Abstract 3392: Ex vivo analysis of RAD51 foci induction in fresh tumor tissue: a promising new tool for the identification of homologous recombination-deficient tumors

Abstract

Abstract Purpose. BRCA1 and BRCA2 related breast and ovarian tumors respond very well to treatment with PARP inhibitors (PARPi) because of their deficiency in homologous recombination (HR). We have indications that a substantial number of ovarian tumors might have deficiencies in HR unrelated to germline mutations in BRCA1 or BRCA2. A functional assay to assess HR efficacy might therefore allow identification of additional ovarian cancer patients that could benefit from PARPi treatment. Experimental procedure. Unselected fresh ovarian tumor samples were collected from patients undergoing surgery or ascites/pleural fluid drainage. Samples were irradiated with ionizing radiation ex vivo, fixed after 2 hours and embedded in paraffin. Sections were stained with hematoxylin and eosin and examined by pathologist. The presence of RAD51 foci in replicating cells as visualized by immunofluorescence was used as a functional read out for HR proficiency. Results. We determined the ability to induce RAD51 foci following radiation in ex vivo cultured ovarian tumor samples that contained sufficient numbers of replicating cells (n = 13). RAD51 foci formation was absent (<20% replicating cells showed RAD51 foci) in 46% of the samples. In one tumor sample that did not form RAD51 foci in replicating cells upon ionizing radiation, a pathogenic mutation in BRCA1 was identified (c.5277+1G>A). Genetic and epigenetic analyses of the other samples are ongoing. Conclusion. The ex vivo analysis of RAD51 foci induction in fresh tumor tissue is a promising new tool to identify HR deficient tumors. Not only is it an important time saving strategy compared to BRCA1/BRCA2 mutation analysis, it may also identify a much broader patient population that will be eligible for the treatment with PARP inhibitors. Citation Format: Maaike Vreeswijk, M. Meijers, K.A.T. Naipal, N.S. Verkaik, R. Kanaar, J.H.J. Hoeijmakers, A. Jager, D.C. van Gent, K.N. Gaarenstroom, V.T.H.B.M. Smit, H. Vrieling. Ex vivo analysis of RAD51 foci induction in fresh tumor tissue: a promising new tool for the identification of homologous recombination-deficient tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3392. doi:10.1158/1538-7445.AM2015-3392