Telomere dynamics in cells with introduced telomerase: a rapid assay for telomerase activity on telomeres

The region upstream of the telomerase reverse transcriptase gene is essential for in planta telomerase complementation

Telomerase is essential for the maintenance of telomeres, structures located at the ends of linear eukaryotic chromosomes that are crucial for genomic stability. Telomerase has been frequently explored in mammals because of its activity in many types of cancers, but knowledge in plants is rather sketchy despite plants representing useful models due to peculiarities in their telomeres and telomerase biology. We studied in planta complementation of telomerase in Arabidopsis thaliana mutant plants with disrupted expression of the gene encoding the telomerase protein subunit (AtTERT) and significantly shortened telomeres. We found that the upstream region of AtTERT, previously identified as a putative minimal promoter, was essential for reconstitution of telomerase function, as demonstrated by the full or partial recovery of the telomere phenotype in mutants. In contrast, transformation by the full length AtTERT gene construct resulted in more progressive telomere shortening in mutants and even in wild type plants, despite the high level of AtTERT transcript and telomerase activity detected by in vitro assay. Thus, the telomerase protein subunit putative promoter is essential for in planta telomerase reconstitution and restoration of its catalytical activity. Contributions from other factors, including those tissue-specific, for proper telomerase function are discussed.

Processive and distributive extension of human telomeres by telomerase under homeostatic and nonequilibrium conditions

Specific information about how telomerase acts in vivo is necessary for understanding telomere dynamics in human tumor cells. Our results imply that, under homeostatic telomere length-maintenance conditions, only one molecule of telomerase acts at each telomere during every cell division and processively adds ∼60 nt to each end. In contrast, multiple molecules of telomerase act at each telomere when telomeres are elongating (nonequilibrium conditions). Telomerase extension is less processive during the first few weeks following the reversal of long-term treatment with the telomerase inhibitor Imetelstat (GRN163L), a time when Cajal bodies fail to deliver telomerase RNA to telomeres. This result implies that processing of telomerase by Cajal bodies may affect its processivity. Overexpressed telomerase is also less processive than the endogenously expressed telomerase. These findings reveal two major distinct extension modes adopted by telomerase in vivo.

Engineered telomere degradation models dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome characterized by cutaneous symptoms, including hyperpigmentation and nail dystrophy. Some forms of DC are caused by mutations in telomerase, the enzyme that counteracts telomere shortening, suggesting a telomere-based disease mechanism. However, mice with extensively shortened telomeres due to telomerase deficiency do not develop the characteristics of DC, raising questions about the etiology of DC and/or mouse models for human telomere dysfunction. Here we describe mice engineered to undergo telomere degradation due to the absence of the shelterin component POT1b. When combined with reduced telomerase activity, POT1b deficiency elicits several characteristics of DC, including hyperpigmentation and fatal bone marrow failure at 4-5 mo of age. These results provide experimental support for the notion that DC is caused by telomere dysfunction, and demonstrate that key aspects of a human telomere-based disease can be modeled in the mouse.

Telomere length homeostasis requires that telomerase levels are limiting

Stabilization of telomere length in germline and highly proliferative human cells is required for long-term survival and for the immortal phenotype of cancer-derived cells. This is achieved through expression of telomerase reverse transcriptase (TERT), which synthesizes telomeric repeats through reverse transcription of its tightly associated RNA template (TR). The telomeric repeat binding factor TRF1 inhibits telomerase at telomeres in cis in a length-dependent manner to achieve telomere length homeostasis. Here we manipulate telomerase activity over a wide range in cancer and primary cells. Concomitant overexpression of TERT and TR was necessary and sufficient to substantially increase telomerase activity. Upon overexpression, more telomerase associated with telomeres and telomeres elongated at a constant rate (up to 0.8 kb/population doubling (PD)) in a length-independent manner. Thus, in less than 50 PDs, the length of telomeres increased 3-8-fold beyond physiological size, while telomere-bound TRF1 and TRF2 increased proportionally to telomere length. Thus, long telomeres do not permanently adopt a structural state that is non-extendible. A low cellular concentration of telomerase is critical to achieve preferential elongation of short telomeres and telomere length homeostasis.

Sister chromatid separation at human telomeric regions

Telomeres are nucleoprotein complexes located at chromosome ends, vital for preserving chromosomal integrity. Telomeric DNA shortens with progressive rounds of cell division, culminating in replicative senescence. Previously we have reported, on the basis of fluorescent in situ hybridization, that several human telomeric regions display solitary signals (singlets) in metaphase cells of presenescent fibroblasts, in comparison to other genomic regions that hybridize as twin signals (doublets). In the current study, we show that an additional 12 out of 12 telomeric regions examined also display metaphase singlet signals in pre-senescent cells, and that excess telomere-metaphase singlets also occur in earlier passage cells harvested from elderly individuals. In cancer cell lines expressing telomerase and in pre-senescent fibroblasts ectopically expressing hTERT, this phenomenon is abrogated. Confocal microscope image analysis showed that the telomere metaphase singlets represent regions that have replicated but not separated; this is presumably because of persistent cohesion. The introduction of mutations that interfere with the normal dissolution of cohesion at the metaphase to anaphase transition induced the cut (chromosomes untimely torn) phenotype in early passage fibroblasts, with predominantly telomeric rather than centromeric DNA, present on the chromatin bridges between the daughter nuclei. These results suggest that telomeric regions in animal cells may potentially be sites of persistent cohesion, and that this cohesion may be the basis for an observed excess of fluorescent in situ hybridization metaphase singlets at telomeres. Persistent cohesion at telomeres may be associated with attempted DNA repair or chromosomal abnormalities, which have been described in pre-senescent cells.

Overexpression of telomerase confers a survival advantage through suppression of TRF1 gene expression while maintaining differentiation characteristics in K562 cells

Leukemic stem cells that expressed endogenous telomerase activity were induced to show overexpression of exogenous hTERT and were analyzed for biological changes in order to assess the possible influence of telomerase gene therapy on the transplantation of normal hematopoietic stem cells. Introduction of hTERT into K562, a telomerase-positive immortal cell line, resulted in a 2.5-fold elevation of telomerase activity and the lengthening of telomeres by 6 kb to 23 kb. Real-time fluorescent PCR, which could perform quantitative analysis of transcripts, revealed a 175-fold increase in hTERT expression, suggesting the posttranscriptional regulation of telomerase. Ectopic expression of hTERT in K562 cells showed a survival advantage during culture in the absence of serum. Expression of mRNA for the telomeric-repeat binding factor 1 (TRF1) and caspase-3 activity were both decreased in hTERT-transfected K562 cells. Transduced cells retained their usual phenotypic characteristics, differentiation ability, and signal transduction response to TPA. These data suggest that ectopic expression of hTERT by normal hematopoietic stem cells may confer a survival advantage without changing their innate biological characteristics.

Telomerase activity and telomerase subunit gene expression levels are not related in prostate cancer: a real-time quantification and in situ hybridization study

Because the mechanisms of telomerase activation in prostate cancer are mainly unknown, we investigated the relationships between telomerase activity and expression levels of human telomerase RNA (hTR) and human telomerase reverse transcriptase (hTERT) mRNA in benign and malignant alterations of the human prostate gland. Using the LightCycler technology, hTERT mRNA expression was quantified in 46 radical prostatectomy and 10 benign prostatic hyperplasia (BPH) cases; hTR expression was quantified in a subset of these tissue samples. Telomerase activity was measured using a quantitative telomeric repeat amplification protocol ELISA assay. Similar to hTR, which was expressed in all tissue samples tested, hTERT mRNA was detected in 98% of the prostate cancer samples and in 30% of the BPH samples. Regarding clinicopathologic variables, telomerase activity was significantly correlated with Gleason score (<7 vs > or =7, p = 0.02). No relationships emerged between normalized hTR or hTERT expression levels and tumor stage, Gleason score, lymph node status, or preoperative serum prostate-specific antigen. Remarkably, one third of all cancer and BPH tissue samples with hTR and hTERT expression lack telomerase activity. Quantitative analyses contradict the assumption that a certain threshold level of hTR or hTERT mRNA is required for telomerase activation, thus indicating that telomerase regulation in prostate cancer occurs more likely on a posttranscriptional level. Nevertheless, the observation that hTR and hTERT mRNA levels are significantly (p < 0.002) correlated suggests some common mechanisms in the up-regulation of hTR and hTERT expression. Because in situ hybridization revealed strong hTERT expression in all cells of the tumor glands but also in high-grade prostatic intraepithelial neoplasia foci, this up-regulation seems to occur early in prostate carcinogenesis.

Elevated telomerase activity, c-MYC-, and hTERT mRNA expression: association with tumour progression in malignant lipomatous tumours

The role of telomerase activity in tumour progression of liposarcomas is not well understood. Therefore, we investigated 72 liposarcomas of different histological subtypes for an association between telomeric lengths and telomerase activity, and assessed the association between the catalytic subunit human telomerase reverse transcriptase (hTERT) and its activator c-MYC. Telomeric repeat fragment lengths were determined using radioactive DNA-fingerprint analysis with the telomere-specific probe (TTAGGG)(3), whereas telomerase activity was ascertained using the non-radioactive TRAP-assay. To evaluate the expression of hTERT and c-MYC, we applied real-time RT-PCR using a LightCycler. Eight tumours were investigated by microdissection. The MIB1-proliferation index and hTERT and c-MYC protein expression were determined immunohistochemically. Genetic alterations showed a high degree of tumour specificity. Highly malignant myxoid/round cell liposarcomas showed the longest telomeres, the strongest telomerase activity, and the highest hTERT and c-MYC expression levels compared with the pure myxoid variants (p < 0.001), which are of low malignancy. Pleomorphic liposarcoma was characterized by zero or low hTERT and c-MYC expression and telomerase activity, but long telomeres, underlining their different pathogenetic pathway. Elevated gene expression was accompanied by protein immunopositivity. MIB1-proliferation index did not correlate with other molecular markers. We conclude that hTERT and c-MYC expression are associated with telomerase activity in liposarcomas. Elevated hTERT and c-MYC expression as well as high telomerase activity play a role in the tumour progression of this sarcoma type. Nevertheless, each histological subtype of liposarcomas is defined by a specific molecular pattern. Telomerase activation is the most common pathway in liposarcomas maintaining telomeric length.

Quantitation of telomerase components and hTERT mRNA splicing patterns in immortal human cells

Telomerase is a reverse transcriptase that adds telomeric repeats to chromosomal ends. In most normal human somatic cells, telomerase is repressed and telomeres progressively shorten, leading to limited proliferative life-span. Telomerase reactivation is associated with cellular immortalization and is a frequent event during tumorigenesis. The telomerase ribonucleoprotein complex consists of two essential components, a catalytic protein subunit [human telomerase reverse transcriptase (hTERT)] and a template RNA (hTR). hTR is constitutively expressed, while hTERT is almost universally absent in telomerase-negative cells. Although repression of telomerase is transcriptional in telomerase-negative cells, post-transcriptional and assembly processes are likely to play important roles in regulating telomerase activity in those that are telomerase-positive. The telomerase transcript can also be alternatively spliced into a variety of non-functional forms. To establish the quantitative relationships between telomerase activity and its various components, we determined the numbers of molecules of hTR and hTERT mRNA, and the levels of alternatively spliced hTERT mRNA variants in normal, in vitro immortalized and cancer cell lines. We report here that there is surprisingly little variation in the proportion of alternatively spliced forms of hTERT in different cell lines. The only variation observed occurred when a change in splicing to non-functional forms appeared in response to conditions that repress telomerase activity in IDH4 cells. We also found that most telomerase-positive cell lines only contain a few molecules of potentially functional hTERT mRNA, and there is a correlation between telomerase activity and the levels of both hTR and hTERT +alpha+beta mRNA.

Telomerase can inhibit the recombination-based pathway of telomere maintenance in human cells

Telomere length can be maintained by telomerase or by a recombination-based pathway. Because individual telomeres in cells using the recombination-based pathway of telomere maintenance appear to periodically become extremely short, cells using this pathway to maintain telomeres may be faced with a continuous state of crisis. We expressed telomerase in a human cell line that uses the recombination-based pathway of telomere maintenance to test whether telomerase would prevent telomeres from becoming critically short and examine the effects that this might have on the recombination-based pathway of telomere maintenance. In these cells, telomerase maintains the length of the shortest telomeres. In some cases, the long heterogeneous telomeres are completely lost, and the cells now permanently contain short telomeres after only 40 population doublings. This corresponds to a telomere reduction rate of 500 base pairs/population doubling, a rate that is much faster than expected for normal telomere shortening but is consistent with the rapid telomere deletion events observed in cells using the recombination-based pathway of telomere maintenance (Murnane, J. P., Sabatier, L., Marder, B. A., and Morgan, W. F. (1994) EMBO J. 13, 4953-4962). We also observed reductions in the fraction of cells containing alternative lengthening of telomere-associated promyelocytic leukemia bodies and extrachromosomal telomere repeats; however, no alterations in the rate of sister chromatid exchange were observed. Our results demonstrate that human cells using the recombination-based pathway of telomere maintenance retain factors required for telomerase to maintain telomeres and that once the telomerase-based pathway of telomere length regulation is engaged, recombination-based elongation of telomeres can be functionally inhibited.

Telomere position effect in human cells

In yeast, telomere position effect (TPE) results in the reversible silencing of genes near telomeres. Here we demonstrate the presence of TPE in human cells. HeLa clones containing a luciferase reporter adjacent to a newly formed telomere express 10 times less luciferase than do control clones generated by random integration. Luciferase expression is restored by trichostatin A, a histone deacetylase inhibitor. Overexpression of a human telomerase reverse transcriptase complementary DNA results in telomere elongation and an additional 2- to 10-fold decrease in expression in telomeric clones but not control clones. The dependence of TPE on telomere length provides a mechanism for the modification of gene expression throughout the replicative life-span of human cells.