PARP1-specific inhibitor displays PARP1-detrapping activity

The successful use of PARP inhibitors against primarily homologous recombination-deficient tumors relies on their ability to inhibit the catalytic activity of PARP1 and PARP2, leading to their retention at sites of DNA damage, known as PARP "trapping." Different PARP inhibitors vary in their trapping ability and thus in their resulting toxicity. Here, we develop an inducible complementation system for the expression of tagged PARP1 variants to assess the impact of different mutations in the PARP1 catalytic site on ADP-ribosylation (ADPr) activity, trapping, and cell survival. Testing these cell lines for their behavior and sensitivity to different PARP inhibitors, we find that saruparib, a first-in-class PARP1-specific inhibitor, promotes the release of certain PARP1 catalytic mutants and resistance to this inhibitor. We also characterize a PARP1 catalytic mutant with intact mono(ADP-ribosyl)ation activity but devoid of poly(ADP-ribosyl)ation, enabling us to demonstrate a significant contribution of mono(ADP-ribosyl)ation toward PARP1 release from sites of DNA damage.