Disruption of a C/EBP binding site in the factor IX promoter is associated with haemophilia B

HAEMOPHILIA B (or Christmas disease) is an inherited, X-linked bleeding disorder caused by mutations in the gene for clotting factor IX. There is a rare class of patients, exemplified by haemophilia B Leyden¹, who suffer from haemophilia B as children but improve after puberty. In these patients, plasma factor IX concentrations are less than 10% of normal during childhood, but after puberty they gradually rise to between 40 and 80% of normal. Mutations clustered around the main transcription start point (defined as +1 (ref. 2)) have been reported in seven of these patients (at -20 (refs 1, 3, 4); -6 (refs 5, 6) and +13 (refs 7, 8)). To determine how these mutations interfere with factor IX expression, we have assayed for transcription factors binding to this area and have identified a nuclear factor-1 liver (NF1-L) binding site⁹ (-99 to -76) and a binding site for the CCAAT/enhancer binding protein (C/EBP)¹⁰ (+1 to +18). We show that the A --> G mutation at +13 prevents the binding of C/EBP to this site. Furthermore, we show that C/EBP is capable of transactivating a cotransfected normal factor IX promoter but not the mutant promoter. This is the first natural mutation to be reported which disrupts a C/EBP binding site and is an illustration of the importance of this transcription factor in humans.