Molecular Recognition of Human Telomeric DNA by Phenanthroline-Based G-Quadruplex Ligands

Abstract

G-quadruplexes (G4) are non-canonical DNA structures assumed by guanine rich sequences. G4 are stabilized by the presence of cations and are characterized by a high degree of structural polymorphism with different patterns of groove, loop arrangement, strand orientations and stoichiometry. G-rich sequences are over-represented in the promoter regions of many oncogenes as well as at human telomeres, d(TTAGGG) repeats, ranging in size from 3 to 15 kb, involved in protecting chromosomal ends. A specialized enzyme, called telomerase, provides a telomere maintenance mechanism by elongating the end of the G-strand and it is activated in the majority of cancer cells. Therefore there are two general strategies of telomerase targeting in cancer treatment. One is a direct targeting of telomerase to cause its inhibition; the other one is the use of G4 stabilizers which block telomerase access to telomere, thus causing an indirect enzyme inhibition. Here, we evaluated the molecular recognition of some phenanthroline-based ligands against four different experimental models of the human telomeric sequence d[AG3(T2AG3)3] by means of docking simulations. Our theoretical analysis was able to reproduce the experimental affinity measurements, with a linear squared correlation factor r2 equal to 0.719 among all the studied models. These findings highlighted the importance to consider the polymorphism of the DNA G4. Interestingly, this correlation resulted always improved with respect to that of the single folds, with the exception of the parallel structure, thus suggesting a key role of this G4 conformation in the interaction network of the tested binders. Moreover, we identified the moieties of the phenanthroline scaffold directly involved in the complex formation. This allowed to rationalize the improved binding affinity always associated with a bis-phenanthroline system and to explain why a phenanthroline substituted with a pyridine ring is favored with respect to the pyrimidine one.