Telomere shortening associated with chromosome instability is arrested in immortal cells which express telomerase activity

The RNA component of human telomerase

Eukaryotic chromosomes are capped with repetitive telomere sequences that protect the ends from damage and rearrangements. Telomere repeats are synthesized by telomerase, a ribonucleic acid (RNA)-protein complex. Here, the cloning of the RNA component of human telomerase, termed hTR, is described. The template region of hTR encompasses 11 nucleotides (5'-CUAACCCUAAC) complementary to the human telomere sequence (TTAGGG)n. Germline tissues and tumor cell lines expressed more hTR than normal somatic cells and tissues, which have no detectable telomerase activity. Human cell lines that expressed hTR mutated in the template region generated the predicted mutant telomerase activity. HeLa cells transfected with an antisense hTR lost telomeric DNA and began to die after 23 to 26 doublings. Thus, human telomerase is a critical enzyme for the long-term proliferation of immortal tumor cells.

Cellular immortality: a late event in the progression of human squamous cell carcinoma of the head and neck associated with p53 alteration and a high frequency of allele loss

Many human tumors contain variant cells that, unlike their normal counterparts, possess indefinite proliferative potential in vitro. However, little is known of the relevance of these immortal cells to human carcinomas in vivo. To investigate immortality in a human tumor system, we established cultures from different stages of head and neck squamous carcinoma (SCC-HN). All the neoplastic cultures were transformed because they showed very low cornification in surface or suspension culture and were partially or completely resistant to suspension-induced death. Immortal variants were not detected in premalignant erythroplakia cultures, but their frequency increased with tumor progression, indicating that immortality is a late event in carcinogenesis. Some late-stage carcinomas still produced senescent cultures, but, significantly, all recurrent tumors were immortal. Immortal but not senescent carcinoma cultures were associated with p53 dysfunction and a high frequency of allele loss, indicative of tumor suppressor gene inactivation. These results show that there are at least two classes of human SCC-HN that are phenotypically and genotypically distinct and that the pathological stage of a given tumor is not necessarily indicative of the kind of cells it contains.

An in vitro assay for Saccharomyces telomerase requires EST1

Telomerase activity was demonstrated in cell-free extracts from S. cerevisiae through the use of a PCR-based assay. As expected, this activity was eliminated by RNase or phenol treatment of the extract and was dependent on dGTP and dTTP. Telomerase was not detected in extracts prepared from cells grown for approximately 30 or more cell divisions in the absence of the EST1 product, Est1p. TLC1 RNA, which determines the sequence of telomeric DNA in vivo, was present in normal amounts in est1 delta cells. Moreover, TLC1 RNA specifically precipitated with epitope-tagged Est1p. These data indicate that Est1p is either a subunit of yeast telomerase or an accessory protein associated with telomerase that is essential in vitro for its activity.

Purification of Tetrahymena telomerase and cloning of genes encoding the two protein components of the enzyme

Telomerase is a ribonucleoprotein DNA polymerase that catalyzes the de novo synthesis of telomeric simple sequence repeats. We describe the purification of telomerase and the cloning of cDNAs encoding two protein subunits from the ciliate Tetrahymena. Two proteins of 80 and 95 kDa copurified and coimmunoprecipitated with telomerase activity and the previously identified Tetrahymena telomerase RNA. The p95 subunit specifically cross-linked to a radiolabeled telomeric DNA primer, while the p80 subunit specifically bound to radiolabeled telomerase RNA. At the primary sequence level, the two telomerase proteins share only limited homologies with other polymerases and polymerase accessory factors.

Origins of G1 arrest in senescent human fibroblasts

Human diploid fibroblasts have a finite proliferative lifespan in culture, at the end of which they are arrested with G1 phase DNA contents. Upon serum stimulation, senescent cells are deficient in carrying out a subset of early signal transduction events such as activation of protein kinase C and induction of c-fos. Later in G1, they uniformly fail to express late G1 genes whose products are required for DNA synthesis, implying that they are unable to pass the R point. Failure to pass the R point may occur because senescent cells are unable to phosphorylate the retinoblastoma protein, owing to the accumulation of inactive complexes of cyclin E/Cdk2 and possibly cyclin D/Cdk4. Senescent cells contain high amounts of p21, a potent cyclin-dependent kinase inhibitor whose levels are also elevated in cells arrested in G1 following DNA damage, suggesting that both arrests might share a common mechanism. Cell aging is accompanied by a progressive shortening of chromosomal telomeres, which could be perceived by the cells as a form of DNA damage that gives rise to the signals that inactive the cell cycle machinery.

Telomerase activity and oncogenesis in giant cell tumor of bone

BACKGROUND

Benign giant cell tumor of bone (GCT) is a primary skeletal neoplasm with an unpredictable pattern of biologic aggressiveness and cytogenetic findings characterized by telomeric associations and telomeric reduction. The role of maintaining telomeric integrity is performed by telomerase. To determine if telomerase activity is present, cell extracts from fibroblasts and tumor cells from five patients with GCT were analyzed and compared with HeLa (a positive control cell line).

METHODS

Telomerase activity was detected by visualizing the extension of radioactive telomeric repeats on DNA sequencing gels. Telomere reduction was assessed using southern blot analyses of the restriction enzyme Hinf I digested DNA with a radio-labeled telomere probe.

RESULTS

Telomerase or telomerase-like activity was detected in the cell extracts from HeLa and tumor cells. However, GCT telomerase activity varied and was less than that observed in HeLa, but no activity was detected from fibroblasts. In addition, telomere reduction was seen in DNA isolated from both HeLa and GCT but not in fibroblasts or age-matched controls.

CONCLUSION

Telomere reduction and telomerase activity may be oncogenic sustaining events required to maintain the transformed phenotype seen in GCT.

A role for genomic instability in cellular radioresistance?

Inherent cellular radioresistance plays a critical role in the failure of radiotherapy. Although the consequences of radioresistance are well known, the molecular, biological, and cellular bases of radioresistance remain a mystery. We propose that genomic instability, the increased rate of acquisition of alterations in the mammalian genome, can directly modulate cells' sensitivity to radiation. In particular, destabilization of chromosomes occurring as a consequence of genomic instability may result in enhanced 'plasticity of the genome'. This increased plasticity of the genome allows cells to better adapt to changes in local environment(s) during tumor progression, or improve cell survival following exposure to DNA damage encountered during radiotherapy protocols, thereby contributing to radioresistant cell populations found in tumors both before and after radiotherapy.

Correlating telomerase activity levels with human neuroblastoma outcomes

Telomerase activity was analysed in 100 neuroblastoma cases. Although telomerase activity was not detected in normal adrenal tissues or benign ganglioneuromas, almost all neuroblastomas (94%) did express it, suggesting an important role for telomerase in neuroblastoma development. Neuroblastomas with high telomerase activity had other genetic changes (for example, N-myc amplification) and an unfavourable prognosis, whereas tumours with low telomerase activity were devoid of such genetic alterations and were associated with a favourable prognosis. Three neuroblastomas lacking telomerase activity regressed (stage IVS). Thus telomerase expression may be required as a critical step in the multigenetic process of tumorigenesis, and two different pathways may exist for the development of neuroblastoma.

Changes in mean telomere length in basal cell carcinomas of the skin

Human telomeres consist of arrays of the sequence TTAGGG up to 15-20 kb in length, which are essential for the maintenance of normal chromosomal stability. It has been suggested that genomic instability observed in tumours may be due to loss of telomere sequences. Somatic cells that are dividing continuously appear to progressively lose telomere sequences, and it would therefore be anticipated that cell type specific differences in mean telomere length may exist within an individual. Previous reports have suggested that mean telomere length may be different in human neoplasia when compared to control. Basal cell carcinomas are epidermal derived tumours and in order therefore to make valid cell type specific comparisons we have measured mean telomere length in 20 basal cell carcinomas as well as in both adjacent epidermis and dermis. Mean telomere length was significantly reduced in epidermis in comparison with dermis, from clinically normal skin immediately adjacent to the tumours (mean difference 2.5 kb). This result is not related to the presence of the tumour as similar results were obtained from skin samples of healthy volunteers. Basal cell carcinomas showed increased mean telomere length in 13/20 samples in comparison with matched epidermis (mean difference 3.1 kb), whereas in 7/20 mean telomere length was reduced (mean difference 2.2 kb). These results showing that mean telomere length varies from cell type to cell type underpin the importance of performing cell type specific controls when assessing changes in tumour telomeres.

Cellular aging and the importance of energetic factors

The in vitro aging of human fibroblasts has become a classical model for studying cellular aging. This model was lately redefined by showing that these cells represent a stem cell system in which they progressively pass through seven morphotypes. Experimental data showed that external conditions that can be considered as stresses for the cells, can modulate the genome expression by speeding up the passage of the cells from one morphotype to the other. In this article, we will interpret these observations from the point of view of the thermodynamics of far from equilibrium open systems, which shows the importance of the production and the use of energy, both responsible for the generation of a given amount of entropy production. In stable systems like these cell morphotypes, such a production is constant but external stresses can prematurely destabilize the steady state of entropy production and, in doing so, accelerate the process of aging. It is also predicted that cells submitted to a stress will use part of their energy in response to the stress. Some experimental data in favor of such an interpretation have been obtained and more will be presented here that show that both cell death and accelerated cell aging under stress are modulated by the level of energy metabolism. All theoretical and experimental arguments presented in this article will show that cellular aging is related to stress and also to energy production through a very elaborate system of regulatory processes necessary for the cell to survive and to perform specific functions according to its differentiated state. This regulatory system also permits the cell to adapt its response according to the intensity of external as well as internal challenges and one of these responses will influence the cellular aging rate.

Telomere length and proliferation potential of hematopoietic stem cells

Hematopoietic stem cells have typically been defined as pluripotent cells with self-renewal capacity. Recent studies have shown striking differences in the mean length of telomeric repeat sequences at the end of chromosomes from human hematopoietic cells at different stages of development. The most likely explanation for these observations is that hematopoietic stem cells, like all other somatic cells studied to date, lose telomeric DNA upon each cell division. In this review, limitations in the replicative potential of hematopoietic stem cells are discussed in the context of possible clinical use of such cells for transplantation and gene therapy.

Specific association of human telomerase activity with immortal cells and cancer

Synthesis of DNA at chromosome ends by telomerase may be necessary for indefinite proliferation of human cells. A highly sensitive assay for measuring telomerase activity was developed. In cultured cells representing 18 different human tissues, 98 of 100 immortal and none of 22 mortal populations were positive for telomerase. Similarly, 90 of 101 biopsies representing 12 human tumor types and none of 50 normal somatic tissues were positive. Normal ovaries and testes were positive, but benign tumors such as fibroids were negative. Thus, telomerase appears to be stringently repressed in normal human somatic tissues but reactivated in cancer, where immortal cells are likely required to maintain tumor growth.

Telomere shortening in leukemic cells is related to their genetic alterations but not replicative capability

We compared telomere length in donor leukemic cells and corresponding established cell lines from three patients with chronic myeloid leukemia (CML) and three with acute lymphoblastic leukemia (ALL) to study the relation between the immortalization capacity of hematologic neoplasms and telomere length. Six of the seven established leukemia cell lines (four CML and two ALL) carried additional chromosome changes and had shorter telomere repeats than those of the donor patients' leukemic cells; the remaining ALL line showed no significant difference in telomere length between fresh leukemic cells and the corresponding cell line. Thus, most established leukemic cells lose effective telomerase activity during the process of establishment, and reduction in telomere length of established leukemic cells appeared to be associated with the presence of additional chromosome changes.

Genetic determination of telomere size in humans: a twin study of three age groups

Reduction of telomere length has been postulated to be a causal factor in cellular aging. Human telomeres terminate in tandemly arranged repeat arrays consisting of the (TTAGGG) motif. The length of these arrays in cells from human mitotic tissues is inversely related to the age of the donor, indicating telomere reduction with age. In addition to telomere length differences between different age cohorts, considerable variation is present among individuals of the same age. To investigate whether this variation can be ascribed to genetic influences, we have measured the size of terminal restriction fragments (TRFs) in HaeIII-digested genomic DNA from 123 human MZ and DZ twin pairs 2-95 years of age. The average rate of telomere shortening was 31 bp/year, which is similar to that observed by others. Statistical analysis in 115 pairs 2-63 years of age indicates a 78% heritability for mean TRF length in this age cohort. The individual differences in mean TRF length in blood, therefore, seem to a large extent to be genetically determined.

Telomere length variation in normal and malignant human tissues

Tissue and tumor specific length variation of telomere (TTAGGG)n repeats was studied in DNAs from various normal and malignant tissues. DNA was isolated from bone marrow and blood cells, malignant tissues, and established tumor cell lines. Nonisotopic Southern hybridization revealed a reduction of telomere repeat arrays in 14 of the 35 tumors analyzed. However, other cases (60%) showed no reduction, or even an increase, in telomeric length. Our finding of elongated telomere stretches in several tumors of different origin compared with normal tissue is in contrast to previous reports describing a general shortening of terminal repeat length in colorectal cancer and neuroblastoma. We tentatively conclude that there is no general tendency to telomere reduction in malignant tissues.

TLC1: template RNA component of Saccharomyces cerevisiae telomerase

Telomeres, the natural ends of linear eukaryotic chromosomes, are essential for chromosome stability. Because of the nature of DNA replication, telomeres require a specialized mechanism to ensure their complete duplication. Telomeres are also capable of silencing the transcription of genes that are located near them. In order to identify genes in the budding yeast Saccharomyces cerevisiae that are important for telomere function, a screen was conducted for genes that, when expressed in high amounts, would suppress telomeric silencing. This screen lead to the identification of the gene TLC1 (telomerase component 1). TLC1 encodes the template RNA of telomerase, a ribonucleoprotein required for telomere replication in a variety of organisms. The discovery of TLC1 confirms the existence of telomerase in S. cerevisiae and may facilitate both the analysis of this enzyme and an understanding of telomere structure and function.

Telomere dynamics in an immortal human cell line

The integration of transfected plasmid DNA at the telomere of chromosome 13 in an immortalized simian virus 40-transformed human cell line provided the first opportunity to study polymorphism in the number of telomeric repeat sequences on the end of a single chromosome. Three subclones of this cell line were selected for analysis: one with a long telomere on chromosome 13, one with a short telomere, and one with such extreme polymorphism that no distinct band was discernible. Further subcloning demonstrated that telomere polymorphism resulted from both gradual changes and rapid changes that sometimes involved many kilobases. The gradual changes were due to the shortening of telomeres at a rate similar to that reported for telomeres of somatic cells without telomerase, eventually resulting in the loss of nearly all of the telomere. However, telomeres were not generally lost completely, as shown by the absence of polymorphism in the subtelomeric plasmid sequences. Instead, telomeres that were less than a few hundred base pairs in length showed a rapid, highly heterogeneous increase in size. Rapid changes in telomere length also occurred on longer telomeres. The frequency of this type of change in telomere length varied among the subclones and correlated with chromosome fusion. Therefore, the rapid changes in telomere length appeared occasionally to result in the complete loss of telomeric repeat sequences. Rapid changes in telomere length have been associated with telomere loss and chromosome instability in yeast and could be responsible for the high rate of chromosome fusion observed in many human tumor cell lines.

Two human breast cancer cell lines showing decreasing telomeric repeat length during early in vitro passaging

Specific DNA repeats serve as a molecular protection shield at the telomeric ends of mammalian chromosomes. The absence of telomerase activity leads to a gradual decrease of telomeric repeat length in normal somatic cells. In contrast, immortalized cells from malignant tumors are usually thought to re-express telomerase to overcome a self-limited growth. Following this hypothesis, re-expression of telomerase is due to a rare mutational event. Herein we describe our results of telomeric length determination in two newly established breast cancer cell lines. During in vitro establishment from pleural effusions, both cell lines showed a marked decrease of the upper border range of telomeric repeat length distribution. The lower border remained within a constant range characteristic for each cell line. In no case was decrease of repeat length accompanied by an increased incidence of telomeric associations or fusions. The results show that a constant telomeric repeat length does not constitute a characteristic feature of immortalized cells. Furthermore, the kinetics of repeat length decrease and the constant range of the lower border reveal that the onset of telomerase activity is not necessarily due to a rare, i.e., mutational, event.

Genomic stability and instability in different neuroepithelial tumors. A role for chromosome structure?

Selected childhood and adult neoplasm exemplify fundamental differences in their propensity for genomic change. DNA replication is essential for the formation of neuroepithelial tumors, probably because the genome can be remodeled. Nonetheless, several differentiated and stable childhood neoplasms retain their nuclear controls for differentiation. In contrast, rapidly arising gliomas often show a variety of phenotypic changes. Genomic plasticity and instability allow gliomas to flexibly adapt to new environments. Gene changes (in DNA) can be limited in childhood tumors whereas more widespread genetic changes in malignant gliomas indicate a fundamental alteration in many chromosome regions. Can such regions be defined? We used one repeated DNA sequence (TTAGGG)n, present at the end of all normal human chromosomes, to investigate chromosome termini in more detail. Pulsed-field gel electrophoresis showed this region can be unusually variable, as several other multilocus probes did not reveal comparable changes. Because telomeres form unique chromosomal structures, and are thought to provide essential signals to position chromosomes in the interphase nucleus, it was pertinent to assess these regions by in situ hybridization. Many telomeric domains localized at variable as well as interior nuclear positions in glioma cells. These positions, which are presumably abnormal, may be generated by the DNA variants observed. Such position changes may contribute to the more general 'disorder' observed in glioma nuclei. Other chromosome domains with a unique DNA-protein structure may define additional genomic loci that are preferentially modified in neoplasia. A fundamental understanding of chromosome structure should clarify the problem of multilocus instability in glioblastoma.

Alteration in length of telomeric repeats in lung cancer

We investigated the relationship between telomere length and various characteristics of tumor cells in 46 lung cancer specimens (40 primary lesions and six metastatic lesions). Three variant patterns of telomere length were observed in 16 cases (34.8%): reduction in 13 cases, elongation in two cases, and convergence in one case. These variant patterns were frequently observed in small cell carcinomas, in metastatic lesions, and in cases which possessed the S-type allele of the L-myc gene. All three cases with telomere elongation or convergence were associated with a poor prognosis. This is compatible with the previous report suggesting that telomerase activity may be an indicator of immortality in vitro. In adenocarcinoma, telomere reduction or elongation was also observed in the early stages with a low percentage of cells in the S-phase, while in cases with other histologic types, these changes were observed only in late stage, in metastatic lesions, or in cancerous tissues with a high percentage of cells in the S-phase. Although the reduction of telomere length in these tissues may be a result of many cell divisions, it may represent another stage of carcinogenesis in early-stage adenocarcinoma.

Dynamics of telomere turnover in Plasmodium berghei

Non-uniform composition in telomeric repeats at the extremities of Plasmodium chromosomes was exploited in order to obtain data on intraclonal diversification of telomeric sequences, relevant for the study of telomere regeneration dynamics. Families of sibling telomeric clones were obtained from several chromosomal ends of Plasmodium berghei, and analysed so as to determine the exact points from which individual clones start to diverge. As much as 90% of the telomeric tract appears to be subject to events causing abrupt changes in the sequence of telomeric repeats. The results are compatible with the hypothesis that breakpoint probability is a continuously increasing function over the entire telomeric tract.

Mammalian telomere dynamics: healing, fragmentation shortening and stabilization

Telomeres are essential for stable chromosome maintenance. The simple G-rich sequence motif d(TTAGGG)n is all that is required in cis for telomere function in mammalian cells, as in other eukaryotes. Using this fact, telomeres have been used to specifically fragment mammalian chromosomes to dissect their structure and function. Telomere length maintenance is altered in cancer cells. Trans-acting factors, such as telomerase and telomere-binding proteins, may determine telomere function in both normal and cancer cells. Current experiments are aimed at understanding the role of telomerase and telomere-binding proteins in cellular senescence and immortalization.

DNA amplification: new insights into its mechanism

DNA amplification is a process whereby a limited part of the genome is increased in copy number with various consequences for the cell. It is frequently observed in cancer cells and it can be induced in mammalian cells grown in culture as well as in tumor cells when these are subjected to growth inhibiting drugs. In recent years new insights into the mechanisms involved in DNA amplification have been obtained; discussion of these will form the major subject of this short review.

Functional reconstitution of wild-type and mutant Tetrahymena telomerase

Telomerase is a ribonucleoprotein that catalyzes telomere elongation in vitro and in vivo. The 159-nucleotide RNA component of Tetrahymena telomerase contains the sequence 5'-CAACCCCAA-3' ("template region"), which serves as a template for the addition of the sequence d(TTGGGG)n to Tetrahymena telomeres. To dissect the Tetrahymena telomerase enzyme mechanism, we developed a functional in vitro reconstitution assay. After removal of the essential telomerase RNA by micrococcal nuclease digestion of partially purified telomerase, the addition of in vitro-transcribed telomerase RNA reconstituted telomerase activity. The reconstituted activity was processive and showed the same primer specificities as native telomerase. Mutants in the RNA template region were tested in reconstitution assays to determine the role of the residues in this region in primer recognition and elongation. Two template mutants, encoding the sequences 5'-UAACCCCAA-3' and 5'-UAACCCUAA-3', specified the incorporation of dATP into the sequence d(TTAGGG). Telomerase reconstituted with a template mutant encoding the sequence 5'-CAACCCUAA-3' did not specify dATP incorporation and elongation by this mutant was not terminated by the addition of ddATP. In addition, a template mutant encoding the sequence 5'-CGGCCCCAA-3' specified the incorporation of ddCTP but not ddTTP while a mutant encoding the sequence 5'-CAACCCCGG-3' specified the incorporation of ddTTP but not ddCTP. These data suggest that only the most 5' six residues of the template region dictate the addition of telomeric repeats.

Reflections on the cost consequences of the new gene technology for health policy

This article presents a preliminary and necessarily tentative and subjective assessment of the impact of new gene technology on health care costs. In the short term, diagnosis and treatment of genetic disease are likely to increase costs. Treatment with nongene therapy will continue to be far less expensive than gene therapy where it is available. Research developments to monitor as indicators of forthcoming cost reductions in genetic therapy are set forth. Some forms of genetic screening may soon reduce health care costs, and an example is provided. Genetically engineered pharmaceuticals are described and their impact on costs reviewed. Conditions under which they are likely to reduce health care costs are indicated.

Dancing around the fountain

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Presence of telomeric sequences on deleted chromosomes and their absence on double minutes in cell line HL-60

The issue of telomeric sequences on deleted chromosomes and double minutes (dmin) was investigated by examining the cell line HL-60 with fluorescence in situ hybridization using a human plasmid DNA sequence with 800 bp TTAGGG repeats. This cell line showed telomeric sequences on the deleted short arms of chromosomes 9 and 10, with the results suggesting that so-called terminal deletion may be, in fact, an interstitial deletion, or that telomeric sequences may be synthesized by telomerase after deletion. On the other hand, numerous dmin showed no evidence of hybridization with the telomeric probe. This suggests that the characteristics unequal distribution of dmin during mitosis may result from the lack of not only centromeres but also telomeres.

Is 'senescence' of diploid cells an ad hoc mechanism suppressing 'replicon' or not?

So-called 'limited life span' of diploid cells does not depend on their unresponsiveness to growth factors from the data obtained by complete protein-free culture of tumor cells. Limited proliferation (called limited life span) of diploid cells is one of the 'ad-hoc' negative regulatory mechanisms in animals, and can not be differentiated from other inhibitory mechanisms called 'tumor suppression' and 'terminal differentiation'. Metabolic imbalance induced by proliferation-dependent time-bomb mechanisms including infidelity of DNA repair is suggested to explain limited proliferation of cells. After maturation, autonomic progression of negative regulators in cells is induced by no prohibition of terminal differentiation, since organisms prepare no programs to stop development and differentiation. It is an attractive hypothesis that a proliferation-dependent time-bomb has been developed to control organogenesis for maturation and to determine body size. It is true that limited proliferation of cells can not explain longevity of individuals, although the so-called longevity genes play roles in ageing. Teleologically, longevity of individuals has been developed to produce genetic heterogeneity according to the selfish gene theory. This inter- and intra-species genetic heterogeneity increases the probability of selfish gene replication in germ cell line. After maturation and reproduction of DNA in germ cell line, individuals as vehicles for the DNA can be discarded by the 'selfish' genes.

Telomeres and telomerases

The ends of eukaryotic chromosomes are defined by specialized nucleoprotein complexes called telomeres. Telomeres impart stability to the genome and are of general interest due to their unique structure and unconventional mode of synthesis. Recent work has identified new components of the telomere complex and expanded our understanding of the role of terminal structures in maintaining cell viability.

An alternative pathway for yeast telomere maintenance rescues est1- senescence

Yeast cells lacking a functional EST1 gene show progressive shortening of the terminal G1-3T telomeric repeats and a parallel increase in the frequency of cell death. Although the majority of the cells in an est1- culture die, a minor subpopulation survives the potentially lethal consequences of the est1 mutation. We show that these est1- survivors arise as a result of the amplification and acquisition of subtelomeric elements (and their deletion derivatives) by a large number of telomeres. Hence, even when the primary pathway for telomere replication is defective, an alternative backup pathway can restore telomere function and keep the cell alive.

Cell immortalization as a key, rate-limiting event in malignant transformation: approaches toward a molecular genetic analysis

Recent advances using somatic cell genetic approaches have provided a convincing body of evidence that the senescence of mammalian cells in culture is controlled by a small group of genes, one or more of which are functionally deleted in the process of immortalization. Microcell-mediated mono-chromosomal transfer methods should permit precise mapping of these genes to specific chromosomal regions. Cloning of senescence genes, using either conventional 'positional cloning' techniques or retroviral insertion mutagenesis, is now a realistic possibility. The leap in our understanding of the molecular genetic events driving the alternative cellular states of limited proliferative capacity and immortality, which such advances should precipitate, will finally permit the question of the role of cell immortalization in cancer to be addressed, and may open the door to the design of new modes of cancer therapy. In addition, the precise mechanism underlying the wide difference in transformability between human and rodent cells, which should also emerge from these investigations, is likely to make a significant contribution towards resolving the key issue of the relevance of rodent tumour induction assays in assessing the potential carcinogenicity of environmental chemicals.

Loss of telomeric DNA during aging of normal and trisomy 21 human lymphocytes

The telomere hypothesis of cellular aging proposes that loss of telomeric DNA (TTAGGG) from human chromosomes may ultimately cause cell-cycle exit during replicative senescence. Since lymphocytes have a limited replicative capacity and since blood cells were previously shown to lose telomeric DNA during aging in vivo, we wished to determine: (a) whether accelerated telomere loss is associated with the premature immunosenescence of lymphocytes in individuals with Down syndrome (DS) and (b) whether telomeric DNA is also lost during aging of lymphocytes in vitro. To investigate the effects of aging and trisomy 21 on telomere loss in vivo, genomic DNA was isolated from peripheral blood lymphocytes of 140 individuals (age 0-107 years), including 21 DS patients (age 0-45 years). Digestion with restriction enzymes HinfI and RsaI generated terminal restriction fragments (TRFs), which were detected by Southern analysis using a telomere-specific probe (32P-(C3TA2)3). The rate of telomere loss was calculated from the decrease in mean TRF length, as a function of donor age. DS patients showed a significantly higher rate of telomere loss with donor age (133 +/- 15 bp/year) compared with age-matched controls (41 +/- 7.7 bp/year) (P < .0005), suggesting that accelerated telomere loss is a biomarker of premature immunosenescence of DS patients and that it may play a role in this process. Telomere loss during aging in vitro was calculated for lymphocytes from four normal individuals, grown in culture for 10-30 population doublings. The rate of telomere loss was approximately 120 bp/cell doubling, comparable to that seen in other somatic cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a pentanucleotide telomeric sequence, (TTAGG)n, in the silkworm Bombyx mori and in other insects

A pentanucleotide repetitive sequence, (TTAGG)n, has been isolated from a silkworm genomic library, using cross-hybridization with a (TTNGGG)5 sequence, which is conserved among most eukaryotic telomeres. Both fluorescent in situ hybridization and Bal 31 exonuclease experiments revealed major clusters of (TTAGG)n at the telomeres of all Bombyx chromosomes. To determine the evolutionary origin of this sequence, two types of telomeric sequence, (TTAGG)5 and a hexanucleotide repetitive sequence, (TTAGGG)4, which is conserved mainly among vertebrate and several invertebrate telomeres so far examined, were hybridized to DNAs from a wide variety of eukaryotic species under highly stringent hybridization conditions. The (TTAGGG)5 oligonucleotide hybridized to genomic DNAs from vertebrates and several nonvertebrate species, as has been reported so far, but not to any DNAs from insects. On the other hand, the Bombyx type of telomere sequence, (TTAGG)n, hybridized to DNAs from 8 of 11 orders of insect species tested but not to vertebrate DNAs, suggesting that this TTAGG repetitive sequence is conserved widely among insects.

Preferential incorporation of 3'-azido-2',3'-dideoxythymidine into telomeric DNA and Z-DNA-containing regions of Chinese hamster ovary cells

3'-Azido-2',3'-dideoxythymidine (azidothymidine; AZT) induces bone marrow toxicity in patients chronically given therapeutic doses of drug and is tumorigenic in rodents, inducing squamous cell tumors in vaginal tissues of mice and rats. In the study reported here, we explored the incorporation of AZT into specific regions of mammalian chromosomal DNA. CHO cells were exposed to AZT for 4 h, allowed to complete at least one cell cycle, and then arrested in metaphase with colchicine. Regions of concentrated AZT incorporation were identified in individual metaphase chromosomes by immunohistochemistry using antiserum specific for AZT and a secondary antiserum with a streptavidin--Texas red end point. These studies demonstrated that most of the intensely staining regions were chromosomal ends or telomeres. When 18 metaphases were examined, all telomeres but one (39 of 40) were positive at least once. Using an anti-Z-DNA antibody, chromosomal regions containing DNA in Z conformation were also localized by immunohistochemistry using a rhodamine-conjugated secondary antibody. When metaphase chromosome spreads were stained for either AZT or Z-DNA, ideograms showing localization of AZT (18 metaphases) and DNA in Z configuration (26 metaphases) were drawn for every chromosome of each metaphase examined. These ideograms demonstrated that 60% of the regions that stained positive for AZT were also positive for Z-DNA. Furthermore, slides incubated with both antibodies, using streptavidin--Texas red to identify AZT and fluorescein to identify Z-DNA, confirmed colocalization of the two markers. Additional experiments exploring the induction of chromatin bridges in AZT-treated cells suggest that the analogue may be able to bind to and disrupt the normal functioning of telomeric DNA.

Telomeres and their possible role in chromosome stabilization

The evidence to date generally supports the hypothesis that telomere capping makes chromosome fragments refractory to subsequent rejoining events, but this control may be somewhat relaxed after chromosome breakage. Cell survival requires that the fragments rejoin before metaphase. Unprotected ends such as those produced by DNA damage are subject to degradation, presumably by endogenous cellular exo- and endonucleases. Telomere repeat sequences may be added to broken chromosome ends to protect the ends from further degradation. That telomeric DNA does not always prevent rejoining raises interesting questions as to what constitutes capping, and how rapidly it occurs after DNA damage in relation to chromosome break rejoining. The prevention of degradation and control of rejoining may be mediated by telomere-specific binding proteins, especially the telomere terminal binding protein [Gualberto et al., 1992; Longtine et al., 1989; Price, 1990; Price and Cech, 1989]. Some of these proteins may be involved in scavenging telomeric DNA when the cell senses that chromosomal breaks have occurred. This mechanism is consistent with the observations of Murnane and Yu [1993], who found that a plasmid with telomere sequences was stably integrated in vivo into a chromosome terminal breakpoint lacking telomere repeats. It is also consistent with the high frequency of interstitial telomere sequences observed in normal cells; a history of DNA damage and repair may be recorded by these sequences (Ijdo et al., 1991]. Although chromosome break rejoining is an efficient process in eukaryotic cells, some breaks are never rejoined and can result in terminal deletions and chromatid and isochromatid deletions at metaphase. It is unclear why these breaks are not rejoined, but it may be due to one or more of the following: 1) chance: broken chromosomes are separated, do not approach sufficiently close to one another, and are consequently physically unable to rejoin; 2) a large number of added telomere repeat sequences indicating to the cell that the chromosome has an authentic telomere; 3) some other DNA modification event that protects DNA ends from degradation, e.g., folding back of DNA ends to form a hairpin, as has been implicated in VDJ recombination [Lieber, 1993].

The dichotomy between germ line and somatic line, and the origin of cell mortality

The germ cells of extant animals are potentially immortal, whereas somatic cells are mortal, that is, they are able to carry out only a finite number of divisions. In this article we propose an evolutionary interpretation of these differences. We assume that germ cells of the earliest metazoans inherited immortality from their unicellular ancestor, while somatic cells acquired mortality by gaining new functions. It follows that cell mortality was under genetic control from the beginning of metazoan life.

Toward a molecular understanding of human breast cancer: a hypothesis

A rate limiting step in most metastatic breast cancers is the development of unlimited proliferative potential by mammary epithelial cells. We describe mechanisms by which these cells can attain this state. The two independent mortality mechanisms controlling fibroblast senescence and immortalization (M1 and M2) are also found in human mammary epithelial cells. However, although both p53 and Rb are involved in the M1 mechanism of fibroblast cellular senescence, in human mammary epithelial cells only p53 is involved. The M1/M2 mechanisms may be induced by the gradual loss of telomere ends that occur as normal cells divide. Loss of telomere ends may result in genomic instability and in altered gene expression due to heterochromatin changes in subtelomeric regions. Events which can abrogate p53 functions are described, as is the current state of knowledge about the function of p53. All these factors are included in a molecular model for the onset of breast cancer.