Chengqi Yi

N6-methyladenosine in nuclear RNA is a major substrate of the obesity-associated FTO.

We report here that FTO (fat mass and obesity-associated protein) exhibits efficient oxidative demethylation activity of abundant N6-methyladenosine (m6A) residues in RNA in vitro. FTO knockdown with siRNA led to an increased level of m6A in mRNA, whereas overexpression of FTO resulted in a decreased level of m6A in human cells. We further show that FTO partially colocalizes with nuclear speckles, supporting m6A in nuclear RNA as a physiological substrate of FTO.

Selective chemical labeling reveals the genome-wide distribution of 5-hydroxymethylcytosine

In contrast to 5-methylcytosine (5-mC), which has been studied extensively, little is known about 5-hydroxymethylcytosine (5-hmC), a recently identified epigenetic modification present in substantial amounts in certain mammalian cell types. Here we present a method for determining the genome-wide distribution of 5-hmC. We use the T4 bacteriophage β-glucosyltransferase to transfer an engineered glucose moiety containing an azide group onto the hydroxyl group of 5-hmC. The azide group can be chemically modified with biotin for detection, affinity enrichment and sequencing of 5-hmC–containing DNA fragments in mammalian genomes. Using this method, we demonstrate that 5-hmC is present in human cell lines beyond those previously recognized. We also find a gene expression level–dependent enrichment of intragenic 5-hmC in mouse cerebellum and an age-dependent acquisition of this modification in specific gene bodies linked to neurodegenerative disorders.

Oxidative demethylation of 3-methylthymine and 3-methyluracil in single-stranded DNA and RNA by mouse and human FTO

The human obesity susceptibility gene, FTO, encodes a protein that is homologous to the DNA repair AlkB protein. The AlkB family proteins utilize iron(II), α‐ketoglutarate (α‐KG) and dioxygen to perform oxidative repair of alkylated nucleobases in DNA and RNA. We demonstrate here the oxidative demethylation of 3‐methylthymine (3‐meT) in single‐stranded DNA (ssDNA) and 3‐methyluracil (3‐meU) in single‐stranded RNA (ssRNA) by recombinant human FTO protein in vitro. Both human and mouse FTO proteins preferentially repair 3‐meT in ssDNA over other base lesions tested. They showed negligible activities against 3‐meT in double‐stranded DNA (dsDNA). In addition, these two proteins can catalyze the demethylation of 3‐meU in ssRNA with a slightly higher efficiency over that of 3‐meT in ssDNA, suggesting that methylated RNAs are the preferred substrates for FTO.

Crystal structures of DNA/RNA repair enzymes AlkB and ABH2 bound to dsDNA

Escherichia coli AlkB and its human homologues ABH2 and ABH3 repair DNA/RNA base lesions by using a direct oxidative dealkylation mechanism. ABH2 has the primary role of guarding mammalian genomes against 1-meA damage by repairing this lesion in double-stranded DNA (dsDNA), whereas AlkB and ABH3 preferentially repair single-stranded DNA (ssDNA) lesions and can repair damaged bases in RNA. Here we show the first crystal structures of AlkB–dsDNA and ABH2–dsDNA complexes, stabilized by a chemical cross-linking strategy. This study reveals that AlkB uses an unprecedented base-flipping mechanism to access the damaged base: it squeezes together the two bases flanking the flipped-out one to maintain the base stack, explaining the preference of AlkB for repairing ssDNA lesions over dsDNA ones. In addition, the first crystal structure of ABH2, presented here, provides a structural basis for designing inhibitors of this human DNA repair protein.

Mapping recently identified nucleotide variants in the genome and transcriptome

Nucleotide variants, especially those related to epigenetic functions, provide critical regulatory information beyond simple genomic sequence, and they define cell status in higher organisms. 5-methylcytosine, which is found in DNA, was until recently the only nucleotide variant studied in terms of epigenetics in eukaryotes. However, 5-methylcytosine has turned out to be just one component of a dynamic DNA epigenetic regulatory network that also includes 5-hydroxymethylcytosine, 5-formylcytosine and 5-carboxylcytosine. Recently, reversible methylation of N6-methyladenosine in RNA has also been demonstrated. The discovery of these new nucleotide variants triggered an explosion of new information in the epigenetics field. This rapid research progress has benefited significantly from timely developments of new technologies that specifically recognize, enrich and sequence nucleotide modifications, as evidenced by the wide application of the bisulfite sequencing of 5-methylcytosine and very recent modifications of bisulfite sequencing to resolve 5-hydroxymethylcytosine from 5-methylcytosine with base-resolution information.

Chemical pulldown reveals dynamic pseudouridylation of the mammalian transcriptome

Pseudouridine (Ψ) is the most abundant post-transcriptional RNA modification, yet little is known about its prevalence, mechanism and function in mRNA. Here, we performed quantitative MS analysis and show that Ψ is much more prevalent (Ψ/U ratio ∼0.2-0.6%) in mammalian mRNA than previously believed. We developed N3-CMC-enriched pseudouridine sequencing (CeU-Seq), a selective chemical labeling and pulldown method, to identify 2,084 Ψ sites within 1,929 human transcripts, of which four (in ribosomal RNA and EEF1A1 mRNA) are biochemically verified. We show that hPUS1, a known Ψ synthase, acts on human mRNA; under stress, CeU-Seq demonstrates inducible and stress-specific mRNA pseudouridylation. Applying CeU-Seq to the mouse transcriptome revealed conserved and tissue-specific pseudouridylation. Collectively, our approaches allow comprehensive analysis of transcriptome-wide pseudouridylation and provide tools for functional studies of Ψ-mediated epigenetic regulation.

Efficient and quantitative high-throughput tRNA sequencing

Despite its biological importance, tRNA has not been adequately sequenced by standard methods because of its abundant post-transcriptional modifications and stable structure, which interfere with cDNA synthesis. We achieved efficient and quantitative tRNA sequencing in HEK293T cells by using engineered demethylases to remove base methylations and a highly processive thermostable group II intron reverse transcriptase to overcome these obstacles. Our method, DM-tRNA-seq, should be applicable to investigations of tRNA in all organisms.

Protein cysteine phosphorylation of SarA/MgrA family transcriptional regulators mediates bacterial virulence and antibiotic resistance

Protein posttranslational modifications (PTMs), particularly phosphorylation, dramatically expand the complexity of cellular regulatory networks. Although cysteine (Cys) in various proteins can be subject to multiple PTMs, its phosphorylation was previously considered a rare PTM with almost no regulatory role assigned. We report here that phosphorylation occurs to a reactive cysteine residue conserved in the staphylococcal accessary regulator A (SarA)/MarR family global transcriptional regulator A (MgrA) family of proteins, and is mediated by the eukaryotic-like kinase-phosphatase pair Stk1-Stp1 in Staphylococcus aureus. Cys-phosphorylation is crucial in regulating virulence determinant production and bacterial resistance to vancomycin. Cell wall-targeting antibiotics, such as vancomycin and ceftriaxone, inhibit the kinase activity of Stk1 and lead to decreased Cys-phosphorylation of SarA and MgrA. An in vivo mouse model of infection established that the absence of stp1, which results in elevated protein Cys-phosphorylation, significantly reduces staphylococcal virulence. Our data indicate that Cys-phosphorylation is a unique PTM that can play crucial roles in bacterial signaling and regulation.

Iron-catalysed oxidation intermediates captured in a DNA repair dioxygenase

Mononuclear iron-containing oxygenases conduct a diverse variety of oxidation functions in biology, including the oxidative demethylation of methylated nucleic acids and histones. Escherichia coli AlkB is the first such enzyme that was discovered to repair methylated nucleic acids, which are otherwise cytotoxic and/or mutagenic. AlkB human homologues are known to play pivotal roles in various processes. Here we present structural characterization of oxidation intermediates for these demethylases. Using a chemical cross-linking strategy, complexes of AlkB–double stranded DNA (dsDNA) containing 1,N6-etheno adenine (εA), N3-methyl thymine (3-meT) and N3-methyl cytosine (3-meC) are stabilized and crystallized, respectively. Exposing these crystals, grown under anaerobic conditions containing iron(II) and α-ketoglutarate (αKG), to dioxygen initiates oxidation in crystallo. Glycol (from εA) and hemiaminal (from 3-meT) intermediates are captured; a zwitterionic intermediate (from 3-meC) is also proposed, based on crystallographic observations and computational analysis. The observation of these unprecedented intermediates provides direct support for the oxidative demethylation mechanism for these demethylases. This study also depicts a general mechanistic view of how a methyl group is oxidatively removed from different biological substrates.

A Non-Heme Iron-Mediated Chemical Demethylation in DNA and RNA

DNA methylation is arguably one of the most important chemical signals in biology. However, aberrant DNA methylation can lead to cytotoxic or mutagenic consequences. A DNA repair protein in Escherichia coli, AlkB, corrects some of the unwanted methylations of DNA bases by a unique oxidative demethylation in which the methyl carbon is liberated as formaldehyde. The enzyme also repairs exocyclic DNA lesions--that is, derivatives in which the base is augmented with an additional heterocyclic subunit--by a similar mechanism. Two proteins in humans that are homologous to AlkB, ABH2 and ABH3, repair the same spectrum of lesions; another human homologue of AlkB, FTO, is linked to obesity. In this Account, we describe our studies of AlkB, ABH2, and ABH3, including our development of a general strategy to trap homogeneous protein-DNA complexes through active-site disulfide cross-linking. AlkB uses a non-heme mononuclear iron(II) and the cofactors 2-ketoglutarate (2KG) and dioxygen to effect oxidative demethylation of the DNA base lesions 1-methyladenine (1-meA), 3-methylcytosine (3-meC), 1-methylguanine (1-meG), and 3-methylthymine (3-meT). ABH3, like AlkB, works better on single-stranded DNA (ssDNA) and is capable of repairing damaged bases in RNA. Conversely, ABH2 primarily repairs lesions in double-stranded DNA (dsDNA); it is the main housekeeping enzyme that protects the mammalian genome from 1-meA base damage. The AlkB-family proteins have moderate affinities for their substrates and bind DNA in a non-sequence-specific manner. Knowing that these proteins flip the damaged base out from the duplex DNA and insert it into the active site for further processing, we first engineered a disulfide cross-link in the active site to stabilize the Michaelis complex. Based on the detailed structural information afforded by the active-site cross-linked structures, we can readily install a cross-link away from the active site to obtain the native-like structures of these complexes. The crystal structures show a distinct base-flipping feature in AlkB and establish ABH2 as a dsDNA repair protein. They also provide a molecular framework for understanding the demethylation reaction catalyzed by these proteins and help to explain their substrate preferences. The chemical cross-linking method demonstrated here can be applied to trap other labile protein-DNA interactions and can serve as a general strategy for exploring the structural and functional aspects of base-flipping proteins.