A Quantitative Assay of In Vivo Cleavage of Von Willebrand Factor By ADAMTS13 Reveals Excessive Cleavage in Type 2B Von Willebrand Disease and Diminished Cleavage in Thrombotic Thrombocytopenic Purpura

Von Willebrand Factor (VWF) is a large glycoprotein that plays a crucial role in hemostasis by enabling platelets to adhere to sites of vascular injury, particularly under conditions of high shear stress. The multimeric structure of VWF is central to its activity; VWF is most active in its ultra-large multimer form (ULVWF). The size of plasma VWF is regulated by the plasma metalloprotease ADAMTS13, which cleaves VWF at the Tyr1605-Met1606 peptide bond within the A2 domain. Abnormal VWF cleavage has been implicated in a wide range of thrombotic and bleeding disorders, including thrombotic thrombocytopenic purpura (TTP) and Type 2 von Willebrand disease (VWD). These two diseases represent extreme phenotypes of VWF proteolysis-the former with too little cleavage and the latter with too much cleavage. ADAMTS13 proteolysis of VWF is currently assessed by either multimer gel analysis or ADAMTS13 activity assays. High-resolution agarose gels give an estimate of the extent to which a particular band represents cleaved vs native VWF. This method, however, is not quantitative and cannot distinguish whether a particular cleavage product is produced by ADAMTS13 or by another protease. Alternatively, ADAMTS13 activity assays are indirect measures of VWF cleavage, used especially to diagnose TTP. These assays measure the extent to which a subject's ADAMTS13 can cleave an exogenous substrate, usually a small peptide, and do not determine the extent of endogenous VWF cleavage. Therefore, we developed a mass spectrometry-based method to directly quantify cleavage of the Tyr1605-Met1606 peptide bond in plasma VWF. Here, we report quantification of VWF cleavage in plasma from normal donors, and TTP and VWD patients.

Methods: VWF was isolated from plasma by immunoprecipitation using polyclonal VWF antibody and then digested with trypsin. Tryptic peptides, including the ADAMTS13-cleaved peptide and a control peptide, were quantified by nano ultra-performance liquid chromatography tandem mass spectrometry (nanoLC-MS/MS) using heavy isotope-labelled peptides as internal standards. The extent of VWF cleavage by ADAMTS13 was determined as the concentration ratio of ADAMTS13-cleaved peptide to a control peptide representative of total VWF, and expressed as percent cleavage.

Results: Given that the plasma concentration of VWF is relatively low, we first optimized conditions for sample preparation and analysis. Using NanoLC-MS/MS with isotopic dilution, we quantified ADAMTS13 cleavage of VWF in plasma samples (10-100 µl) containing 100-500 ng of VWF. We tested the reproducibility of the measurement using pooled normal plasma. The quantification was highly reproducible, with a wide linear range. VWF cleavage in pooled normal plasma was 4.9% ± 0.3% (mean ± SD, n=6), indicating that one VWF monomer in 20 is cleaved, on average. Cleavage was much higher in plasma from type 2B VWD patients (n=5), ranging from 15-30% depending on mutations. In contrast, we observed only 2.1-3.2% (n=6) cleavage in plasma VWF from TTP patients.

Summary: This is the first report of quantification of ADAMTS13-mediated VWF cleavage in plasma. In normal plasma, approximately 5% of VWF A2 domains are cleaved, whereas cleavage was reduced to approximately 60% of normal on average in 6 TTP patients, and increased up to 6 times normal in patients with Type 2B VWD. In TTP, this level of proteolysis would be expected to result in accumulation of ULVWF multimers, as only between 1 in 50 to 1 in 30 monomers were cleaved. In type 2B VWD, by contrast, proteolysis may play a much larger role in the lack of larger multimers than previously appreciated, as up to 1 in 3 monomers were cleaved. This method should also prove useful in conditions such as severe malaria, in which it has been suggested that VWF cleavage is impaired, but this is not necessarily accompanied by low ADAMTS13 activity as measured with a small peptide substrate.