DNA methylation of the TERT promoter and its impact on human cancer
Introduction
Replicative immortality is a hallmark of cancer characterized by the ability of cancer cells to bypass senescence and achieve unlimited proliferative capacity [1]. This characteristic is largely dependent on the activation of a telomere maintenance mechanism (although not always; see Chapter X) and allows cancer cells to form macroscopic tumors as well as relapse or recur after initial treatment.
Telomeres are end-chromosomal DNA–protein complexes consisting primarily of (TTAGGG)n tandem DNA repeats protected by a 6-member protein complex known as shelterin [2]. Telomeres protect chromosomal ends from being recognized by the DNA repair machinery, thus preventing chromosomal end-to-end fusions. Progressive shortening of telomeric sequences occurs after each cell division as a result of the end-replication problem, driving normal somatic cells towards senescence once telomeres reach a critically shortened length [3]. In the absence of cell-cycle checkpoints, cells will continue to proliferate without entering senescence until they enter crisis, a chaotic state of chromosomal end-fusions and mitotic catastrophe. While the majority of cells will be irreversibly committed to apoptosis at this stage, a small population will spontaneously emerge and acquire unlimited replicative potential by actively maintaining their telomere lengths [3,4]. This process is known as cellular immortalization, a crucial step in carcinogenesis and cancer progression [5].
In humans, most somatic cells do not display a telomere maintenance mechanism. However, certain cell types including normal stem cells, germ cells and activated memory lymphocytes maintain telomere length through the activity of the telomerase enzymatic complex [6]. Research in the past few decades has revealed that the telomerase holoenzyme is tightly regulated in normal somatic cells, most importantly by repressing its rate-limiting component, telomerase reverse transcriptase (TERT). To overcome this intrinsic defensive mechanism against uncontrolled proliferation and malignant transformation, most cancer cells acquire alterations in order to aberrantly upregulate TERT expression and eventually enable telomere maintenance.
Genetic alterations that result in aberrant upregulation of TERT expression include TERT amplification [7], TERT rearrangements [8,9], and TERT promoter mutations (TPM) [10,11], which have been identified in several human cancer types. TPMs have been associated with upregulation of TERT expression and poorer clinical outcomes in several cancers including brain [12], thyroid [13], bladder [14••], and skin [15]. However, other common cancers (i.e. breast, prostate, lung, and colon) harbor a low frequency of TPM and/or other known genetic alterations, and the mechanism of telomerase activation in these tumors remains largely unknown.
Some of the understudied aspects in TERT transcriptional regulation are the epigenetic alterations at the TERT promoter. Although the interplay between histone modifications and DNA methylation has not yet been fully explored, numerous studies have provided clinical and mechanistic insights on the role of TERT promoter DNA methylation in cancer [16, 17, 18, 19]. In most cases, promoter DNA methylation of human genes results in downregulated expression or complete gene silencing [20]. Interestingly, the human TERT promoter that is highly enriched for CpG dinucleotides and thus is subject to DNA methylation does not behave in such a simplistic manner. In this review, we discuss recent progress in understanding the role of DNA methylation at the TERT promoter as a telomere maintenance activating mechanism in the context of human cancer, from both biological and clinical perspectives. Furthermore, we review how this knowledge can be implicated in potential telomere-based anti-cancer strategies.
Section snippets
Understanding the DNA methylation landscape of the TERT promoter in human cancer
Initial studies using human cell lines to assess DNA methylation as a form of epigenetic regulation at the TERT promoter have generated inconsistent results. One seminal study reported that non-TERT expressing normal cell lines had unmethylated/hypomethylated promoters, while two-thirds of TERT expressing cancer cell lines exhibited partial or total methylation at the promoter [21]. Contradictory to this, other groups have reported TERT expressing cancer cell lines with hypomethylated TERT
Biological role of cancer-specific DNA methylation within the TERT promoter
Although promoter DNA hypermethylation is generally associated with gene silencing [20], a genome-wide study in prostate cancer demonstrated that a large number of promoter-associated CpG islands that were hypermethylated surprisingly showed transcriptional activation [29]. This is consistent with the observation that TERT promoter (THOR) hypermethylation is associated with upregulation of TERT expression in cancers (Figure 1a). Functional analyses using luciferase reporter plasmid vectors have
Clinical investigation of cancer-specific DNA methylation within the TERT promoter
The prevalence of TERT promoter (THOR) hypermethylation was recently examined and reported to be ≥45% in over 1300 human cancer samples across 11 tumor types screened [27••]. This methylation signature exhibited striking specificity, as >90% of the tumors exceeded the median THOR methylation level of normal samples. Interestingly, THOR hypermethylation is more prevalent (>70%) in cancer types that lack TPMs, such as lung, breast, prostate, and colon cancers, compared to cancer types that
Clinical implications of THOR hypermethylation signature
Since the introduction of UTSS methylation status as a potential biomarker in pediatric brain tumors [19], several clinical studies published in the recent years have focused their analyses on this smaller region within THOR (Figure 2). The mapping of THOR, a larger region consisting of 52 CpG sites that display cancer-specific hypermethylation [27••], may lead to the identification of novel cancer-specific DNA methylation signatures that would serve as predictive and prognostic biomarkers in
Conclusion and future directions
In this review, we highlight the emerging role of epigenetic regulation through DNA methylation of the TERT promoter as a telomere maintenance activating mechanism in cancer. This phenomenon is an attractive target – both biologically and clinically – as it is commonly observed across most malignant human cancer types. Comprehensive understanding of the timing and mechanisms that govern cancer-specific DNA methylation of the TERT promoter during malignant transformation will be crucial in
Conflicts of interest statement
Nothing declared.
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
References (58)
- et al.
Hallmarks of cancer: the next generation
Cell
(2011) - et al.
The telomere deprotection response is functionally distinct from the genomic DNA damage response
Mol Cell
(2013) - et al.
DNA methylation landscapes: provocative insights from epigenomics
Nat Rev Genet
(2008) - et al.
DNA hypermethylation within TERT promoter upregulates TERT expression in cancer
J Clin Invest
(2019) - et al.
CTCF: master weaver of the genome
Cell
(2009) - et al.
Human glioblastoma arises from subventricular zone cells with low-level driver mutations
Nature
(2018) - et al.
Promoter hypermethylation of TERT is associated with hepatocellular carcinoma in the Han Chinese population
Clin Res Hepatol Gastroenterol
(2015) - et al.
Genetic and epigenetic alterations of TERT are associated with inferior outcome in adolescent and young adult patients with melanoma
Sci Rep
(2017) - et al.
Genetic and epigenetic background and protein expression profiles in relation to telomerase activation in medullary thyroid carcinoma
Oncotarget
(2016) - et al.
TERT promoter methylation is significantly associated with TERT upregulation and disease progression in pituitary adenomas
J Neurooncol
(2019)
Involvement of epigenetic modification of TERT promoter in response to all-trans retinoic acid in ovarian cancer cell lines
J Ovarian Res
How shelterin solves the telomere end-protection problem
Cold Spring Harb Symp Quant Biol
Cell death during crisis is mediated by mitotic telomere deprotection
Nature
Senescence and immortalization: role of telomeres and telomerase
Carcinogenesis
Telomerase in T lymphocytes: use it and lose it?
J Immunol
Increased copy number of the TERT and TERC telomerase subunit genes in cancer cells
Cancer Sci
Telomerase activation by genomic rearrangements in high-risk neuroblastoma
Nature
TERT rearrangements are frequent in neuroblastoma and identify aggressive tumors
Nat Genet
TERT promoter mutations in familial and sporadic melanoma
Science
Highly recurrent TERT promoter mutations in human melanoma
Science
TERT promoter mutations: a novel independent prognostic factor in primary glioblastomas
Neuro Oncol
TERT promoter mutations are a major indicator of poor outcome in differentiated thyroid carcinomas
J Clin Endocrinol Metab
Combined genetic and epigenetic alterations of the TERT promoter affect clinical and biological behavior of bladder cancer
Int J Cancer
TERT promoter mutation status as an independent prognostic factor in cutaneous melanoma
J Natl Cancer Inst
Correlation of promoter hypermethylation in hTERT, DAPK and MGMT genes with cervical oncogenesis progression
Oncol Rep
Positive regulation of human telomerase reverse transcriptase gene expression and telomerase activity by DNA methylation in pancreatic cancer
Ann Surg Oncol
hTERT promoter activity and CpG methylation in HPV-induced carcinogenesis
BMC Cancer
Methylation of the TERT promoter and risk stratification of childhood brain tumours: an integrative genomic and molecular study
Lancet Oncol
DNA methylation analysis of the promoter region of the human telomerase reverse transcriptase (hTERT) gene
Cancer Res
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Evolution of Diverse Strategies for Promoter Regulation
2021, Trends in GeneticsCitation Excerpt :Examples show that promoter polymorphism and methylation are associated with diseases, such as PARK2 and colorectal cancer [79] or the association of TNFa promoter polymorphisms with multiple sclerosis [80]. Similarly, the hTERT promoter forms three parallel G-quadruplexes [81] and its mutation and methylation influence human tumorigenesis [82,83]. Methylation is a type of epigenetic modification that we consider in more detail later.
TERT Gene Fusions Characterize a Subset of Metastatic Leydig Cell Tumors
2021, Clinical Genitourinary CancerCitation Excerpt :If the fusions are early events, patients with fusion-positive primary tumors could have increased surveillance. In addition, the TERT promoter mutations, commonly observed in other malignancies (eg, gliomas, bladder, thyroid cancers, and melanoma), were not possible to examine in this study owing to the lack of the gene promoter coverage in the NGS panel available at the time of study.30–33 Finally, there is lack of feedback information on the usefulness of molecular profiling in the treatment of metastatic LCT with potentially actionable findings detected in our cohort (eg, overexpression of Topo1 and AR).
Unveiling the mechanisms and challenges of cancer drug resistance
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