Thrombomodulin-dependent effect of factor V(Leiden) mutation on factor XIII activation

Introduction: Factor V Leiden mutation (FVLeiden) is associated with impaired down-regulation of activated FV procoagulant activity and loss of FV anticoagulant function that result in an increased risk of venous thromboembolism. As the downstream effects of FVLeiden on clot formation and fibrinolyis have only partially been revealed, we investigated its effect on the activation of factor XIII (FXIII) and the cross-linking of fibrin. Methods: In the plasma samples of fifteen healthy individuals with known FV genotypes coagulation was initiated by recombinant human tissue factor and phospholipids with or without recombinant human thrombomodulin (rhTM). FV deficient plasma supplemented with purified wild type FV or FVLeiden were also investigated. Clots were recovered and analyzed by SDS-PAGE and quantitative densitometric evaluation of Western blots. Results: rhTM considerably delayed the activation of FXIII in the plasma from FV wild type individuals. This effect of rhTM was significantly impaired in the plasma from FVLeiden carriers. The results were confirmed in experiments with FV deficient plasma supplemented by FV prepared from wild type individuals or FVLeiden homozygotes. Fibrin γ-chain dimerization was also considerably delayed by rhTM in plasma samples from individuals without Leiden mutation, but not in plasma samples from FVLeiden heterozygotes or homozygotes. The difference between heterozygotes and homozygotes was not statistically significant. Conclusion: The highly diminished delaying effect of TM on FXIII activation and on the cross-linking of fibrin in FVLeiden carriers might represent a novel mechanism contributing to the increased thrombosis risk of these individuals