Chromatin and epigenetic regulation of the telomerase reverse transcriptase gene

CTCF mediates the TERT enhancer-promoter interactions in lung cancer cells: identification of a novel enhancer region involved in the regulation of TERT gene

Telomerase activation is a hallmark of cancer. Although the regulation of the telomerase reverse transcriptase catalytic subunit (TERT), the rate-limiting factor for telomerase activity, has been studied intensively it remains incompletely understood. In cells devoid of telomerase activity, TERT is embedded in a region of condensed chromatin and the chromatin remodeling protein CCCTC-binding factor (CTCF) has been implicated in the inhibition of TERT expression. The importance of TERT activation for cellular immortalization and carcinogenesis is attested by the fact that the gene is expressed in more than 90% of immortal cell lines and tumors and that gain of TERT is the most frequent amplification event in early stage lung cancer. This study was designed to study the mechanisms of regulation of the TERT gene expression by the CTCF transcription factor in three human lung cancer cell lines, A427, A549 and H838. Depletion of CTCF by siRNA resulted in reduced TERT mRNA levels in two (A427 and A549) of the three cell lines. A novel enhancer element was identified approximately 4.5 kb upstream of the TERT transcription start site. Chromatin immunoprecipitation experiments revealed recruitment of CTCF to this enhancer element. Chromosome conformation capture experiments demonstrated the presence of CTCF-dependent chromatin loops between this enhancer element and the TERT proximal promoter in A427 and A549 cell lines. In summary, the results show that CTCF plays an important role in maintaining TERT expression in a subset of human lung cancer cell lines. This role may be due to CTCF-dependent enhancer-promoter interactions.

Epigenetic regulation of the human telomerase reverse transciptase gene: A potential therapeutic target for the treatment of leukemia (Review)

Telomerase activation is a critical step in human carcinogenesis through the maintenance of telomeres. Telomerase activity is primarily regulated by the human telomerase reverse transcriptase gene (hTERT), thus, an improved understanding of the transcriptional control of hTERT may provide potential therapeutic targets for the treatment of leukemia and other forms of cancer. Epigenetic modulation, a significant regulatory process in cell biology, has recently been shown to be involved in the regulation of the hTERT gene. Moreover, several epigenetic modifiers, including DNA methyltransferase (DNMT) and histone deacetylase (HDAC) inhibitors, are now in pre- and early clinical trials of leukemia as monotherapies or in combination with other drugs, and have achieved significant clinical success. In the present review, the epigenetic mechanisms associated with telomerase activity in leukemia, and the therapeutic potential of an antitelomerase strategy that combines epigenetic modifiers with telomerase hTR subunit small molecule inhibitors are discussed.

Alternative mechanisms of telomere lengthening: permissive mutations, DNA repair proteins and tumorigenic progression

Telomeres protect chromosome termini to maintain genomic stability and regulate cellular lifespan. Maintenance of telomere length is required for neoplastic cells after the acquisition of mutations that deregulate cell cycle control and increase cellular proliferation, and can occur through expression of the enzyme telomerase or in a telomerase-independent manner termed alternative lengthening of telomeres (ALT). The precise mechanisms that govern the activation of ALT or telomerase in tumor cells are unknown, although cellular origin may favor one or the other mechanisms. ALT pathways are incompletely understood to date; however, recent publications have increasingly broadened our understanding of how ALT is activated, how it proceeds, and how it influences tumor growth. Specific mutational events influence ALT activation, as mutations in genes that suppress recombination and/or alterations in the regulation of telomerase expression are associated with ALT. Once engaged, ALT uses DNA repair proteins to maintain telomeres in the absence of telomerase; experiments that manipulate the expression of specific proteins in cells using ALT are illuminating some of its mechanisms. Furthermore, ALT may influence tumor growth, as experimental and clinical data suggest that telomerase expression may favor tumor progression. This review summarizes recent findings in mammalian cells and models, as well as clinical data, that identify the genetic mutations permissive to ALT, the DNA repair proteins involved in ALT mechanisms and the importance of telomere maintenance mechanisms for tumor progression. A comprehensive understanding of the mechanisms that permit tumor cell immortalization will be important for identifying novel therapeutic targets in cancer.

Developmental silencing of the AtTERT gene is associated with increased H3K27me3 loading and maintenance of its euchromatic environment

Telomerase, an enzyme responsible for the maintenance of linear chromosome ends, is precisely regulated during plant development. In animals, involvement of the epigenetic state of the telomerase reverse transcriptase (TERT) gene in the complex regulation of telomerase activity has been reported. To reveal whether epigenetic mechanisms participate in the regulation of plant telomerase, the relationship between telomerase activity in tissues of Arabidopsis thaliana and DNA methylation and histone modifications in the A. thaliana TERT (AtTERT) upstream region was studied. As expected, a gradual decrease of telomerase activity during leaf maturation was observed. A different pattern with a more progressive loss of telomerase activity and AtTERT transcription during leaf development was revealed in MET1 gene-knockout mutants. Analysis of DNA methylation in the AtTERT upstream region showed low levels of methylated cytosines without notable differences between telomerase-positive and telomerase-negative wild-type tissues. Surprisingly, a high level of CG methylation was found in the AtTERT coding region, although this type of methylation is a characteristic attribute of constitutively expressed genes. Analysis of chromatin modifications in the AtTERT upstream region and in exon 5 showed increased loading of the H3K27me3 mark in the telomerase-negative mature leaf compared to telomerase-positive seedlings, whereas H3K4me3, H3K9Ac, and H3K9me2 were approximately at the same level. Consistently, the chromatin structure of the AtTERT gene was maintained. These results are discussed in the context of the general involvement of epigenetic mechanisms in the regulation of gene expression and with respect to similar studies performed in animal models.

Telomerase regulation

The intimate connection between telomerase regulation and human disease is now well established. The molecular basis for telomerase regulation is highly complex and entails multiple layers of control. While the major target of enzyme regulation is the catalytic subunit TERT, the RNA subunit of telomerase is also implicated in telomerase control. In addition, alterations in gene dosage and alternative isoforms of core telomerase components have been described. Finally, telomerase localization, recruitment to the telomere and enzymology at the chromosome terminus are all subject to modulation. In this review we summarize recent advances in understanding fundamental mechanisms of telomerase regulation.

Involvement of WRN helicase in immortalization and tumorigenesis by the telomeric crisis pathway (Review)

The repeated replication of cells shortens telomeres, culminating in their instability, after which most cells cease to replicate and die. However, a small fraction of the cells become immortalized by maintaining telomeres with activated telomerase activity. It has been proposed that WRN helicase encoded by the WRN gene, the causative gene of Werner syndrome (WS), is required for immortalization by the telomeric crisis pathway (TCP) in a system that uses lymphoblastoid cell lines transformed by the Epstein-Barr virus. Taken together, these characteristics indicate that WRN helicase is also required for the immortalization of epithelial cells by TCP and consequent carcinogenesis, suggesting that the tumorigenesis of epithelial cells by TCP is suppressed in WS lacking the WRN helicase function. Notably, in WS the pathway of alternative lengthening of telomeres without activation of telomerase activity has been suggested to be involved in immortalization and tumorigenesis. This factor is consistent with the abundance of non-epithelial cancers in WS in that the ratio of epithelial to non-epithelial cancers is approximately 1:1 in WS patients compared to 10:1 in the general population. A hypothetical scheme showing the role of WRN helicase in immortalization by means of the supposed 'breakage-fusion-bridge cycle' of chromosomes at telomeric crisis is described.

A BAC transgenic reporter recapitulates in vivo regulation of human telomerase reverse transcriptase in development and tumorigenesis

Telomerase is tightly regulated in humans relative to mice, owing to the differential regulation of TERT genes. To explore hTERT regulation in vivo, we engineered mice with a 160-kb transgenic bacterial artificial chromosome (BAC) spanning the hTERT locus with a Renilla luciferase (Rluc) cassette downstream of its promoter. Analysis of multiple founder lines revealed that the Rluc expression profile from the transgenic hTERT reporter locus reproduced that of the native hTERT gene in all tissues and organs examined, demonstrating that genetic sequence determined the species-specific developmental regulation of the hTERT gene and that mouse epigenetic and transcription machineries faithfully regulated hTERT transcription. Thus, these mice allowed detailed analyses of developmental hTERT regulation. Both the transgenic hTERT reporter and the endogenous mTERT locus were expressed in early embryonic stages, and their mRNA levels progressively decreased throughout embryonic and postnatal development. Whereas hTERT transcription was much lower than mTERT expression in most organs, it increased significantly during postnatal development of thymus, testis, and ovary. In testis, the Rluc mRNA was enriched in elongating spermatids of seminiferous tubules. In addition, the transcription of transgenic hTERT reporter, but surprisingly not the endogenous mTERT gene, was activated during Wnt1-induced mammary tumorigenesis, allowing the monitoring of tumor development via noninvasive bioluminescent imaging. Collectively, our results demonstrate that the hTERT transgenic reporter system recapitulates the developmental regulation of the hTERT gene in a chromosomal position-independent manner and serves as a legitimate model to explore telomerase regulation in the development of normal and neoplastic tissues in vivo.