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.

Organism Telomeric DNA sequence RNA template sequence______________________________________________H. sapien 5’-T2AG3 3’-UCCCAAUC

T. thermophila 5’-T2G4 3’-AACCCCAA

O. bifaria 5’-T4G4 3’-CCAAAACCCC

A. thaliana 5’-T3AG3 unidentified E. aediculatus 5’-T4G4 3’-CCAAAACCCCAAAAC S. cerevisiae 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 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

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.

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?