ADAPTATION TO THE DNA DAMAGE CHECKPOINT REQUIRES THE REWIRING OF THE CELL CYCLE MACHINERY

The DNA damage checkpoint is a surveillance mechanism evolved to preserve genome integrity in response to DNA damaging agents. The DNA damage checkpoint senses DNA insults and halts the cell cycle providing time and conditions to repair the lesion(s). If the damage is successfully repaired, cells reenter in the cell cycle in a process known as recovery to the DNA damage checkpoint. If the damage is not repaired, cells either undergo programmed cell death or override the checkpoint reentering the cell cycle in the presence of the lesion. This process, known as adaptation to the DNA damage checkpoint, represents an opportunity for cells to repair the damage in the following cell cycle. However, adaptation to the DNA damage checkpoint can be an unsafe event as daughter cells can accumulate genomic aberrations, therefore promoting genomic instability, and, indeed checkpoint adaptation has been described to occur also in cancer cells. Therefore, understanding the molecular mechanisms that drive checkpoint adaptation is a fundamental question to be addressed. The molecular mechanism causing adaptation, as well as the players involved in this process, remains largely unknown. In budding yeast S. cerevisiae, the existence of a crosstalk between the cell cycle machinery and the DNA damage checkpoint have been suggested by two observations. First, the DNA damage checkpoint acts to halt cell cycle progression by directly inhibiting the pathways that control the exit from mitosis, namely the Cdc fourteen early anaphase release (FEAR) network and the mitotic exit network (MEN). Second, the activity of the FEAR network is required for checkpoint adaptation. Indeed, impairing the functions of single components of the FEAR network, namely Cdc5, Spo12 and Slk19, results in cells impaired in the adaptation process. While the molecular events for the DNA damage checkpoint activation have been intensely studied and 17 relatively well characterized, the molecular events that drive cell cycle resumption after checkpoint adaptation are less well understood. In the work presented in this thesis, we used and integrated different approaches, including genetics, single cell analyses, and fluorescence microscopy techniques to tackle this question. Our findings indicate that the FEAR mutants (with the exception of Cdc5) are proficient in switching off the checkpoint but cannot exit mitosis, and suggest a more complex picture. As impairing the activity of single FEAR components does not affect exit from mitosis both in unperturbed conditions, and following checkpoint recovery, our studies unveil checkpoint adaptation as the rewiring of a cell cycle with peculiar features. From our investigations, we expect to elucidate the molecular circuitry underlying the rewiring of the cell cycle in persistent DNA damage conditions.