Poster Presentation 25th Lorne Cancer Conference 2013

The biological function of parallel telomeric G-quadruplexes and their interaction with the cancer associated enzyme telomerase (#415)

Katherine G Zyner 1 , Liana Oganesian 2 , Paul Bonnefin , Scott B Cohen 1 , Roger R Reddel 1 , Tracy M Bryan 1
  1. Children's Medical Research Institute, Westmead, NSW, Australia
  2. The Salk Institute for Biological Sciences, La Jolla, California, USA

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.

  1. Grieder C.W. and Blackburn E.H. (1985). Identification of a specific telomere terminal transferase activity in Tetrahymena extracts. Cell 43; 405-413
  2. Williamson, J.R. (1994). G-quartert Stuctures in Telomeric DNA. An Rev Biophysics & Biomolecular Structures 23; 703-730
  3. Zahler, A. M., Williamson, J. R., Cech, T. R. and Prescott, D. M. (1991). Inhibition of telomerase by G-quartet DMA structures. Nature 350, 718-720.
  4. Neidle, S. (2010) Human telomeric G-quadruplex: the current status of telomeric G-quadruplexes as the therapeutic targets in human cancer. FEBS J. 277, 1118-1125
  5. Oganesian, L., Bryan T.M., Larstfer, M.B. (2006) Extension of G-quadruplex DNA by ciliate telomerase. EMBO 25, 1148-1159
  6. Sen, D. And Gilbert W. (1988) Formation of parallel four-stranded complexes by guanine-rich motifs in DNA and its implications for meiosis Nature 334, 364-366.
  7. Oganesian L., Grahan M.E., Robinson, P.J. and Bryan T.M. Telomerase recognizes G-quadruplex and linear DNA as distinct substrates (2007) ACS Publications Biochemistry, 2007, 46 (40),11279–11290