METTL1 Promotes let-7 MicroRNA Processing via m7G Methylation

Summary

7-methylguanosine (m7G) is present at mRNA caps and at defined internal positions within tRNAs and rRNAs. However, its detection within low-abundance mRNAs and microRNAs (miRNAs) has been hampered by a lack of sensitive detection strategies. Here, we adapt a chemical reactivity assay to detect internal m7G in miRNAs. Using this technique (Borohydride Reduction sequencing [BoRed-seq]) alongside RNA immunoprecipitation, we identify m7G within a subset of miRNAs that inhibit cell migration. We show that the METTL1 methyltransferase mediates m7G methylation within miRNAs and that this enzyme regulates cell migration via its catalytic activity. Using refined mass spectrometry methods, we map m7G to a single guanosine within the let-7e-5p miRNA. We show that METTL1-mediated methylation augments let-7 miRNA processing by disrupting an inhibitory secondary structure within the primary miRNA transcript (pri-miRNA). These results identify METTL1-dependent N7-methylation of guanosine as a new RNA modification pathway that regulates miRNA structure, biogenesis, and cell migration.

Graphical Abstract

Highlights

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  Internal m7G is identified in miRNAs by two independent sequencing techniques

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  Methyltransferase METTL1 mediates m7G modification of specific miRNAs

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  METTL1 promotes miRNA maturation and suppresses lung cancer cell migration

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  m7G promotes processing by antagonizing G-quadruplex structures in miRNA precursors

Abstract

Pandolfini, Barbieri, et al. show that a subgroup of tumor suppressor microRNAs, including let-7e, contain 7-methylguanosine (m7G). Methyltransferase METTL1 is required for m7G modification of miRNAs, their efficient processing, and the inhibition of lung cancer cell migration. Structurally, m7G in miRNA precursors antagonizes RNA secondary structures that would otherwise inhibit their maturation.