Functional interaction between the DNA damage tolerance pathway and the DNA damage checkpoint : implications for genome stability and oncogenesis

Our genome is continuously exposed to DNA damaging agents. In order to preserve the integrity of their genome, cells have evolved a DNA damage signalling pathway known as checkpoint. Some lesions may persist when cells enter the S-phase and halt the progression of replicative DNA polymerases. This can cause prolonged replication forks stalling which threaten the stability of the genome. To preserve the integrity of genetic information, cells have developed a tolerance pathway which involves specialized DNA polymerases, called translesion DNA polymerases (TLS Pols). These polymerases have the unique ability to accommodate the damaged bases thanks to their catalytic site. In this process, PCNA acts as a scaffold for many proteins involved in DNA metabolism. The mechanisms governing the exchanges between different PCNA partners are not well understood. Among the proteins that interact with PCNA, CDT1, p21 and PR-Set7/set8 are characterized by a high binding affinity. These proteins have a particular interaction domain with PCNA, called "PIP degron", which promotes their proteasomal degradation via the E3 ubiquitin ligase CRL4Cdt2. After UV-C irradiation, the replication initiation factor CDT1 is rapidly degraded in a PIP degron-dependent manner. During the first part of my work, we wanted to understand the functional role of this degradation. Our results have shown that inhibition of CDT1 degradation by CRL4Cdt2 in mammalian cells, compromises the relocalisation of TLS Pol eta and Pol kappato nuclear foci after UV-C irradiation. We also found that only the proteins which contain a PIP degron interfere with the formation of Pol eta foci. Mutagenesis experiments on CDT1 PIP degron revealed that a threonine residue conserved among PIP degrons is essential for inhibiting foci formation of at least two TLS polymerases. This results suggest that CRL4Cdt2-dependent degradation of proteins containing PIP degrons regulates the recruitment of TLS polymerases at sites of UV-induced DNA damage.During the second part of my thesis, we studied DNA damage checkpoint regulation during embryogenesis. Indeed, in early embryos, the DNA damage checkpoint is silent until the mid-blastula transition (MBT) due to maternal inhibiting factors. In this work, we have shown, both in vitro and in vivo, that the E3 ubiquitin ligase RAD18, a major regulator of translesion DNA synthesis, is a limiting factor for the checkpoint activation in Xenopus embryos. We have also shown that RAD18 depletion leads to the activation of DNA damage checkpoints by inducing replication fork uncoupling in front of the lesions. Furthermore, we showed that the abundance of RAD18 and PCNA monoubiquitination (PCNAmUb) is regulated during embryonic development. Near the MBT, the increased abundance of DNA limits the availability of RAD18, thereby reducing the amount of PCNAmUb and inducing the de-repression of the checkpoint. Moreover, we have shown that this embryonic-like regulation can be reactivated in somatic mammalian cells by ectopic expression of RAD18, conferring resistance to DNA damaging. Finally, we found high RAD18 levels in glioblastoma cancer stem cells highly resistant to DNA damage. All together, these data propose RAD18 as a critical factor that inhibits DNA damage checkpoint in early embryos and suggests that dysregulation of RAD18 expression may have an unexpected oncogenic potential