Loss of DNA replication control is a potent initiator of gene amplification

Eukaryotic cells use numerous mechanisms to ensure that no segment of their DNA is re-replicated within a single cell cycle. Despite longstanding speculation that such tight regulation is needed to protect cells from genomic alterations, this notion has never been experimentally tested. Here we show that even just transient and limited re-replication in Saccharomyces cerevisiae can strongly induce the critical first step of gene amplification, increasing gene copy number from one to two or more. The amplified units, or amplicons, consist of large internal chromosomal segments that are bounded by Ty repetitive elements and are intrachromosomally arrayed at their endogenous locus in direct head-to-tail orientation. The presence of hybrid Ty elements at inter-amplicon junctions together with the dependence of amplification on RAD52 indicate that in budding yeast these re-replication-induced gene amplifications (RRIGA) are mediated by homologous recombination between re-replicated non-allelic repetitive elements. These results finally establish the importance of stringent replication control for genome stability and suggest that re-replication should now be considered as a possible contributor to gene copy number changes in fields as diverse as cancer biology, evolution, and human genetics. The arrays in this series are primary comparative genomic hybridizations to determine genomic changes after re-replication or other genomic stresses. Genomic DNA was purified from reference Saccharomyces cerevisiae cells or survivors of re-replication or other genomic stresses, differentially labeled with Cy3 and Cy5, and competitively hybridized to a spotted microarray containing ORF and intergenic PCR products. Cy5/Cy3 ratios are normalized so that the average ratio of all elements was 1. A small number of the arrays were used to determine the extent and location of re-replication under different conditions. For those, genomic DNA was purified from non-replicating and re-replicating cells and treated as above. Series contains a total of 203 hybridizations.