A brief review on Telomerase and its importance in aging and cancer.

1. HIMADRI NATHHIMADRI NATH
SEM -1,MSc ZOOLOGYSEM -1,MSc ZOOLOGY
ROLL:BGC/ZOOI NO:14101ROLL:BGC/ZOOI NO:14101
BARASAT GOVT. COLLEGEBARASAT GOVT. COLLEGE

3. Telomeres are the genetic material at the ends of chromosomes andTelomeres are the genetic material at the ends of chromosomes and
are essential for proper chromosome structure and function.are essential for proper chromosome structure and function.
Telomeres are a repeating sequence of base pairs. In humans, andTelomeres are a repeating sequence of base pairs. In humans, and
all vertebrates, the sequence is TTAGGG.all vertebrates, the sequence is TTAGGG.
The sequences form structures that protect the DNA from attack byThe sequences form structures that protect the DNA from attack by
nucleases that degrade the ends of DNA molecules in the cell.nucleases that degrade the ends of DNA molecules in the cell.

4. The Telomere. The DNA in human cells is present in 46 pieces called
chromosomes. Each end is capped with the telomeric repeat sequence
that is made to specifically glow bright green in this photograph.
(Courtesy of Peter Landsdorp).

5.  Without these end caps, chromosomes stick to one
another, and undergo structural changes. These activities
threaten cell survival and the faithful replication of
chromosomes.
 Chromosomes slowly lose base pairs at the end of their
strands during replication. To overcome the end-
replication problem, most eucaryotic cells utilize the
enzyme telomerase that allows the replication of the
lagging strand to be completed.
Ref: Blackburn, E.H.(1991), Structure and function telomeres.

6. THE END REPLICATION PROBLEM

7.  Telomerase is a ribonucleoprotein enzyme capable
of extending chromosome ends consisting mainly of
a protein.
 It also includes a single molecule of RNA that
contains the critical nucleotide template for building
telomeric subunits.

8. Proposed “Rough” Model of Telomerase

9.  Telomerase places one strand of DNA on the RNA,
positioning itself so that the template lies adjacent
to the tip of the chromosome.
 Then, the enzyme adds one DNA nucleotide at a
time until a full telomeric subunit is formed.
 When the subunit is complete, telomerase can
attach another by sliding to the new end of the
chromosome and repeating the synthetic process.

10. Ref:Alberts, B. et al., Molecular Biology of the Cell, 4th Ed., Garland
Science, 2002, p. 264.

13.  Telomerase is synthesized by nearly all organisms
with nucleated cells. The precise makeup differs
from species to species, but each version
possesses a species-specific RNA template.
 Lack of telomerase activity in human cells leads to
telomere shortening. Eventually this leads to an
end that is no longer recognized as a telomere by
the cell.
 For there to be continued cell division, telomere
loss must stop and telomerase must be activated.
Ref: Blackburn, E.H.(1991), Structure and function of telomeres.

14.  Leonard Hayflick first described cellular aging in detail in
1962. He explained that most normal human cells are
mortal because they can divide only a finite number of
times.
 In culture, cells divide from 20 to 100 times before they
stop. The limit of cell reproduction is often referred to as
the “Hayflick Limit.”
 Since Hayflick’s discoveries many theories have been
formed to explain the mechanism behind cell aging.
Ref: Hayflick L (1965). "The limited in vitro lifetime of human diploid cell
strains".

15.  The connection between telomeres and aging first
emerged in 1986 from observations made by Howard
Cooke.
 Cooke noticed that the length of telomeres in reproductive
cells were longer than telomeres in somatic cells. Since
Cooke understood that telomeres shortened each time
that a cell divided, he concluded that somatic cells must
not make telomerase.
Ref: Cooke, H.J. & Smith, B.A. (1986), Variability at the
telomeres of the human X/Y pseudoautosomal region.

16. Telomere Length ChangesTelomere Length Changes in Human Agingin Human Aging
In newborn humans,In newborn humans,
telomeres are approximately 15-20kb in length and shorten gradually throught life,telomeres are approximately 15-20kb in length and shorten gradually throught life,
suggesting that telomere length may serve as a surrogate marker for aging.suggesting that telomere length may serve as a surrogate marker for aging.

17.  Cooke’s discovery coincides with one of the theories used
to describe aging, the telomere hypothesis.
 This theory proposes that as mortal cells divide, they
eventually lose parts of their telomeres until they
ultimately die. Only cells that can continue to reactivate
their telomerase will continue to live. Cells that can not
reactivate their telomerase will stop dividing, or senesce.

18. ◊◊ In 1997, Thomas Cech and colleagues at GeronIn 1997, Thomas Cech and colleagues at Geron
Corporation isolated the human gene for aCorporation isolated the human gene for a
catalytic protein called telomerase reversecatalytic protein called telomerase reverse
transcriptase (hTRT).transcriptase (hTRT).
◊◊ This gene is only found in immortal cells. TheThis gene is only found in immortal cells. The
function of the hTRT is to add the repeatingfunction of the hTRT is to add the repeating
sequences to the chromosomes, lengthening thesequences to the chromosomes, lengthening the
telomeres.telomeres.

19. ◊◊ The group introduced the hTRT into mortal cells.The group introduced the hTRT into mortal cells.
◊◊ The outcome of the experiment was theThe outcome of the experiment was the
production of active telomerase and the extendedproduction of active telomerase and the extended
life of the originally mortal cells.life of the originally mortal cells.
◊◊ These findings show that by reactivatingThese findings show that by reactivating
telomerase activity in cells, cellular aging can betelomerase activity in cells, cellular aging can be
bypassed.bypassed.
Ref: Wright, W. E., and Shay, J. W.(2001) Cellular senescence as a
tumor protection mechanism

20.  There are many different types of cancers varying in
appearance and locations in the body. The unifying
aspect of cancer is uncontrollable growth and
immortality.
 The tissues expand without limit, compromising the
function of organs and threatening the life of the
organism.

21.  Cancers arise when a cell acquires multiple genetic
mutations that together cause the cell to escape from
normal controls on replication and migration.
 As the cell and its offspring multiply uncontrollably, they
can invade and damage nearby tissue. Through
metastasis, cells may invade nearby blood or lymphatic
vessels, establishing new malignancies at distant sites.

22.  In theory, a lack of telomerase would retard the growth of
tumors by causing continually dividing cells to lose their
telomeres and to die before they did any damage.
 Therefore, the removal of telomerase could be a possible
treatment for cancer.
 In regard to this theory, many experiments have been
conducted to detect telomerase in cancers versus normal
somatic tissue.

23. < Role of Telomerase in Normal and Cancer Cells >< Role of Telomerase in Normal and Cancer Cells >
The telomere hypothesis for cellular mortality.The telomere hypothesis for cellular mortality.
Telomere length is maintained in germ cellsTelomere length is maintained in germ cells
by active telomerase.by active telomerase.
In contrast, somatic cells shut telomeraseIn contrast, somatic cells shut telomerase
OFF and lose telomere length until theyOFF and lose telomere length until they
become growth arrested during senescence.become growth arrested during senescence.
Oncogenically transformed cells that lackOncogenically transformed cells that lack
telomerase activity can bypass senescencetelomerase activity can bypass senescence
but then die during crisis.but then die during crisis.
Immortal cells, with telomerase ON,Immortal cells, with telomerase ON,
can continue to proliferate and do not stopcan continue to proliferate and do not stop
growing either in senescence or crisis.growing either in senescence or crisis.
Journal of clinical oncology, Vol 18, Issue 13(Huly), 2000: 2626-2634Journal of clinical oncology, Vol 18, Issue 13(Huly), 2000: 2626-2634
Targeting Telomerase forTargeting Telomerase for Cancer TherapeuticsCancer Therapeutics

24.  In 1994, groups led by Calvin B. Harley and Jerry W. Shay
detected telomerase in 90 of 101 human tumor samples
representing twelve tumor types.
In the 50 samples of normal somatic tissue
representing four tissue types, that were tested, no
telomerase was detected.

25. Studies by Greider's, Bacchetti's and Harley's
laboratories explained why the telomeres were so small.
♠:The teams had induced normal cells from humans to
make a viral protein causing cells to ignore the alarm
signals that usually warn them to stop dividing.
♠:The treated cells continued to proliferate long after they
would normally enter senescence. In most of the cells,
telomeres shortened drastically, and no telomerase was
detected.

26. ♠: The treated cells continued to divide long after they
would normally enter senescence.
♠: In most of the cells, the telomeres shortened drastically
and the cells eventually died. However, some cells
persisted and became immortal.
♠: In these immortal cells, the telomeres were maintained
at a significantly short length.
Ref: Shay, J.W. & Bacchetti, S.(1997) A survey of telomerase
activity in human cancer.

27.  The outcome of this experiment implies that the
reason why telomeres are so short in tumor cells is
because the telomerase is activated after the cell
has began replicating uncontrollably.
 The telomerase stabilizes the incredibly short
lengths of the telomeres.
 These results coincide with Harley’s theory of the
role of telomerase being permissive rather than
having an initiative role for cancer.
Ref: Harley, C.B. et al.(1994) Telomerase, cell immortality, and
cancer.

28.  In the following model of the telomere hypothesis the
terminal restrictive fragment (TRF) is plotted against the
age of replication.
 An estimate of telomere length is measured by digesting
cellular DNA with restriction enzymes having 4-base
recognition sites, so that most of the DNA is reduced to
short fragments. Because telomere repeats lack
restriction sites, they remain as relatively long terminal
restrictive fragments.
Ref:Olovnikov, A.M. 1996. Telomeres, Telomerase, and Aging:
Origin of the Theory.

30.  There has also been a more recent development of a
TRAP (Telomeric Repeat Amplification Protocol) assay that
is able to detect the presence and level of telomerase
activity in small tissue samples.
 This test has allowed the evaluation of telomerase activity
in a wide range of cancers. Because of such a large
variety of cancers containing telomerase activity,
telomerase may be the most widely expressed and
specific cancer marker known.

31. REFERENCES
1. Blackburn, E.H.(1991), Structure and function of telomeres. Nature 350: 569−573.
2. : Hayflick L (1965). "The limited in vitro lifetime of human diploid cell strains".
Experimental Cell Research 37 (3): 614–636
3. Cooke, H.J. & Smith, B.A. (1986), Variability at the telomeres of the human X/Y
pseudoautosomal region. Cold Spring Harbor Symposia on Quantitative Biology
51:213-219
4. Wright, W. E., and Shay, J. W.(2001) Cellular senescence as a tumor protection
mechanism: the essential role of counting. Curr. Opin. Genes Dev., 11: 98–103.
5. Greider CW,(1990), Telomeres, telomerase and senescence. Bioessays.12:363-369.
6. Shay, J.W. and Wright, W.E. (2002) Telomerase: a target for cancer therapeutics.
Cancer Cell 2: 257–265
7. Shay, J.W. & Bacchetti, S.(1997) A survey of telomerase activity in human cancer.
Eur. J. Cancer 33: 777−791 .
8. Harley, C.B. et al.(1994) Telomerase, cell immortality, and cancer. Cold Spring
Harbor Symp. Quant. Biol. 59: 307−315.
9. Olovnikov, A.M. 1996. Telomeres, Telomerase, and Aging: Origin of the Theory. Exp.
Gerontol. 31: 443-448.

32.  Sincere thanks to Dr. Debajyoty Chakraborty, the Head of the P.G. Dept. of
Zoology, Barasat Govt. College for their valuable suggestions and providing
infrastructure facilities.
 Thanks to Dr. Tuhin Saha, Dr. Madhumita Manna, Dr. Debjani Sarkar, Dr.
Sanjoy Poddar, Dr. Srikanta Guria,Dr.Tanaya Dey for providing
encouragements and various help during the entire period of study.
 Thanks to my seniors of SEM-III.
 Grateful thanks also to the non teaching stuffs of our department
 Lastly, the co-operation received from the classmates is also acknowledged.