Novel ADAMTS-13 mutations in an adult with delayed onset thrombotic thrombocytopenic purpura

Laboratory investigation and diagnosis of thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a rare and potentially fatal disease for which rapid diagnosis is crucial for patient outcomes. Deficient activity (< 10%) of the liver enzyme, ADAMTS13, is the pathophysiological hallmark of TTP, and measurement of the enzyme activity can establish the diagnosis of TTP with high accuracy. Thus, along with the clinical history, appropriate laboratory assessment of a suspected case of TTP is essential for diagnosis and treatment. Here, we present a review of the available laboratory tests that can assist clinicians in establishing the diagnosis of TTP, with special focus on ADAMTS13 assays, including the measurement of the antigen and activity, and detection of autoantibodies to ADAMTS13.

Role of ADAMTS13, VWF and F8 genes in deep vein thrombosis

Background

We previously described the association between rare ADAMTS13 single nucleotide variants (SNVs) and deep vein thrombosis (DVT). Moreover, DVT patients with at least one rare ADAMTS13 SNV had a lower ADAMTS13 activity than non-carriers.

Aims

To confirm ADAMTS13 variants association with DVT and reduced plasma ADAMTS13 activity levels in a larger population. To investigate the role of VWF and F8 variants.

Methods

ADAMTS13, VWF and F8 were sequenced using next-generation sequencing in 594 Italian DVT patients and 571 controls. Genetic association testing was performed using logistic regression and gene-based tests. The association between rare ADAMTS13 variants and the respective plasmatic activity, available for 365 cases and 292 controls, was determined using linear regression. All analyses were age-, sex- adjusted.

Results

We identified 48 low-frequency/common and 272 rare variants. Nine low-frequency/common variants had a P<0.05, but a false discovery rate between 0.06 and 0.24. Of them, 7 were found in ADAMTS13 (rs28641026, rs28503257, rs685523, rs3124768, rs3118667, rs739469, rs3124767; all protective) and 2 in VWF (rs1800382 [risk], rs7962217 [protective]). Rare ADAMTS13 variants were significantly associated with DVT using the burden, variable threshold (VT) and UNIQ (P<0.05), but not with C-ALPHA, SKAT and SKAT-O tests. Rare VWF and F8 variants were not associated with DVT. Carriers of rare ADAMTS13 variants had lower ADAMTS13 activity than non-carriers (ß -6.2, 95%CI -11,-1.5). This association was stronger for DVT patients than controls (ß -7.5, 95%CI -13.5,-1.5 vs. ß -2.9, 95%CI -10.4,4.5).

Conclusions

ADAMTS13 and VWF low-frequency/common variants mainly showed a protective effect, although their association with DVT was not confirmed. DVT patients carrying a rare ADAMTS13 variants had slightly reduced ADAMTS13 activity levels, but a higher DVT risk. Rare VWF and FVIII variants were not associated with DVT suggesting that other mechanisms are responsible for the high VWF and FVIII levels measured in DVT patients.

Two novel mutations in ADAMTS13 in a Chinese boy with congenital thrombocytopenic purpura: a case report

BACKGROUND

Mutations in the ADAMTS13 gene family have been reported to cause congenital thrombotic thrombocytopenic purpura (cTTP), a rare disease characterized by thrombocytopenia and hemolytic anemia. Nearly 150 causative mutations in ADAMTS13 have been identified; however, only a few of them were detected in Chinese patients.

CASE PRESENTATION

A 5-year-old Chinese boy presented with history of thrombocytopenic purpura, hemolytic anemia, and renal injury since the neonatal period. Gene analysis revealed two novel mutations in ADAMTS13: a missense mutation 332G > A (p:Gly111Glu) in exon4 and a nonsense mutation 3121C > T (p:Gln1041stop) in exon 24. Genetic analysis of his parents confirmed the heterozygous nature of the mutations.

CONCLUSION

We report two novel mutations in ADAMTS13 (332G > A, 3121C > T) in a Chinese boy. These two mutations may lead to early onset of cTTP and severe symptoms.

Atypical Thrombotic Thrombocytopenic Purpura Presenting as Stroke

Here we report a case of atypical thrombotic thrombocytopenic purpura that presented as an ischemic cerebrovascular accident. A 56-year-old man with multiple cardiovascular risk factors presented with sudden left-sided weakness, slurred speech, and left facial droop. He showed mild improvement when he was treated with thrombolytic therapy according to the hospital stroke protocol. Later in the course, he developed thrombocytopenia followed by schistocytes revealed by peripheral blood smear and other lab abnormalities. Thrombotic thrombocytopenic purpura (TTP) was suspected, and he was treated with total plasma exchange that improved his condition significantly. This case shows that TTP can have unusual and atypical presentations either with the first episode or upon relapse, making diagnosis extremely difficult. Because patients may not present the expected clinical findings, it is important to be aware of variant presentations. In the early stages of the disease, platelet aggregation and thrombus formation may not be widespread, and thrombocytopenia and microangiopathic hemolytic anemia may not be clinically evident. Patients can present soon after the onset of symptoms when the typical laboratory abnormalities may not have had ample time to manifest. Although most other similar cases in the literature had a previous typical presentation of the disease before an atypical presentation, our patient's first presentation was atypical. An atypical presentation of disease in a patient with cardiovascular risk factors may therefore be extremely difficult to diagnose. We believe that TTP should be considered for any patient presenting with stroke and thrombocytopenia.

Hereditary thrombotic thrombocytopenic purpura and the hereditary TTP registry

Hereditary thrombotic thrombocytopenic purpura (TTP), also known as Upshaw-Schulman syndrome, is a rare recessively inherited disease. Underlying is a severe constitutional deficiency of the von Willebrand factor-cleaving protease, ADAMTS13, due to compound heterozygous or homozygous mutations in the ADAMTS13 gene. The clinical picture is variable and more and more patients with an adult-onset are diagnosed.

In the majority of countries the only available treatment is plasma, which when administered regularly can efficiently prevent acute disease bouts. The decision to initiate regular prophylaxis is often not easy, as evidence based guidelines and long term outcome data are lacking. Through the hereditary TTP registry (www.ttpregistry.net, ClinicalTrials.gov identifier: NCT01257269), which was initiated in 2006 and is open to all patients diagnosed with Upshaw-Schulman syndrome and their family members, we aim to gain further information and insights into this rare disease, which eventually will help to improve clinical management of affected patients.

Thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP; also known as Moschcowitz disease) is characterized by the concomitant occurrence of often severe thrombocytopenia, microangiopathic haemolytic anaemia and a variable degree of ischaemic organ damage, particularly affecting the brain, heart and kidneys. Acute TTP was almost universally fatal until the introduction of plasma therapy, which improved survival from <10% to 80-90%. However, patients who survive an acute episode are at high risk of relapse and of long-term morbidity. A timely diagnosis is vital but challenging, as TTP shares symptoms and clinical presentation with numerous conditions, including, for example, haemolytic uraemic syndrome and other thrombotic microangiopathies. The underlying pathophysiology is a severe deficiency of the activity of a disintegrin and metalloproteinase with thrombospondin motifs 13 (ADAMTS13), the protease that cleaves von Willebrand factor (vWF) multimeric strings. Ultra-large vWF strings remain uncleaved after endothelial cell secretion and anchorage, bind to platelets and form microthrombi, leading to the clinical manifestations of TTP. Congenital TTP (Upshaw-Schulman syndrome) is the result of homozygous or compound heterozygous mutations in ADAMTS13, whereas acquired TTP is an autoimmune disorder caused by circulating anti-ADAMTS13 autoantibodies, which inhibit the enzyme or increase its clearance. Consequently, immunosuppressive drugs, such as corticosteroids and often rituximab, supplement plasma exchange therapy in patients with acquired TTP.

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.

ADAMTS13 Secretion and Residual Activity among Patients with Congenital Thrombotic Thrombocytopenic Purpura with and without Renal Impairment

BACKGROUND AND OBJECTIVES

Acute renal impairment is observed in 11%-23% of patients with congenital thrombotic thrombocytopenic purpura (TTP) and deficiency of a disintegrin and metalloprotease with thrombospondin motifs 13 (ADAMTS13, a metalloprotease that cleaves von Willebrand factor [VWF] multimers), a substantial percentage of whom develop CKD during follow-up.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Here we investigated whether, in 18 patients with congenital recruited from 1996 to 2013 who fulfilled inclusion criteria, acute renal involvement occurred during bouts segregated with lower secretion and activity levels of ADAMTS13 mutants. We performed expression studies and a sensitive recombinant VWF (rVWF) A1-A2-A3 cleavage test (detection limit, 0.78% of normal ADAMTS13 activity).

RESULTS

A higher risk of acute renal impairment during bouts was observed in patients with childhood (<18 years) onset (odds ratio [OR], 24.6 [95% confidence interval (CI), 1.11 to 542.44]) or a relapsing (≥1 episode per year) disease (OR, 54.6 [95% CI, 2.25 to 1326.28]) than in patients with adulthood onset or long-lasting remission, respectively. Whatever the age at onset, patients with acute renal impairment had mutations different from those in patients without renal involvement. Moreover, mutations in patients with acute renal impairment compared with those in patients without renal involvement caused lower in vitro rADAMTS13 secretion (1.33% versus 12.5%; P<0.001) and residual activity (0.11% versus 3.47%; P=0.003). rADAMTS13 secretion ≤3.75% and residual activity ≤0.4% best discriminated patients with renal impairment (receiver-operating characteristic curve sensitivity, 100% and 100%; specificity, 100% and 83.3%, respectively; logistic regression OR, 325 [95% CI, 6 to 18339] and 91.7 [95% CI, 3.2 to 2623.5], respectively). All mutations found in patients with childhood onset or relapsing disease were associated with acute renal impairment during bouts, confirming the link between acute renal impairment and early onset or a relapsing course. ADAMTS13 activity levels in vivo, measured in patients' serum by rVWF A1-A2-A3 cleavage test, correlated with in vitro rADAMTS13 mutant activity (r=0.95; P<0.001).

CONCLUSIONS

In congenital TTP, renal impairment and relapsing disease might be predicted by measurements of in vitro rADAMTS13 secretion and activity levels and in vivo serum ADAMTS13 activity.

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.

Late onset and pregnancy-induced congenital thrombotic thrombocytopenic purpura

We report on our patient (case 2) who experienced a first acute episode of thrombotic thrombocytopenic purpura (TTP) at the age of 19 years during her first pregnancy in 1976 which ended in a spontaneous abortion in the 30th gestational week. Treatment with red blood cell concentrates was implemented and splenectomy was performed. After having suffered from several TTP episodes in 1977, possibly mitigated by acetylsalicylic acid therapy, an interruption and sterilization were performed in 1980 in her second pregnancy thereby avoiding another disease flare-up. Her elder sister (case 1) had been diagnosed with TTP in 1974, also during her first pregnancy. She died in 1977 during her second pregnancy from a second acute TTP episode.

DIAGNOSIS

In 2013 a severe ADAMTS13 deficiency of <10% without detectable ADAMTS13 inhibitor was repeatedly found. Investigation of the ADAMTS13 gene showed that the severe ADAMTS13 deficiency was caused by compound heterozygous ADAMTS13 mutations: a premature stop codon in exon 2 (p.Q44X), and a missense mutation in exon 24 (p.R1060W) associated with low but measurable ADAMTS13 activity.

CONCLUSION

Genetic analysis of the ADAMTS13 gene is important in TTP patients of all ages if an ADAMTS13 inhibitor has been excluded.

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.

Partial ADAMTS13 deficiency in atypical hemolytic uremic syndrome

Complement dysregulation leads to atypical hemolytic uremic syndrome (aHUS), while ADAMTS13 deficiency causes thrombotic thrombocytopenic purpura. We investigated whether genetic variations in the ADAMTS13 gene partially explain the reduced activity known to occur in some patients with aHUS. We measured complement activity and ADAMTS13 function, and completed mutation screening of multiple complement genes and ADAMTS13 in a large cohort of aHUS patients. In over 50% of patients we identified complement gene mutations. Surprisingly, 80% of patients also carried at least 1 nonsynonymous change in ADAMTS13, and in 38% of patients, multiple ADAMTS13 variations were found. Six of the 9 amino acid substitutions in ADAMTS13 were common single nucleotide polymorphisms; however, 3 variants-A747V, V832M, and R1096H- were rare, with minor allele frequencies of 0.0094%, 0.5%, and 0.32%, respectively. Reduced complement and ADAMTS13 activity (<60% of normal activity) were found in over 60% and 50% of patients, respectively. We concluded that partial ADAMTS13 deficiency is a common finding in aHUS patients and that genetic screening and functional tests of ADAMTS13 should be considered in these patients.

Atypical thrombotic thrombocytopenic purpura in a middle-aged woman who presented with a recurrent stroke

An atypical clinical presentation makes TTP diagnosis difficult, which prevents prompt management of TTP. This case highlights the importance of early recognition of TTP in patients with atypical presentations who may not have the expected clinical or laboratory findings. The level of suspicion should be especially high in young and middle-aged patients with strokes or acute coronary syndrome who do not have other risk factors for cardiovascular events. This is particularly so for those patients who have a previous episode of TTP or patients with subtle laboratory abnormalities, which may suggest the potential existence of a thrombotic microangiopathic process.

A phenotype-genotype correlation of ADAMTS13 mutations in congenital thrombotic thrombocytopenic purpura patients treated in the United Kingdom

BACKGROUND

ADAMTS13 mutations play a role in thrombotic thrombocytopenic purpura (TTP) pathogenesis.

OBJECTIVES

To establish a phenotype-genotype correlation in a cohort of congenital TTP patients.

PATIENTS/METHODS

Clinical history and ADAMTS13 activity, antigen and anti-ADAMTS13 antibody assays were used to diagnose congenital TTP, and DNA sequencing and in vitro expression were performed to identify the functional effects of the ADAMTS13 mutations responsible.

RESULTS

Seventeen (11 novel) ADAMTS13 mutations were identified in 17 congenital TTP patients. All had severely reduced ADAMTS13 activity and antigen levels at presentation. Six patients with pregnancy-associated TTP and six patients with childhood TTP were homozygous or compound heterozygous for ADAMTS13 mutations located in the metalloprotease (MP), cysteine-rich, spacer and/or distal thrombospondin type 1 domains. The adults had TTP precipitated by pregnancy, and had overall higher antigen levels (median, 30 ng mL(-1) ; range, < 10-57 ng mL(-1) ) than the children (median, 14 ng mL(-1) ; range, < 10-40 ng mL(-1)). Presentation in the neonatal period was associated with more intensive treatment requirements. The two neonates with the most severe phenotype had mutations in the first thrombospondin type 1 motif of ADAMTS13 (p.R398C, p.R409W, and p.Q436H). Using transfected HEK293T cells, we have shown that p.R398C and p.R409W block ADAMTS13 secretion, whereas p.Q436H allows secretion at reduced levels.

CONCLUSIONS

This study confirms the heterogeneity of ADAMTS13 defects and an association between ADAMTS13 genotypes and TTP phenotype.

G, Shapiro A, Okunji C, Kosti I, Shomron N, Grigoryan V, Przytycka TM, Sauna ZE, Salari R, Mandel-Gutfreund Y, Komar AA, Kimchi-Sarfaty C. Characterization of coding synonymous and non-synonymous variants in ADAMTS13 using ex vivo and in silico approaches

Synonymous variations, which are defined as codon substitutions that do not change the encoded amino acid, were previously thought to have no effect on the properties of the synthesized protein(s). However, mounting evidence shows that these “silent” variations can have a significant impact on protein expression and function and should no longer be considered “silent”. Here, the effects of six synonymous and six non-synonymous variations, previously found in the gene of ADAMTS13, the von Willebrand Factor (VWF) cleaving hemostatic protease, have been investigated using a variety of approaches. The ADAMTS13 mRNA and protein expression levels, as well as the conformation and activity of the variants have been compared to that of wild-type ADAMTS13. Interestingly, not only the non-synonymous variants but also the synonymous variants have been found to change the protein expression levels, conformation and function. Bioinformatic analysis of ADAMTS13 mRNA structure, amino acid conservation and codon usage allowed us to establish correlations between mRNA stability, RSCU, and intracellular protein expression. This study demonstrates that variants and more specifically, synonymous variants can have a substantial and definite effect on ADAMTS13 function and that bioinformatic analysis may allow development of predictive tools to identify variants that will have significant effects on the encoded protein.

Unexpected frequency of Upshaw-Schulman syndrome in pregnancy-onset thrombotic thrombocytopenic purpura

Pregnancy may be complicated by a rare but life-threatening disease called thrombotic thrombocytopenic purpura (TTP). Most cases of TTP are due to an acquired autoimmune or hereditary (Upshaw-Schulman syndrome [USS]) severe deficiency of a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13 (ADAMTS13). In the present study, we performed a cross-sectional analysis of the national registry of the French Reference Center for Thrombotic Microangiopathies from 2000-2010 to identify all women who were pregnant at their initial TTP presentation. Among 592 adulthood-onset TTP patients with a severe ADAMTS13 deficiency, 42 patients with a pregnancy-onset TTP were included. Surprisingly, the proportion of USS patients (n = 10 of 42 patients [24%]; confidence interval, 13%-39%) with pregnancy-onset TTP was much higher than that in adulthood-onset TTP in general (less than 5%) and was mostly related to a cluster of ADAMTS13 variants. In the present study, subsequent pregnancies in USS patients not given prophylaxis were associated with very high TTP relapse and abortion rates, whereas prophylactic plasmatherapy was beneficial for both the mother and the baby. Pregnancy-onset TTP defines a specific subgroup of patients with a strong genetic background. This study was registered at www.clinicaltrials.gov as number NCT00426686 and at the Health Authority, French Ministry of Health, as number P051064.

SNPs in ADAMTS13

The multidomain metalloprotease ADAMTS13 limits thrombus formation via the cleavage of large multimeric forms of von Willebrand factor. Deficiency of functional ADAMTS13 is associated with a number of disease pathologies including thrombotic thrombocytopenic purpura, cardiovascular disease and inflammation. To date, deficiency is known to result from mutations in the ADAMTS13 gene or from inhibitory and non-neutralizing antibodies. The exact contributory effect of genetic variation in ADAMTS13 on observable pathology is unclear, and specifically, polymorphisms of ADAMTS13 have not been the focus of much systematic study. Here we have amassed an up-to-date collection of ADAMTS13 polymorphisms described in the literature and from the National Center for Biotechnology Information’s SNP database. This article considers the effect that these polymorphisms may have on the expression and function of ADAMTS13 and speculates on their relevance in future therapies based on pharmacogenomics.

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.

Disulfide bond reduction of von Willebrand factor by ADAMTS-13

BACKGROUND

von Willebrand factor (VWF) released from endothelial cells is rich in ultra-large (UL) multimers that are intrinsically active in binding platelets, whereas plasma-type VWF multimers require shear stress to be activated. This functional difference may be attributed to thiols exposed on the surface of plasma-type VWF multimers, but not on ULVWF multimers. Shear stress induces the exposed thiols to form disulfide bonds between laterally apposed plasma-type VWF multimers, leading to enhanced VWF binding to platelets.

OBJECTIVES

We tested a hypothesis that ADAMTS-13 has a disulfide bond reducing activity that regulates shear-induced thiol-disulfide exchange of VWF.

METHODS

Thiol blocking agents and active thiol bead capturing were used to identify and locate this activity, along with truncated ADAMTS-13 mutants.

RESULTS

ADAMTS-13 contains a disulfide bond reducing activity that primarily targets disulfide bonds in plasma-type VWF multimers induced by high shear stress or formed with thiol beads, but not disulfide bonds in native multimeric structures. Cysteine thiols targeted by this activity are in the VWF C-domain and are known to participate in shear-induced thiol-disulfide exchange. ADAMTS-13 contains cysteine thiols that remain exposed after being subjected to hydrodynamic forces. Blocking these active thiols eliminates this reducing activity and moderately decreases ADAMTS-13 activity in cleaving ULVWF strings anchored to endothelial cells under flow conditions, but not under static conditions. This activity is located in this C-terminal region of ADAMTS-13.

CONCLUSIONS

This novel disulfide-bond-reducing activity of ADAMTS-13 may prevent covalent lateral association and increased platelet adherence of plasma-type VWF multimers induced by high fluid shear stress.

Thrombotic microangiopathies: multimers, metalloprotease, and beyond

The pathophysiology of various types of thrombotic microangiopathies is coming progressively into focus. Therapeutic advances are likely to follow at a quickening pace. This discussion focuses on thrombotic thrombocytopenic purpura (TTP), the hemolytic-uremic syndrome (HUS), thrombotic microangiopathies associated with transplantation-immunosuppression or anti-angiogenesis therapy, and the preeclampsia/hemolysis-elevated liver enzymes and low platelets syndrome (HELLP).

Cysteine residues in CUB-1 domain are critical for ADAMTS13 secretion and stability

Upon stimulation, endothelial cells release von Willebrand factor (VWF) enriched in ultra-large (UL) forms that are rapidly cleaved by ADAMTS13. The zinc metalloprotease fits in the consensus for members of the ADAMTS family, but also contains two unique C-terminal CUB domains. There are five and two cysteine residues in the CUB-1 and CUB-2 domains, respectively, instead of four as deducted from the consensus. In this study, we investigated the role of cysteine residues in the CUB-1 domain in ADAMTS13 synthesis and activity. CUB-1 and cysteine mutations were expressed in mammalian cell lines and examined for synthesis, secretion, stability, and VWF-cleaving activity. When expressed as an isolated domain, CUB-1, but not CUB-2, covalently aggregated. Converting any of the four cysteines that fit in the CUB consensus (C1192, C1213, C1236 and C1254) reduced the secretion of the mutants to the conditioned medium, but not to extracellular matrix. The mutations also resulted in a moderate increase in proteolytic degradation and decrease in cleaving VWF under static, but not flowing conditions. In contrast, replacing C1275, which was found to be in the thiol form, with a serine residue prevented covalent aggregation of CUB-1, but had no effect on secretion and VWF-cleaving activity. C1275S was also markedly resistant to proteolytic degradation. The data illustrate the importance of consensus cysteines in the secretion and proteolytic activity of ADAMTS13. They also identify an ADAMTS-13 mutant that is resistant to proteolytic degradation, while maintaining a normal VWF-cleaving activity.

Polymorphisms and mutations in vWF and ADAMTS13 genes and their correlation with plasma levels of FVIII and vWF in patients with deep venous thrombosis

BACKGROUND

Increased levels of factor VIII (FVIII) are a prevalent and independent risk factor for deep venous thrombosis (DVT) and are affected by von Willebrand factor (vWF) levels.

DESIGN AND METHODS

ADAMTS13 contributes to vWF levels, and we investigated genetic polymorphisms previously described to be associated with decreased levels of these proteins in 435 patients with DVT (126 M and 309 F; median age 37 years, range 18-68 years) and 580 controls (163 M and 417 F; median age 35 years, range 18-68 years). Subsequently, we investigated the relationship between the genotypes and plasma levels of FVIII, vWF, and DVT risk.

RESULTS

Patients with DVT showed higher plasma levels of FVIII:C, FVIII:Ag, and vWF:Ag (P < .001) when compared to controls. Patients and controls heterozygous for the 4751A>G polymorphism in the vWF gene presented decreased levels of vWF:Ag, FVIII:Ag, and FVIII:C (P < .001), but this was not a protective factor for DVT. Individuals heterozygous for 1852C>G polymorphism in ADAMTS13 gene, which is associated with reduced levels of ADAMTS13, had significantly elevated levels of vWF:Ag (P = .001), FVIII:Ag (P = .01), and FVIII:C (P = .02). However, this polymorphism was not a risk factor for DVT in our study. Heterozygosis for a new polymorphism identified in ADAMTS13 gene, 1787-26G>A, was significantly associated with elevated levels of FVIII:C (P = .02) when compared to wild type.

CONCLUSIONS

Despite the tempting assumption that genetic factors that change ADAMTS13 activity might modulate the risk of DVT by altering vWF and FVIII levels, the polymorphisms analyzed in this study did not correlate with DVT risk among patients investigated.

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.

Atypical presentations of thrombotic thrombocytopenic purpura: a review

Thrombotic thrombocytopenic purpura (TTP) is diagnosed by the presence of microangiopathic hemolytic anemia and thrombocytopenia in a patient who frequently presents with central nervous system involvement and, to a lesser extent, renal dysfunction. Recent understanding of the pathophysiology of TTP due to severe deficiency of von Willebrand factor cleaving protease, known as ADAMTS13, has improved diagnosis of TTP. Once the diagnosis is suspected, life-saving therapeutic plasma exchange therapy is initiated. Occasionally, an unusual clinical presentation makes TTP diagnosis difficult, thus resulting in a delay in the management of TTP. This review highlights a variety of atypical TTP presentations described in the literature. It is intended to bring unusual scenarios to the clinician's awareness, so that timely treatment can be delivered.

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.

The first deletion mutation in the TSP1-6 repeat domain of ADAMTS13 in a family with inherited thrombotic thrombocytopenic purpura

The inherited deficiency of ADAMTS13 is usually associated with severe forms of thrombotic thrombocytopenic purpura. Among the mutations identified in the ADAMTS13 gene, none have been described on the TSP1-6 repeat domain. We investigated an Iranian family with a history of chronic recurrent thrombotic thrombocytopenic purpura, severe ADAMTS13 deficiency and a heterogeneous pattern of clinical symptoms among affected members. Genetic analysis revealed a homozygous deletion of nucleotides 2930-2935 (GTGCCC) in exon 23 of ADAMTS13, leading to the replacement of Cys977 by a Trp and the deletion of Ala978 and Arg979 in the TSP1-6 repeat domain. To explore the mechanism of ADAMTS13 deficiency, in vitro expression studies were performed. Western blotting, pulse-chase labeling and immunofluorescence studies demonstrated a secretion pathway defect of the mutant protein, with no intracellular accumulation. This finding is consistent with the severe ADAMTS13 deficiency but does not explain the heterogeneous clinical picture of the 3 siblings carrying the same mutation.

Effects of naturally occurring mutations in CUB-1 domain on synthesis, stability, and activity of ADAMTS-13

INTRODUCTION

Upon stimulation, endothelial cells release von Willebrand factor (VWF) in the unusually large (UL) and hyperactive forms that are rapidly cleaved by ADAMTS-13. Mutations in the ADAMTS13 gene result in ULVWF-mediated thrombosis found in patients with familial thrombotic thrombocytopenia purpura (TTP). ADAMTS-13 fits in the consensus of the ADAMTS family metalloproteases, but also contains two unique C- terminal CUB domains. Studying mutations in CUB domains could provide insights into the functional role of these domains.

METHODS

Three naturally occurring mutations (C1213Y, W1245del and K1256FS) in the CUB-1 domain found in patients with TTP were expressed in Hela cells. The secretion, stability and VWF-cleaving activity of the mutants under static and flow conditions were examined.

RESULTS

The mutations impaired secretion of ADAMTS-13 to apical surface, but not to extracellular matrix of transfected Hela cells. C1213Y and K1256FS also accelerated, whereas W1245del delayed, extracellular degradation of the mutants. The mutations also resulted in a moderate decrease in cleaving plasma VWF under static conditions. However, the mutated ADAMTS-13 bound to VWF substrate similarly as the wild-type metalloprotease and remained active in cleaving (UL)VWF under flow conditions.

CONCLUSIONS

The CUB-1 domain is critical for ADAMTS-13 secretion and stability upon secretion. ADAMTS-13 deficiency found in TTP patients could be resulted from reduced ADAMTS-13 secretion and, in the case of C1213Y and K1256FS accelerated degradation. W1245del is highly resistant to degradation and active in cleaving VWF.

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.

Thrombotic thrombocytopenic purpura: a thrombotic disorder caused by ADAMTS13 deficiency

A serious disorder with characteristic microvascular thrombosis involving the brain and other organs, thrombotic thrombocytopenic purpura (TTP) typically presents with thrombocytopenia, hemolysis with schistocytes on blood smears, and mental changes or seizures. It may progress rapidly to a fatal end if the patient is not treated immediately with plasma. Recent advances have shown that TTP is caused by deficiency of a circulating, von Willebrand factor cleaving metalloprotease, ADAMTS13. This new knowledge will provide clues to improve the diagnosis and management of this intriguing disease.

TTP and ADAMTS13: When Is Testing Appropriate?

The last 10 years witnessed the publication of many studies on the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a life-threatening disease characterized by microangiopathic hemolytic anemia, thrombocytopenia and multiorgan failure. The most important finding was the identification of a novel metalloprotease, named ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin type 1 motives), that is involved in the regulation of the size of von Willebrand factor (VWF), a major modulator of platelet adhesion and aggregation in the microcirculation. Inherited or acquired deficiencies of ADAMTS13 impair VWF cleavage, leading in turn to the disseminated formation of platelet-rich thrombi in the micro-circulation and to symptoms of end-organ ischemia. By measuring ADAMTS13 in plasma, it has been clearly shown that patients with inherited TTP have severe ADAMTS13 deficiency. However, patients with acquired TTP present with clinical and laboratory heterogeneity, and there are unequivocal cases of acquired TTP with measurable plasma levels of ADAMTS13. This heterogeneity poses a challenge for understanding the pathogenesis of TTP and selecting appropriate therapies.