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Table of Contents   
SYSTEMATIC REVIEW  
Year : 2017  |  Volume : 28  |  Issue : 5  |  Page : 574-584
Telomerase: An exploration toward the end of cancer


Department of Oral Pathology and Microbiology, Faculty of Dentistry, JMI, New Delhi, India

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Date of Web Publication25-Oct-2017
 

   Abstract 

Background: The distinguishing feature of cancer cells is their ability to proliferate indefinitely, which is in contrast to the restricted cell multiplication potential for somatic cells. A better understanding of this contrasting behavior was provided in the early 1990s with the discovery of a relationship between telomeres, telomerase, aging, and cancer. Telomeres (tandem repeat DNA sequence TTAGGG) are protective caps at the ends of human chromosomes. Normal human cells experience telomere shortening with each successive cell division. However, in tumor cells, an overexpression of telomerase confers limitless replicative potential to tumor cells by continuous elongation of telomeres. The objective of this review was to systematically assess the data available on telomerase expression in oral cancer, with special reference to its role in diagnosis, prognosis, and treatment. Materials and Methods: A systematic review of studies that investigated the telomerase expression in oral squamous cell carcinoma (OSCC) was registered with PROSPERO. Subsequent to registration, a predetermined search strategy in accordance with PRISMA guidelines was formulated, and a literature search was conducted using online databases along with hand searching. Results: Eighty-nine articles from PubMed, 83 from Scopus, 5 from BioMed Central, 43 from Google Scholar, and 2 from hand search were identified. A total of 21 articles were shortlisted that met strict inclusion and exclusion criteria and quality assessment. Each study was evaluated for the markers under study, type of sample used, study design/methodology, and statistical analysis. The studies were then grouped into three subheads depending on their implications in the diagnosis, prognosis, and treatment of OSCC. Conclusion: This review explains the basic biology and the clinical implications of telomerase-based diagnosis and prognosis, the prospects for its use in anticancer therapy, in the context of oral cancer.

Keywords: hTR, human telomerase reverse transcriptase, human telomerase RNA component, oral cancer, oral squamous cell carcinoma, reverse transcriptase, reverse transcriptase catalytic protein, telomerase, telomerase RNA component

How to cite this article:
Popli DB, Sircar K, Chowdhry A. Telomerase: An exploration toward the end of cancer. Indian J Dent Res 2017;28:574-84

How to cite this URL:
Popli DB, Sircar K, Chowdhry A. Telomerase: An exploration toward the end of cancer. Indian J Dent Res [serial online] 2017 [cited 2020 Sep 18];28:574-84. Available from: http://www.ijdr.in/text.asp?2017/28/5/574/217199

   Introduction Top


The classical hallmark of a malignant cell is its ability to multiply indefinitely.[1] This immortalization of malignant cells has been credited primarily to reactivation of the telomerase enzyme which sustains the telomere length. Telomeres are nucleoprotein structures which cap the ends of eukaryotic chromosomes and show progressive shortening with cellular multiplication. Telomere dysfunction has been known to produce the opposing pathophysiological states of degenerative aging or cancer.[2]

The telomerase enzyme complex consists of two subunits, the reverse transcriptase catalytic protein (telomerase reverse transcriptase [TERT]), and the telomerase RNA component (TERC). Telomerase helps preserve genome stability as well as replication potential in both embryonic stem cells and proliferating progenitor cells derived from quiescent normal stem cells (e.g., male germline spermatocytes), but it is silent in somatic cells, which make up the vast majority of human tissues.[3] Normal somatic cells thus enter replicative senescence and undergo growth arrest or apoptosis. Sporadically, some cells may evade these cellular check points and continue to grow limitlessly. These cells are characterized by maintenance of telomere length and telomerase expression.[4]

Past research has shown that telomerase is activated as a rule in human malignant tissues but not in adjacent normal tissues.[5] In addition, previous studies have shown that the lack of telomerase activity correlates with critically shortened telomeres and frequent spontaneous cancer remission.[6] Thus, the expression of telomerase is important and may be a rate-limiting step for tumor progression.[7] Owing to its selective expression in cancer cells, telomerase targeted cancer therapeutics offer superior specificity, lesser toxicity, and fewer side effects in contrast to conventional chemotherapeutic approaches.[4]

In the past few years, valuable research from various laboratories has provided major insights into telomerase, and telomeres leading to their use as diagnostic and prognostic markers in several types of cancer. This review is an attempt to systematically analyze the data available on telomerase-based diagnosis and prognosis, the prospects for its use in anticancer therapy in the context of oral squamous cell carcinoma (OSCC).


   Materials and Methods Top


This systematic review (PROSPERO registration number CRD42016043162) was carried out as a structured search following guidelines suggested by PROSPERO, to identify all reports of investigations that had been undertaken to assess the role of telomerase in OSCC. A search strategy was finalized utilizing MESH terms, Boolean terminology, and free text terms [Annexure 1] [Additional file 1] with key words Telomerase, hTR, TERT, human telomerase reverse transcriptase (hTERT), TERC, human telomerase RNA component (hTERC), reverse transcriptase catalytic protein, TERC, oral cancer, and OSCC. This search strategy was applied to key databases such as PubMed, Biomed Central, Scopus, and Google scholar and identified articles published from 2006 to 2016 independently by two reviewers, which was cross-checked by the third reviewer. The search was augmented by using the “related articles” link to articles recovered with PubMed. Following this, a search was done of the references cited in these articles to identify additional relevant writings. In addition, hand search of journals was performed for article retrieval. At this stage, all articles were assembled and arranged in reverse chronological order. The articles were screened, and selection of articles to be considered for review was based on stringent inclusion and exclusion criteria.

Eighty-nine articles in PubMed, 83 in Scopus, 5 in BioMed Central, 43 in Google Scholar, and 2 from hand searching were retrieved [Figure 1]. Duplicates were separated from the selection, and titles and abstracts of these selected manuscripts were studied, considering exclusion and inclusion criteria. From the 28 identified articles, a seven had to be excluded (PubMed - 2, Scopus - 1, BioMed Central - 1, and Google Scholar - 3) due to nonavailability of full text. After scrutiny by all reviewers, 21 articles were identified for systematic review [Figure 1].
Figure 1: Flowchart depicting the retrieval of studies for review process

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Inclusion criteria

Participants/population - Human studies, OSCC cell lines, controls, and sample size >15.

Intervention (s), exposure (s) - All original research works done on telomerase/hTERT/hTERC/terminal transferase/TERC in OSCC will be included in the study.

Exclusion criteria

Participants/population - Animal studies, xenograft models, sample size <15, and controls absent.

Intervention (s), exposure (s) - Studies using markers other that telomerase/hTERT/hTERC/terminal transferase/TERC in OSCC. Review articles will not be included.

This was followed by data extraction done independently by two reviewers. The data were recorded in a tabular form based on the following criteria:

  1. Participant characteristics: tissue selected, cell line, site of tissue selection, and sample size
  2. Study characteristics: marker under study, method used for research, and statistical analysis used.


Quality assessment of the articles included in the review was done based on a quality assessment instrument modified and developed from relevant articles in literature[8],[9] given in Annexure 2. [Additional file 2]

The studies were analyzed and inferences were drawn after grouping of observations from relevant studies to arrive at conclusions with diagnostic, prognostic, and therapeutic implications.


   Results Top


Twenty-one research studies analyzing telomerase activity in OSCC were shortlisted for the systematic review. Sixteen studies focused on assessing the expression of reverse transcriptase catalytic protein (TERT/hTERT) or the TERT gene as a marker for telomerase activity. Two studies used fluorescent in situ hybridization to detect TERC (TERC/hTERC) gene amplifications. Three studies used electrochemical telomerase assay to assess the telomerase activity and one study dealt with an association of glutathione S-transferase M1 (GSTM1) polymorphism with telomerase activity.

Data extraction was done, and the studies were grouped under the following subheads: implications in diagnosis [Table 1], implications in prognosis [Table 2], and therapeutic implications [Table 3].
Table 1: Diagnostic implications

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Table 2: Prognostic implications

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Table 3: Therapeutic implications

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It was noted that most studies in [Table 1] and [Table 2] were retrospective studies done on tissues from patients. Nine studies compared OSCC with oral premalignant/dysplastic lesions,[10],[11],[17],[18],[20],[21],[22],[23],[25] whereas five studies compared OSCC with healthy oral mucosa.[10],[13],[17],[20],[23] Four studies were carried out on exfoliated cells[11],[13],[15],[16] and four others were done on OSCC cell lines.[7],[14],[19],[22] Strikingly, most studies from [Table 3] (therapeutic implications) were done on OSCC cell lines[27],[28],[29] and one on tissue microarray of OSCC.[30]


   Discussion Top


Telomeres are the extreme ends of double-stranded eukaryotic chromosomes comprising tandem array of TTAGGG repeats and DNA binding proteins. In humans, it consists of repeats of TTAGGG with a 3' end overhang that helps in the formation of D-loop and T-loop structures. Telomeres protect the chromosomal ends from degradation by exonucleases and prevent recognition as double-stranded DNA breaks, end-to-end fusions, and ring chromosome formation. Thus, telomeres play a vital role in the regulation of gene expression, functional organization of the chromosome, and in controlling the replicative life of cells and entry into senescence.[26]

Progression to malignancy requires that cells overcome senescence and switch to an immortal phenotype. Mammalian cells have an intrinsic program, the Hayflick limit,[31] that limits their multiplication to about 60–70 doublings, at which point they reach a stage of senescence. The cell division limit can be overcome, allowing them to continue doubling until they reach a critical stage (M2 or crisis). At this point, chromosomal instability arises due to end-to-end fusions and/or chromosome breakage. DNA damage checkpoints are activated along with apoptosis. Unless the cell develops a mechanism through which to stabilize telomere length, it will not survive. Cells that escape crisis and become immortalized generally achieve telomeric stability through the reactivation of telomerase.[32]

Telomerase is a ribonucleoprotein that acts to elongate telomeres in cells that possess its activity.[33] This enzyme is expressed during embryonic development, loses its expression during differentiation of somatic cells, and is almost undetectable in most normal human somatic cells.[34] By contrast, telomerase is expressed in ~85% of human cancers.[11],[35] There are a few types of cells that normally express telomerase including germline cells, stem cells, hematopoietic cells, cells lining the intestine, and other rapidly proliferating cells. The widespread expression of telomerase in a variety of human cancers, while being almost undetectable in most normal cells, makes it a very attractive drug target.[32]

Diagnostic potential

The hTERT, the catalytic subunit of telomerase, is strongly associated with telomerase activity implicated in cellular immortalization and tumorigenesis.[33] Most studies in our review employed hTERT to study telomerase activity in OSCC. Immunohistochemical labeling for hTERT, telomerase repeat amplification protocol (TRAP), polymerase chain reaction amplification, and sequencing of TERT gene, ECTA, and interphase FISH were the main methods for the selected studies. Archival biopsy tissues of diagnosed OSCC patients and potentially malignant disorders, exfoliated oral cells, OSCC cell lines, and normal oral mucosal biopsies were used as study samples. The following inferences were drawn from the studies under consideration:

  1. Telomerase activity was highest in patients with OSCC and lowest in normal mucosa.[10],[11] The increased expression of hTERT protein is an early event in oral carcinogenesis and hTERT may be a biomarker for OSCCs[20]
  2. ECTA was shown in three studies to be a promising assay in comparison with TRAP for screening for oral cancer[11],[13],[16]
  3. Physical interaction between heat shock proteins and the hTERT promoter occurs in telomerase-positive cells but not in normal human cells. Heat shock protein association with the hTERT promoter complex may, in part, be responsible for telomerase activation during cellular immortalization[7],[36]
  4. SOX2 and TERC gene amplifications are common in all squamous cell carcinomas and their detection in early stages could be crucial for early detection and more accurate prognosis of OSCC.[15]


Prognostic potential

Telomerase activity as shown by the mean hTERT expression in cells showed a steady increase from normal oral mucosa to oral epithelial dysplasia to OSCC.[8],[23] The selected studies help us draw the following inferences. Oral premalignant lesions could be classified into high-risk and low-risk categories by morphological evaluation together with FISH assessment for hTERC gain.[6] Telomere expression has been linked to Chronic inflammation, progressive epithelial dysplasia, and long-term exposure to inflammatory cytokines which in turn pave the way to malignant transformation and regulates the invasion of certain types of oral cancer cells.[22] Telomere length, telomeric repeat binding factor, and telomerase activation have been strongly linked to the prognosis of oral cancer patients.[26] A high recurrence rate in OSCC patients has been associated with high hTERT labeling indices. On the contrary, work done by Pannone et al. showed that telomere activity could not be linked to classical clinicopathological parameters, as there was no significant relationship between hTERT expression and several clinicopathological parameters such as tumor stage, size, and histological grade.[25] GSTM1 polymorphism has also been linked to determine individual susceptibility toward cancer and telomere-associated changes.[24]

Therapeutic potential

Several telomerase-based immunotherapy strategies have been developed and many are in advanced clinical trials, making this a rapidly progressing field of antitelomerase cancer therapy.[37],[38],[39],[40] Telomerase is an attractive target antigen for cancer immunotherapy because it is expressed almost universally in human cancers and is functionally required to sustain malignant tumor long-term growth.[41]

While telomerase is expressed in some normal tissues,[42],[43],[44] no patients have exhibited serious adverse effects (such as autoimmune disease or bone marrow depletion) indicative of an immune response against normal cells. One explanation is that normal cells express very low levels of hTERT,[44] making them poor targets relative to tumor cells with high levels of hTERT expression.

In OSCC patients, hTERT has been linked to epithelial mesenchymal transition, providing an explanation for the aggressive nature of human tumors and partially explained by activation of the Wnt/β-catenin pathway.[23] Telomerase inhibitors could also be used to sensitize a subset of OSCCs with short telomeres to radiotherapy and for the first time demonstrate that the tumor Anaphase Bridge Index may assist the selection of cancers that would be suitable for such sensitization therapy.[29] Thus, hTERT represents a possible therapeutic target in highly metastatic cancers.[23]


   Conclusion Top


This systematic review is focused on the association of OSCC and telomerase activity with special emphasis on telomerase-based diagnosis, prognosis, and the prospects for its use in anticancer therapy. The literature search and critical review suggested a positive link between increased expression of hTERT protein and oral carcinogenesis. Increased telomerase activity/hTERT expression correlates with poorer prognosis and a high recurrence rate for OSCC. However, there is a need for studies exploring its plausible role as a biomarker in diagnostic immunopathology. The telomere hypothesis of cancer cell immortalization remains an attractive yet not fully understood concept.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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Correspondence Address:
Deepika Bablani Popli
Department of Oral Pathology and Microbiology, Faculty of Dentistry, JMI, New Delhi
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijdr.IJDR_690_16

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