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

Polymeric Properties of Telomeric G-Quadruplex Multimers: Effects of Chemically Inert Crowders

G-quadruplexes are noncanonical DNA structures rather ubiquitous in the human genome, which are thought to play a crucial role in the development of the majority of cancers. Here, we present a novel coarse-grained approach in modeling G-quadruplexes that accounts for their structural flexibility. We apply it to study the polymeric properties of G-quadruplex multimers, with and without crowder molecules, to mimic in vivo conditions. We find that, contrary to some suggestions found in the literature, long G-quadruplex multimers are rather flexible polymeric macromolecules, with a local persistence length comparable to monomer size, exhibiting a chain stiffness variation profile consistent with a real polymer in good solvent. Moreover, in a crowded environment (up to 10% volume fraction), we report that G-quadruplex multimers exhibit an increased propensity for coiling, with a corresponding decrease in the measured chain stiffness.

PET Imaging of Solid Tumors with a G-Quadruplex-Targeting 18F-Labeled Peptide Probe

Positron emission tomography (PET) is a common imaging technique and can provide accurate information about the size, shape, and location of tumors. Recent evidence has shown that G-quadruplex structures (G4s) are identified in human oncogenes, and these special structures are recognized as diagnostic cancer markers and drug targets for anticancer therapies. Although a number of techniques for in vivo imaging of G4s have been developed, achieving sufficient sensitivity and selectivity in vivo remains challenging. Herein, we have engineered and developed a radiolabeled peptide probe [18F]AlF-NOTA-RHAU18 targeting mitochondrial DNA G4s for in vivo PET imaging. The results of the study indicate that this probe is able to visualize and detect solid tumors in living homozygous mice. In addition, the distribution of the probe in cancer cells was investigated using FITC-RHAU18. This work may offer new insights into the development of cancer diagnostic tools by targeting in vivo G4s.

Selective recognition of RNA G-quadruplex in vitro and in cells by L-aptamer-D-oligonucleotide conjugate

RNA Guanine-quadruplexes (rG4s) are important nucleic acid structures that govern vital biological processes. Although numerous tools have been developed to target rG4s, few specific tools are capable of discerning individual rG4 of interest. Herein, we design and synthesize the first L-aptamer-antisense oligonucleotide (ASO) conjugate, L-Apt.4-1c-ASO15nt(APP), with a focus on recognizing the amyloid precursor protein (APP) rG4 region as an example. The L-aptamer module binds with the rG4 structure, whereas ASO hybridizes with flanking sequences. Together, these two modules enhance the precise recognition of APP rG4. We demonstrate that the L-Apt.4-1c-ASO15nt(APP) conjugate can interact with the APP rG4 region with sub-nanomolar binding affinity, and distinguish APP rG4 from other G4s and non-G4s in vitro and in cells. We also show that L-Apt.4-1c-ASO15nt(APP) can inhibit APP protein expression. Notably, we investigate the inhibitory mechanism of this newly developed tool, and reveal that it controls gene expression by hindering DHX36 protein from unraveling the rG4, as well as by promoting translational inhibition and RNase H-mediated mRNA knockdown activity. Our novel L-aptamer-ASO conjugate tool not only enables the specific recognition of rG4 region of interest, but also allows efficient gene control via targeting rG4-containing transcripts in cells.

The transcription of the main gene associated with Treacher-Collins syndrome (TCOF1) is regulated by G-quadruplexes and cellular nucleic acid binding protein (CNBP)

Treacle ribosome biogenesis factor 1 (TCOF1) is responsible for about 80% of mandibular dysostosis (MD) cases. We have formerly identified a correlation between TCOF1 and CNBP (CCHC-type zinc finger nucleic acid binding protein) expression in human mesenchymal cells. Given the established role of CNBP in gene regulation during rostral development, we explored the potential for CNBP to modulate TCOF1 transcription. Computational analysis for CNBP binding sites (CNBP-BSs) in the TCOF1 promoter revealed several putative binding sites, two of which (Hs791 and Hs2160) overlap with putative G-quadruplex (G4) sequences (PQSs). We validated the folding of these PQSs measuring circular dichroism and fluorescence of appropriate synthetic oligonucleotides. In vitro studies confirmed binding of purified CNBP to the target PQSs (both folded as G4 and unfolded) with Kd values in the nM range. ChIP assays conducted in HeLa cells chromatin detected the CNBP binding to TCOF1 promoter. Transient transfections of HEK293 cells revealed that Hs2160 cloned upstream SV40 promoter increased transcription of downstream firefly luciferase reporter gene. We also detected a CNBP-BS and PQS (Dr2393) in the zebrafish TCOF1 orthologue promoter (nolc1). Disrupting this G4 in zebrafish embryos by microinjecting DNA antisense oligonucleotides complementary to Dr2393 reduced the transcription of nolc1 and recapitulated the craniofacial anomalies characteristic of Treacher Collins Syndrome. Both cnbp overexpression and Morpholino-mediated knockdown in zebrafish induced nolc1 transcription. These results suggest that CNBP modulates the transcriptional expression of TCOF1 through a mechanism involving G-quadruplex folding/unfolding, and that this regulation is active in vertebrates as distantly related as bony fish and humans. These findings may have implications for understanding and treating MD.