The
prothrombin gene mutation G20210A is a common risk factor for
thrombosis and has been reported to cause
APC resistance. However, the inhibition of
thrombin formation by APC not only limits
fibrin formation but also stimulates fibrinolysis by reducing TAFI activation. We evaluated the influence of
prothrombin G20210A mutation on the
anticoagulant and fibrinolytic activities of APC (1 microg/ml). Thirty-two heterozygous carriers and 32 non carriers were studied. APC
anticoagulant activity was assessed by aPTT prolongation whereas APC fibrinolytic activity was determined by a microplate clot lysis assay. APC-induced aPTT prolongation was markedly less pronounced in carriers than in non carriers. On the contrary, fibrinolysis time was shortened by APC to a comparable extent in both groups. Accordingly,
prothrombin levels were strongly correlated with APC-induced aPTT prolongation but not with APC-induced shortening of lysis time. The addition of purified
prothrombin to normal plasma (final concentration 150%) caused
APC resistance in the clotting assay over the whole range of tested APC concentrations (0.125-1.5 microg/ml). In the fibrinolytic assay, instead,
prothrombin supplementation made the sample resistant to low but not to high concentrations of APC (>0.5 microg/ml).
Thrombin and TAFIa determination in the presence of 1 microg/ml APC revealed that
hyperprothrombinemia, although capable of enhancing
thrombin generation, was unable to induce detectable TAFIa formation. It is suggested that
APC resistance caused by hyperprothrombinaemia does not translate in impaired fibrinolysis, at least in the presence of high APC levels, because the increase in
thrombin formation is insufficient to activate the amount of TAFI required to inhibit
plasminogen conversion. These data might help to better understand the relationship between
thrombin formation and fibrinolysis down-regulation.