The prevalent DNA modification in higher organisms is the methylation of cytosine to 5-methylcytosine (5mC), which is partially converted to 5-hydroxymethylcytosine (5hmC) by the Tet (ten eleven translocation) family of dioxygenases. Despite their importance in epigenetic regulation, it is unclear how these cytosine modifications are reversed. Here, we demonstrate that 5mC and 5hmC in DNA are oxidized to 5-carboxylcytosine (5caC) by Tet dioxygenases in vitro and in cultured cells. 5caC is specifically recognized and excised by thymine-DNA glycosylase (TDG). Depletion of TDG in mouse embyronic stem cells leads to accumulation of 5caC to a readily detectable level. These data suggest that oxidation of 5mC by Tet proteins followed by TDG-mediated base excision of 5caC constitutes a pathway for active DNA demethylation.

The crystal structure of a ternary complex of human _nununodeficiency virus type 1 reverse transcriptase (HIV-1 RT) heterodimer (p66/p5i), a 19-base/18-base doublestranded DNA template-primer, and a monoclonal antibody Fab fragment has been determined at 3.0 A resolution. (40)(41)(42)(43)(44)(45). The most numerous nucleic acid interactions with protein occur primarily along the sugar-phosphate backbone of the DNA and involve amino acid residues of the palm, thumb, and ringers of p66. Highly conserved regions are located in the p66 palm near the polymerase active site. These structural elements, together with two a-helices of the thumb of p66, act as a clamp to position the template-primer relative to the polymerase active site. The 3'-hydroxyl of the primer terminus is close to the catalytically essential Asp-110, Asp-185, and Asp-186 residues at the active site and is in a position for nudeophilic attack on the a-phosphate of an incoming nucleoside triphosphate. The structure of the HIV-1 RT/DNA/Fab complex should aid our understanding of general mechanisms of nucleic acid polymerization. AIDS therapies may be enhanced by a fuller understanding of drug inhibition and resistance emerging from these studies. A 3.5 A resolution structure of HIV-1 RT complexed with the nonnucleoside inhibitor nevirapine has been reported (8).Although the resolution of the study was not sufficient to determine the position of every amino acid and their side chains, the overall folding of the enzyme was described.We have prepared crystals of a ternary complex (9) of the HIV-1 RT p66/pSl heterodimer, a double-stranded DNA (dsDNA) template-primer, and the antigen-binding fragment (Fab fragment) ofa noninhibiting antibody that diffract x-rays to 2.8 A resolution, and reported the structure ofthis complex at 7 A resolution (10). At this resolution it was possible to determine the location ofthe template-primer and the relative positions of the polymerase and the RNase H active sites. In addition, it was shown that when the nucleic acid substrate was bound to RT a significant portion of the protein moved out of the nucleic acid-binding site.Here we report the structure of the RT-dsDNA-Fab28 complex at 3.0 A resolution. The overall arrangement of the enzyme is similar to that previously reported (8 ITo whom reprint requests should be addressed

Heterozygous mutations in the gene encoding isocitrate dehydrogenase-1 (IDH1) occur in certain human brain tumors, but their mechanistic role in tumor development is unknown. We have shown that tumor-derived IDH1 mutations impair the enzyme’s affinity for its substrate and dominantly inhibit wild-type IDH1 activity through the formation of catalytically inactive heterodimers. Forced expression of mutant IDH1 in cultured cells reduces formation of the enzyme product,α-ketoglutarate (α-KG), and increases the levels of hypoxia-inducible factor subunit HIF-1α, a transcription factor that facilitates tumor growth when oxygen is low and whose stability is regulated by α-KG. The rise in HIF-1α levels was reversible by an α-KG derivative. HIF-1α levels were higher in human gliomas harboring an IDH1 mutation than in tumors without a mutation. Thus, IDH1 appears to function as a tumor suppressor that, when mutationally inactivated, contributes to tumorigenesis in part through induction of the HIF-1 pathway.