Mutation Analysis of the ADAMTS13 Gene in Patients with Congenital Thrombotic Thrombocytopenic Purpura from the rADAMTS13 Phase 3 Study

Background: Congenital thrombotic thrombocytopenic purpura (cTTP) is an ultra-rare, life-threatening, thrombotic microangiopathy caused by a deficiency of the ADAMTS13 enzyme, which may be caused by either a homozygous or compound heterozygous loss-of-function genotype. A Phase 3 study (NCT03393975), the first randomized controlled clinical trial, is investigating the safety and efficacy of recombinant ADAMTS13 (rADAMTS13; Takeda Pharmaceuticals U.S.A., Inc.) as prophylaxis or on-demand treatment in patients with cTTP.

Aims: This study aimed to investigate the landscape of ADAMTS13 pathogenic mutations in patients enrolled in the Phase 3 clinical trial and highlight any novel mutations.

Methods: Eligibility criteria for the Phase 3 study included documented diagnosis of cTTP confirmed by genetic testing and ADAMTS13 activity <10% at screening or documented in the patient’s history. Genetic testing data on mutations was either obtained from the medical history or acquired from analysis conducted by the study’s central laboratory at screening. Source genetic testing reports were manually reviewed to standardize cDNA and amino acid changes using the Human Genome Variation Society nomenclature. Mutations were harmonized against the Ensembl transcript ENST00000371929.7 using the Ensembl Variant Effect Predictor (VEP). Mutation types and in silico pathogenicity predictions were obtained using the VEP, and variant frequency in the general population was obtained from the international Genome Aggregation Database, the Japanese Multi Omics Reference Panel, and the Westlake BioBank for Chinese to distinguish pathogenic variants from benign polymorphisms. Known mutations previously reported in patients with cTTP were identified using a list compiled from the Human Gene Mutation Database, ClinVAR, and review of the cTTP literature.

Results: Forty-eight patients from the Phase 3 study were included in this analysis. Genetic data was sourced from medical history for 24 patients (50%), on-study testing for 7 (15%), and both medical history and on-study testing for 17 (35%). Two mutations were found in 46 patients, with 16 homozygotes and 30 compound heterozygotes. One patient with 4 pathogenic/likely pathogenic mutations was identified, and 1 patient with 1 heterozygous pathogenic mutation and 1 undetermined mutation was identified. A total of 47 distinct pathogenic mutations were identified. The distribution of mutation types across the 98 alleles were 50 missense, 24 frameshift, 10 stop gain, and 10 splice mutations, as well as 1 small in-frame deletion, 1 large deletion, 1 large insertion/deletion, and 1 undetermined mutation. The two most frequent mutations identified were also the most frequently reported in the literature (c.3178C>T [n=12 alleles] and c.4143dupA [n=10 alleles]). However, the third most frequent mutation, c.1584+5G>A (n=6 alleles), was a splice variant previously reported but not described as frequent in patients with cTTP. In the literature, c.3178C>T is associated with pregnancy-related cTTP. In this analysis, this mutation was found in 3 female patients aged ≤18 years at time of diagnosis, and 6 female patients aged >18 years at time of diagnosis. Seven of the 47 pathogenic mutations identified have not, to our knowledge, previously been reported in patients with cTTP: c.781del, c.1615del, c.2119_2150dup, c.3223C>T, c.3728T>C, c.3854G>C, and c.1584+442_1969-338delins2730_2731+660.

Conclusions: This analysis contributes to the genetic epidemiology of cTTP, with 7 novel pathogenic mutations identified, and one mutation described as frequent for the first time in patients with cTTP. Further analyses may uncover genotype-phenotype correlations.