FTO controls reversible m 6 Am RNA methylation during snRNA biogenesis

Small nuclear RNAs (snRNAs) are core spliceosome components and mediate pre-mRNA splicing. Here we show that snRNAs contain a regulated and reversible nucleotide modification causing them to exist as two different methyl isoforms, m 1 and m 2 , reflecting the methylation state of the adenosine adjacent to the snRNA cap. We find that snRNA biogenesis involves the formation of an initial m 1 isoform with a single-methylated adenosine (2′- O -methyladenosine, Am), which is then converted to a dimethylated m 2 isoform ( N 6 ,2′- O -dimethyladenosine, m 6 Am). The relative m 1 and m 2 isoform levels are determined by the RNA demethylase FTO, which selectively demethylates the m 2 isoform. We show FTO is inhibited by the oncometabolite d -2-hydroxyglutarate, resulting in increased m 2 -snRNA levels. Furthermore, cells that exhibit high m 2 -snRNA levels show altered patterns of alternative splicing. Together, these data reveal that FTO controls a previously unknown central step of snRNA processing involving reversible methylation, and suggest that epitranscriptomic information in snRNA may influence mRNA splicing.