DNA lesions occur across the genome and constitute a threat to cell viability; however, damage at specific genomic loci has a relatively greater impact on overall genome stability. The heavily transcribed ribosomal RNA (rDNA) repeats that give rise to the ribosomal RNA are clustered in a unique chromatin structure, the nucleolus (reviewed in Kasselimi et al, 2022 Trends in Biochemical Sciences). Due to its highly repetitive nature and transcriptional activity, the nucleolus is considered a hotspot of genomic instability. Employing targeted double strand breaks in the rDNA we uncovered RASSF1A, a tumour suppressor adaptor protein that is commonly inactivated in cancer as a novel regulator of the nucleolar DNA damage response. We found that RASSF1A gets recruited at rDNA double strand breaks (DSBs) via 53BP1 and facilitates 53BP1 function in local ATM signal amplification. RASSF1A loss of expression, a common event in cancer, results in persistent rDNA breaks, sensitivity in rDNA damage and discrepancies in rDNA copy numbers in lung adenocarcinoma patient cohorts (Tsaridou et al., 2022 EMBO Reports).
Additionally, we developed tools (innovative imaging-based approaches and stable cell lines) to study how the rDNA damage response is regulated in time and space. We employed cell systems where we induced oncogene expression or induced chemically replication stress to explore rDNA as a fragile site. Our data so far supports that rDNA is one of the first genomic sites to accumulate DNA lesions upon induction of replication stress highlighting its contribution to genomic instability in cancer.
