Protein carbamylation predicts mortality in ESRD

Abstract

Traditional risk factors fail to explain the increased risk for cardiovascular morbidity and mortality in ESRD. Cyanate, a reactive electrophilic species in equilibrium with urea, posttranslationally modifies proteins through a process called carbamylation, which promotes atherosclerosis. The plasma level of protein-bound homocitrulline (PBHCit), which results from carbamylation, predicts major adverse cardiac events in patients with normal renal function, but whether this relationship is similar in ESRD is unknown. We quantified serum PBHCit in a cohort of 347 patients undergoing maintenance hemodialysis with 5 years of follow-up. Kaplan-Meier analyses revealed a significant association between elevated PBHCit and death (log-rank P<0.01). After adjustment for patient characteristics, laboratory values, and comorbid conditions, the risk for death among patients with PBHCit values in the highest tertile was more than double the risk among patients with values in the middle tertile (adjusted hazard ratio [HR], 2.4; 95% confidence interval [CI], 1.5-3.9) or the lowest tertile (adjusted HR, 2.3; 95% CI, 1.5-3.7). Including PBHCit significantly improved the multivariable model, with a net reclassification index of 14% (P<0.01). In summary, serum PBHCit, a footprint of protein carbamylation, predicts increased cardiovascular risk in patients with ESRD, supporting a mechanistic link among uremia, inflammation, and atherosclerosis.

Figures

Figure 1.

Scheme of protein carbamylation. Thiocyanate (SCN−) is a naturally occurring pseudohalide found in dietary sources. MPO uses SCN− as cosubstrate with hydrogen peroxide (H2O2) to form cyanate. Urea is elevated in patients with kidney dysfunction and is in equilibrium with cyanate and isocyanate. Carbamylation of nucleophilic amino groups (lysine, for example) modifies protein structure and ultimately causes dysfunction.

Figure 2.

PBHCit concentrations in healthy persons with normal renal function (n=90) and patients undergoing MHD (n=347). Shown are box whisker plots for each group. The box represents the interquartile range, the line within the box is the median level, and the whiskers represent a length equal to 1.5 times the interquartile range (the difference between the 25th and 75th percentiles) or the lowest value per the method of Tukey. The P value represents the Wilcoxon rank sum comparison between groups.

Figure 3.

Kaplan-Meier survival analysis of tertiles of PBHCit in patients with ESRD receiving MHD during a 5-year period. PBHCit tertiles 1, 2, and 3 consisted of 116, 116, and 115 patients, respectively. Tertile cut points (mmol PBHCit/mol Lys): T1, 0.2165.