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Template switching during break-induced replication


DNA double-strand breaks (DSBs) are potentially lethal lesions that arise spontaneously during normal cellular metabolism, as a consequence of environmental genotoxins or radiation, or during programmed recombination processes. Repair of DSBs by homologous recombination generally occurs by gene conversion resulting from transfer of information from an intact donor duplex to both ends of the break site of the broken chromosome1. In mitotic cells, gene conversion is rarely associated with reciprocal exchange and thus limits loss of heterozygosity for markers downstream of the site of repair and restricts potentially deleterious chromosome rearrangements2,3,4,5. DSBs that arise by replication fork collapse or by erosion of uncapped telomeres have only one free end and are thought to repair by strand invasion into a homologous duplex DNA followed by replication to the chromosome end (break-induced replication, BIR)6. BIR from one of the two ends of a DSB would result in loss of heterozygosity, suggesting that BIR is suppressed when DSBs have two ends so that repair occurs by the more conservative gene conversion mechanism. Here we show that BIR can occur by several rounds of strand invasion, DNA synthesis and dissociation. We further show that chromosome rearrangements can occur during BIR if dissociation and reinvasion occur within dispersed repeated sequences. This dynamic process could function to promote gene conversion by capture of the displaced invading strand at two-ended DSBs to prevent BIR.

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Figure 1: Multiple strand invasion model and experimental design to test the model.
Figure 2: Chromosome fragments derived from the wild-type diploid show evidence of template switching.
Figure 3: Formation of non-reciprocal translocations during BIR.
Figure 4: Models for the post-invasion steps of BIR.


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We thank W. K. Holloman and members of the Symington and Dujon laboratories for comments on the manuscript, and T. Petes for providing the haploid strains with restriction site polymorphisms. These studies were supported by grants from the NIH, and by a postdoctoral fellowship from La Ligue Contre Le Cancer to B.L.

Author Contributions C.E.S. characterized template switching between CFV1 or CFV2 and the marked chromosome III homologues. B.L. designed and performed the experiments and analysed the data characterizing chromosome translocations by comparative genome hybridization. L.S.S. designed the experiments, analysed the data and wrote the paper.

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Correspondence to Lorraine S. Symington.

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This file contains Supplementary Methods, Supplementary Figure S1 with Legend and additional references. (PDF 179 kb)

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Smith, C., Llorente, B. & Symington, L. Template switching during break-induced replication. Nature 447, 102–105 (2007).

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