Mechanisms that control the growth of cells in the presence of replication stress

Resumen

During our lifetime, DNA encounters many kinds of damage, both from endogenous and exogenous origin. Most lesions are detected and repaired without compromising cell viability. However, there are special types of damage that can endanger genome integrity. During replication, for instance, a wide range of lesions can occur. The different types of damage that appear during DNA replication give rise to Replication Stress (RS). If the amount of damage is over a certain threshold, cells can activate apoptotic and/or senescence programs, which can compromise the regenerative ability of tissues and lead to ageing related pathologies. This way, the integrity of the genome is a crucial event in the life of every cell. Nevertheless, unlike proteins and other molecules, DNA is not replaced. Thus, proper detection of DNA damage, precise signaling and potent repair machineries are needed. The response that cells establish upon DNA damage is what we call the DNA Damage Response (DDR). Protein kinases such us ATM and ATR are the key activators of this transduction pathway. They start a phosphorylation cascade in order to establish the different cell cycle checkpoints, which will slow down the cell cycle, leaving time for DNA repair. Replication stress is a particular source of DNA damage that has been linked to cancer and ageing, and which is suppressed by the ATR kinase. In mice, reduced ATR levels in a model of the ATR-Seckel Syndrome lead to RS and accelerated ageing. Similarly, ATR-Seckel embryonic fibroblasts (MEF) accumulate RS and undergo cellular senescence. We previously showed that senescence of ATR-Seckel MEF cannot be rescued by p53-deletion. In this thesis work we show that the genetic ablation of the INK4a/ARF locus fully rescues senescence on ATR mutant MEF, but also that induced by other conditions that generate RS, such us low doses of HU or ATR inhibitors. In addition, we show that a persistent exposure to RS leads to increased levels of INK4a/ARF products, revealing that INK4a/ARF behaves as a bona fide RS- checkpoint. Our data revealed an unknown role for INK4a/ARF in limiting the expansion of cells suffering from persistent replication stress, linking this well-know tumor suppressor to the maintenance of genomic integrity.