Mild bleeding diathesis in a boy with combined severe haemophilia B (C¹⁰⁴⁰⁰→T) and heterozygous factor V Leiden

Haemophilia B patients with factor IX (FIX) activity < 1% are usually characterized by severe bleeding episodes early in life. We report a case of sporadic severe haemophilia B, clinically characterized by mild bleeding diathesis. The presence of anamnestic thrombophlebitis in the patient's mother prompted us to investigate a possible associated hypercoagulable condition. Resistance to activated protein C due to factor V R506Q mutation was present in the mother and in the propositus, in the homozygous and heterozygous form, respectively. Molecular analysis of the FIX gene led to the identification of a nonsense mutation resulting in a stop codon at position 50, previously described and usually responsible for a severe pattern of haemophilia B. The implications of this unusual association are discussed.