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What is the Role of Telomerase in Cancer?
Cancer is defined as any tumor or malignant growth that invades other tissues due to abnormal cell
proliferation (Royal Society of Chemistry, 2003). Cancer cells lose their ability to undergo apoptosis
(programmed cell death) and divide uncontrollably. When the DNA of normal somatic cells are damaged,
proteases and nucleases destroy the cell. This destruction decreases the chance of mutated cells from
surviving. Cancer cells bypass senescence (non-dividing state) and divide without restraint. Growth
factors and other extracellular factors regulate cell growth in non-cancerous cells (Royal Society of
Chemistry, 2003). In cancerous cells these factors no longer correctly regulate cell growth. In cancerous
cells, telomerase activity exists because the cell is dividing. Although stem cells and germ-line cells
experience ongoing cell division, normal (non-dividing) somatic cells lack active telomerase because
telomerase is inactive.
Telomerase is a specialized ribonucleoprotein that is present replicating cells such as cancerous
cells. Telomerase is an enzyme that catalyzes the attachment of telomeres, or repeat bases TTAGGG, at
the end of every chromosome (Figure 1). Telomeres protect the DNA of the cell and prevent mutations
from occurring. The presence of telomerase during DNA replication is vital to the proliferation of cancer
cells because telomeres need to be added to the end of the newly formed DNA (Bearss, 2002). The end of
the telomere forms a t-loop and a d-loop which help control telomere length. The t-loop and d-loop have
binding sites where telomere-associated proteins, such as TRF1 and TRF2 specifically bind to control
division. As the telomere increases in length, a t-loop forms and inhibits telomerase, from extending the
telomere. During DNA replication, telomeres decrease in length by 25-200 base pairs of DNA (Bearss,
2002). Cancer cells elongate their shortened telomeres by telomerase activity.
Human telomerase (Figure 2) consists of the catalytic subunit, hTERT and an RNA template hTR.
hTERT catalyses telomere lengthening by serving as a template for the second telomerase component
(hTR) to bind. The hTR component of telomerase is approximately 445 nucleotides with its 11
nucleotides, sequence CAAUCCCAUC identified as the active binding site for hTR. Here base A-U
pairing G-C interactions elongate the telomere (Royal Society of Chemistry, 2003).
Human telomerase is active in sperm, ova, and developing embryos: “Once the body is formed and their
telomeres of sufficient length have been synthesized, the telomerase activity is shut off in the majority of
cells and their telomeres begin to shorten as they reproduce” (Royal Society of Chemistry, 2003). The
research article entitled, “Telomere maintenance mechanisms as a target for drug development”,
telomerase is shown to be active in 85% of human malignancies, and only 0.5% of normal cells (Bearss,
2002). This finding is significant because inhibiting telomerase would specifically target cancerous cells
and have little effect on normal cells.
Viviana Sumi Lee (Dr. Emily Schmitt, advisor)
Department of Math, Science, and Technology
Telomerase Inhibitors in Phase II Clinical Trials
The objective of this experiment conducted by Nair et al. was to determine whether
immunization against hTERT (catalytic reverse transcriptase protein subunit) generated a cytotoxic
T lymphocyte (CTL) immune response against tumors that were distinct from one another (Nair,
2000). The researchers tested and found that the effects of inoculating TERT RNA-transfected
dendritic cells (DC) into mice stimulated CTL. These lymphocytes destroyed and lysed “melanoma
and thymoma tumor cells” and inhibited proliferation of three unrelated tumors in mice of different
genetic backgrounds (Nair, 2000). The purpose in using hTERT RNA-transfected dendritic cells
(DC) was to activate a CTL response, in vitro and in vivo, specific to TERT. “TERT RNAtransfected human DC stimulated TERT-specific CTL in vitro that lysed human tumor cells,
including Epstein Barr virus (EBV)-transformed B cells as well as autologous tumor targets from
patients with renal and prostate cancer” (Nair, 2000). Researchers concluded that, evidently,
cytotoxic T cells destroyed tumor cells taken from renal cancer patients that were placed in culture.
Currently, Geron Corp. of Menlo Park, California developed a potential telomerase vaccine
which is in Phase II trials for prostate cancer (Figure 5). Blood that is cultured from a cancer patient
is surged with telomerase RNA into its dendritic cells. The telomerase vaccine operates in a way
where the injected telomerase RNA dendritic cells are reinjected into the patient. The newly
reinjected dendritic cells signal an immune response (that of cytotoxic T cells) in pursuit of
telomerase. If Phase II trials of telomerase vaccine are successful, the vaccine may be used to treat
many cancers due to active presence of telomerase in cancer cells (Jaffe, 2004).
Figure 2: Structure of Telomerase. Image taken from
http://www.chemsoc.org/exemplarchem/entries/2003/imperial_Burgoine/Fight
Cancer.txt.html
Additional research by Elayadi et al., 2001 discussed the “Inhibition of telomerase by 2′-O-(2
methoxyethyl) RNA oligomers” and observed the effects of length, phosphorothioate substitution and
time of the oligomer inside cells” (Elayadi, 2001). Once telomerase inhibitors were shown to end cell
life cycle in vitro, tests were performed to see what combination of nucleotides would produce a stable
oligonucleotide in vivo. Elayadi et al. 2001 treated DU145 prostate cancer cells with different RNA
sequences to determine which one would be most stable in vivo testing. A 13 base phosphodiester 2′MOE RNA oligonucleotide called ISIS 24691 was found to be most pharmacokinetically stable. Two
agents were found to inhibit cancer cell growth for up to 7 days with one dose (ISIS 24691 and
ISIS113749), but because ISIS24691 is a shorter RNA oligonucleotide, it is more cost effective and
practical to produce
Cannabinoid Effects on MS Symptoms
Practical Application of Telomerase Inhibitors
Figure 1: Structure of telomere from chromosome. Image taken from
http://www.chemsoc.org/exemplarchem/
entries/2003/imperial_Burgoiee/origins.txt.html
An oncogene is defined as a cancer “causing” gene which is simply a “normal” gene involved in
growth or cell division that is mutated: leading to cancer. Two genes involved in cell proliferation that
are commonly mutated in cancer cells are the Rb and p53 genes. If the gene is mutated it is followed by a
(-) sign (Example: p52-). If it is not mutated it is known as wild type and is followed by a + sign.
Telomerase inhibitors work only when the telomeres are completely reduced in size. Complete telomere
shortening does not occur until many cell generations later. This led researchers to conclude that
telomerase inhibition would have no effect on cell proliferation until the telomeres would be completely
shortened (Chen et al, 2003). In their research Chen et al, 2003 demonstrate that there is no significant
loss in cell proliferation with short term treatment. However, with long term treatment telomerase
inhibitor 2’-0-methoxyethyl oligonucleotide binds to the RNA template on telomerase and acts as a
competitive inhibitor. The results of their experiment show that 2’-0-methoxyethyl RNA is effective on
the tested DU145 cell line, which has Rb- and p53- genes, but not effective on the tested LNCaP cell
line, which has Rb+, p53+ gene. This is significant because the antiproliferative effects of the 2’-Omethoxyethyl RNA were specific for the mutated gene, but did not inhibit proliferation of the wild type
cells. This allows for a higher dose of telomerase inhibitors which is specific for destroying cancer cells
but not showing antiproliferative effects on normal non-mutated cells. (Chen et al, 2003)
The Significance of Telomerase Inhibitors
The presence of telomerase is vital to the proliferation and differentiation of cancer cells
because it adds the telomere to the end of replicating DNA (Figure 3 and 4). By inhibiting telomerase
each subsequent cell generation will have shorter telomeres until cell proliferation ceases.
Oligonucleotides that inhibit telomerase are shown to decrease proliferation in cancer cells. Since
“telomeres in cancer cells are hundreds or thousands of bases long, and if telomerase is fully
inhibited, telomeres may erode at a rate of 50 – 200 bases per population doubling, with variability
likely caused by genetic background and growth conditions” (Chen, 2003). Observation of telomerase
activity in tumor cells has become a significant and ever-growing approach to cancer therapeutic
development. To develop a better understanding and approach to cancer therapeutics, several
experiments were conducted to examine the effects of telomerase inhibition by using powerful MOE
(2’-O-methoxyethyl) RNA oligomers. Significant antiproliferative effects were seen (Chen, 2003).
Because there was an increase in telomerase activity in human tumors, a hypothesis was
formed, “that tumor growth requires reactiviation of telomerase and that telomerase inhibitors
represent a class of chemotherapeutic agents” (Herbert et al., 1999). In this experiment, progressive
telomere shortening by 2′-O-MeRNA caused “cell death” in cancer cells. They also set the criteria
for telomerase dependent mechanisms of cell death: i) cell growth rates would not be immediately
reduced because the telomeres would not be shortened immediately, ii) rather the telomeres would be
shortened with every division until iii) cell growth would stop.
.
Figure 3: Fluorescent image of telomeres (orange
dots) located at the end of mouse chromosomes.
Image taken from Campbell and Reece, 2002.
Figure 4: Telomeres and telomerase
activity. Image taken from Campbell and
Reece, 2002
Figure 5: Mechanism for Telomerase Vaccine (bottom right corner). Image taken
from The Scientist, 2004.
Literature Cited
Bearss, D. J., Hurley, L. H., and Von Hoff, D. D. Telomere maintenance mechanisms as a target
for drug development. Oncogene, 19: 6632-6641, 2002.
Brown, J. M., and Wouters, B. G. Apoptosis, p53, and tumor cell sensitivity to anticancer agents.
Cancer Research, 59: 1391-1399, 1999.
Campbell, N. A. and Reece, J. B. Telomeres and telomerase. Biology. Benjamin Cummings,
2002, p.300.
Chen, Z., Corey, D. R., and Koeneman, K. S. 2003. Consequences of Telomerase Inhibition and
Combination Treatments for the Proliferation of Cancer Cells. Cancer Research, 63: 5917-5925.
Elayadi, A. N., Demieville, A., Wancewicz, E. V., Monia, B. P., and Corey, D. R. Inhibition of
telomerase by 2′-O-(2-methoxyethyl) RNA oligomers: effect of length phosphorothioate
modification, and time inside cells. Nucleic Acids Research, 29: 1683-1689, 2001.
Herbert, B-S., Pitts, A. E., Baker, S. I., Hamilton, S. E., Wright, W. E., Shay, J. W., and Corey,
D. R. Inhibition of human telomerase in immortal human cells leads to progressive telomere
shortening and cell death. Proc. Natl. Acad. Sci. USA, 96: 14726-14781, 1999.
Jaffe, S. 2004. Vax Facts. The Scientist. March 15, 2004: 18, 5: p.25.
Nair, S.K., Heiser, A., Boczkowski, D., Majumdar, A., Naoe, M., Lebkowski, J., Vieweg, J., and
Gilboa, E. Induction of cytotoxic T cell responses and tumor immunity against unrelated
tumors using telomerase reverse transcriptase RNA transfected dendritic cells. Nature
Medicine, 6: 1011-1017, 2000.
Royal Society of Chemistry, 2003. Origins of G-Quadruplex DNA. www.Chemsoc.org.
Retrieved 3/11/2005 from
http://www.chemsoc.org/exemplarchem/entries/2003/imperial_Burgoine/origins.txt.html
Stott, B and Wyse, M. Biomedicine – How to Catch Cancer. biotech.ubc.ca. Retrieved 3/11/05
from http://biotech.ubc.ca/Biomedicine/HowToCatchACancer/