A mechanism for the extension and unfolding of parallel telomeric G-quadruplexes by human telomerase at single-molecule resolution

The molecular mechanism for TERRA recruitment and annealing to telomeres

Telomeric repeat containing RNA (TERRA) is a noncoding RNA that is transcribed from telomeres. Previous study showed that TERRA trans anneals by invading into the telomeric duplex to form an R-loop in mammalian cells. Here, we elucidate the molecular mechanism underlying TERRA recruitment and invasion into telomeres in the context of shelterin proteins, RAD51 and RNase H using single molecule (sm) assays. We demonstrate that TERRA trans annealing into telomeric DNA exhibits dynamic movement that is stabilized by TRF2. TERRA annealing to the telomeric duplex results in the formation of a stable triplex structure which differs from a conventional R-loop. We identified that the presence of a sub-telomeric DNA and a telomeric overhang in the form of a G-quadruplex significantly enhances TERRA annealing to telomeric duplex. We also demonstrate that RAD51-TERRA complex invades telomere duplex more efficiently than TERRA alone. Additionally, TRF2 increases TERRA affinity to telomeric duplex and protects it from RNase H digestion. In contrast, TRF1 represses TERRA annealing to telomeric duplex and fails to provide protection against RNase H digestion. Our findings provide an in-depth molecular mechanism underpinning TERRA recruitment and annealing to the telomere.

Opportunities and challenges with G-quadruplexes as promising targets for drug design

INTRODUCTION

G-quadruplexes (G4s) are secondary structures formed in guanine-rich regions of nucleic acids (both DNA and RNA). G4s are significantly enriched at regulatory genomic regions and are associated with important biological processes ranging from telomere homeostasis and genome instability to transcription and translation. Importantly, G4s are related to health and diseases such as cancer, neurological diseases, as well as infections with viruses and microbial pathogens. Increasing evidence suggests the potential of G4s for designing new diagnostic and therapeutic strategies although in vivo studies are still at early stages.

AREAS COVERED

This review provides an updated summary of the literature describing the impact of G4s in human diseases and different approaches based on G4 targeting in therapy.

EXPERT OPINION

Within the G4 field, most of the studies have been performed in vitro and in a descriptive manner. Therefore, detailed mechanistic understanding of G4s in the biological context remains to be deciphered. In clinics, the use of G4s as therapeutic targets has been hindered due to the low selectivity profile and poor drug-like properties of G4 ligands. Future research on G4s may overcome current methodological and interventional limitations and shed light on these unique structural elements in the pathogenesis and treatment of diseases.

Effect of substituents on the ability of nickel Schiff base complexes with four pendant groups to bind to G-quadruplexes

The synthesis of eleven new nickel Schiff base complexes each bearing four pendant groups is reported. The structures of the complexes differ in the identity of the pendant groups and/or diamine moiety. All complexes were characterised by microanalysis, Nuclear Magnetic Resonance (NMR) spectroscopy and Electrospray Ionisation Mass Spectrometry (ESI-MS), while the solid-state structures of two of the molecules were also determined using X-ray crystallographic methods. The DNA binding properties of the nickel complexes with double stranded DNA and a range of G-quadruplex DNA structures was explored using different spectroscopic methods as well as computational techniques. Results from ESI-MS experiments and Fluorescent Indicator Displacement (FID) assays were consistent with each other and indicated that varying the diamine moiety had less influence on DNA affinity than changing the pendant groups. These conclusions were also generally supported by results obtained from UV melting experiments and Fluorescence Resonance Energy Transfer (FRET) assays. The cytotoxicity of selected examples of the new complexes, and close analogues reported recently, towards V79 Chinese hamster lung cancer cells and THP-1 human leukemia cells was measured. All were found to display modest cytotoxicity, with flow cytometry experiments suggesting an apoptotic pathway was the most likely mechanism of cell death.

Formation of left-handed helices by C2'-fluorinated nucleic acids under physiological salt conditions

Recent findings in cell biology have rekindled interest in Z-DNA, the left-handed helical form of DNA. We report here that two minimally modified nucleosides, 2'F-araC and 2'F-riboG, induce the formation of the Z-form under low ionic strength. We show that oligomers entirely made of these two nucleosides exclusively produce left-handed duplexes that bind to the Zα domain of ADAR1. The effect of the two nucleotides is so dramatic that Z-form duplexes are the only species observed in 10 mM sodium phosphate buffer and neutral pH, and no B-form is observed at any temperature. Hence, in contrast to other studies reporting formation of Z/B-form equilibria by a preference for purine glycosidic angles in syn, our NMR and computational work revealed that sequential 2'F…H2N and intramolecular 3'H…N3' interactions stabilize the left-handed helix. The equilibrium between B- and Z- forms is slow in the 19F NMR time scale (≥ms), and each conformation exhibited unprecedented chemical shift differences in the 19F signals. This observation led to a reliable estimation of the relative population of B and Z species and enabled us to monitor B-Z transitions under different conditions. The unique features of 2'F-modified DNA should thus be a valuable addition to existing techniques for specific detection of new Z-binding proteins and ligands.

Unlocking the potential of protein-derived peptides to target G-quadruplex DNA: from recognition to anticancer activity

Noncanonical nucleic acid structures, particularly G-quadruplexes, have garnered significant attention as potential therapeutic targets in cancer treatment. Here, the recognition of G-quadruplex DNA by peptides derived from the Rap1 protein is explored, with the aim of developing novel peptide-based G-quadruplex ligands with enhanced selectivity and anticancer activity. Biophysical techniques were employed to assess the interaction of a peptide derived from the G-quadruplex-binding domain of the protein with various biologically relevant G-quadruplex structures. Through alanine scanning mutagenesis, key amino acids crucial for G-quadruplex recognition were identified, leading to the discovery of two peptides with improved G-quadruplex-binding properties. However, despite their in vitro efficacy, these peptides showed limited cell penetration and anticancer activity. To overcome this challenge, cell-penetrating peptide (CPP)-conjugated derivatives were designed, some of which exhibited significant cytotoxic effects on cancer cells. Interestingly, selected CPP-conjugated peptides exerted potent anticancer activity across various tumour types via a G-quadruplex-dependent mechanism. These findings underscore the potential of peptide-based G-quadruplex ligands in cancer therapy and pave the way for the development of novel therapeutic strategies targeting these DNA structures.

G-quadruplexes in MTOR and induction of autophagy

G-quadruplex (G4) structures have emerged as singular therapeutic targets for cancer and neurodegeneration. Autophagy, a crucial homeostatic mechanism of the cell, is often dysregulated in neurodegenerative diseases and cancers. We used QGRS mapper to identify 470 G4 sequences in MTOR, a key negative regulator of autophagy. We sought to identify a functional context by leveraging the effect of G4-targeting ligands on MTOR G4 sequences. The effect of Bis-4,3, a G4 selective dimeric carbocyanine dye, was compared with the known G4-stabilizing activity of the porphyrin, TMPyP4 in HeLa and SHSY-5Y cells. Our results show that treatment with G4-selective ligands downregulates MTOR RNA and mTOR protein expression levels. This is the first report describing G4 motifs in MTOR. This study indicates a possible role of G4 stabilizing ligands in induction of autophagy by downregulation of mTOR levels, albeit not precluding MTOR independent pathways.

G-Quadruplexes as Sensors of Intracellular Na+/K+ Ratio: Potential Role in Regulation of Transcription and Translation

Data on the structure of G-quadruplexes, noncanonical nucleic acid forms, supporting an idea of their potential participation in regulation of gene expression in response to the change in intracellular Na+i/K+i ratio are considered in the review. Structural variety of G-quadruplexes, role of monovalent cations in formation of this structure, and thermodynamic stability of G-quadruplexes are described. Data on the methods of their identification in the cells and biological functions of these structures are presented. Analysis of information about specific interactions of G-quadruplexes with some proteins was conducted, and their potential participation in the development of some pathological conditions, in particular, cancer and neurodegenerative diseases, is considered. Special attention is given to the plausible role of G-quadruplexes as sensors of intracellular Na+i/K+i ratio, because alteration of this parameter affects folding of G-quadruplexes changing their stability and, thereby, organization of the regulatory elements of nucleic acids. The data presented in the conclusion section demonstrate significant change in the expression of some early response genes under certain physiological conditions of cells and tissues depending on the intracellular Na+i/K+i ratio.

Synthesis and Molecular Dynamic Simulation of Novel Cationic and Non-cationic Pyrimidine Derivatives as Potential G-quadruplex-ligands

BACKGROUND

Drug resistance has been a problem in cancer chemotherapy, which often causes shortterm effectiveness. Further, the literature indicates that telomere G-quadruplex could be a promising anti-cancer target.

OBJECTIVE

We synthesized and characterized two new pyrimidine derivatives as ligands for G-quadruplex DNA.

METHODS

The interaction of novel non-cationic and cationic pyrimidine derivatives (3a, b) with G-quadruplex DNA (1k8p and 3qsc) was explored by circular dichroism (CD) and ultraviolet-visible spectroscopy and polyacrylamide gel electrophoresis (PAGE) methods. The antiproliferative activity of desired compounds was evaluated by the MTT assay. Apoptosis induction was assessed by Propidium iodide (P.I.) staining and flow cytometry. Computational molecular modeling (CMM) and molecular dynamics simulation (MD) were studied on the complexes of 1k8p and 3qsc with the compounds. The van der Waals, electrostatic, polar solvation, solventaccessible surface area (SASA), and binding energies were calculated and analyzed.

RESULTS

The experimental results confirmed that both compounds 3a and 3b interacted with 1k8p and 3qsc and exerted cytotoxic and proapoptotic effects on cancer cells. The number of hydrogen bonds and the RMSD values increased in the presence of the ligands, indicating stronger binding and suggesting increased structural dynamics. The electrostatic contribution to binding energy was higher for the cationic pyrimidine 3b, indicating more negative binding energies.

CONCLUSION

Both experimental and MD results confirmed that 3b was more prone to form a complex with DNA G-quadruplex (1k8p and 3qsc), inhibit cell growth, and induce apoptosis, compared to the non-cationic pyrimidine 3a.

Prospecting the cancer therapeutic edge of chitosan-based gold nanoparticles through conformation selective binding to the parallel G-quadruplex formed by short telomeric DNA sequence: A multi-spectroscopic approach

The biological relevance of G4 structures formed in telomere & oncogenes promoters make them extremely crucial therapeutic target for cancer treatment. Herein, we have synthesized chitosan-based gold nanoparticles (CH-Au NPs) through green method and have investigated their interaction with G4 structures formed by short telomeric sequences to evaluate their potential for targeting G4 structures. Firstly, we have characterized morphological/physical attributes of synthesized CH-Au NPs and salt dependent structural aspects of model G-rich DNA sequence, 12-mer d(T2G4)2 [TETRA] using spectroscopic and biophysical techniques. The molecular interactions between CH-Au NPs and parallel/antiparallel TETRA G4 structures were evaluated using UV-Visible, CD, Fluorescence, CD melting, DLS and Zeta potential studies. The experimental data indicated that CH-Au NPs showed strong binding interactions with Parallel TETRA G4 and provided thermal stabilization to the structure, whereas their interactions with Antiparallel TETRA G4 DNA and Ct-DNA (DNA duplex) were found to be negligible. Further, CH-Au NPs were also investigated for their selectivity aptitude for different G4 structures formed by human telomeric sequences; d(T2AG3)3 [HUM-12] and d(T2AG3)4T [HUM-25]. Our findings suggested that CH-Au NPs exhibited topology specific binding aptitude towards G4 structure, which can be utilized to inhibit/modulate crucial biological functions for potential anticancer activity.

Genetic variations in G-quadruplex forming sequences affect the transcription of human disease-related genes

Guanine-rich DNA strands can fold into non-canonical four-stranded secondary structures named G-quadruplexes (G4s). G4s folded in proximal promoter regions (PPR) are associated either with positive or negative transcriptional regulation. Given that single nucleotide variants (SNVs) affecting G4 folding (G4-Vars) may alter gene transcription, and that SNVs are associated with the human diseases' onset, we undertook a novel comprehensive study of the G4-Vars genome-wide (G4-variome) to find disease-associated G4-Vars located into PPRs. We developed a bioinformatics strategy to find disease-related SNVs located into PPRs simultaneously overlapping with putative G4-forming sequences (PQSs). We studied five G4-Vars disturbing in vitro the folding and stability of the G4s located into PPRs, which had been formerly associated with sporadic Alzheimer's disease (GRIN2B), a severe familiar coagulopathy (F7), atopic dermatitis (CSF2), myocardial infarction (SIRT1) and deafness (LHFPL5). Results obtained in cultured cells for these five G4-Vars suggest that the changes in the G4s affect the transcription, potentially contributing to the development of the mentioned diseases. Collectively, data reinforce the general idea that G4-Vars may impact on the different susceptibilities to human genetic diseases' onset, and could be novel targets for diagnosis and drug design in precision medicine.

Telomeric i-motifs and C-strands inhibit parallel G-quadruplex extension by telomerase

Telomeric C-rich repeated DNA sequences fold into tetrahelical i-motif structures in vitro at acidic pH. While studies have suggested that i-motifs may form in cells, little is known about their potential role in human telomere biology. In this study, we explore the effect of telomeric C-strands and i-motifs on the ability of human telomerase to extend G-rich substrates. To promote i-motif formation at neutral pH, we use telomeric sequences where the cytidines have been substituted with 2'-fluoroarabinocytidine. Using FRET-based studies, we show that the stabilized i-motifs resist hybridization to concomitant parallel G-quadruplexes, implying that both structures could exist simultaneously at telomeric termini. Moreover, through telomerase activity assays, we show that both unstructured telomeric C-strands and telomeric i-motifs can inhibit the activity and processivity of telomerase extension of parallel G-quadruplexes and linear telomeric DNA. The data suggest at least three modes of inhibition by C-strands and i-motifs: direct hybridization to the substrate DNA, hybridization to nascent product DNA resulting in early telomerase dissociation, and interference with the unique mechanism of telomerase unwinding and extension of a G-quadruplex. Overall, this study highlights a potential inhibitory role for the telomeric C-strand in telomere maintenance.

G-quadruplex DNA binding properties of novel nickel Schiff base complexes with four pendant groups

Three new nickel Schiff base complexes were prepared using a two-step procedure that involves initial selective dialkylation of 2,4,6-trihydroxybenzaldehyde, followed by reaction with 1,2-phenylenediamine and nickel(II) acetate. Each of the complexes possessed the same Schiff base core but differed in the identity of the four pendant groups. All complexes were characterised by microanalysis, NMR spectroscopy and ESI mass spectrometry. In addition, two of the complexes were also characterised in the solid state using X-ray crystallography, which confirmed the presence of a square planar geometry around the metal ion. The DNA binding properties of the three nickel complexes with double stranded DNA and a range of G-quadruplex DNA structures were explored using ESI mass spectrometry, CD spectroscopy, UV melting curves, Fluorescence Resonance Energy Transfer (FRET) assays, Fluorescent Indicator Displacement (FID) assays and molecular docking studies. These techniques confirmed the ability of the three nickel complexes to bind to most of the DNA molecules examined, as well as stabilise the latter in several instances. In addition, the results of these investigations provided evidence that pendant groups with morpholine rings generally reduced DNA binding behaviour, whilst pendants featuring piperidine ring systems attached to the Schiff base core by three and not two methylene linkers often showed the greatest extent of binding or DNA stabilisation.

G-quadruplexes and associated proteins in aging and Alzheimer's disease

Aging is a prominent risk factor for many neurodegenerative disorders, such as Alzheimer's disease (AD). Alzheimer's disease is characterized by progressive cognitive decline, memory loss, and neuropsychiatric and behavioral symptoms, accounting for most of the reported dementia cases. This disease is now becoming a major challenge and burden on modern society, especially with the aging population. Over the last few decades, a significant understanding of the pathophysiology of AD has been gained by studying amyloid deposition, hyperphosphorylated tau, synaptic dysfunction, oxidative stress, calcium dysregulation, and neuroinflammation. This review focuses on the role of non-canonical secondary structures of DNA/RNA G-quadruplexes (G4s, G4-DNA, and G4-RNA), G4-binding proteins (G4BPs), and helicases, and their roles in aging and AD. Being critically important for cellular function, G4s are involved in the regulation of DNA and RNA processes, such as replication, transcription, translation, RNA localization, and degradation. Recent studies have also highlighted G4-DNA's roles in inducing DNA double-strand breaks that cause genomic instability and G4-RNA's participation in regulating stress granule formation. This review emphasizes the significance of G4s in aging processes and how their homeostatic imbalance may contribute to the pathophysiology of AD.

Noncanonical DNA structures are drivers of genome evolution

In addition to the canonical right-handed double helix, other DNA structures, termed 'non-B DNA', can form in the genomes across the tree of life. Non-B DNA regulates multiple cellular processes, including replication and transcription, yet its presence is associated with elevated mutagenicity and genome instability. These discordant cellular roles fuel the enormous potential of non-B DNA to drive genomic and phenotypic evolution. Here we discuss recent studies establishing non-B DNA structures as novel functional elements subject to natural selection, affecting evolution of transposable elements (TEs), and specifying centromeres. By highlighting the contributions of non-B DNA to repeated evolution and adaptation to changing environments, we conclude that evolutionary analyses should include a perspective of not only DNA sequence, but also its structure.

Dual Targeting Topoisomerase/G-Quadruplex Agents in Cancer Therapy-An Overview

Topoisomerase (Topo) inhibitors have long been known as clinically effective drugs, while G-quadruplex (G4)-targeting compounds are emerging as a promising new strategy to target tumor cells and could support personalized treatment approaches in the near future. G-quadruplex (G4) is a secondary four-stranded DNA helical structure constituted of guanine-rich nucleic acids, and its stabilization impairs telomere replication, triggering the activation of several protein factors at telomere levels, including Topos. Thus, the pharmacological intervention through the simultaneous G4 stabilization and Topos inhibition offers a new opportunity to achieve greater antiproliferative activity and circumvent cellular insensitivity and resistance. In this line, dual ligands targeting both Topos and G4 emerge as innovative, efficient agents in cancer therapy. Although the research in this field is still limited, to date, some chemotypes have been identified, showing this dual activity and an interesting pharmacological profile. This paper reviews the available literature on dual Topo inhibitors/G4 stabilizing agents, with particular attention to the structure-activity relationship studies correlating the dual activity with the cytotoxic activity.

Telomerase structural biology comes of age

Telomerase is an RNA-protein complex comprising telomerase reverse transcriptase, a non-coding telomerase RNA, and proteins involved in biogenesis, assembly, localization, or recruitment. Telomerase synthesizes the telomeric DNA at the 3'-ends of linear chromosomes. During the past decade, structural studies have defined the architecture of Tetrahymena and human telomerase as well as protein and RNA domain structures, but high-resolution details of interactions remained largely elusive. In the past two years, several sub-4 Å cryo-electron microscopy structures of telomerase were published, including Tetrahymena telomerase at different steps of telomere repeat addition and human telomerase with telomere shelterin proteins that recruit telomerase to telomeres. These and other recent structural studies have expanded our understanding of telomerase assembly, mechanism, recruitment, and mutations leading to disease.

Targeting telomeres: advances in telomere maintenance mechanism-specific cancer therapies

Cancer cells establish replicative immortality by activating a telomere-maintenance mechanism (TMM), be it telomerase or the alternative lengthening of telomeres (ALT) pathway. Targeting telomere maintenance represents an intriguing opportunity to treat the vast majority of all cancer types. Whilst telomerase inhibitors have historically been heralded as promising anticancer agents, the reality has been more challenging, and there are currently no therapeutic options for cancer types that use ALT despite their aggressive nature and poor prognosis. In this Review, we discuss the mechanistic differences between telomere maintenance by telomerase and ALT, the current methods used to detect each mechanism, the utility of these tests for clinical diagnosis, and recent developments in the therapeutic strategies being employed to target both telomerase and ALT. We present notable developments in repurposing established therapeutic agents and new avenues that are emerging to target cancer types according to which TMM they employ. These opportunities extend beyond inhibition of telomere maintenance, by finding and exploiting inherent weaknesses in the telomeres themselves to trigger rapid cellular effects that lead to cell death.

Interaction between non-coding RNAs, mRNAs and G-quadruplexes

G-quadruplexes are secondary helical configurations established between guanine-rich nucleic acids. The structure is seen in the promoter regions of numerous genes under certain situations. Predicted G-quadruplex-forming sequences are distributed across the genome in a non-random way. These structures are formed in telomeric regions of the human genome and oncogenic promoter G-rich regions. Identification of mechanisms of regulation of stability of G-quadruplexes has practical significance for understanding the molecular basis of genetic diseases such as cancer. A number of non-coding RNAs such as H19, XIST, FLJ39051 (GSEC), BC200 (BCYRN1), TERRA, pre-miRNA-1229, pre-miRNA-149 and miR-1587 have been found to contain G-quadraplex-forming regions or affect configuration of these structures in target genes. In the current review, we outline the recent research on the interaction between G-quadruplexes and non-coding RNAs, other RNA transcripts and DNA molecules.

Enhancing Anticancer Potency of a 13-Substituted Berberine Derivative with Cationic Liposomes

Cationic liposomal formulations of the telomeric G-quadruplex stabilizing ligand, 13-(2-naphthylmethoxy)berberine bromide (1), have been developed with the purpose of delivering 1 into the nucleus of cancer cells for potential telomere targeting. Berberine derivative 1 was encapsulated in various cationic lipids 2-4 by the thin film evaporation method; these lipids are cationic after amine protonation. The most appropriate liposomal berberine formulation was that of 1 and the cholesterol derived cationic lipid 4 in a weight ratio of 1:20 with 76.5% encapsulation efficiency of 1. Cellular uptake studies in the HeLa and HT-29 cancer cells line showed that the liposomal berberine derivative uptake in the cells was higher and more stable than for berberine derivative 1 alone while free 1 was completely decomposed in the cells within 60 min exposure to the cells. Anticancer activity of the liposomal berberine derivative 1 based on 4 was greater than that for the free berberine derivative 1 in the MCF-7, HeLa and HT-29 cell line by 2.3-, 4.9- and 5.3-fold, respectively, and also, interestingly, superior to the anticancer drug doxorubicin against the HT29 cancer cell line.

Triazolyl Dibenzo[a,c]phenazines Stabilize Telomeric G-quadruplex and Inhibit Telomerase

We herein report the synthesis and biophysical evaluation of triazolyl dibenzo[a,c]phenazine derivatives as a novel class of G-quadruplex ligands. The aromatic core facilitates π-π interaction and the flexible, protonatable side chains interact with the phosphate backbone of DNA via electrostatic interactions. Förster resonance energy transfer (FRET) melting assay and isothermal titration calorimetry (ITC) studies suggest that these ligands show binding preference for the hTELO G-quadruplex over G-quadruplexes found in the promoter region of various oncogenes and duplex DNA. The in vitro telomeric repeat amplification protocol (Q-TRAP) assay reveals that these ligands reduce telomerase activity in cancer cells.

The Effect of Atypical Nucleic Acids Structures in DNA Double Strand Break Repair: A Tale of R-loops and G-Quadruplexes

The fine tuning of the DNA double strand break repair pathway choice relies on different regulatory layers that respond to environmental and local cues. Among them, the presence of non-canonical nucleic acids structures seems to create challenges for the repair of nearby DNA double strand breaks. In this review, we focus on the recently published effects of G-quadruplexes and R-loops on DNA end resection and homologous recombination. Finally, we hypothesized a connection between those two atypical DNA structures in inhibiting the DNA end resection step of HR.

Characterization of G-Quadruplexes Folding/Unfolding Dynamics and Interactions with Proteins from Single-Molecule Force Spectroscopy

G-quadruplexes (G4s) are stable secondary nucleic acid structures that play crucial roles in many fundamental biological processes. The folding/unfolding dynamics of G4 structures are associated with the replication and transcription regulation functions of G4s. However, many DNA G4 sequences can adopt a variety of topologies and have complex folding/unfolding dynamics. Determining the dynamics of G4s and their regulation by proteins remains challenging due to the coexistence of multiple structures in a heterogeneous sample. Here, in this mini-review, we introduce the application of single-molecule force-spectroscopy methods, such as magnetic tweezers, optical tweezers, and atomic force microscopy, to characterize the polymorphism and folding/unfolding dynamics of G4s. We also briefly introduce recent studies using single-molecule force spectroscopy to study the molecular mechanisms of G4-interacting proteins.

G-quadruplex DNA: a novel target for drug design

G-quadruplex (G4) DNA is a type of quadruple helix structure formed by a continuous guanine-rich DNA sequence. Emerging evidence in recent years authenticated that G4 DNA structures exist both in cell-free and cellular systems, and function in different diseases, especially in various cancers, aging, neurological diseases, and have been considered novel promising targets for drug design. In this review, we summarize the detection method and the structure of G4, highlighting some non-canonical G4 DNA structures, such as G4 with a bulge, a vacancy, or a hairpin. Subsequently, the functions of G4 DNA in physiological processes are discussed, especially their regulation of DNA replication, transcription of disease-related genes (c-MYC, BCL-2, KRAS, c-KIT et al.), telomere maintenance, and epigenetic regulation. Typical G4 ligands that target promoters and telomeres for drug design are also reviewed, including ellipticine derivatives, quinoxaline analogs, telomestatin analogs, berberine derivatives, and CX-5461, which is currently in advanced phase I/II clinical trials for patients with hematologic cancer and BRCA1/2-deficient tumors. Furthermore, since the long-term stable existence of G4 DNA structures could result in genomic instability, we summarized the G4 unfolding mechanisms emerged recently by multiple G4-specific DNA helicases, such as Pif1, RecQ family helicases, FANCJ, and DHX36. This review aims to present a general overview of the field of G-quadruplex DNA that has progressed in recent years and provides potential strategies for drug design and disease treatment.

Decoding the Structural Dynamics and Conformational Alternations of DNA Secondary Structures by Single-Molecule FRET Microspectroscopy

In addition to the canonical double helix form, DNA is known to be extrapolated into several other secondary structural patterns involving themselves in inter- and intramolecular type hydrogen bonding. The secondary structures of nucleic acids go through several stages of multiple, complex, and interconvertible heterogeneous conformations. The journey of DNA through these conformers has significant importance and has been monitored thoroughly to establish qualitative and quantitative information about the transition between the unfolded, folded, misfolded, and partially folded states. During this structural interconversion, there always exist specific populations of intermediates, which are short-lived or sometimes even do not accumulate within a heterogeneous population and are challenging to characterize using conventional ensemble techniques. The single-molecule FRET(sm-FRET) microspectroscopic method has the advantages to overcome these limitations and monitors biological phenomena transpiring at a measurable high rate and balanced stochastically over time. Thus, tracing the time trajectory of a particular molecule enables direct measurement of the rate constant of each transition step, including the intermediates that are hidden in the ensemble level due to their low concentrations. This review is focused on the advantages of the employment of single-molecule Forster's resonance energy transfer (sm-FRET), which is worthwhile to access the dynamic architecture and structural transition of various secondary structures that DNA adopts, without letting the donor of one molecule to cross-talk with the acceptor of any other. We have emphasized the studies performed to explore the states of folding and unfolding of several nucleic acid secondary structures, for example, the DNA hairpin, Holliday junction, G-quadruplex, and i-motif.

Beyond small molecules: targeting G-quadruplex structures with oligonucleotides and their analogues

G-Quadruplexes (G4s) are widely studied secondary DNA/RNA structures, naturally occurring when G-rich sequences are present. The strategic localization of G4s in genome areas of crucial importance, such as proto-oncogenes and telomeres, entails fundamental implications in terms of gene expression regulation and other important biological processes. Although thousands of small molecules capable to induce G4 stabilization have been reported over the past 20 years, approaches based on the hybridization of a synthetic probe, allowing sequence-specific G4-recognition and targeting are still rather limited. In this review, after introducing important general notions about G4s, we aim to list, explain and critically analyse in more detail the principal approaches available to target G4s by using oligonucleotides and synthetic analogues such as Locked Nucleic Acids (LNAs) and Peptide Nucleic Acids (PNAs), reporting on the most relevant examples described in literature to date.

Construction of a Three-Color Prism-Based TIRF Microscope to Study the Interactions and Dynamics of Macromolecules

Simple Summary

Prism-based single-molecule total internal reflection fluorescence (prismTIRF) microscopes are excellent tools for studying macromolecular dynamics and interactions. Here, we provide an easy-to-follow guide for the design, assembly, and operation of a three-color prismTIRF microscope using commercially available components with the hope of assisting those who aim to implement TIRF imaging techniques in their laboratory.

Abstract

Single-molecule total internal reflection fluorescence (TIRF) microscopy allows for the real-time visualization of macromolecular dynamics and complex assembly. Prism-based TIRF microscopes (prismTIRF) are relatively simple to operate and can be easily modulated to fit the needs of a wide variety of experimental applications. While building a prismTIRF microscope without expert assistance can pose a significant challenge, the components needed to build a prismTIRF microscope are relatively affordable and, with some guidance, the assembly can be completed by a determined novice. Here, we provide an easy-to-follow guide for the design, assembly, and operation of a three-color prismTIRF microscope which can be utilized for the study of macromolecular complexes, including the multi-component protein–DNA complexes responsible for DNA repair, replication, and transcription. Our hope is that this article can assist laboratories that aspire to implement single-molecule TIRF techniques, and consequently expand the application of this technology.

2'-Fluoro-arabinonucleic Acid (FANA): A Versatile Tool for Probing Biomolecular Interactions

This Account highlights the structural features that render 2'-deoxy-2'-fluoro-arabinonucleic acid (FANA) an ideal tool for mimicking DNA secondary structures and probing biomolecular interactions relevant to chemical biology.The high binding affinity of FANA to DNA and RNA has had implications in therapeutics. FANA can hybridize to complementary RNA, resulting in a predominant A-form helix stabilized by a network of 2'F-H8(purine) pseudohydrogen bonding interactions. We have shown that FANA/RNA hybrids are substrates of RNase H and Ago2, both implicated in the mechanism of action of antisense oligonucleotides (ASOs) and siRNA, respectvely. This knowledge has helped us study the conformational preferences of ASOs and siRNA as well as crRNA in CRISPR-associated Cas9, thereby revealing structural features crucial to biochemical activity.Additionally, FANA is of particular use in stabilizing noncanonical DNA structures. For instance, we have taken advantage of the anti N-glycosidic bond conformation of FANA monomers to induce a parallel topology in telomeric G-quadruplexes. Subsequent single-molecule FRET studies elucidated the mechanism by which these parallel G-quadruplexes are recognized and extended by telomerase. Similarly, we have utilized FANA to stabilize elusive telomeric i-motifs in the presence of concomitant parallel G-quadruplexes and under physiological conditions, thereby reinforcing their potential relevance to telomere biology. In another study, we adapted microarray technology and used FANA substitutions to enhance the binding affinity of the G-quadruplex thrombin-binding aptamer to its thrombin target.Finally, we discovered that DNA polymerases can synthesize FANA strands from DNA templates. On the basis of this property, other groups demonstrated that FANA, like DNA, can store hereditary information. They did so by engineering polymerases to efficiently transfer genetic information from DNA to FANA and retrieve it back into DNA. Subsequent studies showed that FANA could be evolved to acquire ribozyme-like endonuclease or ligase activity and to form high-affinity aptamers.Overall, the implications of these studies are remarkable because they promise a deeper understanding of human biochemistry for innovative therapeutic avenues. This Account summarizes past achievements and provides an outlook for inspiring the increased use of FANA in biological applications and fostering interdisciplinary collaborations.

Single-Molecular Förster Resonance Energy Transfer Measurement on Structures and Interactions of Biomolecules

Single-molecule Förster resonance energy transfer (smFRET) inherits the strategy of measurement from the effective "spectroscopic ruler" FRET and can be utilized to observe molecular behaviors with relatively high throughput at nanometer scale. The simplicity in principle and configuration of smFRET make it easy to apply and couple with other technologies to comprehensively understand single-molecule dynamics in various application scenarios. Despite its widespread application, smFRET is continuously developing and novel studies based on the advanced platforms have been done. Here, we summarize some representative examples of smFRET research of recent years to exhibit the versatility and note typical strategies to further improve the performance of smFRET measurement on different biomolecules.

G-quadruplexes: a promising target for cancer therapy

DNA and RNA can fold into a variety of alternative conformations. In recent years, a particular nucleic acid structure was discussed to play a role in malignant transformation and cancer development. This structure is called a G-quadruplex (G4). G4 structure formation can drive genome instability by creating mutations, deletions and stimulating recombination events. The importance of G4 structures in the characterization of malignant cells was currently demonstrated in breast cancer samples. In this analysis a correlation between G4 structure formation and an increased intratumor heterogeneity was identified. This suggests that G4 structures might allow breast cancer stratification and supports the identification of new personalized treatment options. Because of the stability of G4 structures and their presence within most human oncogenic promoters and at telomeres, G4 structures are currently tested as a therapeutic target to downregulate transcription or to block telomere elongation in cancer cells. To date, different chemical molecules (G4 ligands) have been developed that aim to target G4 structures. In this review we discuss and compare G4 function and relevance for therapeutic approaches and their impact on cancer development for three cancer entities, which differ significantly in their amount and type of mutations: pancreatic cancer, leukemia and malignant melanoma. G4 structures might present a promising new strategy to individually target tumor cells and could support personalized treatment approaches in the future.

Identifying and validating the presence of Guanine-Quadruplexes (G4) within the blood fluke parasite Schistosoma mansoni

Schistosomiasis is a neglected tropical disease that currently affects over 250 million individuals worldwide. In the absence of an immunoprophylactic vaccine and the recognition that mono-chemotherapeutic control of schistosomiasis by praziquantel has limitations, new strategies for managing disease burden are urgently needed. A better understanding of schistosome biology could identify previously undocumented areas suitable for the development of novel interventions. Here, for the first time, we detail the presence of G-quadruplexes (G4) and putative quadruplex forming sequences (PQS) within the Schistosoma mansoni genome. We find that G4 are present in both intragenic and intergenic regions of the seven autosomes as well as the sex-defining allosome pair. Amongst intragenic regions, G4 are particularly enriched in 3´ UTR regions. Gene Ontology (GO) term analysis evidenced significant G4 enrichment in the wnt signalling pathway (p<0.05) and PQS oligonucleotides synthetically derived from wnt-related genes resolve into parallel and anti-parallel G4 motifs as elucidated by circular dichroism (CD) spectroscopy. Finally, utilising a single chain anti-G4 antibody called BG4, we confirm the in situ presence of G4 within both adult female and male worm nuclei. These results collectively suggest that G4-targeted compounds could be tested as novel anthelmintic agents and highlights the possibility that G4-stabilizing molecules could be progressed as candidates for the treatment of schistosomiasis.

Shaping human telomeres: from shelterin and CST complexes to telomeric chromatin organization

The regulation of telomere length in mammals is crucial for chromosome end-capping and thus for maintaining genome stability and cellular lifespan. This process requires coordination between telomeric protein complexes and the ribonucleoprotein telomerase, which extends the telomeric DNA. Telomeric proteins modulate telomere architecture, recruit telomerase to accessible telomeres and orchestrate the conversion of the newly synthesized telomeric single-stranded DNA tail into double-stranded DNA. Dysfunctional telomere maintenance leads to telomere shortening, which causes human diseases including bone marrow failure, premature ageing and cancer. Recent studies provide new insights into telomerase-related interactions (the 'telomere replisome') and reveal new challenges for future telomere structural biology endeavours owing to the dynamic nature of telomere architecture and the great number of structures that telomeres form. In this Review, we discuss recently determined structures of the shelterin and CTC1-STN1-TEN1 (CST) complexes, how they may participate in the regulation of telomere replication and chromosome end-capping, and how disease-causing mutations in their encoding genes may affect specific functions. Major outstanding questions in the field include how all of the telomere components assemble relative to each other and how the switching between different telomere structures is achieved.

Recent progress in non-native nucleic acid modifications

While Nature harnesses RNA and DNA to store, read and write genetic information, the inherent programmability, synthetic accessibility and wide functionality of these nucleic acids make them attractive tools for use in a vast array of applications.

An Updated Focus on Quadruplex Structures as Potential Therapeutic Targets in Cancer

Non-canonical, four-stranded nucleic acids secondary structures are present within regulatory regions in the human genome and transcriptome. To date, these quadruplex structures include both DNA and RNA G-quadruplexes, formed in guanine-rich sequences, and i-Motifs, found in cytosine-rich sequences, as their counterparts. Quadruplexes have been extensively associated with cancer, playing an important role in telomere maintenance and control of genetic expression of several oncogenes and tumor suppressors. Therefore, quadruplex structures are considered attractive molecular targets for cancer therapeutics with novel mechanisms of action. In this review, we provide a general overview about recent research on the implications of quadruplex structures in cancer, firstly gathering together DNA G-quadruplexes, RNA G-quadruplexes as well as DNA i-Motifs.

G-Quadruplexes at Telomeres: Friend or Foe?

Telomeres are DNA-protein complexes that cap and protect the ends of linear chromosomes. In almost all species, telomeric DNA has a G/C strand bias, and the short tandem repeats of the G-rich strand have the capacity to form into secondary structures in vitro, such as four-stranded G-quadruplexes. This has long prompted speculation that G-quadruplexes play a positive role in telomere biology, resulting in selection for G-rich tandem telomere repeats during evolution. There is some evidence that G-quadruplexes at telomeres may play a protective capping role, at least in yeast, and that they may positively affect telomere maintenance by either the enzyme telomerase or by recombination-based mechanisms. On the other hand, G-quadruplex formation in telomeric DNA, as elsewhere in the genome, can form an impediment to DNA replication and a source of genome instability. This review summarizes recent evidence for the in vivo existence of G-quadruplexes at telomeres, with a focus on human telomeres, and highlights some of the many unanswered questions regarding the location, form, and functions of these structures.