The implications of telomerase biochemistry for human disease

Quantification of telomerase activity in normal oral mucosal tissue and oral squamous cell carcinoma

BACKGROUND AND OBJECTIVE

The role of telomeres and telomerase in oral cancer is an area of much recent interest. The understanding of the role of telomere biology, the end replication problem leading to genomic instability and the reactivation of telomerase, is absolutely critical to our understanding of oral cancer, and more so, to our ability of early diagnosis and developing novel therapies and cancer prevention approaches. The aim of the present study was to quantify telomerase activity (TA) in oral squamous cell carcinoma (OSCC) and normal oral mucosa and assess the role of telomerase as diagnostic and prognostic marker of oral malignancy.

MATERIALS AND METHODS

We quantified TA in 45 patients with OSCC and 20 normal oral mucosal specimens using polymerase chain reaction-based telomeric repeat amplification protocol assay and compared it with the clinical status and grade of malignancy.

RESULTS

TA was detected in 89% of malignant and 5% of normal oral mucosal tissue. The TA levels ranged from 0.28 to 6.91 (mean 2.05, standard deviation [SD] 1.33) in OSCC and 0.21 to 1.09 (mean 0.54, SD 0.27) in normal oral mucosa. There was no relationship between TA levels and clinical stages, site of the lesion, history of adverse habits, or sex of the patient. However, under the WHO classification, there were significant differences (P < 0.00) between Grades I, II, and III. Furthermore, increasing age of the patient significantly correlated with TA.

INTERPRETATION AND CONCLUSION

The results of the present study indicate that activation of TA is frequent in OSCC. Statistically significant difference in quantified telomerase levels of OSCC and normal oral mucosa (P < 0.00) demonstrates the significant clinical usefulness of telomerase activation as a valuable marker for diagnosis while significant correlation of TA with grades of malignancy indicates its effectiveness as marker for prognosis of OSCC.

Impaired telomerase activity hinders proliferation and in vitro transformation of Penaeus monodon lymphoid cells

Retaining terminal transferase activity of telomerase, the ribonucleoprotein enzyme which add telomeric repeats on chromosome end is thought to be required to prevent cellular ageing. Additionally, telomerase considered as a marker for cell proliferation and immortalization in eukaryotes. We examined telomerase activity in tissues and lymphoid cell culture of Penaeus monodon. Along with telomerase activity, telomere repeats and an attempt on identification of telomerase reverse transcriptase (PmTERT) were made. Telomeric repeat amplification protocol revealed that telomerase-dependent telomeric lengthening has been taking place in P. monodon and the adult tissues were retaining this capacity throughout their lifespan with the highest activity in ovary, testis and lymphoid organ. However, telomerase activity could not be detected in lymphoid cells in culture. The canonical telomeric repeats added by telomerase of lymphoid tissue extract were identified as TTAGG, but pentameric repeats GGTTA and AGGTT were also added by the telomerase. PmTERT protein sequence (partial) shared 100 % identity with the TERT sequence of Daphnia pulex, 27 % sequence identity with Purple sea urchin and 24-25 % with Zebra fish. Undetectable telomerase activity in lymphoid cell culture supports the hypothesis that the inadequate telomerase activity or gene expression may be a reason that prevents neoplastic transformation and spontaneous immortalization of the cells in vitro. Thus, it is envisaged that telomerase activation in lymphoid cells may surmount cellular ageing for in vitro transformation and cell line establishment.

Quantum dots thermal stability improves simultaneous phenotype-specific telomere length measurement by FISH-flow cytometry

Telomere length analysis has been greatly simplified by the quantitative flow cytometry technique FISH-flow. In this method, a fluorescein-labeled synthetic oligonucleotide complementary to the telomere terminal repeat sequence is hybridized to the telomere sequence and the resulting fluorescence measured by flow cytometry. This technique has supplanted the traditional laborious Southern blot telomere length measurement techniques in many laboratories, and allows single cell analysis of telomere length in high-throughput sample formats. Nevertheless, the harsh conditions required for telomere probe annealing (82 degrees C) has made it difficult to successfully combine this technique with simultaneous immunolabeling. Most traditional organic fluorescent probes (i.e. fluorescein, phycoerythrin, etc.) have limited thermal stability and do not survive the high temperature annealing process, despite efforts to covalently crosslink the antigen-antibody-fluorophore complex. This loss of probe fluorescence has made it difficult to measure FISH-flow in complex lymphocyte populations, and has generally forced investigators to use fluorescent-activated cell sorting to pre-separate their populations, a laborious technique that requires prohibitively large numbers of cells. In this study, we have substituted quantum dots (nanoparticles) for traditional fluorophores in FISH-flow. Quantum dots were demonstrated to possess much greater thermal stability than traditional low molecular weight and phycobiliprotein fluorophores. Quantum dot antibody conjugates directed against monocyte and T cell antigens were found to retain most of their fluorescence following the high temperature annealing step, allowing simultaneous fluorescent immunophenotyping and telomere length measurement. Since quantum dots have very narrow emission bandwidths, we were able to analyze multiple quantum dot antibody conjugates (Qdot 605, 655 and 705) simultaneously with FISH-flow measurement to assess the age-associated decline in telomere length in both human monocytes and T cell subsets. With quantum dot immunolabeling, the mean decrease rate in telomere length for CD4+ cells was calculated at 41.8 bp/year, very close to previously reported values using traditional flow-FISH and Southern blotting. This modification to the traditional flow-FISH technique should therefore allow simultaneous fluorescent immunophenotyping and telomere length measurement, permitting complex cell subset-specific analysis in small numbers of cells without the requirement for prior cell sorting.

The telomere-telomerase axis and the heart

The preservation of myocyte number and cardiac mass throughout life is dependent on the balance between cell death and cell division. Rapidly emerging evidence indicates that new myocytes can be formed through the activation and differentiation of resident cardiac progenitor cells. The critical issue is the identification of mechanisms that define the aging of cardiac progenitor cells and, ultimately, their inability to replace dying myocytes. The most reliable marker of cellular senescence is the modification of the telomere-telomerase axis, together with the expression of the cell cycle inhibitors p16INK4a and p53. Cellular senescence is characterized by biochemical events that occur within the cell. In this regard, one of the most relevant processes is represented by repeated oxidative stress that may evolve into the activation of the cell death program or result in the development of a senescent phenotype. Thus, the modulation of telomerase activity and the control of telomeric length, together with the attenuation of the formation of reactive oxygen species, may represent important therapeutic tools in regenerative medicine and in prevention of aging and diabetic cardiomyopathies.

Telomerase as an independent prognostic factor in head and neck squamous cell carcinoma

BACKGROUND

Telomerase activity has been found to be associated with many cancers, including head and neck squamous cell carcinoma (HNSCC). We examined the association of telomerase activity with the clinical outcome of patients with HNSCC.

METHODS

A PCR-based enzyme immunoassay method was used to measure telomerase activity in 217 matched (grossly normal and cancerous) tissues from patients with HNSCC. Pearson chi-square test was used to analyze the correlation of telomerase activity with clinicopathologic parameters. Kaplan-Meier method and Cox logistic regression model were used for prognostic analysis.

RESULTS

Of the 217 tissues assayed, 4.1% of the normal and 63.3% of the cancer tissues had high levels of telomerase activity. Telomerase activity was shown to be statistically correlated with extracapsule spreading (ECS) of lymph nodes (p =.005) and overall survival (p =.003). On multivariant analysis, overall stage (p =.007), tumor depth (p =.045), and telomerase activity (p =.008) showed independent variables associated with poor survival.

CONCLUSIONS

Telomerase activity has been shown to be an independent prognostic factor for survival in cases of HNSCC. Telomerase may be a potential molecular target for clinical use in prognostication and therapy in cases of the disease.

Replicative mosaicism might explain the seeming contradictions in the telomere theory of aging

All living organisms are regarded as a mosaic of cells with different replicative histories explaining all the contradictions in the telomere theory of aging. Exhausted proliferative potential of cells in some areas of the organ tissue might be sufficient to promote one of the age-dependent diseases. Thus a combination of these disorders, gradually increasing with age, is aging. Nobody dies because of the age. Nothing other than the age-related diseases occur with age, and if we separate these diseases, we will get not just a healthy old man, but according to Dr Hayflick's new hypothesis (Exp. Gerontol., 33 (1998) 639) a healthy young man, if not a newborn child.

Telomerase activity is frequently found in metaplastic and malignant human nasopharyngeal tissues

Telomerase is a specialized ribonucleoprotein polymerase that directs the synthesis of telomere repeats at chromosome ends. Accumulating evidence has indicated that telomerase is stringently repressed in normal human somatic tissues but reactivated in cancers and immortal cells, suggesting that reactivation of telomerase plays an important role in carcinogenesis. In this study, the status of telomerase activity in diseased human nasopharyngeal lesions was determined by the telomeric repeat amplification protocol (TRAP). Fifty-four patients participated including 17 inflammation or hyperplasia, eight with squamous metaplasia, and 29 with different stages of carcinomas. Telomerase activity was detected in 1 of 17 (5.9%) inflammatory or lymphoid hyperplastic tissues, 3 of 8 (37.5%) squamous metaplastic, and 25 of 29 (86.2%) carcinoma tissues. The differences in telomerase expression in these groups is statistically significant (P < 0.001). Levels of telomerase activity correlated with tumour stage (P = 0.024). These results suggest that telomerase reactivation plays a role in the carcinogenesis of nasopharyngeal cancer. Since telomerase activity is found in the majority of nasopharyngeal cancers and a subset of metaplasia, this enzyme may be served as a reference to monitoring the status of abnormal nasopharyngeal tissues.

The in vitro life-span of human periodontal ligament fibroblasts

The in vitro life-span of human periodontal ligament fibroblasts (PDLF) was studied on clones from periodontium of teeth extracted due to periodontitis and dental caries (69 clones/192 individuals, aged 20-80 years) and from periodontium of teeth extracted for orthodontic reasons (23 clones/26 individuals, aged 15-19 years). In the primary cultures the ratio of the number of cells expressing senescence-associated beta-galactosidase (SA-beta-Gal) to the total number of cells is significantly larger in PDLF (92 clones; 11.1+/-4.9%) than in human gingival fibroblasts (GF) (10 clones; 0.5+/-0.1 %). The finite population doubling numbers (PD) of PDLF are not age-matched and the mean PD of PDLF (7.1+/-2.9) is significantly smaller than GF (28.5+/-3.2), IMR-90 (human lung fibroblasts, 5 clones; 44.3 +/- 2.2), and human osteoblasts (5 clones; 19.7+/-1.4). Comparing the ratio of the number of SA-beta-Gal positive cells to the total number of cells in primary culture, and the finite PD in PDLF cultures: 1) the ratio of 15-19 years old donor group is significantly smaller than in the other donor groups (20-29, 30-39, 40-49, 50-59 and 60-80 years old), and 2) there were no statistically significant differences among the 20-29, 30-39, 40-49 and 50-59 year old donor groups, and the 30-39, 40-49, 50-59 and 60-80 year old donor groups. These findings suggest that the in vitro life-span of PDLF is shorter than other fibroblasts in the connective tissues and that PDLF may undergo senescence in adult clones without relation to donor's age. There may be more aged fibroblasts in periodontium than in other tissues, such as gingiva and lung.

Telomerase targeting in cancer treatment: new developments

Telomerase, a ribonucleoprotein expressed in 85% of advanced cancers but not in most somatic cells, compensates for telomeric DNA erosion and as such stabilizes cell immortality. Telomerase inhibition might restore mortality in tumor cells. Recent progress is illustrated in studies on telomerase and telomere targeting with differentiation induction, reverse transcriptase inhibitors, promoter down regulation, antisense inhibition, and blockage of telomere/telomerase interactions. Also, new developments are described indicating that anti-telomerase treatment can induce apoptosis in tumor cells and can chemosensitize drug-resistant cell lines. Implications of these findings for anti-telomerase-based therapeutic applications, in particular in combination therapies, are discussed. Copyright 1999 Harcourt Publishers Ltd.

Polymerase chain reaction-based enzyme immunoassay for quantitation of telomerase activity: application to colorectal cancers

Telomerase is a specialized reverse transcriptase that synthesizes telomeric sequences onto human chromosomal ends. It appears to be present in the majority of primary human cancer tissues, and may have potential as a universal tumor marker. In this report, we describe a sensitive, non-radioactive, polymerase chain reaction (PCR)-based enzyme immunoassay (EIA) for the quantitation of telomerase activity in human cells. This PCR-EIA is convenient and can be easily completed within 3 h. The correlation coefficient between the results of PCR-EIA and the conventional telomeric repeat amplification protocol (TRAP) method, as measured on 4 different cell lines, was over 0.98. Evaluation of this method for clinical application was conducted with tissues obtained from patients with colorectal cancers and the results were compared with those of the conventional TRAP method. Our data indicate that telomerase activities measured by conventional TRAP and PCR-EIA are highly correlated, and we suggest that the PCR-EIA method can substitute for conventional TRAP.

Cellular senescence and cancer

The proliferative lifespan of normal mammalian cells is limited by intrinsic controls, which desensitize the cell-cycle machinery to extrinsic stimulation after a given number of cell divisions. One underlying clock driving this process of 'replicative senescence' is the progressive erosion of chromosome telomeres, which occurs with each round of DNA replication. This appears to trigger growth inhibition via activation of the tumour suppressor gene (TSG) product, p53, and the consequent up-regulation of the cell-cycle inhibitor p21WAF1. Other inhibitory pathways are also activated (possibly by additional clocks), including the TSG p16INK4a and the less well-defined complementation group genes. Loss of one pathway can be compensated, after a limited extension of lifespan, by further up-regulation of the others, so that to escape mortality a developing tumour must overcome multiple 'proliferative lifespan barriers' (PLBs) by successive genetic events, each conferring a new wave of clonal expansion. This provides one explanation for the existence of multiple genetic abnormalities in human cancers; furthermore, the diversity in the nature and timing of these PLBs between different cell types may explain the variation in the spectrum of abnormalities observed between the corresponding cancers. Even if all senescence pathways are inactivated, immortalization can only be achieved if erosion of telomeres is halted, before their end-protecting function is lost. This usually requires either activation of telomerase during tumour development, if the cell of origin is telomerase-negative, or up-regulation if the normal cell already has some activity, but not enough to prevent erosion. In either case, cancers often maintain near-critical telomere lengths; hence pharmacological inhibition of telomerase remains an attractive approach to the selective killing of tumour cells.

Telomerase in cancer: clinical applications

The biology of telomeres and telomerase has been the subject of intensive investigative effort since it became evident that they play a significant role in two important biological processes, the loss of cellular replicative capacity inherent to organismal ageing and the unrestricted cell proliferation characteristic of carcinogenesis. Telomere shortening in normal cells is a result of DNA replication events, and reduction beyond a critical length is a signal for cellular senescence. One of the cellular mechanisms used to overcome proliferative restriction is the activation of the enzyme telomerase, which replaces the loss of telomeric DNA that occurs at each cell division. Studies have demonstrated that tumours have shorter telomeres than normal tissue and that telomerase is activated in up to 90% of all human cancers while it is present only in a limited range of normal adult tissues. The role of telomerase in the extension of the cellular replicative lifespan has recently been shown by ectopic expression of the enzyme, being consistent with the oncogenesis model whereby the acquisition of an 'immortal' phenotype is a requirement for advanced tumour progression. In this article we review the present knowledge of telomeres and telomerase in cancer and discuss the potential use of this enzyme as a diagnostic and prognostic tumour marker and as a target for cancer therapy.

Reconstitution of human telomerase with the template RNA component hTR and the catalytic protein subunit hTRT

The maintenance of chromosome termini, or telomeres, requires the action of the enzyme telomerase, as conventional DNA polymerases cannot fully replicate the ends of linear molecules. Telomerase is expressed and telomere length is maintained in human germ cells and the great majority of primary human tumours. However, telomerase is not detectable in most normal somatic cells; this corresponds to the gradual telomere loss observed with each cell division. It has been proposed that telomere erosion eventually signals entry into senescence or cell crisis and that activation of telomerase is usually required for immortal cell proliferation. In addition to the human telomerase RNA component (hTR; ref. 11), TR1/TLP1 (refs 12, 13), a protein that is homologous to the p80 protein associated with the Tetrahymena enzyme, has been identified in humans. More recently, the human telomerase reverse transcriptase (hTRT; refs 15, 16), which is homologous to the reverse transcriptase (RT)-like proteins associated with the Euplotes aediculatus (Ea_p123), Saccharomyces cerevisiae (Est2p) and Schizosaccharomyces pombe (5pTrt1) telomerases, has been reported to be a telomerase protein subunit. A catalytic function has been demonstrated for Est2p in the RT-like class but not for p80 or its homologues. We now report that in vitro transcription and translation of hTRT when co-synthesized or mixed with hTR reconstitutes telomerase activity that exhibits enzymatic properties like those of the native enzyme. Single amino-acid changes in conserved telomerase-specific and RT motifs reduce or abolish activity, providing direct evidence that hTRT is the catalytic protein component of telomerase. Normal human diploid cells transiently expressing hTRT possessed telomerase activity, demonstrating that hTRT is the limiting component necessary for restoration of telomerase activity in these cells. The ability to reconstitute telomerase permits further analysis of its biochemical and biological roles in cell aging and carcinogenesis.

Mammalian telomerase: catalytic subunit and knockout mice

For the second time this year random cDNA sequencing, in combination with data from unicellular eukaryotes, has made a significant contribution to the analysis of human telomerase. Two groups have reported mammalian homologues of the Tetrahymena p80 telomerase-associated protein, in both cases the key breakthrough being mammalian cDNA clones with database matches to Tetrahymena p80. This has now been joined by the sequence of a candidate for the human telomerase catalytic subunit. The discovery that its message abundance closely follows telomerase activity could make a major impact on the utility of telomerase as a diagnostic marker for human malignancy. In addition, Blasco et al . report the phenotype of a transgenic mouse deleted for the mTR gene, which encodes the essential RNA component of telomerase. Interestingly tumour formation is unaffected in these mice, strengthening the argument that telomerase expression in mouse tumourigenesis is an innocent bystander rather than a necessary event. However, fundamental differences between the genomic organisation of mouse and human telomeres mean that the mouse is not a straightforward model to critically test the role of telomere loss and telomerase in human malignancy.