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Mécanismes par lesquels la recombinaison homologue prévient les défauts mitotiques induits par le stress réplicatif

Abstract : At each cell cycle, cells undertaking the DNA replication process face several sources of replication stress (RS) compromising the progression of the replicating forks and threatening both chromosome duplication fidelity and their correct segregation during mitosis. Replication stresses emerged as a major source of genetic instability and cancer development. Several mechanisms, among which homologous recombination (HR), operate to buffer the deleterious effects of RS. HR acts as an escort to fork progression and prevents mitotic defects. Nonetheless, the molecular connection between replication stress and mitotic defects remains elusive. A conditional replication fork barrier (RFB) acting in a polar manner was developed in the lab to terminally-arrest fork progression. In this system, HR functions handling replication stress can be assessed independently of its well-known function in double strand break repair. The work described here aims to understanding the mechanism that HR performs to ensure genetic stability in response to replication stress. In general, blocked replication forks can be rescued either by fork convergence or by active HR-mediated fork restart. However, in absence of Rad51 recombinase or it loader Rad52, a single activated RFB is sufficient to induce mitotic abnormalities including anaphase bridges. The involvement of HR factors in fork protection was explored at the molecular and cellular levels. It turns out that terminally-arrested forks are extensively resected by the Exo1 nuclease in the absence of Rad51/Rad52. Interestingly, the excess of ssDNA accumulation at the fork triggers sister chromatid non-disjunction in mitosis despite the arrival of an uncorrupted converging fork to rescue replication. Thus, unprotected replication forks are prone to pathological termination threatening chromosome segregation. HR being involved in fork protection and restart, the use of a Rad51 mutant showed that these two functions are genetically separable. Indeed, protected forks unable to restart by HR do not show any pathological termination. Thus, beyond their ability to restart inactivated forks, HR factors ensure replication completion by maintaining the forks in a suitable conformation for a fusion with the converging fork. Overall, these results shed light on the molecular events engaged by RH to ensure genome stability in response to replication stress.
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Submitted on : Friday, July 20, 2018 - 1:09:05 PM
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Anissia Ait Saada. Mécanismes par lesquels la recombinaison homologue prévient les défauts mitotiques induits par le stress réplicatif. Génétique. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS167⟩. ⟨tel-01845376⟩

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