Retinoblastoma susceptibility genes contain 5' sequences with a high propensity to form guanine-tetrad structures

RNA G-quadruplexes control mitochondria-localized mRNA translation and energy metabolism

Cancer cells rely on mitochondria for their bioenergetic supply and macromolecule synthesis. Central to mitochondrial function is the regulation of mitochondrial protein synthesis, which primarily depends on the cytoplasmic translation of nuclear-encoded mitochondrial mRNAs whose protein products are imported into mitochondria. Despite the growing evidence that mitochondrial protein synthesis contributes to the onset and progression of cancer, and can thus offer new opportunities for cancer therapy, knowledge of the underlying molecular mechanisms remains limited. Here, we show that RNA G-quadruplexes (RG4s) regulate mitochondrial function by modulating cytoplasmic mRNA translation of nuclear-encoded mitochondrial proteins. Our data support a model whereby the RG4 folding dynamics, under the control of oncogenic signaling and modulated by small molecule ligands or RG4-binding proteins, modifies mitochondria-localized cytoplasmic protein synthesis. Ultimately, this impairs mitochondrial functions, affecting energy metabolism and consequently cancer cell proliferation.

Non-canonical DNA structures: Diversity and disease association

A complete understanding of DNA double-helical structure discovered by James Watson and Francis Crick in 1953, unveil the importance and significance of DNA. For the last seven decades, this has been a leading light in the course of the development of modern biology and biomedical science. Apart from the predominant B-form, experimental shreds of evidence have revealed the existence of a sequence-dependent structural diversity, unusual non-canonical structures like hairpin, cruciform, Z-DNA, multistranded structures such as DNA triplex, G-quadruplex, i-motif forms, etc. The diversity in the DNA structure depends on various factors such as base sequence, ions, superhelical stress, and ligands. In response to these various factors, the polymorphism of DNA regulates various genes via different processes like replication, transcription, translation, and recombination. However, altered levels of gene expression are associated with many human genetic diseases including neurological disorders and cancer. These non-B-DNA structures are expected to play a key role in determining genetic stability, DNA damage and repair etc. The present review is a modest attempt to summarize the available literature, illustrating the occurrence of non-canonical structures at the molecular level in response to the environment and interaction with ligands and proteins. This would provide an insight to understand the biological functions of these unusual DNA structures and their recognition as potential therapeutic targets for diverse genetic diseases.

Lipid membranes modulate the activity of RNA through sequence-dependent interactions

RNA is a ubiquitous biomolecule that can serve as both catalyst and information carrier. Understanding how RNA bioactivity is controlled is crucial for elucidating its physiological roles and potential applications in synthetic biology. Here, we show that lipid membranes can act as RNA organization platforms, introducing a mechanism for riboregulation. The activity of R3C ribozyme can be modified by the presence of lipid membranes, with direct RNA-lipid interactions dependent on RNA nucleotide content, base pairing, and length. In particular, the presence of guanine in short RNAs is crucial for RNA-lipid interactions, and G-quadruplex formation further promotes lipid binding. Lastly, by artificially modifying the R3C substrate sequence to enhance membrane binding, we generated a lipid-sensitive ribozyme reaction with riboswitch-like behavior. These findings introduce RNA-lipid interactions as a tool for developing synthetic riboswitches and RNA-based lipid biosensors and bear significant implications for RNA world scenarios for the origin of life.

Inosine Substitutions in RNA Activate Latent G-Quadruplexes

It is well-accepted that gene expression is heavily influenced by RNA structure. For instance, stem-loops and G-quadruplexes (rG4s) are dynamic motifs in mRNAs that influence gene expression. Adenosine-to-inosine (A-to-I) editing is a common chemical modification of RNA which introduces a nucleobase that is iso-structural with guanine, thereby changing RNA base-pairing properties. Here, we provide biophysical, chemical, and biological evidence that A-to-I exchange can activate latent rG4s by filling incomplete G-quartets with inosine. We demonstrate the formation of inosine-containing rG4s (GI-quadruplexes) in vitro and verify their activity in cells. GI-quadruplexes adopt parallel topologies, stabilized by potassium ions. They exhibit moderately reduced thermal stability compared to conventional G-quadruplexes. To study inosine-induced structural changes in a naturally occurring RNA, we use a synthetic approach that enables site-specific inosine incorporation in long RNAs. In summary, RNA GI-quadruplexes are a previously unrecognized structural motif that may contribute to the regulation of gene expression in vivo.

Substituted adenine quartets: interplay between substituent effect, hydrogen bonding, and aromaticity

Adenine, one of the components of DNA/RNA helices, has the ability to form self-organizing structures with cyclic hydrogen bonds (A₄), similar to guanine quartets. Here, we report a computational investigation of the effect of substituents (X = NO₂, Cl, F, H, Me, and NH₂) on the electronic structure of 9H-adenine and its quartets (A₄-N1, A₄-N3, and A₄-N7). DFT calculations were used to show the relationships between the electronic nature of the substituents, strength of H-bonds in the quartets, and aromaticity of five- and six-membered rings of adenine. We demonstrated how the remote substituent X modifies the proton-donating properties of the NH₂ group involved in the H-bonds within quartets and how the position of the substituent and its electronic nature affect the stability of the quartets. We also showed the possible changes in electronic properties of the substituent and aromaticity of adenine rings caused by tetramer formation. The results indicate that the observed relationships depend on the A₄ type. Moreover, the same substituent can both strengthen and weaken intermolecular interactions, depending on the substitution position.

Substituent effects on hydrogen bonds in adenine quartets and aromaticity of adenine rings depend on the quartet type. A₄-N3 and A₄-N7 quartets are more responsive to the electronic nature of substituents than A₄-N1.

Topologies of G-quadruplex: Biological functions and regulation by ligands

G-Quadruplex (G4) is one of the higher-order structures occurring in guanine-rich sequences of nucleic acids, and plays critical roles in biological processes. The G4-forming sequences can generate three kinds of topologies, i.e., parallel, anti-parallel, and hybrid, and these polymorphic structures have an important influence on G4-related biological functions. In this review, we highlight variety of structures generated by G4s containing various sequences and under diverse conditions. We also discuss the G4 ligands which induce specific topologies and/or conversion between different topologies.

Thioflavin T Monitoring of Guanine Quadruplex Formation in the rs689-Dependent INS Intron 1

The human proinsulin gene (INS) contains a thymine-to-adenine variant (rs689) located in the 3′ splice site (3′ ss) recognition motif of the first intron. The adenine at rs689 is strongly associated with type 1 diabetes. By weakening the polypyrimidine tract, the adenine allele reduces the efficiency of intron 1 splicing, which can be ameliorated by antisense oligonucleotides blocking a splicing silencer located upstream of the 3′ ss. The silencer is surrounded by guanine-rich tracts that may form guanine quadruplexes (G4s) and modulate the accessibility of the silencer. Here, we employed thioflavin T (ThT) to monitor G4 formation in synthetic DNAs and RNAs derived from INS intron 1. We show that the antisense target is surrounded by ThT-positive segments in each direction, with oligoribonucleotides exhibiting consistently higher fluorescence than their DNA counterparts. The signal was reduced for ThT-positive oligonucleotides that were extended into the silencer, indicating that flanking G4s have a potential to mask target accessibility. Real-time monitoring of ThT fluorescence during INS transcription in vitro revealed a negative correlation with ex vivo splicing activities of corresponding INS constructs. Together, these results provide a better characterization of antisense targets in INS primary transcripts for restorative strategies designed to improve the INS splicing defect associated with type 1 diabetes.

ALS/FTD-Associated C9ORF72 Repeat RNA Promotes Phase Transitions In Vitro and in Cells

Membraneless RNA granules originate via phase separation events driven by multivalent interactions. As RNA is the defining component of such granules, we examined how RNA contributes to granule assembly. Expansion of hexanucleotide GGGGCC (G4C2) repeats in the first intron of C9ORF72 is the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). We describe a biophysical phenomenon whereby G4C2 RNA (rG4C2) promotes the phase separation of RNA granule proteins in vitro and in cells. The ability of rG4C2 to promote phase separation is dependent on repeat length and RNA structure because rG4C2 must assume a G-quadruplex conformation to promote granule assembly. We demonstrate a central role for RNA in promoting phase separations and implicate rG4C2 G-quadruplex structures in the pathogenesis of C9-ALS/FTD.

Symmetry-adapted perturbation theory potential for the adenine dimer

A new ab initio intermolecular interaction potential for the adenine dimer has been developed.

Far-red fluorescent probes for canonical and non-canonical nucleic acid structures: current progress and future implications

Our review presents the recent progress on far-red fluorescent probes of canonical and non-canonical nucleic acid (NA) structures, critically discusses the design principles, applications, limitations and outline the future prospects of developing newer probes with target-specificity for different NA structures.

In What Ways Do Synthetic Nucleotides and Natural Base Lesions Alter the Structural Stability of G-Quadruplex Nucleic Acids?

Synthetic analogs of natural nucleotides have long been utilized for structural studies of canonical and noncanonical nucleic acids, including the extensively investigated polymorphic G-quadruplexes (GQs). Dependence on the sequence and nucleotide modifications of the folding landscape of GQs has been reviewed by several recent studies. Here, an overview is compiled on the thermodynamic stability of the modified GQ folds and on how the stereochemical preferences of more than 70 synthetic and natural derivatives of nucleotides substituting for natural ones determine the stability as well as the conformation. Groups of nucleotide analogs only stabilize or only destabilize the GQ, while the majority of analogs alter the GQ stability in both ways. This depends on the preferred syn or anti N-glycosidic linkage of the modified building blocks, the position of substitution, and the folding architecture of the native GQ. Natural base lesions and epigenetic modifications of GQs explored so far also stabilize or destabilize the GQ assemblies. Learning the effect of synthetic nucleotide analogs on the stability of GQs can assist in engineering a required stable GQ topology, and exploring the in vitro action of the single and clustered natural base damage on GQ architectures may provide indications for the cellular events.

A genome-inspired, reverse selection approach to aptamer discovery

Limitations of Systematic Evolution of Ligands by Exponential Enrichment (SELEX) and related methods that depend upon combinatorial oligonucleotide libraries have hindered progress in this area. Our laboratory has introduced a new approach to aptamer discovery that uses oligonucleotides with sequences drawn from the human genome to capture proteins from biological samples. Specifically, we have focused on capture of proteins in nuclear extracts from human cell lines using G-quadruplex (G4) forming genomic sequences. Previous studies identified capture of several proteins both in vitro and in live cells by the Pu28-mer sequence from the ERBB2 promoter region. Here we provide a more comprehensive study of protein capture from BT474 and MCF7 human breast cancer cells using G4-forming sequences from the CMYC, RB, VEGF and ERBB2 human oncogene promoter regions. Mass spectrometric analysis and Western blot analysis of protein capture at oligonucleotide-modified surfaces revealed capture of nucleolin by all three of the oligonucleotides in BT474 and MCF7 cells, and also of ribosomal protein L19 (RPL19) in BT474 cells. Chromatin immunoprecipitation (ChIP) analysis confirmed the interaction of nucleolin with all three promoter sequences in MCF7 cells and with RB in BT474 cells. ChIP also revealed interactions of RPL19 with CMYC in BT474 cells and of both RPL19 and ribosomal protein L14 (RPL14) with ERBB2 in BT474 cells. These results offer the basis for development of new aptamers based on the G4 sequences from the CMYC, RB, VEGF, and ERBB2 promoters toward proteins including nucleolin, RPL19 and RPL14. These interactions also may have biological and therapeutic significance.

RNA G-Quadruplexes in Biology: Principles and Molecular Mechanisms

G-quadruplexes (G4s) are extremely stable DNA or RNA secondary structures formed by sequences rich in guanine. These structures are implicated in many essential cellular processes, and the number of biological functions attributed to them continues to grow. While DNA G4s are well understood on structural and, to some extent, functional levels, RNA G4s and their functions have received less attention. The presence of bona fide RNA G4s in cells has long been a matter of debate. The development of G4-specific antibodies and ligands hinted on their presence in vivo, but recent advances in RNA sequencing coupled with chemical footprinting suggested the opposite. In this review, we will critically discuss the biology of RNA G4s focusing on the molecular mechanisms underlying their proposed functions.

Identification of G-quadruplexes in long functional RNAs using 7-deazaguanine RNA

RNA G-quadruplex (G4) structures are thought to affect biological processes, including translation and pre-mRNA splicing, but it is not possible at present to demonstrate that they form naturally at specific sequences in long functional RNA molecules. We developed a new strategy, footprinting of long 7-deazaguanine-substituted RNAs (FOLDeR), that allows the formation of G4s to be confirmed in long RNAs and under functional conditions.

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.

Quadruplex Nucleic Acids as Novel Therapeutic Targets

Quadruplex-forming sequences are widely prevalent in human and other genomes, including bacterial ones. These sequences are over-represented in eukaryotic telomeres, promoters, and 5' untranslated regions. They can form quadruplex structures, which may be transient in many situations in normal cells since they can be effectively resolved by helicase action. Mutated helicases in cancer cells are unable to unwind quadruplexes, which are impediments to transcription, translation, or replication, depending on their location within a particular gene. Small molecules that can stabilize quadruplex structures augment these effects and produce cell and proliferation growth inhibition. This article surveys the chemical biology of quadruplexes. It critically examines the major classes of quadruplex-binding small molecules that have been developed to date and the various approaches to discovering selective agents. The challenges of requiring (and achieving) small-molecule targeted selectivity for a particular quadruplex are discussed in relation to the potential of these small molecules as clinically useful therapeutic agents.

Structure, mechanism and therapeutic utility of immunosuppressive oligonucleotides

Synthetic oligodeoxynucleotides that can down-regulate cellular elements of the immune system have been developed and are being widely studied in preclinical models. These agents vary in sequence, mechanism of action, and cellular target(s) but share the ability to suppress a plethora of inflammatory responses. This work reviews the types of immunosuppressive oligodeoxynucleotide (Sup ODN) and compares their therapeutic activity against diseases characterized by pathologic levels of immune stimulation ranging from autoimmunity to septic shock to cancer (see graphical abstract). The mechanism(s) underlying the efficacy of Sup ODN and the influence size, sequence and nucleotide backbone on function are considered.

Quadruplex formation by both G-rich and C-rich DNA strands of the C9orf72 (GGGGCC)8•(GGCCCC)8 repeat: effect of CpG methylation

Unusual DNA/RNA structures of the C9orf72 repeat may participate in repeat expansions or pathogenesis of amyotrophic lateral sclerosis and frontotemporal dementia. Expanded repeats are CpG methylated with unknown consequences. Typically, quadruplex structures form by G-rich but not complementary C-rich strands. Using CD, UV and electrophoresis, we characterized the structures formed by (GGGGCC)8 and (GGCCCC)8 strands with and without 5-methylcytosine (5mCpG) or 5-hydroxymethylcytosine (5hmCpG) methylation. All strands formed heterogenous mixtures of structures, with features of quadruplexes (at pH 7.5, in K(+), Na(+) or Li(+)), but no feature typical of i-motifs. C-rich strands formed quadruplexes, likely stabilized by G•C•G•C-tetrads and C•C•C•C-tetrads. Unlike G•G•G•G-tetrads, some G•C•G•C-tetrad conformations do not require the N7-Guanine position, hence C9orf72 quadruplexes still formed when N7-deazaGuanine replace all Guanines. 5mCpG and 5hmCpG increased and decreased the thermal stability of these structures. hnRNPK, through band-shift analysis, bound C-rich but not G-rich strands, with a binding preference of unmethylated > 5hmCpG > 5mCpG, where methylated DNA-protein complexes were retained in the wells, distinct from unmethylated complexes. Our findings suggest that for C-rich sequences interspersed with G-residues, one must consider quadruplex formation and that methylation of quadruplexes may affect epigenetic processes.

Sequence-specific activation of the DNA sensor cGAS by Y-form DNA structures as found in primary HIV-1 cDNA

Cytosolic DNA that emerges during infection with a retrovirus or DNA virus triggers antiviral type I interferon responses. So far, only double-stranded DNA (dsDNA) over 40 base pairs (bp) in length has been considered immunostimulatory. Here we found that unpaired DNA nucleotides flanking short base-paired DNA stretches, as in stem-loop structures of single-stranded DNA (ssDNA) derived from human immunodeficiency virus type 1 (HIV-1), activated the type I interferon-inducing DNA sensor cGAS in a sequence-dependent manner. DNA structures containing unpaired guanosines flanking short (12- to 20-bp) dsDNA (Y-form DNA) were highly stimulatory and specifically enhanced the enzymatic activity of cGAS. Furthermore, we found that primary HIV-1 reverse transcripts represented the predominant viral cytosolic DNA species during early infection of macrophages and that these ssDNAs were highly immunostimulatory. Collectively, our study identifies unpaired guanosines in Y-form DNA as a highly active, minimal cGAS recognition motif that enables detection of HIV-1 ssDNA.

An improved model for the hTERT promoter quadruplex

Mutations occur at four specific sites in the hTERT promoter in >75% of glioblastomas and melanomas, but the mechanism by which the mutations affect gene expression remains unexplained. We report biophysical computational studies that show that the hTERT promoter sequence forms a novel G-quadruplex structure consisting of three contiguous, stacked parallel quadruplexes. The reported hTERT mutations map to the central quadruplex within this structure, and lead to an alteration of its hydrodynamic properties and stability.

Remarkable stability of an instability-prone lentiviral vector plasmid in Escherichia coli Stbl3

Large-scale production of plasmid DNA to prepare therapeutic gene vectors or DNA-based vaccines requires a suitable bacterial host, which can stably maintain the plasmid DNA during industrial cultivation. Plasmid loss during bacterial cell divisions and structural changes in the plasmid DNA can dramatically reduce the yield of the desired recombinant plasmid DNA. While generating an HIV-based gene vector containing a bicistronic expression cassette 5′-Olig2cDNA-IRES-dsRed2-3′, we encountered plasmid DNA instability, which occurred in homologous recombination deficient recA1 Escherichia coli strain Stbl2 specifically during large-scale bacterial cultivation. Unexpectedly, the new recombinant plasmid was structurally changed or completely lost in 0.5 L liquid cultures but not in the preceding 5 mL cultures. Neither the employment of an array of alternative recA1 E. coli plasmid hosts, nor the lowering of the culture incubation temperature prevented the instability. However, after the introduction of this instability-prone plasmid into the recA13E. coli strain Stbl3, the transformed bacteria grew without being overrun by plasmid-free cells, reduction in the plasmid DNA yield or structural changes in plasmid DNA. Thus, E. coli strain Stbl3 conferred structural and maintenance stability to the otherwise instability-prone lentivirus-based recombinant plasmid, suggesting that this strain can be used for the faithful maintenance of similar stability-compromised plasmids in large-scale bacterial cultivations. In contrast to Stbl2, which is derived wholly from the wild type isolate E. coli K12, E. coli Stbl3 is a hybrid strain of mixed E. coli K12 and E. coli B parentage. Therefore, we speculate that genetic determinants for the benevolent properties of E. coli Stbl3 for safe plasmid propagation originate from its E. coli B ancestor.

Possible regulatory roles of promoter g-quadruplexes in cardiac function-related genes - human TnIc as a model

G-quadruplexes (G4s) are four-stranded DNA secondary structures, which are involved in a diverse range of biological processes. Although the anti-cancer potential of G4s in oncogene promoters has been thoroughly investigated, the functions of promoter G4s in non-cancer-related genes are not well understood. We have explored the possible regulatory roles of promoter G4s in cardiac function-related genes using both computational and a wide range of experimental approaches. According to our bioinformatics results, it was found that potential G4-forming sequences are particularly enriched in the transcription regulatory regions (TRRs) of cardiac function-related genes. Subsequently, the promoter of human cardiac troponin I (TnIc) was chosen as a model, and G4s found in this region were subjected to biophysical characterisations. The chromosome 19 specific minisatellite G4 sequence (MNSG4) and near transcription start site (TSS) G4 sequence (−80 G4) adopt anti-parallel and parallel structures respectively in 100 mM KCl, with stabilities comparable to those of oncogene G4s. It was also found that TnIc G4s act cooperatively as enhancers in gene expression regulation in HEK293 cells, when stabilised by a synthetic G4-binding ligand. This study provides the first evidence of the biological significance of promoter G4s in cardiac function-related genes. The feasibility of using a single ligand to target multiple G4s in a particular gene has also been discussed.

Thymine quintets and their higher order assemblies studied by electrospray ionization mass spectrometry and theoretical calculation

We previously reported that thymine molecules can specifically form a pentameric magic number cluster named as thymine quintet in the presence of K(+) , Rb(+) and Cs(+) . Actually, thymine decamer and doubly charged thymine 15-mer metaclusters can be observed along with thymine quintet in the ESI mass spectra of thymine with the addition of K(+) , Rb(+) and Cs(+) . The product ion spectra of these metaclusters, especially the 15-mer with hetero central ions, indicate that they are higher order assemblies of thymine quintets. The collision-induced dissociation experiments show that the gas-phase stabilities of these metaclusters depend on the size of the central ions, following the order Cs(+) > Rb(+) > K(+) , while K(+) leads to the highest dissociation energy of a thymine quintet. The optimized structures of thymine quintet and decamer were provided by density functional theory calculations, which showed that thymine quintet is bowl-shaped and its tilting angle increases with the size of the central ion. Furthermore, the chirality of thymine quintet was defined for the first time and the resulting different diastereoisomers of thymine decamers were also revealed by the calculation study. Copyright © 2011 John Wiley & Sons, Ltd.

G-quadruplexes-novel mediators of gene function

Since the famous double-helix model was proposed, chromosomal DNA has been regarded as a rigid molecule containing the genetic information of an organism. It is clear now that DNA can adopt many transient, complex structures that can perform different biological functions. The G4 DNA (also called DNA G-quadruplex or G-tetraplex), a four-stranded DNA structure composed of stacked G-tetrads (guanine tetrads), has attracted much attention during the past two decades due to its ability to adopt a variety of structures and its possible biological functions. This review gives a glimpse on the structural diversity and biophysical properties of these fascinating DNA structures. Common methods that are widely used in investigating biophysical properties and biological functions of G4 DNA are described briefly. Next, bioinformatics studies that indicate evidence of evolutionary selection and potential functions of G4 DNA are discussed. Finally, examples of various biological functions of different G4 DNA are given, and potential roles of G4 DNA in respect of cardiovascular science are discussed.

The HSV-1 ICP27 RGG box specifically binds flexible, GC-rich sequences but not G-quartet structures

Herpes simplex virus 1 (HSV-1) protein ICP27, an important regulator for viral gene expression, directly recognizes and exports viral RNA through an N-terminal RGG box RNA binding motif, which is necessary and sufficient for RNA binding. An ICP27 N-terminal peptide, including the RGG box RNA binding motif, was expressed and its binding specificity was analyzed using EMSA and SELEX. DNA oligonucleotides corresponding to HSV-1 glycoprotein C (gC) mRNA, identified in a yeast three-hybrid analysis, were screened for binding to the ICP27 N-terminal peptide in EMSA experiments. The ICP27 N-terminus was able to bind most gC substrates. Notably, the ICP27 RGG box was unable to bind G-quartet structures recognized by the RGG domains of other proteins. SELEX analysis identified GC-rich RNA sequences as a common feature of recognition. NMR analysis of SELEX and gC sequences revealed that sequences able to bind to ICP27 did not form secondary structures and conversely, sequences that were not able to bind to ICP27 gave spectra consistent with base-pairing. Therefore, the ICP27 RGG box is unique in its recognition of nucleic acid sequences compared to other RGG box proteins; it prefers flexible, GC-rich substrates that do not form stable secondary structures.

Biochemical techniques for the characterization of G-quadruplex structures: EMSA, DMS footprinting, and DNA polymerase stop assay

The proximal promoter region of many human growth-related genes contains a polypurine/polypyrimidine tract that serves as multiple binding sites for Sp1 or other transcription factors. These tracts often contain a guanine-rich sequence consisting of four runs of three or more contiguous guanines separated by one or more bases, corresponding to a general motif known for the formation of an intramolecular G-quadruplex. Recent results provide strong evidence that specific G-quadruplex structures form naturally within these polypurine/polypyrimidine tracts in many human promoter regions, raising the possibility that the transcriptional control of these genes can be modulated by G-quadruplex-interactive agents. In this chapter, we describe three general biochemical methodologies, electrophoretic mobility shift assay (EMSA), dimethylsulfate (DMS) footprinting, and the DNA polymerase stop assay, which can be useful for initial characterization of G-quadruplex structures formed by G-rich sequences.

Putative DNA G-quadruplex formation within the promoters of Plasmodium falciparum var genes

Background

Guanine-rich nucleic acid sequences are capable of folding into an intramolecular four-stranded structure called a G-quadruplex. When found in gene promoter regions, G-quadruplexes can downregulate gene expression, possibly by blocking the transcriptional machinery. Here we have used a genome-wide bioinformatic approach to identify Putative G-Quadruplex Sequences (PQS) in the Plasmodium falciparum genome, along with biophysical techniques to examine the physiological stability of P. falciparum PQS in vitro.

Results

We identified 63 PQS in the non-telomeric regions of the P. falciparum clone 3D7. Interestingly, 16 of these PQS occurred in the upstream region of a subset of the P. falciparum var genes (group B var genes). The var gene family encodes PfEMP1, the parasite's major variant antigen and adhesin expressed at the surface of infected erythrocytes, that plays a key role in malaria pathogenesis and immune evasion. The ability of the PQS found in the upstream regions of group B var genes (UpsB-Q) to form stable G-quadruplex structures in vitro was confirmed using ¹H NMR, circular dichroism, UV spectroscopy, and thermal denaturation experiments. Moreover, the synthetic compound BOQ1 that shows a higher affinity for DNA forming quadruplex rather than duplex structures was found to bind with high affinity to the UpsB-Q.

Conclusion

This is the first demonstration of non-telomeric PQS in the genome of P. falciparum that form stable G-quadruplexes under physiological conditions in vitro. These results allow the generation of a novel hypothesis that the G-quadruplex sequences in the upstream regions of var genes have the potential to play a role in the transcriptional control of this major virulence-associated multi-gene family.

8-Oxo-7,8-dihydro-2'-deoxyguanosine Forms a Relatively Unstable Tetrameric Structure Compared with 2'-Deoxyguanosine

The hydrogen-bonded guanine tetrad, or G-quartet has been implicated in a variety of biological roles, including the function of chromosome telomeres. Here effect of the hydroxylation of guanosine at the 8 position on the G-quartet formation was examined. Electrospray inonization mass (ESI-MS) spectra of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) and 2'-deoxyguanosine (dG) were measured in order to know whether or not 8-oxodG forms a tetrameric structure as 2'-deoxyguanosine forms in teromeres. The ESI-MS spectra of dG shows prominent peaks at m/z 290, m/z 557, and m/z 1092, corresponding to [dG + Na]⁺, [dG2 + Na]⁺, and [dG₄ + Na]⁺ in the presence of 0.1 mM NaCl. On the other hand, the ESI-MS spectra of 8-oxodG in the presence of 0.1 mM NaCl shows prominent peaks at m/z 306 and m/z 589, corresponding to [8-oxodG + Na]⁺ and [8-oxodG₂ + Na]⁺. The results showed that 8-oxodG forms a relatively unstable tetrameric structure compared with dG.

Metastases suppressor NM23-H2 interaction with G-quadruplex DNA within c-MYC promoter nuclease hypersensitive element induces c-MYC expression

Regulatory influence of the G-quadruplex or G4 motif present within the nuclease hypersensitive element (NHE) in the promoter of c-MYC has been noted. On the other hand, association of NM23-H2 to the NHE leads to c-MYC activation. Therefore, NM23-H2 interaction with the G4 motif within the c-MYC NHE presents an interesting mechanistic possibility. Herein, using luciferase reporter assay and chromatin immunoprecipitation we show NM23-H2 mediated c-MYC activation involves NM23-H2-G4 motif binding within the c-MYC NHE. G4 motif complex formation with recombinant NM23-H2 was independently confirmed using fluorescence energy transfer, which also indicated that the G4 motif was resolved to an unfolded state within the protein-bound complex. Taken together, this supports transcriptional role of NM23-H2 via a G4 motif.

Genome-wide computational and expression analyses reveal G-quadruplex DNA motifs as conserved cis-regulatory elements in human and related species

Using a combination of in silico and experimental approaches, we present evidence that the G-quadruplex (G4) motif (an alternative higher-order DNA conformation) has regulatory potential. Genome-wide analyses of 99980 human, chimpanzee, mouse, and rat promoters showed enrichment of sequence with potential to adopt G4 (potential G4 or PG4) motifs near transcription start sites (TSS; P < 0.0001), supporting earlier findings. Interestingly, we found >700 orthologously related promoters in human, mouse, and rat conserve PG4 motif(s). The corresponding genes have enriched (z score > 4.0) tissue-specific expression in 75 of 79 human tissues and are significantly overrepresented in signaling and regulation of cell-cycle (P < 10(-05)). This is supported by results from whole genome expression experiments in human HeLa S3 cells following treatment with TMPyP4 [5,10,15,20-tetra(N-methyl-4-pyridyl) porphine chloride], which is known to bind the G4 motif inside cells. Our results implicate G4-motif mediated regulation as a more general mode of transcription control than currently appreciated.

Purine twisted-intercalating nucleic acids: a new class of anti-gene molecules resistant to potassium-induced aggregation

Sequence-specific targeting of genomic DNA by triplex-forming oligonucleotides (TFOs) is a promising strategy to modulate in vivo gene expression. Triplex formation involving G-rich oligonucleotides as third strand is, however, strongly inhibited by potassium-induced TFO self-association into G-quartet structures. We report here that G-rich TFOs with bulge insertions of (R)-1-O-[4-(1-pyrenylethynyl)-phenylmethyl] glycerol (called twisted intercalating nucleic acids, TINA) show a much lower tendency to aggregate in potassium than wild-type analogues do. We designed purine-motif TINA–TFOs for binding to a regulatory polypurine-polypyrimidine (pur/pyr) motif present in the promoter of the KRAS proto-oncogene. The binding of TINA–TFOs to the KRAS target has been analysed by electrophoresis mobility shift assays and DNase I footprinting experiments. We discovered that in the presence of potassium the wild-type TFOs did not bind to the KRAS target, differently from the TINA analogues, whose binding was observed up to 140 mM KCl. The designed TINA–TFOs were found to abrogate the formation of a DNA–protein complex at the pur/pyr site and to down-regulate the transcription of CAT driven by the murine KRAS promoter. Molecular modelling of the DNA/TINA–TFO triplexes are also reported. This study provides a new and promising approach to create TFOs to target in vivo the genome.

Structures, folding patterns, and functions of intramolecular DNA G-quadruplexes found in eukaryotic promoter regions

In its simplest form, a DNA G-quadruplex is a four-stranded DNA structure that is composed of stacked guanine tetrads. G-quadruplex-forming sequences have been identified in eukaryotic telomeres, as well as in non-telomeric genomic regions, such as gene promoters, recombination sites, and DNA tandem repeats. Of particular interest are the G-quadruplex structures that form in gene promoter regions, which have emerged as potential targets for anticancer drug development. Evidence for the formation of G-quadruplex structures in living cells continues to grow. In this review, we examine recent studies on intramolecular G-quadruplex structures that form in the promoter regions of some human genes in living cells and discuss the biological implications of these structures. The identification of G-quadruplex structures in promoter regions provides us with new insights into the fundamental aspects of G-quadruplex topology and DNA sequence-structure relationships. Progress in G-quadruplex structural studies and the validation of the biological role of these structures in cells will further encourage the development of small molecules that target these structures to specifically modulate gene transcription.

Structural polymorphism exhibited by a homopurine.homopyrimidine sequence found at the right end of human c-jun protooncogene

Homopurine.homopyrimidine (Pu.Py) tracts are likely to play important biological role in eukaryotes. Using circular dichroism, UV-thermal denaturation and gel electrophoresis, we have analyzed the structural polymorphism of a 21-bp Pu.Py DNA segment within human c-jun protooncogene 3'-region, a potential target for triplex formation. Results show that below physiological pH and in the presence of Na+/K+ with Mg2+ the duplex is destabilized/disproportionated, resulting in strand mediated structural transitions to the self-associated structures of G- and C-rich strands separately, identified as G-quadruplex and i-motif species. A significant differential behavior of the monovalent cations was observed, accordingly the presence of Na+ in acidic as well as neutral pH facilitated the duplex formation, while K+ favored the formation of self-associated structures. In Na+ and Mg2+, under acidic and neutral pH conditions, the duplex displayed triphasic and biphasic melting profiles, respectively. This self-association property of oligonucleotides might limit their use as duplex targets in triplex formation. Study is also relevant for understanding structural and biological properties of DNA sequence containing homopurine tracts.

A hitchhiker's guide to G-quadruplex ligands

Over the past decade, nucleic acid chemists have seen the spectacular emergence of molecules designed to interact efficiently and selectively with a peculiar DNA structure named G-quadruplex. Initially derived from classical DNA intercalators, these G-quadruplex ligands progressively became the focal point of new excitement since they appear to inhibit selectively the growth of cancer cells thereby opening interesting perspectives towards the development of novel anti-cancer drugs. The present article aims to help researchers enter this exciting research field, and to highlight recent advances in the design of G-quadruplex ligands.

QuadBase: genome-wide database of G4 DNA--occurrence and conservation in human, chimpanzee, mouse and rat promoters and 146 microbes

Emerging evidence indicates the importance of G-quadruplex motifs as drug targets. [Stuart A. Borman, Ascent of quadruplexes-nucleic acid structures become promising drug targets. Chem. Eng. News, 2007;85, 12-17], which stems from the fact that these motifs are present in a surprising number of promoters wherein their role in controlling gene expression has been demonstrated for a few. We present a compendium of quadruplex motifs, with particular focus on their occurrence and conservation in promoters-QuadBase. It is composed of two parts (EuQuad and ProQuad). EuQuad gives information on quadruplex motifs present within 10 kb of transcription starts sites in 99 980 human, chimpanzee, rat and mouse genes. ProQuad contains quadruplex information of 146 prokaryotes. Apart from gene-specific searches for quadruplex motifs, QuadBase has a number of other modules. 'Orthologs Analysis' queries for conserved motifs across species based on a selected reference organism; 'Pattern Search' can be used to fetch specific motifs of interest from a selected organism using user-defined criteria for quadruplex motifs, i.e. stem, loop size, etc. 'Pattern Finder' tool can search for motifs in any given sequence. QuadBase is freely available to users from non-profit organization at http://quadbase.igib.res.in/.

Intramolecular DNA quadruplexes with different arrangements of short and long loops

We have examined the folding, stability and kinetics of intramolecular quadruplexes formed by DNA sequences containing four G₃ tracts separated by either single T or T₄ loops. All these sequences fold to form intramolecular quadruplexes and 1D-NMR spectra suggest that they each adopt unique structures (with the exception of the sequence with all three loops containing T₄, which is polymorphic). The stability increases with the number of single T loops, though the arrangement of different length loops has little effect. In the presence of potassium ions, the oligonucleotides that contain at least one single T loop exhibit similar CD spectra, which are indicative of a parallel topology. In contrast, when all three loops are substituted with T₄ the CD spectrum is typical of an antiparallel arrangement. In the presence of sodium ions, the sequences with two and three single T loops also adopt a parallel folded structure. Kinetic studies on the complexes with one or two T₄ loops in the presence of potassium ions reveal that sequences with longer loops display slower folding rates.

Photochemical approach to probing different DNA structures

The various conformations of DNA--the A, B, and Z forms, the protein-induced DNA kink, and the G-quartet form--are thought to play important biological roles in processes such as DNA replication, gene expression and regulation, and the repair of DNA damage. The investigation of local DNA conformational changes associated with biological events is therefore essential for understanding the function of DNA. In this Minireview, we discuss the use of photochemical dehalogenation of 5-halouracil-containing DNA to probe the structure of DNA. Hydrogen abstraction by the resultant uracil-5-yl radicals is atom-specific and highly dependent on the structure of the DNA, suggesting that this photochemical approach could be applied as a probe of DNA conformations in living cells.

Effect of G-tract length on the topology and stability of intramolecular DNA quadruplexes

G-Rich sequences are known to form four-stranded structures that are based on stacks of G-quartets, and sequences with the potential to adopt these structures are common in eukaryotic genomes. However, there are few rules for predicting the relative stability of folded complexes that are adopted by sequences with different-length G-tracts or variable-length linkers between them. We have used thermal melting, circular dichroism, and gel electrophoresis to examine the topology and stability of intramolecular G-quadruplexes that are formed by sequences of the type d(GnT)4 and d(GnT2)4 (n = 3-7) in the presence of varying concentrations of sodium and potassium. In the presence of potassium or sodium, d(GnT)4 sequences form intramolecular parallel complexes with the following order of stability: n = 3 > n = 7 > n = 6 > n = 5 > n = 4. d(G3T)4 is anomalously stable. In contrast, the stability of d(GnT2)4 increases with the length of the G-tract (n = 7 > n = 6 > n = 5 > n = 4 > n = 3). The CD spectra for d(GnT)4 in the presence of potassium exhibit positive peaks around 260 nm, consistent with the formation of parallel topologies. These peaks are retained in sodium-containing buffers, but when n = 4, 5, or 6, CD maxima are observed around 290 nm, suggesting that these sequences [especially d(G5T)4] have some antiparallel characteristics. d(G3T2)4 adopts a parallel conformation in the presence of both sodium and potassium, while all the other d(GnT2)4 complexes exhibit predominantly antiparallel features. The properties of these complexes are also affected by the rate of annealing, and faster rates favor parallel complexes.

Solid-phase synthesis of positively charged deoxynucleic guanidine (DNG) oligonucleotide incorporating 7-deazaguanine bases

DNG nucleotides represent a positively charged DNA analog in which the negatively charged phosphodiester linkages of DNA are replaced by positively charged guanidinium linkages. We report herein the synthesis of 3'-end, middle, and 5'-end monomers required for the synthesis of a DNG sequence in which the natural guanine base is replaced by 7-deazaguanine (c(7)G). 7-Deazaguanine nucleobase was chosen because of their unique glycoside bond stability and their ability to prevent G-quartet formation. A facile and high yield two-step synthesis of xylo-7-deazaguanine 7, a key intermediate for introducing 3'-amino functionality, is carried out under Mitsunobu conditions. Subsequently, the 3'-Fmoc-protected thiourea monomers 13 and 19 were prepared from 7 via their corresponding 3'-amino-7-deazaguanines 11 and 18, respectively. The smooth coupling of these thiourea monomers with monomethoxytrityl (MMTr)-protected 3'-end monomer 25, prepared from 5, occurred on solid phase in 3'-->5' direction. The resultant trimeric HO-c(7)Ggc(7)Ggc(7)G-OH (1) has been designed to be included into DNA using standard DNA synthesis technology. The combination of C-c(7)G base pairing and electrostatic association of phosphodiester and guanidinium backbone allows the small synthesized DNG trimer 1 to form 1:1 complex with DNA-C pentamer.

Ethanol is a better inducer of DNA guanine tetraplexes than potassium cations

Guanine tetraplexes are a biologically relevant alternative of the Watson and Crick duplex of DNA. It is thought that potassium or other cations present in the cavity between consecutive guanine tetrads are an integral part of the tetraplexes. Here we show using CD spectroscopy that ethanol induces the guanine tetraplexes like or even better than potassium cations. We present examples of ethanol stabilizing guanine tetraplexes even in cases when potassium cations fail to do so. Hence, besides the A-form or Z-form, ethanol stabilizes another conformation of DNA, i.e., the guanine tetraplexes. We discuss the mechanism of the stabilization. Use of ethanol will permit studies of guanine tetraplexes that cannot be induced by potassium cations or other tetraplex-promoting agents. This work demonstrates that a still broader spectrum of nucleotide sequences can fold into guanine tetraplexes than has previously been thought. Aqueous ethanol may better simulate conditions existing in vivo than the aqueous solutions.

Formation of the G-quadruplex and i-motif structures in retinoblastoma susceptibility genes (Rb)

The formation of G-quadruplex and i-motif structures in the 5′ end of the retinoblastoma (Rb) gene was examined using chemical modifications, circular dichroism (CD) and fluorescence spectroscopy. It was found that substitutions of 8-methylguanine at positions that show syn conformations in antiparallel G-quadruplexes stabilize the structure in the G-rich strand. The complementary C-rich 18mer forms an i-motif structure, as suggested by CD spectroscopy. Based on the C to T mutation experiments, C bases participated in the C–C⁺ base pair of the i-motif structure were determined. Experiments of 2-aminopurine (2-AP) substitution reveal that an increase of fluorescence in the G-quadruplex relative to duplex is attributed to unstacked 2-AP within the loop of G-quadruplex. The fluorescence experiments suggest that formation of the G-quadruplex and i-motif can compete with duplex formation. Furthermore, a polymerase arrest assay indicated that formation the G-quadruplex structure in the Rb gene acts as a barrier in DNA synthesis.

Quadruplex formation by a guanine-rich PNA oligomer

A guanine-rich PNA dodecamer having the sequence H-G4T4G4-Lys-NH2 (G-PNA) hybridizes with a DNA dodecamer of homologous sequence to form a four-stranded quadruplex (Datta, B.; Schmitt, C.; Armitage, B. A. J. Am. Chem. Soc. 2003, 125, 4111-4118). This report describes quadruplex formation by the PNA alone. UV melting curves and fluorescence resonance energy transfer experiments reveal formation of a multistranded structure stabilized by guanine tetrads. The ion dependency of these structures is analogous to that reported for DNA quadruplexes. Electrospray ionization mass spectrometry indicates that both dimeric and tetrameric quadruplexes are formed by G4-PNA, with the dimeric form being preferred. These results have implications for the use of G-rich PNA for homologous hybridization to G-rich targets in chromosomal DNA and suggest additional applications in assembling quadruplex structures within lipid bilayer environments.

Impact of intrinsic DNA structure on processing of plasmids for gene therapy and DNA vaccines

Several non-Watson Crick DNA structures have been discovered to date, which may be incorporated into future plasmid constructs for gene therapy and DNA vaccine products. In this study, intrinsic DNA structures were included at a defined point in a 2.9 kb plasmid, and their effects on cell growth rate, total plasmid yield, and topology (i.e. the relative proportions of supercoiled plasmid, open circular and linear forms), were determined. The stability of the inserted sequences were assessed using gel electrophoresis. Z-DNA was shown to be unstable in a batch Escherichia coli DH1 production system grown in complex medium. Encouragingly other sequences studied (triplex, bend and quadruplex) did not cause spontaneous deletions, and no detrimental effect was found on growth rate or on total plasmid yield; indicating that such sequences could be included in future DNA products without any detrimental effect on plasmid yields; although the intra molecular triplex studied significantly decreased the proportion of supercoiled species.

Kinetic stability of intermolecular DNA quadruplexes

Fluorescently labeled oligodeoxyribonucleotides containing a single tract of four successive guanines have been used to study the thermodynamic and kinetic properties of short intermolecular DNA quadruplexes. When these assemble to form intermolecular quadruplexes the fluorophores are in close proximity and the fluorescence is quenched. On raising the temperature these complexes dissociate and there is a large increase in fluorescence. These complexes are exceptionally stable in potassium-containing buffers, and possess Tm values that are too high to measure. Tm values were determined in sodium-containing buffers for which the rate of reannealing is extremely slow; the melting profiles are effectively irreversible, and the apparent melting temperatures are dependent on the rates of heating. The dissociation kinetics of these complexes was estimated by rapidly increasing the temperature and following the time-dependent changes in fluorescence. From these data we have estimated the half-lives of these quadruplexes at 37 degrees C. Addition of a T to the unlabeled end of the oligonucleotide increases quadruplex stability. In contrast, addition of a T between the fluorophore and the oligonucleotide leads to a decrease in stability.

Design and synthesis of an expanded porphyrin that has selectivity for the c-MYC G-quadruplex structure

Cationic porphyrins are known to bind to and stabilize different types of G-quadruplexes. Recent studies have shown the biological relevance of the intramolecular parallel G-quadruplex as a transcriptional silencer in the c-MYC promoter. TMPyP4 also binds to this G-quadruplex and most likely converts it to a mixed parallel/antiparallel G-quadruplex with two external lateral loops and one internal propeller loop, suppressing c-MYC transcriptional activation. To achieve therapeutic selectivity by targeting G-quadruplexes, it is necessary to synthesize drugs that can differentiate among the different types of G-quadruplexes. We have designed and synthesized a core-modified expanded porphyrin analogue, 5,10,15,20-[tetra(N-methyl-3-pyridyl)]-26,28-diselenasapphyrin chloride (Se2SAP). Se2SAP converts the parallel c-MYC G-quadruplex into a mixed parallel/antiparallel G-quadruplex with one external lateral loop and two internal propeller loops, resulting in strong and selective binding to this G-quadruplex. A Taq polymerase stop assay was used to evaluate the binding of TMPyP4 and Se2SAP to G-quadruplex DNA. Compared to TMPyP4, Se2SAP shows a greater selectivity for and a 40-fold increase in stabilization of the single lateral-loop hybrid. Surface plasmon resonance and competition experiments with duplex DNA and other G-quadruplexes further confirmed the selectivity of Se2SAP for the c-MYC G-quadruplex. Significantly, Se2SAP was found to be less photoactive and noncytotoxic in comparison to TMPyP4. From this study, we have identified an expanded porphyrin that selectively binds with the c-MYC G-quadruplex in the presence of duplex DNA and other G-quadruplexes.