Moderate haemophilia B in a female carrier caused by preferential inactivation of the paternal X chromosome.

The case of a female with moderate haemophilia B is reported. She is the only affected member of her family, and factor IX RFLP analysis shows her to have inherited no maternal markers for polymorphisms located in the first intron and 8 Kb 3' of the polyadenylation signal (DdeI and HhaI, respectively). This clearly indicates a deletion involving at least the last 7 exons of the factor IX gene. Her other factor IX gene inherited from her healthy father is normal as her son is also healthy. This suggests the patient's haemophilia to be due to gross bias in the proportion of factor IX-producing cells with an inactive paternal X chromosome. Methylation studies on the 5' region of the PGK gene show that virtually all the patient's lymphocytes carry a hypermethylated and presumably an inactive paternal X chromosome. The reason for this bias in the activity of her two X chromosomes is not clear, as no chromosomal alterations were found.