Forever Young: Structural Stability of Telomeric Guanine-Quadruplexes in Presence of Oxidative DNA Lesions

Abstract

Human telomeric DNA (h-Telo), in G-quadruplex (G4) conformation, is characterized by a remarkable structural stability that confers it the capacity to resist to oxidative stress producing one or even clustered 8-oxoguanine lesions. We present a combined experimental/computational investigation, by using circular dichroism in aqueous solutions, cellular immunofluorescence assays and molecular dynamics (MD) simulations, that identifies the crucial role of the stability of G4s to oxidative lesions, related also to their biological role as inhibitors of telomerase, an enzyme overexpressed in most cancers associated to oxidative stress.

Impact of G-Quadruplexes on the Regulation of Genome Integrity, DNA Damage and Repair

“…One of the most common and well-documented oxidative guanine lesions is 8-oxo-guanine (8-oxoG) [ 113 ]. Molecular dynamics simulation and CD spectroscopy studies showed that the single guanine-to-8-oxoG substitutions within the quadruplex-forming sequence alter the folding of G4s in a location-dependent manner [ 114 , 115 ], significantly destabilize G4 structure and even lead to its unfolding due to a loss of a Hoogsteen hydrogen bonds within a G-tetrad and/or a steric clash between the proton H7 of 8-oxoG and the amino group of the neighboring guanine [ 116 , 117 ]. The oxidation of those guanosine residues that adopt the syn -conformation in the DNA G4 structure is accompanied by only minor structural changes and, as a consequence, poor G4 destabilization, while the oxidative modification of guanosines in the anti -conformation leads to significant conformational perturbations.…”

“…If there are no “spare” dGs in the sequence that can replace the damaged one, destabilization of a separate G-tract or exclusion of certain guanosines from the G4 structure is possible to preserve the partially formed quadruplex. Complete G4 unfolding usually correlates with destabilization of the central G-tetrad and the concomitant release of both central cations [ 114 ].…”

Never cared for what they do. High structural stability of Guanine-quadruplexes in presence of strand-break damages

“…In this case we can observe a rearrangement of the first peripheral quartet which is accompanied by a leakage of a K + cation, leading to the expulsion of the guanine G8 into a loop region and further the complete destabilization of the peripheral tetrad. In the past, we have already observed that the loss of the cation is an important phenomenon in the destabilization of the G4 structure [31,55]. Although it points out the crucial role played by the metal cations in dictating the stability of the G4 arrangements, this case remains isolated since it is only observed once in all our simulations and can then be considered as a rare event.…”

“…Once again, this feature can be correlated to the biological role of G4s, and the protective role they can play in presence of high oxidative stress. The global stability of G4s to strand-breaks is also resonating with their resilience to the presence of oxidative damages, which we have recently determined [31]. However, it has to be pointed out that the guanine core may be much more sensitive to the damages.…”

“…In particular, oxidative damages have been particularly scrutinized, due to the fact that G4s are inherently composed of guanine-rich sequences and the latter is the most easily oxidized nucleotide, being its most common oxidation product 8-oxo-guanine (8-OxoG) [27][28][29][30]. Although G4s are clearly considered hotspots for oxidative DNA damages, they have shown strong structural resistance to this class of lesion [31], also depending on the amount of oxidative lesions and on their position in the DNA backbone [31][32][33][34]. In addition to base modification or deletion, oxidative stress [35,36] and ionizing radiation [37] are able to induce DNA strandbreak damage.…”