The Intrinsic Pathway in Hemostasis and Thrombosis

Intrinsic pathway mediated amplification of coagulation

The intrinsic pathway of coagulation is involved in both physiological hemostasis and pathological thrombosis [1]. Individuals with congenital deficiencies in  FVIII or FIX (known as hemophilia A and B) have a severe bleeding phenotype whereas individuals with a congenital deficiency for FXI can present with a relatively mild bleeding phenotype [1]. It has been proposed that crosstalk between the extrinsic coagulation pathway (TF:FVIIa) and the intrinsic pathway contribute to these phenotypes [1]. TF:FVIIa has been shown to activate FIX whereas thrombin, generated initially by the extrinsic pathway,  can activate FXI.

We have sought to determine the contribution of these pathways to activation of coagulation using transgenic mouse models. In a novel mouse whole blood thrombin generation assay we observed reduced TF-initiated thrombin generation in FXI deficient and FIX deficient mice [2]. This observation was consistent with the involvement of both TF:FVIIa-mediated activation of FIX and thrombin-mediated activation of FIX in the activation of coagulation. In a complementary approach we assessed the effect of combining FIX or FXI deficiency with low levels of TF expression in mice. We found that, under conditions of lowTF,  FIX deficiency, but not FXI deficiency, resulted in impaired survival [3]. This indicated that TF:FVIIa mediated activation of FIX, but not thrombin mediated activation of FXI, plays a critical role in maintenance of adult hemostasis.

Contribution of FXII and FXI to venous thrombosis

Current anticoagulants are effective in preventing thrombosis but are associated with a significantly increased risk of bleeding. FXII and FXI are attractive targets for the development of novel anticoagulants as deficiencies in these proteins are not associated with a bleeding phenotype, in the case of FXII, or associated with a mild bleeding phenotype, in the case of FXI. A number of agents targeting are under clinical evaluation for prevention of pathological coagulation, in particular venous thrombosis.

There have been conflicting reports in the literature regarding the relative contribution of FXII and FXI deficiency to venous thrombosis in mice [1]. To address this we conducted a comprehensive evaluation of the contribution of FXII and FXI to venous thrombosis in mice in multiple preclinical models [4]. In the inferior vena cava stenosis model and the saphenous vein laser injury model we observed an equivalent degree of protection from thrombosis in FXII-deficient and FXI-deficient mice compared to controls [4]. These findings reinforced the important contribution of both FXII and FXI to venous thrombosis formation suggesting that agents targeting either of these factors could function as effective anticoagulants.