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1
Brault, Marie Eve, and Chantal Autexier. "Telomeric recombination induced by dysfunctional telomeres." Molecular Biology of the Cell 22, no. 2 (January 15, 2011): 179–88. http://dx.doi.org/10.1091/mbc.e10-02-0173.
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Telomere maintenance is essential for cellular immortality, and most cancer cells maintain their telomeres through the enzyme telomerase. Telomeres and telomerase represent promising anticancer targets. However, 15% of cancer cells maintain their telomeres through alternative recombination-based mechanisms, and previous analyses showed that recombination-based telomere maintenance can be activated after telomerase inhibition. We determined whether telomeric recombination can also be promoted by telomere dysfunction. We report for the first time that telomeric recombination can be induced in human telomerase-positive cancer cells with dysfunctional telomeres.
2
Lin, Chi-Ying, Hsih-Hsuan Chang, Kou-Juey Wu, Shun-Fu Tseng, Chuan-Chuan Lin, Chao-Po Lin та Shu-Chun Teng. "Extrachromosomal Telomeric Circles Contribute to Rad52-, Rad50-, and Polymerase δ-Mediated Telomere-Telomere Recombination in Saccharomyces cerevisiae". Eukaryotic Cell 4, № 2 (лютий 2005): 327–36. http://dx.doi.org/10.1128/ec.4.2.327-336.2005.
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ABSTRACT Telomere maintenance is required for chromosome stability, and telomeres are typically replicated by the telomerase reverse transcriptase. In both tumor and yeast cells that lack telomerase, telomeres are maintained by an alternative recombination mechanism. By using an in vivo inducible Cre-loxP system to generate and trace the fate of marked telomeric DNA-containing rings, the efficiency of telomere-telomere recombination can be determined quantitatively. We show that the telomeric loci are the primary sites at which a marked telomeric ring-containing DNA is observed among wild-type and surviving cells lacking telomerase. Marked telomeric DNAs can be transferred to telomeres and form tandem arrays through Rad52-, Rad50-, and polymerase δ-mediated recombination. Moreover, increases of extrachromosomal telomeric and Y′ rings were observed in telomerase-deficient cells. These results imply that telomeres can use looped-out telomeric rings to promote telomere-telomere recombination in telomerase-deficient Saccharomyces cerevisiae.
3
Bechard, Laura H., Bilge D. Butuner, George J. Peterson, Will McRae, Zeki Topcu, and Michael J. McEachern. "Mutant Telomeric Repeats in Yeast Can Disrupt the Negative Regulation of Recombination-Mediated Telomere Maintenance and Create an Alternative Lengthening of Telomeres-Like Phenotype." Molecular and Cellular Biology 29, no. 3 (November 24, 2008): 626–39. http://dx.doi.org/10.1128/mcb.00423-08.
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ABSTRACT Some human cancers maintain telomeres using alternative lengthening of telomeres (ALT), a process thought to be due to recombination. In Kluyveromyces lactis mutants lacking telomerase, recombinational telomere elongation (RTE) is induced at short telomeres but is suppressed once telomeres are moderately elongated by RTE. Recent work has shown that certain telomere capping defects can trigger a different type of RTE that results in much more extensive telomere elongation that is reminiscent of human ALT cells. In this study, we generated telomeres composed of either of two types of mutant telomeric repeats, Acc and SnaB, that each alter the binding site for the telomeric protein Rap1. We show here that arrays of both types of mutant repeats present basally on a telomere were defective in negatively regulating telomere length in the presence of telomerase. Similarly, when each type of mutant repeat was spread to all chromosome ends in cells lacking telomerase, they led to the formation of telomeres produced by RTE that were much longer than those seen in cells with only wild-type telomeric repeats. The Acc repeats produced the more severe defect in both types of telomere maintenance, consistent with their more severe Rap1 binding defect. Curiously, although telomerase deletion mutants with telomeres composed of Acc repeats invariably showed extreme telomere elongation, they often also initially showed persistent very short telomeres with few or no Acc repeats. We suggest that these result from futile cycles of recombinational elongation and truncation of the Acc repeats from the telomeres. The presence of extensive 3′ overhangs at mutant telomeres suggests that Rap1 may normally be involved in controlling 5′ end degradation.
4
Kondratieva, Yu A., and L. P. Mendeleeva. "Characteristics of telomere length in patients with hematological diseases (literature review)." Oncohematology 16, no. 1 (April 14, 2021): 23–30. http://dx.doi.org/10.17650/1818-8346-2021-16-1-23-30.
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Telomeres are protein structures that regulate the process of cellular aging and play the role of a protective “cap” on the end sections of chromosomes. The telomeres of nucleated cells undergo permanent shortening during their lifetime as a result of multiple cycles of DNA replication. The enzyme that provides completion of the missing telomeric repeats at the ends of chromosomes is called “telomerase”. However, recovery of critically short telomeres by telomerase or recombination in somatic cells is limited due to the presence of a large accumulation of unclosed telomeres, which triggers apoptosis. The death of stem cells due to telomere depletion ensures the selection of abnormal cells in which the genome instability contributes to malignant progression. During carcinogenesis, cells acquire mechanisms for maintaining telomeres in order to avoid programmed death. In addition, tumor cells are able to support the telomere's DNA, counteracting its shortening and premature death. Activation of telomere length maintenance mechanisms is a hallmark of most types of cancers. In the modern world, there is an increasing interest in studying the biological characteristics of telomeres. The development of new methods for measuring telomere length has provided numerous studies to understand the relationship between telomere length of human nucleated cells and cancer. Perhaps maintaining telomere length will be an important step, determining the course and prognosis of the disease. The purpose of this review is to provide an analysis of published data of the role and significance of telomere length in patients with hematological malignancies.
5
Basenko, Evelina, Zeki Topcu, and Michael J. McEachern. "Recombination Can either Help Maintain Very Short Telomeres or Generate Longer Telomeres in Yeast Cells with Weak Telomerase Activity." Eukaryotic Cell 10, no. 8 (June 10, 2011): 1131–42. http://dx.doi.org/10.1128/ec.05079-11.
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ABSTRACT Yeast mutants lacking telomerase are able to elongate their telomeres through processes involving homologous recombination. In this study, we investigated telomeric recombination in several mutants that normally maintain very short telomeres due to the presence of a partially functional telomerase. The abnormal colony morphology present in some mutants was correlated with especially short average telomere length and with a requirement for RAD52 for indefinite growth. Better-growing derivatives of some of the mutants were occasionally observed and were found to have substantially elongated telomeres. These telomeres were composed of alternating patterns of mutationally tagged telomeric repeats and wild-type repeats, an outcome consistent with amplification occurring via recombination rather than telomerase. Our results suggest that recombination at telomeres can produce two distinct outcomes in the mutants we studied. In occasional cells, recombination generates substantially longer telomeres, apparently through the roll-and-spread mechanism. However, in most cells, recombination appears limited to helping to maintain very short telomeres. The latter outcome likely represents a simplified form of recombinational telomere maintenance that is independent of the generation and copying of telomeric circles.
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Cook, Brandoch D., Jasmin N. Dynek, William Chang, Grigoriy Shostak, and Susan Smith. "Role for the Related Poly(ADP-Ribose) Polymerases Tankyrase 1 and 2 at Human Telomeres." Molecular and Cellular Biology 22, no. 1 (January 1, 2002): 332–42. http://dx.doi.org/10.1128/mcb.22.1.332-342.2002.
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ABSTRACT Telomere maintenance is essential for the continuous growth of tumor cells. In most human tumors telomeres are maintained by telomerase, a specialized reverse transcriptase. Tankyrase 1, a human telomeric poly(ADP-ribose) polymerase (PARP), positively regulates telomere length through its interaction with TRF1, a telomeric DNA-binding protein. Tankyrase 1 ADP-ribosylates TRF1, inhibiting its binding to telomeric DNA. Overexpression of tankyrase 1 in the nucleus promotes telomere elongation, suggesting that tankyrase 1 regulates access of telomerase to the telomeric complex. The recent identification of a closely related homolog of tankyrase 1, tankyrase 2, opens the possibility for a second PARP at telomeres. We therefore sought to establish the role of tankyrase 1 at telomeres and to determine if tankyrase 2 might have a telomeric function. We show that endogenous tankyrase 1 is a component of the human telomeric complex. We demonstrate that telomere elongation by tankyrase 1 requires the catalytic activity of the PARP domain and does not occur in telomerase-negative primary human cells. To investigate a potential role for tankyrase 2 at telomeres, recombinant tankyrase 2 was subjected to an in vitro PARP assay. Tankyrase 2 poly(ADP-ribosyl)ated itself and TRF1. Overexpression of tankyrase 2 in the nucleus released endogenous TRF1 from telomeres. These findings establish tankyrase 2 as a bona fide PARP, with itself and TRF1 as acceptors of ADP-ribosylation, and suggest the possibility of a role for tankyrase 2 at telomeres.
7
Perera, Omesha N., Alexander P. Sobinoff, Erdahl T. Teber, Ashley Harman, Michelle F. Maritz, Sile F. Yang, Hilda A. Pickett, et al. "Telomerase promotes formation of a telomere protective complex in cancer cells." Science Advances 5, no. 10 (October 2019): eaav4409. http://dx.doi.org/10.1126/sciadv.aav4409.
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Telomerase is a ribonucleoprotein complex that catalyzes addition of telomeric DNA repeats to maintain telomeres in replicating cells. Here, we demonstrate that the telomerase protein hTERT performs an additional role at telomeres that is independent of telomerase catalytic activity yet essential for telomere integrity and cell proliferation. Short-term depletion of endogenous hTERT reduced the levels of heat shock protein 70 (Hsp70-1) and the telomere protective protein Apollo at telomeres, and induced telomere deprotection and cell cycle arrest, in the absence of telomere shortening. Short-term expression of hTERT promoted colocalization of Hsp70-1 with telomeres and Apollo and reduced numbers of deprotected telomeres, in a manner independent of telomerase catalytic activity. These data reveal a previously unidentified noncanonical function of hTERT that promotes formation of a telomere protective complex containing Hsp70-1 and Apollo and is essential for sustained proliferation of telomerase-positive cancer cells, likely contributing to the known cancer-promoting effects of both hTERT and Hsp70-1.
8
Chan, Simon R. W. L., and Elizabeth H. Blackburn. "Telomeres and telomerase." Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences 359, no. 1441 (January 29, 2004): 109–22. http://dx.doi.org/10.1098/rstb.2003.1370.
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Telomeres are the protective DNA–protein complexes found at the ends of eukaryotic chromosomes. Telomeric DNA consists of tandem repeats of a simple, often G–rich, sequence specified by the action of telomerase, and complete replication of telomeric DNA requires telomerase. Telomerase is a specialized cellular ribonucleoprotein reverse transcriptase. By copying a short template sequence within its intrinsic RNA moiety, telomerase synthesizes the telomeric DNA strand running 5' to 3' towards the distal end of the chromosome, thus extending it. Fusion of a telomere, either with another telomere or with a broken DNA end, generally constitutes a catastrophic event for genomic stability. Telomerase acts to prevent such fusions. The molecular consequences of telomere failure, and the molecular contributors to telomere function, with an emphasis on telomerase, are discussed here.
9
Dreesen, Oliver, and George A. M. Cross. "Telomerase-Independent Stabilization of Short Telomeres in Trypanosoma brucei." Molecular and Cellular Biology 26, no. 13 (July 1, 2006): 4911–19. http://dx.doi.org/10.1128/mcb.00212-06.
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ABSTRACT In cancer cells and germ cells, shortening of chromosome ends is prevented by telomerase. Telomerase-deficient cells have a replicative life span, after which they enter senescence. Senescent cells can give rise to survivors that maintain chromosome ends through recombination-based amplification of telomeric or subtelomeric repeats. We found that in Trypanosoma brucei, critically short telomeres are stable in the absence of telomerase. Telomere stabilization ensured genomic integrity and could have implications for telomere maintenance in human telomerase-deficient cells. Cloning and sequencing revealed 7 to 27 TTAGGG repeats on stabilized telomeres and no changes in the subtelomeric region. Clones with short telomeres were used to study telomere elongation dynamics, which differed dramatically at transcriptionally active and silent telomeres, after restoration of telomerase. We propose that transcription makes the termini of short telomeres accessible for rapid elongation by telomerase and that telomere elongation in T. brucei is not regulated by a protein-counting mechanism. Many minichromosomes were lost after long-term culture in the absence of telomerase, which may reflect their different mitotic segregation properties.
10
Kishtagari, Ashwin, and Justin Watts. "Biological and clinical implications of telomere dysfunction in myeloid malignancies." Therapeutic Advances in Hematology 8, no. 11 (October 6, 2017): 317–26. http://dx.doi.org/10.1177/2040620717731549.
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Telomeres at the ends of linear chromosomes protect the genome. Telomeres shorten with each round of cell division, placing a finite limit on cell growth. Telomere attrition is associated with cell senescence and apoptosis. Telomerase, a specialized ribonucleoprotein complex, maintains telomeres homeostasis through repeat addition of telomere sequences to the 3′ telomeric overhang. Telomere biology is closely related to cancer and normal aging. Upregulation of telomerase or activation of the alternative pathway of telomere lengthening is a hallmark of cancer cells, making telomerase an attractive target for cancer therapeutics. In this review, we will discuss telomere biology and the prognostic implications of telomere length in acute myeloid leukemia, and review exciting new investigational approaches using telomerase inhibitors in acute myeloid leukemia and other myeloid malignancies.
11
Mondello, Chiara, and A. Ivana Scovassi. "Telomeres, telomerase, and apoptosis." Biochemistry and Cell Biology 82, no. 4 (August 1, 2004): 498–507. http://dx.doi.org/10.1139/o04-048.
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Telomeres are specialized high-order chromatin structures that cap the ends of eukaryotic chromosomes. In vertebrates, telomeric DNA is composed of repetitions of the TTAGGG hexanucleotide, is bound to a set of specific proteins, and is elongated by the reverse transcriptase enzyme telomerase. Telomerase activity is promptly detected in cells with an indefinite replicative potential, such as cancer cells, while is almost undetectable in normal cells, which are characterized by a limited life span. Mounting evidence indicates that the maintenance of telomere integrity and telomerase protect cells from apoptosis. Disruption of the telomere capping function and (or) telomerase inhibition elicit an apoptotic response in cancer cells, while restoration of telomerase activity in somatic cells confers resistance to apoptosis. The possible mechanisms linking telomeres, telomerase and apoptosis are discussed in this review, together with the impact of this field in anticancer research.Key words: telomeres, telomerase, telomeric proteins, apoptosis, tumorigenesis.
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Marchesini, M., R. Matocci, L. Tasselli, V. Cambiaghi, A. Orleth, L. Furia, C. Marinelli, et al. "PML is required for telomere stability in non-neoplastic human cells." Oncogene 35, no. 14 (June 29, 2015): 1811–21. http://dx.doi.org/10.1038/onc.2015.246.
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Abstract Telomeres interact with numerous proteins, including components of the shelterin complex, whose alteration, similarly to proliferation-induced telomere shortening, initiates cellular senescence. In tumors, telomere length is maintained by Telomerase activity or by the Alternative Lengthening of Telomeres mechanism, whose hallmark is the telomeric localization of the promyelocytic leukemia (PML) protein. Whether PML contributes to telomeres maintenance in normal cells is unknown. We show that in normal human fibroblasts the PML protein associates with few telomeres, preferentially when they are damaged. Proliferation-induced telomere attrition or their damage due to alteration of the shelterin complex enhances the telomeric localization of PML, which is increased in human T-lymphocytes derived from patients genetically deficient in telomerase. In normal fibroblasts, PML depletion induces telomere damage, nuclear and chromosomal abnormalities, and senescence. Expression of the leukemia protein PML/RARα in hematopoietic progenitors displaces PML from telomeres and induces telomere shortening in the bone marrow of pre-leukemic mice. Our work provides a novel view of the physiologic function of PML, which participates in telomeres surveillance in normal cells. Our data further imply that a diminished PML function may contribute to cell senescence, genomic instability, and tumorigenesis.
13
Cohn, Marita, Ahu Karademir Andersson, Raquel Quintilla Mateo, and Mirja Carlsson Möller. "Alternative Lengthening of Telomeres in the Budding Yeast Naumovozyma castellii." G3: Genes|Genomes|Genetics 9, no. 10 (August 19, 2019): 3345–58. http://dx.doi.org/10.1534/g3.119.400428.
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The enzyme telomerase ensures the integrity of linear chromosomes by maintaining telomere length. As a hallmark of cancer, cell immortalization and unlimited proliferation is gained by reactivation of telomerase. However, a significant fraction of cancer cells instead uses alternative telomere lengthening mechanisms to ensure telomere function, collectively known as Alternative Lengthening of Telomeres (ALT). Although the budding yeast Naumovozyma castellii (Saccharomyces castellii) has a proficient telomerase activity, we demonstrate here that telomeres in N. castellii are efficiently maintained by a novel ALT mechanism after telomerase knockout. Remarkably, telomerase-negative cells proliferate indefinitely without any major growth crisis and display wild-type colony morphology. Moreover, ALT cells maintain linear chromosomes and preserve a wild-type DNA organization at the chromosome termini, including a short stretch of terminal telomeric sequence. Notably, ALT telomeres are elongated by the addition of ∼275 bp repeats containing a short telomeric sequence and the subtelomeric DNA located just internally (TelKO element). Although telomeres may be elongated by several TelKO repeats, no dramatic genome-wide amplification occurs, thus indicating that the repeat addition may be regulated. Intriguingly, a short interstitial telomeric sequence (ITS) functions as the initiation point for the addition of the TelKO element. This implies that N. castellii telomeres are structurally predisposed to efficiently switch to the ALT mechanism as a response to telomerase dysfunction.
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Jády, Beáta E., Patricia Richard, Edouard Bertrand, and Tamás Kiss. "Cell Cycle-dependent Recruitment of Telomerase RNA and Cajal Bodies to Human Telomeres." Molecular Biology of the Cell 17, no. 2 (February 2006): 944–54. http://dx.doi.org/10.1091/mbc.e05-09-0904.
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Telomerase is a ribonucleoprotein enzyme that counteracts replicative telomere erosion by adding telomeric sequence repeats onto chromosome ends. Despite its well-established role in telomere synthesis, telomerase has not yet been detected at telomeres. The RNA component of human telomerase (hTR) resides in the nucleoplasmic Cajal bodies (CBs) of interphase cancer cells. Here, in situ hybridization demonstrates that in human HeLa and Hep2 S phase cells, besides accumulating in CBs, hTR specifically concentrates at a few telomeres that also accumulate the TRF1 and TRF2 telomere marker proteins. Surprisingly, telomeres accumulating hTR exhibit a great accessibility for in situ oligonucleotide hybridization without chromatin denaturation, suggesting that they represent a structurally distinct, minor subset of HeLa telomeres. Moreover, we demonstrate that more than 25% of telomeres accumulating hTR colocalize with CBs. Time-lapse fluorescence microscopy demonstrates that CBs moving in the nucleoplasm of S phase cells transiently associate for 10-40 min with telomeres. Our data raise the intriguing possibility that CBs may deliver hTR to telomeres and/or may function in other aspects of telomere maintenance.
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Fernandes, Stina George, Rebecca Dsouza, Gouri Pandya, Anuradha Kirtonia, Vinay Tergaonkar, Sook Y. Lee, Manoj Garg, and Ekta Khattar. "Role of Telomeres and Telomeric Proteins in Human Malignancies and Their Therapeutic Potential." Cancers 12, no. 7 (July 14, 2020): 1901. http://dx.doi.org/10.3390/cancers12071901.
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Telomeres are the ends of linear chromosomes comprised of repetitive nucleotide sequences in humans. Telomeres preserve chromosomal stability and genomic integrity. Telomere length shortens with every cell division in somatic cells, eventually resulting in replicative senescence once telomere length becomes critically short. Telomere shortening can be overcome by telomerase enzyme activity that is undetectable in somatic cells, while being active in germline cells, stem cells, and immune cells. Telomeres are bound by a shelterin complex that regulates telomere lengthening as well as protects them from being identified as DNA damage sites. Telomeres are transcribed by RNA polymerase II, and generate a long noncoding RNA called telomeric repeat-containing RNA (TERRA), which plays a key role in regulating subtelomeric gene expression. Replicative immortality and genome instability are hallmarks of cancer and to attain them cancer cells exploit telomere maintenance and telomere protection mechanisms. Thus, understanding the role of telomeres and their associated proteins in cancer initiation, progression and treatment is very important. The present review highlights the critical role of various telomeric components with recently established functions in cancer. Further, current strategies to target various telomeric components including human telomerase reverse transcriptase (hTERT) as a therapeutic approach in human malignancies are discussed.
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Ji, Hong, Christopher J. Adkins, Bethany R. Cartwright, and Katherine L. Friedman. "Yeast Est2p Affects Telomere Length by Influencing Association of Rap1p with Telomeric Chromatin." Molecular and Cellular Biology 28, no. 7 (January 22, 2008): 2380–90. http://dx.doi.org/10.1128/mcb.01648-07.
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ABSTRACT In Saccharomyces cerevisiae, the sequence-specific binding of the negative regulator Rap1p provides a mechanism to measure telomere length: as the telomere length increases, the binding of additional Rap1p inhibits telomerase activity in cis. We provide evidence that the association of Rap1p with telomeric DNA in vivo occurs in part by sequence-independent mechanisms. Specific mutations in EST2 (est2-LT) reduce the association of Rap1p with telomeric DNA in vivo. As a result, telomeres are abnormally long yet bind an amount of Rap1p equivalent to that observed at wild-type telomeres. This behavior contrasts with that of a second mutation in EST2 (est2-up34) that increases bound Rap1p as expected for a strain with long telomeres. Telomere sequences are subtly altered in est2-LT strains, but similar changes in est2-up34 telomeres suggest that sequence abnormalities are a consequence, not a cause, of overelongation. Indeed, est2-LT telomeres bind Rap1p indistinguishably from the wild type in vitro. Taken together, these results suggest that Est2p can directly or indirectly influence the binding of Rap1p to telomeric DNA, implicating telomerase in roles both upstream and downstream of Rap1p in telomere length homeostasis.
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Natarajan, Shobhana, Cindy Groff-Vindman, and Michael J. McEachern. "Factors Influencing the Recombinational Expansion and Spread of Telomeric Tandem Arrays in Kluyveromyces lactis." Eukaryotic Cell 2, no. 5 (October 2003): 1115–27. http://dx.doi.org/10.1128/ec.2.5.1115-1127.2003.
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ABSTRACT We have previously shown that DNA circles containing telomeric repeats and a marker gene can promote the recombinational elongation of telomeres in Kluyveromyces lactis by a mechanism proposed to involve rolling-circle DNA synthesis. Wild-type cells acquire a long tandem array at a single telomere, while telomerase deletion (ter1-Δ) cells, acquire an array and also spread it to multiple telomeres. In this study, we further examine the factors that affect the formation and spread of telomeric tandem arrays. We show that a telomerase+ strain with short telomeres and high levels of subtelomeric gene conversion can efficiently form and spread arrays, while a telomere fusion mutant is not efficient at either process. This indicates that an elevated level of gene conversion near telomeres is required for spreading but that growth senescence and a tendency to elongate telomeres in the absence of exogenously added circles are not. Surprisingly, telomeric repeats are frequently deleted from a transforming URA3-telomere circle at or prior to the time of array formation by a mechanism dependent upon the presence of subtelomeric DNA in the circle. We further show that in a ter1-Δ strain, long tandem arrays can arise from telomeres initially containing a single-copy insert of the URA3-telomere sequence. However, the reduced rate of array formation in such strains suggests that single-copy inserts are not typical intermediates in arrays formed from URA3-telomere circles. Using heteroduplex circles, we have demonstrated that either strand of a URA3-telomere circle can be utilized to form telomeric tandem arrays. Consistent with this, we demonstrate that 100-nucleotide single-stranded telomeric circles of either strand can promote recombinational telomere elongation.
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Choe, Wonchae, Martin Budd, Osamu Imamura, Laura Hoopes, and Judith L. Campbell. "Dynamic Localization of an Okazaki Fragment Processing Protein Suggests a Novel Role in Telomere Replication." Molecular and Cellular Biology 22, no. 12 (June 15, 2002): 4202–17. http://dx.doi.org/10.1128/mcb.22.12.4202-4217.2002.
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ABSTRACT We have found that the Dna2 helicase-nuclease, thought to be involved in maturation of Okazaki fragments, is a component of telomeric chromatin. We demonstrate a dynamic localization of Dna2p to telomeres that suggests a dual role for Dna2p, one in telomere replication and another, unknown function, perhaps in telomere capping. Both chromatin immunoprecipitation (ChIP) and immunofluorescence show that Dna2p associates with telomeres but not bulk chromosomal DNA in G1 phase, when there is no telomere replication and the telomere is transcriptionally silenced. In S phase, there is a dramatic redistribution of Dna2p from telomeres to sites throughout the replicating chromosomes. Dna2p is again localized to telomeres in late S, where it remains through G2 and until the next S phase. Telomeric localization of Dna2p required Sir3p, since the amount of Dna2p found at telomeres by two different assays, one-hybrid and ChIP, is severely reduced in strains lacking Sir3p. The Dna2p is also distributed throughout the nucleus in cells growing in the presence of double-strand-break-inducing agents such as bleomycin. Finally, we show that Dna2p is functionally required for telomerase-dependent de novo telomere synthesis and also participates in telomere lengthening in mutants lacking telomerase.
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Eberhard, Stephan, Sona Valuchova, Julie Ravat, Jaroslav Fulneček, Pascale Jolivet, Sandrine Bujaldon, Stéphane D. Lemaire, et al. "Molecular characterization of Chlamydomonas reinhardtii telomeres and telomerase mutants." Life Science Alliance 2, no. 3 (June 2019): e201900315. http://dx.doi.org/10.26508/lsa.201900315.
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Telomeres are repeated sequences found at the end of the linear chromosomes of most eukaryotes and are required for chromosome integrity. Expression of the reverse-transcriptase telomerase allows for extension of telomeric repeats to counteract natural telomere shortening. Although Chlamydomonas reinhardtii, a photosynthetic unicellular green alga, is widely used as a model organism in photosynthesis and flagella research, and for biotechnological applications, the biology of its telomeres has not been investigated in depth. Here, we show that the C. reinhardtii (TTTTAGGG)n telomeric repeats are mostly nondegenerate and that the telomeres form a protective structure, with a subset ending with a 3′ overhang and another subset presenting a blunt end. Although telomere size and length distributions are stable under various standard growth conditions, they vary substantially between 12 genetically close reference strains. Finally, we identify CrTERT, the gene encoding the catalytic subunit of telomerase and show that telomeres shorten progressively in mutants of this gene. Telomerase mutants eventually enter replicative senescence, demonstrating that telomerase is required for long-term maintenance of telomeres in C. reinhardtii.
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Underwood, Dana H., Coleen Carroll, and Michael J. McEachern. "Genetic Dissection of the Kluyveromyces lactis Telomere and Evidence for Telomere Capping Defects in TER1 Mutants with Long Telomeres." Eukaryotic Cell 3, no. 2 (April 2004): 369–84. http://dx.doi.org/10.1128/ec.3.2.369-384.2004.
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ABSTRACT In the yeast Kluyveromyces lactis, the telomeres are composed of perfect 25-bp repeats copied from a 30-nucleotide RNA template defined by 5-nucleotide terminal repeats. A genetic dissection of the K. lactis telomere was performed by using mutant telomerase RNA (TER1) alleles to incorporate mutated telomeric repeats. This analysis has shown that each telomeric repeat contains several functional regions, some of which may physically overlap. Mutations in the terminal repeats of the template RNA typically lead to telomere shortening, as do mutations in the right side of the Rap1p binding site. Mutations in the left half of the Rap1p binding site, however, lead to the immediate formation of long telomeres. When mutated, the region immediately 3′ of the Rap1p binding site on the TG-rich strand of the telomere leads to telomeres that are initially short but eventually undergo extreme telomere elongation. Mutations between this region and the 3′ terminal repeat cause elevated recombination despite the presence of telomeres of nearly wild-type length. Mutants with highly elongated telomeres were further characterized and exhibit signs of telomere capping defects, including elevated levels of subtelomeric recombination and the formation of extrachromosomal and single-stranded telomeric DNA. Lengthening caused by some Rap1 binding site mutations can be suppressed by high-copy-number RAP1. Mutated telomeric repeats from a delayed elongation mutant are shown to be defective at regulating telomere length in cells with wild-type telomerase, indicating that the telomeric repeats are defective at telomere length regulation.
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Kelleher, Colleen, Isabel Kurth, and Joachim Lingner. "Human Protection of Telomeres 1 (POT1) Is a Negative Regulator of Telomerase Activity In Vitro." Molecular and Cellular Biology 25, no. 2 (January 15, 2005): 808–18. http://dx.doi.org/10.1128/mcb.25.2.808-818.2005.
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ABSTRACT The telomeric single-strand DNA binding protein protection of telomeres 1 (POT1) protects telomeres from rapid degradation in Schizosaccharomyces pombe and has been implicated in positive and negative telomere length regulation in humans. Human POT1 appears to interact with telomeres both through direct binding to the 3′ overhanging G-strand DNA and through interaction with the TRF1 duplex telomere DNA binding complex. The influence of POT1 on telomerase activity has not been studied at the molecular level. We show here that POT1 negatively effects telomerase activity in vitro. We find that the DNA binding activity of POT1 is required for telomerase inhibition. Furthermore, POT1 is incapable of inhibiting telomeric repeat addition to substrate primers that are defective for POT1 binding, suggesting that in vivo, POT1 likely affects substrate access to telomerase.
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Prescott, John C., and Elizabeth H. Blackburn. "Telomerase RNA Template Mutations Reveal Sequence-Specific Requirements for the Activation and Repression of Telomerase Action at Telomeres." Molecular and Cellular Biology 20, no. 8 (April 15, 2000): 2941–48. http://dx.doi.org/10.1128/mcb.20.8.2941-2948.2000.
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ABSTRACT Telomeric DNA is maintained within a length range characteristic of an organism or cell type. Significant deviations outside this range are associated with altered telomere function. The yeast telomere-binding protein Rap1p negatively regulates telomere length. Telomere elongation is responsive to both the number of Rap1p molecules bound to a telomere and the Rap1p-centered DNA-protein complex at the extreme telomeric end. Previously, we showed that a specific trinucleotide substitution in the Saccharomyces cerevisiae telomerase gene (TLC1) RNA template abolished the enzymatic activity of telomerase, causing the same cell senescence and telomere shortening phenotypes as a complete tlc1 deletion. Here we analyze effects of six single- and double-base changes within these same three positions. All six mutant telomerases had in vitro enzymatic activity levels similar to the wild-type levels. The base changes predicted from the mutations all disrupted Rap1p binding in vitro to the corresponding duplex DNAs. However, they caused two classes of effects on telomere homeostasis: (i) rapid, RAD52-independent telomere lengthening and poor length regulation, whose severity correlated with the decrease in in vitro Rap1p binding affinity (this is consistent with loss of negative regulation of telomerase action at these telomeres; and (ii) telomere shortening that, depending on the template mutation, either established a new short telomere set length with normal cell growth or was progressive and led to cellular senescence. Hence, disrupting Rap1p binding at the telomeric terminus is not sufficient to deregulate telomere elongation. This provides further evidence that both positive and negativecis-acting regulators of telomerase act at telomeres.
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Schaetzlein, S., and K. L. Rudolph. "Telomere length regulation during cloning, embryogenesis and ageing." Reproduction, Fertility and Development 17, no. 2 (2005): 85. http://dx.doi.org/10.1071/rd04112.
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Telomeres are nucleoprotein complexes at the end of eukaryotic chromosomes with an essential role in chromosome capping. Owing to the end-replication problem of DNA polymerase, telomeres shorten during each cell division. When telomeres become critically short, they loose their capping function, which in turn induces a DNA damage-like response. This mechanism inhibits cell proliferation at the senescence stage and there is evidence that it limits the regenerative capacity of tissues and organs during chronic diseases and ageing. The holoenzyme telomerase synthesises telomeric DNA de novo, but, in humans, it is active only during embryogenesis, in immature germ cells and in a subset of stem/progenitor cells during postnatal life. Telomere length can be maintained or increased by telomerase, a process that appears to be regulated by a variety of telomere-binding proteins that control telomerase recruitment and activity at the telomeres. During embryogenesis, telomerase is strongly activated at the morula/blastocyst transition. At this transition, telomeres are significantly elongated in murine and bovine embryos. Early embryonic telomere elongation is telomerase dependent and leads to a rejuvenation of telomeres in cloned bovine embryos. Understanding of the molecular mechanisms underlying this early embryonic telomere elongation programme is of great interest for medical research in the fields of regeneration, cell therapies and therapeutic cloning.
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Henderson, S., R. Allsopp, D. Spector, S. S. Wang, and C. Harley. "In situ analysis of changes in telomere size during replicative aging and cell transformation." Journal of Cell Biology 134, no. 1 (July 1, 1996): 1–12. http://dx.doi.org/10.1083/jcb.134.1.1.
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Telomeres have been shown to gradually shorten during replicative aging in human somatic cells by Southern analysis. This study examines telomere shortening at the single cell level by fluorescence in situ hybridization (FISH). FISH and confocal microscopy of interphase human diploid fibroblasts (HDFs) demonstrate that telomeres are distributed throughout the nucleus with an interchromosomal heterogeneity in size. Analysis of HDFs at increasing population doubling levels shows a gradual increase in spot size, intensity, and detectability of telomeric signal. FISH of metaphase chromosomes prepared from young and old HDFs shows a heterogeneity in detection frequency for telomeres on chromosomes 1, 9, 15, and Y. The interchromosomal distribution of detection frequencies was similar for cells at early and late passage. The telomeric detection frequency for metaphase chromosomes also decreased with age. These observations suggest that telomeres shorten at similar rates in normal human somatic cels. T-antigen transformed HDFs near crisis contained telomere signals that were low compared to nontransformed HDFs. A large intracellular heterogeneity in telomere lengths was detected in two telomerase-negative cell lines compared to normal somatic cells and the telomerase-positive 293 cell line. Many telomerase-negative immortal cells had telomeric signals stronger than those in young HDFs, suggesting a different mechanism for telomere length regulation in telomerase-negative immortal cells. These studies provide an in situ demonstration of interchromosomal heterogeneity in telomere lengths. Furthermore, FISH is a reliable and sensitive method for detecting changes in telomere size at the single cell level.
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Smogorzewska, Agata, Bas van Steensel, Alessandro Bianchi, Stefan Oelmann, Matthias R. Schaefer, Gisela Schnapp, and Titia de Lange. "Control of Human Telomere Length by TRF1 and TRF2." Molecular and Cellular Biology 20, no. 5 (March 1, 2000): 1659–68. http://dx.doi.org/10.1128/mcb.20.5.1659-1668.2000.
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ABSTRACT Telomere length in human cells is controlled by a homeostasis mechanism that involves telomerase and the negative regulator of telomere length, TRF1 (TTAGGG repeat binding factor 1). Here we report that TRF2, a TRF1-related protein previously implicated in protection of chromosome ends, is a second negative regulator of telomere length. Overexpression of TRF2 results in the progressive shortening of telomere length, similar to the phenotype observed with TRF1. However, while induction of TRF1 could be maintained over more than 300 population doublings and resulted in stable, short telomeres, the expression of exogenous TRF2 was extinguished and the telomeres eventually regained their original length. Consistent with their role in measuring telomere length, indirect immunofluorescence indicated that both TRF1 and TRF2 bind to duplex telomeric DNA in vivo and are more abundant on telomeres with long TTAGGG repeat tracts. Neither TRF1 nor TRF2 affected the expression level of telomerase. Furthermore, the presence of TRF1 or TRF2 on a short linear telomerase substrate did not inhibit the enzymatic activity of telomerase in vitro. These findings are consistent with the recently proposed t loop model of telomere length homeostasis in which telomerase-dependent telomere elongation is blocked by sequestration of the 3′ telomere terminus in TRF1- and TRF2-induced telomeric loops.
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Donate, Luis E., and Maria A. Blasco. "Telomeres in cancer and ageing." Philosophical Transactions of the Royal Society B: Biological Sciences 366, no. 1561 (January 12, 2011): 76–84. http://dx.doi.org/10.1098/rstb.2010.0291.
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Telomeres protect the chromosome ends from unscheduled DNA repair and degradation. Telomeres are heterochromatic domains composed of repetitive DNA (TTAGGG repeats) bound to an array of specialized proteins. The length of telomere repeats and the integrity of telomere-binding proteins are both important for telomere protection. Furthermore, telomere length and integrity are regulated by a number of epigenetic modifications, thus pointing to higher order control of telomere function. In this regard, we have recently discovered that telomeres are transcribed generating long, non-coding RNAs, which remain associated with the telomeric chromatin and are likely to have important roles in telomere regulation. In the past, we showed that telomere length and the catalytic component of telomerase, Tert, are critical determinants for the mobilization of stem cells. These effects of telomerase and telomere length on stem cell behaviour anticipate the premature ageing and cancer phenotypes of telomerase mutant mice. Recently, we have demonstrated the anti-ageing activity of telomerase by forcing telomerase expression in mice with augmented cancer resistance. Shelterin is the major protein complex bound to mammalian telomeres; however, its potential relevance for cancer and ageing remained unaddressed to date. To this end, we have generated mice conditionally deleted for the shelterin proteins TRF1, TPP1 and Rap1. The study of these mice demonstrates that telomere dysfunction, even if telomeres are of a normal length, is sufficient to produce premature tissue degeneration, acquisition of chromosomal aberrations and initiation of neoplastic lesions. These new mouse models, together with the telomerase-deficient mouse model, are valuable tools for understanding human pathologies produced by telomere dysfunction.
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Schmidt, Tobias T., Carly Tyer, Preeyesh Rughani, Candy Haggblom, Jeff Jones, Xiaoguang Dai, Kelly A. Frazer, et al. "Abstract 1639: Telomere dynamics in aging and cancer by nanopore long-read sequencing." Cancer Research 84, no. 6_Supplement (March 22, 2024): 1639. http://dx.doi.org/10.1158/1538-7445.am2024-1639.
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Abstract Telomeres are the protective, nucleoprotein structure at the ends of linear eukaryotic chromosomes. The accurate measurement of both telomeric length and composition of individual telomeres in mammalian cells has been challenged by the length and repetitive nature of telomeres. With the advent of third generation sequencing technologies, it is now technically possible to sequence entire telomeres and map them to individual chromosome arms. Here, we report a reliable method to enrich, sequence and analyze human telomeres using Oxford Nanopore Technologies long-read sequencing. To enrich for telomeric sequences we combine the ligation of adapters complementary to the telomeric G-overhang with restriction enzyme digest to sequence the telomeric C-strand and part of the adjacent subtelomere. The subtelomeric information is harvested to map individual telomeric reads to specific chromosome arms and even alleles. We have measured bulk, chromosome-arm-specific telomere length dynamics during cellular aging of cultured primary cells and in a patient-derived aging cohort. To address the impact of the telomere maintenance mechanism on telomere length and composition, we have sequenced matched pairs of fibroblasts and induced pluripotent stem cells, as well as five well-established telomerase- and ALT-positive cancer cell lines. Our results suggest that based on nanopore telomere long-read sequencing ALT-positive cells can be easily discriminated from normal and telomerase-positive cancer cells. Further, telomere sequencing allows to evaluate the methylation status of the subtelomeric CpG islands adjacent to telomeres. In summary, nanopore telomere long-read sequencing allows to measure the length and composition of individual telomeres and their mapping to specific chromosome arms. Telomere long read sequencing methods will be valuable tools to study telomere biology during aging and cancer. Citation Format: Tobias T. Schmidt, Carly Tyer, Preeyesh Rughani, Candy Haggblom, Jeff Jones, Xiaoguang Dai, Kelly A. Frazer, Fred H. Gage, Sissel Juul, Scott Hickey, Jan Karlseder. Telomere dynamics in aging and cancer by nanopore long-read sequencing [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 1639.
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Stock, Carmel J. W., and Elisabetta A. Renzoni. "Telomeres in Interstitial Lung Disease." Journal of Clinical Medicine 10, no. 7 (March 30, 2021): 1384. http://dx.doi.org/10.3390/jcm10071384.
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Interstitial lung diseases (ILD) encompass a group of conditions involving fibrosis and/or inflammation of the pulmonary parenchyma. Telomeres are repetitive DNA sequences at chromosome ends which protect against genome instability. At each cell division, telomeres shorten, but the telomerase complex partially counteracts progressive loss of telomeres by catalysing the synthesis of telomeric repeats. Once critical telomere shortening is reached, cell cycle arrest or apoptosis are triggered. Telomeres progressively shorten with age. A number of rare genetic mutations have been identified in genes encoding for components of the telomerase complex, including telomerase reverse transcriptase (TERT) and telomerase RNA component (TERC), in familial and, less frequently, in sporadic fibrotic ILDs. Defects in telomerase result in extremely short telomeres. More rapidly progressive disease is observed in fibrotic ILD patients with telomere gene mutations, regardless of underlying diagnosis. Associations with common single nucleotide polymorphisms in telomere related genes have also been demonstrated for various ILDs. Shorter peripheral blood telomere lengths compared to age-matched healthy individuals are found in a proportion of patients with fibrotic ILDs, and in idiopathic pulmonary fibrosis (IPF) and fibrotic hypersensitivity pneumonitis (HP) have been linked to worse survival, independently of disease severity. Greater susceptibility to immunosuppressant-induced side effects in patients with short telomeres has been described in patients with IPF and with fibrotic HP. Here, we discuss recent evidence for the involvement of telomere length and genetic variations in the development, progression, and treatment of fibrotic ILDs.
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Hahn, William C. "Role of Telomeres and Telomerase in the Pathogenesis of Human Cancer." Journal of Clinical Oncology 21, no. 10 (May 15, 2003): 2034–43. http://dx.doi.org/10.1200/jco.2003.06.018.
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Specialized nucleoprotein structures, termed telomeres, cap the ends of human chromosomes. These terminal structures, composed of repetitive arrays of guanine-rich hexameric DNA together with specific telomere-binding proteins, play essential roles in protecting the chromosome from damage and degradation. In addition, several lines of evidence implicate telomere maintenance as an important regulator of cell life span. Activation of telomerase, a dedicated reverse transcriptase that synthesizes telomeric sequences, is strongly associated with cancer, and recent observations confirm that telomeres and telomerase perform important roles in both suppressing and facilitating malignant transformation. These dual functions of telomere biology are evident in the clinical manifestations of the multisystem syndrome, dyskeratosis congenita, forms of which display defects in telomerase function. Recent advances in our understanding of telomere biology indicate that the manipulation of telomeres and telomerase will lead to clinically significant applications in the diagnosis, prevention, and treatment of neoplastic disease.
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Natarajan, Shobhana, and Michael J. McEachern. "Recombinational Telomere Elongation Promoted by DNA Circles." Molecular and Cellular Biology 22, no. 13 (July 1, 2002): 4512–21. http://dx.doi.org/10.1128/mcb.22.13.4512-4521.2002.
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ABSTRACT Yeast mutants lacking telomerase are capable of maintaining telomeres by an alternate mechanism that depends on homologous recombination. We show here, by using Kluyveromyces lactis cells containing two types of telomeric repeats, that recombinational telomere elongation generates a repeating pattern common in most or all telomeres in survivors that retain both repeat types. We propose that these patterns arise from small circles of telomeric DNA being used as templates for rolling-circle gene conversion and that the sequence from the lengthened telomere is spread to other telomeres by additional, more typical gene conversion events. Consistent with this, artificially constructed circles of DNA containing telomeric repeats form long tandem arrays at telomeres when transformed into K. lactis cells. Mixing experiments done with two species of telomeric circles indicated that all of the integrated copies of the transforming sequence arise from a single original circular molecule.
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Bessler, Monica, Rachida Bouharich, Shashikant Kulkarni, Sara Freeman, Hong-Yan Du, Philip J. Mason, Arturo Londono-Vallejo, and Fred Goldman. "Accelerated Shortening of Long Telomeres and Accumulation of Short Telomeres in Dyskeratosis Congenita." Blood 106, no. 11 (November 16, 2005): 1053. http://dx.doi.org/10.1182/blood.v106.11.1053.1053.
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Abstract Dyskeratosis congenita (DC) is the first human disease whose pathogenesis has been directly linked to an impairment of telomere maintenance. Telomeres protect chromosome ends from end to end fusion and degradation. Loss of telomere function causes cell cycle arrest or cell death. Telomeres are maintained by the telomerase ribonucloprotein complex whose integral RNA component, the telomerase RNA or TERC RNA, contains the sequences that act as a template for the synthesis of telomeric repeats. Autosomal dominant DC (AD DC), a rare inherited bone marrow failure syndrome, is caused by mutations in TERC, the RNA component of telomerase. Patients with AD DC have very short telomeres. Haploinsufficiency has been proposed to be the mechanism for telomere shortening in TERC gene mutation carriers. Individuals with AD DC not only inherited the TERC gene mutation but also the shortened telomeres from the affected parent. Here we studied the telomere dynamics over 3 generations in a 32-member extended family with AD DC due to a TERC gene deletion. The investigation of telomere length within a single family has the advantage that the molecular lesion responsible for telomere shortening is uniform and that the contribution of other genetic components influencing telomere length is similar. Our analysis shows that peripheral blood cells from family members haploinsufficient for TERC have very short telomeres (6.68 kb, range 5.53–8.45, SD 1.13, normal controls: 9.15 kb rage 8.56–10.77, SD 1.22). In contrast to normal controls, whose telomere lengths shorten with age, the telomere lengths in all individuals carrying the TERC gene deletion are equally short irrespective of their age. To study the inheritance of short telomeres and the effect of TERC haploinsufficiency on specific telomere lengths in affected individuals and their relatives we carried out Q-FISH analysis using polymorphic subtelomeric probes on chromosomes 11p, 7p, and 1p, which are able to distinguish the parental origin of telomeres in this family. Our analysis showed that in children of affected parents who have inherited the gene deletion, paternal and maternal telomeres are similarly short, and similar in length to those of the affected parent. In children of affected parents who have normal TERC genes paternal and maternal telomeres are again similar in length, and similar to those of the unaffected parent. These results are consistent with a model in which telomerase preferentially acts on the shortest telomeres. When TERC is limiting this leads to the accelerated shortening of longer telomeres and the accumulation of short telomeres. The limited amount of active telomerase in TERC RNA haploinsufficiency may not be able to maintain the minimal length of the increasing number of short telomeres. Thus, the number of critically short telomeres and the degree of residual telomerase activity may determine the onset of disease in patients with DC.
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Li, Bibo, and Titia de Lange. "Rap1 Affects the Length and Heterogeneity of Human Telomeres." Molecular Biology of the Cell 14, no. 12 (December 2003): 5060–68. http://dx.doi.org/10.1091/mbc.e03-06-0403.
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Telomere length is controlled in part by cis-acting negative regulators that limit telomere extension by telomerase. In budding yeast, the major telomere length regulator scRap1 binds to telomeric DNA and acts to inhibit telomere elongation in cis. Because the human Rap1 ortholog hRap1 does not bind to telomeric DNA directly but is recruited to telomeres by TRF2, we examined its role in telomere length control. The data are consistent with hRap1 being a negative regulator of telomere length, indicating functional conservation. Deletion mapping confirmed that hRap1 is tethered to telomeres through interaction of its C terminus with TRF2. The telomere length phenotypes of hRap1 deletion mutants implicated both the BRCT and Myb domain as protein interaction domains involved in telomere length regulation. By contrast, scRap1 binds to telomeres with its Myb domains and uses its C terminus to recruit the telomere length regulators Rif1 and Rif2. Together, our data show that although the role of Rap1 at telomeres has been largely conserved, the domains of Rap1 have undergone extensive functional changes during eukaryotic evolution. Surprisingly, hRap1 alleles lacking the BRCT domain diminished the heterogeneity of human telomeres, indicating that hRap1 also plays a role in the regulation of telomere length distribution.
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McEachern, M. J., and J. B. Hicks. "Unusually large telomeric repeats in the yeast Candida albicans." Molecular and Cellular Biology 13, no. 1 (January 1993): 551–60. http://dx.doi.org/10.1128/mcb.13.1.551-560.1993.
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We have identified sequences at the telomeres of the yeast Candida albicans and have found that they are composed of tandem copies of a 23-bp sequence. Through the cloning of native telomeric ends and the characterization and cloning of a "healed" end, we demonstrate that these repeated sequences are sufficient to function as a telomere. All copies of the 23-bp repeat that have been sequenced from a number of C. albicans strains are identical. In contrast, adjacent subtelomeric sequences are variable both between strains and within the WO-1 strain. In the WO-1 strain, the lengths of the telomeres are dependent upon growth temperature and are substantially longer at higher temperatures. Telomere growth is accompanied by increases in the number of the 23-bp repeats present on the telomeric fragments. These results suggest that either telomerase-maintained telomeres can be more complex in structure than was previously imagined or that Candida telomeres are maintained via a telomerase-independent mechanism.
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McEachern, M. J., and J. B. Hicks. "Unusually large telomeric repeats in the yeast Candida albicans." Molecular and Cellular Biology 13, no. 1 (January 1993): 551–60. http://dx.doi.org/10.1128/mcb.13.1.551.
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We have identified sequences at the telomeres of the yeast Candida albicans and have found that they are composed of tandem copies of a 23-bp sequence. Through the cloning of native telomeric ends and the characterization and cloning of a "healed" end, we demonstrate that these repeated sequences are sufficient to function as a telomere. All copies of the 23-bp repeat that have been sequenced from a number of C. albicans strains are identical. In contrast, adjacent subtelomeric sequences are variable both between strains and within the WO-1 strain. In the WO-1 strain, the lengths of the telomeres are dependent upon growth temperature and are substantially longer at higher temperatures. Telomere growth is accompanied by increases in the number of the 23-bp repeats present on the telomeric fragments. These results suggest that either telomerase-maintained telomeres can be more complex in structure than was previously imagined or that Candida telomeres are maintained via a telomerase-independent mechanism.
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Feigon, Juli. "Structural biology of telomerase mechanism and interactions at telomeres." Structural Dynamics 12, no. 2_Supplement (March 1, 2025): A201. https://doi.org/10.1063/4.0000509.
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Telomerase is a unique RNA-containing reverse transcriptase that synthesizes the DNA at the 3’-ends of telomeres, the structures at the ends of linear chromosomes. It is a highly regulated determinant of tumorigenesis, cellular aging, and stem cell renewal. All telomerases contain a catalytic core comprising telomerase reverse transcriptase (TERT) and telomerase RNA (TER), along with other proteins involved in biogenesis, assembly, and activation. TER includes a template complementary to ∼1.5 telomere repeats used by TERT to repetitively synthesize the telomere repeat (dTTGGGG in Tetrahymena, dTTAGGG in human), but other regions of TER are required for activity. Human telomerase is recruited to the 3’-ends of telomeres by TPP1–POT1 to processively synthesize multiple telomeric repeats (G-strand). The complementary C-strand is subsequently synthesized by DNA polymerase a–primase (PolaPrim) in concert with CTC1–STN1–TEN1 (CST). We have used cryo-electron microscopy studies combined with NMR spectroscopy and biochemical methods on Tetrahymena and human telomerase to investigate structure, assembly, and the mechanism of G-strand telomeric DNA repeat addition [1,2], interactions of TPP1/p50 with telomerase for recruitment and activation [1,2], and interactions of CST with telomerase and PolaPrim for C-strand synthesis [3]. For human telomerase, results also reveal that numerous disease mutations would disrupt interactions at the TER–TER interface, highlighting their importance for function [3]. Highlights of this recent work from our laboratory will be presented.
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Datta, Neelabh. "Unravelling the Intricacies of Telomere Replication: A Molecular Conundrum." Canadian Journal for the Academic Mind 2, no. 1 (July 16, 2024): 97–127. http://dx.doi.org/10.25071/2817-5344/74.
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Telomeres are specialized structures at the ends of linear chromosomes that protect them from degradation and fusion. It’s replication is a complex process that involves both DNA polymerases and a specialized enzyme called telomerase which is a ribonucleoprotein complex that synthesizes telomeric DNA by using an internal RNA template. However, telomerase alone cannot fully replicate the telomeric DNA, and requires the cooperation of other factors, such as shelterin, CST, and DNA repair proteins. Telomere replication is tightly regulated by various mechanisms, such as cell cycle checkpoints, telomere length homeostasis, and telomere position effect and dysregulation of it can lead to genomic instability, cellular senescence, and cancer. Therefore, understanding the molecular details of telomere replication is crucial for elucidating the role of telomeres in aging and disease.
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Smith, Christopher D., and Elizabeth H. Blackburn. "Uncapping and Deregulation of Telomeres Lead to Detrimental Cellular Consequences in Yeast." Journal of Cell Biology 145, no. 2 (April 19, 1999): 203–14. http://dx.doi.org/10.1083/jcb.145.2.203.
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Telomeres are the protein–nucleic acid structures at the ends of eukaryote chromosomes. Tandem repeats of telomeric DNA are templated by the RNA component (TER1) of the ribonucleoprotein telomerase. These repeats are bound by telomere binding proteins, which are thought to interact with other factors to create a higher-order cap complex that stabilizes the chromosome end. In the budding yeast Kluyveromyces lactis, the incorporation of certain mutant DNA sequences into telomeres leads to uncapping of telomeres, manifested by dramatic telomere elongation and increased length heterogeneity (telomere deregulation). Here we show that telomere deregulation leads to enlarged, misshapen “monster” cells with increased DNA content and apparent defects in cell division. However, such deregulated telomeres became stabilized at their elongated lengths upon addition of only a few functionally wild-type telomeric repeats to their ends, after which the frequency of monster cells decreased to wild-type levels. These results provide evidence for the importance of the most terminal repeats at the telomere in maintaining the cap complex essential for normal telomere function. Analysis of uncapped and capped telomeres also show that it is the deregulation resulting from telomere uncapping, rather than excessive telomere length per se, that is associated with DNA aberrations and morphological defects.
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Han, Xuesheng, Alice Hirschel, Menelaos Tsapekos, Diego Perez, and David Vollmer. "In Vitro Assessment of Gold Nanoparticles on Telomerase Activity and Telomere Length in Human Fibroblasts." International Journal of Molecular Sciences 24, no. 18 (September 19, 2023): 14273. http://dx.doi.org/10.3390/ijms241814273.
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Telomerase activity coincides with lengthening of the ends of chromosomes known as telomeres. Telomere length is used as a marker for cellular aging. Telomeres shorten over time as cells divide, and certain bioactive compounds such as gold nanoparticles (AuNPs) may slow the shortening of telomeres by increasing telomerase activity. The objective of the present study is to assess the effect of AuNPs on telomerase activity and telomere length in human fibroblasts. Telomerase activity was measured using enzyme-linked immunosorbent assay (ELISA) in primary human lung fibroblasts (IMR90) and using quantitative PCR-based telomeric repeat amplification protocol (Q-TRAP) in primary human dermal fibroblasts, neonatal (HDFn). Telomere length was determined by Telomere Analysis Technology (TAT®)assay in HDFn. In IMR90, all AuNP treatments showed significant increases in telomerase activity when compared to earlier passages. HDFn treated with AuNPs at 0 ppm, 0.05 ppm, 0.5 ppm, or 5 ppm did not show significant differences in telomerase activity compared to the control group. Significant differences in telomere length in HDFn were observed at 2 weeks of 0.05 and 0.5 ppm AuNPs under oxidative culture conditions as compared to the control group. The study showed preliminary evidence that AuNPs may increase telomerase activity and decelerate the shortening of telomeres in human fibroblasts, suggesting its potential anti-aging effects, which warrants further investigation.
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Smolikov, Sarit, and Anat Krauskopf. "The Rap1p-Telomere Complex Does Not Determine the Replicative Capacity of Telomerase-Deficient Yeast." Molecular and Cellular Biology 23, no. 23 (December 1, 2003): 8729–39. http://dx.doi.org/10.1128/mcb.23.23.8729-8739.2003.
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ABSTRACT Telomeres are nucleoprotein structures that cap the ends of chromosomes and thereby protect their stability and integrity. In the presence of telomerase, the enzyme that synthesizes telomeric repeats, telomere length is controlled primarily by Rap1p, the budding yeast telomeric DNA binding protein which, through its C-terminal domain, nucleates a protein complex that limits telomere lengthening. In the absence of telomerase, telomeres shorten with every cell division, and eventually, cells enter replicative senescence. We have set out to identify the telomeric property that determines the replicative capacity of telomerase-deficient budding yeast. We show that in cells deficient for both telomerase and homologous recombination, replicative capacity is dependent on telomere length but not on the binding of Rap1p to the telomeric repeats. Strikingly, inhibition of Rap1p binding or truncation of the C-terminal tail of Rap1p in Kluyveromyces lactis and deletion of the Rap1p-recruited complex in Saccharomyces cerevisiae lead to a dramatic increase in replicative capacity. The study of the role of telomere binding proteins and telomere length on replicative capacity in yeast may have significant implications for our understanding of cellular senescence in higher organisms.
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Calado, Rodrigo T., Solomon A. Graf, and Neal S. Young. "Telomeric Recombination in Lymphocytes Implicates ALT, an Alternative Mechanism for Telomere Length Maintenance, in Normal Human Hematopoietic Cells." Blood 110, no. 11 (November 16, 2007): 1332. http://dx.doi.org/10.1182/blood.v110.11.1332.1332.
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Abstract Telomeres are the very ends of chromosomes and protect the genome from recombination, end-to-end-fusion, and recognition as damaged DNA. Telomeres are eroded with each cell division, eventually reaching such critically short length as to cause cell cycle arrest, apoptosis, or genomic instability. In most highly proliferative cells, including hematopoietic stem cells and T lymphocytes, telomere attrition is countered by telomere extension by telomerase reverse transcriptase complex. The majority of cancer cells also express telomerase, which maintains telomere length and allows indefinite cell proliferation. However, about 10% of tumors maintain telomere length in the absence of telomerase by mechanisms collectively termed alternative lengthening of telomeres (ALT). ALT mainly acts through asymmetrical exchange of telomeric material between chromosomes or sister chromatids, producing one daughter-cell with short telomeres and a limited life-span and its sister with long telomeres and higher proliferative capacity. To date, ALT has only been reported in cancer cells or through genetic engineering of mammalian cells. Here we investigated whether ALT mechanisms were active in hematopoietic cells using chromosome orientation fluorescent in situ hybridization (CO-FISH). In standard FISH, a telomeric probe produces fours signals per chromosome, one at each end of the two chromatids. Using CO-FISH, the newly synthesized DNA strand is fragmented by BrdU incorporation and UV light exposure and then digested by exonucleases. In CO-FISH, a telomeric probe produces two signals only, one at each end of the chromosome; in the presence of telomeric recombination, the telomeric signal is split, generating more than two signals per chromosome. Peripheral blood lymphocytes from three healthy volunteers, normal human fibroblasts, K562 cells, telomerase-positive HeLa cells (known to be negative for ALT),and telomerase-negative VA13 cells (known to be positive for ALT) were investigated for telomeric sister chromatid exchange (t-SCE); at least 20 metaphases per cell type were examined. Cultured peripheral blood lymphocytes and VA13 cells both showed increased levels of telomeric sister chromatid exchange in comparison to the other cells (P=0.0001): telomeric probe generated 2.62±0.11 telomeric signals/chromosome in lymphocytes; 2.23±0.04 in VA13 cells; 2.09±0.01 in HeLa cells; 2.02±0.01 in K562 cells; and 2.02±0.01 in human skin fibroblasts. Staining incorporated-BrdU over 24 hours and evaluation of “harlequin” chromosomes point to a similar rate of genomic sister chromatid exchange in lymphocytes, VA13 cells, and HeLa cells, suggesting that high chromatid exchange is confined to the telomeric region. A physical association between promyelocytic leukemia protein (PML) and telomeres is characteristic of some ALT-positive cells, but confocal microscopy failed to co-localize the telomeric probe and anti-PML monoclonal antibody in peripheral blood lymphocytes, suggesting that t-SCE in lymphocytes is not mediated by PML. This is the first demonstration of ALT activation in normal mammalian cells. ALT may be activated in peripheral blood lymphocytes as a complementary mechanism to maintain telomere length, and may explain the differences in age-related telomere shortening observed between lymphocytes and granulocytes.
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Ogrocká, Anna, Pavla Polanská, Eva Majerová, Zlatko Janeba, Jiří Fajkus, and Miloslava Fojtová. "Compromised telomere maintenance in hypomethylated Arabidopsis thaliana plants." Nucleic Acids Research 42, no. 5 (December 10, 2013): 2919–31. http://dx.doi.org/10.1093/nar/gkt1285.
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Abstract Telomeres, nucleoprotein structures at the ends of linear eukaryotic chromosomes, are important for the maintenance of genomic stability. Telomeres were considered as typical heterochromatic regions, but in light of recent results, this view should be reconsidered. Asymmetrically located cytosines in plant telomeric DNA repeats may be substrates for a DNA methyltransferase enzyme and indeed, it was shown that these repeats are methylated. Here, we analyse the methylation of telomeric cytosines and the length of telomeres in Arabidopsis thaliana methylation mutants (met 1-3 and ddm 1-8), and in their wild-type siblings that were germinated in the presence of hypomethylation drugs. Our results show that cytosine methylation in telomeric repeats depends on the activity of MET1 and DDM1 enzymes. Significantly shortened telomeres occur in later generations of methylation mutants as well as in plants germinated in the presence of hypomethylation drugs, and this phenotype is stably transmitted to the next plant generation. A possible role of compromised in vivo telomerase action in the observed telomere shortening is hypothesized based on telomere analysis of hypomethylated telomerase knockout plants. Results are discussed in connection with previous data in this field obtained using different model systems.
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Souza, Rhonda F., Tisha Lunsford, Ruben D. Ramirez, Xi Zhang, Edward L. Lee, Yuenan Shen, Charles Owen, Jerry W. Shay, Carmela Morales, and Stuart Jon Spechler. "GERD is associated with shortened telomeres in the squamous epithelium of the distal esophagus." American Journal of Physiology-Gastrointestinal and Liver Physiology 293, no. 1 (July 2007): G19—G24. http://dx.doi.org/10.1152/ajpgi.00055.2007.
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Telomeres are repetitive DNA sequences located at the ends of chromosomes. Telomeres are shortened by repeated cell divisions and by oxidative DNA damage, and cells with critically shortened telomeres cannot divide. We hypothesized that chronic gastroesophageal reflux disease (GERD)-induced injury of the esophageal squamous epithelium results in progressive telomeric shortening that eventually might interfere with mucosal healing. To address our hypothesis, we compared telomere length and telomerase activity in biopsy specimens of esophageal squamous epithelium from GERD patients and control patients. Endoscopic biopsies were taken from the esophageal squamous epithelium of 38 patients with GERD [10 long-segment Barrett's esophagus (LSBE), 15 short-segment (SSBE), 13 GERD without Barrett's esophagus] and 16 control patients without GERD. Telomere length was assessed using the terminal restriction fragment assay, and telomerase activity was studied by the PCR-based telomeric repeat amplification protocol assay. Patients with GERD had significantly shorter telomeres in the distal esophagus than controls [8.3 ± 0.5 vs. 10.9 ± 1.5 (SE) Kbp, P = 0.043]. Among the patients with GERD, telomere length in the distal esophagus did not differ significantly in those with and without Barrett's esophagus (LSBE 7.9 ± 0.8, SSBE 8.6 ± 0.9, GERD without BE 8.7 ± 1.0 Kbp). No significant differences in telomerase activity in the distal esophagus were noted between patients with GERD and controls (4.0 ± 0.39 vs. 5.2 ± 0.53 RIUs). Telomeres in the squamous epithelium of the distal esophagus of patients who have GERD, with and without Barrett's esophagus, are significantly shorter than those of patients without GERD despite similar levels of telomerase activity.
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Blackburn, Elizabeth H., Carol W. Greider, Eric Henderson, Margaret S. Lee, Janis Shampay, and Dorothy Shippen-Lentz. "Recognition and elongation of telomeres by telomerase." Genome 31, no. 2 (January 15, 1989): 553–60. http://dx.doi.org/10.1139/g89-104.
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Telomeres stabilize chromosomal ends and allow their complete replication in vivo. In diverse eukaryotes, the essential telomeric DNA sequence consists of variable numbers of tandem repeats of simple, G + C rich sequences, with a strong strand bias of G residues on the strand oriented 5′ to 3′ toward the chromosomal terminus. This strand forms a protruding 3′ overhang at the chromosomal terminus in three different eukaryotes analyzed. Analysis of yeast and protozoan telomeres showed that telomeres are dynamic structures in vivo, being acted on by shortening and lengthening activities. We previously identified and partially purified an enzymatic activity, telomere terminal transferase, or telomerase, from the ciliate Tetrahymena. Telomerase is a ribonucleoprotein enzyme with essential RNA and protein components. This activity adds repeats of the Tetrahymena telomeric sequence, TTGGGG, onto the 3′ end of a single-stranded DNA primer consisting of a few repeats of the G-rich strand of known telomeric, and telomere-like, sequences. The shortest oligonucleotide active as a primer was the decamer G4T2G4. Structural analysis of synthetic DNA oligonucleotides that are active as primers showed that they all formed discrete intramolecular foldback structures at temperatures below 40 °C. Addition of TTGGGG repeats occurs one nucleotide at a time by de novo synthesis, which is not templated by the DNA primer. Up to 8000 nucleotides of G4T2 repeats were added to the primer in vitro. We discuss the implications of this finding for regulation of telomerase in vivo and a model for telomere elongation by telomerase.Key words: chromosome telomeres, telomerase, oligonucleotide repeats.
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Bunch, Jeremy T., Nancy S. Bae, Jessica Leonardi, and Peter Baumann. "Distinct Requirements for Pot1 in Limiting Telomere Length and Maintaining Chromosome Stability." Molecular and Cellular Biology 25, no. 13 (July 1, 2005): 5567–78. http://dx.doi.org/10.1128/mcb.25.13.5567-5578.2005.
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ABSTRACT The fission yeast Pot1 (protection of telomeres) protein binds to the single-stranded extensions at the ends of telomeres, where its presence is critical for the maintenance of linear chromosomes. Homologs of Pot1 have been identified in a wide variety of eukaryotes, including plants, animals, and humans. We now show that Pot1 plays dual roles in telomere length regulation and chromosome end protection. Using a series of Pot1 truncation mutants, we have defined distinct areas of the protein required for chromosome stability and for limiting access to telomere ends by telomerase. We provide evidence that a large portion of Pot1, including the N-terminal DNA binding domain and amino acids close to the C terminus, is essential for its protective function. C-terminal Pot1 fragments were found to exert a dominant-negative effect by displacing endogenous Pot1 from telomeres. Reducing telomere-bound Pot1 in this manner resulted in dramatic lengthening of the telomere tract. Upon further reduction of Pot1 at telomeres, the opposite phenotype was observed: loss of telomeric DNA and chromosome end fusions. Our results demonstrate that cells must carefully regulate the amount of telomere-bound Pot1 to differentiate between allowing access to telomerase and catastrophic loss of telomeres.
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Lister-Shimauchi, Evan H., Benjamin McCarthy, Michael Lippincott, and Shawn Ahmed. "Genetic and Epigenetic Inheritance at Telomeres." Epigenomes 6, no. 1 (March 16, 2022): 9. http://dx.doi.org/10.3390/epigenomes6010009.
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Transgenerational inheritance can occur at telomeres in distinct contexts. Deficiency for telomerase or telomere-binding proteins in germ cells can result in shortened or lengthened chromosome termini that are transmitted to progeny. In human families, altered telomere lengths can result in stem cell dysfunction or tumor development. Genetic inheritance of altered telomeres as well as mutations that alter telomeres can result in progressive telomere length changes over multiple generations. Telomeres of yeast can modulate the epigenetic state of subtelomeric genes in a manner that is mitotically heritable, and the effects of telomeres on subtelomeric gene expression may be relevant to senescence or other human adult-onset disorders. Recently, two novel epigenetic states were shown to occur at C. elegans telomeres, where very low or high levels of telomeric protein foci can be inherited for multiple generations through a process that is regulated by histone methylation.Together, these observations illustrate that information relevant to telomere biology can be inherited via genetic and epigenetic mechanisms, although the broad impact of epigenetic inheritance to human biology remains unclear.
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Liu, Jun, Lihui Wang, Zhiguo Wang, and Jun-Ping Liu. "Roles of Telomere Biology in Cell Senescence, Replicative and Chronological Ageing." Cells 8, no. 1 (January 15, 2019): 54. http://dx.doi.org/10.3390/cells8010054.
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Telomeres with G-rich repetitive DNA and particular proteins as special heterochromatin structures at the termini of eukaryotic chromosomes are tightly maintained to safeguard genetic integrity and functionality. Telomerase as a specialized reverse transcriptase uses its intrinsic RNA template to lengthen telomeric G-rich strand in yeast and human cells. Cells sense telomere length shortening and respond with cell cycle arrest at a certain size of telomeres referring to the “Hayflick limit.” In addition to regulating the cell replicative senescence, telomere biology plays a fundamental role in regulating the chronological post-mitotic cell ageing. In this review, we summarize the current understandings of telomere regulation of cell replicative and chronological ageing in the pioneer model system Saccharomyces cerevisiae and provide an overview on telomere regulation of animal lifespans. We focus on the mechanisms of survivals by telomere elongation, DNA damage response and environmental factors in the absence of telomerase maintenance of telomeres in the yeast and mammals.
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Martin, Aegina Adams, Isabelle Dionne, Raymund J. Wellinger та Connie Holm. "The Function of DNA Polymerase α at Telomeric G Tails Is Important for Telomere Homeostasis". Molecular and Cellular Biology 20, № 3 (1 лютого 2000): 786–96. http://dx.doi.org/10.1128/mcb.20.3.786-796.2000.
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ABSTRACT Telomere length control is influenced by several factors, including telomerase, the components of telomeric chromatin structure, and the conventional replication machinery. Although known components of the replication machinery can influence telomere length equilibrium, little is known about why mutations in certain replication proteins cause dramatic telomere lengthening. To investigate the cause of telomere elongation in cdc17/pol1 (DNA polymerase α) mutants, we examined telomeric chromatin, as measured by its ability to repress transcription on telomere-proximal genes, and telomeric DNA end structures in pol1-17 mutants. pol1-17 mutants with elongated telomeres show a dramatic loss of the repression of telomere-proximal genes, or telomeric silencing. In addition,cdc17/pol1 mutants grown under telomere-elongating conditions exhibit significant increases in single-stranded character in telomeric DNA but not at internal sequences. The single strandedness is manifested as a terminal extension of the G-rich strand (G tails) that can occur independently of telomerase, suggesting thatcdc17/pol1 mutants exhibit defects in telomeric lagging-strand synthesis. Interestingly, the loss of telomeric silencing and the increase in the sizes of the G tails at the telomeres temporally coincide and occur before any detectable telomere lengthening is observed. Moreover, the G tails observed incdc17/pol1 mutants incubated at the semipermissive temperature appear only when the cells pass through S phase and are processed by the time cells reach G1. These results suggest that lagging-strand synthesis is coordinated with telomerase-mediated telomere maintenance to ensure proper telomere length control.
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Grandin, Nathalie, and Michel Charbonneau. "Telomerase- and Rad52-Independent Immortalization of Budding Yeast by an Inherited-Long-Telomere Pathway of Telomeric Repeat Amplification." Molecular and Cellular Biology 29, no. 4 (December 1, 2008): 965–85. http://dx.doi.org/10.1128/mcb.00817-08.
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ABSTRACT In the absence of telomerase, telomeres erode, provoking accumulation of DNA damage and death by senescence. Rare survivors arise, however, due to Rad52-based amplification of telomeric sequences by homologous recombination. The present study reveals that in budding yeast cells, postsenescence survival relying on amplification of the TG1-3 telomeric repeats can take place in the absence of Rad52 when overelongated telomeres are present during senescence (hence its designation ILT, for inherited-long-telomere, pathway). By growth competition, the Rad52-independent pathway was almost as efficient as the Rad51- and Rad52-dependent pathway that predominates in telomerase-negative cells. The ILT pathway could also be triggered by increased telomerase accessibility before telomerase removal, combined with loss of telomere protection, indicating that prior accumulation of recombination proteins was not required. The ILT pathway was dependent on Rad50 and Mre11 but not on the Rad51 recombinase and Rad59, thus making it distinct from both the type II (budding yeast ALT [alternative lengthening of telomeres]) and type I pathways amplifying the TG1-3 repeats and subtelomeric sequences, respectively. The ILT pathway also required the Rad1 endonuclease and Elg1, a replication factor C (RFC)-like complex subunit, but not Rad24 or Ctf18 (two subunits of two other RFC-like complexes), the Dnl4 ligase, Yku70, or Nej1. Possible mechanisms for this Rad52-independent pathway of telomeric repeat amplification are discussed. The effects of inherited long telomeres on Rad52-dependent recombination are also reported.
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Groff-Vindman, Cindy, Anthony J. Cesare, Shobhana Natarajan, Jack D. Griffith, and Michael J. McEachern. "Recombination at Long Mutant Telomeres Produces Tiny Single- and Double-Stranded Telomeric Circles." Molecular and Cellular Biology 25, no. 11 (June 1, 2005): 4406–12. http://dx.doi.org/10.1128/mcb.25.11.4406-4412.2005.
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ABSTRACT Recombinational telomere elongation (RTE) known as alternate lengthening of telomeres is the mechanism of telomere maintenance in up to 5 to 10% of human cancers. The telomeres of yeast mutants lacking telomerase can also be maintained by recombination. Previously, we proposed the roll-and-spread model to explain this elongation in the yeast Kluveromyces lactis. This model suggests that a very small (∼100-bp) circular molecule of telomeric DNA is copied by a rolling circle event to generate a single long telomere. The sequence of this primary elongated telomere is then spread by recombination to all remaining telomeres. Here we show by two-dimensional gel analysis and electron microscopy that small circles of single- and double-stranded telomeric DNA are commonly made by recombination in a K. lactis mutant with long telomeres. These circles were found to be especially abundant between 100 and 400 bp (or nucleotides). Interestingly, the single-stranded circles consist of only the G-rich telomeric strand sequence. To our knowledge this is the first report of single-stranded telomeric circles as a product of telomere dysfunction. We propose that the small telomeric circles form through the resolution of an intratelomeric strand invasion which resembles a t-loop. Our data reported here demonstrate that K. lactis can, in at least some circumstances, make telomeric circles of the very small sizes predicted by the roll-and-spread model. The very small circles seen here are both predicted products of telomere rapid deletion, a process observed in both human and yeast cells, and predicted templates for roll-and-spread RTE.
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Niida, Hiroyuki, Yoichi Shinkai, M. Prakash Hande, Takehisa Matsumoto, Shoko Takehara, Makoto Tachibana, Mitsuo Oshimura, Peter M. Lansdorp, and Yasuhiro Furuichi. "Telomere Maintenance in Telomerase-Deficient Mouse Embryonic Stem Cells: Characterization of an Amplified Telomeric DNA." Molecular and Cellular Biology 20, no. 11 (June 1, 2000): 4115–27. http://dx.doi.org/10.1128/mcb.20.11.4115-4127.2000.
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ABSTRACT Telomere dynamics, chromosomal instability, and cellular viability were studied in serial passages of mouse embryonic stem (ES) cells in which the telomerase RNA (mTER) gene was deleted. These cells lack detectable telomerase activity, and their growth rate was reduced after more than 300 divisions and almost zero after 450 cell divisions. After this growth crisis, survivor cells with a rapid growth rate did emerge. Such survivors were found to maintain functional telomeres in a telomerase-independent fashion. Although telomerase-independent telomere maintenance has been reported for some immortalized mammalian cells, its molecular mechanism has not been elucidated. Characterization of the telomeric structures in one of the survivor mTER −/− cell lines showed amplification of the same tandem arrays of telomeric and nontelomeric sequences at most of the chromosome ends. This evidence implicatescis/trans amplification as one mechanism for the telomerase-independent maintenance of telomeres in mammalian cells.