PO-462 Functional ex vivo assay reveals homologous recombination deficiency in breast cancer beyond brca gene defects

Introduction Tumours of germline BRCA1/2 mutated carriers show homologous recombination (HR) deficiency (HRD), resulting in impaired DNA double strand break (DSB) repair and high sensitivity to Poly-(ADP-Ribose)-Polymerase (PARP) inhibitors. Although this therapy is expected to be effective beyond germline BRCA1/2 mutated carriers, a robust validated test to detect HRD tumours is lacking. In the present study we therefore evaluated a functional HR assay exploiting the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh breast cancers (BrC) tissue: the RECAP test. Material and methods Fresh samples of 170 primary BrC were analysed using the RECAP test. The molecular explanation for the HRD phenotype was investigated by exploring BRCA deficiencies, mutational signatures, amount of tumour infiltrating lymphocytes (TILs) and microsatellite instability (MSI). Results and discussions RECAP was completed successfully in 148 out of 170 samples (87%). 24 tumours showed HRD (16%), while 6 tumours were HR intermediate (HRi) (4%). HRD was explained by BRCA deficiencies (mutations, promoter hypermethylation, deletions) in 16 cases. Several non- BRCA deficient HRD tumours showed BRCAness related mutational signatures, suggesting that they are likely also bona fide HRD cases. HRD tumours showed an increased incidence of high TIL counts (p=0.023) compared to HR proficient (HRP) tumours and MSI was more frequently observed in the HRD group (2/20, 10%) than expected in unselected BrC (1%) (p=0.017). Conclusion The RECAP test is a robust assay detecting both BRCA1/2 deficient and BRCA1/2 proficient HRD tumours. This test identifies approximately 50% more patients that may benefit from PARPi treatment than BRCA gene testing only.