Specific von Willebrand factor-cleaving protease in thrombotic microangiopathies: a study of 111 cases

The proximal carboxyl-terminal domains of ADAMTS13 determine substrate specificity and are all required for cleavage of von Willebrand factor

ADAMTS13 limits platelet-rich thrombosis by cleaving von Willebrand factor at the Tyr(1605)-Met(1606) bond. Previous studies showed that ADAMTS13 truncated after spacer domain remains proteolytically active or hyperactive. However, the relative contribution of each domain within the proximal carboxyl terminus of ADAMTS13 in substrate recognition and specificity is not known. We showed that a metalloprotease domain alone was unable to cleave the Tyr-Met bond of glutathione S-transferase (GST)-VWF73-H substrate in 3 h, but it did cleave the substrate at a site other than the Tyr-Met bond after 16-24 h of incubation. Remarkably, the addition of even one or several proximal carboxyl-terminal domains of ADAMTS13 restored substrate specificity. Full proteolytic activity, however, was not achieved until all of the proximal carboxyl-terminal domains were added. The addition of TSP1 2-8 repeats and two CUB domains did not further increase proteolytic activity. Furthermore, ADAMTS13 truncated after the spacer domain with or without metalloprotease domain bound GST-VWF73-H with a K(d) of approximately 7.0 or 13 nm, comparable with full-length ADAMTS13 (K(d) = 4.6 nm). Metalloprotease domain did not bind GST-VWF73-H detectably, but the disintegrin domain, first TSP1 repeat, Cys-rich domain, and spacer domain bound GST-VWF73-H with K(d) values of 489, 136, 121, and 108 nm, respectively. These proximal carboxyl-terminal domains dose-dependently inhibited cleavage of fluorescent resonance energy transfer (FRETS)-VWF73 by full-length ADAMTS13 and ADAMTS13 truncated after the spacer domain. These data demonstrated that the proximal carboxyl-terminal domains of ADAMTS13 determine substrate specificity and are all required for recognition and cleavage of von Willebrand factor between amino acid residues Asp(1595) and Arg(1668).

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.

The “cutting” edge: von Willebrand factor-cleaving protease activity in thrombotic microangiopathies

The thrombotic microangiopathies (TMAs), including thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), are classically defined by constellations of clinical findings. In the late 1990's, several groups reported that a single test might be able to diagnose TTP and distinguish it from HUS. This test was an assay to detect von Willebrand factor-cleaving protease activity. Although there has been debate in the literature as to the utility of this assay, review of the available data suggests that this test can separate the idiopathic TMAs into three distinct categories: (1) patients who are protease positive with severe renal failure (similar to classic HUS), (2) patients who are protease positive without severe renal failure and (3) patients who are protease negative. While the latter two categories may appear similar to TTP at presentation, the prognosis differs in each of these groups of patients. Through use of the protease assay clinicians will be better able to diagnose, treat, predict outcome and design clinical trials.

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.

The von Willebrand Factor-Cleaving Protease (ADAMTS-13) and the Diagnosis of Thrombotic Thrombocytopenic Purpura (TTP)

Thrombotic thrombocytopenic purpura (TTP) is a life threatening disorder characterized by microangiopathic hemolytic anemia and thrombocytopenia as a result of microvascular platelet clumping often accompanied by ischemic organ dysfunctions such as neurological abnormalities or renal insufficiency, and fever. Until the sixties of the 20th century TTP remained an almost universally fatal disorder. The introduction of plasma exchange therapy (PE) with replacement of fresh frozen plasma has dramatically improved the survival of patients with acute TTP from less than 10% to about 80-90% and is now considered the therapy of choice. Severe deficiency of the von Willebrand factor (VWF)-cleaving protease, now denoted as ADAMTS-13, prevents normal processing of unusually large VWF multimers released from endothelial cells and it is assumed that their persistence is responsible for the formation of platelet thrombi in the microvasculature, a pathophysiological hallmark of acute TTP. An ADAMTS-13 activity of <5% of the normal is a specific finding for acute classical TTP. However, the sensitivity of this finding for the clinical diagnosis of TTP is equivocal with reported prevalences ranging from 33 -100%. Today, two forms of classical TTP are distinguished. Hereditary TTP, also known as Upshaw-Schulman syndrome, is caused by severe constitutionalADAMTS-13 deficiency due to compound heterozygous or homozygous mutations of theADAMTS13 gene and patients often present with a chronic relapsing course. The acquired or sporadic form of TTP is caused by circulating autoantibodies inhibiting ADAMTS-13 activity. Relapses are also frequent in acquired TTP occurring in about 35-50% of survivors of a first bout. Despite improved treatment modalities, patients suffering from acute bouts of TTP constitute a challenge for any clinician as mortality and morbidity rates are still considerably high.

von Willebrand factor cleaving protease (ADAMTS-13) and ADAMTS-13 neutralizing autoantibodies in 100 patients with thrombotic thrombocytopenic purpura

The congenital or acquired deficiency of the von Willebrand factor (VWF) cleaving protease, ADAMTS-13 has been specifically associated with a diagnosis of thrombotic thrombocytopenic purpura (TTP), a microangiopathy characterized by the formation of occlusive platelet thrombi. The mechanisms of TTP were investigated in 100 patients diagnosed on the basis of the presence of at least three of the following: thrombocytopenia, haemolytic anaemia, elevated serum levels of lactate dehydrogenase and neurological symptoms. Plasma levels of ADAMTS-13 were severely reduced (<10% of normal) in 48%, moderately reduced (between 10% and 46%) in 24% and normal (>46%) in 28%. A neutralizing antibody was the cause of the deficiency in 38% of the cases, with a higher prevalence of this mechanism (87%) in the 48 patients with severely reduced ADAMTS-13. Double heterozygosity for a 29 base pair (bp) deletion and a nucleotide insertion and homozygosity for a 6 bp deletion in the ADAMTS13 gene were identified only in two patients born from consanguineous marriages. In conclusion, this study indicated that ADAMTS-13 was normal in nearly one-third of patients with TTP and that ADAMTS-13 deficiency was not associated with the presence of neutralizing antibodies in more than half of the patients.

Outcome of severe adult thrombotic microangiopathies in the intensive care unit

OBJECTIVE

Thrombotic microangiopathies, namely thrombotic thrombocytopenic purpura and hemolytic uremic syndrome, are uncommon microvascular occlusive diseases. Despite the dramatic improvement in the outcome by exogenous plasma supply, either through plasma infusion or through plasma exchange, patients frequently require support in the intensive care unit. In the present study, we evaluated the outcome of a large cohort of patients with severe thrombotic microangiopathies.

DESIGN

A retrospective multicenter study from January 1998 to June 2001.

SETTING

Fourteen French university hospital medical intensive care units.

PATIENTS

Sixty three adult patients with severe thrombotic microangiopathies.

MEASUREMENTS AND RESULTS

Of the 63 patients, 19 had a clinical presentation of thrombotic thrombocytopenic purpura, 18 had hemolytic uremic syndrome and 26 had combined neurologic and renal failures. Infections were the main etiology associated with thrombotic microangiopathies. The mortality rate was 35%. Of the survivors, all achieved complete remission. Whereas neurologic failure assessed through the Glasgow coma scale was an independent predictor of mortality [HR=0.845 (CI 95%: 0.759-0.940), P=0.002], renal impairment did not appear to be an adverse prognostic factor. The use of plasma exchange was independently associated with survival [HR=0.269 (CI 95%: 0.104-0.691), P=0.006].

CONCLUSIONS

Thrombotic microangiopathies with severe organ dysfunctions leading to hospitalization in the intensive care unit are associated with high mortality. Neurologic impairment appears to be the main adverse prognostic factor correlated to mortality, and the study confirms the importance of plasma exchange in the treatment of high-risk patients.

Drug-induced thrombotic thrombocytopenic purpura/hemolytic uremic syndrome: a concise review

An extensive variety of drugs have been associated with thrombotic thrombocytopenic purpura and hemolytic uremic syndrome (TTP/HUS). Although a direct causal effect has usually not been proven, the cumulative evidence linking several drugs with TTP/HUS is strong. This paper reviews several categories of drugs including antineoplastics, immunotherapeutics and anti-platelet agents that have been reported to induce TTP/HUS. The pathogenesis of drug-induced TTP/HUS and the effectiveness of treatment regimens are also reviewed. A consensus on diagnostic criteria to accurately and consistently diagnose drug-induced TTP is needed.

Thrombotic thrombocytopenic purpura with severe ADAMTS-13 deficiency in two patients with primary antiphospholipid syndrome

Arterial thrombotic events, thrombocytopenia, and hemolytic anemia with schistocytes may be encountered in the setting of both thrombotic thrombocytopenic purpura (TTP) and primary antiphospholipid syndrome (APS). We report 2 cases of TTP occurring in patients with definite primary APS. We also describe the results of tests for ADAMTS-13 activity in 20 consecutive patients with primary APS, as well as tests for antiphospholipid antibodies in 26 patients who had TTP, severe ADAMTS-13 deficiency, and ADAMTS-13-inhibiting antibodies. In both of the patients with primary APS and TTP, ADAMTS-13 activity was undetectable, and ADAMTS-13-inhibiting antibodies were present. None of the 26 patients with TTP and severe ADAMTS-13 deficiency was positive for the lupus anticoagulant. One of these patients had a low level of anticardiolipin antibodies (22 IgG phospholipid units). In the 20 patients with primary APS, mean ADAMTS-13 activity was 116% (range 44-250%), and no severe deficiency (< 5%) was observed. Our findings suggest that primary APS must be added to the list of autoimmune disorders that can be complicated by TTP.

Measurement of von Willebrand factor cleaving protease (ADAMTS-13): results of an international collaborative study involving 11 methods testing the same set of coded plasmas

BACKGROUND

ADAMTS-13 is a von Willebrand factor (VFW)-cleaving protease. Its congenital or acquired deficiency is associated with thrombotic thrombocytopenic purpura (TTP) and more rarely with the hemolytic uremic syndrome. We report on a survey evaluating 11 methods for ADAMTS-13 measurement performed in different labs.

DESIGN

Two plasmas, one normal and one from a patient with familial TTP, were mixed at the co-ordinating center to prepare 6 plasmas with 0%, 10%, 20%, 40%, 80% and 100% ADAMTS-13 levels. Each plasma was aliquoted and assembled into sets of 60 (coded from 1 to 60), each containing 10 copies of the original 6 plasmas. Plasmas were frozen and shipped in dry ice to 10 labs with a common frozen reference plasma. Laboratories were asked to measure ADAMTS-13 with their methods. Results were sent to the coordinating center for statistical analysis.

RESULTS

Of the 10 methods performed under static conditions 9 were quantitative and one was semiquantitative. One method performed under flow conditions evaluated the extent of cleavage of endothelial cell-derived ultralarge VWF string-like structures and expressed results as deficient, normal, or borderline. Linearity (expected-vs-observed levels), assessed as the squared correlation coefficient, ranged from 0.98 to 0.39. Reproducibility, expressed as the coefficient of variation for repeated measurements, ranged from < 10% to 83%. The majority of methods were able to discriminate between different ADAMTS-13 levels. The majority were able to detect the plasma with 0% level and some of them to discriminate between 0% and 10%. Overall the best performance was observed for three methods measuring cleaved VWF by ristocetin cofactor, collagen binding, and immunoblotting of degraded multimers of VWF substrate, respectively. The poor interlaboratory agreement of results was hardly affected by the use of the common standard. The method performed under flow conditions identified the plasmas with 0%, 10%, 20% and 40% activity as deficient in 7, 5, 1 and 3 of the 10 replicate measurements. The plasmas with 80% and 100% were identified as normal in all of the 10 replicate measurements.

CONCLUSIONS

The survey shows varied performance, but supports an optimistic view about the reliability of current methods for ADAMTS-13.

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.

Plasma therapy in thrombotic thrombocytopenic purpura: review of the literature and the Bern experience in a subgroup of patients with severe acquired ADAMTS-13 deficiency

Based on clinical studies daily plasma exchange (PE) has become the first-choice therapy for thrombotic thrombocytopenic purpura (TTP) since 1991. Recent findings may explain its effectiveness, which particularly may include supply of ADAMTS-13 and removal of anti-ADAMTS-13 autoantibodies and unusually large von Willebrand factor (VWF) multimers. The most preferable PE regimens as well as replacement fluids are discussed and treatment-related adverse reactions are summarized. Proposals for a potential reduction of their frequency and for improvement of treatment efficiency are given. These suggestions are partially based on the experience of our institution in adult patients with severe ADAMTS-13 deficiency (<5% activity), and include (1) continuous calcium-gluconate infusion during PE in order to reduce citrate-related adverse reactions; (2) the evaluation of solvent/detergent-treated (S/D) plasma as replacement fluid in order to reduce adverse events due to fresh frozen plasma (FFP); (3) the evaluation of immunoadsorption in order to increase procedural efficiency in autoantibody removal; and (4) the substitution of ADAMTS-13 by means of recombinant drug instead of plasma.

von Willebrand factor-cleaving protease (ADAMTS-13) activity determination in the diagnosis of thrombotic microangiopathies: the Swiss experience

Severe deficiency of von Willebrand factor (VWF)-cleaving protease (ADAMTS-13) activity (<5% of normal) is a specific finding for acute idiopathic thrombotic thrombocytopenic purpura (TTP), a disorder that presents as thrombocytopenia, microangiopathic hemolytic anemia, and often organ dysfunction such as neurological disturbances or renal failure, and fever. Between January 2001 and July 2003, ADAMTS-13 activity was determined in plasma samples of 396 consecutive patients referred to our laboratory for diagnostic purposes. Plasma samples with ADAMTS-13 activity less than 5% were in addition tested for the presence of inhibitory antibodies. Patients were assigned to 10 predefined clinical categories according to information provided by the referring clinician: thrombotic microangiopathy (TMA) not further specified; neoplasia- or chemotherapy-associated TMA; TMA following hematopoietic stem cell transplantation; TMA with additional/alternative disorder; idiopathic TTP; hemolytic-uremic syndrome (HUS) not specified; HUS with diarrhea prodrome (D+HUS); atypical HUS; other hematological disorder; and no clinical information available. Severe ADAMTS-13 deficiency was found in 69 (17%) patients, including 42 with acquired idiopathic TTP, either at initial presentation or at relapse, 14 with confirmed or suspected hereditary TTP, 10 with TMA not further specified, two with neoplasia- or chemotherapy-associated TMA, and one in continued clinical remission 3.4 years after splenectomy for plasma-refractory TTP. Forty-three (62%) patients with ADAMTS-13 activity less than 5% displayed inhibitory antibodies. Severe ADAMTS-13 deficiency was found in 60% of patients diagnosed with acute idiopathic TTP, but in none of 130 patients diagnosed with HUS or in any of the 14 patients with hematopoietic stem cell transplantation-associated TMA. Thus, plasma ADAMTS-13 activity less than 5% does not identify all patients clinically diagnosed with TTP, and severe ADAMTS-13 deficiency is not invariably associated with clinical manifestations of microvascular platelet clumping.

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.

Thrombotic microangiopathic haemolytic anaemia and antiphospholipid antibodies

OBJECTIVE

To analyse the clinical and laboratory features of patients with thrombotic microangiopathic haemolytic anaemia (TMHA) associated with antiphospholipid antibodies (aPL).

METHODS

A computer assisted (PubMed) search of the literature was performed to identify all cases of TMHA associated with aPL from 1983 to December 2002.

RESULTS

46 patients (36 female) with a mean (SD) age at presentation of TMHA of 34 (15) years were reviewed. Twenty eight (61%) patients had primary antiphospholipid syndrome (APS). TMHA was the first clinical manifestation of APS in 26 (57%) patients. The clinical presentations were haemolytic-uraemic syndrome (26%), catastrophic APS (23%), acute renal failure (15%), malignant hypertension (13%), thrombotic thrombocytopenic purpura (13%), and HELLP (haemolysis, elevated liver enzymes, and low platelet count in association with eclampsia) syndrome (4%). Lupus anticoagulant was detected in 86% of the episodes of TMHA, and positive anticardiolipin antibodies titres in 89%. Steroids were the most common treatment (69% of episodes), followed by plasma exchange (PE) (62%), anticoagulant or antithrombotic agents (48%), immunosuppressive agents (29%), and immunoglobulins (12%). Recovery occurred in only 10/29 (34%) episodes treated with steroids, and in 19/27 (70%) episodes treated with PE. Death occurred in 10/46 (22%) patients.

CONCLUSIONS

The results emphasise the need for systematic screening for aPL in all patients with clinical and laboratory features of TMHA. The existence of TMHA in association with an APS forces one to rule out the presence of the catastrophic variant of this syndrome. PE is indicated as a first line of treatment for all patients with TMHA associated with aPL.

Simplified assay for VWF cleaving protease (ADAMTS13) activity and inhibitor in plasma

The absence of VWF cleaving protease activity (VWFcp, ADAMTS13) has recently been identified as a central component in the pathogenesis of TTP. Several assays for the measurement of ADAMTS13 activity have been described, but most are cumbersome and employ techniques not easily adapted to routine laboratories. Thus, ADAMTS13 assays are available at only a few reference or research laboratories. Prompt identification of absent or impaired ADAMTS13 activity could prove invaluable to clinical decision-making regarding diagnosis and treatment of suspected TTP patients. We describe an assay for ADAMTS13 that uses a commercial source of VWF substrate and obviates dialysis for sample and substrate preparation. Patient and normal plasma are prepared by desalting over Micro-Spin columns, and barium is added to activate the cleaving protease. Humate-P(R) is prepared over a desalting column and reconstituted in urea prior to use as the exogenous VWF substrate. Desalted plasma is mixed with prepared substrate, and digestion is carried out at 37 degrees C. Maximum sensitivity to very low levels of enzyme activity is achieved by using undiluted plasma and allowing digestion to proceed for 14 hr. The extent of VWF multimer cleavage is then demonstrated by Western blot. We used this assay to analyze VWF cleaving protease activity in plasma samples from 30 patients treated at our institution for TTP. Plasma from patients lacking ADAMTS13 was incubated with PNP and then analyzed by the same methods to determine the presence of an inhibitor. Our results indicate that the assay is suitable for providing timely information regarding ADAMTS13 activity for the diagnosis and treatment of patients with suspected TTP.

Severe ADAMTS13 deficiency in adult idiopathic thrombotic microangiopathies defines a subset of patients characterized by various autoimmune manifestations, lower platelet count, and mild renal involvement

The significance of ADAMTS13 deficiency in adult thrombotic microangiopathy (TMA) remains controversial. In an attempt to define the characteristics of adult TMA with severe ADAMTS13 deficiency, we determined 2 groups of patients on the basis of ADAMTS13 activity (undetectable or detectable). Clinical presentation, laboratory values, autoimmune manifestations, and outcome were compared between the groups. Patients were included retrospectively from 12 centers. All fulfilled the diagnosis criteria of TMA. Patients with a history of transplantation, cancer and chemotherapy, and Centers for Disease Control and Prevention (CDC) stage C human immunodeficiency virus (HIV) infection were not included. Forty-six patients were included. Thirty-one patients had an undetectable ADAMTS13 activity (<5%), and the remaining 15 patients had ADAMTS13 activity of >25%. Severe ADAMTS13 deficiency was associated with a plasmatic inhibitor in 17 cases (55%), suggesting an immune-mediated mechanism. Patients with undetectable ADAMTS13 were more frequently of Afro-Caribbean origin than patients with detectable ADAMTS13 activity (48.4% vs 13.3%, respectively; p = 0.03). As opposed to patients with detectable ADAMTS13 activity, patients with severe ADAMTS13 deficiency displayed various autoimmune manifestations that consisted of nondestructive polyarthritis (4 cases) associated in 1 case with malar rash and extramembranous glomerulonephritis, discoid lupus (3 cases), and autoimmune endocrinopathies, Raynaud phenomenon, and sarcoidosis-like disease (1 case each). In patients with severe ADAMTS13 deficiency, antinuclear antibodies, anti-double-stranded DNA antibodies, and anticardiolipin antibodies were positive in 22 (71%) cases, 3 (9.7%) cases, and 1 (3.2%) case, respectively. One patient fulfilled the criteria for the diagnosis of systemic lupus erythematosus. During follow-up, 1 patient with severe ADAMTS13 deficiency developed antinuclear antibodies, and 3 others developed anti-double-stranded DNA antibodies, in association with neurologic manifestations and anticardiolipin antibodies in 1 case. Patients with severe ADAMTS13 deficiency also had a lower platelet count (12 x 10(9)/L; range, 2-69 x 10(9)/L) and less severe renal failure (estimated glomerular filtration rate: 78 mL/min; range, 9-157 mL/min) than patients with detectable ADAMTS13 activity (49.5 x 10(9)/L; range, 6-103 x 10(9)/L; p = 0.0004, and 15.8 mL/min; range, 5.6-80 mL/min; p < 0.0001, respectively). End-stage renal failure occurred in 1 patient with severe ADAMTS13 deficiency and in 3 patients with detectable ADAMTS13 activity (3.2% vs 21.4%, respectively; p = 0.08). Flare-up and relapse episodes and survival were comparable between the groups. Taken together, these data indicate that adult idiopathic thrombotic thrombocytopenic purpura, as defined by severe ADAMTS13 deficiency, may occur preferentially in a particular ethnic group, and is characterized by severe thrombocytopenia, mild renal involvement, and a wide spectrum of autoimmune manifestations that may be completed during follow-up. Indeed, apparently idiopathic thrombotic thrombocytopenic purpura may be considered a specific autoimmune disease.

Clopidogrel-induced thrombotic thrombocytopenic purpura-hemolytic uremic syndrome after coronary artery stenting

The antiplatelet drug clopidogrel has largely replaced ticlopidine, due to an association between ticlopidine and thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (TTP-HUS). Clopidogrel at first was thought to be void of this potentially fatal adverse effect, but recent case reports have called that assumption into question. Even with proper treatment (plasma exchange), TTP-HUS can persist for weeks. Clinicians should be aware of this possible adverse effect because prompt therapy is imperative for patients' survival. Earlier reports of clopidogrel-related TTP-HUS have involved patients who had received at least 72 hours of therapy. We describe a case of TTP-HUS in a patient who had received only a 300-mg loading dose of clopidogrel.

Thrombotic Thrombocytopenic Purpura: Yesterday, Today, Tomorrow

Although much has been learned about the pathophysiologic process of thrombotic thrombocytopenic purpura (TTP), both diagnostically and therapeutically, since its initial description by Moschcowitz in 1924, its etiology and treatments remain, in many instances, problematic. Thrombotic thrombocytopenic purpura remains a rare entity whose etiology is usually unknown, but several drugs and infections have now been implicated in its development (i.e. Cyclosporine A, Mitomycin-C, Ticlopidine, Simvastatin, Lipitor, Plavix, FK 506, Rapamune (sirolimus), HIV). Although its treatment by plasma exchange has gained worldwide acceptance since the late 1970s, the optimal exchange media is not known, nor the volume and duration of exchange therapy, nor appropriate salvage therapy(ies). Without the benefit of randomized controlled trials, its treatment, to a large extent, remains not evidence-based but 'eminence-based', making the same mistakes with increasing confidence over an impressive number of years.

Ten candidate ADAMTS13 mutations in six French families with congenital thrombotic thrombocytopenic purpura (Upshaw-Schulman syndrome)

ADAMTS13, the specific von Willebrand factor (VWF)-cleaving metalloprotease, prevents the spontaneous formation of platelet thrombi in the microcirculation by degrading the highly adhesive ultralarge VWF multimers into smaller forms. ADAMTS13 severe enzymatic deficiency and mutations have been described in the congenital thrombotic thrombocytopenic purpura (TTP or Upshaw-Schulman syndrome), a rare and severe disease related to multivisceral microvascular thrombosis. We investigated six French families with congenital TTP for ADAMTS13 enzymatic activity and gene mutations. Six probands with congenital TTP and their family were tested for ADAMTS13 activity in plasma using a two-site immunoradiometric assay and for ADAMTS13 gene mutations using polymerase chain reaction and sequencing. ADAMTS13 activity was severely deficient (< 5%) in the six probands and one mildly symptomatic sibling but normal (> 50%) in all the parents and the asymptomatic siblings. Ten novel candidate ADAMTS13 mutations were identified in all families, showing either a compound heterozygous or a homozygous status in all probands plus the previous sibling and a heterozygous status in the parents. The mutations were spread all over the gene, involving the metalloprotease domain (I79M, S203P, R268P), the disintegrin domain (29 bp deletion in intron/exon 8), the cystein-rich domain (acceptor splice exon 12, R507Q), the spacer domain (A596V), the 3rd TSP1 repeat (C758R), the 5th TSP1 repeat (C908S) and the 8th TSP1 repeat (R1096stop). This study emphasizes the role of ADAMTS13 mutations in the pathogenesis of congenital TTP and suggests that several structural domains of this metalloprotease are involved in both its biogenesis and its substrate recognition process.

Effect of plasma exchange on plasma ADAMTS13 metalloprotease activity, inhibitor level, and clinical outcome in patients with idiopathic and nonidiopathic thrombotic thrombocytopenic purpura

Therapeutic plasma exchange is an effective empiric treatment for thrombotic thrombocytopenic purpura (TTP), but how therapy affects the level of a disintegrin and metalloprotease with thrombospondin type 1 motif 13 (ADAMTS13) or inhibitor has not been reported in many patients. We prospectively analyzed ADAMTS13 activity and inhibitor levels in 37 adults with TTP. ADAMTS13 level at presentation was lower than 5% in 16 of 20 patients with idiopathic TTP and in none of 17 patients with TTP associated with hematopoietic stem cell transplantation, cancer, drugs, or pregnancy (P <.00001). Seven of the 16 patients with ADAMTS13 activity lower than 5% ( approximately 44%) had inhibitors. For 8 patients followed serially with ADAMTS13 activity lower than 5% but no inhibitor at presentation, plasma exchange led to complete clinical remission and a rise in ADAMTS13 level. In contrast, 4 patients with low ADAMTS13 activity but high-titer inhibitor (> 5 units/mL) had neither a rise in ADAMTS13 activity nor a reduction in the inhibitor titer: 3 had recurrent disease and 1 died. Among 17 patients with AD-AMTS13 activity at presentation higher than 25%, 10 died. Mortality rate for idiopathic TTP was 15%, whereas mortality for nonidiopathic TTP was 59% (P <.02). We conclude that assays of ADAMTS13 activity and inhibitors in addition to the clinical categories (idiopathic TTP and nonidiopathic TTP) are predictive of outcome and may be useful to tailor patient treatment.

Epitope mapping of ADAMTS13 autoantibodies in acquired thrombotic thrombocytopenic purpura

Severe deficiency of the von Willebrand factor (VWF)-cleaving protease ADAMTS13 can lead to thrombotic thrombocytopenic purpura (TTP), a disease associated with the widespread formation of platelet-rich thrombi in many organs. Autoantibodies that inactivate ADAMTS13 are the most frequent cause of acquired TTP. Little is known about epitope specificity and reactivity of anti-ADAMTS13 antibodies. In this study, a series of ADAMTS13 domains were expressed in Escherichia coli, and the reactivity of purified recombinant fragments with anti-ADAMTS13 auto-antibodies from 25 patients with severe ADAMTS13 deficiency was evaluated in vitro. All TTP plasmas contained antibodies directed against the cysteine-rich spacer (cys-rich/spacer) domain of ADAMTS13. In the plasma of 3 patients, antibodies were detected that reacted exclusively with the cys-rich/spacer domain, underscoring the importance of this region for functional activity of ADAMTS13. In 64% of the plasmas, antibodies reacted with the 2 CUB domains, and in 56% they reacted with the isolated first thrombospondin type 1 (TSP-1) repeat and with the compound fragment consisting of the catalytic, the disintegrin-like, and the TSP1-1 domain. Less frequently, in 28% of the plasmas, antibodies reacted with the TSP1 repeats 2 to 8. Unexpectedly, antibodies reacted with the propeptide region in 20% of the plasmas. In conclusion, this study shows that even though anti-ADAMTS13 autoantibodies react with multiple domains of the protease, the cys-rich/spacer domain is consistently involved in antibody reactivity.

Severe deficiency of VWF-cleaving protease (ADAMTS13) activity defines a distinct population of thrombotic microangiopathy patients

BACKGROUND

Severe deficiency of ADAMTS13 activity is a biologic risk factor for thrombotic microangiopathy (TMA). It was hypothesized that severe ADAMTS13 deficiency is associated with a distinct TMA subpopulation.

STUDY DESIGN AND METHODS

ADAMTS13 activity before treatment was determined retrospectively in 107 adult TMA patients treated with plasma exchange. Patients were not clinically categorized, but divided between severely deficient (n = 50) and nonseverely deficient (n = 57) ADAMTS13 activity. Laboratory and clinical factors before treatment were compared between the groups.

RESULTS

Median PLT counts were 44,000 per micro L in nonseverely deficient ADAMTS13 patients and 13,000 per micro L in severely deficient ADAMTS13 patients (p < 0.001). Median serum creatinine levels were 2.7 mg per dL in nonseverely deficient patients and 1.2 mg per dL in severely deficient patients (p < 0.001). In surviving patients, median plasma exchange procedures were 9 in nonseverely deficient patients and 14.5 in severely deficient patients (p < 0.01). Rates of relapse following remission were 4 of 47 in nonseverely deficient patients and 16 of 46 in severely deficient patients (p < 0.01). Among analyzed factors only mortality rates were not significantly different.

CONCLUSION

In a heterogeneous population of TMA patients treated with plasma exchange, ADAMTS13 activity defined two subpopulations with distinct clinical and laboratory features. These results suggest that TMA with severe ADAMTS13 deficiency is a distinct pathologic process.

Thienopyridine-linked thrombotic microangiopathy: association with endothelial cell apoptosis and activation of MAP kinase signalling cascades

The thienopyridine platelet antagonist ticlopidine is associated with development of thrombotic thrombocytopenic purpura (TTP) but the pathophysiology of this link is unclear. Severe deficiency of disintegrin and metalloproteinase with thrombospondin motif-13 (ADAMTS13), described in familial cases and a significant fraction of idiopathic TTP, has been reported in only a few ticlopidine-linked cases. As ticlopidine can disrupt production of extracellular matrix (ECM) components critical to microvascular endothelial cell (MVEC) integrity in vitro, we explored the hypotheses that ticlopidine and ticlopidine-linked TTP plasmas induce MVEC apoptosis in a manner similar to that of idiopathic TTP plasmas, and that ECM components and related mitogen-activated protein kinase (MAPK) signalling cascades may be involved in this process. Replicating the activity of plasmas from patients with idiopathic TTP, plasma from five ticlopidine-linked TTP patients induced apoptosis of primary human dermal, glomerular and hepatic MVEC, but had no effect on pulmonary MVEC or large vessel endothelial cells (EC). Pharmacological levels of ticlopidine initiated apoptosis with similar EC lineage restriction. In parallel, ticlopidine and plasmas from idiopathic and ticlopidine-TTP patients decreased transcripts for the ECM component thrombospondin-1 in MVEC, but not in large vessel EC. These changes were accompanied by prolonged induction of MAPKs extracellular signal-related kinase (ERK)-1/2 and p38 only in TTP susceptible MVEC. Induction of apoptosis by ticlopidine and TTP plasma was abrogated by inhibitors of ERK-1/2 and p38 phosphorylation. In conclusion, MVEC apoptosis related to altered ECM-MVEC interactions may be a key part of the pathology of ticlopidine-linked and idiopathic TTP.

The activity of the von Willebrand factor cleaving protease ADAMTS-13 in newborn infants

BACKGROUND

Unusually large von Willebrand factor (VWF) multimers have been observed in patients with thrombotic microangiopathies (TMA), and absence of the VWF cleaving protease ADAMTS-13 activity is considered to be involved in the etiology of TMA. Increased amounts of large multimers of VWF have also been identified in neonates.

OBJECTIVE

We assessed ADAMTS-13 activity in healthy neonates, children and adults to establish baseline levels.

PATIENTS AND METHODS

Cord blood was collected from 38 full-term newborns; venous samples were taken from 15 neonates on day 2-3 of life. Seventeen children, 24 healthy adults and seven patients with TMA were studied as well. ADAMTS-13 activity was quantified by the binding of the subjects' plasma VWF to collagen before and after enzyme activation. The multimer distribution of VWF was also determined.

RESULTS

Neonates and children had percentage ADAMTS-13 activity similar to adults. However, two groups were apparent in the cord blood samples: while 28/38 newborns had percentage activity within the normal range of healthy adults (102 +/- 3.0%), 10 had significantly lower percentage activity (53 +/- 1.1%; P < 0.0001) that normalized by day 2-3. The VWF multimer distribution was the same in all cord blood samples and was not different compared with children and adults. High-molecular-weight VWF multimers were significantly increased in the 2-3-day-old neonates and in TMA patients.

CONCLUSIONS

Although ADAMTS-13 activity was similar in neonates compared with adults, 26% of neonates had mildly reduced activity. Further studies are needed to investigate the complex interaction of VWF production and secretion with its size control by ADAMTS-13 in different age groups.

Clinical usefulness of a functional assay for the von Willebrand factor cleaving protease (ADAMTS 13) and its inhibitor in a patient with thrombotic thrombocytopenic purpura

Decreased von Willebrand factor cleaving protease activity (VWFCP, ADAMTS 13) leads to persistence of unusually large multimers of von Willebrand factor that bind to platelets, causing platelet aggregates, microangiopathic hemolysis, and thrombocytopenia in patients with thrombotic thrombocytopenic purpura (TTP). The clinical value of measuring ADAMTS 13 and its inhibitor is not fully defined; the case reported here illustrates the usefulness of the assay to help confirm the clinical diagnosis in a patient with other potential causes for thrombotic microangiopathy; the assay also helped in making treatment decisions. A patient with systemic lupus erythematosis (SLE) presented with fever and abdominal pain, thrombocytopenia, and anemia. Thrombotic microangiopathy was diagnosed by the appearance of schistocytes, decreasing platelet count, and evidence of hemolysis. ADAMTS 13 was decreased and an inhibitor was demonstrated in the patient's initial blood sample within 24 hr of admission. Plasma exchange was initiated, and serial assays showed increased ADAMTS 13 activity and decreased inhibitor after each plasma exchange; there was a rebound in inhibitor and a decrease in ADAMTS 13 activity prior to the next exchange that lessened over time. Increasing levels of protease activity correlated with clinical and laboratory improvement. Measurement of ADAMTS 13 activity and its inhibitor aided in the diagnosis of this complicated case of a patient with other potential causes for microangiopathic hemolysis. Subsequent levels correlated with the clinical course, and disappearance of the inhibitor indicated that long-term plasma exchange or other immunosuppressive treatment was not needed.

Recent advances in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is characterized by microangiopathic hemolytic anemia and thrombocytopenia, accompanied by microvascular thrombosis that causes variable degrees of tissue ischemia and infarction. Intravascular coagulation is not a prominent feature of the disorder. Plasma exchange can induce remissions in approximately 80% of patients with idiopathic TTP, but patients have a much worse prognosis when thrombotic microangiopathy is associated with cancer, certain drugs, infections, or tissue transplantation. Recently, acquired autoimmune deficiency of a plasma metalloprotease named ADAMTS13 was shown to cause many cases of idiopathic TTP. This review describes our current understanding of how to use this knowledge clinically. In Section I, Dr. Joel Moake describes the presentation of thrombotic microangiopathy, emphasizing the pathophysiology of idiopathic TTP. Platelets adhere to ultra-large (or "unusually large") von Willebrand factor (ULVWF) multimers that are immobilized in exposed subendothelial connective tissue and secreted into the circulation in long "strings" from stimulated endothelial cells. ADAMTS13 cleaves ULVWF multimers within growing platelet aggregates under flowing conditions, and this normally limits platelet thrombus formation. If ADAMTS13 is absent, either congenitally or due to acquired autoantibodies, platelet-rich microvascular thrombosis proceeds unchecked and TTP ensues. Plasma exchange is effective therapy for idiopathic TTP, probably because it replenishes the deficient ADAMTS13 and removes some of the pathogenic autoantibodies and endothelial-stimulating cytokines. Some patients have a type of thrombotic microangiopathy after transplantation/chemotherapy but do not have severe ADAMTS13 deficiency. The pathogenesis of their disease must differ but remains poorly understood. In Section II, Dr. Toshiyuki Miyata describes recent advances in assay methods that should facilitate routine laboratory testing of ADAMTS13 for patients with thrombotic microangiopathy. ADAMTS13 cleaves a single Tyr-Met bond in domain A2 of the VWF subunit. ADAMTS13 assays based on the cleavage of plasma VWF multimers have been used extensively but require considerable time and expertise to perform. A recombinant substrate containing 73 amino acid residues of VWF domain A2 has been devised that allows short incubation times and rapid product detection by gel electrophoresis or immunoassay. These results should encourage the development of even simpler assays that can be performed in most clinical laboratories. In Section III, Dr. James George provides an update on the long-term prospective study of thrombotic microangiopathy in the Oklahoma TTP-HUS Registry. At presentation, the clinical distinction between idiopathic TTP, various forms of secondary thrombotic microangiopathy, and even Shiga toxin-associated hemolytic uremic syndrome (HUS) can be problematic because the symptoms and laboratory findings often overlap. Consequently, plasma exchange usually is administered to any patient with thrombotic microangiopathy if there is doubt about the cause. The role of ADAMTS13 testing in choosing therapy remains uncertain, but the results do appear to have prognostic significance. Severe ADAMTS13 deficiency is specific for idiopathic TTP and identifies a subgroup with a high likelihood of response to plasma exchange, and high-titer ADAMTS13 inhibitors correlate strongly with a high risk of relapsing disease. Patients with normal ADAMTS13 activity have a much worse prognosis, although many factors probably contribute to this difference. Longitudinal study of these patients will continue to clarify the relationship of ADAMTS13 deficiency to the clinical course of thrombotic microangiopathy.

Relapsed thrombotic thrombocytopenic purpura presenting as an acute cerebrovascular accident

Thrombotic thrombocytopenic purpura (TTP) is an uncommon but severe disorder that classically presents with microangiopathic hemolytic anemia (MAHA), thrombocytopenia, and fluctuating neurological changes. Previously, it was impossible to make a diagnosis of TTP in the absence of thrombocytopenia or microangiopathic hemolysis (MAHA). We describe two cases of relapsing TTP that presented with acute cerebrovascular accident (CVA) without concurrent thrombocytopenia or MAHA after initial classical presentation of TTP. In both cases, the diagnosis of TTP as the cause of the CVA was attributed to severe deficiency of the von Willebrand factor cleaving protease, ADAMTS13 in plasma (11 and 12%, normal 79-127%). Each patient had a dramatic clinical improvement in response to therapeutic plasma exchange. The experience in these two cases suggests that TTP should be considered as a potential cause among patients presenting with a CVA, particularly if the patients have a history of TTP.

Measurement of von Willebrand factor-cleaving protease (ADAMTS-13) activity in plasma: a multicenter comparison of different assay methods

A severely deficient ADAMTS-13 activity (<5%) is a key laboratory finding confirming the diagnosis of thrombotic thrombocytopenic purpura (TTP), whereas a mildly or moderately decreased activity is found in various other conditions. Laboratory tests for ADAMTS-13 activity must reliably identify a severe deficiency and detect inhibitory antibodies against ADAMTS-13. We carried out a multicenter comparison of different assays for ADAMTS-13 activity in plasma, including the quantitative immunoblotting of degraded von Willebrand factor (VWF) substrate, the residual collagen binding activity and ristocetin cofactor activity of degraded VWF, and an immunoradiometric assay. The main goal was to investigate whether all assays concordantly identified severe ADAMTS-13 deficiency and detected inhibitory antibodies. ADAMTS-13 activity was determined by five laboratories in 30 plasma samples of patients with hereditary and acquired TTP and other conditions. ADAMTS-13 activity values of the samples ranged from <3% to > 100%. Concerning the identification of a severe ADAMTS-13 deficiency, good interassay and interlaboratory agreement was observed with only one false-negative and two false-positive results by two laboratories using a collagen binding assay. For samples with normal or mildly to moderately reduced ADAMTS-13 activity, results were less concordant. There was good agreement for the detection of strong inhibitors. We conclude that all assays investigated are useful as a screening test in suspected TTP. Further assay improvement is needed, however.

Platelets: thrombotic thrombocytopenic purpura

Abnormalities of plasma von Willebrand factor (VWF) have been recognized to be associated with thrombotic thrombocytopenic purpura (TTP) for over 20 years. Patients with chronic, relapsing TTP have VWF multimers that are larger than normal, similar in size to those secreted by cultured endothelial cells. Recent observations have documented that a deficiency of a VWF-cleaving protease (termed ADAMTS13) may be responsible for the presence of these unusually large VWF multimers. Multiple mutations of the ADAMTS13 gene can result in ADAMTS13 deficiency and cause congenital TTP; autoantibodies neutralizing ADAMTS13 protease activity have been associated with acquired TTP. In Section I, Dr. Evan Sadler reviews the structure, biosynthesis, and function of the ADAMTS13 protease. He describes the mutations that have been identified in congenital TTP and describes the relationship of ADAMTS13 deficiency to the development of both congenital and acquired TTP. Dr. Sadler postulates that the development of TTP may be favored by conditions that combine increased VWF secretion, such as during the later stages of pregnancy, and decreased ADAMTS13 activity. In Section II, Dr. Bernhard Lämmle describes the assay methods for determining ADAMTS13 activity. Understanding the complexity of these methods is essential for understanding the difficulty of assay performance and the interpretation of assay data. Dr. Lämmle describes his extensive experience measuring ADAMTS13 activity in patients with TTP as well as patients with acute thrombocytopenia and severe illnesses not diagnosed as TTP. His data suggest that a severe deficiency of ADAMTS13 activity (< 5%) is a specific feature of TTP. However, he emphasizes that, although severe ADAMTS13 deficiency may be specific for TTP, it may not be sensitive enough to identify all patients who may be appropriately diagnosed as TTP and who may respond to plasma exchange treatment. In Section III, Dr. James George describes the evaluation and management of patients with clinically suspected TTP, as well as adults who may be described as having hemolytic-uremic syndrome (HUS). Dr. George presents a classification of TTP and HUS in children and adults. Appropriate evaluation and management are related to the clinical setting in which the diagnosis is considered. A clinical approach is described for patients in whom the diagnosis of TTP or HUS is considered (1) following bone marrow transplantation, (2) during pregnancy or the postpartum period, (3) in association with drugs which may cause TTP either by an acute immune-mediated toxicity or a dose-related toxicity, (4) following a prodrome of bloody diarrhea, (5) in patients with autoimmune disorders, and (6) in patients with no apparent associated condition who may be considered to have idiopathic TTP. Patients with idiopathic TTP appear to have the greatest frequency of ADAMTS13 deficiency and appear to be at greatest risk for a prolonged clinical course and subsequent relapse. Management with plasma exchange has a high risk of complications. Indications for additional immunosuppressive therapy are described.

ADAMTS13 activity in thrombotic thrombocytopenic purpura-hemolytic uremic syndrome: relation to presenting features and clinical outcomes in a prospective cohort of 142 patients

Initial management of patients with thrombotic thrombocytopenic purpura-hemolytic uremic syndrome (TTP-HUS) is difficult because of lack of specific diagnostic criteria, high mortality without plasma exchange treatment, and risks of plasma exchange. Although severe ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type 1 repeats) deficiency may be specific for TTP, the role of ADAMTS13 activity measurements for initial management decisions is unknown. ADAMTS13 was measured before beginning plasma exchange treatment in 142 (88%) of 161 consecutive patients with clinically diagnosed TTP-HUS with assignment to 1 of 4 categories: less than 5% (severe deficiency), 5% to 9%, 10% to 25%, and more than 25%. Eighteen (13%) of 142 patients had severe ADAMTS13 deficiency. Among 6 predefined clinical categories (stem cell transplantation, pregnant/postpartum, drug association, bloody diarrhea, additional/alternative disorder, idiopathic), severe deficiency occurred only among pregnant/postpartum (2 of 10) and idiopathic (16 of 48) patients. The presenting features and clinical outcomes of the 16 patients with idiopathic TTP-HUS who had severe ADAMTS13 deficiency were variable and not distinct from the 32 patients with idiopathic TTPHUS who did not have severe ADAMTS13 deficiency. Many patients in all ADAMTS13 activity categories apparently responded to plasma exchange treatment. Therefore, severe ADAMTS13 deficiency does not detect all patients who may be appropriately diagnosed with TTP-HUS and who may respond to plasma exchange treatment.

Platelets and von Willebrand factor

The interaction of platelets with von Willebrand factor (VWF) is crucial in the initiation of any hemostatic or thrombotic process. VWF enables the platelet, via its surface glycoprotein receptors, to adhere to exposed subendothelium and to respond to shear stress in the blood. Via VWF that is stored and released from platelet alpha-granules and from Weibel-Palade bodies of endothelial cells, the hemostatic system can respond locally to lesions in the vessel wall and can initiate the activation of other platelets. This review describes the molecular structure of VWF, its functions and its interactions with the platelet membrane glycoprotein receptors GP Ib-IX-V and GP IIb-IIIa. As well, the role of VWF in shear-induced platelet adhesion and aggregation is described, and mechanisms are discussed that control the size of VWF multimers and the responsiveness of platelets to multimeric VWF. Finally, the review discusses the role of locally released VWF from platelet alpha-granules and from Weibel-Palade bodies for platelet activation in neonates and adults.

Therapeutic hemapheresis practitioners need to know about the treatment of thrombotic thrombocytopenic purpura

In the field of therapeutic apheresis, thrombotic thrombocytopenic purpura (TTP) makes up a large percentage of the conditions treated by plasma exchange. The implementation of plasma exchange in the treatment of TTP has been shown to reduce the mortality rate for this group of patients significantly. Apheresis operators see many patients with acute symptoms, the "chronic" syndrome of TTP, and those with a "relapsing" TTP. The apheresis practitioner is with the patient for several hours during each treatment, and thus becomes an element of continuity in the care and assessment of the TTP patient. Progress in understanding the pathophysiology of this condition necessitates the frequent review of literature published regarding TTP, and continuing education of staff and physicians.

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.

Use of vincristine and cyclosporine in childhood thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare but life-threatening hematologic disorder in children. Plasmapheresis, the standard therapy for TTP, is effective in achieving remission in most patients. However, some patients become either refractory to or dependent upon plasmapheresis. The authors report three such patients in whom the use of vincristine or vincristine plus cyclosporine resulted in permanent remission. A 12-year-old girl with TTP dependent on plasmapheresis for more than 5 months responded to vincristine with a decrease in the required frequency of plasmapheresis, but the addition of cyclosporine abrogated the need for further plasmapheresis. She subsequently developed serologic evidence of systemic lupus erythematosus. Two 15-year-old boys with TTP (one of them with underlying mixed connective tissue disease) became refractory to plasmapheresis after a brief initial response. The addition of vincristine in one patient and vincristine and cyclosporine in the second (with mixed connective tissue disease) led to complete remission. The authors' experience in this case study of three patients suggests that vincristine and cyclosporine are effective agents in the management of patients with TTP who do not achieve complete remission with plasmapheresis alone.

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

Thrombotic thrombocytopenic purpura (TTP) and the hemolytic uremic syndrome (HUS) are both characterized by thrombocytopenia, microangiopathic hemolysis, and organ dysfunction. Other disorders occasionally present with similar manifestations. Recent studies have demonstrated that deficiency in the von Willebrand factor cleaving protease ADAMTS13, due to genetic mutations or autoimmune inhibitors, causes TTP. Molecular cloning of ADAMTS13 elucidates the structure of the protease, raising the prospect for advances in diagnosis and treatment of the disease. Assay of ADAMTS13 activity distinguishes TTP from HUS and other types of thrombotic microangiopathy (TMA); therefore, the term TTP/HUS should be avoided because it obscures the known or potential differences among the various types of TMA.

A perspective on the measurement of ADAMTS13 in thrombotic thrombocytopaenic purpura

The recent discovery of the von Willebrand Factor (vWF) cleaving protease (ADAMTS13) and the association of its deficiency with thrombotic thrombocytopaenic purpura (TTP) has generated both enormous interest and considerable confusion. Ultra large von Willebrand Factor (UL vWF) multimers are present in the plasmas of patients with chronic relapsing TTP in remission but disappear during an attack. This observation led to the recognition that UL vWF multimers precipitate the thrombotic occlusion of arterioles, a feature that characterizes TTP. Multiple mutations in ADAMTS13 are associated with congenital TTP and neutralizing autoantibodies have been demonstrated in the acquired TTP syndrome. Although a number of functional assays for this enzyme have been described, the more rigorously evaluated assays are difficult to perform outside a research laboratory. There is also an enduring uncertainty about the specificity of ADAMTS13 deficiency for the diagnosis of acquired TTP and a perception that the result does not alter patient management. The cloning of the ADAMTS13 gene has also raised the prospect of recombinant enzyme therapy for the treatment of TTP, and this has heightened the need for a simple assay. In this review, we evaluate the value of measuring this enzyme in the management of TTP.

Thrombotic thrombocytopenic purpura in systemic lupus erythematosus: disease activity and the use of cytotoxic drugs

Thrombotic thrombocytopenic purpura (TTP) is a rare and occasionally fatal haematologic disorder that can coexist with systemic lupus erythematosus (SLE) and other autoimmune diseases. We identified all cases of TTP seen in our institution over a 3 year period using a computerized database. We found that SLE activity (measured by the SLE Disease Activity Index) and TTP activity ran a parallel course in three patients with coexistent SLE and TTP. TTP in these three patients, although refractory to plasmapheresis, responded to cytotoxic therapy. These observations further support an autoimmune contribution to the pathogenesis of some cases of TTP. A literature review revealed that mortality in SLE patients with more severe, refractory TTP treated with plasmapheresis and cytotoxics, may not be higher than in patients responding to plasmapheresis alone (who are likely to have milder disease). These data suggest that cytotoxics may have a role in treatment of patients with active SLE and TTP refractory to plasmapheresis.

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.

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.