Factor XIII is also known as fibrin-stabilizing factor or fibrinoligase. Factor XIII is converted to its active form (XIIIa) by thrombin in the presence of Ca2+. The reaction catalyzed by factor XIIIa is the formation of γ-glutamyl-ε-lysine bonds between fibrin monomers as well as between fibrin and α2-plasmin inhibitor. Other proteins such as fibronectin, actin and myosin have been shown to be substrates for factor XIIIa-induced cross-linking. Factor XIII is a heterotetramer composed of two A chains and two B chains (A2B2) held together by non-covalent bonds. The active site of the complex is within the A subunits and the B subunits are thought to stabilize the A subunits or serve as plasma carrier subunits. Thrombin cleaves a small peptide from the A chains which activates the transamidase activity.
The gene for the A subunit (symbol F13A1) is found on chromosome 6p25.3–p24.3 spanning more than 160 kb and it is composed of 17 exons encoding a 732 amino acid preproprotein. The B subunit gene (symbol F13B) is on chromosome 1q31–q32.1 and is approximately 28 kb in length comprising 13 exons that encode a 661 amino acid preproprotein. Deficiency of factor XIII is inherited as an autosomal recessive trait and can be due to mutation in either the A or B subunit genes.
Deficiency in factor XIII is characterized by delayed bleeding even though primary hemostasis is normal. Typical symptoms include neonatal bleeding from the umbilical cord, intracranial hemorrhage, soft tissue hematomas, recurrent spontaneous miscarriage and abnormal wound healing. Umbilical cord bleeding after birth occurs in over 90% of afflicted individuals. Intracranial bleeding occurs in about 25% of individuals and is the leading cause of mortality in factor XIII deficiency. Patients who lack both the A and B subunits of factor XIII exhibit what is referred to as type I factor XIII deficiency. Most patients with factor XIII deficiency only lack functional subunit A protein with a frequency of around 1 in 5 million individuals. This form of the disorder is referred to as type II factor XIII deficiency. Factor XIII deficiency is normally treated with fresh frozen plasma, cryoprecipitate or crude factor XIII concentrate from placenta.