Background of von Willebrand factor and von Willebrand disease
Definition of von Willebrand factor and von Willebrand disease
Von Willebrand Factor (VWF) is a large multimeric protein with two main functions in hemostasis. It is responsible for the flow-dependent tethering of platelets to the vascular injury sites, specifically adhesion to subendothelial collagen and bridging to other platelets via aggregation to secure platelet plug formation and primary hemostasis. Furthermore, VWF is a carrier protein for coagulation factor VIII (FVIII), which is thereby protected from degradation in plasma. VWF can also interact with fibrinogen and fibrin and can contribute to the clot formation process under certain situations [1].
Von Willebrand disease (VWD) is a bleeding disorder caused by deficiency of and/or dysfunctional VWF. VWD is usually inherited (congenital), but rare acquired forms exist. Congenital VWD is divided into type 1, characterized by quantitative deficiency of VWF, type 2, by dysfunctional VWF disorder, and type 3, by lack of VWF. Type 2 is further subdivided into subtypes 2A, 2B, 2M and 2N, depending on the type of functional deficit in the VWF protein. Spontaneous or tissue injury-related mucocutaneous bleeding events are characteristic of VWD, due to the impaired primary hemostasis. These bleeds are due to non-optimal interaction between vessel wall and platelets, where the role of VWF is crucial.
Introduction to the biochemistry of VWF
VWF is a circulating large glycoprotein with a concentration of approx. 10 mg/L in plasma. In healthy population, there is a 5-fold variability of VWF levels, and 25 % of activities of which is influenced by ABO blood group, 35% by genetics of VWF, and the remaining variability is explained by other genes [2–4] to cause rest of the variation. VWF protein is secreted into plasma from the EC by a continuous constitutive mechanism, whereas VWF from platelets is released only upon platelet activation. EC will release VWF from stores in the Weibel Palade bodies when exposed to various perturbation stimuli, including catecholamines, histamine and fibrin formation.
The plasma form of VWF is a multimeric protein constructed of 2 - 40 dimer subunits of the protomer, resulting in a range of multimers with molecular weights ranging from 500 - 20.000 kDa. The mature VWF protomer hosts several well-characterized binding sites (Figure 1). Most importantly, regarding VWD, one binding site interacts with collagen and another site with GP Ib of the platelet surface contributing to platelet adhesion at the wound site. These particular functions of primary hemostasis depend on blood flow and the structure and molecular weight of VWF multimers, and subsets of low-molecular-weight multimers are regarded too small to provide a sufficient spacer function. The unfolding of the VWF protein is important, however, but this can be detected only under blood flow and is not captured by routine laboratory assays. In addition, the VWF subunit holds binding sites for FVIII, a RGD motif, recognizing the platelet GPIIb/IIIa during aggregation, and a site that interacts with heparins and heparin like -molecules. The specific site for FVIII in VWF provides a protective non-covalent binding, thereby limiting random proteolytic breakdown of FVIII. With our current understanding, VWF multimers are assembled in the Golgi apparatus of EC and if not directly exported, the multimers are retained in the Weibel Palade bodies. It has been suggested that ECs are also involved in the storage of FVIII.
Following the release of VWF from ECs under blood flow, enzymatic exposure of VWF to a metalloprotease (ADAMTS13) reduces the largest VWF multimers in size. Not all cleavage sites will be exposed and the limited proteolysis results in a typical oligomer sub-band pattern (called triplet structure) that can be revealed by multimer analysis from plasma. The lack of VWF and mutations in the FVIII binding sequences of VWF may significantly reduce the plasma level of FVIII. In severe VWD, the FVIII values may be 2-10 % compared to healthy individuals.
Von Willebrand disease
From aforementioned it is well understood that VWD is caused by a defect, inflicting platelet functions, and the major clinical hallmark in VWD is a tendency to mucocutaneous bleeds.
The history of VWD dates back to Erik von Willebrand [5] who reported on a bleeding disorder, with fatal outcomes, denoted pseudohemophilia, occurring equally often in both sexes. Today, we know that VWD is a highly heterogeneous group of bleeding disorders with the common denominator of a quantitative or qualitative deficiency of VWF in circulating plasma, platelets, and endothelium. The basis for diagnosis of VWD relies on patient’s and relatives’ medical history, signifying an increased and objective bleeding tendency, together with a phenotype compatible with a defect of primary hemostasis.
Since many variants of VWD are reported, the ISTH has developed a guideline for classification of VWD, simplifying the hierarchy of subclasses. Table 1 summarizes the current recommendation, including the amendments agreed upon by the ISTH in 2006 [6] and 2021 [7].
| Type | Description |
|---|---|
| 1 | Partial quantitative deficiency of VWF. There is one distinct subtype called type 1C, which is caused by markedly increased VWF clearance. |
| 2 | Dysfunctional (qualitative) VWF. |
| 2A | Decreased VWF-dependent platelet adhesion with a selective deficiency of high molecular weight multimers. |
| 2B | Increased affinity for platelet GPIb. |
| 2M | Decreased VWF-dependent platelet adhesion and/or decreased collagen-binding capacity without a selective deficiency of high molecular weight multimers. |
| 2N | Markedly decreased binding affinity for FVIII. |
| 3 | Virtually complete deficiency of VWF. |
In many variants of type 2 and 3 VWD, genetic defects have been identified, while in many individuals with type 1 the molecular defects remain unknown. Type 3 VWD is an autosomal recessive form with severe bleeding problems. The type 2 VWD expresses variable clinical manifestations but can be quite severe in some situations.
The current classification recognizes four qualitative forms: 2A, 2B, 2M with a dominant inheritance pattern) and 2N that is a recessive disease. Thanks to large multicenter studies on type 1 VWD our knowledge of the complexities causing VWD have been extended [7,8]. Candidate mutations have been identified in approx. 65 % of the index cases with type I VWD whereas in the remaining cases the reduced VWF level may be caused by other factors, with the blood group AB0 gene showing the strongest effect. In later years, genetic data from genome-wide association studies have identified other genes that contribute to the plasma level variation of VWF [4,9]. VWF gene mutations are listed in an international web- based registry hosted by the EAHAD DBS organization (dbs.eahad.org).
Differential diagnosis
Congenital or acquired vascular and connective tissue defects along with platelet function defects are to be considered in the differential diagnosis of the patient presenting with bleeding symptoms, especially if a history of mucocutaneous bleeding is present. Thrombocytopenia and platelet function defects, e.g., Bernard-Soulier syndrome, Glanzmann thrombasthenia, platelet-type VWD, secretion disorders, and acquired causes of dysfunction (e.g., drugs, uremia or hematological disorders) also impair primary hemostasis. In the surgical setting, the distinction must be made between a bleeding diathesis versus bleeding caused by insufficient surgical hemostasis (to be checked from the respective surgical files). A profuse generalized bleeding tendency is typical of coagulopathy, as for example encountered in association with disseminated intravascular coagulation (DIC). The term describes a complex of signs that increase the risk for type 2 diabetes, stroke, and coronary artery disease.