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

Proposing candidate genes under telomeric control based on cross-species position data

In this paper, we present a comprehensive computational framework aimed at suggesting genes whose transcriptional regulation is likely to be influenced by their chromosomal position. This framework provides a user-friendly web interface, enabling researchers to explore the positional properties of all human genes and their orthologs across species, with a focus on their relation to the telomeres. Our approach involves multiple scoring methods, each adjustable by users, representing different features of the genes' positional variation across species. The resulting rankings can be combined to identify candidate genes that may be subject to position effects. Furthermore, the ranking can be tailored to a specific set of reference genes. We evaluate the method within the context of TPE-OLD, a mechanism where telomeres can exert a direct influence on gene expression across considerable genomic distances, and empower researchers to delve deeper into genes of interest, analyzing their position across species and estimating their susceptibility to position effects like TPE-OLD. We also provide simple enrichment analyses of user-provided gene lists in relation to top-ranking candidate genes.

Shaping DNA damage responses: Therapeutic potential of targeting telomeric proteins and DNA repair factors in cancer

Shelterin proteins regulate genomic stability by preventing inappropriate DNA damage responses (DDRs) at telomeres. Unprotected telomeres lead to persistent DDR causing cell cycle inhibition, growth arrest, and apoptosis. Cancer cells rely on DDR to protect themselves from DNA lesions and exogenous DNA-damaging agents such as chemotherapy and radiotherapy. Therefore, targeting DDR machinery is a promising strategy to increase the sensitivity of cancer cells to existing cancer therapies. However, the success of these DDR inhibitors depends on other mutations, and over time, patients develop resistance to these therapies. This suggests the need for alternative approaches. One promising strategy is co-inhibiting shelterin proteins with DDR molecules, which would offset cellular fitness in DNA repair in a mutation-independent manner. This review highlights the associations and dependencies of the shelterin complex with the DDR proteins and discusses potential co-inhibition strategies that might improve the therapeutic potential of current inhibitors.

The role of telomerase reverse transcriptase in the mitochondrial protective functions of Angiotensin-(1-7) in diabetic CD34+ cells

Angiotensin (Ang)-(1-7) stimulates vasoprotective functions of diabetic (DB) CD34+ hematopoietic stem/progenitor cells partly by decreasing reactive oxygen species (ROS), increasing nitric oxide (NO) levels and decreasing TGFβ1 secretion. Telomerase reverse transcriptase (TERT) translocates to mitochondria and regulates ROS generation. Alternative splicing of TERT results in variants α-, β- and α-β-TERT, which may oppose functions of full-length (FL) TERT. This study tested if the protective functions of Ang-(1-7) or TGFβ1-silencing are mediated by mitoTERT and that diabetes decreases FL-TERT expression by inducing splicing. CD34+ cells were isolated from the peripheral blood mononuclear cells of nondiabetic (ND, n = 68) or DB (n = 74) subjects. NO and mitoROS levels were evaluated by flow cytometry. TERT splice variants and mitoDNA-lesions were characterized by qPCR. TRAP assay was used for telomerase activity. Decoy peptide was used to block mitochondrial translocation (mitoXTERT). TERT inhibitor or mitoXTERT prevented the effects of Ang-(1-7) on NO or mitoROS levels in DB-CD34+ cells. FL-TERT expression and telomerase activity were lower and mitoDNA-lesions were higher in DB cells compared to ND and were reversed by Ang-(1-7) or TGFβ1-silencing. The prevalence of TERT splice variants, with predominant β-TERT expression, was higher and the expression of FL-TERT was lower in DB cells (n = 25) compared to ND (n = 30). Ang-(1-7) or TGFβ1-silencing decreased TERT-splicing and increased FL-TERT. Blocking of β-splicing increased FL-TERT and protected mitoDNA in DB-cells. The findings suggest that diabetes induces TERT-splicing in CD34+ cells and that β-TERT splice variant largely contributes to the mitoDNA oxidative damage.

Antiproliferative and Antitelomerase Effects of Silymarin on Human Colorectal and Hepatocellular Carcinoma Cells

Silymarin, a widely-used hepatoprotective agent, has shown antitumor properties in both in vitro and animal studies. Currently, there is limited knowledge regarding silymarin's antitelomerase effects on human colorectal cancer and hepatocyte carcinoma cells. In this study, we investigated the antiproliferative and antitelomerase effects of silymarin on four human colorectal cancer and HepG2 hepatocyte carcinoma cell lines. The cell viability and telomerase activity were assessed using MTT and the telomerase repeat amplification protocol assay, respectively. We also investigated the effects of silymarin on the expression of human telomerase reverse transcriptase and its promoter methylation in HepG2 cells by real-time RT-PCR and methylation-specific PCR, respectively. Silymarin treatment inhibited cell proliferation and telomerase activity in all cancer cells. After 24 h of treatment, silymarin exhibited IC50 values ranging from 19 - 56.3 µg/mL against these cancer cells. A 30-min treatment with silymarin at the IC50 concentration effectively inhibited telomerase activity in cell-free extracts of both colorectal cancer and hepatocyte carcinoma cells. Treatment of HepG2 cells with 10 and 30 µg/mL of silymarin for 48 h resulted in a decrease in human telomerase reverse transcriptase expression to 75 and 35% of the level observed in the untreated control (p < 0.01), respectively. Treatment with silymarin (10, 30, and 60 µg/mL) for 48 h did not affect human telomerase reverse transcriptase promoter methylation in HepG2 cells. In conclusion, our findings suggest that silymarin inhibits cancer cell growth by directly inhibiting telomerase activity and downregulating its human telomerase reverse transcriptase catalytic subunit. However, silymarin did not affect human telomerase reverse transcriptase promoter methylation at the concentrations of 10 - 60 µg/mL used in this study.

Unwrap RAP1's Mystery at Kinetoplastid Telomeres

Although located at the chromosome end, telomeres are an essential chromosome component that helps maintain genome integrity and chromosome stability from protozoa to mammals. The role of telomere proteins in chromosome end protection is conserved, where they suppress various DNA damage response machineries and block nucleolytic degradation of the natural chromosome ends, although the detailed underlying mechanisms are not identical. In addition, the specialized telomere structure exerts a repressive epigenetic effect on expression of genes located at subtelomeres in a number of eukaryotic organisms. This so-called telomeric silencing also affects virulence of a number of microbial pathogens that undergo antigenic variation/phenotypic switching. Telomere proteins, particularly the RAP1 homologs, have been shown to be a key player for telomeric silencing. RAP1 homologs also suppress the expression of Telomere Repeat-containing RNA (TERRA), which is linked to their roles in telomere stability maintenance. The functions of RAP1s in suppressing telomere recombination are largely conserved from kinetoplastids to mammals. However, the underlying mechanisms of RAP1-mediated telomeric silencing have many species-specific features. In this review, I will focus on Trypanosoma brucei RAP1's functions in suppressing telomeric/subtelomeric DNA recombination and in the regulation of monoallelic expression of subtelomere-located major surface antigen genes. Common and unique mechanisms will be compared among RAP1 homologs, and their implications will be discussed.

Olovnikov, Telomeres, and Telomerase. Is It Possible to Prolong a Healthy Life?

The science of telomeres and telomerase has made tremendous progress in recent decades. In this review, we consider it first in a historical context (the Carrel-Hayflick-Olovnikov-Blackburn chain of discoveries) and then review current knowledge on the telomere structure and dynamics in norm and pathology. Central to the review are consequences of the telomere shortening, including telomere position effects, DNA damage signaling, and increased genetic instability. Cell senescence and role of telomere length in its development are discussed separately. Therapeutic aspects and risks of telomere lengthening methods including use of telomerase and other approaches are also discussed.

Exercise as a Therapy to Maintain Telomere Function and Prevent Cellular Senescence

Exercise transiently impacts the expression, regulation, and activity of TERT/telomerase to maintain telomeres and protect the genome from insults. By protecting the telomeres (chromosome ends) and the genome, telomerase promotes cellular survival and prevents cellular senescence. By increasing cellular resiliency, via the actions of telomerase and TERT, exercise promotes healthy aging.

Combining old and new concepts in targeting telomerase for cancer therapy: transient, immediate, complete and combinatory attack (TICCA)

Telomerase can overcome replicative senescence by elongation of telomeres but is also a specific element in most cancer cells. It is expressed more vastly than any other tumor marker. Telomerase as a tumor target inducing replicative immortality can be overcome by only one other mechanism: alternative lengthening of telomeres (ALT). This limits the probability to develop resistance to treatments. Moreover, telomerase inhibition offers some degree of specificity with a low risk of toxicity in normal cells. Nevertheless, only one telomerase antagonist reached late preclinical studies. The underlying causes, the pitfalls of telomerase-based therapies, and future chances based on recent technical advancements are summarized in this review. Based on new findings and approaches, we propose a concept how long-term survival in telomerase-based cancer therapies can be significantly improved: the TICCA (Transient Immediate Complete and Combinatory Attack) strategy.

Dynamics of TERT regulation via alternative splicing in stem cells and cancer cells

Part of the regulation of telomerase activity includes the alternative splicing (AS) of the catalytic subunit telomerase reverse transcriptase (TERT). Although a therapeutic window for telomerase/TERT inhibition exists between cancer cells and somatic cells, stem cells express TERT and rely on telomerase activity for physiological replacement of cells. Therefore, identifying differences in TERT regulation between stem cells and cancer cells is essential for developing telomerase inhibition-based cancer therapies that reduce damage to stem cells. In this study, we measured TERT splice variant expression and telomerase activity in induced pluripotent stem cells (iPSCs), neural progenitor cells (NPCs), and non-small cell lung cancer cells (NSCLC, Calu-6 cells). We observed that a NOVA1-PTBP1-PTBP2 axis regulates TERT alternative splicing (AS) in iPSCs and their differentiation into NPCs. We also found that splice-switching of TERT, which regulates telomerase activity, is induced by different cell densities in stem cells but not cancer cells. Lastly, we identified cell type-specific splicing factors that regulate TERT AS. Overall, our findings represent an important step forward in understanding the regulation of TERT AS in stem cells and cancer cells.

Exploring the Causal Relationship Between Telomere Biology and Alzheimer's Disease

Telomeres, also known as the "protective caps" of our chromosomes, shorten with each cell cycle due to the end replication problem. This process, termed telomere attrition, is associated with many age-related disorders, such as Alzheimer's disease (AD). Despite the numerous studies conducted in this field, the role of telomere attrition in the onset of the disease remains unclear. To investigate the causal relationship between short telomeres and AD, this review aims to highlight the primary factors that regulate telomere length and maintain its integrity, with an additional outlook on the role of oxidative stress, which is commonly associated with aging and molecular damage. Although some findings thus far might be contradictory, telomere attrition likely plays a crucial role in the progression of AD due to its close association with oxidative stress. The currently available treatments for AD are only symptomatic without affecting the progression of the disease. The components of telomere biology discussed in this paper have previously been studied as an alternative treatment option for several diseases and have exhibited promising in vitro and in vivo results. Hence, this should provide a basis for future research to develop a potential therapeutic strategy for AD. (Created with BioRender.com).

Genetic and clinical determinants of telomere length

Many epidemiologic studies have identified important relationships between leukocyte telomere length (LTL) with genetics and health. Most of these studies have been significantly limited in scope by focusing predominantly on individual diseases or restricted to GWAS analysis. Using two large patient populations derived from Vanderbilt University and Marshfield Clinic biobanks linked to genomic and phenomic data from medical records, we investigated the inter-relationship between LTL, genomics, and human health. Our GWAS confirmed 11 genetic loci previously associated with LTL and two novel loci in SCNN1D and PITPNM1. PheWAS of LTL identified 67 distinct clinical phenotypes associated with both short and long LTL. We demonstrated that several diseases associated with LTL were related to one another but were largely independent from LTL genetics. Age of death was correlated with LTL independent of age. Those with very short LTL (<-1.5 standard deviation [SD]) died 10.4 years (p < 0.0001) younger than those with average LTL (±0.5 SD; mean age of death = 74.2 years). Likewise, those with very long LTL (>1.5 SD) died 1.9 years (p = 0.0175) younger than those with average LTL. This is consistent with the PheWAS results showing diseases associating with both short and long LTL. Finally, we estimated that the genome (12.8%) and age (8.5%) explain the largest proportion of LTL variance, whereas the phenome (1.5%) and sex (0.9%) explained a smaller fraction. In total, 23.7% of LTL variance was explained. These observations provide the rationale for expanded research to understand the multifaceted correlations between TL biology and human health over time, leading to effective LTL usage in medical applications.

Non-canonical telomere protection role of FOXO3a of human skeletal muscle cells regulated by the TRF2-redox axis

Telomeric repeat binding factor 2 (TRF2) binds to telomeres and protects chromosome ends against the DNA damage response and senescence. Although the expression of TRF2 is downregulated upon cellular senescence and in various aging tissues, including skeletal muscle tissues, very little is known about the contribution of this decline to aging. We previously showed that TRF2 loss in myofibers does not trigger telomere deprotection but mitochondrial dysfunction leading to an increased level of reactive oxygen species. We show here that this oxidative stress triggers the binding of FOXO3a to telomeres where it protects against ATM activation, revealing a previously unrecognized telomere protective function of FOXO3a, to the best of our knowledge. We further showed in transformed fibroblasts and myotubes that the telomere properties of FOXO3a are dependent on the C-terminal segment of its CR2 domain (CR2C) but independent of its Forkhead DNA binding domain and of its CR3 transactivation domain. We propose that these non-canonical properties of FOXO3a at telomeres play a role downstream of the mitochondrial signaling induced by TRF2 downregulation to regulate skeletal muscle homeostasis and aging.

Epigenetic modifiers as game changers for healthy ageing

Epigenetic alterations during ageing are manifested with altered gene expression linking it to lifespan regulation, genetic instability, and diseases. Diet and epigenetic modifiers exert a profound effect on the lifespan of an organism by modulating the epigenetic marks. However, our understanding of the multifactorial nature of the epigenetic process during ageing and the onset of disease conditions as well as its reversal by epidrugs, diet, or environmental factors is still mystifying. This review covers the key findings in epigenetics related to ageing and age-related diseases. Further, it holds a discussion about the epigenetic clocks and their implications in various age-related disease conditions including cancer. Although, epigenetics is a reversible process how fast the epigenetic alterations can revert to normal is an intriguing question. Therefore, this paper touches on the possibility of utilizing nutrition and MSCs secretome to accelerate the epigenetic reversal and emphasizes the identification of new therapeutic epigenetic modifiers to counter epigenetic alteration during ageing.

Senescence-associated transcriptional derepression in subtelomeres is determined in a chromosome-end-specific manner

Aging is a continuous process leading to physiological deterioration with age. One of the factors contributing to aging is telomere shortening, causing alterations in the protein protective complex named shelterin and replicative senescence. Here, we address the question of the link between this telomere shortening and the transcriptional changes occurring in senescent cells. We found that in replicative senescent cells, the genes whose expression escaped repression are enriched in subtelomeres. The shelterin protein TRF2 and the nuclear lamina factor Lamin B1, both downregulated in senescent cells, are involved in the regulation of some but not all of these subtelomeric genes, suggesting complex mechanisms of transcriptional regulation. Indeed, the subtelomeres containing these derepressed genes are enriched in factors of polycomb repression (EZH2 and H3K27me3), insulation (CTCF and MAZ), and cohesion (RAD21 and SMC3) while being associated with the open A-type chromatin compartment. These findings unveil that the subtelomere transcriptome associated with senescence is determined in a chromosome-end-specific manner according to the type of higher-order chromatin structure.

Transcriptional analysis links B cells and TERT expression to favorable prognosis in head and neck cancer

Telomerase reverse transcriptase (TERT) is a conserved self-tumor antigen overexpressed in ∼85% of tumor cells and is immunogenic in cancer patients. The effect of TERT expression on the regulation of intratumor adaptive immunity has not yet been investigated. We used RNA sequencing data from The Cancer Genome Atlas (TCGA) in 11 solid tumor types to investigate potential interactions between TERT expression, and B and T cell infiltrate in the tumor microenvironment. We found a positive correlation between TERT expression, B and T cells in four cancer types with the strongest association in head and neck squamous cell carcinoma (HSNCC). In HNSCC a Bhigh/TERThigh signature was associated with improved progression-free survival (PFS) (P = 0.0048). This effect was independent of HPV status and not shared in comparable analysis by other conserved tumor antigens (NYESO1, MUC1, MAGE, and CEA). Bhigh/TERThigh HNSCC tumors also harbored evidence of tertiary lymphoid structure (TLS) such as signatures for germinal center (GC) and switched memory B cells, central memory CD4 and effector memory CD8 T cells. Bhigh/TERThigh HNSCC tumors also showed an up-regulation of genes and pathways related to B and T cell activation, proliferation, migration, and cytotoxicity, while factors associated with immunosuppression and cancer cell invasiveness were down-regulated. In summary, our study uncovers a new association between high TERT expression and high B cell infiltrate in HNSCC, suggesting a potential benefit from therapeutic strategies that invigorate intratumor TERT-mediated T-B cooperation.

TERT promoter mutations and methylation for telomerase activation in urothelial carcinomas: New mechanistic insights and clinical significance

Telomerase, an RNA-dependent DNA polymerase synthesizing telomeric TTAGGG sequences, is primarily silent in normal human urothelial cells (NHUCs), but widely activated in urothelial cell-derived carcinomas or urothelial carcinomas (UCs) including UC of the bladder (UCB) and upper track UC (UTUC). Telomerase activation for telomere maintenance is required for the UC development and progression, and the key underlying mechanism is the transcriptional de-repression of the telomerase reverse transcriptase (TERT), a gene encoding the rate-limiting, telomerase catalytic component. Recent mechanistic explorations have revealed important roles for TERT promoter mutations and aberrant methylation in activation of TERT transcription and telomerase in UCs. Moreover, these TERT-featured genomic and epigenetic alterations have been evaluated for their usefulness in non-invasive UC diagnostics, recurrence monitoring, outcome prediction and response to treatments such as immunotherapy. Importantly, the detection of the mutated TERT promoter and TERT mRNA as urinary biomarkers holds great promise for urine-based UC liquid biopsy. In the present article, we review recent mechanistic insights into altered TERT promoter-mediated telomerase activation in UCs and discuss potential clinical implications. Specifically, we compare differences in senescence and transformation between NHUCs and other types of epithelial cells, address the interaction between TERT promoter mutations and other factors to affect UC progression and outcomes, evaluate the impact of TERT promoter mutations and TERT-mediated activation of human endogenous retrovirus genes on UC immunotherapy including Bacillus Calmette-Guérin therapy and immune checkpoint inhibitors. Finally, we suggest the standardization of a TERT assay and evaluation system for UC clinical practice.

Telomeres expand sphere of influence: emerging molecular impact of telomeres in non-telomeric functions

Although the impact of telomeres on physiology stands well established, a question remains: how do telomeres impact cellular functions at a molecular level? This is because current understanding limits the influence of telomeres to adjacent subtelomeric regions despite the wide-ranging impact of telomeres. Emerging work in two distinct aspects offers opportunities to bridge this gap. First, telomere-binding factors were found with non-telomeric functions. Second, locally induced DNA secondary structures called G-quadruplexes are notably abundant in telomeres, and gene regulatory regions genome wide. Many telomeric factors bind to G-quadruplexes for non-telomeric functions. Here we discuss a more general model of how telomeres impact the non-telomeric genome - through factors that associate at telomeres and genome wide - and influence cell-intrinsic functions, particularly aging, cancer, and pluripotency.

Improving the effectiveness of anti-aging modalities by using the constrained disorder principle-based management algorithms

Aging is a complex biological process with multifactorial nature underlined by genetic, environmental, and social factors. In the present paper, we review several mechanisms of aging and the pre-clinically and clinically studied anti-aging therapies. Variability characterizes biological processes from the genome to cellular organelles, biochemical processes, and whole organs' function. Aging is associated with alterations in the degrees of variability and complexity of systems. The constrained disorder principle defines living organisms based on their inherent disorder within arbitrary boundaries and defines aging as having a lower variability or moving outside the boundaries of variability. We focus on associations between variability and hallmarks of aging and discuss the roles of disorder and variability of systems in the pathogenesis of aging. The paper presents the concept of implementing the constrained disease principle-based second-generation artificial intelligence systems for improving anti-aging modalities. The platform uses constrained noise to enhance systems' efficiency and slow the aging process. Described is the potential use of second-generation artificial intelligence systems in patients with chronic disease and its implications for the aged population.

Physical Activity on Telomere Length as a Biomarker for Aging: A Systematic Review

Background

Overall life expectancy continues to rise, approaching 80 years of age in several developed countries. However, healthy life expectancy lags far behind, which has, in turn, contributed to increasing costs in healthcare. One way to improve health and attenuate the socio-economic impact of an aging population is to increase overall fitness through physical activity. Telomere attrition or shortening is a well-known molecular marker in aging. As such, several studies have focused on whether exercise influences health and aging through telomere biology. This systematic review examines the recent literature on the effect of physical activity on telomere length (TL) and/or telomerase activity as molecular markers of aging.

Methods

A focused search was performed in the databases PubMed and Web of Science for retrieving relevant articles over the past ten years. The search contained the following keywords: exercise, sport, physical activity, fitness, sedentary, physical inactivity, telomere, telomere length, t/s ratio, and telomerase. PRISMA guidelines for systematic reviews were observed.

Results

A total of 43 articles were identified and categorized into randomized controlled trials (RCT), observational or interventional studies. RCTs (n = 8) showed inconsistent findings of increased TL length with physical activity in, e.g. obese, post-menopausal women. In comparison with a predominantly sedentary lifestyle, observational studies (n = 27) showed significantly longer TL with exercise of moderate to vigorous intensity; however, there was no consensus on the duration and type of physical activity and training modality. Interventional studies (n = 8) also showed similar findings of significantly longer TL prior to exercise intervention; however, these studies had smaller numbers of enrolled participants (mostly of high-performance athletes), and the physical activities covered a range of exercise intensities and duration. Amongst the selected studies, aerobic training of moderate to vigorous intensity is most prevalent. For telomere biology analysis, TL was determined mainly from leukocytes using qPCR. In some cases, especially in RCT and interventional studies, different sample types such as saliva, sperm, and muscle biopsies were analyzed; different leukocyte cell types and potential genetic markers in regulating telomere biology were also investigated.

Conclusions

Taken together, physical activity with regular aerobic training of moderate to vigorous intensity appears to help preserve TL. However, the optimal intensity, duration of physical activity, as well as type of exercise still need to be further elucidated. Along with TL or telomerase activity, participants’ fitness level, the type of physical activity, and training modality should be assessed at different time points in future studies, with the plan for long-term follow-up. Reducing the amount of sedentary behavior may have a positive effect of preserving and increasing TL. Further molecular characterization of telomere biology in different cell types and tissues is required in order to draw definitive causal conclusions on how physical activity affects TL and aging.

Thyroid carcinoma-featured telomerase activation and telomere maintenance: Biology and translational/clinical significance

BACKGROUND

Telomerase is a ribonucleoprotein complex consisting of a catalytic component telomerase reverse transcriptase (TERT), internal RNA template and other co-factors, and its essential function is to synthesize telomeric DNA, repetitive TTAGGG sequences at the termini of linear chromosomes. Telomerase is silent in normal human follicular thyroid cells, primarily due to the TERT gene being tightly repressed. During the development and progression of thyroid carcinomas (TCs), TERT induction and telomerase activation is in general required to maintain telomere length, thereby conferring TC cells with immortal and aggressive phenotypes.

METHODS

The genomic alterations of the TERT loci including TERT promoter's gain-of-function mutations, copy number gain, fusion and rearrangements, have recently been identified in TCs as mechanisms to induce TERT expression and to activate telomerase. Importantly, numerous studies have consistently shown that TERT promoter mutations and TERT expression occur in all TC subtypes, and are robustly associated with TC malignancy, aggressiveness, treatment failure and poor outcomes. Therefore, the assessment of TERT promoter mutations and TERT expression is highly valuable in TC diagnostics, prognosis, treatment decision, and follow-up design. In addition, the TERT promoter is frequently hypermethylated in TC cells and tumors, which is required to activate TERT transcription and telomerase. Dysregulation of other components in the telomerase complex similarly upregulate telomerase. Moreover, shortened telomeres lead to altered gene expression and metabolism, thereby actively promoting TC aggressiveness. Here we summarize recent findings in TCs to provide the landscape of TC-featured telomere/telomerase biology and discuss underlying implications in TC precision medicine.

CONCLUSION

Mechanistic insights into telomerase activation and TERT induction in TCs are important both biologically and clinically. The TERT gene aberration and expression-based molecular classification of TCs is proposed, and for such a purpose, the standardization of the assay and evaluation system is required. Moreover, the TERT-based system and 2022 WHO TC classification may be combined to improve TC care.

The Molecular Interplay between Human Oncoviruses and Telomerase in Cancer Development

Human oncoviruses are able to subvert telomerase function in cancer cells through multiple strategies. The activity of the catalytic subunit of telomerase (TERT) is universally enhanced in virus-related cancers. Viral oncoproteins, such as high-risk human papillomavirus (HPV) E6, Epstein-Barr virus (EBV) LMP1, Kaposi's sarcoma-associated herpesvirus (HHV-8) LANA, hepatitis B virus (HBV) HBVx, hepatitis C virus (HCV) core protein and human T-cell leukemia virus-1 (HTLV-1) Tax protein, interact with regulatory elements in the infected cells and contribute to the transcriptional activation of TERT gene. Specifically, viral oncoproteins have been shown to bind TERT promoter, to induce post-transcriptional alterations of TERT mRNA and to cause epigenetic modifications, which have important effects on the regulation of telomeric and extra-telomeric functions of the telomerase. Other viruses, such as herpesviruses, operate by integrating their genomes within the telomeres or by inducing alternative lengthening of telomeres (ALT) in non-ALT cells. In this review, we recapitulate on recent findings on virus-telomerase/telomeres interplay and the importance of TERT-related oncogenic pathways activated by cancer-causing viruses.

Combinatorial Effect of Arsenic and Herbal Compounds in Telomerase-Mediated Apoptosis Induction in Liver Cancer

Tumour illness and its resistance against existing anticancer therapies pose a serious health concern globally despite the progressive advancement of therapeutic options. The prevailing treatment of HCC using numerous antitumor agents has inflated long-lived complete remissions, but a percentage of individuals still die due to disease recurrence, indicating a need for further exploration of possible anti-tumour regimes. We aim to boost the effectiveness of the HCC treatment by conducting current investigations evaluating the effect of arsenic trioxide (ATO) with different herbal compounds like quercetin and aloe-emodin against liver tumour via inhibition of telomerase, a pro-cancer enzyme. The anticancer activity of ATO with herbal compounds was investigated in human control liver cell line (Wrl-68) and cancer liver cell line (HepG2) at different time intervals. Viability and cytotoxicity in response to combinatorial drugs were assessed in vitro by trypan blue dye exclusion assay and MTT and WST assay. Apoptosis was analysed by annexin V/PI assay, and the expression of telomerase and apoptosis-regulating proteins was evaluated by immunoblotting and qRT-PCR. Arsenic trioxide in combination with quercetin and aloe-emodin reduced cell viability in cancerous cells compared to normal cells by inducing apoptosis, downregulating telomerase and Bcl-2 (anti-apoptotic protein) and upregulating the expression of Bax (pro-apoptotic protein). ATO exhibited significant anticancer effects due to the synergistic effects of quercetin and aloe-emodin in liver tumour cells. The current study data collectively suggest that a successful inhibition of cancer growth by the combination of ATO and tested herbal medicines against liver tumour growth is via the inhibition of telomerase activity.

Establishment and Characterization of hTERT Immortalized Hutchinson–Gilford Progeria Fibroblast Cell Lines

Hutchinson–Gilford progeria syndrome (HGPS) is a rare premature aging syndrome caused by a dominant mutation in the LMNA gene. Previous research has shown that the ectopic expression of the catalytic subunit of telomerase (hTERT) can elongate the telomeres of the patients’ fibroblasts. Here, we established five immortalized HGP fibroblast cell lines using retroviral infection with the catalytic subunit of hTERT. Immortalization enhanced the proliferative life span by at least 50 population doublings (PDs). The number of cells with typical senescence signs was reduced by 63 + 17%. Furthermore, the growth increase and phenotype improvement occurred with a lag phase of 50–100 days and was not dependent on the degree of telomere elongation. The initial telomeric stabilization after hTERT infection and relatively low amounts of hTERT mRNA were sufficient for the phenotype improvement but the retroviral infection procedure was associated with transient cell stress. Our data have implications for therapeutic strategies in HGP and other premature aging syndromes.

A conserved long-distance telomeric silencing mechanism suppresses mTOR signaling in aging human fibroblasts

Telomeres are repetitive nucleotide sequences at the ends of each chromosome. It has been hypothesized that telomere attrition evolved as a tumor suppressor mechanism in large long-lived species. Long telomeres can silence genes millions of bases away through a looping mechanism called telomere position effect over long distances (TPE-OLD). The function of this silencing mechanism is unknown. We determined a set of 2322 genes with high positional conservation across replicatively aging species that includes known and candidate TPE-OLD genes that may mitigate potentially harmful effects of replicative aging. Notably, we identified PPP2R2C as a tumor suppressor gene, whose up-regulation by TPE-OLD in aged human fibroblasts leads to dephosphorylation of p70S6 kinase and mammalian target of rapamycin suppression. A mechanistic link between telomeres and a tumor suppressor mechanism supports the hypothesis that replicative aging fulfills a tumor suppressor function and motivates previously unknown antitumor and antiaging strategies.

TElomeric repeat-containing RNA (TERRA): Physiological functions and relevance in cancer

Telomeres are complex protective structures located at the ends of linear eukaryotic chromosomes. Their purpose is to prevent genomic instability. Research progress in telomere biology during the past decades has identified a network of telomeric transcripts of which the best-studied is TElomeric Repeat-containing RNA (TERRA). TERRA was shown to be important not only for the preservation of telomere homeostasis and genomic stability but also for the expression of hundreds of genes across the human genome. These findings added a new level of complexity to telomere biology. Herein we provide insights on the telomere transcriptome, its relevance for proper telomere function, and its implications in human pathology. We also discuss possible clinical opportunities of exosomal telomere transcripts detection as a biomarker in cancer precision medicine.

Comprehensive molecular analysis of immortalization hallmarks in thyroid cancer reveals new prognostic markers

BACKGROUND

Comprehensive molecular studies on tumours are needed to delineate immortalization process steps and identify sensitive prognostic biomarkers in thyroid cancer.

METHODS AND RESULTS

In this study, we extensively characterize telomere-related alterations in a series of 106 thyroid tumours with heterogeneous clinical outcomes. Using a custom-designed RNA-seq panel, we identified five telomerase holoenzyme-complex genes upregulated in clinically aggressive tumours compared to tumours from long-term disease-free patients, being TERT and TERC denoted as independent prognostic markers by multivariate regression model analysis. Characterization of alterations related to TERT re-expression revealed that promoter mutations, methylation and/or copy gains exclusively co-occurred in clinically aggressive tumours. Quantitative-FISH (fluorescence in situ hybridization) analysis of telomere lengths showed a significant shortening in these carcinomas, which matched with a high proliferative rate measured by Ki-67 immunohistochemistry. RNA-seq data analysis indicated that short-telomere tumours exhibit an increased transcriptional activity in the 5-Mb-subtelomeric regions, site of several telomerase-complex genes. Gene upregulation enrichment was significant for specific chromosome-ends such as the 5p, where TERT is located. Co-FISH analysis of 5p-end and TERT loci showed a more relaxed chromatin configuration in short telomere-length tumours compared to normal telomere-length tumours.

CONCLUSIONS

Overall, our findings support that telomere shortening leads to a 5p subtelomeric region reorganization, facilitating the transcription and accumulation of alterations at TERT-locus.

Telomere Maintenance and the cGAS-STING Pathway in Cancer

Cancer cells exhibit the unique characteristics of high proliferation and aberrant DNA damage response, which prevents cancer therapy from effectively eliminating them. The machinery required for telomere maintenance, such as telomerase and the alternative lengthening of telomeres (ALT), enables cancer cells to proliferate indefinitely. In addition, the molecules in this system are involved in noncanonical pro-tumorigenic functions. Of these, the function of the cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) pathway, which contains telomere-related molecules, is a well-known contributor to the tumor microenvironment (TME). This review summarizes the current knowledge of the role of telomerase and ALT in cancer regulation, with emphasis on their noncanonical roles beyond telomere maintenance. The components of the cGAS-STING pathway are summarized with respect to intercell communication in the TME. Elucidating the underlying functional connection between telomere-related molecules and TME regulation is important for the development of cancer therapeutics that target cancer-specific pathways in different contexts. Finally, strategies for designing new cancer therapies that target cancer cells and the TME are discussed.

Identification of mechanism of cancer-cell-specific reactivation of hTERT offers therapeutic opportunities for blocking telomerase specifically in human colorectal cancer

Transcriptional reactivation of hTERT is the limiting step in tumorigenesis. While mutations in hTERT promoter present in 19% of cancers are recognized as key drivers of hTERT reactivation, mechanisms by which wildtype hTERT (WT-hTERT) promoter is reactivated, in majority of human cancers, remain unknown. Using primary colorectal cancers (CRC) we identified Tert INTeracting region 2 (T-INT2), the critical chromatin region essential for reactivating WT-hTERT promoter in CRCs. Elevated β-catenin and JunD level in CRC facilitates chromatin interaction between hTERT promoter and T-INT2 that is necessary to turn on hTERTexpression. Pharmacological screens uncovered salinomycin, which inhibits JunD mediated hTERT-T-INT2 interaction that is required for the formation of a stable transcription complex on the hTERT promoter. Our results showed for the first time how known CRC alterations, such as APC, lead to WT-hTERT promoter reactivation during stepwise-tumorigenesis and provide a new perspective for developing cancer-specific drugs.

Acute Exercise Regulates hTERT Gene Expression and Alternative Splicing in the hTERT-BAC Transgenic Mouse Model

INTRODUCTION

Aerobic exercise maintains telomere length through increased human telomerase reverse transcriptase (hTERT) expression and telomerase enzyme activity. The impact of acute exercise on hTERT alternative splicing (AS) is unknown.

PURPOSE

This study aimed to examine hTERT AS in response to acute treadmill running.

METHODS

A bacterial artificial chromosome mouse model containing the 54-kilobase hTERT gene locus inserted into its genome (hTERT-BAC) was utilized. The gastrocnemius, left ventricle, and brain were excised before (Pre), upon cessation (Post), and during recovery (1, 24, 48, and 72 h; n = 5/time point) from treadmill running (30 min at 60% maximum speed). Full-length (FL) hTERT and the "minus beta" (-β) AS variant (skips exons 7 and 8 and does not code for active telomerase) were measured by gel-based and droplet digital reverse transcription-polymerase chain reaction methods. SF3B4 and SRSF2 protein expression were measured by Western blotting.

RESULTS

Compared with Pre, FL hTERT increased at Post before decreasing during recovery in the gastrocnemius (48 and 72 h; P ≤ 0.001) and left ventricle (24 h; P = 0.004). The percentage of FL hTERT in the gastrocnemius also increased during recovery (1 and 72 h; P ≤ 0.017), whereas a decrease was observed in the left ventricle (1, 24, and 48 h; P ≤ 0.041). hTERT decreased in the brain (48 h), whereas FL hTERT percentage remained unaltered. SF3B4 protein expression decreased throughout recovery in the gastrocnemius and tended to be associated with FL hTERT (r = -0.348, P = 0.075) and -β in opposite directions (r = 0.345, P = 0.067).

CONCLUSIONS

Endurance exercise increased hTERT gene expression, and altered FL hTERT splicing in contractile tissues and may maintain telomere length necessary to improve the function and health of the organism.

Emerging mechanisms of telomerase reactivation in cancer

Mutations in the promoter of human telomerase reverse transcriptase (hTERT) result in hyperactivation of hTERT. Notably, all mutations are G>A transitions, frequently found in a wide range of cancer types, and causally associated with cancer progression. Initially, the mutations were understood to reactivate hTERT by generating novel E26 transformation-specific (ETS) binding sites. Recent work reveals the role of DNA secondary structure G-quadruplexes, telomere binding factor(s), and chromatin looping in hTERT regulation. Here, we discuss these emerging findings in relation to the clinically significant promoter mutations to provide a broader understanding of the context-dependent outcomes that result in hTERT activation in normal and pathogenic conditions.

Methylation of Subtelomeric Chromatin Modifies the Expression of the lncRNA TERRA, Disturbing Telomere Homeostasis

The long noncoding RNA (lncRNA) telomeric repeat-containing RNA (TERRA) has been associated with telomeric homeostasis, telomerase recruitment, and the process of chromosome healing; nevertheless, the impact of this association has not been investigated during the carcinogenic process. Determining whether changes in TERRA expression are a cause or a consequence of cell transformation is a complex task because studies are usually carried out using either cancerous cells or tumor samples. To determine the role of this lncRNA in cellular aging and chromosome healing, we evaluated telomeric integrity and TERRA expression during the establishment of a clone of untransformed myeloid cells. We found that reduced expression of TERRA disturbed the telomeric homeostasis of certain loci, but the expression of the lncRNA was affected only when the methylation of subtelomeric bivalent chromatin domains was compromised. We conclude that the disruption in TERRA homeostasis is a consequence of cellular transformation and that changes in its expression profile can lead to telomeric and genomic instability.

Genome-wide screens identify specific drivers of mutant hTERT promoters

Mutations in hTERT promoter are seen in over 19% of human cancers, irrespective of the cancer type. Understanding the molecular players that regulate Mut-hTERT promoters may help the design of effective targeting strategies to inhibit telomerase reactivation specifically in cancer cells. Our work uses genome-wide functional screens to identify 30 specific regulators of Mut-hTERT promoters. These candidates identified from the screening serve as an excellent resource to understand how telomerase is reactivated and as targets for making inhibitors to telomerase, a key driver of cancer.

Cancer-specific hTERT promoter mutations reported in 19% of cancers result in enhanced telomerase activity. Understanding the distinctions between transcriptional regulation of wild-type (WT) and mutant (Mut) hTERT promoters may open up avenues for development of inhibitors which specially block hTERT expression in cancer cells. To comprehensively identify physiological regulators of WT- or Mut-hTERT promoters, we generated several isogenic reporter cells driven by endogenous hTERT loci. Genome-wide CRISPR-Cas9 and small interfering RNA screens using these isogenic reporter lines identified specific regulators of Mut-hTERT promoters. We validate and characterize one of these hits, namely, MED12, a kinase subunit of mediator complex. We demonstrate that MED12 specifically drives expression of hTERT from the Mut-hTERT promoter by mediating long-range chromatin interaction between the proximal Mut-hTERT promoter and T-INT1 distal regulatory region 260 kb upstream. Several hits identified in our screens could serve as potential therapeutic targets, inhibition of which may specifically block Mut-hTERT promoter driven telomerase reactivation in cancers.

Epigenetic aging of the demographically non-aging naked mole-rat

The naked mole-rat (NMR) is an exceptionally long-lived rodent that shows no increase of mortality with age, defining it as a demographically non-aging mammal. Here, we perform bisulfite sequencing of the blood of > 100 NMRs, assessing > 3 million common CpG sites. Unsupervised clustering based on sites whose methylation correlates with age reveals an age-related methylome remodeling, and we also observe a methylome information loss, suggesting that NMRs age. We develop an epigenetic aging clock that accurately predicts the NMR age. We show that these animals age much slower than mice and much faster than humans, consistent with their known maximum lifespans. Interestingly, patterns of age-related changes of clock sites in Tert and Prpf19 differ between NMRs and mice, but there are also sites conserved between the two species. Together, the data indicate that NMRs, like other mammals, epigenetically age even in the absence of demographic aging of this species.

The exceptionally long-lived naked mole-rat is characterized by the lack of increased mortality with aging. Here the authors perform epigenetic studies to show that naked mole-rats epigenetically age despite their non-increasing mortality rate.

Stress and telomere shortening: Insights from cellular mechanisms

Short telomeres confer risk of degenerative diseases. Chronic psychological stress can lead to disease through many pathways, and research from in vitro studies to human longitudinal studies has pointed to stress-induced telomere damage as an important pathway. However, there has not been a comprehensive model to describe how changes in stress physiology and neuroendocrine pathways can lead to changes in telomere biology. Critically short telomeres or the collapse of the telomere structure caused by displacement of telomere binding protein complex shelterin elicit a DNA damage response and lead to senescence or apoptosis. In this narrative review, we summarize the key roles glucocorticoids, reactive oxygen species (ROS) and mitochondria, and inflammation play in mediating the relationship between psychological stress and telomere maintenance. We emphasis that these mediators are interconnected and reinforce each other in positive feedback loops. Telomere length has not been studied across the lifespan yet, but the initial setting point at birth appears to be the most influential point, as it sets the lifetime trajectory, and is influenced by stress. We describe two types of intergenerational stress effects on telomeres - prenatal stress effects on telomeres during fetal development, and 'telotype transmission" -the directly inherited transmission of short telomeres from parental germline. It is clear that the initial simplistic view of telomere length as a mitotic clock has evolved into a far more complex picture of both transgenerational telomere influences, and of interconnected molecular and cellular pathways and networks, as hallmarks of aging where telomere maintenance is a key player interacting with mitochondria. Further mechanistic investigations testing this comprehensive model of stress mediators shaping telomere biology and the telomere-mitochondrial nexus will lead to better understanding from cell to human lifespan aging, and could lead to anti-aging interventions.

Telomeres and Cancer

Telomeres cap the ends of eukaryotic chromosomes and are indispensable chromatin structures for genome protection and replication. Telomere length maintenance has been attributed to several functional modulators, including telomerase, the shelterin complex, and the CST complex, synergizing with DNA replication, repair, and the RNA metabolism pathway components. As dysfunctional telomere maintenance and telomerase activation are associated with several human diseases, including cancer, the molecular mechanisms behind telomere length regulation and protection need particular emphasis. Cancer cells exhibit telomerase activation, enabling replicative immortality. Telomerase reverse transcriptase (TERT) activation is involved in cancer development through diverse activities other than mediating telomere elongation. This review describes the telomere functions, the role of functional modulators, the implications in cancer development, and the future therapeutic opportunities.

Antisense oligonucleotide repress telomerase activity via manipulating alternative splicing or translation

Telomerase is a reverse transcriptase that catalyzes the addition of telomeric repeated DNA onto the 3' ends of linear chromosomes. Telomerase inhibition was broadly used for cancer therapeutics. Here, six antisense oligonucleotides were designed to regulate TERT mRNA alternative splicing and protein translation. To pursue a better stability in vitro, we chemically modified the oligonucleotides into phosphorothioate (PS) backbone and 2'-O-methoxyethyl (2'-MOE PS) version and phosphoroamidate morpholino oligomer (PMO) version. The oligonucleotides were transfected into HEK 293T cells and HeLa cells, and the mRNA expression, protein level and catalytic activity of telomerase were determined. We found the Int8 notably promoted hTERT mRNA exon 7-8 skipping, which greatly reduced telomerase activity, and the 5'-UTR treatment led to an obvious protein translation barrier and telomerase inhibition. These results demonstrate the potential of antisense oligonucleotide drugs targeting hTERT for antitumor therapy. Moreover, two specific antisense oligonucleotides were identified to be effective in reducing telomerase activity.

Regulation of Gene Expression by Telomere Position Effect

Many diseases that involve malignant tumors in the elderly affect the quality of human life; therefore, the relationship between aging and pathogenesis in geriatric diseases must be under-stood to develop appropriate treatments for these diseases. Recent reports have shown that epigenetic regulation caused by changes in the local chromatin structure plays an essential role in aging. This review provides an overview of the roles of telomere shortening on genomic structural changes during an age-dependent shift in gene expression. Telomere shortening is one of the most prominent events that is involved in cellular aging and it affects global gene expression through genome rearrangement. This review provides novel insights into the roles of telomere shortening in disease-affected cells during pathogenesis and suggests novel therapeutic approaches.

Telomerase therapy reverses vascular senescence and extends lifespan in progeria mice

AIMS

Hutchinson-Gilford progeria syndrome (HGPS) is an accelerated ageing syndrome associated with premature vascular disease and death due to heart attack and stroke. In HGPS a mutation in lamin A (progerin) alters nuclear morphology and gene expression. Current therapy increases the lifespan of these children only modestly. Thus, greater understanding of the underlying mechanisms of HGPS is required to improve therapy. Endothelial cells (ECs) differentiated from induced pluripotent stem cells (iPSCs) derived from these patients exhibit hallmarks of senescence including replication arrest, increased expression of inflammatory markers, DNA damage, and telomere erosion. We hypothesized that correction of shortened telomeres may reverse these measures of vascular ageing.

METHODS AND RESULTS

We generated ECs from iPSCs belonging to children with HGPS and their unaffected parents. Telomerase mRNA (hTERT) was used to treat HGPS ECs. Endothelial morphology and functions were assessed, as well as proteomic and transcriptional profiles with attention to inflammatory markers, DNA damage, and EC identity genes. In a mouse model of HGPS, we assessed the effects of lentiviral transfection of mTERT on measures of senescence, focusing on the EC phenotype in various organs. hTERT treatment of human HGPS ECs improved replicative capacity; restored endothelial functions such as nitric oxide generation, acetylated low-density lipoprotein uptake and angiogenesis; and reduced the elaboration of inflammatory cytokines. In addition, hTERT treatment improved cellular and nuclear morphology, in association with a normalization of the transcriptional profile, effects that may be mediated in part by a reduction in progerin expression and an increase in sirtuin 1 (SIRT1). Progeria mice treated with mTERT lentivirus manifested similar improvements, with a reduction in inflammatory and DNA damage markers and increased SIRT1 in their vasculature and other organs. Furthermore, mTERT therapy increased the lifespan of HGPS mice.

CONCLUSION

Vascular rejuvenation using telomerase mRNA is a promising approach for progeria and other age-related diseases.

Cell Biology of Giant Cell Tumour of Bone: Crosstalk between m/wt Nucleosome H3.3, Telomeres and Osteoclastogenesis

Giant cell tumour of bone (GCTB) is a rare and intriguing primary bone neoplasm. Worrisome clinical features are its local destructive behaviour, its high tendency to recur after surgical therapy and its ability to create so-called benign lung metastases (lung 'plugs'). GCTB displays a complex and difficult-to-understand cell biological behaviour because of its heterogenous morphology. Recently, a driver mutation in histone H3.3 was found. This mutation is highly conserved in GCTB but can also be detected in glioblastoma. Denosumab was recently introduced as an extra option of medical treatment next to traditional surgical and in rare cases, radiotherapy. Despite these new insights, many 'old' questions about the key features of GCTB remain unanswered, such as the presence of telomeric associations (TAs), the reactivation of hTERT, and its slight genomic instability. This review summarises the recent relevant literature of histone H3.3 in relation to the GCTB-specific G34W mutation and pays specific attention to the G34W mutation in relation to the development of TAs, genomic instability, and the characteristic morphology of GCTB. As pieces of an etiogenetic puzzle, this review tries fitting all these molecular features and the unique H3.3 G34W mutation together in GCTB.

Genetic, Environmental and Lifestyle Determinants of Accelerated Telomere Attrition as Contributors to Risk and Severity of Multiple Sclerosis

Telomeres are protective structures at the ends of linear chromosomes. Shortened telomere lengths (TL) are an indicator of premature biological aging and have been associated with a wide spectrum of disorders, including multiple sclerosis (MS). MS is a chronic inflammatory, demyelinating and neurodegenerative disease of the central nervous system. The exact cause of MS is still unclear. Here, we provide an overview of genetic, environmental and lifestyle factors that have been described to influence TL and to contribute to susceptibility to MS and possibly disease severity. We show that several early-life factors are linked to both reduced TL and higher risk of MS, e.g., adolescent obesity, lack of physical activity, smoking and vitamin D deficiency. This suggests that the mechanisms underlying the disease are connected to cellular aging and senescence promoted by increased inflammation and oxidative stress. Additional prospective research is needed to clearly define the extent to which lifestyle changes can slow down disease progression and prevent accelerated telomere loss in individual patients. It is also important to further elucidate the interactions between shared determinants of TL and MS. In future, cell type-specific studies and advanced TL measurement methods could help to better understand how telomeres may be causally involved in disease processes and to uncover novel opportunities for improved biomarkers and therapeutic interventions in MS.

Telomeres and Age-Related Diseases

Telomeres are at the non-coding ends of linear chromosomes. Through a complex 3-dimensional structure, they protect the coding DNA and ensure appropriate separation of chromosomes. Aging is characterized by a progressive shortening of telomeres, which compromises their structure and function. Because of their protective function for genomic DNA, telomeres appear to play an important role in the development and progression of many age-related diseases, such as cardiovascular disease (CVD), malignancies, dementia, and osteoporosis. Despite substantial evidence that links telomere length with these conditions, the nature of these observations remains insufficiently understood. Therefore, future studies should address the question of causality. Furthermore, analytical methods should be further improved with the aim to provide informative and comparable results. This review summarize the actual knowledge of telomere biology and the possible implications of telomere dysfunction for the development and progression of age-related diseases. Furthermore, we provide an overview of analytical techniques for the measurement of telomere length and telomerase activity.

Integrated evaluation of telomerase activation and telomere maintenance across cancer cell lines

In cancer, telomere maintenance is critical for the development of replicative immortality. Using genome sequences from the Cancer Cell Line Encyclopedia and Genomics of Drug Sensitivity in Cancer Project, we calculated telomere content across 1299 cancer cell lines. We find that telomerase reverse transcriptase (TERT) expression correlates with telomere content in lung, central nervous system, and leukemia cell lines. Using CRISPR/Cas9 screening data, we show that lower telomeric content is associated with dependency of CST telomere maintenance genes. Increased dependencies of shelterin members are associated with wild-type TP53 status. Investigating the epigenetic regulation of TERT, we find widespread allele-specific expression in promoter-wildtype contexts. TERT promoter-mutant cell lines exhibit hypomethylation at PRC2-repressed regions, suggesting a cooperative global epigenetic state in the reactivation of telomerase. By incorporating telomere content with genomic features across comprehensively characterized cell lines, we provide further insights into the role of telomere regulation in cancer immortality.

TAZ maintains telomere length in TNBC cells by mediating Rad51C expression

Background

Telomere maintenance is crucial for the unlimited proliferation of cancer cells and essential for the “stemness” of multiple cancer cells. TAZ is more extensively expressed in triple negative breast cancers (TNBC) than in other types of breast cancers, and promotes proliferation, transformation and EMT of cancer cells. It was reported that TAZ renders breast cancer cells with cancer stem cell features. However, whether TAZ regulates telomeres is still unclear. In this study, we explored the roles of TAZ in the regulation of telomere maintenance in TNBC cells.

Methods

siRNA and shRNA was used to generate TAZ-depleted TNBC cell lines. qPCR and Southern analysis of terminal restriction fragments techniques were used to test telomere length. Co-immunoprecipitation, Western blotting, immunofluorescence, Luciferase reporter assay and Chromatin-IP were conducted to investigate the underlying mechanism.

Results

By knocking down the expression of TAZ in TNBC cells, we found, for the first time, that TAZ is essential for the maintenance of telomeres in TNBC cells. Moreover, loss of TAZ causes senescence phenotype of TNBC cells. The observed extremely shortened telomeres in late passages of TAZ knocked down cells correlate with an elevated hTERT expression, reductions of shelterin proteins, and an activated DNA damage response pathway. Our data also showed that depletion of TAZ results in overexpression of TERRAs, which are a group of telomeric repeat‐containing RNAs and regulate telomere length and integrity. Furthermore, we discovered that TAZ maintains telomere length of TNBC cells likely by facilitating the expression of Rad51C, a crucial element of homologous recombination pathway that promotes telomere replication.

Conclusions

This study supports the notion that TAZ is an oncogenic factor in TNBC, and further reveals a novel telomere-related pathway that is employed by TAZ to regulate TNBC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01466-z.

Abnormal function of telomere protein TRF2 induces cell mutation and the effects of environmental tumor‑promoting factors (Review)

Recent studies have found that somatic gene mutations and environmental tumor-promoting factors are both indispensable for tumor formation. Telomeric repeat-binding factor (TRF)2 is the core component of the telomere shelterin complex, which plays an important role in chromosome stability and the maintenance of normal cell physiological states. In recent years, TRF2 and its role in tumor formation have gradually become a research hot topic, which has promoted in-depth discussions into tumorigenesis and treatment strategies, and has achieved promising results. Some cells bypass elimination, due to either aging, apoptosis via mutations or abnormal prolongation of the mitotic cycle, and enter the telomere crisis period, where large-scale DNA reorganization occurs repeatedly, which manifests as the precancerous cell cycle. Finally, at the end of the crisis cycle, the mutation activates either the expression level of telomerase or activates the alternative lengthening of telomere mechanism to extend the local telomeres. Under the protection of TRF2, chromosomes are gradually stabilized, immortal cells are formed and the stagewise mutation-driven transformation of normal cells to cancer cells is completed. In addition, TRF2 also shares the characteristics of environmental tumor-promoting factors. It acts on multiple signal transduction pathway-related proteins associated with cell proliferation, and affects peripheral angiogenesis, inhibits the immune recognition and killing ability of the microenvironment, and maintains the stemness characteristics of tumor cells. TRF2 levels are abnormally elevated by a variety of tumor control proteins, which are more conducive to the protection of telomeres and the survival of tumor cells. In brief, the various regulatory mechanisms which tumor cells rely on to survive are organically integrated around TRF2, forming a regulatory network, which is conducive to the optimization of the survival direction of heterogeneous tumor cells, and promotes their survival and adaptability. In terms of clinical application, TRF2 is expected to become a new type of cancer prognostic marker and a new tumor treatment target. Inhibition of TRF2 overexpression could effectively cut off the core network regulating tumor cell survival, reduce drug resistance, or bypass the mutation under the pressure of tumor treatment selection, which may represent a promising therapeutic strategy for the complete eradication of tumors in the clinical setting. Based on recent research, the aim of the present review was to systematically elaborate on the basic structure and functional characteristics of TRF2 and its role in tumor formation, and to analyze the findings indicating that TRF2 deficiency or overexpression could cause severe damage to telomere function and telomere shortening, and induce DNA damage response and chromosomal instability.

External environmental agents influence telomere length and telomerase activity by modulating internal cellular processes: Implications in human aging

External environment affects cellular physiological processes and impact the stability of our genome. The most important structural components of our linear chromosomes which endure the impact by these agents, are the chromosomal ends called telomeres. Telomeres preserve the integrity of our genome by preventing end to end fusions and telomeric loss through by inhibiting DNA damage response (DDR) activation. This is accomplished by the presence of a six membered shelterin complex at telomeres. Further, telomeres cannot be replicated by normal DNA polymerase and require a special enzyme called telomerase which is expressed only in stem cells, few immune cells and germ cells. Telomeres are rich in guanine content and thus become extremely prone to damage arising due to physiological processes like oxidative stress and inflammation. External environmental factors which includes various physical, biological and chemical agents also affect telomere homeostasis by increasing oxidative stress and inflammation. In the present review, we highlight the effect of these external factors on telomerase activity and telomere length. We also discuss how the external agents affect the physiological processes, thus modulating telomere stability. Further, we describe its implication in the development of aging and its related pathologies.

Polymorphic tandem DNA repeats activate the human telomerase reverse transcriptase gene

Multiple independent sequence variants of the hTERT locus have been associated with telomere length and cancer risks in genome-wide association studies. Here, we identified an intronic variable number tandem repeat, VNTR2-1, as an enhancer-like element, which activated hTERT transcription in a cell in a chromatin-dependent manner. VNTR2-1, consisting of 42-bp repeats with an array of enhancer boxes, cooperated with the proximal promoter in the regulation of hTERT transcription by basic helix-loop-helix transcription factors and maintained hTERT expression during embryonic stem-cell differentiation. Genomic deletion of VNTR2-1 in MelJuSo melanoma cells markedly reduced hTERT transcription, leading to telomere shortening, cellular senescence, and impairment of xenograft tumor growth. Interestingly, VNTR2-1 lengths varied widely in human populations; hTERT alleles with shorter VNTR2-1 were underrepresented in African American centenarians, indicating its role in human aging. Therefore, this polymorphic element is likely a missing link in the telomerase regulatory network and a molecular basis for genetic diversities of telomere homeostasis and age-related disease susceptibilities.

Telomerase and Pluripotency Factors Jointly Regulate Stemness in Pancreatic Cancer Stem Cells

To assess the role of telomerase activity and telomere length in pancreatic CSCs we used different CSC enrichment methods (CD133, ALDH, sphere formation) in primary patient-derived pancreatic cancer cells. We show that CSCs have higher telomerase activity and longer telomeres than bulk tumor cells. Inhibition of telomerase activity, using genetic knockdown or pharmacological inhibitor (BIBR1532), resulted in CSC marker depletion, abrogation of sphere formation in vitro and reduced tumorigenicity in vivo. Furthermore, we identify a positive feedback loop between stemness factors (NANOG, OCT3/4, SOX2, KLF4) and telomerase, which is essential for the self-renewal of CSCs. Disruption of the balance between telomerase activity and stemness factors eliminates CSCs via induction of DNA damage and apoptosis in primary patient-derived pancreatic cancer samples, opening future perspectives to avoid CSC-driven tumor relapse. In the present study, we demonstrate that telomerase regulation is critical for the "stemness" maintenance in pancreatic CSCs and examine the effects of telomerase inhibition as a potential treatment option of pancreatic cancer. This may significantly promote our understanding of PDAC tumor biology and may result in improved treatment for pancreatic cancer patients.

Human telomerase is directly regulated by non-telomeric TRF2-G-quadruplex interaction

Human telomerase reverse transcriptase (hTERT) remains suppressed in most normal somatic cells. Resulting erosion of telomeres leads eventually to replicative senescence. Reactivation of hTERT maintains telomeres and triggers progression of >90% of cancers. However, any direct causal link between telomeres and telomerase regulation remains unclear. Here, we show that the telomere-repeat-binding-factor 2 (TRF2) binds hTERT promoter G-quadruplexes and recruits the polycomb-repressor EZH2/PRC2 complex. This is causal for H3K27 trimethylation at the hTERT promoter and represses hTERT in cancer as well as normal cells. Two highly recurrent hTERT promoter mutations found in many cancers, including ∼83% glioblastoma multiforme, that are known to destabilize hTERT promoter G-quadruplexes, showed loss of TRF2 binding in patient-derived primary glioblastoma multiforme cells. Ligand-induced G-quadruplex stabilization restored TRF2 binding, H3K27-trimethylation, and hTERT re-suppression. These results uncover a mechanism of hTERT regulation through a telomeric factor, implicating telomere-telomerase molecular links important in neoplastic transformation, aging, and regenerative therapy.

The Power of Stress: The Telo-Hormesis Hypothesis

Adaptative response to stress is a strategy conserved across evolution to promote survival. In this context, the groundbreaking findings of Miroslav Radman on the adaptative value of changing mutation rates opened new avenues in our understanding of stress response. Inspired by this work, we explore here the putative beneficial effects of changing the ends of eukaryotic chromosomes, the telomeres, in response to stress. We first summarize basic principles in telomere biology and then describe how various types of stress can alter telomere structure and functions. Finally, we discuss the hypothesis of stress-induced telomere signaling with hormetic effects.