Platelet factor V to the rescue

In this issue of Blood, Duckers and colleagues show that residual platelet factor V is sufficient to promote thrombin generation and likely protects against major bleeding in patients with undetectable plasma factor V.

Blood coagulation factor V (FV) plays an essential role in hemostasis and has a profound influence on the production of thrombin. Factor V deficiency (parahemophilia) is an autosomal recessive bleeding disorder first described in the 1940s by Paul Owren in Norway.¹  Deficiency in FV presents a conundrum because the phenotype is variable and unexpectedly poorly correlates with FV levels in plasma.²  Patients with undetectable levels of FV (< 1%) due to homozygous nonsense, frameshift, or missense mutations exhibit the gamut of phenotypes from asymptomatic to severe bleeding. These observations make no sense considering the fundamental role played by FV in coagulation and with findings in the FV knockout mouse, which have a lethal phenotype.³ 

In the current study, Duckers et al show how another source of FV may provide new insight into this paradox.⁴  In whole blood, FV is divided between 2 pools; approximately 80% circulates in plasma while the remaining 20% is found within the α-granules of platelets. Platelet FV is largely derived from endocytosis from the plasma pool, yet it appears to have unique physical and functional characteristics that render it more procoagulant than its plasma counterpart.⁵  In this issue of Blood, the authors evaluate 3 patients with severe FV deficiency, each having undetectable FV activity in plasma. Using a thrombin generation assay with a range of tissue factor concentrations, platelet-poor plasma from each of the patients failed to generate any thrombin. In marked contrast, platelet-rich plasma produced a robust thrombin signal, and preactivation of the platelets with collagen or with a calcium ionophore further enhanced thrombin generation. Thrombin generation could be inhibited with an anti-FV antibody or activated protein C, suggesting that small amounts of FV released from platelets must be responsible. In support of this, the authors were able to detect platelet FV antigen and activity and were able to immunoprecipitate FV from a concentrated platelet suspension.

Do these residual amounts of platelet FV fully explain the whole story? Earlier simulation data show that depending on the tissue factor stimulus, very low levels of FV (< 1%) can indeed generate substantial amounts of thrombin.⁶  However, it turns out that the picture in these FV-deficient patients appears to be more complicated. In the current study and in previous work published in Blood, the authors show that FV-deficient patients also have reduced levels of tissue factor pathway inhibitor (TFPI).^4,7  These low levels of TFPI substantially decrease the FV requirement for minimal thrombin generation. Using platelet-rich plasma from the FV-deficient patients, normalization of plasma TFPI levels profoundly reduced the amount of thrombin formed.

Both of these significant findings help clarify our understanding of severe FV deficiency and also highlight the important role that platelet FV plays in promoting thrombin generation at the activated platelet surface. What remains uncertain, however, is whether residual platelet FV levels correlate with the severity of bleeding in severe FV-deficient patients. Although there has been speculation on this correlation,⁸  no systematic study has been done. Furthermore, mechanistic studies are needed to investigate some remaining questions. The first relates to the origin of residual platelet FV in these severe FV-deficient patients. Are megakaryocytes able to synthesize a very small amount of FV that is protected in the platelet environment or, alternatively, do megakaryocytes have an extremely efficient system to endocytose most of the FV that is being synthesized form the liver in these patients? Recent studies have shown that the majority of platelet FV is endocytosed from the plasma pool by megakaryocytes via a specific receptor-mediated process.⁵  However, this system has yet to be evaluated in sufficient detail to know whether it is efficient enough to take up FV in the context of plasma levels less than 1%. A final area that needs further investigation is an understanding of why TFPI levels are decreased in severe FV deficiency. Although FV was shown to bind to TFPI, it is currently unknown whether FV actually protects TFPI from proteases or other clearance mechanisms. Overall, the quality work of Duckers et al underscores the complexity of a seemingly simple deficiency of one clotting factor and reminds us that a balance of multiple components ultimately contribute to the clinical phenotype.