Genome stability requires tight regulation of DNA replication to ensure that the entire genome of the cell is duplicated once and only once per cell cycle. In mammalian cells, origin activation is controlled in space and time by a cell-specific and robust program called replication timing. About 100,000 potential replication origins form on the chromatin in the gap 1 (G1) phase but only 20–30% of them are active during the DNA replication of a given cell in the synthesis (S) phase. When the progress of replication forks is slowed by exogenous or endogenous impediments, the cell must activate some of the inactive or “dormant” origins to complete replication on time. Thus, the many origins that may be activated are probably key to protect the genome against replication stress. This review aims to discuss the role of these dormant origins as safeguards of the human genome during replicative stress.

A characteristic of cancer development is the acquisition of genomic instability, which results from the inaccurate repair of DNA damage. Among double-strand break repair mechanisms induced by oncogenic stress, the highly mutagenic theta-mediated end-joining (TMEJ) pathway, which requires DNA polymerase theta (POLθ) encoded by the POLQ gene, has been shown to be overexpressed in several human cancers. However, little is known regarding the regulatory mechanisms of TMEJ and the consequence of its dysregulation. In this study, we combined a bioinformatics approach exploring both Molecular Taxonomy of Breast Cancer International Consortium and The Cancer Genome Atlas databases with CRISPR/Cas9-mediated depletion of the zinc finger E-box binding homeobox 1 (ZEB1) in claudin-low tumor cells or forced expression of ZEB1 in basal-like tumor cells, two triple-negative breast cancer (TNBC) subtypes, to demonstrate that ZEB1 represses POLQ expression. ZEB1, a master epithelial-to-mesenchymal transition–inducing transcription factor, interacted directly with the POLQ promoter. Moreover, downregulation of POLQ by ZEB1 fostered micronuclei formation in TNBC tumor cell lines. Consequently, ZEB1 expression prevented TMEJ activity, with a major impact on genome integrity. In conclusion, we showed that ZEB1 directly inhibits the expression of POLQ and, therefore, TMEJ activity, controlling both stability and integrity of breast cancer cell genomes. Significance: These findings uncover an original mechanism of TMEJ regulation, highlighting ZEB1 as a key player in genome stability during cancer progression via its repression of POLQ. See related commentary by Carvajal-Maldonado and Wood, p. 1441

Cytidine deaminase (CDA) functions in the pyrimidine salvage pathway for DNA and RNA syntheses and has been shown to protect cancer cells from deoxycytidine-based chemotherapies. In this study, we observed that CDA was overexpressed in pancreatic adenocarcinoma from patients at baseline and was essential for experimental tumor growth. Mechanistic investigations revealed that CDA localized to replication forks where it increased replication speed, improved replication fork restart efficiency, reduced endogenous replication stress, minimized DNA breaks, and regulated genetic stability during DNA replication. In cellular pancreatic cancer models, high CDA expression correlated with resistance to DNA-damaging agents. Silencing CDA in patient-derived primary cultures in vitro and in orthotopic xenografts in vivo increased replication stress and sensitized pancreatic adenocarcinoma cells to oxaliplatin. This study sheds light on the role of CDA in pancreatic adenocarcinoma, offering insights into how this tumor type modulates replication stress. These findings suggest that CDA expression could potentially predict therapeutic efficacy and that targeting CDA induces intolerable levels of replication stress in cancer cells, particularly when combined with DNA-targeted therapies. Significance: Cytidine deaminase reduces replication stress and regulates DNA replication to confer resistance to DNA-damaging drugs in pancreatic cancer, unveiling a molecular vulnerability that could enhance treatment response.

Maintenance of the human chromosomes stability requires a tight regulation of DNA replication to duplicate once and only once the entire genome of a single cell. In mammalians cells, origin activation is controlled in space and time by a cell specific and robust program called replication timing. About 100 000 of potential origins are loaded onto the chromatin at the G1 phase but only 20-30% are selected and active during the replication of a given cell. When the replication fork is slowed down by exogenous or endogenous sources, the cell need to activate more origins to complete the replication on time. Thus, the large choice of origins that can be activated may be a key player in the protection of the genome. The aim of this review is to discuss about the role of these dormant origins as housekeepers of the human genome in response to replicative stress.