Phenotypic characterization of haemophilia B - Understanding the underlying biology of coagulation factor IX.

Haemophilia B is a recessive, X-linked bleeding disorder due to inherited deficiency in vitamin K-dependent coagulation factor IX (FIX). FIX activity levels, as a basis for the definition of disease severity, do not clearly correlate with bleeding phenotype, likely due to the multiple steps regulating coagulation. Timely, with the availability of extended half-life products and successful steps in gene therapy, haemophilia B therapy is in an active developmental phase. Therefore, increased knowledge of the factors contributing to the variation of haemostatic and clinical outcome and response to therapy is welcomed. FIX acts at the crossroads of both the extrinsic and intrinsic pathways, and on the platelet procoagulant membrane at the site of vascular injury, and therefore, FIX biology is targeted for multiple effectors and regulators. The synthesis, cellular and molecular interactions, and elimination routes of FIX are not as well studied as for FVIII. The specific roles of magnesium in both platelet adhesion and FIX activation, and of vascular collagen at the haemostatic site of platelet adhesion and FIX residence are of particular interest. Biochemical and translational research on these issues should improve our understanding of the mechanisms involved, leading to the development of relevant assays that measure both haemostasis and treatment response. The latter is becoming increasingly important in the new era of haemophilia management and ultimately may lead to improved treatment strategies individually tailored to a patient's needs and cost-efficiency.