Uremic Metabolites Are Associated with Albuminuria and Estimated Glomerular Filtration Rate (eGFR) in Sickle Cell Disease

Introduction: Chronic kidney disease (CKD) is common in patients with sickle cell disease (SCD) and is associated with increased morbidity and mortality. As albuminuria and changes in estimated glomerular filtration rate (eGFR) occur well after substantial functional and structural damage have occurred, identification of novel biomarkers which could predict the development of CKD is necessary. Previous studies have shown an increase in the level of the uremic metabolite indoxyl-sulfate (IS) in urine in patients with SCD. IS and other uremic metabolites are cleared by active secretion in kidney tubule cells and their accumulation in these cells leads to increased oxidative stress, and apoptotic and necrotic cell death. In this single center, cross-sectional study, we investigated the associations between both urinary levels of uremic toxins and biomarkers of kidney tubular injury with persistent albuminuria and eGFR in patients with SCD.

Methods: The participants represent a subset of patients with HbSS or HbSβ⁰ thalassemia enrolled in a prospective cohort study to assess the natural history of kidney disease in SCD. We excluded patients with diabetic nephropathy, cancer, connective tissue disease, other glomerular diseases, known infection with hepatitis (B or C) or HIV, end-stage kidney disease on dialysis and who had undergone bone marrow transplantation. Participants were enrolled during routine clinic visits. Urine levels of uremic toxins, indoxyl sulfate (IS) and trimethylamine-n-oxide (TMAO), and tubular injury biomarkers, monocyte chemoattractant protein-1 (MCP-1; also called CC-chemokine ligand 2 [CCL2]) and neutrophil gelatinase-associated lipocalin (NGAL) were measured using quantitative ELISA assay. All values obtained for urine biomarkers were normalized to urine creatinine. Persistent albuminuria was defined based on at least 2 of 3 spot urine albumin/creatinine ratio (ACR) values ≥30 mg/g of creatinine or a single value ≥100 mg/g. Glomerular filtration rate was estimated using the 2009 creatinine-based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula. As the distribution of the biomarkers were skewed, the data were log-transformed. Associations between both ACR and eGFR with each of the urine biomarkers were assessed using linear regression.

Results: One hundred and sixteen subjects, 26 (22.4%; ACR - median 276.5 mg/g [IQR: 156, 802]) with persistent albuminuria (PA), and 90 (77.6%; ACR - median 13 mg/g [IQR: 6.5, 25.5]) without persistent albuminuria (non-PA) were evaluated. Subjects with PA appeared to be older and had lower hemoglobin and fetal hemoglobin levels, but higher reticulocyte counts, lactate dehydrogenase levels and systolic and diastolic blood pressures than subjects with non-PA (Table 1). Log-transformed urine levels of IS (β=0.52, 95% confidence interval [CI]: 0.08, 0.96, p=0.02), MCP-1 (β=0.65, 95% CI: 0.29, 1.01, p=0.0006) and NGAL (β=0.25, 95% CI: 0.02, 0.49, p=0.03) were positively associated with log-transformed ACR (Table 2). Log-transformed urine levels of IS (β=-8.0, 95% CI: -14.7, -1.3, p=0.02), MCP-1 (β=-11.6, 95% CI: -16.9, -6.3, p<0.0001) and NGAL (β=-6.0, 95% CI: -9.4, -2.6, p=0.0008) were inversely associated with eGFR. Log-transformed urine level of TMAO was not associated with either ACR or eGFR.

Conclusion: This study demonstrates a novel association of IS with albuminuria and eGFR in patients with SCD. In addition, we confirm the association of biomarkers of tubular injury, MCP-1 and NGAL, with persistent albuminuria and eGFR in SCD. Future studies will determine whether uremic metabolites may predict the development of albuminuria in patients with SCD.

Ataga:Biomarin: Consultancy; Roche: Consultancy; Agios Pharmaceuticals: Consultancy; Forma Therapeutics: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Consultancy.