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At least 19 recordsLinked to original sources
PubMed1996

Differential regulation of telomerase activity and telomerase RNA during multi-stage tumorigenesis.

Telomeres consist of short repeated sequences that are synthesized by telomerase, a ribonucleo-protein DNA polymerase. Telomerase activity is present in many tumours and not detected in many normal tissues. Telomere shortening in human and mouse tissues and primary cell cultures may be due to the absence of telomerase activity. To determine when telomerase is activated during tumour development and progression, we examined telomerase activity and expression of the recently cloned mouse telomerase RNA component (mTR) in two different transgenic mouse models of multi-stage tumorigenesis. These mouse models allow examination of many independent tumours from genetically identical individuals. These mice reproducibly develop pancreatic islet cell carcinoma and squamous cell carcinoma of the skin. In both carcinoma types, we detected telomerase activity only in late-stage tumours; in contrast, we found mTR levels were upregulated in the early preneoplastic stages, and further increased during progression. Surprisingly, mTR levels did not parallel the amount of telomerase activity detected and a subset of tumours lacked telomerase activity and yet expressed telomerase RNA. Regulation of telomerase activity may therefore be separable from expression of its RNA component. These results clearly demonstrate telomerase is activated in late stages of tumour progression, and show for the first time that the initial up regulation of telomerase RNA is an early event, concurrent with the hyperproliferation elicited by viral oncogenes.

Animals↗
PubMed1996

Telomerase activation in mouse mammary tumors: lack of detectable telomere shortening and evidence for regulation of telomerase RNA with cell proliferation.

Activation of telomerase in human cancers is thought to be necessary to overcome the progressive loss of telomeric DNA that accompanies proliferation of normal somatic cells. According to this model, telomerase provides a growth advantage to cells in which extensive terminal sequence loss threatens viability. To test these ideas, we have examined telomere dynamics and telomerase activation during mammary tumorigenesis in mice carrying a mouse mammary tumor virus long terminal repeat-driven Wnt-1 transgene. We also analyzed Wnt-1-induced mammary tumors in mice lacking p53 function. Normal mammary glands, hyperplastic mammary glands, and mammary carcinomas all had the long telomeres (20 to 50 kb) typical of Mus musculus and did not show telomere shortening during tumor development. Nevertheless, telomerase activity and the RNA component of the enzyme were consistently upregulated in Wnt-1-induced mammary tumors compared with normal and hyperplastic tissues. The upregulation of telomerase activity and RNA also occurred during tumorigenesis in p53-deficient mice. The expression of telomerase RNA correlated strongly with histone H4 mRNA in all normal tissues and tumors, indicating that the RNA component of telomerase is regulated with cell proliferation. Telomerase activity in the tumors was elevated to a greater extent than telomerase RNA, implying that the enzymatic activity of telomerase is regulated at additional levels. Our data suggest that the mechanism of telomerase activation in mouse mammary tumors is not linked to global loss of telomere function but involves multiple regulatory events including upregulation of telomerase RNA in proliferating cells.

Animals↗
PubMed1996

Reconstitution of human telomerase activity and identification of a minimal functional region of the human telomerase RNA.

Telomerase is a ribonucleoprotein that catalyzes telomere elongation through the addition of TTAGGG repeats in humans. Activation of telomerase is often associated with immortalization of human cells and cancer. To dissect the human telomerase enzyme mechanism, we developed a functional in vitro reconstitution assay. After removal of the essential 445 nucleotide human telomerase RNA (hTR) by micrococcal nuclease digestion of partially purified human telomerase, the addition of in vitro transcribed hTR reconstituted telomerase activity. The activity was dependent upon and specific to hTR. Using this assay, truncations at the 5' and 3' ends of hTR identified a functional region of hTR, similar in size to the full-length telomerase RNAs from ciliates. This region is located between positions 1-203. Furthermore, we found that residues 1-44, 5' to the template region (residues 46-56) are not essential for activity, indicating a minimal functional region is located between residues 44-203. Mutagenesis of full-length hTR between residues 170-179, 180-189 or 190-199 almost completely abolished the ability of the hTR to function in the reconstitution of telomerase activity, suggesting that sequences or structures within this 30 nucleotide region are required for activity, perhaps by binding telomerase protein components.

Animals↗
PubMed1996

Human telomerase RNA and telomerase activity in immortal cell lines and tumor tissues.

Telomerase activity has been detected in many human immortal cells lines and in tumor tissues, whereas it is generally absent from primary cell strains and from many tumor adjacent tissue samples. With the recently cloned human telomerase RNA (hTR), we used Northern analysis to follow the levels of hTR in primary, precrisis, and immortalized cells. It was surprising that the amount of hTR was high in cell strains that lacked telomerase activity, and the levels did not parallel the increases in telomerase activity, which accompanies immortalization. In addition, although the hTR levels were somewhat higher in tumor samples compared to nontumor tissues, the level of hTR in a variety of different human tumors did not predict the level of telomerase activity in the tumor. Thus, whereas hTR was detected in all samples that have telomerase activity, the presence of the RNA was not a good predictor of the presence or amount of telomerase activity.

B-Lymphocytes↗
PubMed1995

Telomerase activity in small-cell and non-small-cell lung cancers.

BACKGROUND: Telomerase is an enzyme that adds hexameric TTAGGG nucleotide repeats onto the ends of vertebrate chromosomal DNAs (i.e., telomeres) to compensate for losses that occur with each round of DNA replication. Somatic cells do not have telomerase activity and stop dividing when the telomeric ends of at least some chromosomes have been shortened to a critical length. It has been suggested that immortalized cells (including some, but probably not all, cancer cells) continue to proliferate indefinitely because they express telomerase. PURPOSE: To investigate whether expression of telomerase is a prerequisite for the development of naturally occurring human cancers, we assayed the levels of telomerase activity in specimens of human lung tumor and adjacent normal tissue. METHODS: Using a polymerase chain reaction-based assay, we examined telomerase activity in 136 primary lung cancer tissues and 68 adjacent noncancerous tissues obtained by surgical resection. We also studied telomerase activity in four primary and 23 metastatic lesions obtained through biopsy, (two patients) or autopsy (10 patients). Relative telomerase activity levels were estimated by serial dilutions of extracts prepared from the specimens. Telomerase activity was also assayed in extracts of cells present in pleural fluids from three patients with adenocarcinoma of the lung. RESULTS: Among surgically resected samples, telomerase activity was detected in 109 (80.1%) of 136 primary lung cancer tissues and in three (4.4%) of 68 normal adjacent tissues. All 11 surgically resected specimens of primary small-cell lung cancer (from 11 patients) revealed high levels of telomerase activity, whereas the activity ranged from undetectable to high levels in the 125 surgically resected specimens of primary non-small-cell lung cancer tissue (from 125 patients). Generally, high levels of telomerase activity were observed in metastatic lesions and tumors with altered telomere length. A few primary and, surprisingly, some metastatic tumors did not appear to have detectable telomerase activity. Telomerase activity was, however, detected in cells present in all tested pleural fluids obtained (from three patients with adenocarcinoma of the lung). CONCLUSION: The subset of non-small-cell lung cancers that exhibits only low or undetectable levels of telomerase activity may contain primarily mortal cancer cells. Cancers that exhibit high levels of telomerase activity, such as all of the small-cell lung cancers examined in this study, are likely to consist mainly of immortal cells. IMPLICATIONS: Telomerase activity may be useful both as a diagnostic marker to detect the existence of immortal lung cancer cells in clinical materials and as a target for therapeutic intervention.

Base Sequence↗
PubMed1996

Development of retinoblastoma in the absence of telomerase activity.

BACKGROUND: The length and stability of telomeres (essential functional structures at the end of eukaryotic chromosomes) have been implicated in the control of cell lifespan. Most somatic cells lack telomerase, the enzyme that synthesizes telomeric DNA, and their telomeres shorten with cell division. Cells immortalized in vitro, on the other hand, express telomerase and maintain their telomeres. Telomerase activity has also been detected in the large majority of tumors from a variety of cancers. These observations have suggested that telomere maintenance is required for unlimited cell proliferation and that telomerase is a marker for cell immortality in vitro and in vivo. PURPOSE: We investigated whether telomerase is activated during the development of retinoblastoma. This is a childhood eye cancer associated with a limited number of mutations in an embryonic tissue and thus likely to develop in cells that have long telomeres. The ease of detection of retinoblastoma makes it possible to screen relatively small tumors before extensive proliferation of the malignant cells. METHODS: We measured telomerase activity in 34 samples of retinoblastoma, four retinoblastoma-derived cell lines, and six cell lines derived from other cancers. Only three of the cell lines from other cancers expressed the retinoblastoma protein. Telomerase activity was assayed by a polymerase chain reaction protocol in extracts prepared from tumors or cell lines. The level of enzyme activity in cell extracts was quantified at several protein concentrations and expressed relative to that in a positive control, after normalization for the amount of protein. Telomere length was measured by Southern blot hybridization of genomic DNA with a telomere-specific probe. Average values of telomere length in telomerase-positive and telomerase-negative tumors and in cell lines were compared by two-sided, two-sample Student's t test. RESULTS: No telomerase activity was detected in 17 (50%) of 34 retinoblastomas. Assays of cell lines derived from other cancers revealed no association between the presence or the level of the enzyme activity and the expression of the retinoblastoma protein. Telomeres were significantly longer in telomerase-negative tumors than in telomerase-positive tumors (P = .0008). CONCLUSIONS: Our results indicate that retinoblastoma can develop when telomeres are still relatively long and in the absence of telomerase. Telomerase activity associated with short telomeres is, however, observed in 50% of the retinoblastomas and in retinoblastoma-derived cell lines. IMPLICATIONS: Telomerase may not be a marker for acquisition of the malignant phenotype in the case of tumors that are derived from cells with long telomeres and that are associated with a low number of mutations.

Base Sequence↗
PubMed1996

Telomerase activity in human breast tumors.

BACKGROUND: The activity of the ribonucleoprotein enzyme telomerase is not detected in normal somatic cells; thus, with each cell division, the ends of chromosomes consisting of the telomeric repeats TTAGGG progressively erode. The current model gaining support is that telomerase activity in germline and immortal cells maintains telomere length and thus compensates for the "end-replication problem." PURPOSE: Our objective was to determine when telomerase activity is reactivated in the progression to malignant breast cancer and if knowledge of telomerase activity may be an indicator for the diagnosis and potential treatment of breast cancer. METHODS: Using a polymerase chain reaction-based telomerase activity assay, we examined telomerase activity in 140 breast cancer specimens (from 140 patients), four phyllodes tumors (from four patients), 38 noncancerous lesions (20 fibroadenomas, 17 fibrocystic diseases, one gynecomastia; from 38 patients), and 55 adjacent noncancerous mammary tissues (from 55 of the 140 breast cancer patients). In addition, 33 fine-needle-aspirated breast samples (from 33 patients) were analyzed. RESULTS: Among surgically resected samples, telomerase activity was detected in 130 (93%) of 140 breast cancers. Telomerase activity was detected in 68% of stage I primary breast cancers, in 73% of cancers smaller than 20 mm, and in 81% of axillary lymph node-negative cancers. Moreover, the activity was detected in more than 95% of advanced stage tumors but in only two (4%) of 55 adjacent noncancerous tissues. While telomerase activity was not detected in any of 17 specimens of fibrocystic disease, surprisingly low levels of telomerase activity were detected in nine (45%) of 20 fibroadenomas. Among samples obtained by fine-needle aspiration, 14 (100%) of 14 patients whose fine-needle-aspirated specimen contained telomerase activity and who subsequently underwent surgery were confirmed to have breast cancer. Multivariate analysis of 125 specimens from patients for whom data were available on age at surgery, stage of disease, tumor size, lymph node status tumor histology, and menopausal status indicated that stage classification exhibited the strongest association with telomerase activity (for stage I versus stages II-IV: odds ratio = 1.0 versus 73.4; 95% confidence interval = 2.0-959.0; P = .02). CONCLUSION: Telomerase activity was detected in more than 95% of advanced stage breast cancers. It was absent in 19%-32% of less advanced cancers. Since a determination of any association between telomerase activity and patient survival is not possible at the present time, it remains to be determined whether lack of telomerase activity predicts for favorable outcome.

Adult↗
PubMed1995

Telomere elongation in immortal human cells without detectable telomerase activity.

Immortalization of human cells is often associated with reactivation of telomerase, a ribonucleoprotein enzyme that adds TTAGGG repeats onto telomeres and compensates for their shortening. We examined whether telomerase activation is necessary for immortalization. All normal human fibroblasts tested were negative for telomerase activity. Thirteen out of 13 DNA tumor virus-transformed cell cultures were also negative in the pre-crisis (i.e. non-immortalized) stage. Of 35 immortalized cell lines, 20 had telomerase activity as expected, but 15 had no detectable telomerase. The 15 telomerase-negative immortalized cell lines all had very long and heterogeneous telomeres of up to 50 kb. Hybrids between telomerase-negative and telomerase-positive cells senesced. Two senescent hybrids demonstrated telomerase activity, indicating that activation of telomerase is not sufficient for immortalization. Some hybrid clones subsequently recommenced proliferation and became immortalized either with or without telomerase activity. Those without telomerase activity also had very long and heterogeneous telomeres. Taken together, these data suggest that the presence of lengthened or stabilized telomeres is necessary for immortalization, and that this may be achieved either by the reactivation of telomerase or by a novel and as yet unidentified mechanism.

Base Sequence↗
PubMed1996

Specific RNA residue interactions required for enzymatic functions of Tetrahymena telomerase.

The ribonucleoprotein enzyme telomerase is a specialized reverse transcriptase that synthesizes telomeric DNA by copying a template sequence within the telomerase RNA. Here we analyze the actions of telomerase from Tetrahymena thermophila assembled in vivo with mutated or wild-type telomerase RNA to define further the roles of particular telomerase RNA residues involved in essential enzymatic functions: templating, substrate alignment, and promotion of polymerization. Position 49 of the telomerase RNA defined the 3' templating residue boundary, demonstrating that seven positions, residues 43 to 49, are capable of acting as templating residues. We demonstrate directly that positioning of the primer substrate involves Watson-Crick base pairing between the primer with telomerase RNA residues. Unexpectedly, formation of a Watson-Crick base pair specifically between the primer DNA and telomerase RNA residue 50 is critical in promoting primer elongation. In contrast, mutant telomerase with the cytosine at position 49 mutated to a G exhibited efficient 3' mispair extension. This work provides new evidence for specific primer-telomerase interactions, as well as base-specific interactions involving the telomerase RNA, playing roles in essential active-site functions of telomerase.

Animals↗
PubMed1995

Increased telomerase activity in mouse skin premalignant progression.

It has been postulated that the expression of the ribonucleoprotein telomerase is necessary to overcome cellular senescence and that malignant tumors must express telomerase to maintain their immortality. In most human adult tissues, telomerase activity is not detected. In contrast, several murine tissues express various levels of telomerase. Mouse skin however, does not show telomerase activity. Using the mouse skin chemical carcinogenesis system, a well-characterized model for studying premalignant and malignant progression, we assayed telomerase activity at various stages of premalignant papilloma progression by means of the recently developed telomeric repeat amplification protocol. We observed that at 10 weeks of promotion, only one mouse skin papilloma of 11 analyzed showed high levels of telomerase activity. The number of papillomas showing higher levels of telomerase activity increased at 20 weeks, and at 30 weeks of promotion, 100% of papillomas expressed significantly higher levels of telomerase. We learned from previous studies that early papillomas are diploid, well-differentiated lesions, whereas late papillomas are aneuploid and very dysplastic. It appears that the progressive increase in telomerase activity is associated with the increased level of genomic instability and the phenotypic progression of these premalignant tumors. It is also possible, however, that the increase in telomerase activity could be in part a consequence of an increase in the proportion of proliferating cells. Nevertheless, the mouse skin system may be a very useful in vivo model for the study and development of anti-telomerase therapeutic strategies.

Animals↗
PubMed1995

Telomerase activity in gastric cancer.

Although many genetic alterations have been reported in gastric cancer, it is not known whether all gastric tumors are capable of indefinite proliferative potential, e.g., immortality. The expression of telomerase and stabilization of telomeres are concomitant with the attainment of immortality in tumor cells; thus, the measurement of telomerase activity in clinically obtained tumor samples may provide important information useful both as a diagnostic marker to detect immortal cancer cells in clinical materials and as a prognostic indicator of patient outcome. Telomerase activity was analyzed in 66 primary gastric cancers with the use of a PCR-based assay. The majority of tumors (85%) displayed telomerase activity, but telomerase was undetectable in 10 tumors (15%), 8 of which were early stage tumors. Most of the tumors with telomerase activity were large and of advanced stages, including metastases. Survival rate of patients of tumors with detectable telomerase activity was significantly shorter than that of those without telomerase activity. Alterations of telomere length (reduced/elongated terminal restriction fragments) were detected in 14 of 66 (21%) gastric cancers, and all 14 had telomerase activity. Cellular DNA contents revealed that all 22 aneuploid tumors had detectable telomerase activity. The present results indicate that telomerase activation may be required as a critical step in the multigenetic process of tumorigenesis, and that telomerase is frequently but not always activated as a late event in gastric cancer progression.

Base Sequence↗
PubMed1995

Differentiation of immortal cells inhibits telomerase activity.

Telomerase, a ribonucleic acid-protein complex, adds hexameric repeats of 5'-TTAGGG-3' to the ends of mammalian chromosomal DNA (telomeres) to compensate for the progressive loss that occurs with successive rounds of DNA replication. Although somatic cells do not express telomerase, germ cells and immortalized cells, including neoplastic cells, express this activity. To determine whether the phenotypic differentiation of immortalized cells is linked to the regulation of telomerase activity, terminal differentiation was induced in leukemic cell lines by diverse agents. A pronounced downregulation of telomerase activity was produced as a consequence of the differentiated status. The differentiation-inducing agents did not directly inhibit telomerase activity, suggesting that the inhibition of telomerase activity is in response to induction of differentiation. The loss of telomerase activity was not due to the production of an inhibitor, since extracts from differentiated cells did not cause inhibition of telomerase activity. By using additional cell lineages including epithelial and embryonal stem cells, down-regulation of telomerase activity was found to be a general response to the induction of differentiation. These findings provide the first direct link between telomerase activity and terminal differentiation and may provide a model to study regulation of telomerase activity.

Animals↗
PubMed1996

Mechanism of telomerase induction during T cell activation.

The progressive shortening of the ends of chromosomes (telomeres) during cell division may serve as a mitotic clock for replicative senescence. Telomerase, a ribonucleoprotein which synthesizes telomeric DNA and maintains telomere length, is absent from most normal somatic cells but is expressed in immortal cells. Low levels of telomerase activity have been detected in peripheral blood mononuclear cells (PBMC) and hematopoietic cells and an increase in telomerase activity during T cell activation has recently been reported. In this study, we show that the increase in telomerase activity during T cell activation was transient and did not prevent the loss of telomeres in long-term T cell cultures. Analysis of the mechanism of telomerase induction showed that the increase in telomerase activity was accompanied by an increase in the levels of hTR, the RNA component of human telomerase. Moreover, telomerase induction occurred in the presence of aphidicolin, indicating that DNA synthesis was not required. Increased telomerase expression was observed when PBMC were activated with phorbol myristate acetate (PMA) and ionomycin, indicating that it was independent of early transmembrane signals. It was, however, linked to the T cell signal transduction pathway, as inhibiting protein kinase C with bisindolylmaleimide prevented the increase in telomerase activity.

Adult↗
PubMed1996

Regulated expression of telomerase activity in human T lymphocyte development and activation.

Telomerase, a ribonucleoprotein that is capable of synthesizing telomeric repeats, is expressed in germline and malignant cells, and is absent in most normal human somatic cells. The selective expression of telomerase has thus been proposed to be a basis for the immortality of the germline and of malignant cells. In the present study, telomerase activity was analyzed in normal human T lymphocytes. It was found that telomerase is expressed at a high level in thymocyte subpopulations, at an intermediate level in tonsil T lymphocytes, and at a low to undetectable level in peripheral blood T lymphocytes. Moreover, telomerase activity is highly inducible in peripheral T lymphocytes by activation through CD3 with or without CD28 costimulation, or by stimulation with phorbol myristate acetate (PMA)/ionomycin. The induction of telomerase by anti-CD3 plus anti-CD28 (anti-CD3/CD28) stimulation required RNA and protein synthesis, and was blocked by herbimycin A, an inhibitor of S pi protein tyrosine kinases. The immunosuppressive drug cyclosporin A selectively inhibited telomerase induction by PMA/ionomycin and by anti-CD3, but not by anti-CD3/CD28. Although telomerase activity in peripheral T lymphocytes was activation dependent and correlated with cell proliferation, it was not cell cycle phase restricted. These results indicate that the expression of telomerase in normal human T lymphocytes is both developmentally regulated and activation induced. Telomerase may thus play a permissive role in T cell development and in determining the capacity of lymphoid cells for cell division and clonal expansion.

Antigens, CD↗
PubMed1996

Telomerase regulation during entry into the cell cycle in normal human T cells.

Telomerase activity is involved in telomere length maintenance. Leukocytes, unlike many human somatic tissues, have detectable telomerase activity. These cells provide a normal human cell type in which to study telomerase. We studied the regulation of telomerase activity and the telomerase RNA component as leukocytes were stimulated to enter the cell cycle. In primary human leukocytes stimulated with phytohemagglutinin, telomerase activity increased > 10-fold as naturally quiescent cells entered the cell cycle. Antibodies to the T cell receptor (TCR)/CD3 complex and the costimulatory CD28 receptor induced telomerase activity in a T cell-enriched population of cells. Rapamycin, an immunosuppressant that blocks TCR/CD3 signal transduction pathways and cdk2 activation, blocked telomerase induction. Hydroxyurea, an inhibitor of S phase, did not block cdk2 kinase activity or telomerase activation. In summary, telomerase is regulated in G1 phase as normal human T cells enter the cell cycle.

Antibodies↗
PubMed1996

Detection of telomerase activity in malignant and nonmalignant skin conditions.

Telomeres are the end regions of linear chromosomes, and in normal somatic cells the lengths of telomeres shorten with successive cell divisions. Telomerase, a ribonucleoprotein enzyme, maintains the length of telomeres in immortal and germline cells. Although present in human fetal tissues, shortly after birth telomerase activity is not detectable except in germline cells, hematopoietic cells, and most human primary tumors. In the present study we show telomerase activity to be present in 73 of 77 basal cell carcinomas, 15 of 18 nonmetastatic cutaneous squamous cell carcinomas, and 6 of 7 cutaneous melanomas, contrasting with extremely low levels detected in sun-protected skin. Sun-damaged skin, psoriatic lesional skin, and skin from lesions of poison ivy dermatitis, however, have increased levels of telomerase activity compared to sun-protected skin, although less than that detected in tumor tissue. Because telomerase activity can be found in inflammatory lesions of the skin, this indicates that telomerase activity does not always correlate with the malignant phenotype. In addition, we show that telomerase activity is localized to the epidermis of newborn foreskin, which suggests that telomerase is expressed constitutively by cells in the epidermis. Finding higher levels of telomerase activity in sun-exposed skin compared to nonexposed skin suggests that environmental factors may modulate telomerase activity.

Adult↗
PubMed1996

Telomerase activity is repressed during differentiation of maturation-sensitive but not resistant human tumor cell lines.

The effects of induced differentiation on telomerase activity were examined in human acute promyelocytic leukemic (NB4) and human embryonal carcinoma (NTERA-2) cells exposed to all-trans-retinoic acid or hexamethylene bisacetamide. Retinoic acid treatment of NB4 and NTERA-2 cells, and hexamethylene bisacetamide treatment of NTERA-2 cells caused a decline in telomerase activity in differentiation-sensitive but not in resistant clones of these cell lines. Changes in telomerase activity as measured by the PCR-based telomeric repeat amplification protocol assay were noted by 24-72 h of exposure to the inducer, suggesting that its regulation may precede terminal differentiation. The degree of telomerase activity decline was greater in NB4 cells than in NTERA-2 cells, probably reflecting in part a more mature state of NB4 cells after 5 days of exposure to the inducer. Mixing of protein extracts from treated and untreated cells did not suggest the presence of diffusible telomerase inhibitors. Expression of the RNA component of telomerase was also examined in NB4 cells, and its decline correlated with the reduced telomerase activity measured by the telomeric repeat amplification protocol assay during induced differentiation of these tumor cells. Taken together, these findings indicate that telomerase is a regulated enzyme system during induced human tumor cell differentiation, showing an inverse relationship between the degree of differentiation and telomerase activity. These models will be be useful to study the regulation and role of telomerase during induced differentiation of human tumor cells.

Acetamides↗
PubMed1996

Telomerase activation in normal B lymphocytes and non-Hodgkin's lymphomas.

Activation of telomerase seems to be a prerequisite for immortalization and is found in permanent cell lines and most malignant tumors. Normal somatic cells are generally telomerase negative, except for bone marrow stem cells. Weak activity is also present in peripheral blood cells. In the present study strong telomerase activity was demonstrated in vivo in normal mature cells of the immune system, as well as in malignant lymphomas. Benign lymph nodes had lower telomerase activity than benign tonsils, which exhibited intermediate to high activity comparable with findings in malignant lymphomas. In benign tonsils the activity seemed to be restricted to germinal center B cells. In benign lymphoid tissues telomerase activity correlated with B-cell numbers and cell proliferation, but this was not observed in the lymphoma group. High-grade lymphomas exhibited higher levels of telomerase compared with low-grade cases. The data showed that in vivo activation of telomerase is a characteristic feature of germinal center B cells. Different signals for activation of telomerase are likely to exist, one of them being immune stimulation. The data suggest that telomerase activity in malignant lymphomas can be explained by an "induction and retention" model, ie, transformation occurs in a normal, mature B cell with reactivated telomerase, which is retained in the neoplastic clone.

B-Lymphocytes↗