Topology of a DNA G-quadruplex structure formed in the HIV-1 promoter: a potential target for anti-HIV drug development

Quadruplex-duplex junction in LTR-III: A molecular insight into the complexes with BMH-21, namitecan and doxorubicin

Quadruplex-Duplex (Q-D) junctions are unique structural motifs garnering increasing interest as drug targets, due to their frequent occurrence in genomic sequences. The viral HIV LTR-III sequence was chosen as a Q-D junction model to study the affinity of the selected compounds BMH-21, namitecan (ST-1968), and doxorubicin (DOXO), all containing a planar polycyclic aromatic moiety, linked to either one short aminoalkyl or an aminoglycosyl group. A multidisciplinary approach that combines NMR spectroscopy, molecular modelling, circular dichroism (CD) and fluorescence spectroscopy was employed. The studied ligands induced moderate but clear stabilization to the Q-D junction by interacting with the interfacial tetrad. DOXO was found to be the best Q-D junction binder. Interestingly, the removal of the aminoglycosyl group significantly changed the pattern of the interactions, indicating that highly polar substituents have a stronger affinity with the exposed regions of the Q-D junction, particularly at the level of the interfacial tetrad.

Identification of a conserved G-quadruplex within the E165R of African swine fever virus (ASFV) as a potential antiviral target

Identification of a conserved G-quadruplex in E165R of ASFVAfrican swine fever virus (ASFV) is a double-stranded DNA arbovirus with high transmissibility and mortality rates. It has caused immense economic losses to the global pig industry. Currently, no effective vaccines or medications are to combat ASFV infection. G-quadruplex (G4) structures have attracted increasing interest because of their regulatory role in vital biological processes. In this study, we identified a conserved G-rich sequence within the E165R gene of ASFV. Subsequently, using various methods, we verified that this sequence could fold into a parallel G4. In addition, the G4-stabilizers pyridostatin and 5,10,15,20-tetrakis-(N-methyl-4-pyridyl) porphin (TMPyP4) can bind and stabilize this G4 structure, thereby inhibiting E165R gene expression, and the inhibitory effect is associated with G4 formation. Moreover, the G4 ligand pyridostatin substantially impeded ASFV proliferation in Vero cells by reducing gene copy number and viral protein expression. These compelling findings suggest that G4 structures may represent a promising and novel antiviral target against ASFV.

DNA G-quadruplexes in the genome of Trypanosoma cruzi as potential therapeutic targets for Chagas disease: Dithienylethene ligands as effective antiparasitic agents

Chagas disease is caused by the parasite Trypanosoma cruzi and affects over 7 million people worldwide. The two actual treatments, Benznidazole (Bzn) and Nifurtimox, cause serious side effects due to their high toxicity leading to treatment abandonment by the patients. In this work, we propose DNA G-quadruplexes (G4) as potential therapeutic targets for this infectious disease. We have found 174 PQS per 100,000 nucleotides in the genome of T. cruzi and confirmed G4 formation of three frequent motifs. We synthesized a family of 14 quadruplex ligands based in the dithienylethene (DTE) scaffold and demonstrated their binding to these identified G4 sequences. Several DTE derivatives exhibited micromolar activity against epimastigotes of four different strains of T. cruzi, in the same concentration range as Bzn. Compounds L3 and L4 presented remarkable activity against trypomastigotes, the active form in blood, of T. cruzi SOL strain (IC50 = 1.5-3.3 μM, SI = 25-40.9), being around 40 times more active than Bzn and displaying much better selectivity indexes.

Systematic Evaluation of Benchmark G4 Probes and G4 Clinical Drugs using three Biophysical Methods: A Guideline to Evaluate Rapidly G4-Binding Affinity

G-quadruplex DNA structures (G4) are proven to interfere with most genetic and epigenetic processes. Small molecules binding these structures (G4 ligands) are invaluable tools to probe G4-biology and address G4-druggability in various diseases (cancer, viral infections). However, the large number of reported G4 ligands (>1000) could lead to confusion while selecting one for a given application. Herein we conducted a systematic affinity ranking of 11 popular G4 ligands vs 5 classical G4 sequences using FRET-melting, G4-FID assays and SPR. Interestingly SPR data globally align with the rankings obtained from the two semi-quantitative assays despite discrepancies due to limits and characteristics of each assay. In the whole, PhenDC3 emerges as the most potent binder irrespective of the G4 sequence. Immediately below PDS, PDC-360A, BRACO19, TMPyP4 and RHPS4 feature strong to medium binding again with poor G4 topology discrimination. More strikingly, the G4 drugs Quarfloxin, CX5461 and c-PDS exhibit weak affinity with all G4s studied. Finally, NMM and Cu-ttpy showed heterogeneous behaviors due, in part, to their physicochemical particularities poorly compatible with screening conditions. The remarkable properties of PhenDC3 led us to propose its use for benchmarking FRET-melting and G4-FID assays for rapid G4-affinity evaluation of newly developed ligands.

Structural elucidation of HIV-1 G-quadruplexes in a cellular environment and their ligand binding using responsive 19F-labeled nucleoside probes

Understanding the structure and recognition of highly conserved regulatory segments of the integrated viral DNA genome that forms unique topologies can greatly aid in devising novel therapeutic strategies to counter chronic infections. In this study, we configured a probe system using highly environment-sensitive nucleoside analogs, 5-fluoro-2'-deoxyuridine (FdU) and 5-fluorobenzofuran-2'-deoxyuridine (FBFdU), to investigate the structural polymorphism of HIV-1 long terminal repeat (LTR) G-quadruplexes (GQs) by fluorescence and 19F NMR. FdU and FBFdU, serving as hairpin and GQ sensors, produced distinct spectral signatures for different GQ topologies adopted by LTR G-rich oligonucleotides. Importantly, systematic 19F NMR analysis in Xenopus laevis oocytes gave unprecedented information on the structure adopted by the LTR G-rich region in the cellular environment. The results indicate that it forms a unique GQ-hairpin hybrid architecture, a potent hotspot for selective targeting. Furthermore, structural models generated using MD simulations provided insights on how the probe system senses different GQs. Using the responsiveness of the probes and Taq DNA polymerase stop assay, we monitored GQ- and hairpin-specific ligand interactions and their synergistic inhibitory effect on the replication process. Our findings suggest that targeting GQ and hairpin motifs simultaneously using bimodal ligands could be a new strategy to selectively block the viral replication.

Base pair dynamics, electrostatics, and thermodynamics at the LTR-III quadruplex:duplex junction

G-quadruplexes (GQs) play key regulatory roles within the human genome and have also been identified to play similar roles in other eukaryotes, bacteria, archaea, and viruses. Human immunodeficiency virus 1, the etiological agent of acquired immunodeficiency syndrome, can form two GQs in its long terminal repeat (LTR) promoter region, each of which act to regulate viral gene expression in opposing manners. The major LTR GQ, called LTR-III, is a distinct hybrid GQ containing a 12-nucleotide duplex loop attached to the quadruplex motif. The resulting quadruplex:duplex junction (QDJ) has been hypothesized to serve as a selective drug targeting site. To better understand the dynamics of this QDJ, we performed conventional and enhanced-sampling molecular dynamics simulations using the Drude-2017 force field. We observed unbiased and reversible formation of additional base pairs in the QDJ, between Ade4:Thy14 and Gua3:Thy14. Both base pairs were electrostatically favored, but geometric constraints within the junction may drive the formation of, and preference for, the Ade4:Thy14 base pair. Finally, we demonstrated that the base pairs are separated only by small energy barriers that may enable transitions between both base-paired states. Together, these simulations provide new insights into the dynamics, electrostatics, and thermodynamics of the LTR-III QDJ.

Naphthalene Diimide-Tetraazacycloalkane Conjugates Are G-Quadruplex-Based HIV-1 Inhibitors with a Dual Mode of Action

Human immunodeficiency virus 1 (HIV-1) therapeutic regimens consist of three or more drugs targeting different steps of the viral life cycle to limit the emergence of viral resistance. In line with the multitargeting strategy, here we conjugated a naphthalene diimide (NDI) moiety with a tetraazacycloalkane to obtain novel naphthalene diimide (NDI)-tetraazacycloalkane conjugates. The NDI inhibits the HIV-1 promoter activity by binding to LTR G-quadruplexes, and the tetraazacycloalkane mimics AMD3100, which blocks HIV entry into cells by interfering with the CXCR4 coreceptor. We synthesized, purified, and tested the metal-free NDI-tetraazacycloalkane conjugate and the two derived metal-organic complexes (MOCs) that incorporate Cu2+ and Zn2+. The NDI-MOCs showed enhanced binding to LTR G4s as assessed by FRET and CD assays in vitro. They also showed enhanced activity in cells where they dose-dependently reduced LTR promoter activity and inhibited viral entry only of the HIV-1 strain that exploited the CXCR4 coreceptor. The time of addition assay confirmed the dual targeting at the different HIV-1 steps. Our results indicate that the NDI-MOC conjugates can simultaneously inhibit viral entry, by targeting the CXCR4 coreceptor, and LTR promoter activity, by stabilizing the LTR G-quadruplexes. The approach of combining multiple targets in a single compound may streamline treatment regimens and improve the overall patient outcomes.

Structure of a DNA G-quadruplex that Modulates SP1 Binding Sites Architecture in HIV-1 Promoter

Nucleic acid sequences containing guanine tracts are able to form non-canonical DNA or RNA structures known as G-quadruplexes (or G4s). These structures, based on the stacking of G-tetrads, are involved in various biological processes such as gene expression regulation. Here, we investigated a G4 forming sequence, HIVpro2, derived from the HIV-1 promoter. This motif is located 60 nucleotides upstream of the proviral Transcription Starting Site (TSS) and overlaps with two SP1 transcription factor binding sites. Using NMR spectroscopy, we determined that HIVpro2 forms a hybrid type G4 structure with a core that is interrupted by a single nucleotide bulge. An additional reverse-Hoogsteen AT base pair is stacked on top of the tetrad. SP1 transcription factor is known to regulate transcription activity of many genes through the recognition of Guanine-rich duplex motifs. Here, the formation of HIVpro2 G4 may modulate SP1 binding sites architecture by competing with the formation of the canonical duplex structure. Such DNA structural switch potentially participates to the regulation of viral transcription and may also interfere with HIV-1 reactivation or viral latency.

G-Quadruplexes in the Viral Genome: Unlocking Targets for Therapeutic Interventions and Antiviral Strategies

G-quadruplexes (G4s) are unique non-canonical four-stranded nucleic acid secondary structures formed by guanine-rich DNA or RNA sequences. Sequences with the potential to form quadruplex motifs (pG4s) are prevalent throughout the genomes of all organisms, spanning from prokaryotes to eukaryotes, and are enriched within regions of biological significance. In the past few years, the identification of pG4s within most of the Baltimore group viruses has attracted increasing attention due to their occurrence in regulatory regions of the genome and the subsequent implications for regulating critical stages of viral life cycles. In this context, the employment of specific G4 ligands has aided in comprehending the intricate G4-mediated regulatory mechanisms in the viral life cycle, showcasing the potential of targeting viral G4s as a novel antiviral strategy. This review offers a thorough update on the literature concerning G4s in viruses, including their identification and functional significance across most of the human-infecting viruses. Furthermore, it delves into potential therapeutic avenues targeting G4s, encompassing various G4-binding ligands, G4-interacting proteins, and oligonucleotide-based strategies. Finally, the article highlights both progress and challenges in the field, providing valuable insights into leveraging this unusual nucleic acid structure for therapeutic purposes.

Guidelines for G-quadruplexes: I. In vitro characterization

Besides the well-known DNA double-helix, non-canonical nucleic acid structures regulate crucial biological activities. Among these oddities, guanine-rich DNA sequences can form unusual four-stranded secondary structures called G-quadruplexes (G4s). G4-prone sequences have been found in the genomes of most species, and G4s play important roles in essential processes such as transcription, replication, genome integrity and epigenetic regulation. Here, we present a short overview of G-quadruplexes followed by a detailed description of the biophysical and biochemical methods used to characterize G4s in vitro. The principles, experimental details and possible shortcomings of each method are discussed to provide a comprehensive view of the techniques used to study these structures. We aim to provide a set of guidelines for standardizing research on G-quadruplexes; these guidelines are not meant to be a dogmatic set of rules, but should rather provide useful information on the methods currently used to study these fascinating motifs.

Unfolding of an RNA G-quadruplex motif in the negative strand genome of porcine reproductive and respiratory syndrome virus by host and viral helicases to promote viral replication

G-quadruplex (G4) is a unique secondary structure formed by guanine-rich nucleic acid sequences. Growing studies reported that the genomes of some viruses harbor G4 structures associated with viral replication, opening up a new field to dissect viral infection. Porcine reproductive and respiratory syndrome virus (PRRSV), a representative member of Arteriviridae, is an economically significant pathogen that has devastated the swine industry worldwide for over 30 years. In this study, we identified a highly conserved G-rich sequence with parallel-type G4 structure (named PRRSV-G4) in the negative strand genome RNA of PRRSV. Pyridostatin (PDS), a well-known G4-binding ligand, stabilized the PRRSV-G4 structure and inhibited viral replication. By screening the proteins interacting with PRRSV-G4 in PRRSV-infected cells and single-molecule magnetic tweezers analysis, we found that two helicases, host DDX18 and viral nsp10, interact with and efficiently unwound the PRRSV-G4 structure, thereby facilitating viral replication. Using a PRRSV reverse genetics system, we confirmed that recombinant PRRSV with a G4-disruptive mutation exhibited resistance to PDS treatment, thereby displaying higher replication than wild-type PRRSV. Collectively, these results demonstrate that the PRRSV-G4 structure plays a crucial regulatory role in viral replication, and targeting this structure represents a promising strategy for antiviral therapies.

Probing the Competition between Duplex, G-Quadruplex and i-Motif Structures of the Oncogenic c-Myc DNA Promoter Region

Development of probe systems that provide unique spectral signatures for duplex, G-quadruplex (GQ) and i-motif (iM) structures is very important to understand the relative propensity of a G-rich-C-rich promoter region to form these structures. Here, we devise a platform using a combination of two environment-sensitive nucleoside analogs namely, 5-fluorobenzofuran-modified 2'-deoxyuridine (FBF-dU) and 5-fluoro-2'-deoxyuridine (F-dU) to study the structures adopted by a promoter region of the c-Myc oncogene. FBF-dU serves as a dual-purpose probe containing a fluorescent and 19 F NMR label. When incorporated into the C-rich sequence, it reports the formation of different iMs via changes in its fluorescence properties and 19 F signal. F-dU incorporated into the G-rich ON reports the formation of a GQ structure whose 19 F signal is clearly different from the signals obtained for iMs. Rewardingly, the labeled ONs when mixed with respective complementary strands allows us to determine the relative population of different structures formed by the c-Myc promoter by the virtue of the probe's ability to produce distinct and resolved 19 F signatures for different structures. Our results indicate that at physiological pH and temperature the c-Myc promoter forms duplex, random coil and GQ structures, and does not form an iM. Whereas at acidic pH, the mixture largely forms iM and GQ structures. Taken together, our system will complement existing tools and provide unprecedented insights on the population equilibrium and dynamics of nucleic acid structures under different conditions.

G-quadruplex in hepatitis B virus pregenomic RNA promotes its translation

Hepatitis B virus (HBV) is a hepatotropic DNA virus that has a very compact genome. Due to this genomic density, several distinct mechanisms are used to facilitate the viral life cycle. Recently, accumulating evidence show that G-quadruplex (G4) in different viruses play essential regulatory roles in key steps of the viral life cycle. Although G4 structures in the HBV genome have been reported, their function in HBV replication remains elusive. In this study, we treated an HBV replication-competent cell line and HBV-infected cells with the G4 structure stabilizer pyridostatin (PDS) and evaluated different HBV replication markers to better understand the role played by the G4. In both models, we found PDS had no effect on viral precore RNA (pcRNA) or pre-genomic RNA (pgRNA), but treatment did increase HBeAg/HBc ELISA reads and intracellular levels of viral core/capsid protein (HBc) in a dose-dependent manner, suggesting post-transcriptional regulation. To further dissect the mechanism of G4 involvement, we used in vitro-synthesized HBV pcRNA and pgRNA. Interestingly, we found PDS treatment only enhanced HBc expression from pgRNA but not HBeAg expression from pcRNA. Our bioinformatic analysis and CD spectroscopy revealed that pgRNA harbors a conserved G4 structure. Finally, we introduced point mutations in pgRNA to disrupt its G4 structure and observed the resulting mutant failed to respond to PDS treatment and decreased HBc level in in vitro translation assay. Taken together, our data demonstrate that HBV pgRNA contains a G4 structure that plays a vital role in the regulation of viral mRNA translation.

G-quadruplex ligands as potent regulators of lysosomes

Guanine-quadruplex structures (G4) are unusual nucleic acid conformations formed by guanine-rich DNA and RNA sequences and known to control gene expression mechanisms, from transcription to protein synthesis. So far, a number of molecules that recognize G4 have been developed for potential therapeutic applications in human pathologies, including cancer and infectious diseases. These molecules are called G4 ligands. When the biological effects of G4 ligands are studied, the analysis is often limited to nucleic acid targets. However, recent evidence indicates that G4 ligands may target other cellular components and compartments such as lysosomes and mitochondria. Here, we summarize our current knowledge of the regulation of lysosome by G4 ligands, underlying their potential functional impact on lysosome biology and autophagic flux, as well as on the transcriptional regulation of lysosomal genes. We outline the consequences of these effects on cell fate decisions and we systematically analyzed G4-prone sequences within the promoter of 435 lysosome-related genes. Finally, we propose some hypotheses about the mechanisms involved in the regulation of lysosomes by G4 ligands.

DNA topoisomerase 1 represses HIV-1 promoter activity through its interaction with a guanine quadruplex present in the LTR sequence

BACKGROUND

Once integrated in the genome of infected cells, HIV-1 provirus is transcribed by the cellular transcription machinery. This process is regulated by both viral and cellular factors, which are necessary for an efficient viral replication as well as for the setting up of viral latency, leading to a repressed transcription of the integrated provirus.

RESULTS

In this study, we examined the role of two parameters in HIV-1 LTR promoter activity. We identified DNA topoisomerase1 (TOP1) to be a potent repressor of this promoter and linked this repression to its catalytic domain. Additionally, we confirmed the folding of a Guanine quadruplex (G4) structure in the HIV-1 promoter and its repressive effect. We demonstrated a direct interaction between TOP1 and this G4 structure, providing evidence of a functional relationship between the two repressive elements. Mutations abolishing G4 folding affected TOP1/G4 interaction and hindered G4-dependent inhibition of TOP1 catalytic activity in vitro. As a result, HIV-1 promoter activity was reactivated in a native chromatin environment. Lastly, we noticed an enrichment of predicted G4 sequences in the promoter of TOP1-repressed cellular genes.

CONCLUSIONS

Our results demonstrate the formation of a TOP1/G4 complex on the HIV-1 LTR promoter and its repressive effect on the promoter activity. They reveal the existence of a new mechanism of TOP1/G4-dependent transcriptional repression conserved between viral and human genes. This mechanism contrasts with the known property of TOP1 as global transcriptional activator and offers new perspectives for anti-cancer and anti-viral strategies.

Helquat dyes targeting G-quadruplexes as a new class of anti-HIV-1 inhibitors

The secondary structure of nucleic acids containing quartets of guanines, termed G-quadruplexes, is known to regulate the transcription of many genes. Several G-quadruplexes can be formed in the HIV-1 long terminal repeat promoter region and their stabilization results in the inhibition of HIV-1 replication. Here, we identified helquat-based compounds as a new class of anti-HIV-1 inhibitors that inhibit HIV-1 replication at the stage of reverse transcription and provirus expression. Using Taq polymerase stop and FRET melting assays, we have demonstrated their ability to stabilize G-quadruplexes in the HIV-1 long-terminal repeat sequence. Moreover, these compounds were not binding to the general G-rich region, but rather to G-quadruplex-forming regions. Finally, docking and molecular dynamics calculations indicate that the structure of the helquat core greatly affects the binding mode to the individual G-quadruplexes. Our findings can provide useful information for the further rational design of inhibitors targeting G-quadruplexes in HIV-1.

Structural Polymorphism of Guanine Quadruplex-Containing Regions in Human Promoters

Intramolecular guanine quadruplexes (G4s) are non-canonical nucleic acid structures formed by four guanine (G)-rich tracts that assemble into a core of stacked planar tetrads. G4-forming DNA sequences are enriched in gene promoters and are implicated in the control of gene expression. Most G4-forming DNA contains more G residues than can simultaneously be incorporated into the core resulting in a variety of different possible G4 structures. Although this kind of structural polymorphism is well recognized in the literature, there remain unanswered questions regarding possible connections between G4 polymorphism and biological function. Here we report a detailed bioinformatic survey of G4 polymorphism in human gene promoter regions. Our analysis is based on identifying G4-containing regions (G4CRs), which we define as stretches of DNA in which every residue can form part of a G4. We found that G4CRs with higher degrees of polymorphism are more tightly clustered near transcription sites and tend to contain G4s with shorter loops and bulges. Furthermore, we found that G4CRs with well-characterized biological functions tended to be longer and more polymorphic than genome-wide averages. These results represent new evidence linking G4 polymorphism to biological function and provide new criteria for identifying biologically relevant G4-forming regions from genomic data.

Can G-quadruplex become a promising target in HBV therapy?

The chronic infection with hepatitis B virus (HBV) is an important health problem that affects millions of people worldwide. Current therapies for HBV always suffer from a poor response rate, common side effects, and the need for lifelong treatment. Novel therapeutic targets are expected. Interestingly, non-canonical structures of nucleic acids play crucial roles in the regulation of gene expression. Especially the formation of G-quadruplexes (G4s) in G-rich strands has been demonstrated to affect many bioprocesses including replication, transcription, and translation, showing great potential as targets in anticancer and antiviral therapies. In this review, we summarize recent antiviral studies about G4s and discuss the potential roles of G4 structures in antiviral therapy for HBV.

Deciphering RNA G-quadruplex function during the early steps of HIV-1 infection

G-quadruplexes (G4s) are four-stranded nucleic acid structures formed by the stacking of G-tetrads. Here we investigated their formation and function during HIV-1 infection. Using bioinformatics and biophysics analyses we first searched for evolutionary conserved G4-forming sequences in HIV-1 genome. We identified 10 G4s with conservation rates higher than those of HIV-1 regulatory sequences such as RRE and TAR. We then used porphyrin-based G4-binders to probe the formation of the G4s during infection of human cells by native HIV-1. The G4-binders efficiently inhibited HIV-1 infectivity, which is attributed to the formation of G4 structures during HIV-1 replication. Using a qRT-PCR approach, we showed that the formation of viral G4s occurs during the first 2 h post-infection and their stabilization by the G4-binders prevents initiation of reverse transcription. We also used a G4-RNA pull-down approach, based on a G4-specific biotinylated probe, to allow the direct detection and identification of viral G4-RNA in infected cells. Most of the detected G4-RNAs contain crucial regulatory elements such as the PPT and cPPT sequences as well as the U3 region. Hence, these G4s would function in the early stages of infection when the viral RNA genome is being processed for the reverse transcription step.

RNA G-quadruplex forming regions from SARS-2, SARS-1 and MERS coronoviruses

COVID-19 (Corona Virus Disease 2019), SARS (Severe Acute Respiratory Syndrome) and MERS (Middle East Respiratory Syndrome) are infectious diseases each caused by coronavirus outbreaks. Small molecules and other therapeutics are rapidly being developed to treat these diseases, but the threat of new variants and outbreaks argue for the identification of additional viral targets. Here we identify regions in each of the three coronavirus genomes that are able to form G-quadruplex (G4) structures. G4s are structures formed by DNA or RNA with a core of two or more stacked planes of guanosine tetrads. In recent years, numerous DNA and RNA G4s have emerged as promising pharmacological targets for the treatment of cancer and viral infection. We use a combination of bioinformatics and biophysical approaches to identify conserved RNA G4 regions from the ORF1A and S sequences of SARS-CoV, SARS-CoV-2 and MERS-CoV. Although a general depletion of G4-forming regions is observed in coronaviridae, the preservation of these selected G4 sequences support a significance in viral replication. Targeting these RNA structures may represent a new antiviral strategy against these viruses distinct from current approaches that target viral proteins.

G-quadruplex-guided RNA engineering to modulate CRISPR-based genomic regulation

It is important to develop small moelcule-based methods to modulate gene editing and expression in human cells. The roles of the G-quadruplex (G4) in biological systems have been widely studied. Here, G4-guided RNA engineering is performed to generate guide RNA with G4-forming units (G4-gRNA). We further demonstrate that chemical targeting of G4-gRNAs holds promise as a general approach for modulating gene editing and expression in human cells. The rich structural diversity of RNAs offers a reservoir of targets for small molecules to bind, thus creating the potential to modulate RNA biology.

RNA G-quadruplex formed in SARS-CoV-2 used for COVID-19 treatment in animal models

The ongoing COVID-19 pandemic has continued to affect millions of lives worldwide, leading to the urgent need for novel therapeutic strategies. G-quadruplexes (G4s) have been demonstrated to regulate life cycle of multiple viruses. Here, we identify several highly conservative and stable G4s in SARS-CoV-2 and clarify their dual-function of inhibition of the viral replication and translation processes. Furthermore, the cationic porphyrin compound 5,10,15,20-tetrakis-(N-methyl-4-pyridyl)porphine (TMPyP4) targeting SARS-CoV-2 G4s shows excellent antiviral activity, while its N-methyl-2-pyridyl positional isomer TMPyP2 with low affinity for G4 has no effects on SARS-CoV-2 infection, suggesting that the antiviral activity of TMPyP4 attributes to targeting SARS-CoV-2 G4s. In the Syrian hamster and transgenic mouse models of SARS-CoV-2 infection, administration of TMPyP4 at nontoxic doses significantly suppresses SARS-CoV-2 infection, resulting in reduced viral loads and lung lesions. Worth to note, the anti-COVID-19 activity of TMPyP4 is more potent than remdesivir evidenced by both in vitro and in vivo studies. Our findings highlight SARS-CoV-2 G4s as a novel druggable target and the compelling potential of TMPyP4 for COVID-19 therapy. Different from the existing anti-SARS-CoV-2 therapeutic strategies, our work provides another alternative therapeutic tactic for SARS-CoV-2 infection focusing on targeting the secondary structures within SARS-CoV-2 genome, and would open a new avenue for design and synthesis of drug candidates with high selectivity toward the new targets.

G-quadruplex structure of the C. elegans telomeric repeat: a two tetrads basket type conformation stabilized by a non-canonical C-T base-pair

The Caenorhabditis elegans model has greatly contributed to the understanding of the role of G-quadruplexes in genomic instability. The GGCTTA repeats of the C. elegans telomeres resemble the GGGTTA repeats of the human telomeres. However, the comparison of telomeric sequences (Homo sapiens, Tetrahymena, Oxytricha, Bombyx mori and Giardia) revealed that small changes in these repeats can drastically change the topology of the folded G-quadruplex. In the present work we determined the structure adopted by the C. elegans telomeric sequence d[GG(CTTAGG)3]. The investigated C. elegans telomeric sequence is shown to fold into an intramolecular two G-tetrads basket type G-quadruplex structure that includes a C-T base pair in the diagonal loop. This work sheds light on the telomeric structure of the widely used C. elegans animal model.

Major Achievements in the Design of Quadruplex-Interactive Small Molecules

Organic small molecules that can recognize and bind to G-quadruplex and i-Motif nucleic acids have great potential as selective drugs or as tools in drug target discovery programs, or even in the development of nanodevices for medical diagnosis. Hundreds of quadruplex-interactive small molecules have been reported, and the challenges in their design vary with the intended application. Herein, we survey the major achievements on the therapeutic potential of such quadruplex ligands, their mode of binding, effects upon interaction with quadruplexes, and consider the opportunities and challenges for their exploitation in drug discovery.

Solution structure of thrombin binding aptamer complex with a non-planar platinum(II) compound

Thrombin Binding Aptamer (TBA) is a monomolecular well-defined two G-tetrads antiparallel G-quadruplex DNA that inhibits the activity of human α-thrombin. In this report, we synthesized a quasi-cross-shaped platinum(II) compound (L'2LPt)...

G-Quadruplexes in Neurobiology and Virology: Functional Roles and Potential Therapeutic Approaches

A G-quadruplex (G4) is a four-stranded nucleic acid secondary structure maintained by Hoogsteen hydrogen bonds established between four guanines. Experimental studies and bioinformatics predictions support the hypothesis that these structures are involved in different cellular functions associated with both DNA and RNA processes. An increasing number of diseases have been shown to be associated with abnormal G4 regulation. Here, we describe the existence of G4 and then discuss G4-related pathogenic mechanisms in neurodegenerative diseases and the viral life cycle. Furthermore, we focus on the role of G4s in the design of antiviral therapy and neuropharmacology, including G4 ligands, G4-based aptamers, G4-related proteins, and CRISPR-based sequence editing, along with a discussion of limitations and insights into the prospects of this unusual nucleic acid secondary structure in therapeutics. Finally, we highlight progress and challenges in this field and the potential G4-related research fields.

G-Quadruplex DNA as a Target in Pathogenic Bacteria: Efficacy of an Extended Naphthalene Diimide Ligand and Its Mode of Action

Guanidine DNA quadruplex (G4-DNA) structures convey a distinctive layer of epigenetic information that is critical for regulating key biological activities and processes as transcription, replication, and repair in living cells. The information regarding their role and use as therapeutic drug targets in bacteria is still scarce. Here, we tested the biological activity of a G4-DNA ligand library, based on the naphthalene diimide (NDI) pharmacophore, against both Gram-positive and Gram-negative bacteria. For the best compound identified, NDI-10, a different action mechanism was described for Gram-positive or negative bacteria. This asymmetric activity profile could be related to the different prevalence of putative G4-DNA structures in each group, the influence that they can exert on gene expression, and the different roles of the G4 structures in these bacteria, which seem to promote transcription in Gram-positive bacteria and repress transcription in Gram-negatives.

Photoresponsive Control of G-Quadruplex DNA Systems

G-quadruplex (G4) oligonucleotide secondary structures have recently attracted significant attention as therapeutic targets owing to their occurrence in human oncogene promoter sequences and the genome of pathogenic organisms. G4s also demonstrate interesting catalytic activities in their own right, as well as the ability to act as scaffolds for the development of DNA-based materials and nanodevices. Owing to this diverse range of opportunities to exploit G4 in a variety of applications, several strategies to control G4 structure and function have emerged. Interrogating the role of G4s in biology requires the delivery of small-molecule ligands that promote its formation under physiological conditions, while exploiting G4 in the development of responsive nanodevices is normally achieved by the addition and sequestration of the metal ions required for the stabilization of the folded structure. Although these strategies prove successful, neither allows the system in question to be controlled externally. Meanwhile, light has proven to be an attractive means for the control of DNA-based systems as it is noninvasive, can be delivered with high spatiotemporal precision, and is orthogonal to many chemical and biological processes. A plethora of photoresponsive DNA systems have been reported to date; however, the vast majority deploy photoreactive moieties to control the stability and assembly of duplex DNA hybrids. Despite the unique opportunities afforded by the regulation of G-quadruplex formation in biology, catalysis, and nanotechnology, comparatively little attention has been devoted to the design of photoresponsive G4-based systems. In this Perspective, we consider the potential of photoresponsive G4 assemblies and examine the strategies that may be used to engineer these systems toward a variety of applications. Through an overview of the main developments in the field to date, we highlight recent progress made toward this exciting goal and the emerging opportunities that remain ripe for further exploration in the coming years.

Probing GFP Chromophore Analogs as Anti-HIV Agents Targeting LTR-III G-Quadruplex

Green fluorescent protein (GFP) chromophore and its congeners draw significant attention mostly for bioimaging purposes. In this work we probed these compounds as antiviral agents. We have chosen LTR-III DNA G4, the major G-quadruplex (G4) present in the long terminal repeat (LTR) promoter region of the human immunodeficiency virus-1 (HIV-1), as the target for primary screening and designing antiviral drug candidates. The stabilization of this G4 was previously shown to suppress viral gene expression and replication. FRET-based high-throughput screening (HTS) of 449 GFP chromophore-like compounds revealed a number of hits, sharing some general structural features. Structure-activity relationships (SAR) for the most effective stabilizers allowed us to establish structural fragments, important for G4 binding. Synthetic compounds, developed on the basis of SAR analysis, exhibited high LTR-III G4 stabilization level. NMR spectroscopy and molecular modeling revealed the possible formation of LTR-III G4-ligand complex with one of the lead selective derivative ZS260.1 positioned within the cavity, thus supporting the LTR-III G4 attractiveness for drug targeting. Selected compounds showed moderate activity against HIV-I (EC50 1.78–7.7 μM) in vitro, but the activity was accompanied by pronounced cytotoxicity.

Insights from Binding on Quadruplex Selective Carbazole Ligands

Polymorphic G-quadruplex (G4) secondary DNA structures have received increasing attention in medicinal chemistry owing to their key involvement in the regulation of the maintenance of genomic stability, telomere length homeostasis and transcription of important proto-oncogenes. Different classes of G4 ligands have been developed for the potential treatment of several human diseases. Among them, the carbazole scaffold with appropriate side chain appendages has attracted much interest for designing G4 ligands. Because of its large and rigid π-conjugation system and ease of functionalization at three different positions, a variety of carbazole derivatives have been synthesized from various natural or synthetic sources for potential applications in G4-based therapeutics and biosensors. Herein, we provide an updated close-up of the literatures on carbazole-based G4 ligands with particular focus given on their detailed binding insights studied by NMR spectroscopy. The structure-activity relationships and the opportunities and challenges of their potential applications as biosensors and therapeutics are also discussed. This review will provide an overall picture of carbazole ligands with remarkable G4 topological preference, fluorescence properties and significant bioactivity; portraying carbazole as a very promising scaffold for assembling G4 ligands with a range of novel functional applications.

Gold(III) porphyrins: Synthesis and interaction with G-quadruplex DNA

G-quadruplex nucleic acids (G4s) are RNA and DNA secondary structures involved in the regulation of multiple key biological processes. They can be found in telomeres, oncogene promoters, RNAs, but also in viral genomes. Due to their unique structural features, very distinct from the canonical duplexes or single-strands, G4s represent promising pharmacological targets for small molecules, namely G4-ligands. Gold(III) penta-cationic porphyrins, as specific G4 ligands, are able to inhibit HIV-1 infectivity and their antiviral activity correlates with their affinity for G4s. Up to now, one of the best antiviral compounds is meso-5,10,15,20-tetrakis[4-(N-methyl-pyridinium-2-yl)phenyl]porphyrinato gold(III) (1). Starting from this compound, we report a structure/affinity relationship study of gold(III) cationic porphyrins to find out the best porphyrin candidate for functionalization, in order to study the antiviral mechanism of action of these gold(III) porphyrins.

Interaction of a Short Peptide with G-Quadruplex-Forming Sequences: An SRCD and CD Study

G-quadruplex (G4) forming DNA sequences were recently found to play a crucial role in the regulation of genomic processes such as replication, transcription and translation, also related to serious diseases. Therefore, systems capable of controlling DNA and RNA G-quadruplex structures would be useful for the modulation of various cellular events. In particular, peptides represent good candidates for targeting G-quadruplex structures, since they are easily tailored to enhance their functionality. In this work, we analyzed, by circular dichroism and synchrotron radiation circular dichroism spectroscopies, the interaction of a 25-residue peptide deriving from RHAU helicases (Rhau25) with three G-quadruplex-forming oligonucleotide sequences, in both sodium- and potassium-containing buffers, the most relevant monovalent cations in physiological conditions. The peptide displayed greater affinity for the G4 sequences adopting a parallel structure. However, it showed the ability to also interact with antiparallel or hybrid G-quadruplex structures, inducing a conformation conversion to the parallel structure. The stability of the oligonucleotide structure alone or in presence of the Rhau25 peptide was studied by temperature melting and UV denaturation experiments, and the data showed that the interaction with the peptide stabilized the conformation of oligonucleotide sequences when subjected to stress conditions.

Potential G-quadruplexes and i-Motifs in the SARS-CoV-2

Quadruplex structures have been identified in a plethora of organisms where they play important functions in the regulation of molecular processes, and hence have been proposed as therapeutic targets for many diseases. In this paper we report the extensive bioinformatic analysis of the SARS-CoV-2 genome and related viruses using an upgraded version of the open-source algorithm G4-iM Grinder. This version improves the functionality of the software, including an easy way to determine the potential biological features affected by the candidates found. The quadruplex definitions of the algorithm were optimized for SARS-CoV-2. Using a lax quadruplex definition ruleset, which accepts amongst other parameters two residue G- and C-tracks, 512 potential quadruplex candidates were discovered. These sequences were evaluated by their in vitro formation probability, their position in the viral RNA, their uniqueness and their conservation rates (calculated in over seventeen thousand different COVID-19 clinical cases and sequenced at different times and locations during the ongoing pandemic). These results were then compared subsequently to other Coronaviridae members, other Group IV (+)ssRNA viruses and the entire viral realm. Sequences found in common with other viral species were further analyzed and characterized. Sequences with high scores unique to the SARS-CoV-2 were studied to investigate the variations amongst similar species. Quadruplex formation of the best candidates were then confirmed experimentally. Using NMR and CD spectroscopy, we found several highly stable RNA quadruplexes that may be suitable therapeutic targets for the SARS-CoV-2.

Unlocking G-Quadruplexes as Antiviral Targets

Guanine-rich DNA and RNA sequences can fold into noncanonical nucleic acid structures called G-quadruplexes (G4s). Since the discovery that these structures may act as scaffolds for the binding of specific ligands, G4s aroused the attention of a growing number of scientists. The versatile roles of G4 structures in viral replication, transcription, and translation suggest direct applications in therapy or diagnostics. G4-interacting molecules (proteins or small molecules) may also affect the balance between latent and lytic phases, and increasing evidence reveals that G4s are implicated in generally suppressing viral processes, such as replication, transcription, translation, or reverse transcription. In this review, we focus on the discovery of G4s in viruses and the role of G4 ligands in the antiviral drug discovery process. After assessing the role of viral G4s, we argue that host G4s participate in immune modulation, viral tumorigenesis, cellular pathways involved in virus maturation, and DNA integration of viral genomes, which can be potentially employed for antiviral therapeutics. Furthermore, we scrutinize the impediments and shortcomings in the process of studying G4 ligands and drug discovery. Finally, some unanswered questions regarding viral G4s are highlighted for prospective future projects. SIGNIFICANCE STATEMENT: G-quadruplexes (G4s) are noncanonical nucleic acid structures that have gained increasing recognition during the last few decades. First identified as relevant targets in oncology, their importance in virology is now increasingly clear. A number of G-quadruplex ligands are known: viral transcription and replication are the main targets of these ligands. Both viral and cellular G4s may be targeted; this review embraces the different aspects of G-quadruplexes in both host and viral contexts.

DNA G-quadruplexes for native mass spectrometry in potassium: a database of validated structures in electrospray-compatible conditions

G-quadruplex DNA structures have become attractive drug targets, and native mass spectrometry can provide detailed characterization of drug binding stoichiometry and affinity, potentially at high throughput. However, the G-quadruplex DNA polymorphism poses problems for interpreting ligand screening assays. In order to establish standardized MS-based screening assays, we studied 28 sequences with documented NMR structures in (usually ∼100 mM) potassium, and report here their circular dichroism (CD), melting temperature (Tm), NMR spectra and electrospray mass spectra in 1 mM KCl/100 mM trimethylammonium acetate. Based on these results, we make a short-list of sequences that adopt the same structure in the MS assay as reported by NMR, and provide recommendations on using them for MS-based assays. We also built an R-based open-source application to build and consult a database, wherein further sequences can be incorporated in the future. The application handles automatically most of the data processing, and allows generating custom figures and reports. The database is included in the g4dbr package (https://github.com/EricLarG4/g4dbr) and can be explored online (https://ericlarg4.github.io/G4_database.html).

De Novo Design of Selective Quadruplex-Duplex Junction Ligands and Structural Characterisation of Their Binding Mode: Targeting the G4 Hot-Spot

Targeting the interface between DNA quadruplex and duplex regions by small molecules holds significant promise in both therapeutics and nanotechnology. Herein, a new pharmacophore is reported, which selectively binds with high affinity to quadruplex-duplex junctions, while presenting a poorer affinity for G-quadruplex or duplex DNA alone. Ligands complying with the reported pharmacophore exhibit a significant affinity and selectivity for quadruplex-duplex junctions, including the one observed in the HIV-1 LTR-III sequence. The structure of the complex between a quadruplex-duplex junction with a ligand of this family has been determined by NMR methods. According to these data, the remarkable selectivity of this structural motif for quadruplex-duplex junctions is achieved through an unprecedented interaction mode so far unexploited in medicinal and biological chemistry: the insertion of a benzylic ammonium moiety into the centre of the partially exposed G-tetrad at the interface with the duplex. Further decoration of the described scaffolds with additional fragments opens up the road to the development of selective ligands for G-quadruplex-forming regions of the genome.

G-Quadruplexes Are Present in Human Coronaviruses Including SARS-CoV-2

The global coronavirus disease 2019 (COVID-19) pandemic is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which is one of seven human coronaviruses. G-quadruplexes are intrinsic obstacles to genome replication. Whether G-quadruplexes are present in human coronaviruses is unknown. In the current study, we have predicted that all seven human coronaviruses harbor G-quadruplex sequences. Conserved G-quadruplex sequences in SARS-CoV and SARS-CoV-2 were analyzed and verified by circular dichroism (CD) spectroscopy and Thioflavin T fluorescence assay. Similar to SARS-CoV, SARS-CoV-2 encodes an nsP3 protein, which is predicted to associate with G-quadruplexes. Targeting G-quadruplex sequences in the SARS-CoV-2 genome by G-quadruplex ligands could be a new way to conquer COVID-19.

Stabilization of human telomeric RNA G-quadruplex by the water-compatible optically pure and biologically-active metallohelices

RNA G-quadruplexes have been suggested to play key roles in fundamental biological processes and are linked to human diseases. Thus, they also represent good potential therapeutic targets. Here, we describe, using the methods of molecular biophysics, interactions of a series of biologically-active supramolecular cationic metallohelices with human telomeric RNA G-quadruplex. We demonstrate that the investigated metallohelices bind with a high affinity to human telomeric RNA G-quadruplex and that their binding selectivity considerably differs depending on the dimensions and overall shape of the metallohelices. Additionally, the investigated metallohelices inhibit DNA synthesis on the RNA template containing four repeats of the human telomeric sequence by stabilizing the RNA G-quadruplex structure. Collectively, the results of this study suggest that stabilization of RNA sequences capable of G-quadruplex formation by metallohelices investigated in this work might contribute to the mechanism of their biological activity.

Synthesis of new riboflavin modified ODNs: Effect of riboflavin moiety on the G-quadruplex arrangement and stability

In the panorama of modified G-quadruplexes (G4s) with interesting proprieties, here, it has been reported the synthesis of new modified d(TGGGAG) sequences forming G-quadruplexes, with the insertion of a riboflavin unit (Rf, vitamin B₂). Exploiting the flavin similarity with the hydrogen bond pattern of guanine and aiming at mimic a typical nucleoside scaffold, the synthesis of the riboflavin building block 3 it has been efficiently carried out. The effect of insertion of riboflavin mimic nucleoside on the G-quadruplex properties has been here, for the first time investigated. A biophysical characterization of Rf-modified sequences (A-D) has been carried out by circular dichroism (CD), fluorescence spectroscopy, differential scanning calorimetry (DSC) and native gel electrophoresis. CD and electrophoresis data have suggested that Rf-modified sequences are able to form parallel tetramolecular G4 structures similar to that of the unmodified sequence. Analysis of the DSC thermograms has revealed that all modified G-quadruplexes have a higher thermal stability compared with the natural sequence, particularly the stabilisation is higher when the Rf residue is introduced at the 3'-end. Further, DSC analysis has revealed that the Rf residues introduced at the 3'-end are able to form additional stabilising interactions, energetically almost comparable to the enthalpic contribution of a G-tetrad. Fluorescence measurement are consistent with this result showing that the Rf residues introduced at 3'-end are able to form stacking interactions with the adjacent bases within the G-quadruplex structure. The whole of data suggested that the introduction of Rf unit can stabilize G-quadruplex structures and can be a promising candidate for future theranostic applications.

G-quadruplex ligands targeting telomeres do not inhibit HIV promoter activity and cooperate with latency reversing agents in killing latently infected cells

Altered telomere maintenance mechanism (TMM) is linked to increased DNA damage at telomeres and telomere uncapping. We previously showed that HIV-1 latent cells have altered TMM and are susceptible to ligands that target G-quadruplexes (G4) at telomeres. Susceptibility of latent cells to telomere targeting could potentially be used to support approaches to eradicate HIV reservoirs. However, G4 ligands also target G-quadruplexes in promoters blocking gene transcription. Since HIV promoter sequence can form G-quadruplexes, we investigated whether G4 ligands interfere with HIV-1 promoter activity and virus reactivation from latency, and whether telomere targeting could be combined with latency reversing agents (LRAs) to promote elimination of HIV reservoirs. Our results indicate that Sp1 binding region in HIV-1 promoter can adopt G4 structures in duplex DNA, and that in vitro binding of Sp1 to G-quadruplex is blocked by G4 ligand, suggesting that agents targeting telomeres interfere with virus reactivation. However, our studies show that G4 agents do not affect HIV-1 promoter activity in cell culture, and do not interfere with latency reversal. Importantly, primary memory CD4 + T cells infected with latent HIV-1 are more susceptible to combined treatment with LRAs and G4 ligands, indicating that drugs targeting TMM may enhance killing of HIV reservoirs. Using a cell-based DNA repair assay, we also found that HIV-1 infected cells have reduced efficiency of DNA mismatch repair (MMR), and base excision repair (BER), suggesting that altered TMM in latently infected cells could be associated with accumulation of DNA damage at telomeres and changes in telomeric caps.

Diazapyrenes: interaction with nucleic acids and biological activity

The review summarizes data on the practical aspects of the interaction of nucleic acids with diazapyrene derivatives. The information on biological activity is given and the probable mechanisms underlying the action of diazapyrenes are analyzed. It contains 119 references.

A dual-app nucleoside probe reports G-quadruplex formation and ligand binding in the long terminal repeat of HIV-1 proviral genome

We have developed a dual-app nucleoside analog, 5-selenophene-modified 2'-deoxyuridine (^SedU), to probe the structure and ligand-binding properties of a G-rich segment present in the long terminal repeat (LTR) of the HIV-1 proviral DNA promoter region. The nucleoside probe is made of an environment-responsive fluorophore and X-ray crystallography phasing label (Se atom). ^SedU incorporated into LTR-IV sequence, fluorescently reports the formation of G-quadruplex (GQ) structure without affecting the native fold. Further, using the environment sensitivity of the probe, a fluorescence assay was designed to estimate the binding affinity of small molecule ligands to the GQ motif. An added feature of this probe system is that it would enable direct correlation of structure and recognition properties in solution and atomic level by using a combination of fluorescence and X-ray crystallography techniques.

Epigenetic Modulation of Chromatin States and Gene Expression by G-Quadruplex Structures

G-quadruplexes are four-stranded helical nucleic acid structures formed by guanine-rich sequences. A considerable number of studies have revealed that these noncanonical structural motifs are widespread throughout the genome and transcriptome of numerous organisms, including humans. In particular, G-quadruplexes occupy strategic locations in genomic DNA and both coding and noncoding RNA molecules, being involved in many essential cellular and organismal functions. In this review, we first outline the fundamental structural features of G-quadruplexes and then focus on the concept that these DNA and RNA structures convey a distinctive layer of epigenetic information that is critical for the complex regulation, either positive or negative, of biological activities in different contexts. In this framework, we summarize and discuss the proposed mechanisms underlying the functions of G-quadruplexes and their interacting factors. Furthermore, we give special emphasis to the interplay between G-quadruplex formation/disruption and other epigenetic marks, including biochemical modifications of DNA bases and histones, nucleosome positioning, and three-dimensional organization of chromatin. Finally, epigenetic roles of RNA G-quadruplexes in post-transcriptional regulation of gene expression are also discussed. Undoubtedly, the issues addressed in this review take on particular importance in the field of comparative epigenetics, as well as in translational research.

A DNA G-quadruplex/i-motif hybrid

DNA can form many structures beyond the canonical Watson–Crick double helix. It is now clear that noncanonical structures are present in genomic DNA and have biological functions. G-rich G-quadruplexes and C-rich i-motifs are the most well-characterized noncanonical DNA motifs that have been detected in vivo with either proscribed or postulated biological roles. Because of their independent sequence requirements, these structures have largely been considered distinct types of quadruplexes. Here, we describe the crystal structure of the DNA oligonucleotide, d(CCAGGCTGCAA), that self-associates to form a quadruplex structure containing two central antiparallel G-tetrads and six i-motif C–C⁺ base pairs. Solution studies suggest a robust structural motif capable of assembling as a tetramer of individual strands or as a dimer when composed of tandem repeats. This hybrid structure highlights the growing structural diversity of DNA and suggests that biological systems may harbor many functionally important non-duplex structures.

Viral G-quadruplexes: New frontiers in virus pathogenesis and antiviral therapy

Viruses are the most abundant organisms on our planet, affecting all living beings: some of them are responsible for massive epidemics that concern health, national economies and the overall welfare of societies. Although advances in antiviral research have led to successful therapies against several human viruses, still some of them cannot be eradicated from the host and most of them do not have any treatment available. Consequently, innovative antiviral therapies are urgently needed. In the past few years, research on G-quadruplexes (G4s) in viruses has boomed, providing powerful evidence for the regulatory role of G4s in key viral steps. Comprehensive bioinformatics analyses have traced putative G4-forming sequences in the genome of almost all human viruses, showing that their distribution is statistically significant and their presence highly conserved. Since the genomes of viruses are remarkably variable, high conservation rates strongly suggest a crucial role of G4s in the viral replication cycle and evolution, emphasizing the possibility of targeting viral G4s as a new pharmacological approach in antiviral therapy. Recent studies have demonstrated the formation and function of G4s in pathogens responsible for serious diseases, such as HIV-1, Hepatitis B and C, Ebola viruses, to cite a few. In this chapter, we present the state of the art on the structural and functional characterization of viral G4s in RNA viruses, DNA viruses and retroviruses. We also present the G4 ligands that provide further details on the viral G4 role and which, showing promising antiviral activity, which could be exploited for the development of innovative antiviral agents.

A dynamic i-motif with a duplex stem-loop in the long terminal repeat promoter of the HIV-1 proviral genome modulates viral transcription

I-motifs are non-canonical nucleic acids structures characterized by intercalated H-bonds between hemi-protonated cytosines. Evidence on the involvement of i-motif structures in the regulation of cellular processes in human cells has been consistently growing in the recent years. However, i-motifs within non-human genomes have never been investigated. Here, we report the characterization of i-motifs within the long terminal repeat (LTR) promoter of the HIV-1 proviral genome. Biophysical and biochemical analysis revealed formation of a predominant i-motif with an unprecedented loop composition. One-dimensional nuclear magnetic resonance investigation demonstrated formation of three G-C H-bonds in the long loop, which likely improve the structure overall stability. Pull-down experiments combined with mass spectrometry and protein crosslinking analysis showed that the LTR i-motif is recognized by the cellular protein hnRNP K, which induced folding at physiological conditions. In addition, hnRNP K silencing resulted in an increased LTR promoter activity, confirming the ability of the protein to stabilize the i-motif-forming sequence, which in turn regulates the LTR-mediated HIV-1 transcription. These findings provide new insights into the complexity of the HIV-1 virus and lay the basis for innovative antiviral drug design, based on the possibility to selectively recognize and target the HIV-1 LTR i-motif.

Probing G-quadruplex topologies and recognition concurrently in real time and 3D using a dual-app nucleoside probe

Comprehensive understanding of structure and recognition properties of regulatory nucleic acid elements in real time and atomic level is highly important to devise efficient therapeutic strategies. Here, we report the establishment of an innovative biophysical platform using a dual-app nucleoside analog, which serves as a common probe to detect and correlate different GQ structures and ligand binding under equilibrium conditions and in 3D by fluorescence and X-ray crystallography techniques. The probe (SedU) is composed of a microenvironment-sensitive fluorophore and an excellent anomalous X-ray scatterer (Se), which is assembled by attaching a selenophene ring at 5-position of 2'-deoxyuridine. SedU incorporated into the loop region of human telomeric DNA repeat fluorescently distinguished subtle differences in GQ topologies and enabled quantify ligand binding to different topologies. Importantly, anomalous X-ray dispersion signal from Se could be used to determine the structure of GQs. As the probe is minimally perturbing, a direct comparison of fluorescence data and crystal structures provided structural insights on how the probe senses different GQ conformations without affecting the native fold. Taken together, our dual-app probe represents a new class of tool that opens up new experimental strategies to concurrently investigate nucleic acid structure and recognition in real time and 3D.