Reduced Topoisomerase 1-dependent R-loops Promote Aberrant Replication Fork Acceleration Underlying Oncogene-induced Genomic Instability

DNA replication is a complex process tightly regulated to ensure faithful genome duplication, and thus replication perturbation leads to DNA damage and genome instability. Oncogene expression or treatment with replication inhibitors generate replication stress characterized by fork slowing or even stalling and activation of dormant origins. However, whether oncogenes could induce other forms of replication perturbation causing genomic instability remains unknown. Here we show that activation of mutated RAS leads to a non-canonical form of replication stress, characterized by accelerated replication rate, resulting in DNA damage. Investigating the molecular mechanism underlying this replication stress revealed that mutated RAS increases topoisomerase 1 (TOP1) expression and reduces levels of RNA-DNA hybrids (R-loops). Restoration of the perturbed replication either by restoration of TOP1 levels or directly by mild replication inhibition, leads to a dramatic reduction in the DNA damage. Furthermore, decreased R-loop accumulation leads to fork acceleration. Altogether, these results reveal a novel form of oncogene-induced replication stress, leading to deleterious acceleration of replication rate driving genomic instability. Furthermore, the results indicate a novel role of TOP1 in the regulation of DNA replication dynamics by TOP1-dependent R-loop levels.