The factor V light chain mediates the binding and endocytosis of plasma-derived factor V by megakaryocytes

In vitro and ex vivo rescue of a nonsense mutation responsible for severe coagulation factor V deficiency

BACKGROUND

Coagulation factor V (FV) deficiency is a rare bleeding disorder that is usually managed with fresh-frozen plasma. Patients with nonsense mutations may respond to treatment with readthrough agents.

OBJECTIVES

To investigate whether the F5 p.Arg1161Ter mutation, causing severe FV deficiency in several patients, would be amenable to readthrough therapy.

METHODS

F5 mRNA and protein expression were evaluated in a F5 p.Arg1161Ter-homozygous patient. Five readthrough agents with different mechanisms of action, i.e. G418, ELX-02, PTC-124, 2,6-diaminopurine (2,6-DAP), and Amlexanox, were tested in in vitro and ex vivo models of the mutation.

RESULTS

The F5 p.Arg1161Ter-homozygous patient showed residual F5 mRNA and functional platelet FV, indicating detectable levels of natural readthrough. COS-1 cells transfected with the FV-Arg1161Ter cDNA expressed 0.7% FV activity compared to wild-type. Treatment with 0-500 μM G418, ELX-02, and 2,6-DAP dose-dependently increased FV activity up to 7.0-fold, 3.1-fold, and 10.8-fold, respectively, whereas PTC-124 and Amlexanox (alone or in combination) were ineffective. These findings were confirmed by thrombin generation assays in FV-depleted plasma reconstituted with conditioned media of treated cells. All compounds except ELX-02 showed some degree of cytotoxicity. Ex vivo differentiated megakaryocytes of the F5 p.Arg1161Ter-homozygous patient, which were negative at FV immunostaining, turned positive after treatment with all 5 readthrough agents. Notably, they were also able to internalize mutant FV rescued with G418 or 2,6-DAP, which would be required to maintain the crucial platelet FV pool in vivo.

CONCLUSION

These findings provide in vitro and ex vivo proof-of-principle for readthrough-mediated rescue of the F5 p.Arg1161Ter mutation.

Endocytosis of exogenous factor V by ex-vivo differentiated megakaryocytes from patients with severe parahaemophilia

Although human megakaryocytes can synthesize factor V (FV), platelet FV derives largely from endocytosis of plasma FV. Recently, it has been shown that plasma transfusions can replenish the platelet FV pool in parahaemophilic patients. Here we corroborate this finding by showing FV endocytosis by ex vivo differentiated megakaryocytes derived from patients with inherited parahaemophilia. Mononuclear stem cells isolated from peripheral blood of healthy subjects and of three patients with severe parahaemophilia were cultured in the presence of thrombopoietin and interleukin-3 and differentiated into CD41-positive polynucleated megakaryocytes. Exogenous purified FV was added to the culture medium to evaluate FV endocytosis. Immunofluorescence staining revealed abundant FV expression in megakaryocytes derived from healthy donors, but no FV expression in those derived from patients with severe parahaemophilia. However, after the addition of purified FV to the culture medium, megakaryocytes from parahaemophilia patients became positive upon FV immunostaining, suggesting endocytosis of exogenous FV. Endocytosed FV retained factor Xa-co-factor activity as assessed by a prothrombin time-based functional test in megakaryocyte lysates. Addition of exogenous FV to culture medium can restore the FV content of megakaryocytes derived from patients with severe FV defects. This rescue mechanism can have important clinical implications in the management of parahaemophilia patients.

Mechanisms Regulating Acquisition of Platelet-Derived Factor V/Va by Megakaryocytes

Factor Va serves as the nonenzymatic protein cofactor for the prothrombinase complex, which converts prothrombin to thrombin in the events leading to formation of a hemostatic plug. Several observations support the concept that platelet-derived factor V/Va is physically and functionally distinct and plays a more important role in thrombin generation at sites of vascular injury as compared to its plasma counterpart. Platelet-derived factor V/Va is generated following endocytosis of the plasma-derived molecule by the platelet precursor cells, megakaryocytes, via a two receptor system consisting of low density lipoprotein (LDL) receptor-related protein-1 (LRP-1) and an unidentified specific "binding site". More recently, it was suggested that a cell surface-expressed β-galactoside binding protein, galectin-8, was involved in factor V endocytosis. Endocytosed factor V is trafficked through the cell and retailored prior to its storage in α-granules. Given the essential role of platelet-derived factor Va in clot formation, understanding the cellular and molecular mechanisms that regulate how platelets acquire this molecule will be important for the treatment of excessive bleeding or clotting.

Coagulation function and mechanisms in various clinical phenotypes of patients with acquired factor V inhibitors

BACKGROUND

The clinical phenotype of individuals with acquired factor V (A-FV) inhibitors varies from asymptomatic (non-B group) to life-threatening bleeding (B group), but the mechanism(s) underlying this variation in hemorrhagic phenotype are poorly understood.

OBJECTIVE

To investigate coagulation mechanistically in a range of patients with A-FV antibodies.

METHODS AND RESULTS

Ten cases of A-FV inhibitors in the non-B (n = 5) and B groups (n = 5) were studied. Thrombin generation assays in these plasmas revealed little thrombin generation, despite similar FV activity levels in both groups. However, prothrombin time-based clot waveform analysis revealed that the clot times were significantly prolonged and the maximum velocity and acceleration of coagulation were lower in the B group than in the non-B group, suggesting that this technique might be useful for predicting and monitoring hemorrhagic symptoms. A-FV inhibitors from the non-B group recognized predominantly the FV heavy chain, whereas those from the B group recognized the light chain. Purified anti-FV autoantibodies (autoAbs) from the B group inhibited FV binding to phospholipid by 60-90%, whereas there was little effect on this reaction in the non-B group. In addition, anti-FV autoAbs from the non-B group impaired the activated protein C (APC) cofactor activity of FV in FVIIIa inactivation mechanisms, and delayed APC-catalyzed cleavage of FVa at Arg306, but not at Arg506, indicating the presence of APC resistance in the non-B group.

CONCLUSIONS

The results suggest that the different hemorrhagic phenotypes in A-FV inhibitors depend on the specific epitope of anti-FV autoAbs, and appear to be associated with an imbalance of procoagulant and anticoagulant function.