Human von Willebrand Disease Mutations in Mouse von Willebrand Factor Alter Its Cleavage by ADAMTS13.

Von Willebrand Factor (VWF) is a large multimeric glycoprotein that mediates platelet adhesion to the damaged blood vessel wall and subsequent platelet aggregation at the site of vascular injury. The size of VWF multimers in plasma is regulated by the specific VWF cleaving protease, ADAMTS13, that cleaves VWF at the Y1605-M1606 bond in the VWF A2 domain. The adhesive properties of VWF is directly related to the multimer size, with loss of high molecular weight VWF leading to the bleeding phenotype in Type 2A von Willebrand disease (VWD) and ultra high molecular weight VWF multimers observed in the prothrombotic state of thrombotic thrombocytopenic purpura (TTP). VWF mutations leading to multimer changes have not been examined in an animal model. Although the human and mouse VWF genes are highly conserved, mutations in the two genes have not been compared experimentally. To that end, we have made mutations in the mouse VWF (mVWF) cDNA to compare the ADAMTS13 cleavage patterns seen with human VWF mutations in two in vitro assays. Recombinant multimerized full-length mVWF was digested with different concentrations of recombinant mouse ADAMTS13 (mADAMTS13) and 1.5M urea, and analyzed via multimer migration distance. The GST and histidine-tagged G1554-T1668 A2 domain region of mVWF (mVWF115) was assayed via ELISA as follows: the mVWF115 was bound to anti-GST coated plates, digested with mADAMTS13, and intact mVWF115 detected via HRP-labeled anti-histidine tag antibody. We examined R1597W (VWD Type 2A), R1306W, (VWD Type 2B), Y1584C (VWD Type 1), as well as two previously described changes that drastically lower VWF cleavage, Y1605A/M1606A, and D1614A/E1615A/K1617A. Y1584C showed a statistically insignificant 41% decrease in cleavage in the full-length assay, but a 20% increase in the mVWF115 assay. Conversely, R1597W had a 64% decrease in the full-length assay, but a 28% increase in the mVWF115 assay. Y1605A/M1606A showed a dramatic loss of cleavage, with no observable loss of high molecular weight multimers at 32 units/ml of mADAMTS13, and a 126-fold higher mADAMTS13 concentration than wild type in the mVWF115 assay. This contrasted with the 8-fold increase observed with D1614A/E1615A/K1617A. The R1306W A1 domain mutation showed a significant increase in cleavage, with 63% less ADAMTS13 necessary in the full-length assay. The triple mutant R1306W/Y1605A/M1606A did not cleave at the highest concentration, 32 U/ml, similar to that of Y1605A/M1606A, showing that the R1306W mutation was not able to increase cleavage of the Y1605A/M1606A change. These results show a strong correlation between the mouse mutations described here and previously reported human VWF mutations in their susceptibility to ADAMTS13 cleavage. These results provide a rationale for the development of mouse models of type 1, 2A, and 2B VWD, as well as mVWF mutations that might lead to a prothrombotic state similar to TTP in humans.