Hyperprothrombinemia is associated with enhanced TAFI-mediated inhibition of fibrinolysis

Abstract number: P0791

Colucci^* M., Binetti^* B. M., Asti† D., Tripodi† A., Semeraro^* N.

†A. Bianchi Bonomi Hemophilia and Thrombosis Center, Italy ^*Department Biomedical Sciences, University of Bari, Italy;

The 20210 G to A mutation of the prothrombin gene is associated with increased levels of circulating prothrombin and is a risk factor for venous thrombosis. Since thrombin is the physiologic activator of TAFI (thrombin activatable fibrinolysis inhibitor), the precursor of an anti-fibrinolytic carboxypeptidase (TAFIa), we studied the influence of hyperprothrombinemia on TAFI-mediated inhibition of fibrinolysis. Fibrinolysis was evaluated by measuring the time to 50% lysis (referred to as lysis time) of plasma clots by t-PA (25 ng mL^-1) in the absence and in the presence of a specific inhibitor of TAFIa (PTI, 50 mg mL^-1). The influence of TAFI activation on the rate of lysis was calculated as the ratio between the lysis times in the absence and in the presence of PTI (PTI ratio). When normal plasma was supplemented with purified human prothrombin, clot lysis time increased in parallel with prothrombin concentration (48, 55, and 63 min at prothrombin concentrations of 100, 125, and 150%, respectively). This effect was not seen in the presence of PTI (giving rise to PTI-ratio values of 1.77, 2.03, and 2.3, respectively) or when normal plasma was replaced by TAFI-depleted plasma, indicating that the anti-fibrinolytic effect of prothrombin elevation was TAFI-mediated. To assess the influence of G20210A mutation on fibrinolysis, we tested plasma samples from 15 subjects carrying the mutation (all heterozygous) and 15 normal subjects. While the mean clot lysis time was similar in the two groups (48.3 ± 10.2 min vs. 48.7 ± 12.6 min) the PTI ratio was significantly higher in the 20210A carriers (1.91 ± 0.06 vs. 1.71 ± 0.06; P = 0.037). Mean TAFIa activity (PTI-sensitive arginine carboxypeptidase activity) generated 30 min after the start of clot lysis, was also higher in the 20210A group (11.7 ± 7.1 vs. 8.0 ± 4.8), though not statistically significant (P = 0.06), likely because of the small sample size. Moreover, TAFIa generation during clot lysis was significantly correlated with PTI ratio (r = 0.37, P = 0.04), further confirming that the response to PTI was related to the extent of TAFI activation. These data suggest that hyperprothrombinemia may enhance the thrombotic risk through the up-regulation of TAFI-mediated inhibition of the fibrinolytic process.