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.

PARP1 is implicated in the detection and repair of unligated Okazaki fragment intermediates, highlighting these structures as a potential source of genome breakage induced by PARP inhibition. In agreement with this, we show here that PARP1 activity is greatly elevated in chicken and human S phase cells in which FEN1 nuclease is genetically deleted, and that PARP activity is highest tens of kilobases behind DNA replication forks. Importantly, 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 post-replicative single-strand gaps within individual DNA fibres. Collectively, these data show that PARP inhibitors impede the maturation of Okazaki fragments in nascent DNA, implicating these canonical DNA replication intermediates in the cytotoxicity of these compounds.

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