Extravascular factor IX: a semicentennial narrative review

Coagulation factor IX (FIX) is a serine protease that is crucial for normal hemostasis in vertebrates. Unlike other soluble vitamin K-dependent procoagulant enzymes, there is an "extravascular reservoir" of FIX in various tissues. The extravasation of FIX and its subsequent maintenance outside of circulation have long been attributed to its ability to bind collagen IV. This property is conferred by a short, specialized region of its phospholipid-binding gamma-carboxyglutamic acid (Gla) domain. Collagen IV is a nonfibrillar extracellular protein found primarily in basement membranes that assembles into sheet-like supportive structures around blood vessels and epithelia, providing a convenient scaffold for this "hemostatic barrier." Early seminal investigations from 1980 to 2000 suggested the existence of a large extravascular reservoir containing the majority of a vertebrate organism's FIX. The subsequent development of a FIX-deficient mouse model, identification of the amino acid residues responsible for FIX's collagen binding (K5 and V10), and development of enzyme mutants with high and low affinities for collagen IV afforded new insights into the hemostatic importance of this property from 2000 through 2020. Recent interest on the topic has been catalyzed by therapeutic implications for hemophilia treatment. For example, studies have examined the hemostatic and pharmacokinetic effects of enhanced collagen binding on the hyperactive FIX variant (Padua) currently in trials for hemophilia B gene therapy. In this review, we highlight research updates on extravascular FIX since 2020 and present remaining questions to help guide future investigations.