TP53 pathogenic variants with low allele fraction in germline genetic testing.

10600 Background: Blood or saliva DNA generally considered to be representative of germline genome in genetic cancer risk assessment. However white blood cells from these samples may also include somatic origin DNA due to postzygotic variation or, most commonly, clonal hematopoiesis (CH). Low variant allele fraction (VAF) found in germline genetic testing suggest the possibility of somatic variant and may lead to misinterpretation of genetic risk. TP53, of which germline pathogenic variants are associated with Li-Fraumeni syndrome (LFS), is frequently mutated in CH. This analysis investigated characteristics of TP53 pathogenic variants with low VAF. Methods: We reviewed the prevalence and distribution of TP53 pathogenic variants(PVs) detected in 11,277 advanced cancer patients who underwent clinical testing with a clinical NGS pan-cancer panel. Potential somatic PVs were defined as variants with low VAF from 10% to 35%. The VAF were evaluated in matched tumor tissue samples if available. Results: TP53 pathogenic variants were detected in 36 (0.32%) patients from blood or saliva samples, VAF between 10% and 35% were identified in 8(22.22%) patients and 7 of them were performed NGS sequencing in matched tumor tissue samples. The average VAF of tissue samples were 9.31times lower than blood or saliva samples(21.69% vs 3.88%). Conclusions: TP53 pathogenic variants with low allele fraction in blood or saliva samples indicate the possibility of somatic variant, a reduced VAF in matched tumor tissue samples may contribute to confirmation for suspicion of somatic origin.