Hemophilia B results from a mutation of the F.IX B gene, causing a decrease in the ability of the F.IX protein to function in coagulation. At the age of eighteen months, a female child (individual II.2 in the pedigree in FIGURE 3-11) fell while playing with her fraternal twin sister and hit her knee on a table leg. She suffered a serious bleeding event in the injured knee joint that required hospital therapy. At that time, she was found to have a F.IX level 2 percent of normal and was diagnosed as being a female with hemophilia B. She required therapy with recombinant F.IX. The family history indicated that her father (individual I.1 in the pedigree) also has the bleeding disease hemophilia B. Because his F.IX activity is less than 1 percent of normal levels, he requires therapy with human recombinant F.IX produced using genetic engineering technology in animal cells grown in tissue culture. If not treated, he suffers from spontaneous bleeding into his joints (hemarthrosis), which ultimately can result in severe joint damage. He is also at risk for life-threatening hemorrhages in his brain. His wife (individual I.2) has no bleeding tendencies and has normal levels of F.IX (about 100 percent of normal). She has no family history of bleeding diseases. The parents have two offspring (II.1 and II.3) in addition to II.2. The older sibling (II.1) is a male with normal levels of F.IX. The younger siblings are female fraternal (not identical) twins. The sister (II.3) has not suffered from bleeding and has a F.IX level of 60 percent, which is sufficient to provide normal coagulation. (Bleeding is usually only noted when F.IX levels are below about 20 percent of normal.) As far as could be determined, the karyotype of all members of the family (including II.2) was normal.
FIGURE 3-11
Discussion
The normal level of F.IX in the male sibling is expected as he would have had to inherit his father’s Y chromosome (to be male) and one of the mother’s two normal X chromosomes. However, because hemophilia B is a recessive sex-linked disorder, all daughters of an affected father are obligate carriers (both individuals II.2 and II.3). They must inherit their father’s single X chromosome, which carries the defective gene. Even as carriers, they would be expected to be normal (in terms of hemophilia B), having inherited one of their mother’s normal X chromosomes as well as their father’s mutated X chromosome. However, individual II.2 is not normal.
Female hemophiliacs do occur occasionally in families in which defects of the Factor IX gene are found. There are two general reasons this might occur.
1. Suppose the mother was actually not normal, but a carrier for hemophilia B. In such a case daughter II.2 would have received her father’s affected X chromosome and had a 50 percent chance of inheriting her mother’s single affected X chromosome. If she had, II.2 would be homozygous for a mutation in the F.IX gene and expected to be affected. In this case, this is unlikely because the mother’s family did not demonstrate a history of bleeding disease and her normal level of F.IX strongly argues against her being an unknown carrier. However, females with homozygous hemophilia are known to occur (although very rarely).
2. A more likely reason for the low level of F.IX in II.2 is nonrandom X chromosome inactivation. As noted in this chapter, about half the cells in a female should have the paternally derived X chromosome inactivated and half the maternally derived chromosome. However, random inactivation does not always occur. A number of defects in the structure of the X chromosome (such as certain deletions and other chromosomal abnormalities) can result in the faulty chromosome being selectively inactivated. In such a case, if the structurally defective chromosome was from the mother (and hence normal in terms of the F.IX gene), the father’s chromosome (carrying the hemophilia B gene) might be selectively expressed.
To determine whether nonrandom X inactivation did occur, the distribution of active maternal and paternal X chromosomes in the patient was examined using an X-linked SNP, which could differentiate the maternal and paternal X chromosomes of the patient. Ninety-seven percent of X chromosome product bearing the SNP in II.2 was from the paternal (affected) chromosome. In the unaffected twin sibling (II.3), the distribution was nearly random (about 40 percent was from the affected paternal chromosome).
Etiology and Pathogenesis
The most likely cause of female hemophilia in II.2 was extreme nonrandom X inactivation resulting in overrepresentation of the paternal X chromosome bearing a mutated Factor IX gene. However, this conclusion does not explain why this occurred in II.2 but not her sister, II.3. The great majority of female hemophiliacs result from unbalanced nonrandom X chromosome inactivation with no evidence of defects in the structure of the X chromosomes.
Questions
1. Suppose the mother in the family was a carrier of hemophilia B. How would this change the probability of disease in the offspring (and in future offspring)?
2. If the twins (II.2 and II.3) were identical, would this change your explanation for the occurrence of female hemophilia?
3. It is noted that recombinant human F.IX was used in therapy for affected individuals in the family. Why might recombinant human F.IX be preferable to F.IX isolated from pools of human blood?