A Family Pedigree with an Ambiguous Variant
A couple seeks counseling. The woman is 34 and healthy; her brother has a severe early-onset neurodegenerative disorder diagnosed at age 8, and their father died at 52 with the same diagnosis. The mother is healthy at 70. The woman is pregnant and wants to know the risk to her child. Genetic testing finds a missense variant in a gene associated with autosomal dominant inheritance of this disorder, but the variant is classified as a "variant of uncertain significance" (VUS) in the public database. The woman tests positive for the VUS; her affected brother also carries it; her healthy mother does not. The woman asks whether her fetus should be tested and whether the VUS "proves" her child is at risk.
1. Construct the most likely mode of inheritance from the pedigree and explain how the VUS result should be interpreted in light of it.
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The pedigree shows an affected father and an affected son (the woman's brother), with a healthy mother. Affected individuals in two consecutive generations, with an unaffected mother, is most consistent with autosomal dominant inheritance with the father transmitting the variant. The fact that the father died at 52 with the same diagnosis and the brother was diagnosed at 8 suggests variable age of onset or anticipation, but dominant transmission fits: each child of an affected parent has a 50% chance of inheriting the variant.
However, the VUS must be interpreted cautiously. A VUS is not evidence of pathogenicity; it is a variant whose effect is unknown. The fact that the affected brother and the woman both carry it is consistent with segregation, but a single family with two carriers (one affected, one unaffected at 34) does not establish causation - the woman could be a non-penetrant carrier, or the variant could be a benign polymorphism in linkage with the true cause. Co-segregation in one family is weak evidence. The correct interpretation: the pedigree strongly suggests dominant inheritance, but the VUS does not "prove" pathogenicity. Additional evidence (segregation in more affected relatives, functional studies, population frequency, in silico predictions) is needed to reclassify it.
2. The woman asks whether her fetus should be tested. Outline the counseling points you would give, including the limits of testing a VUS and the option of testing the affected brother instead.
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Key counseling points:
The VUS is not actionable as a diagnosis. Testing the fetus for a VUS can produce a result that cannot be interpreted - a positive result would not confirm disease, and a negative result would not rule it out. This uncertainty must be explained before any testing.
Clarify the true cause first if possible. The most useful step is to test the affected brother (and, if available, stored samples from the affected father) more comprehensively - for example, exome or genome sequencing - to identify a pathogenic variant. If a pathogenic variant is found in the affected relatives and absent in the healthy mother, the woman can be tested for that specific variant, and only then would prenatal testing be informative.
Explain the 50% prior risk if the condition is dominant and she carries the causal variant, while noting that the VUS status means her actual risk is uncertain until the variant is resolved.
Discuss options: prenatal diagnosis (chorionic villus sampling or amniocentesis) if a causal variant is identified; preimplantation genetic testing; or deferring fetal testing until the variant is clarified. Emphasize that testing a VUS prenatally could lead to difficult decisions based on uninterpretable information.
Offer genetic counseling resources and psychosocial support, and document the discussion. The bottom line: do not treat the VUS as proof; pursue variant resolution in affected relatives first.