Endotrophin, a collagen type VI-derived matrikine, reflects the degree of renal fibrosis in patients with IgA nephropathy and in patients with ANCA-associated vasculitis

Abstract

Background: Renal fibrosis is the hallmark of chronic kidney disease (CKD) and is characterized by an imbalanced extracellular matrix remodelling. Endotrophin (ETP) is a signalling molecule released from collagen type VI (COL VI). ETP can be measured by the PRO-C6 assay, which quantifies the levels of COL VI formation. ETP levels were previously associated with mortality and disease progression in patients with CKD. We hypothesized that serum and urinary ETP levels correlate with the degree of interstitial fibrosis in kidney biopsies from patients with immunoglobulin A nephropathy (IgAN) and patients with anti-neutrophil cytoplasmic antibody-associated vasculitis (AAV).

Methods: We examined a cohort of 49 IgAN and 47 AAV patients. A validation cohort of 85 IgAN patients was included. ETP was measured in serum (S-ETP) and urine (U-ETP/Cr) samples, taken on the same day before renal biopsy was performed, using the enzyme-linked immunosorbent assay PRO-C6. The biopsies were evaluated for interstitial fibrosis and tubular atrophy according to the Banff and MEST-C scores.

Results: S-ETP and U-ETP/Cr levels correlated with kidney function, increased CKD severity, correlated with the extent of interstitial fibrosis and gradually increased with increasing degree of interstitial fibrosis and tubular atrophy. ETP outperformed the known fibrosis biomarker Dickkopf-3 for discrimination of patients with high fibrotic burden. The association of S-ETP and U-ETP/Cr with the level of kidney fibrosis was confirmed in the validation cohort.

Conclusions: We demonstrated that high levels of circulating and excreted ETP are not only indicative of lower kidney function, but also reflect the burden of fibrosis in the kidneys.

Keywords: ANCA-associated vasculitis; IgA nephropathy; biomarkers; chronic kidney disease; interstitial fibrosis.

Graphical Abstract

FIGURE 1

ETP and DKK-3 levels according to CKD stages, interstitial fibrosis and tubular atrophy, and glomerular morphology classes. Serum and urinary levels of ETP (A) and DKK-3 (B) in IgAN and AAV patients according to CKD stages (CKD1–5). Serum and urinary levels of ETP (C) and DKK-3 (D) in IgAN and AAV patients according to Banff score (ci0–ci3). Serum and urinary levels of ETP (E) and DKK-3 (F) in IgAN patients according to T-score (T0–T2). Serum and urinary levels of ETP (G) and DKK-3 (H) in AAV patients divided into different glomerular morphology classes (focal, mixed, crescentic or sclerotic). Data are presented on a log₁₀ scale as median with IQR and statistical differences were assessed by Kruskal–Wallis test; *P < 0.05, **P < 0.01, ***P < 0.001, ****P < 0.0001.

FIGURE 2

Spearman's rank correlations of serum and urinary levels of ETP with fibrosis levels in the combined IgAN and AAV discovery cohort. ETP levels are presented on a log₁₀ scale.

FIGURE 3

ETP levels in the validation cohort according to CKD stages (A), Banff scores (B) and Spearman's rank correlations with fibrosis levels (C). (A and B) Data are presented on a log₁₀ scale as median with IQR and statistical differences were assessed by Kruskal–Wallis test; *P < 0.05, ***P < 0.001, ****P < 0.0001. (C) ETP levels are presented on a log₁₀ scale.

FIGURE 4

Baseline ETP levels in patients with AAV divided into groups based on changes in CKD stage from baseline to the 3-year follow-up. Serum (A) and urinary (B) ETP levels. Data are presented on a linear (A) and log₁₀ scale (B) as median with IQR and statistical differences were assessed by Kruskal–Wallis test; *P < 0.05.