The CNDP1 (CTG)5 Polymorphism Is Associated with Biopsy-Proven Diabetic Nephropathy, Time on Hemodialysis, and Diabetes Duration

Abstract

Considering that the homozygous CNDP1 (CTG)₅ genotype affords protection against diabetic nephropathy (DN) in female patients with type 2 diabetes, this study assessed if this association remains gender-specific when applying clinical inclusion criteria (CIC-DN) or biopsy proof (BP-DN). Additionally, it assessed if the prevalence of the protective genotype changes with diabetes duration and time on hemodialysis and if this occurs in association with serum carnosinase (CN-1) activity. Whereas the distribution of the (CTG)₅ homozygous genotype in the no-DN and CIC-DN patients was comparable, a lower frequency was found in the BP-DN patients, particularly in females. We observed a significant trend towards high frequencies of the (CTG)₅ homozygous genotype with increased time on dialysis. This was also observed for diabetes duration but only reached significance when both (CTG)₅ homo- and heterozygous patients were included. CN-1 activity negatively correlated with time on hemodialysis and was lower in (CTG)₅ homozygous patients. The latter remained significant in female subjects after gender stratification. We confirm the association between the CNDP1 genotype and DN to be likely gender-specific. Although our data also suggest that (CTG)₅ homozygous patients may have a survival advantage on dialysis and in diabetes, this hypothesis needs to be confirmed in a prospective cohort study.

Figure 1

Flow diagram for patient recruitment and group allocation. DN: diabetic nephropathy, NDRD: nondiabetic renal disease, NPDR: nonproliferative diabetic retinopathy, CIC: clinical inclusion criteria, BP: biopsy-proven, DR: diabetic retinopathy, ESRD: end-stage renal disease.

Figure 2

CNDP1 (CTG)_n genotype distribution in T2DM patients. Genotype distribution is depicted as homozygosity for the (CTG)₅ allele (10 leucine) versus all other genotypes (≥11 leucine). Planned comparisons were carried out between T2DM patients without DN and with either CIC- or BP-defined nephropathy. (a) No significant difference in genotype distribution was observed between T2DM patients with DN and without DN when applying CIC. The frequency of patients homozygous for the (CTG)₅ allele decreased when BP-DN was considered. However, this difference did not hold after Bonferroni correction. ((b) and (c)) Gender stratification ((b) male patients, (c) female patients) showed no significant difference in the frequency of homozygosity for the CNDP1 (CTG)₅ allele between T2DM with and without DN when applying CIC. When DN was confirmed through biopsy, however, the frequency of CNDP1 (CTG)₅ homozygosity significantly decreased in female T2DM patients, which remained significant after Bonferroni adjustment.

Figure 3

The CNDP1 (CTG)₅ genotype distribution is changed with time on dialysis and diabetes duration. To assess if the frequencies change over time, the χ² test for trend (Cochran-Armitage test for trend) was carried out. (a) The frequency of the homozygous CNDP1 (CTG)₅ genotype (10 leucine) significantly increased with time on hemodialysis. ((b) and (c)) Although the observed frequency of the homozygous (b) CNDP1 (CTG)₅ genotype uniformly increased with diabetes duration, this trend did not reach statistical significance. Yet, if patients with a heterozygous (c) CNDP1 (CTG)₅ genotype (one 5 allele) were included as well, a significant trend towards high frequencies with increasing diabetes duration was found. N.S.: not significant.

Figure 4

CN-1 activities decrease with time on dialysis. (a) Serum CN-1 activity was assessed in 175 hemodialysis patients and plotted against the log-transformed duration since hemodialysis was initiated. A significant correlation between serum CN-1 activity and log-transformed hemodialysis duration was found in all patients. ((b) and (c)) After stratification in T2DM (b) and other causes of renal failure (c), the correlation remained significant. HD: hemodialysis.

Figure 5

CN-1 activity correlates with CNDP1 genotype in hemodialysis patients. (a) CN-1 activity in (CTG)₅ homozygous hemodialysis patients is significantly lower than that in patients carrying other genotypes. ((b) and (c)) After gender stratification differences in CN-1 activity between the (CTG)₅ homozygous, all other genotypes remained but only reached statistical significance in females (c).