Thrombosis Caused by Factor XI Gly397Ser Mutation with Enhanced Procoagulant Activity

Area of Science:

• Hematology
• Molecular Biology
• Genetics

Background:

• Elevated Factor XI (FXI) levels are linked to increased venous thromboembolism (VTE) risk.
• Genetic mutations causing elevated FXI levels have not been previously identified.

Purpose of the Study:

• To investigate a novel pathogenic FXI mutation, Gly397Ser (G397S), identified in a patient with deep venous thrombosis (DVT).
• To elucidate the molecular mechanisms by which the FXI G397S mutation contributes to thrombotic events.

Main Methods:

• Patient genetic sequencing to identify FXI mutations.
• In vitro expression and functional characterization of the FXI G397S mutant protein.
• Enzyme kinetic assays (Km, kcat) and activation studies using polyanions, thrombin, and FXIIa.
• Assays for FXI's activity against Factor IX (FIX) and inhibition by Protease Nexin 2 (PN2).
• Clot lysis assays to assess fibrinolytic resistance.

Main Results:

• The FXI G397S mutant showed a 2-fold increase in FXI activity (FXI:C) relative to antigen (FXI:Ag).
• Mutant FXIa exhibited significantly increased catalytic efficiency (kcat) and enhanced activation by thrombin in the presence of heparin.
• The G397S mutation conferred resistance to clot lysis, dependent on thrombin-activatable fibrinolysis inhibitor (TAFI).

Conclusions:

• The FXI G397S mutation enhances FXIa activity primarily through increased activation, leading to sustained thrombin generation.
• This hypercoagulable state, coupled with fibrinolytic resistance, underlies the patient's predisposition to DVT.
• Identifies a novel genetic cause for FXI-related thrombophilia.