Stable G quadruplex formed in promoter region of oncogene RET in physiological solution conditions

The promoter region of proto-oncogene RET has continuous guanine-rich regions I–V, which form G-quadruplex (G4) structures. Reported RET20T G4 in the regions II–V has parallel folding in potassium-ion solutions; however, this G4-DNA is not an ideal drug target, as its formation is limited to a low concentration potassium-ion solution. Here, we demonstrate that RET-21mer in the regions I–IV forms a mixed parallel/anti-parallel G4 in either 50 mM K+ or 100 mM Na+ solutions. It is stable, with a melting temperature of 74.3°C in 100 mM Na+ solution and 91.2°C in 100 mM K+ solution. A loop forms a special G⋅G base-pair plane in K+ solution or an unconventional A⋅G⋅G plane in Na+ solution, stacking with the 3′ end G tetrad, stabilizing its conformation. We also show that RET-21mer G4 could exist under physiological conditions. Therefore, RET-21mer G4s are likely a major G4-DNA, providing a research direction for anti-cancer drug screening.