SOMATIC MOSAICISM AND FEMALE-TO-FEMALE TRANSMISSION IN A KINDRED WITH HEMOPHILIA-B (FACTOR-IX DEFICIENCY)

Studies have shown that hemophilia B (Christmas disease; factor IX deficiency) results from many different mutations in the factor IX gene, of which >95% are single nucleotide substitutions. This study has identified a previously unreported form of hemophilia B in a patient who was somatic mosaic for a genuine-to-cytosine transversion at nucleotide 31,170 in the factor IX gene. This point mutation changes the codon for residue 350 in the catalytic domain of factor IX from a cysteine to a serine. We used differential termination of primer extension to confirm and measure the degree of mosaicism. Our study shows that a varying proportion of cells from hepatic, renal, smooth muscle, and hematopoietic populations possessed normal as well as mutant factor IX sequences. These results indicate that the mutation in this patient occurred either as an uncorrected half-chromatid mutation in the female gamete or as a replication or postreplication error in the initial mitotic divisions of the zygote preceding implantation. In addition, this kindred also contains two females in successive generations who have moderately severe factor IX deficiency. The molecular pathogenesis of this latter phenomenon has been studied and seems to relate to the unaccompanied expression of the mutant factor IX gene consequent upon a second, as yet undefined, genetic event that has prevented inactivation of sequences including the mutant factor IX gene on the X chromosome inherited from the affected male.