Telomestatin

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.

Telomestatin, a strong telomerase inhibitor with G-quadruplex stabilizing activity, is a potential therapeutic agent for treating cancers. Difficulties in isolating telomestatin from microbial cultures and in chemical synthesis are bottlenecks impeding the wider use. Therefore, improvement in telomestatin production and structural diversification are required for further utilization and application. Here, we discovered the gene cluster responsible for telomestatin biosynthesis, and achieved production of telomestatin by heterologous expression of this cluster in the engineered Streptomyces avermitilis SUKA strain. Utilization of an optimal promoter was essential for successful production. Gene disruption studies revealed that the tlsB, tlsC, and tlsO-T genes play key roles in telomestatin biosynthesis. Moreover, exchanging TlsC core peptide sequences resulted in the production of novel telomestatin derivatives. This study sheds light on the expansion of chemical diversity of natural peptide products for drug development.

Purpose: Glioma stem cells (GSC) are a critical therapeutic target of glioblastoma multiforme (GBM). Experimental design: The effects of a G-quadruplex ligand, telomestatin, were evaluated using patient-derived GSCs, non-stem tumor cells (non-GSC), and normal fetal neural precursors in vitro and in vivo. The molecular targets of telomestatin were determined by immunofluorescence in situ hybridization (iFISH) and cDNA microarray. The data were then validated by in vitro and in vivo functional assays, as well as by immunohistochemistry against 90 clinical samples. Results: Telomestatin impaired the maintenance of GSC stem cell state by inducing apoptosis in vitro and in vivo. The migration potential of GSCs was also impaired by telomestatin treatment. In contrast, both normal neural precursors and non-GSCs were relatively resistant to telomestatin. Treatment of GSC-derived mouse intracranial tumors reduced tumor sizes in vivo without a noticeable cell death in normal brains. iFISH revealed both telomeric and non-telomeric DNA damage by telomestatin in GSCs but not in non-GSCs. cDNA microarray identified a proto-oncogene, c-Myb, as a novel molecular target of telomestatin in GSCs, and pharmacodynamic analysis in telomestatin-treated tumor-bearing mouse brains showed a reduction of c-Myb in tumors in vivo. Knockdown of c-Myb phenocopied telomestatin-treated GSCs both in vitro and in vivo, and restoring c-Myb by overexpression partially rescued the phenotype. Finally, c-Myb expression was markedly elevated in surgical specimens of GBMs compared with normal tissues. Conclusions: These data indicate that telomestatin potently eradicates GSCs through telomere disruption and c-Myb inhibition, and this study suggests a novel GSC-directed therapeutic strategy for GBMs.