Parallel G-quadruplexes formed by guanine-rich microsatellite repeats inhibit human topoisomerase I

Using UV and CD spectroscopy, we studied the thermodynamic stability and folding topology of G-quadruplexes (G4), formed by G-rich fragments in human microsatellites that differ in the number of guanosines within the repeating unit. The oligonucleotides d(GGGT)4 and d(GGT)4 were shown to form propeller-type parallel-stranded intramolecular G-quadruplexes. The G4 melting temperature is dramatically decreased (by more than 45°C) in the transition from the tri-G-tetrad to the bi-G-tetrad structure. d(GT)n-repeats do not form perfect G-quadruplexes (one-G-tetrad); folded G4-like conformation is not stable at room temperature and is not stabilized by monovalent metal ions. The minimum concentration of K+ that promotes quadruplex folding of d(GGT)4 was found to depend on the supporting Na+ concentration. It was demonstrated for the first time that the complementary regions flanking G4-motifs (as in d(CACTGG-CC-(GGGT)4-TA-CCAGTG)) cannot form a double helix in the case of a parallel G4 due to the steric remoteness, but instead destabilize the structure. Additionally, we investigated the effect of the described oligonucleotides on the activity of topoisomerase I, one of the key cell enzymes, with a focus on the relationship between the stability of the formed quadruplexes and the inhibition degree of the enzyme. The most active inhibitor with IC50 = 0.08 µM was the oligonucleotide d(CACTGG-CC-(GGGT)4-TA-CCAGTG), whose flanking G4-motif sequences reduced the extreme stability of G-quadruplex formed by d(GGGT)4.