Tumor-Only Sequencing Reveals Suspected Germline TP53 Variant
A 42-year-old woman with high-grade serous ovarian carcinoma undergoes tumor-normal paired sequencing using a 500-gene panel. The tumor sample is from a paraffin-embedded omental biopsy with 30% tumor cellularity. The normal sample is peripheral blood. The pipeline reports a TP53 missense variant, c.733G>A (p.G245S), with a variant allele fraction (VAF) of 48% in the tumor and 47% in the blood. The patient has no personal history of other cancers, but her mother had breast cancer at 52 and her maternal aunt had ovarian cancer at 60. The variant is classified as pathogenic in ClinVar. The laboratory's policy is to report somatic variants with VAF ≥5% and to flag potential germline variants when VAF in tumor is ≥30% and in normal is ≥30%. The report is being finalized by a molecular genetic pathology fellow.
1. What is the most likely explanation for the VAF findings, and what confirmatory testing is indicated?
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The VAF of ~48% in tumor and ~47% in blood is consistent with a heterozygous germline TP53 variant. In a tumor sample with 30% tumor cellularity, a somatic variant would typically have a VAF much lower than 50% (e.g., if heterozygous in tumor cells, VAF ≈ 15% after accounting for stromal admixture). The near-50% VAF in both tumor and blood strongly suggests a germline origin. Confirmatory testing should include Sanger sequencing or a second orthogonal method on a new blood sample to rule out sample mix-up or contamination. Additionally, the patient should be referred for genetic counseling and testing for Li-Fraumeni syndrome, given the family history of early-onset breast cancer and ovarian cancer. The variant should be reported as a likely germline pathogenic variant with a recommendation for germline confirmation.
2. How should the laboratory report this finding, and what are the ethical and regulatory considerations?
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The laboratory should report the variant as a likely germline pathogenic variant in TP53, with a clear recommendation for germline confirmation and genetic counseling. Because the variant was initially identified in a tumor-normal paired test, the report must distinguish between somatic and germline findings. Under CLIA and CAP guidelines, the laboratory must ensure that the report is clear about the limitations of tumor-only or tumor-normal testing for germline detection. Ethical considerations include the duty to warn the patient about potential hereditary cancer risk and the implications for family members. Regulatory considerations include compliance with HIPAA and state laws regarding genetic information, and possibly the need for a separate germline consent if germline testing was not explicitly consented. The laboratory should also ensure that the variant classification follows ACMG/AMP guidelines for germline variants.