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Vitamin C, a micronutrient known for its anti-scurvy activity in humans, promotes the generation of induced pluripotent stem cells (iPSCs) through the activity of histone demethylating dioxygenases. TET hydroxylases are also dioxygenases implicated in active DNA demethylation. Here we report that TET1 either positively or negatively regulates somatic cell reprogramming depending on the absence or presence of vitamin C. TET1 deficiency enhances reprogramming, and its overexpression impairs reprogramming in the context of vitamin C by modulating the obligatory mesenchymal-to-epithelial transition (MET). In the absence of vitamin C, TET1 promotes somatic cell reprogramming independent of MET. Consistently, TET1 regulates 5-hydroxymethylcytosine (5hmC) formation at loci critical for MET in a vitamin C-dependent fashion. Our findings suggest that vitamin C has a vital role in determining the biological outcome of TET1 function at the cellular level. Given its benefit to human health, vitamin C should be investigated further for its role in epigenetic regulation.

DNA hydroxymethylation and its mediated DNA demethylation are critical for multiple cellular processes, for example, nuclear reprogramming, embryonic development, and many diseases. Here, we demonstrate that a vital nutrient ascorbic acid (AA), or vitamin C (Vc), can directly enhance the catalytic activity of Tet dioxygenases for the oxidation of 5-methylcytosine (5mC). As evidenced by changes in intrinsic fluorescence and catalytic activity of Tet2 protein caused by AA and its oxidation-resistant derivatives, we further show that AA can uniquely interact with the C-terminal catalytic domain of Tet enzymes, which probably promotes their folding and/or recycling of the cofactor Fe(2+). Other strong reducing chemicals do not have a similar effect. These results suggest that AA also acts as a cofactor of Tet enzymes. In mouse embryonic stem cells, AA significantly increases the levels of all 5mC oxidation products, particularly 5-formylcytosine and 5-carboxylcytosine (by more than an order of magnitude), leading to a global loss of 5mC (∼40%). In cells deleted of the Tet1 and Tet2 genes, AA alters neither 5mC oxidation nor the overall level of 5mC. The AA effects are however restored when Tet2 is re-expressed in the Tet-deficient cells. The enhancing effects of AA on 5mC oxidation and DNA demethylation are also observed in a mouse model deficient in AA synthesis. Our data establish a direct link among AA, Tet, and DNA methylation, thus revealing a role of AA in the regulation of DNA modifications.