DNA N(6)-methyladenine (6mA) modification is commonly found in microbial genomes and plays important functions in regulating numerous biological processes in bacteria. However, whether 6mA occurs and what its potential roles are in higher-eukaryote cells remain unknown. Here, we show that 6mA is present in Drosophila genome and that the 6mA modification is dynamic and is regulated by the Drosophila Tet homolog, DNA 6mA demethylase (DMAD), during embryogenesis. Importantly, our biochemical assays demonstrate that DMAD directly catalyzes 6mA demethylation in vitro. Further genetic and sequencing analyses reveal that DMAD is essential for development and that DMAD removes 6mA primarily from transposon regions, which correlates with transposon suppression in Drosophila ovary. Collectively, we uncover a DNA modification in Drosophila and describe a potential role of the DMAD-6mA regulatory axis in controlling development in higher eukaryotes.

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. ■ INTRODUCTIONDNA demethylation remarkably contributes to the dynamics of the epigenetic marker 5-methylcytosine (5mC) in mammals and is critical for multiple biological processes, including animal cloning, 1 nuclear reprogramming, 2,3 development, 4−8 and highly locus-specific regulation of gene activities. 9−11 DNA demethylation can be initiated by the oxidation of 5mC and the formation of 5-hydroxymethylcytosine (5hmC), which are catalyzed by ten eleven translocation (Tet) family dioxygenases. 12−15 The formed 5hmC can be diluted by DNA replication, suggesting a passive DNA demethylation pathway. 16 Moreover, the 5hmC can be further oxidized by Tet proteins to form 5-formylcytosine (5fC) and 5-carboxylcytosine (5caC), which can be excised by thymine DNA glycosylase (TDG) followed by the reintroduction of unmethylated cytosine through the base-excision repair (BER) pathway. 14,15 This important pathway for active DNA demethylation has been thought to be involved in a number of prominent biological processes. 5,6,10,11 Early and recent studies suggested that active and replication-independent DNA demethylation might be a rapid process. 10,11 The radically altered methylation, as observed in replication-independent demethylation of the paternal genome in zygotes, may complete within hours. 5,6,17−19 However, the observed levels of the active DNA demethylation intermediates, 5fC and 5caC in the cultured cells, were 100-fold less than the primary product 5hmC. 13−15,20−22 Biochemically, the Tet-mediated DNA dem...

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