Urinary collagen-derived peptides as sensitive markers for bone resorption and bisphosphonate treatment in kidney transplant patients

Kidney transplant recipients (KTR) are at increased risk of fractures. Total urinary hydroxyproline excretion used to be a marker for bone resorption (BR) but faded into the background when more specific markers like Beta-CrossLaps (CTX) became available. Proteomic studies identified numerous hydroxyproline-containing urinary collagen peptides but their origin remains unknown. We followed the hypothesis that some of the urinary collagen peptides are associated with BR and are markers for pathophysiological changes in bone metabolism of KTR. Clinical and laboratory data including serum levels of CTX in 96 KTR from two French centers (Strasbourg, n=38; Paris-Necker, n=58) were correlated with the signal intensity of urinary peptides identified by capillary electrophoresis (CE) coupled to mass spectrometry (MS) and tandem-MS. The effect of oral bisphosphonates on urinary peptides was studied in an independent group of 11 KTR. Eighty-two urinary peptides were identified to be significantly correlated with serum CTX levels in both cohorts. Statistical association with parameters other than BR markers were not significant. Collagen alpha-1(I) chain (COL1A1) was the most frequently identified peptide source. COL1A1 peptides associated with BR were significantly more hydroxylated than those showing no association (55.9% versus 45,2%, p<0.0003 by a chi2-test). From the 82 urinary peptides correlated to CTX, 17 were significantly associated with bisphosphonate treatment. All of these 17 peptides showed a marked reduction in their excretion levels after 410 +- 344 days of bisphosphonate treatment compared to baseline levels. We studied the cleavage sites of these COL1A1 peptides and observed a signature of Cathepsin K and Matrix Metallopeptidase 9. This study provides strong evidence for the occurrence of collagen peptides in the urine of KTR that are associated with BR and that are sensitive to bisphosphonate treatment. Their assessment might become a valuable tool to monitor bone status in KTR.