Abstract Number: OC 05.1
Meeting: ISTH 2020 Congress
Theme: Coagulation and Natural Anticoagulants » Contact Pathway
Background: Factor XII (FXII) zymogen activation requires cleavage in the activation loop, which can be executed by plasma kallikrein (PKa), activated FXII (autoactivation) or plasmin. Human mutations in FXII cause acute- (T309K/R) or chronic inflammatory disease (W268R). The former mutations result in inappropriate truncation events, while the latter mutation disturbs protein conformation. In both cases, the activation loop of FXII becomes exposed in the absence a contact surface.
Aims: To identify the domain that shields the activation loop.
Methods: Wild-type FXII (WT-FXII), 7 consecutive N-terminal domain-deleted FXII variants (Fig.1), as well as active-site incapacitated (S544A) or T309K variants were expressed in HEK293F cells. We studied enzyme activation/activity and surface binding in chromogenic substrate assays and by Western blotting.
Results: FXII mutants that lack the Fibronectin type II domain (FnII; Δ1-71 and onwards), generate activation fragments and display spontaneous amidolytic activity. This results from autoactivation, since matching FXII S544A variants display no fragmentation. Furthermore, deletion of the FnII domain enhances FXII activation by PKa or plasmin.
Pull-down experiments show that the first EGF-like domain is essential for binding to polyphosphate or kaolin. Surface binding of WT-FXII that contains the FnII domain increases activation by PKa (~3x). Without the FnII domain, surface binding no longer enhances FXII activation by PKa.
Human FXII mutation T309K causes acute inflammation by enabling enzymatic release of the protease domain. Full-length FXII-T309K develops increased activity compared to WT-FXII after exposure to plasmin. Conversely, in the absence of the FnII domain, the T309K mutation (FXII-T309K-Δ1-71) fails to further increase activity after exposure to plasmin. This suggests that the pathogenic gain of function of T309K mutation is caused by enzymatic elimination of the FnII domain.
Conclusions: Cumulatively, our data suggest that the FnII domain shields the activation loop of FXII, ensuring zymogen quiescence. Pathogenic mutations that interfere with this function accelerate FXII activation.
[Fig. 1 Factor XII domain organization and truncation strategy]
To cite this abstract in AMA style:
Clark CC, Hofman ZLM, Sanrattana W, den Braven L, de Maat S, Maas C. The Fibronectin Type II Domain of Factor XII Prevents Inappropriate Zymogen Activation [abstract]. Res Pract Thromb Haemost. 2020; 4 (Suppl 1). https://abstracts.isth.org/abstract/the-fibronectin-type-ii-domain-of-factor-xii-prevents-inappropriate-zymogen-activation/. Accessed March 21, 2024.« Back to ISTH 2020 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/the-fibronectin-type-ii-domain-of-factor-xii-prevents-inappropriate-zymogen-activation/