Clement Lin

The Major G-Quadruplex Formed in the Human BCL-2 Proximal Promoter Adopts a Parallel Structure with a 13-nt Loop in K+ Solution

The human BCL-2 gene contains a 39-bp GC-rich region upstream of the P1 promoter that has been shown to be critically involved in the regulation of BCL-2 gene expression. Inhibition of BCL-2 expression can decrease cellular proliferation and enhance the efficacy of chemotherapy. Here we report the major G-quadruplex formed in the Pu39 G-rich strand in this BCL-2 promoter region. The 1245G4 quadruplex adopts a parallel structure with one 13-nt and two 1-nt chain-reversal loops. The 1245G4 quadruplex involves four nonsuccessive G-runs, I, II, IV, V, unlike the previously reported bcl2 MidG4 quadruplex formed on the central four G-runs. The parallel 1245G4 quadruplex with the 13-nt loop, unexpectedly, appears to be more stable than the mixed parallel/antiparallel MidG4. Parallel-stranded structures with two 1-nt loops and one variable-length middle loop are found to be prevalent in the promoter G-quadruplexes; the variable middle loop is suggested to determine the specific overall structure and potential ligand recognition site. A limit of 7 nt in loop length is used in all quadruplex-predicting software. Thus, the formation and high stability of the 1245G4 quadruplex with a 13-nt loop is significant. The presence of two distinct interchangeable G-quadruplexes in the overlapping region of the BCL-2 promoter is intriguing, suggesting a novel mechanism for gene transcriptional regulation and ligand modulation.

Solution structure of a 2:1 complex of anticancer drug XR5944 with TFF1 estrogen response element: insights into DNA recognition by a bis-intercalator

XR5944, a deoxyribonucleic acid (DNA) bis-intercalator with potent anticancer activity, can bind the estrogen response element (ERE) sequence to inhibit estrogen receptor-α activities. This novel mechanism of action may be useful for overcoming drug resistance to currently available antiestrogen treatments, all of which target the hormone-receptor complex. Here we report the nuclear magnetic resonance solution structure of the 2:1 complex of XR5944 with the naturally occurring TFF1-ERE, which exhibits important and unexpected features. In both drug–DNA complexes, XR5944 binds strongly at one intercalation site but weakly at the second site. The sites of intercalation within a native promoter sequence appear to be context and sequence dependent. The binding of one drug molecule influences the binding site of the second. Our structures underscore the fact that the DNA binding of a bis-intercalator is directional and different from the simple addition of two single intercalation sites. Our study suggests that improved XR5944 bis-intercalators targeting ERE may be designed through optimization of aminoalkyl linker and intercalation moieties at the weak binding sites.

Abstract 5232: The molecular basis for specific recognition of the biologically relevant hybrid-2 type human telomeric G-quadruplex by epiberberine

G-quadruplex structures have been shown to form in human telomeres, and the formation of G-quadruplexes can inhibit telomerase, which plays a key role in cancers by stabilizing telomere length and integrity thereby granting limitless replicative potential. The human telomeric G-quadruplex is thus considered to be a potential target for cancer therapeutics. In physiologically relevant potassium solution, human telomeric DNA sequences form two equilibrating hybrid-type G-quadruplex structures, with the hybrid-2 structure being the predominant form in extended sequences. We discovered that epiberberine (EPI), a naturally occurring protoberberine alkaloid, can specifically bind the hybrid-2 human telomeric G-quadruplex and can induce the conversion to the hybrid-2 structure. Using nuclear magnetic resonance (NMR) spectroscopy, we determined the solution structure of the 1:1 complex of EPI and hybrid-2 human telomeric G-quadruplex. Our NMR structure shows EPI bound at the 5’ end of the hybrid-2 telomeric quadruplex with an unexpectedly large drug-induced conformational change in the flanking and loop regions, creating a very well-defined drug binding pocket with extensive capping structures. The EPI molecule and 5’ flanking adenine form an induced quasi-triad plane which is intercalated between the external 5’ tetrad and capping structures. Two layers of new capping structures are formed with the 5’ flanking segment and the second TTA lateral loop to cover the “intercalated quasi-triad”. Notably, the well-defined EPI-induced multi-layer binding-site arrangement is only possible in the hybrid-2 folding topology of telomeric DNA. Our results provide important insights into specific targeting of the physiologically relevant hybrid-2 human telomeric G-quadruplex by a small molecule compound; EPI’s asymmetric crescent-shape and the positioning of its dioxolane moiety, as well as the hybrid-2 folding and the loop and flanking bases in the human telomeric DNA sequence, all contribute toward the specific recognition of EPI. In addition, we have conducted polymerase stop assays, which confirmed that EPI can stabilize the human telomeric G-quadruplex to inhibit polymerase activity. Citation Format: Clement Lin, Guanhui Wu, Yong Shao, Danzhou Yang. The molecular basis for specific recognition of the biologically relevant hybrid-2 type human telomeric G-quadruplex by epiberberine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5232. doi:10.1158/1538-7445.AM2017-5232

Abstract 4853: BMVC specifically binds the major G-quadruplex structure formed in the c-MYC promoter to lower c-MYC levels

The c-MYC proto-oncogene is one of the most deregulated genes in human cancers. Transcriptional repression of c-MYC is considered as one of the attractive strategies in targeting c-MYC. Previous studies revealed that the c-MYC promoter nuclease hypersensitive element (NHE) III1 region, which regulates 80-95% of total c-MYC transcription, can form DNA G-quadruplex (G4) and that stabilization of the c-MYC G4 could repress c-MYC gene expression. We have previously determined the molecular structure of the major G-quadruplex formed in the c-MYC promoter NHE (MycG4), which is an intramolecular parallel-stranded structure. BMVC, a carbazole derivative, was designed as a fluorescent probe for recognizing telomeric G4. However, we found that BMVC binds to MycG4 with much greater affinity and specificity. Cellular studies showed that BMVC was enriched in the nucleus at 48 hr after treatment, leading to markedly reduced c-MYC expression levels. Promoter-driven luciferase assays revealed that BMVC could enhance the inhibitory effects of MycG4, suggesting BMVC can repress c-MYC expression in vivo through stabilization of MycG4. To understand the molecular interactions between BMVC and MycG4, we have determined the molecular structure of the complexes of BMVC with MycG4 by NMR. BMVC appears to bind the 5’-end of the MycG4 with very high affinity, as shown by the slow-exchange binding on the NMR timescale. The structure of the 5’-end complex of BMVC and MycG4 shows that BMVC is paired to the flanking adenine with specific hydrogen bonding interactions, while the BMVC-adenine plane stacks nicely on the 5’-end G-tetrad of MycG4. Compared to the previous MycG4 complex with a quindoline compound, the tighter and more specific binding of BMVC appears to be imparted by the novel arched shape of BMVC and specific pairing recognition of BMVC with the flanking bases of MycG4. BMVC can also bind to the 3’-end of MycG4 with lower affinity to form a second complex. However, the 3’-end complex shows much more dynamic conformation and less specific interactions. Our results indicated that the cellular targets of BMVC include not only telomeric G4, but also the c-MYC promoter G4. Because of the inherent fluorescence and novel binding mode of BMVC, our study provides useful information for future design of improved carbazole-based anticancer agents or specific cellular fluorescent probes, as well as insights into specific recognition of MycG4 by small molecules. Citation Format: Wenting Liu, Guanhui Wu, Clement Lin, Buket Onel, Ding Chen, Ta-Chau Chang, Danzhou Yang. BMVC specifically binds the major G-quadruplex structure formed in the c-MYC promoter to lower c-MYC levels. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4853.

Abstract 3090: Molecular structure of the major G-quadruplex formed in the PDGFR-β promoter nuclease hypersensitivity element and its binding with small molecules

Proceedings: AACR 107th Annual Meeting 2016; April 16-20, 2016; New Orleans, LA Overexpression of platelet-derived growth factor receptor β (PDGFR-β) is associated with multiple cancers, making PDGFR-β an attractive target for anticancer drugs. Few strategies other than molecular targeting of the PDGFR-β protein or its cognate ligand have been reported for developing inhibitors for the PDGFR-β signaling pathway. DNA G-quadruplexes (G4s) formed in the GC-rich nuclease hypersensitivity element of the human PDGFR-β gene promoter have been found to inhibit PDGFR-β transcriptional activity, and stabilization of these G4s by small-molecule compounds could serve as a mechanism for cancer therapeutics. We have shown that the major G4 formed in the PDGFR-β promoter is from the central four G-runs, adopting an intramolecular parallel-stranded structure with a broken G-strand and contains three 1-nucleotide (nt) chain-reversal loops and one lateral loop. The novel folding of the PDGFR-β G4 highlights the inherent stability of the 1-nt loops in parallel-stranded G4 structures. Elucidating the structure of the major PDGFR-β promoter G4 is important for designing small-molecule drugs to specifically target this structure to inhibit gene transcription. Using nuclear magnetic resonance (NMR) spectroscopy, we have determined the potassium solution structure of this major G4 formed in the PDGFR-β promoter. Our structure showed a unique 3’ capping structure involving three guanine nucleotides in the lateral loop. This novel capping structure is determined by the specific loop sequence as well as the broken-stranded PDGFR-β G4 folding pattern. The unique 3’ capping structure could be specifically recognized by proteins and small molecule ligands. Unrestrained molecular dynamic calculations showed that this major PDGFR-β G4 and its 3’ capping structure are stable in aqueous environment on the ns time-scale. We also investigated the binding of small-molecules to the PDGFR-β G4 using a combination of NMR, circular dichroism (CD), and fluorescence based methods to identify compounds that can selectively bind the PDGFR-β G4 over other classic parallel-stranded G-quadruplexes, such as the c-MYC promoter G4. Our study demonstrates the structural diversity in promoter G4s which may enable specific recognition and the modulation of gene expression by small-molecule drugs. Citation Format: Clement Lin, Prashansa Agrawal, Yuwei Chen, Salil Kalarn, Nanjie Deng, Laurence Hurley, Danzhou Yang. Molecular structure of the major G-quadruplex formed in the PDGFR-β promoter nuclease hypersensitivity element and its binding with small molecules. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3090.

Abstract 2445: DNA recognition by XR5944, a novel bis-intercalator and potent anticancer drug

DNA is a major target for drugs used in cancer therapy. However, DNA interactive chemotherapeutics are limited by adverse effects due to their poor selectivity. The bis-phenazine compound XR5944 is a DNA bis-intercalator and potent anticancer drug which reached phase I clinical trials. The DNA binding mode and mechanism of XR5944 are unique, allowing it to recognize and bind the estrogen response element (ERE) sequence to inhibit ERα activity and making it highly effective against ER-positive breast cancer in vitro. Understanding XR59449s novel mechanism of DNA recognition may allow the further development of anti-cancer agents capable of overcoming drug resistance to current anti-estrogen treatments by directly targeting transcriptional control, bypassing the hormone-receptor complex currently targeted by anti-estrogen treatments. In this study, we used nuclear magnetic resonance (NMR) spectroscopy to determine the structure of XR5944 in complex with its preferred DNA duplex sequence identified its binding mode. The structure of the 2:1 complex of XR5944 with the naturally occurring TFF1-ERE exhibits several unexpected features. Our NMR structure shows the two phenazine moieties are well stacked and the carboxamide amino linker lies in the major groove of DNA. At both binding sites in the 2:1 complex, XR5944 intercalates strongly at one site but weakly at the other. The binding sites within a native promoter sequence appear to be context- and sequence- dependent. Taken together, our results highlight that the DNA binding of a bis-intercalator is different from the simple addition of two single intercalation sites. Our study suggests that improved XR5944 derivatives targeting EREs may be designed through optimization of aminoalkyl linker and intercalation moieties. Citation Format: Clement Lin, Raveendra I. Mathad, Neil Sidell, Danzhou Yang. DNA recognition by XR5944, a novel bis-intercalator and potent anticancer drug. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2445. doi:10.1158/1538-7445.AM2015-2445