Physiology, Von Willebrand Factor

Cellular Level

The initial synthesis of vWF, transcribed from chromosome 12, occurs in endothelial cells and megakaryocytes as a 2813 amino acid pre-pro-polypeptide of repeating domain sequences. The first 22 amino serve as a signal peptide to initiate post-translational processing.[6] The signal peptide is then cleaved to allow the pro-peptide monomers to dimerize.[7] Von Willebrand factor is rich in cysteine, a neutral yet polar amino acid, to facilitate multimerization and disulfide bridging necessary for its higher-order structure. Multimerization can combine different amounts of pro-vWF subunits, resulting in a heterogeneous mix of multimers ranging from as few as 2 to as many as 60 or more pro-vWF subunits. Post-translational glycosylation, sialylation, sulfation, and folding occur in the endoplasmic reticulum and Golgi apparatus. Once assembled, the pro-peptide domains (D1-D2, 741 amino acids), essential to aligning pro-vWF dimers, are cleaved to form the mature 2050 amino acid wWF. Once complete, vWF is transported for storage in alpha-granules of megakaryocytes and platelets and packaged into Weibel-Palade bodies (WPBs) within endothelial cells.[1]

Function

The 2 main functions of vWF in hemostasis include platelet adhesion and Factor VIII stabilization. During primary hemostasis, vascular injury exposes vWF-bound to subendothelial collagen. Then, glycoprotein 1b (GP1b) receptors on the surface of nearby platelets adhere to the exposed vWF, triggering platelet activation and a cascade of events, which includes the release of platelet storage granule content such as vWF from alpha granules and the recruitment of more platelets to form a plug at the site of damaged endothelium.[7] 

Plasma vWF supports the intrinsic coagulation cascade by stabilizing factor VIII, increasing its circulating half-life. During the intrinsic coagulation pathway, thrombin cleaves the factor VIII binding site with vWF, allowing the release (activation) of factor VIIIa to continue the clotting process.[6] By serving as a carrier for factor VIII, vWF influences the common coagulation pathway and the generation of thrombin and fibrin.[8] Without vWF, or more specifically, without the D’D3 domains that facilitate vWF and factor VIII binding, factor VIII is rapidly cleared from the circulation, hence the clinical similarities between some forms of vWD and hemophilia A.[9]

Mechanism

VWF multimers have repeating domain sequences with specific functions for each domain. Large vWF multimers, by nature of having more sites for adhesion, are more effective during hemostasis than the smaller varieties with less binding capacity.[6] The generally acknowledged domain structure is arranged SP-D1-D2-D’-D3-A1-A2-A3-D4-C1-C6-CK.[1][10] Each domain has multiple roles; however, the key functions of the domains are as follows:

• SP is the signal peptide that initiates post-translational processing
• D1 and D2 are the vWF propeptide domains that interact with D’ to allow proper alignment during vWF dimerization.
• D’ and D3 interact with D1 and D2 during dimerization and interdimeric cross-linking, p-selectin on the surface of platelets and endothelial cells, and factor VIII within WBP and the circulation.[1]
• The A1 domain binds GP1b on platelets and collagen in the subendothelial matrix, as well as several other proteins, such as histones, heparin, osteoprotegerin, and beta-2 integrin receptors on leukocytes.
• A2 domain is crucial in regulating vWF activity; the mechanosensitive switch unfolds when exposed to shear stress to expose other vWF binding domains. A2 is also the cleavage site for the zinc protease ADAMTS13, which regulates the size and pro-thrombotic activity of vWF.[7]
• A3 domain binds extracellular matrix collagens.
• C domains bind with fibrin plasma complement proteins and interact with GPIIb/IIIa during platelet adhesion.[11]
• CK (cysteine knot) domain is important for multimerization and post-translational folding.[1]

Platelet activation stimulates alpha granules to release vWF to facilitate hemostasis, while endothelial cells utilize a mixture of basal secretion and induced release. In response to vascular injury, WPB undergoes exocytosis, and vWF multimers exposed to shear stress unfold at the A2 domain. Unfolded vWF multimers form long bundles that anchor to subendothelial collagen and expose binding sites for nearby platelets. The A1 domain of vWF interacts with platelet GP1b receptors to initiate platelet aggregation and activation, which leads to alpha granule content release.[1] In addition to releasing more vWF for further platelet aggregation and fibrin mesh formation, alpha granules also release a protein called thrombospondin-1, which binds to the A2-A3 domains of vWF and competitively inhibits cleavage of vWF by ADAMTS13.[12] As mentioned, larger vWF multimers are more effective in thrombus formation; therefore, ADAMTS13 is critical to preventing unregulated pro-thrombotic activity.[6]

Plasma vWF is a molecular carrier and binds with factor VIII at its D’D3 domains. This prolongs the half-life of factor VIII, making it more readily available to participate in the intrinsic coagulation cascade. Clearance of vWF complexes is multifactorial and varies between individuals based on age, weight, and blood type. It occurs through endothelial cells, hepatocytes, and macrophages in the liver and spleen binding and internalizing the vWf complex, thereby removing it from the circulation.[1][9]

Related Testing

Disorders involving vWF can present on a spectrum of severity, described in more detail below. Relevant testing to assess the specific defect or degree of deficiency can further delineate qualitative versus qualitative platelet or coagulation disorders.[13]

Complete blood count

• Platelet count
• Red blood cell count
• Peripheral smear

Bleeding time

Coagulation tests

• Prothrombin time (PT/INR)
• Partial thromboplastin time (PTT)

vWF assays (quantity and activity)

• vWF antigen
• vWF-collagen-binding
• vWF-ristocetin cofactor activity
• vWF-factor VIII binding
• vWF genotyping

Factor VIII assays (quantity and activity)

ADAMTS13 assays (quantity and activity) [14][10]

Pathophysiology

Because vWF is a large, complex, heavily modified glycoprotein with more than 1 role in hemostasis, disorders involving defective or deficient vWF can present with a spectrum of severity. The most common genetic bleeding disorder, vWD, encompasses 4 subtypes inherited in an autosomal dominant pattern.[10][6] Clinical presentation includes a family history of a bleeding disorder, easy bruising, gum bleeding, heavy menses, prolonged bleeding after minor trauma, excess bleeding after major trauma, and symptoms of anemia such as fatigue. 

Type 1 vWD is the most common, representing about 80% of cases. It is caused by a decreased quantity of otherwise normal functioning vWF from missense mutations or null alleles. Type 1 vWD is considered the most mild, and the main symptom is mucocutaneous bleeding, which is exacerbated by taking non-steroidal anti-inflammatory drugs.

Type 2 vWD can be either autosomal dominant or autosomal recessive in its inheritance pattern. It is due to a vWF functional defect and is further subdivided into type IIA, IIB, IIM, and IIN.

• Type IIA vWD - decreased vWF-platelet binding due to loss of high molecular weight vWF multimers
• Type IIB vWD - abnormal vWF A1 domain causing increased vWF-platelet affinity
• Type IIM - decreased vWF-platelet binding activity despite normal amounts of high molecular weight vWF multimers
• Type IIN - decreased vWF-factor VIII binding due to defective vWF D’ domain

Type 3 vWD is the most severe, caused by the complete absence of vWF, and follows an autosomal recessive inheritance pattern.[15][9][6]

Lab findings include decreased vWF, normal platelet count, increased bleeding time, normal prothrombin time, and normal or decreased factor VIII corresponding with normal or increased partial thromboplastin time, depending on which domain is affected.[15] One specific lab test to assess vWF-platelet binding is the ristocetin cofactor assay. Ristocetin cofactor binds to both vWF and platelets, inducing the A1 domain of vWF to bind Gp1b surface receptors on platelets, leading to platelet aggregation. In patients with vWD, agglutination is decreased with ristocetin cofactor assay except for type IIB vWD.[6] Treatment options for vWD are primarily replacement therapy such as vWF and factor VIII plasma concentrates and recombinant vWF. Desmopressin may also treat insufficient vWF quantity by stimulating the release of endothelial vWF from WPBs.[10]

Clinical Significance

TTP, a rare form of thrombotic microangiopathy, is caused by defective or deficient ADAMTS13, a vWF-cleaving plasma protease. Mutations in the ADAMTS13 gene (congenital TTP, autosomal recessive) or aberrant autoimmune activity (acquired TTP or immune TTP) can impede enzymatic function such that large vWF multimers accumulate and dramatically increase thrombus formation and platelet consumption.[5][16] Acquired TTP does not have a single inciting cause but has been associated with pregnancy, HIV, malignancy, lupus, certain medications, and infectious processes.[17][12]

• Clinical presentation is often described as a triad of neurologic symptoms due to impaired cerebral blood flow, thrombocytopenia, and microangiopathic hemolytic anemia or a pentad of the aforementioned triad plus fever and renal problems. Given its namesake, skin findings such as petechiae and purpura also manifest TTP. Ultimately, microthrombi formed due to this condition can affect the perfusion of any organ system, so the scope of possible symptoms is broad.
• Lab findings include low levels of ADAMTS13 activity, antibodies against ADAMTS13 (specific to acquired TTP, not found in congenital TTP), evidence of hemolytic anemia such as elevated lactate dehydrogenase, elevated reticulocyte count, elevated unconjugated bilirubin, and decreased haptoglobin, in addition to the presence of schistocytes, significantly decreased platelet count and increased bleeding time.[18] Bleeding time is increased in TTP because platelets are consumed in microvascular thrombus formation faster than they are produced, and primary hemostasis depends on the formation of a platelet plug.
• Standard treatment for TTP is plasmapheresis and steroids; however, in cases of acquired TTP, rituximab prevents relapse when ADAMTS13 activity levels are especially low by inhibiting the production of ADAMTS13 autoantibodies.[16][19]]

Bernard-Soulier syndrome, a rare genetic platelet disorder with an autosomal recessive inheritance pattern, is caused by abnormally large platelets with impaired surface expression of Gp1b, which causes defective platelet adhesion to vWF.

• Clinical presentation typically includes excessive bleeding, epistaxis, easy bruising, petechiae, and (in females) heavy menses.
• Lab findings include normal or decreased platelet counts, increased platelet size, increased bleeding time, normal prothrombin time, normal partial thromboplastin time, and a failure of platelet aggregation when exposed to ristocetin cofactor due to Gp1b deficiency.
• Treatment for Bernard-Soulier syndrome is supportive, and genetic counseling is recommended. In some scenarios, antifibrinolytic therapy may be administered to reduce post-procedural bleeding.[20]

Coronavirus disease 2019 (COVID-19) is associated with hypercoagulability; the risk of microthrombosis correlates with disease severity. One proposed mechanism is a quantitative imbalance between vWF and ADAMTS13 activity. Similar to the hypercoagulable state of patients with sepsis, inflammatory microthrombogenesis in severe COVID-19 is facilitated by excessive release of vWF, which outpaces ADAMTS13 activity.[5]