RAD51 up- or down regulation is highly toxic to human transformed cells through the induction of replication stress, G2/M arrest and apoptosis

Abstract Homologous recombination (HR) plays a central role in DNA repair and replication, and consequently in the maintenance of genome stability. Although most HR genes are essential in non-malignant proliferating cells, many of them are mutated in cancers. This implies that either actively proliferating cancer cells have adapted to cope with HR defects or HR proteins became dispensable in this context. Here, we observed that prolonged inhibition of HR results in severe toxicity in a variety of transformed human cells, dismissing the latter hypothesis. We then show that HR deficient cells experience a massive replication stress, and that their cell cycle is blocked in late G2 before entry into mitosis. This cell cycle arrest is not mediated by the activation of the ATR/CHK1 axis but by downregulation of Cyclin B1 and then leads to apoptosis. Intriguingly, RAD51, which plays a pivotal role in HR, is overexpressed rather than inactivated in cancers. We show that overexpression of wild-type RAD51 also impedes replication and triggers apoptosis in transformed cells, which points to a unifying mechanism of cell death upon HR dysregulation. Our results underscore the importance of tight control of HR for maintaining cell viability and also imply that HR dysregulated tumor cells have adapted to escape cell cycle arrest and induction of apoptosis.