Molecular and functional characterization of a natural homozygous Arg67His mutation in the prothrombin gene of a patient with a severe procoagulant defect contrasting with a mild hemorrhagic phenotype

In a patient who presented with a severe coagulation deficiency in plasma contrasting with a very mild hemorrhagic diathesis a homozygous Arg67His mutation was identified in the prothrombin gene. Wild-type (factor lla [Flla]-WT) and mutant Arg67His thrombin (Flla-MT67) had similar amidolytlc activity. By contrast, the k_cat/K_m value of fibrinopeptide A hydrolysis by Flla-WT and Flla-MT67 was equal to 2.1 × 10⁷ M^-1s^-1 and 9 × 10⁵ M^-1s^-1. Decreased activation of protein C (PC) correlated with the 33-fold decreased binding affinity for thrombomodulin (TM; K_d = 65.3 nM vs 2.1 nM, in Flla-MT67 and In Flla-WT, respectively). In contrast, hydrolysis of PC in the absence of TM was normal. The Arg67His mutation had a dramatic effect on the cleavage of protease-activated G protein-coupled receptor 1 (PAR-1) 38-60 peptide (k_cat/K_m = 4 × 10⁷ M^-1s^-1 to 1.2 x 10⁶ M^-1s^-1). Flla-MT67 showed a weaker platelet activating capacity, attributed to a defective PAR-1 interaction, whereas the interaction with glycoprotein lb was normal. A drastic decrease (up to 500-fold) of the second-order rate constant pertaining to heparin cofactor II (HCII) interaction, especially in the presence of dermatan sulfate, was found for the Flla-MT67 compared with Flla-WT, suggesting a severe impairment of thrombin inhibition by HCII in vivo. Finally, the Arg67His mutation was associated with a 5-fold decrease of prothrombin activation by the factor Xa-factor Va complex, perhaps through impairment of the prothrombin-factor Va interaction. These experiments show that the Arg67His substitution affects drastically both the procoagulant and the anticoagulant functions of thrombin as well as its inhibition by HCII. The mild hemorrhagic phenotype might be explained by abnormalities that ultimately counterbalance each other.