Regular Article THROMBOSIS AND HEMOSTASIS Analysis of the role of von Willebrand factor, platelet glycoprotein VI-, and a2b1-mediated collagen binding in thrombus formation

von Willebrand factor (VWF) plays an important role in coagulation as a carrier protein for factor VIII (FVIII) and in primary hemostasis with binding of VWF to exposed subendothelial collagens. The most hemostatically important forms of collagen include the fibrillar collagens I and III, and the microfibrillar collagen VI. VWF binds to collagen I and III via theVWFA1andA3domainswhereas only the A1 domain binds collagen VI. Although the relative hemostatic importance of the A3 and A1 collagen-binding domains is controversial, they likely have a complementary role and blood flowmediated shear stress is important for the conformation and function of both. Once immobilized by collagen, the VWF platelet glycoprotein Iba (GPIba)–binding site within the A1 domain is exposed and the high affinity, rapid, and reversible interaction between VWF and GPIba tethers platelets to the endothelium where they roll until they are immobilized by direct platelet-collagen binding which has slower bindingkinetics. This ismediated by2 collagen receptors onplatelets, GPVI and the integrin a2b1 (or GPIa/IIa), which lead to irreversible adhesion and activation of platelets. von Willebrand disease (VWD) is a highly heterogeneous mucocutaneous bleeding disorder marked by either quantitative or qualitative pathologies of VWF. The qualitative disorders representing type 2 VWD are further classified into variants that affect VWF-platelet interactions (types 2A, 2B, and 2M VWD) or affect VWF-FVIII binding (2N). The diagnostic algorithm for the evaluation of VWD recommended by the National Heart, Lung, and Blood Institute (NHLBI) VWD guidelines includes the measurement of VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo, a measure of VWF-GpIba activity), and factor VIII coagulant activity (FVIII:C). The evaluation of collagen binding is infrequently incorporated into diagnostic testing and the omission of this assay will miss cases where rare mutations affect only the VWF collagen-binding function (also classified as type 2M VWD). Several enzyme-linked immunosorbent assay (ELISA)–based VWF collagen-binding assays (VWF:CB) which use type I, type III, or a mixture of the 2 collagens, are commercially available and have been previously evaluated and compared. Because the VWF:CB assay is sensitive to the loss of high-molecular-weight multimer (HMWM) VWF in addition to detecting a defect of collagen binding, studies have focused on optimizing this assay to discriminate between deficiencies of VWF (type 1 VWD) from dysfunction (type 2A/B VWD), potentially replacing the need for VWF:RCo or multimer analysis. On the other hand, an abnormal CB:Ag ratio of ,0.7, with a normal multimer profile indicates