N6-Methyladenosine in nuclear RNA is a major substrate of the obesity-associated FTO

N6-Methyladenosine is an abundant nucleoside in cellular mRNA that undergoes demethylation under physiological conditions by fat mass and obesity-associated protein (FTO). This new pathway suggests that RNA modifications can be reversible and potentially have an impact on RNA metabolism. We report here that fat mass and obesity-associated protein (FTO) has efficient oxidative demethylation activity targeting the abundant N6-methyladenosine (m6A) residues in RNA in vitro. FTO knockdown with siRNA led to increased amounts of m6A in mRNA, whereas overexpression of FTO resulted in decreased amounts of m6A in human cells. We further show the partial colocalization of FTO with nuclear speckles, which supports the notion that m6A in nuclear RNA is a major physiological substrate of FTO.