A prothrombotic von Willebrand factor variant

In this issue of Blood, Schneppenheim et al report the first gain-of-function (GOF) variant located in the C4 domain of von Willebrand factor (VWF) associated with an increased risk of arterial thrombosis.¹  This study is accompanied by another study in this issue by Xu et al, who present the solution structure of this VWF C domain, providing important insights into the structure-function relationship of VWF.²  The combined impact of these 2 reports is unique in that they address molecular aspects, structural analysis, functional consequences, and thrombosis risk of a GOF VWF mutation.

VWF is a large multimeric glycoprotein that facilitates platelet adhesion and aggregation at the site of vascular injury. It plays an important role in platelet plug formation, the first step of the hemostatic mechanism. Reduced VWF antigen or activity levels are found in von Willebrand disease (VWD), the most common inherited bleeding disorder, resulting in mainly mucocutaneous bleeding.³  Most mutations in the VWF gene are associated with reduced VWF antigen levels and/or VWF activity leading to VWD. A few mutations in VWF are associated with a GOF, such as mutations in the A2 domain of VWF, resulting in increased platelet aggregation, as observed in type 2B VWD.³  A GOF VWF variant associated with increased platelet aggregation and increased risk of thrombosis has not previously been reported.

The dual role of VWF in hemostasis is well known: Low VWF levels are associated with bleeding, and high levels of VWF are associated with thrombosis, especially arterial thrombosis, including ischemic stroke and myocardial infarction (MI).⁴  It remains controversial whether the association of VWF antigen levels and thrombosis is causal. Many studies performed so far have been case-control studies and do not provide evidence on causality. However, large prospective epidemiological studies are hampered by the fact that other environmental factors, including endothelial dysfunction, increasing age, stress, and inflammation, increase VWF antigen levels, which also increase the risk of thrombotic events.⁵  The relationship between several single nucleotide polymorphisms and mutations in VWF gene, VWF levels, and MI and stroke has previously been investigated by many groups.⁶  Many of these studies only considered VWF antigen levels and did not take function of VWF into account. The only indirect evidence that VWF is causally related to thrombotic events is the finding that patients with VWD with VWF levels <30% of normal have a reduced risk of ischemic stroke and MI compared with the general population.⁷ 

Xu et al investigated the solution structure of the C4 domain of VWF, located at the C-terminus of VWF. They present the first structure of 6 type C domains of the VWF protein. This part is known to bind to the integrin α_IIbβ₃ on platelets and is important for VWF functionality. They showed that the substitution of Phe to Tyr at position 2561 is more likely to have an effect on the arrangement of the C4 domain with neighboring domains, rather than impairing platelet integrin binding. This reveals important insight in the structure-function of VWF.

The study by Schneppenheim et al provides evidence that a GOF mutant of VWF affecting the C4 domain, which contains the α_IIbβ₃ binding site, leads to enhanced platelet aggregation, and thereby to an increased risk of cardiovascular disease. In addition, they studied in greater detail the functional consequences of this variant. The variant c.7682T>A; rs 35335161 in exon 45 of the VWF gene is characterized by the substitution of Phe at position 2561 by a Tyr. This is a variant with a frequency in the general population of ∼10%. They studied the occurrence of this variant in the Ludwigshafen Risk and Cardiovascular Health Study, a case-control study including >1700 patients with coronary heart disease (CAD) or MI.⁸  In the total patient group, an increased CAD risk was not found in carriers of the Tyr2561, but in a subgroup of women <55 years, the variant was associated with a significant increased risk of MI. Tyr 2561 carriers also had an increased risk of a recurrent cardiovascular event. In a life table analysis, Tyr2561 carriers had a significant earlier onset of MI. Based on these epidemiological findings, the authors also studied the functionality and pathophysiology of this variant with state-of-the-art techniques using static VWF-platelet receptor binding assays and shear-based assays. Using recombinant Tyr 2561–VWF, they showed that the Tyr 2561 variant resulted in enhanced α_IIbβ₃ binding and platelet aggregation. Circular dichroism spectra from Tyr 2561 suggest an increased shear sensitivity of VWF.

This is the first study that reports a functional variant of VWF to be associated with an increased risk of arterial thrombosis. This is an exciting and novel observation further pointing toward an important role of VWF in arterial thrombosis. As the authors indicate themselves, these results need to be confirmed in larger epidemiological studies. So far, screening for this variant to identify individuals at high risk of cardiovascular disease is not justified, especially since the increased risk was only found in specific subgroups of patients.