von Willebrand factor (vWF) is a multimeric plasma
glycoprotein that mediates platelet adhesion to the subendothelium via binding to
platelet glycoprotein Ib (GPIb) and to components of the vessel wall. Recently, missense mutations that cause
type IIB von Willebrand disease (vWD) were described, clustered within a
disulfide loop in the A1 domain of vWF that has binding sites for GPIb,
collagen, and
heparin. In type IIB vWD, plasma vWF exhibits increased affinity for platelet GPIb, but decreased binding to
collagen and
heparin. The effect was studied of a type IIB vWD mutation, Arg578-->Gln, on the interaction of vWF with GPIb,
collagen, and
heparin. Recombinant wild type rvWF and mutant rvWF(R578Q) were expressed in COS-7 cells.
Ristocetin-induced binding of rvWF(R578Q) to GPIb was markedly increased compared with rvWF, confirming that the Arg578-->Gln mutation causes the characteristic gain-of-function abnormality of type IIB vWD;
botrocetin-induced binding was only slightly increased. Binding to
collagen type III and
heparin-agarose was compared for rvWF(R578Q) and plasma vWF from patients with four different type IIB mutations: Arg543-->Trp, Arg545-->Cys, Val553-->Met, Arg578-->Gln. For all of the plasma samples, binding to
collagen and to
heparin was reduced compared with normal plasma. In contrast, binding of rvWF(R578Q) to
collagen and
heparin was normal compared with wild type rvWF. Therefore, the Arg578-->Gln mutation increases the affinity of vWF for GPIb but does not directly impair vWF interaction with
collagen or
heparin. Arg578 may therefore be necessary to prevent normal vWF from interacting with GPIb. In type IIB vWD, the defective binding of plasma vWF to
collagen and
heparin may be secondary to post-synthetic modifications that occur in vivo, such as the loss of high molecular weight vWF multimers.