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Immune TTP: interplay between haplotypes & environment
aromatization of androgens to estrogens, but also higher leptin levels, in this population of patients.71,76 In addition, obese healthy individuals have an increased prevalence of non-inhibitory ADAMTS13 auto-Abs, although no differ- ences in ADAMTS13 activity and antigen levels were found with lean people.77 Taken together, these findings stress the role of modifying factors including hormones, cytokines and other mediators involved in the loss of tol- erance against ADAMTS13; these require further study to be better understood mechanistically. There is also a need to specify whether iTTP patients with associated condi- tions, including pregnancy, connective tissue diseases or obesity, also have specific genetic risk factors as compared to patients with idiopathic iTTP.
Von Willebrand Factor (VWF) is another important modifier of TTP susceptibility, either in the congenital or in the autoimmune form. Raised VWF levels are required to induce TTP in Adamts13-/- mice, but varying the concen- tration between 20 and 120 U/mL does not appear to affect the occurrence or severity of the disease, suggesting that a threshold level of VWF is sufficient, and that higher levels confer little additional risk. However, humans appear to be more sensitive to changes in VWF levels than mice. Women with inherited ADAMTS13 deficiency fre- quently develop TTP during pregnancy, which probably results from the progressive increase in VWF levels throughout the gestation period. Moreover, obese individ- uals have higher levels of VWF, providing further evidence for an association between obesity and TTP. Thus,
changes in VWF secretion, multimer distribution, and plasma level may trigger TTP.78 Among additional modi- fiers of TTP, blood group O was found over-represented in iTTP cohorts, suggesting that blood group O could be a risk factor for TTP.79
ADAMTS13 conformation in iTTP
The precise mechanism triggering the formation of anti- ADAMTS13 Abs in previously healthy individuals still has to be identified. As mentioned earlier, the majority of anti- spacer auto-Abs that develop in patients with iTTP is directed toward an exosite composed of residues R568, F592, R660, Y661 and Y665.30 Under quiescent conditions, the exosite within the spacer domain is covered by the CUB1/2 domains maintaining ADAMTS13 in a so-called closed conformation.80-82 Binding of VWF to the carboxy- terminal CUB domains induces a conformational activa- tion of ADAMTS13 which results in the exposure of the spacer domain exosite composed of residues R568, F592, R660, Y661 and Y665 which now becomes available for binding to the unfolded VWF A2 domain.83 Apart from VWF, murine monoclonal Abs can induce a conversion of ADAMTS13 from a closed to an open conformation.80,82 Conservative mutations of the spacer domain exosite cre- ated a variant with increased proteolytic activity toward its substrate VWF.84 Recently, it was shown that this so- called gain-of-function (GoF) variant is present in an open conformation, which promotes binding of the spacer domain exosite to the unfolded A2 domain of VWF.81
haematologica | 2018; 103(7)
Figure 4. Conformational changes of ADAMTS13. [Mp: metalloprotease (red); Dis: disintegrin-like domain (green); Cys-rich: cys- teine-rich domain (orange); TSRs: throm- bospondin-type 1 repeats (light and dark gray); CUB1 domain (cyan); CUB2 domain (magen- ta)]. (A) In the blood circulation of healthy indi- viduals and immune-mediated thrombotic thrombocytopenic purpura (iTTP) patients in remission ADAMTS13 is present in closed con- formation with CUB1/2 domains covering the spacer domain. Binding of plasma factors (e.g. activating anti-ADAMTS13 auto-Abs against the TSR5-8-CUB1/2 domains80,82) (orange part of the anti-ADAMTS13 antibodies) results in a conformational change: the opening of the ADAMTS13.83,100 (B) Open conformation of ADAMTS13 exposes the B-cell epitope localized in the spacer domain, making it accessible for additional anti-ADAMTS13 Abs.
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