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Keywords:

  • desmopressin;
  • inherited bleeding disorders;
  • von Willebrand disease;
  • von Willebrand factor

Abstract

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

Summary.  Von Willebrand disease (VWD) is characterized by a wide heterogeneity of clinical and laboratory phenotypes. The complexity of the phenotype is further increased by a highly variable removal rate of von Willebrand factor (VWF) released by desmopressin, which is independent of post-infusion peak level. After the initial demonstration that a reduced VWF survival is present in patients with R1205H mutation (VWD Vicenza), several other mutations, mostly occurring in the VWF D3 domain, have been shown to be associated with accelerated removal of released VWF. Normal subjects with O blood group show reduced survival after desmopressin, underlining the role of different VWF glycosylation present in ABO blood group. Recent evidence suggests that liver and spleen macrophages are responsible for VWF clearance through uptake and endocellular degradation, but it is still not known why some VWF mutants are more prone to increased clearance.

The level of von Willebrand factor (VWF) in plasma is the result of its production and clearance. Recent studies suggest that in some patients with von Willebrand disease (VWD), increased clearance of the released VWF may contribute to explain the disease-associated phenotype, together with abnormalities in synthesis and/or secretion. A significant role in physiologic and perhaps pathologic VWF clearance seems to be exerted by the ABO blood group system, which is present in the VWF. The mechanism(s) by which VWF clearance is increased in some VWF mutants is (are) still obscure and require(s) a better understanding to elucidate further aspects on VWF physiology in health and disease. In this article, we will review the recent advances on the features of increased VWF clearance.

VWF, VWF propeptide and ABO blood groups

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

VWF is a large glycoprotein, which serves as factor VIII carrier and mediates platelet adhesion to subendothelium and subsequent platelet aggregation [1]. VWF is synthesized in megakaryocytes and endothelial cells with a 22-amino acid signal peptide, 741-amino acid propeptide and 2050-amino acid mature VWF. In cellular compartments, after extensive intracellular modifications [2], VWF is noncovalently associated with its propeptide (VWFpp), which is simultaneously released into circulation from Weibel–Palade bodies in 1 : 1 molar ratio with mature VWF [3]. The VWFpp has a half-life of 2–3 h, while mature VWF has a half-life of 8–12 h [3]. As VWFpp half-life is constant, an increased ratio VWFpp/VWF at steady state has been suggested to reflect an increased clearance of mature VWF [4].

VWF level in plasma depends on several factors. Subjects with O blood group have consistently a lower level, showing an average 25% reduction compared to non-O blood group [5]. The O allele does not code for a glycosyltransferase which in A and B blood group, adds N-acetylgalactosamine and d-galactose to the common H determinant. ABO antigens are added to the N-linked oligosaccharide chains present in mature VWF, but not in the VWFpp. In animal models, the removal of sialyc acid induces rapid VWF removal as sialyc acid prevents premature clearance by receptors, which recognize non-sialylated terminal galactose residues (e.g. hepatic asialoglycoprotein receptor). Recombinant VWF lacking carbohydrates is proeolysed more rapidly than the glycosylated ones [6]. Thus, carbohydrate content seems to be able to protect VWF structure as well as its half-life [reviewed in ref. 6]. The role of this protective mechanism could explain the lower levels of VWF in O group subjects. Moreover, O-blood group subjects have consistently elevated VWFpp/VWF:Ag ratio and a shorter VWF survival after demopressin, again suggesting increased clearance [7].

VWF and ADAMTS-13

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

When released into circulation, mature VWF shows a proportion of ultra-large high molecular weight multimers, which are able to bind spontaneously to glycoprotein 1b receptors on platelet surface. Dangerous formation of VWF-platelet aggregates induced by these highly efficient multimers is prevented by physiologic proteolysis at Tyr1605–Met1606 bond within the A2 domain by the metallo-protease ADAMTS-13. Normal subjects with O blood group show a faster rate of ADAMTS-13-dependent VWF proteolysis leading to relate this increased clearance to ADAMTS-13-dependent proteolysis [8]. Furthermore, the presence of a Tyr to Cys polymorphism at position 1584 results in a more rapid VWF proteolysis by ADAMTS-13 and the concomitance of O blood group and Y1584C shows the strongest effect, suggesting interaction of the polymorphism with ABO blood groups [9]. However, experimental results do not support this hypothesis. In fact, patients with VWD Vicenza R1205H who show increased clearance have a normal proteolysis by ADAMTS-13 [10]. Furthermore, a similar clearance rate of wild type VWF and type 2 A VWD variants highly susceptible to ADAMTS-13 cleavage has been observed in a rat model [11]. On the other hand, VWF mutants associated with an increased clearance (see below) were not associated with an increased ADAMTS-13-dependent proteolysis [12]. Thus, it appears that reduced survival of VWF is modulated by other factors in addition to the activity of ADAMTS-13.

The cellular basis of VWF clearance

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

Until recently, little was known on the cellular basis of VWF clearance. Van Schooten et al. [13] evaluated the spleen and liver tissue sections of VWF-deficient mice, infused with recombinant VWF or recombinant FVIII, and showed that both proteins were targeted to cells of macrophage origin. The binding to macrophages was followed by a rapid uptake and subsequent degradation of internalized protein. The chemical inactivation of these cells resulted in a 2-fold prolonged VWF survival. It is still unclear why only some mutants VWF have a preferential increased uptake by macrophages.

VWF mutants and increased clearance

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

The pattern of response to desmopressin in these patients may significantly vary in terms of magnitude of increase and half-life of elicited VWF post-desmopressin [14]. De la Fuente et al. [15] first demonstrated that patients with type 1 VWD, similarly responsive, could have two different pattern of VWF response: the first, with a return to baseline of the raised FVIII and VWF not reached even after 6 h, and the second, with a fast return to baseline values occurring after 2 h. Casonato et al. [16] showed that patients with VWD Vicenza, characterized by severely reduced plasma FVIII and VWF levels, the presence of ultra-large VWF multimers in plasma, and R1205H mutation in D3 domain, had a marked increase of FVIII and VWF after desmopressin, followed by a rapid disappearance from circulation. An additional amino acid VWF change (M740I) has been identified in some patients with R1205H from the Vicenza area [17]. Although its causative role has not been fully elucidated yet, it appears that its presence does not cause a VWF survival different from that observed with R1205H alone [10,18,19]. An elegant confirmation of these findings was represented by the demonstration that purified recombinant R1205H VWF infused in VWF-deficient mice had a very short residence time (0.3 h) compared with wild-type VWF (2.8 h) [10]. Subsequently, Haberichter et al. [4] showed that two additional mutations (W1144G in the D3 domain and S2179F in D4 domain) were associated with reduced VWF survival after desmopressin, while a normal survival after infusion of a FVIII/VWF concentrate was observed (Table 1). Most importantly, these Authors clearly showed that increased clearance could be predicted by an increased steady-state VWFpp/VWF:Ag ratio, which differentiated affected individuals from unaffected individuals. Recently, it has been demonstrated that C1130F and C1149H mutations, both located in the D3 domain, and C2671Y mutation, located in the CK domain residue, had about 4-fold shorter half-life of mutant recombinant VWF infused into VWF-deficient mice compared with Wild Type VWF, but 2-fold longer than that of R1205H [7]. Similarly, VWF half-life in the patients after desmopressin was 4.5-fold shorter compared with a control group. Consistent with previous results [4,18], also these patients showed an increased VWFpp/VWF:Ag ratio. Millar et al. [19] in a cohort of 26 patients with type 1 VWD showed that as a whole the clearance of VWF after desmopressin was 3-fold increased. This pattern however was neither VWF consistently related to basal VWF:Ag level nor with an increased ratio of VWFpp/VWF:Ag, probably becasuse VWFpp at steady-state was reduced in several patients. Thus, caution should be exercised in extrapolating post-desmopressin VWF clearance to steady-state plasma levels (Table 1). Again, no association between the pattern of VWF clearance and ADAMTS-13-mediated proteolysis was evident. In addition to six patients with significantly shortened VWF:Ag half-lives post-desmopressin, patients with a normal VWFpp/VWF:Ag ratio pre-infusion could have a shortened survival, suggesting that an increased clearance may be associated to a normal ratio. In addition to the R1205H mutation, two novel mutations at the same amino acid position were associated with an increased clearance phenotype (R1205S and R1205C). Interestingly, the shortest survival after desmopressin and the greatest steady-state VWFpp/VWF:Ag ratio was observed in R1205H mutation suggesting a significant role of His1205 in fostering VWF clearance. An additional novel mutation (I1416N in the A1 domain) with a typical type 1 phenotype also showed increased clearance [19]. In a recent study [14], an increased clearance after desmopressin was probably observed in R1315C, R1379C and K1794E mutations in A1–A3 domain and in three patients with compound heterozygosity (Y1146C/S1378F, V1485fs/Y1584C, R2464C/Y1584C), suggesting that increased VWF clearance in VWD is more frequent than previously believed. Some of the mutations associated with this phenotype have been included among type 2 A (e.g. Y1146C) or 2 M (e.g. R1315C) VWD as some abnormalities of VWF multimeric pattern are detectable by using sophisticated methods [20]. Thus, increased clearance of VWF neither is always associated with a typical type 1 VWD multimeric pattern nor exclusively with mutations in the D3 domain.

Table 1.   Phenotypes and genotypes of VWD patients with shortened VWF survival
AuthorsVWF mutationVWF domainPlasma VWF:Ag level (IU/dL) VWFpp/VWF:Ag ratioVWF:Ag T1/2 after desmopressin (hours)Multimeric pattern
  1. *Denotes type 2 A, subtype IIE of the older classification [19]. Type 1 according to Haberichter et al. [4] or 2 A(IIE) according to Budde et al. [19]. All these patients had M740I amino acid change on the same R1205H allele. §All patients are heterozygotes, apart from C2671Y in compound heterozygosity with a VWF gene deletion.

van Schooten et al. [12]C1130FD310–203-fold increased1.62 A (IIE)*
Haberichter et al. [18]C1130FD315.75.22.32 A (IIE)*
Haberichter et al. [18]C1130GD385.20.92 A (IIE)*
Haberichter et al. [18]C1130RD3234.62.72 A (IIE)*
Haberichter et al. [4]W1144GD312–592.1–5.11.4–3.61 or 2 A (IIE)
van Schooten et al. [12]C1149RD310–353-fold increased1.52 A
Casonato et al. [16]R1205HD3<10ND1.24 ± 0.25Vicenza
Haberichter et al. [18]R1205HD35.7–8.410.1–11.11–1.7Vicenza
Millar et al. [19]R1205SD3250.15 (increased)1.91
Millar et al. [19]R1205CD3130.45 (increased)3.31
Millar et al. [19]I1416NA114–180.49–0.55 (increased)3–3.51
Haberichter et al. [4]S2179FD49–213.2–6.41–3.11
van Schooten et al. [12]C2671Y/del§CK103-fold increased1.31

Why some mutants are more prone to reduced survival remains unclear and different mechanisms could be responsible. For example, although all localized in D3 domain and all having a reduced VWF survival, C1130F, C1149R and R1205H have a different impact on VWF functions [12]. Unlike R1205H, C1130F and C1149R display impaired FVIII binding and do not multimerize normally (type 2 A VWD), probably because of a defective binding of VWFpp [12]. On the contrary, an additional mutation associated with reduced survival (C2671Y) showed none of these abnormalities suggesting that this part of VWF could contribute to interactions with clearance receptors.

Clinical implications of a reduced VWF survival in VWD

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

The interaction of Y1584C with O blood group could have an impact on the diagnosis of VWD, particularly in those subjects with doubtful bleeding history and VWF levels of 30–40%. The identification of decreased VWF survival could suggest that perhaps the treatment of these patients would differ from those with a normal VWF survival after desmopressin [4]. No data are available about differences in the bleeding tendency compared to patients with similar low VWF levels and normal clearance and it remains unclear whether the presence of a reduced survival demands a different treatment approach. We have prospectively evaluated the clinical history of 60 patients with VWD Vicenza and 23 patients with C1130F mutation, characterized by severe VWF reduction and increased clearance, during a 6-year follow-up study [21]. During follow-up, 35% and 39% of patients with VWD Vicenza or C1130F respectively did not require treatment for bleeding or prior to invasive procedures. Ten patients with VWD Vicenza and five with C1130F had one to three tooth extraction instances successfully treated with a single desmopressin infusion plus oral tranexamic acid. Four women with VWD Vicenza and six with C1130F had one or two pregnancies during the study period, and successfully completed delivery by using two or three desmopressin infusions over 24–48 h after vaginal delivery. Most minor surgical interventions and post-traumatic bleedings were successfully covered with desmopressin while major surgery, including heart surgery, was easily managed with FVIII/VWF concentrates. Even though the results have not been compared with those of patients with normal VWF survival, it appears that the bleeding tendency in this large group of patients with reduced VWF survival is not significantly severe. Furthermore, despite the short FVIII and VWF half-life post-infusion, most of the clinical situations can be successfully managed with desmopressin, as it usually occurs for other type 1 VWD patients. More studies are needed to clarify definitely if in some clinical circumstances these patients would require a approach different from that for ‘classical’ type 1 patients.

Conclusions

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References

Increasing evidence suggests that a reduced VWF survival is rather frequent in VWD and is associated with a variety of VWF mutations. The exact mechanism by which a rapid removal of particular variants occurs, especially those in D3 domain, requires further studies. However, it appears that desmopressin still plays a major role in the treatment of these patients.

References

  1. Top of page
  2. Abstract
  3. VWF, VWF propeptide and ABO blood groups
  4. VWF and ADAMTS-13
  5. The cellular basis of VWF clearance
  6. VWF mutants and increased clearance
  7. Clinical implications of a reduced VWF survival in VWD
  8. Conclusions
  9. Disclosure of Conflict of Interests
  10. References
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