Replication Fork Stability in Mammalian Cells

Abstract: Maintaining replication fork integrity is vital to preserve genomic stability and avoid cancer. Physical DNA damage and altered nucleotide or protein pools represent replication obstacles, generating replicative stress. Numerous cellular responses have evolved to ensure faithful DNA replication despite such challenges. Understanding those responses is essential to understand and prevent or treat replication-associated diseases, such as cancer.Re-priming is a mechanism to allow resumption of DNA synthesis past a fork-stalling lesion. This was recently suggested in yeast and explains the formation of gaps during DNA replication on damaged DNA. Using a combination of assays, we indicate the existence of re-priming also in human cells following UV irradiation.The gap left behind a re-primed fork must be stabilised to avoid replication-associated collapse. Our results show that the checkpoint signalling protein CHK1 is dispensable for stabilisation of replication forks after UV irradiation, despite its role in replication fork progression on UV-damaged DNA. It is not known what proteins are necessary for collapse of an unsealed gap or a stalled fork. We exclude one, previously suggested, endonuclease from this mechanism in UV-irradiated human fibroblasts. We also show that focus formation of repair protein RAD51 is not necessarily associated with cellular sensitivity to agents inducing replicative stress, in rad51d CHO mutant cells.Multiple factors are required for replication fork stability, also under unperturbed conditions. We identify the histone methyltransferase SET8 as an important player in the maintenance of replication fork stability. SET8 is required for replication fork progression, and depletion of SET8 led to the formation of replication-associated DNA damage.In summary, our results increase the knowledge about mechanisms and signalling at replication forks in unperturbed cells and after induction of replicative stress.