Platelet-Type von Willebrand Disease: New Insights into the Molecular Pathophysiology of a Unique Platelet Defect

Compared with coagulation factor defects, little attention is given to defects of platelet function as causes of rare bleeding disorders. Platelet-type von Willebrand disease (PT-VWD) is an autosomal dominant bleeding disorder and is unique among platelet disorders because it is characterized by platelet hyperresponsiveness rather than decreased function. The disease is caused by gain-of-function mutations in the platelet GP1BA gene, which codes for the platelet von Willebrand factor (VWF) receptor, GPIb. Only five mutations (four missense and one deletion) have so far been reported. Affected patients suffer from mild to moderate mucocutaneous bleeding, low VWF activity compared with antigen, decreased high-molecular-weight VWF multimers, variable degree of thrombocytopenia and typically platelet aggregation in response to low concentrations of ristocetin. All reported PT-VWD missense mutations occur within the R-loop of GPIb and it was speculated that the introduction of short branched chain mutations such as Val in PT-VWD stabilized the extended -hairpin. Examination of this theory by surveying all the available GPIb structures showed that a distinct conformation predominates for the R-loop when GPIb is not bound to VWF-A1 and this provides the framework of a new hypothesis for the molecular basis of PT-VWD. Worldwide efforts to improve diagnosis of PT-VWD continue, and international systematic studies are required to further our understanding of the phenotype and the influence of the hyperresponsive GPIb beyond hemostasis.