Destabilization of DNA G-Quadruplexes by Chemical Environment Changes during Tumor Progression Facilitates Transcription

Abstract

DNA G-quadruplex formation is highly responsive to surrounding conditions, particularly K⁺ concentration. Malignant cancer cells have a much lower K⁺ concentration than normal cells because of overexpression of a K⁺ channel; thus, G-quadruplexes may be unstable in cancer cells. Here, we physicochemically investigated how changes in intracellular chemical environments in vitro and in cells influence G-quadruplex formation and transcription during tumor progression. In vitro, the stable G-quadruplex formation inhibits transcription in a solution containing 150 mM KCl (normal condition). As K⁺ concentration decreases, which decreases G-quadruplex stability, transcript production from templates with G-quadruplex-forming potential increases. In normal cells, the trend in transcript productions was similar to that in in vitro experiments, with transcription efficiency inversely correlated with G-quadruplex stability. Interestingly, higher transcript levels were produced from templates with G-quadruplex-forming potential in Ras-transformed and highly metastatic breast cancer cells (MDA-MB-231) than in nontransformed and control MCF-7 cells. Moreover, the amount of transcript produced from G-quadruplex-forming templates decreased upon addition of siRNA targeting KCNH1 mRNA, which encodes a potassium voltage-gated channel subfamily H member 1 (K_V10.1). Importantly, G-quadruplex dissociation during tumor progression was observed by immunofluorescence using a G-quadruplex-binding antibody in cells. These results suggest that in normal cells, K⁺ ions attenuate the transcription of certain oncogenes by stabilizing G-quadruplex structures. Our findings provide insight into the novel mechanism of overexpression of certain G-rich genes during tumor progression.