Differential Effects of the G-Quadruplex Ligand 360A in Human Normal and Cancer Cells

Abstract

Telomeres are essential for chromosome replication and genome integrity. The 3’ singlestranded overhang of human telomere may adopt particular conformations such as T-loops and G-quadruplexes. Reactivated in most tumors, telomerase, a specific reverse transcriptase that elongates the telomeres, is thought to enable cancer cells to proliferate in an unlimited manner, thereby correcting the normal telomere erosion that occurs during cell division. The level of interest in G-quadruplex has increased due to their ability to inhibit telomerase activity. We have investigated chromosomal binding and the cellular effects induced by pyridine derivative G-quadruplex ligand in human normal and tumour cells. We show from our analysis that this G-quadruplex ligand preferentially binds the terminal regions of chromosomes in both normal and tumour cells. This compound also induces DNA damage signals in a strictly ATM-dependent manner, inhibits cell proliferation and induces apoptosis. We further observed by telo-FISH and Chromosome Orientation-FISH that this compound induces specific telomere aberrations either during or after replication and mainly consisting of sister telomere fusions and recombination events principally involving the lagging strand telomeres. We have also observed that ATM (Ataxia Telangiectasia Mutated) and ATR (Ataxia Telangiectasia Related) reduce telomere instability independently of apoptosis suggesting its direct role in preventing inappropriate DNA repair at the telomeres. We have further demonstrated that, even at elevated concentrations, G-quadruplex ligand has limited effects on proliferation of normal cells, and does not induce apoptosis or telomere aberrations. Interestingly, we observed induction of reversible premature senescence in primary fibroblasts. Taken together, our results suggest that the protein composition and/or organization of the telomeres differ markedly between normal and cancer cells, and provide higher telomere stability to normal cells.