RNA demethylase that mediates oxidative demethylation of different RNA species, such as mRNAs, tRNAs and snRNAs, and acts as a regulator of fat mass, adipogenesis and energy homeostasis (PubMed:17991826, PubMed:18775698, PubMed:28002401). Specifically demethylates N(6)-methyladenosine (m6A) RNA, the most prevalent internal modification of messenger RNA (mRNA) in higher eukaryotes (PubMed:28002401). M6A demethylation by FTO affects mRNA expression and stability (By similarity). Also able to demethylate m6A in U6 small nuclear RNA (snRNA) (By similarity). Mediates demethylation of N(6),2'-O-dimethyladenosine cap (m6A(m)), by demethylating the N(6)-methyladenosine at the second transcribed position of mRNAs and U6 snRNA (PubMed:28002401). Demethylation of m6A(m) in the 5'-cap by FTO affects mRNA stability by promoting susceptibility to decapping (By similarity). Also acts as a tRNA demethylase by removing N(1)-methyladenine from various tRNAs (By similarity). Has no activity towards 1-methylguanine (By similarity). Has no detectable activity towards double-stranded DNA (By similarity). Also able to repair alkylated DNA and RNA by oxidative demethylation: demethylates single-stranded RNA containing 3-methyluracil, single-stranded DNA containing 3-methylthymine and has low demethylase activity towards single-stranded DNA containing 1-methyladenine or 3-methylcytosine (PubMed:17991826, PubMed:18775698). Ability to repair alkylated DNA and RNA is however unsure in vivo (PubMed:17991826, PubMed:18775698). Involved in the regulation of fat mass, adipogenesis and body weight, thereby contributing to the regulation of body size and body fat accumulation (PubMed:19234441, PubMed:19680540, PubMed:21076408, PubMed:23817550, PubMed:23300482). Involved in the regulation of thermogenesis and the control of adipocyte differentiation into brown or white fat cells (PubMed:19234441, PubMed:19680540). Regulates activity of the dopaminergic midbrain circuitry via its ability to demethylate m6A in mRNAs (PubMed:23817550). 2-oxoglutarate + a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in mRNA + O2 = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in mRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + an N(6)-methyladenosine in mRNA + O2 = an adenosine in mRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + N(6)-methyladenosine in U6 snRNA + O2 = adenosine in U6 snRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(N(6),2'-O-dimethyladenosine) in U6 snRNA + O2 = a 5'-end (N(7)-methyl 5'-triphosphoguanosine)-(2'-O-methyladenosine) in U6 snRNA + CO2 + formaldehyde + succinate 2-oxoglutarate + an N(1)-methyladenosine in tRNA + O2 = an adenosine in tRNA + CO2 + formaldehyde + succinate Binds 1 Fe(2+) ion per subunit. Activated by ascorbate (PubMed:17991826). Inhibited by N-oxalylglycine, fumarate and succinate (PubMed:17991826). Optimum pH is 5.5-6. Monomer (PubMed:19680540). May also exist as homodimer (PubMed:19680540). Localizes mainly in the nucleus, where it is able to demethylate N(6)-methyladenosine (m6A) and N(6),2'-O-dimethyladenosine cap (m6A(m)) in U6 small nuclear RNA (snRNA), N(1)-methyladenine from tRNAs and internal m6A in mRNAs. In the cytoplasm, mediates demethylation of m6A and m6A(m) in mRNAs and N(1)-methyladenine from tRNAs. Ubiquitous. Detected in brain, brain cortex, hypothalamus, cerebellum, liver, pancreas, heart, kidney, white adipose tissue and skeletal muscle. Most abundant in the brain, particularly in hypothalamic nuclei governing energy balance. Down-regulated in fasting animals. The 3D-structure of the Fe2OG dioxygenase domain is similar to that of the Fe2OG dioxygenase domain found in the bacterial DNA repair dioxygenase alkB and its mammalian orthologs, but sequence similarity is very low. As a consequence, the domain is not detected by protein signature databases. Elevated perinatal mortality (PubMed:19234441). Mice have normal body weight at birth, but show growth retardation from day 2 onwards, resulting in a weight reduction of 30-40% after 6 weeks, both in males and females (PubMed:19234441). In addition, animals display reduced nose to anus length (PubMed:19234441). Fat mass is reduced by 60% in males and by 23% in females (PubMed:19234441). Lean body mass is reduced by 26% in males and 19% in females (PubMed:19234441). White adipose tissue decreases more and more over time, while brown adipose tissue is not affected (PubMed:19234441). Serum leptin levels are decreased, while serum levels of adiponectin are increased (PubMed:19234441). Mice exhibit significant hyperphagia after correction for body weight (PubMed:19234441). They show increased oxygen consumption, carbon dioxide production and heat generation, indicating increased energy expenditure, in spite of reduced spontaneous locomotor activity (PubMed:19234441). Plasma adrenaline concentrations are significantly increased (PubMed:19234441). Overall glucose metabolism appears normal (PubMed:19234441). Conditional deletion in the adult affects body composition and metabolism, and causes a small reduction in food intake and weight gain (PubMed:23300482). Mice with conditional deletion in dopaminergic neurons show abnormal dopamine signaling pathways, including impaired dopamine receptor type 2 (D2R) and type 3 (D3R) signaling and the related locomotion function (PubMed:23817550). Deficient mice show increased N(6)-methyladenosine (m6A) in a subset of mRNAs important for neuronal signaling, including many in the dopaminergic signaling pathway (PubMed:23817550). Knockout cells show strongly increased levels of N(6),2'-O-dimethyladenosine cap (m6A(m)) mRNAs (PubMed:28002401). Belongs to the fto family. Many publications have reported a critical role of Fto in regulating fat mass, adipogenesis and total body weight (PubMed:19234441, PubMed:19680540, PubMed:21076408, PubMed:23817550, PubMed:23300482). However, some reports suggest that some effects are indirect and caused by impaired expression of adjacent genes such as Irx3 and Rpgrip1l (PubMed:24807221, PubMed:24646999). Extended N-terminus.