A large number of signalling pathways converge on p53 to induce different cellular stress responses that aim to promote cell cycle arrest and repair or, if the damage is too severe, to induce irreversible senescence or apoptosis. The differentiation of p53 activity towards specific cellular outcomes is tightly regulated via a hierarchical order of post-translational modifications and regulated protein-protein interactions. The mechanisms governing these processes provide a model for how cells optimize the genetic information for maximal diversity. The p53 mRNA also plays a role in this process and this review aims to illustrate how protein and RNA interactions throughout the p53 mRNA in response to different signalling pathways control RNA stability, translation efficiency or alternative initiation of translation. We also describe how a p53 mRNA platform shows riboswitch-like features and controls the rate of p53 synthesis, protein stability and modifications of the nascent p53 protein. A single cancer-derived synonymous mutation disrupts the folding of this platform and prevents p53 activation following DNA damage. The role of the p53 mRNA as a target for signalling pathways illustrates how mRNA sequences have co-evolved with the function of the encoded protein and sheds new light on the information hidden within mRNAs.

Physiological and pathological conditions that affect the folding capacity of the endoplasmic reticulum (ER) provoke ER stress and trigger the unfolded protein response (UPR). The UPR aims to either restore the balance between newly synthesized and misfolded proteins or if the damage is severe, to trigger cell death. However, the molecular events underlying the switch between repair and cell death are not well understood. The ER-resident chaperone BiP governs the UPR by sensing misfolded proteins and thereby releasing and activating the three mediators of the UPR: PERK, IRE1 and ATF6. PERK promotes G2 cell cycle arrest and cellular repair by inducing the alternative translated p53 isoform p53ΔN40 (p53/47), which activates 14-3-3σ via suppression of p21^CDKN1A. Here we show that prolonged ER stress promotes apoptosis via a p53-dependent inhibition of BiP expression. This leads to the release of the pro-apoptotic BH3-only BIK from BiP and activation of apoptosis. Suppression of bip mRNA translation is mediated via the specific binding of p53 to the first 346-nt of the bip mRNA and via a p53 trans-suppression domain located within the first seven N-terminal amino acids of p53ΔN40. This work shows how p53 targets BiP to promote apoptosis during severe ER stress and further illustrates how regulation of mRNA translation has a key role in p53-mediated regulation of gene expression during the UPR.

Background: The ATM kinase constitutes a master regulatory hub of DNA damage and activates the p53 response pathway by phosphorylating the MDM2 protein, which develops an affinity for the p53 mRNA secondary structure. Disruption of this interaction prevents the activation of the nascent p53. The link of the MDM2 protein—p53 mRNA interaction with the upstream DNA damage sensor ATM kinase and the role of the p53 mRNA in the DNA damage sensing mechanism, are still highly anticipated. Methods: The proximity ligation assay (PLA) has been extensively used to reveal the sub-cellular localisation of the protein—mRNA and protein–protein interactions. ELISA and co-immunoprecipitation confirmed the interactions in vitro and in cells. Results: This study provides a novel mechanism whereby the p53 mRNA interacts with the ATM kinase enzyme and shows that the L22L synonymous mutant, known to alter the secondary structure of the p53 mRNA, prevents the interaction. The relevant mechanistic roles in the DNA Damage Sensing pathway, which is linked to downstream DNA damage response, are explored. Following DNA damage (double-stranded DNA breaks activating ATM), activated MDMX protein competes the ATM—p53 mRNA interaction and prevents the association of the p53 mRNA with NBS1 (MRN complex). These data also reveal the binding domains and the phosphorylation events on ATM that regulate the interaction and the trafficking of the complex to the cytoplasm. Conclusion: The presented model shows a novel interaction of ATM with the p53 mRNA and describes the link between DNA Damage Sensing with the downstream p53 activation pathways; supporting the rising functional implications of synonymous mutations altering secondary mRNA structures.

P53 is an intrinsically disordered protein with a large number of post-translational modifications and interacting partners. The hierarchical order and subcellular location of these events are still poorly understood. The activation of p53 during the DNA damage response (DDR) requires a switch in the activity of the E3 ubiquitin ligase MDM2 from a negative to a positive regulator of p53. This is mediated by the ATM kinase that regulates the binding of MDM2 to the p53 mRNA facilitating an increase in p53 synthesis. Here we show that the binding of MDM2 to the p53 mRNA brings ATM to the p53 polysome where it phosphorylates the nascent p53 at serine 15 and prevents MDM2-mediated degradation of p53. A single synonymous mutation in p53 codon 22 (L22L) prevents the phosphorylation of the nascent p53 protein and the stabilization of p53 following genotoxic stress. The ATM trafficking from the nucleus to the p53 polysome is mediated by MDM2, which requires its interaction with the ribosomal proteins RPL5 and RPL11. These results show how the ATM kinase phosphorylates the p53 protein while it is being synthesized and offer a novel mechanism whereby a single synonymous mutation controls the stability and activity of the encoded protein.

Cancer is the second leading cause of death globally. One of the main hallmarks in cancer is the functional deregulation of crucial molecular pathways via driver genetic events that lead to abnormal gene expression, giving cells a selective growth advantage. Driver events are defined as mutations, fusions and copy number alterations that are causally implicated in oncogenesis. Molecular analysis on tissues that have originated from a wide range of anatomical areas has shown that mutations in different members of several pathways are implicated in different cancer types. In recent decades, significant efforts have been made to incorporate this knowledge into daily medical practice, providing substantial insight towards clinical diagnosis and personalized therapies. However, since there is still a strong need for more effective drug development, a deep understanding of the involved signaling mechanisms and the interconnections between these pathways is highly anticipated. Here, we perform a systemic analysis on cancer patients included in the Pan-Cancer Atlas project, with the aim to select the ten most highly mutated signaling pathways (p53, RTK-RAS, lipids metabolism, PI-3-Kinase/Akt, ubiquitination, b-catenin/Wnt, Notch, cell cycle, homology directed repair (HDR) and splicing) and to provide a detailed description of each pathway, along with the corresponding therapeutic applications currently being developed or applied. The ultimate scope is to review the current knowledge on highly mutated pathways and to address the attractive perspectives arising from ongoing experimental studies for the clinical implementation of personalized medicine.