Von Willebrand Disease: Overview, Prevalence, Treatment, and Current Research

Dylan Djani

BIOSC 467 Principles of Hematology

April 26, 2013

Abstract

Von Willebrand disease involves a decrease in the blood levels of von Willebrand factor,

which represents a clotting disorder due to the lack of platelet aggregation and corresponding

diminished blood levels of Factor VIII. Von Willebrand disease is more often caused by

inherited mutations than issues secondary to other disease processes, and specific details of the

inherited mutations and subsequent phenotypic effects characterize types and subtypes of the

disease. Types of hereditary von Willebrand disease and treatment options are relatively

consistent between humans and dogs. Treatments include drug and transfusion therapy aimed at

increasing circulating levels of von Willebrand factor and Factor VIII, but other options are

available in order to control bleeding issues. Current clinical research is being conducted to

determine the best ways to manage surgical cases in patients with von Willebrand disease,

although each individual has a unique disease presentation in that different drugs and transfusion

products may show more efficacy than others.

Introduction

Blood is a type of connective tissue in the animal body in that blood consists of cells

suspended in a matrix of plasma and includes erythrocytes, leukocytes, and thrombocytes or

platelets (1). The importance of blood is inherently illustrated in its function in transportation:

blood is responsible for transporting nutrients, wastes, gases, hormones, heat, cells, and proteins

throughout the entire animal body (2). Every single cell of the animal body requires nutrients

and generates metabolic waste products and thus must be accessible by blood in order to

continue living. Blood gains access to tissues of the body by existing and circulating through

specific vessels in the body that, along with the heart, make up the vascular system (3). Starting

at the heart blood flows into arteries, which branch into smaller arterioles and eventually the

characteristically thin capillaries. Blood flows out of the capillaries into larger venules and

eventually veins, which feed the blood back into the heart. Transport and exchange between

cells of the body and the blood occur at the level of the capillary. The structure of the capillary

reflects its function in that the vessel wall is the thinnest of all vessels, consisting of a single

layer of cells. The walls of all vessels in the vascular system facing the lumen are generally

referred to as the vascular endothelium and also have multiple critical functions. Maintaining

blood circulation in the closed system of the vasculature at an adequate rate and volume is

quintessential for the survival of the cells that make up the body’s tissues, organs, and organ

systems. The animal body has homeostatic mechanisms in place to prevent blood from escaping

the vasculature when damaged, as well as ways to shut down such mechanisms so as not to

overcompensate (4). Damage to the vascular system results in hemorrhage and, in severe cases

such as trauma or disease, compromises the accessibility of blood to body tissues due to

decreased intravascular blood volume, termed hypovolemic shock. Hemostasis is the

homeostatic mechanism involved in preventing blood loss from the vasculature. This

mechanism involves positive feedback and must be strictly regulated in order to prevent

excessive hemostasis or thrombosis, which can also compromise blood’s accessibility to body

tissues by directly blocking circulation.

The overall goal of hemostasis is to regulate blood flow in order to prevent its escape

from the vasculature so that proper physiology can be conserved for survival (5). Hemostasis is

divided into two systems: primary hemostasis and secondary hemostasis (3). Primary hemostasis

involves local vasoconstriction to limit blood flow to the site of damaged vasculature and the

action of platelets to help initially stop blood from flowing out of the vasculature. Secondary

hemostasis results in the formation of a clot at the injured site via a cascade of plasma proteins

called clotting factors that results in blood coagulation. Blood coagulation inherently

incorporates proteins involved in degrading the blood clot after repair of the vascular tissue in

order to resume normal blood flow. The balance of pro-coagulation and anti-coagulation

mechanisms in the body contributes to overall homeostasis.

Von Willebrand Disease is defined as a bleeding disorder associated with the von

Willebrand factor plasma protein, either due to insufficient levels in circulation or a dysfunction

or defect in the factor (6). The negative impacts on hemostasis seen in von Willebrand disease

reflect a lack of the main functions of the von Willebrand factor and are due to inadequate

platelet adhesion or due to a corresponding reduction in the levels of circulating Factor VIII.

Von Willebrand disease is the most commonly inherited bleeding disorder in both humans and

dogs and is caused by an inherited mutation in the VWF gene (7, 8). On the other hand, acquired

von Willebrand disease is rarer than the inherited disease and results in a qualitative deficiency

of circulating levels of von Willebrand factor (9). Potential mechanisms for triggering acquired

von Willebrand disease are not fully understood and may involve autoimmune antibodies

targeting von Willebrand factor or tumor cells adsorbing the factor, which diminishes its levels

in circulation and interferes with its roles in hemostasis (10). Subtypes of inherited von

Willebrand disease are used to classify the degree of severity of the disease in both humans and

dogs (6, 8). Considerations in affected patients depend on severity of the disease and are

important in determining how to best manage the disease. Treatments usually vary based on

disease subtype and include drug or transfusion therapy (3).

Primary Hemostasis

Primary hemostasis is a result of the exposure of extravascular collagen as a result of

endothelial damage (11). Initial vasoconstriction occurs due to the removal of normal products

synthesized by healthy endothelium during times of damage, including nitric oxide. Platelets in

circulation undergo platelet adhesion and bind to the exposed collagen through specific platelet

surface receptors and ligands based on the shear force of circulation at the damaged site. If the

damaged site is located at a point in circulation with high shear force, platelet adhesion primarily

occurs via interactions between the collagen, a protein called von Willebrand factor (vWF), and

platelet receptors specific for vWF; however, if the damaged site is at a location with lower shear

force, platelet adhesion occurs mainly through interactions of the proteins fibronectin and

laminin with the exposed collagen. The von Willebrand factor is synthesized and stored by

endothelial cells and platelets, in Weibel-Palade bodies and alpha granules respectively, and

exposure of underlying collagen is one factor that stimulates vWF release. After being released

vWF either stays in circulation or binds to the exposed collagen, the latter of which results in the

release of blood protein Factor VIII and platelet adhesion by virtue of binding vWF-specific

receptors on the platelets. Platelets express a glycoprotein complex called GP Ib-IX-V complex,

which adheres platelets to collagen-bound vWF. Adhered platelets, whether via vWF or

fibronectin/laminin, then undergo the process of platelet activation, resulting in a shape change

such that the platelets acquire pseudopodia, releasing of granule contents, and modification of

cell surface expression of molecules (2). However, platelet activation can actually be triggered

in vivo prior to any adhesion in cases of inflammation via the blood protein thrombin, which is

also involved in the coagulation cascade in secondary hemostasis.

Platelets activated by adhering to collagen induce activation and aggregation in more

platelets by releasing stores of ADP and by synthesizing thromboxane A2 and platelet-activating

factor, all of which induce activation by increasing intracellular calcium in the target platelet;

although, the most effective platelet activating has been demonstrated to be thrombin, indicating

the overlap of primary and secondary hemostasis (5). Thromboxane A2 and platelet-activating

factor also further contribute to local vasoconstriction. Activated platelets have an altered GP Ib-

IX-V complex such that an active binding site for the protein fibrinogen is revealed, which is

accompanied by the expression of the GP IIb-IIIa complex due to the shape changes from

activation. These shape changes also result in the exposure of negatively charged phospholipids

on the surface of platelets – mainly phosphatidylinositol and phosphatidylserine. The negative

surfaces of the exposed phospholipids on the surface of platelets as well as damaged endothelial

cells provide a stabilizing microenvironment on which the coagulation cascade of secondary

hemostasis begins (12). After activation, platelets express both GP Ib-IX-V and GP IIb-IIIa

complexes that have the ability to bind vWF and fibrinogen, resulting in the aggregation of

platelets at the damaged site via the formation of vWF and fibrinogen bridges between platelets,

regardless of whether vWF was initially involved in platelet adhesion.

The aggregation of platelets results in the formation of a platelet plug that contributes to

hemostasis, albeit being unstable. Once the platelet plug has completely sealed the damaged site,

healthy endothelial cells surrounding the platelet plug are induced to produce prostacyclin, which

has vasodilation effects and inhibitory effects on platelet aggregation. Adhered and activated

platelets can also bind leukocytes passing by in circulation via cross binding of many receptor

complexes, including vWF and the corresponding platelet receptors (13).

Secondary Hemostasis

The ultimate goal of secondary hemostasis is to stabilize the platelet plug via formation

and cross-linking of a fibrin clot at the site of the plug (3). The overlapping extrinsic and

intrinsic pathways of blood coagulation involve protease cascades culminating in the conversion

of prothrombin to thrombin via activation of Factor X and the subsequent cleavage of fibrinogen

into fibrin, resulting in the formation of a fibrin clot (4). The extrinsic pathway of coagulation

involves the release of tissue factor from damaged endothelium, which cleaves and activates

Factor VII to VIIa while also forming complexes with VIIa. The tissue factor-VIIa complex

amplifies the activation of Factor IX to IXa in the intrinsic pathway of coagulation and

contributes the extrinsic tenase complex along with the calcium ion and Factor X on an exposed

phosphatidylserine on the surface of an activated platelet in the platelet plug. The tenase

complex cleaves and activates Factor X to Xa, an activation step common to both the extrinsic

and intrinsic pathways.

The intrinsic pathway involves components of the kallikrein-kinin system and begins

when a spontaneous cleavage event of prekallikrein to kallikrein occurs and is stabilized by the

negative charges of the exposed phospholipids on platelets in the platelet plug, along with

Factors XII, XI, and high molecular weight kininogen (12). The stabilization of these factors is

termed contact activation and is referred to as the contact system (5). Kallikrein cleaves and

activates Factor XII to XIIa, which amplifies the conversion of prekallikrein to kallikrein and

thus the entire intrinsic pathway. Factor XIIa also cleaves and activates Factor XI to XIa and

cleaves the molecule bradykinin off of the compound high molecular weight kininogen.

Bradykinin acts as a local vasodilator in order to bring more plasma clotting factors to the site of

coagulation. Factor XIa in conjunction with the calcium ion cleaves and activates Factor IX to

IXa, which is further carried out by the tissue factor-VIIa complex from the extrinsic pathway.

Along with the extrinsic pathway’s tenase complex, the intrinsic pathway forms a version of the

tenase complex as well via Factor IXa, the calcium ion, Factor X, and the activated Factor VIIIa.

Factor VIIIa is a product of the activation of Factor VIII by minute amounts of thrombin and

contributes to the intrinsic tenase complex; increasing amounts of thrombin inactivates VIIIa and

illustrates the self-limiting capacity of thrombin to help regulate the entire coagulation cascade.

The von Willebrand factor also contributes to the stabilization of Factor VIII and serves as a

plasma carrier for this clotting factor (6). The intrinsic tenase complex also results in the

activation of Factor X to Xa, although the intrinsic pathway as a whole in healthy individuals

seems to mainly function in amplifying Factor X activation and having implications in

fibrinolysis.

Both the extrinsic and intrinsic pathways of blood coagulation overlap at the formation of

the tenase comlex and the subsequent activation of Factor X to Xa and contribute to the

conversion of prothrombin to thrombin via the formation of the prothrombinase complex (5).

Similar to the tenase complex, the prothrombinase complex also forms on the negative

phospholipid exposed on the surface of activated platelets; however, the prothrombinase

complex consists of the calcium ion, Factor Xa, Factor Va, and prothrombin. Factor V, like

Factor VIII, is activated by minute amounts of thrombin, but is inactivated through the activation

of Protein C to limit thrombin formation (4). The binding of prothrombin is the last event that

occurs in the formation of the prothrombinase complex and results in its cleavage via Factor Xa

into thrombin. Thrombin cleaves soluble plasma fibrinogen into fibrin monomers that

spontaneously arrange to form the initial fibrin clot at the location of the platelet plug. Thrombin

also cleaves and activates Factor XIII to XIIIa, which is a transglutaminase enzyme that

covalently cross-links fibrin monomers to form a stabilized fibrin clot, or thrombus. Thrombin

also has other regulatory functions, such as those described in platelet activation and activation

of other clotting factors. Levels of circulating active thrombin are specifically controlled via

feedback inhibition and the action of thrombin inhibitors, including antithrombin, α2-

macroglobulin, heparin cofactor II, and α1-antitrypsin. Furthermore, thrombin regulation is

achieved via the complexing of thrombin with thrombomodulin, which activates protein C and

serves to inhibit various clotting factors along with protein S. The importance of protein C and

protein S is evident in corresponding genetic deficiencies, which can lead to venous thrombosis.

The formation of fibrin clots in secondary hemostasis is homeostatically balanced

through the process of fibrinolysis (4). Fibrinolysis is mediated through the activity of plasmin,

which is a serine protease responsible for the degradation of fibrin. The inactive form of

plasmin, plasminogen, circulates and binds to fibrin as clots are laid down, such that the

plasminogen must only be activated in order for clot dissolution to occur. Plasminogen

activators include Factor XIIa, tissue plasminogen activator, urokinase, and kallikrein, which are

all kept in check through various activating and inhibiting proteins in order to maintain

coagulation and clot dissolution homeostasis. The intrinsic pathway of coagulation inherently

contains more plasminogen activators than the extrinsic pathway, thus the intrinsic pathway is

responsible for proportionately more fibrin degradation (12). Upon degradation, fibrin is cleaved

to form various fibrin degradation products, some of which are of clinical significance.

Etiology and Epidemiology of von Willebrand Disease

Etiologies of von Willebrand disease can be categorized as either hereditary or acquired

based on whether the underlying cause is genetic. The prevalence rate of hereditary von

Willebrand disease is about 1% of the general population in humans, whereas acquired von

Willebrand disease is thought to be very rare due to the low number of documented cases (10).

The VWF gene encodes for the von Willebrand factor and is located towards the distal end of the

short arm of the autosomal chromosome number 12 (6). The large size and boundaries of introns

and exons reflect the nature of the von Willebrand factor, namely it’s large size with multiple

domains that grant the protein its multiple functions. Illustrations of the von Willebrand factor’s

domains emphasize the location of binding sites for Factor VIII, GPIb, heparin, collagen, and GP

IIb-IIIa. Synthesis of the von Willebrand factor involves the initial translation of pre-pro-VWF,

which is cleaved into pro-VWF and eventually VWF before its release from endothelial cells or

platelets (7). Hereditary von Willebrand disease can result from differing mutations along the

entire span of the VWF gene, resulting in a phenotypic spread of severity of the presenting

disease and subsequent difficulty in assigning a single pattern of inheritance. The multiple

possibilities for VWF gene mutations and their inheritance patterns results in various

homozygous and heterozygous genotypes, which means that in some cases people or animals can

be carriers of the disease.

Human hereditary von Willebrand disease is classified into three types based on

phenotypic disease presentation, with type 2 having four subtypes (6). Type I is characterized by

a partial deficiency in the circulating amounts of von Willebrand factor and the highest

prevalence rate compared to other types. Type I is associated with an autosomal dominant

inheritance pattern of a mutation that affects intracellular transport of von Willebrand factor or

promotes its rapid clearance from the blood; autosomal dominant meaning that affected

individuals are either homozygous dominant or heterozygous. The lowered levels of plasma von

Willebrand factor may be clinically noted; however, the amount of von Willebrand factor present

is generally enough to maintain platelet adhesion and stabilization of Factor VIII properly, thus

patients with Type I von Willebrand disease are at a low risk for bleeding issues. Type II von

Willebrand disease is characterized by a structural defect in the von Willebrand factor due to a

mutation at a specific location on the VWF gene. The phenotypic results based on the location of

the mutation allow for subtyping of Type II based on the affected domain: IIA results in

decreased platelet adhesion and deficiency of specific multimers, IIB results in increased binding

affinity for GP Ib, IIM results in decreased platelet adhesion without any multimer deficiencies,

and IIN results in a substantial decrease in binding affinity for Factor VIII. In general type II

shows an autosomal dominant pattern of inheritance, but in some cases an autosomal recessive

pattern is seen. The severity of type II is higher than type I in that the propensity to bleed

excessively is increased, which indicates the need for additional medical care by a hematologist.

Type III von Willebrand disease is characterized by virtually undetectable circulating levels of

von Willebrand factor, accompanied by extremely low levels of circulating Factor VIII. Type III

is the rarest of all types and shows an autosomal recessive mode of inheritance. The extreme

disease presentation of type III can often result in severe bleeding episodes and are usually

caused by mutations throughout the VWF gene, commonly sourcing from nonsense or frameshift

mutations.

Canine hereditary von Willebrand disease is classified into three subtypes that correspond

with the types of human hereditary von Willebrand disease, but may be interpreted slightly

differently (8). Type I describes low concentrations of von Willebrand factor in the blood with

the presence of all forms of the multimer. Breeds in which type I have been reported include

Dobermans, dachshunds, German shepherds, golden retrievers, and greyhounds. Type II

describes a lowered plasma concentration of von Willebrand factor with a disproportionate

amount of von Willebrand multimers in the blood. Reported breeds for type II include German

shorthaired pointers and wirehaired pointers. Type III describes undetectable amounts of von

Willebrand factor in circulation and has been reported in Scottish terriers and Shetland

sheepdogs. As with human hereditary von Willebrand disease, the severity of the canine

hereditary von Willebrand disease generally increases from type I to type III. Inheritance is

autosomal recessive for types II and III; however, evidence for incomplete dominant and

recessive patterns exists for type I. In most cases type I shows an incomplete dominant

inheritance pattern, with homozygous dominant phenotypes only rarely seen due to periparturient

deaths of the fetus (14).

Acquired von Willebrand disease is often referred to as a syndrome due to its general

associations with underlying diseases such as lymphoproliferative and myeloproliferative

disorders, immunological disorders, and cancers (9). Patients with acquired von Willebrand

disease have the wild-type VWF gene with no mutations with laboratory findings similar to

patients with the hereditary form of the disease, namely a decreased circulating amount of von

Willebrand factor with a lesser degree of decreased circulating Factor VIII levels. The main

pathophysiological mechanisms associated with the acquired von Willebrand disease are an

increased intravascular proteolysis of von Willebrand factor, the formation of autoantibodies

against von Willebrand factor, immunoadsorption of von Willebrand factor – Factor VIII

complexes to cancer cells, or a decreased synthesis of von Willebrand factor somehow linked to

hypothyroidism. In humans, acquired von Willebrand disease normally presents more often in

elderly patients, which differs from hereditary forms of the disease (10). In dogs, acquired von

Willebrand disease is also linked to hypothyroidism due to the influence of thyroid hormone

regulation on the synthesis of von Willebrand factor (14). Furthermore, dogs that are carriers of

the von Willebrand disease gene show exacerbated disease signs after the onset of hypothyroid

events, which may also be caused in part by thrombocytopenia secondary to hypothyroidism.

Current Clinical Treatments and Therapies

Patients presenting with bleeding episodes, regardless of whether von Willebrand disease

is the cause, should be initially stabilized and treated for any issues associated with blood loss,

such as anemia or hypovolemic shock (8). Considerations in treating von Willebrand disease

patients include avoiding invasive surgery and the use of drugs with anti-platelet activity due to

the compromised hemostasis in diseased patients. Initial treatment of von Willebrand disease

includes drug therapy and transfusion therapy in both humans and dogs, with the main goal of

increasing circulating levels of von Willebrand factor; however, treatments involved in

stimulating hemostasis without manipulating blood von Willebrand factor levels may also be

used (6).

The drug desmopressin acetate is used in treating mild to moderate von Willebrand

disease due to its stimulatory effects on endogenous von Willebrand factor release from

endothelial cells (6). The drug’s complete name is deamino 8 D-arginine vasopressin (DDAVP)

and functions as a vasopressin analog by agonizing V2 receptors expressed on endothelial cells,

which increases intracellular cAMP levels and triggers the release of Weibel-Palade bodies. The

release of endothelial Weibel-Palade bodies results in the release of von Willebrand factor,

Factor VIII, and tissue plasminogen activator, but circulating levels of plasminogen inhibitors

prevent the release of the latter from inhibiting hemostasis. However, the use of DDAVP should

be avoided in patients at risk of cardiovascular disease due to complications involving

thrombosis.

The administration of interleukin-11 (IL-11) has been shown to increase plasma von

Willebrand factor and Factor VIII levels in mouse models with and without von Willebrand

disease, as well as in dogs (15, 16). The studies performed on dogs illustrated that IL-11 and

DDAVP increase circulating levels of von Willebrand factor through different mechanisms in

vivo, which suggest future potential use of IL-11 as supplemental therapy for von Willebrand

disease patients.

Transfusion therapies involve replacing blood lost from bleeding events and introducing

exogenous von Willebrand factor and Factor VIII into the patients (8). Exogenous

administration of von Willebrand factor and Factor VIII concentrates has shown to improve

signs associated with disease and are also used in diseased patients who require invasive surgery

to promote hemostasis (6). Manufactured products of specific ratios of von Willebrand

factor/Factor VIII concentrates include Humate-P and Alphanate SD/HT. Humate-P also

contains fibrinogen and albumin and is indicated for treating spontaneous bleeding or bleeding

from traumatic injuries when the use of DDAVP is contraindicated or not completely effective.

Alphanate SD/HT also contains other plasma proteins and is indicated, along with Humate-P, in

invasive surgeries on diseased patients as prophylaxis. Alphanate SD/HT, however, is not

indicated in patients with type III von Willebrand disease who require major surgery.

Other treatments and therapies for von Willebrand disease patients involve the

administration of anti-fibrinolytic drugs and the application of topical agents to promote

hemostasis (6). Anti-fibrinolytic drugs include aminocaproic and tranexamic acids, which

inhibit the activation of plasmin. Such drugs are contraindicated in patients with disseminated

intravascular coagulation or bleeding of the genitourinary tract due to downstream complications

of the drugs. Topical agents include bovine thrombin, fibrin sealants, collagen sponges, and a

zeolite mineral product called QuickClotTM. Fibrin sealants are especially helpful in minor

surgical situations, such as dental surgery.

Current Research into Treatments and Therapies: Clinical Trials

Current clinical trials seem to be aimed at testing the effectiveness of various ratios of

von Willebrand factor and Factor VIII concentrates in diseased patients undergoing surgery,

which is pragmatic and geared towards improving survival rates of these patients in surgery.

One study is analyzing the effectiveness of Alphanate concentrates in patients with type III von

Willebrand disease undergoing major surgery (17). The use of Alphanate is currently

contraindicated on patients with type III von Willebrand disease, possibly because studies on the

safety and efficacy of its use have not yet been completed. Another study is analyzing the

efficacy and safety of the product Wilate on patients with hereditary von Willebrand disease who

also require invasive surgeries (18). The product Wilate represents another combination of von

Willebrand factor and Factor VIII to help promote hemostasis during surgery.

Other clinical trials involved with von Willebrand disease include attempting to better

understand how our laboratory methods of measuring hemostasis function in patients with von

Willebrand disease. One such study involves measuring changes in thromboelastography

throughout the menstrual cycle in women (19). Thromboelastography is a newer method of

measuring blood coagulation and has shown better results than the traditional PT and aPTT

clotting assays because the latter tests are based on older models of hemostasis.

The last group of current clinical trials involving von Willebrand disease involves the

effectiveness and safety of IL-11 in von Willebrand disease patients. An example of such studies

includes the phase II study of the use of IL-11 in treating type I von Willebrand disease (24).

Another study is analyzing the use of IL-11 in cases of von Willebrand disease that are

unresponsive to DDAVP (20).

Conclusion

Von Willebrand disease involves a decrease in the blood levels of von Willebrand factor

and Factor VIII. The von Willebrand factor is involved in primary and secondary hemostasis by

acting as a ligand for platelet aggregation and by serving as an intravascular carrier protein for

Factor VIII. Diminished levels of von Willebrand factor thus decreases platelet aggregation and

blood levels of Factor VIII, which negatively impacts both primary and secondary hemostasis

and has the potential to cause severe bleeding issues in diseased individuals. Hereditary and

acquired forms of von Willebrand disease have been clinically documented in humans and dogs,

with 1% of the human population having a hereditary form of the disease. However, acquired

forms are thought to be considerably rarer than hereditary forms. Types and subtypes of

hereditary von Willebrand disease are classified based on the specific mutation involved in

decreasing plasma von Willebrand factor levels and the corresponding phenotypic effects of the

mutation. Treatments involve increasing the levels of von Willebrand factor in the blood in

order to promote both primary and secondary hemostasis, which is accomplished through drugs

or through transfusion therapy of fresh plasma or concentrated products. Other treatments

involved in von Willebrand disease are involved in controlling episodes of bleeding, which is of

a more immediate concern in order to stabilize patients presenting with bleeding. The use of

transfusion therapy is largely used in promoting hemostasis in von Willebrand disease patients

who require major surgery, and the focus of many current clinical trials is to determine the

effectiveness of various combinations of von Willebrand factor, Factor VIII, and other plasma

protein products to improve survival rates of diseased patients undergoing invasive procedures.

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