Nucleotide depletion reveals the impaired ribosome biogenesis checkpoint as a barrier against DNA damage.

Marta Garcia‐Cajide,Ramon Salazar,Oscar Yanes,Sara Samino,Constanza Cortés,Caroline Mauvezin,Ricard Marcos,Neus Agell,Antonio Gentilella,Ana Domostegui,Sara C Kozma,Joffrey Pelletier,Sonia Feu,Ferran Riaño‐Canalias,Eugènia Almacellas,George Thomas,Sandra Menoyo,Albert Tauler

doi:10.15252/embj.2019103838

Abstract

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint (IRBC) and the DNA damage response (DDR) have been implicated in p53 activation following nucleotide depletion. However, it is difficult to reconcile the two checkpoints operating together, as the IRBC induces p21‐mediated G1 arrest, whereas the DDR requires that cells enter S phase. Gradual inhibition of inosine monophosphate dehydrogenase (IMPDH), an enzyme required for de novo GMP synthesis, reveals a hierarchical organization of these two checkpoints. We find that the IRBC is the primary nucleotide sensor, but increased IMPDH inhibition leads to p21 degradation, compromising IRBC‐mediated G1 arrest and allowing S phase entry and DDR activation. Disruption of the IRBC alone is sufficient to elicit the DDR, which is strongly enhanced by IMPDH inhibition, suggesting that the IRBC acts as a barrier against genomic instability.

Highlights

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation
This study demonstrates a hierarchy of p53-dependent checkpoints activated by the inhibition of de novo guanine nucleotide synthesis
We have focused on inosine monophosphate dehydrogenase (IMPDH) inhibitors given their clinical approval in other disease settings and the recent exciting pre-clinical findings concerning their application in specific cancer types (Valvezan et al, 2017; Huang et al, 2018); it is most likely that other clinically approved non-genotoxic nucleotide inhibitors operate in a similar manner

Summary

Introduction

Many oncogenes enhance nucleotide usage to increase ribosome content, DNA replication, and cell proliferation, but in parallel trigger p53 activation. Both the impaired ribosome biogenesis checkpoint (IRBC) and the DNA damage response (DDR) have been implicated in p53 activation following nucleotide depletion. It is difficult to reconcile the two checkpoints operating together, as the IRBC induces p21-mediated G1 arrest, whereas the DDR requires that cells enter S phase. We find that the IRBC is the primary nucleotide sensor, but increased IMPDH inhibition leads to p21 degradation, compromising IRBC-mediated G1 arrest and allowing S phase entry and DDR activation. Disruption of the IRBC alone is sufficient to elicit the DDR, which is strongly enhanced by IMPDH inhibition, suggesting that the IRBC acts as a barrier against genomic instability

Methods

Results

Conclusion