Potential role of activated factor VIII (FVIIIa) in FVIIa/tissue factor-dependent FXa generation in initiation phase of blood coagulation

The role of proteolytic cleavage at Arg336 and Arg372 of the A1 domain in factor VIIa/tissue factor-catalyzed reactions of B domain-deleted factor VIII

BACKGROUND

We previously demonstrated that factor (F)VIII was rapidly activated through proteolytic cleavage at Arg372 and Arg740 by activated FVII (FVIIa)/tissue factor (TF) in very early coagulation phase, followed by inactivation by cleavage at Arg336. The influence of the absence of FVIII B domain in this series of FVIIa/TF-catalyzed reaction remains unclear, however.

AIM

To examine the FVIIa/TF-catalyzed reaction of B domain-deleted (BDD-)FVIII.

METHODS AND RESULTS

The FVIII activity (FVIII:C) of commercial full-length (FL-)FVIII and BDD-FVIII increased by ∼1.7-fold within 0.5 min after addition of FVIIa/TF (1 nM/0.1 nM).

FVIII

C decreased to initial levels with inactivation rate constant (k; ∼0.035) within 15 min of FL-FVIII activation, but decreased gradually to initial levels (k; ∼0.017) within 30 min of BDD-FVIII activation. SDS-PAGE analyses demonstrated that the FVIIa/TF-catalyzed cleavage of BDD-FVIII occurred at Arg336 within 0.5 min in parallel with elevation of FVIII:C, but cleavage at Arg372 was not evident. FVIIa/TF-catalyzed activation of both recombinant BDD-FVIII R336A and R372A mutants that were prepared, were similar to that of wild-type (WT) BDD-FVIII. However, FVIII:C returned to initial levels (k; ∼0.046) within 30 min of R336A mutant activation, but little reduction of FVIII:C was observed with R372A mutant (k; ∼0.0046). SDS-PAGE analysis indicated that FVIIa/TF-catalyzed cleavage of WT and R372A mutant was predominant at Arg336, whereas that of R336A mutant was observed at Arg372.

CONCLUSIONS

FVIIa/TF-catalyzed activation of BDD-FVIII was initiated by cleavage at Arg336, and the FVIII B domain appeared to control FVIIa/TF-catalyzed reactions by altering pattern of cleavage at Arg336 and Arg372.

Differences in wild-type- and R338L-tenase complex formation are at the root of R338L-factor IX assay discrepancies

Adeno-associated virus (AAV) gene therapy has the potential to functionally cure hemophilia B by restoring factor (F)IX concentrations into the normal range. Next-generation AAV therapies express a naturally occurring gain-of-function FIX variant, FIX-Padua (R338L-FIX), that increases FIX activity (FIX:C) by approximately eightfold compared with wild-type FIX (FIX-WT). Previous studies have shown that R338L-FIX activity varies dramatically across different clinical FIX:C assays, which complicates the monitoring and management of patients. To better understand mechanisms that contribute to R338L-FIX assay discrepancies, we characterized the performance of R338L-FIX in 13 1-stage clotting assays (OSAs) and 2 chromogenic substrate assays (CSAs) in a global field study. This study produced the largest R338L-FIX assay dataset to date and confirmed that clinical FIX:C assay results vary over threefold. Both phospholipid and activating reagents play a role in OSA discrepancies. CSA generated the most divergent FIX:C results. Manipulation of FIX:C CSA kits demonstrated that specific activity gains for R338L-FIX were most profound at lower FIX:C concentrations and that these effects were enhanced during the early phases of FXa generation. Supplementing FX into CSA had the effect of dampening FIX-WT activity relative to R338L-FIX activity, suggesting that FX impairs WT tenase formation to a greater extent than R338L-FIX tenase. Our data describe the scale of R338L-FIX assay discrepancies and provide insights into the causative mechanisms that will help establish best practices for the measurement of R338L-FIX activity in patients after gene therapy.

A Pathophysiological Perspective on the SARS-CoV-2 Coagulopathy

Recent evidence is focusing on the presence of a hypercoagulable state with development of both venous and arterial thromboembolic complications in patients infected with SARS-CoV-2. The ongoing activation of coagulation related to the severity of the illness is further characterized by thrombotic microangiopathy and endotheliitis. These microangiopathic changes cannot be classified as classical disseminated intravascular coagulation (DIC). In this short review we describe the interaction between coagulation and inflammation with focus on the possible mechanisms that might be involved in SARS-CoV-2 infection associated coagulopathy in the critically ill.

Novel Insights and New Developments Regarding Coagulation Revealed by Studies of the Anti-Factor IXa (Activated Factor IX)/Factor X Bispecific Antibody, Emicizumab

Emicizumab is a humanized anti-FIXa/FX (factor IXa/X) bispecific monoclonal antibody that mimics FVIIIa (activated factor VIII) cofactor function. The hemostatic efficacy of emicizumab has been confirmed in clinical studies of patients with hemophilia A, irrespective of the presence of FVIII inhibitors. Emicizumab differs in some properties from FVIIIa molecule. Emicizumab requires no activation by thrombin and is not inactivated by activated protein C, but emicizumab-mediated coagulation is regulatable and maintains hemostasis. A small amount of FIXa (activated factor IX) is required to initiate emicizumab-mediated hemostasis, whereas tissue factor/FVIIa (activated factor VII)-mediated FXa (activated factor X) and thrombin activation initiates FVIIIa-mediated hemostasis. Fibrin formation, followed by fibrinolysis, appears to be similar between emicizumab- and FVIIIa-mediated hemostasis. These results suggest possible future uses of emicizumab for treating hemorrhagic diseases other than hemophilia A and reveal previously unobservable behaviors of procoagulation and anticoagulation factors in conventional hemostasis. Here, we have reviewed novel insights and new developments regarding coagulation highlighted by emicizumab.