ADAMTS13 phenotype in plasma from normal individuals and patients with thrombotic thrombocytopenic purpura

Red blood cell-derived arginase release in hemolytic uremic syndrome

BACKGROUND

Hemolysis is a cardinal feature of hemolytic uremic syndrome (HUS) and during hemolysis excess arginase 1 is released from red blood cells. Increased arginase activity leads to reduced L-arginine, as it is converted to urea and L-ornithine, and thereby reduced nitric oxide bioavailability, with secondary vascular injury. The objective of this study was to investigate arginase release in HUS patients and laboratory models and correlate arginase levels to hemolysis and kidney injury.

METHODS

Two separate cohorts of patients (n = 47 in total) with HUS associated with Shiga toxin-producing enterohemorrhagic E. coli (EHEC) and pediatric controls (n = 35) were investigated. Two mouse models were used, in which mice were either challenged intragastrically with E. coli O157:H7 or injected intraperitoneally with Shiga toxin 2. An in vitro model of thrombotic microangiopathy was developed in which Shiga toxin 2- and E. coli O157 lipopolysaccharide-stimulated human blood cells combined with ADAMTS13-deficient plasma were perfused over glomerular endothelial cells. Two group statistical comparisons were performed using the Mann-Whitney test, multiple groups were compared using the Kruskal-Wallis test followed by Dunn's procedure, the Wilcoxon signed rank test was used for paired data, or linear regression for continuous variables.

RESULTS

HUS patients had excessively high plasma arginase 1 levels and activity (conversion of L-arginine to urea and L-ornithine) during the acute phase, compared to remission and controls. Arginase 1 levels correlated with lactate dehydrogenase activity, indicating hemolysis, as well as the need for dialysis treatment. Patients also exhibited high levels of plasma alpha-1-microglobulin, a heme scavenger. Both mouse models exhibited significantly elevated plasma arginase 1 levels and activity. Plasma arginase 1 levels correlated with lactate dehydrogenase activity, alpha-1-microglobulin and urea levels, the latter indicative of kidney dysfunction. In the in vitro model of thrombotic microangiopathy, bioactive arginase 1 was released and levels correlated to the degree of hemolysis.

CONCLUSIONS

Elevated red blood cell-derived arginase was demonstrated in HUS patients and in relevant in vivo and in vitro models. The excessively high arginase levels correlated to the degree of hemolysis and kidney dysfunction. Thus, arginase inhibition should be investigated in HUS.

High prevalence of hereditary thrombotic thrombocytopenic purpura in central Norway: from clinical observation to evidence

Essentials The population prevalence of hereditary thrombotic thrombocytopenic purpura (TTP) is unknown. We studied the prevalence of hereditary TTP and population frequencies of two ADAMTS-13 mutations. A high frequency of hereditary TTP related to ADAMTS-13 mutation c.4143_4144dupA was found. Vicinity of ABO blood group and ADAMTS-13 loci may facilitate screening of ADAMTS-13 mutations.

SUMMARY

Background Hereditary thrombotic thrombocytopenic purpura (TTP) caused by ADAMTS-13 mutations is a rare, but serious condition. The prevalence is unknown, but it seems to be high in Norway. Objectives To identify all patients with hereditary TTP in central Norway and to investigate the prevalence of hereditary TTP and the population frequencies of two common ADAMTS-13 mutations. Patients/Methods Patients were identified in a cross-sectional study within the Central Norway Health Region by means of three different search strategies. Frequencies of ADAMTS-13 mutations, c.4143_4144dupA and c.3178 C>T (p.R1060W), were investigated in a population-based cohort (500 alleles) and in healthy blood donors (2104 alleles) by taking advantage of the close neighborhood of the ADAMTS-13 and ABO blood group gene loci. The observed prevalence of hereditary TTP was compared with the rates of ADAMTS-13 mutation carriers in different geographical regions. Results We identified 11 families with hereditary TTP in central Norway during the 10-year study period. The prevalence of hereditary TTP in central Norway was 16.7 × 10(-6) persons. The most prevalent mutation was c.4143_4144dupA, accounting for two-thirds of disease causing alleles among patients and having an allelic frequency of 0.33% in the central, 0.10% in the western, and 0.04% in the southeastern Norwegian population. The allelic frequency of c.3178 C>T (p.R1060W) in the population was even higher (0.3-1%), but this mutation was infrequent among patients, with no homozygous cases. Conclusions We found a high prevalence of hereditary TTP in central Norway and an apparently different penetrance of ADAMTS-13 mutations.

Congenital thrombotic thrombocytopenic purpura: Upshaw-Schulman syndrome: a cause of neonatal death and review of literature

Thrombotic thrombocytopenic purpura (TTP) is a rare disorder in children characterized by microangiopathic hemolytic anemia (MAHA) and thrombocytopenia. The classic Moschcowitz Pentads of TTP include hemolytic anemia, with fragmentation of erythrocytes, thrombocytopenia, diffuse and non-focal neurologic findings, decrease renal function and fever. We report a newborn who was diagnosed with congenital TTP. The newborn was admitted at age of 40 h, in our hospital, in view of respiratory distress with impending respiratory failure and red colored urine. On examination, the newborn was febrile, tachypneic, had deep icterus, pallor and no hepatosplenomegaly. Family history was significant with one unexplained neonatal death at age of 24 with symptoms of red colored urine. Examination of peripheral smear was diagnostic with the presence of fragmented RBCS, giant but fewer platelets consistent with a diagnosis of MAHA. The diagnosis of TTP was confirmed with low ADAMTS activity and gene analysis showed c 2203 G > T-p.Glu735X (domain TSP1-2) mutation in exon 18 of ADAMTS 13 gene. The newborn had rapid deterioration, with respiratory distress and refractory shock leading to death. Post-mortem bone marrow done showed marrow hyperplasia.

ADAMTS13 gene variants and function in women with preeclampsia: a population- based nested case- control study from the HUNT Study

INTRODUCTION

Known genetic variants with reference to preeclampsia only explain a proportion of the heritable contribution to the development of this condition. The association between preeclampsia and the risk of cardiovascular disease later in life has encouraged the study of genetic variants important in thrombosis and vascular inflammation also in relation to preeclampsia. The von Willebrand factor-cleaving protease, ADAMTS13, plays an important role in micro vascular thrombosis, and partial deficiencies of this enzyme have been observed in association with cardiovascular disease and preeclampsia. However, it remains unknown whether decreased ADAMTS13 levels represent a cause or an effect of the event in placental and cardiovascular disease.

METHODS

We studied the distribution of three functional genetic variants of ADAMTS13, c.1852C>G (rs28647808), c.4143_4144dupA (rs387906343), and c.3178C>T (rs142572218) in women with preeclampsia and their controls in a nested case-control study from the second Nord-Trøndelag Health Study (HUNT2). We also studied the association between ADAMTS13 activity and preeclampsia, in serum samples procured unrelated in time of the preeclamptic pregnancy.

RESULTS

No differences were observed in genotype, allele or haplotype frequencies of the different ADAMTS13 variants when comparing cases and controls, and no association to preeclampsia was found with lower levels of ADAMTS13 activity.

CONCLUSION

Our findings indicate that ADAMTS13 variants and ADAMTS13 activity do not contribute to an increased risk of preeclampsia in the general population.

Biological variations of ADAMTS13 and von Willebrand factor in human adults

Background:

The ultra-large von Willebrand factor (vWF) multimers are very active and must be degraded by ADAMTS13 for optimal activity. A severe functional deficiency of ADAMTS13 has been associated with thrombotic thrombocytopenic purpura. The correct interpretation of patient vWF and ADAMTS13 plasma levels requires an understanding of the biological variation associated with these analytes. In the present paper, we aimed to determine the biological variation of ADAMTS13 and vWF in human adults.

Materials and methods:

Blood samples were collected weekly from 19 healthy subjects for 5 consecutive weeks. vWF activity and antigenicity were determined using aggregometric and immunoturbidimetric methods. ADAMTS13 antigenicity and activity were determined by ELISA.

Results:

The within-subject biological variations for vWF activity and antigenicity were 8.06% and 14.37%, respectively, while the between-subject biological variations were 18.5% and 22.59%, respectively. The index of individuality for vWF activity was 0.44, while vWF antigenicity was 0.64. Similarly, ADAMTS13 activity and antigenicity within-subject biological variations were 12.73% and 9.75%, respectively, while between-subject biological variations were 9.63% and 6.28%, respectively. The ADAMTS13 indexes of individuality were 1.32 and 1.55, respectively.

Conclusion:

We report high biological variation and individuality in vWF antigenicity and activity levels. However, ADAMTS13 antigenicity and activity displayed high biological variation, but low individuality. Thus, population-based reference intervals may be useful for monitoring ADAMTS13 antigenicity and activity, but not for vWF, which displays high individuality. These findings should be considered when determining the reference interval and other clinical variables associated with ADAMTS13 and vWF levels.

Proteolytic processing of von Willebrand factor by adamts13 and leukocyte proteases

ADAMTS13 is a 190 kDa zinc protease encoded by a gene located on chromosome 9q34. This protease specifically hydrolyzes von Willebrand factor (VWF) multimers, thus causing VWF size reduction. ADAMTS13 belongs to the A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats (ADAMTS) family, involved in proteolytic processing of many matrix proteins. ADAMTS13 consists of numerous domains including a metalloprotease domain, a disintegrin domain, several thrombospondin type 1 (TSP1) repeats, a cysteine-rich domain, a spacer domain and 2 CUB (Complement c1r/c1s, sea Urchin epidermal growth factor, and Bone morphogenetic protein) domains. ADAMTS13 cleaves a single peptide bond (Tyr1605-Met1606) in the central A2 domain of the VWF molecule. This proteolytic cleavage is essential to reduce the size of ultra-large VWF polymers, which, when exposed to high shear stress in the microcirculation, are prone to form with platelets clumps, which cause severe syndromes called thrombotic microangiopathies (TMAs). In this review, we a) discuss the current knowledge of structure-function aspects of ADAMTS13 and its involvement in the pathogenesis of TMAs, b) address the recent findings concerning proteolytic processing of VWF multimers by different proteases, such as the leukocyte-derived serine and metallo-proteases and c) indicate the direction of future investigations.

Complement activation associated with ADAMTS13 deficiency in human and murine thrombotic microangiopathy

This study addressed the contribution of ADAMTS13 deficiency to complement activation in thrombotic thrombocytopenic purpura (TTP). Renal tissue and blood samples were available from 12 TTP patients. C3 and C5b-9 deposition were demonstrated in the renal cortex of two TTP patients, by immunofluorescence and immunohistochemistry, respectively. C3 was also demonstrated in the glomeruli of Shiga toxin-2-treated Adamts13(-/-) mice (n = 6 of 7), but less in mice that were not Shiga toxin-2 treated (n = 1 of 8, p < 0.05) or wild-type mice (n = 0 of 7). TTP patient plasma (n = 9) contained significantly higher levels of complement-coated endothelial microparticles than control plasma (n = 13), as detected by flow cytometry. Exposure of histamine-stimulated primary glomerular endothelial cells to platelet-rich plasma from patients, or patient platelet-poor plasma combined with normal platelets, in a perfusion system, under shear, induced C3 deposition on von Willebrand factor-platelet strings (on both von Willebrand factor and platelets) and on endothelial cells. Complement activation occurred via the alternative pathway. No C3 was detected when cells were exposed to TTP plasma that was preincubated with EDTA or heat-inactivated, or to control plasma. In the perfusion system, patient plasma induced more release of C3- and C9-coated endothelial microparticles compared with control plasma. The results indicate that the microvascular process induced by ADAMTS13 deficiency triggers complement activation on platelets and the endothelium, which may contribute to formation of thrombotic microangiopathy.

Hypercoagulability and cardiovascular disease in diabetic nephropathy

Diabetic nephropathy is the leading cause of end stage renal disease (ESRD) and an important risk factor for cardiovascular disease. Recent studies have shown that increased plasma levels of Von Willebrand factor (VWF) and reduced plasma levels of enzyme ADAMTS13 are associated with diabetic nephropathy and an increased risk of developing cardiovascular disease, suggesting that these markers of hypercoagulability may contribute to an increased risk of cardiovascular disease in diabetic patients with impaired renal function. However, it is still not clear whether VWF and ADAMTS13 are only markers of cardiovascular events or whether they play an active role in the development of these events. It is also unclear how renal injury may affect ADAMTS13 levels, leading consequently to hypercoagulability. The association of diabetic nephropathy, atherosclerotic cardiovascular disease and these hypercoagulability markers is discussed in this review. Insights on the role that renal dysfunction and other possible mechanisms may have in ADAMTS13 metabolism, leading to reduced levels of this enzyme and increased hypercoagulability are also presented.

ADAMTS13 and Von Willebrand factor in patients undergoing hemodialysis

Hemodialysis (HD) is associated with increasing thrombotic trend. Vascular access thrombosis (VAT) increases morbidity in HD patients. The aim of this study was to evaluate ADAMTS13 and VWF plasma levels from patients undergoing HD as putative biomarkers of the hypercoagulability state, as well the association between these markers and VAT occurrence. This study included 195 patients on HD for more than 6 months. HD patients were allocated into two groups according to the occurrence or not of previous episode of VAT; HD with VAT (N = 46) and HD without VAT (N = 149). ADAMTS13 and VWF were performed by ELISA. There was no significant difference between HD patients with and without VAT for ADAMTS13 and VWF levels. However, VWF levels were higher (P < 0.001) and ADAMTS13 were lower (P < 0.001) in HD patients, comparing to the control group composed by healthy subjects without kidney disease, age and sex-matched (N = 80). Taken together our data suggest a potential role of the kidneys function compromised on ADAMTS13 synthesis or metabolism, regardless other known sources of ADAMTS13. The imbalance between ADAMTS13 and VWF levels does not explain the development of VAT in HD patients by itself, although it should contribute for the hypercoagulability state. Therefore, additional studies to identify other risk factors are warranted and essential for better management of HD patients.

ADAMTS13, FVIII, von Willebrand factor, ABO blood group assessment in preeclampsia

INTRODUCTION

Preeclampsia (PE) is a multifactorial disease characterized by high blood pressure and proteinuria after the 20th week of pregnancy. PE is associated with fibrin deposition in placental microcirculation and intrauterine fetal growth retardation. We evaluated FVIII activity, VWF and ADAMTS13 plasma levels, according to O and "non O" blood groups, in women with severe PE (sPE).

METHODS

This case-control study included 140 women; 55 pregnant with sPE, 35 normotensive pregnant and 50 non-pregnant women. VWF and ADAMTS13 antigen levels were assessed by ELISA (American Diagnostica). FVIII activity was measured by automated coagulometric method (Dade Behring) and ABO blood groups phenotyping was performed by indirect technique.

RESULTS

FVIII activity and VWF levels were significantly higher comparing either sPE to normotensive pregnant (P=0.01; P=0.05) and to non-pregnant women (P=0.00 in both cases) or normotensive pregnant and non-pregnant women (P=0.00 in both cases). A significant decrease in ADAMTS13 levels was observed comparing either sPE to normotensive pregnant (P=0.02) and non-pregnant women (P=0.00) or normotensive pregnant and non-pregnant women (P=0.00). FVIII activity and VWF levels were associated to O and "non O" blood groups only in non-pregnant women.

CONCLUSIONS

The increase of FVIII activity and VWF levels and the decrease of ADAMTS13 in sPE are not associated to O and "non O" blood groups. These alterations in hemostatic markers in sPE largely surpass those physiologically determined by ABO blood groups influence and may have masked the effect of O and "non O" groups in this disease. A concomitant analysis of VWF levels and ADAMTS13 activity and antigenic levels will be important to clarify the imbalance between these parameters found in sPE in the present study.

Structure and proteolytic properties of ADAMTS13, a metalloprotease involved in the pathogenesis of thrombotic microangiopathies

ADAMTS13 is a 190-kDa zinc protease encoded by a gene located on chromosome 9q34. This protease specifically hydrolyzes von Willebrand factor (VWF) multimers, thus causing VWF size reduction. ADAMTS13 belongs to the A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats (ADAMTS) family, involved in proteolytic processing of many matrix proteins. ADAMTS13 consists of numerous domains, including a metalloprotease domain, a disintegrin domain, several thrombospondin type 1 (TSP1) repeats, a cysteine-rich domain, a spacer domain, and two CUB (Complement c1r/c1s, sea Urchin epidermal growth factor, and Bone morphogenetic protein) domains. ADAMTS13 cleaves a single peptide bond (Tyr(1605)-Met(1606)) in the central A2 domain of the VWF molecule. This proteolytic cleavage is essential to reduce the size of ultralarge VWF polymers, which, when exposed to high shear stress in the microcirculation, are prone to form platelets clumps, which cause severe syndromes called thrombotic microangiopathies (TMAs). In this chapter, we (a) discuss the current knowledge of structure-function aspects of ADAMTS13 and its involvement in the pathogenesis of TMAs, (b) address the ongoing controversies, and (c) indicate the direction of future investigations.

Natural history of Upshaw-Schulman syndrome based on ADAMTS13 gene analysis in Japan

Upshaw-Schulman syndrome (USS) is an extremely rare hereditary deficiency of ADAMTS13 activity, termed congenital TTP. The clinical signs are usually mild during childhood, often with isolated thrombocytopenia. But their symptoms become more evident when patients have infections or get pregnant. We identified 43 USS-patients in Japan, who ranged in age from early childhood to 79 years of age. Analysing the natural history of these USS patients based on ADAMTS13 gene mutations may help characterise their clinical phenotypes. Severe neonatal jaundice that requires exchange blood transfusion, a hallmark of USS, was found in 18 of 43 patients (42%). During childhood, 25 of 43 patients were correctly diagnosed with USS without gender disparity. These 25 patients were categorised as having 'the early-onset phenotype'. Between 15 and 45 years of age, 15 were correctly diagnosed, and, interestingly, they were all female. The remaining three patients were male and were diagnosed when they were older than 45 years of age, suggesting that they were 'the late-onset phenotype'. Two of these three males developed sudden overt TTP when they were 55 and 63 years old, respectively. These two men had two different homozygous ADAMTS13 gene mutations, p.R193W/p.R193W and p.C1024R/p.C1024R, respectively. Both of which were not discovered in the US or Western countries. In vitro expression studies showed that these two proteins were consistently secreted into the culture medium but to a lesser extent and with reduced activity compared to the wild-type protein. Our results indicate that 'the late-onset phenotype' of USS is formed with ethnic specificity.

Phenotypic expression of ADAMTS13 in glomerular endothelial cells

Background

ADAMTS13 is the physiological von Willebrand factor (VWF)-cleaving protease. The aim of this study was to examine ADAMTS13 expression in kidneys from ADAMTS13 wild-type (Adamts13^+/+) and deficient (Adamts13^−/−) mice and to investigate the expression pattern and bioactivity in human glomerular endothelial cells.

Methodology/Principal Findings

Immunohistochemistry was performed on kidney sections from ADAMTS13 wild-type and ADAMTS13-deficient mice. Phenotypic differences were examined by ultramorphology. ADAMTS13 expression in human glomerular endothelial cells and dermal microvascular endothelial cells was investigated by real-time PCR, flow cytometry, immunofluorescence and immunoblotting. VWF cleavage was demonstrated by multimer structure analysis and immunoblotting. ADAMTS13 was demonstrated in glomerular endothelial cells in Adamts13^+/+ mice but no staining was visible in tissue from Adamts13^−/− mice. Thickening of glomerular capillaries with platelet deposition on the vessel wall was detected in Adamts13^−/− mice. ADAMTS13 mRNA and protein were detected in both human endothelial cells and the protease was secreted. ADAMTS13 activity was demonstrated in glomerular endothelial cells as cleavage of VWF.

Conclusions/Significance

Glomerular endothelial cells express and secrete ADAMTS13. The proteolytic activity could have a protective effect preventing deposition of platelets along capillary lumina under the conditions of high shear stress present in glomerular capillaries.

Escherichia coli Shiga Toxin Mechanisms of Action in Renal Disease

Shiga toxin-producing Escherichia coli is a contaminant of food and water that in humans causes a diarrheal prodrome followed by more severe disease of the kidneys and an array of symptoms of the central nervous system. The systemic disease is a complex referred to as diarrhea-associated hemolytic uremic syndrome (D⁺HUS). D⁺HUS is characterized by thrombocytopenia, microangiopathic hemolytic anemia, and acute renal failure. This review focuses on the renal aspects of D⁺HUS. Current knowledge of this renal disease is derived from a combination of human samples, animal models of D⁺HUS, and interaction of Shiga toxin with isolated renal cell types. Shiga toxin is a multi-subunit protein complex that binds to a glycosphingolipid receptor, Gb3, on select eukaryotic cell types. Location of Gb3 in the kidney is predictive of the sites of action of Shiga toxin. However, the toxin is cytotoxic to some, but not all cell types that express Gb3. It also can cause apoptosis or generate an inflammatory response in some cells. Together, this myriad of results is responsible for D⁺HUS disease.

Pathophysiology of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disorder with characteristic von Willebrand factor (VWF)-rich microthrombi affecting the arterioles and capillaries of multiple organs. The disorder frequently leads to early death unless the patients are treated with plasma exchange or infusion. Studies in the last decade have provided ample evidence to support that TTP is caused by deficiency of a plasma metalloprotease, ADAMTS13. When exposed to high shear stress in the microcirculation, VWF and platelets are prone to form aggregates. This propensity of VWF and platelet to form microvascular thrombosis is mitigated by ADAMTS13, which cleaves VWF before it is activated by shear stress to cause platelet aggregation in the circulation. Deficiency of ADAMTS13, due to autoimmune inhibitors in patients with acquired TTP and mutations of the ADAMTS13 gene in hereditary cases, leads to VWF-platelet aggregation and microvascular thrombosis of TTP. In this review, we discuss the current knowledge on the pathogenesis, diagnosis and management of TTP, address the ongoing controversies, and indicate the directions of future investigations.

Biologically active ADAMTS13 is expressed in renal tubular epithelial cells

ADAMTS13 mRNA, which encodes the von Willebrand factor-cleaving protease, has been detected in a variety of tissues, including the kidney. The aim of our study was to characterize tubular expression and bioactivity of ADAMTS13. ADAMTS13 mRNA was detected in cultured primary human renal tubular epithelial cells (HRTEC) and in A498 cells, a human renal carcinoma cell line, by real-time PCR. Protein was detected using immunofluorescence and immunoblotting. Immunoblots demonstrated that the protein was secreted. The protease was proteolytically active in both cell lysates and cleaved the FRETS–VWF73 substrate. ADAMTS13 was demonstrated in situ in the renal cortex by immunohistochemistry. Protease was detected in both the proximal and distal renal tubules in normal renal tissue (n=3) as well as in patients with tubular disorders (n=3). Immunoblotting revealed that ADAMTS13 was present in the urine of patients with tubulopathy (n=5) but not in normal urine. ADAMTS13 in urine had a molecular size similar to that in plasma, which indicates that the protease originates in the tubuli because such large proteins do not normally pass the glomerular filter. In conclusion, human renal tubular epithelial cells synthesize biologically active ADAMTS13 which may, after release from tubuli, regulate hemostasis in the local microenvironment.

von Willebrand factor-cleaving protease ADAMTS13 reduces ischemic brain injury in experimental stroke

Stroke is a leading cause of death and disability. The only therapy available is recombinant tissue plasminogen activator, but side effects limit its use. Platelets play a crucial role during stroke, and the inflammatory reaction promotes neurodegeneration. von Willebrand factor (VWF), an adhesion molecule for platelets, is elevated in patients with acute stroke. The activity of VWF is modulated by ADAMTS13 (a disintegrin-like and metalloprotease with thrombospondin type I repeats-13) that cleaves VWF to smaller less-active forms. We recently documented that ADAMTS13 negatively regulates both thrombosis and inflammation. We report that deficiency or reduction of VWF reduces infarct volume up to 2-fold after focal cerebral ischemia in mice, thus showing the importance of VWF in stroke injury. In contrast, ADAMTS13 deficiency results in larger infarctions, but only in mice that have VWF. Importantly, infusion of a high dose of recombinant human ADAMTS13 into a wild-type mouse immediately before reperfusion reduces infarct volume and improves functional outcome without producing cerebral hemorrhage. Furthermore, recombinant ADAMTS13 did not enhance bleeding in a hemorrhagic stroke model. Our findings show the importance of VWF in regulating infarction and suggest that recombinant ADAMTS13 could be considered as a new therapeutic agent for prevention and/or treatment of stroke.

Thrombotic microangiopathy in haematopoietic stem cell transplantation: diagnosis and treatment

Each year in the US, more than 10 000 patients benefit from allogeneic haematopoietic stem cell transplantation (HSCT), a modality that offers an excellent chance of eradicating malignancy but confers a higher risk of treatment-related mortality. An uncommon but devastating consequence of HSCT is transplantation-associated thrombotic microangiopathy (TA-TMA). The incidence of TA-TMA ranges from 0.5% to 76%, with a mortality rate of 60-90% despite treatment. Although there appears to be a consistent treatment approach to idiopathic thrombotic thrombocytopenic purpura (TTP) using plasma exchange, corticosteroids and rituximab, the treatment strategies for TA-TMA are perplexing, in part, because the literature regarding this complex condition does not provide true consensus for incidence, aetiology, diagnostic criteria, classification and optimal therapy. The classic definition of idiopathic TTP includes schistocytes on the peripheral blood smear, thrombocytopenia and increased serum lactate dehydrogenase. Classic idiopathic TTP has been attributed to deficient activity of the metalloproteinase responsible for cleaving ultra-large von Willebrand factor multimers. This protease is a member of the 'a disintegrin and metalloprotease with thrombospondin type 1 motif' family and is subsequently named ADAMTS-13. Severely deficient ADAMTS-13 activity (<5% of normal) is associated with idiopathic TTP in 33-100% of patients. In constrast to the pathophysiology of idiopathic TTP, patients with TA-TMA have >5% ADAMTS-13 serum activity. These data may explain why plasma exchange, a standard treatment modality for idiopathic TTP that restores ADAMTS-13 activity, is not effective in TA-TMA. TA-TMA has a multifactorial aetiology of endothelial damage induced by intensive conditioning therapy, irradiation, immunosuppressants, infection and graft-versus-host disease. Treatment consists of substituting calcineurin inhibitors with an alternative immunosuppressive agent that possesses another mode of action. One candidate may be daclizumab, especially in those with mild to moderate TMA. Rituximab therapy or the addition of defibrotide may also be beneficial. In general, plasma exchange is not recommended.

Molecular basis of ADAMTS13 dysfunction in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a thrombotic microangiopathic disorder characterized by thrombocytopenia, hemolytic anemia, neurological and renal manifestations, and fever. It is associated with dysfunctional von Willebrand factor (VWF) proteolysis and the occurrence of VWF- and platelet-rich thrombi in the microcirculation of multiple organs, including the kidneys. Von Willebrand factor is a large glycoprotein that circulates in plasma as a series of multimers, and it plays a major role in primary hemostasis by inducing the formation of platelet plugs at sites of vascular injury and high-shear stress. Its activity is dependent on the sizes of the multimers, with ultra-large (UL) VWF multimers being biologically very potent. The ULVWF multimers are rapidly degraded upon their secretion from endothelial cells in normal individuals but not in the circulation of TTP patients, causing the formation of disseminated thrombi in the latter. The defective breakdown of VWF is attributed to a severely deficient activity of the VWF-cleaving protease ADAMTS13, a plasma metalloprotease synthesized in the liver, kidneys, and endothelium. This protease rapidly degrades VWF-platelet strings under flow by proteolytic cleavage of the VWF subunit, thereby regulating the size of the platelet thrombus. Congenital TTP occurs due to ADAMTS13 mutations, with the usual debut occurring during the first years of life, while acquired TTP is associated with auto-antibodies against ADAMTS13.

Atypical haemolytic uremic syndrome complicated by microangiopathic antiphospholipid-associated syndrome

A 4-year-old boy with an atypical course of haemolytic uremic syndrome (HUS), who developed microangiopathic antiphospholipid-associated syndrome (MAPS) with signs of multiple organ failure during the course of his disease, is reported. Early and aggressive treatment with intravenous gammaglobulin, pulse methylprednisolone and plasmapheresis resulted in an excellent clinical recovery. Our patient showed a concomitant presence of multiple factors that could precipitate atypical HUS, including positive antiphospholipid antibodies, decreased level of factor H and positive anti-ADAMTS-13 antibodies. We suggest that, along with infections, autoimmune conditions or defined genetic abnormalities of complement regulatory genes, MAPS should be considered among the pathogenic mechanisms in patients with atypical HUS.

Thrombotic thrombocytopenic purpura related to severe ADAMTS13 deficiency in children

Thrombotic thrombocytopenic purpura (TTP) related to a severely deficient activity of the von Willebrand factor cleaving protease, ADAMTS (A Disintegrin And Metalloprotease with ThromboSpondin type 1 repeats) 13, is a life-threatening event, the onset of which may occur as early as childhood. TTP is either inherited (Upshaw-Schulman syndrome) via ADAMTS13 gene mutations (neonatal onset) or acquired via anti-ADAMTS13 autoantibodies (childhood onset). TTP is due to platelet- and von-Willebrand-factor-rich thrombi of the microvasculature, inducing mechanical hemolytic anemia, consumption thrombocytopenia, and multivisceral ischemia. Clinical course consists of relapsing acute events triggered mostly by infections, associated icterus and hyperbilirubinemia, severe hemolytic anemia with schistocytosis and a negative Coombs test, severe thrombocytopenia, and sometimes symptoms related to visceral ischemia (renal failure, central nervous system vascular events, other organ failure). The recently available ADAMTS13 laboratory investigation combining measurement of ADAMTS13 activity in plasma, search for an ADAMTS13 circulating inhibitor, and anti-ADAMTS13 IgG and ADAMTS13 gene sequencing is a crucial addition to TTP diagnosis. Plasma exchanges are first-line treatment of acquired TTP, combined with steroids and immunosuppressive drugs. Curative treatment of acute events in Upshaw-Schulman syndrome relies on plasma infusions (provider of active ADAMTS13). Guidelines for preventive treatment of relapses are not clearly established but should associate plasmatherapy and caution to triggers of relapses. Therapeutic perspectives are focused on the development of concentrated plasma-derived ADAMTS13 or recombinant ADAMTS13.

Podocytes express ADAMTS13 in normal renal cortex and in patients with thrombotic thrombocytopenic purpura

Congenital thrombotic thrombocytopenic purpura (TTP) is associated with ADAMTS13 mutations. The major site of ADAMTS13 synthesis is the liver. Expression in other tissues, and in TTP, has not been shown. In this study, ADAMTS13 protein expression was investigated in normal kidney and in renal tissue from two TTP patients, with a compound heterozygous mutation (P353L and P457L) and a homozygous mutation (4143insA). Real-time polymerase chain reaction demonstrated ADAMTS13 mRNA in normal kidney. ADAMTS13 was detected in the glomeruli and tubuli of normal and TTP kidney using anti-ADAMTS13 antibodies. In the glomeruli, expression was localised to podocytes (as demonstrated by counterstaining with two podocyte markers) and endothelium. Similar distribution was detected in the TTP kidneys. Electron microscopy detected ADAMTS13 in podocytes, endothelium and glomerular basement membrane. Cultured human podocytes expressed ADAMTS13 mRNA and protein, and podocyte lysate exhibited von Willebrand factor-cleaving activity. Mutation expression studies of the P353L and P457L mutations showed partially impaired secretion and lower activity of the secreted mutants. Impaired secretion has previously been shown for the 4143insA mutation. Podocyte-derived ADAMTS13 may offer local protection in the high-shear microcirculation of the glomerulus. The mutations in the two TTP patients studied enabled protein expression in the podocytes but affected protease secretion.