AN INEFFICIENT RESOLUTION OF UNDER-REPLICATED DNA IN MITOSIS TRIGGER GENOMIC INSTABILITY

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

Congreso; Reunión conjunta 2020 SAIB-SAMIGE; 2020

SAIB-SAMIGE

The S phase is the period of the cell cycle in which genomes are entirely duplicated. But in organisms with gigabase-sized genomes, such as humans, cells routinely enter the G2 phase with stretches of under-replicated DNA (UR-DNA). Such DNA regions can be duplicated in mitosis by a specialized DNA replication pathway known as mitotic DNA synthesis (MiDAS). Failure to replicate UR-DNA during mitosis results in aberrant chromosome segregation, which subsequently leads to genomic instability. Genomic instability is defined as a persistent and high rate of mutations and is a hallmark of cancer; it contributes 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 downregulation of Checkpoint Kinase 1 (Chk1), a key mediator of the S-phase checkpoint and whose inhibitors are undergoing clinical evaluation across a variety of cancers, induces UR-DNA and MiDAS. But in apparent contrast with the idea that MiDAS completes DNA duplication and hence safeguards genomic stability, our data show that MiDAS in Chk1-deficient cells induces chromosome mis-segregation. Importantly, we unveil the molecular basis of aberrant MiDAS. Upon Chk1 loss, mitotic DNA replication intermediates in mitosis stall due to nucleotide shortage. Stalled DNA replication intermediates are then cleaved by the structure specific endonuclease Mus81-Eme1, and these mitotic DSBs culminate in chromosome mis-segregation. Intriguingly, both MiDAS abrogation, Mus81-Eme1 down-regulation and MiDAS upregulation by nucleosides reverts the genomic instability caused by Chk1 depletion. Such observation indicates that genomic instability is the consequence of incomplete duplication of UR-DNA by MiDAS. Our work unveils a novel molecular pathway leading to genomic instability in cancer cells. Given the interest in avoiding genomic instability during oncologic treatments, our study provides tools to develop novel anti-cancer strategies.