Recent studies suggest that PARP1 inhibitors, several of which are currently in clinical trial, may selectively kill BRCA1/2 mutant cancers cells. It is thought that the success of this therapy is based on immitigable lethal DNA damage in the cancer cells resultant from the concurrent loss or inhibition of two DNA damage repair pathways: single-strand break (SSB) repair and homologous recombination repair (HRR). Presumably, inhibition of PARP1 activity obstructs the repair of SSBs and during DNA replication, these lesions cause replication fork collapse and are transformed into substrates for HRR. In fact, several previous studies have indicated a hyper-recombinogenic phenotype in the absence of active PARP1 in vitro or in response to DNA damaging agents. In this study, we demonstrate an increased frequency of spontaneous HRR in vivo in the absence of PARP1 using the p un assay. Furthermore, we found that the HRR events that occur in Parp1 nullizygous mice are associated with a significant increase in large, clonal events, as opposed to the usually more frequent single cell events, suggesting an effect in replicating cells. In conclusion, our data demonstrates that PARP1 inhibits spontaneous HRR events, and supports the model of DNA replication transformation of SSBs into HRR substrates.
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