5-Fluorouracil sensitizes colorectal tumor cells towards double stranded DNA breaks by interfering with homologous recombination repair

// Upadhyayula Sai Srinivas 1 , Jerzy Dyczkowski 2 , Tim Beisbarth 2 , Jochen Gaedcke 3 , Wael Y. Mansour 4,5 , Kerstin Borgmann 4 and Matthias Dobbelstein 1 1 Institute of Molecular Oncology, University Medical Center Gottingen, Germany 2 Department of Medical Stastics, University Medical Center Gottingen, Germany 3 Department of General, Visceral and Pediatric Surgery, University Medical Center Gottingen, Germany 4 Laboratory of Radiobiology & Experimental Radiooncology, University Medical Center Hamburg-Eppendorf, Hamburg, Germany 5 Tumor Biology Department, National Cancer Institute, Cairo University, Cairo, Egypt Correspondence to: Matthias Dobbelstein, email: // Keywords : colorectal cancer, radiochemotherapy, homologous recombination repair, 5-fluorouracil, Rad51 Received : November 19, 2014 Accepted : March 11, 2015 Published : March 30, 2015 Abstract Malignant tumors of the rectum are treated by neoadjuvant radiochemotherapy. This involves a combination of 5-fluorouracil (5-FU) and double stranded DNA-break (DSB)-inducing radiotherapy. Here we explored how 5-FU cooperates with DSB-induction to achieve sustainable DNA damage in colorectal cancer (CRC) cells. After DSB induction by neocarzinostatin, phosphorylated histone 2AX (γ-H2AX) rapidly accumulated but then largely vanished within a few hours. In contrast, when CRC cells were pre-treated with 5-FU, gammaH2AX remained for at least 24 hours. GFP-reporter assays revealed that 5-FU decreases the efficiency of homologous recombination (HR) repair. However, 5-FU did not prevent the initial steps of HR repair, such as the accumulation of RPA and Rad51 at nuclear foci. Thus, we propose that 5-FU interferes with the continuation of HR repair, e. g. the synthesis of new DNA strands. Two key mediators of HR, Rad51 and BRCA2, were found upregulated in CRC biopsies as compared to normal mucosa. Inhibition of HR by targeting Rad51 enhanced DNA damage upon DSB-inducing treatment, outlining an alternative way of enhancing therapeutic efficacy. Taken together, our results strongly suggest that interfering with HR represents a key mechanism to enhance the efficacy when treating CRC with DNA-damaging therapy.