651
|
Lustig AJ. DNA dynamics: different means to a common end? Curr Biol 1998; 8:R161-4. [PMID: 9501060 DOI: 10.1016/s0960-9822(98)70100-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/06/2023]
Abstract
A ribosomal frameshift is required for the synthesis of an essential component of the yeast telomerase pathway; this and other findings on telomerases from many species raise interesting questions regarding the evolutionary relationship between telomerases and retrotransposons lacking long terminal repeats.
Collapse
Affiliation(s)
- A J Lustig
- Department of Biochemistry, Tulane University Medical Center, New Orleans, Louisiana 70012, USA
| |
Collapse
|
652
|
Abstract
Telomeres of most investigated species terminate with short repeats and are elongated by telomerase. Short repeats have never been detected in dipteran species which have found other solutions to end a chromosome. Whereas in Drosophila melanogaster retroelements are added onto the termini, chironomids have long complex repeats at their chromosome ends. We review evidence that these units are terminal and probably have evolved from short telomeric repeats. In Chironomus pallidivittatus the units have been shown to belong to different subfamilies which have specific inter- and intrachromosomal distribution, the most terminal subfamily of repeats being characterized by pronounced secondary structures for the single strand. The complex repeats are efficiently homogenized both within and between different chromosome ends. Gene conversion is probably an important component in the coordinate evolution of the repeats but it is not known whether it is used for net synthesis of DNA. RNA is used as an intermediate in telomere elongation both by organisms having chromosomes terminating with short repeats and by D. melanogaster. It is therefore interesting that the terminal repeats in chironomids are transcribed.
Collapse
Affiliation(s)
- I Kamnert
- Department of Genetics, University of Lund, Sweden
| | | | | | | |
Collapse
|
653
|
Abstract
The recent characterisation of subtelomeric regions from a variety of organisms from yeast to man has led to the realisation that all chromosome ends are similar in structure although maintenance of the terminus varies. The mosaic of repeats and proteins associated with telomeres has an architectural role which divides the genome into two domains, allowing for the adaptive use of the region as well as the evolution of non-telomerase-mediated telomere maintenance.
Collapse
Affiliation(s)
- F E Pryde
- Institut of Molecular Medicine, John Radcliffe Hospital, Oxford UK
| | | | | |
Collapse
|
654
|
Chute I, Le Y, Ashley T, Dobson MJ. The telomere-associated DNA from human chromosome 20p contains a pseudotelomere structure and shares sequences with the subtelomeric regions of 4q and 18p. Genomics 1997; 46:51-60. [PMID: 9403058 DOI: 10.1006/geno.1997.5007] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The human chromosome 20p telomere has been cloned on a yeast artificial chromosome (YAC). The telomere-associated DNA contains an interstitial tract of (TTAGGG)n telomeric repeats 60 kb in from the chromosome end. Frequent truncation of the YAC clone was observed due to resolution of the internal telomeric array into a telomere. The 20p internal telomeric repeat tract is flanked on its centromeric side by telomere-associated repeated sequences that have previously been found adjacent to terminal telomeric repeat arrays. The pseudotelomere structure of the 20p subtelomeric region is similar to the structure of some yeast subtelomeric regions where these sequences act as substrates for recombinational repair of chromosome ends that have lost their terminal telomeric repeat arrays. Sequences flanking the telomeric end of the internal (TTAGGG)n repeat array on 20p are found adjacent to three other subtelomeric (TTAGGG)n tracts on 4q, 18p, and an unknown chromosome end, respectively. These shared sequences provide evidence of exchange between nonhomologous chromosomes in humans.
Collapse
MESH Headings
- Chromosomes, Artificial, Yeast/genetics
- Chromosomes, Human, Pair 18/genetics
- Chromosomes, Human, Pair 20/genetics
- Chromosomes, Human, Pair 4/genetics
- Cloning, Molecular
- DNA/analysis
- DNA/genetics
- Gene Dosage
- Humans
- Mitosis
- Molecular Sequence Data
- Repetitive Sequences, Nucleic Acid/genetics
- Restriction Mapping
- Sequence Homology, Nucleic Acid
- Telomere/genetics
Collapse
Affiliation(s)
- I Chute
- Department of Biochemistry, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
| | | | | | | |
Collapse
|
655
|
|
656
|
Marusíc L, Anton M, Tidy A, Wang P, Villeponteau B, Bacchetti S. Reprogramming of telomerase by expression of mutant telomerase RNA template in human cells leads to altered telomeres that correlate with reduced cell viability. Mol Cell Biol 1997; 17:6394-401. [PMID: 9343401 PMCID: PMC232491 DOI: 10.1128/mcb.17.11.6394] [Citation(s) in RCA: 63] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
Telomerase synthesizes telomeric DNA by copying the template sequence of its own RNA component. In Tetrahymena thermophila and yeast (G. Yu, J. D. Bradley, L. D. Attardi, and E. H. Blackburn, Nature 344:126-131, 1990; M. McEachern and E. H. Blackburn, Nature 376:403-409, 1995), mutations in the template domain of this RNA result in synthesis of mutant telomeres and in impaired cell growth and survival. We have investigated whether mutant telomerase affects the proliferative potential and viability of immortal human cells. Plasmids encoding mutant or wild-type template RNAs (hTRs) of human telomerase and the neomycin resistance gene were transfected into human cells to generate stable transformants. Expression of mutant hTR resulted in the appearance of mutant telomerase activity and in the synthesis of mutant telomeres. Transformed cells were not visibly affected in their growth and viability when grown as mass populations. However, a reduction in plating efficiency and growth rate and an increase in the number of senescent cells were detected in populations with mutant telomeres by colony-forming assays. These results suggest that the presence of mutant telomerase, even if coexpressed with the wild-type enzyme, can be deleterious to cells, likely as a result of the impaired function of hybrid telomeres.
Collapse
Affiliation(s)
- L Marusíc
- Department of Pathology, McMaster University, Hamilton, Ontario, Canada
| | | | | | | | | | | |
Collapse
|
657
|
Bryan TM, Englezou A, Dalla-Pozza L, Dunham MA, Reddel RR. Evidence for an alternative mechanism for maintaining telomere length in human tumors and tumor-derived cell lines. Nat Med 1997; 3:1271-4. [PMID: 9359704 DOI: 10.1038/nm1197-1271] [Citation(s) in RCA: 901] [Impact Index Per Article: 33.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The gradual loss of DNA from the ends of telomeres has been implicated in the control of cellular proliferative potential. Telomerase is an enzyme that restores telomeric DNA sequences, and expression of its activity was thought to be essential for the immortalization of human cells, both in vitro and in tumor progression in vivo. Telomerase activity has been detected in 50-100% of tumors of different types, but not in most normal adult somatic tissues. It has also been detected in about 70% of human cell lines immortalized in vitro and in all tumor-derived cell lines examined to date. It has previously been shown that in vitro immortalized telomerase-negative cell lines acquire very long and heterogeneous telomeres in association with immortalization presumably via one or more novel telomere-lengthening mechanisms that we refer to as ALT (alternative lengthening of telomeres). Here we report evidence for the presence of ALT in a subset of tumor-derived cell lines and tumors. The maintenance of telomeres by a mechanism other than telomerase, even in a minority of cancers, has major implications for therapeutic uses of telomerase inhibitors.
Collapse
Affiliation(s)
- T M Bryan
- Children's Medical Research Institute, Sydney, NSW, Australia
| | | | | | | | | |
Collapse
|
658
|
Morii K, Tanaka R, Onda K, Tsumanuma I, Yoshimura J. Expression of telomerase RNA, telomerase activity, and telomere length in human gliomas. Biochem Biophys Res Commun 1997; 239:830-4. [PMID: 9367854 DOI: 10.1006/bbrc.1997.7562] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
To understand the mechanisms of telomere maintenance in human gliomas, telomerase activity, telomerase RNA expression and telomere length of surgically excised glioma samples were analyzed. Sixty-five percent of gliomas exhibited telomerase activity, the occurrence of which was not related to their histological malignancy scale. Not only the telomerase-positive gliomas, but also the telomerase-negative gliomas and normal brain expressed telomerase RNA, suggesting that the presence of telomerase RNA component does not indicate the presence of telomerase activity. Compared with telomerase-positive gliomas, telomerase-negative gliomas had long heterogeneous telomeric terminal restriction fragments. These data suggest that in addition to the telomerase-dependent mechanism, a telomerase-independent mechanism for telomere maintenance may be present in human gliomas.
Collapse
Affiliation(s)
- K Morii
- Department of Neurosurgery, Niigata University, Japan
| | | | | | | | | |
Collapse
|
659
|
Affiliation(s)
- V A Zakian
- Department of Molecular Biology, Princeton University, New Jersey 08544-1014, USA
| |
Collapse
|
660
|
Blasco MA, Lee HW, Hande MP, Samper E, Lansdorp PM, DePinho RA, Greider CW. Telomere shortening and tumor formation by mouse cells lacking telomerase RNA. Cell 1997; 91:25-34. [PMID: 9335332 DOI: 10.1016/s0092-8674(01)80006-4] [Citation(s) in RCA: 1556] [Impact Index Per Article: 57.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
To examine the role of telomerase in normal and neoplastic growth, the telomerase RNA component (mTR) was deleted from the mouse germline. mTR-/- mice lacked detectable telomerase activity yet were viable for the six generations analyzed. Telomerase-deficient cells could be immortalized in culture, transformed by viral oncogenes, and generated tumors in nude mice following transformation. Telomeres were shown to shorten at a rate of 4.8+/-2.4 kb per mTR-/- generation. Cells from the fourth mTR-/- generation onward possessed chromosome ends lacking detectable telomere repeats, aneuploidy, and chromosomal abnormalities, including end-to-end fusions. These results indicate that telomerase is essential for telomere length maintenance but is not required for establishment of cell lines, oncogenic transformation, or tumor formation in mice.
Collapse
Affiliation(s)
- M A Blasco
- Cold Spring Harbor Laboratory, New York 11724, USA
| | | | | | | | | | | | | |
Collapse
|
661
|
Roth CW, Kobeski F, Walter MF, Biessmann H. Chromosome end elongation by recombination in the mosquito Anopheles gambiae. Mol Cell Biol 1997; 17:5176-83. [PMID: 9271395 PMCID: PMC232368 DOI: 10.1128/mcb.17.9.5176] [Citation(s) in RCA: 70] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023] Open
Abstract
One of the functions of telomeres is to counteract the terminal nucleotide loss associated with DNA replication. While the vast majority of eukaryotic organisms maintain their chromosome ends via telomerase, an enzyme system that generates short, tandem repeats on the ends of chromosomes, other mechanisms such as the transposition of retrotransposons or recombination can also be used in some species. Chromosome end regression and extension were studied in a medically important mosquito, the malaria vector Anopheles gambiae, to determine how this dipteran insect maintains its chromosome ends. The insertion of a transgenic pUChsneo plasmid at the left end of chromosome 2 provided a unique marker for measuring the dynamics of the 2L telomere over a period of about 3 years. The terminal length was relatively uniform in the 1993 population with the chromosomes ending within the white gene sequence of the inserted transgene. Cloned terminal chromosome fragments did not end in short repeat sequences that could have been synthesized by telomerase. By late 1995, the chromosome ends had become heterogeneous: some had further shortened while other chromosomes had been elongated by regenerating part of the integrated pUChsneo plasmid. A model is presented for extension of the 2L chromosome by recombination between homologous 2L chromosome ends by using the partial plasmid duplication generated during its original integration. It is postulated that this mechanism is also important in wild-type telomere elongation.
Collapse
Affiliation(s)
- C W Roth
- Developmental Biology Center, University of California, Irvine 92697, USA
| | | | | | | |
Collapse
|
662
|
Affiliation(s)
- H Riethman
- The Wistar Institute, Molecular Genetics, Philadelphia, Pennsylvania 19104, USA
| |
Collapse
|
663
|
Nakamura TM, Morin GB, Chapman KB, Weinrich SL, Andrews WH, Lingner J, Harley CB, Cech TR. Telomerase catalytic subunit homologs from fission yeast and human. Science 1997; 277:955-9. [PMID: 9252327 DOI: 10.1126/science.277.5328.955] [Citation(s) in RCA: 1649] [Impact Index Per Article: 61.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
Catalytic protein subunits of telomerase from the ciliate Euplotes aediculatus and the yeast Saccharomyces cerevisiae contain reverse transcriptase motifs. Here the homologous genes from the fission yeast Schizosaccharomyces pombe and human are identified. Disruption of the S. pombe gene resulted in telomere shortening and senescence, and expression of mRNA from the human gene correlated with telomerase activity in cell lines. Sequence comparisons placed the telomerase proteins in the reverse transcriptase family but revealed hallmarks that distinguish them from retroviral and retrotransposon relatives. Thus, the proposed telomerase catalytic subunits are phylogenetically conserved and represent a deep branch in the evolution of reverse transcriptases.
Collapse
Affiliation(s)
- T M Nakamura
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
| | | | | | | | | | | | | | | |
Collapse
|
664
|
Higashiyama T, Noutoshi Y, Fujie M, Yamada T. Zepp, a LINE-like retrotransposon accumulated in the Chlorella telomeric region. EMBO J 1997; 16:3715-23. [PMID: 9218812 PMCID: PMC1169995 DOI: 10.1093/emboj/16.12.3715] [Citation(s) in RCA: 65] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
Six copies of insertion elements accumulate in the subtelomeric region immediately proximal to the telomeric repeats on Chlorella chromosome I. The elements, designated Zepps, bear the characteristic features of non-viral (LINE-like) retrotransposons, including a poly(A) tail, 5'-truncations, a retroviral reverse transcriptase-like ORF and flanking target duplications. Detailed sequence analysis of the Chlorella subtelomeric region revealed a novel mechanism of Zepp transposition; successive insertions of each Zepp element into another Zepp as a target, leaving a tandem array of their 3'-regions with poly(A) tracts facing toward the centromere. Only the most distal Zepp copy was inverted to connect its poly(A) tail with the telomeric repeats. A similar Zepp cluster but without the telomeric repeats was also found at the terminus of another Chlorella chromosome. These structures contrast with that proposed for the addition of HeT-A and TART elements to Drosophila telomeres. Expression of Zepp elements is induced by heat shock treatment. Possible roles of the subtelomeric retrotransposons in formation and maintenance of telomeres are discussed.
Collapse
Affiliation(s)
- T Higashiyama
- Faculty of Engineering, Hiroshima University, Higashi-Hiroshima, Japan
| | | | | | | |
Collapse
|
665
|
Eid JE, Sollner-Webb B. ST-2, a telomere and subtelomere duplex and G-strand binding protein activity in Trypanosoma brucei. J Biol Chem 1997; 272:14927-36. [PMID: 9169464 DOI: 10.1074/jbc.272.23.14927] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
From Trypanosoma brucei, we identified ST-2, a protein complex that interacts with telomeric DNA and exhibits novel features. It binds specifically to the double-stranded telomere repeats (TTAGGG) and more tightly to the subtelomere 29-base pair elements that separate the telomere repeats from their proximal telomere-associated sequences. Interestingly, ST-2 showed still greater affinity for the G-rich strand of the telomere present either as an overhang or in a single-stranded form, but it exhibited the highest affinity for the G-rich strand of the subtelomere repeats. The binding characteristics of ST-2 are complementary to those of ST-1, a 39-kDa polypeptide we previously identified in T. brucei (Eid, J., and Sollner-Webb, B. (1995) Mol. Cell. Biol. 15, 389-397) that binds preferentially to the C-rich strands of the subtelomere and telomere repeats. UV cross-linking revealed five polypeptides of ST-2 that bind directly to the G-rich strand of the DNA, one of which is phosphorylated. Furthermore, the presence of ST-1 is critical for ST-2 complex binding both to the G-rich strand and to the duplex DNA, evidently as part of the ST-2 complex. This indicates that when binding to the duplex subtelomere and telomere repeats, ST-2 may act as a protein bridge with its ST-1 subunit binding to the C-rich strand and its five other cross-linkable polypeptides binding to the G-rich strand. Such an association could serve to hold the genomic subtelomeric and telomeric sequences in a partially single-stranded configuration to facilitate the recombinational events in this region that are crucial to the parasite.
Collapse
Affiliation(s)
- J E Eid
- Department of Biological Chemistry, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA
| | | |
Collapse
|
666
|
Pati D, Keller C, Groudine M, Plon SE. Reconstitution of a MEC1-independent checkpoint in yeast by expression of a novel human fork head cDNA. Mol Cell Biol 1997; 17:3037-46. [PMID: 9154802 PMCID: PMC232156 DOI: 10.1128/mcb.17.6.3037] [Citation(s) in RCA: 84] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
A novel human cDNA, CHES1 (checkpoint suppressor 1), has been isolated by suppression of the mec1-1 checkpoint mutation in Saccharomyces cerevisiae. CHES1 suppresses a number of DNA damage-activated checkpoint mutations in S. cerevisiae, including mec1, rad9, rad24, dun1, and rad53. CHES1 suppression of sensitivity to DNA damage is specific for checkpoint-defective strains, in contrast to DNA repair-defective strains. Presence of CHES1 but not a control vector resulted in G2 delay after UV irradiation in checkpoint-defective strains, with kinetics, nuclear morphology, and cycloheximide resistance similar to those of a wild-type strain. CHES1 can also suppress the lethality, UV sensitivity, and G2 checkpoint defect of a mec1 null mutation. In contrast to this activity, CHES1 had no measurable effect on the replication checkpoint as assayed by hydroxyurea sensitivity of a mec1 strain. Sequence analysis demonstrates that CHES1 is a novel member of the fork head/Winged Helix family of transcription factors. Suppression of the checkpoint-defective phenotype requires a 200-amino-acid domain in the carboxy terminus of the protein which is distinct from the DNA binding site. Analysis of CHES1 activity is most consistent with activation of an alternative MEC1-independent checkpoint pathway in budding yeast.
Collapse
Affiliation(s)
- D Pati
- Texas Children's Cancer Center, Department of Pediatrics, Baylor College of Medicine, Houston 77030, USA
| | | | | | | |
Collapse
|
667
|
Bryan TM, Marusic L, Bacchetti S, Namba M, Reddel RR. The telomere lengthening mechanism in telomerase-negative immortal human cells does not involve the telomerase RNA subunit. Hum Mol Genet 1997; 6:921-6. [PMID: 9175740 DOI: 10.1093/hmg/6.6.921] [Citation(s) in RCA: 145] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
According to the telomere hypothesis of senescence, the progressive shortening of telomeres that occurs upon division of normal somatic cells eventually leads to cellular senescence. The immortalisation of human cells is associated with the acquisition of a telomere maintenance mechanism which is usually dependent upon expression of the enzyme telomerase. About one third of in vitro immortalised human cell lines, however, have no detectable telomerase but contain telomeres that are abnormally long. The nature of the alternative telomere maintenance mechanism (referred to as ALT, for Alternative Lengthening of Telomeres) that must exist in these telomerase-negative cells has not been elucidated. It has previously been shown that abnormal lengthening of yeast telomeres may occur due to mutations in the yeast telomerase RNA gene. That this is not the mechanism of the abnormally long telomeres in ALT cell lines was demonstrated by the finding that seven of seven ALT lines have wild-type human telomerase RNA (hTR) sequence, including a novel polymorphism that is present in 30% of normal individuals. We found that two ALT cell lines have no detectable expression of the hTR gene. This shows that the ALT mechanism in these cell lines is not dependent on hTR. Expression of exogenous hTR via infection of these cells with a recombinant hTR-adenovirus vector did not result in telomerase activity, indicating that their lack of telomerase activity is not due to absence of hTR expression. We conclude that the ALT mechanism is not dependent on the expression of hTR, and does not involve mutations in the hTR sequence.
Collapse
Affiliation(s)
- T M Bryan
- Children's Medical Research Institute, Sydney, NSW, Australia
| | | | | | | | | |
Collapse
|
668
|
Abstract
Abstract
We review the present knowledge of telomeres and telomerase with special attention to their role in cell proliferation, cellular senescence, and human aging. We summarize the functional aspects of telomerase in cancer, as well as its role as a useful diagnostic and prognostic tumor marker, and discuss possible approaches to telomerase inhibition as a target for cancer therapy.
Collapse
|
669
|
Lingner J, Hughes TR, Shevchenko A, Mann M, Lundblad V, Cech TR. Reverse transcriptase motifs in the catalytic subunit of telomerase. Science 1997; 276:561-7. [PMID: 9110970 DOI: 10.1126/science.276.5312.561] [Citation(s) in RCA: 881] [Impact Index Per Article: 32.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023]
Abstract
Telomerase is a ribonucleoprotein enzyme essential for the replication of chromosome termini in most eukaryotes. Telomerase RNA components have been identified from many organisms, but no protein component has been demonstrated to catalyze telomeric DNA extension. Telomerase was purified from Euplotes aediculatus, a ciliated protozoan, and one of its proteins was partially sequenced by nanoelectrospray tandem mass spectrometry. Cloning and sequence analysis of the corresponding gene revealed that this 123-kilodalton protein (p123) contains reverse transcriptase motifs. A yeast (Saccharomyces cerevisiae) homolog was found and subsequently identified as EST2 (ever shorter telomeres), deletion of which had independently been shown to produce telomere defects. Introduction of single amino acid substitutions within the reverse transcriptase motifs of Est2 protein led to telomere shortening and senescence in yeast, indicating that these motifs are important for catalysis of telomere elongation in vivo. In vitro telomeric DNA extension occurred with extracts from wild-type yeast but not from est2 mutants or mutants deficient in telomerase RNA. Thus, the reverse transcriptase protein fold, previously known to be involved in retroviral replication and retrotransposition, is essential for normal chromosome telomere replication in diverse eukaryotes.
Collapse
Affiliation(s)
- J Lingner
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA
| | | | | | | | | | | |
Collapse
|
670
|
Takahashi H, Okazaki S, Fujiwara H. A new family of site-specific retrotransposons, SART1, is inserted into telomeric repeats of the silkworm, Bombyx mori. Nucleic Acids Res 1997; 25:1578-84. [PMID: 9092665 PMCID: PMC146635 DOI: 10.1093/nar/25.8.1578] [Citation(s) in RCA: 77] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Abstract
The telomeres of the silkworm, Bombyx mori, consist of pentanucleotide repeats (TTAGG)n . We previously characterized the non-LTR element TRAS1, which terminates with oligo (A) in a head to tail orientation at the exact position (between A and C) of the (CCTAA) n repeats. Here we characterized another family of telomere-specific non-LTR retrotransposon named SART1. The SART1 family was inserted at another site of the (TTAGG) n in a reverse orientation from that of TRAS1. The complete unit of SART1, 6.7 kb in length with a poly (A) stretch, contains two open reading frames encoding putative gag and pol products, overlapping by 54 bp in the -1 reading frame. Most of the 600 SART1 copies in the silkworm haploid genome are completely conserved in structure without 5'truncation. All SART1 sequences analyzed were inserted at the same position (between T and A) within the (TTAGG) n repeats. Fluorescence in situ hybridization showed that many of the SART1 copies were localized in the chromosomal ends. A phylogenetic tree showed that the SART1, TRAS1 and two other site-specific elements, R1 and RT, which insert into 28S ribosomal RNA genes in insects, belong to the same group. Based on the orientation for the chromosomal insertion and structural similarities, these elements could be further classified into two subgroups, R1/TRAS1 and RT/SART1, suggesting that the target specificity of the two telomere-associated elements was changed independently.
Collapse
Affiliation(s)
- H Takahashi
- Department of Biological Sciences, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan
| | | | | |
Collapse
|
671
|
Abstract
The sequence organisation of the telomeric regions is extremely similar for all eukaryotes examined to date. Subtelomeric areas may contain large sequence arrays of middle repetitive, complex elements that sometimes have similarities to retrotransposons. In between and within these complex sequences are short, satellite-like repeats. These areas contain very few genes and are thought to be organised into a heterochromatin-like domain. The terminal regions almost invariably consist of short, direct repeats. These repeats usually contain clusters of 2-4 G residues and the strand that contains these clusters (the G strand) always forms the extreme 3'-end of the chromosome. Thus, most telomeric repeats are clearly related to each other which in turn suggests a common evolutionary origin. A number of different structures can be formed by single-stranded telomeric G strand repeats and, as has been suggested recently, by the G strand. Since the main mechanism for the maintenance of telomeric repeats predicts the occurrence of single-stranded extensions of the G strand, the propensity of G-rich DNA to fold into alternative DNA structures may have implications for telomere biology.
Collapse
Affiliation(s)
- R J Wellinger
- Faculté de Médecine, Department de Microbiologie et Infectiologie, Université de Sherbrooke, QC, Canada
| | | |
Collapse
|
672
|
Abstract
This article reviews the current understanding of the involvement of telomerase in in vitro immortalisation of human cells. In vitro immortalisation with DNA tumour viruses or chemicals usually occurs in two phases. The first stage is an extension of lifespan beyond that at which cells would normally senescence, after which the culture enters a period of crisis. The second stage involves the escape from crisis of a rare cell in the culture, which goes on to proliferate indefinitely. The hypothesis that telomere shortening acts as a signal for senescence and crisis, and that cells need to activate telomerase to survive these states, gained support from early studies examining telomere behaviour and telomerase activity in immortalised cell lines. In many cases, telomeres were found to continue to shorten during the phase of extended lifespan, and no telomerase was detectable. Cells which survived crisis had activated telomerase and had stable or lengthened telomerase. However, it is now clear that this model does not apply to all cell lines. Approximately a quarter of in vitro immortalised cell lines so far examined have no detectable telomerase activity, yet have very long and heterogeneous telomeres. These cell lines have acquired a novel mechanism for lengthening their telomeres, named ALT (Alternative Lengthening of Telomeres). The nature of ALT is not yet understood, but may involve non-reciprocal recombination between telomeres. ALT is not merely a phenomenon of in vitro immortalised cell lines, but has also been found in tumours and tumour-derived cell lines. Furthermore, there are a number of cell lines which have been shown to have low levels of telomerase prior to crisis while telomere shortening is still occurring, and the function of these low levels of telomerase activity is unknown.
Collapse
Affiliation(s)
- T M Bryan
- Children's Medical Research Institute, Wentworthville, N.S.W., Australia
| | | |
Collapse
|
673
|
Abstract
Aging is a near universal process, yet the molecular mechanisms that underlie cellular senescence have remained elusive. Recent progress in determining the roles of various genetic influences in controlling the rate of cellular aging has made this an exciting time in aging research. Genetic screens designed to isolate long-lived mutants in Saccharomyces cerevisiae and Caenorhabditis elegans have implicated factors involved in transcriptional silencing and the dauer pathway in the control of aging. The gene responsible for Werner's syndrome, a disease with symptoms of premature aging, was isolated and found to be a member of the RecQ subfamily of DNA helicases. The regulation of telomere length and its role in senescence and cellular immortalization has been found to be more complex than expected. In C. elegans, mutations have been isolated in maternal-effect genes that presumably control its biological clocks and can dramatically extend its lifespan. Indeed, aging research within the past year has implicated a variety of mechanisms ranging from the control of gene expression, stress resistance, and DNA metabolism to the overall 'rate of living'.
Collapse
Affiliation(s)
- T Smeal
- Department of Biology, Massachusetts Institute of Technology, 31 Ames St, Building 68-280, Cambridge, Massachusetts 02139, USA.
| | | |
Collapse
|
674
|
Cooper JP, Nimmo ER, Allshire RC, Cech TR. Regulation of telomere length and function by a Myb-domain protein in fission yeast. Nature 1997; 385:744-7. [PMID: 9034194 DOI: 10.1038/385744a0] [Citation(s) in RCA: 392] [Impact Index Per Article: 14.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Telomeres, the specialized nucleoprotein structures that comprise the ends of eukaryotic chromosomes, are essential for complete replication, and regulation of their length has been a focus of research on tumorigenesis. In the budding yeast Saccharomyces cerevisiae, the protein Rap1p binds to telomeric DNA and functions in the regulation of telomere length. A human telomere protein, hTRF (human TTAGGG repeat factor) binds the telomere sequence in vitro and localizes to telomeres cytologically, but its functions are not yet known. Here we use a genetic screen to identify a telomere protein in fission yeast, Taz1p (telomere-associated in Schizosaccharomyces pombe), that shares homology to the Myb proto-oncogene DNA-binding domain with hTRF. Disruption or deletion of the taz1+ gene causes a massive increase in telomere length. Taz1p is required for the repression of telomere-adjacent gene expression and for normal meiosis or sporulation. It may be a negative regulator of the telomere-replicating enzyme, telomerase, or may protect against activation of telomerase-independent pathways of telomere elongation.
Collapse
Affiliation(s)
- J P Cooper
- Howard Hughes Medical Institute, Department of Chemistry and Biochemistry, University of Colorado, Boulder 80309, USA
| | | | | | | |
Collapse
|
675
|
Prescott J, Blackburn EH. Telomerase RNA mutations in Saccharomyces cerevisiae alter telomerase action and reveal nonprocessivity in vivo and in vitro. Genes Dev 1997; 11:528-40. [PMID: 9042865 DOI: 10.1101/gad.11.4.528] [Citation(s) in RCA: 127] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
The ribonucleoprotein enzyme telomerase adds telomeric DNA to chromosomal ends. In most eukaryotes the telomeric repeat units are repeated precisely, consistent with the action of a telomerase that faithfully copies its RNA template. In contrast, Saccharomyces cerevisiae telomeric repeats are degenerate, suggesting that its telomerase has unusual mechanistic properties. We mutated the S. cerevisiae telomerase RNA (TLC1) with a series of 3-base (GUG) substitutions in and next to the 17-nucleotide templating domain. All mutant telomerases were active in TLC1/tlc1 diploids and synthesized patterns of mixed wild-type and mutant telomeric repeats into telomeric DNA, consistent with nonprocessive action. Telomerase isolated from cells containing each mutated tlc1 allele by itself had altered reaction properties in vitro. One mutant template enzyme, 476GUG, was active in vivo and in vitro in the presence of wild-type TLC1 RNA but lacked detectable activity in its absence. Haploid tlc1-476GUG cells containing only this mutant tlc1 allele underwent senescence. Other tlc1 template region mutations allowed maintenance of shortened telomeres in vivo but altered specific enzymatic properties of telomerase in vitro, including induction of primer-template slippage (472GUG) or alteration of the 5' boundary of the template (467GUG). These data demonstrate that telomerase RNA bases influence enzyme activity profoundly, suggesting that their roles are not confined to serving simply as the template for this specialized reverse transcriptase.
Collapse
Affiliation(s)
- J Prescott
- Department of Microbiology and Immunology, University of California at San Francisco, 94143-0414, USA
| | | |
Collapse
|
676
|
Gotta M, Gasser SM. Nuclear organization and transcriptional silencing in yeast. EXPERIENTIA 1996; 52:1136-47. [PMID: 8988257 DOI: 10.1007/bf01952113] [Citation(s) in RCA: 46] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Transcriptional repression at the yeast silent mating type loci requires the formation of a nucleoprotein complex at specific cis-acting elements called silencers, which in turn promotes the binding of a histone-associated Sir-protein complex to adjacent chromatin. A similar mechanism of long-range transcriptional repression appears to function near telomeric repeat sequences, where it has been demonstrated that Sir3p is a limiting factor for the propagation of silencing. A combined immunofluorescence/in situ hybridization method for budding yeast was developed that maintains the three-dimensional structure of the nucleus. In wild-type cells the immunostaining of Sir3p, Sir4p and Rap1 colocalizes with Y' subtelomeric sequences detected by in situ hybridization. All three antigens and the subtelomeric in situ hybridization signals are clustered in foci, which are often adjacent to, but not coincident with, nuclear pores. This colocalization of Rap1, Sir3p and Sir4p with telomeres is lost in sir mutants, and also when Sir4p is overexpressed. To test whether the natural positioning of the two HM loci, located roughly 10 and 25 kb from the ends of chromosome III, is important for silencer function, a reporter gene flanked by wild-type silencer elements was integrated at various internal sites on other yeast chromosomes. We find that integration at internal loci situated far from telomeres abrogates the ability of silencers to repress the reporter gene. Silencing can be restored by creation of a telomere at 13 kb from the reporter construct, or by insertion of 340 bp of yeast telomeric repeat sequence at this site without chromosomal truncation. Elevation of the internal nuclear pools of Sir1p, Sir3p and Sir4p can relieve the lack of repression at the LYS2 locus in an additive manner, suggesting that in wild-type cells silencer function is facilitated by its juxtaposition to a pool of highly concentrated Sir proteins, such as those created by telomere clustering.
Collapse
Affiliation(s)
- M Gotta
- Swiss Institute for Experimental Cancer Research (ISREC), Epalinges/Lausanne, Switzerland
| | | |
Collapse
|
677
|
Abstract
A combination of classical genetic, biochemical, and molecular biological approaches have generated a rather detailed understanding of the structure and function of Saccharomyces telomeres. Yeast telomeres are essential to allow the cell to distinguish intact from broken chromosomes, to protect the end of the chromosome from degradation, and to facilitate the replication of the very end of the chromosome. In addition, yeast telomeres are a specialized site for gene expression in that the transcription of genes placed near them is reversibly repressed. A surprisingly large number of genes have been identified that influence either telomere structure or telomere function (or both), although in many cases the mechanism of action of these genes is poorly understood. This article reviews the recent literature on telomere biology and highlights areas for future research.
Collapse
Affiliation(s)
- V A Zakian
- Department of Molecular Biology, Princeton University, New Jersey 08544-1014, USA
| |
Collapse
|
678
|
Zou S, Kim JM, Voytas DF. The Saccharomyces retrotransposon Ty5 influences the organization of chromosome ends. Nucleic Acids Res 1996; 24:4825-31. [PMID: 8972872 PMCID: PMC146320 DOI: 10.1093/nar/24.23.4825] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023] Open
Abstract
Retrotransposons are ubiquitous components of eukaryotic genomes suggesting that they have played a significant role in genome organization. In Saccharomyces cerevisiae, eight of 10 endogenous insertions of the Ty5 retrotransposon family are located within 15 kb of chromosome ends, and two are located near the subtelomeric HMR locus. This genomic organization is the consequence of targeted transposition, as 14 of 15 newly transposed Ty5 elements map to telomeric regions on 10 different chromosomes. Nine of these insertions are within 0.8 kb and three are within 1.5 kb of the autonomously replicating consensus sequence in the subtelomeric X repeat. This suggests that the X repeat plays an important role in directing Ty5 integration. Analysis of endogenous insertions from S.cerevisiae and its close relative S.paradoxus revealed that only one of 12 insertions has target site duplications, indicating that recombination occurs between elements. This is further supported by the observation that Ty5 insertions mark boundaries of sequence duplications and rearrangements in these species. These data suggest that transposable elements like Ty5 can shape the organization of chromosome ends through both transposition and recombination.
Collapse
Affiliation(s)
- S Zou
- Department of Zoology and Genetics, Iowa State University, Ames 50011, USA
| | | | | |
Collapse
|
679
|
Affiliation(s)
- V Lundblad
- Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas 77030, USA
| | | |
Collapse
|
680
|
Hamilton SE, Corey DR. Telomerase: anti-cancer target or just a fascinating enzyme? CHEMISTRY & BIOLOGY 1996; 3:863-7. [PMID: 8939712 DOI: 10.1016/s1074-5521(96)90173-8] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Telomerase activity is upregulated in most types of malignant tumor. Highly selective small molecule inhibitors will be needed to understand the biological basis for this observation and to determine if telomerase is a viable target for chemotherapy.
Collapse
Affiliation(s)
- S E Hamilton
- Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235, USA
| | | |
Collapse
|
681
|
Nugent CI, Hughes TR, Lue NF, Lundblad V. Cdc13p: a single-strand telomeric DNA-binding protein with a dual role in yeast telomere maintenance. Science 1996; 274:249-52. [PMID: 8824190 DOI: 10.1126/science.274.5285.249] [Citation(s) in RCA: 460] [Impact Index Per Article: 16.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The CDC13 gene has previously been implicated in the maintenance of telomere integrity in Saccharomyces cerevisiae. With the use of two classes of mutations, here it is shown that CDC13 has two discrete roles at the telomere. The cdc13-2est mutation perturbs a function required in vivo for telomerase regulation but not in vitro for enzyme activity, whereas cdc13-1ts defines a separate essential role at the telomere. In vitro, purified Cdc13p binds to single-strand yeast telomeric DNA. Therefore, Cdc13p is a telomere-binding protein required to protect the telomere and mediate access of telomerase to the chromosomal terminus.
Collapse
Affiliation(s)
- C I Nugent
- Department of Molecular and Human Genetics and Cell and Molecular Biology Program, Baylor College of Medicine, Houston, TX 77030, USA
| | | | | | | |
Collapse
|
682
|
Abstract
Telomeres cap and protect the ends of chromosomes from degradation and illegitimate recombination. The termini of a linear template cannot, however, be completely replicated by conventional DNA-dependent DNA polymerases, and thus in the absence of a mechanisms to counter this effect, telomeres of eukaryotic cells shorten every round of DNA replication. In humans and possibly other higher eukaryotes, telomere shortening may have been adopted to limit the life span of somatic cells. Human somatic cells have a finite proliferative capacity and enter a viable growth arrested state called senescence. Life span appears to be governed by cell division, not time. The regular loss of telomeric DNA could therefore serve as a mitotic clock in the senescence programme, counting cell divisions. In most eukaryotic organisms, however, telomere shortening can be countered by the de novo addition of telomeric repeats by the enzyme telomerase. Cells which are "immortal' such as the human germ line or tumour cell lines, established mouse cells, yeast and ciliates, all maintain a stable telomere length through the action of telomerase. Abolition of telomerase activity in such cells nevertheless results in telomere shortening, a process that eventually destabilizes the ends of chromosomes, leading to genomic instability and cell growth arrest or death. Therefore, loss of terminal DNA sequences may limit cell life span by two mechanisms: by acting as a mitotic clock and by denuding chromosomes of protective telomeric DNA necessary for cell viability.
Collapse
Affiliation(s)
- C M Counter
- Whitehead Institute for Biomedical Research (Weinberg Lab), Nine Cambridge Center, MA 02142-1479, USA.
| |
Collapse
|
683
|
Gagos S, Iliopoulos D, Tseleni-Balafouta S, Agapitos M, Antachopoulos C, Kostakis A, Karayannakos P, Skalkeas G. Cell senescence and a mechanism of clonal evolution leading to continuous cell proliferation, loss of heterozygosity, and tumor heterogeneity: studies on two immortal colon cancer cell lines. CANCER GENETICS AND CYTOGENETICS 1996; 90:157-65. [PMID: 8830727 DOI: 10.1016/s0165-4608(96)00049-0] [Citation(s) in RCA: 19] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Extensive karyotypic analysis was performed on early and late passages of two continuous human cell lines, SW480 and SW620, that were derived from the same colon cancer patient. We cultivated these two cell lines in vitro for a period of 24 months and periodically examined their chromosome constitution. SW480 cells, from passage 138, were injected subcutaneously into 20 nude mice. The tumors that grew in nude mice were then cultivated in vitro for several passages to compare histopathologic findings and tumor growth patterns with clonal chromosomal profiles. Despite some karyotypic diversity, the two cell lines exhibited common marker chromosomes and followed similar patterns of evolution. During subsequent passages, acquisition of new chromosomal abnormalities gave rise to sidelines with a near-diploid genome that frequently underwent endoreduplication. Genomic instability seemed to play an important role in the emergence, growth, and subsequent elimination of the heterogenous sidelines by selection, clonal expansion, and cell death by senescence. Despite continuous growth, both the cell lines occasionally showed telomeric associations and random dicentric and multicentric formations. These lesions were considered evidence of cell senescence and were related to the disappearance of particular sidelines through evolution. Successful evolutionary steps were characterized by elimination of pre-existing marker chromosomes that were subsequently replaced in the karyotype by their cytologically intact homologous chromosomes possibly after selective endoreduplication. Frequent loss of heterozygosity for the chromosomes taking part in this process is postulated. We suggest that one of the mechanisms by which cancer cells bypass senescence may be related to their potential for continuous clonal diversification.
Collapse
Affiliation(s)
- S Gagos
- Laboratory for Experimental Surgery, Athens Medical School, University of Athens, Greece
| | | | | | | | | | | | | | | |
Collapse
|
684
|
Small MB, Hubbard K, Pardinas JR, Marcus AM, Dhanaraj SN, Sethi KA. Maintenance of telomeres in SV40-transformed pre-immortal and immortal human fibroblasts. J Cell Physiol 1996; 168:727-36. [PMID: 8816928 DOI: 10.1002/(sici)1097-4652(199609)168:3<727::aid-jcp26>3.0.co;2-u] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
Shortening of telomeres has been hypothesized to contribute to cellular senescence and may play a role in carcinogenesis of human cells. Furthermore, activation of telomerase has frequently been demonstrated in tumor-derived and in vitro immortalized cells. In this study, we have assessed these phenomena during the life span of simian virus 40 (SV40)-transformed preimmortal and immortal human fibroblasts. We observed progressive reduction in telomere length in preimmortal transformed cells with extended proliferative capacity, with the most dramatic shortening at late passage. Telomere lengths became stabilized (or increased) in immortal fibroblasts accompanied, in one case, by the activation of telomerase. However, an independent immortal cell line that displayed stable telomeres did not have detectable telomerase activity. Furthermore, we found significant telomerase activity in two preimmortal derivatives. Our results provide further evidence for maintenance of telomeres in immortalized human fibroblasts, but they suggest a lack of causal relationship between telomerase activation and immortalization.
Collapse
Affiliation(s)
- M B Small
- Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, Newark 07103-2714, USA.
| | | | | | | | | | | |
Collapse
|
685
|
McEachern MJ, Blackburn EH. Cap-prevented recombination between terminal telomeric repeat arrays (telomere CPR) maintains telomeres in Kluyveromyces lactis lacking telomerase. Genes Dev 1996; 10:1822-34. [PMID: 8698241 DOI: 10.1101/gad.10.14.1822] [Citation(s) in RCA: 187] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023]
Abstract
Deletion of the telomerase RNA gene (TER1) in the yeast Kluyveromyces lactis results in gradual loss of telomeric repeats and progressively declining cell growth capability (growth senescence). We show that this initial growth senescence is characterized by abnormally large, defectively dividing cells and is delayed when cells initially contain elongated telomeres. However, cells that survive the initial catastrophic senescence emerge relatively frequently, and their subsequent growth without telomerase is surprisingly efficient. Survivors have lengthened telomeres, often much longer than wild type, but that are still subject to gradual shortening. Production of these postsenescence survivors is strongly dependent on the RAD52 gene. We propose that shortened, terminal telomeric repeat tracts become uncapped, promoting recombinational repair between them to regenerate lengthened telomeres in survivors. This process, which we term telomere cap-prevented recombination (CPR) may be a general alternative telomere maintenance pathway in eukaryotes.
Collapse
Affiliation(s)
- M J McEachern
- Department of Microbiology and Immunology, University of California, San Francisco 94143-0414, USA
| | | |
Collapse
|
686
|
Abstract
While there has been substantial experimental evidence in support of the telomere-telomerase hypothesis of aging and cancer, it has been suggested that the theory has been oversimplified to the exclusion of alternative mechanisms in the progression of cancer. This review strives to present an overview of some of the areas that have not been well explained and to indicate where multiple interpretations of the data are possible.
Collapse
Affiliation(s)
- S E Holt
- Department of Cell Biology and Neuroscience, University of Texas Southwestern Medical Center, Dallas 75235-9039, USA
| | | | | |
Collapse
|
687
|
Abstract
The study of eukaryotic telomeres at the molecular level began with the discovery of short, tandem repeats at Tetrahymena chromosome ends. In the following two decades, major insights about telomere structure and function have come from investigations of telomerase, the DNA polymerase that synthesizes these repeats. In the past year, three areas of telomerase research have been particularly intense: assays of telomerase activity, isolation of telomerase components, and studies of the regulation of telomerase and telomere length in vivo.
Collapse
Affiliation(s)
- K Collins
- Department of Molecular and Cell Biology, University of California, Berkeley 94720-3204, USA.
| |
Collapse
|
688
|
Abstract
The strand of telomeric DNA that runs 5'-3' toward a chromosome end is typically G rich. Telomerase-generated G tails are expected at one end of individual DNA molecules. Saccharomyces telomeres acquire TG1-3 tails late in S phase. Moreover, the telomeres of linear plasmids can interact when the TG1-3 tails are present. Molecules that mimic the structures predicted for telomere replication intermediates were generated in vitro. These in vitro generated molecules formed telomere-telomere interactions similar to those on molecules isolated from yeast, but only if both ends that interacted had a TG1-3 tail. Moreover, TG1-3 tails were generated in vivo in cells lacking telomerase. These data suggest a new step in telomere maintenance, cell cycle-regulated degradation of the C1-3A strand, which can generate a potential substrate for telomerase and telomere-binding proteins at every telomere.
Collapse
Affiliation(s)
- R J Wellinger
- Department of Microbiology, Faculty of Medicine, Université de Sherbrooke, Quebec, Canada
| | | | | | | |
Collapse
|
689
|
Steiner BR, Hidaka K, Futcher B. Association of the Est1 protein with telomerase activity in yeast. Proc Natl Acad Sci U S A 1996; 93:2817-21. [PMID: 8610124 PMCID: PMC39716 DOI: 10.1073/pnas.93.7.2817] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
The est1 mutant was previously identified because it is defective in telomere maintenance and displays a senescent phenotype. To see if Est1 might be a component of yeast telomerase, we examined immunoprecipitated Est1. The yeast telomerase RNA Tlc1 specifically coprecipitated with Est1. Furthermore, the Est1 immunoprecipitates contained a telomerase-like activity. As expected for yeast telomerase, the activity elongated telomeric primers, it required dGTP and dTTP but not dATP or dCTP, and it was sensitive to RNase A. Further evidence suggesting that the activity was telomerase was obtained from experiments using a TLC1-1 mutant strain, which has a mutant telomerase template containing dG residues. The activity immunoprecipitated from TLC1-1 mutant strains incorporated 32P-labeled dCTP, while activity from TLC1 strains did not. Use of different telomeric primer substrates revealed two distinguishable telomerase-like activities: one was dependent on TLC1, and one was not. The TLC1-independent activity may be due to a second yeast telomerase RNA, or it may be some other kind of activity.
Collapse
Affiliation(s)
- B R Steiner
- Cold Spring Harbor Labortory, New York 11724, USA
| | | | | |
Collapse
|
690
|
Zou S, Ke N, Kim JM, Voytas DF. The Saccharomyces retrotransposon Ty5 integrates preferentially into regions of silent chromatin at the telomeres and mating loci. Genes Dev 1996; 10:634-45. [PMID: 8598292 DOI: 10.1101/gad.10.5.634] [Citation(s) in RCA: 155] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
The nonrandom integration of retrotransposons and retroviruses suggests that chromatin influences target choice. Targeted integration, in turn, likely affects genome organization. In Saccharomyces, native Ty5 retrotransposons are located near telomeres and the silent mating locus HMR. To determine whether this distribution is a consequence of targeted integration, we isolated a transposition-competent Ty5 element from S. paradoxus, a species closely related to S. cerevisiae. This Ty5 element was used to develop a transposition assay in S. cerevisiae to investigate target preference of de novo transposition events. Of 87 independent Ty5 insertions, approximately 30% were located on chromosome III, indicating this small chromosome (approximately 1/40 of the yeast genome) is a highly preferred target. Mapping of the exact location of 19 chromosome III insertions showed that 18 were within or adjacent to transcriptional silencers flanking HML and HMR or the type X subtelomeric repeat. We predict Ty5 target preference is attributable to interactions between transposition intermediates and constituents of silent chromatin assembled at these sites. Ty5 target preference extends the link between telomere structure and reverse transcription as carried out by telomerase and Drosophila retrotransposons.
Collapse
MESH Headings
- Amino Acid Sequence
- Base Sequence
- Blotting, Northern
- Chromatin/genetics
- Chromosome Mapping
- Chromosomes, Fungal
- Gene Expression Regulation, Fungal
- Mating Factor
- Models, Genetic
- Molecular Sequence Data
- Peptides/genetics
- RNA, Fungal/analysis
- RNA, Messenger/analysis
- Repetitive Sequences, Nucleic Acid
- Retroelements/genetics
- Saccharomyces/genetics
- Saccharomyces cerevisiae/genetics
- Sequence Analysis, DNA
- Sequence Homology, Amino Acid
- Species Specificity
- Telomere/genetics
- Transcription, Genetic
Collapse
Affiliation(s)
- S Zou
- Department of Zoology and Genetics, Iowa State University, Ames, 50011, USA
| | | | | | | |
Collapse
|
691
|
Abstract
The enzyme, telomerase, may be switched on in tumor cells. Inhibitors of this enzyme might constitute a new class of anti-cancer drugs.
Collapse
|
692
|
|
693
|
Strahl C, Blackburn EH. Effects of reverse transcriptase inhibitors on telomere length and telomerase activity in two immortalized human cell lines. Mol Cell Biol 1996; 16:53-65. [PMID: 8524329 PMCID: PMC230978 DOI: 10.1128/mcb.16.1.53] [Citation(s) in RCA: 258] [Impact Index Per Article: 9.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023] Open
Abstract
The ribonucleoprotein telomerase, a specialized cellular reverse transcriptase, synthesizes one strand of the telomeric DNA of eukaryotes. We analyzed telomere maintenance in two immortalized human cell lines: the B-cell line JY616 and the T-cell line Jurkat E6-1, and determined whether known inhibitors of retroviral reverse transcriptases could perturb telomere lengths and growth rates of these cells in culture. Dideoxyguanosine (ddG) caused reproducible, progressive telomere shortening over several weeks of passaging, after which the telomeres stabilized and remained short. However, the prolonged passaging in ddG caused no observable effects on cell population doubling rates or morphology. Azidothymidine (AZT) caused progressive telomere shortening in some but not all T- and B-cell cultures. Telomerase activity was present in both cell lines and was inhibited in vitro by ddGTP and AZT triphosphate. Prolonged passaging in arabinofuranyl-guanosine, dideoxyinosine (ddI), dideoxyadenosine (ddA), didehydrothymidine (d4T), or phosphonoformic acid (foscarnet) did not cause reproducible telomere shortening or decreased cell growth rates or viabilities. Combining AZT, foscarnet, and/or arabinofuranyl-guanosine with ddG did not significantly augment the effects of ddG alone. Strikingly, with or without inhibitors, telomere lengths were often highly unstable in both cell lines and varied between parallel cell cultures. We propose that telomere lengths in these T- and B-cell lines are determined by both telomerase and telomerase-independent mechanisms.
Collapse
Affiliation(s)
- C Strahl
- Department of Microbiology and Immunology, University of California, San Francisco 94143-0414, USA
| | | |
Collapse
|
694
|
Abstract
Telomeres are the protein-DNA structures at the ends of eukaryotic chromosomes. In yeast, and probably most other eukaryotes, telomeres are essential. They allow the cell to distinguish intact from broken chromosomes, protect chromosomes from degradation, and are substrates for novel replication mechanisms. Telomeres are usually replicated by telomerase, a telomere-specific reverse transcriptase, although telomerase-independent mechanisms of telomere maintenance exist. Telomere replication is both cell cycle- and developmentally regulated, and its control is likely to be complex. Because telomere loss causes the kinds of chromosomal changes associated with cancer and aging, an understanding of telomere biology has medical relevance.
Collapse
Affiliation(s)
- V A Zakian
- Department of Molecular Biology, Princeton University, NJ 08544, USA
| |
Collapse
|
695
|
Abstract
Yeast chromosome ends are similar in structure and function to chromosome ends in most, if not all, eukaryotic organisms. There is a G-rich terminal repeat at the ends which is maintained by telomerase. In addition to the classical functions of protecting the end from degradation and end-to-end fusions, and completing replication, yeast telomeres have several interesting properties including: non-nucleosomal chromatin structure; transcriptional position effect variegation for genes with adjacent telomeres; nuclear peripheral localization; apparent physical clustering; non-random recombinational interactions. A number of genes have been identified that are involved in modifying one or more of these properties. These include genes involved in general DNA metabolism, chromatin structure and telomere maintenance. Adjacent to the terminal repeat is a mosaic of middle repetitive elements that exhibit a great deal of polymorphism both between individual strains and among different chromosome ends. Much of the sequence redundancy in the yeast genome is found in the sub-telomeric regions (within the last 25 kb of each end). The sub-telomeric regions are generally low in gene density, low in transcription, low in recombination, and they are late replicating. The only element which appears to be shared by all chromosome ends is part of the previously defined X element containing an ARS consensus. Most of the 'core' X elements also contain an Abf1p binding site and a URS1-like element, which may have consequences for the chromatin structure, nuclear architecture and transcription of native telomeres. Possible functions of sub-telomeric repeats include: fillers for increasing chromosome size to some minimum threshold level necessary for chromosome stability; barrier against transcriptional silencing; a suitable region for adaptive amplification of genes; secondary mechanism of telomere maintenance via recombination when telomerase activity is absent.
Collapse
Affiliation(s)
- E J Louis
- Yeast Genetics, Institute of Molecular Medicine, John Radcliffe Hospital, Oxford, UK.
| |
Collapse
|
696
|
Walter MF, Jang C, Kasravi B, Donath J, Mechler BM, Mason JM, Biessmann H. DNA organization and polymorphism of a wild-type Drosophila telomere region. Chromosoma 1995; 104:229-41. [PMID: 8565699 DOI: 10.1007/bf00352254] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Telomeres at the ends of linear chromosomes of eukaryotes protect the chromosome termini from degradation and fusion. While telomeric replication/elongation mechanisms have been studied extensively, the functions of subterminal sequences are less well understood. In general, subterminal regions can be quite polymorphic, varying in size from organism to organism, and differing among chromosomes within an organism. The subterminal regions of Drosophila melanogaster are not well characterized today, and it is not known which and how many different components they contain. Here we present the molecular characterization of DNA components and their organization in the subterminal region of the left arm of chromosome 2 of the Oregon RC wild-type strain of D. melanogaster, including a minisatellite with a 457bp repeat length. Two distinct polymorphic arrangements at 2L were found and analyzed, supporting the Drosophila telomere elongation model by retrotransposition. The high incidence of terminal chromosome deficiencies occurring in natural Drosophila populations is discussed in view of the telomere structure at 2L.
Collapse
Affiliation(s)
- M F Walter
- Developmental Biology Center, University of California, Irvine, CA 92717, USA
| | | | | | | | | | | | | |
Collapse
|
697
|
Macina RA, Morii K, Hu XL, Negorev DG, Spais C, Ruthig LA, Riethman HC. Molecular cloning and RARE cleavage mapping of human 2p, 6q, 8q, 12q, and 18q telomeres. Genome Res 1995; 5:225-32. [PMID: 8593610 DOI: 10.1101/gr.5.3.225] [Citation(s) in RCA: 21] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
Large terminal fragments of human chromosomes 2p, 6p, 8q, 12q, and 18q were cloned using yeast artificial chromosomes (YACs). RecA-assisted restriction endonuclease (RARE) cleavage analysis of genomic DNA samples from II unrelated individuals using YAC-derived probes confirmed the telomeric localizations of the half-YACs studied. The cloned fragments provide telomeric closure of maps for the respective chromosome arms and will supply the reagents needed for analyzing and sequencing these distal subtelomeric regions.
Collapse
MESH Headings
- Base Sequence
- Chromosomes, Artificial, Yeast
- Chromosomes, Human/genetics
- Chromosomes, Human/ultrastructure
- Chromosomes, Human, Pair 12/genetics
- Chromosomes, Human, Pair 12/ultrastructure
- Chromosomes, Human, Pair 18/genetics
- Chromosomes, Human, Pair 18/ultrastructure
- Chromosomes, Human, Pair 2/genetics
- Chromosomes, Human, Pair 2/ultrastructure
- Chromosomes, Human, Pair 6/genetics
- Chromosomes, Human, Pair 6/ultrastructure
- Chromosomes, Human, Pair 8/genetics
- Chromosomes, Human, Pair 8/ultrastructure
- Cloning, Molecular
- Humans
- In Situ Hybridization, Fluorescence
- Molecular Sequence Data
- Polymerase Chain Reaction
- Restriction Mapping
- Telomere/genetics
Collapse
Affiliation(s)
- R A Macina
- Wistar Institute, Philadelphia, Pennsylvania 19104, USA
| | | | | | | | | | | | | |
Collapse
|
698
|
Rogan EM, Bryan TM, Hukku B, Maclean K, Chang AC, Moy EL, Englezou A, Warneford SG, Dalla-Pozza L, Reddel RR. Alterations in p53 and p16INK4 expression and telomere length during spontaneous immortalization of Li-Fraumeni syndrome fibroblasts. Mol Cell Biol 1995; 15:4745-53. [PMID: 7651392 PMCID: PMC230718 DOI: 10.1128/mcb.15.9.4745] [Citation(s) in RCA: 193] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
Normal cells have a strictly limited growth potential and senesce after a defined number of population doublings (PDs). In contrast, tumor cells often exhibit an apparently unlimited proliferative potential and are termed immortalized. Although spontaneous immortalization of normal human cells in vitro is an extremely rare event, we observed this in fibroblasts from an affected member of a Li-Fraumeni syndrome kindred. The fibroblasts were heterozygous for a p53 mutation and underwent senescence as expected at PD 40. In four separate senescent cultures (A to D), there were cells that eventually recommenced proliferation. This was associated with aneuploidy in all four cultures and either loss (cultures A, C, and D) or mutation (culture B) of the wild-type (wt) p53 allele. Loss of wt p53 function was insufficient for immortalization, since cultures A, B, and D subsequently entered crisis from which they did not escape. Culture C has continued proliferating beyond 400 PDs and thus appears to be immortalized. In contrast to the other cultures, the immortalized cells have no detectable p16INK4 protein. A culture that had a limited extension of proliferative potential exhibited a progressive decrease in telomere length with increasing PD. In the culture that subsequently became immortalized, the same trend occurred until PD 73, after which there was a significant increase in the amount of telomeric DNA, despite the absence of telomerase activity. Immortalization of these cells thus appears to be associated with loss of wt p53 and p16INK4 expression and a novel mechanism for the elongation of telomeres.
Collapse
Affiliation(s)
- E M Rogan
- Cancer Research Group, Children's Medical Research Institute, Westmead, Sydney, New South Wales, Australia
| | | | | | | | | | | | | | | | | | | |
Collapse
|
699
|
McEachern MJ, Blackburn EH. Runaway telomere elongation caused by telomerase RNA gene mutations. Nature 1995; 376:403-9. [PMID: 7630414 DOI: 10.1038/376403a0] [Citation(s) in RCA: 257] [Impact Index Per Article: 8.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The ribonucleoprotein enzyme telomerase adds telomeric DNA onto chromosome ends and is normally regulated so that telomeric DNA lengths are kept within defined bounds. In the telomerase RNA gene from the yeast Kluyveromyces lactis, specific mutations that alter telomeric DNA sequences result in telomeres elongating to up to 100 times their normal length and impair cell growth. Some mutations cause immediate elongation whereas others behave like genetic time bombs, causing elongation only after a latent period of hundreds of generations.
Collapse
Affiliation(s)
- M J McEachern
- Department of Microbiology and Immunology, University of California at San Francisco 94143, USA
| | | |
Collapse
|
700
|
Okazaki S, Ishikawa H, Fujiwara H. Structural analysis of TRAS1, a novel family of telomeric repeat-associated retrotransposons in the silkworm, Bombyx mori. Mol Cell Biol 1995; 15:4545-52. [PMID: 7623845 PMCID: PMC230694 DOI: 10.1128/mcb.15.8.4545] [Citation(s) in RCA: 84] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
We characterized TRAS1, a retrotransposable element which was inserted into the telomeric repetitive sequence (CCTAA)n of the silkworm, Bombyx mori. The complete sequence of TRAS1, a stretch of 7.8 kb with a poly(A) tract at the 3' end, was determined. No long terminal repeat (LTR) was found at the termini of the element. TRAS1 contains gag- and pol-like open reading frames (ORFs) which are similar to those of non-LTR retrotransposons. The two ORFs overlap but are one nucleotide out of frame (+1 frameshift). Most of the approximately 250 copies of TRAS1 elements in the genome were highly conserved in the structure. Chromosomal in situ hybridization showed that TRAS1 elements are clustered at the telomeres of Bombyx chromosomes. A phylogenetic analysis using the amino acid sequence of the reverse transcriptase domain within the pol-like ORF revealed that TRAS1 falls into one lineage with R1, which is a family of non-LTR retrotransposons inserted into the same site within the 28S ribosomal DNA unit in most insects. TRAS1 may have been derived from R1 and changed the target specificity so that TRAS1 inserts into the telomeric repetitive sequence (CCTAA)n. Southern hybridization and Bal 31 exonuclease analyses showed that TRAS1 elements are clustered proximal to the terminal long tract of (CCTAA)n. TRAS1 is a novel family of non-LTR retrotransposons which are inserted into the telomeric repetitive sequences as target sites.
Collapse
Affiliation(s)
- S Okazaki
- Zoological Institute, Graduate School of Science, University of Tokyo, Japan
| | | | | |
Collapse
|