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Hereditary thrombotic thrombocytopenic purpura (hTTP) is a rare thrombotic microangiopathy characterized by severe congenital ADAMTS13 deficiency and recurring acute episodes causing morbidity and premature death. Information on the annual incidence and severity of acute episodes in patients with hTTP is largely lacking. This study reports prospective data on 87 patients from the Hereditary TTP Registry (clinicaltrials.gov #NCT01257269) for survival, frequency, and severity of acute episodes from enrollment until December 2019. The 87 patients, followed up for a median of 4.2 years (range, 0.01-15 years), had a median age at overt disease onset and at clinical diagnosis of 4.6 years and 18 years (range, 0.0-70 years for both), respectively. Forty-three patients received regular plasma prophylaxis, whereas 22 did not, and treatment changed over time or was unknown in the remaining 22. Forty-three patients experienced 131 acute episodes, of which 91 (69%) occurred in patients receiving regular prophylaxis. This resulted in an annual incidence of acute episodes of 0.36 (95% confidence interval [CI], 0.29-0.44) with regular plasma treatment and of 0.41 (95% CI, 0.30-0.56) without regular plasma treatment. More than one-third of acute episodes (n = 51) were documented in children 40 years (1.18 [95% CI, 0.88-1.55] vs 0.14 [95% CI, 0.08-0.23]). The prophylactic plasma infusion regimens used were insufficient to prevent acute episodes in many patients. Such regimens are burdensome, and caregivers, patients, and their guardians are reluctant to start regular plasma infusions, from which children particularly would benefit.

Congenital thrombotic thrombocytopenic purpura is an autosomal recessive inherited disease with a clinically heterogeneous course and an incompletely understood genotype-phenotype correlation. In 2006, the Hereditary TTP Registry started recruitment for a study which aimed to improve the understanding of this ultra-rare disease. The objective of this study is to present characteristics of the cohort until the end of 2017 and to explore the relationship between overt disease onset and ADAMTS13 activity with emphasis on the recurring ADAMTS13 c.4143_4144dupA mutation. Diagnosis of congenital thrombotic thrombocytopenic purpura was confirmed by severely deficient ADAMTS13 activity (≤10% of normal) in the absence of a functional inhibitor and the presence of ADAMTS13 mutations on both alleles. By the end of 2017, 123 confirmed patients had been enrolled from Europe (n=55), Asia (n=52, 90% from Japan), the Americas (n=14), and Africa (n=2). First recognized disease manifestation occurred from around birth up to the age of 70 years. Of the 98 different ADAMTS13 mutations detected, c.4143_4144dupA (exon 29; p.Glu1382Argfs*6) was the most frequent mutation, present on 60 of 246 alleles. We found a larger proportion of compound heterozygous than homozygous carriers of ADAMTS13 c.4143_4144dupA with overt disease onset at < 3 months of age (50% vs. 37%), despite the fact that ADAMTS13 activity was

Introduction: Congenital thrombotic thrombocytopenic purpura (cTTP) is an autosomal recessive disease characterised by acute episodes of thrombotic microangiopathy. cTTP has a clinically heterogeneous course. Currently there are more than 150 disease-causing ADAMTS13 gene mutations reported, though a genotype-phenotype correlation of this ultra-rare disease remains incompletely understood. Characterization of mutations of the ADAMTS13 gene will be useful for carrier detection and genetic counselling. In 2006, the Hereditary TTP Registry (ClinicalTrials.gov NCT01257269; www.ttpregistry.net) was initiated in order to achieve this objective by identifying possible triggers of acute episodes of TTP and to document individual clinical courses and treatment requirements. Methods: The Hereditary TTP Registry is an international cohort study. Individual data were analysed from 121 cTTP patients, who were enrolled between 2006 and the end of 2017. Diagnosis of cTTP was confirmed by a severely deficient ADAMTS13 activity Results: The cTTP cohort consists of 54 and 52 patients from Europe and Asia (90% from Japan), respectively, 13 patients from the Americas and 2 patients from Africa. For 69% of the patients regular treatment was reported, which consisted predominantly of fresh frozen plasma infusion (84%). More compound heterozygous (n=76) than homozygous mutation carriers (n=45) were included. The female to male ratio was 1:1, and the median age at first diagnosis was 25.8 years (range from 0 to 81.3 years). Prior arterial thromboembolic events (mainly myocardial infarction, stroke, TIA) were present in 27% of patients at enrolment and at the age of 50 years more than half of the patients had experienced at least one such event. In the 121 patients, we identified 98 different ADAMTS13 mutations (missense 57; nonsense 12; frameshift due to deletions or insertions 21; splice site 8). The most frequent observed mutations were c.4143_4144dupA (exon 29; p.Glu1382Argfs*6; n=58 alleles), c.3178C>T (exon 24; p.Arg1060Trp; n=13), and c.577C>T (exon 6; p.Arg193Trp; n=11). Nineteen mutations have not been previously reported (8 missense, 3 splice site, 2 nonsense, and 6 frameshift mutations). Mutations are located across the whole ADAMTS13 gene and affect all domains (there was no "hotspot"). We did not find a correlation of the mutations with clinical or laboratory features, except for ADAMTS13 c.4143_4144dupA. For this mutation, the age at initial diagnosis was younger (median age 4.1 years, range 1.8 - 52.1) in compound heterozygous compared to homozygous patients (median 23.3 years, range 0.7 - 47.5,) (p=0.040). Patient numbers were equally distributed (p = 0.51); 16 patients were compound heterozygotes and 21 were homozygotes. All patients were of Scandinavian or Central European ancestry. For the other assessed clinical parameters, no significant difference between compound heterozygotes and homozygotes was observed. Conclusion: At present, the Hereditary TTP Registry is the largest cohort on cTTP, to our knowledge. The large number of different mutations as well as confounding factors, including multi-ethnic and geographical factors in our internationally compiled patient cohort make unravelling the genotype-phenotype correlation in cTTP challenging. Yet, for c.4143_4144dupA, we can conclude that homozygous patients with ADAMTS13 c.4143_4144dupA are clinically less severely affected than compound heterozygotes due to the later onset of the disease in homozygotes, whereas the clinical characteristics are similar between compound heterozygotes and homozygotes. Why homozygous carriers of c.4143_4144dupA, despite having no ADAMTS13 activity in plasma, may have a rather late onset of acute TTP remains to be further elucidated. Disclosures Friedman: Shire: Membership on an entity's Board of Directors or advisory committees; CSL Behring: Membership on an entity's Board of Directors or advisory committees. Knöbl:Ablynx: Consultancy, Other: Member of Advisory Board. Schneppenheim:CSL Behring: Consultancy; SHIRE: Consultancy. Kremer Hovinga Strebel:Ablynx: Other: Advisory Board; SHIRE: Other: Advisory Board, Research Funding.

Several ligand blotting or immunoblotting assays for the detection of single-chain and proteolytically cleaved two-chain high molecular weight kininogen (HK) in whole plasma have been described. Since they may suffer from poor sensitivity for the light chain species of cleaved HK on reduced blots, an antiserum against the reduced and alkylated 47 kDa light chain of HK was raised in rabbits allowing improved immunodetection of HK species on blots of reduced electropherograms. This immunoblotting method is highly specific and sensitive, permitting detection of 0.2 ng single-chain HK or the light chains of 2 ng proteolytically cleaved HK in whole plasma. Thus, this immunoblotting technique is at least 50-100 times more sensitive than ligand blotting with radiolabelled factor XI overlay. A similar cleavage pattern was observed following in vitro activation of normal human plasma by dextran sulphate and after plasma kallikrein-induced proteolysis of purified HK. However, bands of different molecular weights were generated after HK had been cleaved by purified leukocyte elastase. During acute attack in a patient with hereditary angioedema, high levels of in vivo cleaved HK were noticed, whereas concentration of cleaved HK in plasma samples and synovial fluids from patients suffering from various inflammatory conditions were not substantially higher than those in normal plasma. During in vitro cold activation of plasma samples of pregnant women concomitant HK cleavage and plasma kallikrein generation were observed.