A pre-rRNA positive feedback loop drives malignant ribosome biogenesis
Altered nucleoli are a well-established hallmark of cancer1, but how oncogenic signalling remodels the nucleolus remains poorly understood. Here we used an inducible mouse model of pancreatic ductal adenocarcinoma (PDAC)2 to generate spatially resolved proteomic and phosphoproteomic maps of the nucleolus upon RAS oncogene activation. We identify a phosphorylation programme initiated by translocation of the Casein Kinase 2 (CK2) holoenzyme to the nucleolus. This programme amplifies rRNA synthesis and malignant ribosome biogenesis by phosphorylating factors that control RNA polymerase I transcription and early ribosomal RNA (rRNA) processing. Preventing the nucleolar activity of CK2 inhibits oncogene-induced rRNA production, while constitutive nucleolar trapping of CK2 is sufficient to activate rRNA synthesis in the absence of RAS oncogene. Mechanistically, CK2 accumulation in the nucleolus is mediated by direct binding to the 3 External Transcribed Sequence (3ETS) of nascent precursor rRNA, creating an RNA-dependent self-amplifying feedback loop. Nucleolar CK2 accumulation is conserved across diverse human cancers, and its disruption synergises with inhibition of oncogenic RAS signalling to suppress anchorage-independent growth and tumourigenesis. Our study reveals 3ETS as a CK2 signalling scaffold that amplifies oncogenic ribosome biogenesis, and defines a druggable nucleolar vulnerability that can be exploited by targeting this process.