Cytolethal Distending Toxin from Aggregatibacter actinomycetemcomitans Induces DNA Damage, S/G2 Cell Cycle Arrest, and Caspase-Independent Death in a Saccharomyces cerevisiae Model

Cytolethal distending toxin (CDT) is a bacterial toxin that induces G(2)/M cell cycle arrest, cell distension, and/or apoptosis in mammalian cells. It is produced by several Gram-negative species and may contribute to their pathogenicity. The catalytic subunit CdtB has homology with DNase I and may act as a genotoxin. However, the mechanism by which CdtB leads to cell death is not yet clearly understood. Here, we used Saccharomyces cerevisiae as a model to study the molecular pathways involved in the function of CdtB from Aggregatibacter actinomycetemcomitans, a cause of aggressive periodontitis. We show that A. actinomycetemcomitans CdtB (AaCdtB) expression induces S/G(2) arrest and death in a DNase-catalytic residue and nuclear localization-dependent manner in haploid yeasts. Yeast strains defective in homologous recombination (HR) repair, but not other DNA repair pathways, are hypersensitive to AaCdtB, suggesting that HR is required for survival upon CdtB expression. In addition, yeast does not harbor the substrate for the other activity proposed for CdtB function, which is phosphatidylinositol-3,4,5-triphosphate phosphatase. Thus, these results suggest that direct DNA-damaging activity alone is sufficient for CdtB toxicity. To investigate how CdtB induces cell death, we examined the effect of CdtB in yeast strains with mutations in apoptotic regulators. Our results suggest that yeast death occurs independently of the yeast metacaspase gene YCA1 and the apoptosis-inducing factor AIF1 but is partially dependent on histone H2B serine 10 phosphorylation. Therefore, we report here the evidence that AaCdtB causes DNA damage that leads to nonapoptotic death in yeast and the first mutation that confers resistance to CdtB.