Augusto B. Federici

Platelet aggregation, which contributes to bleeding arrest and also to thrombovascular disorders, is thought to initiate after signaling-induced activation. We found that this paradigm does not apply under blood flow conditions comparable to those existing in stenotic coronary arteries. Platelets interacting with immobilized von Willebrand factor (VWF) aggregate independently of activation when soluble VWF is present and the shear rate exceeds 10 000 s(-1) (shear stress = 400 dyn/cm(2)). Above this threshold, active A1 domains become exposed in soluble VWF multimers and can bind to glycoprotein Ibalpha, promoting additional platelet recruitment. Aggregates thus formed are unstable until the shear rate approaches 20 000 s(-1) (shear stress = 800 dyn/cm.(2)). Above this threshold, adherent platelets at the interface of surface-immobilized and membrane-bound VWF are stretched into elongated structures and become the core of aggregates that can persist on the surface for minutes. When isolated dimeric A1 domain is present instead of native VWF multimers, activation-independent platelet aggregation occurs without requiring shear stress above a threshold level, but aggregates never become firmly attached to the surface and progressively disaggregate as shear rate exceeds 6000 s(-1). Platelet and VWF modulation by hydrodynamic force is a mechanism for activation-independent aggregation that may support thrombotic arterial occlusion.

Type 1 von Willebrand disease (VWD) is characterized by a personal and family history of bleeding coincident with reduced levels of normal plasma von Willebrand factor (VWF). The molecular basis of the disorder is poorly understood. The aims of this study were to determine phenotype and genotype and their relationship in patients historically diagnosed with type 1 VWD. Families were recruited in 9 European countries based on previous type 1 VWD diagnosis. Bleeding symptoms were recorded, plasma phenotype analyzed, and VWF mutation analysis performed in all index cases (ICs). Phenotypic and molecular analysis stratified patients into those with or without phenotypes suggestive of qualitative VWF defects (abnormal multimers) and with or without mutations. A total of 105 of 150 ICs (70%) had mutations identified. A subgroup with abnormal multimers (38% of ICs, 57 of 150) showed a high prevalence of VWF gene mutations (95% of ICs, 54 of 57), whereas in those with qualitatively normal VWF, fewer mutations were identified (55% of ICs, 51 of 93). About one third of the type 1 VWD cases recruited could be reconsidered as type 2. The remaining group could be considered "true" type 1 VWD, although mutations were found in only 55%.

The acquired von Willebrand syndrome (AVWS) is a bleeding disorder that is frequently unrecognized or is misdiagnosed as von Willebrand disease. AVWS is characterized by structural or functional defects of von Willebrand factor (VWF) that are secondary to autoimmune, lymphoproliferative or myeloproliferative, malignant, cardiovascular, or other disorders. VWF abnormalities in these disorders can result from (1) antibody-mediated clearance or functional interference, (2) adsorption to surfaces of transformed cells or platelets, or (3) increased shear stress and subsequent proteolysis. Diagnosis can be challenging as no single test is usually sufficient to prove or exclude AVWS. Furthermore, there are no evidence-based guidelines for management. Treatments of the underlying medical condition, including chemo/radiotherapy, surgery, or immunosuppressants can result in remission of AVWS, but is not always feasible and successful. Because of the heterogeneous mechanisms of AVWS, more than one therapeutic approach is often required to treat acute bleeds and for prophylaxis during invasive procedures; the treatment options include, but are not limited to, desmopressin, VWF-containing concentrates, intravenous immunoglobulin, plasmapheresis or recombinant factor VIIa. Here, we review the management of AVWS with an overview on the currently available evidence and additional considerations for typical treatment situations.

Type 2B von Willebrand disease (VWD2B) is caused by an abnormal von Willebrand factor (VWF) with increased affinity for the platelet receptor glycoprotein Ib-alpha (GPIb-alpha) that may result in moderate to severe thrombocytopenia. We evaluated the prevalence and clinical and molecular predictors of thrombocytopenia in a cohort of 67 VWD2B patients from 38 unrelated families characterized by VWF mutations. Platelet count, mean platelet volume, and morphologic evaluations of blood smear were obtained at baseline and during physiologic (pregnancy) or pathologic (infections, surgeries) stress conditions. Thrombocytopenia was found in 20 patients (30%) at baseline and in 38 (57%) after stress conditions, whereas platelet counts were always normal in 16 patients (24%) from 5 families carrying the P1266L/Q or R1308L mutations. VWF in its GPIb-alpha-binding conformation (VWF-GPIb-alpha/BC) was higher than normal in all except the 16 cases without thrombocytopenia (values up to 6-fold higher than controls). The risk of bleeding was higher in patients with thrombocytopenia (adjusted hazard ratio = 4.57; 95% confidence interval, 1.17-17.90) and in those with the highest tertile of bleeding severity score (5.66; 95% confidence interval, 1.03-31.07). Prediction of possible thrombocytopenia in VWD2B by measuring VWF-GPIb-alpha/BC is important because a low platelet count is an independent risk factor for bleeding.