Unusual distribution of DNA methylation within the hTERT CpG island in tissues and cell lines

Telomeres and telomerase in mesothelioma: Pathophysiology, biomarkers and emerging therapeutic strategies (Review)

Malignant mesothelioma (MM) is a rare but aggressive cancer linked to asbestos exposure and characterized by advanced‑stage disease at presentation. Despite advances in treatment, prognosis remains abysmal, highlighting the imperative for the development of novel biomarkers and treatment approaches. Telomere biology plays a pivotal role in the tumorigenic process and has emerged as a key area in oncology research. Short telomeres have been associated with genomic instability, and substantially shorter telomere length (TL) has been identified in MM, showcasing the potential of TL in risk assessment, early detection, and disease progression monitoring. MM predominantly maintains TL through telomerase activity (TA), which in research has been identified in >90% of MM cases, underscoring the potential of TA as a biomarker in MM. Telomerase reverse transcriptase (TERT) polymorphisms may serve as valuable biomarkers, with research identifying associations between single nucleotide polymorphisms (SNPs) and the risk and prognosis of MM. Additionally, TERT promoter mutations have been associated with poor prognosis and advanced‑stage disease, with the non‑canonical functions of TERT hypothesized to contribute to the development of MM. TERT promoter mutations occur in ~12% of MM cases; C228T, C250T and A161C are the most common, while the distribution and frequency differ depending on histological subtype. Research reveals the promise of the various approaches therapeutically targeting telomerase, with favorable results in pre‑clinical models and inconclusive findings in clinical trials. The present review examines the role of telomere biology in MM and its implications in diagnosis, prognosis, and therapy.

Targeting oncogenic TERT promoter variants by allele-specific epigenome editing

BACKGROUND

Activation of dominant oncogenes by small or structural genomic alterations is a common driver mechanism in many cancers. Silencing of such dominantly activated oncogenic alleles, thus, is a promising strategy to treat cancer. Recently, allele-specific epigenome editing (ASEE) has been described as a means to reduce transcription of genes in an allele-specific manner. In cancer, specificity to an oncogenic allele can be reached by either targeting directly a pathogenic single-nucleotide variant or a polymorphic single-nucleotide variant linked to the oncogenic allele. To investigate the potential of ASEE in cancer, we here explored this approach by targeting variants at the TERT promoter region. The TERT promoter region has been described as one of the most frequently mutated non-coding cancer drivers.

RESULTS

Sequencing of the TERT promoter in cancer cell lines showed 53% (41/77) to contain at least one heterozygous sequence variant allowing allele distinction. We chose the hepatoblastoma cell line Hep-G2 and the lung cancer cell line A-549 for this proof-of-principle study, as they contained two different kinds of variants, namely the activating mutation C228T in the TERT core promoter and the common SNP rs2853669 in the THOR region, respectively. These variants were targeted in an allele-specific manner using sgRNA-guided dCas9-DNMT3A-3L complexes. In both cell lines, we successfully introduced DNA methylation specifically to the on-target allele of the TERT promoter with limited background methylation on the off-target allele or an off-target locus (VEGFA), respectively. We observed a maximum CpG methylation gain of 39% and 76% on the target allele when targeting the activating mutation and the common SNP, respectively. The epigenome editing translated into reduced TERT RNA expression in Hep-G2.

CONCLUSIONS

We applied an ASEE-mediated approach to silence TERT allele specifically. Our results show that the concept of dominant oncogene inactivation by allele-specific epigenome editing can be successfully translated into cancer models. This new strategy may have important advantages in comparison with existing therapeutic approaches, e.g., targeting telomerase, especially with regard to reducing adverse side effects.

Cell Survival, Death, and Proliferation in Senescent and Cancer Cells: the Role of (Poly)phenols

Cellular senescence has long been considered a permanent state of cell cycle arrest occurring in proliferating cells subject to different stressors, used as a cellular defense mechanism from acquiring potentially harmful genetic faults. However, recent studies highlight that senescent cells might also alter the local tissue environment and concur to chronic inflammation and cancer risk by secreting inflammatory and matrix remodeling factors, acquiring a senescence-associated secretory phenotype (SASP). Indeed, during aging and age-related diseases, senescent cells amass in mammalian tissues, likely contributing to the inevitable loss of tissue function as we age. Cellular senescence has thus become one potential target to tackle age-associated diseases as well as cancer development. One important aspect characterizing senescent cells is their telomere length. Telomeres shorten as a consequence of multiple cellular replications, gradually leading to permanent cell cycle arrest, known as replicative senescence. Interestingly, in the large majority of cancer cells, a senescence escape strategy is used and telomere length is maintained by telomerase, thus favoring cancer initiation and tumor survival. There is growing evidence showing how (poly)phenols can impact telomere maintenance through different molecular mechanisms depending on dose and cell phenotypes. Although normally, (poly)phenols maintain telomere length and support telomerase activity, in cancer cells this activity is negatively modulated, thus accelerating telomere attrition and promoting cancer cell death. Some (poly)phenols have also been shown to exert senolytic activity, thus suggesting both antiaging (directly eliminating senescent cells) and anticancer (indirectly, via SASP inhibition) potentials. In this review, we analyze selective (poly)phenol mechanisms in senescent and cancer cells to discriminate between in vitro and in vivo evidence and human applications considering (poly)phenol bioavailability, the influence of the gut microbiota, and their dose-response effects.

THOR is a targetable epigenetic biomarker with clinical implications in breast cancer

BACKGROUND

Breast cancer (BC) is the most frequently diagnosed cancer and a leading cause of death among women worldwide. Early BC is potentially curable, but the mortality rates still observed among BC patients demonstrate the urgent need of novel and more effective diagnostic and therapeutic options. Limitless self-renewal is a hallmark of cancer, governed by telomere maintenance. In around 95% of BC cases, this process is achieved by telomerase reactivation through upregulation of the human telomerase reverse transcriptase (hTERT). The hypermethylation of a specific region within the hTERT promoter, termed TERT hypermethylated oncological region (THOR) has been associated with increased hTERT expression in cancer. However, its biological role and clinical potential in BC have never been studied to the best of our knowledge. Therefore, we aimed to investigate the role of THOR as a biomarker and explore the functional impact of THOR methylation status in hTERT upregulation in BC.

RESULTS

THOR methylation status in BC was assessed by pyrosequencing on discovery and validation cohorts. We found that THOR is significantly hypermethylated in malignant breast tissue when compared to benign tissue (40.23% vs. 12.81%, P < 0.0001), differentiating malignant tumor from normal tissue from the earliest stage of disease. Using a reporter assay, the addition of unmethylated THOR significantly reduced luciferase activity by an average 1.8-fold when compared to the hTERT core promoter alone (P < 0.01). To further investigate its biological impact on hTERT transcription, targeted THOR demethylation was performed using novel technology based on CRISPR-dCas9 system and significant THOR demethylation was achieved. Cells previously demethylated on THOR region did not develop a histologic cancer phenotype in in vivo assays. Additional studies are required to validate these observations and to unravel the causality between THOR hypermethylation and hTERT upregulation in BC.

CONCLUSIONS

THOR hypermethylation is an important epigenetic mark in breast tumorigenesis, representing a promising biomarker and therapeutic target in BC. We revealed that THOR acts as a repressive regulatory element of hTERT and that its hypermethylation is a relevant mechanism for hTERT upregulation in BC.

Acute type adult T-cell leukemia cells proliferate in the lymph nodes rather than in peripheral blood

A massive increase in the number of mature CD4⁺ T-cells in peripheral blood (PB) is a defining characteristic of acute type of adult T-cell leukemia (ATL). To date, the site of proliferation of ATL cells in the body has been unclear. In an attempt to address this question, we examined the expression of the proliferation marker, Ki-67, in freshly isolated ATL cells from PB and lymph nodes (LNs) of patients with various types of ATL. Our findings reveal that LN-ATL cells display higher expression of the Ki-67 antigen than PB-ATL cells in acute type patients. The gene expression of T-cell quiescence regulators such as Krüppel-like factor 2/6 and forkhead box protein 1 was substantially high in acute type PB-ATL cells. The expression of human telomerase reverse transcriptase, which is involved in T-cell expansion, was significantly low in PB-ATL cells from acute type patients, similar to that in normal resting T-cells. These findings suggest that ATL cells proliferate in the LNs rather than in PB.

Targeted Long-Read Bisulfite Sequencing Identifies Differences in the TERT Promoter Methylation Profiles between TERT Wild-Type and TERT Mutant Cancer Cells

Background: TERT promoter methylation, located several hundred base pairs upstream of the transcriptional start site, is cancer specific and correlates with increased TERT mRNA expression and poorer patient outcome. Promoter methylation, however, is not mutually exclusive to TERT activating genetic alterations, as predicted for functionally redundant mechanisms. To annotate the altered patterns of TERT promoter methylation and their relationship with gene expression, we applied a Pacific Biosciences-based, long-read, bisulfite-sequencing technology and compared the differences in the methylation marks between wild-type and mutant cancers in an allele-specific manner. Results: We cataloged TERT genetic alterations (i.e., promoter point mutations or structural variations), allele-specific promoter methylation patterns, and allele-specific expression levels in a cohort of 54 cancer cell lines. In heterozygous mutant cell lines, the mutant alleles were significantly less methylated than their silent, mutation-free alleles (p < 0.05). In wild-type cell lines, by contrast, both epialleles were equally methylated to high levels at the TERT distal promoter, but differentially methylated in the proximal regions. ChIP analysis showed that epialleles with the hypomethylated proximal and core promoter were enriched in the active histone mark H3K4me2/3, whereas epialleles that were methylated in those regions were enriched in the repressive histone mark H3K27me3. Decitabine therapy induced biallelic expression in the wild-type cancer cells, whereas the mutant cell lines were unaffected. Conclusions: Long-read bisulfite sequencing analysis revealed differences in the methylation profiles and responses to demethylating agents between TERT wild-type and genetically altered cancer cell lines. The causal relation between TERT promoter methylation and gene expression remains to be established.

Promoter CpG methylation inhibits Krüppel-like factor 2 (KLF2)-Mediated repression of hTERT gene expression in human T-cells

Constitutive expression of human telomerase reverse transcriptase (hTERT) with DNA methylation of its promoter is a common phenomenon in tumor cells. We recently found that the transcriptional factor Krüppel-like factor 2 (KLF2) binds to the CpG sequences in the hTERT promoter and inhibits hTERT gene expression in normal resting T-cells. The human T-cell line Kit 225 in the resting phase induced by the deprivation of interleukin (IL)-2 showed no decrease in the expression of hTERT, despite the high expression of KLF2. To elucidate the mechanisms of deregulation of hTERT expression in T-cells, we examined the relationship between DNA methylation and KLF2 binding to the hTERT promoter. The hTERT promoter was methylated in Kit 225 cells, resulting in the inhibition of the binding of KLF2 to the promoter. DNA demethylation by the reagent Zebularine recovered KLF2 binding to the hTERT promoter, followed by the downregulation of its gene expression. These findings indicate that the repressive effect of KLF2 on hTERT gene expression is abolished by DNA methylation in T-cell lines.

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Unlike normal T-cells, the hTERT promoter is highly methylated in a leukemic T-cell line.

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DNA methylation of the promoter induces hTERT gene expression.

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Association between the promoter and the repressor KLF2 is inhibited by DNA methylation.

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Demethylation promotes binding of KLF2 and silencing of hTERT expression.

Re-Sensitizing Tumor Cells to Cancer Drugs with Epigenetic Regulators

Cancer drug resistance is a major problem for cancer therapy. While many drugs can be effective in first-line treatments, cancer cells can become resistant due to genetic (mutations and chromosomal aberrations) but also epigenetic changes. Hence, many research studies addressed epigenetic drugs in circumventing resistance to conventional therapeutics in different tumor entities and in increasing the efficiency of immune checkpoint therapies. Furthermore, repositioning of already approved drugs in combination with epigenetic modifiers could potentiate their efficacy and thus could be an attractive strategy for cancer treatment. Summarizing, we recapitulate current data on epigenetic drugs and their targets in modulating sensitivity towards conventional and immune therapies, providing evidence that altering expression profiles by epigenetic modifiers holds great potential to improve the clinical outcome of cancer patients.

TERT promoter methylation and protein expression as predictive biomarkers for recurrence risk in patients with serous borderline ovarian tumours

Epithelial ovarian neoplasms can be divided into three distinct clinicopathological groups: benign, malignant and borderline tumours. Borderline tumours are less aggressive than epithelial carcinomas, with an indolent clinical course and delayed recurrence. However, a subset of these cases can progress to malignancy and relapse, and death from recurrent disease can occasionally occur. Telomerase activation is a critical element in cellular immortalisation and cancer. The enzyme telomerase comprises a catalytic subunit (TERT) expressed in various types of cancers and regulated by promoter methylation mainly in epithelial tumours. The aim of this study was to investigate the promoter methylation status and the expression of TERT in 50 serous borderline tumours (SBTs) and their correlation with clinicopathological features and outcome. TERT methylation was analysed by bisulfite pyrosequencing and TERT expression by immunohistochemistry. Methylation of TERT promoter was only observed in four SBTs. A good correlation with immunostochemistry was found: nuclear positivity for TERT expression was observed in the methylated cases, whereas no expression was detected in unmethylated tumours. One of these patients had a recurrence after 7 years and another patient died from the disease. SBTs with hypomethylated tumours and absence of TERT expression showed a good clinical behaviour. Our study highlights the low presence of TERT methylation in SBTs, confirming that these tumours have a different biology than serous carcinomas. Furthermore, the concordance between TERT promoter methylation and TERT expression and their association with clinical outcomes leads to consider TERT alteration as a potential predictive biomarker for recurrence risk identifying patients who should undergo a careful and prolonged follow-up.

Pervasive promoter hypermethylation of silenced TERT alleles in human cancers

BACKGROUND

In cancers, maintenance of telomeres often occurs through activation of the catalytic subunit of telomerase, encoded by TERT. Yet, most cancers show only modest levels of TERT gene expression, even in the context of activating hotspot promoter mutations (C228T and C250T). The role of epigenetic mechanisms, including DNA methylation, in regulating TERT gene expression in cancer cells is as yet not fully understood.

METHODS

Here, we have carried out the most comprehensive characterization to date of TERT promoter methylation using ultra-deep bisulfite sequencing spanning the CpG island surrounding the core TERT promoter in 96 different human cell lines, including primary, immortalized and cancer cell types, as well as in control and reference samples.

RESULTS

In general, we observed that immortalized and cancer cell lines were hypermethylated in a region upstream of the recurrent C228T and C250T TERT promoter mutations, while non-malignant primary cells were comparatively hypomethylated in this region. However, at the allele-level, we generally found that hypermethylation of promoter sequences in cancer cells is associated with repressed expression, and the remaining unmethylated alleles marked with open chromatin are largely responsible for the observed TERT expression in cancer cells.

CONCLUSIONS

Our findings suggest that hypermethylation of the TERT promoter alleles signals transcriptional repression of those alleles, leading to attenuation of TERT activation in cancer cells. This type of fine tuning of TERT expression may account for the modest activation of TERT expression in most cancers.

Interleukin-6, -8, and TGF-β Secreted from Mesenchymal Stem Cells Show Functional Role in Reduction of Telomerase Activity of Leukemia Cell Via Wnt5a/β-Catenin and P53 Pathways

Purpose: The effect of mesenchymal stem cells (MSCs) on the immortality features of malignant cells, such as hematologic cancerous cells, are controversial, and the associated mechanisms are yet to be well understood. The aim of the present study was to investigate the in vitro effect of bone marrow-derived MSCs (BMSCs) on the chronic myeloid leukemia cell line K562 through telomere length measurements, telomerase activity assessments, and hTERT gene expression. The possible signaling pathways involved in this process, including Wnt-5a/β-catenin and P53, were also evaluated. Methods: Two cell populations (BMSCs and K562 cell line) were co-cultured on transwell plates for 7 days. Next, K562 cells were collected and subjected to quantitative real-time PCR, PCR-ELISA TRAP assay, and the ELISA sandwich technique for telomere length, hTERT gene expression, telomerase activity assay, and cytokine measurement, respectively. Also, the involvement of the mentioned signaling pathways in this process was reported by real-time PCR and Western blotting through gene and protein expression, respectively. Results: The results showed that BMSCs caused significant decreases in telomere length, telomerase activity, and the mRNA level of hTERT as a regulator of telomerase activity. The significant presence of interleukin (IL)-6, IL-8, and transforming growth factor beta (TGF-β) was obvious in the co-cultured media. Also, BMSCs significantly decreased and increased the gene and protein expression of β-catenin and P53, respectively. Conclusion: It was concluded that the mentioned effects of IL-6, IL-8, and TGF-β cytokines secreted from MSCs on K562 cells as therapeutic agents were applied by Wnt-5a/β-catenin and P53 pathways.

Characterization of human telomerase reverse transcriptase promoter methylation and transcription factor binding in differentiated thyroid cancer cell lines

Telomerase reverse transcriptase (TERT) activation plays an important role in cancer development by enabling the immortalization of cells. TERT regulation is multifaceted, and its promoter methylation has been implicated in controlling expression through alteration in transcription factor binding. We have characterized TERT promoter methylation, transcription factor binding, and TERT expression levels in five differentiated thyroid cancer (DTC) cell lines and six normal thyroid tissue samples by targeted bisulfite sequencing, ChIP-qPCR, and qRT-PCR. DTC cell lines express varying levels of TERT and exhibit TERT promoter methylation patterns similar to patterns seen in other telomerase positive cancer cell lines. The minimal promoter immediately surrounding the transcription start site is hypomethylated, while further upstream portions show dense methylation. In contrast, the TERT promoter in normal thyroid tissue is largely unmethylated throughout and expresses TERT minimally. Transcription factor binding is also affected by TERT mutation status. The E-twenty-six (ETS) factor GABPA exhibits TERT binding in the TERT mutant DTC cells only, and allele-specific methylation patterns at the minimal promoter were observed as well, which may indicate allele-specific factor recruitment at the minimal promoter. Furthermore, we identified binding sites for activators MYC and GSC in the hypermethylated upstream region, pointing to its possible importance in TERT regulation. Overall, TERT expression and telomerase activity depend on the interplay of multiple regulatory mechanisms including TERT promoter methylation, mutation status, and recruitment of transcription factors. This work explores of the interplay between these regulatory mechanisms and offers insight into cellular control of active telomerase in human cancer.

Involvement of epigenetic modification of TERT promoter in response to all-trans retinoic acid in ovarian cancer cell lines

BACKGROUND

All-trans retinoic acid (ATRA) is currently being used to treat hematological malignancies, given the ability to inhibit cell proliferation. This effect seems to be related to epigenetic changes of the TERT (Telomerase Reverse Transcriptase) promoter. When hypomethylated, ATRA-inducible TERT repressors can bind the promoter, repressing transcription of TERT, the rate-limiting component of telomerase. Ovarian carcinomas are heterogeneous tumors characterized by several aberrantly methylated genes among which is TERT. We recently found a hypomethylation of TERT promoter in about one third of serous carcinoma, the most lethal histotype. Our aim was to investigate the potential role of ATRA as an anticancer drug in a sub-group of ovarian carcinoma where the TERT promoter was hypomethylated.

METHODS

The potential antiproliferative and cytotoxic effect of ATRA was investigated in seven serous ovarian carcinoma and one teratocarcinoma cell lines and the results were compared to the methylation status of their TERT promoter.

RESULTS

The serous ovarian carcinoma cell line OVCAR3, harboring a hypomethylated TERT promoter, was the best and fastest responder. PA1 and SKOV3, two cell lines with an intermediate methylated promoter, revealed a weaker and delayed response. On the contrary, the other 5 cell lines with a highly methylated promoter did not respond to ATRA, indicative of ATRA-resistant cells.

CONCLUSIONS

Our results demonstrate an inverse correlation between the methylation level of TERT promoter and ATRA efficacy in ovarian carcinoma cell lines. Although these results are preliminary, ATRA treatment could become a new powerful, personalized therapy in serous ovarian carcinoma patients, but only in those with tumors harboring a hypomethylated TERT promoter.

Diverse regulatory manners of human telomerase reverse transcriptase

Human telomerase reverse transcriptase (hTERT) is the core subunit of human telomerase and plays important roles in human cancers. Aberrant expression of hTERT is closely associated with tumorigenesis, cancer cell stemness maintaining, cell proliferation, apoptosis inhibition, senescence evasion and metastasis. The molecular basis of hTERT regulation is highly complicated and consists of various layers. A deep and full-scale comprehension of the regulatory mechanisms of hTERT is pivotal in understanding the pathogenesis and searching for therapeutic approaches. In this review, we summarize the recent advances regarding the diverse regulatory mechanisms of hTERT, including the transcriptional (promoter mutation, promoter region methylation and histone acetylation), post-transcriptional (mRNA alternative splicing and non-coding RNAs) and post-translational levels (phosphorylation and ubiquitination), which may provide novel perspectives for further translational diagnosis or therapeutic strategies targeting hTERT.

Non-duplex G-Quadruplex Structures Emerge as Mediators of Epigenetic Modifications

The role of non-duplex DNA, the guanine-quadruplex structure in particular, is becoming widely appreciated. Increasing evidence in the last decade implicates quadruplexes in important processes such as transcription and replication. Interestingly, more recent work suggests roles for quadruplexes, in association with quadruplex-interacting proteins, in epigenetics through both DNA and histone modifications. Here, we review the effect of the quadruplex structure on post-replication epigenetic memory and quadruplex-induced promoter DNA/histone modifications. Furthermore, we highlight the epigenetic state of the telomerase promoter where quadruplexes could play a key regulatory role. Finally, we discuss the possibility that DNA structures such as quadruplexes, within a largely duplex DNA background, could act as molecular anchors for locally induced epigenetic modifications.

Possible Mechanisms of Green Tea and Its Constituents against Cancer

A number of epidemiological, clinical, and experimental researches have indicated that administration of green tea appears to have anti-cancer activity. According to findings of laboratory cell culture studies, a diverse mechanism has been observed underlying the effects of green tea catechins against cancer. These mechanisms include anti-oxidant activity, cell cycle regulation, receptor tyrosine kinase pathway inhibition, immune system modulation, and epigenetic modification control. This review discusses the results of these studies to provide more insight into the effects of green tea administration on cancers observed to date in this research field.

hTERT promoter methylation status in peripheral blood leukocytes as a molecular marker of head and neck cancer progression

Cancer cells, including head and neck cancer cell carcinoma (HNSCC), are characterized by an increased telomerase activity. This enzymatic complex is active in approximately 80-90% of all malignancies, and is regulated by various factors, including methylation status of hTERT gene promoter. hTERT methylation pattern has been thoroughly studied so far. It was proved that hTERT is aberrantly methylated in tumor tissue versus healthy counterparts. However, such effect has not yet been investigated in PBLs (peripheral blood leukocytes) of cancer patients. The aim of this study was to analyze the hTERT gene promoter methylation status in blood leukocytes. DNA was extracted from PBL of 92 patients with histologically diagnosed HNSCC and 53 healthy controls. Methylation status of whole hTERT promoter fragment with independent analysis of each 19 CpG sites was performed using bisulfide conversion technique followed by sequencing of PCR products. Not significant (p = 0.0532) differences in the general frequency of hTERT CpG sites methylation were detected between patients and healthy controls. However, it was discovered that some of analyzed positions (CpG islands: 1 [p = 0.0235], 5 [p = 0.0462], 8 [p = 0.0343]) are significantly more often methylated in HNSCC patients than in controls. The opposite finding was observed in case of CpG position 2 (p = 0.0210). Furthermore, closer analysis of single CpG positions revealed differences in methylation status dependent on anatomical site and TNM classification. To conclude, hTERT promoter methylation status (general or single CpG sites) would be considered as a molecular markers of HNSCC diagnostics.

ALV Integration-Associated Hypomethylation at the TERT Promoter Locus

Avian leukosis virus (ALV) is a simple retrovirus that can induce B-cell lymphoma in chicken(s) and other birds by insertional mutagenesis. The promoter region of telomerase reverse transcriptase (TERT) has been identified as an important integration site for tumorigenesis. Tumors with TERT promoter integrations are associated with increased TERT expression. The mechanism of this activation is still under investigation. We asked whether insertion of proviral DNA perturbs the epigenome of the integration site and, subsequently, impacts the regulation of neighboring genes. DNA cytosine methylation, which generally acts to suppress transcription, is one major form of epigenetic regulation. In this study, we examine allele-specific methylation patterns of genomic DNA from chicken tumors by bisulfite sequencing. We observed that alleles with TERT promoter integrations are associated with decreased methylation in the host genome near the site of integration. Our observations suggest that insertion of ALV in the TERT promoter region may induce expression of TERT through inhibition of maintenance methylation in the TERT promoter region.

Allele-Specific DNA Methylation and Its Interplay with Repressive Histone Marks at Promoter-Mutant TERT Genes

A mutation in the promoter of the Telomerase Reverse Transcriptase (TERT) gene is the most frequent noncoding mutation in cancer. The mutation drives unusual monoallelic expression of TERT, allowing immortalization. Here, we find that DNA methylation of the TERT CpG island (CGI) is also allele-specific in multiple cancers. The expressed allele is hypomethylated, which is opposite to cancers without TERT promoter mutations. The continued presence of Polycomb repressive complex 2 (PRC2) on the inactive allele suggests that histone marks of repressed chromatin may be causally linked to high DNA methylation. Consistent with this hypothesis, TERT promoter DNA containing 5-methyl-CpG has much increased affinity for PRC2 in vitro. Thus, CpG methylation and histone marks appear to collaborate to maintain the two TERT alleles in different epigenetic states in TERT promoter mutant cancers. Finally, in several cancers, DNA methylation levels at the TERT CGI correlate with altered patient survival.

Zinc sulfate contributes to promote telomere length extension via increasing telomerase gene expression, telomerase activity and change in the TERT gene promoter CpG island methylation status of human adipose-derived mesenchymal stem cells

The use of mesenchymal stem cells (MSCs) for cell therapy and regenerative medicine has received widespread attention over the past few years, but their application can be complicated by factors such as reduction in proliferation potential, the senescent tendency of the MSCs upon expansion and their age-dependent decline in number and function. It was shown that all the mentioned features were accompanied by a reduction in telomerase activity and telomere shortening. Furthermore, the role of epigenetic changes in aging, especially changes in promoter methylation, was reported. In this study, MSCs were isolated from the adipose tissue with enzymatic digestion. In addition, immunocytochemistry staining and flow cytometric analysis were performed to investigate the cell-surface markers. In addition, alizarin red-S, sudan III, toluidine blue, and cresyl violet staining were performed to evaluate the multi-lineage differentiation of hADSCs. In order to improve the effective application of MSCs, these cells were treated with 1.5 × 10-8 and 2.99 × 10-10 M of ZnSO4 for 48 hours. The length of the absolute telomere, human telomerase reverse transcriptase (hTERT) gene expression, telomerase activity, the investigation of methylation status of the hTERT gene promoter and the percentage of senescent cells were analyzed with quantitative real-time PCR, PCR-ELISA TRAP assay, methylation specific PCR (MSP), and beta-galactosidase (SA-β-gal) staining, respectively. The results showed that the telomere length, the hTERT gene expression, and the telomerase activity had significantly increased. In addition, the percentage of senescent cells had significantly decreased and changes in the methylation status of the CpG islands in the hTERT promoter region under treatment with ZnSO4 were seen. In conclusion, it seems that ZnSO4 as a proper antioxidant could improve the aging-related features due to lengthening of the telomeres, increasing the telomerase gene expression, telomerase activity, decreasing aging, and changing the methylation status of hTERT promoter; it could potentially beneficial for enhancing the application of aged-MSCs.

DNA hypermethylation enhanced telomerase reverse transcriptase expression in human-induced pluripotent stem cells

During reprogramming into human induced pluripotent stem cells (iPSCs), several stem cell marker genes are induced, such as OCT-4, NANOG, SALL4, and TERT. OCT-4, NANOG, and SALL4 gene expression can be regulated by DNA methylation. Their promoters become hypomethylated in iPSCs during reprogramming, leading to their induced expression. However, epigenetic regulation of the TERT gene remains unclear. In this study, we focused on epigenetic regulation of the human TERT gene and identified a differentially methylated region (DMR) at a distal region in the TERT promoter between human iPSCs and their parental somatic cells. Interestingly, the TERT-DMR was highly methylated in iPSCs, but low-level methylation was observed in their parental somatic cells. Region-specific, methylated-promoter assays showed that the methylated TERT-DMR up-regulated the promoter activity in iPSCs. In addition, Lamin B1 accumulated at the TERT-DMR in iPSCs, but not in their parent somatic cells. These results suggested that the TERT transcription was enhanced by DNA methylation at the TERT-DMR via binding to nuclear lamina during reprogramming. Our findings shed light on a new functional aspect of DNA methylation in gene expression.

The methylation status and expression of human telomerase reverse transcriptase is significantly high in oral carcinogenesis

Telomerase activity is present in most cancers and is tightly regulated by the expression of human telomerase reverse transcriptase (hTERT). Hypermethylation in the promoter region of hTERT contributes to the regulation of hTERT expression. In this study, we investigated the methylation and expression of hTERT in oral squamous cell carcinoma (OSCC), oral leukoplakia, and normal oral mucosa. Furthermore, we investigated the significance of hTERT to the clinicopathological findings of OSCC. 35 OSCC, 50 oral leukoplakia (epithelial dysplasia n = 25, squamous cell hyperplasia n = 25), and 10 normal oral mucosa samples were investigated through methylation-specific PCR. Immunohistochemistry was analyzed in 35 OSCC, 50 oral leukoplakia, and 4 normal oral mucosa samples. The methylation and expression of hTERT increased from normal oral mucosa to oral leukoplakia to OSCC. In OSCC, all samples were methylated. However, partial methylation (20%) or unmethylation (80%), but never complete methylation, was observed in normal oral mucosa. Additionally, hTERT expression correlated with cervical lymph node metastasis. These results suggested that the methylation and expression of hTERT is high in oral carcinogenesis and may play an important role in oral cancer. hTERT expression may also be predictive of cervical lymph node metastasis.

Human telomerase reverse transcriptase regulation by DNA methylation, transcription factor binding and alternative splicing (Review)

The catalytic subunit of telomerase, human telomerase reverse transcriptase (hTERT), plays an essential role in telomere maintenance to oppose cellular senescence and, is highly regulated in normal and cancerous cells. Regulation of hTERT occurs through multiple avenues, including a unique pattern of CpG promoter methylation and alternative splicing. Promoter methylation affects the binding of transcription factors, resulting in changes in expression of the gene. In addition to expression level changes, changes in promoter binding can affect alternative splicing in a cotranscriptional manner. The alternative splicing of hTERT results in either the full length transcript which can form the active telomerase complex with hTR, or numerous inactive isoforms. Both regulation strategies are exploited in cancer to activate telomerase, however, the exact mechanism is unknown. Therefore, unraveling the link between promoter methylation status and alternative splicing for hTERT could expose yet another level of hTERT regulation. In an attempt to provide insight into the cellular control of active telomerase in cancer, this review will discuss our current perspective on CpG methylation of the hTERT promoter region, summarize the different forms of alternatively spliced variants, and examine examples of transcription factor binding that affects splicing.

Telomerase Activation in Hematological Malignancies

Telomerase expression and telomere maintenance are critical for cell proliferation and survival, and they play important roles in development and cancer, including hematological malignancies. Transcriptional regulation of the rate-limiting subunit of human telomerase reverse transcriptase gen (hTERT) is a complex process, and unveiling the mechanisms behind its reactivation is an important step for the development of diagnostic and therapeutic applications. Here, we review the main mechanisms of telomerase activation and the associated hematologic malignancies.

Differential timing of oxidative DNA damage and telomere shortening in hepatitis C and B virus-related liver carcinogenesis

In viral hepatitis, inflammation is correlated with chronic oxidative stress, one of the biological events leading to DNA damage and hepatocellular carcinoma (HCC) development. Aim of this study was to investigate the complex molecular network linking oxidative damage to telomere length and telomerase activity and regulation in hepatitis C and B virus-related liver carcinogenesis. We investigated 142 patients: 21 with HCC (in both tumor and peritumor tissues) and 121 with chronic viral hepatitis in different stages. We evaluated 8-hydroxydeoxyguanosine (8-OHdG), marker of oxidative DNA damage, OGG1 gene polymorphism, telomere length, telomerase activity, TERT promoter methylation, and mitochondrial TERT localization. In hepatitis C-related damage, 8-OHdG levels increased since the early disease stages, whereas hepatitis B-related liver disease was characterized by a later and sharper 8-OHdG accumulation (P = 0.005). In C virus-infected patients, telomeres were shorter (P = 0.03), whereas telomerase activity was higher in tumors than that in the less advanced stages of disease in both groups (P = 0.0001, P = 0.05), with an earlier increase in hepatitis C. Similarly, TERT promoter methylation was higher in tumor and peritumor tissues in both groups (P = 0.02, P = 0.0001). Finally, TERT was localized in mitochondria in tumor and peritumor samples, with 8-OHdG levels significantly lower in mitochondrial than those in genomic DNA (P = 0.0003). These data describe a pathway in which oxidative DNA damage accumulates in correspondence with telomere shortening, telomerase activation, and TERT promoter methylation with a different time course in hepatitis B and C virus-related liver carcinogenesis. Finally, TERT localizes in mitochondria in HCC, where it lacks a canonical function.

Diagnostic Value of Methylated Human Telomerase Reverse Transcriptase in Human Cancers: A Meta-Analysis

Human telomerase reverse transcriptase (hTERT) plays a critical role in the pathogenesis of human malignancies. Overexpression of hTERT is essential in controlling the propagation of cancer cells. The CpG island located at hTERT promoter region is subjected to methylation modification in human cancer. In this perspective article, we discussed the diagnostic value of methylated hTERT in human cancers. The definitive diagnosis of most solid tumors is based on histological and immunohistochemical features. Under certain circumstances, however, the use of methylated hTERT might be useful in overcoming the limitation of the conventional methods. Methylated hTERT showed a good diagnostic power in discriminating cancer from benign or normal tissues. Nevertheless, differences in detection method, methylation site, cancer type, and histological subtype of cancer make it difficult to evaluate the actual diagnostic accuracy of methylated hTERT. Therefore, we performed subgroup analysis to assess the effects of these factors on the diagnostic efficiency of methylated hTERT. We demonstrated that quantitative MSP (qMSP) assay offers the highest discriminative power between normal and cancer in comparison with different detection methods. In addition, the methylated sites selected by different studies had an impact on the detection performance. Moreover, the diagnostic power of methylated hTERT was affected by cancer type and histological subtype. In conclusion, the existing evidence demonstrated that methylated hTERT is effective in cancer detection. Detailed profiling of the methylation sites to local the common methylation hotspot across human cancers is warranted to maximize the diagnostic value of methylated hTERT in cancer detection.

Chemoprevention of Breast Cancer by Dietary Polyphenols

The review will discuss in detail the effects of polyphenols on breast cancer, including both the advantages and disadvantages of the applications of these natural compounds. First, we focus on the characterization of the main classes of polyphenols and then on in vitro and in vivo experiments carried out in breast cancer models. Since the therapeutic effects of the administration of a single type of polyphenol might be limited because of the reduced bioavailability of these drugs, investigations on combination of several polyphenols or polyphenols with conventional therapy will also be discussed. In addition, we present recent data focusing on clinical trials with polyphenols and new approaches with nanoparticles in breast cancer. Besides the clinical and translational findings this review systematically summarizes our current knowledge about the molecular mechanisms of anti-cancer effects of polyphenols, which are related to apoptosis, cell cycle regulation, plasma membrane receptors, signaling pathways and epigenetic mechanisms. At the same time the effects of polyphenols on primary tumor, metastasis and angiogenesis in breast cancer are discussed. The increasing enthusiasm regarding the combination of polyphenols and conventional therapy in breast cancer might lead to additional efforts to motivate further research in this field.

DNA methylation analysis of cancer-related genes in oral epithelial cells of healthy smokers

AIM

The aim of this study was to investigate the smoking habit influence on DNA methylation status in the promoters of the cancer related-genes MLH1, hTERT and TP53 in oral epithelial cells of healthy subjects.

MATERIALS AND METHODS

DNA methylation analysis was performed using methylation-sensitive restriction enzymes (MSRE) in oral epithelial cells from non-smokers, smokers and ex-smokers.

RESULTS

The investigated CpG dinucleotides located at HhaI and HpaII sites in the MLH1 gene promoter were observed to be fully methylated in the majority of DNA samples from the smoker group and statistical differences were found between non-smokers and smokers and between smokers and ex-smokers (p<0.05). The same was observed in the hTERT gene promoter at HhaI sites (p<0.05) and for HpaII sites the unmethylated condition was more frequent in smokers in comparison to non-smokers (p<0.05). For TP53, no differences were found among groups (p>0.05), with the fully methylated condition found to be a common event in healthy oral epithelial cells.

CONCLUSION

We conclude that smoking may induce changes in DNA methylation status in cancer-related genes of oral epithelial cells and that the cessation of smoking is capable of reversing this process. Based on our data, we suggest that DNA methylation status of the hTERT and MLH1 gene promoters are promising markers for screening a set of smoking-related alterations in oral cells.

Expression profile of shelterin components in plasma cell disorders. Clinical significance of POT1 overexpression

The core complex of telomere-associated proteins, named the shelterin complex, plays a critical role in telomere protection and telomere length (TL) homeostasis. In this study, we have explored changes in the expression of telomere-associated genes POT1, TIN2, RAP1 and TPP1, in patients with monoclonal gammopathy of undetermined significance (MGUS) and multiple myeloma (MM). A total of 154 patients: 70 with MGUS and 84 with MM were studied. Real-time quantitative PCR was used to quantify gene expression. TL was evaluated by Terminal Restriction Fragments. Our data showed increased expression of POT1, TPP1, TIN2 and RAP1 in MM with respect to MGUS patients, with significant differences for POT1 gene (p=0.002). In MM, the correlation of gene expression profiles with clinical characteristics highlighted POT1 for its significant association with advanced clinical stages, high calcium and β2-microglobulin levels (p=0.02) and bone lesions (p=0.009). In multivariate analysis, POT1 expression (p=0.04) was a significant independent prognostic factor for overall survival as well as the staging system (ISS) (p<0.02). Our findings suggest for the first time the participation of POT1 in the transformation process from MGUS to MM, and provide evidence of this gene as a useful prognostic factor in MM as well as a possible molecular target to design new therapeutic strategies.

Methylation of the hTERT promoter: a novel cancer biomarker for leptomeningeal metastasis detection in cerebrospinal fluids

PURPOSE

The diagnosis of leptomeningeal metastases is usually confirmed by the finding of malignant cells by cytologic examination in the cerebrospinal fluid (CSF). More sensitive and specific cancer biomarkers may improve the detection of tumor cells in the CSF. Promoter methylation of the human telomerase reverse transcriptase (hTERT) gene characterizes most cancer cells. The aim of this study was to develop a sensitive method to detect hTERT methylation and to explore its use as a cancer biomarker in CSF.

EXPERIMENTAL DESIGN

In 77 CSF specimens from 67 patients, hTERT promoter methylation was evaluated using real-time methylation-sensitive high-resolution melting (MS-HRM) and real-time TaqMan PCR and MS-HRM in a single-tube assay.

RESULTS

Real-time MS-HRM assay was able to detect down to 1% hTERT-methylated DNA in a background of unmethylated DNA. PCR products were obtained from 90% (69/77) of CSF samples. No false positive hTERT was detected in the 21 non-neoplastic control cases, given to the method a specificity of 100%. The sensitivity of the real-time MS-HRM compared with the cytologic gold standard analysis was of 92% (11/12). Twenty-six CSFs from 22 patients with an hTERT-methylated primary tumor showed cytologic results suspicious for malignancy; in 17 (65%) of them, a diagnosis of leptomeningeal metastases could be confirmed by the hTERT methylation test.

CONCLUSION

The hTERT real-time MS-HRM approach is fast, specific, sensitive, and could therefore become a valuable tool for diagnosis of leptomeningeal metastases as an adjunct to the traditional examination of CSF.

hTERT promoter methylation and telomere length in childhood acute lymphoblastic leukemia: associations with immunophenotype and cytogenetic subgroup

Telomere maintenance, important for long-term cell survival and malignant transformation, is directed by a multitude of factors, including epigenetic mechanisms, and has been implicated in outcomes for patients with leukemia. In the present study, the objective was to investigate the biological and clinical significance of telomere length and promoter methylation of the human telomerase reverse transcriptase gene in childhood acute lymphoblastic leukemia. A cohort of 169 childhood acute lymphoblastic leukemias was investigated for telomere length, human telomerase reverse transcriptase gene promoter methylation status, genomic aberrations, immunophenotype, and clinical outcomes. Methylation of the core promoter of the human telomerase reverse transcriptase (hTERT) gene was demonstrated in 24% of diagnostic samples, with a significant difference between B-cell precursor (n = 130) and T-cell acute lymphoblastic leukemia (ALL) (n = 17) cases (18% and 72%, respectively; p < 0.001). No remission sample demonstrated hTERT promoter methylation (n = 40). Within the B-cell precursor group, t(12;21)(p13;q22) [ETV6/RUNX1] cases (n = 19) showed a much higher frequency of hTERT methylation than high-hyperdiploid (51-61 chromosomes) ALL (n = 44) (63% and 7%, respectively; p < 0.001). hTERT messenger RNA levels were negatively associated with methylation status and, in the t(12;21) group, methylated cases had shorter telomeres (p = 0.017). In low-risk B-cell precursor patients (n = 101), long telomeres indicated a worse prognosis. The collected data from the present study indicate that the telomere biology in childhood ALL has clinical implications and reflects molecular differences between diverse ALL subgroups.

Human telomerase expression regulation

Since telomerase has been recognized as a relevant factor distinguishing cancer cells from normal cells, it has become a very promising target for anti-cancer therapy. A correlation between short telomere length and increased mortality was revealed in many studies. The telomerase expression/activity appears to be one of the most crucial factors to study to improve cancer therapy and prevention. However, this multisubunit enzymatic complex can be regulated at various levels. Thus, several strategies have been proposed to control telomerase in cancer cells such as anti-sense technology against TR and TERT, ribozymes against TERT, anti-estrogens, progesterone, vitamin D, retinoic acid, quadruplex stabilizers, telomere and telomerase targeting agents, modulation of interaction with other proteins involved in the regulation of telomerase and telomeres, etc. However, the transcription control of key telomerase subunits seems to play the crucial role in whole complexes activity and cancer cells immortality. Thus, the research of telomerase regulation can bring significant insight into the knowledge concerning stem cells metabolism but also ageing. This review summarizes the current state of knowledge of numerous telomerase regulation mechanisms at the transcription level in human that might become attractive anti-cancer therapy targets.

Cancer chemoprevention by dietary polyphenols: promising role for epigenetics

Epigenetics refers to heritable changes that are not encoded in the DNA sequence itself, but play an important role in the control of gene expression. In mammals, epigenetic mechanisms include changes in DNA methylation, histone modifications and non-coding RNAs. Although epigenetic changes are heritable in somatic cells, these modifications are also potentially reversible, which makes them attractive and promising avenues for tailoring cancer preventive and therapeutic strategies. Burgeoning evidence in the last decade has provided unprecedented clues that diet and environmental factors directly influence epigenetic mechanisms in humans. Dietary polyphenols from green tea, turmeric, soybeans, broccoli and others have shown to possess multiple cell-regulatory activities within cancer cells. More recently, we have begun to understand that some of the dietary polyphenols may exert their chemopreventive effects in part by modulating various components of the epigenetic machinery in humans. In this article, we first discuss the contribution of diet and environmental factors on epigenetic alterations; subsequently, we provide a comprehensive review of literature on the role of various dietary polyphenols. In particular, we summarize the current knowledge on a large number of dietary agents and their effects on DNA methylation, histone modifications and regulation of expression of the non-coding miRNAs in various in vitro and in vivo models. We emphasize how increased understanding of the chemopreventive effects of dietary polyphenols on specific epigenetic alterations may provide unique and yet unexplored novel and highly effective chemopreventive strategies for reducing the health burden of cancer and other diseases in humans.

Epigenetic targets of bioactive dietary components for cancer prevention and therapy

The emergent interest in cancer epigenetics stems from the fact that epigenetic modifications are implicated in virtually every step of tumorigenesis. More interestingly, epigenetic changes are reversible heritable changes that are not due to the alteration in DNA sequence but have potential to alter gene expression. Dietary agents consist of many bioactive ingredients which actively regulate various molecular targets involved in tumorigenesis. We present evidence that numerous bioactive dietary components can interfere with various epigenetic targets in cancer prevention and therapy. These agents include curcumin (turmeric), genistein (soybean), tea polyphenols (green tea), resveratrol (grapes), and sulforaphane (cruciferous vegetables). These bioactive components alter the DNA methylation and histone modifications required for gene activation or silencing in cancer prevention and therapy. Bioactive components mediate epigenetic modifications associated with the induction of tumor suppressor genes such as p21(WAF1/CIP1) and inhibition of tumor promoting genes such as the human telomerase reverse transcriptase during tumorigenesis processes. Here, we present considerable evidence that bioactive components and their epigenetic targets are associated with cancer prevention and therapy which should facilitate novel drug discovery and development. In addition, remarkable advances in our understanding of basic epigenetic mechanisms as well as the rapid progress that is being made in developing powerful new technologies, such as those for sensitive and quantitative detection of epigenetic and epigenomic changes in cancer biology, hold great promise for novel epigenetic approaches to cancer prevention and therapy.

[Telomeres and telomerase as targeted therapies in cancer treatment]

Advances in chromosome dynamics have increased our understanding of the significant role of telomeres and telomerase in cancer. Telomerase is expressed in almost all cancer cells but is inactive in most normal somatic cells. Therefore, telomerase is an important target for the design of therapeutic agents that might have minimal side effects. Herein, we evaluate current approaches to telomerase/telomere-targeted therapy, discuss the benefits and disadvantages, and speculate on the future direction of telomerase inhibitors as cancer therapeutics.

BORIS/CTCFL-mediated transcriptional regulation of the hTERT telomerase gene in testicular and ovarian tumor cells

Telomerase activity, not detectable in somatic cells but frequently activated during carcinogenesis, confers immortality to tumors. Mechanisms governing expression of the catalytic subunit hTERT, the limiting factor for telomerase activity, still remain unclear. We previously proposed a model in which the binding of the transcription factor CTCF to the two first exons of hTERT results in transcriptional inhibition in normal cells. This inhibition is abrogated, however, by methylation of CTCF binding sites in 85% of tumors. Here, we showed that hTERT was unmethylated in testicular and ovarian tumors and in derivative cell lines. We demonstrated that CTCF and its paralogue, BORIS/CTCFL, were both present in the nucleus of the same cancer cells and bound to the first exon of hTERT in vivo. Moreover, exogenous BORIS expression in normal BORIS-negative cells was sufficient to activate hTERT transcription with an increasing number of cell passages. Thus, expression of BORIS was sufficient to allow hTERT transcription in normal cells and to counteract the inhibitory effect of CTCF in testicular and ovarian tumor cells. These results define an important contribution of BORIS to immortalization during tumorigenesis.

Unrestricted somatic stem cells (USSC) from human umbilical cord blood display uncommitted epigenetic signatures of the major stem cell pluripotency genes

Unrestricted somatic stem cells (USSC) from human cord blood display a broad differentiation potential for ectodermal, mesodermal, and endodermal cell types. The molecular basis for these stem cell properties is unclear and unlike embryonic stem cells (ESC) none of the major stem cell factors OCT4, SOX2, and NANOG exhibits significant expression in USSC. Here, we report that these key stem cell genes hold an epigenetic state in between that of an ESC and a terminally differentiated cell type. DNA methylation analysis exhibits partial demethylation of the regulatory region of OCT4 and a demethylated state of the NANOG and SOX2 promoter/enhancer regions. Further genome-wide DNA methylation profiling identified a partially demethylated state of the telomerase gene hTERT. Moreover, none of the pluripotency factors exhibited a repressive histone signature. Notably, SOX2 exhibits a bivalent histone signature consisting of the opposing histone marks dimeH3K4 and trimeH3K27, which is typically found on genes that are "poised" for transcription. Consequently, ectopic expression of OCT4 in USSC led to rapid induction of expression of its known target gene SOX2. Our data suggest that incomplete epigenetic repression and a "poised" epigenetic status of pluripotency genes preserves the USSC potential to be able to react adequately to distinct differentiation and reprogramming cues.

Epigenetic regulation of telomerase expression in HIV-1-specific CD8+ T cells

Telomerase activity in HIV-1-specific CD8(+) T cells from controllers contributes to the maintenance of highly functional cytotoxic T cell responses against HIV-1. Here, we show that high expression of telomerase in controllers is associated with hypermethylation at the distal and hypomethylation at the proximal human telomerase catalytic subunit promoter, whereas HIV-1-specific CD8(+) T cells from progressors showed an inverse pattern with distal promoter hypomethylation and proximal promoter hypermethylation. These data suggest distinct epigenetic signatures in HIV-1-specific T cells in progressors and controllers.

hTERT promoter activity and CpG methylation in HPV-induced carcinogenesis

Background

Activation of telomerase resulting from deregulated hTERT expression is a key event during high-risk human papillomavirus (hrHPV)-induced cervical carcinogenesis. In the present study we examined hTERT promoter activity and its relation to DNA methylation as one of the potential mechanisms underlying deregulated hTERT transcription in hrHPV-transformed cells.

Methods

Using luciferase reporter assays we analyzed hTERT promoter activity in primary keratinocytes, HPV16- and HPV18-immortalized keratinocyte cell lines and cervical cancer cell lines. In the same cells as well as cervical specimens we determined hTERT methylation by bisulfite sequencing analysis of the region spanning -442 to +566 (relative to the ATG) and quantitative methylation specific PCR (qMSP) analysis of two regions flanking the hTERT core promoter.

Results

We found that in most telomerase positive cells increased hTERT core promoter activity coincided with increased hTERT mRNA expression. On the other hand basal hTERT promoter activity was also detected in telomerase negative cells with no or strongly reduced hTERT mRNA expression levels. In both telomerase positive and negative cells regulatory sequences flanking both ends of the core promoter markedly repressed exogenous promoter activity.

By extensive bisulfite sequencing a strong increase in CpG methylation was detected in hTERT positive cells compared to cells with no or strongly reduced hTERT expression. Subsequent qMSP analysis of a larger set of cervical tissue specimens revealed methylation of both regions analyzed in 100% of cervical carcinomas and 38% of the high-grade precursor lesions, compared to 9% of low grade precursor lesions and 5% of normal controls.

Conclusions

Methylation of transcriptionally repressive sequences in the hTERT promoter and proximal exonic sequences is correlated to deregulated hTERT transcription in HPV-immortalized cells and cervical cancer cells. The detection of DNA methylation at these repressive regions may provide an attractive biomarker for early detection of cervical cancer.

Advances in research of the epigenetic regulation of hTERT expression

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of the telomerase, is the rate-limiting component for telomerase activity. Epigenetic regulation of gene transcription does not change DNA sequences but depends on chemical modification of either DNA or histones or non-coding RNAs. Epigenetic regulation is inheritable and plays an important role in controlling gene expression. The expression of hTERT may also be subjected to epigenetic regulation, such as DNA methylation, histone acetylation and methylation, and non-coding RNAs.

Analysis of DNA methylation status of the promoter of human telomerase reverse transcriptase in gastric carcinogenesis

BACKGROUND AND AIMS

Telomerase is expressed in normal somatic cells and reactivated in majority of tumor cells. Human telomerase reverse transcriptase (hTERT), a catalytic subunit of telomerase, is a rate-limiting factor of telomerase activity. Evidence has shown that gastric cancer is the result of genetics and epignomics. DNA methylation is one of the most important research fields in epigenomics. It is one of the mechanisms resulting in gene silencing in carcinogenesis.

METHODS

Genomic DNAs were extracted from normal gastric mucosa, precancerous lesions and gastric cancer samples and were modified by sodium bisulfite. The modified genomic DNAs were amplified by PCR with primers that did not contain CpG sites. Each PCR product was sequenced. By matching the sequencing results and the original sequence, the status of each sample was obtained. PCR was carried out to identify hTERT expression.

RESULTS

The promoter of hTERT in gastric cancer was more methylated than in the precancerous lesions and normal gastric mucosa (p<0.05). hTERT was absent in normal gastric mucosa and its positive rate was higher in gastric cancer than in precancerous lesions (p<0.05).

CONCLUSIONS

hTERT promoter in gastric cancer was more methylated than in the precancerous lesions and normal gastric mucosa. This may suggest that the degree of methylation of the hTERT promoter was increased during gastric carcinogenesis and may be a potential biological maker in early diagnosis of gastric cancer. During gastric carcinogenesis, expression of hTERT was increased. This may suggest that methylation of hTERT may influence expression of hTERT.

Epigenetic plasticity of hTERT gene promoter determines retinoid capacity to repress telomerase in maturation-resistant acute promyelocytic leukemia cells

The expression of hTERT gene, encoding the catalytic subunit of telomerase, is a feature of most cancer cells. Changes in the chromatin environment of its promoter and binding of transcriptional factors have been reported in differentiating cells when its transcription is repressed. However, it is not clear whether these changes are directly involved in this repression or only linked to differentiation. In a maturation-resistant acute promyelocytic leukemia (APL) cell line (NB4-LR1), we have previously identified a new pathway of retinoid-induced hTERT repression independent of differentiation. Using a variant of this cell line (NB4-LR1(SFD)), which resists to this repression, we show that although distinct patterns of histone modifications and transcription factor binding at the proximal domain of hTERT gene promoter could concur to modulate its expression, this region is not sufficient to the on/off switch of hTERT by retinoids. DNA methylation analysis of the hTERT promoter led to the identification of two distinct functional domains, a proximal one, fully unmethylated in both cell lines, and a distal one, significantly methylated in NB4-LR1(SFD) cells, whose methylation was further re-enforced by retinoid treatment. Interestingly, we showed that the binding to this distal domain of a known hTERT repressor, WT1, was defective only in NB4-LR1(SFD) cells. We propose that epigenetic modifications targeting this distal region could modulate the binding of hTERT repressors and account either for hTERT reactivation and resistance to retinoid-induced hTERT downregulation.

Transcriptional activation of hTERT, the human telomerase reverse transcriptase, by nuclear factor of activated T cells

Telomerase is essential for telomere maintenance, and its activation is thought to be a critical step in cellular immortalization and tumorigenesis. Human telomerase reverse transcriptase (hTERT) is a major component of telomerase activity. We show here that hTERT is expressed soon after lymphocyte activation and that its expression is inhibited by rapamycin, wortmannin, and FK506, which was the most potent inhibitor. These results suggest a potential role for the transcription factor nuclear factor of activated T cells (NFAT) in the regulation of hTERT expression. Five putative NFAT-binding sites were identified in the hTERT promoter. In luciferase assays, the hTERT promoter was activated by overexpressed NFAT1. Moreover, serial deletions revealed that the promoter activation was mainly due to a -40 NFAT1-binding site flanked by two SP1-binding sites. Mutation of the -40 NFAT-binding site caused a 53% reduction in the transcriptional activity of hTERT promoter. Simultaneous mutations of the -40 NFAT-responsive element together with one or both SP1-binding sites led to a more dramatic decrease in luciferase activity than single mutations, suggesting a functional synergy between NFAT1 and SP1 in hTERT transcriptional regulation. NFAT1 overexpression in MCF7 and Jurkat cell lines induced an increase in endogenous hTERT mRNA expression. Inversely, its down-regulation was induced by NFAT1 silencing. Furthermore, chromatin immunoprecipitation assay demonstrated that NFAT1 directly binds to two sites (-40 and -775) in the endogenous hTERT promoter. Thus, we show for the first time the direct involvement of NFAT1 in the transcriptional regulation of hTERT.

TSA-induced DNMT1 down-regulation represses hTERT expression via recruiting CTCF into demethylated core promoter region of hTERT in HCT116

Trichostatin A (TSA), an inhibitor of histone deacetylase, is a well-known antitumor agent that effectively and selectively induces tumor growth arrest and apoptosis. Recently, it was reported that hTERT is one of the primary targets for TSA-induced apoptosis in cancer cells but the mechanism of which has not yet been elucidated. In the present study, to better understand the epigenetic regulation mechanism responsible for the repression of hTERT by TSA, we examined expression of hTERT in the HCT116 colon cancer cell line after treatment with TSA and performed site-specific CpG methylation analysis of the hTERT promoter. We found that TSA-induced the demethylation of site-specific CpGs on the promoter of hTERT, which was caused by down-regulation of DNA methyltransferase 1 (DNMT1). Among the demethylated region, the 31st-33rd CpGs contained a binding site for CTCF, an inhibitor of hTERT transcription. ChIP analysis revealed that TSA-induced demethylation of the 31st-33rd CpGs promoted CTCF binding on hTERT promoter, leading to repression of hTERT. Taken together, down-regulation of DNMT1 by TSA caused demethylation of a CTCF binding site on the hTERT promoter, the result of which was repression of hTERT via recruitment of CTCF to the promoter.

Specific association between the methyl-CpG-binding domain protein 2 and the hypermethylated region of the human telomerase reverse transcriptase promoter in cancer cells

Human telomerase reverse transcriptase (hTERT) is expressed in most cancer cells. Paradoxically, its promoter is embedded in a hypermethylated CpG island. A short region escapes to this alteration, allowing a basal level of transcription. However, the methylation of adjacent regions may play a role in the maintenance of low hTERT expression. It is now well established that methyl-CpG binding domain proteins mediate the transcriptional silencing of hypermethylated genes. The potential involvement of these proteins in the control of hTERT expression was firstly investigated in HeLa cells. Chromatin immunoprecipitation assays showed that only methyl-CpG-binding domain protein 2 (MBD2) associated the hypermethylated hTERT promoter. In MBD2 knockdown HeLa cells, constitutively depleted in MBD2, neither methyl CpG binding protein 2 (MeCP2) nor MBD1 acted as substitutes for MBD2. MBD2 depletion by transient or constitutive RNA interference led to an upregulation of hTERT transcription that can be downregulated by expressing mouse Mbd2 protein. Our results indicate that MBD2 is specifically and directly involved in the transcriptional repression of hTERT in HeLa cells. This specific transcriptional repression was also observed in breast, liver and neuroblastoma cancer cell lines. Thus, MBD2 seems to be a general repressor of hTERT in hTERT-methylated telomerase-positive cells.