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Molecular and Cellular Biology, August 2003, p. 5706-5715, Vol. 23, No. 16
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.16.5706-5715.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Pathways of DNA Double-Strand Break Repair during the Mammalian Cell Cycle

Kai Rothkamm,1 Ines Krüger,1 Larry H. Thompson,2 and Markus Löbrich1*

Fachrichtung Biophysik, Universität des Saarlandes, D-66421 Homburg/Saar, Germany,1 Biology and Biotechnology Research Program, Lawrence Livermore National Laboratory, Livermore, California 945502

Received 26 December 2002/ Returned for modification 9 May 2003/ Accepted 16 May 2003

Little is known about the quantitative contributions of nonhomologous end joining (NHEJ) and homologous recombination (HR) to DNA double-strand break (DSB) repair in different cell cycle phases after physiologically relevant doses of ionizing radiation. Using immunofluorescence detection of {gamma}-H2AX nuclear foci as a novel approach for monitoring the repair of DSBs, we show here that NHEJ-defective hamster cells (CHO mutant V3 cells) have strongly reduced repair in all cell cycle phases after 1 Gy of irradiation. In contrast, HR-defective CHO irs1SF cells have a minor repair defect in G1, greater impairment in S, and a substantial defect in late S/G2. Furthermore, the radiosensitivity of irs1SF cells is slight in G1 but dramatically higher in late S/G2, while V3 cells show high sensitivity throughout the cell cycle. These findings show that NHEJ is important in all cell cycle phases, while HR is particularly important in late S/G2, where both pathways contribute to repair and radioresistance. In contrast to DSBs produced by ionizing radiation, DSBs produced by the replication inhibitor aphidicolin are repaired entirely by HR. irs1SF, but not V3, cells show hypersensitivity to aphidicolin treatment. These data provide the first evaluation of the cell cycle-specific contributions of NHEJ and HR to the repair of radiation-induced versus replication-associated DSBs.

* Corresponding author. Mailing address: Fachrichtung Biophysik, Universität des Saarlandes, D-66421 Homburg/Saar, Germany. Phone: 49-6841-1626202. Fax: 49-6841-1626160. E-mail: markus.loebrich{at}uniklinik-saarland.de.

Molecular and Cellular Biology, August 2003, p. 5706-5715, Vol. 23, No. 16
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.16.5706-5715.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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