m6Am sequesters PCF11 to suppress premature termination and drive neuroblastoma differentiation

N 6 ,2′-O-dimethyladenosine (m 6 Am) is an abundant mRNA modification that impacts multiple diseases, but its function remains controversial because the m 6 Am reader is unknown. Using quantitative proteomics, we identified transcriptional terminator premature cleavage factor II (PCF11) as a m 6 Am-specific reader in human cells. Direct quantification of mature versus nascent RNAs reveals that m 6 Am does not regulate mRNA stability but promotes nascent transcription. Mechanistically, m 6 Am functions by sequestering PCF11 away from proximal RNA polymerase II (RNA Pol II). This suppresses PCF11 from dissociating RNA Pol II near transcription start sites, thereby promoting full-length transcription of m 6 Am-modified RNAs. m 6 Am’s unique relationship with PCF11 means m 6 Am function is enhanced when PCF11 is reduced, which occurs during all- trans -retinoic-acid (ATRA)-induced neuroblastoma-differentiation therapy. Here, m 6 Am promotes expression of ATF3, which represses neuroblastoma biomarker MYCN. Depleting m 6 Am suppresses MYCN repression in ATRA-treated neuroblastoma and maintains their tumor-stem-like properties. Collectively, we characterize m 6 Am as an anti-terminator RNA modification that suppresses premature termination and modulates neuroblastoma’s therapeutic response.