Resumen de Molecular causes and mechanisms of genomic instability in G1- deregulated cell cycle
Eukaryotic DNA replication initiates at numerous sites called origins and is tightly regulated so that chromosomes are accurately replicated only once per cell cycle. Absence of active cyclin-dependent kinase (CDK) complexes from late mitosis and during G1 allows the licensing of origins for their potential activation later in S phase. At the G1/S transition, rising levels of CDK activity block additional origin licensing and promote the activation of a subset of licensed origins, together with Dbf4-dependent kinase (DDK) activity. Origin activation follows a spatiotemporal program during S phase that is highly conserved in cell populations but partially stochastic in individual cells, and presumed to largely influence the timely completion of DNA replication due to exceeding numbers of licensed origins available to counteract hindered forks. Replication completion is critical to the genome integrity, as cells allowed to enter mitosis with on-going forks might suffer from chromosome breaks upon premature segregation during anaphase.
In both yeast and mammals, genomic instability arises when the G1/S transition is deregulated, as cells escaping this control have proliferative advantages and a mutator phenotype. Indeed, cancer cells often show mutations in G1/S regulators and perturbed DNA replication; furthermore, genomic instability is a hallmark of cancer. However, the molecular mechanism by which G1-phase deregulation causes genomic instability remains poorly understood. To study this question, we used budding yeast cells lacking the CDK inhibitor Sic1, a central regulator of the G1 phase and orthologous to p27Kip1 in mammals, as eukaryotic model of oncogenic cell cycles. Here we show that in the absence of Sic1, cells loose functional origin redundancy that directly causes chromosomal instability. Moreover, we report that the differential loss of origin redundancy along the genome delays the completion of DNA synthesis at specific chromosomal regions. Importantly, these defects at sites containing elements delaying fork-progression commit chromosomes to fragility. Finally, we show that chromosomal instability in cells lacking Sic1 can be supressed by retaining cells prior to anaphase entry without alleviating G1-phenotype and origin activity defects, consistent with uncoupled DNA replication completion and mitosis entry. We conclude that in G1/S deregulated cells, chromosomal regions with an irregular distribution of inefficient origins are delayed in completing replication by lacking of functional origin redundancy that causes genomic instability. Moreover, additional obstacles to fork elongation at these regions may impede DNA replication completion and commit these sites to fragility by resulting in chromosome breaks during mitosis, which is considered a driving force of oncogenesis.
