N6-methyladenosine marks primary microRNAs for processing

The addition of the N6-methyladenosine (m6A) mark to primary microRNAs by METTL3 in mammalian cells is found to promote the recognition of these microRNA precursors by DGCR8, a component of the microprocessor complex. While N6-methyl-adenosine (m6A) modification of RNA has been known since the 1970s, it was only recently, with the transcriptome-wide characterization of its pervasiveness, that much attention turned to its role in regulating cellular events. Sohail Tavazoie and colleagues have found that the addition of an m6A mark to primary microRNAs (miRNAs) by methyltransferase-like 3 (METTL3) is required for their recognition by DGCR8, the hairpin-binding component of the microprocessor complex. Consequently, in the absence of METTL3, the biogenesis of mature miRNAs is impeded globally in a cell-type-independent manner. One possible role of such a nuclear m6A mark is to distinguish genuine precursors from random RNAs with secondary structure. Altered METTL3 expression has been reported in various human cancers, suggesting a possible link to aberrant expression of miRNAs in cancerous cells. The first step in the biogenesis of microRNAs is the processing of primary microRNAs (pri-miRNAs) by the microprocessor complex, composed of the RNA-binding protein DGCR8 and the type III RNase DROSHA1,2,3,4. This initial event requires recognition of the junction between the stem and the flanking single-stranded RNA of the pri-miRNA hairpin by DGCR8 followed by recruitment of DROSHA, which cleaves the RNA duplex to yield the pre-miRNA product5. While the mechanisms underlying pri-miRNA processing have been determined, the mechanism by which DGCR8 recognizes and binds pri-miRNAs, as opposed to other secondary structures present in transcripts, is not understood. Here we find in mammalian cells that methyltransferase-like 3 (METTL3) methylates pri-miRNAs, marking them for recognition and processing by DGCR8. Consistent with this, METTL3 depletion reduced the binding of DGCR8 to pri-miRNAs and resulted in the global reduction of mature miRNAs and concomitant accumulation of unprocessed pri-miRNAs. In vitro processing reactions confirmed the sufficiency of the N6-methyladenosine (m6A) mark in promoting pri-miRNA processing. Finally, gain-of-function experiments revealed that METTL3 is sufficient to enhance miRNA maturation in a global and non-cell-type-specific manner. Our findings reveal that the m6A mark acts as a key post-transcriptional modification that promotes the initiation of miRNA biogenesis.