Insights into von Willebrand factor proteolysis: clinical implications

Influence of ABO blood group on von Willebrand factor tests in healthy Saudi blood donors

: Von Willebrand disease is a common bleeding disorder. The wide variation in von Willebrand factor (VWF) levels between and within normal individuals highlights the clinical challenge of defining its cutoff value. Although studies on the influence of ethnicity on ABO phenotypes and the levels of VWF have been carried out on different ethnicities, there is a lack of such data among Arab population. We aimed to evaluate the correlation of ABO phenotypes with all the parameters of the minimal test panel of VWF including VWF antigen, VWF activity using the ristocetin cofactor and the collagen binding activity assays, and factor VIII coagulant activity (VWF:Ag, VWF:RCo, VWF:CB and FVIII:C) tested in a normal Arab population, and to estimate ABO-specific normal reference range. Blood samples were collected from 87 healthy donors in Riyadh to determine levels of factor VIII and VWF panel between the various ABO phenotypes. The highest mean values of factor VIII : C (128 U/dl), VWF : Ag (125 U/dl), VWF : RCo (109 U/dl) and VWF : CB (91 U/dl) were observed with type AB and the lowest mean values of factor VIII : C (81 U/dl), VWF : Ag (85 U/dl), VWF : RCo (73 U/dl) and VWF : CB (70 U/dl) corresponded to type O. ABO phenotypes significantly influence plasma levels of VWF parameters in Arab nations as seen with other ethnicity. Hence, ABO-specific normal ranges of the minimal test panel of VWF and factor VIII : C are essential for the appropriate prediction of mild von Willebrand disease. Further study including a larger categorized sample size is required to generalize the test panel on the Arab population.

Association between Von Willebrand factor, disintegrin and metalloproteinase with thrombospondin type 1 motif member 13, d-Dimer and cystatin C levels with retinopathy in type 1 diabetes mellitus

BACKGROUND

We evaluated the association between plasma levels of VWF, ADAMTS13 and d-Dimer, which consist on endothelial dysfunction and hypercoagulability biomarkers, and cystatin C with retinopathy in type 1 diabetic patients.

METHODS

Patients were classified according to presence (n=55) or absence (n=70) of retinopathy. Plasma levels of VWF, ADAMTS13, d-Dimer and cystatin C were evaluated by ELISA and ADAMTS13 activity was evaluated by FRET.

RESULTS

Plasma levels of VWF (p=0.033), ADAMTS13 activity (p=0.014), d-Dimer (p=0.002) and cystatin C (p<0.001) were elevated in diabetic patients with retinopathy compared to those without this complication. The multivariate logistic regression analysis showed that ADAMTS13 activity (p=0.031) d-Dimer (p=0.015) and cystatin C (p=0.001) remained associated with retinopathy after adjustment for age, diabetes duration, use of statin, use of ACEi or angiotensin antagonist, use of acetylsalicylic acid and glomerular filtration rate.

CONCLUSION

ADAMTS13 activity, d-Dimer and cystatin C are associated with retinopathy in type 1 diabetic patients and are promising biomarkers for the diagnosis and monitoring of diabetic retinopathy.

ADAMTS13 content and VWF multimer and triplet structure in commercially available VWF/FVIII concentrates

ADAMTS13 is a metalloproteinase that cleaves von Willebrand factor (VWF) into smaller multimers in vivo. This cleavage creates both the typical multimeric size distribution and the characteristic triplet band distribution of VWF. Here we analysed ADAMTS13 content, VWF multimeric size distribution and VWF triplet structure in five commercial VWF/factor VIII (FVIII) concentrates. The relative distribution of ADAMTS13 activity values corresponded well to the ADAMTS13 antigen values for all examined concentrates except Haemate HS®, which had markedly higher ADAMTS13 antigen/activity ratio, with Fanhdi® and Haemate HS® displaying the most intense ADAMTS13 signal. Interestingly, ADAMTS13 levels did not correlate with the high molecular weight multimer content of the concentrates, but did correlate with VWF triplet distribution. Densitometric quantification showed that Wilate®, Immunate® and Willfact® displayed human plasma-like VWF triplet distribution, whereas Fanhdi® and Haemate HS® showed enhanced content of the faster migrating triplet band, which corresponded well to their higher ADAMTS13 content. In summary, Immunate®, Willfact® and Wilate® had lower levels of ADAMTS13 antigen and activity and exhibited a plasma-like VWF triplet structure. Fanhdi® and Haemate HS® had higher ADAMTS13 content and an altered triplet structure. The possible impact of these observations on function and clinical efficacy of VWF/FVIII concentrates is discussed.

An update on type 2B von Willebrand disease

Type 2B von Willebrand disease (VWD) accounts for fewer than 5% of all VWD patients. In this disease, mutations in the A1 domain result in increased von Willebrand factor (VWF) binding to platelet GPIbα receptors, causing increased platelet clearance and preferential loss of high molecular weight VWF multimers. Diagnosis is complicated because of significant clinical variations even among patients with identical mutations. Platelet transfusion often provides suboptimal results since transfused platelets may be aggregated by the patients' abnormal VWF. Desmopressin may cause a transient decrease in platelet count that could lead to an increased risk of bleeding. Replacement therapy with factor VIII/VWF concentrates is the most effective approach to prevention and treatment of bleeding in type 2B VWD.

ABO blood group polymorphisms and risk for ischemic stroke and peripheral arterial disease

Recent studies have demonstrated association between ABO blood system and thrombosis, indicating that individuals belonging to non-O blood groups (A, B or AB) present an increased risk of venous thrombosis, heart disease, and ischemic stroke (IS) as compared to O blood group carriers. In this study, we investigated the frequency of ABO blood group polymorphisms and its association with IS and peripheral arterial disease. Significant differences were observed for O1 (OR 0.57, 95% CI 0.35-0.95, p < 0.05) and O2 (OR 3.47, 95% CI 1.15-10.28, p < 0.05) alleles among IS patients while significant differences were observed for B phenotype (26.3 vs 9.5%, OR 3.42, 95% CI 1.32-8.76, p = 0.01, patients vs controls, respectively) and alleles A1 (OR 0.31, 95% CI 0.11-0.84, p < 0.05), O2 (OR 4.61, 95% CI 1.59-13.23, p < 0.01) and B (OR 3.42, 95% CI 1.62-7.13, p < 0.001) alleles for PAD patients. O1 allele was an independent variable (OR 0.27, 95% CI 0.12-0.57, p < 0.001) for IS patients. These data suggest the relationship of non-O blood groups in pathogenesis of thrombosis events and a possible protective effect of O blood group.

The molecular genetics of von Willebrand disease

Quantitative and/or qualitative deficiency of von Willebrand factor (vWF) is associated with the most common inherited bleeding disease von Willebrand disease (vWD). vWD is a complex disease with clinical and genetic heterogeneity. Incomplete penetrance and variable expression due to genetic and environmental factors contribute to its complexity. vWD also has a complex molecular pathogenesis. Some vWF gene mutations are associated with the affected vWF biosynthesis and multimerization, whereas others are associated with increased clearance and functional impairment. Moreover, in addition to a particular mutation, type O blood may result in the more severe phenotype. The present review aimed to provide a summary of the current literature on the molecular genetics of vWD.

Conflict of interest:None declared.

Hypercoagulability and cardiovascular disease in diabetic nephropathy

Diabetic nephropathy is the leading cause of end stage renal disease (ESRD) and an important risk factor for cardiovascular disease. Recent studies have shown that increased plasma levels of Von Willebrand factor (VWF) and reduced plasma levels of enzyme ADAMTS13 are associated with diabetic nephropathy and an increased risk of developing cardiovascular disease, suggesting that these markers of hypercoagulability may contribute to an increased risk of cardiovascular disease in diabetic patients with impaired renal function. However, it is still not clear whether VWF and ADAMTS13 are only markers of cardiovascular events or whether they play an active role in the development of these events. It is also unclear how renal injury may affect ADAMTS13 levels, leading consequently to hypercoagulability. The association of diabetic nephropathy, atherosclerotic cardiovascular disease and these hypercoagulability markers is discussed in this review. Insights on the role that renal dysfunction and other possible mechanisms may have in ADAMTS13 metabolism, leading to reduced levels of this enzyme and increased hypercoagulability are also presented.

[Acquired von Willebrand's disease type 2A following arteriovenous fistula for haemodialysis?]

Acquired von Willebrand's disease (aVWD) is considered to be an underestimated cause of unexplained bleeding. Adsorption of von Willebrand factor (VWF) to tumour cells or hydroxyethyl starch and elimination of VWF by autoantibodies as well as shear stress-induced mechanical alteration of VWF with concomitant cleavage by enzymes may lead to an acquired deficiency of VWF and a bleeding disorder. We report a 39-year-old woman who developed spontaneous bleeding five years after surgical creation of an arteriovenous fistula (AVF) for haemodialysis treatment. AVWD type 2A was diagnosed after successful renal transplantation. One year after surgical closure of the AVF, the aVWD could not be verified again. Thus, the aVWD may have developed because of altered blood flow and shear stress inside the arteriovenous fistula.

Type 1 von Willebrand disease: application of emerging data to clinical practice

There has been much recent data published on type 1 von Willebrand disease (VWD) predominantly from three multi-centre cohort studies. These data have influenced a revision of the classification of type 1 VWD and have important implications for the management of this disorder. Patients with low von Willebrand factor (VWF) levels tend to have VWF mutations and VWD is transmitted predictably within families. In patients with VWF levels close to the lower end of the normal range, candidate mutations are found less often, ABO blood group is a more important factor and the disease has variable heritability within families. The importance of bleeding symptoms, in addition to VWF levels, in the diagnosis of type 1 VWD has been highlighted.

C1584 in von Willebrand factor is necessary for enhanced proteolysis by ADAMTS13 in vitro

The cysteine variant of the amino acid change tyrosine/cysteine 1584 (Y/C1584) in von Willebrand factor (VWF) has previously been shown to cosegregate with increased susceptibility of VWF to proteolysis by ADAMTS13. It is not known whether C1584 itself confers increased proteolysis or is linked to a causative change elsewhere in VWF. To address whether C1584 underlies enhanced susceptibility of VWF to ADAMTS13-mediated proteolysis, a single family comprising two heterozygous Y/C1584 individuals and four homozygous Y/Y1584 individuals was investigated. The essential regions of the VWF gene were sequenced in all six individuals and ADAMTS13-mediated proteolysis of plasma VWF was assessed for each individual. Comparison of the VWF coding sequences for the Y/C1584 individuals with those for the Y/Y1584 individuals revealed that two amino acid variants were unique to the heterozygotes: R484 and C1584. The plasma VWF of the two heterozygotes showed increased susceptibility to proteolysis in vitro compared with that of the four homozygotes. In the present study we demonstrate that R484, in the absence of C1584, does not influence VWF proteolysis. Enhanced proteolysis occurred only in the presence of Cys1584. Thus, Cys1584 is necessary for increased susceptibility of VWF to proteolysis by ADAMTS13.

Platelet dysfunction and a high bone mass phenotype in a murine model of platelet-type von Willebrand disease

The platelet glycoprotein Ib-IX receptor binds surface-bound von Willebrand factor and supports platelet adhesion to damaged vascular surfaces. A limited number of mutations within the glycoprotein Ib-IX complex have been described that permit a structurally altered receptor to interact with soluble von Willebrand factor, and this is the molecular basis of platelet-type von Willebrand disease. We have developed and characterized a mouse model of platelet-type von Willebrand disease (G233V) and have confirmed a platelet phenotype mimicking the human disorder. The mice have a dramatic increase in splenic megakaryocytes and splenomegaly. Recent studies have demonstrated that hematopoetic cells can influence the differentiation of osteogenic cells. Thus, we examined the skeletal phenotype of mice expressing the G233V variant complex. At 6 months of age, G233V mice exhibit a high bone mass phenotype with an approximate doubling of trabecular bone volume in both the tibia and femur. Serum measures of bone resorption were significantly decreased in G233V animals. With decreased bone resorption, cortical thickness was increased, medullary area decreased, and consequently, the mechanical strength of the femur was significantly increased. Using ex vivo bone marrow cultures, osteoclast-specific staining in the G233V mutant marrow was diminished, whereas osteoblastogenesis was unaffected. These studies provide new insights into the relationship between the regulation of megakaryocytopoiesis and bone mass.

The significance of evaluating conventional inflammatory markers in Von Willebrand factor measurement

BACKGROUND

The appropriate clinical interpretation of Von Willebrand factor (VWF) measurements is crucial since inherited or acquired deficiencies are responsible for a potentially life threatening bleeding disorder, whereas increased plasma concentrations may be associated with an increased thrombotic risk. Besides age and blood group, the variability introduced by the presence of an acute phase response might contribute to decrease the clinical usefulness of this measurement.

METHODS

To investigate the relationship between conventional inflammatory markers and VWF, 387 consecutive unselected patients undergoing surgical procedures at our University Hospital were systematically investigated for routine laboratory testing, including also VWF Antigen (VWF:Ag), C-reactive protein (CRP) and the erythrocyte sedimentation rate (ESR) as parts of a routine preoperative screening program.

RESULTS

After stratifying the study population according to the upper limit of the reference range of the two inflammatory markers, a significantly increased median VWF:Ag plasma concentration was observed for subjects with increased value of both CRP (CRP>5 mg/L=VWF:Ag>120 UI/dL; CRP<5 mg/L=VWF:Ag 98 UI/dL, p=0.003) and ESR (ESR>30 mm/hg=VWF:Ag>120 UI/dL; ESR<30 mm/hg=VWF:Ag 96 UI/dL, p<0.001). A significantly different frequency distribution of VWF:Ag concentrations was also observed in subjects with values of inflammatory markers above the upper limits of the reference ranges when compared to those with normal values. In particular, the prevalence of subjects with VWF:Ag levels>120 UI/dL was six- and nearly three-times higher in subjects with abnormal values of CRP (55.8% versus 9.3%, p<0.001) and ESR (54.5% versus 18.7%, p<0.001), respectively.

CONCLUSIONS

Results of this investigation demonstrate that the evaluation of conventional inflammatory markers would represent a valuable tool to enhance the clinical usefulness of VWF measurements, especially in patients with transitory and subclinical inflammation. Since VWF and factor VIII are likely to increase in parallel, we suggest that their measurements should be repeated at least after 6 months to one year in the presence of concomitantly increased values of inflammatory markers, such as ESR and/or CRP.

Effect of von Willebrand factor Y/C1584 on in vivo protein level and function and interaction with ABO blood group

Blood group O and the cysteine allele of the Y/C1584 change in von Willebrand factor (VWF) are enriched in type 1 VWD, but neither causes disease. We investigated the effect of C1584, alone and in combination with the ABO blood group, on the level and properties of plasma VWF. A cohort of 5052 blood donors was recruited: 50 donors were heterozygous for Y/C1584 and 5002 were homozygous for Y/Y1584. Mean VWF antigen (VWF:Ag) for heterozygotes (82 +/- 35 IUdL(-1)) was significantly lower than for homozygotes (111 +/- 37 IUdL(-1)) (P < .001). Foreach ABO blood group, VWF:Ag was decreased among Y/C1584 heterozygotes compared with Y/Y1584 homozygotes; a larger decrease was observed for group O. Among donors with VWF:Ag levels of 50 IUdL(-1) or lower, Y/C1584 heterozygosity was markedly enriched (18%) compared with the entire cohort (1.5%). Blood group O was enriched to a lesser extent (2.4%), but Y/C1584 in conjunction with group O was strikingly enriched (34.8%). VWF collagen binding activity (VWF:CB) and ristocetin cofactor activity (VWF:RCo) were significantly lower for Y/C1584 heterozygotes than for Y/Y1584 homozygotes, and a qualitative difference in Y/C1584 plasma VWF multimer profile was observed compared with that for Y/Y1584 VWF. The data support a multifactorial basis for low VWF levels in some individuals.

ADAMTS13 assays and ADAMTS13-deficient mice

PURPOSE OF REVIEW

Thrombotic thrombocytopenic purpura can be induced by acquired or congenital deficiency of the plasma von Willebrand factor-cleaving protease, ADAMTS13. Measurement of ADAMTS13 activity is important for the diagnosis and treatment of microangiopathies including thrombotic thrombocytopenic purpura. Phenotypic analysis of mice lacking the Adamts13 gene is valuable for understanding the pathogenesis of microangiopathies.

RECENT FINDINGS

The minimum substrate for ADAMTS13 activity was identified as 73 amino acid residues in the A2 domain of von Willebrand factor, called VWF73. Several new assays have been developed using this sequence. The VWF73-based assays are rapid, quantitative, and easy to handle, and are well correlated with the measures from previous assays. Mice lacking the Adamts13 gene were produced. The mice were viable and fertile. They showed a prothrombotic state but no symptoms of spontaneous thrombocytopenia, hemolytic anemia, or microvascular thrombosis were observed.

SUMMARY

VWF73-based ADAMTS13 assays will significantly facilitate the accurate diagnosis of microangiopathies and contribute to the improved clinical treatment of these diseases. Accumulated clinical information on patients with ADAMTS13 deficiency and mice lacking the Adamts13 gene indicates that additional environmental or genetic susceptibility factors are required to trigger thrombotic thrombocytopenic purpura.

Severe limb necrosis: primary thrombotic microangiopathy or “seronegative” catastrophic antiphospholipid syndrome? A diagnostic dilemma

An episode of gastroenteritis triggered severe necrosis of all extremities in a previously asymptomatic male. Hepatic and renal involvement were also manifest, while the hematological picture was one of thrombotic microangiopathic hemolytic anemia. Antiphospholipid antibodies were negative. He responded well to a combination of plasma exchange, anticoagulation (heparin), parenteral steroids, and antibiotics, as well as vasodilators (prostacycline) and hyperbaric oxygen, but died because of a cerebral hemorrhage. The differential diagnosis included thrombotic thrombocytopenic purpura/hemolytic-uremic syndrome, or seronegative catastrophic antiphospholipid (Asherson's) syndrome. The dangers of administering such a combination of therapies with anticoagulation, as well as vasodilatation (prostacycline) and hyperbaric oxygen, are highlighted by the case report and emphasized.