Introduction: BRCA1 and BRCA2 are involved in double-strand break (DSBs) repair via homologous recombination (HR). Clinically, BRCA1/2 mutant tumors show sensitivity to cisplatin, a cross-linking agent, but only mild sensitivity to ionizing radiation (IR). The addition of cisplatin to IR has been one of the largest advances in improving patient outcomes with radiotherapy in the past 50 years. Recently, BRCAness has been described wherein sporadic deficits in HR are observed in the absence of germline BRCA1/2 mutations. We hypothesize that BRCAness tumors also exhibit sensitivity to the combination of cisplatin + IR, and are a significant driver of the observed clinical benefit of combination therapy.

Methods: Sensitivity to cisplatin + IR was examined in vitro and correlated with clinical data. Functional HR deficiency (HRD) was assessed in isogenic BRCA1 (H1299; lung) and BRCA2 (DLD1; colon) knockout pairs using 3 assays: the Rad51 foci assay, a flow-based DRGFP assay (DSB reporter assay) and clonogenic survival assays (CSA) with cisplatin +/- IR. Cancer Cell Line Encyclopedia (CCLE) data was used to identify breast cancer cell lines with sporadic, non-BRCA1/2 mutations. Functional HRD in 4 breast cancer cell lines was assessed with the same 3 assays used for the BRCA1/2 isogenic pairs. We also examined 14 triple-negative breast cancer (TNBC) patients treated with neoadjuvant cisplatin + IR. Nine of the 14 had available tissue, which was stained for Rad51 foci.

Results: Functional HRD is observed in the BRCA1/2 isogenic pairs, as demonstrated by the Rad51 foci and DRGFP assay results. Increased sensitivity to cisplatin + IR is observed in the BRCA1/2 knockout lines in CSA results compared to isogenic controls. A genomic scar score, LST (large scale transition), was generated for each line based on CCLE data, with higher LST scores suggesting increased genomic instability and therefore HRD. Four cell lines were selected for further study. Similar functional HRD and cisplatin + IR sensitivity is observed in 2 breast cancer cell lines with high LST scores, as compared to 2 with low LST scores. Epistasis was observed upon siBRCA1 knockdown in the 2 cell lines with high LST scores. Half of the 14 TNBC patients demonstrated pathological complete response to neoadjuvant cisplatin + IR. Nine of the 14 patients had available tissue, of which 4 demonstrated functional HRD by the Rad51 foci assay.

Conclusions: Multiple in vitro assays suggest that HRD tumors are much more sensitive to the widely used combination of cisplatin + IR as compared with HR proficient tumors. Data from our small cohort of triple-negative breast cancer patients appear concordant with these findings. Reliable determination of “BRCAness” tumors should identify a subpopulation of non-germline BRCA1/2-mutated breast cancer patients who may benefit from targeted therapy with cisplatin + IR.

Citation Format: Jennifer Ma, Andrew C. Bell, Jeremy Setton, Justin Haseltine, Marcher Thompson, Rachna Shah, Benjamin H. Lok, Robert Delsite, Rebecca Aft, Nadeem Riaz, Simon Powell. In vitro characterization and clinical correlation of BRCAness as a personalized biomarker for radiosensitization with homologous recombination-directed therapies [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 832. doi:10.1158/1538-7445.AM2017-832