cGAS restricts PARP1-mediated microhomology-mediated end joining by suppressing poly-ADP-ribosylation.

Repair of DNA double-strand breaks (DSBs) is essential for cells to maintain genome stability and cell survival. While cyclic GMP-AMP synthase (cGAS) is best known for its role in innate immunity, emerging evidence reveals that it plays regulatory roles in DNA damage response. In this study, we demonstrate that cGAS suppresses PARP1-mediated poly(ADP-ribose) (PAR) formation at both double-stranded DNA (dsDNA) and DNA:RNA hybrids. As a result, cGAS deficiency enhances PARP1-mediated microhomology-mediated end joining (MMEJ). Since MMEJ competes with transcription-coupled homologous recombination (TC-HR) at actively transcribed genomic regions, cGAS-mediated suppression of MMEJ consequently leads to efficient TC-HR with increased recruitment of RAD52 and RAD51. Mechanistically, the zinc finger domain of cGAS binds PAR, inhibiting PARP1 activation. In prostate cancer cells, loss of cGAS increases dependency on PARP1, rendering them more sensitive to PARP inhibitors. Collectively, our findings uncover a noncanonical role for cGAS in negatively regulating PARP1-mediated MMEJ and suggest its potential as a therapeutic biomarker in prostate cancer.