<p>BRCA1 promoter hypermethylation and RNA in situ hybridization by RNAscope. M248, M253, M112 and M232 clearly showed presence of BRCA1 RNA, whereas in M94 (BRCA1 deletion) no BRCA1 RNA was present. M131 and M277 showed heterogeneous presence of BRCA1 RNA. PPIB probe = positive control</p>
ARTICLE ABSTRACT
Tumors 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 PARP inhibitors. Although this therapy is expected to be effective beyond germline BRCA1/2 mutated carriers, a robust validated test to detect HRD tumors is lacking. In this study, we therefore evaluated a functional HR assay exploiting the formation of RAD51 foci in proliferating cells after ex vivo irradiation of fresh breast cancer tissue: the recombination REpair CAPacity (RECAP) test.
Fresh samples of 170 primary breast cancer were analyzed using the RECAP test. The molecular explanation for the HRD phenotype was investigated by exploring BRCA deficiencies, mutational signatures, tumor-infiltrating lymphocytes (TIL), and microsatellite instability (MSI).
RECAP was completed successfully in 125 of 170 samples (74%). Twenty-four tumors showed HRD (19%), whereas six tumors were HR intermediate (HRi; 5%). HRD was explained by BRCA deficiencies (mutations, promoter hypermethylation, deletions) in 16 cases, whereas seven HRD tumors were non-BRCA related. HRD tumors showed an increased incidence of high TIL counts (P = 0.023) compared with HR proficient (HRP) tumors and MSI was more frequently observed in the HRD group (2/20, 10%) than expected in breast cancer (1%; P = 0.017).
RECAP is a robust functional HR assay detecting both BRCA1/2-deficient and BRCA1/2-proficient HRD tumors. Functional assessment of HR in a pseudo-diagnostic setting is achievable and produces robust and interpretable results.
