Cell cycle checkpoints: reversible when possible, irreversible when needed

Cell cycle checkpoints are reversible in nature, and can prevent progression into the next cell cycle phase if needed. In the case of DNA damage, cells can prevent progression from G1 into S phase, and from G2 into mitosis in the presence of DNA double strand breaks. Following DNA repair, these checkpoints can be silenced, allowing further progression through the cell cycle. In chapter 2 we review recent literature regarding activation and silencing of the DNA damage response. We describe how checkpoint activation and its reversal are tuned to each phase of the cell cycle, despite shared mechanisms of DNA damage detection. We also describe mechanisms that contribute to the maintenance of a reversible checkpoint following DNA damage in G2 phase, which is the main topic of this thesis. Chapter 3 addresses the reversibility of cell cycle checkpoints. We discovered that a DNA-damage-induced arrest in G1 remains reversible for much longer compared to an arrest in G2. This finding led us to identify the sequence of events leading to an irreversible cell cycle withdrawal following excessive DNA damage in G2 phase. In chapter 4 we describe how normal wiring of the cell cycle machinery, after DNA damage, results in loss of DNA damage checkpoint reversibility in prophase. As a result, cells are hypersensitive to DNA damage during prophase, causing cells to exit the cell cycle from prophase in response to relatively mild DNA damage. We show that this mechanism helps to limit the proliferation of cells that have undergone DNA-replication stress. Chapter 5 describes the identification of a novel mechanism contributing to the maintenance of DNA damage checkpoint reversibility in G2 phase. Finally, in chapter 6, we confirm the existence of a mitotic timer that can limit the proliferation of daughter cells when the completion of mother cell mitosis exceeds a certain time-window. We show that time-dependent accumulation of the tumor suppressor p53 in mitosis precedes a cell cycle arrest in G1, and find that mitotic cells lack Mdm2. This suggests that the mitotic timer is controlled through time-dependent accumulation of p53 in mitosis.