Template-Assembled Synthetic G-Quartets (TASQs): multiTASQing Molecular Tools for Investigating DNA and RNA G-Quadruplex Biology

Fluorescence Detection of DNA/RNA G-Quadruplexes (G4s) by Twice-as-Smart Ligands

Fluorescence detection of DNA and RNA G-quadruplexes (G4s) is a very efficient strategy to assess not only the existence and prevalence of cellular G4s but also their relevance as targets for therapeutic interventions. Among the fluorophores used to this end, turn-on probes are the most interesting since their fluorescence is triggered only upon interaction with their G4 targets, which ensures a high sensitivity and selectivity of detection. We reported on a series of twice-as-smart G4 probes, which are both smart G4 ligands (whose structure is reorganized upon interaction with G4s) and smart fluorescent probes (whose fluorescence is turned on upon interaction with G4s). The fine mechanistic details behind the excellent properties of the best prototype N-TASQ remain to be deciphered: to investigate this, we report here on the synthesis and studies of two analogues, TzN-TASQ and AlkN-TASQ, and on a careful analysis of their G4-interacting properties, investigated both in vitro and in silico. Our results show that fine-tuning their constitutive structural elements allows for increasing the efficiency of both their 'off' (i. e., a conformation with a low fluorescence) and 'on' states (i. e., a conformation with a high fluorescence), which opens interesting ways for the design of more efficient fluorogenic G4 probes.

Identification of G-quadruplex nucleic acid structures by high-throughput sequencing: A review

G-quadruplexes (G4s) are non-canonical nucleic acid secondary structures formed by guanine-rich DNA or RNA sequences. These structures play pivotal roles in cellular processes, including DNA replication, transcription, RNA splicing, and protein translation. High-throughput sequencing has significantly advanced the study of G4s by enabling genome-wide mapping and detailed characterization. This review provides a comprehensive overview of current methods for G4 identification using high-throughput sequencing, focusing on key techniques such as G4-seq, G4-ChIP-seq, G4-CUT&Tag, LiveG4ID-seq, G4assess, HepG4-seq, rG4-seq, RT-stop profiling with DMS-m7G footprinting, G4RP-seq, Keth-seq, and SHALIPE-seq. We discuss the principles, advantages, limitations, and applications of these methods, highlighting their contribution to our understanding of G4 biology. The review also emphasizes the need for improved tools to explore the dynamic behavior of G4s, particularly in living organisms.

Protocol for cellular RNA G-quadruplex profiling using G4RP.v2

The isolation of G-quadruplexes (G4s) from human cells using specific molecular tools constitutes an invaluable step forward in uncovering the biology of these higher-order DNA and RNA structures. Here, we present an improved version of the G4-RNA precipitation (G4RP) protocol developed to identify RNA G4s from human cancer cells. We describe steps for cell treatment and lysis, chemoprecipitation of G4s using TASQ tools, go/no-go steps, and quantitative reverse-transcription PCR (RT-qPCR) quantification and analysis. For complete details on the use and execution of this protocol, please refer to Mitteaux et al.1.

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.

G-quadruplexes in long non-coding RNAs and their interactions with proteins

Long non-coding RNAs (lncRNAs) have emerged as crucial regulators of cellular processes, with their dysregulation linked to various disease states. Among the structural motifs in lncRNAs, RNA G-quadruplexes (rG4s) have gained increasing attention due to their diverse roles in cellular function and disease pathogenesis. This review provides an updated and comprehensive overview of rG4s in lncRNAs, elucidating their formation, interaction with proteins, and distinctive roles in cellular processes. We discuss current methodologies for experimentally probing RNA G4s, including the use of specific small molecules, biomolecular ligands and fluorescent probes. The commonly found RNA G4-interacting protein domains are summarised along with potential strategies for disrupting lncRNA G4-protein interactions from a therapeutic perspective.

A Cyanine Dye for Highly Specific Recognition of Parallel G-Quadruplex Topology and Its Application in Clinical RNA Detection for Cancer Diagnosis

G-quadruplex (G4), an unconventional nucleic acid structure, shows polymorphism in its topological morphology. The parallel G4 topology is the most prevalent form in organisms and plays a regulatory role in many biological processes. Designing fluorescent probes with high specificity for parallel G4s is important but challenging. Herein, a supramolecular assembly of the anionic cyanine dye SCY-5 is reported, which selectively identifies parallel G4 topology. SCY-5 can clearly distinguish parallel G4s from other G4s and non-G4s, even including hybrid-type G4s with parallel characteristics. The high specificity mechanism of SCY-5 involves a delicate balance between electrostatic repulsion and π-π interaction between SCY-5 and G4s. Using SCY-5, cellular RNA extracted from peripheral venous blood was quantitatively detected, and a remarkable increase in RNA G4 content in cancer patients compared to healthy volunteers was confirmed for the first time. This study provides new insights for designing specific probes for parallel G4 topology and opens a new path for clinical cancer diagnosis using RNA G4 as a biomarker.

RNA G-quadruplex in functional regulation of noncoding RNA: Challenges and emerging opportunities

G-quadruplexes (G4s) are stable, noncanonical structures formed in guanine (G)-rich sequences of DNA/RNA. G4 structures are reported to play a regulatory role in various cellular processes and, recently, a considerable number of studies have attributed new biological functions to these structures, especially in RNA. Noncoding RNA (ncRNA), which does not translate into a functional protein, is widely expressed and has been shown to play a key role in shaping cellular activity. There has been growing evidence of G4 formation in several ncRNA classes, and it has been identified as a key part for diverse biological functions and physio-pathological contexts in neurodegenerative diseases and cancer. This review discusses RNA G4s (rG4s) in ncRNA, focusing on the molecular mechanism underlying its function. This review also aims to highlight potential and emerging opportunities to identify and target the rG4s in ncRNA to understand its function and, ultimately, treat many diseases.

PhpC modulates G-quadruplex-RNA landscapes in human cells

Stabilizing DNA/RNA G-quadruplexes (G4s) using small molecules (ligands) has proven an efficient strategy to decipher G4 biology. Quite paradoxically, this search has also highlighted the need for finding molecules able to disrupt G4s to tackle G4-associated cellular dysfunctions. We report here on both qualitative and quantitative investigations that validate the G4-RNA-destabilizing properties of the leading compound PhpC in human cells.

The multivalent G-quadruplex (G4)-ligands MultiTASQs allow for versatile click chemistry-based investigations

Chemical biology hinges on multivalent molecular tools that can specifically interrogate and/or manipulate cellular circuitries from the inside. The success of many of these approaches relies on molecular tools that make it possible to visualize biological targets in cells and then isolate them for identification purposes. To this end, click chemistry has become in just a few years a vital tool in offering practically convenient solutions to address highly complicated biological questions. We report here on two clickable molecular tools, the biomimetic G-quadruplex (G4) ligands MultiTASQ and ^azMultiTASQ, which benefit from the versatility of two types of bioorthogonal chemistry, CuAAC and SPAAC (the discovery of which was very recently awarded the Nobel Prize of chemistry). These two MultiTASQs are used here to both visualize G4s in and identify G4s from human cells. To this end, we developed click chemo-precipitation of G-quadruplexes (G4-click-CP) and in situ G4 click imaging protocols, which provide unique insights into G4 biology in a straightforward and reliable manner.

Side-by-side comparison of G-quadruplex (G4) capture efficiency of the antibody BG4 versus the small-molecule ligands TASQs

The search for G-quadruplex (G4)-forming sequences across the genome is motivated by their involvement in key cellular processes and their putative roles in dysregulations underlying human genetic diseases. Sequencing-based methods have been developed to assess the prevalence of DNA G4s genome wide, including G4-seq to detect G4s in purified DNA (in vitro) using the G4 stabilizer PDS, and G4 chromatin immunoprecipitation sequencing (G4 ChIP-seq) to detect G4s in in situ fixed chromatin (in vivo) using the G4-specific antibody BG4. We recently reported on G4-RNA precipitation and sequencing (G4RP-seq) to assess the in vivo prevalence of RNA G4 landscapes transcriptome wide using the small molecule BioTASQ. Here, we apply this technique for mapping DNA G4s in plants (rice) and compare the efficiency of this new technique, G4-DNA precipitation and sequencing, G4DP-seq, to that of BG4-DNA-IP-seq that we developed for mapping of DNA G4s in rice using BG4. By doing so, we compare the G4 capture ability of small-sized ligands (BioTASQ and BioCyTASQ) versus the antibody BG4.