Dioxygenase that catalyzes the conversion of the modified genomic base 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC), and thereby plays a role in active DNA demethylation. Also mediates subsequent conversion of 5hmC into 5-formylcytosine (5fC), and conversion of 5fC to 5-carboxylcytosine (5caC). 2-oxoglutarate + a 5-methyl-2'-deoxycytidine in DNA + O2 = a 5-hydroxymethyl-2'-deoxycytidine in DNA + CO2 + succinate 2-oxoglutarate + a 5-hydroxymethyl-2'-deoxycytidine in DNA + O2 = a 5-formyl-2'-deoxycytidine in DNA + CO2 + H2O + succinate 2-oxoglutarate + a 5-formyl-2'-deoxycytidine in DNA + O2 = a 5-carboxyl-2'-deoxycytidine in DNA + CO2 + H(+) + succinate Binds 1 Fe(2+) ion per subunit. Has good structural similarity to 2-oxoglutarate-dependent dioxygenases of the TET and the alkB families, in spite of very low overall sequence similarity.