Telomere dysfunction and tumour suppression: the senescence connection

In the paper

“Activation of telomerase results in telomere elongation, abrogation of replicative senescence, a normal karyotype and cellular immortalization.”

What the sources say

• Not checked yetcites [3] Bodnar, A. G. et al. Extension of life-span by introduction of telomerase into normal human cells. Science 279, 349–352 (1998). source
• Not checked yetcites [4] Vaziri, H. & Benchimol, S. Reconstitution of telomerase activity in normal human cells leads to elongation of telomeres and extended replicative life span. Curr. Biol. 8, 279–282 (1998). source

In the paper

“Once activated, these kinases phosphorylate downstream factors including CHK1 and CHK2 that in turn phosphorylates p5329. Phosphorylation of p53 results in the displacement of the murine double minute 2 (MDM2) protein, liberating p53 from degradation and stimulation of the expression of the cyclin-dependent kinase inhibitor p21. p21 inhibits cell cycle progression by inhibiting cyclin-dependent”

What the sources say

• Not checked yetcites [29] Gire, V., Roux, P., Wynford-Thomas, D., Brondello, J. M. & Dulic, V. DNA damage checkpoint kinase Chk2 triggers replicative senescence. EMBO J. 23, 2554–2563 (2004). source

In the paper

“These fusions are the basis of genomic instability associated with telomere dysfunction as they lead to anaphase bridging, subsequent breakage and that requires further repair. Repetition of this fusion-bridge-breakage cycle leads to aneuploidy, further fusions and depending on how such DNA damage is resolved, it has been hypothesized that loss of heterozygosity or gene amplification could result”

What the sources say

• Not checked yetcites [5] Maser, R. S. & DePinho, R. A. Connecting chromosomes, crisis, and cancer. Science 297, 565–569 (2002). source
• Not checked yetcites [21] DePinho, R. A. & Polyak, K. Cancer chromosomes in crisis. Nature Genet. 36, 932–934 (2004). source

In the paper

“After 60-80 population doublings in culture, HDFs stop dividing and adopt a phenotype characterized by large, flat cell size, a vacuolated morphology, inability to synthesize DNA, and the presence of the senescence-associated β-galactosidase (SA-β-gal) marker2.”

What the sources say

• Not checked yetcites [1] Hayflick, L. & Moorhead, P. S. The serial cultivation of human diploid cell strains. Exp. Cell Res. 25, 585–621 (1961). A classic paper demonstrating that human cells have finite proliferative capacity in vitro (the 'Hayflick limit'). source
• Not checked yetcites [2] Dimri, G. P. et al. A biomarker that identifies senescent human cells in culture and in aging skin in vivo. Proc. Natl Acad. Sci. USA 92, 9363–9367 (1995). This paper reports on the SA-β-gal assay as a way to mark senescent cells. source

In the paper

“Mammalian telomeres associate with the shelterin complex of six core proteins — telomeric-repeat binding factor 1 (TRF1), TRF2, TRF1 interacting protein 2 (TIN2), protection of telomeres 1 (POT1), the POT1 and TIN2 interacting protein TPP1 and the transcriptional repressor/activator protein RAP115, 16”

What the sources say

• Not checked yetcites [15] de Lange, T. Shelterin: the protein complex that shapes and safeguards human telomeres. Genes Dev. 19, 2100–2110 (2005). source
• Not checked yetcites [16] Liu, D., O'Connor, M. S., Qin, J. & Songyang, Z. Telosome, a mammalian telomere-associated complex formed by multiple telomeric proteins. J. Biol. Chem. 279, 51338–51342 (2004). source

In the paper

“These results clearly demonstrate that telomere length determines the proliferative lifespan of HDFs, and that upregulation of telomerase activity (hence telomere length) restores proliferative capacity.”

In the paper

“These results suggest that both critically short telomeres and direct disruption of the shelterin structure can initiate telomere dysfunction, trigger a DDR and chromosomal fusions.”

In the paper

“Considering that human telomeres are only 8-12 kb in length or less at birth, this degree of attrition is significant over the natural lifespan of humans, and may be responsible to elicit certain aspects of human aging phenotypes11.”

What the sources say

• Not checked yetcites [11] Blasco, M. A. Telomere length, stem cells and aging. Nature Chem. Biol. 3, 640–649 (2007). source

In the paper

“Telomeres that can no longer exert end-protective functions are said to be dysfunctional, and these telomeres could arise either from progressive telomere attrition, or when components of the shelterin complex are perturbed, resulting in inappropriate chromosomal end-to-end fusions through the non-homologous end joining (NHEJ) or homologous recombination (HR) DNA repair pathways 17.”

What the sources say

• Not checked yetcites [17] Verdun, R. E. & Karlseder, J. Replication and protection of telomeres. Nature 447, 924–931 (2007). source

In the paper

“Telomerase expression is low or absent in most human somatic tissues6, while it is robust in early proliferative progenitor germ and stem cells 7, 8.”

What the sources say

• Not checked yetcites [6] Masutomi, K. et al. Telomerase maintains telomere structure in normal human cells. Cell 114, 241–253 (2003). source
• Not checked yetcites [7] Wright, W. E., Piatyszek, M. A., Rainey, W. E., Byrd, W. & Shay, J. W. Telomerase activity in human germline and embryonic tissues and cells. Dev. Genet. 18, 173–179 (1996). source
• Not checked yetcites [8] Harley, C. B., Futcher, A. B. & Greider, C. W. Telomeres shorten during ageing of human fibroblasts. Nature 345, 458–460 (1990). This report links increasing telomere attrition with increased cell divisions and advancing age, suggesting that telomere shortening may be the underlying mechanism of the Hayflick limit. source

In the paper

“These results suggest that dysfunctional telomeres are recognized as DSBs that impinge upon p53 and/or pRB tumor suppressor pathways, to induce expression of their regulators p21 and p16 Ink4a to initiate replicative senescence24-26.”

What the sources say

• Not checked yetcites [24] Wright, W. E. & Shay, J. W. The two-stage mechanism controlling cellular senescence and immortalization. Exp. Gerontol. 27, 383–389 (1992). This paper documents that DNA damage checkpoint proteins such as p53 and RB are required for cells with shortened telomeres to undergo cellular senescence. Elimination of these proteins enables these cells to immortalize. source
• Not checked yetcites [25] Kamijo, T. et al. Tumor suppression at the mouse INK4a locus mediated by the alternative reading frame product p19ARF. Cell 91, 649–659 (1997). source
• Not checked yetcites [26] Sage, J., Miller, A. L., Perez-Mancera, P. A., Wysocki, J. M. & Jacks, T. Acute mutation of retinoblastoma gene function is sufficient for cell cycle re-entry. Nature 424, 223–228 (2003). source

In the paper

“Biochemical analyses revealed that HDFs that entered replicative senescence display molecular markers characteristic of cells bearing DNA double-strand breaks (DSBs), suggesting that dysfunctional telomeres elicit a potent DNA damage response (DDR)22.”

What the sources say

• Not checked yetcites [22] d'Adda di Fagagna, F. et al. A DNA damage checkpoint response in telomere-initiated senescence. Nature 426, 194–198 (2003). source

In the paper

“These markers of the DDR include phosphorylated γ-H2AX, 53BP1, NBS1, MDC1 and CHK2, among others. Many of these proteins localize directly to dysfunctional telomeres to form dysfunctional telomere-induced foci (TIFs), and their inactivation in senescent cells restores cell cycle progression into S-phase22, 23.”

What the sources say

• Not checked yetcites [22] d'Adda di Fagagna, F. et al. A DNA damage checkpoint response in telomere-initiated senescence. Nature 426, 194–198 (2003). source
• Not checked yetcites [23] Takai, H., Smogorzewska, A. & de Lange, T. DNA damage foci at dysfunctional telomeres. Curr. Biol. 13, 1549–1556 (2003). References 22 and 23 report that dysfunctional telomeres activate a DDR, resulting in the accumulation of DDR proteins at telomeres. Cells with dysfunctional telomeres enter into senescence by activating a p53-dependent checkpoint response. source

In the paper

“Analysis of telomeres from a diverse array of organisms revealed that the telomere 3′ single-stranded overhang can invade the double-stranded telomeric tracts, displacing the homologous strand of the same telomere. Consequently, a telomere forms a lasso-like structure, termed the ‘t-loop’, with a displacement (D) loop at the invasion site12, 13”

What the sources say

• Not checked yetcites [12] Griffith, J. D. et al. Mammalian telomeres end in a large duplex loop. Cell 97, 503–514 (1999). source
• Not checked yetcites [13] de Lange, T. T-loops and the origin of telomeres. Nature Rev. Mol. Cell Biol. 5, 323–329 (2004). source

In the paper

“late generation mTerc−/−, p21−/− mice do not show increased chromosomal instability nor do they succumb to increased tumorigenesis61.”

What the sources say

• Not checked yetcites [61] Choudhury, A. R. et al. Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation. Nature Genet. 39, 99–105 (2007). source

In the paper

“The demonstration that dysfunctional telomeres could engage a senescence program to suppress tumorigenesis in vivo suggests possible future therapeutic applications63.”

What the sources say

• Not checked yetcites [63] Harley, C. B. Telomerase and cancer therapeutics. Nature Rev. Cancer 8, 167–179 (2008). source

In the paper

“These results suggest that progression from micro to macroadenomas was inhibited in G6 mTerc−/−ApcMin mice, presumably due to the activation of p53-dependent tumor suppressive pathways by dysfunctional telomeres.”

In the paper

“Together, these observations support the view that replicative senescence and crisis provide potent barriers to tumor development and, by extension, that proper telomere maintenance is an essential aspect of full malignant transformation.”

In the paper

“However, similar treatment of G6-mTerc−/−;Ink4a−/− mice with short dysfunctional telomeres yielded a dramatic reduction in tumor incidence (from 64% to 31%) and much longer survival49.”

What the sources say

• Not checked yetcites [49] Greenberg, R. A. et al. Short dysfunctional telomeres impair tumorigenesis in the INK4aΔ2/3 cancer-prone mouse. Cell 97, 515–525 (1999). source

In the paper

“One explanation is that while telomere induced cellular senescence is capable of suppressing spontaneous tumorigenesis from mesenchymal tumors, it is insufficient to suppress cancer formation of epithelial tissues such as the skin.”

In the paper

“In addition to promoting cell cycle arrest, dysfunctional telomeres can also activate p53-dependent apoptosis, since mice bearing dysfunctional telomeres show increased apoptosis in proliferative cells31-33.”

What the sources say

• Not checked yetcites [31] Lee, H. W. et al. Essential role of mouse telomerase in highly proliferative organs. Nature 392, 569–574 (1998). source
• Not checked yetcites [32] Rudolph, K. L. et al. Longevity, stress response, and cancer in aging telomerase-deficient mice. Cell 96, 701–712 (1999). source
• Not checked yetcites [33] Rajaraman, S. et al. Telomere uncapping in progenitor cells with critical telomere shortening is coupled to S-phase progression in vivo. Proc. Natl Acad. Sci. USA 104, 17747–17752 (2007). source

In the paper

“Importantly, removal of p53 function rescued many of the cellular defects despite the presence of critically short telomeres, including growth arrest in cell culture, testicular atrophy, intestinal and germ cell apoptosis46.”

What the sources say

• Not checked yetcites [46] Chin, L. et al. p53 deficiency rescues the adverse effects of telomere loss and cooperates with telomere dysfunction to accelerate carcinogenesis. Cell 97, 527–538 (1999). source

In the paper

“These results suggest that dysfunctional telomeres could activate a cellular senescence pathway to suppress tumorigenesis in the absence of apoptosis to suppress tumorigenesis, while this pathway was not elicited in the tumors with competent telomeres.”

In the paper

“suggesting it is possible that p53-dependent but senescence- and apoptosis-independent effects of dysfunctional telomeres might account for some of the observed tumor suppression in vivo.”

In the paper

“The telomerase deficient mouse lacks the critical RNA subunit mTerc, is viable, fertile and has no significant morphological abnormalities42.”

What the sources say

• Not checked yetcites [42] Blasco, M. A. et al. Telomere shortening and tumor formation by mouse cells lacking telomerase RNA. Cell 91, 25–34 (1997). source

In the paper

“Instead, loss of p21 extended organismal lifespan and rescued cellular proliferative defects due to dysfunctional telomeres, presumably due to reduced entry of proliferative cells into cellular senescence.”

What the sources say

• Not checked yetcites [61] Choudhury, A. R. et al. Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation. Nature Genet. 39, 99–105 (2007). source

In the paper

“GRN163L is a modified oligonucleotide complementary to hTERC and a potent and specific telomerase antagonist66.”

What the sources say

• Not checked yetcites [66] Dikmen, Z. G. et al. In vivo inhibition of lung cancer by GRN163L: a novel human telomerase inhibitor. Cancer Res. 65, 7866–7873 (2005). source

In the paper

“Together, these findings demonstrate that telomere dysfunction promotes the development of cancer in the setting of p53 deficiency, and further reinforce the importance of an intact p53 pathway in tumor inhibition in the setting of telomere dysfunction.”

In the paper

“Cells harboring this mutation (p53P/P) are incapable of activating p53-dependent apoptosis59, but the p53-dependent cellular senescence pathway is intact as induction of telomere dysfunction through overexpression of TRF2-DN in these cells led to dramatic reduction of cellular proliferation and the appearance of senescent cells that stain positive for SA-β-Gal58.”

What the sources say

• Not checked yetcites [58] Cosme-Blanco, W. et al. Telomere dysfunction suppresses spontaneous tumorigenesis in vivo by initiating p53-dependent cellular senescence. EMBO Rep. 8, 497–503 (2007). Using telomerase-knockout mouse models in a setting in which the apoptotic function of p53 is eliminated, references 57 and 58 document for the first time that activation of the cellular senescence programme could potently inhibit tumour initiation and progression in vivo. source
• Not checked yetcites [59] Liu, G. et al. Chromosome stability, in the absence of apoptosis, is critical for suppression of tumorigenesis in Trp53 mutant mice. Nature Genet. 36, 63–68 (2004). source

In the paper

“However, transplantation of G5/6 mTerc−/−, Eμ-myc, Bcl-2 bone marrow failed to produce any tumors 100 days post-transplantation.”

In the paper

“Interestingly, TIF formation correlated with delocalization of POT1 and was antagonized by overexpression of POT1 or TRF2.”

What the sources say

• Not checked yetcites [71] Salvati, E. et al. Telomere damage induced by the G-quadruplex ligand RHPS4 has an antitumor effect. J. Clin. Invest. 117, 3236–3247 (2007). source

In the paper

“Similar findings were observed in vivo, where the progressive decline in telomere function correlated with increased tumor incidence and decreased survival.”

In the paper

“In contrast, iG1 mTerc−/− p53P/+ and iG1 mTerc−/− p53P/P cells possessed a 6-8 fold increase in p-p arm fusions and a 3-fold increase in the formation of anaphase bridges, both hallmarks of telomere dysfunction.”

In the paper

“The upregulation of telomerase in most human cancers and its requirement for proliferation makes anti-telomerase compounds a potential means to induce telomere shortening and initiation of a senescence program in tumor cells64.”

What the sources say

• Not checked yetcites [64] Shay, J. W. & Keith, W. N. Targeting telomerase for cancer therapeutics. Br. J. Cancer 98, 677–683 (2008). source

In the paper

“Therefore, it is not surprising that p53 loss results in a permissive environment that favors proliferation and survival of genomically damaged cells and the eventual progression to cancer.”

In the paper

“Crisis serves as the second checkpoint in tumor suppression and is characterized by multiple chromosomal fusions36.”

What the sources say

• Not checked yetcites [36] Counter, C. M. et al. Telomere shortening associated with chromosome instability is arrested in immortal cells which express telomerase activity. EMBO J. 11, 1921–1929 (1992). source

In the paper

“treatment of HT1080 fibrosarcoma cell lines with the telomerase inhibitor BIBR1532 results in progressive telomere shortening, induction of a senescence phenotype and inhibition of tumor growth when transplanted into recipient mice65.”

What the sources say

• Not checked yetcites [65] Damm, K. et al. A highly selective telomerase inhibitor limiting human cancer cell proliferation. EMBO J. 20, 6958–6968 (2001). source

In the paper

“GRN163L effectively inhibits telomerase activity of various human cancer cell lines67-70, resulting in progressive telomere shortening and induction of cellular senescence to suppress tumor cell growth in vitro.”

What the sources say

• Not checked yetcites [67] Djojosubroto, M. W. et al. Telomerase antagonists GRN163 and GRN163L inhibit tumor growth and increase chemosensitivity of human hepatoma. Hepatology 42, 1127–1136 (2005). source
• Not checked yetcites [68] Hochreiter, A. E. et al. Telomerase template antagonist GRN163L disrupts telomere maintenance, tumor growth, and metastasis of breast cancer. Clin. Cancer Res. 12, 3184–3192 (2006). source
• Not checked yetcites [69] Jackson, S. R. et al. Antiadhesive effects of GRN163L — an oligonucleotide N3′→P5′ thio-phosphoramidate targeting telomerase. Cancer Res. 67, 1121–1129 (2007). source
• Not checked yetcites [70] Ozawa, T. et al. Antitumor effects of specific telomerase inhibitor GRN163 in human glioblastoma xenografts. Neuro Oncol. 6, 218–226 (2004). source

In the paper

“Similar results were observed in p53-competent mouse models of hepatocellular carcinoma, in which dysfunctional telomeres served to initiate tumor growth, but also limited the size of these tumors (progression) by activating apoptosis53.”

What the sources say

• Not checked yetcites [53] Farazi, P. A. et al. Differential impact of telomere dysfunction on initiation and progression of hepatocellular carcinoma. Cancer Res. 63, 5021–5027 (2003).

In the paper

“Virally transformed human cells that eliminate p53 and/or pRB function can escape crisis at very low frequencies, and invariably adopt telomere-maintenance programs: 80-90% of human tumors possess some telomerase activity, while the remainder maintain telomeres via a recombination-mediated process termed ALT (for Alternative Lengthening of Telomeres)37-40.”

What the sources say

• Not checked yetcites [37] Shay, J. W., Van Der Haegen, B. A., Ying, Y. & Wright, W. E. The frequency of immortalization of human fibroblasts and mammary epithelial cells transfected with SV40 large T-antigen. Exp. Cell Res. 209, 45–52 (1993). source
• Not checked yetcites [38] Kim, N. W. et al. Specific association of human telomerase activity with immortal cells and cancer. Science 266, 2011–2015 (1994). source
• Not checked yetcites [39] Bryan, T. M., Englezou, A., Dalla-Pozza, L., Dunham, M. A. & Reddel, R. R. Evidence for an alternative mechanism for maintaining telomere length in human tumors and tumor-derived cell lines. Nature Med. 3, 1271–1274 (1997). source

In the paper

“In carcinomas, perhaps activation of both senescence and apoptotic pathways are required to enforce tumor suppression.”

In the paper

“One must also be concerned about the possibility that tumor cells rendered senescent would yield secretory products that might itself be tumor promoting77.”

What the sources say

• Not checked yetcites [77] Krtolica, A., Parrinello, S., Lockett, S., Desprez, P. Y. & Campisi, J. Senescent fibroblasts promote epithelial cell growth and tumorigenesis: a link between cancer and aging. Proc. Natl Acad. Sci. USA 98, 12072–12077 (2001). source

In the paper

“Finally, this therapy will not work in ALT tumors which lack functional telomerase.”

In the paper

“Recent observations indicate that both telomere dysfunction73, 74 and the DDR75, 76 are activated at the earliest stages in many human carcinomas.”

What the sources say

• Not checked yetcites [73] Chin, K. et al. In situ analyses of genome instability in breast cancer. Nature Genet. 36, 984–988 (2004). source
• Not checked yetcites [74] Meeker, A. K. et al. Telomere length abnormalities occur early in the initiation of epithelial carcinogenesis. Clin. Cancer Res. 10, 3317–3326 (2004). source
• Not checked yetcites [75] Bartkova, J. et al. DNA damage response as a candidate anti-cancer barrier in early human tumorigenesis. Nature 434, 864–870 (2005). source
• Not checked yetcites [76] Gorgoulis, V. G. et al. Activation of the DNA damage checkpoint and genomic instability in human precancerous lesions. Nature 434, 907–913 (2005). source