Regulation of the Human Telomerase Gene TERT by Telomere Position Effect-Over Long Distances (TPE-OLD): Implications for Aging and Cancer

The role of p21/CIP1/WAF1 (p21) in the negative regulation of the growth hormone/growth hormone receptor and epidermal growth factor/epidermal growth factor receptor pathways, in growth hormone transduction defect

PURPOSE

Growth hormone transduction defect (GHTD) is characterized by severe short stature, impaired STAT3 (signal transducer and activator of transcription-3) phosphorylation and overexpression of the cytokine inducible SH2 containing protein (CIS) and p21/CIP1/WAF1. To investigate the role of p21/CIP1/WAF1 in the negative regulation of the growth hormone (GH)/GH receptor and Epidermal Growth Factor (EGF)/EGF Receptor pathways in GHTD.

METHODS

Fibroblast cultures were developed from gingival biopsies of 1 GHTD patient and 1 control. The protein expression and the cellular localization of p21/CIP1/WAF1 was studied by Western immunoblotting and immunofluorescence, respectively: at the basal state and after induction with 200-μg/L human GH (hGH) (GH200), either with or without siRNA CIS (siCIS); at the basal state and after inductions with 200-μg/L hGH (GH200), 1,000-μg/L hGH (GH1000) or 50-ng/mL EGF.

RESULTS

After GH200/siCIS, the protein expression and nuclear localization of p21 were reduced in the patient. After successful induction of GH signaling (control, GH200; patient, GH1000), the protein expression and nuclear localization of p21 were reduced. After induction with EGF, p21 translocated to the cytoplasm in the control, whereas in the GHTD patient it remained located in the nucleus.

CONCLUSION

In the GHTD fibroblasts, when CIS is reduced, either after siCIS or after a higher dose of hGH (GH1000), p21's antiproliferative effect (nuclear localization) is also reduced and GH signaling is activated. There also appears to be a positive relationship between the 2 inhibitors of GH signaling, CIS and p21. Finally, in GHTD, p21 seems to participate in the regulation of both the GH and EGF/EGFR pathways, depending upon its cellular location.

Telomere length-dependent transcription and epigenetic modifications in promoters remote from telomere ends

Telomere-binding proteins constituting the shelterin complex have been studied primarily for telomeric functions. However, mounting evidence shows non-telomeric binding and gene regulation by shelterin factors. This raises a key question—do telomeres impact binding of shelterin proteins at distal non-telomeric sites? Here we show that binding of the telomere-repeat-binding-factor-2 (TRF2) at promoters ~60 Mb from telomeres depends on telomere length in human cells. Promoter TRF2 occupancy was depleted in cells with elongated telomeres resulting in altered TRF2-mediated transcription of distal genes. In addition, histone modifications—activation (H3K4me1 and H3K4me3) as well as silencing marks (H3K27me3)—at distal promoters were telomere length-dependent. These demonstrate that transcription, and the epigenetic state, of telomere-distal promoters can be influenced by telomere length. Molecular links between telomeres and the extra-telomeric genome, emerging from findings here, might have important implications in telomere-related physiology, particularly ageing and cancer.

Telomeres (special DNA-protein assemblies that protect chromosome ends) affect ageing and diseases such as cancer. Although this has been recognized for many years, biological processes that connect telomeres to ageing, cancer and other cellular functions remain to be fully understood. Certain proteins, believed to be only telomere-associated, engage DNA outside telomeres. This raises an interesting question. Does telomere length influence how telomere-binding proteins associate with DNA at regions distal from telomeres. If so, how does this impact function? Motivated by these questions, in the present studies we tested if extra-telomeric binding of the well-known telomere-repeat-binding-actor-2 (TRF2) depends on telomere length. Our results show that the level of DNA-bound TRF2 at telomere-distal sites changes as telomeres shorten or elongate. Consequently, TRF2-mediated gene regulation affects many genes. Notably, histone modifications that dictate chromatin compaction and access to regulatory factors, at sites distant from telomere ends also depended on telomere length. Together, this links the state of telomeres to gene regulation and epigenetics directly in ways not previously appreciated that might impact a more complete understanding of molecular processes underlying ageing and cancer.

Effect of therapies-mediated modulation of telomere and/or telomerase on cancer cells radiosensitivity

Cancer is one of the leading causes of death in the world. Many strategies of cancer treatment such as radiotherapy which plays a key role in cancer treatment are developed and used nowadays. However, the side effects post-cancer radiotherapy and cancer radioresistance are two major causes of the limitation of cancer radiotherapy effectiveness in the cancer patients. Moreover, reduction of the limitation of cancer radiotherapy effectiveness by reducing the side effects post-cancer radiotherapy and cancer radioresistance is the aim of several radiotherapy-oncologic teams. Otherwise, Telomere and telomerase are two cells components which play an important role in cancer initiation, cancer progression and cancer therapy resistance such as radiotherapy resistance. For resolving the problems of the limitation of cancer radiotherapy effectiveness especially the cancer radio-resistance problems, the radio-gene-therapy strategy which is the use of gene-therapy via modulation of gene expression combined with radiotherapy was developed and used as a new strategy to treat the patients with cancer. In this review, we summarized the information concerning the implication of telomere and telomerase modulation in cancer radiosensitivity.

NOVA1 regulates hTERT splicing and cell growth in non-small cell lung cancer

Alternative splicing is dysregulated in cancer and the reactivation of telomerase involves the splicing of TERT transcripts to produce full-length (FL) TERT. Knowledge about the splicing factors that enhance or silence FL hTERT is lacking. We identified splicing factors that reduced telomerase activity and shortened telomeres using a siRNA minigene reporter screen and a lung cancer cell bioinformatics approach. A lead candidate, NOVA1, when knocked down resulted in a shift in hTERT splicing to non-catalytic isoforms, reduced telomerase activity, and progressive telomere shortening. NOVA1 knockdown also significantly altered cancer cell growth in vitro and in xenografts. Genome engineering experiments reveal that NOVA1 promotes the inclusion of exons in the reverse transcriptase domain of hTERT resulting in the production of FL hTERT transcripts. Utilizing hTERT splicing as a model splicing event in cancer may provide new insights into potentially targetable dysregulated splicing factors in cancer.

TERT expression in gastric carcinogenesis: Correlation with clinical features

AIM

To investigate the expression of telomerase reverse transcriptase (TERT) mRNA in gastric cancer and precancerous lesions, and to analyze the relationship between TERT expression and clinicopathologic features and prognosis in gastric cancer.

METHODS

From September 2011 to October 2016, 102 patients with gastric cancer, 32 patients with precancerous lesions, and 30 patients with chronic non-atrophic gastritis were treated at the First Affiliated Hospital of Jiamus University. The expression of TERT mRNA in tissues was detected by real-time quantitative PCR, and the correlation of expression of TERT with clinicopathologic features and prognosis in gastric cancer was analyzed.

RESULTS

The expression of TERT in gastric cancer and precancerous lesions was significantly higher than that in chronic non-atrophic gastritis (P < 0.05). TERT expression was significantly correlated with the depth of invasion, TNM stage, vascular invasion, and lymph node metastasis (P < 0.05). The overall survival rate of gastric cancer patients with high expression of TERT was significantly lower than that of patients with low expression of TERT (HR = 0.420, 95%CI: 0.264-0.668, P < 0.001). The progression-free survival rate of gastric cancer patients with high expression of TERT was also significantly lower than that of patients with low TERT expression (HR = 0.649, 95%CI: 0.421-0.999, P = 0.049). Cox multivariate survival analysis showed that TERT expression was an independent prognostic factor for overall survival in gastric cancer.

CONCLUSION

TERT is highly expressed in gastric cancer and precancerous lesions, and high expression of TERT is significantly associated with the clinicopathologic features and prognosis of gastric cancer patients.

The role of p38 MAPK pathway in p53 compromised state and telomere mediated DNA damage response

There is an intricate balance of DNA damage response and repair which determines the homeostasis of human genome function. p53 protein is widely known for its role in cell cycle regulation and tumor suppressor activity. In case of several cancers where function of p53 gene gets compromised either by mutation or partial inactivation, the role of p53 in response to DNA damage needs to be supplemented by another molecule or pathway. Due to sedentary lifestyle and exposure to genotoxic agents, genome is predisposed to chronic stress, which ultimately leads to unrepaired or background DNA damage. p38 MAPK signaling pathway is strongly activated in response to various environmental and cellular stresses. DNA damage response and the repair options have crucial links with chromosomal integrity. Telomere that regulates integrity of genome is protected by a six member shielding unit called shelterin complex which communicates with other pathways for functionality of telomeres. There are evidences that p38 gets activated through ATM in response to DNA damage. Dysfunctional telomere leads to activation of ATM which subsequently activates p38 suggesting a crosstalk between p38, ATM and shelterin complex. This review focuses on activation of p38 in response to genotoxic stress induced DNA damage in p53 mutated or compromised state and its possible cross talk with telomere shelterin proteins. Thus p38 may act as an important target to treat various diseases and in majority of cancers in p53 mutated state.

Genomic Instabilities, Cellular Senescence, and Aging: In Vitro, In Vivo and Aging-Like Human Syndromes

As average life span and elderly people prevalence in the western world population is gradually increasing, the incidence of age-related diseases such as cancer, heart diseases, diabetes, and dementia is increasing, bearing social and economic consequences worldwide. Understanding the molecular basis of aging-related processes can help extend the organism’s health span, i.e., the life period in which the organism is free of chronic diseases or decrease in basic body functions. During the last few decades, immense progress was made in the understanding of major components of aging and healthy aging biology, including genomic instability, telomere attrition, epigenetic changes, proteostasis, nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and intracellular communications. This progress has been made by three spear-headed strategies: in vitro (cell and tissue culture from various sources), in vivo (includes diverse model and non-model organisms), both can be manipulated and translated to human biology, and the study of aging-like human syndromes and human populations. Herein, we will focus on current repository of genomic “senescence” stage of aging, which includes health decline, structural changes of the genome, faulty DNA damage response and DNA damage, telomere shortening, and epigenetic alterations. Although aging is a complex process, many of the “hallmarks” of aging are directly related to DNA structure and function. This review will illustrate the variety of these studies, done in in vitro, in vivo and human levels, and highlight the unique potential and contribution of each research level and eventually the link between them.

Epigenetic Mechanisms Impacting Aging: A Focus on Histone Levels and Telomeres

Aging and age-related diseases pose some of the most significant and difficult challenges to modern society as well as to the scientific and medical communities. Biological aging is a complex, and, under normal circumstances, seemingly irreversible collection of processes that involves numerous underlying mechanisms. Among these, chromatin-based processes have emerged as major regulators of cellular and organismal aging. These include DNA methylation, histone modifications, nucleosome positioning, and telomere regulation, including how these are influenced by environmental factors such as diet. Here we focus on two interconnected categories of chromatin-based mechanisms impacting aging: those involving changes in the levels of histones or in the functions of telomeres.

The role of telomeres in the mechanisms and evolution of life-history trade-offs and ageing

Evolutionary biology and biomedicine have seen a surge of recent interest in the possibility that telomeres play a role in life-history trade-offs and ageing. Here, I evaluate alternative hypotheses for the role of telomeres in the mechanisms and evolution of life-history trade-offs and ageing, and highlight outstanding challenges. First, while recent findings underscore the possibility of a proximate causal role for telomeres in current-future trade-offs and ageing, it is currently unclear (i) whether telomeres ever play a causal role in either and (ii) whether any causal role for telomeres arises via shortening or length-independent mechanisms. Second, I consider why, if telomeres do play a proximate causal role, selection has not decoupled such a telomere-mediated trade-off between current and future performance. Evidence suggests that evolutionary constraints have not rendered such decoupling impossible. Instead, a causal role for telomeres would more plausibly reflect an adaptive strategy, born of telomere maintenance costs and/or a function for telomere attrition (e.g. in countering cancer), the relative importance of which is currently unclear. Finally, I consider the potential for telomere biology to clarify the constraints at play in life-history evolution, and to explain the form of the current-future trade-offs and ageing trajectories that we observe today.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Why are there associations between telomere length and behaviour?

Individual differences in telomere length are associated with individual differences in behaviour in humans and birds. Within the human epidemiological literature this association is assumed to result from specific behaviour patterns causing changes in telomere dynamics. We argue that selective adoption-the hypothesis that individuals with short telomeres are more likely to adopt specific behaviours-is an alternative worthy of consideration. Selective adoption could occur either because telomere length directly affects behaviour or because behaviour and telomere length are both affected by a third variable, such as exposure to early-life adversity. We present differential predictions of the causation and selective adoption hypotheses and describe how these could be tested with longitudinal data on telomere length. Crucially, if behaviour is causal then it should be associated with differential rates of telomere attrition. Using smoking behaviour as an example, we show that the evidence that smoking accelerates the rate of telomere attrition within individuals is currently weak. We conclude that the selective adoption hypothesis for the association between behaviour and telomere length is both mechanistically plausible and, if anything, more compatible with existing empirical evidence than the hypothesis that behaviour is causal.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Ring chromosomes: from formation to clinical potential

Ring chromosomes (RCs) are circular DNA molecules, which occur rarely in eukaryotic nuclear genomes. Lilian Vaughan Morgan first described them in the fruit fly. Human embryos very seldom have RCs, about 1:50,000. Carriers of RCs may have varying degrees of symptoms, from healthy phenotype to serious pathologies in physical and intellectual development. Many authors describe common symptoms of RC presence: short stature and some developmental delay that could be described as a "ring chromosome syndrome." As a rule, RCs arise de novo through the end-joining of two DNA double-strand breaks, telomere-subtelomere junction, or inv dup del rearrangement in both meiosis and mitosis. There are family cases of RC inheritance. The presence of RCs causes numerous secondary chromosome rearrangements in vivo and in vitro. RCs can change their size, become lost, or increase their copy number and cause additional deletions, duplication, and translocations, affecting both RCs and other chromosomes. In this review, we examine RC inheritance, instability, mechanisms of formation, and potential clinical applications of artificially created RCs for large-scale chromosome rearrangement treatment.

Whole genome diversity of inherited chromosomally integrated HHV-6 derived from healthy individuals of diverse geographic origin

Human herpesviruses 6-A and -B (HHV-6A, HHV-6B) are ubiquitous in human populations worldwide. These viruses have been associated with several diseases such as multiple sclerosis, Hodgkin's lymphoma or encephalitis. Despite of the need to understand the genetic diversity and geographic stratification of these viruses, the availability of complete viral sequences from different populations is still limited. Here, we present nine new inherited chromosomally integrated HHV-6 sequences from diverse geographical origin which were generated through target DNA enrichment on lymphoblastoid cell lines derived from healthy individuals. Integration with available HHV-6 sequences allowed the assessment of HHV-6A and -6B phylogeny, patterns of recombination and signatures of natural selection. Analysis of the intra-species variability showed differences between A and B diversity levels and revealed that the HHV-6B reference (Z29) is an uncommon sequence, suggesting the need for an alternative reference sequence. Signs of geographical variation are present and more defined in HHV-6A, while they appear partly masked by recombination in HHV-6B. Finally, we conducted a scan for signatures of selection in protein coding genes that yielded at least 6 genes (4 and 2 respectively for the A and B species) showing significant evidence for accelerated evolution, and 1 gene showing evidence of positive selection in HHV-6A.

Insights about genome function from spatial organization of the genome

Over the last 15 years, development of chromosome conformation capture (3C) and its subsequent high-throughput variants in conjunction with the fast development of sequencing technology has allowed investigators to generate large volumes of data giving insights into the spatial three-dimensional (3D) architecture of the genome. This huge data has been analyzed and validated using various statistical, mathematical, genomics, and biophysical tools in order to examine the chromosomal interaction patterns, understand the organization of the chromosome, and find out functional implications of the interactions. This review summarizes the data generated by several large-scale high-throughput chromosome conformation capture studies and the functional implications obtained from the data analyses. We also discuss emerging results on factors (both CCCTC binding factor (CTCF) related and CTCF independent) that could contribute to looping interactions.

Longitudinal changes in telomere length and associated genetic parameters in dairy cattle analysed using random regression models

Telomeres cap the ends of linear chromosomes and shorten with age in many organisms. In humans short telomeres have been linked to morbidity and mortality. With the accumulation of longitudinal datasets the focus shifts from investigating telomere length (TL) to exploring TL change within individuals over time. Some studies indicate that the speed of telomere attrition is predictive of future disease. The objectives of the present study were to 1) characterize the change in bovine relative leukocyte TL (RLTL) across the lifetime in Holstein Friesian dairy cattle, 2) estimate genetic parameters of RLTL over time and 3) investigate the association of differences in individual RLTL profiles with productive lifespan. RLTL measurements were analysed using Legendre polynomials in a random regression model to describe TL profiles and genetic variance over age. The analyses were based on 1,328 repeated RLTL measurements of 308 female Holstein Friesian dairy cattle. A quadratic Legendre polynomial was fitted to the fixed effect of age in months and to the random effect of the animal identity. Changes in RLTL, heritability and within-trait genetic correlation along the age trajectory were calculated and illustrated. At a population level, the relationship between RLTL and age was described by a positive quadratic function. Individuals varied significantly regarding the direction and amount of RLTL change over life. The heritability of RLTL ranged from 0.36 to 0.47 (SE = 0.05-0.08) and remained statistically unchanged over time. The genetic correlation of RLTL at birth with measurements later in life decreased with the time interval between samplings from near unity to 0.69, indicating that TL later in life might be regulated by different genes than TL early in life. Even though animals differed in their RLTL profiles significantly, those differences were not correlated with productive lifespan (p = 0.954).

Short telomeres - A hallmark of heritable cardiomyopathies

Cardiovascular diseases are the leading cause of death worldwide and the incidence increases with age. Genetic testing has taught us much about the pathogenic pathways that drive heritable cardiomyopathies. Here we discuss an unexpected link between shortened telomeres, a molecular marker of aging, and genetic cardiomyopathy. Positioned at the ends of chromosomes, telomeres are DNA repeats which serve as protective caps that shorten with each cell division in proliferative tissues. Cardiomyocytes are an anomaly, as they are largely non-proliferative post-birth and retain relatively stable telomere lengths throughout life in healthy individuals. However, there is mounting evidence that in disease states, cardiomyocyte telomeres significantly shorten. Moreover, this shortening may play an active role in the development of mitochondrial dysfunction central to the etiology of dilated and hypertrophic cardiomyopathies. Elucidation of the mechanisms that underlie the telomere-mitochondrial signaling axis in the heart will provide fresh insights into our understanding of genetic cardiomyopathies, and could lead to the identification of previously uncharacterized modes of therapeutic intervention.

Long-range telomere regulation of gene expression: Telomere looping and telomere position effect over long distances (TPE-OLD)

The human cellular reverse transcriptase, telomerase, is very tightly regulated in large long-lived species. Telomerase is expressed during early human fetal development, is turned off in most adult tissues, and then becomes reactivated in almost all human cancers. However, the exact mechanism regulating these switches in expression are not known. We recently described a phenomenon where genes are regulated by telomere length dependent loops (telomere position effects over long distances; TPE-OLD). The hTERT gene is ~ 1.2Mb from the human chromosome 5p end. We observed that when telomeres are long hTERT gene expression is repressed and a probe next to the 5p telomere and the hTERT locus are spatially co-localized. When telomeres are short at least one of the hTERT alleles is spatially separated from the telomere, developing more active histone marks and changes in DNA methylation in the hTERT promoter region. These findings have implications for how cells turn off telomerase when telomeres are long during fetal development and how cancer cells reactivate telomerase in cells that have short telomeres. In addition to TPE-OLD, in proliferating stem cells such as activated T-lymphocytes, telomerase can be reversibly activated and silenced by telomere looping. In telomerase positive cancer cells that are induced to differentiate and downregulate telomerase, telomere looping correlates with silencing of the hTERT gene. These studies and others support a role of telomeres in regulating gene expression via telomere looping that may involve interactions with internal telomeric sequences (ITS). In addition to telomere looping, TPE-OLD may be one mechanism of how cells time changes in physiology without initiating a DNA damage response.

Genomic origin and nuclear localization of TERRA telomeric repeat-containing RNA: from Darkness to Dawn

Long noncoding RNAs, produced from distinct regions of the chromosomes, are emerging as new key players in several important biological processes. The long noncoding RNAs add a new layer of complexity to cellular regulatory pathways, from transcription to cellular trafficking or chromatin remodeling. More than 25 years ago, the discovery of a transcriptional activity at telomeres of protozoa ended the long-lasting belief that telomeres were transcriptionally silent. Since then, progressively accumulating evidences established that production of TElomeric Repeat-containing RNA (TERRA) was a general feature of eukaryotic cells. Whether TERRA molecules always originate from the telomeres or whether they can be transcribed from internal telomeric repeats as well is however still a matter of debate. Whether TERRA transcripts always localize to telomeres and play similar roles in all eukaryotic cells is also unclear. We review the studies on TERRA localization in the cell, its composition and some aspects of its transcriptional regulation to summarize the current knowledge and controversies about the genomic origin of TERRA, with a focus on human and mouse TERRA.

Regulation of human and mouse telomerase genes by genomic contexts and transcription factors during embryonic stem cell differentiation

Differential regulation of telomerase reverse transcriptase (TERT) genes contribute to distinct aging and tumorigenic processes in humans and mice. To study TERT regulation, we generated mouse embryonic stem cell (ESC) lines containing single-copy bacterial artificial chromosome (BAC) reporters, covering hTERT and mTERT genes and their neighboring loci, via recombinase-mediated BAC targeting. ESC lines with chimeric BACs, in which two TERT promoters were swapped, were also generated. Using these chromatinized BACs, we showed that hTERT silencing during differentiation to embryoid bodies (EBs) and to fibroblast-like cells was driven by the human-specific genomic context and accompanied by increases of repressive epigenetic marks, H3K9me3 and H3K27me3, near its promoter. Conversely, the mouse genomic context did not repress TERT transcription until late during differentiation. The hTERT promoter was more active than its mouse counterpart when compared in the same genomic contexts. Mutations of E-box and E2F consensus sites at the promoter had little effect on hTERT transcription in ESCs. However, the mutant promoters were rapidly silenced upon EB differentiation, indicating that transcription factors (TFs) bound to these sites were critical in maintaining hTERT transcription during differentiation. Together, our study revealed a dynamic hTERT regulation by chromatin environment and promoter-bound TFs during ESC differentiation.

Transcription regulation of CDKN1A (p21/CIP1/WAF1) by TRF2 is epigenetically controlled through the REST repressor complex

We observed extra-telomeric binding of the telomere repeat binding factor TRF2 within the promoter of the cyclin-dependent kinase CDKNIA (p21/CIP1/WAF1). This result in TRF2 induced transcription repression of p21. Interestingly, p21 repression was through engagement of the REST-coREST-LSD1-repressor complex and altered histone marks at the p21 promoter in a TRF2-dependent fashion. Furthermore, mutational analysis shows p21 repression requires interaction of TRF2 with a p21 promoter G-quadruplex. Physiologically, TRF2-mediated p21 repression attenuated drug-induced activation of cellular DNA damage response by evading G2/M arrest in cancer cells. Together these reveal for the first time role of TRF2 in REST- repressor complex mediated transcription repression.

The Telomeric Response to Viral Infection

The ends of linear genomes, whether viral or cellular, can elicit potent DNA damage and innate immune signals. DNA viruses entering the nucleus share many features with telomeres in their ability to either suppress or co-opt these pathways. Here, we review some of the common mechanisms that viruses and telomeres use to manage the DNA damage and innate immune response pathways. We highlight recent studies on the role of the telomere repeat-containing RNA (TERRA) in response to viral infection. We discuss how TERRA can be activated through a p53-response element embedded in a retrotransposon-like repeat found in human subtelomeres. We consider how TERRA can function as a danger signal when secreted in extracellular vesicles to induce inflammatory cytokines in neighboring cells. These findings suggest that TERRA may be part of the innate immune response to viral infection, and support the hypothesis that telomeres and viruses utilize common mechanisms to maintain genome integrity and regulate innate immunity.

Chromosomally Integrated Human Herpesvirus 6: Models of Viral Genome Release from the Telomere and Impacts on Human Health

Human herpesvirus 6A and 6B, alongside some other herpesviruses, have the striking capacity to integrate into telomeres, the terminal repeated regions of chromosomes. The chromosomally integrated forms, ciHHV-6A and ciHHV-6B, are proposed to be a state of latency and it has been shown that they can both be inherited if integration occurs in the germ line. The first step in full viral reactivation must be the release of the integrated viral genome from the telomere and here we propose various models of this release involving transcription of the viral genome, replication fork collapse, and t-circle mediated release. In this review, we also discuss the relationship between ciHHV-6 and the telomere carrying the insertion, particularly how the presence and subsequent partial or complete release of the ciHHV-6 genome may affect telomere dynamics and the risk of disease.

Alternative Splicing of hTERT Pre-mRNA: A Potential Strategy for the Regulation of Telomerase Activity

The activation of telomerase is one of the key events in the malignant transition of cells, and the expression of human telomerase reverse transcriptase (hTERT) is indispensable in the process of activating telomerase. The pre-mRNA alternative splicing of hTERT at the post-transcriptional level is one of the mechanisms for the regulation of telomerase activity. Shifts in splicing patterns occur in the development, tumorigenesis, and response to diverse stimuli in a tissue-specific and cell type–specific manner. Despite the regulation of telomerase activity, the alternative splicing of hTERT pre-mRNA may play a role in other cellular functions. Modulating the mode of hTERT pre-mRNA splicing is providing a new precept of therapy for cancer and aging-related diseases. This review focuses on the patterns of hTERT pre-mRNA alternative splicing and their biological functions, describes the potential association between the alternative splicing of hTERT pre-mRNA and telomerase activity, and discusses the possible significance of the alternative splicing of the hTERT pre-mRNA in the diagnosis, therapy, and prognosis of cancer and aging-related diseases.