p53-dependent polyploidisation after DNA damage in G2 phase

Cell cycle progression in the presence of damaged DNA can lead to accumulation of mutations and pose a risk for tumour development. In response to DNA damage in G2 phase, human cells can be forced to exit the cell cycle in a p53-p21- and APC/CCdh1-dependent manner. Cells that exit the cell cycle in G2 phase become senescent, but it is unclear what determines this commitment and whether other cell fates occur. We find that a subset of immortalised RPE-1 cells and primary human fibroblasts spontaneously initiate DNA re-replication several days after forced cell cycle exit in G2 phase. By combining single cell tracking for more than a week with quantitative immunofluorescence, we find that the resulting polyploid cells contain increased levels of damaged DNA and frequently exit the cell cycle again in the next G2 phase. Subsequently, these cells either enter senescence or commit to another round of DNA re-replication, further increasing the ploidy. At least a subset of the polyploid cells show abnormal centrosome numbers or localisation. In conclusion, cells that are forced to exit the cell cycle in G2 phase face multiple choices that lead to various phenotypes, including propagation of cells with different ploidies. Our findings suggest a mechanism by which p53-positive cells can evade senescence that risks genome integrity. Main points-Cell cycle exit from G2 phase does not necessarily lead to senescence -Resumption of proliferation after G2 phase cell cycle exit starts with DNA replication -Successive cell cycle exits lead to propagation of cells with different ploidies -A p53-dependent mechanism allows eventual proliferation after DNA damage