Recombinant factor XIII and congenital factor XIII deficiency: an update from human and animal studies Aida InbalThrombosis and Hemostasis Unit, Hematology Institute, Beilinson Hospital, Rabin Medical Center, Petach Tikva, and Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, IsraelAbstract: Factor XIII (FXIII) is a protransglutaminase composed of two catalytic A subunits and two carrier B subunits. An intracellular form of FXIII is present in monocytes/macrophages and platelets as a homodimer of two A subunits. Following activation by thrombin, FXIII becomes plasma transglutaminase, which crosslinks γ-glutamyl-ε-lysine residues of fibrin chains and thereby stabilizes the fibrin clot. FXIII deficiency results in a moderate to severe hemorrhagic disorder, abnormal wound healing in about 30% of patients, and recurrent abortion in homozygous females. More than 800 cases of FXIII deficiency have been reported, most of them due to mutation in the FXIII-A gene, resulting in FXIII-A deficiency. Among mutations causing FXIII-A deficiency, 50% are missense mutations. Only 16 mutations in the FXIII-B gene have been published. Routine laboratory tests are normal in patients with FXIII deficiency, and the diagnosis is established by demonstration of decreased FXIII activity and antigen. Plasma-derived, virus-inactivated factor XIII concentrate is the treatment of choice. The low plasma levels of FXIII (about 5%) required to control bleeding and its long half-life make monthly prophylactic therapy feasible. Recently, recombinant FXIII concentrate with a half-life similar to that of native FXIII has been developed and tested in a multinational clinical study. This new product appears to be safe and appropriate for lifelong prophylactic treatment of patients with FXIII-A deficiency.Keywords: recombinant FXIII concentrate, FXIII deficiency