Summary
Haemophilia A carriers have historically been thought to exhibit normal haemostasis. However, recent data demonstrates that, despite normal factor VIII (FVIII), haemophilia A carriers ...demonstrate an increased bleeding tendency. We tested the hypothesis that obligate haemophilia carriers exhibit an increase in clinically relevant bleeding. A cross‐sectional study was performed comparing haemophilia A carriers to normal women. Questionnaire assessment included a general bleeding questionnaire, condensed MCMDM‐1VWD bleeding assessment tool and Pictorial Bleeding Assessment Chart (PBAC). Laboratory assessment included complete blood count, prothrombin time, activated partial thromboplastin time, fibrinogen activity, FVIII activity (FVIII:C), von Willebrand factor antigen level, ristocetin cofactor, platelet function analyser‐100TM and ABO blood type. Forty‐four haemophilia A carriers and 43 controls were included. Demographic features were similar. Laboratory results demonstrated a statistically significant difference only in FVIII:C (82·5 vs. 134%, P < 0·001). Carriers reported a higher number of bleeding events, and both condensed MCMDM‐1 VWD bleeding scores (5 vs. 1, P < 0·001) and PBAC scores (423 vs. 182·5, P = 0·018) were significantly higher in carriers. Haemophilia A carriers exhibit increased bleeding symptoms when compared to normal women. Further studies are necessary to fully understand the bleeding phenotype in this population and optimize clinical management.
Activation of coagulation factor XI (FXI) may play a role in hemostasis. The primary substrate of activated FXI (FXIa) is FIX, leading to FX activation (FXa) and thrombin generation. However, recent ...studies suggest the hemostatic role of FXI may not be restricted to the activation of FIX. We explored whether FXI could interact with and inhibit the activity of tissue factor pathway inhibitor (TFPI). TFPI is an essential reversible inhibitor of activated factor X (FXa) and also inhibits the FVIIa-TF complex. We found that FXIa neutralized both endothelium- and platelet-derived TFPI by cleaving the protein between the Kunitz (K) 1 and K2 domains (Lys86/Thr87) and at the active sites of the K2 (Arg107/Gly108) and K3 (Arg199/Ala200) domains. Addition of FXIa to plasma was able to reverse the ability of TFPI to prolong TF-initiated clotting times in FXI- or FIX-deficient plasma, as well as FXa-initiated clotting times in FX-deficient plasma. Treatment of cultured endothelial cells with FXIa increased the generation of FXa and promoted TF-dependent fibrin formation in recalcified plasma. Together, these results suggest that the hemostatic role of FXIa may be attributed not only to activation of FIX but also to promoting the extrinsic pathway of thrombin generation through inactivation of TFPI.
•Activated factor XI binds and proteolyzes tissue factor pathway inhibitor.•Activated factor XI promotes factor X activation generation and fibrin formation through the inactivation of tissue factor pathway inhibitor from platelets and on endothelial cells.
Inherited deficiency of the trypsin‐like protease factor (F) XI is associated with a mild to moderate bleeding diathesis. In most cases, FXI protein is reduced in plasma, and examples of ...dysfunctional circulating FXI variants are rare. We characterized the defect in one such variant with a proline to leucine substitution at residue 520. FXI Pro520 corresponds to chymotrypsin Pro161, and is conserved in most members of the chymotrypsin protease family. Recombinant FXI containing this substitution will be referred to as FXIP161L. kcat for cleavage of chromogenic substrates and for activation of the natural FXIa substrate FIX is ∼3‐fold lower for activated FXIP161L (FXIaP161L) than for wild‐type FXIa (FXIaWT), consistent with an abnormal protease active site. Inhibition of FXIaP161L by diisopropyl fluorophosphate is 2.4‐fold slower than for FXIaWT, suggesting distortion of the protease oxyanion hole. Binding to p‐aminobenzamidine, a probe for the integrity of the S1 substrate‐binding site, was similar for FXIaWT and FXIaP161L. Rates of carbamylation of Ile16 were also similar for FXIaWT and FXIaP161L, indicating that the critical salt bridge between Ile16 and Asp194 forms normally during protease activation. Cumulatively, the data demonstrate that Pro161 is required for normal active site oxyanion hole conformation in FXIa. Examination of the FXIa crystal structure and modeling studies indicate that Pro161 forms several hydrophobic contacts with adjacent amino acids that stabilize active site conformation. Leucine can be incorporated at position 161 in FXIa, but would not form the extensive stabilizing network of hydrophobic interactions formed by Pro161.
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