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.     

Post-appendectomy thrombotic thrombocytopenic purpura: a case report and review of the literature

Thrombotic thrombocytopenic purpura (TTP) is a syndrome characterized by microangiopathic hemolytic anemia and thrombocytopenia with varying degrees of renal dysfunction, neurologic signs and symptoms, and fever. Evidence has supported that a large proportion of cases of acquired TTP are due to the accumulation of ultralarge von Willebrand factor (vWF) multimers due to an acquired deficiency in the vWF cleaving protease, ADAMTS-13. TTP is rare in the post-surgical setting but is best described after cardiothoracic and vascular surgeries. We present a case of postoperative TTP first presenting with microangiopathic hemolytic anemia and thrombocytopenia 9 days after emergent appendectomy for a ruptured appendix. ADAMTS-13 and factor H levels returned normal and an ADAMTS-13 inhibitor was not identified. To our knowledge, this is the first case report of postoperative TTP after an appendectomy and the first report with correlative ADAMTS-13 data. Plasma exchange with fresh frozen plasma followed by cryopoor plasma, along with steroids resulted in eventual remission of TTP in our patient. Early postoperative diagnosis and aggressive management with consideration of initiation of plasma exchange is imperative to decrease the morbidity and morality associated with TTP.     

Rituximab for acute plasma-refractory thrombotic thrombocytopenic purpura. A case report and concise review of the literature

Thrombotic thrombocytopenic purpura (TTP) is a rare disease which responds well to plasma exchange treatment in the majority of patients. We report on a patient with acute TTP caused by severe autoantibody-mediated ADAMTS-13 deficiency, in whom remission was not achieved by initial treatment consisting of plasma exchange (PE), plasma infusion and corticosteroids, followed by vincristine and splenectomy. In view of the ongoing activity of TTP, treatment was initiated with rituximab, a chimaeric monoclonal antibody directed against the CD 20 antigen present on B lymphocytes. The patient received 4 weekly infusions of 375 mg/m2, each administered after the daily PE session and withholding PE until 48 hours later. Three weeks after the last infusion of rituximab a complete clinical and laboratory remission of this first episode of acute refractory TTP was documented. A concise review of the literature on the role of rituximab in patients with a first episode of acute plasma-refractory TTP suggests that rituximab in that situation may produce clinical remission in a significant proportion of patients, result in a lowered plasma requirement and avoid the complications of salvage immunosuppressive therapy. The use of rituximab in acute refractory TTP appears to be safe, with no excess infectious complications. We conclude that rituximab should be considered in TTP patients with acquired ADAMTS-13 deficiency who fail to respond clinically after 7-14 days of standard treatment with daily PE and glucocorticoids.     

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.     

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.     

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.     

Remission in acute refractory and relapsing thrombotic thrombocytopenic purpura following rituximab is associated with a reduction in IgG antibodies to ADAMTS-13

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening disorder and plasma exchange (PEX) remains the primary treatment modality. Twenty-five patients with acute refractory/relapsing idiopathic TTP received rituximab in conjunction with PEX because of progressive clinical disease or deterioration in laboratory parameters, despite intensive standard therapy. In relapsing TTP, rituximab was started if antibody to ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin motif-13) was demonstrated during previous episodes. All 25 patients attained complete clinical and laboratory remission in a median of 11 d after initiating rituximab. In 21 cases, ADAMTS-13 activity was within the normal range following rituximab. Inhibitors were detected in 24/25 patients by mixing studies and/or immunoglobulin G (IgG) antibodies to ADAMTS-13 pre-rituximab. There was no evidence of inhibitors and/or IgG activity <10% in 23/25 patients following rituximab. In acute refractory cases, the median number of PEX pre-rituximab and following the first rituximab infusion was 13 and 9, respectively. There have been no infectious complications, despite low CD 19 levels and no relapses. In patients with acute refractory/relapsing idiopathic TTP, rituximab appears to be a safe, effective, targeted therapy with a significant reduction in the requirement for PEX.     

Successful management of pregnancy in women with a history of thrombotic thrombocytopaenic purpura.

Pregnancy is an initiating event for acute thrombotic thrombocytopaenic purpura (TTP). There is a high risk of relapse during pregnancy and of foetal morbidity. We describe five cases with successful maternal and foetal outcomes in patients with a history of TTP. Cases 1 and 2 presented with TTP in their first pregnancy and had second-trimester foetal losses. Case 3 had four TTP relapses and soon after achievement of clinical remission became pregnant. Case 4 presented with TTP and left-sided stroke in pregnancy. ADAMTS-13 activity was less than 5% at presentation in four patients and prior to therapy during pregnancy in cases 1-4. Case 5, who had a single acute episode of TTP and became pregnant 6 years later, had normal ADAMTS-13 activity throughout pregnancy. Only case 3 had evidence of an inhibitor on mixing studies. All five patients underwent close haematological and obstetric monitoring and continued low-dose aspirin throughout pregnancy. Patients 1-4 had regular plasma exchange and received low molecular weight heparin during pregnancy. Patient 4 also received rituximab during the third trimester with no observed maternal or neonatal toxicity. Live healthy infants were delivered in all five cases in the third trimester. These findings suggest that successful pregnancy outcome is achievable in patients with a history of TTP and that patients with severely reduced ADAMTS-13 activity at the onset of pregnancy, necessitates regular plasma exchange during pregnancy.     

Cleavage of von Willebrand factor by ADAMTS-13 on endothelial cells

When histamine-stimulated endothelial cells are perfused with platelets in buffer, the platelets form long beads-on-a-string structures on the surface of the cells. The strands connecting the platelets are composed of ultralarge multimers of von Willebrand factor (ULVWF) and can be rapidly cleaved when perfused with normal plasma or purified ADAMTS-13 metalloprotease, but not with plasma from patients with either congenital or acquired thrombotic thrombocytopenic purpura (TTP). These ULVWF strings anchor to the surface of the endothelial cell at least partly through P-selectin, as their formation is prevented by either soluble P-selectin or P-selectin antibodies. They are also able to support the attachment of beads coated with ADAMTS-13. The metalloprotease binds to both the A1 and A3 domains of VWF, the latter with higher affinity. This attachment docks the enzyme close to its substrate site within the A2 domain. We propose that attachment of newly released ULVWF to the endothelial cell surface facilitates its cleavage by ADAMTS-13 by allowing tensile force to be applied to the A2 domain, thereby exposing the ADAMTS-13 cleavage site.     

Plasma exchange with solvent/detergent-treated plasma of resistant thrombotic thrombocytopenic purpura

A patient with unremitting thrombotic thrombocytopenic purpura (TTP), with circulating von Willebrand factor (VWF) multimers of unusually high molecular weight, and refractory to standard plasma exchange therapy, was treated with solvent detergent (S/D) plasma. The patient achieved a sustained clinical and haematological remission, with normal VWF multimeric profile. Spontaneous remission of this patient's condition could not be excluded but would appear unlikely. S/D plasma was efficacious and potentially safe for repeated large-volume plasma exchange with respect to viral safety and reduction of anaphylactoid reactions. We have assayed coagulation factors and physiological inhibitors of haemostasis in S/D plasma, which were comparable to normal plasma except in the distribution of VWF multimers.
The use of S/D plasma, previously reported in paediatric chronic relapsing TTP, should be assessed in further cases of adult TTP in the context of a clinical trial.     

Expression and characterization of recombinant human ADAMTS-13

Thrombotic thrombocytopenic purpura (TTP) is a severe disease associated with unusually large, hemostatically hyperactive von Willebrand factor (VWF) and severe deficiency in ADAMTS-13, the protease responsible for the proteolytic degradation of VWF in plasma. ADAMTS-13 prevents inappropriate microvascular platelet aggregation by cleaving VWF between Tyr1605 and Met1606 thereby producing dimers of 176 kd and 140 kd and smaller multimers. Identification of the ADAMTS13 gene and cloning of the corresponding cDNA allowed for the application of recombinant techniques, such as genetic engineering of ADAMTS13 cDNA, cell culture expression, and in vitro activity studies to analyze the functional relationship between ADAMTS-13 and the pathophysiology of ADAMTS-13 deficiency. In vitro expression and characterization of recombinant ADAMTS-13 (rADAMTS-13) clearly established that ADAMTS-13 is deficient in congenital TTP and inhibited in acquired TTP. Recent studies have contributed greatly to our current understanding of the molecular mechanism leading to congenital and acquired TTP. Apart from being a useful tool, availability of rADAMTS-13 raised the prospect of developing a recombinant substitution therapy to improve TTP treatment and allowing present diagnostic assays to be simplified. Here we report on recent advances in cell culture expression and functional characterization of human rADAMTS-13.     

Assays of ADAMTS-13 activity

Various assays for determination of ADAMTS-13 activity in plasma have been developed, all comprising two steps. The first step consists of proteolyzing a substrate by ADAMTS-13. Substrates were either exogenous von Willebrand factor (VWF) (purified from human plasma concentrates or recombinant VWF [rVWF]), purified VWF fragments, or endogenous VWF (from the tested plasma sample). All assays required a step in which substrate unfolding is performed using either urea or guanidine. Test plasmas were used at various dilutions and ADAMTS-13 was activated in most cases by divalent cations. The second step consists of quantifying the digestion products or the residual VWF remaining after proteolysis. Cleavage of VWF was thus estimated using electrophoresis (generation of proteolytic fragments or decrease of the size of multimers), functional methods (decrease of the collagen binding activity or of ristocetin cofactor activity), or immunological methods (enzyme-linked immunosorbent assay [ELISA] or immunoradiometric assay (IRMA) using selected monoclonal antibodies to VWF). Since 1998, all of these assays have been used to demonstrate the relevance of a deficient ADAMTS-13 activity in thrombotic thrombocytopenic purpura (TTP). However, improvements are required, as these methods remain cumbersome, time-consuming, and too remote from physiology to be routinely helpful for rapid laboratory diagnosis.     

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.     

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.     

Cleavage of ultralarge multimers of von Willebrand factor by C-terminal-truncated mutants of ADAMTS-13 under flow

A disintegrin-like and metalloprotease with thrombospondin type 1-motif 13 (ADAMTS-13) cleaves the A2 domain of von Willebrand factor (VWF), converting the ultralarge (UL) and hyperactive VWF multimers freshly released from endothelial cells to smaller and less active forms found in plasma. Recombinant ADAMTS-13 lacking the C-terminal region is active under static conditions, but its functions under flow conditions have not been determined. Here, we show that VWF-cleaving activity measured under flow was preserved in an ADAMTS-13 mutant lacking the second to eighth thrombospondin-1 motifs and the complement components C1r/C1s, Uegf sea urchin fibropellins, and bone morphogenic protein 1 (CUB) domains, but was severely deficient in a mutant that was further truncated to remove the spacer domain. We also show that the mutant lacking the TSP-1 and CUB domains was hyperactive under flow, suggesting that the C-terminal region may negatively regulate ADAMTS-13 activity. The wild type and the mutant without the spacer were more active in the presence of plasma, raising the possibility of ADAMTS-13 cofactors in plasma.     

Efficacy of rituximab in acute refractory or chronic relapsing non-familial idiopathic thrombotic thrombocytopenic purpura: a systematic review with pooled data analysis

Idiopathic thrombotic thrombocytopenic purpura (TTP) occurs primarily due to the formation of autoantibody against ADAMTS13, a specific von Willebrand factor-cleaving protease, resulting in low ADAMTS13 activity and subsequent accumulation of large vWF multimers, platelet aggregation and thrombus formation in the microvasculature of tissues. Limited clinical data suggest that the administration of anti-CD20 antibody (rituximab) may be useful in treating acute refractory or chronic relapsing idiopathic TTP. We carried out a systematic review with pooled data analysis using individual patient data to evaluate the efficacy of rituximab in these settings. Fifteen case series and 16 case reports comprising 100 patients were eligible for the study. Median age was 39?years. Male constituted 31?% and female 69?%. Complete remission was seen in 98?%, non-response in 2?% and relapse after complete remission in 9?%. For patients with complete remission, median follow-up was 13?months. Median platelet recovery from the first dose of rituximab was 14?days. ADAMTS13 inhibitor positivity and severe ADAMTS13 deficiency were highly predictive of the response to rituximab, implying that these can be useful markers in predicting response to rituximab in acute refractory or chronic relapsing idiopathic TTP.     

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.     

Hematology Morphology Forum

Introduction: Thrombotic thrombocytopenic purpura (TTP) is characterized by a reduction in the von Willebrand cleavage protein ADAMTS-13, mainly as a consequence of autoimmunity. Plasma exchange (PEx) is standard, achieving complete remission (CR) in 77–83% of cases, but rates are variable depending on ADAMTS-13 activity and relapse is frequent in patients with <10%. Thus, an effective front-line immunosuppressive treatment is needed.

Materials and methods: We administered PEx daily until CR and rituximab 100 mg/dose/week for 4 consecutive weeks to 10 patients with a first TTP episode and 1 relapsed patient (8 females (72%) and 3 males (28%)). Median age was 34 years (15–46) and laboratory parameters at diagnosis were as follows: platelets 11?×?10⁹/l (range 7–27.4?×?10⁹/l), lactate dehydrogenase 1822?U/l (range 705–8220?U/l, normal 70–180?U/l), and haemoglobin 6?g/dl (range 4.2–11.8?g/dl). ADAMTS-13 activity was determined in eight patients and was <10% in all. ADAMTS-13 autoantibody titre was determined in seven patients and was >15?units/ml in all (ref: negative <12, undetermined 12–15, positive >15?units/ml); Shiga toxin was negative in all patients. The median number of PEx until CR was 7 (range 4–12); prednisone 1?mg/kg was administered to six patients.

Results: The median follow-up was 22 months (range 4–49) and the estimated 2-year relapse-free survival was 89%; one HIV+ patient relapsed at 8 months follow-up. No complications related to PEx or rituximab were reported.

Conclusions: Our study suggests that low-dose rituximab and PEx are effective as front-line treatment for acute TTP; however, a prospective trial is needed to demonstrate whether low-dose rituximab is as effective as the conventional dose.     

The course of ADAMTS-13 activity and inhibitor titre in the treatment of thrombotic thrombocytopenic purpura with plasma exchange and vincristine

The therapeutic efficacy of plasma exchange (PE) in thrombotic thrombocytopenic purpura (TTP) is attributed to the restoration in ADAMTS-13 (a disintegrin and metalloproteinase with thrombospondin motif-13) activity by substitution of the enzyme and removal of ADAMTS-13-neutralizing autoantibodies. We explored this rationale by analysing ADAMTS-13 activity and corresponding inhibitor levels during PE-treatment in 27 episodes from 23 adults with TTP. All patients with an initial episode of TTP (n = 14) and nine of 11 patients with a relapse showed severe ADAMTS-13 deficiency. ADAMTS-13 inhibitors were detected in 81% of these patients. Twenty-one patients responded to PE-therapy and two patients died. For patients with severe ADAMTS-13 deficiency, 15 patients (71%) showed a PE-induced recovery in ADAMTS-13 activity and six patients (29%) had persistent severe ADAMTS-13 deficiency despite clinical response. Three patients with recurrent TTP demonstrated a permanent increase in inhibitor titre during therapy. Six patients (43%) with an initial episode of TTP displayed a transient increase in inhibitor titre during PE-therapy, which was associated with deterioration in clinical and haematological symptoms of TTP. Treatment with vincristine induced an immediate increase in platelet count and ADAMTS-13 activity in seven of eight patients. We conclude that ADAMTS-13 activity and inhibitor levels, as measured using current methodology, do not solely determine the clinical course of TTP.     

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.