Nucleotide deprivation triggers aberrant processing of newly synthesized DNA during mitosis leading to Mus81-Eme1-mediated chromosome missegregation

CALZETTA, N. L.; GONZÁLEZ BESTEIRO, M. A.; GOTTIFREDI, V.

On-line

Congreso; XV Congresso da MutaGen-Brasil; 2021

MutaGen - Brasil

During the replication phase of the cell cycle (S phase), genomes must be entirely duplicated. Such an endeavor can be challenging when duplicating gigabase-sized genomes, such as the one from humans. As a matter of fact, human cells routinely enter the G2 phase with stretches of under-replicated DNA (UR-DNA). There is a last chance to replicate such DNA regions during mitosis when a specialized DNA replication pathway known as mitotic DNA synthesis (MiDAS) is activated. Failure to replicate UR-DNA during mitosis results in aberrant chromosome segregation, which subsequently leads to genomic instability. Genomic instability increases the rate of mutations and is a hallmark of cancer; contributing to intratumoral genetic heterogeneity, which implies the risk of developing resistance to oncologic treatments. Thus, elucidating the mechanisms that trigger genomic instability is of utmost importance to cancer research. Herein, we show that the downregulation of Checkpoint Kinase 1 (Chk1), a master regulator of DNA replication whose inhibitors are undergoing clinical evaluation across various cancers, induces accumulation of UR-DNA and MiDAS. Significantly, however, Chk1-depletion mediated MiDAS does not safeguards genomic stability and, in striking contrast to such a notion, it triggers chromosome missegregation. Importantly, we unveil the molecular basis of such an aberrant MiDAS. Seemingly, the nucleotide shortage caused by Chk1 loss is key to such a change in the fate of MiDAS events. Because of limiting availability of nucleotides, DNA replication intermediates stall in mitosis to become substrates of the structure-specific endonuclease Mus81-Eme1. The resulting mitotic DSBs then prompt chromosome missegregation. Intriguingly, both MiDAS abrogation, Mus81-Eme1 down-regulation, and MiDAS upregulation by nucleosides reverts the acute genomic instability caused by Chk1 depletion. Such observations suggest that after Chk1 depletion, the genomic instability is triggered by incomplete MiDAS events. Such results unveils a molecular pathway that selective leads to the genomic instability of cancer cells. Given the potential benefit that may be reached when avoiding genomic instability during oncologic treatments, our study reveal novel tools to develop potent anti-cancer strategies.