PARP inhibitors affect the replisome and replication fork progression during a single S phase

Poly(ADP-ribosyl)ation (PARylation) is a protein modification mostly synthesised and degraded by PARP1/2 and PARG enzymes, respectively. PARylation is involved in many covalent and non-covalent protein-protein interactions in the nucleus, making it a powerful form of molecular signaling in DNA metabolism. PARP inhibitors (PARPi) have shown efficacy in the treatment of Homologous Recombination (HR)-deficient cancers, yet the full range of molecular mechanisms underlying the activity of these drugs is not fully understood. Here, we decipher the direct consequences of PARPi-induced loss of PARylation on DNA replication. First, PARPi treatment during a single S phase induces replicative stress, delays S phase progression and causes genome-wide replication timing changes of disease-associated regions. These DNA replication alterations appear to be caused by an accumulation of SSBs in the DNA replicative template when PARylation is inhibited, which then seems to lead to one-ended DSBs and fork collapse during S phase. Second, PARPi deplete FANCD2-I, BRCA, monoubiquitinated PCNA and RAD18 proteins from the replisomes. The two PARPi tested then appear to modulate the choice of fork restart mechanisms and influence replisome dynamics in different ways. Taken together, our study highlights the common and unique primary effects of two PARPi on unperturbed DNA replication in human cells.