The Cys22-Cys27 disulfide bond of factor X (FX) protease domain is not conserved among coagulation factors and its contribution to the physiological haemostasis and implication in the pathogenesis of haemostatic and thrombotic disorders remain to be elucidated. Mutation p.Cys27Ser was identified in a pedigree of congenital FX deficiency and fluorescence labelling study of transiently transfected HEK293 cells showed accumulation of FX p.Cys27Ser within cell, indicating incompetent secretion partially responsible for the FX deficiency. The clotting activity of FX p.Cys27Ser was decreased to about 90% of wild-type, while amidolytic and pro-thrombinase activities (kcat/Km) determined with recombinant FXa mutant were 1.33- and 4.77-fold lower. Molecular dynamic simulations revealed no major change in global structure between FXa p.Cys27Ser and wild-type FXa; however, without the Cys22-Cys27 disulfide bond, the insertion of newly formed N terminal of catalytic domain after the activation cleavage is hindered, perturbing the conformation transition from zymogen to enzyme. The crystal structure of FXa shows that this disulfide bond is solvent accessible, indicating that its stability might be subject to the oxidation/reduction balance. As demonstrated with FX p.Cys27Ser here, Cys22-Cys27 disulfide bond may modulate FX clotting activity, with reduced FX pertaining less pro-coagulant activity. F.L. performed molecular dynamic simulations and analysis; C.C. performed protein expression collection and purification and enzyme kinetical analysis; S.Q. and M.Z. contributed to discussion and manuscript revision of MD results; X.W. performed clinical data acquisition; X.X. and L.L. performed phenotype and genetic analysis; X.W. and Q.D. supervised studies conducted in Ruijin Hospital, Shanghai Jiao Tong University; D.W. supervised studies in School of Life Sciences and Biotechnology, Shanghai Jiao Tong university; and Q.X. and W.W. designed experiments, analysed the data and wrote the manuscript. All authors approved the final version of this manuscript. ^* Fang Li and Changming Chen equally contributed to this work.

Background Venous thromboembolism (VTE) is predisposed by thrombotic mutations in patients with hereditary thrombophilia. Although prothrombin deficiencies caused by homozygous or compound heterozygous mutations are associated with bleeding diathesis, rare cases have shown a correlation between heterozygous prothrombin mutations and thrombosis. Materials and Methods We surveyed genetic variants involved in thrombosis and hemostasis in 347 patients with unprovoked VTE or having a positive family history of thrombosis. For patients identified with heterozygous prothrombin mutations, we conducted family investigations and performed a thrombin generation test (TGT) to elucidate the thrombotic risk. Novel mutants were expressed and subjected to functional assays to clarify the underlying thrombotic mechanisms. Results Heterozygous prothrombin mutations were identified in 3.5% of patients (12/347), including three novel mutations Phe382Ser, Phe382Leu, and Asp597Tyr found in one patient each, as well as previously reported Arg541Trp mutation in four patients and Arg596Gln mutation in five patients. A total of 42 mutation carriers were identified within the 12 pedigrees, among whom 64.3% (27/42) had experienced thrombotic events. TGT results demonstrated hypercoagulability for carriers of the five mutations, with Arg596Gln showing the highest thrombin generation potential followed by Arg541Trp. The Phe382-associated mutations severely impaired thrombomodulin-binding ability of thrombin, resulting in obviously reduced protein C (PC) activation. The Asp597Tyr mutation exhibited a mild reduction in both inactivation by antithrombin and PC activation reactions. Conclusion The presence of heterozygous prothrombin mutations represents a potential genetic predisposition for VTE. All thrombosis-associated mutations potentiate coagulation activity by either conferring antithrombin resistance and/or impairing PC pathway activity.

Key Points: Deleterious F8 mutations do not necessarily lead to the incidence of inhibitors in hemophilia A patients receiving replacement therapy. Maternal chimeric cells migrated into a fetus with hemophilia A during pregnancy could induce tolerance toward FVIII.