PLATELET ACTIVATION AND AGGREGATION INDUCED BY RECOMBINANT VON-WILLEBRAND-FACTORS REPRODUCING 4 TYPE 2B VON-WILLEBRAND-DISEASE MISSENSE MUTATIONS

Type 2B of von Willebrand disease (vWD) refers to qualitative variants with increased affinity of von Willebrand factor (vWF) for platelet g lycoprotein Ib (GPIb). All the mutations responsible for type 2B vWD h ave been located in the Al domain of vWF. In this study various recomb inant von Willebrand factors (rvWF) reproducing four type 2B vWD misse nse mutations were compared to wild-type rvWF (WT-rvWF) for their spon taneous binding to platelets and their capacity to induce platelet act ivation and aggregation. Our data show that the multimeric pattern of each mutated rvWF is similar to that of WT-rvWF but the extent of spon taneous binding and the capacity to induce platelet activation and agg regation are more important for the R543Q and V553M mutations than for the L697V and A698V mutations. Both the binding of mutated rvWFs to p latelets and platelet aggregation induced by type 2B rvWFs are inhibit ed by monoclonal anti-GPIb and anti-vWF antibodies, inhibitors of vWF binding to platelets in the presence of ristocetin, as well as by auri n tricarboxylic acid. On the other hand, EDTA and a monoclonal antibod y directed against GPIIb/IIIa only inhibit platelet aggregation. Furth ermore, the incubation of type 2B rvWFs with platelets, under stirring conditions, results in the decrease in high molecular weight vWF mult imers in solution, the extent of which appears correlated with that of plasma vWF from type 2B vWD patients harboring the corresponding miss ense mutation. This study supports that the binding of different mutat ed type 2B vWFs onto platelet GPIb induces various degrees of platelet activation and aggregation and thus suggests that the phenotypic hete rogeneity of type 2B vWD may be related to the nature and/or location of the causative point mutation.