Distinct urinary lipid profile in children with focal segmental glomerulosclerosis

Abstract

Background: Focal segmental glomerulosclerosis (FSGS) accounts for the majority of new-onset end-stage renal disease (ESRD) during adolescence. FSGS treatment is a great challenge for pediatric nephrologists due to intertwined molecular pathways underlining its complex pathophysiology. There is emerging evidence showing that perturbed lipid metabolism plays a role in the pathophysiology of FSGS.

Methods: We postulate that the nephrotic milieu in FSGS differs from minimal change disease (MCD) and that urinary lipidomics can be used as a tool for early diagnosis of FSGS. We explored the urinary lipid profile of patients with FSGS and MCD using an unbiased metabolomics approach.

Results: We discovered a unique lipid signature characterized by increased concentration of fatty acid (FA) and lysophosphatidylcholines (LPC) and a decrease in urinary concentration of phosphatidylcholine (PC) in patients with FSGS. These findings indicate increased metabolism of membrane phospholipid PC by phospholipase A2 (PLA2), resulting in higher urinary concentrations of LPC and FA.

Conclusions: We propose that increased PC by-products can be used as a biomarker to diagnose FSGS and shed light on the mechanism of tubular and podocyte damage. Validation of identified urinary lipids as a biomarker in predicting the diagnosis and progression of FSGS in a larger patient population is warranted.

Keywords: Biomarker; Children; Focal segmental glomerulosclerosis; Metabolomics; Minimal change disease; Phospholipase A2; Urinary lipidomics.

Conflict of interest statement

Conflict of interest The authors declare there is no conflict of interest.

Figures

Fig. 1

Comparisons between focal segmental glomerulosclerosis (FSGS) and minimal-change disease (MCD) patient samples. Comparisons of all metabolites from urine of patients with FSGS (n=8) or MCD (n=10). The y axis is the negative log10 of p values (a higher value indicates greater significance), and the xaxis is the difference in signal intensity between two experimental groups as measured in log2 space. The volcano plot displays the relationship between fold change and significance between the two studied groups. Metabolites significantly differentiated are color coded, and important ones are labeled (p value<0.05, fold change>2).

Fig. 2

Representative putative biomarkers (four metabolites) of healthy control, focal segmental glomerulosclerosis (FSGS), and minimal-change disease (MCD) groups (a) and FSGS and MCD groups (b). Lipidomic analysis of urine samples displayed an increase in urinary fatty acids (FA) 16:00, FA 22:4, and LPC 18:1 and a decrease in urinary phosphatidylcholine (PC) 38:4 levels in FSGS

Fig. 3

Heat map displaying abundances in major lipid classes in patient groups and healthy controls. Hierarchical clustered Pearson correlations between samples and (sum) abundance of major lipid classes; 20 differentiated lipid metabolite features (a). Urinary acylcarnitine 12:0 level was significantly decreased in patients with focal segmental glomerulosclerosis (FSGS) (b). MCD minimal change disease, CE cholesterol ester, DG diacylglycerol, FA fatty acid, LPC lysophosphatidylcholines, PC phosphatidylcholine, SM sphingomyelin

Fig. 4

Urinary fatty acid (FA) and acylcarnitine levels in patients with focal segmental glomerulosclerosis (FSGS) with normal and low glomerular filtration rate (GFR). Urinary FA 16:0 was higher in patients with low GFR but did not reach statistical significance, most likely due to the small patient population (n=4 in each group). Urinary acylcarnitine C12 was lower in patients with low GFR (p<0.05).

Fig. 5

Proposed pathway for cellular damage caused by urinary milieu in focal segmental glomerulosclerosis (FSGS): Increased intracellular phospholipase A2 (PLA2) activity results in intracellular production of lysophosphatidylcholines (LPC) and fatty acids (FA). LPC and FA are released from damaged cells into the urinary space and can be used as a biomarker of specific pathway to tubular/podocyte injury in FSGS