In contrast to the limited growth of normal human cells, cancer cells divide without limit. At least 85% of human cancers rely on the ribonucleoprotein enzyme telomerase, which extends chromosome ends (also known as telomeres), to sustain their unlimited proliferation [1]. Telomerase is absent in most normal tissues and therefore this enzyme and its interaction with telomeric DNA represents a potentially effective and specific target for future cancer therapy.
Due to their guanine-rich nature, the 3’ overhang of telomeres can form compact secondary nucleic acid structures called G-quadruplexes [2]. In 1991 it was shown that intramolecular telomeric G-quadruplexes could inhibit telomere extension by telomerase.[3]. As a result, many laboratories are currently developing small molecules for use as anti-cancer therapeutics due to their ability to lock telomeric sequences in G-quadruplex conformations and subsequently inhibit telomerase activity in vivo.[4]. However, the biologically significant G-quadruplex telomeric conformations and their roles within human cells and tumours have not yet been identified. As such, global telomeric G-quadruplex stabilisation may result in unintended consequences during clinical use.
Our lab has shown that intermolecular parallel telomeric G-quadruplex structures from both ciliate [5] and human species can be extended by telomerase, suggesting the importance of this conformation in vivo. This particular G-quadruplex has been hypothesised to facilitate homologous chromosome alignment and telomere clustering during prophase I of meiosis [6]. To investigate this hypothesis and further analyse the biological relevance of telomerase’s interaction with the parallel quadruplex, we have utilised the ciliated protozoan Tetrahymena thermophila, which is a valuable model system for studying both telomeres and meiosis. A mutant T. thermophila strain was created using the telomerase TERT mutation K538A, which allows telomerase to elongate linear telomeric sequences but not parallel G-quadruplexes [7]. Cells expressing mutant telomerase exhibit a “monster cell” phenotype and multiple nuclei, suggesting that the G-quadruplex/telomerase interaction is of critical importance for mitotic division.