PARP inhibition impedes the maturation of nascent DNA strands during DNA replication

Biochemical and cell-based assays reveal that PARP inhibitors impede the maturation of nascent DNA strands during DNA replication, and implicate unligated Okazaki fragments and other nascent strand discontinuities in the cytotoxicity of these anti-cancer compounds. Poly(ADP-ribose) polymerase 1 (PARP1) is implicated in the detection and processing of unligated Okazaki fragments and other DNA replication intermediates, highlighting such structures as potential sources of genome breakage induced by PARP inhibition. Here, we show that PARP1 activity is greatly elevated in chicken and human S phase cells in which FEN1 nuclease is genetically deleted and is highest behind DNA replication forks. PARP inhibitor reduces the integrity of nascent DNA strands in both wild-type chicken and human cells during DNA replication, and does so in FEN1(-/-) cells to an even greater extent that can be detected as postreplicative single-strand nicks or gaps. Collectively, these data show that PARP inhibitors impede the maturation of nascent DNA strands during DNA replication, and implicate unligated Okazaki fragments and other nascent strand discontinuities in the cytotoxicity of these compounds.

Automated summary

PARP inhibitors hinder the maturation of nascent DNA strands during DNA replication, particularly unligated Okazaki fragments and other discontinuities, resulting in cytotoxic effects. The activation of PARP1 is elevated in cells lacking the FEN1 nuclease, suggesting its involvement in the detection and processing of these DNA replication intermediates. PARP inhibitors disrupt the integrity of nascent DNA strands in both normal and FEN1(-/-) cells, leading to the formation of single-strand nicks or gaps. These findings highlight the importance of unligated Okazaki fragments and other discontinuities in the cytotoxicity of PARP inhibitors.