Two affected brothers, a normal mother and a negative DNA test
Two unaffected parents bring their two sons, aged 6 and 3, to a district hospital. Both boys bruise easily and have prolonged bleeding after minor cuts; the older boy had joint swelling after a fall. Results: platelet count normal, bleeding time normal, prothrombin time (PT) normal, activated partial thromboplastin time (aPTT) markedly prolonged, and factor VIII activity 4% in the older boy and 2% in the younger. The mother's aPTT and factor VIII level are normal. Her brother has a similar bleeding history and uses factor VIII concentrate; her father and paternal uncle are unaffected. The father's family has no bleeding history. The couple asks why both sons are affected, the chance for a future child, and whether their 8-year-old daughter could pass the condition to her children. The mother's leukocyte DNA is then tested, and no F8 mutation is found.
1. Deduce the most likely mode of inheritance. Explain how PT, aPTT and factor VIII results support it, and assign genotypes to the parents and sons.
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Isolated prolongation of aPTT with normal PT points to the intrinsic pathway, and a low factor VIII with a normal platelet count and bleeding time identifies haemophilia A. The pedigree is consistent with X-linked recessive inheritance: only males are affected, the mother's brother is affected, the father's family is clear, and the mother is clinically unaffected.
PT is normal because factors VII, X, V, II and fibrinogen are intact; aPTT is prolonged because factor VIII is needed for the tenase complex. The father contributes his Y to each son, so his X-linked status does not affect his sons; the mother contributes the X. If the mother is a carrier, her genotype is X^H X^h, the father is X^H Y, and each son is X^h Y. The older son's 4% and the younger son's 2% factor VIII are both severe.
The mother's normal factor VIII does not exclude carriage: X-inactivation is random, and about half of her liver endothelial cells may express the normal allele, giving roughly half-normal plasma activity; some carriers have normal levels. The daughter has a 1/2 chance of being a carrier if her mother is a carrier. The negative leukocyte DNA test changes this calculation, as discussed next.
2. The mother's leukocyte DNA shows no F8 mutation, yet both sons carry the same mutation. Explain how this is possible and revise the counselling for future children and for the daughter.
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The most likely explanation is germline mosaicism: the mutation occurred in a cell ancestral to part of the mother's germ line, so ova can carry it while blood leukocytes do not. A postzygotic mutation in the mother's early development can also produce low-level somatic mosaicism that is missed in blood. The finding does not mean the mutation arose independently twice, because both sons carry the same mutation; recurrence via mosaicism is more economical.
The empirical recurrence risk for a future son is therefore not 1/2 but a few percent (commonly quoted around 5–10% for haemophilia A). A daughter's carrier risk is in the same few-percent range, not zero. Offer targeted testing of the daughter for the familial F8 variant once the sons' mutation is characterised; if she is negative, her risk is very low, though not absolutely zero because of the possibility of low-level mosaicism. For future pregnancies, offer prenatal diagnosis by chorionic villus sampling with F8 testing, and discuss preimplantation genetic testing. Also check the mother's factor VIII before any surgery.
3. Why is the mother's factor VIII normal, and what clinically important point follows for carrier women?
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Factor VIII is encoded on the X chromosome. In a carrier female, one X carries a normal allele and the other a mutation. X-chromosome inactivation in each cell is random, so about half of her cells express the normal allele. Because factor VIII is secreted into plasma, the normal gene product from the active-X-normal cells can circulate and partly compensate; many carriers therefore have factor VIII in the normal range.
However, if inactivation is skewed against the normal allele, a carrier can have low factor VIII and a bleeding tendency. This is why a normal factor VIII does not rule out carrier status and why carrier women should have factor VIII measured before surgery, dental extraction or childbirth, with desmopressin or factor replacement if low. In this family, the mother's normal level is compatible with carrier status and is not evidence against the pedigree. The same reasoning applies to the daughter if she proves to be a carrier.