From single-strand breaks to double-strand breaks during S-phase: a new mode of action of theEscherichia coli Cytolethal Distending Toxin

Abstract

SummaryThe 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 1 mg ml -1 ) of CDT and late after treatment (8-24 h). We provide here evidence that the Escherichia coli CDT (EcCDT) -at low dose (50 pg ml -1 or LD50) and early after treatment (3-6 h) -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.

Bacterial genotoxins: The long journey to the nucleus of mammalian cells

“…This subunit is functionally and structurally homologous to mammalian DNase I, and causes DNA strand breaks once internalised within the nuclear compartment [13][14][15][16].…”

“…16 internalization, considering that the OMVs-mediated delivery offers several advantages such as: i) protection from extracellular proteases, increasing thereby the half life and allowing intoxication of distant targets; ii) delivery of higher toxin concentration to a specific destination, decreasing thereby the effective amount that needs to be produced, thus saving energy (reviewed in [53,54]). Thus, this issue deserves further investigations.…”

Bacterial genotoxins

“…This observation was confirmed in the mammalian system by Fedor et al, who demonstrated that iRNA knockdown of RAD51 in HeLa cells strongly promotes cell death upon exposure to EcolCDT [57]. While the study in S. cerevisae could not conclusively demonstrate a contribution of the NHEJ repair pathway to the CdtB-induced DNA lesion [142], Fahrer et al provided evidence that both HR and NHEJ protect cells from DNA lesions induced by HducCDT [143].…”

“…More recently, Fedor and colleagues have performed a detailed kinetics analysis of the CDT-induced DNA lesions and demonstrated that low doses of EcolCDT-I (50 pg/ml) at early points in time after toxin exposure (3-6 h) induces DNA single strand breaks (SSBs), which are further converted into DNA double strand breaks (DSBs) in the S phase of the cell cycle due to the inhibition of the progression of the replicative fork as a consequence of unrepaired SSBs [57]. Conversely, in the same time frame, higher toxin doses (above 75 ng/ml) induced the formation of DSBs, possibly due to the exceeding formation of SSBs on opposite strands, independently of the cell cycle phase.…”

When our genome is targeted by pathogenic bacteria

“…In yeast, homologous recombination is the predominant DNA repair mechanism for protection against CDT-induced lesions [42], whereas in human cells, both HR and NHEJ repair pathways protect against CDT-induced DSBs [40]. At low doses, CDT primarily generates SSBs which are converted into DSBs during the S-phase [43,44]. It was recently shown that chronic exposure to CDT could induce genomic instability in rat fibroblasts andcolorectal carcinoma cell lines [44].…”