Thrombomodulin Arg403Lys mutation impairs protein C activation and increases thrombosis risk

Thrombomodulin (TM) is an important regulator in various physiological processes, including coagulation, fibrinolysis and inflammation. TM exerts its cofactor activities for protein C (PC) activation and thrombin-activatable fibrinolysis inhibitor (TAFI) activation through its binding with thrombin. Numerous studies have reported that mutations of TM are associated with thromboembolic diseases. Here we report a heterozygous substitution of arginine to lysine at amino acid residue 403 (Arg403Lys, R403K) within the fourth epidermal growth factor (EGF)-like domain of TM, which was identified in five unrelated patients with thrombophilia. To elucidate the role of the R403K mutation in TM, we constructed expression plasmids for both the full-length (residues Met1 to Leu575) and extracellular fragment (residues Met1 to Ser515) of TM to study its functions on surface of endothelial cells and in vitro system as well. We observed a reduced cofactor activity of PC activation by the TM-R403K variant across several assays and demonstrated a weakened interaction between the mutant and PC as well. These results indicate that the R403K mutation in TM induces detrimental alterations in the anticoagulant function of TM, which might elucidate the thrombotic predisposition observed in patients. This study provides further compelling evidence suggesting that mutation at the terminus of the EGF4 domain in TM may be associated with thrombophilia, thereby enhancing our comprehension of the physiological role of TM.