Anti-ADAMTS13 Antibodies and a Novel Heterozygous p.R1177Q Mutation in a Case of Pregnancy-Onset Immune-Mediated Thrombotic Thrombocytopenic Purpura.

An-Sofie Schelpe,Jan Voorberg,Karen Vanhoorelbeke,Simon De Meyer,Aline Vandenbulcke,Claudia Tersteeg,Daan Dierickx,Hans Deckmyn,Elien Roose,Nele Vandeputte,Inge Pareyn,Ruth Demeersseman,Louis Deforche

doi:10.1055/s-0037-1615252

An-Sofie Schelpe, Jan Voorberg + Show 11 more

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https://doi.org/10.1055/s-0037-1615252

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Abstract

In this study, we investigated a case of pregnancy-onset thrombotic thrombocytopenic purpura (TTP). The patient had severely decreased ADAMTS13 ( a d isintegrin a nd m etalloprotease with t hrombo s pondin type 1 motif, member 13) activity levels during acute phase and the presence of inhibitory anti-ADAMTS13 autoantibodies was demonstrated, which led to the diagnosis of immune-mediated TTP. However, ADAMTS13 activity was only mildly restored during remission, although inhibitory anti-ADAMTS13 antibodies were no longer detected. We hypothesized that genetic abnormalities could account for this discrepancy between ADAMTS13 activity and antigen. Genetic analysis revealed the presence of two heterozygous substitutions on the same allele: a single nucleotide polymorphism (SNP) c.2699C > T (p.A900V), located in the beginning of the T5 domain, and a mutation c.3530G > A (p.R1177Q) located in the third linker region of ADAMTS13. In vitro testing of those substitutions by expression of recombinant proteins revealed a normal secretion but a reduced ADAMTS13 activity by the novel p.R1177Q mutation, which could partially explain the subnormal activity levels found during remission. Although changes in the linker region might induce conformational changes in ADAMTS13, the p.R1177Q mutation in the third linker region of ADAMTS13 did not expose a cryptic epitope in the metalloprotease domain. In conclusion, we report on an immune-mediated pregnancy-onset TTP patient who had inhibitory anti-ADAMTS13 autoantibodies during acute phase, but not during remission. Genetic analysis confirmed the diagnosis of immune-mediated TTP and revealed the novel p.R1177Q mutation which mildly impaired ADAMTS13 activity.

Highlights

The rare and life-threatening disease thrombotic thrombocytopenic purpura (TTP) is caused by a severe deficiency in the metalloprotease ADAMTS13.[1]
ADAMTS13 is encoded by 29 exons which results in a multidomain protein containing a signal peptide, propeptide, metalloprotease domain, disintegrin-like domain, a first thrombospondin type 1 repeat (T1), cysteine-rich domain, spacer domain, seven additional T domains (T2-T8), and two CUB domains.[2]
We investigated a case of pregnancy-onset TTP using both the plasma of the patient and recombinant proteins to unequivocally determine the diagnosis of immune-mediated or congenital TTP

Summary

Introduction

The rare and life-threatening disease thrombotic thrombocytopenic purpura (TTP) is caused by a severe deficiency in the metalloprotease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motif, member 13).[1]. UL-VWF is hyper reactive and spontaneously binds platelets This leads to the formation of microthrombi that obstruct the microvasculature, resulting in organ failure, thrombocytopenia, and hemolytic anemia or even death when left untreated.[1] In more than 95% of the patients, TTP is caused by the presence of anti-ADAMTS13 autoantibodies (immune-mediated TTP) that inhibit ADAMTS13 activity or accelerate its clearance from circulation.[3,4] Far less patients (< 5%) suffer from congenital TTP (Upshaw-Schulman syndrome), in which ADAMTS13 deficiency is due to genetic mutations within the ADAMTS13 gene.[5] Those mutations can lead to a secretion deficiency or impaired activity of the enzyme.[6] The onset of TTP is not caused by an ADAMTS13 deficiency alone, but an additional trigger like infection or pregnancy is typically needed to initiate an acute TTP episode

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