Genetic polymorphism in angiotensinogen promotor region affects progression of IgA nephropathy

Abstract

The purpose of the present study was to evaluate the role of angiotensinogen (AGT) gene polymorphisms in the progression of IgA nephropathy (IgAN). The association of the haplotypes defined by A-20C and M235T, which correspond to G-6 A polymorphisms, with the severity of glomerular or interstitial lesions and renal prognosis were investigated. Patients with histologically proven IgAN were recruited after informed consent was obtained for the genetic study, which was approved by the ethics committee of our institute. Genomic DNA from each patient was prepared from peripheral blood leucocytes using an automatic DNA isolation system (NA-100, Kurabo, Osaka, Japan). A–C transition at nucleotide −20, G–A transition at −6, C–T transition at +68 and M235T variant at exon two in the AGT gene were determined as described previously.1,2 The renal survival rate was analysed for 114 IgAN patients with a creatinine clearance (Ccr) level of 70 mL/min or greater and followed up more than 24 months. The patients with haplotype T235 and C-20 (n = 63) were compared with those without T235 and C-20 (n = 51) for age, sex, blood pressure, proteinuria, serum Cr, Ccr at the time of renal biopsy, drugs administered, and severity of renal histopathologic lesion. Light microscopic evaluation of all specimens was performed in a double blind fashion according to the grading classification described previously, including glomerular cellular proliferation, mesangial matrix increase, global or segmental sclerosis, endocapillary proliferation, leucocyte exudation, duplication of glomerular basement membrane, crescent formation, and tufts adhesion to Bowman’s capsule as well as tubulointerstitial lesions.3 The Kaplan-Meier method and the Cox proportional hazards regression model analysed the time course from renal biopsy to end point (initiation of dialysis or sCr level doubled after the time of diagnosis). Several covariates were selected by a stepwise backward method and the effects of these covariates were expressed by a hazard ratio. P < 0.05 was considered statistically significant. The genotype distributions in the present study were not different from that in Hardy–Weinberg equilibrium. The genotype and allele frequencies of AGT variants were compatible with previous studies for Japanese population.1,4 Haplotype analysis showed a complete linkage disequilibrium between M235T and A-20C alleles (linkage-disequilibrium coefficient: D′, 1.00). Because the AGT gene variant at −20 was observed only in a subset of the 235T alleles, the following haplotypes were determined: T235 and C-20; T235 and A-20; and M235 and A-20. The incidence of hypertension was not different between each haplotype of AGT. An association study of histopathological findings revealed that patients with haplotype T235 and C-20 had significantly higher grading scores in crescent formation (P = 0.017) than those without haplotype T235 and C-20. Renal survival rate was significantly lower in the patients with haplotype T235 and C-20 (Fig. 1, P = 0.003). The Cox proportional hazards regression model showed an increased hazard ratio (HR) for haplotype T235 and C–20 of 5.9 from multivariate analysis (Fig. 2; 95% CI, 2.1–16.7; P < 0.001). Furthermore, in IgAN patients without administration of angiotensin I converting enzyme inhibitors (n = 77), renal survival rate was significantly lower in those with haplotype T235 and C–20 (P = 0.011). Figure 1. Effect of AGT haplotype on the renal survival rate in IgAN patients. Solid and dashed lines represent the renal survival rate in the patients with and without haplotype T235 and C–20, respectively. Download figure to PowerPoint Figure 2. Cox proportional hazards regression model to test the predictors for renal survival time in multivariate analysis. Covariates were selected by stepwise backward analysis. U-protein, urinary protein; HT at renal biopsy, hypertensives at the time of renal biopsy; ACEI, angiotensin I converting enzyme inhibitor. The bars represent the 95% confidence intervals of hazard ratio. Download figure to PowerPoint This work provides the evidence that the C–20 gene polymorphism of AGT, a subset of 235T alleles, is associated with histopathological severity of glomerular injury, and progression of renal dysfunction in Japanese patient with IgAN.