PATHOPHYSIOLOGY Protein carbamylation and uremic cardiomyopathy Anders Hayden Berg, Boston The mortality in patients with end stage renal disease (ESRD) is unacceptably high, 15–20 % per year. Most of these patients die from cardiac causes and are at especially high risk for heart failure and cardiac sudden death. Recent studies have suggested that one significant contributor to heart disease in patients with kidney disease is a urea-derived protein modification called carbamylation [1]. CONFERENCES O H 2 N NH 2 urea cyanate .. competitive carbamylation .. NH 2 NH 2 .. R Protein lysyl N-carbamylation NH 4 + N C OH O Free amino acid N/S-carbamylation Figure 1: Competitive carbamylation of proteins vs. free AA scavengers. OH Urea occasionally dissociates into a reactive electrophile called cyanate, which covalently bonds with primary amines (i.e. lysine side chain groups on the -amino groups of free amino acids (AAs) (Figure 1) [2]. Carbamylation of free AAs and proteins has been extensively studied in the context of kidney failure [3–17]. Studies suggest that carbamylation may cause functional depletion of AAs and exacerbate AA deficiencies that are common in kidney patients [18]. In addition to their effects on amino acids, carbamylation modifications may disrupt the normal function of proteins, interfering with ligand-receptor interactions or creating neo-ligands that lead to novel gain-of-function signaling [10, 19]. Intriguingly, experiments in atherosclerosis-prone ApoE-null mice found that adding physiologic amounts of pure urea to their diet resulted in doubling of the size of aortic atherosclerotic lesions compared to controls on normal diet, dramatically demonstrating the specific cardiovascular toxicity of urea, and that protein carbamylation has a direct role in accelerating atherogenesis in uremia [8]. The signaling mechanisms mediating urea’s myocardial damage, and the targets of carbamylation in cardiac myocytes require further investigation, however. Carbamylation and clinical monitoring of kidney disease Because carbamylated proteins represent a measure of time-averaged urea concentrations, there is growing interest in measurement of carbamylated proteins as clinical biomarkers in patients with kidney disease. Currently available clinical tests of uremia such as blood urea nitrogen and creatinine levels, glomerular filtration rate, and measures of hemodialysis dose sufficiency (equilibrated Kt/V, urea reduction rate) have not proven to be reliable predictors of outcomes. Protein carbamylation provides an index of time-averaged blood urea concentrations, analogous to the relationship between serum glucose and glycated hemoglobin (hemoglobin A1c for uremia) [20, 21]. Towards this end, our lab has developed high throughput methods for LC-MS/MS measurement of carbamy lated and non-carbamylated albumin at this specific site of modification [3]. Using this assay, we began studying the clinical significance of C-Alb in patients with kidney disease and its association with outcomes. In our first study, C-Alb values were measured at baseline in hemodialysis patients from the Accelerated Mortality in Renal Replacement (ArMORR) 26
PATHOPHYSIOLOGY study. We found that C-Alb values were significantly associated with 1-year risk of death (hazard ratio 3.76 [95 % CI: 2.20–6.43], p
