Abstract 977: Germline BRCA1/2 mutations detected by circulating tumor DNA testing in breast cancer patients: A retrospective mutiinstitutional analysis

Abstract Background: The use of circulating tumor (ct) DNA testing allows to detect resistant and actionable somatic alterations and it could be associated with the incidental identification of germline mutations. An univocal variant allele frequency (VAF) threshold to distinguish germinal form somatic mutations has not been assessed yet. We aimed to correlate BRCA1/2 ctdNA detected mutations with germline mutational status to determine potential VAF cutoff. Materials and methods:We retrospectively analyzed the incidence of both somatic (s) and germinal (g) BRCA1/2 alterations in a multiinstitutional retrospective breast cancer (BC) cohort to assess the VAF threshold for the likelihood of germline mutations’ detection by ctDNA next-generation sequencing (Guardant 360) testing. Clinical variables were analyzed using descriptive analyses and receiving operating characteristic (ROC) curves were generated to determine the VAF cutoff. Results: Two hundred and fourteen patients (pts) with sBRCA1/2 mutations (including variant of uncertain significant and synonymous) detected by ctDNA testing, referred to the enrolling centers between January 2015 to May 2023 were included in the analysis. The median age at diagnosis was 50 years old (Interquartile range (IQR)43,5-61.5), 43% had a family history of cancer. Hormone receptor positive/HER2 negative was the most represented subtype (68%) followed by the HER2 positive (16%) and triple negative (16%) ones. At ctDNA baseline, 95.8% of pts had a metastatic disease; the main sites of metastases were bone (67.4%) and visceral (59.3%). Ninetyfour (43.9%) and 137 (64%) pts had a sBRCA1 and sBRCA2 alterations respectively, while 17 (7.9%) had a co-mutation in BRCA1 and BRCA2. The mean VAF value for sBRCA1 alterations was 10.1% (standard deviation (SD) 19.5%, range [0.04%-84.3%]) while for sBRCA2 alterations was 10.8% (SD 18.9%, range [0.21%-80.3%]). The germinal testing, performed per standard of care, was available for 100 pts (46.7%). Among these, 42 (42%) had at least a pathogenic variant detected. A gBRCA1 mutation was present in 11 (11%) pts while 25 (25%) had a gBRCA2 mutation. Comparing the somatic and germinal testing, 9/100 pts had a concordance for BRCA1 detection, the 2 discordant cases had both a low allele frequency sBRCA2 alterations; the concordance for BRCA2 detection was instead 100%. An optimal cutoff of 38.4% (AUC 0.98) for BRCA1 and 19.5% (AUC 0.96) for BRCA2 was assessed by ROC analysis as the likelihood of a germline meaning of a somatic mutation detected by ctDNA analysis. Conclusion: The identification of BRCA1/2 alterations in ctDNA could guide the use of germline testing. The VAF cutoff identified for the likelihood of a germinal mutation is lower than expected, suggesting that a wider population should be screened for germinal mutation with relevant impact on the therapeutic choices and family screening. Citation Format: Letizia Pontolillo, Carolina Reduzzi, Andrew A. Davis, Arielle J. Medford, Annika Putur, Lorenzo Gerratana, Katherine Clifton, Whitney L. Hensing, Marko Velimirovic, Surbhi Warrior, Mara S. Serafini, Eleonora Nicolò, Laura Munoz-Arcos, Jeannine Donahue, Charles S. Dai, Jennifer C. Keenan, Amir Behdad, William J. Gradishar, Diana Giannarelli, Emilio Bria, Cynthia X. Ma, Aditya Bardia, Massimo Cristofanilli. Germline BRCA1/2 mutations detected by circulating tumor DNA testing in breast cancer patients: A retrospective mutiinstitutional analysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 977.