Book Description

The cell cycle, the group of processes involved in the duplication and division of a cell in two daughter cells is essential for all organism existence. The correct regulation of these processes is crucial to guarantee genome integrity and cell survival. From the different cell cycle phases, the S phase is the most vulnerable to the acquisition of DNA damage since it is the phase in which the DNA is replicated. Alterations in DNA replication dynamics result in the accumulation of replication stress, one of the major sources of genomic instability, a hallmark of cancer. In this sense, cells have developed complex surveillance mechanisms to ensure stabilization and repair of forks, to coordinate these functions with cell cycle, and thus, to prevent cell division in the presence of unreplicated or damaged DNA. By doing so, these mechanisms will try to overcome the damage, and if so, the DNA replication stress response will promote replication resumption. By contrast, in the cases of persistent damage, cells are withdrawal from the cell cycle either by apoptosis or senescence. The correct activation and regulation of all these mechanisms is essential to prevent the acquisition of genomic instability and the oncogenic transformation. The pathways involved in DNA damage detection and signaling have been extensively studied in tumor cells. However, the response to replication stress, especially in non-transformed human cells, is still poorly understood. Therefore, in order to gain a better understanding of the pathways involved in this response, the main objective of this thesis has been to study and characterize new mechanisms involved in the DNA replication stress response of non-transformed human cells, as well as to analyze their contribution towards safeguarding genome integrity. Combining cellular and molecular approaches, together with several replication stress inducing agents, we have characterized new DNA replication stress response mechanisms that prevent replication resumption upon severe replication stress. For instance, we have described that APC/CCdh1 ubiquitin ligase is prematurely activated in S phase, to prevent new origin firing, in response to a prolonged DNA replication inhibition that results in the processing of replication forks into double strand breaks. Additionally, using an approach that has allowed us to define the changes at replication fork level between an acute and prolonged replication stress, we have seen that replication forks suffer several remodeling and processing events that abrogate their ability to restart after severe replication stress. Notably, our results suggest that this loss in the ability to resume replication under these conditions may act as a mechanism to safeguard genome integrity in non-transformed human cells. Collectively, the results of this thesis contribute to have a better understanding of the mechanisms involved in the DNA replication stress response of non-transformed human cells, opening new doors for the development of future therapies.