mRNA circularization by METTL3-eIF3h enhances translation and promotes oncogenesis.

N⁶-methyladenosine (m⁶A) modification of mRNA is emerging as an important regulator of gene expression that affects different developmental and biological processes, and altered m⁶A homeostasis is linked to cancer^1-5. m⁶A modification is catalysed by METTL3 and enriched in the 3' untranslated region of a large subset of mRNAs at sites close to the stop codon⁵. METTL3 can promote translation but the mechanism and relevance of this process remain unknown¹. Here we show that METTL3 enhances translation only when tethered to reporter mRNA at sites close to the stop codon, supporting a mechanism of mRNA looping for ribosome recycling and translational control. Electron microscopy reveals the topology of individual polyribosomes with single METTL3 foci in close proximity to 5' cap-binding proteins. We identify a direct physical and functional interaction between METTL3 and the eukaryotic translation initiation factor 3 subunit h (eIF3h). METTL3 promotes translation of a large subset of oncogenic mRNAs-including bromodomain-containing protein 4-that is also m⁶A-modified in human primary lung tumours. The METTL3-eIF3h interaction is required for enhanced translation, formation of densely packed polyribosomes and oncogenic transformation. METTL3 depletion inhibits tumorigenicity and sensitizes lung cancer cells to BRD4 inhibition. These findings uncover a mechanism of translation control that is based on mRNA looping and identify METTL3-eIF3h as a potential therapeutic target for patients with cancer.

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