An In-Frame Deletion of Six Amino Acids and a Point Mutation in the Disintegrin Domain of ADAMTS-13 Associates with a Case of Congenital Thrombotic Thromocytopenic Purpura.

Thrombotic thrombocytopenic purpura (TTP) is characterized by severe thrombocytopenia, hemolytic anemia, and diffuse and non-focal neurological findings. Microthrombi found in these patients are predominantly composed of platelets and von Willebrand factor (VWF). Recent studies suggest that the systemic thrombosis in TTP is mostly due to the congenital or acquired deficiency of the VWF-cleaving metalloprotease ADAMTS-13, which cleaves the ultra-large and hyperreactive VWF to smaller and less active form found in plasma. Most congenital cases of TTP have so far been identified in children. Here, we report a 60-year-old Caucasian man with a history of chronic relapsing TTP over thirty years, requiring plasma transfusion every 24 days in recent years. Repeat assays showed no ADAMTS13 activity in patient’s plasma under both static and flow conditions without inhibitors being detected. We therefore examined the possible genetic defects in the ADAMTS-13 gene of this patient. Genomic DNA was extracted from patient’s white blood cells and exons of ADAMTS-13 gene were amplified by polymerase chain reaction. The amplified DNA fragments were then screened for mutations by direct DNA sequencing. We identified a deletion of 18 base pairs from G1095 to G1112 (GTGCTCCAAGGGTCGCTG) in the exon10 of ADAMTS-13 gene, resulting in a deletion of six amino acids (C366 to C371) in the disintegrin domain of the metalloprotease. A point mutation (W365C) occurred immediately before the deletion due to a nucleotide realignment. The patient is heterozygote for the deletion. This is the first report of a deletion mutant (without frame shift and truncations) in the disintegrin region that has previously been demonstrated as critical for the ADAMTS-13 function by in vitro mutagenesis, epitope mapping of autoantibody to the metalloprotease in patients with adult acquired TTP, and identification of natural occurring mutations in patients with congenital TTP. Our ongoing studies are to determine the impact of these two mutations on the synthesis, release, and cleavage of ADAMTS-13.