Structural Insights into the Quadruplex-Duplex 3' Interface Formed from a Telomeric Repeat: A Potential Molecular Target

Development of a multi-targeted chemotherapeutic approach based on G-quadruplex stabilisation and carbonic anhydrase inhibition

A novel class of compounds designed to hit two anti-tumour targets, G-quadruplex structures and human carbonic anhydrases (hCAs) IX and XII is proposed. The induction/stabilisation of G-quadruplex structures by small molecules has emerged as an anticancer strategy, disrupting telomere maintenance and reducing oncogene expression. hCAs IX and XII are well-established anti-tumour targets, upregulated in many hypoxic tumours and contributing to metastasis. The ligands reported feature a berberine G-quadruplex stabiliser scaffold connected to a moiety inhibiting hCAs IX and XII. In vitro experiments showed that our compounds selectively stabilise G-quadruplex structures and inhibit hCAs IX and XII. The crystal structure of a telomeric G-quadruplex in complex with one of these ligands was obtained, shedding light on the ligand/target interaction mode. The most promising ligands showed significant cytotoxicity against CA IX-positive HeLa cancer cells in hypoxia, and the ability to stabilise G-quadruplexes within tumour cells.

Enhanced Recognition of a Herbal Compound Epiberberine by a DNA Quadruplex-Duplex Structure

The small molecule epiberberine (EPI) is a natural alkaloid with versatile bioactivities against several diseases including cancer and bacterial infection. EPI can induce the formation of a unique binding pocket at the 5' side of a human telomeric G-quadruplex (HTG) sequence with four telomeric repeats (Q4), resulting in a nanomolar binding affinity (KD approximately 26 nM) with significant fluorescence enhancement upon binding. It is important to understand (1) how EPI binding affects HTG structural stability and (2) how enhanced EPI binding may be achieved through the engineering of the DNA binding pocket. In this work, the EPI-binding-induced HTG structure stabilization effect was probed by a peptide nucleic acid (PNA) invasion assay in combination with a series of biophysical techniques. We show that the PNA invasion-based method may be useful for the characterization of compounds binding to DNA (and RNA) structures under physiological conditions without the need to vary the solution temperature or buffer components, which are typically needed for structural stability characterization. Importantly, the combination of theoretical modeling and experimental quantification allows us to successfully engineer Q4 derivative Q4-ds-A by a simple extension of a duplex structure to Q4 at the 5' end. Q4-ds-A is an excellent EPI binder with a KD of 8 nM, with the binding enhancement achieved through the preformation of a binding pocket and a reduced dissociation rate. The tight binding of Q4 and Q4-ds-A with EPI allows us to develop a novel magnetic bead-based affinity purification system to effectively extract EPI from Rhizoma coptidis (Huang Lian) extracts.

A Phenotypic Approach to the Discovery of Potent G-Quadruplex Targeted Drugs

G-quadruplex (G4) sequences, which can fold into higher-order G4 structures, are abundant in the human genome and are over-represented in the promoter regions of many genes involved in human cancer initiation, progression, and metastasis. They are plausible targets for G4-binding small molecules, which would, in the case of promoter G4s, result in the transcriptional downregulation of these genes. However, structural information is currently available on only a very small number of G4s and their ligand complexes. This limitation, coupled with the currently restricted information on the G4-containing genes involved in most complex human cancers, has led to the development of a phenotypic-led approach to G4 ligand drug discovery. This approach was illustrated by the discovery of several generations of tri- and tetra-substituted naphthalene diimide (ND) ligands that were found to show potent growth inhibition in pancreatic cancer cell lines and are active in in vivo models for this hard-to-treat disease. The cycles of discovery have culminated in a highly potent tetra-substituted ND derivative, QN-302, which is currently being evaluated in a Phase 1 clinical trial. The major genes whose expression has been down-regulated by QN-302 are presented here: all contain G4 propensity and have been found to be up-regulated in human pancreatic cancer. Some of these genes are also upregulated in other human cancers, supporting the hypothesis that QN-302 is a pan-G4 drug of potential utility beyond pancreatic cancer.

A dual DNA-binding conjugate that selectively recognizes G-quadruplex structures

G-quadruplex (G4) structures play roles in various biological processes, but the challenge lies in specific targeting. To address this, we synthesized a conjugate capable of recognizing the G4 structure and its proximal duplex. Our conjugate can enable recognition of specific G4s in the human genome to understand and target those structures.

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.

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.

Structure-activity relationships for the G-quadruplex-targeting experimental drug QN-302 and two analogues probed with comparative transcriptome profiling and molecular modeling

The tetrasubstituted naphthalene diimide compound QN-302 binds to G-quadruplex (G4) DNA structures. It shows high potency in pancreatic ductal adenocarcinoma (PDAC) cells and inhibits the transcription of cancer-related genes in these cells and in PDAC animal models. It is currently in Phase 1a clinical evaluation as an anticancer drug. A study of structure-activity relationships of QN-302 and two related analogues (CM03 and SOP1247) is reported here. These have been probed using comparisons of transcriptional profiles from whole-genome RNA-seq analyses, together with molecular modelling and molecular dynamics simulations. Compounds CM03 and SOP1247 differ by the presence of a methoxy substituent in the latter: these two compounds have closely similar transcriptional profiles. Whereas QN-302 (with an additional benzyl-pyrrolidine group), although also showing down-regulatory effects in the same cancer-related pathways, has effects on distinct genes, for example in the hedgehog pathway. This distinctive pattern of genes affected by QN-302 is hypothesized to contribute to its superior potency compared to CM03 and SOP1247. Its enhanced ability to stabilize G4 structures has been attributed to its benzyl-pyrrolidine substituent fitting into and filling most of the space in a G4 groove compared to the hydrogen atom in CM03 or the methoxy group substituent in SOP1247.

Steric hindrance and structural flexibility shape the functional properties of a guanine-rich oligonucleotide

Ligand/protein molecular recognition involves a dynamic process, whereby both partners require a degree of structural plasticity to regulate the binding/unbinding event. Here, we present the characterization of the interaction between a highly dynamic G-rich oligonucleotide, M08s-1, and its target protein, human α-thrombin. M08s-1 is the most active anticoagulant aptamer selected thus far. Circular dichroism and gel electrophoresis analyses indicate that both intramolecular and intermolecular G-quadruplex structures are populated in solution. The presence of thrombin stabilises the antiparallel intramolecular chair-like G-quadruplex conformation, that provides by far the main contribution to the biological activity of the aptamer. The crystal structure of the thrombin-oligonucleotide complex reveals that M08s-1 adopts a kinked structural organization formed by a G-quadruplex domain and a long duplex module, linked by a stretch of five purine bases. The quadruplex motif hooks the exosite I region of thrombin and the duplex region is folded towards the surface of the protein. This structural feature, which has never been observed in other anti-exosite I aptamers with a shorter duplex motif, hinders the approach of a protein substrate to the active site region and may well explain the significant increase in the anticoagulant activity of M08s-1 compared to the other anti-exosite I aptamers.

Conformational Plasticity of Parallel G-Quadruplex─Implications on Duplex-Quadruplex Motifs

DNA G-quadruplexes are essential motifs in molecular biology performing a wide range of functions enabled by their unique and diverse structures. In this study, we focus on the conformational plasticity of the most abundant and biologically relevant parallel G-quadruplex topology. A multipronged approach of structure survey, solution-state NMR spectroscopy, and molecular dynamics simulations unravels subtle yet essential features of the parallel G-quadruplex topology. Stark differences in flexibility are observed for the nucleotides depending upon their positioning in the tetrad planes that are intricately correlated with the conformational sampling of the propeller loop. Importantly, the terminal nucleotides in the 5'-end versus the 3'-end of the parallel quadruplex display differential dynamics that manifests their ability to accommodate a duplex on either end of the G-quadruplex. The conformational plasticity characterized in this study provides essential cues toward biomolecular processes such as small molecular binding, intermolecular quadruplex stacking, and implications on how a duplex influences the structure of a neighboring quadruplex.

The Potent G-Quadruplex-Binding Compound QN-302 Downregulates S100P Gene Expression in Cells and in an In Vivo Model of Pancreatic Cancer

The naphthalene diimide compound QN-302, designed to bind to G-quadruplex DNA sequences within the promoter regions of cancer-related genes, has high anti-proliferative activity in pancreatic cancer cell lines and anti-tumor activity in several experimental models for the disease. We show here that QN-302 also causes downregulation of the expression of the S100P gene and the S100P protein in cells and in vivo. This protein is well established as being involved in key proliferation and motility pathways in several human cancers and has been identified as a potential biomarker in pancreatic cancer. The S100P gene contains 60 putative quadruplex-forming sequences, one of which is in the promoter region, 48 nucleotides upstream from the transcription start site. We report biophysical and molecular modeling studies showing that this sequence forms a highly stable G-quadruplex in vitro, which is further stabilized by QN-302. We also report transcriptome analyses showing that S100P expression is highly upregulated in tissues from human pancreatic cancer tumors, compared to normal pancreas material. The extent of upregulation is dependent on the degree of differentiation of tumor cells, with the most poorly differentiated, from more advanced disease, having the highest level of S100P expression. The experimental drug QN-302 is currently in pre-IND development (as of Q1 2023), and its ability to downregulate S100P protein expression supports a role for this protein as a marker of therapeutic response in pancreatic cancer. These results are also consistent with the hypothesis that the S100P promoter G-quadruplex is a potential therapeutic target in pancreatic cancer at the transcriptional level for QN-302.

Homopurine guanine-rich sequences in complex with N-methyl mesoporphyrin IX form parallel G-quadruplex dimers and display a unique symmetry tetrad

DNA can fold into G-quadruplexes (GQs), non-canonical secondary structures formed by π-π stacking of G-tetrads. GQs are important in many biological processes, which makes them promising therapeutic targets. We identified a 42-nucleotide long, purine-only G-rich sequence from human genome, which contains eight G-stretches connected by A and AAAA loops. We divided this sequence into five unique segments, four guanine stretches each, named GA1-5. In order to investigate the role of adenines in GQ structure formation, we performed biophysical and X-ray crystallographic studies of GA1-5 and their complexes with a highly selective GQ ligand, N-methyl mesoporphyrin IX (NMM). Our data indicate that all variants form parallel GQs whose stability depends on the number of flexible AAAA loops. GA1-3 bind NMM with 1:1 stoichiometry. The Ka for GA1 and GA3 is modest, ∼0.3 μM -1, and that for GA2 is significantly higher, ∼1.2 μM -1. NMM stabilizes GA1-3 by 14.6, 13.1, and 7.0 °C, respectively, at 2 equivalents. We determined X-ray crystal structures of GA1-NMM (1.98 Å resolution) and GA3-NMM (2.01 Å). The structures confirm the parallel topology of GQs with all adenines forming loops and display NMM binding at the 3' G-tetrad. Both complexes dimerize through the 5' interface. We observe two novel structural features: 1) a 'symmetry tetrad' at the dimer interface, which is formed by two guanines from each GQ monomer and 2) a NMM dimer in GA1-NMM. Our structural work confirms great flexibility of adenines as structural elements in GQ formation and contributes greatly to our understanding of the structural diversity of GQs and their modes of interaction with small molecule ligands.

High-affinity binding at quadruplex-duplex junctions: rather the rule than the exception

Quadruplex-duplex (Q-D) junctions constitute unique structural motifs in genomic sequences. Through comprehensive calorimetric as well as high-resolution NMR structural studies, Q-D junctions with a hairpin-type snapback loop coaxially stacked onto an outer G-tetrad were identified to be most effective binding sites for various polycyclic quadruplex ligands. The Q-D interface is readily recognized by intercalation of the ligand aromatic core structure between G-tetrad and the neighboring base pair. Based on the thermodynamic and structural data, guidelines for the design of ligands with enhanced selectivity towards a Q-D interface emerge. Whereas intercalation at Q-D junctions mostly outcompete stacking at the quadruplex free outer tetrad or intercalation between duplex base pairs to varying degrees, ligand side chains considerably contribute to the selectivity for a Q-D target over other binding sites. In contrast to common perceptions, an appended side chain that additionally interacts within the duplex minor groove may confer only poor selectivity. Rather, the Q-D selectivity is suggested to benefit from an extension of the side chain towards the exposed part of the G-tetrad at the junction. The presented results will support the design of selective high-affinity binding ligands for targeting Q-D interfaces in medicinal but also technological applications.

Stability and Existence of Noncanonical I-motif DNA Structures in Computer Simulations Based on Atomistic and Coarse-Grained Force Fields

Cytosine-rich DNA sequences are able to fold into noncanonical structures, in which semi-protonated cytosine pairs develop extra hydrogen bonds, and these bonds are responsible for the overall stability of a structure called the i-motif. The i-motif can be formed in many regions of the genome, but the most representative is the telomeric region in which the CCCTAA sequences are repeated thousands of times. The ability to reverse folding/unfolding in response to pH change makes the above sequence and i-motif very promising components of nanomachines, extended DNA structures, and drug carriers. Molecular dynamics analysis of such structures is highly beneficial due to direct insights into the microscopic structure of the considered systems. We show that Amber force fields for DNA predict the stability of the i-motif over a long timescale; however, these force fields are not able to predict folding of the cytosine-rich sequences into the i-motif. The reason is the kinetic partitioning of the folding process, which makes the transitions between various intermediates too time-consuming in atomistic force field representation. Application of coarse-grained force fields usually highly accelerates complex structural transitions. We, however, found that three of the most popular coarse-grained force fields for DNA (oxDNA, 3SPN, and Martini) were not able to predict the stability of the i-motif structure. Obviously, they were not able to accelerate the folding of unfolded states into an i-motif. This observation must be strongly highlighted, and the need to develop suitable extensions of coarse-grained force fields for DNA is pointed out. However, it will take a great deal of effort to successfully solve these problems.

Indoloquinoline Ligands Favor Intercalation at Quadruplex-Duplex Interfaces

Quadruplex‐duplex (Q‐D) junctions are increasingly considered promising targets for medicinal and technological applications. Here, a Q‐D hybrid with a hairpin‐type snapback loop coaxially stacked onto the quadruplex 3’‐outer tetrad was designed and employed as a target structure for the indoloquinoline ligand SYUIQ‐5. NMR spectral analysis demonstrated high‐affinity binding of the ligand at the quadruplex‐duplex interface with association constants determined by isothermal titration calorimetry of about 10⁷ M⁻¹ and large exothermicities ΔH° of −14 kcal/mol in a 120 mM K⁺ buffer at 40 °C. Determination of the ligand‐bound hybrid structure revealed intercalation of SYUIQ‐5 between 3’‐outer tetrad and the neighboring CG base pair, maximizing π–π stacking as well as electrostatic interactions with guanine carbonyl groups in close vicinity to the positively charged protonated quinoline nitrogen of the tetracyclic indoloquinoline. Exhibiting considerable flexibility, the SYUIQ‐5 sidechain resides in the duplex minor groove. Based on comparative binding studies with the non‐substituted N5‐methylated indoloquinoline cryptolepine, the sidechain is suggested to confer additional affinity and to fix the alignment of the intercalated indoloquinoline aromatic core. However, selectivity for the Q‐D junction mostly relies on the geometry and charge distribution of the indoloquinoline ring system. The presented results are expected to provide valuable guidelines for the design of ligands specifically targeting Q‐D interfaces.

Impact of a Single Nucleotide Change or Non-Nucleoside Modifications in G-Rich Region on the Quadruplex-Duplex Hybrid Formation

In this paper, a method to discriminate between two target RNA sequences that differ by one nucleotide only is presented. The method relies on the formation of alternative structures, i.e., quadruplex–duplex hybrid (QDH) and duplex with dangling ends (Dss), after hybridization of DNA or RNA G-rich oligonucleotides with target sequences containing 5′–GGGCUGG–3′ or 5′–GGGCGGG–3′ fragments. Using biophysical methods, we studied the effect of oligonucleotide types (DNA, RNA), non-nucleotide modifications (aliphatic linkers or abasic), and covalently attached G4 ligand on the ability of G-rich oligonucleotides to assemble a G-quadruplex motif. We demonstrated that all examined non-nucleotide modifications could mimic the external loops in the G-quadruplex domain of QDH structures without affecting their stability. Additionally, some modifications, in particular the presence of two abasic residues in the G-rich oligonucleotide, can induce the formation of non-canonical QDH instead of the Dss structure upon hybridization to a target sequence containing the GGGCUGG motif. Our results offer new insight into the sequential requirements for the formation of G-quadruplexes and provide important data on the effects of non-nucleotide modifications on G-quadruplex formation.

Structure of i-Motif/Duplex Junctions at Neutral pH

We report here the three-dimensional structure of an i-motif/duplex junction, determined by NMR methods at neutral pH. By including a minor groove tetrad at one side of the C:C⁺ stack of a monomeric i-motif, and a stem/loop hairpin at the other side, we have designed stable DNA constructs in which i-DNA and B-DNA regions coexist in a wide range of experimental conditions. This study demonstrates that i- and B-DNA are structurally compatible, giving rise to a distinctive fold with peculiar groove shapes. The effect of different residues at the i-motif/duplex interface has been explored. We also show that these constructs can be adapted to sequences of biological relevance, like that found in the promoter region of the KRAS oncogene.

Spatial Matching Selectivity and Solution Structure of Organic-Metal Hybrid to Quadruplex-Duplex Hybrid

The sequence-dependent DNA secondary structures possess structure polymorphism. To date, studies on regulated ligands mainly focus on individual DNA secondary topologies, while lack focus on quadruplex-duplex hybrids (QDHs). Here, we design an organic-metal hybrid ligand L¹ Pt(dien), which matches and selectively binds one type of QDHs with lateral duplex stem-loop (QLDH) with high affinity, while shows poor affinity for other QDHs and individual G4 or duplex DNA. The solution structure of QLDH MYT1L-L¹ Pt(dien) complex was determined by NMR. The structure reveals that L¹ Pt(dien) presents a chair-type conformation, whose large aromatic "chair surface" intercalates into the G-quadruplex-duplex interface via π-π stacking and "backrest" platinum unit interacts with duplex region through hydrogen bonding and electrostatic interactions, showing a highly matched lock-key binding mode. Our work provided guidance for spatial matching design of selectively targeting ligands to QDH structures.

Protonation of Cytosine-Rich Telomeric DNA Fragments by Carboxylated Carbon Nanotubes: Insights from Computational Studies

In this work, we studied, using computational methods, the protonation reactions of telomeric DNA fragments being due to interaction with carboxylated carbon nanotubes. The applied computational methodology is divided into two stages. (i) Using classical molecular dynamics, we generated states in which carboxyl groups are brought to the vicinity of nitrogen atoms within the cytosine rings belonging to the DNA duplex. (ii) From these states, we selected two systems for systematic quantum chemical studies aimed at the analysis of proton-transfer reactions between the carboxyl groups and nitrogen atoms within the cytosine rings. Results of molecular dynamics calculations led to the conclusion that sidewall-functionalized carbon nanotubes deliver carboxyl groups slightly more effectively than the on-tip-functionalized ones. The latter can provide carboxyl groups in various arrangements and more diverse quality of approach of carboxyl groups to the cytosines; however, the differences between various arrangements of carboxyl groups are still not big. It was generally observed that narrow nanotubes can access the cytosine pocket easier than wider ones. Quantum chemical calculations led however to the conclusion that a direct proton transfer from the carboxyl group to the nitrogen atom within the cytosine ring is impossible under normal conditions. Precisely, we detected either very high activation barrier for the proton-transfer reaction or instability of the reaction product, i.e., its spontaneous decomposition toward reaction substrates.

Opposite Effects of Potassium Ions on the Thermal Stability of i-Motif DNA in Different Buffer Systems

i-motifs are noncanonical DNA structures formed via the stack of intercalating hemi-protonated C⁺: C base pairs in C-rich DNA strands and play essential roles in the regulation of gene expression. Here, we systematically investigated the impacts of K⁺ on i-motif DNA folding using different buffer systems. We found that i-motif structures display very different T _m values at the same pH and ion strength in different buffer systems. More importantly, K⁺ disrupts the i-motif formed in the MES and Bis-Tris buffer; however, K⁺ stabilizes the i-motif in phosphate, citrate, and sodium cacodylate buffers. Next, we selected phosphate buffer and confirmed by single-molecule fluorescence resonance energy transfer that K⁺ indeed has the stabilizing effect on the folding of i-motif DNA from pH 5.8 to 8.0. Nonetheless, circular dichroism spectra further indicate that the structures formed by i-motif sequences at high K⁺ concentrations at neutral and alkaline pH are not i-motif but other types of higher-order structures and most likely C-hairpins. We finally proposed the mechanisms of how K⁺ plays the opposite roles in different buffer systems. The present study may provide new insights into our understanding of the formation and stability of i-motif DNA.

Bulges in left-handed G-quadruplexes

G-quadruplex (G4) DNA structures with a left-handed backbone progression have unique and conserved structural features. Studies on sequence dependency of the structures revealed the prerequisites and some minimal motifs required for left-handed G4 formation. To extend the boundaries, we explore the adaptability of left-handed G4s towards the existence of bulges. Here we present two X-ray crystal structures and an NMR solution structure of left-handed G4s accommodating one, two and three bulges. Bulges in left-handed G4s show distinct characteristics as compared to those in right-handed G4s. The elucidation of intricate structural details will help in understanding the possible roles and limitations of these unique structures.

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.

Coexistence of two quadruplex-duplex hybrids in the PIM1 gene

The triple-negative breast cancer (TNBC), a subtype of breast cancer which lacks of targeted therapies, exhibits a poor prognosis. It was shown recently that the PIM1 oncogene is highly related to the proliferation of TNBC cells. A quadruplex-duplex hybrid (QDH) forming sequence was recently found to exist near the transcription start site of PIM1. This structure could be an attractive target for regulation of the PIM1 gene expression and thus the treatment of TNBC. Here, we present the solution structures of two QDHs that could coexist in the human PIM1 gene. Form 1 is a three-G-tetrad-layered (3+1) G-quadruplex containing a propeller loop, a lateral loop and a stem-loop made up of three G•C Watson-Crick base pairs. On the other hand, Form 2 is an anti-parallel G-quadruplex comprising two G-tetrads and a G•C•G•C tetrad; the structure has three lateral loops with the middle stem-loop made up of two Watson-Crick G•C base pairs. These structures provide valuable information for the design of G-quadruplex-specific ligands for PIM1 transcription regulation.

Biophysical and X-ray structural studies of the (GGGTT)3GGG G-quadruplex in complex with N-methyl mesoporphyrin IX

The G-quadruplex (GQ) is a well-studied non-canonical DNA structure formed by G-rich sequences found at telomeres and gene promoters. Biological studies suggest that GQs may play roles in regulating gene expression, DNA replication, and DNA repair. Small molecule ligands were shown to alter GQ structure and stability and thereby serve as novel therapies, particularly against cancer. In this work, we investigate the interaction of a G-rich sequence, 5'-GGGTTGGGTTGGGTTGGG-3' (T1), with a water-soluble porphyrin, N-methyl mesoporphyrin IX (NMM) via biophysical and X-ray crystallographic studies. UV-vis and fluorescence titrations, as well as a Job plot, revealed a 1:1 binding stoichiometry with an impressively tight binding constant of 30-50 μM-1 and ΔG298 of -10.3 kcal/mol. Eight extended variants of T1 (named T2 -T9) were fully characterized and T7 was identified as a suitable candidate for crystallographic studies. We solved the crystal structures of the T1- and T7-NMM complexes at 2.39 and 2.34 Å resolution, respectively. Both complexes form a 5'-5' dimer of parallel GQs capped by NMM at the 3' G-quartet, supporting the 1:1 binding stoichiometry. Our work provides invaluable details about GQ-ligand binding interactions and informs the design of novel anticancer drugs that selectively recognize specific GQs and modulate their stability for therapeutic purposes.

Quadruplex-Duplex Junction: A High-Affinity Binding Site for Indoloquinoline Ligands

A parallel quadruplex derived from the Myc promoter sequence was extended by a stem-loop duplex at either its 5'- or 3'-terminus to mimic a quadruplex-duplex (Q-D) junction as a potential genomic target. High-resolution structures of the hybrids demonstrate continuous stacking of the duplex on the quadruplex core without significant perturbations. An indoloquinoline ligand carrying an aminoalkyl side chain was shown to bind the Q-D hybrids with a very high affinity in the order K_a ≈10⁷  m^-1 irrespective of the duplex location at the quadruplex 3'- or 5'-end. NMR chemical shift perturbations identified the tetrad face of the Q-D junction as specific binding site for the ligand. However, calorimetric analyses revealed significant differences in the thermodynamic profiles upon binding to hybrids with either a duplex extension at the quadruplex 3'- or 5'-terminus. A large enthalpic gain and considerable hydrophobic effects are accompanied by the binding of one ligand to the 3'-Q-D junction, whereas non-hydrophobic entropic contributions favor binding with formation of a 2:1 ligand-quadruplex complex in case of the 5'-Q-D hybrid.

Duplex formation in a G-quadruplex bulge

Beyond the consensus definition of G-quadruplex-forming motifs with tracts of continuous guanines, G-quadruplexes harboring bulges in the G-tetrad core are prevalent in the human genome. Here, we study the incorporation of a duplex hairpin within a bulge of a G-quadruplex. The NMR solution structure of a G-quadruplex containing a duplex bulge was resolved, revealing the structural details of the junction between the duplex bulge and the G-quadruplex. Unexpectedly, instead of an orthogonal connection the duplex stem was observed to stack below the G-quadruplex forming a unique quadruplex–duplex junction. Breaking up of the immediate base pair step at the junction, coupled with a narrowing of the duplex groove within the context of the bulge, led to a progressive transition between the quadruplex and duplex segments. This study revealed that a duplex bulge can be formed at various positions of a G-quadruplex scaffold. In contrast to a non-structured bulge, the stability of a G-quadruplex slightly increases with an increase in the duplex bulge size. A G-quadruplex structure containing a duplex bulge of up to 33 nt in size was shown to form, which was much larger than the previously reported 7-nt bulge. With G-quadruplexes containing duplex bulges representing new structural motifs with potential biological significance, our findings would broaden the definition of potential G-quadruplex-forming sequences.

Effects of Length and Loop Composition on Structural Diversity and Similarity of (G3TG3NmG3TG3) G-Quadruplexes

A G-rich sequence containing three loops to connect four G-tracts with each ≥2 guanines can possibly form G-quadruplex structures. Given that all G-quadruplex structures comprise the stacking of G-quartets, the loop sequence plays a major role on their folding topology and thermal stability. Here circular dichroism, NMR, and PAGE are used to study the effect of loop length and base composition in the middle loop, and a single base difference in loop 1 and 3 on G-quadruplex formation of (G₃HG₃N_mG₃HG₃) sequences with and without flanking nucleotides, where H is T, A, or C and N is T, A, C, or G. In addition, melting curve for G-quadruplex unfolding was used to provide relatively thermal stability of G-quadruplex structure after the addition of K⁺ overnight. We further studied the effects of K⁺ concentration on their stability and found structural changes in several sequences. Such (G₃HG₃N_mG₃HG₃) configuration can be found in a number of native DNA sequences. The study of structural diversity and similarity from these sequences may allow us to establish the correlation between model sequences and native sequences. Moreover, several sequences upon interaction with a G-quadruplex ligand, BMVC, show similar spectral change, implying that structural similarity is crucial for drug development.

Cyclic Automated Model Building (CAB) Applied to Nucleic Acids

Obtaining high-quality models for nucleic acid structures by automated model building programs (AMB) is still a challenge. The main reasons are the rather low resolution of the diffraction data and the large number of rotatable bonds in the main chains. The application of the most popular and documented AMB programs (e.g., PHENIX.AUTOBUILD, NAUTILUS and ARP/wARP) may provide a good assessment of the state of the art. Quite recently, a cyclic automated model building (CAB) package was described; it is a new AMB approach that makes the use of BUCCANEER for protein model building cyclic without modifying its basic algorithms. The applications showed that CAB improves the efficiency of BUCCANEER. The success suggested an extension of CAB to nucleic acids—in particular, to check if cyclically including NAUTILUS in CAB may improve its effectiveness. To accomplish this task, CAB algorithms designed for protein model building were modified to adapt them to the nucleic acid crystallochemistry. CAB was tested using 29 nucleic acids (DNA and RNA fragments). The phase estimates obtained via molecular replacement (MR) techniques were automatically submitted to phase refinement and then used as input for CAB. The experimental results from CAB were compared with those obtained by NAUTILUS, ARP/wARP and PHENIX.AUTOBUILD.

The DNA secondary structures at telomeres and genome instability

Telomeric DNA are TTAGGG tandem repeats, which are susceptible for oxidative DNA damage and hotspot regions for formation of DNA secondary structures such as t-loop, D-loop, G-quadruplexes (G4), and R-loop. In the past two decades, unique DNA or RNA secondary structures at telomeres or some specific regions of genome have become promising therapeutic targets. G-quadruplex and R-loops at telomeres or transcribed regions of genome have been considered as the potential targets for cancer therapy. Here we discuss the potentials to target the secondary structures (G4s and R-loops) in genome as therapy approaches.

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.

Interaction of Human Telomeric i-Motif DNA with Single-Walled Carbon Nanotubes: Insights from Molecular Dynamics Simulations

This work deals with molecular dynamics simulations of human telomeric i-motif DNA interacting with functionalized single-walled carbon nanotubes. We study two kinds of i-motifs differing by the protonation state of cytosines, i.e., unprotonated ones representative to neutral pH and with half of the cytosines protonated and representative to acidic conditions. These i-motifs interact with two kinds of carbon nanotubes differing mainly in chirality (diameter), i.e., (10, 0) and (20, 0). Additionally, these nanotubes were on-tip functionalized by amino groups or by guanine- containing residues. We found that protonated i-motif adsorbs strongly, although not specifically, on the nanotube surfaces with its 3' and 5' ends directed toward the surface and that adsorption does not affect the i-motif shape and hydrogen bonds existing between C:C⁺ pairs. The functional groups on the nanotube tips have minimal effect either on position of i-motif or on its binding strength. Unprotonated i-motif, in turn, deteriorates significantly during interaction with the nanotubes and its binding strength is rather high as well. We found that (10, 0) nanotubes destroy the i-motif shape faster than (20, 0). Moreover the i-motif either tries to wrap the nanotube or migrates to its tip and becomes immobilized due to interaction with guanine residue localized on the nanotube tip and attempts to incorporate its 3' end into the nanotube interior. No hydrogen bonds exist within the unprotonated i-motif prior to and after adsorption on the nanotube. Thus, carbon nanotubes do not improve the stability of unprotonated i-motif due to simple adsorption or just physical interactions. We hypothesize that the stabilizing effect of carbon nanotubes reported in the literature is due to proton transfer from the functional group in the nanotube to cytosines and subsequent formation of C:C⁺ pairs.

Structural properties and influence of solvent on the stability of telomeric four-stranded i-motif DNA

Repetitive cytosine rich i-motif forming sequences are abundant in the telomere, centromere and promoters of several oncogenes and in some instances are known to regulate transcription and gene expression. The in vivo existence of i-motif structures demands further insight into the factors affecting their formation and stability and development of better understanding of their gene regulatory functions. Most prior studies characterizing the conformational dynamics of i-motifs are based on i-motif forming synthetic constructs. Here, we present a systematic study on the stability and structural properties of biologically relevant i-motifs of telomeric and centromeric repeat fragments. Our results based on molecular dynamics simulations and quantum chemical calculations indicate that along with base pairing interactions within the i-motif core the overall folded conformation is associated with the stable C-HO sugar "zippers" in the narrow grooves and structured water molecules along the wide grooves. The stacked geometry of the hemi-protonated cytosine pairs within the i-motif core is mainly governed by the repulsive base stacking interaction. The loop sequence can affect the structural dynamics of the i-motif by altering the loop motion and backbone conformation. Overall this study provides microscopic insight into the i-motif structure that will be helpful to understand the structural aspect of mechanisms of gene regulation by i-motif DNA.

Several structural motifs cooperate in determining the highly effective anti-thrombin activity of NU172 aptamer

Despite aptamers are very promising alternative to antibodies, very few of them are under clinical trials or are used as drugs. Among them, NU172 is currently in Phase II as anticoagulant in heart disease treatments. It inhibits thrombin activity much more effectively than TBA, the best-known thrombin binding aptamer. The crystal structure of thrombin-NU172 complex reveals a bimodular duplex/quadruplex architecture for the aptamer, which binds thrombin exosite I through a highly complementary surface involving all three loops of the G-quadruplex module. Although the duplex domain does not interact directly with thrombin, the features of the duplex/quadruplex junction and the solution data on two newly designed NU172 mutants indicate that the duplex moiety is important for the optimization of the protein-ligand interaction and for the inhibition of the enzyme activity. Our work discloses the structural features determining the inhibition of thrombin by NU172 and put the basis for the design of mutants with improved properties.

Submolecular dissection reveals strong and specific binding of polyamide-pyridostatin conjugates to human telomere interface

To modulate biological functions, G-quadruplexes in genome are often non-specifically targeted by small molecules. Here, specificity is increased by targeting both G-quadruplex and its flanking duplex DNA in a naturally occurring dsDNA-ssDNA telomere interface using polyamide (PA) and pyridostatin (PDS) conjugates (PA-PDS). We innovated a single-molecule assay in which dissociation constant (Kd) of the conjugate can be separately evaluated from the binding of either PA or PDS. We found Kd of 0.8 nM for PA-PDS, which is much lower than PDS (Kd ∼ 450 nM) or PA (Kd ∼ 35 nM). Functional assays further indicated that the PA-PDS conjugate stopped the replication of a DNA polymerase more efficiently than PA or PDS. Our results not only established a new method to dissect multivalent binding into actions of individual monovalent components, they also demonstrated a strong and specific G-quadruplex targeting strategy by conjugating highly specific duplex-binding molecules with potent quadruplex ligands.

Recent Progress of Targeted G-Quadruplex-Preferred Ligands Toward Cancer Therapy

A G-quadruplex (G4) is a well-known nucleic acid secondary structure comprising guanine-rich sequences, and has profound implications for various pharmacological and biological events, including cancers. Therefore, ligands interacting with G4s have attracted great attention as potential anticancer therapies or in molecular probe applications. To date, a large variety of DNA/RNA G4 ligands have been developed by a number of laboratories. As protein-targeting drugs face similar situations, G-quadruplex-interacting drugs displayed low selectivity to the targeted G-quadruplex structure. This low selectivity could cause unexpected effects that are usually reasons to halt the drug development process. In this review, we address the recent research on synthetic G4 DNA-interacting ligands that allow targeting of selected G4s as an approach toward the discovery of highly effective anticancer drugs.

Binding of Telomestatin, TMPyP4, BSU6037, and BRACO19 to a Telomeric G-Quadruplex-Duplex Hybrid Probed by All-Atom Molecular Dynamics Simulations with Explicit Solvent

A promising anticancer therapeutic strategy is the stabilization of telomeric G-quadruplexes using G-quadruplex-binding small molecules. Although many G-quadruplex-specific ligands have been developed, their low potency and selectivity to G-quadruplexes over duplex remains unsolved. Recently, a crystal structure of a telomeric 3' quadruplex-duplex hybrid was reported and the quadruplex-duplex interface was suggested to a good target to address the issues. However, there are no high-resolution complex structures reported for G-quadruplex ligands except for a docked BSU6037. In this study, molecular dynamic (MD) binding simulations with a free ligand were used to study binding poses and dynamics of four representative ligands: telomestatin, TMPyP4, BSU6037, and BRACO19. The MD data showed that BSU6037 was able to fully intercalate into the interface whereas TMPyP4 and BRACO19 could only maintain partial intercalation into the interface and telomestatin only binds at the quadruplex and duplex ends. Both linear ligands, BSU6037 and BRACO19, were able to interact with the interface, yet they were not selective over duplex DNA. The DNA geometry, binding modes, and binding pathways were systematically characterized, and the binding energy was calculated and compared for each system. The interaction of the ligands to the interface was by the means of an induced-fit binding mechanism rather than a lock-key mechanism, consisting of the DNA unfolding at the interface to allow entrance of the drug and then the refolding and repacking of the DNA and the ligand to further stabilize the G-quadruplex. On the basis of the findings in this study, modifications were suggested to optimize the interface binding for TMPyp4 and telomestatin.

Ligand Design to Acquire Specificity to Intended G-Quadruplex Structures

A G-quadruplex is a nucleic acid secondary structure that is adopted by guanine-rich sequences, and is considered to be relevant in various pharmacological and biological contexts. G-Quadruplexes have also attracted great attention in the field of DNA nanotechnology because of their extremely high thermal stability and the availability of many defined structures. To date, a large repertory of DNA/RNA G-quadruplex-interactive ligands has been developed by numerous laboratories. Several relevant reviews have also been published that have helped researchers to grasp the full scope of G-quadruplex research from its outset to the present. This review focuses on the G-quadruplex ligands that allow targeting of specific G-quadruplexes. Moreover, unique ligands, successful methodologies, and future perspectives in relation to specific G-quadruplex recognition are also addressed.

Principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign metal ligands

The characterization of the interactions between biological macromolecules (proteins and nucleic acids) and metal-based drugs is a fundamental prerequisite for understanding their mechanisms of action. X-ray crystallography enables the structural analysis of such complexes with atomic level detail. However, this approach requires the preparation of highly diffracting single crystals, the measurement of diffraction patterns and the structural analysis and interpretation of macromolecule-metal interactions from electron density maps. In this review, we describe principles and methods used to grow and optimize crystals of protein-metallodrug adducts, to determine metal binding sites and to assign and validate metal ligands. Examples from the literature and experience in our own laboratory are provided and key challenges are described, notably crystallization and molecular model refinement against the X-ray diffraction data.

G-quadruplexes in Prague: A Bohemian Rhapsody

The Sixth International Meeting on Quadruplex Nucleic Acids was held at the Hotel Internationale in Prague, Czech Republic from 31 May - 3 June 2017. A vibrant interdisciplinary community of over 300 scientists gathered to share their newest results in this exciting field and exchange ideas for further investigations.

Interaction of a gold(i) dicarbene anticancer drug with human telomeric DNA G-quadruplex: solution and computationally aided X-ray diffraction analysis

Solution and solid-state data give a quite clear picture for a bis carbene gold(i) complex having perspective anticancer properties.

Role of the Benzodioxole Group in the Interactions between the Natural Alkaloids Chelerythrine and Coptisine and the Human Telomeric G-Quadruplex DNA. A Multiapproach Investigation

The binding properties toward the human telomeric G-quadruplex of the two natural alkaloids coptisine and chelerythrine were studied using spectroscopic techniques, molecular modeling, and X-ray diffraction analysis. The results were compared with reported data for the parent compounds berberine and sanguinarine. Spectroscopic studies showed modest, but different rearrangements of the DNA-ligand complexes, which can be explained considering particular stereochemical features for these alkaloids, in spite of the similarity of their skeletons. In fact, the presence of a dioxolo moiety rather than the two methoxy functions improves the efficiency of coptisine and sanguinarine in comparison to berberine and chelerythrine, and the overall stability trend is sanguinarine > chelerythrine ≈ coptisine > berberine. Accordingly, the X-ray diffraction analysis confirmed the involvement of the benzodioxolo groups in the coptisine/DNA binding by means of π···π, O···π, and CH···O interactions. Similar information is provided by modeling studies, which, additionally, evidenced reasons for the quadruplex vs double-helix selectivity shown by these alkaloids. Thus, the analyses shed light on the key role of the benzodioxolo moieties in strengthening the interaction with the G4-folded human telomeric sequence and indicated the superior G4 stabilizing properties of the benzophenanthridine scaffold with respect to the protoberberine one and conversely the better G4 vs dsDNA selectivity profile of coptisine over the other alkaloids.

G-Quadruplex Induction by the Hairpin Pyrrole-Imidazole Polyamide Dimer

The G-quadruplex (G4) is one type of higher-order structure of nucleic acids and is thought to play important roles in various biological events such as regulation of transcription and inhibition of DNA replication. Pyrrole-imidazole polyamides (PIPs) are programmable small molecules that can sequence-specifically bind with high affinity to the minor groove of double-stranded DNA (dsDNA). Herein, we designed head-to-head hairpin PIP dimers and their target dsDNA in a model G4-forming sequence. Using an electrophoresis mobility shift assay and transcription arrest assay, we found that PIP dimers could induce the structural change to G4 DNA from dsDNA through the recognition by one PIP dimer molecule of two duplex-binding sites flanking both ends of the G4-forming sequence. This induction ability was dependent on linker length. This is the first study to induce G4 formation using PIPs, which are known to be dsDNA binders. The results reported here suggest that selective G4 induction in native sequences may be achieved with PIP dimers by applying the same design strategy.

Involvement of G-triplex and G-hairpin in the multi-pathway folding of human telomeric G-quadruplex

G-quadruplex (G4) can be formed by G-rich DNA sequences that are widely distributed throughout the human genome. Although G-triplex and G-hairpin have been proposed as G4 folding intermediates, their formation still requires further investigation by experiments. Here, we employed single-molecule FRET to characterize the folding dynamics of G4 from human telomeric sequence. First, we observed four states during G4 folding initially assigned to be anti-parallel G4, G-triplex, G-hairpin and unfolded ssDNA. Then we constructed putative intra-strand G-triplex, G-hairpin structures and confirmed their existences in both NaCl and KCl. Further studies revealed those structures are going through dynamic transitions between different states and show relatively weak dependence on cations, unlike G4. Based on those results and molecular dynamics simulations, we proposed a multi-pathway folding mechanism for human telomeric G4. The present work may shed new light on our current understanding about the existence and stability of G4 intermediate states.