Background
Coronary artery disease (CAD) is characterized as a chronic inflammatory disease and represents one of the main causes of worldwide morbidity and mortality [
1]. An impressive body of evidence has proven the role of hypertension in development of CAD [
2]. CAD and hypertension are believed to be multifactorial diseases [
3‐
7]. Furthermore, ample evidence has revealed that the metabolic, genetic, and environmental factors are also involved in susceptibility to CAD and hypertension [
8,
9]. Recent advances in disease biology and understanding of their molecular mechanisms have led to reporting various susceptible genes involved in incidence of these diseases. In this regard, identification of these genes along with environmental factors is essentially important [
10,
11].
Klotho has been recently evaluated in atherosclerosis and is assumed to be an age-regulating protein [
12,
13]. This gene is located on human chromosome 13 (13q12), spanning about 50 kb in length and consisting of 5 exons and 4 introns [
14]. Genetic studies have shown that Klotho gene variants are associated with atherosclerotic disease [
15,
16], and only one study reported a significant correlation between Klotho plasma levels and a reduction in the risk of cardiovascular disease (CVD) [
17]. Previous studies showed a complex syndrome in Klotho-deficient mice in which two remarkable changes are arteriosclerosis and endothelial dysfunction [
18,
19]. Furthermore, it has been reported that adenovirus-mediated Klotho gene delivery increased nitric oxide (NO) synthesis, improved vascular endothelial dysfunction, and decreased blood pressure (BP) [
20]. Therefore, based on the previous findings, Klotho gene is said to contribute to regulation of endothelial function through pathways mediated by NO.
So far, over 10 single nucleotide polymorphisms (SNPs) have been reported in human Klotho gene [
12], and a growing body of evidence has shown the relationship between Klotho SNPs, susceptibility to CAD and endothelial dysfunction. [
16]. Recently, it has been demonstrated that Klotho C1818T (rs564481) polymorphism, located in the exon 4 of the human Klotho gene, is associated with the risk of CAD, bone mineral density, systolic BP, and vascular dysfunction [
12,
20,
21]. Therefore, Klotho gene may be involved in the pathophysiology of CAD. In spite of genetic-related studies to identify the links of Klotho SNPs with changes in related disease risk, the exact mechanisms have not yet been fully elucidated. Since endothelial dysfunction is the one of the etiological factors of CAD and hypertension, it was hypothesized that the polymorphism of the human Klotho gene is involved in CAD and hypertension.
In this content, the objective of this study was to investigate the possible association of the Klotho gene polymorphism with hypertension and CAD.
Discussion
Cardiovascular disorders, considered as multifactorial diseases, might be caused by environmental and genetic factors, and bring about high rates of worldwide morbidity and mortality [
28]. Currently, little is known about the complex interplay between the genetic factors and CVDs. Previous genetic studies conducted on humans have implied the possible association of Klotho gene polymorphisms with longevity and CVDs. Nonetheless, they only succeeded in yielding a limited number of results. Thus, as the need was strongly felt, this study evaluated the Klotho gene polymorphism in Iranian subjects to reveal its possible association with CAD, hypertension, and CAD combined with hypertension. In the present study, Klotho C1818T gene polymorphism was attributed to the decreased risk of hypertension and CAD combined with hypertension.
The association of Klotho gene polymorphism with hypertension and CAD was assessed. At the rs564481 locus, the Klotho C1818T gene polymorphism was associated with decreased risk of hypertension in all subjects category as well as a decreased risk of association with CAD combined with hypertension in those subjects older than 57 years. However, no significant association was observed in subjects younger than 57 years subgroups. In the previous reports, the frequency of T allele carriers were 0.41 [
29], 0.17 [
12] and 0.22 [
19] in Caucasians, Japanese healthy subjects and ischemic stroke Korean females, respectively, which resembled those of the present study, indicating a lower frequency of T alleles among the Asian population. The difference between Caucasian and the Asian populations was most likely due to the racial-ethnic discrepancies. In the Asian population, the Klotho C1818T gene polymorphism was associated with an increased risk of CADs, hypertension, glucose metabolism, and lipid levels [
12,
15,
20,
30]. Results of a study conducted by Rhee et al. showed that subjects with CC genotype had almost three-fold higher risk of CAD compared to those with T allele carriers [
20], suggesting the CC genotype as a predictor for CAD in the Korean population. However, the present study showed that T allele carriers decreased the hypertension and/or CAD risk. Moreover, in stark contrast to the present study, a research conducted in China reported the TT genotype of Klotho C1818T as a susceptibility factor for coronary heart disease [
31]. In a systematic review and meta-analysis conducted by Zhang et al., the association of five Klotho SNPs with CVDs was assessed [
32], revealing that the Klotho C1818T and G-395A SNPs are the risk factors for coronary heart disease (CHD). However, no significant association was reported between the risk of ischemic stroke and Klotho C1818T SNP, as determined within Indian and Korean populations [
18,
19]. In spite of the association of Klotho C1818T gene polymorphism and other Klotho polymorphisms with CHD and CADs as above-mentioned [
13,
16,
32], the present study revealed an inverse relationship merely in patients with CAD combined with hypertension. Moreover, the results of the present study are not consistent with the previous studies which indicated the association of Klotho gene polymorphisms with increased risk of hypertension [
32]. Contrarily, it showed the association of Klotho gene polymorphisms with decreased risk of hypertension. The discrepancies witnessed here might be because of the differences in the genetic constitution and non-genetic or environmental properties through life. The other illustration possibly endorsing genetic heterogeneity, gene-environment and gene-gene interactions in various populations.
The Klotho’s anti-aging effects and differential effects of age in the association of Klotho polymorphism with CVDs have been well established. Hence, in the present study, the subjects were divided into two subgroups, namely, younger and older than 57. According to the study by Rhee et al. [
20], the effects of Klotho C1818T SNP on CAD were enhanced in the subjects younger than 60 years. Considering the identical age-cutoff (60-year olds in Rhee et al., study vs. 57-year olds in the present study), it is safe to claim that the results of the current study strikingly contrasted with those of Rhee et al., suggesting the significant association of Klotho C1818T gene polymorphism in patients over 57 years old suffering from CAD combined with hypertension. On the other hand, they reported the CC genotype as a strong predictor for CAD, a finding not coinciding with that of the present study, in which subjects having CAD combined with hypertension after the age of 57 demonstrated a lower prevalence of TT genotype. Therefore, it is safe to assume that Klotho C1818T polymorphism might be associated with the late atherosclerosis. In spite of hypothetical nature of this deduction, present findings along with further researches to be conducted could clarify the exact mechanism of Klotho SNPs in humans. Furthermore, regarding lack of studies on these age subgroup divisions and important effects of Klotho, a peptide hormone with anti-aging activities, it is recommended that further studies be performed on a larger population to explore the possible age effects.
Some limitations can be considered in interpretation of the results yielded by the study. Firstly, the present study’s participants were restricted to the Iranian southeast populations. Hence, the present findings could not be generalizable to other racial/ethnic populations. Secondly, the results were adjusted for each variable to show the reliable statistical power of our association study by using multinomial regression model analysis. Finally, “H-Tens” group selection was one of the important current study limitations as all H-Tens patients were susceptible to coronary angiography. So, selection of hypertensive patients without any critical of coronary vessel stenosis was a tough and demanding task. Hence a relatively small sample size resulted. However, the present study enjoys some strong points as well. The CAD was diagnosed by coronary angiograms while other studies confirmed the CAD only with an electrocardiogram or a thallium scan. Moreover, to the best of the author’s knowledge, this is the first study reporting the association of Klotho gene polymorphisms with the risk of CAD, hypertension, and CAD combined with hypertension.
Conclusion
In conclusion, the findings of the present case-controlled study revealed the importance of genetic influences of Klotho polymorphism on hypertension and/or CAD susceptibility among the Iranian southeast population. Furthermore, TT genotype carriers of the Klotho C1818T gene polymorphism decreased the hypertension risk, and aging enhanced the positive effects of the Klotho polymorphism on CAD combined with hypertension, improving the odds that Klotho gene might play a part in the age-related incidence of CAD along with hypertension. Nevertheless, larger studies with different ethnic populations are required to uncover the exact role of Klotho gene in the pathogenesis of CAD, hypertension, and CAD combined with hypertension.
Acknowledgments
Authors would like to express their deep gratitude towards participants who provided us with their precious assistance in performing this study. This study was granted by Kerman University of Medical Sciences, Kerman, Iran (Grant No. 292-93) and is a part of M.Sc. thesis.