Mapping the tRNA Binding Site on the Surface of Human DNMT2 Methyltransferase

Restricted tRNA methylation by intermolecular disulfide bonds in DNMT2/TRDMT1

Reportedly, DNMT2/TRDMT1 mainly methylates tRNAs at C38 and prevents them from the cleavage under stress. It also plays an essential role in the survival and physiological homeostasis of organisms. Nevertheless, DNMT2/TRDMT1 exhibits much weaker tRNA methylation activity in vitro than other tRNA methyltransferases, TrmD and Trm5. Here, we explored the restricted tRNA methylation mechanism by DNMT2/TRDMT1. In the current study, the optimized buffer C at 37 °C was the best condition for DNMT2/TRDMT1 activation. Of note, Dithiothreitol (DTT) was an indispensable component for this enzyme catalysis. Moreover, reductants took similar effects on the conformation change and oligomeric formation of DNMT2/TRDMT1. Ultimately, LC-MS/MS result revealed that C292-C292 and C292-C287 were predominant intermolecular disulfide bonds in recombinant DNMT2/TRDMT1. Notably, DNMT2/TRDMT1 existed primarily as dimers via intermolecular disulfide bonds C79-C24, C292-C292, and C222-C24 in HEK293T cells. GSSG stress enhanced tRNA methylation level in the early stage of stress, whereas the DNMT2/TRDMT1 activity might be unfavorable along with this enzyme accumulation in the nucleus. Excitingly, GSH stress downregulated the DNMT2/TRDMT1 expression and promoted tRNA methylation in cells, probably through breaking intermolecular disulfide bonds in this enzyme. Thus, our findings demonstrated restricted tRNA methylation by disulfide bonds in DNMT2/TRDMT1, and will provide important implications for redox stress related-diseases.

Determinants of DNMT2/TRDMT1 preference for substrates tRNA and DNA during the evolution

RNA methyltransferase DNMT2/TRDMT1 is the most conserved member of the DNMT family from bacteria to plants and mammals. In previous studies, we found some determinants for tRNA recognition of DNMT2/TRDMT1, but the preference mechanism of this enzyme for substrates tRNA and DNA remains to be explored. In the present study, CFT-containing target recognition domain (TRD) and target recognition extension domain (TRED) in DNMT2/TRDMT1 play a crucial role in the substrate DNA and RNA selection during the evolution. Moreover, the classical substrate tRNA for DNMT2/TRDMT1 had a characteristic sequence CUXXCAC in the anticodon loop. Position 35 was occupied by U, making cytosine-38 (C38) twist into the loop, whereas C, G or A was located at position 35, keeping the C38-flipping state. Hence, the substrate preference could be modulated by the easily flipped state of target cytosine in tRNA, as well as TRD and TRED. Additionally, DNMT2/TRDMT1 cancer mutant activity was collectively mediated by five enzymatic characteristics, which might impact gene expressions. Importantly, G155C, G155V and G155S mutations reduced enzymatic activities and showed significant associations with diseases using seven prediction methods. Altogether, these findings will assist in illustrating the substrate preference mechanism of DNMT2/TRDMT1 and provide a promising therapeutic strategy for cancer.

RNA nucleotide methylation: 2021 update

Among RNA modifications, transfer of methylgroups from the typical cofactor S-adenosyl-l-methionine by methyltransferases (MTases) to RNA is by far the most common reaction. Since our last review about a decade ago, the field has witnessed the re-emergence of mRNA methylation as an important mechanism in gene regulation. Attention has then spread to many other RNA species; all being included into the newly coined concept of the "epitranscriptome." The focus moved from prokaryotes and single cell eukaryotes as model organisms to higher eukaryotes, in particular to mammals. The perception of the field has dramatically changed over the past decade. A previous lack of phenotypes in knockouts in single cell organisms has been replaced by the apparition of MTases in numerous disease models and clinical investigations. Major driving forces of the field include methylation mapping techniques, as well as the characterization of the various MTases, termed "writers." The latter term has spilled over from DNA modification in the neighboring epigenetics field, along with the designations "readers," applied to mediators of biological effects upon specific binding to a methylated RNA. Furthermore "eraser" enzymes effect the newly discovered oxidative removal of methylgroups. A sense of reversibility and dynamics has replaced the older perception of RNA modification as a concrete-cast, irreversible part of RNA maturation. A related concept concerns incompletely methylated residues, which, through permutation of each site, lead to inhomogeneous populations of numerous modivariants. This review recapitulates the major developments of the past decade outlined above, and attempts a prediction of upcoming trends. This article is categorized under: RNA Processing > RNA Editing and Modification.

New substrates and determinants for tRNA recognition of RNA methyltransferase DNMT2/TRDMT1

Methylation is a common post-transcriptional modification of tRNAs, particularly in the anticodon loop region. The cytosine 38 (C38) in tRNAs, such as tRNAAsp-GUC, tRNAGly-GCC, tRNAVal-AAC, and tRNAGlu-CUC, can be methylated by human DNMT2/TRDMT1 and some homologs found in bacteria, plants, and animals. However, the substrate properties and recognition mechanism of DNMT2/TRDMT1 remain to be explored. Here, taking into consideration common features of the four known substrate tRNAs, we investigated methylation activities of DNMT2/TRDMT1 on the tRNAGly-GCC truncation and point mutants, and conformational changes of mutants. The results demonstrated that human DNMT2/TRDMT1 preferred substrate tRNAGly-GCC in vitro. L-shaped conformation of classical tRNA could be favourable for DNMT2/TRDMT1 activity. The complete sequence and structure of tRNA were dispensable for DNMT2/TRDMT1 activity, whereas T-arm was indispensable to this activity. G19, U20, and A21 in D-loop were identified as the important bases for DNMT2/TRDMT1 activity, while G53, C56, A58, and C61 in T-loop were found as the critical bases. The conserved CUXXCAC sequence in the anticodon loop was confirmed to be the most critical determinant, and it could stabilize C38-flipping to promote C38 methylation. Based on these tRNA properties, new substrates, tRNAVal-CAC and tRNAGln-CUG, were discovered in vitro. Moreover, a single nucleotide substitute, U32C, could convert non-substrate tRNAAla-AGC into a substrate for DNMT2/TRDMT1. Altogether, our findings imply that DNMT2/TRDMT1 relies on a delicate network involving both the primary sequence and tertiary structure of tRNA for substrate recognition.

RNA 5-methylcytosine modification and its emerging role as an epitranscriptomic mark

5-methylcytosine (m⁵C) is identified as an abundant and conserved modification in various RNAs, including tRNAs, mRNAs, rRNAs, and other non-coding RNAs. The application of high-throughput sequencing and mass spectrometry allowed for the detection of m⁵C at a single-nucleotide resolution and at a global abundance separately; this contributes to a better understanding of m⁵C modification and its biological functions. m⁵C modification plays critical roles in diverse aspects of RNA processing, including tRNA stability, rRNA assembly, and mRNA translation. Notably, altered m⁵C modifications and mutated RNA m⁵C methyltransferases are associated with diverse pathological processes, such as nervous system disorders and cancers. This review may provide new sights of molecular mechanism and functional importance of m⁵C modification.

Structural insights into the stimulation of S. pombe Dnmt2 catalytic efficiency by the tRNA nucleoside queuosine

Dnmt2 methylates cytosine at position 38 of tRNA^Asp in a variety of eukaryotic organisms. A correlation between the presence of the hypermodified nucleoside queuosine (Q) at position 34 of tRNA^Asp and the Dnmt2 dependent C38 methylation was recently found in vivo for S. pombe and D. discoideum. We demonstrate a direct effect of the Q-modification on the methyltransferase catalytic efficiency in vitro, as V_max/K_0.5 of purified S. pombe Dnmt2 shows an increase for in vitro transcribed tRNA^Asp containing Q34 to 6.27 ∗ 10^–3 s⁻¹ µM⁻¹ compared to 1.51 ∗ 10^–3 s⁻¹ µM⁻¹ for the unmodified substrate. Q34tRNA^Asp exhibits an only slightly increased affinity for Dnmt2 in comparison to unmodified G34tRNA. In order to get insight into the structural basis for the Q-dependency, the crystal structure of S. pombe Dnmt2 was determined at 1.7 Å resolution. It closely resembles the known structures of human and E. histolytica Dnmt2, and contains the entire active site loop. The interaction with tRNA was analyzed by means of mass-spectrometry using UV cross-linked Dnmt2-tRNA complex. These cross-link data and computational docking of Dnmt2 and tRNA^Asp reveal Q34 positioned adjacent to the S-adenosylmethionine occupying the active site, suggesting that the observed increase of Dnmt2 catalytic efficiency by queuine originates from optimal positioning of the substrate molecules and residues relevant for methyl transfer.

Linking epigenetic function to electrostatics: The DNMT2 structural model example

The amino acid sequence of DNMT2 is very similar to the catalytic domains of bacterial and eukaryotic proteins. However, there is great variability in the region of recognition of the target sequence. While bacterial DNMT2 acts as a DNA methyltransferase, previous studies have indicated low DNA methylation activity in eukaryotic DNMT2, with preference by tRNA methylation. Drosophilids are known as DNMT2-only species and the DNA methylation phenomenon is a not elucidated case yet, as well as the ontogenetic and physiologic importance of DNMT2 for this species group. In addition, more recently study showed that methylation in the genome in Drosophila melanogaster is independent in relation to DNMT2. Despite these findings, Drosophilidae family has more than 4,200 species with great ecological diversity and historical evolution, thus we, therefore, aimed to examine the drosophilids DNMT2 in order to verify its conservation at the physicochemical and structural levels in a functional context. We examined the twenty-six DNMT2 models generated by molecular modelling and five crystallographic structures deposited in the Protein Data Bank (PDB) using different approaches. Our results showed that despite sequence and structural similarity between species close related, we found outstanding differences when they are analyzed in the context of surface distribution of electrostatic properties. The differences found in the electrostatic potentials may be linked with different affinities and processivity of DNMT2 for its different substrates (DNA, RNA or tRNA) and even for interactions with other proteins involved in the epigenetic mechanisms.

DNA methyltransferase 1 rs16999593 genetic polymorphism decreases risk in patients with transposition of great arteries

Complete transposition of the great arteries (TGA) is the most frequent cyanotic heart defect diagnosed in neonates. However, the exact etiology of TGA is unknown. The aim of the present study was to assess the association of TGA pathogenesis with single nucleotide polymorphisms (SNPs) in DNA methyltransferases (DNMTs)-1 and 3a- in Chinese children. We genotyped 5 SNPs (rs16999593, rs16999358, and rs2228611 in DNMT1; and rs2276599 and rs2276598 in DNMT3A) in 206 patients with complete TGA and 252 healthy children. Statistical analysis was performed to explore the association of the 5 SNPs with complete TGA susceptibility. Compared with the T/T and C/C genotypes, the heterozygous genotype C/T of rs16999593 correlated with a decreased risk for complete TGA under codominant (OR=0.46; 95% CI=0.29-0.72), dominant (OR=0.58; 95% CI=0.38-0.88), and overdominant (OR=0.44; 95% CI=0.28-0.68) models. In contrast, the genotype C/C of rs16999593 correlated with a higher risk for TGA under a recessive model (OR=3.15; 95% CI=1.14-8.68) compared with the T/T and C/T genotypes. Furthermore, the TGC, TGT, CGC, and CGT haplotypes of DNMT1 did not differ significantly between the two groups, whereas the frequency of the TAC haplotype was lower in the case group (OR<1; P=0.002). No significant differences in the frequencies of the TC, CC, TT, and CT haplotypes of DNMT3A were found between the two groups. Furthermore, logistic regression showed that sex and the rs16999358 SNP were two independent risk factors for complete TGA. Overall, the C/T genotype of the rs16999593 SNP in DNMT1 might decrease the risk of complete TGA pathogenesis in the Southern Chinese population.

Cross-Talk between Dnmt2-Dependent tRNA Methylation and Queuosine Modification

Enzymes of the Dnmt2 family of methyltransferases have yielded a number of unexpected discoveries. The first surprise came more than ten years ago when it was realized that, rather than being DNA methyltransferases, Dnmt2 enzymes actually are transfer RNA (tRNA) methyltransferases for cytosine-5 methylation, foremost C38 (m⁵C38) of tRNA^Asp. The second unanticipated finding was our recent discovery of a nutritional regulation of Dnmt2 in the fission yeast Schizosaccharomyces pombe. Significantly, the presence of the nucleotide queuosine in tRNA^Asp strongly stimulates Dnmt2 activity both in vivo and in vitro in S. pombe. Queuine, the respective base, is a hypermodified guanine analog that is synthesized from guanosine-5’-triphosphate (GTP) by bacteria. Interestingly, most eukaryotes have queuosine in their tRNA. However, they cannot synthesize it themselves, but rather salvage it from food or from gut microbes. The queuine obtained from these sources comes from the breakdown of tRNAs, where the queuine ultimately was synthesized by bacteria. Queuine thus has been termed a micronutrient. This review summarizes the current knowledge of Dnmt2 methylation and queuosine modification with respect to translation as well as the organismal consequences of the absence of these modifications. Models for the functional cooperation between these modifications and its wider implications are discussed.

The RNA methyltransferase Dnmt2 methylates DNA in the structural context of a tRNA

The amino acid sequence of Dnmt2 is very similar to the catalytic domains of bacterial and eukaryotic DNA-(cytosine 5)-methyltransferases, but it efficiently catalyzes tRNA methylation, while its DNA methyltransferase activity is the subject of controversial reports with rates varying between zero and very weak. By using composite nucleic acid molecules as substrates, we surprisingly found that DNA fragments, when presented as covalent DNA-RNA hybrids in the structural context of a tRNA, can be more efficiently methylated than the corresponding natural tRNA substrate. Furthermore, by stepwise development of tRNA^Asp, we showed that this natural Dnmt2 substrate could be engineered to employ RNAs that act like guide RNAs in vitro. The 5’-half of tRNA^Asp was able to efficiently guide methylation toward a single stranded tRNA fragment as would result from tRNA cleavage by tRNA specific nucleases. In a more artificial setting, a composite system of guide RNAs could ultimately be engineered to enable the enzyme to perform cytidine methylation on single stranded DNA in vitro.

Mechanism and biological role of Dnmt2 in Nucleic Acid Methylation

A group of homologous nucleic acid modification enzymes called Dnmt2, Trdmt1, Pmt1, DnmA, and Ehmet in different model organisms catalyze the transfer of a methyl group from the cofactor S-adenosyl-methionine (SAM) to the carbon-5 of cytosine residues. Originally considered as DNA MTases, these enzymes were shown to be tRNA methyltransferases about a decade ago. Between the presumed involvement in DNA modification-related epigenetics, and the recent foray into the RNA modification field, significant progress has characterized Dnmt2-related research. Here, we review this progress in its diverse facets including molecular evolution, structural biology, biochemistry, chemical biology, cell biology and epigenetics.

Investigation of the C-terminal domain of the bacterial DNA-(adenine N6)-methyltransferase CcrM

CcrM-related DNA-(adenine N6)-methyltransferases play very important roles in the biology of Caulobacter crescentus and other alpha-proteobacteria. These enzymes methylate GANTC sequences, but the molecular mechanism by which they recognize their target sequence is unknown. We carried out multiple sequence alignments and noticed that CcrM enzymes contain a conserved C-terminal domain (CTD) which is not present in other DNA-(adenine N6)-methyltransferases and we show here that deletion of this part abrogates catalytic activity and DNA binding of CcrM. A mutational study identified 7 conserved residues in the CTD (out of 13 tested), mutation of which led to a strong reduction in catalytic activity. All of these mutants showed altered DNA binding, but no change in AdoMet binding and secondary structure. Some mutants exhibited reduced DNA binding, but others showed an enhanced DNA binding. Moreover, we show that CcrM does not specifically bind to DNA containing GANTC sequences. Taken together, these findings suggest that the specific CcrM-DNA complex undergoes a conformational change, which is endergonic but essential for catalytic activity and this step is blocked by some of the mutations. Moreover, our data indicate that the CTD of CcrM is involved in DNA binding and recognition. This suggests that the CTD functions as target recognition domain of CcrM and, therefore, CcrM can be considered the first example of a δ-type DNA-(adenine N6)-methyltransferase identified so far.

Somatic cancer mutations in the DNMT2 tRNA methyltransferase alter its catalytic properties

Methylation of tRNA is an important post-transcriptional modification and aberrations in tRNA modification has been implicated in cancer. The DNMT2 protein methylates C38 of tRNA-Asp and it has a role in cellular physiology and stress response and its expression levels are altered in cancer tissues. Here we studied whether DNMT2 somatic mutations found in cancer tissues affect the activity of the enzyme. We have generated 13 DNMT2 variants and purified the corresponding proteins. All proteins were properly folded as determined by circular dichroism spectroscopy. We tested their RNA methylation activity using in vitro generated tRNA-Asp. One of the mutations (E63K) caused a twofold increase in activity, while two of them led to a strong (over fourfold) decrease in activity (G155S and L257V). Two additional mutant proteins were almost inactive (R371H and G155V). The strong effect of some of the somatic cancer mutations on DNMT2 activity suggests that these mutations have a functional role in tumorigenesis.

The Dnmt2 RNA methyltransferase homolog of Geobacter sulfurreducens specifically methylates tRNA-Glu

Dnmt2 enzymes are conserved in eukaryotes, where they methylate C38 of tRNA-Asp with high activity. Here, the activity of one of the very few prokaryotic Dnmt2 homologs from Geobacter species (GsDnmt2) was investigated. GsDnmt2 was observed to methylate tRNA-Asp from flies and mice. Unexpectedly, it had only a weak activity toward its matching Geobacter tRNA-Asp, but methylated Geobacter tRNA-Glu with good activity. In agreement with this result, we show that tRNA-Glu is methylated in Geobacter while the methylation is absent in tRNA-Asp. The activities of Dnmt2 enzymes from Homo sapiens, Drosophila melanogaster, Schizosaccharomyces pombe and Dictyostelium discoideum for methylation of the Geobacter tRNA-Asp and tRNA-Glu were determined showing that all these Dnmt2s preferentially methylate tRNA-Asp. Hence, the GsDnmt2 enzyme has a swapped transfer ribonucleic acid (tRNA) specificity. By comparing the different tRNAs, a characteristic sequence pattern was identified in the variable loop of all preferred tRNA substrates. An exchange of two nucleotides in the variable loop of murine tRNA-Asp converted it to the corresponding variable loop of tRNA-Glu and led to a strong reduction of GsDnmt2 activity. Interestingly, the same loss of activity was observed with human DNMT2, indicating that the variable loop functions as a specificity determinant in tRNA recognition of Dnmt2 enzymes.

Target recognition, RNA methylation activity and transcriptional regulation of the Dictyostelium discoideum Dnmt2-homologue (DnmA)

Although the DNA methyltransferase 2 family is highly conserved during evolution and recent reports suggested a dual specificity with stronger activity on transfer RNA (tRNA) than DNA substrates, the biological function is still obscure. We show that the Dictyostelium discoideum Dnmt2-homologue DnmA is an active tRNA methyltransferase that modifies C38 in tRNA^Asp(GUC)in vitro and in vivo. By an ultraviolet-crosslinking and immunoprecipitation approach, we identified further DnmA targets. This revealed specific tRNA fragments bound by the enzyme and identified tRNA^Glu(CUC/UUC) and tRNA^Gly(GCC) as new but weaker substrates for both human Dnmt2 and DnmA in vitro but apparently not in vivo. Dnmt2 enzymes form transient covalent complexes with their substrates. The dynamics of complex formation and complex resolution reflect methylation efficiency in vitro. Quantitative PCR analyses revealed alterations in dnmA expression during development, cell cycle and in response to temperature stress. However, dnmA expression only partially correlated with tRNA methylation in vivo. Strikingly, dnmA expression in the laboratory strain AX2 was significantly lower than in the NC4 parent strain. As expression levels and binding of DnmA to a target in vivo are apparently not necessarily accompanied by methylation, we propose an additional biological function of DnmA apart from methylation.

DNA methylation and cancer diagnosis

DNA methylation is a major epigenetic modification that is strongly involved in the physiological control of genome expression. DNA methylation patterns are largely modified in cancer cells and can therefore be used to distinguish cancer cells from normal tissues. This review describes the main technologies available for the detection and the discovery of aberrantly methylated DNA patterns. It also presents the different sources of biological samples suitable for DNA methylation studies. We discuss the interest and perspectives on the use of DNA methylation measurements for cancer diagnosis through examples of methylated genes commonly documented in the literature. The discussion leads to our consideration for why DNA methylation is not commonly used in clinical practice through an examination of the main requirements that constitute a reliable biomarker. Finally, we describe the main DNA methylation inhibitors currently used in clinical trials and those that exhibit promising results.

Mapping and significance of the mRNA methylome

Internal methylation of eukaryotic mRNAs in the form of N6-methyladenosine (m(6)A) and 5-methylcytidine (m(5)C) has long been known to exist, but progress in understanding its role was hampered by difficulties in identifying individual sites. This was recently overcome by high-throughput sequencing-based methods that mapped thousands of sites for both modifications throughout mammalian transcriptomes, with most sites found in mRNAs. The topology of m(6)A in mouse and human revealed both conserved and variable sites as well as plasticity in response to extracellular cues. Within mRNAs, m(5)C and m(6)A sites were relatively depleted in coding sequences and enriched in untranslated regions, suggesting functional interactions with post-transcriptional gene control. Finer distribution analyses and preexisting literature point toward roles in the regulation of mRNA splicing, translation, or decay, through an interplay with RNA-binding proteins and microRNAs. The methyltransferase (MTase) METTL3 'writes' m(6)A marks on mRNA, whereas the demethylase FTO can 'erase' them. The RNA:m(5)C MTases NSUN2 and TRDMT1 have roles in tRNA methylation but they also act on mRNA. Proper functioning of these enzymes is important in development and there are clear links to human disease. For instance, a common variant of FTO is a risk allele for obesity carried by 1 billion people worldwide and mutations cause a lethal syndrome with growth retardation and brain deficits. NSUN2 is linked to cancer and stem cell biology and mutations cause intellectual disability. In this review, we summarize the advances, open questions, and intriguing possibilities in this emerging field that might be called RNA modomics or epitranscriptomics.