Mitotic processing of DNA replication and repair intermediates in cancer

Abstract

Genomic instability is one of the hallmarks of cancer. Genome integrity is safeguarded by a complex network of DNA repair pathways and cell cycle checkpoints. Cells duplicate their genome during S-phase, followed by the separation of the sister chromatids into two daughter cells during mitosis. Failure to complete DNA replication before the onset of mitosis, for example due to excessive DNA damage or dysregulated DNA replication, can lead to mitotic aberrancies and loss of genetic information. Recent evidence suggests that DNA lesions that remain unresolved during S-phase can be transferred into mitosis. In this thesis, we aimed to characterise the pathways responsible for the mitotic processing of unresolved replication-associated DNA lesions. We focussed on tumours with mutations in DNA repair genes BRCA1 or BRCA2, which are frequently treated with PARP inhibitors. We found that PARP inhibitor-induced DNA lesions are transferred into mitosis, and processed to give rise to sister chromatid exchanges (SCEs). The SCEs were accompanied by deletions, consistent with mutational signatures observed in BRCA1/2-deficient tumours. DNA replication intermediates that fail to be resolved during the early stages of mitosis, lead to formation of ultrafine DNA bridges (UFBs). The processing of UFBs is currently still poorly understood. Using a genetic loss-of-function screen, we identified C1orf112/FIRRM as a novel player in the maintenance of replication fork dynamics, therefore preventing UFB formation. Overall, our findings may aid in the development of anti-cancer therapies that selectively target mitotic DNA repair pathways.