Rad51 inhibition sensitizes non-replicating quiescent cells to UVB radiation and transcription stress

DNA damage induced by environmental, occupational, and chemotherapeutic compounds lead to a variety of cellular responses that are potentially impacted by the proliferation status of the cell. Using small molecule inhibitors of various DNA doublestrand break (DSB) repair pathways in non-replicating, quiescent human cells exposed to UVB radiation, we unexpectedly observed a major role for the recombination protein Rad51 in promoting cell survival. In contrast to a previous report indicating a requirement for nucleotide excision repair (NER) in DSB formation after UV exposure in quiescent cells, we observed DSB formation and Rad51 function to be independent of NER. Moreover, our analyses of DNA damage response kinase signaling in quiescent cells identified protein substrates that were either dependent or independent of both NER and apoptotic signaling. Finally, we observed that Rad51 inhibition sensitized nonreplicating quiescent cells to diverse genotoxic stressors, including those inducing DNA- RNA hybrids and inhibiting transcription. Thus, these findings clarify the mechanisms by which DSBs arise in non-replicating cells and highlight the important role of Rad51 in promoting quiescent cell survival in response to general genotoxic stress. Summary statementDNA double strand breaks are generated independent of nucleotide excision repair in UV-irradiated quiescent cells and require the recombination protein Rad51 to promote cell survival.