Structures of human m⁶A RNA demethylases: FTO and ALKBH5

Methylation at the N6 position of adenosine in mRNA (m⁶A) was first observed around 40 years ago but little was known about the function of this modification at the time of discovery. Research over the last ten years has led to the identification of methyltransferases that produce or `write' the m<sup>6</sup>A RNA mark and recently demethylases that remove or `erase' m⁶A marks, thus indicating that the m⁶A modification is reversible. Recent analyses have identified the motif and regions where m⁶A marks are present within RNA transcripts as well as m⁶A binding protein `readers'. These studies have revealed a role for m⁶A methylation in mRNA stability and demonstrated that sequence specific subsets of transcripts can be regulated in this manner. Like the histone lysine JmjC demethylases, nucleic acid demethylases are members of the 2-oxoglutarate and iron dependent oxygenase superfamily. The human nucleic acid oxygenases (NAOXs) with known structures include the TET enzyme that produces 5-hydroxymethylcytosine in DNA, ALKBH2 and 3 that demethylate 3-methylcytosine and 1-methyladenine in DNA, and ALKBH8 that modifies tRNA. We identified the fat mass and obesity associated protein, FTO, as a NAOX¹ and m⁶A in RNA was recently reported to be a bona-fide substrate of FTO in in-vivo. Subsequently, ALKBH5 was also shown to be an m⁶A demethylase. Here we present structural studies of FTO² and ALKBH5³ as well as biochemical work that has led to structurally informed inhibitor design of the m⁶A demethylases for the development of small molecules to probe biological function and for potential use as therapeutics.