The EHMT2-MBLAC2 axis suppresses ribosomal DNA transcription in response to nucleolar DNA damage.

The induction of DNA double-strand breaks (DSBs) within actively transcribed ribosomal DNA (rDNA) arrays triggers transcriptional suppression and drives nucleolar reorganization, including the formation of nucleolar caps that facilitate the engagement of DSBs with canonical DSB signaling and repair proteins. Although these nucleolar responses are critical for rDNA stability, the components that orchestrate these responses remain unclear. In this study, we identified euchromatic histone-lysine N-methyltransferase 2 (EHMT2) as a novel regulator that is essential for rDNA DSB-induced transcriptional suppression, while functioning independently of ATM-mediated nucleolar responses. We found that EHMT2 is required for the repair of rDNA DSBs and the maintenance of rDNA stability, and its deficiency can result in cellular hypersensitivity to rDNA DSBs. Global proteomic analysis revealed that EHMT2 interacts with MBLAC2 to repress rDNA transcription upon rDNA DSBs. The depletion of EHMT2 or MBLAC2 sensitized colorectal cancer cells to ribosomal stress. Furthermore, we uncovered that EHMT2 promotes colorectal tumorigenesis, revealing a novel mechanistic link between rDNA transcriptional regulation and tumor promotion. Together, our findings established the EHMT2-MBLAC2 axis as a pivotal regulator of mammalian rDNA DSB-induced transcriptional silencing that coordinates rDNA DSB repair and the maintenance of rDNA integrity during nucleolar damage.