Identification of TP53 germline variants in pediatric patients undergoing tumor testing: strategy and prevalence

Background TP53 alterations are common in certain pediatric cancers, making identification of putative germline variants through tumor genomic profiling crucial for patient management. Methods We analyzed TP53 alterations in 3123 tumors from 2788 pediatric patients sequenced using tumor-only or tumor-normal paired panels. Germline confirmatory testing was performed when indicated. Somatic and germline variants were classified following published guidelines. Results In 248 tumors from 222 patients, 284 Tier 1/2 TP53 sequence and small copy number variants were detected. Following germline classification, 73.9% of 142 unique variants were pathogenic/likely pathogenic (P/LP). Confirmatory testing on 118 patients revealed germline TP53 variants in 28 patients (23 P/LP and 5 uncertain significance), suggesting a minimum Li-Fraumeni syndrome (LFS) incidence of 0.8% (23/2788) in this cohort, 10.4% (23/222) in patients with TP53 variant-carrying tumors, and 19.5% (23/118) with available normal samples. About 25% (7/28) of patients with germline TP53 variants did not meet LFS diagnostic/testing criteria while 20.9% (28/134) with confirmed or inferred somatic origins did. TP53 biallelic inactivation occurred in 75% of germline carrier tumors and was also prevalent in other groups, causing an elevated tumor-observed variant allelic fraction (VAF). However, somatic evidence including low VAF correctly identified only 27.8% (25/90) of patients with confirmed somatic TP53 variants. Conclusion The high incidence and variable phenotype of LFS in this cohort highlights the importance of assessing germline status of TP53 variants identified in all pediatric tumors. Without clear somatic evidence, distinguishing somatic from germline origins is challenging. Classifying germline and somatic variants should follow appropriate guidelines.