Telomerase Telomere Terminal Transferase and its Role in Cancer Brian R. Keppler February 27, 2003
The Telomere • Found on the ends of eukaryotic chromosomes. • Characterized by a 3’-overhang of single-stranded DNA. • Forms a T-loop with the help of specialized proteins to protect itself from exonuclease activity. • Shortening of telomeres results in the loss of genetic information and ultimately cell death.
DNA Replication • Semi-conservative • Bi-directional • Occurs in the 5’ to 3’ direction only • Leading strand is synthesized continuously • Lagging strand is synthesized discontinuously - Can’t replicate to entirety - Gradually shortens Lodish, H. et al., Molecular Cell Biolgy, 4th Ed., Freeman Publishing, 2000, p.461.
Telomerase • A specialized form of reverse transcriptase that carries its own internal RNA template to direct DNA synthesis. • Elongates the lagging strand template from its 3’-OH end. • Adds short, repeated, guanosine-rich sequences to the ends of chromosomes. ______________________________________________ RNA template sequence Organism Telomeric DNA sequence H. sapien 5’-T2AG33’-UCCCAAUC T. thermophila 5’-T2G4 3’-AACCCCAA O. bifaria 5’-T4G43’-CCAAAACCCC A. thaliana 5’-T3AG3 unidentified E. aediculatus 5’-T4G4 3’-CCAAAACCCCAAAAC S. cerevisiae 5’-T1-6GTG2-33’-CACACACCCACACCAC
Proposed “Rough” Model of Telomerase Alberts, B. et al., Molecular Biology of the Cell, 4th Ed., Garland Science, 2002, p. 264.
Mechanism of Telomerase Action Lodish, H. et al., Molecular Cell Biolgy, 4th Ed., Freeman Publishing, 2000, p.466.
Telomerase and Cancer • Telomerase is up-regulated in the vast majority of human cancers and serves to halt the progressive telomere shortening that ultimately blocks would-be cancer cells from achieving a full malignant phenotype. • Telomerase is active in the germ line and in renewable cells (e.g. bone marrow). • Most human somatic cells lack telomerase activity. • Telomere shortening • Senescence • End-to-end chromosome fusion and cell death • Most tumors regain the ability to produce telomerase. - Cells become immortal Artandi, SE et al., Proc Natl Acad Sci U S A 2002 May 28
How Do Cancer Cells Activate Telomerase Activity? • Some cancer cells with telomerase activity arise from mutant precursor cells that have avoided telomere shortening. - These cells have never encountered a telomeric limit to cell division. • Telomerase can be activated after a genetic catastrophe (e.g. p53 loss). - Loss of checkpoint controls, uncontrolled proliferation, accumulation of mutations, chromosomal instability...But, telomeres are also shortening with each successive generation of cells. - The cell up-regulates telomerase expression in order to regain stability and survive. Alberts, B. et al., Molecular Biology of the Cell, 4th Ed., Garland Science, 2002, p. 1348.
Targeting Telomerase as an Anticancer Drug Approach • Targeting the protein/RNA subunit • Mouse models • Specificity?