Conformation and stability of intramolecular telomeric G-quadruplexes: sequence effects in the loops

Structural and Functional Classification of G-Quadruplex Families within the Human Genome

G-quadruplexes (G4s) are short secondary DNA structures located throughout genomic DNA and transcribed RNA. Although G4 structures have been shown to form in vivo, no current search tools that examine these structures based on previously identified G-quadruplexes and filter them based on similar sequence, structure, and thermodynamic properties are known to exist. We present a framework for clustering G-quadruplex sequences into families using the CD-HIT, MeShClust, and DNACLUST methods along with a combination of Starcode and BLAST. Utilizing this framework to filter and annotate clusters, 95 families of G-quadruplex sequences were identified within the human genome. Profiles for each family were created using hidden Markov models to allow for the identification of additional family members and generate homology probability scores. The thermodynamic folding energy properties, functional annotation of genes associated with the sequences, scores from different prediction algorithms, and transcription factor binding motifs within a family were used to annotate and compare the diversity within and across clusters. The resulting set of G-quadruplex families can be used to further understand how different regions of the genome are regulated by factors targeting specific structures common to members of a specific cluster.

Variation in G-quadruplex sequence and topology differentially impacts human DNA polymerase fidelity

G-quadruplexes (G4s), a type of non-B DNA, play important roles in a wide range of molecular processes, including replication, transcription, and translation. Genome integrity relies on efficient and accurate DNA synthesis, and is compromised by various stressors, to which non-B DNA structures such as G4s can be particularly vulnerable. However, the impact of G4 structures on DNA polymerase fidelity is largely unknown. Using an in vitro forward mutation assay, we investigated the fidelity of human DNA polymerases delta (δ4, four-subunit), eta (η), and kappa (κ) during synthesis of G4 motifs representing those in the human genome. The motifs differ in sequence, topology, and stability, features that may affect DNA polymerase errors. Polymerase error rate hierarchy (δ4 < κ < η) is largely maintained during G4 synthesis. Importantly, we observed unique polymerase error signatures during synthesis of VEGF G4 motifs, stable G4s which form parallel topologies. These statistically significant errors occurred within, immediately flanking, and encompassing the G4 motif. For pol δ4, the errors were deletions, insertions and complex errors within the G4 or encompassing the G4 motif and surrounding sequence. For pol η, the errors occurred in 3' sequences flanking the G4 motif. For pol κ, the errors were frameshift mutations within G-tracts of the G4. Because these error signatures were not observed during synthesis of an antiparallel G4 and, to a lesser extent, a hybrid G4, we suggest that G4 topology and/or stability could influence polymerase fidelity. Using in silico analyses, we show that most polymerase errors are predicted to have minimal effects on predicted G4 stability. Our results provide a unique view of G4s not previously elucidated, showing that G4 motif heterogeneity differentially influences polymerase fidelity within the motif and flanking sequences. Thus, our study advances the understanding of how DNA polymerase errors contribute to G4 mutagenesis.

Phen-DC3 Induces Refolding of Human Telomeric DNA into a Chair-Type Antiparallel G-Quadruplex through Ligand Intercalation

Human telomeric G-quadruplex DNA structures are attractive anticancer drug targets, but the target's polymorphism complicates the drug design: different ligands prefer different folds, and very few complexes have been solved at high resolution. Here we report that Phen-DC3 , one of the most prominent G-quadruplex ligands in terms of high binding affinity and selectivity, causes dTAGGG(TTAGGG)3 to completely change its fold in KCl solution from a hybrid-1 to an antiparallel chair-type structure, wherein the ligand intercalates between a two-quartet unit and a pseudo-quartet, thereby ejecting one potassium ion. This unprecedented high-resolution NMR structure shows for the first time a true ligand intercalation into an intramolecular G-quadruplex.

Guiding the folding of G-quadruplexes through loop residue interactions

A G-rich sequence was designed to allow folding into either a stable parallel or hybrid-type topology. With the parent sequence featuring coexisting species, various related sequences with single and double mutations and with a shortened central propeller loop affected the topological equilibrium. Two simple modifications, likewise introduced separately to all sequences, were employed to lock folds into one of the topologies without noticeable structural alterations. The unique combination of sequence mutations, high-resolution NMR structural information, and the thermodynamic stability for both topological competitors identified critical loop residue interactions. In contrast to first loop residues, which are mostly disordered and exposed to solvent in both propeller and lateral loops bridging a narrow groove, the last loop residue in a lateral three-nucleotide loop is engaged in stabilizing stacking interactions. The propensity of single-nucleotide loops to favor all-parallel topologies by enforcing a propeller-like conformation of an additional longer loop is shown to result from their preference in linking two outer tetrads of the same tetrad polarity. Taken together, the present studies contribute to a better structural and thermodynamic understanding of delicate loop interactions in genomic and artificially designed quadruplexes, e.g. when employed as therapeutics or in other biotechnological applications.

Thermodynamic Stability of G-Quadruplexes: Impact of Sequence and Environment

G-quadruplexes have attracted growing interest in recent years due to their occurrence in vivo and their possible biological functions. In addition to being promising targets for drug design, these four-stranded nucleic acid structures have also been recognized as versatile tools for various technological applications. Whereas a large number of studies have yielded insight into their remarkable structural diversity, our current knowledge on G-quadruplex stabilities as a function of sequence and environmental factors only gradually emerges with an expanding collection of thermodynamic data. This minireview provides an overview of general rules that may be used to better evaluate quadruplex thermodynamic stabilities but also discusses present challenges in predicting most stable folds for a given sequence and environment.

Targeting working memory to modify emotional reactivity in adult attention deficit hyperactivity disorder: a functional magnetic resonance imaging study

Understanding the neural mechanisms of emotional reactivity in Attention-Deficit/Hyperactivity Disorder (ADHD) may help develop more effective treatments that target emotion dysregulation. In adult ADHD, emotion regulation problems cover a range of dimensions, including emotional reactivity (ER). One important process that could underlie an impaired ER in ADHD might be impaired working memory (WM) processing. We recently demonstrated that taxing WM prior to the exposure of emotionally salient stimuli reduced physiological and subjective reactivity to such cues in heavy drinkers, suggesting lasting effects of WM activation on ER. Here, we investigated neural mechanisms that could underlie the interaction between WM and ER in adult ADHD participants. We included 30 male ADHD participants and 30 matched controls. Participants performed a novel functional magnetic resonance imaging paradigm in which active WM-blocks were alternated with passive blocks of negative and neutral images. We demonstrated group-independent significant main effects of negative emotional images on amygdala activation, and WM-load on paracingulate gyrus and dorsolateral prefrontal cortex activation. Contrary to earlier reports in adolescent ADHD, no impairments were found in neural correlates of WM or ER. Moreover, taxing WM did not alter the neural correlates of ER in either ADHD or control participants. While we did find effects on the amygdala, paCG, and dlPFC activation, we did not find interactions between WM and ER, possibly due to the relatively unimpaired ADHD population and a well-matched control group. Whether targeting WM might be effective in participants with ADHD with severe ER impairments remains to be investigated.

Mitochondrial genetic variation is enriched in G-quadruplex regions that stall DNA synthesis in vitro

As the powerhouses of the eukaryotic cell, mitochondria must maintain their genomes which encode proteins essential for energy production. Mitochondria are characterized by guanine-rich DNA sequences that spontaneously form unusual three-dimensional structures known as G-quadruplexes (G4). G4 structures can be problematic for the essential processes of DNA replication and transcription because they deter normal progression of the enzymatic-driven processes. In this study, we addressed the hypothesis that mitochondrial G4 is a source of mutagenesis leading to base-pair substitutions. Our computational analysis of 2757 individual genomes from two Italian population cohorts (SardiNIA and InCHIANTI) revealed a statistically significant enrichment of mitochondrial mutations within sequences corresponding to stable G4 DNA structures. Guided by the computational analysis results, we designed biochemical reconstitution experiments and demonstrated that DNA synthesis by two known mitochondrial DNA polymerases (Pol γ, PrimPol) in vitro was strongly blocked by representative stable G4 mitochondrial DNA structures, which could be overcome in a specific manner by the ATP-dependent G4-resolving helicase Pif1. However, error-prone DNA synthesis by PrimPol using the G4 template sequence persisted even in the presence of Pif1. Altogether, our results suggest that genetic variation is enriched in G-quadruplex regions that impede mitochondrial DNA replication.

Duplex-tetraplex equilibria in guanine- and cytosine-rich DNA

Noncanonical four-stranded DNA structures, including G-quadruplexes and i-motifs, have been discovered in the cell and are implicated in a variety of genomic regulatory functions. The tendency of a specific guanine- and cytosine-rich region of genomic DNA to adopt a four-stranded conformation depends on its ability to overcome the constraints of duplex base-pairing by undergoing consecutive duplex-to-coil and coil-to-tetraplex transitions. The latter ability is determined by the balance between the free energies of participating ordered and disordered structures. In this review, we present an overview of the literature on the stability of G-quadruplex and i-motif structures and discuss the extent of duplex-tetraplex competition as a function of the sequence context of the DNA and environmental conditions including temperature, pH, salt, molecular crowding, and the presence of G-quadruplex-binding ligands. We outline how the results of in vitro studies can be expanded to understanding duplex-tetraplex equilibria in vivo.

The Relation Between Position and Chemical Composition of Bis-Indole Substituents Determines Their Interactions with G-Quadruplex DNA

G-quadruplex (G4) DNA structures are linked to fundamental biological processes and human diseases, which has triggered the development of compounds that affect these DNA structures. However, more knowledge is needed about how small molecules interact with G4 DNA structures. This study describes the development of a new class of bis-indoles (3,3-diindolyl-methyl derivatives) and detailed studies of how they interact with G4 DNA using orthogonal assays, biophysical techniques, and computational studies. This revealed compounds that strongly bind and stabilize G4 DNA structures, and detailed binding interactions which for example, show that charge variance can play a key role in G4 DNA binding. Furthermore, the structure-activity relationships generated opened the possibilities to replace or introduce new substituents on the core structure, which is of key importance to optimize compound properties or introduce probes to further expand the possibilities of these compounds as tailored research tools to study G4 biology.

Modulation of DNA structure formation using small molecules

Genome integrity is essential for proper cell function such that genetic instability can result in cellular dysfunction and disease. Mutations in the human genome are not random, and occur more frequently at "hotspot" regions that often co-localize with sequences that have the capacity to adopt alternative (i.e. non-B) DNA structures. Non-B DNA-forming sequences are mutagenic, can stimulate the formation of DNA double-strand breaks, and are highly enriched at mutation hotspots in human cancer genomes. Thus, small molecules that can modulate the conformations of these structure-forming sequences may prove beneficial in the prevention and/or treatment of genetic diseases. Further, the development of molecular probes to interrogate the roles of non-B DNA structures in modulating DNA function, such as genetic instability in cancer etiology are warranted. Here, we discuss reported non-B DNA stabilizers, destabilizers, and probes, recent assays to identify ligands, and the potential biological applications of these DNA structure-modulating molecules.

Loop permutation affects the topology and stability of G-quadruplexes

G-quadruplexes are unusual DNA and RNA secondary structures ubiquitous in a variety of organisms including vertebrates, plants, viruses and bacteria. The folding topology and stability of intramolecular G-quadruplexes are determined to a large extent by their loops. Loop permutation is defined as swapping two or three of these regions so that intramolecular G-quadruplexes only differ in the sequential order of their loops. Over the past two decades, both length and base composition of loops have been studied extensively, but a systematic study on the effect of loop permutation has been missing. In the present work, 99 sequences from 21 groups with different loop permutations were tested. To our surprise, both conformation and thermal stability are greatly dependent on loop permutation. Loop permutation actually matters as much as loop length and base composition on G-quadruplex folding, with effects on T_m as high as 17°C. Sequences containing a longer central loop have a high propensity to adopt a stable non-parallel topology. Conversely, sequences containing a short central loop tend to form a parallel topology of lower stability. In addition, over half of interrogated sequences were found in the genomes of diverse organisms, implicating their potential regulatory roles in the genome or as therapeutic targets. This study illustrates the structural roles of loops in G-quadruplex folding and should help to establish rules to predict the folding pattern and stability of G-quadruplexes.

G-quadruplex-forming oligodeoxyribonucleotides activate leukotriene synthesis in human neutrophils

Human polymorphonuclear leukocytes (PMNLs, neutrophils) play a major role in the immune response to bacterial and fungal infections and eliminate pathogens through phagocytosis. During phagocytosis of microorganisms, the 5-lipoxygenase (5-LOX) pathway is activated resulting in generation of leukotrienes, which mediate host defense. In this study, a library of oligodeoxyribonucleotides (ODNs) with varying numbers of human telomeric repeats (d(TTAGGG)_n) and their analogues with phosphorothioate internucleotide linkages and single-nucleotide substitutions was designed. These ODNs with the potential to fold into G-quadruplex structures were studied from structural and functional perspectives. We showed that exogenous G-quadruplex-forming ODNs significantly enhanced 5-LOX metabolite formation in human neutrophils exposed to Salmonella Typhimurium bacteria. However, the activation of leukotriene synthesis was completely lost when G-quadruplex formation was prevented by substitution of guanosine with 7-deazaguanosine or adenosine residues at several positions. To our knowledge, this study is the first to demonstrate that G-quadruplex structures are potent regulators of 5-LOX product synthesis in human neutrophils in the presence of targets of phagocytosis. Communicated by Ramaswamy H. Sarma.

A G-quadruplex motif in an envelope gene promoter regulates transcription and virion secretion in HBV genotype B

HBV genotypes differ in pathogenicity. In addition, genotype-specific differences in the regulation of transcription and virus replication exist in HBV, but the underlying mechanisms are unknown. Here, we show the presence of a G-quadruplex motif in the promoter of the preS2/S gene; this G-quadruplex is highly conserved only in HBV genotype B but not in other HBV genotypes. We demonstrate that this G-quadruplex motif forms a hybrid intramolecular G-quadruplex structure. Interestingly, mutations disrupting the G-quadruplex in HBV genotype B reduced the preS2/S promoter activity, leading to reduced hepatitis B surface antigen (HBsAg) levels. G-quadruplex ligands stabilized the G-quadruplex in genotype B and enhanced the preS2/S promoter activity. Furthermore, mutations disrupting the G-quadruplex in the full-length HBV genotype B constructs were associated with impaired virion secretion. In contrast to typical G-quadruplexes within promoters which are negative regulators of transcription the G-quadruplex in the preS2/S promoter of HBV represents an unconventional positive regulatory element. Our findings highlight (a) G-quadruplex mediated enhancement of transcription and virion secretion in HBV and (b) a yet unknown role for DNA secondary structures in complex genotype-specific regulatory mechanisms in virus genomes.

A triple stranded G-quadruplex formation in the promoter region of human myosin β(Myh7) gene

Regulatory regions in human genome, enriched in guanine-rich DNA sequences have the propensity to fold into G-quadruplex structures. On exploring the genome for search of G-tracts, it was interesting to find that promoter of Human Myosin Gene (MYH7) contains a conserved 23-mer G-rich sequence (HM-23). Mutations in this gene are associated with familial cardiomyopathy. Enrichment of MYH7 gene in G-rich sequences could possibly play a critical role in its regulation. We used polyacrylamide gel electrophoresis (PAGE), UV-Thermal denaturation (UV-Tm) and Circular Dichroism (CD), to demonstrate the formation of a G-quadruplex by 23-mer G-rich sequence HM23 in promoter location of MYH7 gene. We observed that the wild G-rich sequence HM23 containing consecutive G₅ stretch in two stacks adopt G-quadruplexes of diverse molecularity by involvement of four-strand, three-strand and two-strands with same parallel topology. Interestingly, the mutated sequence in the absence of continuous G₅ stretch obstructs the formation of three-stranded G-quadruplex. We demonstrated that continuous G₅ stretch is mandatory for the formation of a unique three-stranded G-quadruplex. Presence of various transcription factors (TF) in vicinity of the sequence HM23 leave fair possibility of recognition by TF binding sites, and so modulate gene expression. These findings may add on our understanding about the effect of base change in the formation of varied structural species in similar solution condition. This study may give insight about structural polymorphism arising due to recognition of non-Watson-Crick G-quadruplex structures by cellular proteins and designing structure specific molecules.

Clustered abasic lesions profoundly change the structure and stability of human telomeric G-quadruplexes

Ionizing radiation produces clustered damage to DNA which is difficult to repair and thus more harmful than single lesions. Clustered lesions have only been investigated in dsDNA models. Introducing the term 'clustered damage to G-quadruplexes' we report here on the structural effects of multiple tetrahydrofuranyl abasic sites replacing loop adenines (A/AP) and tetrad guanines (G/AP) in quadruplexes formed by the human telomere d[AG3(TTAG3)3] (htel-22) and d[TAG3(TTAG3)3TT] (htel-25) in K+ solutions. Single to triple A/APs increased the population of parallel strands in their structures by stabilizing propeller type loops, shifting the antiparallel htel-22 into hybrid or parallel quadruplexes. In htel-25, the G/APs inhibited the formation of parallel strands and these adopted antiparallel topologies. Clustered G/AP and A/APs reduced the thermal stability of the wild-type htel-25. Depending on position, A/APs diminished or intensified the damaging effect of the G/APs. Taken together, clustered lesions can disrupt the topology and stability of the htel quadruplexes and restrict their conformational space. These in vitro results suggest that formation of clustered lesions in the chromosome capping structure can result in the unfolding of existing G-quadruplexes which can lead to telomere shortening.

Structure, Properties, and Biological Relevance of the DNA and RNA G-Quadruplexes: Overview 50 Years after Their Discovery

G-quadruplexes (G4s), which are known to have important roles in regulation of key biological processes in both normal and pathological cells, are the most actively studied non-canonical structures of nucleic acids. In this review, we summarize the results of studies published in recent years that change significantly scientific views on various aspects of our understanding of quadruplexes. Modern notions on the polymorphism of DNA quadruplexes, on factors affecting thermodynamics and kinetics of G4 folding–unfolding, on structural organization of multiquadruplex systems, and on conformational features of RNA G4s and hybrid DNA–RNA G4s are discussed. Here we report the data on location of G4 sequence motifs in the genomes of eukaryotes, bacteria, and viruses, characterize G4-specific small-molecule ligands and proteins, as well as the mechanisms of their interactions with quadruplexes. New information on the structure and stability of G4s in telomeric DNA and oncogene promoters is discussed as well as proof being provided on the occurrence of G-quadruplexes in cells. Prominence is given to novel experimental techniques (single molecule manipulations, optical and magnetic tweezers, original chemical approaches, G4 detection in situ, in-cell NMR spectroscopy) that facilitate breakthroughs in the investigation of the structure and functions of G-quadruplexes.

Phenanthroline-bis-oxazole ligands for binding and stabilization of G-quadruplexes

BACKGROUND

G-quadruplexes (G4) are found at important genome regions such as telomere ends and oncogene promoters. One prominent strategy to explore the therapeutic potential of G4 is stabilized it with specific ligands.

METHODS

We report the synthesis of new phenanthroline, phenyl and quinoline acyclic bisoxazole compounds in order to explore and evaluate the targeting to c-myc and human telomeric repeat 22AG G4 using FRET-melting, CD-melting, NMR, fluorescence titrations and FID assays.

RESULTS

The design strategy has led to potent compounds (Phen-1 and Phen-2) that discriminate different G4 structures (human telomeric sequences and c-myc promoter) and selectively stabilize G4 over duplex DNA. CD studies show that Phen-2 binds and induces antiparallel topologies in 22AG quadruplex and also binds c-myc promotor, increasing their T_m in about 12°C and 30°C respectively. In contrast, Phen-1 induces parallel topologies in 22AG and c-myc, with a moderate stabilization of 4°C for both sequences. Consistent with a CD melting study, Phen-2 binds strongly (K=10⁶ to 10⁷M^-1) to c-myc and 22AG quadruplexes.

CONCLUSIONS

Phen-1 and Phen-2 discriminated among various quadruplex topologies and exhibited high selectivity for quadruplexes over duplexes. Phen-2 retains antiparallel topologies for quadruplex 22AG and does not induce conformational changes on the parallel c-myc quadruplex although Phen-1 favors the parallel topology. NMR studies also showed that the Phen-2 binds to the c-myc quadruplex via end stacking.

GENERAL SIGNIFICANCE

Overall, the results suggest the importance of Phen-2 as a scaffold for the fine-tuning with substituents in order to enhance binding and stabilization to G4 structures. This article is part of a Special Issue entitled "G-quadruplex" Guest Editor: Dr. Concetta Giancola and Dr. Daniela Montesarchio.

A Thermodynamic Study of Adenine and Thymine Substitutions in the Loops of the Oligodeoxyribonucleotide HTel

Guanine-rich DNA oligodeoxyribonucleotides (ODN) can form four-stranded structures named quadruplexes (G4s), which are stabilized via the association of four guanine bases. Quadruplexes have a high level of conformational diversity depending on the molecularity, sequence, and the cation conditions of the G4 formation. Monomolecular G4 structures have nonguanine loops that usually consist of between one and four adenine and thymine residues. In the work reported here, we systematically modified the nucleotides in the loops of the 22 nucleotide ODN, HTel, which contains four repeats of the human telomeric sequence, GGGTTA. We studied the effect of different types of bases in the loops on the stability and topology of the G4s formed. We show that lower steric hindrance of pyrimidine residues increases the stability of G4s with a major enthalpic contribution. Stacking of the loop bases onto tetrads could compensate for the loss of rotational freedom. In addition, in the presence of sodium, the stabilities of the G4s are loop dependent. In the presence of potassium, the stability of G4 depend on the sequences of each loop. Lastly, in the presence of potassium ions, the modified HTel ODNs may exist in equilibrium of the two types of the hybrid topology, and these structures are stabilized by the second loop. Modifications of the bases in this loop change the topology and stability of the folded structures.

The oxidative damage to the human telomere: effects of 5-hydroxymethyl-2'-deoxyuridine on telomeric G-quadruplex structures

As part of the genome, human telomeric regions can be damaged by the chemically reactive molecules responsible for oxidative DNA damage. Considering that G-quadruplex structures have been proven to occur in human telomere regions, several studies have been devoted to investigating the effect of oxidation products on the properties of these structures. However only investigations concerning the presence in G-quadruplexes of the main oxidation products of deoxyguanosine and deoxyadenosine have appeared in the literature. Here, we investigated the effects of 5-hydroxymethyl-2'-deoxyuridine (5-hmdU), one of the main oxidation products of T, on the physical-chemical properties of the G-quadruplex structures formed by two human telomeric sequences. Collected calorimetric, circular dichroism and electrophoretic data suggest that, in contrast to most of the results on other damage, the replacement of a T with a 5-hmdU results in only negligible effects on structural stability. Reported results and other data from literature suggest a possible protecting effect of the loop residues on the other parts of the G-quadruplexes.

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.

Solution structure of a DNA quadruplex containing ALS and FTD related GGGGCC repeat stabilized by 8-bromodeoxyguanosine substitution

A prolonged expansion of GGGGCC repeat within non-coding region of C9orf72 gene has been identified as the most common cause of familial amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), which are devastating neurodegenerative disorders. Formation of unusual secondary structures within expanded GGGGCC repeat, including DNA and RNA G-quadruplexes and R-loops was proposed to drive ALS and FTD pathogenesis. Initial NMR investigation on DNA oligonucleotides with four repeat units as the shortest model with the ability to form an unimolecular G-quadruplex indicated their folding into multiple G-quadruplex structures in the presence of K⁺ ions. Single dG to 8Br-dG substitution at position 21 in oligonucleotide d[(G₄C₂)₃G₄] and careful optimization of folding conditions enabled formation of mostly a single G-quadruplex species, which enabled determination of a high-resolution structure with NMR. G-quadruplex structure adopted by d[(G₄C₂)₃GG^BrGG] is composed of four G-quartets, which are connected by three edgewise C-C loops. All four strands adopt antiparallel orientation to one another and have alternating syn-anti progression of glycosidic conformation of guanine residues. One of the cytosines in every loop is stacked upon the G-quartet contributing to a very compact and stable structure.

Extended molecular dynamics of a c-kit promoter quadruplex

The 22-mer c-kit promoter sequence folds into a parallel-stranded quadruplex with a unique structure, which has been elucidated by crystallographic and NMR methods and shows a high degree of structural conservation. We have carried out a series of extended (up to 10 μs long, ∼50 μs in total) molecular dynamics simulations to explore conformational stability and loop dynamics of this quadruplex. Unfolding no-salt simulations are consistent with a multi-pathway model of quadruplex folding and identify the single-nucleotide propeller loops as the most fragile part of the quadruplex. Thus, formation of propeller loops represents a peculiar atomistic aspect of quadruplex folding. Unbiased simulations reveal μs-scale transitions in the loops, which emphasizes the need for extended simulations in studies of quadruplex loops. We identify ion binding in the loops which may contribute to quadruplex stability. The long lateral-propeller loop is internally very stable but extensively fluctuates as a rigid entity. It creates a size-adaptable cleft between the loop and the stem, which can facilitate ligand binding. The stability gain by forming the internal network of GA base pairs and stacks of this loop may be dictating which of the many possible quadruplex topologies is observed in the ground state by this promoter quadruplex.

Structure-based virtual screening of novel natural alkaloid derivatives as potential binders of h-telo and c-myc DNA G-quadruplex conformations

Several ligands can bind to the non-canonical G-quadruplex DNA structures thereby stabilizing them. These molecules can act as effective anticancer agents by stabilizing the telomeric regions of DNA or by regulating oncogene expression. In order to better interact with the quartets of G-quadruplex structures, G-binders are generally characterized by a large aromatic core involved in π-π stacking. Some natural flexible cyclic molecules from Traditional Chinese Medicine have shown high binding affinity with G-quadruplex, such as berbamine and many other alkaloids. Using the structural information available on G-quadruplex structures, we performed a high throughput in silico screening of commercially available alkaloid derivative databases by means of a structure-based approach based on docking and molecular dynamics simulations against the human telomeric sequence d[AG₃(T₂AG₃)₃] and the c-myc promoter structure. We identified 69 best hits reporting an improved theoretical binding affinity with respect to the active set. Among them, a berberine derivative, already known to remarkably inhibit telomerase activity, was related to a better theoretical affinity versusc-myc.

Targeting unimolecular G-quadruplex nucleic acids: a new paradigm for the drug discovery?

INTRODUCTION

G-quadruplexes (G4s) are targets of great interest because of their roles in crucial biological processes, such as aging and cancer. G4s are based on the formation of G-quartets, stabilised by Hoogsteen-type hydrogen bonds and by interaction with cations between the tetrads. These biologically relevant conformations were first discovered in eukaryotic chromosomal telomeric DNA, but have also been found in the proximal location of promoters in a number of human genes. Therefore, the extensive analysis of an intriguing target could move towards the rational drug design of new selective anticancer agents.

AREAS COVERED

The authors review G4 structural characterisation, with detailed insight related to the polymorphism issue. The authors describe the topologically distinct G4 structural forms and the factors involved in their interconversion mechanisms, such as the sequence of the oligonucleotides, the strand stoichiometry and orientation, the syn-anti conformation of the guanine glycosidic bonds and the G4 loop types and the environmental factors. Furthermore, the authors report several studies related to folding and unfolding kinetic profiles in order to understand the conformational view of monomolecular G4 formations.

EXPERT OPINION

G4 unimolecular nucleic acids can be considered as valid targets for the rational drug development of novel anticancer agents. Structural biology represents an essential link between the biology and medicinal chemistry knowledge in this field. In silico methods have already been demonstrated to be useful, especially if well integrated with biophysical tests. If this proves successful, the G4-targeting paradigm could also be extended to drug discovery beyond neoplastic pathologies.

Interactions of cyclic and non-cyclic naphthalene diimide derivatives with different nucleic acids

Recently, strategy based on stabilization of G-quadruplex telomeric DNA by small organic molecule has been realized by naphthalene diimide derivatives (NDIs). At the same time NDIs bind to DNA duplex as threading intercalators. Here we present cyclic derivative of naphthalene diimide (ligand 1) as DNA-binding ligand with ability to recognition of different structures of telomeric G-quadruplexes and ability to bis-intercalate to double-stranded helixes. The results have been compared to non-cyclic derivative (ligand 2) and revealed that preferential binding of ligands to nucleic acids strongly depends on their topology and structural features of ligands.