Targeting Human Telomeric Higher-Order DNA: Dimeric G-Quadruplex Units Serve as Preferred Binding Site

A SERS/fluorescence dual-mode nanosensor based on the human telomeric G-quadruplex DNA: Application to mercury (II) detection

DNA-metal nanoparticle conjugates have been increasingly exploited for sensing purposes over the past decades. However, most of the existing strategies are operated with canonical DNA structures, such as single-stranded forms, stem-loop structures, and double helix structures. There is intense interest in the development of nano-system based on high order DNA secondary structures. Herein, we propose a SERS/fluorescence dual-mode nanosensor, where the signal transduction mechanism is based on the conformational switching of the human telomeric G-quadruplex DNA. The nanosensor exhibits excellent SERS/fluorescence responses to the complementary strands of G-quadruplexes. Based on T-Hg(2+)-T coordination chemistry, this sensor is effectively applied to determination of Hg(2+) in buffer solution and real samples. It achieves a limit of detection (LOD) as low as 1ppt, which is ~100 times more sensitive than conventional optical sensors. We anticipate that the proposed G-quadruplex-based nanosensor could be applied to the analysis of other metal ions and small molecules in environmental samples and biological systems.

Selective lighting up of epiberberine alkaloid fluorescence by fluorophore-switching aptamer and stoichiometric targeting of human telomeric DNA G-quadruplex multimer

Aptamers, that exist naturally in living cells as functional elements and can switch nonfluorescent natural targets to fluorophores, are very useful in developing highly sensitive and selective biosensors and screening functional agents. This work demonstrates that human telomeric G-quadruplex (HTG) can serve as a potential fluorophore-switching aptamer (FSA) to target a natural isoquinoline alkaloid. We found that, among the G-quadruplexes studied here and the various structurally similar alkaloids including epiberberine (EPI), berberine (BER), palmatine (PAL), jatrorrhizine (JAT), coptisine (COP), worenine (WOR), sanguinarine (SAN), chelerythrine (CHE), and nitidine (NIT), only the HTG DNA, especially with a 5'-TA-3' residue at the 5' end of the G-quadruplex tetrad (5'-TAG3(TTAG3)3-3', TA[Q]) as the minimal sequence, is the most efficient FSA to selectively light up the EPI fluorescence. Compared to the 5' end flanking sequences, the 3' end flanking sequences of the tetrad contribute significantly less to the recognition of EPI. The binding affinity of EPI to TA[Q] (K(d) = 37 nM) is at least 20 times tighter than those of the other alkaloids. The steady-state absorption, steady-state/time-resolved fluorescence, and NMR studies demonstrate that EPI most likely interact with the 5' end flanking sequence substructure beyond the core [Q] and the G-quadruplex tetrad in a much more specific manner than the other alkaloids. The highly selective and tight binding of EPI with the FSA and significantly enhanced fluorescence suggest the potential development of a selective EPI sensor (detection limit of 10 nM). More importantly, EPI, as the brightest FSA emitter among the alkaloids, can also serve as an efficient conformation probe for HTG DNA and discriminate the DNA G-quadruplex from the RNA counterpart. Furthermore, EPI can bind stoichiometrically to each G-quadruplex unit of long HTG DNA multimer with the most significant fluorescence enhancement, which has not been achieved by the previously reported probes. Our work suggests the potential use of EPI as a bioimaging probe and a therapeutic DNA binder.

Cationic azacryptands as selective three-way DNA junction binding agents

Azacryptands are promising candidates for assessing the therapeutic potential of three-way DNA junctions.

Photoreactivity of the linker region of two consecutive G-quadruplexes formed by human telomeric DNA

The photoreaction method was applied to probe the linker region of two consecutive G-quadruplexes.

Interaction of G-quadruplexes in the full-length 3' human telomeric overhang

The 3' human telomeric overhang provides ample opportunities for the formation and interaction of G-quadruplexes, which have shown impacts on many biological functions including telomerase activities in the telomere region. However, in the few investigations on DNA constructs that approach to the full length of the human telomeric overhang, the presence of higher-order quadruplex-quadruplex interactions is still a subject of debate. Herein, we employed dynamic splint ligation (DSL) to prepare a DNA construct, 5'-(TTAGGG)24 or 24G, which has the length comparable to the full stretch of 3' human telomeric overhang. Using mechanical unfolding assays in laser tweezers, we observed a minor population (∼5%) of higher-order interactions between G-quadruplexes, while the majority of the quadruplexes follow the bead-on-a-string model. Analyses on the noninteracting G-quadruplexes in the 24G construct showed features similar to those of the stand-alone G-quadruplexes in the 5'-(TTAGGG)4 (4G) construct. As each 24G construct contains as many as six G-quadruplexes, this method offers increased throughput for the time-consuming mechanical unfolding experiments of non-B DNA structures.

Recognition of chelerythrine to human telomeric DNA and RNA G-quadruplexes

A study on binding of antitumor chelerythrine to human telomeric DNA/RNA G-quadruplexes was performed by using DNA polymerase stop assay, UV-melting, ESI-TOF-MS, UV-Vis absorption spectrophotometry and fluorescent triazole orange displacement assay. Chelerythrine selectively binds to and stabilizes the K(+)-form hybrid-type human telomeric DNA G-quadruplex of biological significance, compared with the Na(+)-form antiparallel-type DNA G-quadruplex. ESI-TOF-MS study showed that chelerythrine possesses a binding strength for DNA G-quadruplex comparable to that of TMPyP4 tetrachloride. Both 1:1 and 2:1 stoichiometries were observed for chelerythrine's binding with DNA and RNA G-quadruplexes. The binding strength of chelerythrine with RNA G-quadruplex is stronger than that with DNA G-quadruplex. Fluorescent triazole orange displacement assay revealed that chelerythrine interacts with human telomeric RNA/DNA G-quadruplexes by the mode of end- stacking. The relative binding strength of chelerythrine for human telomeric RNA and DNA G-quadruplexes obtained from ESI-TOF-MS experiments are respectively 6.0- and 2.5-fold tighter than that with human telomeric double-stranded hairpin DNA. The binding selectivity of chelerythrine for the biologically significant K(+)-form human telomeric DNA G-quadruplex over the Na(+)-form analogue, and binding specificity for human telomeric RNA G-quadruplex established it as a promising candidate in the structure-based design and development of G-quadruplex specific ligands.

A base-modified PNA-graphene oxide platform as a turn-on fluorescence sensor for the detection of human telomeric repeats

Given the biological and therapeutic significance of telomeres and other G-quadruplex forming sequences in human genome, it is highly desirable to develop simple methods to study these structures, which can also be implemented in screening formats for the discovery of G-quadruplex binders. The majority of telomere detection methods developed so far are laborious and use elaborate assay and instrumental setups, and hence, are not amenable to discovery platforms. Here, we describe the development of a simple homogeneous fluorescence turn-on method, which uses a unique combination of an environment-sensitive fluorescent nucleobase analogue, the superior base pairing property of PNA, and DNA-binding and fluorescence quenching properties of graphene oxide, to detect human telomeric DNA repeats of varying lengths. Our results demonstrate that this method, which does not involve a rigorous assay setup, would provide new opportunities to study G-quadruplex structures.

Two cationic porphyrin isomers showing different multimeric G-quadruplex recognition specificity against monomeric G-quadruplexes

Ligands that can interact specifically with telomeric multimeric G-quadruplexes could be developed as promising anticancer drugs with few side effects related to other G-quadruplex-forming regions. In this paper, a new cationic porphyrin derivative, m-TMPipEOPP, was synthesized and characterized. Its multimeric G-quadruplex recognition specificity under molecular crowding conditions was compared to its isomer p-TMPipEOPP. The slight structural difference accounts for different multimeric G-quadruplex recognition specificity for the two isomers. p-TMPipEOPP can barely discriminate between multimeric and monomeric G-quadruplexes. By contrast, m-TMPipEOPP can bind with multimeric but not with monomeric G-quadruplexes. p-TMPipEOPP might bind to multimeric G-quadruplexes by two modes: sandwich-like end-stacking mode and pocket-dependent intercalative mode. Increasing the pocket size between adjacent two G-quadruplex uints is beneficial for the latter mode. m-TMPipEOPP might bind to multimeric G-quadruplexes by a side binding mode, which confers m-TMPipEOPP with higher multimeric G-quadruplex recognition specificity compared to p-TMPipEOPP. m-TMPipEOPP increases the stability of multimeric G-quadruplex under both dilute and molecular crowding conditions but its G-quadruplex-stabilizing ability is a little weaker than p-TMPipEOPP. These results provide important information for the design of highly specific multimeric G-quadruplex ligands. Another interesting finding is that pocket size is an important factor in determining the stability of multimeric G-quadruplexes.

Design and synthesis of a new dimeric xanthone derivative: enhancement of G-quadruplex selectivity and telomere damage

Dimerization of the xanthone core greatly enhances G-quadruplex binding and biological activity.