Truncated G-Quadruplex Isomers Cross-Talk with the Transcription Factors To Maintain Homeostatic Equilibria in c-MYC Transcription

Molecular dynamics simulations reveal the parallel stranded d(GGGA)3GGG DNA quadruplex folds via multiple paths from a coil-like ensemble

G-quadruplexes (G4s) are non-canonical nucleic acid structures that fold through complex processes. Characterization of the G4 folding landscape may help to elucidate biological roles of G4s but is challenging both experimentally and computationally. Here, we achieved complete folding of a three-quartet parallel DNA G4 with (GGGA)3GGG sequence using all-atom explicit-solvent enhanced-sampling molecular dynamics (MD) simulations. The simulations suggested early formation of guanine stacks in the G-tracts, which behave as semi-rigid blocks in the folding process. The folding continues via the formation of a collapsed compact coil-like ensemble. Structuring of the G4 from the coil then proceeds via various cross-like, hairpin, slip-stranded and two-quartet ensembles and can bypass the G-triplex structure. Folding of the parallel G4 does not appear to involve any salient intermediates and is a multi-pathway process. We also carried out an extended set of simulations of parallel G-hairpins. While parallel G-hairpins are extremely unstable when isolated, they are more stable inside the coil structure. On the methodology side, we show that the AMBER DNA force field predicts the folded G4 to be less stable than the unfolded ensemble, uncovering substantial force-field issues. Overall, we provide unique atomistic insights into the folding landscape of parallel-stranded G4 but also reveal limitations of current state-of-the-art MD techniques.

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-quadruplex structural dynamics at MAPK12 promoter dictates transcriptional switch to determine stemness in breast cancer

P38γ (MAPK12) is predominantly expressed in triple negative breast cancer cells (TNBC) and induces stem cell (CSC) expansion resulting in decreased survival of the patients due to metastasis. Abundance of G-rich sequences at MAPK12 promoter implied the functional probability to reverse tumorigenesis, though the formation of G-Quadruplex (G4) structures at MAPK12 promoter is elusive. Here, we identified two evolutionary consensus adjacent G4 motifs upstream of the MAPK12 promoter, forming parallel G4 structures. They exist in an equilibria between G4 and duplex, regulated by the binding turnover of Sp1 and Nucleolin that bind to these G4 motifs and regulate MAPK12 transcriptional homeostasis. To underscore the gene-regulatory functions of G4 motifs, we employed CRISPR-Cas9 system to eliminate G4s from TNBC cells and synthesized a naphthalene diimide (NDI) derivative (TGS24) which shows high-affinity binding to MAPK12-G4 and inhibits MAPK12 transcription. Deletion of G4 motifs and NDI compound interfere with the recruitment of the transcription factors, inhibiting MAPK12 expression in cancer cells. The molecular basis of NDI-induced G4 transcriptional regulation was analysed by RNA-seq analyses, which revealed that MAPK12-G4 inhibits oncogenic RAS transformation and trans-activation of NANOG. MAPK12-G4 also reduces CD44High/CD24Low population in TNBC cells and downregulates internal stem cell markers, arresting the stemness properties of cancer cells.

G-quadruplexes in cancer-related gene promoters: from identification to therapeutic targeting

INTRODUCTION

Guanine-rich DNA sequences can fold into four-stranded noncanonical secondary structures called G-quadruplexes (G4s) which are widely distributed in functional regions of the human genome, such as telomeres and gene promoter regions. Compelling evidence suggests their involvement in key genome functions such as gene expression and genome stability. Notably, the abundance of G4-forming sequences near transcription start sites suggests their potential involvement in regulating oncogenes.

AREAS COVERED

This review provides an overview of current knowledge on G4s in human oncogene promoters. The most representative G4-binding ligands have also been documented. The objective of this work is to present a comprehensive overview of the most promising targets for the development of novel and highly specific anticancer drugs capable of selectively impacting the expression of individual or a limited number of genes.

EXPERT OPINION

Modulation of G4 formation by specific ligands has been proposed as a powerful new tool to treat cancer through the control of oncogene expression. Actually, most of G4-binding small molecules seem to simultaneously target a range of gene promoter G4s, potentially influencing several critical driver genes in cancer, thus producing significant therapeutic benefits.

Why to target G-quadruplexes using peptides: Next-generation G4-interacting ligands

Guanine-rich oligonucleotides existing in both DNA and RNA are able to fold into four-stranded DNA secondary structures via Hoogsteen type hydrogen-bonding, where four guanines self-assemble into a square planar arrangement, which, when stacked upon each other, results in the formation of higher-order structures called G-quadruplexes. Their distribution is not random; they are more frequently present at telomeres, proto-oncogenic promoters, introns, 5'- and 3'-untranslated regions, stem cell markers, ribosome binding sites and so forth and are associated with various biological functions, all of which play a pivotal role in various incurable diseases like cancer and cellular ageing. Several studies have suggested that G-quadruplexes could not regulate biological processes by themselves; instead, various proteins take part in this regulation and can be important therapeutic targets. There are certain limitations in using whole G4-protein for therapeutics purpose because of its high manufacturing cost, laborious structure prediction, dynamic nature, unavailability for oral administration due to its degradation in the gut and inefficient penetration to reach the target site because of the large size. Hence, biologically active peptides can be the potential candidates for therapeutic intervention instead of the whole G4-protein complex. In this review, we aimed to clarify the biological roles of G4s, how we can identify them throughout the genome via bioinformatics, the proteins interacting with G4s and how G4-interacting peptide molecules may be the potential next-generation ligands for targeting the G4 motifs located in biologically important regions.

Taq-Polymerase Stop Assay to Determine Target Selectivity of G4 Ligands in Native Promoter Sequences of MYC, TERT, and KIT Oncogenes

Computational and high-throughput experimental methods predict thousands of potential quadruplex sequences (PQSs) in the human genome. Often these PQSs contain more than four G-runs, which introduce additional uncertainty into the conformational polymorphism of the G4 DNA. G4-specific ligands, which are currently being actively developed as potential anticancer agents or tools for studying G4 structures in genomes, may preferentially bind to specific G4 structures over the others that can be potentially formed in the extended G-rich genomic region. We propose a simple technique that identifies the sequences that tend to form G4 in the presence of potassium ions or a specific ligand. Thermostable DNA Taq-polymerase stop assay can detect the preferential position of the G4 -ligand binging within a long PQS-rich genomic DNA fragment. This technique was tested for four G4 binders PDS, PhenDC3, Braco-19, and TMPyP4 at three promoter sequences of MYC, KIT, and TERT that contain several PQSs each. We demonstrate that the intensity of polymerase pausing reveals the preferential binding of a ligand to particular G4 structures within the promoter. However, the strength of the polymerase stop at a specific site does not always correlate with the ligand-induced thermodynamic stabilization of the corresponding G4 structure.

Identification and characterization of a flexile G-quadruplex in the distal promoter region of stemness gene REX1

We have identified a parallel G-quadruplex (R1WT) in the distal promoter region (-821 base-pairs upstream of the TSS) of the pluripotent gene REX1. Through biophysical and biochemical approach, we have characterized the G-quadruplex (GQ) as a potential molecular switch that may control REX1 promoter activity to determine the transcriptional fate. Small- molecule interactive study of the monomeric form of R1WT (characterized as R1mut2) with TMPyP4 and BRACO-19 revealed GQ destabilization upon interaction with TMPyP4 and stabilization upon interaction with BRACO-19. This distinctive drug interactivity suggests the in cellulo R1WT to be a promising drug target. The endogenous existence of R1WT was confirmed by BG4 antibody derived chromatin immunoprecipitation experiment. Here in, we also report the endogenous interaction of GQ specific transcription factors (TFs) with R1WT region in the human chromatin of cancer cell. The wild-type G-quadruplex was found to interact with four important transcription factors, (i) specificity protein (Sp1) (ii) non-metastatic cell 2 (NM23-H2): a diphosphatase (iii) cellular nucleic acid binding protein (CNBP) and (iv) heterogenous nuclear ribonucleoprotein K (hnRNPK) in the REX1 promoter. In contrast, nucleolin protein (NCL) binding was found to be low to the said G-quadruplex. The flexibility of R1WT between folded and unfolded states, obtained from experimental and computational analysis strongly suggests R1WT to be an important gene regulatory element in the genome. It controls promoter DNA relaxation with the coordinated interaction of transcription factors, the deregulation of which seeds stemness characteristic in cancer cells for further metastatic progression.

Sequence driven interaction of amino acids in de-novo designed peptides determines c-Myc G-quadruplex unfolding inducing apoptosis in cancer cells

c-MYC proto-oncogene harbors a putative G-quadruplex structure (Pu27) at the NHEIII₁ domain, which can shuffle between transcriptional inhibitor quadruplex and transcriptionally active duplex. In cancer cells this quadruplex destabilization is preferred and NHEIII₁ domain assume a duplex topology thereby inducing c-MYC overexpression and tumorigenesis. Hence, the c-MYC quadruplex acts as an excellent target for anti-cancer therapy. Though researcher have tried to develop G-quadruplex targeted small molecules, work with G-quadruplex targeting peptides is very limited. Here we present a peptide that can bind to c-MYC quadruplex, destabilize the tetrad core, and permit the formation of a substantially different structure from the quartet core seen in the canonical G-quadruplexes. Such conformation potentially acted as a roadblock for transcription factors thereby reducing cMYC expression. This event sensitizes the cancer cell to activate apoptotic cascade via the c-MYC-VEGF-A-BCL2 axis. This study provides a detailed insight into the peptide-quadruplex interface that encourages better pharmacophore design to target dynamic quadruplex structure. We believe that our results will contribute to the development, characterization, and optimization of G-quadruplex binding peptides for potential clinical application.

Curcumin arrests G-quadruplex in the nuclear hyper-sensitive III1 element of c-MYC oncogene leading to apoptosis in metastatic breast cancer cells

c-MYC is deregulated in triple negative breast cancer (TNBC) pointing to be a promising biomarker for breast cancer treatment. Precise level of MYC expression is important in the control of cellular growth and proliferation. Designing of c-MYC-targeted antidotes to restore its basal level of cellular expression holds an optimistic approach towards anti-cancer treatment. MYC transcription is dominantly controlled by Nuclear Hypersensitive Element III-1 (NHE_III1) upstream of the promoter region possessing G-Quadruplex silencer element (Pu-27). We have investigated the selective binding-interaction profile of a natural phytophenolic compound Curcumin with native MYC G-quadruplex by conducting an array of biophysical experiments and in silico based Molecular Docking and Molecular Dynamic (MDs) simulation studies. Curcumin possesses immense anti-cancerous properties. We have observed significantly increased stability of MYC-G Quadruplex and thermodynamic spontaneity of Curcumin-MYC GQ binding with negative ΔG value. Transcription of MYC is tightly regulated by a complex mechanism involving promoters, enhancers and multiple transcription factors. We have used Curcumin as a model drug to understand the innate mechanism of controlling deregulated MYC back to its basal expression level. We have checked MYC-expression at transcriptional and translational level and proceeded for Chromatin Immuno-Precipitation assay (ChIP) to study the occupancy level of SP1, Heterogeneous nuclear ribonucleoprotein K (hnRNPK), Nucleoside Diphosphate Kinase 2 (NM23-H2) and Nucleolin at NHE_III1 upon Curcumin treatment of MDA-MB-231 cells. We have concluded that Curcumin binding tends to drive the equilibrium towards stable G-quadruplex formation repressing MYC back to its threshold-level. On retrospection of the synergistic effect of upregulated c-MYC and BCL-2 in cancer, we have also reported a new pathway [MYC-E2F-1-BCL-2-axis] through which Curcumin trigger apoptosis in cancer cells.Communicated by Ramaswamy H. Sarma.

Effects of G-Quadruplex-Binding Plant Secondary Metabolites on c-MYC Expression

Guanine-rich DNA sequences tending to adopt noncanonical G-quadruplex (G4) structures are over-represented in promoter regions of oncogenes. Ligands recognizing G4 were shown to stabilize these DNA structures and drive their formation regulating expression of corresponding genes. We studied the interaction of several plant secondary metabolites (PSMs) with G4s and their effects on gene expression in a cellular context. The binding of PSMs with G4s formed by the sequences of well-studied oncogene promoters and telomeric repeats was evaluated using a fluorescent indicator displacement assay. c-MYC G4 folding topology and thermal stability, as well as the PMS influence on these parameters, were demonstrated by UV-spectroscopy and circular dichroism. The effects of promising PSMs on c-MYC expression were assessed using luciferase reporter assay and qPR-PCR in cancer and immortalized cultured cells. The ability of PMS to multi-targeting cell signaling pathways was analyzed by the pathway-focused gene expression profiling with qRT-PCR. The multi-target activity of a number of PSMs was demonstrated by their interaction with a set of G4s mimicking those formed in the human genome. We have shown a direct G4-mediated down regulation of c-MYC expression by sanguinarine, quercetin, kaempferol, and thymoquinone; these effects being modulated by PSM’s indirect influence via cell signaling pathways.

Targeting Oncogene Promoters and Ribosomal RNA Biogenesis by G-Quadruplex Binding Ligands Translate to Anticancer Activity

G-Quadruplex (GQ) nucleic acids are promising therapeutic targets in anticancer research due to their structural robustness, polymorphism, and gene-regulatory functions. Here, we presented the structure-activity relationship of carbazole-based monocyanine ligands using region-specific functionalization with benzothiazole (TCA and TCZ), lepidine (LCA and LCZ), and quinaldine (QCA and QCZ) acceptor moieties and evaluated their binding profiles with different oncogenic GQs. Their differential turn-on fluorescence emission upon GQ binding confirmed the GQ-to-duplex selectivity of all carbazole ligands, while the isothermal titration calorimetry results showed selective interactions of TCZ and TCA to c-MYC and BCL-2 GQs, respectively. The aldehyde group in TCA favors stacking interactions with the tetrad of BCL-2 GQ, whereas TCZ provides selective groove interactions with c-MYC GQ. Dual-luciferase assay and chromatin immunoprecipitation (ChIP) showed that these molecules interfere with the recruitment of specific transcription factors at c-MYC and BCL-2 promoters and stabilize the promoter GQ structures to inhibit their constitutive transcription in cancer cells. Their intrinsic turn-on fluorescence response with longer lifetimes upon GQ binding allowed real-time visualization of GQ structures at subcellular compartments. Confocal microscopy revealed the uptake of these ligands in the nucleoli, resulting in nucleolar stress. ChIP studies further confirmed the inhibition of Nucleolin occupancy at multiple GQ-enriched regions of ribosomal DNA (rDNA) promoters, which arrested rRNA biogenesis. Therefore, carbazole ligands act as the "double-edged swords" to arrest c-MYC and BCL-2 overexpression as well as rRNA biogenesis, triggering synergistic inhibition of multiple oncogenic pathways and apoptosis in cancer cells.

G4-quadruplex-binding proteins: review and insights into selectivity

There are over 700,000 putative G4-quadruplexes (G4Qs) in the human genome, found largely in promoter regions, telomeres, and other regions of high regulation. Growing evidence links their presence to functionality in various cellular processes, where cellular proteins interact with them, either stabilizing and/or anchoring upon them, or unwinding them to allow a process to proceed. Interest in understanding and manipulating the plethora of processes regulated by these G4Qs has spawned a new area of small-molecule binder development, with attempts to mimic and block the associated G4-binding protein (G4BP). Despite the growing interest and focus on these G4Qs, there is limited data (in particular, high-resolution structural information), on the nature of these G4Q-G4BP interactions and what makes a G4BP selective to certain G4Qs, if in fact they are at all. This review summarizes the current literature on G4BPs with regards to their interactions with G4Qs, providing groupings for binding mode, drawing conclusions around commonalities and highlighting information on specific interactions where available.

The human AP-endonuclease 1 (APE1) is a DNA G-quadruplex structure binding protein and regulates KRAS expression in pancreatic ductal adenocarcinoma cells

Pancreatic ductal adenocarcinoma (PDAC), one of the most aggressive types of cancer, is characterized by aberrant activity of oncogenic KRAS. A nuclease-hypersensitive GC-rich region in KRAS promoter can fold into a four-stranded DNA secondary structure called G-quadruplex (G4), known to regulate KRAS expression. However, the factors that regulate stable G4 formation in the genome and KRAS expression in PDAC are largely unknown. Here, we show that APE1 (apurinic/apyrimidinic endonuclease 1), a multifunctional DNA repair enzyme, is a G4-binding protein, and loss of APE1 abrogates the formation of stable G4 structures in cells. Recombinant APE1 binds to KRAS promoter G4 structure with high affinity and promotes G4 folding in vitro. Knockdown of APE1 reduces MAZ transcription factor loading onto the KRAS promoter, thus reducing KRAS expression in PDAC cells. Moreover, downregulation of APE1 sensitizes PDAC cells to chemotherapeutic drugs in vitro and in vivo. We also demonstrate that PDAC patients' tissue samples have elevated levels of both APE1 and G4 DNA. Our findings unravel a critical role of APE1 in regulating stable G4 formation and KRAS expression in PDAC and highlight G4 structures as genomic features with potential application as a novel prognostic marker and therapeutic target in PDAC.

Folding dynamics of polymorphic G-quadruplex structures

G-quadruplexes (G4), found in numerous places within the human genome, are involved in essential processes of cell regulation. Chromosomal DNA G4s are involved for example, in replication and transcription as first steps of gene expression. Hence, they influence a plethora of downstream processes. G4s possess an intricate structure that differs from canonical B-form DNA. Identical DNA G4 sequences can adopt multiple long-lived conformations, a phenomenon known as G4 polymorphism. A detailed understanding of the molecular mechanisms that drive G4 folding is essential to understand their ambivalent regulatory roles. Disentangling the inherent dynamic and polymorphic nature of G4 structures thus is key to unravel their biological functions and make them amenable as molecular targets in novel therapeutic approaches. We here review recent experimental approaches to monitor G4 folding and discuss structural aspects for possible folding pathways. Substantial progress in the understanding of G4 folding within the recent years now allows drawing comprehensive models of the complex folding energy landscape of G4s that we herein evaluate based on computational and experimental evidence.

G-quadruplex DNA: a novel target for drug design

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

What's new about CNBP? Divergent functions and activities for a conserved nucleic acid binding protein

BACKGROUND

Cellular nucleic acid binding protein (CNBP) is a conserved single-stranded nucleic acid binding protein present in most eukaryotes, but not in plants. Expansions in the CNBP gene cause myotonic dystrophy type 2. Initially reported as a transcriptional regulator, CNBP was then also identified acting as a translational regulator.

SCOPE OF REVIEW

The focus of this review was to link the CNBP structural features and newly reported biochemical activities with the recently described biological functions, in the context of its pathological significance.

MAJOR CONCLUSIONS

Several post-translational modifications affect CNBP subcellular localization and activity. CNBP participates in the transcriptional and translational regulation of a wide range of genes by remodeling single-stranded nucleic acid secondary structures and/or by modulating the activity of trans-acting factors. CNBP is required for proper neural crest and heart development, and plays a role in cell proliferation control. Besides, CNBP has been linked with neurodegenerative, inflammatory, and congenital diseases, as well as with tumor processes.

GENERAL SIGNIFICANCE

This review provides an insight into the growing functions of CNBP in cell biology. A unique and robust mechanistic or biochemical connection among these roles has yet not been elucidated. However, the ability of CNBP to dynamically integrate signaling pathways and to act as nucleic acid chaperone may explain most of the roles and functions identified so far.

Promoter G-quadruplex folding precedes transcription and is controlled by chromatin

BACKGROUND

Four-stranded G-quadruplexes (G4s) are DNA secondary structures in the human genome that are primarily found in active promoters associated with elevated transcription. Here, we explore the relationship between the folding of promoter G4s, transcription and chromatin state.

RESULTS

Transcriptional inhibition by DRB or by triptolide reveals that promoter G4 formation, as assessed by G4 ChIP-seq, does not depend on transcriptional activity. We then show that chromatin compaction can lead to loss of promoter G4s and is accompanied by a corresponding loss of RNA polymerase II (Pol II), thus establishing a link between G4 formation and chromatin accessibility. Furthermore, pre-treatment of cells with a G4-stabilising ligand mitigates the loss of Pol II at promoters induced by chromatin compaction.

CONCLUSIONS

Overall, our findings show that G4 folding is coupled to the establishment of accessible chromatin and does not require active transcription.

Parallel reaction pathways accelerate folding of a guanine quadruplex

G-quadruplexes (G4s) are four-stranded, guanine-rich nucleic acid structures that can influence a variety of biological processes such as the transcription and translation of genes and DNA replication. In many cases, a single G4-forming nucleic acid sequence can adopt multiple different folded conformations that interconvert on biologically relevant timescales, entropically stabilizing the folded state. The coexistence of different folded conformations also suggests that there are multiple pathways leading from the unfolded to the folded state ensembles, potentially modulating the folding rate and biological activity. We have developed an experimental method for quantifying the contributions of individual pathways to the folding of conformationally heterogeneous G4s that is based on mutagenesis, thermal hysteresis kinetic experiments and global analysis, and validated our results using photocaged kinetic NMR experiments. We studied the regulatory Pu22 G4 from the c-myc oncogene promoter, which adopts at least four distinct folded isomers. We found that the presence of four parallel pathways leads to a 2.5-fold acceleration in folding; that is, the effective folding rate from the unfolded to folded ensembles is 2.5 times as large as the rate constant for the fastest individual pathway. Since many G4 sequences can adopt many more than four isomers, folding accelerations of more than an order of magnitude are possible via this mechanism.

The Molecular Tête-à-Tête between G-Quadruplexes and the i-motif in the Human Genome

G-Quadruplex (GQ) and i-motif structures are the paradigmatic examples of nonclassical tetrastranded nucleic acids having multifarious biological functions and widespread applications in therapeutics and material science. Recently, tetraplexes emerged as promising anticancer targets due to their structural robustness, gene-regulatory roles, and predominant distribution at specific loci of oncogenes. However, it is arguable whether the i-motif evolves in the complementary single-stranded region after GQ formation in its opposite strand and vice versa. In this review, we address the prerequisites and significance of the simultaneous and/or mutually exclusive formation of GQ and i-motif structures at complementary and sequential positions in duplexes in the cellular milieu. We discussed how their dynamic interplay Sets up cellular homeostasis and exacerbates carcinogenesis. The review gives insights into the spatiotemporal formation of GQ and i-motifs that could be harnessed to design different types of reporter systems and diagnostic platforms for potential bioanalytical and therapeutic intervention.

A Thermodynamic Perspective on Potential G-Quadruplex Structures as Silencer Elements in the MYC Promoter

Multiple G‐tracts within the promoter region of the c‐myc oncogene may fold into various G‐quadruplexes with the recruitment of different tracts and guanosine residues for the G‐core assembly. Thermodynamic profiles for the folding of wild‐type and representative truncated as well as mutated sequences were extracted by comprehensive DSC experiments. The unique G‐quadruplex involving consecutive G‐tracts II–V with formation of two one‐nucleotide and one central two‐nucleotide propeller loop, previously proposed to be the biologically most relevant species, was found to be the most stable fold in terms of its Gibbs free energy of formation at ambient temperatures. Its stability derives from its short propeller loops but also from the favorable type of loop residues. Whereas quadruplex folds with long propeller loops are significantly disfavored, a snap‐back loop structure formed by incorporating a 3’‐terminal guanosine into the empty position of a tetrad seems highly competitive based on its thermodynamic stability. However, its destabilization by extending the 3’‐terminus questions the significance of such a species under in vivo conditions.

Inosine 5'-diphosphate, a molecular decoy rescues Nucleoside diphosphate kinase from c-MYC G-Quadruplex unfolding

BACKGROUND

The transcription-inhibitory G-Quadruplex(Pu27-GQ) at c-MYC promoter is challenging to target due to structural heterogeneity. Nucleoside diphosphate kinase (NM23-H2) specifically binds and unfolds Pu27-GQ to increase c-MYC transcription. Here, we used Inosine 5'-diphosphate (IDP) to disrupt NM23-H2-Pu27-GQ interactions and arrest c-MYC transcription without compromising NM23-H2-mediated kinase properties.

METHODS

Site-directed mutagenesis,³¹P-NMR and STD-NMR studies delineate the epitope of NM23-H2-IDP complex and characterize specific amino acids in NM23-H2 involved in Pu27-GQ and IDP interactions. Immunoprecipitations and phosphohistidine-immunoblots reveal how IDP blocks NM23-H2-Pu27 association to downregulate c-MYC transcription in MDAMB-231 cells exempting NM23-H2-mediated kinase properties.

RESULTS

NMR studies show that IDP binds to the Guanosine diphosphate-binding pocket of NM23-H2 (K_D = 5.0 ± 0.276 μM). Arg88-driven hydrogen bonds to the terminal phosphate of IDP restricts P-O-P bond-rotation increasing its pKa (∆pKa = 0.85 ± 0.0025).9-inosinyl moiety of IDP is stacked over Phe60 phenyl ring driving trans-conformation of inosine and axial geometry of pyrophosphates. Chromatin immunoprecipitations revealed that these interactions rescue NM23-H2-driven Pu27-GQ unfolding, which triggers Nucleolin recruitment and lowers Sp1 occupancy at c-MYC promoter stabilizing Pu27-GQ. This silences c-MYC transcription that reduces c-MYC-Sp1 association amplifying Sp1 recruitment across P21 promoter stimulating P21 transcription and G₂/M arrest.

CONCLUSIONS

IDP synergizes the effects of Pu27-GQ-interacting compounds to abrogate c-MYC transcription and induce apoptosis in MDAMB-231 cells by disrupting NM23-H2-Pu27-GQ interactions without affecting NM23-H2-mediated kinase properties.

GENERAL SIGNIFICANCE

Our study provides a pragmatic approach for developing NM23-H2-targeting regulators to rescue NM23-H2 binding at structurally ambiguous Pu27-GQ that synergizes the anti-tumorigenic effects of GQ-based therapeutics with minimized off-target effects.

To probe the binding pathway of a selective compound (D089-0563) to c-MYC Pu24 G-quadruplex using free ligand binding simulations and Markov state model analysis

D089-0563 is a highly promising anti-cancer compound that selectively binds the transcription-silencing G-quadruplex element (Pu27) at the promoter region of the human c-MYC oncogene; however, its binding mechanism remains elusive.

Conformational Dynamics of Strand Register Shifts in DNA G-Quadruplexes

The complex folding energy landscape of DNA G-quadruplexes leads to numerous conformations for this functionally important class of noncanonical DNA structures. A new layer of conformational heterogeneity comes from sequences with different numbers of G-nucleotides in each of the DNA G-strands that form the four-stranded G-quartet core. The mechanisms by which G-quadruplexes transition from one folded conformation to another are currently unknown. To address this question, we studied two different G-quadruplexes, selecting a single conformation by blocking hydrogen bonding with photolabile protection groups. Upon irradiation, the block can be released and the kinetics of re-equilibration to the native conformational equilibrium can be determined by time-resolved NMR. We compared the NMR-derived refolding kinetics with data derived from thermal hysteresis folding kinetic experiments and found excellent agreement. The outlined methodological approach allows separation of K⁺-induced G-quadruplex formation and subsequent refolding and provides key insight into rate-limiting steps of G-quadruplex conformational dynamics.