Hemophilia B, also known as factor IX deficiency or Christmas disease, is an X-linked recessive bleeding disorder caused by defects in the vitamin K-dependent enzyme factor IX of the clotting cascade. Factor IX is also known as the Christmas factor, hence the association of hemophilia B with the term Christmas disease. Activated Factor IX (factor IXa) is the enzyme responsible for the activation of factor X to Xa in a reaction referred to as the "tenase" complex. A cofactor in this reaction is factor VIII, deficiencies in which result in hemophilia A. The factor IX gene (symbol: F9) resides on chromosome Xq27.1-q27.2 and is composed of 8 exons that encode a 461 amino acid preproprotein. The F9 gene resides near the fragile X locus. The gene spans 34 kilobases (kb) and is composed of 8 exons. Multiple mutations have been identified leading to hemophilia B. A complete compilation of all of the mutations can be found at the hemophilia B database. Hemophilia B results in a bleeding disorder that is clinically indistinguishable from the more common hemophilia A. Hemophilia B occurs in approximately 1 in 30,000 male births.
The major symptom of hemophilia B is spontaneous bleeding into the joints and the soft tissues of the body. The disease exhibits three levels of manifestation. The severe disease results in patients with <1% of normal factor IX protein in the circulation, moderate disease results from 1%–5% of normal factor IX protein and mild disease is found when patients have 6%–30% of normal factor IX. The most common clinical symptom of hemophilia B that requires therapeutic intervention is soft tissue hemorrhage and hemarthroses (bleeding in the joints). The leading cause of morbidity and mortality in hemophilia B patients is intracranial hemorrhage. Severely affected patients are usually diagnosed before age 1. The most effective treatment for hemophilia B patients is the administration of highly purified factor IX concentrates or recombinant factor IX.