Activated protein C cofactor function of protein S: a critical role for Asp95 in the EGF1-like domain

Protein S has an established role in the protein C anticoagulant pathway, where it enhances the factor Va (FVa) and factor VIIIa (FVIIIa) inactivating property of activated protein C (APC). Despite its physiological role and clinical importance, the molecular basis of its action is not fully understood. To clarify the mechanism of the protein S interaction with APC, we have constructed and expressed a library of composite or point variants of human protein S, with residue substitutions introduced into the Gla, thrombin-sensitive region (TSR), epidermal growth factor 1 (EGF1), and EGF2 domains. Cofactor activity for APC was evaluated by calibrated automated thrombography (CAT) using protein S–deficient plasma. Of 27 variants tested initially, only one, protein S D95A (within the EGF1 domain), was largely devoid of functional APC cofactor activity. Protein S D95A was, however, γ-carboxylated and bound phospholipids with an apparent dissociation constant (Kdapp) similar to that of wild-type (WT) protein S. In a purified assay using FVa R506Q/R679Q, purified protein S D95A was shown to have greatly reduced ability to enhance APC-induced cleavage of FVa Arg306. It is concluded that residue Asp95 within EGF1 is critical for APC cofactor function of protein S and could define a principal functional interaction site for APC

Inherited risk factors for venous thromboembolism

Venous thromboembolism (VTE) has important heritable components. In the past 20 years, knowledge in this field has greatly increased with the identification of a number of gene variants causing hypercoagulability. The two main mechanisms are loss-of-function of anticoagulant proteins and gain-of-function of procoagulants, the latter owing to increased synthesis or impaired downregulation of a normal protein or, more rarely, to synthesis of a functionally hyperactive molecule. Diagnosis of thrombophilia is useful to determine the causes of VTE, recognizing that this multifactorial disease can also be influenced by various acquired factors including cancer, surgery, trauma, prolonged immobilization, or reproduction-associated risk factors. Diagnosis of inherited thrombophilia rarely affects the acute or long-term management of VTE. However, the risk of recurrent VTE is increased in anticoagulant-deficient patients and in homozygotes for gain-of-function mutations. Screening for inherited thrombophilia in thrombosis-free individuals is indicated only for relatives of a proband who is anticoagulant-deficient or has a family history of VTE. In families with thrombophilia and VTE, primary antithrombotic prophylaxis during risk situations lowers the rate of incident VTE. In this Review, we discuss the main causes of inherited thrombophilia, the associated clinical manifestations, and the implications for antithrombotic prophylaxis in the affected individuals