Defective binding of factor XI-N248 to activated human platelets

Area of Science:

• Hematology
• Molecular Biology
• Biochemistry

Background:

• Factor XI (FXI) is a crucial coagulation factor involved in thrombin generation.
• Genetic variants in FXI can lead to bleeding disorders.
• Specific substitutions, Q226R and S248N, were identified in individuals with excessive bleeding.

Purpose of the Study:

• To investigate the functional consequences of FXI variants Q226R and S248N.
• To compare the biochemical properties of recombinant FXI-R226 and FXI-N248 with wild-type FXI.
• To assess the impact of these variants on factor IX activation and platelet binding.

Main Methods:

• Recombinant expression of FXI variants (FXI-R226, FXI-N248) and wild-type FXI.
• Enzymatic assays to measure factor IX activation kinetics (K(m), k(cat)).
• Platelet binding assays using iodinated FXI variants.
• Coagulation assays, including activated partial thromboplastin time (aPTT).

Main Results:

• FXI-R226 exhibited a 5-fold higher K(m) for factor IX activation but similar catalytic efficiency due to increased k(cat).
• FXI-N248 showed a >5-fold higher dissociation constant for activated platelet binding (55 nM vs. 10 nM).
• FXI-N248 activation by thrombin on platelets was impaired, yet it showed normal factor IX activation in purified systems and aPTT assays.

Conclusions:

• FXI variants Q226R and S248N have distinct functional alterations affecting coagulation.
• FXI-N248 demonstrates impaired platelet interaction, a defect not detectable by standard aPTT assays.
• These findings highlight the importance of specific FXI domains in coagulation and suggest limitations of routine clinical assays for certain FXI defects.