Research Papers:

Impaired 53BP1/RIF1 DSB mediated end-protection stimulates CtIP-dependent end resection and switches the repair to PARP1-dependent end joining in G1

Ali Bakr, Sabrina Köcher, Jennifer Volquardsen, Cordula Petersen, Kerstin Borgmann, Ekkehard Dikomey, Kai Rothkamm and Wael Y. Mansour _

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Oncotarget. 2016; 7:57679-57693. https://doi.org/10.18632/oncotarget.11023

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Ali Bakr1, Sabrina Köcher1, Jennifer Volquardsen1, Cordula Petersen2, Kerstin Borgmann1, Ekkehard Dikomey1, Kai Rothkamm1 and Wael Y. Mansour1,3

1 Laboratory of Radiobiology & Experimental Radiooncology, University Medical Center Hamburg–Eppendorf, Hamburg, Germany

2 Department of Radiotherapy and Radiooncology, University Medical Center Hamburg–Eppendorf, Hamburg, Germany

3 Tumor Biology Department, National Cancer Institute, Cairo University, Egypt

Correspondence to:

Wael Y. Mansour, email:

Keywords: double strand break ends processing, 53BP1, RIF1, ATM, A-EJ

Received: July 07, 2016 Accepted: July 23, 2016 Published: August 02, 2016


End processing at DNA double strand breaks (DSB) is a decisive step in repair pathway selection. Here, we investigated the role of 53BP1/RIF1 in limiting BRCA1/CtIP-mediated end resection to control DSB repair pathway choice. ATM orchestrates this process through 53BP1 phosphorylation to promote RIF1 recruitment. As cells enter S/G2-phase, end resection is activated, which displaces pATM from DSB sites and diminishes 53BP1 phosphorylation and RIF1 recruitment. Consistently, the kinetics of ATM and 53BP1 phosphorylation in S/G2-phase concur. We show that defective 53BP1/RIF1-mediated DSB end-protection in G1-phase stimulates CtIP/MRE11-dependent end-resection, which requires Polo-like kinase 3. This end resection activity in G1 was shown to produce only short tracks of ssDNA overhangs, as evidenced by the findings that in 53BP1 depleted cells, (i) RPA focus intensity was significantly lower in G1 compared to that in S/G2 phase, and (ii) EXO1 knockdown did not alter either number or intensity of RPA foci in G1 but significantly decreased the RPA focus intensity in S/G2 phase. Importantly, we report that the observed DSB end resection in G1 phase inhibits DNA-PK-dependent nonhomologous end joining but is not sufficient to stimulate HR. Instead, it switches the repair to the alternative PARP1-dependent end joining pathway.

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