Identification of homologous recombination deficiency (HRD) by RAD51 in a tumor molecular profiling program for precision medicine.

Abstract

3137 Background: Tumor molecular profiling by panel sequencing helps to identify candidate patients for precision oncology. Mutations in genes of the homologous recombination repair (HRR) pathway confer DNA repair deficiency and sensitivity to DNA damaging agents, such as PARP inhibitors (PARPi) or platinum-based drugs. RAD51 nuclear foci is a biomarker of functional HRR deficiency (HRD). We aimed to incorporate the RAD51 test in an academic molecular profiling program to identify HRD tumors beyond genetic testing. Methods: We included patients with metastatic breast cancer (TNBC, ER+ <60y, 3rd line HER2+ BC), newly diagnosed high-grade epithelial ovarian cancer (HGOC) and metastatic and/or castration-resistant prostate adenocarcinoma (PC) undergoing tumor/germline genetic testing. A customized capture-based targeted sequencing of 450 genes was performed in FFPE tumor samples. RAD51 was manually stained by immunofluorescence and functional HRD was defined as RAD51 score ≤10%. Results: Between January 2021 and January 2022, tumor panel sequencing was performed in 279 tumors. Panel sequencing was informative in 264/279 (95%) samples and the RAD51 test was evaluable in 81/90 (90%). In total, 77 samples were evaluable for both tumor/germline sequencing and RAD51, namely 42 BC, 16 HGOC and 19 PC (Table 1). Functional HRD by RAD51 was observed in 15/77 (20%) cases. Panel sequencing identified 17/77 (22%) cases carrying HRR gene alterations, of which nine were confirmed as HRD by RAD51. In BC, 2/6 tumors with HRD by RAD51 did not carry a germline or tumor mutation in HRR genes. HRD tumor profiling triggered the germline analysis of one hereditary BRCA2 BC patient. Four g BRCA1/2 BC with HRD by RAD51 became HRR proficient (HRP) at PARPi progression. In addition, the RAD51 test identified 2/5 HGOC and 2/4 PC tumors as HRD despite not carrying a tumor HRR gene mutation. Conclusions: RAD51 testing is feasible in an established molecular profiling program and complements gene panel sequencing results by providing evidence of functional HRR status. In particular, the RAD51 test extends the identification of HRD tumors beyond those with HRR gene mutations and captures HRR restoration after PARPi treatment. We aim to expand the analysis of RAD51 to other tumor types.[Table: see text]