Summary
Patients with the haemophilia B Leyden phenotype show a distinct pattern of factor
IX expression characterized by a post-pubertal increase in FIX levels and the remission
of clinical symptoms in adult life. This phenotype has previously been linked to single
base mutations within transcription factor binding sites in a region of ∼40 bp around
the major start point of transcription of the FIX gene. Here we report a novel mutation
in this region within the transcription factor C/EBP binding site at +1 to +18. The
mutation is a single base pair deletion from a triplet of thymine residues at +6 to
+8. We show that the extent to which this mutation disrupts the binding of C/EBP to
its binding site is less marked than the disruption caused by the +13 A→G mutation
of FIX Norwich (1). This correlates with age-matched phenotypic data we have available
for the patient reported here and that of FIX Norwich.
