Abstract 364: The RAD51-FFPE test rapidly and reliably identifies homologous recombination deficient ovarian and endometrial carcinomas
Authors: van Wijk Lise M. , Kramer Claire J. , Vermeulen Sylvia , ter Haar Natalja T. , de Kroon Cor D. , de Jonge Marthe M. , Gaarenstroom Katja N. , Vrieling Harry , Serra Violeta , Bosse Tjalling , Vreeswijk Maaike P. 
Published: July 2021

Sensitivity to PARP inhibitors is related to tumor-specific defects in homologous recombination (HR) and extends beyond BRCA1/2-related deficiencies. Therefore, there is great interest in a robust method to identify HR-deficient (HRD) carcinomas in routine diagnostics. DNA-based assays that are developed to identify HRD are relatively complex, costly and time-consuming. As an alternative to DNA-based assays we developed the REcombination CAPacity (RECAP) test exploiting the accumulation of RAD51 protein after irradiation of fresh tumor tissue as a biomarker for HR proficiency. The RECAP test reliably identified HRD ovarian (OC) and endometrial (EC) carcinomas including those not related to BRCA1/2 deficiency. However, the implementation of the RECAP test in clinical diagnostics is hampered by dependency on ex vivo irradiation of fresh tumor tissue. We therefore explored the use of diagnostic formalin-fixed paraffin-embedded (FFPE) tumor tissue for the functional analysis of HR and aimed to determine the performance of RAD51 assessment on diagnostic FFPE blocks as compared to RECAP test.

FFPE tumor tissue from tumors for which previously published RECAP scores were available (OC n = 49; EC n = 25) were subjected to a three-step quality assessment: 1) Tumor tissue quality analysis of hematoxylin and eosin stained slides by an expert pathologist, 2) Evaluation of yH2AX foci to confirm the presence of DNA damage and 3) Evaluation of the presence of a sufficient number of geminin positive (GMN+) tumor cells. When all quality criteria were met, the capacity of tumor cells to accumulate RAD51 protein at sites of endogenous DNA damage was analyzed using co-immunofluorescent staining. The RAD51-FFPE score was determined as the percentage of GMN+ cells with RAD51 foci. The performance of the RAD51-FFPE test was determined using corresponding RECAP scores.

Seventy-four FFPE tumor tissues were subjected to the quality assessment. In total, 58 out of 74 (78%) met our tissue quality criteria. RAD51-FFPE scores correlated significantly with RECAP scores (p= &lt;0.0001). Of the 16 tumors (including ten BRCA-related tumors) that were classified as HRD in the RECAP test (RECAP score ≤20% GMN+/RAD51+ cells), 15 showed a RAD51-FFPE score below 16%. Thirty-one out of 38 (82%) RECAP HR-proficient tumors (≥50% GMN+/RAD51+ cells) had RAD51-FFPE scores above 16%. The four tumors with RECAP scores in the HR intermediate group (20-50% GMN+/RAD51+ cells) showed RAD51-FFPE scores between 3-38%.

We demonstrated that the RAD51-FFPE test on diagnostic FFPE tumor tissue is an attractive alternative to DNA-based tests to identify HRD in OC and EC. Although the optimal RAD51-FFPE threshold to identify HRD tumors needs to be established using larger cohorts with genomic and clinical trial outcome data, the RAD51-FFPE test is a promising candidate to serve as a low-cost routine diagnostic HRD test in the clinic.

Citation Format: Lise M. van Wijk, Claire J. Kramer, Sylvia Vermeulen, Natalja T. ter Haar, Cor D. de Kroon, Marthe M. de Jonge, Katja N. Gaarenstroom, Harry Vrieling, Violeta Serra, Tjalling Bosse, Maaike P. Vreeswijk. The RAD51-FFPE test rapidly and reliably identifies homologous recombination deficient ovarian and endometrial carcinomas [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 364.