A Negative Carrier Test in an X-linked Dystrophy Family
A couple's 6-year-old son has proximal muscle weakness, calf pseudohypertrophy, and a creatine kinase level of 18,000 U/L. MLPA of his DMD gene shows a deletion of exons 45–52. The mother's brother died at 19 years of age with the same clinical diagnosis. The mother's CK is 320 U/L (reference <200). MLPA on her blood DNA does not detect the deletion. She is 12 weeks pregnant with a male fetus. The laboratory reports that 5–10% of apparently non-carrier mothers of isolated DMD boys have germline mosaicism, and that 65–70% of DMD cases are inherited. The father has no family history and his CK is normal. The couple wants to know whether the mother is a carrier and whether the male fetus is affected.
1. Construct the most likely pedigree and explain how the mother could be a carrier despite a negative blood test.
Read the worked answer
Pedigree: the maternal grandmother is an obligate carrier because she has an affected son, the mother's brother. The mother had a 50% prior chance of inheriting the deletion, and her affected son confirms transmission in her branch. Her CK of 320 U/L is mildly elevated, which is consistent with carrier status but not diagnostic. A negative MLPA on blood DNA can occur with germline mosaicism: the deletion is present in some oocytes but not in blood leukocytes, or it is present below the assay's detection limit. Skewed X-inactivation can also modify CK and muscle symptoms. Therefore the negative blood test does not exclude carrier status.
2. Calculate the risk that the male fetus is affected, stating your assumptions.
Read the worked answer
Use a Bayesian calculation. Prior P(carrier) from her mother is about 0.5. If she is a carrier, the probability of an affected son is 0.5; if she is not, a new mutation in the son is possible but unlikely when the brother is also affected. After the affected son, the posterior carrier probability is roughly 0.6–0.7. A negative blood MLPA has a sensitivity of about 90–95% for carriers; allowing for 5–10% germline mosaicism, the posterior carrier probability falls to roughly 10–20%. If she is a carrier, a male fetus has a 50% chance of inheriting the deletion. Therefore the fetal risk is roughly 5–10%, not zero. The exact figure depends on the assumed prior and test sensitivity. Because the risk is nonzero, prenatal testing is indicated.
3. Recommend prenatal testing and describe how the result should be interpreted if the fetus carries the deletion.
Read the worked answer
Offer chorionic villus sampling at 11–14 weeks or amniocentesis after 15 weeks, with MLPA or deletion/duplication analysis of fetal DNA; fetal sex can be determined by ultrasound or cell-free DNA. If the male fetus carries the same deletion, the diagnosis is essentially certain, although phenotype can vary. If the deletion is absent, the residual risk is low but not zero because of technical limitations and possible other mutations. Genetic counselling should cover the 5–10% germline mosaicism, the option of preimplantation diagnosis in future pregnancies, and the fact that a negative maternal blood test does not guarantee a non-carrier fetus.