Telomeres and Telomerase
The ends of eukaryotic linear chromosomes end in repetitive DNA repeats called telomeres. In humans, chromosomes end in ~5-15 kilobase pairs of the tandem repeat 5′-TTAGGG-3′; this is terminated with a few hundred nucleotides of single-stranded TTAGGG repeats, leaving a 3′-overhang. Telomeric DNA is further bound with sequence-specific binding proteins in a six-protein complex termed ‘shelterin’.
Together, these structural features provide a protective cap to distinguish the end of the chromosome from an internal double-strand break, thereby protecting chromosome ends from aberrant DNA repair and recombination.
A dynamic feature of telomeres is that they shorten during every cycle of DNA replication and cell division; this shortening is a consequence of the inability of the DNA replication machinery to fully replicate to the end of a linear template. In humans, telomeres shorten by ~50-150 base pairs through each cycle of DNA replication. When telomeres have shortened sufficiently, the cellular senescence program is activated, resulting in permanent withdrawal from the cell cycle. Telomere shortening is a powerful and essential tumour suppressor mechanism; unless nascent premalignant cells can prevent it from occurring, they will eventually stop dividing and be unable to form a tumour. Cells that have high or unlimited proliferative capacity must have a mechanism to counteract telomere shortening.
Telomerase is the ribonucleoprotein enzyme complex that catalyses the addition of telomeric DNA repeats. The catalytic protein component, Te
ranscriptase (hTERT in human telomerase), catalyses nucleotide addition onto the 3′-end of the telomere and contains the conserved reverse transcriptase motifs common to retroviral RTs. The RNA component (hTR in human telomerase) directs nucleotide addition by providing a template for reverse transcription that is complementary to the telomeric repeat sequence.
Once nucleotide addition has reached the end of the template, the enzyme translocates a distance of six nucleotides to repeat the nucleotide addition cycle. Thus, telomerase is able to replace the telomeric DNA repeats that are lost during the process of DNA replication.
A telomere maintenance mechanism is essential for a cell to bypass senescence as a response to short telomeres. While most normal cells lack a telomere maintenance mechanism, excessive and dysregulated telomerase activity is strongly associated with cancer. By expressing abnormally high levels of telomerase, cancer cells are able to avoid telomere shortening and thus bypass senescence, thereby becoming immortal. Telomerase is expressed in ≥85% of all human cancers and is considered a prime target for anticancer therapeutics through the development of telomerase inhibitors.
Research in the CBU
Our research projects are collectively directed at understanding the properties of telomerase at the cellular and biochemical level, including association with its telomeric DNA substrate, active recruitment of telomerase to the telomere, telomerase enzymology, and structure, with the long-term aim of using this knowledge to rationally design small-molecule inhibitors of telomerase as potential anti-cancer therapeutics. Much of our research is enabled by our ability to over-express and purify human telomerase on large-scale.
Below are descriptions of specific projects on-going in the Cell Biology Unit for fellow researchers that may be interested in collaboration, or prospective students wishing to carry out their Honours or PhD research degrees in the CBU: