Telomere maintenance as a target for anticancer drug discovery

Abstract

The ends of the chromosomes in all eukaryotic species have specialized, non-coding DNA sequences that, together with associated proteins, are known as telomeres. These have developed to protect the chromosome ends from a range of otherwise catastrophic events. Telomeric DNA comprises simple tandem repeats of guanine-rich sequences, typified by the hexanucleotide repeat d(TTAGGG)_n in vertebrates. The extreme 3′ end of eukaryotic telomeric DNA is single-stranded, typically being 100–200 bases long. In normal cells, telomeres progressively decrease in length with each successive round of cell division owing to the inability of endogenous DNA polymerase to fully replicate the lagging telomeric DNA strand. Once a critical level of telomere shortening is reached, cells enter an irreversible phase of growth arrest, when they cease to divide and might then be directed to apoptotic cell death. In striking contrast, telomeres of cancer cells do not shorten on replication, but remain constant in length on succeeding generations owing to the activity of the enzyme telomerase, an RNA-dependent DNA polymerase that uses its own associated RNA template to catalyse the addition of telomeric DNA repeats to the 3′ end of the single-stranded DNA telomere. Telomerase is not expressed in normal human tissue, but is present in at least 80–85% of tumour cells, suggesting it might be both a diagnostic marker and a target for the design of broad-spectrum anti-cancer drugs. Proof-of-principle experiments have shown that telomerase inhibition leads to telomere length reduction, tumour-cell senescence and ultimately apoptosis. Three approaches to telomerase inhibition are the subject of current investigation: targeting the active site of the polymerase with small molecules; antisense oligonucleotides directed at the RNA template; and stabilization of the single-stranded telomeric DNA into quadruplex structures formed from stacked guanine quartets, which prevents the necessary hybridization to the RNA template. Owing to their mode of action, quadruplex-binding ligands might offer the advantages of rapid onset of action, and activity against the ∼15% of tumour cells that maintain their telomeres through a telomerase-independent mechanism.