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Platelet Biology & its Disorders
Mechanisms of thrombocytopenia in platelet-type von Willebrand disease
Ferrata Storti Foundation
Haematologica 2019 Volume 104(7):1473-1481
Loredana Bury,1 Alessandro Malara,2,3 Stefania Momi,1 Eleonora Petito,1 Alessandra Balduini2,3 and Paolo Gresele1
1Department of Medicine, Section of Internal and Cardiovascular Medicine, University of Perugia; 2Department of Molecular Medicine, University of Pavia and 3Biotechnology Research Laboratories, IRCCS San Matteo Foundation, Pavia, Italy
ABSTRACT
Platelet-type von Willebrand disease is an inherited platelet disorder characterized by thrombocytopenia with large platelets caused by gain-of-function variants in GP1BA leading to enhanced GPIba-von Willebrand factor (vWF) interaction. GPIba and vWF play a role in megakaryocytopoiesis, thus we aimed to investigate megakaryocyte differ- entiation and proplatelet-formation in platelet-type von Willebrand disease using megakaryocytes from a patient carrying the Met239Val variant and from mice carrying the Gly233Val variant. Platelet-type von Willebrand dis- ease megakaryocytes bound vWF at an early differentiation stage and gen- erated proplatelets with a decreased number of enlarged tips compared to control megakaryocytes. Moreover, they formed proplatelets upon contact with collagen, differently from normal megakaryocytes. Similarly, collagen triggered megakaryocytes showed defective activation of the RhoA-MLC2 axis, which prevents proplatelet formation, and increased phosphorylation of Lyn, which acts as a negative regulator of GPVI signaling, thus preventing ectopic proplatelet-formation on collagen. Consistently, human and murine bone marrow contained an increased number of extravascular platelets compared to controls. In addition, platelet survival of mutant mice was shortened compared to control mice, and the administration of desmo- pressin, raising circulating vWF, caused a marked drop in platelet count. Taken together, these results show for the first time that thrombocytopenia in platelet-type von Willebrand disease is due to the combination of differ- ent pathogenic mechanisms, i.e. the formation of a reduced number of platelets by megakaryocytes, the ectopic release of platelets in the bone marrow, and the increased clearance of platelet/vWF complexes.
Introduction
Platelet-type von Willebrand disease (PT-vWD) is an autosomal dominant inher- ited bleeding disorder caused by gain-of-function variants of GP1BA conferring enhanced affinity for von Willebrand factor (vWF) to platelet integrin GPIba.1 This disorder is characterized by a mild to moderate bleeding phenotype2 associated with fluctuating thrombocytopenia, which is conventionally explained by the for- mation of vWF-platelet complexes that are rapidly cleared from the circulation.1 Findings in support of this interpretation are the presence of circulating vWF- platelet complexes in a murine model of PT-vWD3 and the increased platelet clear- ance of mice with type 2B-vWD, a condition in which mutated vWF shows enhanced affinity for GPIba.4 No studies, however, have shown enhanced platelet clearance in either patients or mice with PT-vWD.
On the other hand, GPIba is an important regulator of megakaryocytopoiesis, as shown by defective proplatelet formation (PPF) by megakaryocytes incubated with GPIba-blocking antibodies5 and by megakaryocytes from Bernard-Soulier syn- drome patients.6,7 Indeed, GPIba regulates the polarization of the megakaryocyte demarcation membrane system and transendothelial platelet biogenesis through intracellular signals that involve the small GTPases Cdc42/RhoA.8
von Willebrand factor also plays a role in megakaryocytopoiesis, by accelerating and boosting PPF9 and by increasing platelet production upon megakaryocyte expo-
Correspondence:
PAOLO GRESELE
paolo.gresele@unipg.it
Received: June 22, 2018. Accepted: January 11, 2019. Pre-published: January 17, 2019.
doi:10.3324/haematol.2018.200378
Check the online version for the most updated information on this article, online supplements, and information on authorship & disclosures: www.haematologica.org/content/104/7/1473
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