Preferential binding of a G-quadruplex ligand to human chromosome ends

The G-overhangs of telomeres are thought to adopt particular conformations, such as T-loops or G-quadruplexes. It has been suggested that G-quadruplex structures could be stabilized by spe-cific ligands in a new approach to cancer treatment consisting in inhibition of telomerase, an enzyme involved in telomere maintenance and cell immor-tality. Although the formation of G-quadruplexes was demonstrated in vitro many years ago, it has not been definitively demonstrated in living human cells. We therefore investigated the chromosomal binding of a tritiated G-quadruplex ligand, 3H-360A (2,6-N,N 0-methyl-quinolinio-3-yl)-pyridine dicarbox-amide [methyl-3H]. We verified the in vitro selectivity of 3H-360A for G-quadruplex structures by equilib-rium dialysis. We then showed by binding experi-ments with human genomic DNA that 3H-360A has a very potent selectivity toward G-quadruplex struc-tures of the telomeric 30-overhang. Finally, we per-formed autoradiography of metaphase spreads from cells cultured with 3H-360A. We found that 3H-360A was preferentially bound to chromosome terminal regions of both human normal (peripheral blood lymphocytes) and tumor cells (T98G and CEM1301). In conclusion, our results provide evid-ence that a specific G-quadruplex ligand interacts with the terminal ends of human chromosomes. They support the hypothesis that G-quadruplex ligands induce and/or stabilize G-quadruplex structures at telomeres of human cells