The nuclear RNAi factor, NRDE2, prevents the accumulation of DNA damage during mitosis in stressful growth conditions
Organisms have evolved multiple mechanisms to prevent and repair DNA damage to protect the integrity of the genome, particularly under stressful conditions. Unrepaired DNA damage leads to genomic instability, aneuploidy, and an increased risk for cancer. Before the cell can divide, it must repair damaged DNA and it is thought that this process requires global silencing of most transcription. In C. elegans, NRDE-2, in complex with other nuclear factors and guided by small RNA, directs heterochromatin formation and transcriptional silencing of targeted genes. Additionally, when C. elegans are cultivated at high temperatures, NRDE-2 is required to maintain germ line immortality. However, the role of NRDE-2 in maintaining the physical integrity of the genome is not understood. We show here that loss of NRDE2 in either nematode or human cells induces the accumulation of DNA damage specifically under conditions of stress, such as cultivation at a high temperature in C. elegans or Aurora B Kinase oncogenic overexpression in the MCF10A epithelial breast cell line. In addition, we found that NRDE2 interacts with {beta}-actin in unstressed mammalian cells. This interaction is dramatically reduced upon DNA damage. The oligomerization state of nuclear actin alters its association with targets, which in turn, regulates their function. Monomeric nuclear actin binds to heterochromatin remodeling factors and transcriptional activators while filamentous actin has been implicated in DNA repair processes. Here we show that NRDE2 associates with actin only when DNA is intact and the bulk of nuclear actin is monomeric. Thus, NRDE2 may dissociate from actin when it becomes filamentous as a result of DNA damage. This implies that, NRDE2, in its role as a heterochromatin factor, binds to monomeric actin, protecting the genome from DNA damage in stressful conditions. In this way, heterochromatin factors may associate with the actin dependent DNA repair process to allow appropriate mitotic progression and maintain genomic integrity.