Relevance of ADAMTS13 to liver transplantation and surgery

A disintegrin-like and metalloproteinase with thrombospondin type-1 motifs 13 (ADAMTS13) specifically cleaves unusually-large von Willebrand factor (VWF) multimers under high shear stress, and down-regulates VWF function to form platelet thrombi. Deficiency of plasma ADAMTS13 activity induces a life-threatening systemic disease, termed thrombotic microangiopathy (TMA) including thrombotic thrombocytopenic purpura (TTP). Children with advanced biliary cirrhosis due to congenital biliary atresia sometimes showed pathological features of TMA, with a concomitant decrease of plasma ADAMTS13 activity. Disappearance of their clinical findings of TTP after successful liver transplantation suggested that the liver is a major organ producing plasma ADAMTS13. In situ hybridization analysis showed that ADAMTS13 was produced by hepatic stellate cells. Subsequently, it was found that ADADTS13 was not merely responsible to development of TMA and TTP, but also related to some kinds of liver dysfunction after liver transplantation. Ischemia-reperfusion injury and acute rejection in liver transplant recipients were often associated with marked decrease of ADAMTS13 and concomitant formation of unusually large VWF multimers without findings of TMA/TTP. The similar phenomenon was observed also in patients who underwent hepatectomy for liver tumors. Imbalance between ADAMTS13 and VWF in the hepatic sinusoid might cause liver damage due to microcirculatory disturbance. It can be called as “local TTP like mechanism” which plays a crucial role in liver dysfunction after liver transplantation and surgery.     

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.     

Pediatric thrombotic thrombocytopenic purpura

Child‐onset thrombotic thrombocytopenic purpura (TTP) is a rare entity of thrombotic microangiopathy (TMA). The pathophysiology of the disease is based on a severe functional deficiency of ADAMTS13 (activity <10%), the specific von Willebrand factor (VWF)‐cleavage protease. This deficiency may be either acquired (associated anti‐ADAMTS13 autoantibodies) or congenital (resulting from biallelic mutations of ADAMTS13 gene). ADAMTS13 deficiency is responsible for the accumulation of high molecular weight multimers of VWF and the formation of platelet thrombi in the microcirculation. Consequently, microangiopathic hemolytic anemia and consumption thrombocytopenia are associated with organ ischemia. The differential diagnosis with other TMAs, autoimmune cytopenias or hematological malignancies may be challenging. The exploration of ADAMTS13 (activity, antibodies, antigen, ADAMTS13 gene) supports the diagnosis of TTP. The first‐line treatment of the acute phase of TTP is based on plasmatherapy. In congenital TTP, patients with a chronic disease benefit from a prophylactic plasmatherapy. In autoimmune TTP, steroids and B‐cells depleting therapies increasingly are used together with plasma exchange. Long‐term follow‐up including the monitoring of ADAMTS13 activity is mandatory. A severe decrease in ADAMTS13 activity (<10%) may predict relapses and preemptive B‐cell depletion with rituximab can be used to prevent relapses.     

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.     

FRETS-VWF73, a first fluorogenic substrate for ADAMTS13 assay

A plasma metalloprotease, ADAMTS13, cleaves von Willebrand factor (VWF) multimers and downregulates their activity in platelet aggregation. Functional ADAMTS13 deficiency leads to the accumulation of hyperactive large VWF multimers, inducing a life-threatening disease, thrombotic thrombocytopenic purpura (TTP). Although measuring ADAMTS13 activity is important in TTP diagnosis, existing methods require time and skill. Here, we report a fluorescence resonance energy transfer (FRET) assay for ADAMTS13 activity. We developed a synthetic 73-amino-acid peptide, FRETS-VWF73. Cleavage of this substrate between two modified residues relieves the fluorescence quenching in the intact peptide. Incubation of FRETS-VWF73 with normal human plasma quantitatively increased fluorescence over time, while ADAMTS13-deficient plasma had no effect. Quantitative analysis could be achieved within a 1-h period using a 96-well format in commercial plate readers with common filters. The FRETS-VWF73 assay will be useful for the characterization of thrombotic microangiopathies like TTP and may clarify the importance of ADAMTS13 activity as a predictive marker for various thrombotic diseases.     

von Willebrand factor, ADAMTS-13, and thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a severe, occlusive, microvascular "thrombotic microangiopathy" characterized by systemic platelet aggregation, organ ischemia, profound thrombocytopenia, and erythrocyte fragmentation. Failure to degrade "unusually large" (UL) von Willebrand factor (VWF) multimers as they are secreted from endothelial cells probably causes most cases of familial TTP, acquired idiopathic TTP, thienopyridine-related TTP, and pregnancy-associated TTP. The emphasis in this review is the pathophysiology of familial and acquired idiopathic TTP. In each of these entities, there is a severe defect in the function of a plasma enzyme, VWF-cleaving metalloprotease (ADAMTS-13), that normally cleaves hyper-reactive ULVWF multimers into smaller and less adhesive VWF forms. In familial TTP, mutations in the ADAMTS13 gene cause absent or severely reduced plasma VWF-cleaving metalloprotease activity. Acquired idiopathic TTP, in contrast, is the result in many patients of the production of autoantibodies that inhibit the function of ADAMTS-13. Established, evolving, and some of the unresolved issues in TTP pathophysiology will be summarized.     

Genetic defects leading to hereditary thrombotic thrombocytopenic purpura

In patients with thrombotic thrombocytopenic purpura (TTP), unusually large multimers of von Willebrand factor (VWF) circulate in the plasma. This is caused by a functional deficiency of VWF-cleaving protease, ADAMTS-13. Although TTP usually occurs as an acquired form due to autoantibodies against ADAMTS-13, the condition may be inherited in an autosomal recessive fashion. Thus far, genomic DNA from 23 patients with hereditary TTP and their families has been analyzed and 33 causative mutations identified in the ADAMTS13 gene: 19 missense, five nonsense, five frameshift, and four splice mutations. Common missense polymorphisms have been also found, one of which significantly reduces ADAMTS-13 activity. No cases have been found without mutations in the ADAMTS13 gene, suggesting that genetic defects in ADAMTS13 are the dominant cause of hereditary TTP. Further analysis may reveal the genetic background associated with acquired TTP and other thrombotic diseases.     

Dissociation between the level of von Willebrand factor-cleaving protease activity and disease in a patient with congenital thrombotic thrombocytopenic purpura

Decreased von Willebrand factor (VWF)-cleaving protease activity (<5%) has been implicated in patients with congenital thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (Upshaw-Schulman syndrome) and associated with mutations within the ADAMTS13 gene. In this report, we describe longitudinal studies in a patient with congenital TTP who ultimately developed end-stage renal failure and required plasma therapy from infancy. The patient was deficient in plasma high molecular weight (HMW)-VWF multimers during acute disease but had increased amounts of the HMW-VWF multimers during periods of remission. DNA analysis of this patient detected homozygosity for the R692C mutation on exon 17 of the ADAMTS13 gene, previously linked to congenital TTP. The level of VWF-cleaving protease activity in the patient was remarkably low (<5%) throughout her disease, even after she entered complete remission. However, despite no improvement in the level of VWF-cleaving protease activity, this patient had complete resolution of disease following splenectomy and commencing hemodialysis, without need for ongoing plasma therapy. The patient has remained in remission for over 4 years. These observations suggest that there are other factors in conjunction with severe deficiency of VWF protease activity that participate in the platelet-mediated thrombotic complications and other disease manifestations of congenital TTP. In addition, it is possible that splenectomy could be an effective treatment option for some patients with severe, congenital TTP.     

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.     

Interplay between ADAMTS13 and von Willebrand factor in inherited and acquired thrombotic microangiopathies

The presence of unusually large multimers of von Willebrand factor (VWF) is thought to be a major pathogenic factor for thrombotic thrombocytopenic purpura (TTP). ADAMTS13 is a protease that regulates the multimeric size and function of VWF by cleaving VWF. Hence, congenital or acquired deficiency of ADAMTS13 causes life-threatening illness of TTP. Mutations in the ADAMTS13 gene cause inherited TTP, and the development of autoantibodies that inhibit ADAMTS13 activity frequently are associated with acquired TTP. ADAMTS13 consists of 1,427 amino acid residues and is composed of multiple structural and functional domains, containing a signal peptide, a propeptide, a reprolysin-like metalloprotease domain, a disintegrin-like domain, a thrombospondin type-1 (Tsp1) motif, a cysteine-rich domain, a spacer domain, seven additional Tsp1 repeats, and two CUB domains. In particular, the cysteine-rich/spacer domains are essential for VWF cleavage and are the principal epitopes recognized by autoantibodies in patients with acquired TTP. Therefore, it is likely that these domains are involved in the recognition and binding of ADAMTS13 to VWF. ADAMTS13 circulates in the blood in an active state, and efficiently cleaves unfold form of VWF induced under shear stress caused by blood flow, preventing the accumulation of pathogenic unusually large VWF multimers (ULVWF). Thus, ADAMTS13 helps maintain vascular homeostasis by preventing the excess thrombus formation.     

ADAMTS13 gene mutation in congenital thrombotic thrombocytopenic purpura with previously reported normal VWF cleaving protease activity

Deficiency of von Willebrand factor (VWF) cleaving protease ADAMTS13 is associated with the development of thrombotic thrombocytopenic purpura (TTP). A case of congenital TTP that was previously reported to have normal ADAMTS13 activity was analyzed at the molecular level. Reanalysis of plasma VWF cleaving protease activity using a different assay revealed that the patient had less than 0.1 U/L ADAMTS13 protease activity, while the parents were both partially deficient. Sequence analysis of DNA amplified by polymerase chain reaction showed that the patient was homozygous for a novel TT deletion in exon 15 of the ADAMTS13 gene resulting in a frameshift, while both parents were heterozygous for the same mutation. Taken together with other recent reports, all the cases of hereditary TTP studied by DNA sequence analysis to date appear to be due to mutations within the ADAMTS13 gene.     

Prospective study on the behaviour of the metalloprotease ADAMTS13 and of von Willebrand factor after bone marrow transplantation

Thrombotic microangiopathies (TMAs) are rare but serious complications of bone marrow transplantation (BMT). Clinical manifestations are similar to those of thrombotic thrombocytopenic purpura (TTP), but prognosis is generally poorer despite plasma exchange. The enzymatic activity of the plasma metalloprotease ADAMTS13, which cleaves ultralarge thrombogenic multimers of von Willebrand factor (VWF) derived from activated endothelial cells, is very low or undetectable in patients with classic TTP, and protease deficiency is thought to play a mechanistic role in the formation of platelet thrombi in the microcirculation. This is the first prospective study to evaluate the incidence of TMA in 46 consecutively recruited patients undergoing autologous or allogeneic BMT and explore in parallel the behaviour of ADAMTS13, VWF antigen and VWF multimer size. The incidence of post-BMT TMA was 6% (three of 46); all cases occurred after allogeneic BMT. Compared with baseline values plasma ADAMTS13 activity was significantly reduced in patients undergoing BMT, particularly after the conditioning regimen (mean values: 50 +/- 22 vs. 77 +/- 32%; P < 0.0001). In the three patients who developed TMA, ADAMTS13 decreased after conditioning, but was very low in one case only (8%). VWF antigen levels progressively increased after the conditioning regimen (228 +/- 75 vs. 178 +/- 76% at baseline, P = 0.002). The mean proportion of high-molecular weight VWF multimers did not change in the various stages of BMT, even though ultralarge multimers were transiently found in same cases with and without TMA. Hence, the measurements evaluated in this study are not clinically useful to predict the occurrence of post-BMT TMA.     

Thrombotic thrombocytopenic purpura--then and now

Thrombotic thrombocytopenic purpura (TTP) is a potentially life-threatening disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, and formation of microthrombi in several organs. The disease may manifest once in a lifetime or may relapse after complete recovery of the initial episode; in these recurrent cases, death or neurological sequelae are common final outcomes. Accumulation of unusually large (UL) von Willebrand factor (VWF) multimers was described in the plasma of patients with TTP. Such ULVWF multimers are synthesized in endothelial cells and megakaryocytes and are secreted into the blood upon stimulation. However, in healthy individuals ULVWF multimers do not normally circulate because they are rapidly reduced into smaller multimers soon after their secretion due to cleavage by a plasma metalloprotease, ADAMTS13. Deficiency of ADAMTS13 has been reported consistently in patients with TTP. Such defect may be constitutive, due to homozygous or double heterozygous mutations in the corresponding gene, or acquired, due to the presence of circulating inhibitory antibodies. It follows that in TTP patients, the absent or severely depressed plasma ADAMTS13 activity limits the cleavage of ULVWF multimers, which remain anchored to the endothelial cells in long strings. Particularly under conditions of high shear stress, the multimers may promote the adhesion of circulating platelets, initiating thrombus formation. The clinical implications of these findings to the diagnosis and treatment of TTP are discussed.     

Rituximab for chronic recurring thrombotic thrombocytopenic purpura: a case report and review of the literature

Deficiency of von Willebrand factor (VWF) cleaving protease ADAMTS13 has been demonstrated to be the proximate cause of a subset of thrombotic microangiopathic haemolytic anaemias (MAHA) typical for thrombotic thrombocytopenic purpura (TTP). ADAMTS13 gene mutations cause the hereditary form; acquired deficiency has been attributed to presence of an autoantibody, which may represent a specific subset of MAHA best termed 'autoimmune thrombotic thrombocytopenic purpura'. We describe a patient with relapsing TTP because of ADAMTS13 inhibitors, who failed to achieve sustained remission despite therapies with plasma exchange, steroids, vincristine, staphylococcal protein A and splenectomy. The ADAMTS13 inhibitor titre remained elevated and clinical stability was only maintained by plasma exchange every 2-3 d over a period of 268 d. The patient then received rituximab therapy (eight doses of 375 mg/m2 weekly), during which she required five plasma exchanges in the first 10 d, two exchanges in the next 3 weeks, and none thereafter for 450 d and ongoing. The ADAMTS13 inhibitor titre decreased and enzyme activity increased. We compared this case with that of seven previously reported TTP cases also treated with rituximab; experience suggests that rituximab therapy deserves further investigation for patients with either refractory or relapsing TTP caused by ADAMTS13 inhibitors.     

A new mouse model mimicking thrombotic thrombocytopenic purpura: correction of symptoms by recombinant human ADAMTS13

Deficiency of a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13), a VWF-cleaving protease, is the key factor in the pathogenesis of thrombotic thrombocytopenic purpura (TTP), a life-threatening thrombotic microangiopathy. It is well established that ADAMTS13 deficiency results in elevated plasma levels of ultra-large VWF multimers (ULVWF), which are prone to induce platelet aggregation; however, the actual trigger of TTP development remains uncertain. Here we describe a new animal model in which some TTP-like symptoms can be triggered in ADAMTS13 knockout mice by challenge with 2000 units/kg body weight of recombinant human VWF containing ULVWF multimers. Animals rapidly showed clinical symptoms and developed severe thrombocytopenia. Schistocytosis, a decrease in hematocrit, and elevated serum lactate dehydrogenase levels were observed. The heart was identified as the most sensitive target organ with rapid onset of extensive platelet aggregation in the ventricles and myocardial necrosis. Prophylactic administration of 200 units/kg recombinant human ADAMTS13 protected ADAMTS13 knockout mice from developing TTP. Therapeutic administration of 320 units/kg rhADAMTS13 reduced the incidence and severity of TTP findings in a treatment interval-dependent manner. We therefore consider this newly established mouse model of thrombotic microangiopathy highly predictive for investigating the efficacy of new treatments for TTP.     

Platelet factor 4 inhibits ADAMTS13 activity and regulates the multimeric distribution of von Willebrand factor

The efficiency of von Willebrand factor (VWF) in thrombus formation is related to its multimeric size, which is controlled by the protease ADAMTS13. However, it is not clear what regulates ADAMTS13 activity. In this study, we investigated whether PF4 could bind to VWF and inhibit ADAMTS13 activity. We found that PF4 binds to VWF and protects against ADAMTS13 activity. We also found that VWF-PF4 complexes circulate in patients with thrombotic thrombocytopenic purpura (TTP). Our data provides the first evidence that PF4 may have a novel role in regulating VWF multimers during primary haemostasis and thrombosis.     

Congenital thrombotic thrombocytopenic purpura caused by new compound heterozygous mutations of the ADAMTS13 gene

Upshaw–Schulman syndrome (USS) is due to severe congenital deficiency of von Willebrand factor (VWF)‐cleaving protease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 domains, nr 13) activity resulting in the presence of unusually large forms of VWF in the circulation, causing intravascular platelet clumping and thrombotic microangiopathy. Our patient, a 26‐year‐old man, had attacks of thrombotic thrombocytopenic purpura (TTP) with thrombocytopenia and a urine dipstick positive for hemoglobin (4+), often as the only sign of hemolytic activity. He had ADAMTS13 activity of <1% of normal plasma without the presence of inhibitors of ADAMTS13. ADAMTS13 deficiency was caused by two new mutations of the ADAMTS13 gene: a deletion of a single nucleotide in exon17 (c. 2042 delA) leading to a frameshift (K681C fs X16), and a missense mutation in exon 25 (c.3368G>A) leading to p.R1123H. This case report confirms the importance of the analysis of the ADAMTS13 activity and its inhibitor in patients who have episodes of TTP, with a very low platelet count and sometimes without the classic biochemical signs of hemolysis.     

Congenital thrombotic thrombocytopenic purpura in association with a mutation in the second CUB domain of ADAMTS13

Severe deficiency of the von Willebrand Factor (VWF)-cleaving proteinase, ADAMTS13, is associated with the development of thrombotic thrombocytopenic purpura (TTP). Several mutations spread across the ADAMTS13 gene have been identified in association with a deficiency of VWF-cleaving proteinase activity in patients with congenital TTP. The spread of these dysfunctional mutations and the domain structure of ADAMTS13 are suggestive of a complex interaction between the enzyme and its substrate. We have studied a patient with congenital TTP who is a compound heterozygote for the Thr196Ile mutation in the metalloproteinase domain and a frameshift mutation (4143-4144insA) in the second CUB domain that results in loss of the last 49 amino acids of the protein. The VWF-cleaving proteinase activity of the truncated enzyme was comparable to that of the wild-type enzyme but its secretion from transfected COS-7 cells was about 14% of the wild type.     

The combined roles of ADAMTS13 and VWF in murine models of TTP, endotoxemia, and thrombosis

Ultralarge von Willebrand factor (UL-VWF) multimers are thought to play a central role in pathogenesis of the disease thrombotic thrombocytopenic purpura (TTP); however, experimental evidence in support of this hypothesis has been difficult to establish. Therefore, to examine directly the requirement for VWF in TTP pathogenesis, we generated ADAMTS13-deficient mice on a TTP-susceptible genetic background that were also either haploinsufficient (Vwf+/-) or completely deficient (Vwf-/-) in VWF. Absence of VWF resulted in complete protection from shigatoxin (Stx)-induced thrombocytopenia, demonstrating an absolute requirement for VWF in this model (Stx has been shown previously to trigger TTP in ADAMTS13-deficient mice). We next investigated the requirements for ADAMTS13 and VWF in a murine model of endotoxemia. Unlike Stx-induced TTP findings, LPS-induced thrombocytopenia and mortality were not affected by either VWF or ADAMTS13 deficiency, suggesting divergent mechanisms of thrombocytopenia between these 2 disorders. Finally, we show that VWF deficiency abrogates the ADAMTS13-deficient prothrombotic state, suggesting VWF as the only relevant ADAMTS13 substrate under these conditions. Together, these findings shed new light on the potential roles played by ADAMTS13 and VWF in TTP, endotoxemia, and normal hemostasis.     

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.