Variant Type 2B von Willebrand Disease; No Responsible Mutations atthe von Willebrand Factor A1 Domain and the Surrounding Region

We encountered a patient with variant of type 2B von Willebrand disease (vWD) who showed increased ristocetin-induced platelet aggregation, persistent thrombocytopenia, spontaneous platelet aggregation and normal vWF multimeric pattern in his plasma. However, we could not find any responsible mutations in or around the 505-695 disulfide loop (A1 loop) of the von Willebrand factor (vWF), at which all type 2B vWD candidate mutations have been reported to be clustered. This suggests that an unidentified vWF functional domain facilitating binding to platelet GPIb may exist at a site other than the A1 loop and the surrounding region.