Molecular studies of complications in end stage renal disease : focus on expression and variations of candidate susceptibility genes

Abstract

End stage renal disease (ESRD) is a complex multifactorial disorder, where both environmental and genetic factors are contributing to the phenotype. The most common complications seen in ESRD patients are malnutrition, inflammation and atherosclerosis and the coexistence of these complications is denoted the MIA syndrome, which is associated with inexplicably high mortality. However, due to big inter-individual differences not all patients have the same risk of developing the different complications of the MIAsyndrome.

The objective of this thesis is to contribute to the development of an approach on how to use genetics as an instrument for identifying "high risk" ESRD patients at an early stage and to provide the tools that can help to develop more accurate and individually tailored treatment strategies in the future. With this aim, several susceptibility genes and proteins have been investigated with relation to the MIA syndrome and mortality in ESRD. They are presented in the following papers:

Paper I; Myeloperoxidase (MPO) is suggested to be one link between inflammation, oxidative stress and endothelial dysfunction in uremic patients. The effect of a functional single nucleotide polymorphism (SNP) -463 G/A, that induces a 25-old transcription enhancement, was analyzed in relation to the MIA complications and oxidative stress in ESRD patients. The G/G genotype was associated with higher prevalence of inflammation and CVD, as well as with higher levels of an oxidative stress marker, a finding that replicates other studies.

Paper II; The role of the anti-atherosclerotic protein adiponectin was studied in ESRD and the impact of variations of the adiponectin gene (ApM 1) on plasma adiponectin levels was investigated. In this study we found that plasma levels of adiponectin were markedly increased in ESRD patients. Additionally, patients with insulin dependent diabetes mellitus (DM) had even higher levels as compared to both non diabetic and non insulin dependent DM patients. Among the four SNPs that were studied (-11391G/A, -11377C/Q 45T/G, 276G/T) only the - 1377 C/C genotype was significantly associated with a lower prevalence of CVD as well as lower triglyceride levels. The low impact of the individual SNPs on plasma levels and presence of the MIA-complications shows the importance of analyzing and identifying risk-haplotypes that may have greater impact on the phenotype.

Paper III; Since adiponectin levels are markedly elevated in ESRD, we analyzed the expression of ApM1 in fat tissue from patients with a high prevalence of MIA complications, patients who had few complications and healthy matched controls. We found that the ApM1 gene expression was markedly decreased in ESRD patients as compared to controls, but no significant difference was noted between the two groups of patients. The decreased ApM1 expression could be explained by a negative feedback regulation due to the high levels of circulating adiponectin.

Paper IV; Plasma levels of interleukine-6 (IL-6) and C-reactive protein (CRP) are highly elevated in ESRD patients and predict mortality in both healthy and uremic individuals. Hence putative SNPs that may regulate plasma levels of these proteins were investigated (IL-6 SNPs: -597G/A, -174G/C, 5014A/G, Phe201Phe C/T and CRP SNPs: -286C/T/A, 1059G/C). We found that a genotype combination (-597G, 174G, 5014A) was associated with inflammation in ESRD patients. However, the lack evident impact on plasma levels, which is seen in other studies, may be explained by the strong effect of a promoter haplotype. Hence, further studies are necessary to evaluate how genotype combinations regulate the plasma levels of IL6 and CRP.

Paper V; Resistin is a newly discovered protein that in rodents may promote insulin resistance. Resistin may have a direct proinflammatory effect on vascular endothelial cells, which could enhance the development of atherosclerosis. The role of resistin on insulin resistance was investigated in ESRD patients as well as the impact of a functional -180 C/G SNP on resistin plasma levels. We found that circulating levels of resistin were highly elevated in patients and that plasma levels correlated with glomerual filtrations rate. However, resistin did not seem to be associated with insulin resistance. Plasma resistin was associated with both adhesion molecules and inflammatory markers. Our findings suggest that resistin plasma levels should always be corrected for GFR in all study populations.

The recent development in the field of genetics has made it possible to understand the impact of genotype in disease development and progress. It seems conceivable that in near future, prognostic or predictive multigene DNA assays will provide the nephrological community with a more precise approach for the identification of "high risk" ESRD patients and the development of accurate individual treatment strategies.