Background: Hereditary angioedema(HAE) is a rare dominant disease resulting from an increased generation of bradykinin upon activation of the plasma contact pathway. HAE-FXII is caused by mutations in F12; the most prevalent, c.1032C>A p.Thr309Lys, disrupts an O-glycosylation resulting in a smaller FXII with increased sensitivity to the fluid phase activation by poorly understood mechanisms.

Aims: To study the functionality of the aberrant FXII p.Thr309Lys in plasma.

Methods: Thirty-three HAE-FXII^T309K patients, 25 healthy controls(NHP) and 53 patients with congenital disorder of glycosylation(CDG) were included. FXII and (pre)kallikrein(PK) were assessed by western-blot in basal conditions, and after treatment with neuraminidase, PNGaseF, plasmin, thrombin, dextran sulfate and silica. FXII amidolytic activity was measured by chromogenic methods. The generation and spontaneous or tPA-induced degradation of fibrin on FXII was evaluated. mRNA from peripheral mononuclear cells was sequenced.

Results: Aberrant FXII from all HAE-FXII^T309K had similar electrophoretic mobility (reducing SDS-PAGE) than FXII lacking an N-glycan from CDG patients and asialo-FXII generated by neuraminidase. Under non-reducing conditions or after PNGaseF this aberrant FXII was even faster. In silico predictions and F12 mRNA sequencing ruled out aberrant splicing. FXII^309K had increased sensitivity to activation upon low-dose triggers, generating an additional smaller band of approximately 37KDa. Plasmin activated similarly FXII^309K and FXII^T309. However, exogenous or endogenous thrombin cleaved only FXII^309K, releasing a 37KDa fragment that enhanced its further activation of the fluid phase.

Conclusions: Aberrant FXII from HAE-FXII^T309K has unexpected electrophoretic mobility, that might be explained by a less compact structure. It would facilitate a sequential proteolytic activation process firstly involving thrombin priming of FXII^309K followed by kallikrein cleavage, generating the active β-FXIIa. This and the fact that angioedema episodes occur during hyperocagulable situations suggest a key role for thrombin. The benefit of an antithrombotic prophylaxis should be further evaluated. CIBERER (ACCI18-04; ER19P5AC765/2019) SETH: grupos emergentes.

[Activation of plasma FXII (left panels) or PK (right panels) with different doses of dextran sulphate (DXS) and silica, analysis by western blotting a]

[Effect of thrombin incubation on FXII and PK activation.]

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/relevance-of-thrombin-in-the-activation-of-the-fluid-contact-phase-in-patients-with-hereditary-angioedema-carrying-the-f12-p-thr309lys-variant/