Thrombosis Caused by Factor XI Gly397Ser Mutation with Enhanced Procoagulant Activity

Elevated FXI levels are associated with increased risk of venous thromboembolism, yet genetic mutations contributing to elevated FXI levels have not been reported yet.We described a patient with a history of deep venous thrombosis (DVT) who tested negative on routine laboratory screening but was found to carry a novel pathogenic FXI mutation, Gly397Ser (G397S). FXI mutant protein was expressed, and functional assays were conducted in vitro to explore the underlying thrombotic mechanism.Consistent with FXI activity (FXI:C) and antigen (FXI:Ag) determined in the patient, the FXI G397S mutant exhibited a 2-fold increase of FXI:C/FXI:Ag. FXIa hydrolysis assay demonstrated that the Km value for G397S mutant was comparable to the wild-type FXIa (186.2~281.9 μM V.S. 141.6~242.6 μM), whereas the mutant FXIa displayed approximately 3-fold increases in kcat values. FXI activation by polyanions was more pronounced in the G397S mutant. Notably, this activation was more significant when triggered by thrombin compared with FXIIa in the presence of heparin. Otherwise, the G397S FXI mutant exhibited similar hydrolytic activity of FXIa against FIX, and equivalent inhibition of FXIa by Protease nexin 2 (PN2). Clot lysis assay revealed that the lysis time was longer in G397S mutant than the wild-type and the enhanced fibrinolytic resistance was thrombin-activatable fibrinolysis inhibitor-dependent. In summary, the FXI G397S mutation exhibited higher FXIa activity mainly because of the enhanced activation of FXI, which sustained thrombin generation and manifested resistance to fibrinolysis, ultimately leading to the development of DVT.