G-quadruplex preferentially forms at the very 3′ end of vertebrate telomeric DNA

Abstract

Human chromosome ends are protected with kilobases repeats of TTAGGG. Telomere DNA shortens at replication. This shortening in most tumor cells is compensated by telomerase that adds telomere repeats to the 3′ end of the G-rich telomere strand. Four TTAGGG repeats can fold into G-quadruplex that is a poor substrate for telomerase. This property has been suggested to regulate telomerase activity in vivo and telomerase inhibition via G-quadruplex stabilization is considered a therapeutic strategy against cancer. Theoretically G-quadruplex can form anywhere along the long G-rich strand. Where G-quadruplex forms determines whether the 3′ telomere end is accessible to telomerase and may have implications in other functions telomere plays. We investigated G-quadruplex formation at different positions by DMS footprinting and exonuclease hydrolysis. We show that G-quadruplex preferentially forms at the very 3′ end than at internal positions. This property provides a molecular basis for telomerase inhibition by G-quadruplex formation. Moreover, it may also regulate those processes that depend on the structure of the very 3′ telomere end, for instance, the alternative lengthening of telomere mechanism, telomere T-loop formation, telomere end protection and the replication of bulky telomere DNA. Therefore, targeting telomere G-quadruplex may influence more telomere functions than simply inhibiting telomerase.

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

“…Interaction of Gquadruplex ligand and the telomeric single-strand overhang have been reported for telomestatin (Gomez et al, 2004). In agreement with this, DMS footprinting and exonuclease hydrolysis has revealed that G-quadruplex preferentially forms at the very 3' end of the telomeric DNA (Tang et al, 2008). Using this method, it will be interesting in the future to analyze the position of the G-quadruplex stabilized by 360A.…”

Single Molecule Studies of Physiologically Relevant Telomeric Tails Reveal POT1 Mechanism for Promoting G-quadruplex Unfolding

“…The presence of loops with various lengths on the tetraplex sides can potentially lead to irregularities in G4 structure and consequently cause structure destabilization. Current literature suggests that loop length and composition strongly influence the quadruplex stability, and quadruplexes formed by (TTAGGG) 5 with a 9-nucleotide loop were less stable than quadruplexes formed from four consecutive repeats (41).…”

NSC746364, a G-Quadruplex-Stabilizing Agent, Suppresses Cell Growth of A549 Human Lung Cancer Cells Through Activation of the ATR/Chk1-Dependent Pathway

“…In the ALT mechanism, the longer telomeres can be a template of DNA polymerase-mediated telomere elongation after that was invaded by a short telomeric strand of a second chromosome (41). The telomeric G-quadruplex structure preferentially formatted at the farthest 3′ end of the telomeric single-strand overhang, which will remain a shorter single-strand tail that cannot be an effective template for the telomerase or the ALT-mediated telomere extension (42,43). However, several proteins have been identified to be involved in the unfolding or destabilization of the telomeric G-quadruplex structure (44 -46), which suggested that the coordination between telomeric G-quadruplex formation and its unfolding may determine the availability of telomere extension.…”