Summary
Elevated plasma prothrombin levels, due to the prothrombin 20210 G/A mutation or to
acquired causes, area risk factor for venous thrombosis,partly because of prothrombin-mediated
inhibition of the protein C anticoagulant pathway and consequent activated proteinC
(APC) resistance. We determined the effect of plasma prothrombin concentration on
the APC resistance phenotype and evaluated the role of protein S levels asa modulating
variable. The effect of prothrombin and protein S levels on APC resistance was investigated
in reconstituted plasma systems and in a population of healthy individuals using both
the aPTT-based and the thrombin generation-based APC resistance tests. In reconstituted
plasma, APC resistance increased at increasing prothrombin concentration in both assays.
Enhanced APC resistance was caused by the effect of prothrombin on the clotting time
in the absence of APC in the aPTT-based test, and on thrombin formation in the presence
of APC in the thrombin generation-based test. In plasma from healthy individuals prothrombin
levels were highly correlated to protein S levels. Since prothrombin and proteinS
had opposite effects on the APC resistance phenotype, the prothrombin/protein S ratio
was a better predictor of APC resistance than the levels of either protein alone.
Prothrombin titrations in plasmas containing different amounts of proteinS confirmed
that proteinS levels modulate the ability of prothrombin to induce APC resistance.
These findings suggest that carriers of the prothrombin 20210 G/A mutation, who have
a high prothrombin/protein S ratio, may experience a higher thrombosis risk than non-carriers
with comparable prothrombin levels.
Keywords
Prothrombin - protein S - thrombin generation and APC resistance