In-line probing of RNA G-quadruplexes

Pathogenic SNPs Affect Both RNA and DNA G-Quadruplexes' Responses to Ligands

Single nucleotide polymorphisms (SNPs) are common genetic variations that are present in over 1% of the population and can significantly modify the structures of both DNA and RNA. G-quadruplex structures (G4) are formed by the superposition of tetrads of guanines. To date, the impact of SNPs on both G4 ligands' binding efficacies and specificities has not been investigated. Here, using a bioinformatically predicted G4 and SNPs found in the α-synuclein gene as a proof-of-concept, it was demonstrated that SNPs can modulate both DNA and RNA G4s' responses to ligands. Specifically, six widely recognized ligands (Phen-DC3, PDS, 360A, RHPS4, BRACO19, and TMPyP4) were shown to differentially affect both the structure and the polymerase stalling of the different SNPs. This work highlights the importance of choosing the appropriate G4 ligand when dealing with an SNP identified in a G-rich gene.

The structural features of the ligand-free moaA riboswitch and its ion-dependent folding

Riboswitches are structural elements of mRNA involved in the regulation of gene expression by responding to specific cellular metabolites. To fulfil their regulatory function, riboswitches prefold into an active state, the so-called binding competent form, that guarantees metabolite binding and allows a consecutive refolding of the RNA. Here, we describe the folding pathway to the binding competent form as well as the ligand free structure of the moaA riboswitch of E. coli. This RNA proposedly responds to the molybdenum cofactor (Moco), a highly oxygen-sensitive metabolite, essential in the carbon and sulfur cycles of eukaryotes. K⁺- and Mg²⁺-dependent footprinting assays and spectroscopic investigations show a high degree of structure formation of this RNA already at very low ion-concentrations. Mg²⁺ facilitates additionally a general compaction of the riboswitch towards its proposed active structure. We show that this fold agrees with the earlier suggested secondary structure which included also a long-range tetraloop/tetraloop-receptor like interaction. Metal ion cleavage assays revealed specific Mg²⁺-binding pockets within the moaA riboswitch. These Mg²⁺ binding pockets are good indicators for the potential Moco binding site, since in riboswitches, Mg²⁺ was shown to be necessary to bind phosphate-carrying metabolites. The importance of the phosphate and of other functional groups of Moco is highlighted by binding assays with tetrahydrobiopterin, the reduced and oxygen-sensitive core moiety of Moco. We demonstrate that the general molecular shape of pterin by its own is insufficient for the recognition by the riboswitch.

Efficient translation of Eggplant mottled dwarf nucleorhabdovirus N and X genes requires both 5' and 3' UTRs

Background

Circularization of RNA mediated by association of translation factors and RNA elements in 5′ and 3′ UTRs is a common feature for translation control in eukaryotes. There is no information about translation in plant rhabdoviruses and little information is known in animal rhabdoviruses.

Methods

The role of 5′ and 3′ UTRs in two genes of EMDV in translation were studied using luciferase constructs and RNA structures of these sequences were analyzed by SHAPE and Inline probing.

Results

We have found that efficient translation of N and X mRNAs of nucleorhabdovirus Eggplant mottled dwarf virus (EMDV) requires elements present in both 5′ and 3′ UTRs. Luciferase reporter constructs containing precise 5′ and 3′ UTRs of the N and X genes had substantially higher translational activity compared with constructs containing only the 5′ or 3′ UTR. The 3′UTR of carmovirus Turnip crinkle virus, which contains a well-characterized cap-independent translation enhancer, was unable to complement the lack of EMDV 3′ UTR. Addition of cap analog to luciferase constructs containing the UTRs of the N gene did not restore translation, and translation of the reporter construct in the absence of the 5′ cap was higher than the capped construct. No RNA-RNA interactions between 5′ and 3′ UTRs were detected by EMSA or in-line cleavage structural assays. Deletion of 11 nucleotides from the 3′ terminus negated the synergistic activity of the 3′UTR.

Conclusions

The results with RNA-RNA interaction suggesting that translational synergy between the UTRs may utilize alternative means. Mutation analysis in 3′UTR suggesting that the polyadenylation signal sequence contained in this location may play a critical role in translation.

Guanine Nucleotide-Binding Protein-Like 1 (GNL1) binds RNA G-quadruplex structures in genes associated with Parkinson's disease

RNAs are highly regulated at the post-transcriptional level in neurodegenerative diseases and just a few mutations can significantly affect the fate of neuronal cells. To date, the impact of G-quadruplex (G4) regulation in neurodegenerative diseases like Parkinson’s disease (PD) has not been analysed. In this study, in silico potential G4s located in deregulated genes related to the nervous system were initially identified and were found to be significantly enriched. Several G4 sequences found in the 5ʹ untranslated regions (5ʹUTR) of mRNAs associated with Parkinson’s disease were demonstrated to in fact fold in vitro by biochemical assays. Subcloning of the full-length 5ʹUTRs of these candidates upstream of a luciferase reporter system led to the demonstration that the G4s of both Parkin RBR E3 Ubiquitin Protein Ligase (PRKN) and Vacuolar Protein Sorting-Associated Protein 35 (VPS35) significantly repressed the translation of both genes in SH-SY5Y cells. Subsequently, a strategy of using label-free RNA affinity purification assays with either of these two G4 sequences as bait isolated the Guanine Nucleotide-Binding Protein-Like 1 (GNL1). The latter was shown to have a higher affinity for the G4 sequences than for their mutated version. This study sheds light on new RNA G-quadruplexes located in genes dysregulated in Parkinson disease and a new G4-binding protein, GNL1.

An RNA guanine quadruplex regulated pathway to TRAIL-sensitization by DDX21

DDX21 is a newly discovered RNA G-quadruplex (rG4) binding protein with no known biological rG4 targets. In this study we used label-free proteomic MS/MS to identify 26 proteins that are expressed at significantly different levels in cells expressing an rG4-binding deficient DDX21 (M4). MS data are available via ProteomeXchange with identifier PXD013501. From this list we validate MAGED2 as a protein that is regulated by DDX21 through rG4 in its 5'-UTR. MAGED2 protein levels, but not mRNA levels, are reduced by half in cells expressing DDX21 M4. MAGED2 has a repressive effect on TRAIL-R2 expression that is relieved under these conditions, resulting in elevated TRAIL-R2 mRNA and protein in MCF-7 cells, rendering them sensitive to TRAIL-mediated apoptosis. Our work identifies the role of DDX21 in regulation at the translational level through biologically relevant rG4 and shows that MAGED2 protein levels are regulated, at least in part, by the potential to form rG4 in their 5'-UTRs.

Whole genome experimental maps of DNA G-quadruplexes in multiple species

Genomic maps of DNA G-quadruplexes (G4s) can help elucidate the roles that these secondary structures play in various organisms. Herein, we employ an improved version of a G-quadruplex sequencing method (G4-seq) to generate whole genome G4 maps for 12 species that include widely studied model organisms and also pathogens of clinical relevance. We identify G4 structures that form under physiological K+ conditions and also G4s that are stabilized by the G4-targeting small molecule pyridostatin (PDS). We discuss the various structural features of the experimentally observed G-quadruplexes (OQs), highlighting differences in their prevalence and enrichment across species. Our study describes diversity in sequence composition and genomic location for the OQs in the different species and reveals that the enrichment of OQs in gene promoters is particular to mammals such as mouse and human, among the species studied. The multi-species maps have been made publicly available as a resource to the research community. The maps can serve as blueprints for biological experiments in those model organisms, where G4 structures may play a role.

G-quadruplexes formation in the 5'UTRs of mRNAs associated with colorectal cancer pathways

RNA G-quadruplexes (rG4) are stable non-canonical secondary structures composed of G-rich sequences. Many rG4 structures located in the 5'UTRs of mRNAs act as translation repressors due to their high stability which is thought to impede ribosomal scanning. That said, it is not known if these are mRNA-specific examples, or if they are indicative of a global expression regulation mechanism of the mRNAs involved in a common pathway based on structure folding recognition. Gene-ontology analysis of mRNAs bearing a predicted rG4 motif in their 5'UTRs revealed an enrichment for mRNAs associated with the colorectal cancer pathway. Bioinformatic tools for rG4 prediction, and experimental in vitro validations were used to confirm and compare the folding of the predicted rG4s of the mRNAs associated with dysregulated pathways in colorectal cancer. The rG4 folding was confirmed for the first time for 9 mRNAs. A repressive effect of 3 rG4 candidates on the expression of a reporter gene was also measured in colorectal cancer cell lines. This work highlights the fact that rG4 prediction is not yet accurate, and that experimental characterization is still essential in order to identify the precise rG4 folding sequences and the possible common features shared between the rG4 overrepresented in important biological pathways.

Structural analysis reveals the formation and role of RNA G-quadruplex structures in human mature microRNAs

Here we identify hundreds of RNA G-quadruplex (rG4) candidates in microRNAs (miRNAs), characterize the miRNA structure and miRNA-mRNA interactions on several mammalian-conserved miRNAs, and reveal the formation of rG4s in miRNAs. Notably, we study the effect of these rG4s in cells and uncover the role of rG4s in miRNA-mediated post-transcriptional regulation.

Detecting RNA G-Quadruplexes (rG4s) in the Transcriptome

RNA G-quadruplex (rG4) secondary structures are proposed to play key roles in fundamental biological processes that include the modulation of transcriptional, co-transcriptional, and posttranscriptional events. Recent methodological developments that include predictive algorithms and structure-based sequencing have enabled the detection and mapping of rG4 structures on a transcriptome-wide scale at high sensitivity and resolution. The data generated by these studies provide valuable insights into the potentially diverse roles of rG4s in biology and open up a number of mechanistic hypotheses. Herein we highlight these methodologies and discuss the associated findings in relation to rG4-related biological mechanisms.

G-Quadruplexes influence pri-microRNA processing

RNA G-Quadruplexes (G4) have been shown to possess many biological functions, including the regulation of microRNA (miRNA) biogenesis and function. However, their impact on pri-miRNA processing remains unknown. We identified G4 located near the Drosha cleavage site in three distinct pri-miRNAs: pri-mir200c, pri-mir451a, and pri-mir497. The folding of the potential G4 motifs was determined in solution. Subsequently, mutations disrupting G4 folding led to important changes in the mature miRNAs levels in cells. Moreover, using small antisense oligonucleotides binding to the pri-miRNA, it was possible to modulate, either positively or negatively, the mature miRNA levels. Together, these data demonstrate that G4 motifs could contribute to the regulation of pri-mRNA processing, a novel role for G4. Considering that bio-informatics screening indicates that between 9% and 50% of all pri-miRNAs contain a putative G4, these structures possess interesting potential as future therapeutic targets.

RNA G-quadruplexes: emerging mechanisms in disease

RNA G-quadruplexes (G4s) are formed by G-rich RNA sequences in protein-coding (mRNA) and non-coding (ncRNA) transcripts that fold into a four-stranded conformation. Experimental studies and bioinformatic predictions support the view that these structures are involved in different cellular functions associated to both DNA processes (telomere elongation, recombination and transcription) and RNA post-transcriptional mechanisms (including pre-mRNA processing, mRNA turnover, targeting and translation). An increasing number of different diseases have been associated with the inappropriate regulation of RNA G4s exemplifying the potential importance of these structures on human health. Here, we review the different molecular mechanisms underlying the link between RNA G4s and human diseases by proposing several overlapping models of deregulation emerging from recent research, including (i) sequestration of RNA-binding proteins, (ii) aberrant expression or localization of RNA G4-binding proteins, (iii) repeat associated non-AUG (RAN) translation, (iv) mRNA translational blockade and (v) disabling of protein–RNA G4 complexes. This review also provides a comprehensive survey of the functional RNA G4 and their mechanisms of action. Finally, we highlight future directions for research aimed at improving our understanding on RNA G4-mediated regulatory mechanisms linked to diseases.

3' UTR G-quadruplexes regulate miRNA binding

MicroRNAs (miRNAs) are small noncoding RNAs that repress the translation of their target genes. It has previously been shown that a target's availability to miRNA can be affected by its structure. G-quadruplexes (G4) are noncanonical structures adopted by G-rich nucleic acids that have been shown to have multiple biological functions. In this study, whether or not G4 structures' presence in the 3' UTRs of mRNAs can hinder miRNA binding was investigated. Putative G4 overlapping with predicted miRNAs' binding sites was searched for, and 44,294 hits were found in humans. The FADS2 mRNA/mir331-3p pair was selected as a model example. In-line probing and G4-specific fluorescent ligand experiments binding were performed and confirmed the presence of a G4 near the predicted miRNA binding site. Subsequent luciferase assays showed that the presence of the G4 prevents the binding of mir331-3p in cellulo. Together, these results served as proof of concept that a G4 structure present in a 3' UTR sequence should be taken into consideration when predicting miRNA binding sites.

G-Quadruplexes: Prediction, Characterization, and Biological Application

Guanine (G)-rich sequences in nucleic acids can assemble into G-quadruplex structures that involve G-quartets linked by loop nucleotides. The structural and topological diversity of G-quadruplexes have attracted great attention for decades. Recent methodological advances have advanced the identification and characterization of G-quadruplexes in vivo as well as in vitro, and at a much higher resolution and throughput, which has greatly expanded our current understanding of G-quadruplex structure and function. Accumulating knowledge about the structural properties of G-quadruplexes has helped to design and develop a repertoire of molecular and chemical tools for biological applications. This review highlights how these exciting methods and findings have opened new doors to investigate the potential functions and applications of G-quadruplexes in basic and applied biosciences.

Dawn of the in vivo RNA structurome and interactome

RNA is one of the most fascinating biomolecules in living systems given its structural versatility to fold into elaborate architectures for important biological functions such as gene regulation, catalysis, and information storage. Knowledge of RNA structures and interactions can provide deep insights into their functional roles in vivo For decades, RNA structural studies have been conducted on a transcript-by-transcript basis. The advent of next-generation sequencing (NGS) has enabled the development of transcriptome-wide structural probing methods to profile the global landscape of RNA structures and interactions, also known as the RNA structurome and interactome, which transformed our understanding of the RNA structure-function relationship on a transcriptomic scale. In this review, molecular tools and NGS methods used for RNA structure probing are presented, novel insights uncovered by RNA structurome and interactome studies are highlighted, and perspectives on current challenges and potential future directions are discussed. A more complete understanding of the RNA structures and interactions in vivo will help illuminate the novel roles of RNA in gene regulation, development, and diseases.

Do we know whether potential G-quadruplexes actually form in long functional RNA molecules?

The roles of deoxyribonucleic acid (DNA) G-quadruplex structures in gene expression and telomere maintenance have been well characterized. Recent results suggest that such structures could also play pivotal roles in ribonucleic acid (RNA) biology, such as splicing or translation regulation. However, it has been difficult to show that RNA G-quadruplexes (G4s) exist in specific long RNA sequences, such as precursor messenger RNA, in a functional or cellular context. Most current methods for identifying G4s involve the use of short, purified RNA sequences in vitro, in the absence of competition with secondary structures or protein binding. Therefore, novel methods need to be developed to allow the characterization of G4s in long functional RNAs and in a cellular context. This need has in part been met by our recent development of a method based on a comparison of RNA and 7-deaza-RNA that provides a test for identifying RNA G4s in such conditions.

Irregular G-quadruplexes Found in the Untranslated Regions of Human mRNAs Influence Translation

G-quadruplex structures are composed of coplanar guanines and are found in both DNA and RNA. They are formed by the stacking of two or more G-quartets that are linked together by three loops. The current belief is that RNA G-quadruplexes include loops of l to 7 nucleotides in length, although recent evidence indicates that the central loop (loop 2) can be longer if loops 1 and 3 are limited to a single nucleotide each. With the objective of broadening the definition of irregular RNA G-quadruplexes, a bioinformatic search was performed to find potential G-quadruplexes located in the untranslated regions of human mRNAs (i.e. in the 5′ and 3′-UTRs) that contain either a long loop 1 or 3 of up to 40 nucleotides in length. RNA molecules including the potential sequences were then synthesized and examined in vitro by in-line probing for the formation of G-quadruplex structures. The sequences that adopted a G-quadruplex structure were cloned into a luciferase dual vector and examined for their ability to modulate translation in cellulo. Some irregular G-quadruplexes were observed to either promote or repress translation regardless of the position or the size of the long loop they possessed. Even if the composition of a RNA G-quadruplex is not quite completely understood, the results presented in this report clearly demonstrate that what defines a RNA G-quadruplex is much broader than what we previously believed.

Structural Analysis using SHALiPE to Reveal RNA G-Quadruplex Formation in Human Precursor MicroRNA

RNA G‐quadruplex (rG4) structures are of fundamental importance to biology. A novel approach is introduced to detect and structurally map rG4s at single‐nucleotide resolution in RNAs. The approach, denoted SHALiPE, couples selective 2′‐hydroxyl acylation with lithium ion‐based primer extension, and identifies characteristic structural fingerprints for rG4 mapping. We apply SHALiPE to interrogate the human precursor microRNA 149, and reveal the formation of an rG4 structure in this non‐coding RNA. Additional analyses support the SHALiPE results and uncover that this rG4 has a parallel topology, is thermally stable, and is conserved in mammals. An in vitro Dicer assay shows that this rG4 inhibits Dicer processing, supporting the potential role of rG4 structures in microRNA maturation and post‐transcriptional regulation of mRNAs.

Amyloid Precursor Protein Translation Is Regulated by a 3'UTR Guanine Quadruplex

A central event in Alzheimer’s disease is the accumulation of amyloid β (Aβ) peptides generated by the proteolytic cleavage of the amyloid precursor protein (APP). APP overexpression leads to increased Aβ generation and Alzheimer’s disease in humans and altered neuronal migration and increased long term depression in mice. Conversely, reduction of APP expression results in decreased Aβ levels in mice as well as impaired learning and memory and decreased numbers of dendritic spines. Together these findings indicate that therapeutic interventions that aim to restore APP and Aβ levels must do so within an ideal range. To better understand the effects of modulating APP levels, we explored the mechanisms regulating APP expression focusing on post-transcriptional regulation. Such regulation can be mediated by RNA regulatory elements such as guanine quadruplexes (G-quadruplexes), non-canonical structured RNA motifs that affect RNA stability and translation. Via a bioinformatics approach, we identified a candidate G-quadruplex within the APP mRNA in its 3’UTR (untranslated region) at residues 3008–3027 (NM_201414.2). This sequence exhibited characteristics of a parallel G-quadruplex structure as revealed by circular dichroism spectrophotometry. Further, as with other G-quadruplexes, the formation of this structure was dependent on the presence of potassium ions. This G-quadruplex has no apparent role in regulating transcription or mRNA stability as wild type and mutant constructs exhibited equivalent mRNA levels as determined by real time PCR. Instead, we demonstrate that this G-quadruplex negatively regulates APP protein expression using dual luciferase reporter and Western blot analysis. Taken together, our studies reveal post-transcriptional regulation by a 3’UTR G-quadruplex as a novel mechanism regulating APP expression.

Directly lighting up RNA G-quadruplexes from test tubes to living human cells

RNA G-quadruplexes (G4s) are one of the key components of the transcriptome that act as efficient post-transcriptional regulatory elements in living cells. To conduct further studies of the unique biological functions of RNA G4s, techniques need to be developed that can efficiently recognize RNA G4 structures under various conditions, in fixed cells and living cells, as well as in vitro. This paper presents the development of such a method, a new technique using a cyanine dye called CyT, which can detect both canonical and non-canonical RNA G4 structures from test tubes to living human cells. The ability of CyT to distinguish between G4 and nonG4 RNA offers a promising tool for future RNA G4-based biomarker discovery and potential diagnostic applications.

CRISPRscan: designing highly efficient sgRNAs for CRISPR-Cas9 targeting in vivo

CRISPR-Cas9 technology provides a powerful system for genome engineering. However, variable activity across different single guide RNAs (sgRNAs) remains a significant limitation. We analyzed the molecular features that influence sgRNA stability, activity and loading into Cas9 in vivo. We observed that guanine enrichment and adenine depletion increased sgRNA stability and activity, whereas differential sgRNA loading, nucleosome positioning and Cas9 off-target binding were not major determinants. We also identified sgRNAs truncated by one or two nucleotides and containing 5' mismatches as efficient alternatives to canonical sgRNAs. On the basis of these results, we created a predictive sgRNA-scoring algorithm, CRISPRscan, that effectively captures the sequence features affecting the activity of CRISPR-Cas9 in vivo. Finally, we show that targeting Cas9 to the germ line using a Cas9-nanos 3' UTR led to the generation of maternal-zygotic mutants, as well as increased viability and decreased somatic mutations. These results identify determinants that influence Cas9 activity and provide a framework for the design of highly efficient sgRNAs for genome targeting in vivo.

Aven recognition of RNA G-quadruplexes regulates translation of the mixed lineage leukemia protooncogenes

G-quadruplexes (G4) are extremely stable secondary structures forming stacks of guanine tetrads. DNA G4 structures have been extensively studied, however, less is known about G4 motifs in mRNAs, especially in their coding sequences. Herein, we show that Aven stimulates the mRNA translation of the mixed lineage leukemia (MLL) proto-oncogene in an arginine methylation-dependent manner. The Aven RGG/RG motif bound G4 structures within the coding regions of the MLL1 and MLL4 mRNAs increasing their polysomal association and translation, resulting in the induction of transcription of leukemic genes. The DHX36 RNA helicase associated with the Aven complex and was required for optimal translation of G4 mRNAs. Depletion of Aven led to a decrease in synthesis of MLL1 and MLL4 proteins resulting in reduced proliferation of leukemic cells. These findings identify an Aven-centered complex that stimulates the translation of G4 harboring mRNAs, thereby promoting survival of leukemic cells.

DOI:http://dx.doi.org/10.7554/eLife.06234.001

To make a protein, the DNA sequence that encodes it is first copied to make a molecule of messenger RNA (or mRNA for short). The mRNA is then used as a set of instructions to assemble a protein in a process called translation. Both DNA and RNA molecules can fold into particular shapes. One such structure is known as a G-quartet and involves the DNA or RNA folding back on itself to form a highly stable planar structure. Stacks of G-quartets can form structures known as G-quadruplexes, but little is known about the G-quadruplexes that form in mRNA molecules.

Leukemia affects cells in the bone marrow and causes blood cells to develop abnormally. A protein called Aven is often found in increased amounts in certain types of leukemic cells, but it was not clear how Aven affects how leukemia develops. Thandapani, Song et al. have now found that in leukemic cells, Aven binds to G-quadruplexes found in two mRNA molecules that encode proteins that are linked to leukemia. This binding increases the translation of these mRNAs, with translation occurring most efficiently when a particular enzyme called a helicase—which remodels RNA—also bound to Aven. Reducing the amount of Aven in cells caused fewer of the leukemic proteins to be produced, which also reduced the growth and multiplcation of leukemic cells.

These findings raise the possibility that drugs that disrupt how Aven works could form part of treatments for leukemia. The next challenge will be to identify the signaling pathways that communicate with Aven and to define all the G-quadruplex mRNAs that are regulated by Aven.

DOI:http://dx.doi.org/10.7554/eLife.06234.002

Targeted Detection of G-Quadruplexes in Cellular RNAs

The G-quadruplex (G4) is a non-canonical nucleic acid structure which regulates important cellular processes. RNA G4s have recently been shown to exist in human cells and be biologically significant. Described herein is a new approach to detect and map RNA G4s in cellular transcripts. This method exploits the specific control of RNA G4-cation and RNA G4-ligand interactions during reverse transcription, by using a selective reverse transcriptase to monitor RNA G4-mediated reverse transcriptase stalling (RTS) events. Importantly, a ligation-amplification strategy is coupled with RTS, and enables detection and mapping of G4s in important, low-abundance cellular RNAs. Strong evidence is provided for G4 formation in full-length cellular human telomerase RNA, offering important insights into its cellular function.

A stable RNA G-quadruplex within the 5'-UTR of Arabidopsis thaliana ATR mRNA inhibits translation

Guanine quadruplex structures (GQSs) play important roles in the regulation of gene expression and cellular processes. Recent studies provide strong evidence for the formation and function of DNA and RNA GQSs in human cells. However, whether GQSs form and are functional in plants remains essentially unexplored. On the basis of circular dichroism (CD)-detected titration, UV-detected melting, in-line probing (ILP) and reporter gene assay studies, we report the first example of a plant RNA GQS that inhibits translation. This GQS is located within the 5'-UTR of the ATAXIA TELANGIECTASIA-MUTATED AND RAD3-RELATED (ATR) mRNA of Arabidopsis thaliana (mouse-ear cress). We show that this GQS is highly stable and is thermodynamically favoured over a competing hairpin structure in the 5'-UTR at physiological K⁺ and Mg²⁺ concentrations. Results from ILP reveal the secondary structure of the RNA and support formation of the GQS in vitro in the context of the complete 5'-UTR. Transient reporter gene assays performed in living plants reveal that the GQS inhibits translation but not transcription, implicating this GQS as a translational repressor in vivo. Our results provide the first complete demonstration of the formation and function of a regulatory RNA GQS in plants and open new avenues to explore potential functional roles of GQS in the plant kingdom.

Small antisense oligonucleotides against G-quadruplexes: specific mRNA translational switches

G-quadruplexes (G4) are intricate RNA structures found throughout the transcriptome. Because they are associated with a variety of biological cellular mechanisms, these fascinating structural motifs are seen as potential therapeutic targets against many diseases. While screening of chemical compounds specific to G4 motifs has yielded interesting results, no single compound successfully discriminates between G4 motifs based on nucleotide sequences alone. This level of specificity is best attained using antisense oligonucleotides (ASO). Indeed, oligonucleotide-based strategies are already used to modulate DNA G4 folding in vitro. Here, we report that, in human cells, the use of short ASO to promote and inhibit RNA G4 folding affects the translation of specific mRNAs, including one from the 5'UTR of the H2AFY gene, a histone variant associated with cellular differentiation and cancer. These results suggest that the relatively high specificity of ASO-based strategies holds significant potential for applications aimed at modulating G4-motif folding.

The folding of 5'-UTR human G-quadruplexes possessing a long central loop

G-quadruplexes are widespread four-stranded structures that are adopted by G-rich regions of both DNA and RNA and are involved in essential biological processes such as mRNA translation. They are formed by the stacking of two or more G-quartets that are linked together by three loops. Although the maximal loop length is usually fixed to 7 nt in most G-quadruplex-predicting software, it has already been demonstrated that artificial DNA G-quadruplexes containing two distal loops that are limited to 1 nt each and a central loop up to 30 nt long are likely to form in vitro. This report demonstrates that such structures possessing a long central loop are actually found in the 5'-UTRs of human mRNAs. Firstly, 1453 potential G-quadruplex-forming sequences (PG4s) were identified through a bioinformatic survey that searched for sequences respecting the requirement for two 1-nt long distal loops and a long central loop of 2-90 nt in length. Secondly, in vitro in-line probing experiments confirmed and characterized the folding of eight candidates possessing central loops of 10-70 nt long. Finally, the biological effect of several G-quadruplexes with a long central loop on mRNA expression was studied in cellulo using a luciferase gene reporter assay. Clearly, the actual definition of G-quadruplex-forming sequences is too conservative and must be expanded to include the long central loop. This greatly expands the number of expected PG4s in the transcriptome. Consideration of these new candidates might aid in elucidating the potentially important biological implications of the G-quadruplex structure.

New scoring system to identify RNA G-quadruplex folding

G-quadruplexes (G4s) are non-canonical structures involved in many important cellular processes. To date, the prediction of potential G-quadruplex structures (PG4s) has been based almost exclusively on the sequence of interest agreeing with the algorithm G_x-N-_1–7-G_x-N_1–7-G_x-N_1–7-G_x (where x ≥ 3 and N = A, U, G or C). However, many sequences agreeing with this algorithm do not form G4s and are considered false-positive predictions. Here we show the RNA PG4 candidate in the 3′-untranslated region (UTR) of the TTYH1 gene to be one such false positive. Specifically, G4 folding was observed to be inhibited by the presence of multiple-cytosine tracks, located in the candidate’s genomic context, that adopted a Watson–Crick base-paired structure. Clearly, the neighbouring sequences of a PG4 may influence its folding. The secondary structure of 12 PG4 motifs along with either 15 or 50 nucleotides of their upstream and downstream genomic contexts were evaluated by in-line probing. Data permitted the development of a scoring system for the prediction of PG4s taking into account the effect of the neighbouring sequences. The accuracy of this scoring system was assessed by probing 14 other novel PG4 candidates retrieved in human 5′-UTRs. This new scoring system can be used, in combination with the standard algorithm, to better predict the folding of RNA G4s.