Platelet dysfunction in platelet-type von Willebrand disease due to the constitutive triggering of the Lyn-PECAM1 inhibitory pathway

Platelet-type von Willebrand disease (PT-VWD) is an inherited platelet disorder. It is characterized by macrothrombocytopenia and mucocutaneous bleeding, of variable severity, due to gain-of-function variants of GP1BA conferring to glycoprotein Ib alpha (GPIb alpha) enhanced affinity for von Willebrand factor (VWF). The bleeding tendency is conventionally attributed to thrombocytopenia and large VWF-multimer depletion. However, while some indications suggest that platelet dysfunction may contribute to the bleeding phenotype, no information on its characteristics and causes are available. The aim of the present study was to characterize platelet dysfunction in PT-VWD and shed light on its mechanism. Platelets from a PT-VWD patient carrying the p.M239V variant, and from PT-VWD mice carrying the p.G233V variant, showed a remarkable platelet function defect, with impaired aggregation, defective granule secretion and reduced adhesion under static and flow conditions. VWF-binding to GPIb alpha is known to trigger intracellular signaling involving Src-family kinases (SFK). We found that constitutive phosphorylation of the platelet SFK Lyn induces a negative-feedback loop downregulating platelet activation through phosphorylation of PECAM1 on Tyr686 and that this is triggered by the constitutive binding of VWF to GPIb alpha. These data show, for the first time, that the abnormal triggering of inhibitory signals mediated by Lyn and PECAM1 may lead to platelet dysfunction. In conclusion, our study unravels the mechanism of platelet dysfunction in PT-VWD caused by deranged inhibitory signaling. This is triggered by the constitutive binding of VWF to GPIb alpha which may significantly contribute to the bleeding phenotype of these patients.