Type 2B von Willebrand disease associated with the release of platelet agglutinates from megakaryocytes in the bone marrow

2B von Willebrand disease diagnosis: Considerations reflecting on 2021 multisociety guidelines

The recent American Society of Hematology/ISTH/National Hemophilia Foundation/World Federation of Hemophilia 2021 guidelines on the diagnosis of von Willebrand disease (VWD) is an outstanding effort to unify the diagnosis of VWD. However, as mentioned in the guidelines, there are limitations due to the low certainty in the evidence identified for most questions. The panel encouraged critical review of the guidelines. Compared to other subtypes, there is considerable complexity with diagnosis of type 2B VWD, a type that results from a gain-of-function mutation in the VWF gene. Additionally, the discrimination from its phenocopy platelet-type VWD, representing a gain-of-function mutation in the GP1BA gene, is crucial as this determines treatment decisions. In this forum, we highlight the complexities of a type 2B VWD diagnosis; discuss important issues with respect to these complexities: genotype/phenotype/clinical correlations, challenges with platelet aggregation and ristocetin-induced platelet agglutination testing, platelet count, and thrombocytopathy; and, finally, suggest the consideration of some of these complexities in future iterations of the VWD guidelines.

Abnormal VWF modifies megakaryocytopoiesis: studies of platelets and megakaryocyte cultures from patients with von Willebrand disease type 2B

von Willebrand factor (VWF) is an essential mediator of platelet adhesion to the vessel wall, but little is known about its role in megakaryocytopoiesis. VWF and its platelet receptor, glycoprotein Ibalpha (GPIbalpha), are both expressed during megakaryocyte (MK) maturation. This study was designed to evaluate whether the enhanced VWF-GPIbalpha interactions typical of patients with von Willebrand disease type 2B (VWD2B) modify platelet production. Platelets from 9 patients with VWD2B with 7 different gain-of-function mutations were examined by electron microscopy (EM) and immunofluorescence labeling. For the patients with VWD2B, EM characteristically showed variable numbers of structurally abnormal giant platelets, sometimes in agglutinates. Cultures of MKs from controls performed with or without purified VWF confirmed a positive influence of VWF on platelet production with specific inhibition by an antibody blocking VWF binding to GPIbalpha. VWD2B MK cultures examined by EM showed a disorganized demarcation membrane system and abnormal granule distribution. They produced platelets with structural abnormalities typical of VWD2B. Confocal examination of MK revealed limited extension of pseudopods with few large proplatelets. These results confirm that megakaryocytopoiesis is modified by the enhanced VWF-GPIbalpha interactions. These data obtained for controls and patients with VWD2B suggest a novel regulatory role of VWF-GPIbalpha interactions in platelet production.

Altered megakaryocytopoiesis in von Willebrand type 2B disease

Type 2B von Willebrand disease (VWD2B) is caused by gain-of-function amino acid substitutions in the von Willebrand factor (VWF) A1 domain. These allow facilitated binding of mutated VWF to platelet GPIbalpha with prolonged lifetimes of VWF bonds and enhanced ADAMTS-13 cleavage of large VWF multimers. A bleeding rather than prothrombotic syndrome is due to: (i) decreased large VWF multimers in plasma; (ii) limited thrombus formation; and (iii) thrombocytopenia affecting some but not all patients. Accumulating evidence points to an altered megakaryocytopoiesis in VWD2B with the production of enlarged or giant platelets showing an abnormal ultrastructure and, in a cohort of patients, the presence of circulating platelet agglutinates. In fact, evidence from in vitro cultures and marrow aspirates suggests that the upregulated VWF function can lead to abnormal VWF trafficking in megakaryocytes, a modified platelet production with interacting proplatelets, and the presence or even release of platelet agglutinates in the bone marrow.