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Molecular and Cellular Biology, July 2004, p. 5776-5787, Vol. 24, No. 13
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.13.5776-5787.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

The Structure-Specific Endonuclease Ercc1-Xpf Is Required To Resolve DNA Interstrand Cross-Link-Induced Double-Strand Breaks

Laura J. Niedernhofer,1* Hanny Odijk,1 Magda Budzowska,1 Ellen van Drunen,1 Alex Maas,1 Arjan F. Theil,1 Jan de Wit,1 N. G. J. Jaspers,1 H. Berna Beverloo,1 Jan H. J. Hoeijmakers,1 and Roland Kanaar1,2

Departments of Cell Biology,1 Genetics and Radiation Oncology, Erasmus Medical Center, 3000 DR Rotterdam, The Netherlands2

Received 6 December 2003/ Returned for modification 7 January 2004/ Accepted 6 April 2004

Interstrand cross-links (ICLs) are an extremely toxic class of DNA damage incurred during normal metabolism or cancer chemotherapy. ICLs covalently tether both strands of duplex DNA, preventing the strand unwinding that is essential for polymerase access. The mechanism of ICL repair in mammalian cells is poorly understood. However, genetic data implicate the Ercc1-Xpf endonuclease and proteins required for homologous recombination-mediated double-strand break (DSB) repair. To examine the role of Ercc1-Xpf in ICL repair, we monitored the phosphorylation of histone variant H2AX ({gamma}-H2AX). The phosphoprotein accumulates at DSBs, forming foci that can be detected by immunostaining. Treatment of wild-type cells with mitomycin C (MMC) induced {gamma}-H2AX foci and increased the amount of DSBs detected by pulsed-field gel electrophoresis. Surprisingly, {gamma}-H2AX foci were also induced in Ercc1–/– cells by MMC treatment. Thus, DSBs occur after cross-link damage via an Ercc1-independent mechanism. Instead, ICL-induced DSB formation required cell cycle progression into S phase, suggesting that DSBs are an intermediate of ICL repair that form during DNA replication. In Ercc1–/– cells, MMC-induced {gamma}-H2AX foci persisted at least 48 h longer than in wild-type cells, demonstrating that Ercc1 is required for the resolution of cross-link-induced DSBs. MMC triggered sister chromatid exchanges in wild-type cells but chromatid fusions in Ercc1–/– and Xpf mutant cells, indicating that in their absence, repair of DSBs is prevented. Collectively, these data support a role for Ercc1-Xpf in processing ICL-induced DSBs so that these cytotoxic intermediates can be repaired by homologous recombination.


* Corresponding author. Present address: University of Pittsburgh Cancer Institute, Hillman Cancer Center 2.6, 5117 Centre Ave., Pittsburgh, PA 15213-1863. Phone: (412) 623-7763. Fax: (412) 623-7761. E-mail: niedernhoferl{at}upmc.edu.


Molecular and Cellular Biology, July 2004, p. 5776-5787, Vol. 24, No. 13
0022-538X/04/$08.00+0     DOI: 10.1128/MCB.24.13.5776-5787.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.




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