PARP1 promotes replication-independent DNA double-strand break formation after acute DNA-methylation damage.

Poly-ADP-Ribose Polymerase 1 (PARP1) is a potent regulator of DNA damage response signaling through the recruitment of DNA damage repair proteins to damage sites, and its catalytic function of converting Nicotinamide adenine dinucleotide (NAD+) into poly-ADP-ribose (PAR) which covalently modifies hundreds of protein substrates in a process known as PARylation. However, PARP1's role in the recognition, processing, and intracellular signaling downstream of DNA damage in cells remains incompletely understood, especially in a replication-independent context. Here, we show that cells exposed to high doses of the methylating agent Methyl Methanesulfonate (MMS) generate DNA double-strand breaks (DSBs) in a base excision repair (BER)-dependent and DNA replication-independent manner. The capacity of cells to generate DSBs after MMS exposure relies heavily on intracellular NAD+ availability and PARP1's catalytic production of PAR. In our experimental system, we show that acute MMS exposure causes NAD+ exhaustion in a PARP1-dependent manner, which results in a temporal-dependent loss of downstream PARP1 activity. This functional loss of PARP1 signaling in later timepoints leads to the loss of BER-dependent single-strand break (SSB)-to-DSB conversion, as well as silencing of ATR-Chk1 signaling in both cycling and non-cycling cells, demonstrating a novel PARP1-dependent regulatory mechanism for both ATR-Chk1 signaling and BER-associated processes following methylation challenge. Additionally, we provide experimental evidence supporting the role of PARP1 and NAD+ in promoting the exonuclease-mediated SSB-to-DSB conversion. These findings support a previously uncharacterized mechanism of PARP1-mediated replication-independent DSB generation and provide insight into checkpoint signaling by integrating DDR with PARP1's consumption of NAD+ and production of PAR.