Targeting human telomerase for cancer therapeutics

The enzyme telomerase is involved in the replication of telomeres, specialized structures that cap and protect the ends of chromosomes. Its activity is required for maintenance of telomeres and for unlimited lifespan, a hallmark of cancer cells. Telomerase is overexpressed in the vast majority of human cancer cells and therefore represents an attractive target for therapy. Several approaches have been developed to inhibit this enzyme through the targeting of its RNA or catalytic components as well as its DNA substrate, the single-stranded 3'-telomeric overhang. Telomerase inhibitors are chemically diverse and include modified oligonucleotides as well as small diffusable molecules, both natural and synthetic. This review presents an update of recent investigations pertaining to these agents and discusses their biological properties in the context of the initial paradigm that the exposure of cancer cells to these agents should lead to progressive telomere shortening followed by a delayed growth arrest response.

Stabilization of the c-myc gene promoter quadruplex by specific ligands' inhibitors of telomerase

A parallel G-quadruplex structure was recently identified in the NHE III(1) element of the c-myc gene promoter that functioned as a transcriptional repressor. Different series of telomeric G-quadruplex interacting ligands reported to block telomerase activity were evaluated in a new PCR stop assay on the c-myc quadruplex (Pu22myc). Results indicated that the cationic porphyrin TMPyP4 previously described to stabilize c-myc quadruplex and to cause transcription inhibition efficiently inhibited the assay but with a narrow selectivity when parallel experiments were performed with an oligonucleotide (Pu22mu) containing mutations in the guanine repeat which is unable to form a quadruplex. Other ligands presented potent inhibitory properties with IC(50) in the submicromolar range. 307A, a new 2,6-pyridin-dicarboxamide derivative was found to present the highest selectivity as compared to Pu22mu oligonucleotide (>90-fold). Comparison with telomeric G-quadruplex using TRAP-G4 and PCR stop assays also indicated that ligands 307A, telomestatin, and TMPyP4 are equipotent against both c-myc and telomeric sequences while other ligands displayed some partial selectivity (2- to 6-fold) towards one of these sequences. This work provides evidence that G-quadruplex ligands reported as telomerase inhibitors efficiently stabilized c-myc promoter intramolecular quadruplex and may also potentially be used to inhibit c-myc gene transcription in tumor cells.

Interaction of Telomestatin with the Telomeric Single-strand Overhang

The extremities of chromosomes end in a G-rich single-stranded overhang that has been implicated in the onset of the replicative senescence. The repeated sequence forming a G-overhang is able to adopt a peculiar four-stranded DNA structure in vitro called a G-quadruplex, which is a poor substrate for telomerase. Small molecule ligands that selectively stabilize the telomeric G-quadruplex induce telomere shortening and a delayed growth arrest. Here we show that the G-quadruplex ligand telomestatin has a dramatic effect on the conformation of intracellular G-overhangs. Competition experiments indicate that telomestatin strongly binds in vitro and in vivo to the telomeric overhang and impairs its single-stranded conformation. Long-term treatment of cells with telomestatin greatly reduces the G-overhang size, as evidenced by specific hybridization or telomeric oligonucleotide ligation assay experiments, with a concomitant delayed loss of cell viability. In vivo protection experiments using dimethyl sulfate also indicate that telomestatin treatment alters the dimethyl sulfate effect on G-overhangs, a result compatible with the formation of a local quadruplex structure at telomeric overhang. Altogether these experiments strongly support the hypothesis that the telomeric G-overhang is an intracellular target for the action of telomestatin.

Telomerase downregulation induced by the G-quadruplex ligand 12459 in A549 cells is mediated by hTERT RNA alternative splicing

Ligand 12459, a potent G-quadruplex-interacting agent that belongs to the triazine series, was previously shown to downregulate telomerase activity in the human A549 lung carcinoma cell line. We show here that the downregulation of telomerase activity is caused by an alteration of the hTERT splicing pattern induced by 12459, i.e. an almost complete disappearance of the active (+alpha,+beta) transcript and an over-expression of the inactive -beta transcript. Spliced intron 6 forming the -beta hTERT transcript contained several tracks of G-rich sequences able to form G-quadruplexes. By using a specific PCR-stop assay, we show that 12459 is able to stabilize the formation of these G-quadruplex structures. A549 cell line clones selected for resistance to 12459 have been analyzed for their hTERT splicing pattern. Resistant clones are able to maintain the active hTERT transcript under 12459 treatment, suggesting the appearance of mechanisms able to bypass the 12459-induced splicing alterations. In contrast to 12459, telomestatin and BRACO19, two other G-quadruplex-interacting agents, have no effect on the hTERT splicing pattern in A549 cells, are cytotoxic against the A549-resistant clones and display a lower efficiency to stabilize hTERT G-quadruplexes. These results lead us to propose that 12459 impairs the splicing machinery of hTERT through stabilization of quadruplexes located in the hTERT intron 6. Differences of selectivity between 12459, BRACO19 and telomestatin for these hTERT quadruplexes may be important to explain their respective activity and inactivity against hTERT splicing.

Resistance to the short term antiproliferative activity of the G-quadruplex ligand 12459 is associated with telomerase overexpression and telomere capping alteration

Ligands that stabilize the telomeric G-rich single-stranded DNA overhang into G-quadruplex can be considered as potential antitumor agents that block telomere replication. Ligand 12459, a potent G-quadruplex ligand that belongs to the triazine series, has been previously shown to induce both telomere shortening and apoptosis in the human A549 cell line as a function of its concentration and time exposure. We show here that A549 clones obtained after mutagenesis and selected for resistance to the short term effect of ligand 12459 frequently displayed hTERT transcript overexpression (2-6-fold). Overexpression of hTERT was also characterized in two resistant clones (JFD10 and JFD18) as an increase in telomerase activity, leading to an increase in telomere length. An increased frequency of anaphase bridges was also detected in JFD10 and JFD18, suggesting an alteration of telomere capping functions. Transfection of either hTERT or DN-hTERT cDNAs into A549 cells did not confer resistance or hypersensitivity to the short term effect of ligand 12459, indicating that telomerase expression is not the main determinant of the antiproliferative effect of ligand 12459. In contrast, transfection of DN-hTERT cDNA into resistant JFD18 cells restored sensitivity to apoptotic concentrations of ligand 12459, suggesting that telomerase does participate in the resistance to this G-quadruplex ligand. This work provides evidence that telomerase activity is not the main target for the 12459 G-quadruplex ligand but that hTERT functions contribute to the resistance phenotype to this class of agents.

[G-quadruplex DNA: myth or reality?]

The peculiar sequence of telomeric DNA, composed of repetitions of the GGTTAG motif allows the formation of an unusual DNA conformation based on guanine-quadruplex (G-quadruplex). Small molecules that bind and stabilize telomeric DNA under its G-quadruplex conformation are able to impair telomerase activity. Several recent reports have shown that G-quadruplex ligands could block telomerase activity in cancer cells and represent a new experimental approach to limit cancer growth. The intracellular existence of G-quadruplex structure is still controversial, since no direct proof allowed to establish its reality. Many sequences of nucleic acids in the mammalian genome are able to form a G-quadruplex in vitro and several proteins have been described to interact in vitro with G-quadruplex. These data indicated that G-quadruplex are members of a family of target structures larger than that initially described at telomeres, and raised the question of the selectivity and therapeutic index of their ligands in the context of an antitumor therapy.