Genotype-Phenotype Correlation in Congenital TTP: New Insights from a Multicentre Study with 121 Patients

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 <10 % of normal in the absence of a functional inhibitor and the presence of ADAMTS13 mutations on both alleles. Genetic analysis was performed through ADAMTS13 gene amplification and sequencing all 29 exons with flanking intron-exon boundaries using multiplexed PCR reactions with published primers. Clinical and laboratory data were recorded for each patient retrospectively up to enrolment, and prospectively during annual follow up by physicians responsible for patient care. In this abstract, we report the results of a descriptive analysis on the identified mutations and related clinical data at enrolment.

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.