INTRODUCTION:

Telomere:

Telomeres play a central role in cell fate and aging by adjusting the cellular response to stress and growth stimulation on the basis of previous cell divisions and DNA damage. A telomere is a region of repetitive nucleotide sequences at each end of a chromatid, which protects the end of the chromosome from deterioration or from fusion with neighboring chromosomes. 'During chromosome replication, the enzymes that duplicate DNA cannot continue their duplication all the way to the end of a chromosome, so in each duplication the end of the chromosome is shortened^[1] (this is because the synthesis of Okazaki fragments requires RNA primers attaching ahead on the lagging strand). The telomeres are disposable buffers at the ends of chromosomes which are truncated during cell division; their presence protects the genes before them on the chromosome from being truncated instead. Over time, due to each cell division, the telomere ends become shorter.^[2] They are replenished by an enzyme, telomerase reverse transcriptase.

STRUCTURE AND FUNCTIO:

Telomeres are repetitive nucleotide sequences located at the termini of linear chromosomes of most eukaryotic organisms. For vertebrates, the sequence of nucleotides in telomeres is TTAGGG. Most prokaryotes, lacking this linear arrangement, do not have telomeres. Telomeres compensate for incomplete semi-conservative DNA replication at chromosomal ends. A protein complex known as shelterin serves as protection against double-strand break (DSB) repair by homologous recombination (HR) and non-homologous end joining (NHEJ).^[37,38]

In most prokaryotes, chromosomes are circular and, thus, do not have ends to suffer premature replication termination. A small fraction of bacterial chromosomes (such as those in Streptomyces and Borrelia) are linear and possess telomeres, which are very different from those of the eukaryotic chromosomes in structure and functions. The known structures of bacterial telomeres take the form of proteins bound to the ends of linear chromosomes, or hairpin loops of single-stranded DNA at the ends of the linear chromosomes.^[39]While replicating DNA, the eukaryotic DNA replication enzymes (the DNA polymerase protein complex) cannot replicate the sequences present at the ends of the chromosomes (or more precisely the chromatid fibres). Hence, these sequences and the information they carry may get lost. This is the reason telomeres are so important in context of successful cell division: They "cap" the end-sequences and themselves get lost in the process of DNA replication. But the cell has an enzyme called telomerase, which carries out the task of adding repetitive nucleotide sequences to the ends of the DNA. Telomerase, thus, "replenishes" the telomere "cap" of the DNA. In most multicellular eukaryotic organisms, telomerase is active only in germ cells, some types of stem cells such as embryonic stem cells, and certain white blood cells. Telomerase can be re activated and telomeres reset back to an embryonic state by somatic cell nuclear transfer.^[39]There are theories that claim that the steady shortening of telomeres with each replication in somatic (body) cells may have a role in senescence and in the prevention of cancer. This is because the telomeres act as a sort of time-delay "fuse", eventually running out after a certain number of cell divisions and resulting in the eventual loss of vital genetic information from the cell's chromosome with future divisions. Telom ere length varies greatly between species, from approximately 300 base pairs in yeast^[39]to many kilo bases in humans, and usually is composed of arrays of guanine-rich, six- to eight-base-pair-long repeats. Eukaryotic telomeres normally terminate with 3′ single-stranded-DNA overhang, which is essential for telomere maintenance and capping. Multiple proteins binding single- and double-stranded telomere DNA have been identified.^[40]These function in both telomere maintenance and capping. Telomeres form large loop structures called telomere loops, or T-loops. Here, the single-stranded DNA curls around in a long circle stabilized by telomere-binding proteins.^[41]At the very end of the T-loop, the single-stranded telomere DNA is held onto a region of double-stranded DNA by the telomere strand disrupting the double-helical DNA and base pairing to one of the two strands. This triple-stranded structure is called a displacement loop or D-loop.^[41]Telomere shortening in humans can induce replicative senescence, which blocks cell division. This mechanism appears to prevent genomic instability and development of cancer in human aged cells by limiting the number of cell divisions. However, shortened telomeres impair immune function that might also increase cancer susceptibility.^[42]If telomeres become too short, they have the potential to unfold from their presumed closed structure. The cell may detect this uncapping as DNA damage and then either stop growing, enter cellular old age (senescence), or begin programmed cell self-destruction (apoptosis) depending on the cell's genetic background (p53 status). Uncapped telomeres also result in chromosomal fusions. Since this damage cannot be repaired in normal somatic cells, the cell may even go into apoptosis. Many aging-related diseases are linked to shortened telomeres. Organs deteriorate as more and more of their cells die off or enter cellular senescence. At the very distal end of the telomere is a 300 bp single-stranded portion, which forms the T-Loop. This loop is analogous to a knot, which stabilizes the telomere, preventing the telomere ends from being recognized as break points by the DNA repair machinery. Should non-homologous end joining occur at the telomeric ends, chromosomal fusion will result. The T-loop is held together by several proteins, the most notable ones being TRF1, TRF2, POT1, TIN1, and TIN2, collectively referred to as the shelterin complex. In humans, the shelterin complex consists of six proteins identified as TRF1, TRF2, TIN2, POT1, TPP1, and RAP1.^[37]

REDUCTION IN TELOMERE LENGTH:

Telomeres shorten in part because of the end replication problem that is exhibited during DNA replication in eukaryotes only. Because DNA replication does not begin at either end of the DNA strand, but starts in the center, and considering that all known DNA polymerases move in the 5' to 3' direction, one finds a leading and a lagging strand on the DNA molecule being replicated. On the leading strand, DNA polymerase can make a complementary DNA strand without any difficulty because it goes from 5' to 3'. However, there is a problem going in the other direction on the lagging strand. To counter this, short sequences of RNA acting asprimers attach to the lagging strand a short distance ahead of where the initiation site was. The DNA polymerase can start replication at that point and go to the end of the initiation site. This causes the formation of Okazaki fragments. More RNA primers attach further on the DNA strand and DNA polymerase comes along and continues to make a new DNA strand. Eventually, the last RNA primer attaches, and DNA polymerase, RNA nuclease, and DNA ligase come along to convert the RNA (of the primers) to DNA and to seal the gaps in between the Okazaki fragments. But, in order to change RNA to DNA, there must be another DNA strand in front of the RNA primer. This happens at all the sites of the lagging strand, but it does not happen at the end where the last RNA primer is attached. Ultimately, that RNA is destroyed by enzymes that degrade any RNA left on the DNA. Thus, a section of the telomere is lost during each cycle of replication at the 5' end of the lagging strand's daughter.

However, in vitro studies have shown that telomeres are highly susceptible to oxidative stress, and Richter and Zglinicki presented evidence that oxidative stress-mediated DNA damage is an important determinant of telomere shortening.^[3] Telomere shortening due to free radicals explains the difference between the estimated loss per division because of the end-replication problem (ca. 20 bp) and actual telomere shortening rates (50-100 bp), and has a greater absolute impact on telomere length than shortening caused by the end-replication problem. Population-based studies have also indicated an interaction between anti-oxidant intake and telomere length. In the Long Island Breast Cancer Study Project (LIBCSP), authors found a moderate increase in breast cancer risk among women with the shortest telomeres and lower dietary intake of beta carotene, vitamin C or E.^[4]These results suggest that cancer risk due to telomere shortening may interact with other mechanisms of DNA damage, specifically oxidative stress. Telomere shortening is associated with ageing, mortality and ageing-related diseases. In 2003, Richard Cawthondiscoved that those with longer telomeres lead longer lives than those with short telomeres ^[5] However, it is not known whether short telomeres are just a sign of cellular age or actually contribute to the aging process.

ELONGATION OF TELOMERE:

The phenomenon of limited cellular division was first observed by Leonard Hayflick, and is now referred to as the Hayflick limit.^[6][7]. The cloning of the catalytic component of telomerase enabled experiments to test whether the expression of telomerase at levels sufficient to prevent telomere shortening was capable of immortalizing human cells. Telomerase was demonstrated in a 1998 publication in Science to be capable of extending cell lifespan, and now is well-recognized as capable of immortalizing human somatic cells.^[8]It is becoming apparent that reversing shortening of telomeres through temporary activation of telomerase may be a potent means to slow aging. They reason that this would extend human life because it would extend the Hayflick limit. Three routes have been proposed to reverse telomere shortening:

· drugs,

· gene therapy, or

· metabolic suppression, so-called, torpor/hibernation.

So far these ideas have not been proven in humans, but it has been demonstrated that telomere shortening is reversed in hibernation and aging is slowed (Turbill, et al. 2012 and 2013) and that hibernation prolongs life-span (Lyman et al. 1981). It has been hypothesized that longer telomeres and especially telomerase activation might cause increased cancer (e.g. Weinstein and Ciszek, 2002).However, longer telomeres might also protect against cancer, because short telomeres are associated with cancer. It has also been suggested that longer telomeres might cause increased energy consumption.^[9]

Furthermore, Gomes et al. found, in a study of the comparative biology of mammalian telomeres, that telomere length of different mammalian species correlates inversely, rather than directly, with lifespan, and they concluded that the contribution of telomere length to lifespan remains controversial.^[10] Harris et al. found little evidence that, in humans, telomere length is a significant biomarker of normal aging with respect to important cognitive and physical abilities.^[11] Gilley and Blackburn tested whether cellular senescence in paramecium is caused by telomere shortening, and found that telomeres were not shortened during senescence.^[12]

TELOMERE IN CANCER:

Cancer cells require a mechanism to maintain their telomeric DNA in order to continue dividing indefinitely (immortalization). A mechanism for telomere elongation or maintenance is one of the key steps in cellular immortalization and can be used as a diagnostic marker in the clinic. Telomerase, the enzyme complex responsible for elongating telomeres, is activated in approximately 90% of tumors. However, a sizeable fraction of cancerous cells employ alternative lengthening of telomeres (ALT),^[13] a non-conservative telomere lengthening pathway involving the transfer of telomere tandem repeats between sister-chromatids.^[14]Telomerase is the natural enzyme that promotes telomere repair. It is active in stem cells, germ cells, hair follicles, and 90 percent of cancer cells, but its expression is low or absent in somatic cells. Telomerase functions by adding bases to the ends of the telomeres. Cells with sufficient telomerase activity are considered immortal in the sense that they can divide past the Hayflick limit without entering senescence or apoptosis. For this reason, telomerase is viewed as a potential target for anti-cancer drugs (such as Geron'sImetelstat currently in human clinical trials and telomestatin).^[15]

Telomeres are critical for maintaining genomic integrity and studies show that telomere dysfunction or shortening is commonly acquired during the process of tumor development^[16] Short telomeres can lead to genomic instability, chromosome loss and the formation of non-reciprocal translocations; and telomeres in tumor cells and their precursor lesions are significantly shorter than surrounding normal tissue.^[17][18]Observational studies have found shortened telomeres in many cancers: including pancreatic, bone, prostate, bladder, lung, kidney, and head and neck. In addition, people with many types of cancer have been found to possess shorter leukocyte telomeres than healthy controls.^[19] Recent meta-analyses suggest 1.4 to 3.0 fold increased risk of cancer for those with the shortest vs. longest telomeres.^[20][21] However the increase risk varies by age, sex, tumor type and differences in lifestyle factors. Some of the same lifestyle factors which increase risk of developing cancer have also been associated with shortened telomeres: including smoking, physical inactivity and diet high in refined sugars^[22] Diet and physical activity influence inflammation and oxidative stress. These factors are known to influence telomere maintenance.^[23] Psychologic stress has also been linked to accelerated cell aging, as reflected by decreased telomerase activity and short telomeres.^[24] It has been suggested that a combination of lifestyle modifications, including healthy diet, exercise and stress reduction, have the potential to increase telomere length, reverse cellular aging, and reduce the risk for aging-related diseases. In a recent clinical trial for early-stage prostate cancer patients, comprehensive lifestyle changes resulted in a short-term increase in telomerase activity and long-term modification in telomere length.^[25][26] Lifestyle modifications have to potential to naturally regulate telomere maintenance without promoting tumorgenesis, as traditional mechanisms of telomere lengthening involve the use of telomerase activating agents.

TELOMERE IN AGING:

Aging can be defined as the progressive functional decline of tissue function that eventually results in mortality. Such functional decline can result from the loss or diminished function of postmitotic cells or from failure to replace such cells by a functional decline in the ability of (stem) cells to sustain replication and cell divisions. Aging is not a disease, and the biology of aging, which varies between individuals, is best understood in the context of evolution ^[27]. This model proposes that an increase in longevity in mammals is due to a concomitant reduction in the rates of growth and reproduction and an increase in the accuracy of synthesis of macromolecules. The notion that the fidelity of DNA repair is subject to selective forces and not necessarily better than (strictly) needed for a particular cell type, tissue, or species is not easily grasped. Differences in the fidelity of DNA repair pathways between cells of the germ-line and somatic (stem) cells and between comparable somatic cells from small, short-lived animals and large, long-lived species greatly complicate generalizations about the molecular mechanism of aging across different species. Limitations in the use of model organisms to study the role of telomeres in human aging are perhaps best illustrated by the different consequences of telomerase deficiencies in humans and various model organisms. In laboratory mice (Musmusculus), Baker’s yeast (Saccharomyces cerevisiae)^[28], mustard plants (Arabidopsis) ^[29], and roundworms (Caenorhabditiselegans)^[30], complete loss of telomerase is tolerated for at least several generations. In contrast, a modest twofold reduction in telomerase levels in humans (e.g., resulting from haploinsufficiency for one of the telomerase genes) is now known to cause severe clinical symptoms including aplastic anemia, immune deficiencies, and pulmonary fibrosis after one to three generations. The indirect relation between clinical phenotype and mutations in genes that affect telomere length or telomere maintenance has been confusing to many and certainly has greatly complicated genetic linkage analysis. As a result, the involvement of abnormalities in telomeres and telomere biology in human disease is probably underestimated. A major objective of this review is to set the stage for future studies of telomeres and telomerase in relation to (stem) cell turnover, tissue function, and aging. The progressive loss of telomeric DNA in human somatic (stem) cells is believed to act as a tumor suppressor mechanism that limits clonal proliferation, preventsclonal dominance, and ensures a polyclonal composition of (stem) cells in large, long-lived multicellular organisms. Unfortunately, limits to the clonal expansion of somatic (stem) cells also provide strong selection for cells that can ignore or by pass the “telomere” checkpoint^[31], e.g., because their DNA damage responses are defective. Such cells can continue to grow despite the presence of dysfunctional telomeres. The loss of telomere function in such cells results in chromosome fusions, broken chromosomes, break-fusion bridge cycles, translocations, and aneuploidy. This genetic instability allows selection of cells with abnormal growth characteristics and also facilitates rapid acquisition of genetic alterations that provide further growth advantages^[32,33]. Thus, while telomere loss may act as a tumor suppressor mechanism, it also promotes tumor growth by driving selection of cells with defective DNA damage responses (e.g., loss of p53)^[34]. The aneuploidy and genomic rearrangements in cells with short telomeres and defective DNA damage responses complicate the analysis of the molecular changes that are most relevant for tumor growth initiation and progression. The fact that loss of telomere function has consequences both for aging and carcinogenesis^[35]explains much of the current interest in telomeres. The interconnections between normal and dysfunctional telomeres and intracellular signaling pathways involved in DNA damage responses and DNA repair involving proteins such as ATM, ATR, and p53 ^[36] support a view of telomeres as pivotal dynamic elements required for genome stability that determine how a cell responds to stress and growth stimulation.

REFFERENCE:

1. "A tandemly repeated sequence at the termini of the extra chromosomal ribosomal RNA genes in Tetrahymena". J. Mol. Biol. 120 (1): 33–53.

2. Passarge, Eberhard. Color atlas of genetics, 2007.

3. Richter T, von Zglinicki T (2007). A continuous correlation between oxidative stress and telomere shortening in fibroblasts. Exp Gerontol 42(11):1039-1042.

4. Cawthon RM, Smith KR, O'Brien E, Sivatchenko A, Kerber RA (2003). Association between telomere length in blood and mortality in people aged 60 years or older. Lancet 361:393-395.

5. Shen J, Gammon MD, Terry MB, Wang Q, Bradshaw P, Teitelbaum SL, Neugut AI, Santella RM. Telomere length, oxidative damage, antioxidants and breast cancer risk. Int J Cancer. 2009 Apr 1; 124(7):1637-43

6. Hayflick L, Moorhead PS (1961). "The serial cultivation of human diploid cell strains". Exp Cell Res 25 (3): 585–621

7. Hayflick L. (1965). "The limited in vitro lifetime of human diploid cell strains". Exp. Cell Res. 37 (3): 614–636

8. Bodnar, A.G., M. Ouellette, M. Frolkis, S.E. Holt, C.P. Chiu, G.B. Morin, C.B. Harley, J.W. Shay, S. Lichtsteiner, and W.E. Wright. 1998. Extension of life-span by introduction of telomerase into normal human cells" Science 279:349–352.

9. Eisenberg DTA (2011). "An evolutionary review of human telomere biology: The thrifty telomere hypothesis and notes on potential adaptive paternal effects". American Journal of Human Biology 23 (2): 149–167.

10. Gomes NM, Ryder OA, Houck ML, Charter SJ, Walker W, Forsyth NR, Austad SN, Venditti C, Pagel M, Shay JW, Wright WE (2011). Comparative biology of mammalian telomeres: hypotheses on ancestral states and the roles of telomeres in longevity determination. Aging Cell 10(5) 761-768.

11. Harris SE, Martin-Ruiz C, von Zglinicki T, Starr JM, Deary IJ (2010). Telomere length and aging biomarkers in 70-year-olds: the Lothian Birth Cohort 1936. Neurobiol Aging 33(7) 1486.

12. Gilley D, Blackburn EH (1994). Lack of telomere shortening during senescence in Paramecium" ProcNatlAcadSci U S A 91(5) 1955-1958.

13. Henson JD, Neumann AA, Yeager TR, Reddel RR (2002). "Alternative lengthening of telomeres in mammalian cells". Oncogene 21 (4): 598–610.

14. Chris Molenaar, KarienWiesmeijer, Nico P. Verwoerd, ShadiKhazen, Roland Eils, Hans J. Tanke, and Roeland W. Dirks (2003-12-15). "Visualizing telomere dynamics in living mammalian cells using PNA probes". The EMBO Journal (The European Molecular Biology Organization) 22 (24): 6631–6641.

15. Philippi C, Loretz B, Schaefer UF, Lehr CM. (April 2010). "Telomerase as an emerging target to fight cancer - Opportunities and challenges for nanomedicine". Journal of Controlled Releases 146 (2): 228–40.

16. Raynaud CM, Sabatier L, Philipot O, Olaussen KA, Soria JC (2008) Telomere length, telomeric proteins and genomic instability during the multistep carcinogenic process. Crit Rev OncolHematol 66: 99–117.

17. Blasco MA, Lee HW, Hande MP, Samper E, Lansdorp PM, et al. (1997) Telomere shortening and tumor formation by mouse cells lacking telomerase RNA. Cell 91: 25–34.

18. Artandi SE, Chang S, Lee SL, Alson S, Gottlieb GJ, et al. (2000) Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice. Nature 406: 641–645.

19. Willeit Peter, Willeit Johann, Mayr Anita, Weger Siegfried, Oberhollenzer Friedrich, Brandstätter Anita, Kronenberg Florian, Kiechl Stefan (2010). "Telomere length and risk of incident cancer and cancer mortality". JAMA 304 (1): 69–75.

20. Ma H, Zhou Z, Wei S, et al. Shortened telomere length is associated with increased risk of cancer: a meta-analysis. PLoS One. 2011;6(6):e20466.

21. Wentzensen IM, Mirabello L, Pfeiffer RM, Savage SA. The association of telomere length and cancer: a meta-analysis. Cancer Epidemiol Biomarkers Prev. 2011; 20(6):1238–1250.

22. Wentzensen IM, Mirabello L, Pfeiffer RM, Savage SA. The association of telomere length and cancer: a meta-analysis. Cancer Epidemiol Biomarkers Prev. 2011; 20(6):1238–1250.

23. Paul L. Diet, nutrition and telomere length. J NurtBiochem. 2011 Oct;22(10):895-901. doi: 10.1016/j.jnutbio.2010.12.001. Epub 2011 Mar 22.

24. Epel ES, Lin J, Wilhelm FH, Wolkowitz OM, Cawthon R, Adler NE, Dolbier C, Mendes WB, Blackburn EH. Cell aging in relation to stress arousal and cardiovascular disease risk factors. Psychoneuroendocrinology. 2006 April; 31(3):277-87.

25. Ornish D, Lin J, Chan JM, Epel E, Kemp C, Weidner G, Marlin R, Frenda SJ, Magbanua MJ, Daubenmier J, Estay I, Hills NK, Chainani-Wu N, Carroll PR, Blackburn EH. Effect of comprehensive lifestyle changes on telomerase activity and telomere length in men with biopsy-proven low-risk prostate cancer: 5-year follow-up of a descriptive pilot study. Lancet Oncol. 2013 Oct;14(11):1112-20.Epub 2013 Sep 17.

26. Ornish D, Lin J, Daubenmier J, Weidner G, Epel E, Kemp C, Magbanua MJ, Marlin R, Yglecias L, Carroll PR, Blackburn EH. Increased telomerase activity and comprehensive lifestyle changes: a pilot study. Lancet Oncol. 2008 Nov;9(11):1048-57. doi: 10.1016/S1470-2045(08)70234-1. Epub 2008 Sep 15.

27. Kirkwood TB, Holliday R. The evolution of ageing and longevity. Proc R SocLond B BiolSci205: 531–546, 1979.

28. Lundblad V, Blackburn EH. RNA-dependent polymerase motifs in EST1: tentative identification of a protein component of an essential yeast telomerase. Cell 60: 529–530, 1990.

29. Riha K, McKnight TD, Griffing LR, Shippen DE. Living with genome instability: plant responses to telomere dysfunction. Science 291: 1797–1800, 2001.

30. Cheung I, Schertzer M, Rose A, Lansdorp PM. High incidence of rapid telomere loss in telomerase-deficient Caenorhabditiselegans. Nucleic Acids Res 34: 96–103, 2006.

31. Verfaillie CM, Pera MF, Lansdorp PM. Stem cells: hype and reality. Hematology 369–391, 2002.

32. De Lange T. Telomere dynamics and genome instability in human cancer. In: Telomeres, edited by Blackburn EH, Greider CW. Plainview, NY: Cold Spring Harbor Laboratory, 1995, p. 265–293.

33. Sharpless NE, DePinho RA. Telomeres, stem cells, senescence, cancer. J Clin Invest 113: 160–168, 2004.

34. Artandi SE, Chang S, Lee SL, Alson S, Gottlieb GJ, Chin L, DePinho RA. Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice. Nature 406: 641–645,2000.

35. Leen AM, Rooney CM, Foster AE. Improving T cell therapy for cancer. Annu Rev Immunol25: 243–265, 2007.

36. Vousden KH, Lane DP. p53 in health and disease. Nat Rev Mol Cell Biol8: 275–283, 2007.

37. Blasco, Maria; Paula Martínez (21 Jun 2010). "Role of shelterin in cancer and aging". Aging Cell 9 (5): 653–666.

38. Lundblad, 2000; Ferreira et al., 2004.

39. Shampay , Szostak J.W., Blackburn E.H. (1984). "DNA sequences of telomeres maintained in yeast". Nature 310 (5973): 154–157.

40. Blackburn, 2001; Smogorzewska and de Lange, 2004; Cech, 2004; De Lange et al., 2005; Kota and Runge, 1999 Griffith J, Comeau L, Rosenfield S, Stansel R, Bianchi A, Moss H, de Lange T (1999). "Mammalian telomeres end in a large duplex loop". Cell 97 (4): 503–14.

41. Burge S, Parkinson G, Hazel P, Todd A, Neidle S (2006). "Quadruplex DNA: sequence, topology and structure". Nucleic Acids Res 34 (19): 5402–15.

42. Eisenberg DTA (2011). "An evolutionary review of human telomere biology: The thrifty telomere hypothesis and notes on potential adaptive paternal effects". American Journal of Human Biology 23 (2): 149–167.

Received on 05.11.2014 Modified on 05.09.2017

Research J. Pharm. and Tech 2017; 10(11): 4051-4056.

DOI: 10.5958/0974-360X.2017.00735.1