From single-strand breaks to double-strand breaks during S-phase: A new mode of action of the Escherichia coli Cytolethal Distending Toxin

Abstract

The Cytolethal Distending Toxin (CDT) is a genotoxin produced by several pathogenic bacteria. It is generally admitted that CDT induces double-strand breaks (DSB) and cell cycle arrest in G2/M-phase, in an ATM-dependent manner. Most of these results were obtained at high dose (over 1μg ml-1) of CDT and late after treatment (8-24h). We provide here evidence that the Escherichia coli CDT (EcCDT) - at low dose (50pg ml-1 or LD50) and early after treatment (3-6h) - progressively induces DNA DSB, mostly in S-phase. DSB formation is related to the single-strand breaks induction by CDT, converted into DSB during the S-phase. We also show that homologous recombination is mobilized to these S-phase-associated DSB. This model unveils a new mechanism for CDT genotoxicity that may play a role in cells partly deficient in homologous recombination. The Cytolethal Distending Toxin (CDT), virulence factor produced by several pathogenic bacteria, induces DNA double-strand breaks (DSB) and cell cycle arrest in G2/M phase. We demonstrate that Escherichia coli CDT progressively induces DSB, mostly in S-phase cells, at low doses (50 pg.ml-1 or LD50). These are preceeded by single-strand breaks, converted into DSB through a replicative stress. Furthermore, the homologous recombination Rad51 protein is mobilized to these S-phase-associated DSB, unveiling a new mechanism of action for the CDT genotoxin. © 2012 Blackwell Publishing Ltd.