The dynamic coupling between metabolic signaling and genome surveillance mechanisms
Faithful and complete duplication of genome relies on the precise regulation of DNA replication and protection of newly synthesized DNA. Globally, DNA replication is determined by the frequency of replication initiation and the speed of individual forks. While the series of events required for initiation are well understood, it is entirely unknown how – or if at all – replication fork progression is controlled. To this end, we identified mammalian TIMELESS as an active regulator of replication fork speed under physiological conditions. We found that TIMELESS is actively removed from the fork under even very mild replication stress, e.g., partial depletion of dNTPs, to slow down replication forks. Perturbation of ribonucleotide reductase (RNR) in humans liberates mild reactive oxygen species (ROS) that are detected by peroxiredoxin 2 (PRDX2), which enforces
the displacement of TIMELESS from the replisome. This process instantly slows replication fork progression. These findings uncover a fundamental physiological mechanism to rapidly adjust fork speed to dynamic metabolic fluctuations and thereby limit replication-borne DNA lesions. Strikingly, cancer cells exploit this pathway to increase their adaptability to adverse metabolic conditions.
Next, we explored hypoxia, which is inevitably associated with solid tumors, and its extent
correlates with aggressiveness of the disease. Reduced O2 availability due to poor vascularization in the growing tumor induces metabolic adaptation by switching from oxidative phosphorylation to aerobic glycolysis (the so-called “Warburg effect”), which results in the sustained accumulation of ROS. Notably, hypoxia-induced metabolic re-programing and genome instability are among the known key hallmarks of cancer. Whether and how these two major pathologies are causally connected is currently unknown. Based on our recent findings of ROS as a second messenger that couples metabolic fluctuations with the dynamics of DNA replication, we set out to investigate whether hypoxia-induced prolonged metabolic adaptation impacts on the replicating genome. Here, we find that hypoxia-induced metabolic alterations generate persistent signaling-competent ROS in
cell nuclei. Nuclear ROS directly activate the ATM kinase to unleash the MRE11 nuclease, which degrades nascent DNA in actively replicating cells. Degradation is initiated at daughter-strand DNA gaps (DSGs), which accumulate in cells deficient in BRCA2 or other tumor suppressors involved in homology-directed DNA repair (HDR). Hence, the pathophysiological degradation of newly synthesized DNA is the long sought-after nexus between the metabolic rewiring and genome instability in oxygen-starved cells. The resulting permanent chromosomal alterations may fuel cancer evolution.
Jiri Lukas laboratory Novo Nordisk Foundation Center for Protein Research University of Copenhagen Denmark
Domain 1 - UMR 3348 - Genotoxic Stress and Cancer