Biomedical subjects
C W Greider
Publications and source records attributed to C W Greider.
Short dysfunctional telomeres impair tumorigenesis in the INK4a(delta2/3) cancer-prone mouse.
Maintenance of telomere length is predicted to be essential for bypass of senescence and crisis checkpoints in cancer cells. The impact of telomere dysfunction on tumorigenesis was assessed in successive generations of mice doubly null for the telomerase RNA (mTR) and the INK4a tumor suppressor genes. Significant reductions in tumor formation in vivo and oncogenic potential in vitro were observed in late generations of telomerase deficiency, coincident with severe telomere shortening and associated dysfunction. Reintroduction of mTR into cells significantly restored the oncogenic potential, indicating telomerase activation is a cooperating event in the malignant transformation of cells containing critically short telomeres. The results described here demonstrate that loss of telomere function in a cancer-prone mouse model possessing intact DNA damage responses impairs, but does not prevent, tumor formation.
p53 deficiency rescues the adverse effects of telomere loss and cooperates with telomere dysfunction to accelerate carcinogenesis.
Maintenance of telomere length and function is critical for the efficient proliferation of eukaryotic cells. Here, we examine the interactions between telomere dysfunction and p53 in cells and organs of telomerase-deficient mice. Coincident with severe telomere shortening and associated genomic instability, p53 is activated, leading to growth arrest and/or apoptosis. Deletion of p53 significantly attenuated the adverse cellular and organismal effects of telomere dysfunction, but only during the earliest stages of genetic crisis. Correspondingly, the loss of telomere function and p53 deficiency cooperated to initiate the transformation process. Together, these studies establish a key role for p53 in the cellular response to telomere dysfunction in both normal and neoplastic cells, question the significance of crisis as a tumor suppressor mechanism, and identify a biologically relevant stage of advanced crisis, termed genetic catastrophe.
Telomerase activation. One step on the road to cancer?
Ever since the discovery that telomeres are short in cancer cells and telomerase is activated in immortal cells, telomerase has been an oncogene wannabe. Oncogenes have been the glamour genes of molecular biology for 20 years, garnering flashy headlines and name recognition. More recently, tumor-suppressor genes have joined oncogenes on center stage. Recent evidence has shown that MYC upregulates the catalytic subunit of telomerase, TERT, and that TERT cooperates with HPV E7 in cell immortalization. This evidence now supports the placement of telomerase among the cancer gene elite.
RAD50 and RAD51 define two pathways that collaborate to maintain telomeres in the absence of telomerase.
Telomere length is maintained by the de novo addition of telomere repeats by telomerase, yet recombination can elongate telomeres in the absence of telomerase. When the yeast telomerase RNA component, TLC1, is deleted, telomeres shorten and most cells die. However, gene conversion mediated by the RAD52 pathway allows telomere lengthening in rare survivor cells. To further investigate the role of recombination in telomere maintenance, we assayed telomere length and the ability to generate survivors in several isogenic DNA recombination mutants, including rad50, rad51, rad52, rad54, rad57, xrs2, and mre11. The rad51, rad52, rad54, and rad57 mutations increased the rate of cell death in the absence of TLC1. In contrast, although the rad50, xrs2, and mre11 strains initially had short telomeres, double mutants with tlc1 did not affect the rate of cell death, and survivors were generated at later times than tlc1 alone. While none of the double mutants of recombination genes and tlc1 (except rad52 tlc1) blocked the ability to generate survivors, a rad50 rad51 tlc1 triple mutant did not allow the generation of survivors. Thus RAD50 and RAD51 define two separate pathways that collaborate to allow cells to survive in the absence of telomerase.
Essential role of mouse telomerase in highly proliferative organs.
We have investigated the role of the enzyme telomerase in highly proliferative organs in successive generations of mice lacking telomerase RNA. Late-generation animals exhibited defective spermatogenesis, with increased programmed cell death (apoptosis) and decreased proliferation in the testis. The proliferative capacity of haematopoietic cells in the bone marrow and spleen was also compromised. These progressively adverse effects coincided with substantial erosion of telomeres (the termini of eukaryotic chromosomes) and fusion and loss of chromosomes. These findings indicate an essential role for telomerase, and hence telomeres, in the maintenance of genomic integrity and in the long-term viability of high-renewal organ systems.
Telomeres and senescence: the history, the experiment, the future.
Telomere length has been proposed to signal entry into cellular senescence. Expression of the human telomerase subunit, hTERT, in primary cells now shows that these cells bypass senescence.
Mutational analysis of the Tetrahymena telomerase RNA: identification of residues affecting telomerase activity in vitro.
Telomere-specific repeat sequences are essential for chromosome end stability. Telomerase maintains telomere length by adding sequences de novo onto chromosome ends. The template domain of the telomerase RNA component dictates synthesis of species-specific telomeric repeats and other regions of the RNA have been suggested to be important for enzyme structure and/or catalysis. Using enzyme reconstituted in vitro with RNAs containing deletions or substitutions we identified nucleotides in the RNA component that are important for telomerase activity. Although many changes to conserved features in the RNA secondary structure did not abolish enzyme activity, levels of activity were often greatly reduced, suggesting that regions other than the template play a role in telomerase function. The template boundary was only altered by changes in stem II that affected the conserved region upstream of the template, not by changes in other regions, such as stems I, III and IV, consistent with a role of the conserved region in defining the 5' boundary of the template. Surprisingly, telomerase RNAs with substitutions or deletion of residues potentially abolishing the conserved pseudoknot structure had wild-type levels of telomerase activity. This suggests that this base pairing interaction may not be required for telomerase activity per se but may be conserved as a regulatory site for the enzyme in vivo.
The mouse telomerase RNA 5"-end lies just upstream of the telomerase template sequence.
Telomerase is a ribonucleoprotein enzyme with an essential RNA component. Embedded within the telomerase RNA is a template sequence for telomere synthesis. We have characterized the structure of the 5' regions of the human and mouse telomerase-RNA genes, and have found a striking difference in the location of the template sequence: Whereas the 5'-end of the human telomerase RNA lies 45 nt from the telomerase-RNA template sequence, the 5'-end of the mouse telomerase RNA lies just 2 nt from the telomerase-RNA template sequence. Analysis of genomic sequences flanking the 5'-end of the human and mouse telomerase RNA-coding sequences reveals similar promoter-element arrangements typical of mRNA-type promoters: a TATA box-like element and an upstream region containing a consensus CCAAT box. This putative promoter structure contrasts with that of the ciliate telomerase-RNA genes whose structure resembles RNA polymerase III U6 small nuclear RNA (snRNA) promoters. These and other comparisons suggest that, during evolution, both the RNA-polymerase specificity of telomerase RNA-gene promoters and, more recently, the position of the template sequence in the telomerase RNA changed.
Telomerase activity, cell proliferation, and cancer.
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Telomerase activity in human gliomas.
OBJECTIVE: Telomerase activity, which is undetectable in most mature normal tissues, has been identified in many types of human cancers, including neuroblastomas and oligodendrogliomas. These findings suggest that a novel mechanism in addition to activation of oncogenes and inactivation of tumor suppressor genes may play an important role in tumorigenesis. The goal of the present study was to assess and correlate the telomerase activity in astrocytic gliomas of different grades. METHODS: Telomere repeat amplification protocol and Southern blot hybridization with telomere-specific probes were used to detect telomerase activity and to measure terminal restriction fragment length, respectively. RESULTS: Telomerase activity was detected in 3 of 9 (33%) low-grade astrocytomas (World Health Organization Grade II), 5 of 11 (45%) anaplastic astrocytomas (World Health Organization Grade III), 36 of 41 (89%) glioblastomas multiforme (World Health Organization Grade IV), 3 of 4 (75%) oligodendrogliomas, and none of 4 normal brain specimens. CONCLUSION: We demonstrated that telomerase activity is absent in normal brain tissues while present in most glioma samples (72%). The frequency of such activity increases with malignancy. These results suggest that telomerase activity may be used as a tumor marker and that the activation of telomerase may correlate with initiation and malignant progression of astrocytic tumors.
Telomere shortening and tumor formation by mouse cells lacking telomerase RNA.
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.
Mouse models for the study of telomerase.
The ends of chromosomes, or telomeres, consist of short repeated sequences that are synthesized by a ribonucleoprotein-DNA polymerase called telomerase. The RNA component of telomerase is essential for enzyme activity. The maintenance of telomere length by telomerase has been proposed to be essential for cellular viability and to play an important role in cellular senescence and immortalization. We are interested in using the mouse as a model system for the study of telomerase. We studied telomerase activity and expression of the mouse telomerase RNA component (mTR) in two different transgenic mouse models of multistage tumorigenesis: models of islet cell carcinoma and squamous cell carcinoma. In both tumour models, telomerase activity was detected only in late-stage tumours, whereas the telomerase RNA was present at higher than normal levels in pre-neoplastic stages and increased further in late-stage tumours. However, the RNA levels did not parallel the amounts of telomerase activity detected, suggesting that regulation of telomerase activity does not correlate with the regulation of its RNA component. These results establish a direct correlation between progression to late-stage tumours and induction of telomerase activity, and suggest that the initial upregulation of telomerase RNA is an early event. To address the role of telomerase during normal mouse development and tumour formation, we have constructed a knockout mouse for the mouse telomerase RNA, mTR-/-. These mice and the cell lines derived from them are telomerase deficient.
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.
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.
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.
Telomeres, telomerase and cancer.
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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.