Incidental Finding in a Prenatal Microarray With a Balanced Parental Karyotype
A 34-year-old G2P1 woman undergoes amniocentesis at 20 weeks for ultrasound findings of mild ventriculomegaly and a single umbilical artery. Chromosomal microarray on amniocytes reports a 620-kb deletion at 15q11.2 (BP1-BP2 region, including TUBGCP5, CYFIP1, NIPA1, NIPA2). Parental microarray testing shows the mother carries the identical deletion and is phenotypically normal. The laboratory classifies the finding as a variant of uncertain significance (VUS) with a note that it is maternally inherited. The couple asks whether this explains the ultrasound findings and whether the pregnancy is 'safe.'
1. How should the laboratory and clinician interpret a maternally inherited 15q11.2 BP1-BP2 deletion in the context of the fetal phenotype?
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The 15q11.2 BP1-BP2 deletion is a recurrent CNV with reduced penetrance and variable expressivity; it is enriched in cohorts with developmental delay, learning difficulties, and behavioral features but is also found in unaffected adults, including parents. Maternal inheritance from a phenotypically normal mother is strong evidence against this CNV being a highly penetrant cause of the fetal ventriculomegaly and single umbilical artery. It does not 'explain' the structural anomalies; those findings should prompt continued evaluation (detailed anatomy scan, consideration of additional genetic testing such as exome sequencing, and infectious/metabolic workup as clinically indicated). The correct communication is probabilistic: the CNV confers a modestly increased risk for neurodevelopmental features that cannot be quantified precisely, and it is not a diagnosis. Counseling should be nondirective and avoid language implying the pregnancy is either 'safe' or 'doomed.'
2. The report labels the CNV a VUS. Is that classification appropriate, and how would you reconcile it with the ACMG/ClinGen framework for CNVs?
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Calling it a VUS is defensible but arguably imprecise. Under the ClinGen/ACMG technical standards for constitutional CNVs, a recurrent, well-characterized CNV with established association but reduced penetrance is typically classified as a pathogenic or likely pathogenic copy-number loss with variable expressivity-not a VUS-provided the recurrence and gene content are well documented. The key distinction is that 'pathogenic' for a CNV does not mean 'fully penetrant' or 'diagnostic in this patient.' The laboratory should apply the semiquantitative CNV scoring (section criteria for gene content, inheritance, case-control evidence, and literature) and state the classification plus the penetrance caveat explicitly. The report should separate the technical classification (pathogenic/likely pathogenic CNV) from the clinical interpretation (low-penetrance susceptibility, does not account for the current phenotype). This avoids both over-reassurance and over-medicalization.
3. What quality and reporting safeguards apply to prenatal CNV interpretation when a parental sample is available?
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Safeguards include: (1) confirming the fetal CNV with an orthogonal method (e.g., targeted qPCR, MLPA, or FISH) and confirming parental status by the same validated platform to exclude a parental artifact; (2) documenting the array's validated resolution and the minimum probe count supporting the call (a 620-kb deletion should be supported by multiple consecutive probes and reported with genomic coordinates and genome build); (3) following ACMG/ClinGen reporting guidance for prenatal arrays, including a clear statement of penetrance and the limits of phenotype prediction; (4) ensuring the report distinguishes clearly between the technical classification and the clinical significance; (5) offering genetic counseling by a provider experienced in prenatal genetics; (6) maintaining a policy for reporting variants of uncertain significance in the prenatal setting, recognizing that some laboratories restrict prenatal VUS reporting to clinically actionable findings. The report should also note that a normal parental phenotype does not exclude a low-penetrance effect in the fetus.