Structure of von Willebrand factor-cleaving protease (ADAMTS13), a metalloprotease involved in thrombotic thrombocytopenic purpura

Thrombotic microangiopathies: an update

Thrombotic microangiopathies (TMA) are microvascular occlusive disorders characterized by hemolytic anemia caused by fragmentation of erythrocytes and thrombocytopenia due to increased platelet aggregation and thrombus formation, eventually leading to disturbed microcirculation with reduced organ perfusion. Although several disease states may manifest as TMA, the two most relevant conditions associated with TMA are thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), characterized by prominent brain or renal lesions, respectively. However, occasionally the clinical distinction between these two conditions can be difficult. In this review, we focus on the epidemiologic and diagnostic criteria as well as on the most recent insights into the pathophysiology and treatment of these two conditions.

Platelet-neutrophil-interactions: Linking hemostasis and inflammation

Platelets are essential for primary hemostasis, but they also play an important pro-inflammatory role. Platelets normally circulate in a quiescent state. Upon activation, platelets can secrete and present various molecules, change their shape as well as the expression pattern of adhesion molecules. These changes are associated with the adhesion of platelets to leukocytes and the vessel wall. The interaction of platelets with neutrophils promotes the recruitment of neutrophils into inflammatory tissue and thus participates in host defense. This interaction of neutrophils with platelets is mainly mediated through P-selectin and beta(2) and beta(3) integrins (CD11b/CD18, CD41/CD61). Platelets can also interact with endothelial cells and monocytes. Adherent platelets promote the 'secondary capture' of neutrophils and other leukocytes. In addition, platelets secrete neutrophil and endothelial activators inducing production of inflammatory cytokines. Thus, platelets are important amplifiers of acute inflammation.

Effect of von Willebrand disease type 2B and type 2M mutations on the susceptibility of von Willebrand factor to ADAMTS-13

BACKGROUND

von Willebrand disease (VWD) type 2 is associated with mutations in von Willebrand factor (VWF) that affect its secretion, multimeric pattern, affinity for platelet receptors and clearance of the protein. While increased proteolysis by a disintegrin-like and metalloprotease with thrombospondin type 1 motifs-13 (ADAMTS-13) has been clearly established for VWF type 2A, only little is known about VWF types 2B and 2M in this regard.

OBJECTIVES

Sensitivity of wild-type (WT) and mutated recombinant (r) VWF to proteolysis by ADAMTS-13 was investigated to better understand the role of this process in the pathophysiology of VWD.

METHODS

We used human rADAMTS-13-WT to digest 11 full-length recombinant forms of VWF carrying molecular abnormalities identified in patients with VWD type 2A (E1638K and P1648S), type 2B (InsM1303, R1306W, R1308P and V1314F) and type 2M (G1324A, E1359K, K1362T, R1374H and I1425F).

RESULTS

Using low ionic strength conditions, all mutations induced increased proteolysis of rVWF by rADAMTS-13 as compared with rVWF-WT. The susceptibility of mutants decreased in the following order: type 2A > type 2B > type 2M > rVWF-WT. At physiological salt concentration (150 mm NaCl) the sensitivity of all rVWF to rADAMTS-13 was significantly decreased. However, type 2A and type 2B mutants still exhibited a significantly higher susceptibility to rADAMTS-13 than rVWF-WT, whereas type 2M mutants normalized.

CONCLUSIONS

Type 2M mutants and rVWF-WT exhibit a similar sensitivity to rADAMTS-13-mediated proteolysis, in agreement with the normal multimeric pattern in vivo. In VWD type 2B, the spontaneous binding to platelets and excessive degradation by ADAMTS-13 of VWF high-molecular-weight multimers may account for their clearance from plasma.

Journey in reverse: TTP from bedside to blood bank to bench

Thrombotic thrombocytopenic purpura (TTP) is the most extensive and dangerous intravascular platelet clumping disorder. For more than a half-century after its initial recognition, mortality was near 100% and the etiology totally obscure. Then, in the late 1970s to early 1980s, empiric, but successful, therapy by a few clinician/blood bank partnerships was followed by sudden laboratory insight into pathophysiology. The discussion that follows was prepared in conjunction with the 2006 Francis Morrison, M.D., Memorial Lecture at the 27th Annual Meeting of the American Society for Apheresis.

Changes of clotting factors (7, 9 and 10) and hepatocyte growth factor in patients with thrombotic microangiopathy after bone marrow transplantation

To investigate risk factors for thrombotic microangiopathy (TMA) after bone marrow transplantation (BMT), the levels of three clotting factors (7, 9 and 10) and hepatocyte growth factor (HGF) were measured. Among 46 consecutive patients who underwent BMT, six developed TMA and 40 did not. The levels of the clotting factors and HGF did not differ significantly between the six patients with TMA and the 40 patients without it. In two patients who developed TMA during the earlyperiod after BMT, however, the levels of the three clotting factors were significantly decreased even before BMT, along with a significant increase of HGF. These findings suggest that patients with severe hepatic dysfunction before BMT, especially those with impaired protein synthesis, had an increased risk of developing TMA soon after BMT. It was also suggested that measurement of clotting factors (7, 9 and 10) and HGF may be useful to predict the occurrence of TMA in the early period after BMT.

Decreased ADAMTS13 activity in plasma from patients with thrombotic thrombocytopenic purpura

The ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type I domain 13) activity was measured by a fluorescence resonance energy transfer (FRET) assay in the plasma of healthy volunteers and thrombotic thrombocytopenic purpura (TTP) patients to examine its usefulness in the diagnosis of TTP. The plasma levels of the ADAMTS13 activity did not show a normal distribution. Its median value was 107% (range: 55-170%) in healthy volunteers, but was significantly lower in patients with TTP (acquired or familial) and in patients with hematopoietic stem cell transplantation. However, it was not significantly lower in patients with antiphospholipid syndrome (APS). The ADAMTS13 activity by a FRET assay was closely correlated with that by the ADAMTS13 multimer method (r=0.816; p<0.001). In 18 patients with less than 10% of ADAMTS13 activity by FRET assay, less than 10% of that by multimer assay was 16, thus suggesting a good correlation for a low level of ADAMTS13. These findings suggest that the ADAMTS13 FRET assay correlates well with the ADAMTS13 multimer method and it is therefore useful for making a diagnosis of TTP.

TTP and ADAMTS13: When Is Testing Appropriate?

The last 10 years witnessed the publication of many studies on the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a life-threatening disease characterized by microangiopathic hemolytic anemia, thrombocytopenia and multiorgan failure. The most important finding was the identification of a novel metalloprotease, named ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motives), that is involved in the regulation of the size of von Willebrand factor (VWF), a major modulator of platelet adhesion and aggregation in the microcirculation. Inherited or acquired deficiencies of ADAMTS13 impair VWF cleavage, leading in turn to the disseminated formation of platelet-rich thrombi in the micro-circulation and to symptoms of end-organ ischemia. By measuring ADAMTS13 in plasma, it has been clearly shown that patients with inherited TTP have severe ADAMTS13 deficiency. However, patients with acquired TTP present with clinical and laboratory heterogeneity, and there are unequivocal cases of acquired TTP with measurable plasma levels of ADAMTS13. This heterogeneity poses a challenge for understanding the pathogenesis of TTP and selecting appropriate therapies.

Normal levels of ADAMTS13 and factor H are present in the pharmaceutically licensed plasma for transfusion (Octaplas®) and in the universally applicable plasma (Uniplas) in development

BACKGROUND AND OBJECTIVES

The pathomechanism of thrombotic thrombocytopenic purpura (TTP) and atypical haemolytic uraemic syndrome (aHUS) is associated with a severe deficiency of ADAMTS13 and factor H. The aim of this study was to quantify the levels of ADAMTS13 and factor H in the pharmaceutically licensed plasma for transfusion, Octaplas, and the universally applicable plasma, Uniplas (development product, working title). Furthermore, Octaplas batches of blood groups A, B, O, AB, and plasmas derived from different sources were compared.

MATERIALS AND METHODS

Twenty-four Octaplas and three Uniplas batches were selected for the study. ADAMTS13 activities were measured by fluorescence resonance energy transfer assay, ADAMTS13 antigen levels were quantified using enzyme-linked immunosorbent assay test kit, while factor H antigen levels were detected using radial immunodiffusion (RID) methods. In addition, von Willebrand factor (vWF) multimeric analyses were performed.

RESULTS

Both Octaplas, produced from US and European plasma of different blood groups, and Uniplas contain ADAMTS13 antigen and activity levels as well as factor H concentrations at normal levels without significant differences. In addition, Octaplas and Uniplas show a vWF multimeric pattern comparable to normal plasma.

CONCLUSION

The study revealed that Octaplas and Uniplas contain normal levels of ADAMTS13 at low batch-to-batch variations. Therefore, both products can substitute the missing or neutralized protease activity in TTP patients and thereby limit vWF-dependent (platelet-related) thrombosis. In addition, both plasma products contain factor H at a physiological level, and, thus can be used efficiently in the treatment of aHUS patients, which have been shown to benefit from plasma administration.

Exosite interactions contribute to tension-induced cleavage of von Willebrand factor by the antithrombotic ADAMTS13 metalloprotease

Von Willebrand factor (VWF) is a multimeric protein that mediates platelet adhesion at sites of vascular injury, and ADAMTS13 (a disintegrin and metalloprotease with thrombospondin)is a multidomain metalloprotease that limits platelet adhesion by a feedback mechanism in which fluid shear stress induces proteolysis of VWF and prevents disseminated microvascular thrombosis. Cleavage of the Tyr(1605)-Met(1606) scissile bond in the VWF A2 domain depends on a Glu(1660)-Arg(1668) segment in the same domain and on the noncatalytic spacer domain of ADAMTS13, suggesting that extensive enzyme-substrate interactions facilitate substrate recognition. Based on mutagenesis and kinetic analysis, we find that the ADAMTS13 spacer domain binds to an exosite near the C terminus of the VWF A2 domain. Deleting the spacer domain from ADAMTS13 or deleting the exosite from the VWF substrate reduced the rate of cleavage approximately 20-fold. A cleavage product containing the exosite was a hyperbolic mixed-type inhibitor of ADAMTS13 proteolysis of either VWF multimers or model peptide substrates but only if the ADAMTS13 enzyme contained the spacer domain. The specificity of this unique mechanism depends on tension-induced unfolding of the VWF A2 domain, which exposes the scissile bond and exosite for interaction with complementary sites on ADAMTS13.

ABO blood group determines plasma von Willebrand factor levels: a biologic function after all?

For many years, an association between ABO histo-blood group and risk of thrombosis has been recognized. Blood group non-O (A, B, and AB) individuals have consistently been found to demonstrate increased incidence of both arterial and venous thrombotic disease, compared to group O individuals. This increased risk is attributable to the fact that ABO blood group influences plasma levels of a coagulation glycoprotein named von Willebrand factor (VWF). VWF levels are 25 percent higher in non-O compared to group O individuals. The mechanism by which ABO group determines plasma VWF levels has not been determined. ABO(H) carbohydrate antigenic determinants, however, are expressed on the N-linked glycan chains of circulating plasma VWF. This review will focus on the carbohydrate structures of VWF and recent studies suggesting that subtle variations in these structures (particularly differences in ABO blood group antigen expression) may have clinically significant effects on VWF proteolysis and clearance.

High von Willebrand Factor Levels Increase the Risk of First Ischemic Stroke

Background and Purpose— Elevated von Willebrand factor (vWF) concentrations are associated with an increased risk of ischemic heart disease. Several factors influence vWF antigen levels and activity, including blood group, genetic variability, acute-phase response, and proteolysis by A D isintegrin a nd M etalloprotease with T hrombo S pondin motif (ADAMTS13), a determinant of proteolytic cleavage of vWF. We assessed how these factors affect the relation between vWF and the occurrence of stroke to understand the underlying mechanism.

Methods— In a case-control study of 124 first-ever ischemic stroke patients and 125 age- and sex-matched controls, we studied vWF antigen (vWF:Ag), vWF ristocetin cofactor activity (vWF:RCo), ADAMTS13 activity, the −1793 C/G polymorphism in the vWF gene, and C-reactive protein.

Results— vWF antigen and activity levels were significantly higher in cases than in controls. The relative risk of ischemic stroke was highest in individuals in the upper quartile of vWF:Ag (odds ratio, 3.2; 95% CI, 1.4 to 7.5) and vWF:RCo (odds ratio, 2.1; 95% CI, 0.9 to 4.8) compared with individuals in the lowest quartiles. In individuals with ADAMTS13 in the lowest quartile, the relative risk of stroke was 1.7 (95% CI, 0.7 to 3.9) compared with the highest quartile. C-reactive protein, ADAMTS13, and genetic variation did not affect the association between vWF and the relative risk of stroke, whereas blood group did affect the association.

Conclusions— vWF antigen and activity are associated with the occurrence of acute ischemic stroke. This relation is unaffected by the severity of the acute-phase response or by genetic variation or degradation.

Multiple B-cell clones producing antibodies directed to the spacer and disintegrin/thrombospondin type-1 repeat 1 (TSP1) of ADAMTS13 in a patient with acquired thrombotic thrombocytopenic purpura

BACKGROUND

The cysteine-rich/spacer domains of ADAMTS13 contain a major binding site for antibodies in patients with acquired thrombotic thrombocytopenic purpura (TTP).

OBJECTIVE

To study the heterogeneity of the antibody response towards these domains an immunoglobulin V-gene phage-display library was constructed to isolate monoclonal anti-ADAMTS13 antibodies from the immunoglobulin repertoire of a patient with acquired TTP.

METHODS

Combined variable heavy chain (VH) and variable light chain (VL) segments, expressed as single-chain Fv fragments (scFv), were selected for binding to an ADAMTS13 fragment consisting of the disintegrin/thrombospondin type-1 repeat 1 (TSP1)/cysteine-rich/spacer domains.

RESULTS

Seven different scFv antibody clones were identified that were assigned to four groups based on their homology to VH germline gene segments. Epitope-mapping revealed that scFv I-9 (VH1-69), I-26 (VH1-02), and I-41 (VH3-09) bind to an overlapping binding site in the ADAMTS13 spacer domain, whereas scFv I-16 (VH3-07) binds to the disintegrin/TSP1 domains. The affinity of scFv for the disintegrin/TSP1/cysteine-rich/spacer domain was determined by surface plasmon resonance analysis and the dissociation constants ranged from 3 to 254 nM. The scFv partially inhibited ADAMTS13 activity. However, full-length IgG prepared from the variable domains of scFv I-9 inhibited ADAMTS13 activity more profoundly. Plasma of six patients with acquired TTP competed for binding of scFv I-9 to ADAMTS13.

CONCLUSION

Our data indicate that multiple B-cell clones producing antibodies directed against the spacer domain are present in the patient analyzed in this study. Our findings also suggest that antibodies with a similar epitope specificity as scFv I-9 are present in plasma of other patients with acquired TTP.

ADAMTS13 and microvascular thrombosis

Interaction between platelet and von Willebrand factor, a circulating adhesive glycoprotein, is essential for hemostasis under the high shear environments of arterioles and capillaries. If unregulated, this interaction may lead to unwarranted platelet thrombosis. ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, number 13), a plasma zinc metalloprotease synthesized primarily in the stellate cells of the liver, cleaves shear stress-activated von Willebrand factor, thereby preventing the occurrence of von Willebrand factor-platelet interaction in the circulation. A profound deficiency of ADAMTS13, due to genetic mutations or autoimmune inhibition, results in intravascular von Willebrand factor platelet aggregation and widespread microvascular thrombosis characteristic of thrombotic thrombocytopenic purpura. Cloning of ADAMTS13 and structure-function analyses of the enzyme are leading to exciting advances in the diagnosis and therapy of this hitherto mysterious disease.

ADAMTS13 and von Willebrand factor and the risk of myocardial infarction in men

Von Willebrand factor (VWF) mediates the tethering/adhesion of platelets at sites of vascular injury. This function depends on its multimeric size, which is controlled by ADAMTS13. We measured plasma ADAMTS13 and VWF antigen levels by enzyme-linked immunosorbent assay (ELISA) in a large population-based case-control study (Study of Myocardial Infarctions Leiden [SMILE]), consisting of 560 men with a first myocardial infarction (MI) and 646 control subjects. Although ABO blood groups influenced VWF levels, they had no influence on ADAMTS13. Furthermore, there was no relationship between plasma ADAMTS13 and VWF levels. Similar to VWF, the estimated risk of MI was increased for every quartile of ADAMTS13 when compared to the lowest quartile (odds ratio, 1.5-1.6). If confirmed, the association of ADAMTS13 with MI may suggest an unexpected mechanistic action of ADAMTS13.

Apical sorting of ADAMTS13 in vascular endothelial cells and Madin-Darby canine kidney cells depends on the CUB domains and their association with lipid rafts

ADAMTS13 biosynthesis appeared to occur mainly in hepatic stellate cells, but detection of ADAMTS13 mRNA in many other tissues suggests that vascular endothelium may also produce ADAMTS13. We showed that ADAMTS13 mRNA and protein were detectable in human umbilical vein endothelial cells, aortic endothelial cells, and endothelium-derived cell line (ECV304). ADAMTS13 in cell lysate or serum-free conditioned medium cleaved von Willebrand factor (VWF) specifically. ADAMTS13 and VWF were localized to the distinct compartments of endothelial cells. Moreover, ADAMTS13 was preferentially sorted into apical domain of ECV304 and Madin-Darby canine kidney (MDCK) cells. Apical sorting of ADAMTS13 depended on the CUB domains and their association with lipid rafts. A mutation in the second CUB domain of ADAMTS13 (4143-4144insA), naturally occurring in patients with inherited thrombotic thrombocytopenic purpura, resulted in a significant reduction of ADAMTS13 secretion and a reversal of its polarity in MDCK cells. These data demonstrated that ADAMTS13 is synthesized and secreted from endothelial cells; the apically secreted ADAMTS13 from endothelial cells may contribute significantly to plasma ADAMTS13 proteases. The data also suggest a critical role of the CUB domains and a novel cargo-selective mechanism for apical sorting of a soluble ADAMTS protease in polarized cells.

Novel monoclonal antibody-based enzyme immunoassay for determining plasma levels of ADAMTS13 activity

BACKGROUND

ADAMTS13 specifically cleaves unusually large von Willebrand factor (VWF) multimers, which induce platelet thrombi formation under high shear stress. ADAMTS13 activity is deficient in patients with thrombotic thrombocytopenic purpura (TTP). The determination of plasma levels of ADAMTS13 activity is a prerequisite for a differential diagnosis of thrombotic microangiopathies. Here, a unique and highly sensitive enzyme immunoassay (EIA) of ADAMTS13 activity is described.

STUDY DESIGN AND METHODS

ADAMTS13 hydrolyzes the peptide bond between Y1605 and M1606 of VWF. In this assay, a recombinant fusion protein (GST-VWF73-His) is used as a substrate. A panel of mouse monoclonal antibodies (MoAbs) that specifically recognizes Y1605, which is the C-terminal edge residue of the VWF-A2 domain and is generated by the enzymatic cleavage, has been produced. These antibodies were prepared with a synthetic decapeptide, termed N-10 (1596-DREQAPNLVY-1605), as the immunogen. Twenty-six clones specific to N10 were obtained, and one anti-N10 MoAb was used in this study.

RESULTS

With horseradish peroxidase-conjugated anti-N10 MoAb, a standard enzyme assay was established. This assay was highly sensitive, and the detection limit was 0.5 percent of the normal. Further, an inhibitor of ADAMTS13 was measured to a level of 0.1 Bethesda units per mL. ADAMTS13 activity was measured in 20 patients with Upshaw-Schulman syndrome, a congenital TTP, and 61 acquired TTP patients. The activity measured by this assay and by the classic VWF multimer assay showed high correlation.

CONCLUSION

A convenient and highly sensitive EIA for ADAMTS13 activity has been established. This assay can be introduced for routine laboratory work in transfusion medicine.

Expression of ADAMTS-8, a secreted protease with antiangiogenic properties, is downregulated in brain tumours

Angiogenesis and extracellular matrix degradation are key events in tumour progression, and factors regulating stromal-epithelial interactions and matrix composition are potential targets for the development of novel anti-invasive/antiangiogenic therapies. Here, we examine the expression of ADAMTS-8, a secreted protease with antiangiogenic properties, in brain tissues. Using quantitative RT-polymerase chain reaction (PCR), high, equivalent expression of ADAMTS-8 was found in normal whole brain, cerebral cortex, frontal lobe, cerebellum and meninges. ADAMTS-8 expression in 34 brain tumours (including 22 high-grade gliomas) and four glioma cell lines indicated at least two-fold reduction in mRNA compared to normal whole brain in all neoplastic tissues, and no detectable expression in 14 out of 34 (41%) tumours or four out of four (100%) cell lines. In contrast, differential expression of TSP1 and VEGF was seen in nine out of 15 (60%) and seven out of 13 (54%) tumours, with no relationship in the expression of these genes. Immunohistochemistry and Western analysis indicated downregulation of ADAMTS-8 protein in >77% tumours. Methylation-specific PCR analysis of ADAMTS-8 indicated promoter hypermethylation in one out of 24 brain tumours (a metastasis) and three out of four glioma cell lines suggesting an alternative mechanism of downregulation. These data suggest a role for ADAMTS-8 in brain tumorigenesis, warranting further investigation into its role in regulation of tumour angiogenesis and local invasion.

Novel ADAMTS-13 mutations in an adult with delayed onset thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is associated with congenital and acquired deficiency of ADAMTS-13, a metalloprotease that cleaves von Willebrand factor (VWF) and reduces its adhesive activity. Mutations throughout the ADAMTS13 gene have been identified in congenital TTP patients, most of whom have initial episodes during infancy or in early childhood.

PATIENTS AND METHODS

We report the case of an adult male who was diagnosed with idiopathic thrombocytopenic purpura at age 34, and with TTP 14 years later. The patient was compound heterozygous for an 18 bp in-frame deletion (C365del) in the disintegrin domain and a point mutation of R1060W in the seventh thrombospondin domain of the ADAMTS-13 gene.

CONCLUSIONS

In vitro studies found that C365del and R1060W severely impair ADAMTS-13 synthesis in transfected Hela cells, whereas the deletion mutant also failed to cleave VWF under static and flow conditions.

Inhibitory autoantibodies against ADAMTS-13 in patients with thrombotic thrombocytopenic purpura bind ADAMTS-13 protease and may accelerate its clearance in vivo

BACKGROUND

Many patients with acquired thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies that may bind and/or inhibit ADAMTS-13 proteolytic activity and accelerate its clearance in vivo.

METHODS

To test this hypothesis, we determined ADAMTS-13 activity and antigen levels in parallel plasma samples from patients clinically diagnosed with TTP. Collagen binding, GST-VWF73 and FRETS-VWF73 assays were used to determine ADAMTS-13 activity and to detect inhibitory autoantibodies. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation plus Western blotting (IP/WB) were used to detect total anti-ADAMTS-13 IgG (inhibitory and non-inhibitory).

RESULTS

Among 40 patients with TTP (21 idiopathic and 19 non-idiopathic), inhibitory autoantibodies were detected (by FRETS-VWF73) in 52% of idiopathic and 0% of non-idiopathic TTP patients. In contrast, non-inhibitory IgG autoantibodies were detected in 29% of idiopathic and 50% of non-idiopathic TTP patients. The concentration of inhibitory IgG autoantibody in idiopathic TTP patients was significantly higher than that of non-inhibitory IgG in either idiopathic or non-idiopathic TTP patients. Idiopathic TTP patients demonstrated significantly reduced ADAMTS-13 activity compared with non-idiopathic patients, but only slightly lower ADAMTS-13 antigen levels. Interestingly, patients with inhibitory autoantibodies exhibited significantly lower ADAMTS-13 antigen levels than those with only non-inhibitory IgG autoantibodies or no autoantibody. Serial plasma exchanges increased levels of ADAMTS-13 activity and antigen concurrently in patients with inhibitory autoantibodies.

CONCLUSION

The identification of severe ADAMTS-13 deficiency and autoantibodies or inhibitors appears to be assay-dependent; the inhibitory IgG autoantibodies, in addition to binding and inhibiting ADAMTS-13 proteolytic activity, may accelerate ADAMTS-13 clearance in vivo.

Assay of the von Willebrand factor (VWF) propeptide to identify patients with type 1 von Willebrand disease with decreased VWF survival

Type 1 von Willebrand disease (VWD) is characterized by a partial quantitative deficiency of von Willebrand factor (VWF). Few VWF gene mutations have been identified that cause dominant type 1 VWD. The decreased survival of VWF in plasma has recently been identified as a novel mechanism for type 1 VWD. We report 4 families with moderately severe type 1 VWD characterized by low plasma VWF:Ag and FVIII:C levels, proportionately low VWF:RCo, and dominant inheritance. A decreased survival of VWF in affected individuals was identified with VWF half-lives of 1 to 3 hours, whereas the half-life of VWF propeptide (VWFpp) was normal. DNA sequencing revealed a single (heterozygous) VWF mutation in affected individuals, S2179F in 2 families, and W1144G in 2 families, neither of which has been previously reported. We show that the ratio of steady-state plasma VWFpp to VWF:Ag can be used to identify patients with a shortened VWF half-life. An increased ratio distinguished affected from unaffected individuals in all families. A significantly increased VWFpp/VWF:Ag ratio together with reduced VWF:Ag may indicate the presence of a true genetic defect and decreased VWF survival phenotype. This phenotype may require an altered clinical therapeutic approach, and we propose to refer to this phenotype as type-1C VWD.

An alternative spliced transcript of ADAMTS4 is present in human synovium from OA patients

The major proteoglycan of articular cartilage aggrecan is a substrate for ADAMTS4. RT-PCR analysis of human osteoarthritic (OA) synovial co-cultures using oligonucleotide primers designed to amplify across the exon 8/9 junction of human ADAMTS4 resulted in the amplification of two products, the expected product and a smaller product missing 161 bp from the 5' end of exon 9, the result of alternative splicing in which exon 8 joins to a cryptic 3' splice site within exon 9. The protein produced would be identical to human ADAMTS4 up to Arg(696), and would have a new C-terminal domain with no commonality with the ADAMTS4 spacer domain. Changes in the C-terminal domain of ADAMTS4 may alter its substrate specificity.

Elevated levels of von Willebrand Factor in cirrhosis support platelet adhesion despite reduced functional capacity

Cirrhosis of the liver is frequently accompanied by complex alterations in the hemostatic system, resulting in a bleeding tendency. Although many hemostatic changes in liver disease promote bleeding, compensatory mechanisms also are found, including high levels of the platelet adhesive protein von Willebrand Factor (VWF). However, conflicting reports on the functional properties of VWF in cirrhosis have appeared in literature. We have measured a panel of VWF parameters in plasma from patients with cirrhosis of varying severity and causes. Furthermore, we assessed the contribution of VWF to platelet adhesion, by measuring the ability of plasma from patients with cirrhosis to support adhesion of normal or patient platelets under flow conditions. VWF antigen levels were strongly increased in patients with cirrhosis. In contrast, the relative collagen binding activity, as well as the relative ristocetin cofactor activity, was significantly lower in patients as compared with controls, indicating loss of function. Accordingly, patients had a reduced fraction of high-molecular-weight VWF multimers. Both strongly elevated and reduced activity and antigen levels of the VWF cleaving protease ADAMTS13 were found in individual patients. Adhesion of either normal or patient platelets to a collagen surface was substantially increased when these platelets were resuspended in plasma of patients with cirrhosis, as compared with control plasma. In conclusion, highly elevated levels of VWF in patients with cirrhosis contribute to the induction of primary hemostasis despite reduced functional properties of the molecule. This phenomenon might compensate for defects in platelet number and function in patients with cirrhosis.

Insights into von Willebrand factor proteolysis: clinical implications

The proteolysis of von Willebrand factor (VWF) by the recently discovered metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin repeats), is a normal processing step in VWF biochemistry. Emerging data indicate that this step may be influenced by a variety of factors, some of which favour increased proteolysis and some of which compromise proteolysis. The former may predispose to bleeding, whilst the latter appears to be the underlying mechanism for thrombotic thrombocytopenic purpura (TTP). The new insights support the concept of "risk" in bleeding, particularly in the case of type 1 von Willebrand disease (VWD), in much the same way that risk is considered in venous thrombosis. This review presents relevant current knowledge of VWF proteolysis by ADAMTS13, and a novel model of how this may be implicated in type 1 VWD is proposed, based on events at the vessel wall at a time of haemostatic challenge.

A classification of hemolytic uremic syndrome and thrombotic thrombocytopenic purpura and related disorders

The diagnostic terms hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP) are based on historical and overlapping clinical descriptions. Advances in understanding some of the causes of the syndrome now permit many patients to be classified according to etiology. The increased precision of a diagnosis based on causation is important for considering logical approaches to treatment and prognosis. It is also essential for research. We propose a classification that accommodates both a current understanding of causation (level 1) and clinical association in cases for whom cause of disease is unclear (level 2). We tested the classification in a pediatric disease registry of HUS. The revised classification is a stimulus to comprehensive investigation of all cases of HUS and TTP and is expected to increase the proportion of cases in whom a level 1 etiological diagnosis is confirmed.

ADAMTS-13 plasma level determination uncovers antigen absence in acquired thrombotic thrombocytopenic purpura and ethnic differences

BACKGROUND

The recently discovered plasma enzyme ADAMTS-13 cleaves the A2-domain of von Willebrand factor (VWF). A defective cleaving protease results in unusually large VWF multimers, which cause thrombotic thrombocytopenic purpura (TTP).

AIM

Analysis of the ADAMTS-13 antigen levels in TTP patients compared with normal donors.

METHODS

An antigen ELISA test was built, based on high affinity anti-ADAMTS-13 monoclonal antibodies, which were generated using genetic immunization.

RESULTS

Specificity of the ADAMTS-13 antigen test was confirmed, as (i) plasma from a patient with acquired TTP but presenting without inhibitor did not contain antigen and (ii) the binding of recombinant ADAMTS-13 was inhibited by increasing amounts of normal plasma. The assay is sensitive as it can detect antigen levels as low as 1.6% of normal. The concentration in normal pooled human plasma was determined (1.03 +/- 0.15 microg mL(-1)) and arbitrarily set to 1 U mL(-1). The antigen levels in congenital TTP samples (34 +/- 21 mU mL(-1), n = 2), as well as in samples from patients with acquired TTP (231 +/- 287 mU mL(-1), n = 11), were clearly reduced when compared with normal Caucasian donors (951 +/- 206 mU mL(-1), n = 16). Remarkably, normal Chinese donors have a significantly lower antigen titer (601 +/- 129 mU mL(-1), n = 15), when compared with normal Caucasians.

CONCLUSIONS

Our results show that acquired TTP patients suffer mainly from ADAMTS-13 antigen depletion, thereby indicating the importance of ADAMTS-13 antigen determination in diagnosis and patient follow-up.

Treatment of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP), characterized by thrombocytopenia and microangiopathic haemolytic anaemia, was almost universally fatal until the introduction of plasma exchange (PE) therapy in the 1970s. Based on clinical studies, daily PE has become the first-choice therapy since 1991. Recent findings may explain its effectiveness, which may include, in particular, the removal of anti-ADAMTS13 autoantibodies and unusually large von Willebrand factor multimers and/or supply of ADAMTS13 in acquired idiopathic or congenital TTP. Based on currently available data, the favoured PE regimen is daily PE [involving replacement of 1-1.5 times the patient's plasma volume with fresh-frozen plasma (FFP)] until remission. Adverse events of treatment are mainly related to central venous catheters. The potential reduction of plasma related side-effects, such as transfusion-related acute lung injury (TRALI) or febrile transfusion reactions by use of solvent-detergent treated (S/D) plasma instead of FFP is not established by controlled clinical studies. Uncontrolled clinical observations and the hypothesis of an autoimmune process in a significant part of the patients with acquired idiopathic TTP suggest a beneficial effect of adjunctive therapy with corticosteroids. Other immunosuppressive treatments are not tested in controlled trials and should be reserved for refractory or relapsing disease. There is no convincing evidence for the use of antiplatelet agents. Supportive treatment with transfusion of red blood cells or platelets has to be evaluated on a clinical basis, but the transfusion trigger for platelets should be very restrictive. Further controlled, prospective studies should consider the different pathophysiological features of thrombotic microangiopathies, address the prognostic significance of ADAMTS13 and explore alternative exchange fluids to FFP, the role of immunosuppressive therapies and of new plasma saving approaches as recombinant ADAMTS13 and protein A immunoadsorption.

A practical approach to genetic testing for von Willebrand disease

von Willebrand disease (vWD) is the most commonly diagnosed congenital bleeding disorder. The laboratory diagnosis of type 2 variants and type 3 vWD is reasonably well defined, and characterization of the von Willebrand factor (vWF) gene has facilitated definition of their molecular basis. However, for type 1 vWD, the laboratory diagnosis poses a diagnostic dilemma, and knowledge of its molecular basis is evolving. Characterization of the vWF gene and refinement of genetic techniques have led to an evolving repertoire of genetic tests. Genetic testing is costly, and thus judicious use will be increasingly important for appropriate genetic-counseling of patients with vWD and their family members. This article provides a practical approach to utilization of genetic testing in vWD.

Increased ADAMTS-13 proteolytic activity in rat hepatic stellate cells upon activation in vitro and in vivo

INTRODUCTION

ADAMTS-13 is a member of A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats (ADAMTS) family, primarily synthesized in hepatic stellate cells (HSCs), one of the major cell types transdifferentiating into myofibroblasts during liver fibrosis. However, the association between ADAMTS-13 expression and HSC activation or liver fibrosis is not known.

METHODS

In this study, we determined the ADAMTS-13 mRNA, protein, and activity in isolated primary HSCs upon activation on a plastic dish and in liver after administration of carbon tetrachloride (CCl(4)) in rats.

RESULTS

We showed that ADAMTS-13 antigen and proteolytic activity in the activated rat HSCs were dramatically increased, whereas ADAMTS-13 mRNA in these cells was only minimally altered. Similarly, the ADAMTS-13 antigen and proteolytic activity in rat liver after CCl(4) injury were also significantly increased, whereas the ADAMTS-13 mRNAs in these liver tissues were only slightly increased compared with normal. Surprisingly, despite the dramatic up-regulation of ADAMTS-13 protein synthesis in the activated HSCs after CCl(4) administration, the plasma levels of ADAMTS-13 protease in rats did not increase concordantly.

CONCLUSION

We conclude that the up-regulation of ADAMTS-13 protein expression in rat HSCs during activation in vitro and in vivo suggests the possibility of ADAMTS-13 proteolysis, an important part of function of the activated HSCs, perhaps through modulation of liver regeneration or formation of liver fibrosis after various injuries. The data also suggest the minimal contribution of the activated HSCs in regulation of plasma levels of ADAMTS-13 protease.

Von Willebrand factor and thrombosis

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.

Systemic antithrombotic effects of ADAMTS13

The metalloprotease ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type I repeats 13) cleaves highly adhesive large von Willebrand factor (VWF) multimers after their release from the endothelium. ADAMTS13 deficiency is linked to a life-threatening disorder, thrombotic thrombocytopenic purpura (TTP), characterized by platelet-rich thrombi in the microvasculature. Here, we show spontaneous thrombus formation in activated microvenules of Adamts13^−/− mice by intravital microscopy. Strikingly, we found that ADAMTS13 down-regulates both platelet adhesion to exposed subendothelium and thrombus formation in injured arterioles. An inhibitory antibody to ADAMTS13 infused in wild-type mice prolonged adhesion of platelets to endothelium and induced thrombi formation with embolization in the activated microvenules. Absence of ADAMTS13 did not promote thrombi formation in αIIbβ3 integrin-inhibited blood. Recombinant ADAMTS13 reduced platelet adhesion and aggregation in histamine-activated venules and promoted thrombus dissolution in injured arterioles. Our findings reveal that ADAMTS13 has a powerful natural antithrombotic activity and recombinant ADAMTS13 could be used as an antithrombotic agent.

Novel compound heterozygote mutations (H234Q/R1206X) of the ADAMTS13 gene in an adult patient with Upshaw–Schulman syndrome showing predominant episodes of repeated acute renal failure

BACKGROUND

Unlike acquired thrombotic thrombocytopenic purpura or haemolytic uraemic syndrome, which are often intractable, thrombotic microangiopathy in patients with Upshaw-Schulman syndrome (USS)--a congenital deficiency of von Willebrand factor-cleaving protease (ADAMTS13) activity--responds very well to plasma infusion and does not even require plasma exchange. However, the symptoms significantly vary in each individual and thus clinicians often overlook this diagnosis.

METHODS

A 31-year-old adult male patient with thrombotic microangiopathy, which was complicated with repeated episodes of acute renal failure, is reported. We suspected that the patient had USS and performed assays of ADAMT13 activity and its inhibitor, followed by ADAMTS13 gene analysis of the patient and his parents.

RESULTS

The patient had extremely low ADAMTS13 activity and has no inhibitors of ADAMTS13. Through an ADAMTS13 gene analysis of this family, we found two novel mutations responsible for the disease: a missense mutation in exon 7 [702 C --> A (H234Q)] from the father and a nonsense mutation in exon 26 [3616 C --> T (R1206X)] from the mother.

CONCLUSIONS

Our experience appears to indicate the importance of assays of ADAMTS13 activity and its inhibitor in patients who have episodes of renal insufficiency in association with thrombotic microangiopathy, for diagnosis and choice of treatment.

Thrombotic Microangiopathy after Lung Transplantation

BACKGROUND

Thrombotic microangiopathy (TMA) is a well-recognized complication after transplantation. The purpose of this study was to describe our center's experience with this complication after lung transplantation.

METHODS

We retrospectively reviewed cases of TMA among patients who underwent lung transplantation between January 1, 1999 and December 31, 2003 (n = 257). The cases were characterized and the outcomes were analyzed. Univariate and multivariate Cox regression models were constructed to identify potential risk factors for TMA.

RESULTS

Twenty-four cases of TMA developed in 20 recipients. Thirteen cases occurred in the setting of another illness and 11 cases were isolated complications. Multivariate Cox regression models identified female gender, history of TMA, and the immunosuppressive regimen as independent predictors of TMA. Maintenance immunosuppression with the combination of a calcineurin inhibitor and sirolimus carried a significantly higher risk of TMA than a calcineurin inhibitor alone. After the diagnosis of TMA, calcineurin inhibitors were stopped in 18 cases; however, in 6 cases in which the onset of TMA coincided with the addition of sirolimus to a calcineurin inhibitor, only sirolimus was discontinued. Plasmapheresis was performed for severe cases (n = 10). TMA remitted in all cases, and an alternate calcineurin inhibitor was introduced in 14 cases. TMA recurred in 4 recipients, a median 253 days after the initial episode. The median survival after the onset of TMA was 377 days.

CONCLUSION

TMA is a serious complication after lung transplantation, and the risk is highest when sirolimus is used in combination with a calcineurin inhibitor.

A case of mixed connective tissue disease complicated with thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare complication of mixed connective tissue disease (MCTD). In this report, we describe the case of a 73-year-old Japanese woman with MCTD who developed fever, thrombocytopenia, and microangiopathic hemolytic anemia and was diagnosed with MCTD together with TTP. The activity of von Willebrand factor (vWF) cleaving metalloprotease ADAMTS13 was low and considered to have contributed to the disease activity of TTP. The patient died despite intensive treatment of plasma exchange (PEX) and steroid pulse therapy. Autopsy results revealed that the kidneys had platelet and fibrin thrombi, which occluded capillaries and arterioles. These findings were compatible with TTP and the decreased activity of ADAMTS13 was considered to be associated with the disease activity of TTP.

Mechanisms of the interaction between twoADAMTS13 gene mutations leading to severe deficiency of enzymatic activity

The inherited deficiency of the von Willebrand factor-cleaving protease ADAMTS13 is associated with rare forms of thrombotic thrombocytopenic purpura (TTP). We investigated a woman with a family history of chronic recurrent TTP and undetectable plasma levels of ADAMTS13 activity. Genetic analysis revealed two missense mutations in the heterozygous state: p.Val88Met substitution in the metalloprotease domain and p.Gly1239Val substitution in the first CUB domain of ADAMTS13. To explore the mechanism of ADAMTS13 deficiency in this patient, the wild type (WT; ADAMT13(WT)) and each mutant construct (ADAMTS13(Val88Met), ADAMTS13(Gly1239Val)) were transiently expressed in HEK 293 and COS-7 cells. To recapitulate the compound heterozygous state of the patient, both mutant ADAMTS13 proteins were also expressed together. The p.Val88Met mutation led to a defect of secretion of the protease associated with a reduction of enzymatic activity, the p.Gly1239Val mutation led to a secretion defect causing intracellular accumulation of the protease. The mechanistic effects of the mutations were further explored by means of differential immunofluorescence, that demonstrated an homogeneous distribution of ADAMTS13(WT) in the Cis-Golgi and endoplasmic reticulum (ER) compartments, a reduction of ADAMTS13(Val88Met) in both compartments, while ADAMTS13(Gly1239Val) failed to reach the Cis-Golgi compartment and remained in the ER.

Thrombotic thrombocytopenic purpura: a moving target

Almost 80 years after Eli Moschcowitz published the first description of the disease, most patients with idiopathic thrombotic thrombocytopenic purpura (TTP) were found to have acquired autoantibody inhibitors of the ADAMTS13 metalloprotease. Plasma ADAMTS13 normally cleaves von Willebrand factor within nascent platelet-rich thrombi, and ADAMTS13 deficiency allows unchecked thrombus growth to cause microangiopathic hemolysis, thrombocytopenia, and tissue infarction. At present, ADAMTS13 deficiency with a high-titer inhibitor level appears to be associated with an increased risk of early death and subsequent relapse. Thus, acquired ADAMTS13 deficiency identifies a specific mechanism of TTP and is a potential biomarker of disease activity or risk. At present, two major clinical questions in the field may be summarized as follows. First, by emphasizing TTP caused by ADAMTS13 deficiency, are we in danger of neglecting other causes that should be treated with plasma exchange? Second, should we treat asymptomatic patients who have severe ADAMTS13 deficiency to prevent future disease, and if so, how?

Analysis on the Molecular Species and Concentration of Circulating ADAMTS13 in Blood

ADAMTS13 is the metalloprotease responsible for the proteolytic degradation of von Willebrand factor (VWF). A severe deficiency of this VWF-cleaving protease activity causes thrombotic thrombocytopenic purpura. This protease, comprising 1,427 amino acid residues, is composed of multiple domains, i.e., a preproregion, a metalloprotease domain, a disintegrin-like domain, a thrombospondin type-1 motif (Tsp1), a cysteine-rich domain, a spacer domain, seven Tsp1 repeats, and two CUB domains. We prepared one polyclonal and seven monoclonal antibodies recognizing distinct epitopes spanning the entire ADAMTS13 molecule. Of these antibodies, two of the monoclonal ones, which recognize the disintegrin-like and cysteine-rich/spacer domains, respectively, abolished the hydrolytic activity of ADAMTS13 toward both a synthetic substrate, FRETS-VWF73, and the natural substrate, VWF. In addition, these antibodies blocked the binding of ADAMTS13 to VWF. These results revealed that the region between the disintegrin-like and cysteine-rich/spacer domains interacts with VWF. Employing these established polyclonal and monoclonal antibodies, we examined the molecular species of ADAMTS13 circulating in the blood by immunoprecipitation followed by Western blot analysis, and estimated the plasma concentration of ADAMTS13 by enzyme-linked immunosorbent assay. These studies indicated that the major fraction of ADAMTS13 in blood plasma consisted of the full-length form. The concentration of ADAMTS13 in normal plasma was approximately 0.5-1 microg/ml.

The pathogenicity of von Willebrand factor in thrombotic thrombocytopenic purpura: reconsideration of treatment with cryopoor plasma

New developments in the understanding of thrombotic thrombocytopenic purpura (TTP) provide opportunities for improved patient care. A widely held historical model of TTP microvascular thrombosis implicated circulating ultra large von Willebrand factor (ULVWF) in causing spontaneous platelet (PLT) aggregation. From this pathogenic model, concerns about ULVWF in fresh-frozen plasma (FFP) used to treat patients led to widespread use of cryopoor plasma (CPP) as an alternative. There is scant evidence, however, that circulating ULVWF contributes to microvascular thrombosis in TTP. New evidence suggests that the formation of PLT aggregates in TTP may be mediated by VWF in the process of being released from endothelium. Moreover, clinical studies do not demonstrate superior efficacy of CPP compared to FFP in the treatment of TTP. Because CPP may have reduced concentrations of factors important in the treatment of TTP, including ADAMTS13 metalloprotease, a reappraisal of the use of CPP in the treatment of TTP is warranted.

Current concepts in thrombotic thrombocytopenic purpura

Recent advances have demonstrated that thrombotic thrombocytopenic purpura (TTP), characterized by widespread thrombosis in the arterioles and capillaries, is caused by deficiency of a circulating zinc metalloprotease, ADAMTS13. Two types of TTP are recognized: autoimmune TTP, caused by inhibitory antibodies of ADAMTS13, and hereditary TTP, caused by genetic mutations of ADAMTS13. This article reviews the characteristics and function of ADAMTS13, the mechanism by which ADAMTS13 deficiency may lead to thrombosis, and the causes of ADAMTS13 deficiency. It also discusses how the new knowledge may improve the diagnosis and treatment of this previously mysterious disorder.

Advances in the pathogenesis, diagnosis and treatment of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

The thrombotic microangiopathies are microvascular occlusive disorders characterized by hemolytic anemia caused by fragmentation of erythrocytes and thrombocytopenia due to increased platelet aggregation and thrombus formation, eventually leading to disturbed microcirculation with reduced organ perfusion. Depending on whether brain or renal lesions prevail, two different entities have been described: thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS). However, not rarely the clinical distinctions between these two conditions remain questionable. Recent studies have contributed greatly to our current understanding of the molecular mechanisms leading to TTP and HUS. In this review, we briefly focus on the most important advances in the pathophysiology, diagnosis and treatment of these two thrombotic microangiopathies.

Complete deficiency in ADAMTS13 is prothrombotic, but it alone is not sufficient to cause thrombotic thrombocytopenic purpura

ADAMTS13 is a plasma metalloproteinase that regulates platelet adhesion and aggregation through cleavage of von Willebrand factor (VWF) multimers. In humans, genetic or acquired deficiency in ADAMTS13 causes thrombotic thrombocytopenic purpura (TTP), a condition characterized by thrombocytopenia and hemolytic anemia with microvascular platelet thrombi. In this study, we report characterization of mice bearing a targeted disruption of the Adamts13 gene. ADAMTS13-deficient mice were born in the expected mendelian distribution; homozygous mice were viable and fertile. Hematologic and histologic analyses failed to detect any evidence of thrombocytopenia, hemolytic anemia, or microvascular thrombosis. However, unusually large VWF multimers were observed in plasma of homozygotes. Thrombus formation on immobilized collagen under flow was significantly elevated in homozygotes in comparison with wild-type mice. Thrombocytopenia was more severely induced in homozygotes than in wild-type mice after intravenous injection of a mixture of collagen and epinephrine. Thus, a complete lack of ADAMTS13 in mice was a prothrombotic state, but it alone was not sufficient to cause TTP-like symptoms. The phenotypic differences of ADAMTS13 deficiencies between humans and mice may reflect differences in hemostatic system functioning in these species. Alternatively, factors in addition to ADAMTS13 deficiency may be necessary for development of TTP.

Molecular biology of ADAMTS13 and diagnostic utility of ADAMTS13 proteolytic activity and inhibitor assays

ADAMTS13, a reprolysin-like metalloprotease, limits platelet-rich thrombus formation in the small arteries by cleaving von Willebrand factor (vWF) at the Tyr1605-Met1606 peptide bond. Deficiency of plasma ADAMTS13 activity, due to either an inherited or an acquired etiology, may lead to a potentially lethal syndrome, thrombotic thrombocytopenic purpura (TTP). Molecular cloning and characterization of the ADAMTS13 gene have provided further insight into the structure-function relationships, biosynthesis, and regulation of the ADAMTS13 protease, in addition to understanding the pathogenesis of TTP and perhaps other thrombotic disorders. ADAMTS13 consists of a short propeptide, a typical reprolysin-like metalloprotease domain, followed by a disintegrin-like domain, first thrombospondin type 1 (TSP1) repeat, Cys-rich domain, and spacer domain. The carboxyl terminus of ADAMTS13 has seven more TSP1 repeats and two CUB domains. ADAMTS13 is synthesized mainly in hepatic stellate cells, but also in vascular endothelial cells. Recognition and cleavage of vWF require the proximal carboxyl terminal domains, but not the middle and distal carboxyl terminal domains. Cleavage of vWF appears to be modulated by shear force, binding to platelet or platelet glycoprotein-1balpha, heparin, inflammatory cytokine (interleukin-6), and chloride ion. At the site of thrombus formation, the ADAMTS13 may be inactivated by thrombin, plasmin, and factor Xa. Having a sensitive and specific assay for ADAMTS13 activity is not only critical to understand the basic biology of ADAMTS13 protease, but also to facilitate a more timely and accurate clinical diagnosis of TTP, and to initiate potentially life-saving plasma exchange therapy. Although many assays have been developed and tested for clinical applications, the fluorescent resonance energy transfer-vWF73 assay appears to be the simplest and most promising assay to date.

Analysis of full length ADAMTS6 transcript reveals alternative splicing and a role for the 5' untranslated region in translational control

The ADAMTS (A Disintegrin and metalloproteinase with thrombospondin-1 type repeats) family of enzymes have been implicated in turnover of extracellular matrix. We previously showed that levels of ADAMTS6 mRNA in ARPE-19 cells were markedly increased following treatment with tumour necrosis factor alpha (TNFalpha). This study shows that the ADAMTS6 transcript contains unusually large untranslated regions (UTRs) at both the 5' and 3'end. The 5'UTR contains 11 AUG codons upstream of the predicted ADAMTS6 start codon and potently inhibits translation of a downstream reporter gene. However some translation can be restored by truncating the 5'UTR from the 5'end. The 5'UTR was tested for internal ribosome entry site activity using a bicistronic luciferase reporter plasmid, but none was detected. Using the 5' and 3'UTR sequences to screen the GenBank database we identified a full length ADAMTS6 cDNA of 7262 bp. This transcript is alternatively spliced at the 3'end of the open reading frame (ORF), resulting in an extended ORF containing 3 additional tsp-1 type repeats. Quantitative RT-PCR showed that the long and short form of the ADAMTS6 ORF are co-expressed in ARPE-19 cells, but the relative levels of the two forms is modulated by TNFalpha. The region of the transcript encoding the catalytic domain also contains several notable differences compared to the previously published ADAMTS6 cDNA sequence, including a redefinition of the predicted active site motif.

Size regulation of von Willebrand factor-mediated platelet thrombi by ADAMTS13 in flowing blood

The metalloproteinase ADAMTS13 regulates the size of released von Willebrand factor (VWF) multimers bound to endothelial cells, but it is unknown whether it can cleave plasma VWF during thrombogenesis. To address this issue, we perfused blood over immobilized VWF and used videomicroscopy to visualize an activation-independent platelet aggregation process mediated by soluble VWF at shear rates greater than 10 000 s(-1). At normal Ca2+ concentration, platelets formed rolling as well as surface-attached clusters that grew larger during the first 5 minutes but then lost more than 70% of their mass by 10 minutes. In contrast, platelet clusters were stable in size when metal ions were chelated, anti-ADAMTS13 IgG were added, or washed blood cells were perfused with purified VWF but no plasma. In the latter case, addition of recombinant ADAMTS13 reduced platelet cluster size by more than 70%. Incubating ADAMTS13 with VWF before perfusion did not prevent the initial platelet clustering, indicating that the enzyme may act on platelet-bound VWF under shear stress. At the concentrations tested, ADAMTS13 had no effect on platelet aggregates formed upon blood perfusion over collagen fibrils. ADAMTS13, therefore, may regulate thrombus size preferentially when the cohesion between platelets depends on VWF binding induced by pathologically elevated shear stress.