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Molecular and Cellular Biology, August 2005, p. 7158-7169, Vol. 25, No. 16
0270-7306/05/$08.00+0     doi:10.1128/MCB.25.16.7158-7169.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Spontaneous Homologous Recombination Is Induced by Collapsed Replication Forks That Are Caused by Endogenous DNA Single-Strand Breaks

Nasrollah Saleh-Gohari ,1,{dagger},{ddagger} Helen E. Bryant,1,{dagger} Niklas Schultz,2 Kayan M. Parker,1 Tobias N. Cassel,1,2 and Thomas Helleday1,2*

Institute for Cancer Studies, University of Sheffield Medical School, Beech Hill Road, Sheffield S10 2RX, United Kingdom,1 Department of Genetics, Microbiology and Toxicology, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden2

Received 23 December 2004/ Returned for modification 2 February 2005/ Accepted 16 May 2005

Homologous recombination is vital to repair fatal DNA damage during DNA replication. However, very little is known about the substrates or repair pathways for homologous recombination in mammalian cells. Here, we have compared the recombination products produced spontaneously with those produced following induction of DNA double-strand breaks (DSBs) with the I-SceI restriction endonuclease or after stalling or collapsing replication forks following treatment with thymidine or camptothecin, respectively. We show that each lesion produces different spectra of recombinants, suggesting differential use of homologous recombination pathways in repair of these lesions. The spontaneous spectrum most resembled the spectra produced at collapsed replication forks formed when a replication fork runs into camptothecin-stabilized DNA single-strand breaks (SSBs) within the topoisomerase I cleavage complex. We found that camptothecin-induced DSBs and the resulting recombination repair require replication, showing that a collapsed fork is the substrate for camptothecin-induced recombination. An SSB repair-defective cell line, EM9 with an XRCC1 mutation, has an increased number of spontaneous {gamma}H2Ax and RAD51 foci, suggesting that endogenous SSBs collapse replication forks, triggering recombination repair. Furthermore, we show that {gamma}H2Ax, DSBs, and RAD51 foci are synergistically induced in EM9 cells with camptothecin, suggesting that lack of SSB repair in EM9 causes more collapsed forks and more recombination repair. Furthermore, our results suggest that two-ended DSBs are rare substrates for spontaneous homologous recombination in a mammalian fibroblast cell line. Interestingly, all spectra showed evidence of multiple homologous recombination events in 8 to 16% of clones. However, there was no increase in homologous recombination genomewide in these clones nor were the events dependent on each other; rather, we suggest that a first homologous recombination event frequently triggers a second event at the same locus in mammalian cells.

* Corresponding author. Mailing address: Institute for Cancer Studies, University of Sheffield Medical School, Beech Hill Road, Sheffield S10 2RX, United Kingdom. Phone: 44 114 271 29 93. Fax: 44 114 271 38 92. E-mail: t.helleday{at}sheffield.ac.uk.

{dagger} These authors contributed equally to this work.

{ddagger} Present address: Afzalipour Hospital, Kerman University of Medical Science, Kerman, Iran.

Molecular and Cellular Biology, August 2005, p. 7158-7169, Vol. 25, No. 16
0022-538X/05/$08.00+0     doi:10.1128/MCB.25.16.7158-7169.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



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