Transcript The Telomere

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
T. thermophila
O. bifaria
A. thaliana
E. aediculatus
S. cerevisiae
5’-T2AG3
3’-UCCCAAUC
5’-T2G4
3’-AACCCCAA
5’-T4G4
3’-CCAAAACCCC
5’-T3AG3
unidentified
5’-T4G4
3’-CCAAAACCCCAAAAC
5’-T1-6GTG2-3
3’-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?