Abstract 2054: Whole-genome sequencing based homologous recombination deficiency testing for precision oncology of breast cancers

Abstract Breast cancers (BCs) with defects in homologous recombination repair (HRR) pathway depend on the Poly (ADP-ribose) polymerase enzymes (PARP) for their survival and thus are sensitive to PARP inhibitors (PARPi). To screen out BCs with homologous recombination deficiency (HRD), germline mutations in BRCA genes have been tested, which constitute approximately 5% of unselected BC patients. However, a substantial proportion of BCs show HRD phenotypes despite the absence of BRCA inactivating mutations in germline. Herein, we apply whole-genome sequencing (WGS) and whole-transcriptome sequencing (WTS) in 332 samples from 325 BC patients for characterizing their HRD phenotypes for PARPi treatments. Of 325 BC patients, 75 cases (23.1%) showed remarkable HRD phenotypes, based on mutational signatures of HRR abrogation. Intriguingly, 51 cases do not harbor any germline mutations in HR-related genes including BRCA1 and BRCA2. The whole-genome portrait of these samples was similar to that of HRD-positive BCs with germline mutation in HRR pathway, but showed markedly distinctive features from HRD-negative BCs. With a notable exception of BRCA1, the second most common somatic mutation in HRD-positive BCs without germline mutation, the mutational landscape of the whole study samples was primarily determined by hormone receptor and HER2 positivity, not by HRD status. Likewise, WTS-based clusters determined in an unsupervised manner were highly dependent on the histological subtype of BC, not on HRD status or germline mutations in the HRR pathway. Notably, differentially expressed gene and gene set enrichment analysis revealed that HRD-positive BCs had an increased inflammatory activity compared to BCs with proficient HR. Overall, our analysis demonstrate that WGS enables better classification of the HR status in BC than conventional BRCA germline testing, suggesting WGS as a rapid and sensitive tool for screening breast cancers for PARPi treatment. Citation Format: Ryul Kim, Seongyeol Park, Joonoh Lim, Boram Yi, Jaemo Koo, Sangmoon Lee, Jeong Seok Lee, Yeon Hee Park, Young Seok Ju. Whole-genome sequencing based homologous recombination deficiency testing for precision oncology of breast cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2054.