Prenatal Exome After a Normal Microarray: Deciding What to Report
A couple at 22 weeks' gestation has a fetus with multiple anomalies on ultrasound (cardiac defect, shortened long bones, and echogenic bowel). Chromosomal microarray on amniocytes is normal. The couple consents to prenatal exome sequencing. The laboratory returns a likely pathogenic variant in a gene associated with a severe, childhood-onset neurodegenerative condition, inherited in an autosomal recessive pattern. Only one variant is found; a second cannot be detected. Separately, the lab identifies a variant of uncertain significance in a different gene related to adult-onset cardiomyopathy, and a carrier status for a recessive condition unrelated to the phenotype. The couple is overwhelmed and asks what all of this means for the pregnancy and for future children.
1. How do you interpret and communicate the single likely pathogenic variant in an autosomal recessive gene?
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Reasoning:
One variant is not a diagnosis. For an autosomal recessive condition, two pathogenic variants (in trans) are needed. A single likely pathogenic variant may be incidental, may be part of a missed second variant (e.g., in a region not covered, a deletion, or a deep intronic change), or may not explain the phenotype.
Explain the uncertainty honestly: The result is suggestive but not confirmatory. Discuss options: targeted deletion/duplication analysis, RNA studies, or reanalysis, depending on the gene and lab capability.
Phenotype match matters: Ask whether the fetal findings are consistent with the gene's known spectrum. If not, the variant may be unrelated.
Implications for future children: If the couple are both carriers, recurrence risk is 25%. Carrier testing of the partner can clarify. If only one parent carries, the finding may be de novo or mosaic.
Avoid overpromising: Do not tell the couple the fetus 'has' the condition. Frame as 'a finding that raises concern but does not yet establish a diagnosis.'
Document and offer follow-up with the laboratory and a multidisciplinary team.
2. What is your approach to the secondary findings (adult-onset cardiomyopathy VUS and unrelated carrier status) in the prenatal setting?
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Key points:
- Prenatal secondary findings are handled differently from adult genomics. The focus is the fetal phenotype. Adult-onset findings in the fetus are generally not reported unless the parents opted in under a specific policy, and even then, a VUS in an adult-onset gene is usually not reported because it is not actionable in a fetus.
- Carrier status for an unrelated recessive condition may be reported to parents because it can inform reproductive planning, but this depends on the lab's policy and consent. Explain that carrier status does not affect the fetus's health.
- Consent and expectations: Review what the couple agreed to. Many prenatal exome consent forms specify that only findings related to the indication are returned.
- Psychosocial care: The volume of information is itself a burden. Offer to stage the discussion, prioritize the phenotype-relevant result, and schedule follow-up.
- Avoid conflating fetal and parental findings: A fetal carrier result implies parental carrier status; handle with care and offer parental testing if relevant.