Stefanie Kellner

RNA cytosine methylation by Dnmt2 and NSun2 promotes tRNA stability and protein synthesis

The function of cytosine-C5 methylation, a widespread modification of tRNAs, has remained obscure, particularly in mammals. We have now developed a mouse strain defective in cytosine-C5 tRNA methylation, by disrupting both the Dnmt2 and the NSun2 tRNA methyltransferases. Although the lack of either enzyme alone has no detectable effects on mouse viability, double mutants showed a synthetic lethal interaction, with an underdeveloped phenotype and impaired cellular differentiation. tRNA methylation analysis of the double-knockout mice demonstrated complementary target-site specificities for Dnmt2 and NSun2 and a complete loss of cytosine-C5 tRNA methylation. Steady-state levels of unmethylated tRNAs were substantially reduced, and loss of Dnmt2 and NSun2 was further associated with reduced rates of overall protein synthesis. These results establish a biologically important function for cytosine-C5 tRNA methylation in mammals and suggest that this modification promotes mouse development by supporting protein synthesis.

Aberrant methylation of tRNAs links cellular stress to neuro-developmental disorders

Mutations in the cytosine‐5 RNA methyltransferase NSun2 cause microcephaly and other neurological abnormalities in mice and human. How post‐transcriptional methylation contributes to the human disease is currently unknown. By comparing gene expression data with global cytosine‐5 RNA methylomes in patient fibroblasts and NSun2‐deficient mice, we find that loss of cytosine‐5 RNA methylation increases the angiogenin‐mediated endonucleolytic cleavage of transfer RNAs (tRNA) leading to an accumulation of 5′ tRNA‐derived small RNA fragments. Accumulation of 5′ tRNA fragments in the absence of NSun2 reduces protein translation rates and activates stress pathways leading to reduced cell size and increased apoptosis of cortical, hippocampal and striatal neurons. Mechanistically, we demonstrate that angiogenin binds with higher affinity to tRNAs lacking site‐specific NSun2‐mediated methylation and that the presence of 5′ tRNA fragments is sufficient and required to trigger cellular stress responses. Furthermore, the enhanced sensitivity of NSun2‐deficient brains to oxidative stress can be rescued through inhibition of angiogenin during embryogenesis. In conclusion, failure in NSun2‐mediated tRNA methylation contributes to human diseases via stress‐induced RNA cleavage.

Detection of RNA modifications

RNA nucleotide modifications are typically of low abundance and frequently go unnoticed by standard detection methods of molecular biology and cell biology. With a burst of knowledge intruding from such diverse areas as genomics, structural biology, regulation of gene expression and immunology, it becomes increasingly clear that many exciting functions of nucleotide modifications remain to be explored. It follows in turn that the biology of nucleotide modification and editing is a field poised to rapidly gain importance in a variety of fields. The detection and analysis of nucleotide modifications present a clear limitation in this respect. Here, various methods for detection of nucleotide modifications are discussed based on three discriminating principles, namely physicochemical properties, enzymatic turnover, and chemical reactivity. Because the full extent of nucleotide modification across the various RNA species remains ill understood, emphasis is placed on high-throughput techniques with a potential to screen entire transcriptomes.

Related haloarchaeal pleomorphic viruses contain different genome types

Archaeal viruses have been the subject of recent interest due to the diversity discovered in their virion architectures. Recently, a new group of haloarchaeal pleomorphic viruses has been discovered. It is distinctive in terms of the virion morphology and different genome types (ssDNA/dsDNA) harboured by rather closely related representatives. To date there are seven isolated viruses belonging to this group. Most of these share a cluster of five conserved genes, two of which encode major structural proteins. Putative proviruses and proviral remnants containing homologues of the conserved gene cluster were also identified suggesting a long-standing relationship of these viruses with their hosts. Comparative genomic analysis revealed three different ways of the genome organization, which possibly reflect different replication strategies employed by these viruses. The dsDNA genomes of two of these viruses were shown to contain single-strand interruptions. Further studies on one of the genomes suggested that the interruptions are located along the genome in a sequence-specific manner and exhibit polarity in distribution.

Absolute and relative quantification of RNA modifications via biosynthetic isotopomers

In the resurging field of RNA modifications, quantification is a bottleneck blocking many exciting avenues. With currently over 150 known nucleoside alterations, detection and quantification methods must encompass multiple modifications for a comprehensive profile. LC–MS/MS approaches offer a perspective for comprehensive parallel quantification of all the various modifications found in total RNA of a given organism. By feeding 13C-glucose as sole carbon source, we have generated a stable isotope-labeled internal standard (SIL-IS) for bacterial RNA, which facilitates relative comparison of all modifications. While conventional SIL-IS approaches require the chemical synthesis of single modifications in weighable quantities, this SIL-IS consists of a nucleoside mixture covering all detectable RNA modifications of Escherichia coli, yet in small and initially unknown quantities. For absolute in addition to relative quantification, those quantities were determined by a combination of external calibration and sample spiking of the biosynthetic SIL-IS. For each nucleoside, we thus obtained a very robust relative response factor, which permits direct conversion of the MS signal to absolute amounts of substance. The application of the validated SIL-IS allowed highly precise quantification with standard deviations <2% during a 12-week period, and a linear dynamic range that was extended by two orders of magnitude.

A multifunctional bioconjugate module for versatile photoaffinity labeling and click chemistry of RNA

A multifunctional reagent based on a coumarin scaffold was developed for derivatization of naive RNA. The alkylating agent N3BC [7-azido-4-(bromomethyl)coumarin], obtained by Pechmann condensation, is selective for uridine. N3BC and its RNA conjugates are pre-fluorophores which permits controlled modular and stepwise RNA derivatization. The success of RNA alkylation by N3BC can be monitored by photolysis of the azido moiety, which generates a coumarin fluorophore that can be excited with UV light of 320 nm. The azidocoumarin-modified RNA can be flexibly employed in structure-function studies. Versatile applications include direct use in photo-crosslinking studies to cognate proteins, as demonstrated with tRNA and RNA fragments from the MS2 phage and the HIV genome. Alternatively, the azide function can be used for further derivatization by click-chemistry. This allows e.g. the introduction of an additional fluorophore for excitation with visible light.

Partial methylation at Am100 in 18S rRNA of baker's yeast reveals ribosome heterogeneity on the level of eukaryotic rRNA modification.

Ribosome heterogeneity is of increasing biological significance and several examples have been described for multicellular and single cells organisms. In here we show for the first time a variation in ribose methylation within the 18S rRNA of Saccharomyces cerevisiae. Using RNA-cleaving DNAzymes, we could specifically demonstrate that a significant amount of S. cerevisiae ribosomes are not methylated at 2′-O-ribose of A100 residue in the 18S rRNA. Furthermore, using LC-UV-MS/MS of a respective 18S rRNA fragment, we could not only corroborate the partial methylation at A100, but could also quantify the methylated versus non-methylated A100 residue. Here, we exhibit that only 68% of A100 in the 18S rRNA of S.cerevisiae are methylated at 2′-O ribose sugar. Polysomes also contain a similar heterogeneity for methylated Am100, which shows that 40S ribosome subunits with and without Am100 participate in translation. Introduction of a multicopy plasmid containing the corresponding methylation guide snoRNA gene SNR51 led to an increased A100 methylation, suggesting the cellular snR51 level to limit the extent of this modification. Partial rRNA modification demonstrates a new level of ribosome heterogeneity in eukaryotic cells that might have substantial impact on regulation and fine-tuning of the translation process.

Eukaryotic rRNA Modification by Yeast 5-Methylcytosine-Methyltransferases and Human Proliferation-Associated Antigen p120

Modified nucleotide 5-methylcytosine (m5C) is frequently present in various eukaryotic RNAs, including tRNAs, rRNAs and in other non-coding RNAs, as well as in mRNAs. RNA:m5C-methyltranferases (MTases) Nop2 from S. cerevisiae and human proliferation-associated nucleolar antigen p120 are both members of a protein family called Nop2/NSUN/NOL1. Protein p120 is well-known as a tumor marker which is over-expressed in various cancer tissues. Using a combination of RNA bisulfite sequencing and HPLC-MS/MS analysis, we demonstrated here that p120 displays an RNA:m5C- MTase activity, which restores m5C formation at position 2870 in domain V of 25S rRNA in a nop2Δ yeast strain. We also confirm that yeast proteins Nop2p and Rcm1p catalyze the formation of m5C in domains V and IV, respectively. In addition, we do not find any evidence of m5C residues in yeast 18S rRNA. We also performed functional complementation of Nop2-deficient yeasts by human p120 and studied the importance of different sequence and structural domains of Nop2 and p120 for yeast growth and m5C-MTase activity. Chimeric protein formed by Nop2 and p120 fragments revealed the importance of Nop2 N-terminal domain for correct protein localization and its cellular function. We also validated that the presence of Nop2, rather than the m5C modification in rRNA itself, is required for pre-rRNA processing. Our results corroborate that Nop2 belongs to the large family of pre-ribosomal proteins and possesses two related functions in pre-rRNA processing: as an essential factor for cleavages and m5C:RNA:modification. These results support the notion of quality control during ribosome synthesis by such modification enzymes.

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

ABSTRACT 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 tRNAAsp, we showed that this natural Dnmt2 substrate could be engineered to employ RNAs that act like guide RNAs in vitro. The 5’-half of tRNAAsp 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.

Variable presence of 5-methylcytosine in commercial RNA and DNA

Nucleoside methylations and other nucleic acid modifications have recently encountered a surge in interest, prompted, among other things, by the detection of methylation and active demethylation of DNA and mRNA by similar mechanisms. In DNA, deoxycytidine methylation by Dnmt enzymes generates 5-methyldeoxycytidine,1 an important epigenetic mark that typically causes inactivation of transcription of the methylated promoter region. Recent exciting developments have shown that these marks are not concrete-cast, but can be actively removed by the oxidative action of TET enzymes,2 which generate, through a series of 2-electron oxidations, first hydroxymethylcytidine (hm5C), then formyldeoxycytidine (f5C),3 and finally carboxydeoxycytidine (ca5C), which may eventually regenerate deoxycytidine by decarboxylation. The apparent functional homolog in mRNA is m6A, which appears to reduce translation efficiency. Here, too, the methylation can be removed by TET-related enzymes generating first hydroxymethyladenosine (hm6A), then formyladenosine (f6A).4,5,6 Also, the presence of 5-methylcytidine in mRNA has been reported early on, and has recently raised renewed interest, although its function is as yet unclear and putative conversion into hydroxymethylcytidine is yet to be demonstrated.32 These developments are enhanced by the development of highly sensitive detection methods7,8 including the adaptation of the so-called bisulfite sequencing from DNA, where it is well established,9 to RNA, where its application has significantly contributed to the present high level of interest.10,11,12 However, bisulfite sequencing alone does not yield unassailable results13,14 and we have thus looked to expand the limits of detection of m5C in both DNA and RNA by LC-MS/MS. Here we report a straightforward regimen that provides values for the limit of quantification (LOQ) in the triple digit attomol range. Its application to presumed negative controls, namely synthetic oligonucleotides, surprisingly detected significant amounts of m5C in both types of synthetic nucleic acids.