Co-occurrence of germline pathogenic variants in a Brazilian cohort of patients with breast cancer.

e22532 Background: The estimated prevalence of germline pathogenic variants (PV) or likely pathogenic variants (LPV) in high and moderate penetrance genes is around 7-10% among women with breast cancer (BC). The emergence of next-generation sequencing (NGS) technology allowed multi-gene panel testing (MGPT) analysis at a lower cost. However, the detection of incidental findings with unknown significance (VUS) and challenges with variant interpretation increase substantially with MGPT. The effect of co-existing PV/LPV in two or more genes is still understudied. Here, we present the prevalence of concurrent germline LPV/PV in a cohort of Brazilian women with BC who underwent a MGPT in a single reference laboratory. Methods: Retrospective observational study using a laboratory cohort (Oncoclínicas Precision Medicine). This study includes women with BC who performed a germline MGPT (able to call SNVs, indels and CNVs) from 2019 to 2023. All patients were tested in a single reference laboratory (OC Precision Medicine) and originated from several Brazilian centers within the Oncoclínicas network. Two distinct germline NGS assays were used: breast and ovarian cancer (BOC) panel (36 genes) and a pan-cancer (PC) panel (105 genes). Results: In the overall cohort, 2,651 women with BC were included in this analysis. The median age at genetic testing was 51 years. 1,867 pts (70.4%) underwent a PC panel testing and the remaining 784 pts (29.6%) a BOC panel.Overall, 314 (12%) pts carrier of LP/PV in high- and/or moderate-penetrance genes. Two or more heterozygous LP/PV were found in 35 pts (1.32%), including 8 pts (1.02%) in the BOC panel and 27 pts (1.44%) in the PC panel subgroups. There were no differences in age of onset of BC in single vs co-occurring double mutants. The most frequent co-existing PV/LPV were found in BRCA2 (11) / BRCA1 (8) / PALB2 (4) / TP53 (3) and MUTYH (7) / CHEK2 (3) in the high and moderate penetrance genes, respectively. Six pts (0.23%) had a double heterozygous involving high and/or moderate penetrance genes. Interestingly, there was a case with four heterozygous variants in TP53, MUTYH, MUTYH and APC. Conclusions: In times of rapid incorporation of MGPT in the clinical practice, our real-world data showed that a small proportion of BC patients could be carriers of two or more germline heterozygous LP/PV in cancer genes. However, this represents a challenging scenario for patient and family management, especially in a co-occurrence of high and moderate penetrance genes.