Active demethylation of 5-methylcytosine (5mC) can be realized through ten-eleven translocation (TET) dioxygenase-mediated oxidation of 5mC to 5-hydroxymethylcytosine (5hmC), 5-formylcytosine (5fC), and 5-carboxylcytosine (5caC), followed by thymine DNA glycosylase (TDG)-initiated base excision repair (BER). The TDG-BER pathway may lead to the generation of DNA strand breaks, potentially compromising genome integrity. Alternatively, direct decarboxylation of TET-produced 5caC is highly attractive because this mechanism allows for conversion of 5mC to cytosine without the formation of DNA strand breaks. However, cleavage of the C-C bond in 5caC in human cells remains an open question. We examined this reaction in cell extract and live cells using 5caC-carrying hairpin DNA substrate. After incubation with whole-cell protein extract or transfection into human cells, we monitored the transformation of 5caC to cytosine through direct decarboxylation or BER using liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses at both the mononucleotide and oligodeoxynucleotide levels. Our results clearly showed the direct conversion of 5caC to cytosine in human cells, providing evidence to support a novel pathway for active DNA demethylation.

RNA contains diverse modifications that exert an important influence in a variety of cellular processes. So far, more than 150 modifications have been identified in various RNA species, mainly in ribosomal RNA (rRNA), transfer RNA (tRNA), and messenger RNA (mRNA). In contrast to rRNA, tRNA, and mRNA, the known modifications in small RNA species have been primarily limited to 2'-O-ribose methylation in plants and inosine in mammals. The methylation of small RNAs in mammals is still unclear. Current methods widely used in the characterization of small RNAs are mainly based on the strategy of nucleic acid hybridization and sequencing, which cannot characterize modifications in small RNAs. Herein, we have systematically investigated modifications in small RNAs composed of 16-28 nucleotides (nt) by establishing an effective isolation and neutral enzymatic digestion of small RNAs in combination with liquid chromatography/electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS). This method allowed us to simultaneously detect 57 different types of nucleoside modification. By using this approach, we revealed 24 modifications in small RNAs comprising 16-28 nt from human cells. In addition, we found that the obesity-associated protein (FTO) may demethylate N -methyladenosine (m A) and N ,2'-O-dimethyladenosine (m Am) in small RNAs of 16-28 nt. Our study demonstrates the existence of diverse modifications in small RNAs composed of 16-28 nt, which may promote in-depth understanding of the regulatory roles of noncoding RNAs.

RNA molecules harbor diverse modifications that play important regulatory roles in a variety of biological processes. Over 150 modifications have been identified in RNA molecules. N6-methyladenosine (m6A) and 1-methyladenosine (m1A) are prevalent modifications occurring in various RNA species of mammals. Apart from the single methylation of adenosine (m6A and m1A), dual methylation modification occurring in the nucleobase of adenosine, such as N6,N6-dimethyladenosine (m6,6A), also has been reported to be present in RNA of mammals. Whether there are other forms of dual methylation modification occurring in the nucleobase of adenosine other than m6,6A remains elusive. Here, we reported the existence of a novel adenosine dual methylation modification, i.e. 1,N6-dimethyladenosine (m1,6A), in tRNAs of living organisms. We confirmed that m1,6A is located at position 58 of tRNAs and is prevalent in mammalian cells and tissues. The measured level of m1,6A ranged from 0.0049% to 0.047% in tRNAs. Furthermore, we demonstrated that TRMT6/61A could catalyze the formation of m1,6A in tRNAs and m1,6A could be demethylated by ALKBH3. Collectively, the discovery of m1,6A expands the diversity of RNA modifications and may elicit a new tRNA modification-mediated gene regulation pathway.

DNA cytosine methylation (5-methylcytosine, 5mC) is the most important epigenetic mark in higher eukaryotes. 5mC in genomes is dynamically controlled by the writers and erasers. DNA (cytosine-5)-methyltransferases (DNMTs) are responsible...