Mechanism underlying severe deficiency of plasma ADAMTS-13 activity in immune thrombotic thrombocytopenic purpura

BACKGROUND

Immune-mediated thrombotic thrombocytopenic purpura is caused by autoantibodies against ADAMTS-13, a plasma enzyme that cleaves von Willebrand factor. However, the mechanism resulting in severe deficiency of plasma ADAMTS-13 activity remains controversial.

OBJECTIVES

To determine the mechanism of autoantibody-mediated severe deficiency of plasma ADAMTS13 activity in immune-mediated thrombotic thrombocytopenic purpura.

METHODS

Fluorescence resonance energy transfer-VWF73 was used to determine plasma ADAMTS-13 activity. Enzyme-linked immunosorbent assay (ELISA) was used to determine anti-ADAMTS-13 immunoglobulin G. ELISA and capillary electrophoresis-based Western blotting were employed to assess plasma ADAMTS-13 antigen.

RESULTS

We showed that plasma ADAMTS-13 antigen levels varied substantially in the samples collected on admission despite all showing plasma ADAMTS-13 activity of <10 IU/dL (or <10% of normal level) using either ELISA or Western blotting. More severe deficiency of plasma ADAMTS-13 antigen (<10%) was detected in admission samples by ELISA than by capillary Western blotting. There was a significant but moderate correlation between plasma ADAMTS-13 activity and ADAMTS-13 antigen by either assay method, suggesting that severe deficiency of plasma ADAMTS-13 activity is not entirely associated with low levels of ADAMTS-13 antigen.

CONCLUSION

We conclude that severe deficiency of plasma ADAMTS-13 activity primarily resulted from antibody-mediated inhibition, but the accelerated clearance of plasma ADAMTS-13 antigen via immune complexes may also contribute significantly to severe deficiency of plasma ADAMTS-13 activity in a subset of patients with acute immune-mediated thrombotic thrombocytopenic purpura.

Optimization of anti-ADAMTS13 antibodies for the treatment of ADAMTS13-related bleeding disorder in patients receiving circulatory assist device support

ADAMTS13 (a disintegrin-like and metalloproteinase with thrombospondin type-1 motif 13)-related bleeding disorder has been frequently observed as a life-threatening clinical complication in patients carrying a circulatory assist device. Currently, treatment modalities for the bleeding disorder are very limited and not always successful. To address the unmet medical need, we constructed humanized antibodies of mouse anti-ADAMTS13 antibody A10 (mA10) by using complementarity-determining region (CDR) grafting techniques with human antibody frameworks, 8A7 and 16E8. The characteristics of the two humanized A10 antibodies, namely A10/8A7 and A10/16E8, were assessed in vitro and in silico. Among the two humanized A10 antibodies, the binding affinity of A10/16E8 to ADAMTS13 was comparable to that of mA10 and human-mouse chimeric A10. In addition, A10/16E8 largely inhibited the ADAMTS13 activity in vitro. The results indicated that A10/16E8 retained the binding affinity and inhibitory activity of mA10. To compare the antibody structures, we performed antibody structure modeling and structural similarity analysis in silico. As a result, A10/16E8 showed higher structural similarity to mA10, compared with A10/8A7, suggesting that A10/16E8 retains a native structure of mA10 as well as its antigen binding affinity and activity. A10/16E8 has great potential as a therapeutic agent for ADAMTS13-related bleeding disorder.

Evaluation of a rapid turn-over, fully-automated ADAMTS13 activity assay: a method comparison study

Thrombotic thrombocytopenic purpura (TTP) is a life-threatening thrombotic microangiopathy caused by severely reduced activity of the von-Willebrand factor-cleaving protease ADAMTS13, mainly caused by anti-ADAMTS-13 antibodies. Although several test systems for ADAMTS13 measurement exist, long turn-around times hamper the usability in daily practice. We performed a method comparison study for two commercially available ADAMTS13 assays and evaluated the agreement between the fully-automated rapid turn-over HemosIL AcuStar ADAMTS13 Activity assay and the manually performed TECHNOZYM ADAMTS-13 Activity assay. Twenty-four paired test samples derived from 10 consecutively recruited patients (n = 8, acquired TTP; n = 1, atypical hemolytic uremic syndrome; n = 1, control), of which nine test samples were collected in case of clinically apparent TTP and 13 samples were collected from TTP patients in clinical remission were included. Overall correlation between the TECHNOZYM and AcuStar assay was good with a Pearson R of 0.93 (p < 0.001). Agreement between the assays assessed with the Passing-Bablok analysis showed high agreement with an Intercept of  - 2.56 (95% confidence interval [CI], - 5.07 to  - 0.86) and Slope of 1.04 (95% CI 0.84-1.17). The absolute mean bias was 2.54% (standard difference [SD], 6.38%; 95% CI to 10.0-15.05%). Intra-method reliability was high with an absolute mean bias of - 0.13% (SD 3.21%; 95% CI to 6.42-6.16%). The observer agreement for categorial thresholds (> or < 10% ADAMTS3 activity) was kappa = 0.82 (95% CI 0.59-1.0). Conclusively, overall agreement between the testing methods was sufficient and we support previously published data suggesting the AcuStar assay being a valuable and accurate tool for ADAMTS13 activity testing and TTP diagnostics.

Clinical and laboratory diagnosis of TTP: an integrated approach

Thrombotic thrombocytopenia purpura (TTP) is a rare, life-threatening disease with an incidence of approximately 2 persons per million per year. It is characterized by severe deficiency of the von Willebrand cleaving protease, ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13), leading to formation of platelet-rich thrombi in the microvasculature. Prompt initiation of appropriate therapy, particularly plasma exchange, may be life-saving. Diagnosis of TTP is challenging because of its diverse clinical manifestations, overlap in clinical presentation with other thrombotic microangiopathies, and limited availability of ADAMTS13 testing. Clinical prediction scores have been developed to estimate the pretest probability of severe ADAMTS13 deficiency and may be used as an adjunct to clinical judgment to guide initial management decisions. An ADAMTS13 activity level of less than 10% supports the diagnosis of TTP in appropriate clinical contexts, but many centers do not offer testing in-house and must send out the test to a reference laboratory with a turnaround time of several days. In such instances, initial management decisions must be made without the benefit of laboratory testing. In patients with TTP, inhibitor tests may be useful for distinguishing immune-mediated from congenital TTP. In this article, we review the epidemiology, natural history, and clinical presentation of TTP and laboratory assays for TTP including ADAMTS13 activity and inhibitor assays. We also describe an evidence-based approach to the evaluation of a patient with suspected TTP that integrates clinical and laboratory assessment.

ADAMTS13 Testing Methodologies and Thrombotic Thrombocytopenic Purpura (TTP): Conflicting Results Can Pose a Clinical Dilemma

ADAMTS13 testing plays a critical role in confirming the clinical diagnosis of acquired idiopathic thrombotic thrombocytopenic purpura (TTP) and distinguishing it from other forms of thrombotic microangiopathies (TMA). Serial measurements of ADAMTS13 activity and inhibitor levels are also helpful in determining response to treatment and/or subsequent relapses. Numerous ADAMTS13 assays have been developed recently, including some with rapid turnaround times. Despite the good inter-assay correlation of different ADAMTS13 methodologies in published case studies, discrepancies have been shown to occur. Here we present a case where discrepant results were obtained using two different assays, posing a clinical treatment dilemma.

Pathophysiology of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Thrombotic microangiopathies are rare disorders characterized by the concomitant occurrence of severe thrombocytopenia, microangiopathic hemolytic anemia, and a variable degree of ischemic end-organ damage. The latter particularly affects the brain, the heart, and the kidneys. The primary forms, thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), although their clinical presentations often overlap, have distinctive pathophysiologies. TTP is the consequence of a severe ADAMTS-13 deficiency, either immune-mediated as a result of circulating autoantibodies, or caused by mutations in ADAMTS-13. HUS develops following an infection with Shiga-toxin producing bacteria, or as the result of excessive activation of the alternative pathway of the complement system because of mutations in genes encoding complement system proteins.

An open conformation of ADAMTS-13 is a hallmark of acute acquired thrombotic thrombocytopenic purpura

Essentials Conformational changes in ADAMTS-13 are part of its mode-of-action. The murine anti-ADAMTS-13 antibody 1C4 discriminates between folded and open ADAMTS-13. ADAMTS-13 conformation is open in acute acquired thrombotic thrombocytopenic purpura (TTP). Our study forms an important basis to fully elucidate the pathophysiology of TTP.

SUMMARY

Background Acquired thrombotic thrombocytopenic purpura (aTTP) is an autoimmune disorder characterized by absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies. Recently, it was shown that ADAMTS-13 adopts a folded or an open conformation. Objectives As conformational changes in self-antigens play a role in the pathophysiology of different autoimmune diseases, we hypothesized that the conformation of ADAMTS-13 changes during acute aTTP. Methods Antibodies recognizing cryptic epitopes in the spacer domain were generated. Next, the conformation of ADAMTS-13 in 40 healthy donors (HDs), 99 aTTP patients (63 in the acute phase versus 36 in remission), 12 hemolytic-uremic syndrome (HUS) patients and 63 sepsis patients was determined with ELISA. Results The antibody 1C4 recognizes a cryptic epitope in ADAMTS-13. Therefore, we were able to discriminate between a folded and an open ADAMTS-13 conformation. We showed that ADAMTS-13 in HDs does not bind to 1C4, indicating that ADAMTS-13 circulates in a folded conformation. Similar results were obtained for HUS and sepsis patients. In contrast, ADAMTS-13 of acute aTTP patients bound to 1C4 in 92% of the cases, whereas, in most cases, this binding was abolished during remission, showing that the conformation of ADAMTS-13 is open during an acute aTTP episode. Conclusions Our study shows that, besides absent ADAMTS-13 activity and the presence of anti-ADAMTS-13 autoantibodies, an open ADAMTS-13 conformation is also a hallmark of acute aTTP. Demonstrating this altered ADAMTS-13 conformation in acute aTTP will help to further unravel the pathophysiology of aTTP and lead to improved therapy and diagnosis.

External validation of the PLASMIC score: a clinical prediction tool for thrombotic thrombocytopenic purpura diagnosis and treatment

Essentials Severe ADAMTS-13 deficiency is key to thrombotic thrombocytopenic purpura (TTP) diagnosis. PLASMIC score predicts ADAMTS-13 deficiency in suspected TTP with high discrimination. PLASMIC score is more generalizable with fewer missing data than alternative clinical scores. PLASMIC score identifies a subgroup of patients lacking significant response to plasma exchange.

SUMMARY

Background The PLASMIC score was recently published to distinguish patients with severe ADAMTS-13 deficiency from those without for early identification of thrombotic thrombocytopenia purpura (TTP). Objective We performed an independent external validation of the PLASMIC score for clinical prediction of severe ADAMTS-13 deficiency. Patients/Methods We studied an independent cohort of 112 consecutive hospitalized patients with suspected thrombotic microangiopathy and appropriate ADAMTS-13 testing (including 21 patients with TTP diagnosis). Results The PLASMIC score model predicted severe ADAMTS-13 deficiency with a c statistic of 0.94 (0.88-0.98). When dichotomized at high (score 6-7) vs. low-intermediate risk (score 0-5), the model predicted severe ADAMTS-13 deficiency with positive predictive value of 72%, negative predictive value of 98%, sensitivity of 90% and specificity of 92%. In the low-intermediate risk group (score 0-5) there was no significant improvement in overall survival associated with plasma exchange. Conclusions The PLASMIC score model had excellent applicability, discrimination and calibration for predicting severe ADAMTS-13 deficiency. The clinical algorithm allowed identification of a subgroup of patients who lacked a significant response to empiric treatment.

An enzyme immunoassay of ADAMTS13 distinguishes patients with thrombotic thrombocytopenic purpura from normal individuals and carriers of ADAMTS13 mutations

Recent studies demonstrate that assay of ADAMTS13, a circulating zinc metalloprotease that cleaves von Willebrand factor (VWF) at the Y1605-M1606 bond, is an important tool in the diagnosis of thrombotic thrombocytopenic purpura (TTP). In order to develop a method that could be adapted for general use, we describe an enzyme immunoassay (EIA)-based method for measuring the activity of ADAMTS13 in patient plasma samples. A monomeric peptide consisting of the amino acid residues D1596-R1668 of VWF was produced that spanned the ADAMTS13 cleavage site and was franked by glutathione s-transferase (GST) and a 6His sequences at the amino and carboxyl termini respectively. When probed with either anti-GST or anti-6His, the VWF fragment appeared as a 38.1-kDa band. After incubation with normal plasma, the VWF fragment was replaced by a 30.4-kDa band, which was recognized by antiGST but not by anti-6His, consistent with the expected cleavage at the Y1605-M1606 bond. EDTA or plasma samples from patients with TTP inhibited this cleavage. After incubation with normal plasma, the VWF fusion protein immobilized onto antiGST coated microtiter plate wells lost its anti-6His binding activity in a timeand plasma concentration-dependent manner. By using this EIA, the ADAMTS13 activity level was less than 0.12 U/mL in patients with acquired or hereditary TTP, distinguishing these patients from normal individuals or carriers of one copy of mutant ADAMTS13 allele. These results suggest the EIA method based on the VWF fusion protein is a simple but promising alternative for measuring ADAMTS13 activity.

Relation between ADAMTS13 activity and ADAMTS13 antigen levels in healthy donors and patients with thrombotic microangiopathies (TMA)

We have established a new, enzyme-linked immunosorbent assay (ELISA) for the detection of ADAMTS13 antigen using purified polyclonal rabbit anti-human ADAMTS13 IgG. Normal plasma ADAMTS13 antigen levels span a concentration of 740 –1420ng/ml (median 1080ng/ml) resulting in an ADAMTS13 activity to antigen ratio of 0.48 to 1.68 U/µg. In a cohort of HUS patients, ADAMTS13 antigen was in the normal range, whereas in hereditary TTP patients antigen levels were low to undetectable, in concordance with severe deficient ADAMTS13 activity. Plasma of acquired TTP patients was found to contain free as well as autoantibody-bound ADAMTS13. We also present evidence for circulating anti-ADAMTS13 antibody/ADAMTS13 antigen immune complexes not only in acutely ill or actively treated patients but also in patients who have already achieved clinical remission. This new developed ADAMTS13 antigen ELISA assay allows rapid determination of ADAMTS13 antigen levels in human plasma but is of limited predictive value for the diagnosis or treatment of acquired TTP due to the detection of ADAMTS13 in antibody complexes.

Derivation and external validation of the PLASMIC score for rapid assessment of adults with thrombotic microangiopathies: a cohort study

BACKGROUND

Among the syndromes characterised by thrombotic microangiopathy, thrombotic thrombocytopenic purpura is distinguished by a severe deficiency in the ADAMTS13 enzyme. Patients with this disorder need urgent treatment with plasma exchange. Because ADAMTS13 activity testing typically requires prolonged turnaround times and might be unavailable in resource-poor settings, a method to rapidly assess the likelihood of severe ADAMTS13 deficiency is needed.

METHODS

All consecutive adult patients presenting to three large academic medical centres in Boston, MA, USA, with thrombotic microangiopathy and a possible diagnosis of thrombotic thrombocytopenic purpura between Jan 8, 2004, and Dec 6, 2015, were included in an ongoing multi-institutional registry (the Harvard TMA Research Collaborative). Univariate analysis was used to identify covariates for a logistic regression model predictive of severe ADAMTS13 deficiency (≤10% activity). A clinical point score was generated, and its diagnostic performance was assessed using internal and external validation cohorts and compared to clinical assessment alone.

FINDINGS

214 patients with thrombotic microangiopathy were included in the derivation cohort. A seven-component clinical prediction tool, termed the PLASMIC score, was developed and found to reliably assess the pretest probability of severe ADAMTS13 deficiency (C statistic 0·96, 95% CI 0·92-0·98). Our diagnostic model was reproducibly accurate in both the internal (0·95, 0·91-0·98) and external (0·91, 0·85-0·95) validation cohorts. The scoring system also more consistently diagnosed thrombotic microangiopathy due to severe ADAMTS13 deficiency than did standard clinical assessment, as measured by C statistic (0·96, 95% CI 0·92-0·98 for PLASMIC vs 0·83, 0·77-0·88 for clinical assessment; p<0·0001) and mean Brier score (0·065 for PLASMIC vs 0·111 for clinical assessment; mean paired difference 0·05, 95% CI 0·01-0·08; p<0·0001). When utilised in addition to clinical assessment, the PLASMIC score contributed significant discriminatory power (integrated discrimination improvement 0·24, 95% CI 0·11-0·37).

INTERPRETATION

We have developed and validated a clinical prediction tool-the PLASMIC score-to stratify patients with thrombotic microangiopathy according to their risk of having severe ADAMTS13 deficiency. We have shown that this scoring system is superior to standard clinical assessment in addressing the diagnostic challenge presented by thrombotic microangiopathy. Its use, together with clinical judgment, may facilitate treatment decisions in patients for whom timely results of ADAMTS13 activity testing are unavailable.

FUNDING

The Luick Family Fund of Massachusetts General Hospital.

[Siblings with congenital thrombotic thrombocytopenic purpura]

Congenital thrombotic thrombocytopenic purpura (TTP) is a rare hereditary deficiency of ADAMTS13 (von Willebrand factor-cleaving protease) characterized by thrombocytopenia and microangiopathic hemolytic anemia. The spectrum of the clinical phenotype is wide, ranging from asymptomatic episodes of thrombocytopenia to life-threatening multiorgan failure. Reportedly, some patients develop isolated thrombocytopenia during childhood. We herein report sibling cases of congenital TTP. An 11-year-old boy with thrombocytopenia accompanied by influenza virus infection was referred to our hospital. He had a history of severe neonatal jaundice. His 15-year-old brother also had recurrent thrombocytopenia with approximately 10 episodes of recurrence since 3 years of age. Their ADAMTS13 activities were low and ADAMTS13 inhibitors were negative, and a gene analysis confirmed the diagnosis of congenital TTP. Notably, congenital TTP should be included in the differential diagnosis, and it is essential to determine the ADAMTS13 activity for pediatric patients with thrombocytopenia of unknown etiology.

[New developments in treatment modalities of thrombotic thrombocytopenic purpura]

Although thrombotic thrombocytopenic purpura (TTP) is a life-threatening disease, appropriate diagnosis and treatment result in the higher survival rate of >80%. TTP is usually suspected with thrombocytopenia and hemolytic anemia and is confirmed by a reduced activity of a disintegrin-like and metalloprotease with thrombospondin type 1 motif 13 (ADAMTS13) <10%. TTP is classified as acquired if a patient tests positive for anti-ADAMTS13 autoantibodies, and as congenital if ADAMTS13 gene abnormalities are identified. In patients with congenital TTP, fresh frozen plasma (FFP) transfusion for the supplementation of ADAMTS13 is performed. On the other hand, in patients with acquired TTP, plasma exchange therapy using FFP is conducted to supplement ADAMTS13 and remove anti-ADAMTS13 autoantibodies. Besides, corticosteroid therapy is often administered in conjunction with plasma exchange to suppress autoantibody production. In 2017, we aim to provide "diagnostic and treatment guidelines for TTP 2017 in Japan". In this review, we describe new developments in diagnosis and management of TTP, which are not discussed in the guidelines.

Recombinant thrombomodulin for secondary thrombotic thrombocytopenic purpura

In the pathogenesis of thrombotic thrombocytopenic purpura (TTP), reductions in the enzyme activity of ADAMTS13, which cuts ultralarge von Willebrand multimers, generates shear stress on the microvascular endothelium, leading to platelet aggregation and the formation of a thrombus. ADAMTS13 activity is markedly decreased in typical TTP, but is only mildly reduced in secondary TTP, which concomitantly develops with primary disease. The latter develops with septic disseminated intravascular coagulation (DIC) and often causes organ failure. Recombinant thrombomodulin (rTM) is a drug that is used to treat DIC and may also remit TTP because it improves vascular endothelial dysfunction. Therefore, we herein investigated the efficacy of rTM in patients treated for the pathology of secondary TTP. Patients who were admitted to the Emergency and Critical Care Center of our hospital and met the following conditions were extracted and retrospectively analyzed: hemolytic anemia accompanied by fragmented red blood cells (Hb < 12 g/dL or lower); thrombocytopenia (<100 × 10/μL); and ADAMTS13 activity <50%. Sixteen patients were included and accompanied by Kidney Disease: Improving Global Outcomes (KDIGO) stage 2 or more severe nephropathy and DIC. Eleven and 5 patients treated with and without rTM (the rTM and non-rTM treatment groups, respectively) were compared, and no significant difference was noted in their basic characteristics, such as background disease and severity. No significant difference was observed in survival rates; however, the platelet count, which is an important outcome of treatments for TTP, significantly increased in the rTM treatment group: 3.3 ± 2.6→11.3 ± 14.6 versus 3.5 ± 3.7→5.7 ± 3.9 (×1000/μL) (P = 0.034). Thrombotic thrombocytopenic purpura originally requires invasive treatments and its prognosis is not favorable. Blood thrombomodulin levels also markedly increase due to vascular endothelial dysfunction, whereas rTM alleviates vascular endothelial dysfunction in TTP patients with high blood TM levels, suggesting the importance of administering rTM. Thus, rTM may be effective for secondary TTP and may be adopted as adjuvant therapy.

[A Case of Thrombotic Thrombocytopenic Purpura in a Patient Undergoing FOLFOX6 plus Panitumumab Therapy for Unresectable Recurrent Rectal Cancer with a Rapidly Progressive Course]

A 71-year-old male patient began FOLFOX6 plus panitumumab treatment for unresectable recurrent rectal cancer. He developed thrombocytopenia after 2 courses of treatment and therefore a platelet transfusion was performed. The day after transfusion, the patient developed jaundice and hematuria. His lactate dehydrogenase levels had increased and a peripheral blood smear review revealed the presence of schistocytes. Anti-ADAMTS13 antibodies were present, and there was a reduction in ADAMTS13 activity. The patient was diagnosed with thrombotic thrombocytopenic purpura and treated with a plasma exchange. The day after the plasma exchange, his clinical condition rapidly worsened and he died. Thrombocytopenia due to chemotherapy often appears as myelosuppression. If conditions such as jaundice, indirect bilirubinemia, or hematuria appear during the course of chemotherapy, this condition must be considered as a differential diagnosis.

Acquired thrombotic thrombocytopenic purpura: new therapeutic options and their optimal use

Advances in our understanding of the pathophysiology of both congenital and acquired thrombotic thrombocytopenic purpura (TTP) have led to both an increased understanding of the disease and novel approaches to therapy. The efficacy of rituximab in acquired TTP has led to consideration of rituximab as a prophylactic therapy to prevent relapse of TTP. Novel therapies that target the A1 domain of von Willebrand factor (VWF) to block the formation of microthrombotic disease have also entered clinical study and have demonstrated promise as potential therapeutic options. Additionally, a recombinant ADAMTS13 protease has been developed which may be an important therapeutic option for both congenital and acquired TTP. The development of these new therapeutic options for patients diagnosed with TTP has increased the importance of conducting prospective, randomized studies with these agents to both confirm their efficacy and more importantly understand their most appropriate role in the treatment of patients with TTP.

Acute renal failure is prevalent in patients with thrombotic thrombocytopenic purpura associated with low plasma ADAMTS13 activity

BACKGROUND

Among patients with thrombotic microangiopathies, acute kidney injury (AKI) is the hallmark of hemolytic uremic syndrome (HUS) and is largely underestimated in patients with thrombotic thrombocytopenic purpura (TTP).

OBJECTIVE

We sought to report AKI features and outcomes in patients with TTP.

METHODS

We conducted a retrospective study of 92 patients with TTP assessed by low ADAMTS13 activity (< 10%) between 2001 and 2013. A logistic regression identified variables independently associated with AKI.

RESULTS

Among the 92 patients, 54 (58.7%) presented with AKI, including 25 (46.3%) with stage 3 AKI. Fourteen (27.4%) patients had a nephrotic-range proteinuria and 21 (45.6%) had hemoglobinuria. Hematuria and leucocyturia were detected in 19 (41.3%) and 16 patients (36.4%), respectively. Renal replacement therapy (RRT) was required in 14 patients (25.9%). Six months after TTP remission, RRT-free patients had median (IQR) MDRD (Modification of Diet in Renal Disease formula estimating the glomerular filtration rate) of 93 mL min(-1) per 1.73 m(2) (68.8-110) and three patients required long-term dialysis. Mild or moderate chronic renal disease occurred in 23/54 (42.6%) AKI patients. By multivariate analysis, serum level of complement component 3 at admission was the only factor independently associated with AKI (OR per 0.25 unit decrease of C3, 0.85; CI, 1.82-8.33; P = 0.001).

CONCLUSIONS

In patients with TTP, AKI is present in more than half the patients, and half of those will have lasting renal effects. Further studies to better understand the pathophysiology of renal involvement in patients with TTP and to identify a subset of patients with TTP syndrome overlapping HUS are warranted.

The novel ADAMTS13-p.D187H mutation impairs ADAMTS13 activity and secretion and contributes to thrombotic thrombocytopenic purpura in mice

BACKGROUND

Congenital thrombotic thrombocytopenic purpura (TTP) is characterized by mutations in the ADAMTS13 gene, which either impair protein secretion or influence ADAMTS13 (A Disintegrin-like And Metalloprotease domain with ThromboSpondin type-1 motif, member 13) activity. Phenotypic consequences of these mutations have not yet been evaluated in animal models for TTP.

OBJECTIVES

To identify the in vitro effect of a novel ADAMTS13 mutation and to investigate whether this mutation induces TTP in vivo.

METHODS

All 29 ADAMTS13 exons with exon-intron boundaries of a patient with pregnancy-onset TTP were sequenced. Wild-type and mutant ADAMTS13 proteins were both transiently and stably expressed in human embryonic kidney cells, and their activity was evaluated in vitro using fluorescence resonance energy transfer and flow assays. Molecular dynamics simulations were performed to study Ca(2+) stability. Adamts13(-/-) mice were hydrodynamically injected with wild-type and mutant expression plasmids and triggered with recombinant human von Willebrand factor.

RESULTS

We identified a novel heterozygous c.559G>C mutation in exon 6 of the proposita's ADAMTS13 gene. This mutation resulted in a p.Asp187His substitution (p.D187H), which was located in the high affinity Ca(2+) -binding site in the metalloprotease domain of ADAMTS13. The homozygous p.D187H mutation down-regulated ADAMTS13 activity in vitro. Impaired proteolytic activity was linked to unstable Ca(2+) binding as visualized using a molecular dynamics simulation. In addition, the p.D187H mutation affects protein secretion in vitro. In Adamts13(-/-) mice, the homozygous p.D187H mutation reduced ADAMTS13 secretion and activity and contributed to TTP when these mice were triggered with recombinant human von Willebrand factor.

CONCLUSIONS

Our data indicate that the p.D187H mutation impairs ADAMTS13 activity and secretion and is responsible for TTP onset in mice.

Low incidence of ADAMTS13 missense mutation R1060W in adult Egyptian patients with thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is an acute life-threatening disorder, characterized by thrombocytopenia, microangiopathic hemolytic anemia, widespread microvascular thrombi and consequent clinical sequelae due to ischemic organ damage. TTP is most commonly associated with deficiency or inhibition of von Willebrand factor-cleaving protease (ADAMTS13) activity. ADAMTS13 mutations and polymorphisms have been reported in childhood congenital TTP, but their significance in adult-onset TTP is still under investigation. Two mutations stand out: the single base insertion 4143insA in exon 29 and the missense mutation R1060W in exon 24 have both been observed in several unrelated families, mainly in adult-onset TTP, and over a wide geographic area. Our objective in this study is to identify the prevalence of R1060W missense mutation in exon 24 ADAMTS13 in a sample of adult Egyptian TTP patients. Thirty-one adult-onset TTP patients were included in this study, with a male/female ratio of 1:4. Twenty-six cases (84%) presented with acute idiopathic TTP, 2 cases were drug abusers and 3 cases were pregnant. None of the study cases provided a history of suspicious TTP symptoms during childhood (2 cases gave a history of episodes of thrombocytopenia during childhood). All cases showed statistically significant decreased ADAMTS13 activity compared to normal controls (p < 0.001). The study revealed a high statistical difference regarding the ADAMTS13 inhibitor level in primary versus secondary cases (p = 0.003). None of our Egyptian cases or of the healthy normal controls are positive for exon 24 missense mutation. Larger studies and regional and national TTP registries are recommended.

[Thrombotic thrombocytopenic purpura in a child with low ADAMTS13 enzyme level]

Thrombotic thrombocytopenic purpura (TTP) is a rare condition, but important to consider in case of thrombocytopenia and haemolysis. It is imperative to proceed with the correct treatment, in order to ensure a satisfactory outcome. TTP is either acquired or idiopathic. This case report shows that a 14-year-old boy has acquired TTP due to an infection with Campylobacter jejuni and Samonella szentes. Plasma exchange plays an essential role in the treatment of TTP.

Atypical hemolytic uremic syndrome and thrombotic thrombocytopenic purpura: clinically differentiating the thrombotic microangiopathies

The increased understanding of the pathophysiology of both atypical hemolytic uremic syndrome (aHUS) and thrombotic thrombocytopenic purpura (TTP) in recent years has led to significant therapeutic advances for both conditions. These advances have placed an increased emphasis on a more rapid differentiation of both disorders which remain clinical diagnoses. In particular, recent data demonstrating the effectiveness of complement inhibition in patients with aHUS have increased the need for a more rapid and accurate differentiation of aHUS and TTP. Previously utilized criteria have used the presence or absence of neurologic or renal injury and the pretreatment ADAMTS13 activity to differentiate aHUS from TTP. The use of presenting clinical symptoms and findings alone to differentiate these conditions is problematic given their overlapping clinical presentations. Similarly, the use of the pretreatment ADAMTS13 activity alone to differentiate aHUS from TTP is also problematic, and could lead to the inappropriate witholding of plasma exchange (PEX) therapy. However, when used collectively, the pretreatment clinical findings (symptoms and laboratory data) and ADAMTS13 activity in the context of the patient's response to PEX therapy can allow for a more effective differentiation of these two disorders in a timely fashion that will allow for the prompt initiation of the most appropriate therapy.

Crystal structure and enzymatic activity of an ADAMTS-13 mutant with the East Asian-specific P475S polymorphism

BACKGROUND

An East Asian-specific P475S polymorphism in the gene encoding ADAMTS-13 causes an approximately 16% reduction in plasma ADAMTS-13 activity.

OBJECTIVES

To demonstrate the impact of this dysfunctional polymorphism by characterizing the structure and activity of the P475S mutant protein.

METHODS

We determined the crystal structure of the P475S mutant of ADAMTS-13-DTCS (DTCS-P475S, residues 287-685) and compared it with the wild-type structure. We determined the enzymatic parameters of ADAMTS-13-MDTCS (residues 75-685) and MDTCS-P475S, and further examined the effects of denaturants and reaction temperature on their activity. We also examined the cleavage of shear-treated von Willebrand factor (VWF) by MDTCS-P475S.

RESULTS

MDTCS-P475S showed a reaction rate similar to that of wild-type MDTCS, but showed two-fold lower affinity for the peptidyl substrate, indicating that the Pro475-containing V-loop (residues 474-481) in the CA domain is a substrate-binding exosite. Structural analysis showed that the conformation of the V-loop was significantly different in DTCS-P475S and the wild type, where no obvious interactions of Ser475 with other residues were observed. This explains the higher susceptibility of the enzymatic activity of MDTCS-P475S to reaction environments such as denaturants and high temperature. MDTCS-P475S can moderately cleave shear-treated VWF.

CONCLUSIONS

We have provided structural evidence that the P475S polymorphism in ADAMTS-13 leads to increased local structural instability, resulting in lowered affinity for the substrate without changing the reaction rate. The moderate activity of ADAMTS-13-P475S for shear-treated VWF is sufficient to prevent thrombotic thrombocytopenic purpura (TTP) onset.

Thrombotic thrombocytopenic purpura with unusual 33 recurrences: a case report

The hereditary or acquired deficiency of ADAMTS-13 activity leads to an excess of high molecular weight von Willebrand factor multimers in plasma, leading to platelet aggregation and diffuse intravascular thrombus formation, resulting in thrombotic thrombocytopenic purpura (TTP). We report a 36 year old male with a long history of TTP associated with 33 relapses. As a result of early transfusions, the patient acquired Hepatitis C. This time, the patient presented with a TTP relapse after a 10 year remission, following PEG-interferon-Alpha (IFA) therapy for Hepatitis C. Since IFA has been reported to activate autoimmune reactions, it may have augmented production of ADAMTS-13 antibody.

Clinical experience in treatment of thrombotic thrombocytopenic purpura--hemolytic uremic syndrome with 28 patients

BACKGROUND

Neither optimal treatment nor significance of ADAMTS13 (A Desintegrin And Metalloprotease with ThromboSpondin type 1 repeats) activity for diagnosis and therapy of thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS) have not been defined yet. The aim of the report is to analyze response to different volumes of plasma exchange (PE), and relationship to ADAMTS13.

DESIGN AND METHODS

28 patients clinically diagnosed with idiopathic TTP (n = 18), secondary TTP (n = 4), atypical HUS (n = 3) and typical HUS (n = 3) manifested 31 acute episodes. Patients were treated with PE in 26, and with plasma transfusion in 5 episodes with additional different therapies.

RESULTS

PE volumes were as follows: 1 in 7, 1.5 in 3, 2 in 14, and intensifying schedule (1 to 1.5) in 2 episodes. Procedure number was lower in patients treated with 2 and 1.5 (p = 0.019) than in those treated with 1 volume exchange and PE intensifying, respectively (p = 0.010). PE response rate was 25/26 (96.15%). Exacerbation frequency was higher in idiopathic TTP patients (3/19) treated with 1 compared with patients treated with > 1 volume exchange (p = 0.003). Survival rate was 25/28 (89.29%). ADAMTS13 activity was reduced in 22 with severe deficiency in 14 patients.

CONCLUSION

Patients responded to different treatments regardless of ADAMTS13 activity, requiring less PEs with larger volume exchanges.

ADAMTS-13 activity and autoantibodies classes and subclasses as prognostic predictors in acquired thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is a rare life-threatening disease. Of surviving patients, 45% develops an exacerbation or a late recurrence. Severe ADAMTS-13 deficiency, both during the acute episode and remission, is a well-established predictor of recurrence. The predictive value of anti-ADAMTS-13 antibodies, their inhibitory activity and Ig class subtype for disease recurrence is still to be established.

OBJECTIVES

To analyze ADAMTS-13-related biomarkers (ADAMTS-13 and anti-ADAMTS-13 immunoglobulins, classes and subclasses) and their potential relationship with prognosis.

PATIENTS/METHODS

In 115 patients with TTP, we assessed the association between levels of these biomarkers and the severity of acute episodes; we analysed also the hazard ratio (HR) and 95% confidence interval (CI) of recurrence in association with biomarkers levels retrieved at the previous acute episode or during remission, using Cox regression models.

RESULTS

During the acute phase, higher IgA, IgG1 and IgG3 titers showed the strongest association with acute episode severity. In the survival analyzes, the only biomarker significantly associated with a high hazard of recurrence after an acute episode was the presence of IgG. Conversly, low ADAMTS-13 activity or antigen levels (<10%), the presence of ADAMTS-13 inhibitor or IgG during remission were all significantly associated with a higher hazard of recurrence.

CONCLUSIONS

Both the Ig class and subclass are of predictive value for acute episode severity in patients with TTP. Although markers that could predict the risk of recurrence in the acute phase are limited, a thorough assessment of ADAMTS-13-related parameters during remission is warranted.

ADAMTS-13 assays in thrombotic thrombocytopenic purpura

ADAMTS-13, the thirteenth member of the ADAMTS (A Disintegrin And Metalloprotease with Thrombo-Spondin 1 repeats) family, is the plasma metalloprotease responsible for regulating the multimeric structure of VWF. In congenital or acquired deficiency it is actively involved in the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a rare but life threatening disease characterized by microangiopathic haemolytic anaemia and consumptive thrombocytopenia leading to disseminated microvascular thrombosis and variable signs and symptoms of organ ischemia and damage. In the last few years, a number of in house and commercial laboratory assays for ADAMTS-13 and its autoantibodies have been developed. The features and clinical utility of ADAMTS-13 assays are summarized in this review.

Rituximab as pre-emptive treatment in patients with thrombotic thrombocytopenic purpura and evidence of anti-ADAMTS13 autoantibodies

Thrombotic thrombocytopenic purpura (TTP) is a rare and severe disease characterized by thrombocytopenia, microangiopathic haemolytic anemia, neurological and renal involvement associated with deficiency of the von Willebrand factor-cleaving protease, ADAMTS13. Persistence of high titers of anti-ADAMTS13 autoantibodies predisposes to relapsing TTP. Since relapses are associated with high morbidity and mortality rates, the optimal therapeutic option should be a pre-emptive treatment able to deplete anti-ADAMTS13 autoantibodies and avoid relapses. Five patients who presented with persistence of undetectable ADAMTS13 activity and high titers of autoantibodies, were treated with rituximab as pre-emptive therapy during remission. Four of them were affected by relapsing TTP and one was treated after the first episode. ADAMTS13 activity ranging from 15% to 75% with disappearance of inhibitors was achieved after three months in all patients, and persisted >20% without inhibitors at six months. In three patients disease-free status is still ongoing after 29, 24 and six months, respectively. Relapses were documented in two patients during follow-up: in one patient remission lasted 51 months; while in the other patient relapse occurred after 13 months. Results demonstrated that rituximab used as pre-emptive treatment may be effective in maintaining a sustained remission in patients with anti-ADAMTS13 antibodies in whom other treatments failed to limit the production of inhibitors, and suggests that re-treatment with rituximab should be considered when ADAMTS13 activity decreases and inhibitors reappear into the circulation, to avoid a new relapse.

ADAMTS13 in health and disease

We raised a set of monoclonal antibodies (mAb) against recombinant human ADAMTS13, constructed the first antigen test fully based on mAbs and compared ADAMTS13 antigen and enzymatic activity levels in a large set of plasma samples collected from different patients and healthy controls. Assessing both ADAMTS13 antigen and activity helps to understand whether or not the protease is fully active in pathological conditions, e.g. liver cirrhosis, inflammatory bowel disease, cardiac surgery, pregnancy and oral contraceptive intake, in the neonatal state and in normal individuals. Our ADAMTS13 antigen assay showed less variability than the collagen binding-based activity assay. Antigen values correlated well with activity in normal individuals, but differed to various degrees in neonates, pregnancies at later maternal age and cardiac surgery. No discrepancies were noted in liver cirrhosis and inflammatory bowel disease, which were both associated with low plasma levels of ADAMTS13. In conclusion, parallel measurement of ADAMTS13 activity and antigen provides a new tool for understanding the behavior of the VWF-cleaving protease in health and disease.

[Thrombotic thrombocytopenic purpura: experience in 18 cases and literature review]

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is characterized by anemia, thrombocytopenia, neurological and renal involvement of variable severity and it has a dismal prognosis. Platelet-derived von Willebrand Factor-cleaving metalloprotease ADAMTS-13 activity may orient the diagnosis, but normal levels do not discard it. The most effective therapy thus known is plasmapheresis.

AIM

To report the experience in 18 patients with TTP.

MATERIAL AND METHODS

Retrospective assessment of 11 patients and prospective assessment of seven subjects with TTP, aged 15 to 81 years.

RESULTS

All presented with anemia, thrombocytopenia and LDH elevation. Sixteen had neurological symptoms, five had fever, four had macroscopic urinary excretion of pigments, four had petechiae, and two had nosebleeds. Haptoglobin was low in 10 of 11 patients in whom it was measured. ADAMTS-13 had low activity in 15 of 17 patients (in 11, the inhibitor was found). Seventeen patients were treated with plasmapheresis and nine received steroids also. Seven patients died due to shock with respiratory involvement or multiple organic failure.

CONCLUSIONS

TTP has heterogeneous modes of presentation. If the diagnosis is strongly suspected, plasmapheresis can be started without laboratory confirmation. An ADAMTS-13 activity below 6% is almost exclusive of TTP .

[New marker in thrombotic thrombocytopenic purpura]

Thrombotic microangiopathy can be caused by several conditions which are difficult to diagnose from the clinical presentation alone. Deficient enzyme activity of a newly-discovered enzyme, ADAMTS-13, can lead to thrombotic thrombocytopenic purpura (TTP). Lack of ADAMTS-13 activity causes increased concentrations of high molecular weight von Willebrand factor forms and increased platelet aggregation. Measurement of ADAMTS-13 activity is useful for the diagnosis of TTP and may also be relevant as a prognostic test for recurrent TTP.

Prevalence of the ADAMTS-13 missense mutation R1060W in late onset adult thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura (TTP) is most commonly associated with deficiency or inhibition of von Willebrand factor-cleaving protease (ADAMTS-13) activity. ADAMTS-13 mutations and polymorphisms have been reported in childhood congenital TTP, but their significance in adult onset TTP remains unclear.

OBJECTIVES

We sought to identify common ADAMTS-13 mutations in adults with late onset TTP and to investigate whether they may predispose acute clinical episodes of the disorder in adulthood.

PATIENTS/METHODS/RESULTS

We detected a missense mutation (C3178T) in exon 24 of ADAMTS-13 in 6/53 (11.3%) adult onset TTP patients, but no normal controls (n = 100). Three of the patients had pregnancy-associated TTP; three had chronic relapsing acute idiopathic TTP. C3178T encodes an arginine to tryptophan (R1060W) substitution in the TSP1-7 domain of ADAMTS-13. In vitro expression of mutant and wild-type ADAMTS-13 demonstrated that R1060W caused severe intracellular retention of ADAMTS-13 (<5% secretion) without affecting its metalloprotease activity. One homozygous and five heterozygous patients were identified. No other causative mutations were discovered, yet all six patients had ADAMTS-13 activity levels <5% at presentation (normal: 66-126%). Antibodies/inhibitors to ADAMTS-13 were detected in three/five heterozygous patients, and all six patients had subnormal antigen levels. Six asymptomatic first-degree relatives, including those of two probands with antibodies, were also heterozygous for C3178T; all but one had subnormal ADAMTS-13 activity.

CONCLUSION

The high prevalence of R1060W ADAMTS-13 in adult onset TTP, together with its absence in childhood congenital TTP cases reported elsewhere, suggests it may be a factor in the development of late onset TTP.

ADAMTS13 activity and the presence of acquired inhibitors in human immunodeficiency virus-related thrombotic thrombocytopenic purpura

BACKGROUND

Little is known about the pathophysiology of human immunodeficiency virus (HIV)-related thrombotic thrombocytopenic purpura (TTP). It is generally assumed that acquired ADAMTS13 deficiency is due to the presence of autoantibody inhibitors, but limited data are available regarding ADAMTS13 activity and inhibitors in such patients.

STUDY DESIGN AND METHODS

By use of a collagen-binding assay, ADAMTS13 activity was analyzed at presentation in 20 patients with HIV-related TTP. The presence of inhibitors in patients with reduced ADAMTS13 activity was assessed with mixing studies. The correlation between ADAMTS13 activity and inhibitors and other laboratory and clinical parameters was assessed.

RESULTS

The patients fell clearly into two groups with regard to ADAMTS13 activity. Six patients (30%) had activity within the normal range, whereas the remaining 14 patients had severely reduced levels. Of the patients with reduced activity, only 5 patients had a detectable inhibitor whereas 8 showed no evidence of an inhibitor. There was significant correlation between normal ADAMTS13 activity and lower CD4 counts (p = 0.049). von Willebrand factor (VWF) antigen levels were significantly higher in patients with reduced ADAMTS13 activity (p = 0.03). Low activity in the absence of a detectable inhibitor was associated with significantly higher D-dimer levels (p = 0.01) and worse clinical outcome.

CONCLUSION

The heterogeneity with regard to ADAMTS13 activity and the absence of inhibitors in the majority of patients indicate that other factors are important in the pathogenesis of HIV-related TTP. VWF release and localized coagulation activation due to direct viral or cytokine-mediated endothelial cell injury is likely to be playing a major role.

[Clinical application of a method for evaluating von Willebrand factor cleaving protease activity]

OBJECTIVE

To establish a method for evaluating the activity of von Willebrand factor cleaving protease (vWF-cp) and evaluate its clinical application.

METHODS

Purified von Willebrand factor polymer was isolated by gel filtration from human fresh-frozen plasma as the enzyme substrate. SDS-PAGE, Western blotting, and luminographic detection were used to evaluate vWF-cp activity of 60 healthy adults, 28 patients with cerebral infarction (CI) and 7 with thrombotic thrombocytopenic purpura (TTP).

RESULTS

In the subjects involved, the method for evaluating vWF-cp activity had intra- and inter-batch coefficient of variation(CV) of 4.81% (n=8) and 8.63% (n=6), respectively. According to this method, the plasma vWF-cp activity in the 60 healthy adults was significantly higher than that in the CI patients [(86.53-/+17.49)% vs (77.15-/+16.72)%, P<0.05]. In TTP patients before plasma replacement, the vWF-cp activity was (9.06-/+7.17)% and increased significantly to (47.00-/+6.27)% 24 h after plasma replacement, respectively, but still significantly lower than that of healthy adults (P<0.01), whereas in the convalescent stage, the activity approached the normal level [(83.18-/+8.83)%, P>0.05].

CONCLUSIONS

According to the described method, which allows accurate vWF-cp activity measurement with good sensitivity, specificity and reproducibility, vWF-cp activity is lower in CI patients and even more so in TTP patients than that of healthy adults. Plasma replacement can effectively increase the vWF-cp activity in TTP patients.

An IAP retrotransposon in the mouse ADAMTS13 gene creates ADAMTS13 variant proteins that are less effective in cleaving von Willebrand factor multimers

Severe deficiency of ADAMTS13, a von Willebrand factor (VWF)-cleaving metalloprotease, causes thrombotic thrombocytopenic purpura. When analyzed with VWF multimers, but not with an abbreviated VWF peptide (VWF73) as the substrate, the plasma ADAMTS13 activity levels of mouse strains segregated into a high and a low group that differed by approximately 10 fold. Low ADAMTS13 activity was detected in mice containing 2 alleles of intracisternal A-type particle (IAP) retrotransposon sequence in the ADAMTS13 gene. Molecular cloning of mouse ADAMTS13 identified 2 truncated variants (IAP-a and IAP-b) in the low-activity mice. Both of the IAP variants lacked the 2 carboxyl terminus thrombospondin type 1 repeat (TSR) and CUB domains of full-length ADAMTS13. The IAP-b variant also had splicing abnormalities affecting the spacer domain sequence and had miniscule enzymatic activity. Compared with full-length ADAMTS13, the IAP-a variant was approximately one ninth as active in cleaving VWF multimers but was only slightly less active in cleaving VWF73 peptide. Recombinant human ADAMTS13 was also less effective in cleaving VWF multimers than VWF73 when the C-terminal TSR sequence was deleted. In summary, the carboxyl terminus TSR sequence is important for cleaving VWF multimers. Assay results should be interpreted with caution when peptide substrates are used for analysis of variant ADAMTS13 proteins.

[The role of von Willebrand protease in aetiopathogenesis of thrombotic thrombocytopenic purpura]

Thrombotic thrombocytopenic purpura (TTP) is a disseminated form of thrombotic microangiopathy characterised by thrombocytopenia, microangiopathic haemolysis, a variety of neurologic disturbances, fever and renal dysfunction. Endothelial lesion, activation of apoptosis and factors which induce platelet adhesion and aggregation could play a role in pathogenesis of TTP. Recent studies have demonstrated the inhibiting autoantibody against von Willebrand factor (vWF) cleaving protease in plasma of a great majority of TTP patients. A rare congenital deficiency of protease due to genetic mutations on both alleles has also been described. The accumulation of the ultra-large vWF multimers, due to either congenital or acquired deficiency of protease, exposed to the fluid shear stress in the microcirculation, induces formation of the platelet thrombi thus causing TTP. Cloning and synthesis of the molecular structure of protease raise the prospect for advances in diagnosis and treatment of the disease. Namely, in the absence of the inexpensive, more accurate and commonly available test, a diagnosis is based on the diagnostic dyad: thrombocytopenia and microangiopathic haemolytic anaemia in the absence of other cause. Also, in the absence of the enzyme replacement therapy, a therapeutic plasma exchange remains a gold standard in the treatment of TTP patients. Current assay methods for vWF protease could distinguish TTP from haemolytic uraemic syndrome and other microangiopathic syndromes. All current assays differ in the substrate nature, denaturant, incubation time or the detection method.

ADAMTS13 and microvascular thrombosis

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

In-vitro and in-vivo consequences of mutations in the von Willebrand factor cleaving protease ADAMTS13 in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disease characterized by microvascular thrombosis, often associated with deficiency of the vonWillebrand factor (VWF) cleaving protease ADAMTS13. We investigated the spectrum of ADAMTS13 gene mutations in patients with TTP and congenital ADAMTS13 deficiency to establish the consequences on ADAMTS13 processing and activity. We describe five missense (V88M, G1239V, R1060W, R1123C and R1219W), 1 nonsense (W1016Stop) and 1 insertion (82_83insT) mutations. In two patients no mutation was identified despite undetectable protease activity. Expression in HEK293 mammalian cells (V88M, G1239V, R1123C and R1219W) documented that three missense mutants were not secreted, whereas theV88M was secreted at low levels and with reduced activity. We also provide evidence that impaired secretion of ADAMTS13 mutants observed in vitro translates into severely reduced ADAMTS13 antigen levels in patients in vivo. To evaluate whether the small amounts of mutant protease present in the circulation of patients had VWF cleaving activity, WT and mutant rADAMTS13 were stably expressed in Drosophila S2 cells under the influence of the Drosophila BiP protein signal sequence, which allows protein secretion. Drosophila expression system showed a 40-60% protease activity in the mutants. Several single nucleotide polymorphisms (SNPs) within exons and intron boundaries were found in patients, suggesting that the interplay of SNPs could at least in part account for ADAMTS13 functional abnormalities in patients without mutations. In conclusion, defective secretion and impaired activity of the mutants concur to determine an almost complete deficiency of ADAMTS13 activity in patients with a homozygous or two heterozygous ADAMTS13 mutations.

ADAMTS-13 regulates platelet adhesion under flow. A new method for differentiation between inherited and acquired thrombotic thrombocytopenic purpura

ADAMTS-13 cleaves large and ultra-large von Willebrand factor multimers normally secreted by endothelial cells. Severe deficiency of this enzyme leads to thrombotic thrombocytopenic purpura (TTP). We applied the Impact-R system [Cone and plate(let) Analyzer, CPA] to determine optimal conditions for ADAMTS-13 function, to assess it's activity in TTP patients and to distinguish inherited TTP (inTTP) from acquired TTP (acTTP). The role of ADAMTS-13 in platelet adhesion under different conditions was investigated applying recombinant forms of VWF and ADAMTS-13. rVWF was first treated by rADAMTS-13 either in solution or when immobilized on the surface of the well, under static or flow conditions, in the presence or absence of BaCl2. The resulting cleaved VWF fragments were then immobilized and served to assess type 3 von Willebrand disease whole blood platelet adhesion under flow. Maximal effect of the rADAMTS-13 (decrease of platelet adhesion in the absence compared to the presence of BaCl2), was observed when the rVWF was pre-immobilized and the cleavage step took place under flow (85%). Mixing plasma ofTTP patients with normal blood (1:3) yielded a 1.6- to 2-fold increase of platelet adhesion under flow compared to mixing normal plasma with normal blood, at shear rate range of 1,800 - 2,500 s(-1) . Maximal increase of platelet adhesion was observed under 2,050 s(-1) . Under these conditions, the extent of adhesion was similarly higher in patients with inTTP and acTTP versus control [surface coverage (SC) 14.5 +/- 2.8% and 14.6 +/- 2.5% vs.7.4 +/- 1.7%, respectively]. ADAMTS-13 activity measured by collagen-binding test was similarly low (4.2 +/- 3.8% and 3.5 +/- 2.4% vs. 72.2 +/- 8.0%, respectively). An inverse correlation between SC and ADAMTS-13 activity was observed in a patient with inTTP assayed eight times during plasma infusion treatment. Addition of BaCl2 to the mixture of TTP plasma and normal whole blood yielded a decrease in platelet adhesion in inTTP (by 51%) but not in acTTP. The lack of reduction of platelet adhesion in acTTP could presumably be due to the presence of ADAMTS-13 inhibitor in these patients. These results suggest that VWF immobilization and high shear flow yielded optimal conditions for ADAMTS-13 activity, and that introduction of BaCl2 in the Impact-R (CPA) test may be useful for differentiation between inherited and acquired TTP.

Prise en charge par échanges plasmatiques d'un purpura thrombotique thrombocytopénique au cours d'une maladie de Still : à propos d'un cas

We report the seventh published case of thrombotic thrombocytopenic purpura (TTP) associated to a Still's disease. We confirmed the secondary character of TTP using the measurement of ADAMTS 13 protease. Because of clinical and biological improvement, daily plasmapheresis was stopped after eight days of ICU treatment. Unfortunately, early (24 hours) relapse occurred resulting in daily plasmapheresis resumption for 11 more days. Main therapeutic goals and aggressive treatment duration of PTT associated with a Still's disease remain to be determined.

Thrombotic Thrombocytopenic Purpura Associated with von Willebrand Factor-Cleaving Protease (ADAMTS13) Deficiency in Children

The physiopathology of thrombotic thrombocytopenic purpura (TTP) has been clarified since 1998, when it was shown that TTP in adults was most often associated with an acquired deficiency of von Willebrand factor-cleaving protease (ADAMTS13) due to autoantibodies, whereas TTP in children was most often associated with a hereditary autosomal recessive severe deficiency of ADAMTS13. The hereditary form of TPP (Upshaw-Schulman syndrome) is a very rare but life-threatening disease if adequate treatment (plasma therapy) is not administered. First manifestations occur before age 10 in two thirds of cases and as soon as birth in most cases. The subsequent course is characterized by recurrent hemolytic and thrombocytopenic crises, with intervals between relapses from every 3 to 4 weeks in two thirds of cases to several months or years in one third of cases. TTP crises are associated with cerebral vascular accidents in at least 30% of patients, with a risk of neurologic sequelae in approximately 20% of patients. Renal involvement includes frequent acute renal failure due to hemoglobinuria and/or thrombotic microangiopathy during hemolytic crisis and progressive renal deterioration in approximately 50% of cases, leading to chronic or end-stage renal failure in approximately 20% of patients. The clinical phenotype may vary from the typical congenital recurrent TTP. Some mild forms are limited to a fluctuating thrombocytopenia and may be misdiagnosed as idiopathic thrombocytopenic purpura. Phenotypic variability may be observed within a single family, which suggests a role of modifier genes. Fresh frozen plasma (FFP) replaces active ADAMTS13. Ten milliliters per kilogram FFP every 2 to 4 weeks suffices to maintain remission. FFP infusions are best used preventively, given that rescue infusions may not prevent central nervous system and renal involvement. It is hoped that plasmatic or recombinant purified ADAMTS13 will be available in the years to come.

von Willebrand Factor-Cleaving Protease (ADAMTS13) in the Course of Stem Cell Transplantation

Transplantation-associated microangiopathy (TAM) is a severe complication of stem cell transplantation. Although TAM shares many features with idiopathic thrombotic thrombocytopenic purpura or hemolytic uremic syndrome, the prognosis of TAM is worse. Clinical similarities and the observation that uncleaved ultralarge von Willebrand factor (ULVWF) multimers are found in the circulation of patients suffering from TAM suggest a defect in VWF proteolysis that may be due to a deficiency in ADAMTS13 activity. In this study the course of 28 consecutive patients, who received an allogeneic stem cell transplant was correlated to ADAMTS13 activity. Before stem cell transplantation, mean ADAMTS13 activity was within normal range. Within the first 8 weeks, mean activity declined to less than half the baseline activity. Furthermore, most of the patients showed normalization of ADAMTS13 activity. Low levels of ADAMTS13 activity were not correlated with clinical signs of thrombotic microangiopathy. However, two patients with clinical TAM had the lowest activity of all patients when suffering a severe bout of microangiopathy. Plasma exchange was not able to normalize ADAMTS13 deficiency in these patients, suggesting inactivation or consumption of ADAMTS13 activity in TAM.