Influence of antioxidant genotype and antioxidant status on progression of chronic kidney disease

Chronic kidney disease (CKD) is a significant public health issue that affects an estimated 11 percent of adults over the age of 25 years in Australia. Slowing the progression of kidney disease is a major therapeutic challenge for nephrologists. Oxidative stress is an imbalance between reactive oxygen species (ROS) and antioxidants levels in cells or tissues. It has been linked to a number of diseases, including cardiovascular disease (CVD) and CKD. Decreased antioxidant levels can be caused by a number of factors, such as, genetic mutations that reduce the effectiveness of the antioxidants, or toxins that deplete the concentration of antioxidant enzymes. Antioxidants may be separated into two distinct groups, exogenous and endogenous compounds. Important endogenous antioxidant enzymes include superoxide dismutase (SOD), glutathione peroxidase (GPx) and catalase which together form the primary defence system against ROS. In recent years, the relationship between single nucleotide polymorphisms (SNPs) of antioxidant enzymes and diseases associated with oxidative stress has been a topic of significant interest. SNPs of the antioxidant enzymes SOD2 Alal6Val, GPx1 Pro197Leu and catalase C-262T have been associated with the pathogenesis of cancer, cardiovascular disease and diabetes. However, links between SNPs of these enzymes and CKD have yet to be investigated. Thus, the overall aim of the present study was to determine associations between antioxidant enzyme SNPs, antioxidant activities and renal function at baseline and changes thereof over a year in CKD patients. Two major studies will be presented. The first uses a case-control design and baseline data from the CKD patient cohort (n=230) and control subjects (n=224) to determine (a) the frequency of specific SNPs, present in GPx, SOD and catalase genes and whether and control subjects (n=224), and (b) associations between antioxidant genotypes and renal function. The second study uses baseline and one year data from 185 CKD patients from the cohort and investigated whether the progression of kidney disease (i.e, decline in eGFR) was (a) associated with specific genotypes resulting from the aforementioned GPx, SOD and/or catalase SNPs, and (b) associated with altered plasma GPx, RBC GPx, RBC SOD and/or RBC catalase activities. In the case-control analysis, significantly (p=0.023) more CKD patients had the GPx Leu/Leu genotype (n=5) compared to controls (n=0). Although not statistically significant, patients with the GPx1 Leu/Leu or SOD2 Ala/Ala genotypes had reduced eGFR compared with the GPx1 Pro/Leu and SOD2 ValNal genotypes. CKD patients had significantly lower plasma GPx and RBC catalase activities compared to controls (p<0.0001). In contrast, both RBC GPx and RBC SOD activities were significantly higher in CKD patients (p<0.0001). In addition, in CKD patients, a significant positive association was found between eGFR and plasma GPx activity (p<0.0001). Plasma GPx increased (p<0.0001) and RBC GPx decrease (p<0.05) with disease progression from Stages 1 to 5. Interestingly, when the controls were stratified according to eGFR (and hence, Stage of CKD), 13% (30 subjects) were found to be in Stage 3 according to Kidney Disease Outcomes Initiative (KDOQI) guidelines. The second study showed that eGFR declined over 12 months in both SOD2 Ala/Vat and Val/Val patients, indicating a more rapid progression of kidney disease compared to patients with the Ala/Ala genotype (Ala/Val compared with Ala/Ala: p=0.001; VaVVal compared with Ala/Ala: p=0.005). The progression of CKD did not appear to be influenced by SNPs of GPx1 or catalase. There was a direct relationship between the rate in change of plasma GPx activity and the rate of change of eGFR over the 12 month period (p=0.025). In summary, this is one of the largest case-control studies comparing antioxidant enzyme genotypes and activities in CKD patients conducted to date. In addition, this study is the first (cohort) study to investigate the role of antioxidant enzyme SNPs in the progression of CKD. The data suggest that CKD is associated with impaired plasma GPx and catalase activities and enhanced RBC GPx and SOD activities when compared to controls. Secondly, although genotype frequencies were similar for patients and controls, lower eGFR was associated with the GPx1 Leu/Leu genotype. Thirdly, CKD patients with the SOD2 Ala/Val or Val/Val genotype had a greater decline in kidney function over the 12 month study period when compared to patients with SOD2 Ala/Ala genotype. These findings suggest that SOD and GPx therapies that enhance the activities of these antioxidants may slow the progression of CICD. In addition, as 13% of the control participants were found to have impaired kidney function (eGFR <60mL/min/1.73m2), there appears to be significant undiagnosed CKD in Northern Tasmania, that may warrant routine assessment of kidney function in the general population.