Abstract 4839: Targeting glioma stem cells by pharmacologic stabilization of G-quadruplexes

Abstract Among heterogeneous cell population in a tumor, cancer stem cells (or tumor-propagating cells) are defined as a cell fraction that shows self-renewal, multipotency, high tumorigenicity, and resistance to chemo/radiotherapy. Because cancer stem cells are supposed to drive initiation, metastasis, and recurrence of the disease, identification of their therapeutic targets would have a profound implication for eradication of cancer. G-quadruplex (G4) is an atypical four-stranded nucleic acid structure that can be formed at guanine-rich sequences, such as the telomeric TTAGGG repeats. Accumulating evidence supports that G4s are widely distributed through the genome in living cells and affect various intracellular events, including DNA replication, gene expression and translation, whereas pharmacologic stabilization of G4s has been implicated for cancer intervention. However, precise mechanisms for the efficacy and target cancer types remain elusive. Here we demonstrate that G4 stabilization by chemical compounds, called G4 ligands, preferentially inhibits the growth of glioma stem cells in culture and in vivo. While the established glioma stem cells maintain stemness under the serum-free sphere culture conditions, serum stimulation induces their differentiation into non-stem glioma cells. We found that these glioma stem cells are highly sensitive to G4 ligands, such as a natural compound telomestatin and its newly synthesized derivative, and a bisquinolinium compound Phen-DC3, all of which can stabilize G4s. Upon the ligand treatment, glioma stem cells activated the replication stress pathway more potently than non-stem glioma cells, which could partly explain the selectivity of the deleterious effect. Consistently, these ligands induced DNA damage response in glioma stem cells but not in non-stem glioma cells. In glioma stem cells, about 20% to 30% of DNA damage foci were telomeric (i.e., telomere dysfunction-induced foci: TIFs) whereas the rest were non-telomeric, suggesting that the G4 ligands recognized both telomeric and non-telomeric G4s. Temozolomide, a clinical alkylating agent for brain tumors, induced DNA damage response but did not preferentially attack telomeres. As a potential pharmacodynamic biomarker, the immunofluorescence intensities of the nuclear G4 foci were enhanced by G4 ligands. Furthermore, these ligands directly inhibited in vitro transcription and translation of mRNAs that contained G4-forming sequences, suggesting additional mode of action. While G4 ligands inhibit telomerase activity, classical telomerase inhibitors without G4-stabilizing activity had no preferential impact on glioma stem cells. Together, these observations suggest that G4 is a promising target for pinpointing intractable glioma stem cells. Citation Format: Sachiko Okabe, Takahiro Nakamura, Daiki Hasegawa, Reina Kojima, Keiji Okamoto, Ichiro Nakano, Kazuo Shin-ya, Kazuo Nagasawa, Hiroyuki Seimiya. Targeting glioma stem cells by pharmacologic stabilization of G-quadruplexes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4839.