TET2 and major depressive disorder: Ascorbic acid can also directly enhance the catalytic activity of Tet methylcytosine dioxygenase 2 (TET2) in the oxidation of 5-methylcytosine (5mC), promote the folding and/or recycling of the cofactor Fe (2+) for TET2, and improve symptoms of depression (Ma et al., 2024; Yin et al., 2013).