Redundant but essential functions of PARP1 and PARP2 in DNA ligase I-independent DNA replication.

While DNA ligase I (LigI) joins most Okazaki fragments, a backup pathway involving poly(ADP-ribose) synthesis, XRCC1 and DNA ligase IIIα (LigIIIα) functions along with the LigI-dependent pathway and is also capable of supporting DNA replication in the absence of LigI. Here we have addressed for the first time the roles of PARP1 and PARP2 in this pathway using isogenic null derivatives of mouse CH12F3 cells. While single and double null mutants of the parental cell line and single mutants of LIG1 null cells were viable, loss of both PARP1 and PARP2 was synthetically lethal with LigI deficiency. Thus, PARP1 and PARP2 have a redundant essential role in LigI-deficient cells. Interestingly, higher levels of PARP2 but not PARP1 associated with newly synthesized DNA in the LIG1 null cells and there was a much higher increase in PARP2 chromatin retention in LIG1 null cells incubated with the PARP inhibitor olaparib with this effect occurring independently of PARP1. Together our results suggest that PARP2 plays a major role in specific cell types that are more dependent upon the backup pathway to complete DNA replication and that PARP2 retention at unligated Okazaki fragments likely contributes to the side effects of current clinical PARP inhibitors.