Catalysis of the reaction: 5-methylcytosine (5mC) in DNA + 2-oxoglutarate + O2 = 5-hydroxymethylcytosine (5hmC) in DNA + succinate + CO2. This reaction is the first step in the removal of cytosine methylated on position 5 in double-stranded DNA. This activity can iteratively oxidize 5hmC to 5-formylcytosine (5fC) and to 5-carboxylcytosine (5caC).
methylcytosine dioxygenase activity