Applications of telomerase research in the fight against cancer

Telomerase therapeutics for cancer: challenges and new directions

It has been approximately a decade since telomerase was described as an almost universal marker for human cancer. Most human tumours not only express telomerase but also have very short telomeres, whereas telomerase activity is either reduced or absent in normal tissues, making the inhibition of telomerase an attractive target for cancer therapeutics. Here we review the current status of telomerase therapeutics and discuss future opportunities and challenges for telomerase research, including a possible relationship with cancer stem cells that could be a source of chemo-/radioresistance development in many advanced cancers.

Real-time PCR hTERT mRNA pattern in tumor core, edge, resection margin, and lymph nodes in laryngeal tumors: relation to proliferative index and impact on prognosis

BACKGROUND

The immortalizing enzyme, telomerase, has been linked to carcinogenesis and is being targeted as a novel molecular marker. This study investigated the pattern of human telomerase reverse transcriptase (hTERT) mRNA by real-time polymerase chain reaction (PCR) in tumor core, edge, and safety margin of laryngeal tumors and related lymph nodes, as well as their relation to the proliferative index (PI) and their impact on prognosis.

MATERIAL AND METHODS

This prospective study included 35 patients with laryngeal carcinoma who were surgically treated by total or partial laryngectomy. Collected tissues from tumor core, edge, resection margin, and lymph nodes were examined by flow cytometry and real-time PCR quantification of hTERT mRNA.

RESULTS

The PI showed a statistically significant difference between the contrasted parts (P = 0.04). The highest median for PI was found in the tumor edge samples (31.3), followed by the tumor core (27.5), the resection margin (21.4), and finally the lymph nodes (10.3). There was no significant difference with regard to the ploidy patterns or the synthetic phase fraction (SPF) among the different tissue parts. Real-time PCR hTERT mRNA patterns showed that the highest median for hTERT mRNA level (copies/mug RNA) was in the tumor core (70.5), followed by the tumor edge (36.5), the resection margin (17.3), and finally the lymph nodes (4.4). There was a statistically significant difference with regard to the hTERT mRNA patterns among the different tissue parts (P = 0.04). Tumor differentiation, tumor edge ploidy, and tumor edge hTERT levels were significantly different between survivors and non-survivors, while tumor core S-phase fraction and tumor core PI significantly altered the disease-free survival rate.

CONCLUSION

hTERT mRNA may potentially be used for molecular reevaluation of the safety margin for conservational laryngeal surgery, and as a prognostic factor for overall survival especially if determined in samples taken from the growing edge of tumors.

Prognostic value of telomerase and DNA ploidy in laryngeal carcinoma

Prognosis of carcinoma of the larynx is currently based on the morphologic analysis of the tumor extent, differentiation grading and type of microscopic invasion. The DNA ploidy status and the cell proliferation activity may give complementary information about the prognosis. Telomerase may be used as a measure of tumor proliferation. The aim of this work was to investigate the probable application of telomerase as a measure of cell immortalization as well as DNA ploidy as a measure of cell proliferation in the prognosis of patients with laryngeal carcinoma. The work was carried out as a prospective observational study that included 35 patients with laryngeal carcinoma who were treated surgically at Ain Shams University Hospitals, Cairo, Egypt. Flowcytometric analysis and biochemical assay of telomerase using human Telomerase Reverse Transcriptase (hTERT) mRNA assay by RT-PCR in resected specimens were done. The proliferative indices in different parts of the tumor were found to be significantly different from each other, with those of tumor edges being the highest. The ploidy status at the edge of the tumor was significantly different between surviving and non-surviving patients, while the tumor core proliferative index was found to be significantly lower in patients without recurrence than those with recurrence. A gradient of telomerase levels was noticed, with levels from tumor cores being the highest followed by tumor edges, resection margins and the lowest levels in lymph nodes. Multivariate analysis showed hTERT levels in tumor edges to significantly affect survival. Both Flowcytometric parameters and telomerase activity may be useful as a prognostic factor for survival in patients with laryngeal carcinoma. Telomerase activity may be used for biologic demarcation for conservational laryngeal surgery, as well as a 'spectral' marker.

Application of Targeted Radiotherapy/Gene Therapy to Bladder Cancer Cell Lines

OBJECTIVES

A targeted radiotherapy/gene therapy strategy for transitional cell carcinoma of bladder is described, using [131I]meta-iodobenzylguanidine ([131I]MIBG), a radionuclide combined with a tumour-seeking drug. The aim is to decrease side effects from radiation toxicity, while increasing radiation dose to tumour. This tumour cell kill approach is augmented by radiological bystander effects.

METHODS

The bladder cancer cell line EJ138 was transfected with a gene encoding the noradrenaline transporter (NAT) under the control of tumour-specific telomerase promoters. Resulting uptake of [131I]MIBG was assessed by gamma-counting of cell lysates, and NAT transgene expression by real-time RT-PCR. Cell kill of monolayers and disaggregated spheroids, dosed with [131I]MIBG, was assessed by clonogenic assay.

RESULTS

NAT gene transfected cells exhibited a significantly increased active uptake of [131I]MIBG, leading to dose-dependent cell kill. Clonogenic assay of disaggregated spheroids, a three-dimensional model, suggested cell kill via bystander effects.

CONCLUSIONS

Expression of a functional NAT after in vitro transfection of bladder cancer cells with the NAT gene under the control of telomerase promoters leads to active uptake of [131I]MIBG and dose-dependent cell kill. This strategy could produce a promising new treatment option for bladder cancer.

Combining a targeted radiotherapy and gene therapy approach for adenocarcinoma of prostate

A targeted radiotherapy/gene therapy approach for prostate cancer, using the radiopharmaceutical [(131)I]meta-iodobenzylguanidine ([(131)I]MIBG), would restrict the effects of radiotherapy to malignant cells, thereby increasing efficacy and decreasing morbidity of radiotherapy. Prostate cancer cells were transfected with a transgene encoding the noradrenaline transporter (NAT) under the control of tumour-specific telomerase promoters, enabling them to actively take up [(131)I]MIBG. This led to tumour-specific cell kill. This strategy has the advantage of generating a radiological bystander effect, leading to the destruction of neighbouring tumour cells that have escaped transfection. This targeted approach could be a promising tumour-specific treatment option for prostate cancer.

Real-time quantitative RT-PCR for human telomere elongation reverse transcriptase in chronic myeloid leukemia

Telomeres cap chromosome ends and are pivotal for DNA stability. Deregulation of the telomere stabilising enzyme telomerase in malignancy has implications in diagnosis, prognosis and therapeutics of cancer. Quantification of the expression of the telomerase catalytic subunit, hTERT, using the LightCycler TeloTAGGG hTERT Quantification kit is not optimal for analysis of chronic myeloid leukemia (CML) samples. The internal control, porphobilinogen deaminase (PBGD) is amplified in a separate tube to hTERT and has an unstable genomic localisation of 11q23. Our laboratory thus developed a real-time reverse transcriptase polymerase chain reaction which co-amplifies hTERT and either mitochondrial single-stranded DNA binding protein 1 (ssBP1) or ubiquitin C (UBC).

Correlation between polyamines and apoptosis among Egyptian breast cancer patients

OBJECTIVES

The polyamines putrescine, spermidine, and spermine, play an important role in cell proliferation, differentiation, and transformation. The aim of this study is to correlate the polyamines with apoptosis and clinico-pathologic events in Egyptian breast cancer patients.

METHODS

PUT, SPD, and SPN were investigated using thin layer chromatography (TLC) and apoptosis in fresh frozen tissue specimens obtained from 40 patients suffering from breast cancer, as well as 20 patients with benign breast lesions.

RESULTS

The levels of PUT, SPD, and SPN were higher in breast cancer tissues than in benign breast lesions (p < 0.001). Polyamines were correlated well with apoptosis. Moreover, PUT was an independent prognostic factor for relapse. Also, SPD and SPN correlated significantly with early tumor grades. ROC curves were used to choose the best cut-off values for polyamines (70, 135, and 290 mmol/g tissue) for PUT, SPD, and SPN, respectively. At these cut-off values, the sensitivities were (75%, 60%, and 70%), and the specificities were (80%, 95%, and 95%) for PUT, SPD, and SPN, respectively.

CONCLUSION

Polyamines may be used as additional markers for detection of malignant transformation in breast tissue. Moreover, because of their ability to induce apoptosis in malignant tissues, polyamines are suitable targets for therapeutic intervention that is specifically directed to induce apoptosis.

Telomerase: a target for cancer therapeutics

The continuous growth of advanced malignancies almost universally correlates with the reactivation of telomerase. While there is still a great deal of basic and applied research to be done, telomerase remains a very attractive novel target for cancer therapeutics. In this review, we will discuss the challenges and the pros and cons of the most promising antitelomerase approaches currently being investigated.

Nucleolar localization of hTERT protein is associated with telomerase function

Telomerase is a ribonucleoprotein (RNP) complex that prevents telomeric erosion in eukaryotic cells. Although there are also other associated proteins in this complex, the catalytic activity of this complex is composed of two components. One is a reverse transcriptase subunit, TERT (telomerase reverse transcriptase); another is an RNA template subunit, TR (telomerase RNA). However, where these two parts are assembled in mammalian cells is unclear. In the present study, we investigated the intracellular distribution of human TERT (hTERT) protein and observed that hTERT protein in individual cells could concentrate in or be excluded from the nucleolus. Further we have identified a nucleolar targeting signal in the hTERT protein. Point mutations that disrupted this signal region interrupted telomerase RNP complex formation, decreased telomerase activity, and caused telomere shortening in cells transfected with mutated hTERT. Our results indicate that the amino acid sequence of the extreme N-terminus (1-15) of hTERT, which targets nucleolar localization of the protein, is required for full telomerase function.

Telomerase: biological function and potential role in cancer management

Telomeres are the specialized ends of eukaryotic chromosomes, thought to have many functions, most importantly serving as a clock signaling entry into cellular senescence. These structures are maintained by the reverse transcriptase telomerase, a peculiar enzyme in both structure, since it contains its own template RNA and function, since it is inactivated in most normal tissues but activated in the vast majority of malignant tumors. These features have made telomerase a subject of intense investigation, both to understand its cellular role and regulation and to exploit its activation in cancer to develop drugs or diagnostic methods based on telomerase. This work gathers all the information currently available in the biological and clinical fields of telomerase research.

Telomerase in endocrine and endocrine-dependent tumors

Telomerase, the ribonucleoprotein enzyme that elongates chromosomal ends, or telomeres, is repressed in most normal somatic cells but reactivated in transformed cells to compensate for the progressive erosion of the telomeres during cell divisions. In accordance with this hypothesis, the presence of telomerase activity has been reported in more than 90% of human cancers, whereas most normal tissues or benign tumors contain low or undetectable telomerase activity. Reactivation of telomerase has also been widely reported in endocrine neoplasms and in hormone-related cancers. In the present study, we review the most recent publications on telomerase in these types of tumors. The hormonal regulation of telomerase activity and the possible strategies for cancer therapy based on the inhibition of telomerase has also been discussed.

Lack of tumor recognition by hTERT peptide 540-548-specific CD8(+) T cells from melanoma patients reveals inefficient antigen processing

Telomerase is a ribonucleoprotein complex responsible for the maintenance of the length of the telomeres during cell division, which is active in germ-line cells as well as in the vast majority of tumors but not in most normal tissues. The wide expression of the human telomerase catalytic subunit (hTERT) in tumors makes it an interesting candidate vaccine for cancer. hTERT-derived peptide 540-548 (hTERT(540)) has been recently shown to be recognized in an HLA-A*0201-restricted fashion by T cell lines derived from peptide-stimulated peripheral blood mononuclear cells (PBMC) from healthy donors. As a first step to the inclusion of this peptide in immunotherapy clinical trials, it is crucial to assess hTERT(540)-specific T cell reactivity in cancer patients as well as the ability of hTERT-specific CD8(+) T lymphocytes to recognize and lyse hTERT-expressing target cells. Here, we have analyzed the CD8(+) T cell response to peptide hTERT(540) in HLA-A*0201 melanoma patients by using fluorescent HLA-A*0201/hTERT(540) peptide tetramers. HLA-A*0201/hTERT(540) tetramer(+) CD8(+) T cells were readily detected in peptide-stimulated PBMC from a significant proportion of patients and could be isolated by tetramer-guided cell sorting. hTERT(540)-specific CD8(+) T cells were able to specifically recognize HLA-A*0201 cells either pulsed with peptide or transiently transfected with a minigene encoding the minimal epitope. In contrast, they failed to recognize hTERT-expressing HLA-A*0201(+) target cells. Furthermore, in vitro proteasome digestion studies revealed inadequate hTERT processing. Altogether, these results raise questions on the use of hTERT(540) peptide for cancer immunotherapy.

Telomerase activity, and tissue polypeptide specific antigen (TPS) in Egyptian breast cancer patients

OBJECTIVES

Breast cancer is the most common malignancy among Egyptian women. The aim of this study is to evaluate the role of both telomerase and TPS estimation in assessment of breast cancer.

METHODS

The study included 40 patients with breast cancer, and 20 patients with benign breast diseases. Telomerase activity in breast tissues was assessed using TRAP assay. TPS was measured in sera of the patients by ELISA.

RESULTS

Telomerase positivity was 15% in benign group vs. 60% in malignant group (p = 0.0009). It was significantly correlated to stage, and lymph node status (p < 0.02). Telomerase positivity showed significant correlation to tumor recurrence (p = 0.0076) in a follow-up period of 36 months. Mean rank of TPS was significantly higher in malignant than benign groups (p < 0.001), and in telomerase positive than telomerase negative patients (p < 0.001). In malignant group, mean rank of TPS was significantly higher in late stages (p < 0.002), in higher grade (p < 0.05), in larger tumor size (p < 0.01), and in lymph node positive patients (p < 0.001). ROC curve was utilized to choose the best cutoff for serum TPS (88 U/L). At this cutoff, the sensitivity was 95%, and the specificity was 75%. At a higher cutoff (109 U/L), TPS positivity was significantly correlated to stage, grade, lymph node status, and telomerase positivity (p < 0.05).

CONCLUSION

Telomerase positivity and serum TPS might be used as additional markers for assessment of breast cancer.

Inhibition of telomerase by 2'-O-(2-methoxyethyl) RNA oligomers: effect of length, phosphorothioate substitution and time inside cells

2'-O-(2-methoxyethyl) (2'-MOE) RNA possesses favorable pharmocokinetic properties that make it a promising option for the design of oligonucleotide drugs. Telomerase is a ribonucleoprotein that is up-regulated in many types of cancer, but its potential as a target for chemotherapy awaits the development of potent and selective inhibitors. Here we report inhibition of human telomerase by 2'-MOE RNA oligomers that are complementary to the RNA template region. Fully complementary oligomers inhibited telomerase in a cell extract with IC(50) values of 5-10 nM at 37 degrees C. IC(50) values for mismatch-containing oligomers varied with length and phosphorothioate substitution. After introduction into DU 145 prostate cancer cells inhibition of telomerase activity persisted for up to 7 days, equivalent to six population doublings. Inside cells discrimination between complementary and mismatch-containing oligomers increased over time. Our results reveal two oligomers as especially promising candidates for initiation of in vivo preclinical trials and emphasize that conclusions regarding oligonucleotide efficacy and specificity in cell extracts do not necessarily offer accurate predictions of activity inside cells.

Telomerase, immortality and cancer

Replication of eukaryotic linear chromosomes is incomplete and leaves terminal gaps. The evolutionary widely distributed solution to this "end replication" is twofold: chromosome ends are capped with telomeres, bearing multiple copies of redundant telomeric sequences, and the telomerase enzyme can add (lost) telomeric repeats. Telomerase in humans, as in all mammals, is ubiquitous in all embryonic tissues. In adults, telomerase remains active in germs cells, and, although down-regulated in most somatic tissues, telomerase is active in regenerative tissues and notably, in tumor cells. Telomerase activity is linked to cellular proliferation, and its activation seems to be a mandatory step in carcinogenesis. In contrast to mammals, indeterminately growing multicellular organisms, like fish and crustaceae, maintain unlimited growth potential or 'immortality' in all somatic tissues throughout their entire life. Also this cell immortalization is brought about by maintaining telomerase expression. Disease prognosis for human tumors includes evaluation of cell proliferation, based on the detection of proliferation markers with monoclonal antibodies. The significance of the classical marker Ki-67, and of a novel marker repp-86 are compared with semiquantitative telomerase assays. For tumor therapy, telomerase inhibitors are attractive tools. Results with telomerase knock-out mice have revealed promise, but also risk of this approach. On the other side, telomerase stimulation is attractive for expanding the potential of cellular proliferation in vitro, with possible applications for transplantation of in vitro expanded human cells, for immortalizing primary human cells as improved tissue models, and for the isolation of otherwise intractable products, like genuine human monoclonal antibodies.

Telomeric repeat amplification, without shortening or lengthening of the telomerase products: a method to analyze the processivity of telomerase enzyme

The Telomeric Repeat Amplification Protocol (TRAP) and its modified versions (including ours, TP-TRAP) change the size and/or the ratio of the telomerase products in the amplification stage of the assay. Based on our recently published method we developed a new TRAP. This method ensures that the number of telomeric repeats present in the original telomerase products does not change on PCR amplification. The usefulness of the method was proved with amplification of chemically synthesized telomerase products and a newly designed telomerase substrate oligonucleotide. This is the first report in which the PCR products directly reflect the size distribution of telomerase products generated by the enzyme.

Applications of biochip and microarray systems in pharmacogenomics

A DNA microarray system is usually comprised of DNA probes formatted on a microscale on a glass surface (chip), plus the instruments needed to handle samples (automated robotics), to read the reporter molecules (scanners) and analyse the data (bioinformatic tools). Biochips are formed by in situ (on chip) synthesis of oligonucleotides or peptide nucleic acids (PNAs) or spotting of DNA fragments. Hybridisation of RNA- or DNA-derived samples on chips allows the monitoring of expression of mRNAs or the occurrence of polymorphisms in genomic DNA. Basic types of DNA chips are the sequencing chip, the expression chip and chips for comparative genomic hybridisation. Advanced technologies used in automated microarray production are photolithography, mechanical microspotting and ink jets. Bioelectronic microchips contain numerous electronically active microelectrodes with specific DNA capture probes linked to the electrodes through molecular wires. Several biosensors have been used in combination with biochips. PNA biosensors commonly rely on the immobilisation of a single-stranded DNA sequence (the 'probe') onto a transducer surface for hybridisation with the complementary ('target') strand to give a suitable electrical signal. Other sensors are cell-based immunobiosensors with engineered molecular recognition, integrated biosensors based on phototransistor integrated circuits and sensors based on surface plasmon resonance. Microarray technologies offer enormous savings in time and labour as compared to standard gel-based microsatellite methods. Reading of the information and its management by bioinformatics is necessary because of the enormous amount of data generated by the various technologies using microarrays. Standardised procedures are essential for compatible data production, quality control and analysis. Expression monitoring is the most biologically informative application of this technology at present. Microarray technology has important applications in pharmacogenomics: drug discovery and development, drug safety and molecular diagnostics. DNA chips will facilitate the integration of diagnosis and therapeutics, as well as the introduction of personalised medicines.

Role of telomerase in normal and cancer cells

Shortening of the telomeric DNA at chromosome ends is postulated to limit the lifespan of human cells. In contrast, activation of telomerase, the enzyme that synthesizes telomeric DNA, is proposed to be an essential step in cancer cell immortalization and cancer progression. This review discusses the structure and function of telomeres and telomerase, the role of telomerase in cell immortalization, and the effects of telomerase inactivation on normal and cancer cells. Moreover, data on the experimental use of telomerase assays for cancer detection and diagnosis are reviewed. Finally, the review considers the evidence regarding whether telomerase inhibitors could be used to treat human cancers.

Clinical applications of telomerase in cancer treatment

Telomerase activity has been found in most cancer cells, but not in the majority of normal differentiated tissues. Therefore, telomerase has been considered a relatively selective and widely expressed tumor marker to be used as a diagnostic tool, and in some cases, as a potential prognostic indicator. Telomerase activity can also be used to evaluate chemosensitivity of neoplastic cells obtained from cancer patients, by measuring residual telomerase activity after drug treatment. Finally, telomerase has been considered to represent a suitable target for designing new anticancer strategies. This review focuses on present and future clinical applications of telomerase studies in cancer management. Copyright 2000 Harcourt Publishers Ltd.

Telomerase repeat amplification protocol (TRAP): A new molecular marker for parathyroid carcinoma

Telomerase results to be active in human germ, stem cells, several malignant cell tumors and in immortalized cell lines. In order to investigate if molecular mechanisms other than Rb gene inactivation can be helpful to diagnose malignancy of parathyroid tumors, we decided to investigate the presence of active telomerase in homogenates from different pathological parathyroid tissues (hyperplastic, adenomatous, carcinomatous, and normal) and primary cell cultures. The TRAP assay was performed to detect this activity in histologically characterized normal, hyperplastic, adenomatous, and carcinomatous human parathyroid tissues, primary cell lines, and one metastatic tissue from parathyroid carcinoma. Only malignant parathyroid glands and the metastatic tissue were TRAP positive. Our findings suggest that telomerase expression could represent an important molecular mechanism underlying the acquisition and progression of an aggressive phenotype of epithelial parathyroid cells and it may help to predict their malignant potential. The TRAP assay is easy to perform and it could become an additional tool to be included in the harmamentarium for the molecular diagnosis of parathyroid carcinoma.