by the glucose oxidase method using Kit of Biotrol on Bayer/opeRA analyser. Serum total-cholesterol was measured using commercial kit of Biotrol; HDL-cholesterol using commercial Randox's kit; LDL-cholesterol was calculated by the formula of Friedewald and triglyceride determination was made by the method of lipase/glycerol kinase UV endpoint on opeRA analyser.
The quantification of body fat
At first, lean body mass (LBM) was calculated by the formula depicted by Hume [36], for
Results
The frequencies of the human eNOS gene E298D genotypes were given in Table 1. The eNOS gene E298D polymorphism frequencies for 298E/E, 298E/D and 298D/D genotypes were respectively 41.1%, 44.6%, 14.3% in subjects with eHT and 52.8%, 38.9%, 8.3% in non-HT group. The genotype frequencies between the study groups for E298D polymorphism were not significant (p = 0.49). In detail, the most frequently observed genotype was found to be 298E/D (heterozygotes) in the eHT group, whereas 298E/E (wild-type)
Discussion
In Caucasians the Glu298Asp Glu/Glu, Glu/Asp and Asp/Asp genotype frequencies were reported to be 43.3%, 37% and 19.7% respectively [38]. Srivastava et al. [39] reported Glu298Asp Glu/Glu, Glu/Asp, Asp/Asp genotype frequencies respectively as, 71.22%, 28.06%, 0.72% in 139 healthy Indians, and did not find any significant difference among group with respect to Glu298Asp genotypes. Sawada et al. [40] reported the frequencies of Glu/Glu, Glu/Asp, and Asp/Asp genotypes as 85.9%, 13.4% and 0.7%,
Conflict of interest
There is no conflict of interest in this study.
Acknowledgement
This work was supported by the Marmara University Scientific Research Projects Commission (Project No: FEN-A-040609-0174, 2009).
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These authors participated equally to this publication.