Paired Tumor-Normal Sequencing Provides Insights into TP53-Related Cancer Spectrum in Li-Fraumeni Patients.

BACKGROUND Li-Fraumeni syndrome (LFS) genetic testing is performed using blood specimens from patients selected based on phenotype-dependent guidelines. This approach is problematic for understanding LFS clinical spectrum, because patients with non-classical presentations are missed, clonal hematopoiesis (CH)-related somatic blood mutations cannot be distinguished from germline variants, and unrelated tumors cannot be differentiated from those driven by germline TP53 defects. METHODS To provide insights into LFS-related cancer spectrum, we analyzed paired tumor-blood DNA sequencing results in 17,922 cancer patients, and distinguished CH-related, mosaic, and germline TP53 variants. Loss-of-heterozygosity (LOH) and TP53 mutational status were assessed in tumors, followed by immunohistochemistry for p53 expression on a subset to identify those lacking biallelic TP53 inactivation. RESULTS Pathogenic/likely pathogenic TP53 variants were identified in 50 patients, 12 (24.0%) of which were CH-related and four (8.0%) were mosaic. Twelve (35.3%) of 34 patients with germline TP53 variants did not meet LFS testing criteria. LOH of germline TP53 variant was observed in 96.0% (95% CI = 79.7-99.9%) of core LFS-spectrum type tumors versus 45.5% (95% CI = 16.8-76.6%) of other tumors, and 91.3% (95% CI = 72.0-98.9%) of tumors from patients who met LFS testing criteria versus 61.5% (95% CI = 31.6-86.1%) of tumors from patients who did not. Tumors retaining wild-type TP53 allele exhibited wild-type p53 expression. CONCLUSIONS Our results indicate that some TP53 variants identified in blood-only sequencing are not germline and a substantial proportion of LFS patients are missed by current testing guidelines. Additionally, a subset of tumors from LFS patients do not have biallelic TP53 inactivation and may represent cancers unrelated to their germline TP53 defect.