53BP1 mediated recruitment of RASSF1A at ribosomal DNA breaks facilitates local ATM signal amplification

DNA lesions occur across the genome and constitute a threat to cell viability; however, damage at specific genomic loci has a disproportionally greater impact on the overall genome stability. The ribosomal RNA gene repeats (rDNA) are emerging fragile sites due to repetitive nature, clustering and high transcriptional activity. Recent progress in understanding how the rDNA damage response is organized has highlighted the key role of adaptor proteins in the response. Here we identify that the scaffold and tumor suppressor, RASSF1A is recruited at sites of damage and enriched at rDNA breaks. Employing targeted nucleolar DNA damage, we find that RASSF1A recruitment requires ATM activity and depends on 53BP1. At sites of damage RASSF1A facilitates local ATM signal establishment and rDNA break repair. RASSF1A silencing, a common epigenetic event during malignant transformation, results in persistent breaks, rDNA copy number alterations and decreased cell viability. Meta-analysis of a lung adenocarcinoma cohort showed that RASSF1A epigenetic silencing leads in rDNA copy number discrepancies. Overall, we present evidence that RASSF1A acts as a DNA repair factor and offer mechanistic insight in how the nucleolar DNA damage response is organized.