Vascular access dysfunction, causing considerable morbidity and mortality, is one of the most important complications in hemodialysis (HD) patients1-4). In addition, this dysfunction is common; it occurs in up to 30% of patients in the early period following access placement operations4-7). The major pathophysiologic mechanism is venous thrombosis, following stenosis near the anastomosis site5-7). Several studies have reported that vascular access dysfunction is related to clinical factors (diabetes, old age), biochemical factors (abnormalities of cytokines, cholesterol, apolipoproteins, hemostasis-derived factors), and vascular factors (vessel size, decreased blood flow rate, intimal hyperplasia, atherosclerosis)8-12). In addition, some studies have shown that point mutations in proteins involved in the coagulation system and proinflammatory cytokines associated with endothelial dysfunction and vessel wall proliferation are related to thrombosis and atherosclerosis9-12).

The overall biological effect of the klotho gene is suppression of the aging process; mice that over-express the klotho gene have an increased life span13). Kuro-o et al.14) reported that in an experimental mouse model, klotho gene defects induced premature aging processes, including osteoporosis, infertility, senile atrophy of the gonads, thymus, and skin, physical inactivity, and pulmonary emphysema. They also suggested its involvement in progressive vascular atherosclerosis and vascular calcification14). Other studies in humans have demonstrated that the functional variant of the klotho gene (KLVS) is associated with longevity and early-onset occult coronary artery diseases (CADs)15, 16). This variant of the klotho gene has been shown to be associated with high-density lipoprotein (HDL) cholesterol, systolic blood pressure, and stroke, suggesting an association of this genetic variation with vascular atherosclerosis15). In a Korean population, Rhee et al17) showed that some klotho gene polymorphisms were related to coronary artery disease and hypertension. Kim et al18) also reported that klotho gene polymorphisms were risk factors for ischemic stroke. Notably, the klotho gene G-395A polymorphism was shown to be related to these atherosclerotic diseases in both studies. The present study was conducted to investigate the relationship between the klotho G-395A polymorphism and early vascular access dysfunction in Korean HD patients.

The results of this study indicated that the klotho G-395A polymorphism was related to early vascular access dysfunction in Korean HD patients. Early dysfunction of vascular access occurred in 27.0% of the patients; dysfunction was more common and occurred earlier in A allele carriers than in noncarriers. The A allele frequency of the G-395A polymorphism in the promoter of the klotho gene was 0.155, approximately midway between the values quoted from the Japanese (0.143) and Korean literature (0.170). These values in Asian populations are lower than those reported in Caucasian populations (0.196)17, 19). Previous genetic studies on vascular access failure have documented that several polymorphisms involved in the coagulation system, endothelial dysfunction, and vessel wall proliferation are related to thrombosis and endothelial dysfunction of vascular access10, 12).

Since a novel mouse mutant, klotho, was discovered incidentally by Kuro-o et al.14) many studies have been performed to uncover the secret of the aging process because the gene defect in mice leads to phenotypes resembling human premature aging processes, such as a short life span, growth retardation, senile atrophy of the genitals, thymus, and skin, defective hearing, osteoporosis, and pulmonary emphysema. Notably, mutant mice show impressive features of atherosclerosis, including medial calcification and intimal hyperplasia throughout the vasculature14). Additionally, some studies have reported that this gene influences the development of atherosclerosis through decreased endothelial NO production in mutant mice or reduced expression of the klotho gene in the kidney in hypertensive, nephrectomized, and OLETF (Otsuka Long-Evans Tokushima Fatty) rats20). Furthermore, Saito et al21) suggested that the klotho gene regulates endothelial function and atherosclerosis, based on the finding that klotho gene transfer increased NO production and prevented perivascular fibrosis in OLETF rats, an experimental animal model of atherosclerosis. Another report noted that plasminogen activator inhibitor-1, a factor for the development of thrombosis, is highly expressed in a klotho-deficient mouse22). Some studies inhumans have demonstrated that a functional variant of the klotho gene, KL-VS, is related to high-density lipoprotein (HDL) cholesterol, systolic blood pressure, longevity, coronary artery disease, and their risk factors15, 16). Based on these results, functional defects in the klotho gene may affect vascular access dysfunction caused by thrombosis and stenosis. Kawano et al19) showed that the G395A polymorphism is common and has functional relevance in a Japanese population. Additionally, two previous studies in Korean populations have suggested that this polymorphism can affect vascular diseases17, 18). In this study, the rate of early dysfunction of vascular access was higher in A allele carriers than in noncarriers (p=0.003; Table 2), and this difference was consistent with that of other important variables in the binary logistic regression analysis (p=0.011, odds ratio=3.331; Table 3). This result is an important finding in the G-395A polymorphism's involvement in vascular disease. Another key finding is that A allele carriers suffered access dysfunction earlier than noncarriers. This suggests that this polymorphism could be used to detect patients with a higher risk of early dysfunction. Further studies are warranted on this point.

Failure of vascular access in HD patients is very common and clinically important1, 2). Previous studies have shown that early dysfunction of vascular access ranged from 20% to 30%, although the definition of 'early' varied from 3 to 12 months after the operation4-7). In this study, 34 patients (27.0%) suffered early dysfunction of vascular access, defined as a situation requiring any angiographic or surgical intervention for malfunctioning access within 1 year10). The patients in this study were older (59.6 years old) than those in the two previous similar studies (49.2 and 53.0 years old) in Korea, but other clinical characteristics were similar6, 7). The proportion of males tended to be higher in A allele carriers than in noncarriers, but the difference was not significant p=0.101; Table 1).

While many studies5-9) have reported no association between gender and vascular complications, some have stated that female gender appears to be a risk factor for early failure23-25). In this study, unexpectedly, early dysfunction was associated with male gender (p=0.016, odds ratio=3.306; Table 3). Differences in vessel diameter have been suggested to account for an increase in access complications in female patients. However, as preoperative measurements of vessel diameter were made by venography in all patients at our center, adverse effects of vessel diameter can be avoided. Thus, differences in access placement practice between the United States and Korea seem to limit the extent to which results can be generalized for the Korean population. A second explanation for our finding is that male patients had more A alleles, the most powerful risk factor in our study. Moreover, the effects of the klotho gene on osteoporosis19), nonverbal reasoning26) and cardioembolic strokes18) in human diseases have been suggested to be altered by female hormones27). However, these suggestions have not been further examined or confirmed. Finally, the number of antihypertensive drugs being taken was higher in males than females (3.41±1.61 vs. 2.62±1.54; p=0.006), although the differences in systolic (146.43±17.54 vs. 143.93±19.48 mm Hg; p=0.451) and diastolic (83.87±13.09 vs. 85.38±12.46 mm Hg, p=0.511) blood pressures according to gender were nonsignificant. These findings suggest that a greater atherosclerotic load may occur in the vasculature of male patients compared to females, although other specific tests for load were not performed. Additionally, no difference in clinical or laboratory results was found between the two groups, divided according to the presence of early access dysfunction, in this study.

The primary limitations of our study were its small sample size and retrospective nature. The small sample size may have contributed to the difference in the ratio of males to females between A allele carriers and noncarriers (1.92:1 vs. 0.98:1, p=0.101), although the same ratio was reported in a previous study with a larger sample size21). The retrospective nature of this study may have prevented the collection of important clinical data, such as specific tests for atherosclerotic load, showing vessel wall stiffness, anatomical changes in blood flow, and vessel size, several biochemical factors (anticardiolipin antibody, lipoprotein (a), plasminogen activator inhibitor, monocyte chemoattractant-1), and history of smoking, which could affect the number of antihypertensive drugs being taken. Future prospective studies with larger sample sizes are needed to further evaluate the effect of klotho gene polymorphisms on early access dysfunction.

In conclusion, the G-395A polymorphism in the promoter region of the klotho gene may be associated with the early dysfunction of vascular access in HD patients. In particular, A allele carriers may encounter vascular access dysfunction earlier than noncarriers. This study provides the basis for clarifying the role of the klotho gene in the pathogenesis of vascular diseases.