A Multicentre Study on 100 Cases with Thrombotic Thrombocytopenic Purpura: The Role of Adamts-13 Activity and Anti-Adamts-13 Antibodies.

Background: The metalloprotease ADAMTS-13 disposes physiologically of the unusually large molecular forms of the von Willebrand factor (VWF), that have the property to aggregate platelets in the microcirculation in the presence of high shear forces. The congenital or acquired deficiency of ADAMTS-13 has been specifically associated with a diagnosis of thrombotic thrombocytopenic purpura (TTP), a rare microangiopathy characterised by the massive formation of occlusive platelet thrombi in the microcirculation.

Aim of the study: To investigate the pathogenic mechanism of TTP in 100 patients referred from 6 different countries and diagnosed on the basis of the presence of at least 3 of the followings laboratory abnormalities or clinical signs: thrombocytopenia, hemolytic anemia, elevated serum level of lactadehydrogenase and neurological symptoms compatible with focal ischemia.

Materials and methods: ADAMTS-13 levels were measured in all patients’ plasma by Collagen Binding assay as previously described (Gerritsen et al., Thromb Haemost 1999); anti-ADAMTS-13 neutralizing antibodies was measured only in patients with low ADAMTS-13 levels, whereas candidate gene mutations were searched only in patients who had low ADAMTS-13 levels (less than 20%) not explained by detectable anti-ADAMTS-13 antibodies, whether or not there was a family history of TTP.

Results: Plasma levels of ADAMTS-13 were severely reduced (<10% of normal) in 48% of the cases, moderately reduced (between 10 and 46%) in 24% and normal (>46%) in 28%. An antibody was identified as the cause of the deficiency in 38% the cases: the great majority of the patients with detectable inhibitor (87%) had severe ADAMTS-13 deficiency but 5 (13%) had moderately reduced ADAMTS-13 activity (ranging from 11 to 25%). Mutation analysis of the ADAMTS13 gene, carried on 15 patients, revealed a genetic variant in only two patients born from consanguineous marriages. The first case was a double heterozygosity for a 29 base pair deletion in exon 3 (291–319del) and a single nucleotide insertion in exon 29 (4143insA). The 290–319del and 4143insA mutations lead to premature stops at codons 368 and 1387 respectively. The second one consisted in a homozygosity for a 6 base pair deletion in exon 23 (2930–2935del). This deletion leads to a substitution of Cys 977 by Trp, and a deletion of 2 aminoacids, Ala and Arg, at residues 978 and 979 respectively.

Conclusion: The study of this large series of patients with TTP indicates that in nearly one third of patients ADAMTS-13 levels were normal. In more than half of the patients with low plasma level of the protease, ADAMTS-13 deficiency was not due to the presence of inactivating antibodies or gene abnormalities. An unexpected observation was that an inhibitor was also found in 5 patients with moderately reduced levels of ADAMTS-13.