Acquired thrombotic thrombocytopenic purpura (TTP) is an autoimmune disorder due to autoantibodies against ADAMTS13. The case of identical twins suffering from acute acquired TTP with severe autoantibody-mediated ADAMTS13 deficiency (

Studt JD et al.,
) led us to speculate on unknown genetic determinants of acquired TTP and on the immunological basis of ADAMTS13 autoimmunity. We investigated 37 patients (pts) with acute TTP with acquired severe ADAMTS13 deficiency of <10% of the normal, as well as 60 healthy controls. We measured ADAMTS13 activity, anti-ADAMTS13 IgG by ELISA and searched for a functional ADAMTS13 inhibitor. The ADAMTS13 gene was sequenced and 15 short tandem repeats (STR) at loci not linked to the ADAMTS13 gene were analyzed. Genotypic data of 25 polymorphic markers on the ADAMTS13 gene were used for haplotype inference. Inter-allelic diversity was assessed by summing up the number of individual heterozygous base positions for the polymorphic markers on the ADAMTS13 gene and the STR loci separately. Identified ADAMTS13 mutations were expressed in COS-7 cells and their effect on biosynthesis, secretion and activity was determined. We found that 4 of 37 pts with severe acquired ADAMTS13 deficiency were heterozygous carriers of an ADAMTS13 mutation. The mutation P457L, found in two pts, is a known mutation reported in hereditary TTP. R1096H and A1145T are new mutations. Expression studies revealed that P457L and A1145T mutants were secreted normally, but showed reduced activity of 44% and 11%, respectively. Secretion of the R1096H mutant, however, was reduced. A particular ADAMTS13 haplotype (designated H6) inferred from single nucleotide polymorphism (SNP) data was significantly more frequent in pts than in controls (p<0.0001). Moreover, individual ADAMTS13 inter-allelic diversity was significantly higher in pts than in controls (p=0.012). Primarily, non-synonymous SNPs, which encode amino acid exchanges, contributed to this phenomenon (p=0.0014). The increased inter-allelic diversity in pts was found to be restricted to and specific for the ADAMTS13 gene, as individual inter-allelic diversity at 15 neutral STR markers was similar in pts and controls. In conclusion, heterozygous ADAMTS13 missense mutations are found in a high frequency in pts with acquired TTP. The mutations P457L, R1096H and A1145T, reduce baseline ADAMTS13 activity in vitro by at least 50% and may render heterozygous carriers more susceptible for TTP. In addition, ADAMTS13 mutations, such as R1096H, may promote autoantibody production due to increased intracellular degradation and MHC presentation. The ADAMTS13 haplotype H6 and particularly the increased inter-allelic diversity represent risk factors for autoimmunity towards ADAMTS13 and acute acquired TTP. ADAMTS13 autoimmunity as a result of increased inter-allelic diversity represents the first example of genetic out-breeding depression in humans. As autoantigens in general seem to have increased numbers of SNPs, increased inter-allelic diversity may represent an important pathophysiological feature in other autoimmune disorders as well.

Author notes

Disclosure:Consultancy: Johanna A. Kremer Hovinga and Bernhard Lammle have received a consultancy fee from Baxter Healthcare.