IntroductionPlatelet plug formation at the site of vascular injury is initiated by von Willebrand factor (VWF) interacting with the subendothelial matrix, followed by its binding to platelet glycoprotein (GP) Ib 1,2 and subsequent platelet activation and aggregation. VWF is synthesized by endothelial cells and megakaryocytes, 3,4 and one of its main particular features is a polymer structure ranging in size from 500 000 to more than 20 million Dalton, 5 the largest forms being hemostatically the most efficient. 6 A broad range of values characterizes plasma VWF levels, which average around 10 g/mL. Acquired and inherited factors both modulate plasma VWF levels, and twin studies have demonstrated that 66% of all variations in plasma VWF are genetically determined, while 30% of them depend on ABO blood group, 7 O blood group individuals having plasma VWF levels 25% lower than non-O subjects. 8 ABO group genotyping shows that O 1 O 1 subjects have the lowest VWF levels, and non-O group individuals heterozygous for the O 1 allele have significantly lower VWF levels than AA, AB, or BB subjects. 9,10 Glycosylation accounts for 19% of VWF by weight, and ABO determinants identified on the N-linked oligosaccharide chains are part of this glycosylation process. 11,12 ABO groups are added to the N-linked glycan chains of VWF in the post-Golgi compartment of endothelial cells before VWF secretion, albeit with the variable contribution of the endothelial cells from different vascular beds. 13 The carbohydrate moiety plays an important part in VWF polymerization and function, 14 and also affects the liver-mediated clearance of VWF. In animal models, the removal of sialic acid has been shown to induce an increase in VWF clearance, 15 and the half-life of VWF is halved in mice characterized by an aberrantly glycosylated VWF (due to the absence of the enzyme ST3Gal-IV). 16 Moreover, recombinant VWF, lacking in carbohydrate, is cleared from the circulation faster than its glycosylated counterpart, 16 and posttranslational changes in VWF induced by Galgt2, aberrantly expressed in endothelial cells, lead to a 20-fold increase in VWF clearance. 17 ABO group determinants may also regulate the susceptibility of VWF to the proteolytic action of ADAMTS13, proteolysis being faster in the case of the O blood group. 18 A different susceptibility to cleavage by ADAMTS13 may thus be one of the ways in which ABO group affects VWF removal from the circulation and consequent VWF levels.Although the mechanisms behind ABO blood group and VWF levels have yet to be fully clarified, it has been clearly demonstrated that the effects are mediated by the ABO antigen structures on the N-linked oligosaccharide chains of circulating VWF, and particularly by H antigen expression. 19 Understanding these mechanisms is of clinical relevance: non-O individuals have been shown to carry a significantly greater risk of venous thromboembolism, ischemic heart disease, and peripheral vascular disease, 21-23 while the O blood group is much more common in von Willebra...

Type Vicenza variant of von Willebrand disease (VWD) is characterized by a low plasma von Willebrand factor (VWF) level and supranormal VWF multimers. Two candidate mutations, G2470A and G3864A at exons 17 and 27, respectively, of the VWF gene were recently reported to be present in this disorder. Four additional families, originating from northeast Italy, with both mutations of type Vicenza VWD are now described. Like the original type Vicenza subjects, they showed a mild bleeding tendency and a significant decrease in plasma VWF antigen level and ristocetin cofactor activity but normal platelet VWF content. Unlike the original patients, ristocetin-induced platelet aggregation was found to be normal. Larger than normal VWF multimers were also demonstrated in the plasma. Desmopressin (DDAVP) administration increased factor VIII (FVIII) and VWF plasma levels, with the appearance of even larger multimers. However, these forms, and all VWF oligomers, disappeared rapidly from the circulation. The half-life of VWF antigen release and of elimination was significantly shorter than that in healthy counterparts, so that at 4 hours after DDAVP administration, VWF antigen levels were close to baseline. Similar behavior was demonstrated by VWF ristocetin cofactor activity and FVIII. According to these findings, it is presumed that the low plasma VWF levels of type Vicenza VWD are mainly attributed to reduced survival of the VWF molecule, which, on the other hand, is normally synthesized. In addition, because normal VWF-platelet GPIb interaction was observed before or after DDAVP administration, it is proposed that type Vicenza VWD not be considered a 2M subtype. (Blood. 2002;99:180-184)

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SummaryReduced von Willebrand factor (VWF) half-life has been suggested as a new pathogenic mechanism in von Willebrand disease (VWD). The usefulness of VWF propeptide (VWFpp) in exploring VWF half-life was assessed in 22 type 1 and 14 type Vicenza VWD patients, and in 30 normal subjects, by comparing the findings on post-Desmopressin (DDAVP) VWF t 1/2 elimination (t 1/2el ). The VWFpp/VWF antigen ratio (VWFpp ratio) was dramatically increased in type Vicenza VWD (13AE02 ± 0AE49) when compared to normal subjects (1AE45 ± 0AE06), whereas it appeared to be normal in all type 1 VWD patients (1AE56 ± 0AE7), except for the four carrying the C1130F mutation (4AE69 ± 0AE67). A very short VWF t 1/2el was found in type Vicenza VWD (1AE3 ± 0AE2 h), while all type 1 VWD patients had a t 1/2el similar to that of the controls (11AE6 ± 1AE4 and 15AE4 ± 2AE5 h respectively), except for the four patients carrying the C1130F mutation, who had a significantly shorter VWF survival (4AE1 ± 0AE2 h). A significant inverse correlation emerged between VWFpp ratio and VWF t 1/2el in both VWD patients and normal subjects. The VWFpp ratio thus seemed very useful for distinguishing between type 1 VWD cases with a normal and a reduced VWF survival, as well as for identifying type Vicenza VWD.

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SummaryCushing syndrome (CS) features high-glucocorticoid secretion and an associated hypercoagulable state often involving an increase in von Willebrand factor (VWF). To identify any influence of VWF promoter on glucocorticoid haemostatic effects, four polymorphic positions (-3267, -2708, -2659 and -2525) segregating as haplotypes 1 (GCAG) or 2 (CTGA) were analysed in 50 CS patients with high VWF (group I) and normal VWF (group II) levels, divided by ABO group. Genotype distribution differed significantly between the two groups: in group I, 25AE8% had genotype 1/1, 22AE6% had 2/2 and 38AE7% had 1/2; in group II, 0% had genotype 1/1, 57AE9% had 2/2 and 31AE6% had 1/2 (P = 0AE03). Patients' genotypes also differed from those of controls (P = 0AE003 for group I, P = 0AE03 for group II). Haplotype 1 was prevalent in group I, haplotype 2 in group II (P = 0AE002), both with frequencies differing from controls (P < 0AE001 and P = 0AE009). By odds ratio analysis, genotype 1/1 carried a 12 times greater risk of high-VWF levels than genotype 2/2, and haplotype 1 carried a five times greater risk than haplotype 2. Our findings suggest that VWF promoter haplotypes influence the corticosteroid-mediated increase in VWF.

The defective FVIII carrier function of von Willebrand factor (VWF) identifies type 2N von Willebrand disease (VWD), a variant with a pattern resembling hemophilia A. Type 2N characterization is based on the evaluation of the capacity of VWF to bind exogenous FVIII (VWF:FVIIIB). Here we report on a retrospective evaluation of hemostatic laboratory parameters most useful in detecting type 2N carriers. The diagnostic capacity of aPTT, FVIII, VWF:Ag, FVIII/VWF:Ag ratio, VWF:FVIIIB and VWF:FVIIIB/VWF:Ag ratio was evaluated in 21 type 2N VWD carriers. Twenty subjects were heterozygous for the R854Q mutation, one was heterozygous for the R760C missense mutation, which interferes with cleavage of the VWF propeptide. We found that prolongation of aPTT and decrease in FVIII and FVIII/VWF:Ag ratio were not frequent findings in type 2N carriers. The same was true for VWF:FVIIIB which was not always abnormal. On the contrary, VWF:FVIIIB/VWF:Ag ratio was always defective and its values were not related with FVIII and FVIII/VWF:Ag ratio or influenced by plasma VWF concentration. Given these results, we attribute the greatest significance to VWF:FVIIIB/VWF:Ag ratio in the diagnosis of type 2N defects, and only search for type 2N mutations, to validate the diagnosis, if the ratio proves abnormal.