EFFECT OF TYPE IIB VON-WILLEBRAND DISEASE MUTATION ARG(545)CYS ON PLATELET GLYCOPROTEIN IB BINDING-STUDIES WITH RECOMBINANT VON-WILLEBRAND-FACTOR

Type IIB von Willebrand disease (vWD) is characterized by a selective loss of high molecular weight von Willebrand factor (vWF) multimers in plasma due to their abnormally enhanced reactivity with platelets. Se veral missense mutations in the platelet glycoprotein Ib (GPIb) bindin g domain of vWF were recently characterized that cause type IIB vWD. T he effect of type IIB mutation Arg(545)Cys on VWF binding to platelet GPIb was studied using recombinant wild type (rvWFWT) and mutant rvWFR 545C expressed in COS-7 cells. In the absence of ristocetin, 50% of rv WFR545C bound spontaneously to platelet GPIb and the binding increased to 70% in the presence of 0.2 mg/ml ristocetin; rvWFWT did not bind s ignificantly under either condition. Botrocetin-induced binding of rvW FR545C was only slightly increased compared to rvWFWT. These data demo nstrate that the Arg(545)Cys mutation increases the affinity of vWF fo r GPIb, resulting in the characteristic gain-of-function type IIB vWD phenotype.