G-quadruplex, Friend or Foe: The Role of the G-quartet in Anticancer Strategies

G-quadruplex DNA structures and their relevance in radioprotection

BACKGROUND

DNA, the genetic material of most of the organisms, is the crucial element of life. Integrity of DNA needs to be maintained for transmission of genetic material from one generation to another. All organisms are constantly challenged by the environmental conditions which can lead to the induction of DNA damage. Ionizing radiation (IR) has been known to induce DNA damage and IR sensitivity varies among different organisms. The causes for differential radiosensitivity among various organisms have not been studied in great detail.

SCOPE OF REVIEW

We discuss DNA secondary structure formation, GC content of the genome, role of G-quadruplex formation, and its relationship with radiosensitivity of the genome.

MAJOR CONCLUSION

In Deinococcus radiodurans, the bacterium that exhibits maximum radio resistance, multiple G-quadruplex forming motifs are reported. In human cells, G-quadruplex formation led to differential radiosensitivity. In this article, we have discussed, the role of secondary DNA structure formation like G-quadruplex in shielding the genome from radiation and its implications in understanding evolution of radio protective effect of an organism. We also discuss role of GC content and its correlation with radio resistance.

GENERAL SIGNIFICANCE

This review provides an insight into the role of G-quadruplexes in providing differential radiosensitivity at different site of the genome and in different organisms. It further discusses the possibility of higher GC content contributing towards reduced radiosensitivity in different organisms, evolution of radiosensitivity, and regulation of multiple cellular processes.

The i-Motif as a Molecular Target: More Than a Complementary DNA Secondary Structure

Stretches of cytosine-rich DNA are capable of adopting a dynamic secondary structure, the i-motif. When within promoter regions, the i-motif has the potential to act as a molecular switch for controlling gene expression. However, i-motif structures in genomic areas of repetitive nucleotide sequences may play a role in facilitating or hindering expansion of these DNA elements. Despite research on the i-motif trailing behind the complementary G-quadruplex structure, recent discoveries including the identification of a specific i-motif antibody are pushing this field forward. This perspective reviews initial and current work characterizing the i-motif and providing insight into the biological function of this DNA structure, with a focus on how the i-motif can serve as a molecular target for developing new therapeutic approaches to modulate gene expression and extension of repetitive DNA.

Structural properties and anticoagulant/cytotoxic activities of heterochiral enantiomeric thrombin binding aptamer (TBA) derivatives

The thrombin binding aptamer (TBA) possesses promising antiproliferative properties. However, its development as an anticancer agent is drastically impaired by its concomitant anticoagulant activity. Therefore, suitable chemical modifications in the TBA sequence would be required in order to preserve its antiproliferative over anticoagulant activity. In this paper, we report structural investigations, based on circular dichroism (CD) and nuclear magnetic resonance spectroscopy (NMR), and biological evaluation of four pairs of enantiomeric heterochiral TBA analogues. The four TBA derivatives of the d-series are composed by d-residues except for one l-thymidine in the small TT loops, while their four enantiomers are composed by l-residues except for one d-thymidine in the same TT loop region. Apart from the left-handedness for the l-series TBA derivatives, CD and NMR measurements have shown that all TBA analogues are able to adopt the antiparallel, monomolecular, ‘chair-like’ G-quadruplex structure characteristic of the natural D-TBA. However, although all eight TBA derivatives are endowed with remarkable cytotoxic activities against colon and lung cancer cell lines, only TBA derivatives of the l-series show no anticoagulant activity and are considerably resistant in biological environments.

G-quadruplex-R-loop interactions and the mechanism of anticancer G-quadruplex binders

Genomic DNA and cellular RNAs can form a variety of non-B secondary structures, including G-quadruplex (G4) and R-loops. G4s are constituted by stacked guanine tetrads held together by Hoogsteen hydrogen bonds and can form at key regulatory sites of eukaryote genomes and transcripts, including gene promoters, untranslated exon regions and telomeres. R-loops are 3-stranded structures wherein the two strands of a DNA duplex are melted and one of them is annealed to an RNA. Specific G4 binders are intensively investigated to discover new effective anticancer drugs based on a common rationale, i.e.: the selective inhibition of oncogene expression or specific impairment of telomere maintenance. However, despite the high number of known G4 binders, such a selective molecular activity has not been fully established and several published data point to a different mode of action. We will review published data that address the close structural interplay between G4s and R-loops in vitro and in vivo, and how these interactions can have functional consequences in relation to G4 binder activity. We propose that R-loops can play a previously-underestimated role in G4 binder action, in relation to DNA damage induction, telomere maintenance, genome and epigenome instability and alterations of gene expression programs.

The Molecular 'Myc-anisms' Behind Myc-Driven Tumorigenesis and the Relevant Myc-Directed Therapeutics

MYC, a well-studied proto-oncogene that is overexpressed in >20% of tumors across all cancers, is classically known as “undruggable” due to its crucial roles in cell processes and its lack of a drug binding pocket. Four decades of research and creativity led to the discovery of a myriad of indirect (and now some direct!) therapeutic strategies targeting Myc. This review explores the various mechanisms in which Myc promotes cancer and highlights five key therapeutic approaches to disrupt Myc, including transcription, Myc-Max dimerization, protein stability, cell cycle regulation, and metabolism, in order to develop more specific Myc-directed therapies.

Enhanced G-quadruplex selectivity of flavonoid glycoside rutin over quercetin

In drug discovery, ligand-mediated stabilization of G-quadruplexes is pursued for regulating gene expression and key cellular processes. Electrospray ionization mass spectrometry (ESI-MS) has been optimized for screening putative DNA-binding small molecules of natural and synthetic origin. Several flavonoids were reported to interact with G-quadruplex, and quercetin is among them. In this contribution, the interaction with G-quadruplex DNA of rutin, a glycoside of quercetin extracted from flower buds of Styphnolobium japonicum (L.) Schott, was investigated by means of ESI-MS and molecular docking. While rutin and quercetin showed similar G-quadruplex binding affinity values, rutin was characterized by enhanced selectivity for G-quadruplex over double stranded DNA. Moreover, collision-induced dissociation (CID) assays demonstrated that rutin stabilizes the G-quadruplex arrangement more efficiently, and molecular docking predicted stacking as the preferential interaction pattern.

Targeted Treatment of Colon Cancer with Aptamer-Guided Albumin Nanoparticles Loaded with Docetaxel

Purpose

Chemotherapy of colon cancer needs improvement to mitigate the severe adverse effects (AEs) associated with the cytotoxic drugs. The aim of this study is to develop a novel targeted drug delivery system (TDDS) with practical application potential for colon cancer treatment.

Methods

The TDDS was built by loading docetaxel (DTX) in albumin nanoparticles (NPs) that were functionalized with nucleolin-targeted aptamers (AS1411).

Results

The TDDS (Apt-NPs-DTX) had an average size of 62 nm and was negatively charged with a zeta potential of −31.2 mV. DTX was released from the albumin NP with a typical sustained release profile. Aptamer-guided NPs were preferentially ingested by nucleolin-expressing CT26 colon cancer cells vs the control cells. In vitro cytotoxicity study showed that Apt-NPs-DTX significantly enhanced the killing of CT26 colon cancer cells. Importantly, compared with non-targeted drug delivery, Apt-NPs-DTX treatment significantly improved antitumor efficacy and prolonged the survival of CT26-bearing mice, without raising systemic toxicity.

Conclusion

The results suggest that Apt-NPs-DTX has potential in the targeted treatment of colon cancer.

Drug-like biimidazole derivatives dually target c-MYC/BCL-2 G-quadruplexes and inhibit acute myeloid leukemia

Chemotherapy is the main approach for treating acute myeloid leukemia (AML). However, this therapy can cause severe side effects as well as drug resistance, hence calling for new therapeutic strategies. As c-MYC and BCL-2 are often overexpressed in AML, and synergism between c-MYC and BCL-2 promotes tumorigenesis, therefore, dual targeting of c-MYC/BCL-2 promoter G-quadruplexes (G4s) and then inhibiting the targeted gene expression would be a potential strategy in ALM treatment. In this work, in the search of dual ligands, we performed a screening assay with an in-house, imidazole-based compound library. Consequently, two drug-like biimidazole derivatives were identified as selective c-MYC/BCL-2 G4 binders, of which, BIM-2 was selected as the candidate for inhibiting AML cell growth. Then, BIM-2 was demonstrated to downregulate both c-MYC and BCL-2 expression, and thereby cause cell cycle arrest at G0/G1 phase and apoptosis in AML cells. Furthermore, the possible end-stacking binding modes between BIM-2 and c-MYC/BCL-2 G4s were revealed by NMR and molecular docking studies. Accordingly, this study provides a new class of drug-like dual-selective c-MYC/BCL-2 G4 ligands for the potential treatment of AML.

Oligonucleotides and microRNAs Targeting Telomerase Subunits in Cancer Therapy

Telomerase provides cancer cells with replicative immortality, and its overexpression serves as a near-universal marker of cancer. Anti-cancer therapeutics targeting telomerase have garnered interest as possible alternatives to chemotherapy and radiotherapy. Oligonucleotide-based therapies that inhibit telomerase through direct or indirect modulation of its subunits, human telomerase reverse transcriptase (hTERT) and human telomerase RNA gene (hTERC), are a unique and diverse subclass of telomerase inhibitors which hold clinical promise. MicroRNAs that play a role in the upregulation or downregulation of hTERT and respective progression or attenuation of cancer development have been effectively targeted to reduce telomerase activity in various cancer types. Tumor suppressor miRNAs, such as miRNA-512-5p, miRNA-138, and miRNA-128, and oncogenic miRNAs, such as miRNA-19b, miRNA-346, and miRNA-21, have displayed preclinical promise as potential hTERT-based therapeutic targets. Antisense oligonucleotides like GRN163L and T-oligos have also been shown to uniquely target the telomerase subunits and have become popular in the design of novel cancer therapies. Finally, studies suggest that G-quadruplex stabilizers, such as Telomestatin, preserve telomeric oligonucleotide architecture, thus inhibiting hTERC binding to the telomere. This review aims to provide an adept understanding of the conceptual foundation and current state of therapeutics utilizing oligonucleotides to target the telomerase subunits, including the advantages and drawbacks of each of these approaches.