Ann Hematol (2006) 85: 415–423 DOI 10.1007/s00277-006-0085-5

REVIEW ARTICLE

Massimo Franchini . Giuseppe Lippi

Von Willebrand factor and thrombosis

Received: 29 December 2005 / Accepted: 29 December 2005 / Published online: 28 March 2006 # Springer-Verlag 2006

Abstract There is increasing evidence that von Willebrand factor (VWF), an adhesive multimeric protein that has an important function in primary hemostasis and as a carrier of factor VIII, has a pivotal role in thrombogenesis. In fact, while the presence in plasma of unusually large VWF multimers due to a congenital or acquired deficiency of a VWF-cleaving metalloprotease has been implicated in the pathogenesis of thrombotic thrombocytopenic purpura (TTP), high plasma levels of VWF have been associated with a slightly increased risk of arterial thrombosis. With regard to the association between VWF and venous thrombosis, clear conclusions cannot yet be drawn from the conflicting published data. Patients with von Willebrand disease, an inherited hemorrhagic disorder, may also paradoxically experience thrombotic events as a result of interactions among multiple prothrombotic risk factors. After a description of the structure and physiology of VWF, all these aspects are discussed in the present review. Keywords von Willebrand factor . Thrombosis . Thrombotic thrombocytopenic purpura . von Willebrand disease

lets, endothelial cells, and the subendothelium. It supports primary hemostasis and serves as a carrier for factor VIII (FVIII), protecting this coagulation factor from proteolysis by the activated protein C system [1, 2]. While an inherited deficiency or abnormality of VWF is associated with a bleeding disorder named von Willebrand disease (VWD), high plasma concentrations of VWF have been associated with an increased risk of thrombosis [3]. However, paradoxically, thrombotic complications may also occur in VWD patients [4]. Other thrombotic events in which VWF is involved have also been described. In fact, a congenital or acquired deficiency of the VWF-cleaving metalloprotease ADAMTS-13, the major regulator of the size of VWF in plasma, causes the presence of unusually large multimers of VWF and thus, massive intravascular formation of plateletrich thrombi in patients with thrombotic thrombocytopenic purpura (TTP) [5]. All these thrombotic events are discussed in this review. Data were identified by searches of the published literature, including PubMed (without time limits) and references from reviews.

Structure and function of von Willebrand factor Introduction Von Willebrand factor (VWF), the largest human plasma protein, is an adhesive multimeric protein present in plateM. Franchini (*) Servizio di Immunoematologia e Trasfusione, Ospedale Policlinico, Azienda Ospedaliera di Verona, Piazzale L. Scuro, 10, 37134 Verona, Italy e-mail: [email protected] Tel.: +39-45-8073610 Fax: +39-45-8073612 G. Lippi Istituto di Chimica e Microscopia Clinica, Dipartimento di Scienze Biomediche e Morfologiche, Università di Verona, Verona, Italy

Von Willebrand factor is a large multimeric glycoprotein synthesized by endothelial cells and megakaryocytes. It mediates the adhesion of platelets to sites of vascular lesions and being the carrier for FVIII, is required for normal survival of this coagulation factor in the circulation [6]. The gene coding for VWF is large, being composed of 178 kb; it is located on the short arm of chromosome 12 and contains 52 exons. The primary product of the VWF gene is a 2,813 amino acid protein made of a signal peptide of 22 amino acids (also called a pre-peptide), a large pro-peptide of 741 amino acids and a mature VWF molecule of 270 kDa containing 2,050 amino acids [7, 8]. The pro-peptide and mature subunit constitute the pro-VWF monomer, composed of four types of repeated domains (D1, D2, D′, D3, A1, A2, A3, D4, B, C1, C2 from the N- to C-terminal region) of cDNA which are responsible for the different binding functions of the molecule (see Fig. 1). The building block of VWF multimers is a dimer

416 Fig. 1 Schematic representation of the mature von Willebrand factor subunit, including the various domains responsible for the different binding functions of the molecule

formed in the endoplasmic reticulum and made up of two single-chain pro-VWF molecules, joined through disulphide bonds within their C-terminal region. The pro-VWF dimers are then transported to the Golgi apparatus where they are polymerized into very large molecules, with molecular weights of up to 20,000 kDa, through disulphide bonds connecting the two N-terminal ends of each dimer [9]. The newly synthesized VWF is either secreted constitutively or is targeted to storage granules, the Weibel–Palade bodies in endothelial cells or α-granules in megakaryocytes [10]. These storage granules contain the larger, more hemostatic forms of VWF that are released upon stimulation by agonists such as thrombin, epinephrine, and fibrin. The storage granules also contain ultralarge multimers (ULVWF), which are not usually found in the circulation [11]. Indeed, a specific plasma protease, known as ADAMTS-13 (a disintegrin and metalloprotease, with thrombospondin-1-like domains), acts on VWF multimers secreted from endothelial cells, rapidly cleaving the VWF subunit on the endothelial surface between the amino acids residues Tyr1605 and Met1606 in the A2 domain, thus, reducing the size of plasma VWF and creating the full spectrum of circulating VWF species, ranging from the single dimer to about 20 dimers in each VWF multimer [9, 12]. Besides being found in endothelial cells, megakaryocytes and platelets, VWF is also present in the subendothelial matrix, where it is bound, through specific regions in its A1 and A3 domains, to different types of collagen. VWF has two major functions in hemostasis [6, 13]. First, it is essential for platelet-subendothelium adhesion and platelet-to-platelet interactions and also causes platelet aggregation in vessels in which rapid blood flow results in elevated shear stress. Adhesion is promoted by the interaction of a region of the A1 domain of VWF with platelet-receptor glycoprotein Ibα (GpIbα) on the platelet membrane. It is thought that high shear stress activates the A1 domain of the VWF bound to collagen (through the A3 domain) by stretching VWF multimers into their filamentous form [14]. Furthermore, GPIbα and VWF are also necessary for plateletto-platelet interactions [15]. The interaction between GPIbα and VWF can be mimicked in platelet-rich plasma by addition of the antibiotic ristocetin, which promotes binding of VWF to GPIbα of fresh or formalin-fixed platelets. The Arg-Gly-Asp (RGD) sequence within the VWF C1 domain can bind to activated integrin αIIbβ3 (GPIIb-IIIa) on activated platelets, leading to the subsequent steps of platelet spreading and aggregation. Both these binding activities of VWF are highly expressed by the largest VWF multimers. The second major function of VWF in hemostasis is as the specific carrier of FVIII in plasma [16]. Each VWF monomer

has two binding domains (D′ and D3 domains) which can non-covalently bind one FVIII molecule, thus, protecting the coagulation factor from proteolytic degradation, prolonging its half-life in the circulation and efficiently localizing it at the site of vascular injury.

Von Willebrand factor as a risk factor for arterial and venous thrombosis Particularly at high shear forces, which are physiologic in the arterioles and also occur in pathologic conditions at sites of severe stenosis of large arteries caused by the formation of atherosclerotic plaques, VWF supports the initial adhesion of platelets by functioning as a ligand between platelet membrane glycoproteins and the subendothelium and plays an essential role in platelet aggregation [1, 17, 18]. The high shear forces increase VWF secretion by vascular endothelium, thus, stimulating platelet adhesion and aggregation at the site of damaged arterial walls and leading to thrombus formation. However, plasma VWF levels depend on various factors including the individual’s genetic background, chronic conditions, and transient events. With regard to the genetic factors determining VWF levels, individuals with a non-O blood group, females, and black races have higher VWF levels (and consequently higher FVIII levels) than do individuals with blood group O, males, and white people [19, 20]. Conditions causing chronically raised levels of VWF include older age, obesity, diabetes, chronic inflammation, cancer, liver, and renal diseases. Conditions causing a transient rise in VWF levels include pregnancy, surgery, exercise, and agents such as epinephrine, vasopressin, and desmopressin [3, 21]. Since the early 1990s many attempts have been made to elucidate whether high plasma concentrations of VWF are associated with an increased risk of thrombosis [3]. Several prospective studies on the role of VWF in arterial and venous thrombosis have been performed in healthy individuals and patients with cardiovascular diseases. The results of these studies are presented in the next two sections. Von Willebrand factor and arterial thrombosis Among the prospective clinical studies published on the role of VWF in coronary heart disease and stroke, the Northwick Park Heart Study [22] examined the relation of FVIII, VWF, and ABO blood groups with the incidence of ischemic heart disease in 1,393 healthy men aged between 40 and 64 years

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who experienced 178 first major episodes of ischemic heart disease during an average follow-up of 16.1 years. After adjustment for ABO blood groups, a multivariate analysis showed that higher levels of VWF and FVIII were associated with fatal ischemic heart disease. In 1995, Thompson and colleagues reported the results from the multicenter, prospective European Concerted Action on Thrombosis (ECAT) Study [23], which was conducted on 3,043 patients with angina pectoris followed for 2 years. Selected hemostatic factors of the coagulation and fibrinolytic systems were analyzed in relation to the development of myocardial infarction or sudden coronary death, and after adjustment for many confounding factors (age, sex, blood group, diabetes, hypertension, smoking, etc.), only VWF and tissue plasminogen activator were found to be independent predictors of subsequent acute coronary events. The Atherosclerosis Risk In Communities (ARIC) study recruited 14,477 adults, aged between 45 and 64 years, who were initially free of coronary heart disease [24], and monitored them over a mean period of 5 years for the occurrence of new onset coronary heart disease. Among the various plasma hemostatic factors measured, fibrinogen, FVIII, and VWF were associated with an increased risk of ischemic heart disease, but in a multivariate analysis that included classical cardiovascular risk factors, this relationship was attenuated for fibrinogen and disappeared for VWF and FVIII. In a subsequent update, the same study [25] did, however, show a significantly higher risk of stroke in individuals with high VWF and FVIII levels. Like the previous study, the Edinburgh Artery Study [26], conducted on 1,592 individuals who were free of coronary heart disease and were monitored for 5 years, failed to find an association between VWF levels and ischemic heart disease. Other investigators, in the Caerphilly Heart Study [27], studied 1,997 men aged 49–65 years for 5 years and found a positive association between FVIII, VWF, and ischemic heart disease. However, VWF and FVIII levels were mutually dependent, as the statistical significance of the relative risk for ischemic heart disease was lost in multivariate analysis after adjustment for each other. Jansson and colleagues [28] followed a cohort of 123 survivors of myocardial infarction for up to 10 years and found that VWF and tissue plasminogen activator levels were independent predictors of cardiovascular mortality. In a prospective study conducted by Whincup and colleagues [29] on 625 male patients with major coronary events and 1,266 male controls followed up for 16 years for fatal coronary heart disease and non-fatal myocardial infarction, VWF values in the upper tertile were associated with a higher rate of incident coronary heart disease compared to values in the lower tertile (odds ratio, 1.83; 95% confidence interval, 1.43 to 2.35). The same authors also conducted a meta-analysis on all the relevant populationbased prospective studies published by that time and found a similar combined odds ratio of 1.5 (95% confidence interval, 1.1 to 2.0). However, among the studies included in this analysis, the predictive ability of VWF depended strongly on the variables controlled for, in particular,

markers of inflammation. Thus, the prognostic relevance of VWF, which is an acute-phase reactant, could simply be as a marker of the inflammatory process, which is known to play a major role in coronary heart disease, rather than an as independent risk factor. This issue had been raised by the aforementioned ECAT study [23] which showed in subjects with angina pectoris that the independent relative risk of cardiovascular mortality associated with VWF disappeared after adjustment for variables related to inflammation, i.e., C-reactive protein and fibrinogen. In contrast, the Hoorn Study [30], which investigated a cohort of 631 middle-aged diabetic and nondiabetic individuals followed for 5 years, found that increased levels of VWF and C-reactive protein were independently associated with cardiovascular and all-cause mortality in both groups. The Prospective Epidemiological Study of Myocardial Infarction (PRIME) examined the association of VWF and coronary heart disease (CHD) in 9,758 healthy men aged 50–59 years old who were followed up for 5 years [31]. Individuals with VWF levels in the upper quartile had a threefold higher risk of ischemic heart disease compared to those with levels in the lowest quartile. Moreover, adjustment for inflammatory markers (C-reactive protein, interleukin-6, and fibrinogen) did not alter the value of VWF as an independent risk factor for coronary heart disease. Recently, in order to investigate the association between the genetic variability of VWF and the risk of coronary artery disease, the Rotterdam Study prospectively examined the association of the −1,793 C/G polymorphism in the VWF gene with coronary heart disease in 1,088 subjects with and without advanced atherosclerosis [32]. The study concluded that this VWF gene polymorphism was associated with an increased risk of coronary heart disease but only in individuals with advanced atherosclerosis. The largest prospective studies of VWF and coronary heart disease are reported in Table 1. Although these epidemiologic studies differ profoundly from each other in terms of design, patient populations enrolled, and end-points evaluated, collectively, they do provide some evidence that high VWF levels are associated with a moderately increased risk of arterial thrombosis [29]. However, a recent study documented that ultrasonographically measured intima–media thickness in atherosclerotic plaques in the carotid and femoral arteries of patients with severe VWD was not different from that in healthy controls [33]. The fact that patients with severe VWD are not protected against atherosclerosis, together with similar findings in animal models, suggests that VWF does not play a role in the atherosclerotic process but only in occlusive arterial thrombosis. Von Willebrand factor and venous thrombosis Fewer studies have been published with regard to the relationship between plasma VWF levels and venous thrombosis [3]. The belief, derived from a few studies conducted in the 1980s [34, 35], that VWF may represent a

418 Table 1 Prospective studies of von Willebrand factor and coronary heart disease Study (reference)

Year No. of subjects Risk ratio (95% confidence interval)*

NPHS (19)

1994 1,393

ECAT Study (20)

1995 3,043

ARIC Study (21)

1997 14,477

Edinburgh Heart Study (23) 1997 1,592 Caerphilly Heart Study (24) 1999 1,997 Hoorn Study (27)

1999

631

Whincup et al. (26)

2002 1,891

PRIME Study (28)

2004 9,758

Rotterdam Study (29)

2004 1,088

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

NPHS Northwick Park Heart Study, ECAT European Concerted Action on Thrombosis, ARIC Atherosclerosis Risk in Communities, PRIME Prospective Epidemiological Study of Myocardial Infarction *The risk ratios and 95% confidence intervals reported are adjusted for the other hemostatic and classical vascular risk factors analyzed in the various studies

risk factor for venous thrombosis was challenged by the results of the Leiden Thrombophilia Study [36], in which VWF was not associated with an increased risk of venous thrombosis. In fact, although the odds ratio for venous thrombosis in patients with VWF levels above 150 IU/dl compared to those with levels lower than 100 IU/dl was 3.0 (95% confidence interval, 1.8 to 4.9) in the univariate analysis, when the results were adjusted for blood group and FVIII in the multivariate analysis, the odds ratio was 1.2 (95% confidence interval, 0.6 to 2.1), suggesting that the effect of VWF on the risk of venous thrombosis was fully explained by FVIII levels. In contrast, the Longitudinal Investigation of Thromboembolism Etiology (LITE) study [37], which followed 19,237 healthy individuals older than 45 years for a mean period of 7.8 years, showed that VWF and FVIII were independently associated with venous thromboembolism in a dose-dependent manner. Another study on a small series of patients found a positive association between VWF and deep vein thrombosis, but it was not established whether the levels were raised prior to the thrombosis or were merely due to a secondary reaction (FVIII levels were not measured) [38]. Finally, based on the fact that VWF as a carrier of FVIII is an important determinant of FVIII levels, other researchers have investigated whether genetic variations in the FVIII and VWF genes could influence FVIII levels. However, none of the several polymorphisms in the VWF and FVIII genes analyzed was found to be associated with VWF, FVIII levels or venous thrombotic risk [39]. Thus, while the analysis of the above data documents that FVIII is associated with an increased risk of venous

thrombosis, less information is available with regard to VWF and venous thrombosis, making such an association uncertain.

Von Willebrand factor and thrombotic thrombocytopenic purpura Since the first description of TTP in 1924, numerous hypotheses on the etiology and pathogenesis of this condition have been proposed. In particular, abnormalities of plasma VWF have been recognized to be associated with TTP for more than 20 years [40]. In fact in 1985, Asada and colleagues used immunohistochemical techniques to show that intravascular thrombi and subendothelial hyaline deposits in TTP react positively for VWF antigen and negatively for fibrin [41]. As reported above, both endothelial cells and megakaryocytes produce multimers of VWF that are larger (ULVWF) and have stronger affinity for platelet glycoprotein receptors than the VWF normally present in the plasma. In vivo, when a vascular lesion occurs in areas of blood flow, ULVWF multimers secreted from endothelial cells adhere to subendothelial collagen and become extended under the high shear stress forces, thus, localizing the platelets, bound to VWF through the GPIb-IX-V complex, to the site of the injury and arresting the bleeding. Newly secreted ULVWF multimers are immediately cleaved on the endothelial surface into smaller multimers by the metalloprotease ADAMTS13, which is produced predominantly in the liver and prevents the accumulation of pathogenic ULWVF multi-

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mers, thus, protecting against uncontrolled platelet adhesion [5, 13, 42, 43]. Although a VWF-cleaving protease was hypothesized to be the cause of TTP in 1982 by Moake and colleagues [44], its existence was demonstrated only in 1996 independently by Furlan [45] and Tsai [46]. These two groups subsequently published evidence that the activity of this protease was consistently and severely reduced in patients with TTP, either because of a congenital deficiency or an acquired deficiency caused by an autoantibody [47–49]. In fact, in two retrospective studies on a large number of patients with acute TTP [48, 49], these researchers showed that most patients had severely deficient VWF-cleaving protease activity, which in the majority of cases of acute sporadic TTP was caused by circulating IgG autoantibodies inhibiting the VWF-cleaving protease activity. In most patients with acute acquired idiopathic TTP who achieved a remission, the VWFcleaving protease activity normalized and the inhibiting autoantibodies disappeared. In contrast, patients with a hereditary form of TTP remained severely deficient in VWF-cleaving protease activity, even in remission, and their plasma contained no inhibitory autoantibodies. These results were confirmed by further studies. In particular, Veyradier and colleagues [50], in a large study on 111 patients with thrombotic microangiopathy, found that VWF-cleaving protease activity was deficient in the idiopathic form of the microangiopathy and in most subsets of TTP and that an inhibitor to metalloprotease could be detected in about half of the TTP patients. In 2001, VWF-cleaving protease was purified from normal plasma and was characterized as a member of the so-called ADAMTS family of proteases [51–54]. In the same year, through a genomic analysis of patients with hereditary TTP and their relatives, Levy and colleagues [55] detected the gene responsible, ADAMTS13, on chromosome 9q34. The authors identified several mutations of this gene as being presumably responsible for the severely deficient ADAMTS13 activity and disease in homozygous or compound heterozygous carriers of mutated alleles, whereas, family members with a heterozygous ADAMTS13 mutation had approximately 50% protease activity and were clinically asymptomatic. Various groups of researchers have since reported many new mutations throughout the ADAMTS13 gene and to date, more than 40 missense and nonsense mutations have been identified [5].

Von Willebrand disease and thrombosis It is easy to appreciate from the foregoing that VWF plays an important role in thrombogenesis. It is, however, decidedly less intuitive to imagine thrombotic complications of VWD, a disease that is expected to be associated with a clinical picture of bleeding manifestations [4]. However, in some cases, the coexistence of acquired and/or inherited prothrombotic risk factors may overcome the bleeding tendency and lead to the development of thrombotic complications [56–60]. These thrombotic events, which occur only rarely in VWD patients [61], may be associated

or not with the infusion of FVIII/VWF concentrates; events associated with such infusions are more frequently venous, whereas, events not related to infusions are more frequently arterial. However, before discussing these issues more fully, it is worth focusing briefly on a particular subtype of VWD— type 2B—as the recent elucidation of the physiopathological mechanisms of this subtype has led to a better understanding of the relationship between VWF and thrombosis [62]. Why do patients with type 2B WVD suffer from bleeding rather than thrombosis? Although it may seem paradoxical that patients with VWD can develop thrombotic complications, it is equally paradoxical that the 2B subtype of VWD is characterized by bleeding rather than thrombotic events. In fact, the VWF in type 2B VWD is structurally abnormal causing enhanced binding to the platelet glycoprotein Ib (GPIb) receptor. As a consequence of this functional alteration, the concentration of the largest VWF multimers in plasma decreases, and the platelet count may be episodically (e.g., after desmopressin infusion or after surgery) decreased as a result of microaggregation [61]. Although it may be surprising that the presence of a hyperfunctional adhesive molecule in the blood causes a bleeding tendency instead of thrombosis, this is only apparently paradoxical and can be explained starting from the physiology of VWF [62]. Under normal conditions, large VWF multimers circulate in the blood in an inactive form in which the A1 domains (where the GPIbα binding site is located) are protected from interactions with platelet GPIb. Interactions between platelets, mediated by the VWF–GPIbα bond, occur in blood but only rarely and are reversible [63]. This situation is radically different at the site of a wound, where VWF multimers bind through A3 domains to the exposed collagen of the extracellular matrix and become extended and immobilized, thus, exposing the A1 domains. The consequent adhesion of platelets to the immobilized VWF is initially transient but becomes irreversible once additional bonds with other matrix components have been established [63]. The mutations in type 2B VWD enhance the binding of the VWF A1 domain with platelet GP1b, thus, promoting a more stable interaction with circulating platelets [64]. Thus, the occupancy of platelet GP1b receptors in the circulation makes the interaction of platelets with VWF at a wound site, where it is needed for normal hemostasis, less efficient [62]. Moreover, in parallel with VWF-dependent platelet aggregation in the circulation, ADAMTS13-dependent proteolysis of VWF subunits is markedly increased, and the functionally important large VWF multimers are removed from plasma. Thus, there is in fact, a good explanation for how a gain-of-function mutation that increases VWF–GPIb binding, and might be expected to cause microvascular thrombosis, is actually associated with a loss-of-function phenotype causing a bleeding tendency [62].

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Thrombotic complications in VWD patients after infusion of clotting factor concentrates Most reports of thrombotic complications in VWD patients concern those after infusion of coagulation factor concentrates. In fact, cases of thrombosis in VWD patients after administration of various FVIII/VWF concentrates have been reported [64–67], and nowadays, an evaluation of postinfusion thombotic complications is included in the reported safety profile of every factor concentrate product [66, 67]. Thrombosis in this setting may be attributed to various factors, the most important being high post-treatment levels of FVIII (>150 UI/dl), an established risk factor for venous thromboembolism [68], reached when FVIII/VWF concentrates are administered at short intervals. In fact, in this setting, exogenous FVIII infused with the concentrate adds to the endogenously synthesized FVIII which is stabilized by the infused VWF [64]. Makris and colleagues [65] described four cases of venous thromboembolism after treatment with the intermediate purity FVIII concentrate Haemate P but all of them had additional risk factors (older age, surgery, estrogen intake). Mannucci reported the case of a patient with type 3 VWD who developed a non-fatal pulmonary embolism 12 days after hip replacement surgery. A postoperative check of coagulation parameters documented very high FVIII levels (up to 400%) [69]. Recently, the same author [64] carried out a questionnaire survey on the occurrence of venous thromboembolism in patients with VWD treated with FVIII/VWF concentrates in the last 10 years in 52 hemophilia centers and found a low incidence of thromboembolic events (seven cases in 12,640 treatments over 10 years), although higher than that observed in patients with hemophilia A (two cases in 141,250 treatments). On the basis of these results, the author suggested that FVIII plasma levels should be measured daily in VWD patients treated with FVIII/VWF concentrates to avoid excessive levels of the clotting factor, and that thromboprophylaxis should be given to those patients undergoing major surgical procedures, especially if other risk factors for venous thromboembolism are present (i.e., old age, previous thrombosis, presence of prothrombotic gene mutations, orthopedic surgery, obesity, immobility, hormone replacement therapy). Moreover, both authors (Makris and Mannucci) outlined the importance of using the FVIII/VWF concentrate with the highest ratio between VWF:RCof and FVIII:C, to correct the VWF defect without increasing FVIII:C plasma levels excessively. Two venous thrombotic complications were recorded in a multicenter, prospective study evaluating a high purity FVIII/VWF concentrate in 81 patients with VWD [67]. Finally, a myocardial infarct after administration of recombinant activated factor VII was described in a patient with type 2A VWD [70]. Thrombotic complications in VWD patients not associated with clotting factor concentrate use Rare cases of thrombotic events not associated with the infusion of clotting factor concentrates have been reported in

patients with VWD. The largest series was published in 1982 by Goodnough and colleagues who, reviewing personal and literature data, reported 23 cases of myocardial infarction or arterial thrombosis in VWD patients [71]. In 1989, Dulhoste and colleagues reported the cases of three patients with VWD (two mild and one severe) who developed atherosclerotic lesions and thrombosis [72]. More recently, Fragasso, et al. [63] reported an acute myocardial infarct occurring in a 61-year old man with type 1 VWD, successfully treated with a thrombolytic agent (recombinant tissue plasminogen activator). Other studies have reported thrombosis in VWD patients with concomitant inherited prothrombotic risk factors. This association was first described in 1986 by Girolami, et al., who noted that the concomitant presence of VWD in a patient with antithrombin deficiency could have a protective action against thrombotic manifestations [73]. Bowen, et al. [60] described a patient with type III VWD and concomitant protein C and antithrombin deficiencies who experienced deep venous thrombosis and pulmonary embolism. Although two additional cases have recently been reported by us [61, 62], the association between VWD and inherited prothrombotic risk factors remains a controversial issue which requires further studies on large populations of patients to confirm or refute these preliminary findings. Moreover, as the presence of prothrombotic factors may modulate the clinical phenotype of severe hemophilia [74], some authors have found that the severity of bleeding symptoms in type 1 VWD depends on functional defects in platelet aggregation [75] or DNA polymorphisms in the platelet membrane proteins integrin α2, αIIb and GPVI [76]. Finally, although only a few cases of myocardial infarction have been described in patients with congenital or acquired bleeding conditions treated with desmopressin [77–80], this drug should be used cautiously in elderly VWD patients with atherosclerotic disease [6].

Conclusions Greater understanding of the structure and function of VWF and the mechanisms that underlie its interactions with vascular and platelet surfaces can aid the elucidation of important aspects of normal hemostasis and pathological processes leading to thrombosis. The prospective studies so far published provide some evidences of an involvement, although at a moderate level, of VWF in the process of arterial thrombosis. However, further investigations are warranted to assess the exact role of VWF in the spectrum of multiple thrombotic risk factors. Finally, literature data show that thrombotic complications of VWD are rare and are often, if not always, associated with multiple acquired and/or inherited prothrombotic risk factors. The thrombotic event occurs when these risk factors balance or even outweigh the bleeding tendency of the disease. Future studies should be aimed at identifying these risk factors as precisely as possible to be able to prevent the development of thrombotic episodes in VWD patients in particular situations, such as major surgery.

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