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

Von Willebrand factor

The adhesive protein von Willebrand factor contributes to platelet function by mediating the initiation and progression of thrombus formation at sites of vascular injury. In the last 2 years, there has been considerable progress in explaining the biologic properties of von Willebrand factor. The three-dimensional structure of specific domains has been explained, with the demonstration of distinct conformational changes in the A1 domain caused by single amino acid substitutions associated with enhanced binding to platelets. The structural and functional properties of the interaction between the von Willebrand factor A1 domain and glycoprotein Ibalpha have also been elucidated in greater detail, bringing researchers closer to understanding how this adhesive bond can oppose the fluid dynamic effects of rapidly flowing blood to initiate thrombus formation and, concurrently, contribute to platelet activation. Because hemodynamic forces greatly influence platelet responses to vascular injury in stenosed and partially occluded arteries, a detailed description of how von Willebrand factor interacts with tissues and platelets may help in the design of more specific therapeutic inhibitors of arterial thrombosis. Moreover, enlightening findings have been obtained on the link between regulation of von Willebrand factor multimer size and microvascular thrombosis. This progress in basic research has provided critical information to define with greater precision the role of von Willebrand factor in vascular biology and pathology.

Severe deficiency of the specific von Willebrand factor-cleaving protease (ADAMTS 13) activity in a subgroup of children with atypical hemolytic uremic syndrome

OBJECTIVE

The von Willebrand factor-cleaving protease (VWF-cp) activity has been reported to be deficient in adults with thrombotic thrombocytopenic purpura (TTP) and generally normal in adults with hemolytic uremic syndrome (HUS). The goal of this study was to determine VWF-cp activity in children with typical postdiarrheal (d+) HUS or atypical non-postdiarrheal (d-) HUS. Study design We measured VWF-cp activity in the plasma of 64 children with either (d+) HUS (n = 41) or (d-) HUS (n = 23).

RESULTS

In the acute phase of HUS, VWF-cp activity was normal (>50%) in 54 children and undetectable (<5%) in one (d+) HUS and in 6 (d-) HUS children. After a 3-month remission, the (d+) HUS patient recovered a 100% VWF-cp activity, and the 6 (d-) HUS patients kept an undetectable level. In these 6 (d-) HUS patients, the disease was characterized by a neonatal onset and several relapses (hemolytic anemia, thrombocytopenia, transient acute renal failure, cerebral ischemia), and sometimes the development of arterial hypertension or end stage renal failure.

CONCLUSION

A subgroup of pediatric patients with atypical (d-) HUS, with hematologic symptoms starting at birth and a recurrent course progressively involving kidney and brain, is related to VWF-cp deficiency and actually corresponds to Upshaw-Schulman syndrome revisited as congenital TTP.

ADAMTS13 gene defects in two brothers with constitutional thrombotic thrombocytopenic purpura and normalization of von Willebrand factor-cleaving protease activity by recombinant human ADAMTS13

Genetic analysis of the ADAMTS13 locus identified six mutations in the ADAMTS13 genes of two brothers suffering from constitutional thrombotic thrombocytopenic purpura (TTP): a stop codon leading to a truncated protein on the paternal ADAMTS13 allele and five amino acid exchanges on the maternal allele, three of which were single nucleotide polymorphisms. The other two mutations, not detected in 230 sequenced alleles of healthy control subjects, are, therefore, probably responsible, alone or as part of a combination, for the severe ADAMTS13 deficiency. We also investigated the feasibility of using recombinant ADAMTS13 (rADAMTS13) for normalization of von Willebrand factor-cleaving protease (VWF-cp) activity in plasma of the two congenitally deficient patients. Addition of rADAMTS13 to their plasma restored the VWF-processing pattern to normal, suggesting the potential usefulness of rADAMTS13 for therapy and prophylaxis of familial TTP.

Platelet activation and the formation of the platelet plug: deficiency of ADAMTS13 causes thrombotic thrombocytopenic purpura

In the circulation, a plasma metalloprotease, ADAMTS13, cleaves von Willebrand factor (vWF) in a shear-dependent manner. This article reviews the role of this cleavage in regulating vWF-platelet interaction and proposes a scheme for understanding how a deficiency of ADAMTS13 results in the development of microthrombi in patients with thrombotic thrombocytopenic purpura.

von Willebrand factor cleaving protease and ADAMTS13 mutations in childhood TTP

Thrombotic thrombocytopenic purpura (TTP) is caused by the persistence of the highly reactive high-molecular-weight multimers of von Willebrand factor (VWF) due to deficiency of the specific VWF-cleaving protease (VWF-CP) ADAMTS13, resulting in microangiopathic disease. The acquired form is caused by autoantibodies against VWF-CP, whereas homozygous or compound heterozygous mutations of ADAMTS13 are responsible for recessively inherited TTP. We investigated 83 children with hemolytic or thrombocytopenic episodes with or without additional neurologic symptoms or renal failure. The presumed diagnosis was chronic idiopathic thrombocytopenic purpura (ITP; n = 50), TTP (n = 8), hemolytic uremic syndrome (HUS; n = 24), and Evans syndrome (n = 1). A severe deficiency of VWF-CP (< or = 5%) was found in all investigated patients with TTP and in none of those with HUS. Additionally, 2 of 50 patients with a prior diagnosis of ITP were deficient for VWF-CP. Antibodies against VWF-CP were found in 4 children. Mutation analysis of the ADAMTS13 gene in the patients deficient in VWF-CP by direct sequencing of all 29 exons identified 8 different mutations, suggesting the hereditary form of TTP in 1 patient with ITP, in the patient with Evans syndrome, and in 5 of the 8 patients with TTP. The phenotype of TTP in childhood can be rather variable. Besides the classical clinical picture, oligosymptomatic forms may occur that can delay the identification of patients at risk.

Low resolution structure determination shows procollagen C-proteinase enhancer to be an elongated multidomain glycoprotein

Procollagen C-proteinase enhancer (PCPE) is an extracellular matrix glycoprotein that can stimulate the action of tolloid metalloproteinases, such as bone morphogenetic protein-1, on a procollagen substrate, by up to 20-fold. The PCPE molecule consists of two CUB domains followed by a C-terminal NTR (netrin-like) domain. In order to obtain structural insights into the function of PCPE, the recombinant protein was characterized by a range of biophysical techniques, including analytical ultracentrifugation, transmission electron microscopy, and small angle x-ray scattering. All three approaches showed PCPE to be a rod-like molecule, with a length of approximately 150 A. Homology modeling of both CUB domains and the NTR domain was consistent with the low-resolution structure of PCPE deduced from the small angle x-ray scattering data. Comparison with the low-resolution structure of the procollagen C-terminal region supports a recently proposed model (Ricard-Blum, S., Bernocco, S., Font, B., Moali, C., Eichenberger, D., Farjanel, J., Burchardt, E. R., van der Rest, M., Kessler, E., and Hulmes, D. J. S. (2002) J. Biol. Chem. 277, 33864-33869) for the mechanism of action of PCPE.

Changes in von Willebrand factor-cleaving protease (ADAMTS13) activity after infusion of desmopressin

von Willebrand factor-cleaving protease (ADAMTS13) cleaves von Willebrand factor (VWF) and regulates its physiologic function. To investigate the relation between ADAMTS13 activity and VWF, we compared ADAMTS13 activity with the VWF-related parameters VWF antigen (VWF:Ag), VWF collagen-binding activity (VWF:CBA), VWF-propeptide, proVWF, and VWF multimeric composition in 10 healthy volunteers and 3 patients with type 1 von Willebrand disease before and after infusing 0.3 microg/kg desmopressin. The VWF-related parameters in the volunteers increased 60 minutes after start of infusion by 3.7-fold for VWF:Ag, 7.2-fold for propeptide, and 2.2-fold for VWF:CBA. Unusually large VWF multimers and traces of proVWF appeared. The ADAMTS13 activity decreased to about half the initial value. After 24 hours values returned to baseline. Patients with type 1 von Willebrand disease showed similar results. We conclude that the inverse correlation between ADAMTS13 and VWF-related parameters suggests a consumption of ADAMTS13 after the desmopressin-induced release of higher multimers of VWF.

Metalloproteinases: their role in arthritis and potential as therapeutic targets

Irreversible degradation of articular cartilage is a major feature of the arthritides, and its prevention is a therapeutic goal which has been difficult to achieve. Enzymes from the matrix metalloproteinase and ADAMTS (a disintegrin, a metalloproteinase, and thrombospondin motif) families are key mediators of cartilage extracellular matrix destruction. Inhibition of metalloproteinase activity is therefore a conceptually attractive therapeutic strategy, although clinical efficacy has not yet been demonstrated. This review outlines the biology behind metalloproteinases as drug targets in the arthritides, and poses important questions for the future design of such therapies.

Transfusion medicine: looking to the future

The evolution of transfusion medicine into a clinically oriented discipline emphasising patient care has been accompanied by challenges that need to be faced as specialists look to the future. Emerging issues that affect blood safety and blood supply, such as pathogen inactivation and more stringent donor screening questions, bring new pressures on the availability of an affordable blood supply. Imminent alternatives for management of anaemia, such as oxygen carriers, hold great promise but, if available, will require close oversight. With current estimates of HIV or hepatitis C viral (HCV) transmission approaching one in 2000000 units transfused, keeping to a minimum bacterial contamination of platelet products (one in 2000) and errors in transfusion, with its estimated one in 800000 mortality rate, assume great urgency. Finally, serious difficulties in blood safety and availability for poor, developing countries require innovative strategies and commitment of resources.

ADAMTS-13 activity in patients with brain and prostate tumors is mildly reduced, but not correlated to stage of malignancy and metastasis

Impaired Von Willebrand factor cleaving activity of ADAMTS-13 was demonstrated in patients with metastasizing and malignant tumors. To investigate the relevance of ADAMTS-13 for tumor progression, we determined ADAMTS-13 activity and VWF:Ag in 80 patients with various malignancies: 30 patients with benign brain tumors, 30 patients with malignant brain tumors, 10 patients with local prostate tumors and 10 patients with metastatic prostate tumors. We found mild ADAMTS-13 deficiency in 17/80 tumor patients, but there was no significant difference in ADAMTS-13 activity between the age- and sex-matched patients with benign and malignant brain tumors nor between the age matched patients with local and metastatic prostate tumors. Patients with malignant brain tumors could only be distinguished from patients with benign tumors by elevated levels of VWF:Ag (p=0.018). Our data are hence in variance to previous studies, which found a distinct inverse correlation between ADAMTS-13 activity and level of metastasis and/or malignancy in diverse cancer-types.

ADAMTS-13 rapidly cleaves newly secreted ultralarge von Willebrand factor multimers on the endothelial surface under flowing conditions

Thrombotic thrombocytopenic purpura (TTP) is a devastating thrombotic disorder caused by widespread microvascular thrombi composed of platelets and von Willebrand factor (VWF). The disorder is associated with a deficiency of the VWF-cleaving metalloprotease, ADAMTS-13, with consequent accumulation of ultralarge (UL) VWF multimers in the plasma. ULVWF multimers, unlike plasma forms of VWF, attach spontaneously to platelet GP Ibalpha, a component of the GP Ib-IX-V complex. We have found that ULVWF multimers secreted from stimulated endothelial cells (ECs) remained anchored to the endothelial surface where platelets and Chinese hamster ovary cells expressing the GP Ib-IX-V complex attached to form long beads-on-a-string structures in the presence of fluid shear stresses in both the venous (2.5 dyne/cm(2)) and arterial (20 and 50 dyne/cm(2)) ranges. Although measurement of the activity of the ADAMTS-13 VWF-cleaving metalloprotease in vitro requires prolonged incubation of the enzyme with VWF under nonphysiologic conditions, EC-derived ULVWF strings with attached platelets were cleaved within seconds to minutes in the presence of normal plasma (containing approximately 100% ADAMTS-13 activity) or in the presence of partially purified ADAMTS-13. By contrast, the strings persisted for the entire period of perfusion (10 minutes) in the presence of plasma from patients with TTP containing 0% to 10% ADAMTS-13 activity. These results suggest that cleavage of EC-derived ULVWF multimers by ADAMTS-13 is a rapid physiologic process that occurs on endothelial cell surfaces.

Successful treatment of a young infant who developed high-titer inhibitors against VWF-cleaving protease (ADAMTS-13): important discrimination from Upshaw-Schulman syndrome

We report herein the case of a 9-month-old female infant with acquired thrombotic thrombocytopenic purpura (TTP), which was initially suspected to be either Upshaw-Schulman syndrome (USS or a congenital TTP) or hemolytic uremic syndrome (HUS) because of onset of clinical signs in infancy and accompanying diarrhea. She received combination therapy of plasma exchange, steroid pulse, and high-dose intravenous immunoglobulin infusion that was initiated before the definitive diagnosis, which resulted in excellent clinical improvement. The retrograde analysis of plasma ADAMTS-13 activity and its inhibitor showed a lack of this enzyme activity and the presence of a high-titer IgG inhibitor (200-320 Bethesda units/mL) to this enzyme activity. From our experience, it was suggested that we should recognize the possibility of the patient with acquired TTP in infancy and the importance of plasma exchange therapy for management of its clinical symptoms.

Cloning, expression, and functional characterization of the von Willebrand factor-cleaving protease (ADAMTS13)

Deficient von Willebrand factor (VWF) degradation has been associated with thrombotic thrombocytopenic purpura (TTP). In hereditary TTP, the specific VWF-cleaving protease (VWF-cp) is absent or functionally defective, whereas in the nonfamilial, acquired form of TTP, an autoantibody inhibiting VWF-cp activity is found transiently in most patients. The gene encoding for VWF-cp has recently been identified as a member of the metalloprotease family and designated ADAMTS13, but the functional activity of the ADAMTS13 gene product has not been verified. To establish the functional activity of recombinant VWF-cp, we cloned the complete cDNA sequence in a eukaryotic expression vector and transiently expressed the encoded recombinant ADAMTS13 in HEK 293 cells. The expressed protein degraded VWF multimers and proteolytically cleaved VWF to the same fragments as those generated by plasma VWF-cp. Furthermore, recombinant ADAMTS13-mediated degradation of VWF multimers was entirely inhibited in the presence of plasma from a patient with acquired TTP. These data show that ADAMTS13 is responsible for the physiologic proteolytic degradation of VWF multimers.

Thrombotic thrombocytopenic purpura and the hemolytic uremic syndrome

OBJECTIVE

To evaluate the usefulness and feasibility of measuring plasma von Willebrand factor (vWF)-cleaving metalloprotease activity (ADAMTS 13) in the differential diagnosis of thrombotic thrombocytopenic purpura (TTP), the hemolytic uremic syndrome, and other thrombotic microangiopathies.

DATA SOURCES

Articles published in the medical literature.

DATA EXTRACTION AND SYNTHESIS

In TTP, a multimeric form of vWF that is larger than that ordinarily found in the plasma may cause systemic platelet aggregation under the high-shear conditions of the microcirculation. ADAMTS 13 is a divalent cation-activated, vWF-cleaving metalloprotease that converts unusually large vWF multimers derived from endothelial cells into smaller vWF forms in normal plasma. ADAMTS 13 is severely reduced or absent in most patients with TTP. The vWF-cleaving metalloprotease is present in fresh-frozen plasma, cryoprecipitate-depleted plasma (cryosupernatant), and in plasma that has been treated with solvent and detergent. The enzyme is defective in children with chronic relapsing TTP. Infusion of any of the plasma products that contain the vWF-cleaving metalloprotease stops or prevents (for about 3 weeks) TTP episodes in these patients. An immunoglobulin (Ig) G autoantibody to the vWF-cleaving metalloprotease is found transiently in many adult patients with acquired acute idiopathic, recurrent, and ticlopidine/clopidogrel-associated TTP. Patients with acquired TTP require plasma exchange, that is, both infusion of a plasma product containing vWF-cleaving metalloprotease and removal of autoantibody and/or unusually large vWF multimers by plasmapheresis. The pathophysiology of platelet aggregation in bone marrow transplantation/chemotherapy-associated thrombotic microangiopathy, as well as in hemolytic uremic syndrome, is not established. In neither condition is there a severe decrease in plasma vWF-cleaving metalloprotease activity, as there is in TTP.

CONCLUSIONS

The presently available lengthy and complicated procedure for estimation of plasma vWF-cleaving metalloprotease activity is not yet practical for rapid diagnostic use. This test has supplanted the equally lengthy and difficult, less specific analysis of plasma vWF multimeric pattern. If the clinical distinction between TTP and hemolytic uremic syndrome is uncertain, it is appropriate to acquire (before therapy) a citrate-plasma sample for the ultimate determination of vWF-cleaving metalloprotease activity.

Mutations and common polymorphisms in ADAMTS13 gene responsible for von Willebrand factor-cleaving protease activity

von Willebrand factor (VWF) is synthesized primarily in vascular endothelial cells and secreted into the plasma as unusually large VWF multimers. Normally, these multimers are quickly degraded into smaller forms by a plasma metalloproteinase, VWF-cleaving protease (VWF-CP). Decreases in the activity of this enzyme result in congenital and acquired thrombotic thrombocytopenic purpura (TTP). The human VWF-CP has recently been purified. Cloning of the corresponding cDNA revealed that the 1,427-aa polypeptide is a member of the ADAMTS gene family, termed ADAMTS13. Twelve rare mutations in this gene have been identified in patients with congenital TTP. Here, we report missense and nonsense mutations in two Japanese families with Upshaw-Schulman syndrome, congenital TTP with neonatal onset and frequent relapses. The comparison of individual ADAMTS13 genotypes and plasma VWF-CP activities indicated that the R268P, Q449stop, and C508Y mutations abrogated activity of the enzyme, whereas the P475S mutant retained low but significant activity. The effects of these mutations were further confirmed by expression analysis in HeLa cells. Recombinant VWF-CP containing either the R268P or C508Y mutations was not secreted from cells. In contrast, Q449stop and P475S mutants were normally secreted but demonstrated minimal activity. Genotype analysis of 364 Japanese subjects revealed that P475S is heterozygous in 9.6% of individuals, suggesting that approximately 10% of the Japanese population possesses reduced VWF-CP activity. We report on a single-nucleotide polymorphism associated with alterations in VWF-CP activity; it will be important to assess this single-nucleotide polymorphism as a risk factor for thrombotic disorders.

Control of von Willebrand factor multimer size and implications for disease

Plasma von Willebrand factor (vWF) is a multimeric protein that mediates adhesion of platelets to sites of vascular injury, however only the very large vWF multimers are haemostatically competent. Plasma vWF derives predominantly from the vascular endothelium and is of a smaller average multimer size to that found in the sub-endothelial matrix. The existence of plasma factors that control the size of vWF multimers and account for this difference has long been suspected. vWF cleaving protease (vWFCP), a metalloproteinase that regulates vWF multimer size by proteolytic cleavage, and thrombospondin-1 (TSP-1), a trimeric glycoprotein that uncouples multimers by reducing the disulfide-bonds which link individual subunits, are two such factors that have recently been identified. The large and ultra large vWF multimers play a central role in arterial thrombosis and agents that regulate their size hold promise as novel anti-thrombotic drugs.

ADAMTS13 and TTP

Thrombotic thrombocytopenic purpura (TTP) has been a mysterious and deadly disease that often could be treated effectively by plasma exchange, but without real understanding of the underlying pathophysiology. Recent advances now suggest that deficiency of a specific von Willebrand factor (VWF) cleaving protease promotes tissue injury in TTP. VWF multimers participate in the formation of platelet thrombi. Proteolytic cleavage of VWF multimers normally limits platelet thrombus growth, and failure to cleave VWF appears to encourage microvascular thrombosis. The VWF cleaving protease proves to be a new member of the ADAMTS family of metalloproteases, designated ADAMTS13. Autoantibodies that inhibit ADAMTS13 cause sporadic TTP, and mutations in the ADAMTS13 gene cause an autosomal recessive form of chronic relapsing TTP. Further studies of ADAMTS13 seem likely to change our approach to the diagnosis and treatment of TTP and other thrombotic microangiopathies.

von Willebrand factor cleaving protease (ADAMTS13) is deficient in recurrent and familial thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Whether measurement of ADAMTS13 activity may enable physicians to distinguish thrombotic thrombocytopenic purpura (TTP) from hemolytic uremic syndrome (HUS) is still a controversial issue. Our aim was to clarify whether patients with normal or deficient ADAMTS13 activity could be distinguished in terms of disease manifestations and multimeric patterns of plasma von Willebrand factor (VWF). ADAMTS13 activity, VWF antigen, and multimeric pattern were evaluated in patients with recurrent and familial TTP (n = 20) and HUS (n = 29). Results of the collagen-binding assay of ADAMTS13 activity were confirmed in selected samples by testing the capacity of plasma to cleave recombinant VWF A1-A2-A3. Most patients with TTP had complete or partial deficiency of ADAMTS13 activity during the acute phase, and in some the defect persisted at remission. However, complete ADAMTS13 deficiency was also found in 5 of 9 patients with HUS during the acute phase and in 5 patients during remission. HUS patients with ADAMTS13 deficiency could not be distinguished clinically from those with normal ADAMTS13. In a subgroup of patients with TTP or HUS, the ADAMTS13 defect was inherited, as documented by half-normal levels of ADAMTS13 in their asymptomatic parents, consistent with the heterozygous carrier state. In patients with TTP and HUS there was indirect evidence of increased VWF fragmentation, and this occurred also in patients with ADAMTS13 deficiency. In conclusion, deficient ADAMTS13 activity does not distinguish TTP from HUS, at least in the recurrent and familial forms, and it is not the only determinant of VWF abnormalities in these conditions.

Von Willebrand factor-cleaving protease (ADAMTS13) in thrombocytopenic disorders: a severely deficient activity is specific for thrombotic thrombocytopenic purpura

A severe deficiency in von Willebrand factor-cleaving protease (ADAMTS13) activity (< 5% that in normal plasma) has been observed in most patients with a diagnosis of thrombotic thrombocytopenic purpura (TTP) but not in those with a diagnosis of hemolytic uremic syndrome. However, ADAMTS13 deficiency has been claimed not to be specific for TTP, since it was observed in various thrombocytopenic and other conditions. We studied 68 patients with thrombocytopenia due to severe sepsis or septic shock (n = 17), heparin-induced thrombocytopenia (n = 16), idiopathic thrombocytopenic purpura (n = 10), or other hematologic (n = 15) or miscellaneous conditions (n = 10). Twelve of the 68 patients had subnormal levels of ADAMTS13 activity (<or= 30%), but none had less than 10%. Thus, the study showed that ADAMTS13 activity is decreased in a substantial proportion of patients with thrombocytopenia of various causes. A severe deficiency of ADAMTS13 (< 5%), identified in more than 120 patients during 1996 to 2001 in our laboratory, is specific for a thrombotic microangiopathy commonly labeled TTP.

The genetics and pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura

PURPOSE OF REVIEW

In recent years there has been a substantial increase in the understanding of the genetics and pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura.

RECENT FINDINGS

In diarrhoeal associated haemolytic uraemic syndrome it has been established that the virulence of Escherichia coli O157 is related to intimin adhesion and the transport of verocytotoxin on polymorphonuclear cells. It has been shown that early changes in the coagulation pathway predate the onset of diarrhoeal haemolytic uraemic syndrome. Mutations in factor H, a fluid-phase regulator of the alternative complement pathway, have been identified in 10-20% of patients with both familial and sporadic (non-diarrhoeal-associated) haemolytic uraemic syndrome. The mutations mainly cluster in the C terminal part of factor H, a region that is important for both binding to C3b and also polyanionic structures on cell surfaces. The identification of antibodies against a plasma metalloproteinase responsible for cleaving ultralarge von Willebrand factor multimers in thrombotic thrombocytopenic purpura has been followed by the elucidation of the identity of the proteinase. It has been shown to be a member of the ADAMTS family, and mutations have been identified in the gene in families with inherited thrombotic thrombocytopenic purpura.

SUMMARY

The molecular pathogenesis of haemolytic uraemic syndrome and thrombotic thrombocytopenic purpura is an exciting and rapidly evolving field. These recent advances will lead to logical, targetted changes in the management of these conditions.

Ultralarge multimers of von Willebrand factor form spontaneous high-strength bonds with the platelet glycoprotein Ib-IX complex: studies using optical tweezers

Ultralarge von Willebrand factor (ULVWF) multimers have been implicated in the pathogenesis of the catastrophic microangiopathic disorder, thrombotic thrombocytopenic purpura. Spontaneous ULVWF binding to platelets has been ascribed to increased avidity due to the greatly increased number of binding sites for platelets (the A1 domain) per molecule. To address the mechanism of enhanced ULVWF binding to platelets, we used optical tweezers to study the unbinding forces from the glycoprotein Ib-IX (GP Ib-IX) complex of plasma VWF, ULVWF, and isolated A1 domain. The unbinding force was defined as the minimum force required to pull ligand-coated beads away from their attachment with GP Ib-IX-expressing cells. Beads coated with plasma VWF did not bind to the cells spontaneously, requiring the modulators ristocetin or botrocetin. The force required to break the ristocetin- and botrocetin-induced plasma VWF-GP Ib-IX bonds occurred in integer multiples of 6.5 pN and 8.8 pN, respectively, depending on the number of bonds formed. In contrast, beads coated with either ULVWF or A1 domain bound the cells in the absence of modulators, with bond strengths in integer multiples of approximately 11.4 pN for both. Thus, in the absence of shear stress, ULVWF multimers form spontaneous high-strength bonds with GP Ib-IX, while plasma VWF requires exogenous modulators. The strength of individual bonds formed with GP Ib-IX was similar for both ULVWF and the isolated A1 domain and greater than those of plasma VWF induced by either modulator. Therefore, we suggest that the conformational state of ULVWF multimers is more critical than their size for interaction with platelets.

Predicting response to plasma exchange in patients with thrombotic thrombocytopenic purpura with measurement of vWF-cleaving protease activity

BACKGROUND

Severe deficiency of vWF-cleaving protease (vWF-CPase) activity was recently found in patients with thrombotic thrombocytopenic purpura (TTP). Although the survival of patients with TTP has been dramatically improved with plasma exchange (PE), there are still many patients who are refractory to PE and immunosuppressive therapy.

STUDY DESIGN AND METHODS

The activities of vWF-CPase and its inhibitor were measured in 27 patients with nonfamilial TTP and hemolytic-uremic syndrome (HUS) to examine the relationship between the clinical variables and vWF-CPase activity.

RESULTS

Eight of nine patients with HUS had more than 40 percent of vWF-CPase activity, whereas one had 28 percent of the normal level at the acute phase. Ten of 12 TTP patients with a good outcome had a severe deficiency of vWF-CPase activity and its inhibitor, whereas four of six patients with a poor outcome had a moderate deficiency of vWF-CPase activity along with a lack of the inhibitor. PE produced normalization of the vWF-CPase activity and neutralization of the inhibitor in TTP patients with a good outcome; however, some TTP patients with vWF-CPase inhibitor had relapsed and required an immunosuppressive therapy. The response to the combination therapy with PE and immunosuppressive treatment was poor in TTP patients without a severe deficiency of vWF-CPase activity.

CONCLUSION

Assays of vWF-CPase activity and its inhibitor may be useful for predicting the response to therapy and the outcome of patients with TTP. In some patients, nonfamilial TTP with a poor prognosis may not be caused by a constitutional or acquired deficiency of vWF-CPase with its inhibitor. Although PE and immunosuppressive therapy are effective in patients with nonfamilial TTP and a vWF-CPase inhibitor, other therapeutic modalities may be needed for nonfamilial TTP with unknown etiology.

Rituximab therapy for refractory thrombotic thrombocytopenic purpura

While most patients of thrombotic thrombocytopenic purpura (TTP) respond to plasma exchange and achieve remission, a subset of the patients experience multiple relapses or develop a persistent disease that may be debilitating and life threatening. We report the experience of rituximab treatment in three consecutive patients who had required periodic plasma exchange for greater than 50-180 days after failing other modalities of treatments. Two patients each received eight doses of rituximab infusion and had clinical remissions that have lasted for 23 months and ongoing in one patient and for 17 months before relapse in the other. The third patient received four doses of rituximab infusion and had improvement of disease for 60 days until her death from unrelated causes. Analysis of the von Willebrand factor-cleaving metalloprotease activity (ADAMTS13) and its inhibitor in two patients showed that rituximab treatment was associated with a rise of the protease level and a decrease of the inhibitor titers in one patient who achieved clinical remission and a decrease of the inhibitor titer but no increase in protease level in the other patient who had a partial response. Based on the responses to rituximab and its mild toxicity in these three patients, and the lack of effective alternative treatments, additional exploration of the role of rituximab in the treatment of refractory TTP is warranted.

Cloning, expression analysis, and structural characterization of seven novel human ADAMTSs, a family of metalloproteinases with disintegrin and thrombospondin-1 domains

ADAMTS (A Disintegrin And Metalloproteinase domain, with ThromboSpondin type-1 modules) is a recently described family of zinc-dependent proteases which play important roles in a variety of normal and pathological conditions, including arthritis and cancer. In this work, we report the identification and cloning of cDNAs encoding seven new human ADAMTSs. These novel enzymes have been called ADAMTS-13, -14, -15, -16, -17, -18, and -19. All of them show a domain organization similar to that of previously characterized family members, consisting of a signal sequence, a propeptide, a metalloproteinase domain, a disintegrin-like domain, a cysteine-rich region, and a variable number of TS-1 repeats. Expression analysis revealed that these ADAMTS genes are mainly expressed in fetal tissues, especially in lung (ADAMTS14, ADAMTS16, ADAMTS17, ADAMTS18, and ADAMTS19), kidney (ADAMTS14, ADAMTS15, and ADAMTS16), and liver (ADAMTS13, ADAMTS15 and ADAMTS18). Reverse transcriptase--polymerase chain reaction analysis also revealed the expression of some of these new ADAMTSs in different human adult tissues, such as prostate (ADAMTS13, ADAMTS17, and ADAMTS18), and brain (ADAMTS13, ADAMTS16, ADAMTS17, and ADAMTS18). High levels of ADAMTSs transcripts were also observed in some tumor biopsies and cells lines, including osteosarcomas (ADAMTS19), melanoma and colon carcinoma cells (ADAMTS13). Chromosomal location analysis indicated that the seven identified ADAMTS genes are dispersed in the human genome mapping to 9q34, 10q21, 11q25, 5p15, 15q24, 16q23, and 5q31, respectively. According to these results, together with a comparative analysis of ADAMTSs in other eukaryotic organisms, we conclude that these enzymes, with at least 18 distinct members encoded within the human genome, represent an example of a widely expanded protease family during metazoan evolution.

Molecular and cellular biology of von Willebrand factor

von Willebrand factor (VWF) is a plasma protein that performs 2 main functions in hemostasis: it mediates platelet adhesion to the injured vessel wall, and it carries and protects coagulation factor VIII. VWF is synthesized through a multistep process in endothelial cells and megakaryocytes as a very large polymer composed of identical disulfide-linked 250-kd subunits. In endothelial cells, VWF not only directs the formation of its own storage granules, the Weibel-Palade bodies, but it also acts as a chaperone molecule to direct other proteins, such as P-selectin, into these granules. Upon stimulation of the endothelium, the Weibel-Palade bodies will be translocated to the plasma membrane, and their contents will be secreted into the plasma milieu. The expression of VWF can be regulated at different levels by a number of genetic and environmental factors, resulting in control of its activity. New roles for VWF, especially in inflammatory processes, have recently been suggested, indicating that some aspects of this well-studied protein remain to be investigated.

Von Willebrand factor-cleaving protease and Upshaw-Schulman syndrome

Vascular endothelial cell (EC)-produced plasma von Willebrand factor (vWF) plays a critical role in primary hemostasis through its action of anchoring platelets onto the injured denuded subendothelial matrices under high shear stress. Unusually large vWF multimers (UL-vWFMs), present in plasma immediately after release from ECs, are most biologically active, but they are soon cleaved and degraded into smaller vWFMs by a specific plasma protease, termed vWF-cleaving protease (vWF-CPase), in normal circulation. Recent studies on the relationship between UL-vWFMs and vWF-CPase, together with its autoantibody (inhibitor) have brought about a clear discrimination between thrombotic thrombocytopenic purpura and hemolytic uremic syndrome. Furthermore, a congenital deficiency of this enzyme activity has been shown to cause Upshaw-Schulman syndrome, a complex constitutional bleeding diathesis. Successful purification of vWF-CPase revealed that this enzyme is composed of a single polypeptide with a molecular mass of approximately 190 kd, and its complementary DNA cloning unambiguously indicated that it is uniquely produced in the liver and its gene is located on chromosome 9q34. The messenger RNA of vWF-CPase had a span of 4.6 kb, and its enzyme was designated ADAMTS 13. The predicted complete amino acid sequence of this enzyme consisted of 1427 residues, including a signal peptide, a short propeptide terminating in the sequence RQRR, a reprolysin-like metalloprotease domain, a disintegrin-like domain, a thrombospondin-1 repeat (TSP1), a cysteine-rich domain, an ADAMTS spacer, 7 additional TSP1 repeats, and 2 CUB domains.