Telomerase recruitment requires both TCAB1 and Cajal bodies independently

RIOK2 transcriptionally regulates TRiC and dyskerin complexes to prevent telomere shortening

Telomere shortening is a prominent hallmark of aging and is emerging as a characteristic feature of Myelodysplastic Syndromes (MDS) and Idiopathic Pulmonary Fibrosis (IPF). Optimal telomerase activity prevents progressive shortening of telomeres that triggers DNA damage responses. However, the upstream regulation of telomerase holoenzyme components remains poorly defined. Here, we identify RIOK2, a master regulator of human blood cell development, as a critical transcription factor for telomere maintenance. Mechanistically, loss of RIOK2 or its DNA-binding/transactivation properties downregulates mRNA expression of both TRiC and dyskerin complex subunits that impairs telomerase activity, thereby causing telomere shortening. We further show that RIOK2 expression is diminished in aged individuals and IPF patients, and it strongly correlates with shortened telomeres in MDS patient-derived bone marrow cells. Importantly, ectopic expression of RIOK2 alleviates telomere shortening in IPF patient-derived primary lung fibroblasts. Hence, increasing RIOK2 levels prevents telomere shortening, thus offering therapeutic strategies for telomere biology disorders.

Small Molecules Blocking the Assembly of TCAB1 and Telomerase Complexes: Lead Discovery and Biological Activity

The vast majority of tumor cells maintain the length of the telomeres through a telomerase-dependent maintenance mechanism, allowing for unlimited proliferation. TCAB1 is indispensable for the correct assembly of telomerase complexes and the delivery of telomerase to the telomere. Therefore, this study aimed to explore small molecules capable of interfering with the assembly of TCAB1 and the telomerase complex as novel efficient telomerase inhibitors. Through virtual screening, biological evaluation, and the confirmation of target engagement, the potential ligands of TCAB1 effectively inhibiting telomerase activity were discovered. Among them, compound 9 exhibited telomerase inhibitory activity at a two-digit nanomolar level (IC50 = 0.03 μM), which was dramatically enhanced in comparison with the previously reported telomerase inhibitors. This research, based on the blockage of telomerase assembly through disturbing TCAB1, provides a novel strategy and a potential target for telomerase inhibitor discovery.

Selenomethionine suppresses head and neck squamous cell carcinoma progression through TopBP1/ATR and TCAB1 signaling

OBJECTIVE

Head and neck squamous cell carcinoma (HNSCC) is a histological type of cancer originating from the head and neck. Selenium complexes have been considered as a potential treatment for HNSCC. Therefore, the present work focused on probing the mechanism of L-selenomethionine (SeMet) in HNSCC treatment.

METHODS

MTT and colony formation assays were carried out to analyze the survival rate and proliferation of HNSCC cells, respectively. TUNEL staining was performed to examine apoptosis of HNSCC cells. Additionally, qRT-PCR and Western blotting assays were performed to measure mRNA and protein levels, separately.

RESULTS

SeMet treatment significantly hindered the survival and promoted the apoptosis of HNSCC cells in a dose- and time-dependently. SeMet administration promoted expression of TopBP1, ATR, H2AX, p-ATR and γ-H2AX, and suppressed that of TCAB1. Importantly, SeMet treatment suppressed the proliferation and facilitated the apoptosis of HNSCC cells, which were partly reversed by down-regulation of TopBP1 or up-regulation of TCAB1. The activation of SeMet to TopBP1/ATR signaling was rescued by TCAB1 up-regulating, and the inhibition of SeMet to TCAB1 expression was rescued by TopBP1 silencing.

CONCLUSION

Our findings show that SeMet inhibits the proliferation of HNSCC cells and promotes their apoptosis by targeting TopBP1/ATR and TCAB1 signaling. SeMet is a potential method for HNSCC treatment.

Molecular and Evolutionary Analysis of RNA-Protein Interactions in Telomerase Regulation

Telomerase is an enzyme involved in the maintenance of telomeres. Telomere shortening due to the end-replication problem is a threat to the genome integrity of all eukaryotes. Telomerase inside cells depends on a myriad of protein-protein and RNA-protein interactions to properly assemble and regulate the function of the telomerase holoenzyme. These interactions are well studied in model eukaryotes, like humans, yeast, and the ciliated protozoan known as Tetrahymena thermophila. Emerging evidence also suggests that deep-branching eukaryotes, such as the parasitic protist Trypanosoma brucei require conserved and novel RNA-binding proteins for the assembly and function of their telomerase. In this review, we will discuss telomerase regulatory pathways in the context of telomerase-interacting proteins, with special attention paid to RNA-binding proteins. We will discuss these interactors on an evolutionary scale, from parasitic protists to humans, to provide a broader perspective on the extensive role that protein-protein and RNA-protein interactions play in regulating telomerase activity in eukaryotes.

2.7 Å cryo-EM structure of human telomerase H/ACA ribonucleoprotein

Telomerase is a ribonucleoprotein (RNP) enzyme that extends telomeric repeats at eukaryotic chromosome ends to counterbalance telomere loss caused by incomplete genome replication. Human telomerase is comprised of two distinct functional lobes tethered by telomerase RNA (hTR): a catalytic core, responsible for DNA extension; and a Hinge and ACA (H/ACA) box RNP, responsible for telomerase biogenesis. H/ACA RNPs also have a general role in pseudouridylation of spliceosomal and ribosomal RNAs, which is critical for the biogenesis of the spliceosome and ribosome. Much of our structural understanding of eukaryotic H/ACA RNPs comes from structures of the human telomerase H/ACA RNP. Here we report a 2.7 Å cryo-electron microscopy structure of the telomerase H/ACA RNP. The significant improvement in resolution over previous 3.3 Å to 8.2 Å structures allows us to uncover new molecular interactions within the H/ACA RNP. Many disease mutations are mapped to these interaction sites. The structure also reveals unprecedented insights into a region critical for pseudouridylation in canonical H/ACA RNPs. Together, our work advances understanding of telomerase-related disease mutations and the mechanism of pseudouridylation by eukaryotic H/ACA RNPs.

Advances in understanding telomerase assembly

Telomerase is a complex ribonucleoprotein scaffolded by the telomerase RNA (TR). Telomere lengthening by telomerase is essential to maintain the proliferative potential of stem cells and germ cells, and telomerase is inappropriately activated in the majority of cancers. Assembly of TR with its 12 protein co-factors and the maturation of the 5'- and 3'-ends of TR have been the focus of intense research efforts over the past two decades. High-resolution Cryo-EM structures of human telomerase, high-throughput sequencing of the 3' end of TR, and live cell imaging of various telomerase components have significantly advanced our understanding of the molecular mechanisms that govern telomerase biogenesis, yet many important questions remain unaddressed. In this review, we will summarize these recent advances and highlight the remaining key questions with the ultimate goal of targeting telomerase assembly to suppress telomere maintenance in cancer cells or to promote telomerase activity in patients affected by telomere shortening disorders.

Coilin and Cajal bodies

The nucleus of higher eukaryotes contains a number of structures that concentrate specific biomolecules and play distinct roles in nuclear metabolism. In recent years, the molecular mechanisms controlling their formation have been intensively studied. In this brief review, I focus on coilin and Cajal bodies. Coilin is a key scaffolding protein of Cajal bodies that is evolutionarily conserved in metazoans. Cajal bodies are thought to be one of the archetypal nuclear structures involved in the metabolism of several short non-coding nuclear RNAs. Yet surprisingly little is known about the structure and function of coilin, and a comprehensive model to explain the origin of Cajal bodies is also lacking. Here, I summarize recent results on Cajal bodies and coilin and discuss them in the context of the last three decades of research in this field.

TCAB1 prevents nucleolar accumulation of the telomerase RNA to facilitate telomerase assembly

Localization of a variety of RNAs to non-membrane-bound cellular compartments such as nucleoli and Cajal bodies is critical for their stability and function. The molecular mechanisms that underly the recruitment and exclusion of RNAs from these phase-separated organelles is incompletely understood. Telomerase is a ribonucleoprotein composed of the reverse transcriptase protein telomerase reverse transcriptase (TERT), the telomerase RNA (TR), and several auxiliary proteins, including TCAB1. Here we show that in the absence of TCAB1, a large fraction of TR is tightly bound to the nucleolus, while TERT is largely excluded from the nucleolus, reducing telomerase assembly. This suggests that nuclear compartmentalization by the non-membrane-bound nucleolus counteracts telomerase assembly, and TCAB1 is required to retain TR in the nucleoplasm. Our work provides insight into the mechanism and functional consequences of RNA recruitment to organelles formed by phase separation and demonstrates that TCAB1 plays an important role in telomerase assembly.

Post-Transcriptional and Post-Translational Modifications in Telomerase Biogenesis and Recruitment to Telomeres

Telomere length is associated with the proliferative potential of cells. Telomerase is an enzyme that elongates telomeres throughout the entire lifespan of an organism in stem cells, germ cells, and cells of constantly renewed tissues. It is activated during cellular division, including regeneration and immune responses. The biogenesis of telomerase components and their assembly and functional localization to the telomere is a complex system regulated at multiple levels, where each step must be tuned to the cellular requirements. Any defect in the function or localization of the components of the telomerase biogenesis and functional system will affect the maintenance of telomere length, which is critical to the processes of regeneration, immune response, embryonic development, and cancer progression. An understanding of the regulatory mechanisms of telomerase biogenesis and activity is necessary for the development of approaches toward manipulating telomerase to influence these processes. The present review focuses on the molecular mechanisms involved in the major steps of telomerase regulation and the role of post-transcriptional and post-translational modifications in telomerase biogenesis and function in yeast and vertebrates.

Functional genomics for curation of variants in telomere biology disorder associated genes: A systematic review

PURPOSE

Patients with an underlying telomere biology disorder (TBD) have variable clinical presentations, and they can be challenging to diagnose clinically. A genomic diagnosis for patients presenting with TBD is vital for optimal treatment. Unfortunately, many variants identified during diagnostic testing are variants of uncertain significance. This complicates management decisions, delays treatment, and risks nonuptake of potentially curative therapies. Improved application of functional genomic evidence may reduce variants of uncertain significance classifications.

METHODS

We systematically searched the literature for published functional assays interrogating TBD gene variants. When possible, established likely benign/benign and likely pathogenic/pathogenic variants were used to estimate the assay sensitivity, specificity, positive predictive value, negative predictive value, and odds of pathogenicity.

RESULTS

In total, 3131 articles were screened and 151 met inclusion criteria. Sufficient data to enable a PS3/BS3 recommendation were available for TERT variants only. We recommend that PS3 and BS3 can be applied at a moderate and supportive level, respectively. PS3/BS3 application was limited by a lack of assay standardization and limited inclusion of benign variants.

CONCLUSION

Further assay standardization and assessment of benign variants are required for optimal use of the PS3/BS3 criterion for TBD gene variant classification.

Emerging Implications of Phase Separation in Cancer

In eukaryotic cells, biological activities are executed in distinct cellular compartments or organelles. Canonical organelles with membrane-bound structures are well understood. Cells also inherently contain versatile membrane-less organelles (MLOs) that feature liquid or gel-like bodies. A biophysical process termed liquid-liquid phase separation (LLPS) elucidates how MLOs form through dynamic biomolecule assembly. LLPS-related molecules often have multivalency, which is essential for low-affinity inter- or intra-molecule interactions to trigger phase separation. Accumulating evidence shows that LLPS concentrates and organizes desired molecules or segregates unneeded molecules in cells. Thus, MLOs have tunable functional specificity in response to environmental stimuli and metabolic processes. Aberrant LLPS is widely associated with several hallmarks of cancer, including sustained proliferative signaling, growth suppressor evasion, cell death resistance, telomere maintenance, DNA damage repair, etc. Insights into the molecular mechanisms of LLPS provide new insights into cancer therapeutics. Here, the current understanding of the emerging concepts of LLPS and its involvement in cancer are comprehensively reviewed.

Recent advances in understanding telomere diseases

Germline genetic defects impairing telomere length maintenance may result in severe medical conditions in humans, from aplastic anemia and myeloid neoplasms to interstitial lung disease and liver cirrhosis, from childhood (dyskeratosis congenita) to old age (pulmonary fibrosis). The molecular mechanisms underlying these clinically distinct disorders are pathologically excessive telomere erosion, limiting cell proliferation and differentiation, tissue regeneration, and increasing genomic instability. Recent findings also indicate that telomere shortening imbalances stem cell fate and is associated with an abnormal inflammatory response and the senescent-associated secretory phenotype. Bone marrow failure is the most common phenotype in patients with telomere diseases. Pulmonary fibrosis is a typical phenotype in older patients, and disease progression appears faster than in pulmonary fibrosis not associated with telomeropathies. Liver cirrhosis may present in isolation or in combination with other phenotypes. Diagnosis is based on clinical suspicion and may be confirmed by telomere length measurement and genetic testing. Next-generation sequencing (NGS) techniques have improved genetic testing; today, at least 16 genes have been implicated in telomeropathies. NGS also allows tracking of clonal hematopoiesis and malignant transformation. Patients with telomere diseases are at high risk of developing cancers, including myeloid neoplasms and head and neck cancer. However, treatment options are still limited. Transplant modalities (bone marrow, lung, and liver) may be definitive to the respective organ involvement but limited by donor availability, comorbidities, and impact on other affected organs. In clinical trials, androgens elongate telomeres of peripheral blood leukocytes and improve hematopoiesis. Further understanding of how telomere erosion impairs organ function and how somatic mutations evolve in the hematopoietic tissue may help develop new strategies to treat and prevent telomere diseases.

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.

TGS1 mediates 2,2,7-trimethyl guanosine capping of the human telomerase RNA to direct telomerase dependent telomere maintenance

Pathways that direct the selection of the telomerase-dependent or recombination-based, alternative lengthening of telomere (ALT) maintenance pathway in cancer cells are poorly understood. Using human lung cancer cells and tumor organoids we show that formation of the 2,2,7-trimethylguanosine (TMG) cap structure at the human telomerase RNA 5′ end by the Trimethylguanosine Synthase 1 (TGS1) is central for recruiting telomerase to telomeres and engaging Cajal bodies in telomere maintenance. TGS1 depletion or inhibition by the natural nucleoside sinefungin impairs telomerase recruitment to telomeres leading to Exonuclease 1 mediated generation of telomere 3′ end protrusions that engage in RAD51-dependent, homology directed recombination and the activation of key features of the ALT pathway. This indicates a critical role for 2,2,7-TMG capping of the RNA component of human telomerase (hTR) in enforcing telomerase-dependent telomere maintenance to restrict the formation of telomeric substrates conductive to ALT. Our work introduces a targetable pathway of telomere maintenance that holds relevance for telomere-related diseases such as cancer and aging.

Telomerase protects chromosome ends in stem cells and cancer cells. Here the authors show that Trimethylguaonsine Synthase 1 (TGS-1) – dependent trimethylguanosine capping of the RNA component of the human telomerase complex has an important role in directing telomere dependent telomere maintenance and suppressing the ALT pathway in cancer cells.

A point mutation in human coilin prevents Cajal body formation

Coilin is a conserved protein essential for integrity of nuclear membrane-less inclusions called Cajal bodies. Here, we report an amino-acid substitution (p.K496E) found in a widely-used human EGFP-coilin construct that has a dominant negative effect on Cajal body formation. We show that this coilin-K496E variant fails to rescue Cajal bodies in cells lacking endogenous coilin, whereas the wild-type construct restores Cajal bodies in mouse and human coilin-knockout cells. In cells containing endogenous coilin, both the wild-type and K496E variant proteins accumulate in Cajal bodies. However, high-level overexpression of coilin-K496E causes Cajal body disintegration. Thus, a mutation in the C-terminal region of human coilin can disrupt Cajal body assembly. Caution should be used when interpreting data from coilin plasmids that are derived from this variant (currently deposited at Addgene).

Structural basis of human telomerase recruitment by TPP1-POT1

Telomerase maintains genome stability by extending the 3' telomeric repeats at eukaryotic chromosome ends, thereby counterbalancing progressive loss caused by incomplete genome replication. In mammals, telomerase recruitment to telomeres is mediated by TPP1, which assembles as a heterodimer with POT1. We report structures of DNA-bound telomerase in complex with TPP1 and with TPP1-POT1 at 3.2- and 3.9-angstrom resolution, respectively. Our structures define interactions between telomerase and TPP1-POT1 that are crucial for telomerase recruitment to telomeres. The presence of TPP1-POT1 stabilizes the DNA, revealing an unexpected path by which DNA exits the telomerase active site and a DNA anchor site on telomerase that is important for telomerase processivity. Our findings rationalize extensive prior genetic and biochemical findings and provide a framework for future mechanistic work on telomerase regulation.

Molecular insight of dyskeratosis congenita: Defects in telomere length homeostasis

BACKGROUND

Dyskeratosis congenita (DC) is a rare disease and is a heterogenous disorder, with its inheritance patterns as autosomal dominant, autosomal recessive, and X-linked recessive. This disorder occurs due to faulty maintenance of telomeres in stem cells. This congenital condition is diagnosed with three symptoms: oral leukoplakia, nail dystrophy, and abnormal skin pigmentation. However, because it has a wide range of symptoms, it may have phenotypes similar to other diseases. For this reason, it is necessary to use methods of measuring the Telomere Length (TL) and determining the shortness of the telomere in these patients so that it can be distinguished from other diseases. Today, the Next Generation Sequencing technique accurately detects mutations in the target genes.

AIM

This work aims to review and summarize how each of the DC genes is involved in TL, and how to diagnose and differentiate the disease using clinical signs and methods to measure TL. It also offers treatments for DC patients, such as Hematopoietic Stem Cell Transplantation and Androgen therapy.

RELEVANCE FOR PATIENTS

In DC patients, the genes involved in telomere homeostasis are mutated. Because these patients may have an overlapping phenotype with other diseases, it is best to perform whole-exome sequencing after genetics counseling to find the relevant mutation. As DC is a multi-systemic disease, we need to monitor patients frequently through annual lung function tests, ultrasounds, gynecological examinations, and skin examinations.

Analysis of RNA conformation in endogenously assembled RNPs by icSHAPE

The majority of the mammalian genome is transcribed into non-coding RNAs, many of which co-evolve with RNA-binding proteins (RBPs) to function as biochemically defined and tractable ribonucleoproteins (RNPs). Here, we applied icSHAPE- a robust and versatile RNA structural probing pipeline- to endogenous RNPs purified from nuclei, providing base-resolution structural rationale for RNP activity and subcellular localization. Combining with genetic and biochemical reconstitutions, structural and functional alternations can be directly attributed to a given RBP without ambiguity. For complete details on the use and execution of this protocol, please refer to Chen et al. (2018).

Effects of renal denervation on the expression profile of circular RNA in the serum of patients with resistant hypertension

OBJECTIVE

Renal denervation (RDN) is a new treatment option for resistant hypertension (RH), although it has been shown that reduced sympathetic nerve activity after RDN is the main cause of blood pressure decline. In view of the possible correlation between circRNA and hypertension and the metabolic state of the body after RDN, we investigated the potential role of circRNA in RDN treatment of RH.

METHODS

Serum samples of patients with RH were collected before and 48 h after RDN. We explored the mechanism underlying RDN with high-throughput integration of circRNA data.

RESULTS

There were 338 circRNAs that were differentiated before and after RDN; 170 were upregulated and 168 were downregulated (≥1.2-fold, P < 0.05), and the expression of five of them changed significantly (≥1.5-fold, P < 0.05). We used reverse transcription-quantitative polymerase chain reaction to confirm these results in 13 other patients with RH. hsa_circRNA_000367 was upregulated and hsa_circRNA_405119 was downregulated after RDN. We predicted their downstream miRNA-mRNA network and analyzed their putative function via the circRNA-miRNA-mRNA pathway. GO/KEGG analysis showed that their functional annotation may be related to nerve injury and hypertension. We used the Venn Diagram Generator to obtain the intersection of predicted target and sympathetic nerve-related genes (from GeneCards website).

CONCLUSION

The mechanism underlying RDN may be closely related to upregulated hsa_circRNA_000367 or downregulated hsa_circRNA_405119 and involve regulated multiple pathways and multiple cellular and molecular biological processes. These circRNAs may potentially be used as treatment effect biomarkers in RDN.

Telomere Length and Oxidative Stress and Its Relation with Metabolic Syndrome Components in the Aging

A great amount of scientific evidence supports that Oxidative Stress (OxS) can contribute to telomeric attrition and also plays an important role in the development of certain age-related diseases, among them the metabolic syndrome (MetS), which is characterised by clinical and biochemical alterations such as obesity, dyslipidaemia, arterial hypertension, hyperglycaemia, and insulin resistance, all of which are considered as risk factors for type 2 diabetes mellitus (T2DM) and cardiovascular diseases, which are associated in turn with an increase of OxS. In this sense, we review scientific evidence that supports the association between OxS with telomere length (TL) dynamics and the relationship with MetS components in aging. It was analysed whether each MetS component affects the telomere length separately or if they all affect it together. Likewise, this review provides a summary of the structure and function of telomeres and telomerase, the mechanisms of telomeric DNA repair, how telomere length may influence the fate of cells or be linked to inflammation and the development of age-related diseases, and finally, how the lifestyles can affect telomere length.

Targeting telomerase for cancer therapy

Telomere maintenance via telomerase reactivation is a nearly universal hallmark of cancer cells which enables replicative immortality. In contrast, telomerase activity is silenced in most adult somatic cells. Thus, telomerase represents an attractive target for highly selective cancer therapeutics. However, development of telomerase inhibitors has been challenging and thus far there are no clinically approved strategies exploiting this cancer target. The discovery of prevalent mutations in the TERT promoter region in many cancers and recent advances in telomerase biology has led to a renewed interest in targeting this enzyme. Here we discuss recent efforts targeting telomerase, including immunotherapies and direct telomerase inhibitors, as well as emerging approaches such as targeting TERT gene expression driven by TERT promoter mutations. We also address some of the challenges to telomerase-directed therapies including potential therapeutic resistance and considerations for future therapeutic applications and translation into the clinical setting. Although much work remains to be done, effective strategies targeting telomerase will have a transformative impact for cancer therapy and the prospect of clinically effective drugs is boosted by recent advances in structural models of human telomerase.

Full length RTEL1 is required for the elongation of the single-stranded telomeric overhang by telomerase

Telomeres cap the ends of eukaryotic chromosomes and distinguish them from broken DNA ends to suppress DNA damage response, cell cycle arrest and genomic instability. Telomeres are elongated by telomerase to compensate for incomplete replication and nuclease degradation and to extend the proliferation potential of germ and stem cells and most cancers. However, telomeres in somatic cells gradually shorten with age, ultimately leading to cellular senescence. Hoyeraal-Hreidarsson syndrome (HHS) is characterized by accelerated telomere shortening and diverse symptoms including bone marrow failure, immunodeficiency, and neurodevelopmental defects. HHS is caused by germline mutations in telomerase subunits, factors essential for its biogenesis and recruitment to telomeres, and in the helicase RTEL1. While diverse phenotypes were associated with RTEL1 deficiency, the telomeric role of RTEL1 affected in HHS is yet unknown. Inducible ectopic expression of wild-type RTEL1 in patient fibroblasts rescued the cells, enabled telomerase-dependent telomere elongation and suppressed the abnormal cellular phenotypes, while silencing its expression resulted in gradual telomere shortening. Our observations reveal an essential role of the RTEL1 C-terminus in facilitating telomerase action at the telomeric 3' overhang. Thus, the common etiology for HHS is the compromised telomerase action, resulting in telomere shortening and reduced lifespan of telomerase positive cells.

Telomerase Biogenesis and Activities from the Perspective of Its Direct Interacting Partners

Telomerase reverse transcriptase (TERT)—the catalytic subunit of telomerase—is reactivated in up to 90% of all human cancers. TERT is observed in heterogenous populations of protein complexes, which are dynamically regulated in a cell type- and cell cycle-specific manner. Over the past two decades, in vitro protein–protein interaction detection methods have discovered a number of endogenous TERT binding partners in human cells that are responsible for the biogenesis and functionalization of the telomerase holoenzyme, including the processes of TERT trafficking between subcellular compartments, assembly into telomerase, and catalytic action at telomeres. Additionally, TERT have been found to interact with protein species with no known telomeric functions, suggesting that these complexes may contribute to non-canonical activities of TERT. Here, we survey TERT direct binding partners and discuss their contributions to TERT biogenesis and functions. The goal is to review the comprehensive spectrum of TERT pro-malignant activities, both telomeric and non-telomeric, which may explain the prevalence of its upregulation in cancer.

Regulation of human telomerase in homeostasis and disease

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

Combining conservation and species-specific differences to determine how human telomerase binds telomeres

Telomerase catalyzes telomeric DNA synthesis at chromosome ends to allow for continued cell division. The telomeric protein TPP1 is essential for enhancing the processivity of telomerase and recruiting the enzyme to telomeres. The telomerase interaction surface on human TPP1 has been mapped to 2 regions of the N-terminal oligosaccharide/oligonucleotide-binding (OB) domain, namely the TPP1 glutamate (E) and leucine (L)-rich (TEL) patch and the N terminus of TPP1-oligosaccharide/oligonucleotide-binding (NOB) region. To map the telomerase side of the interface, we exploited the predicted structural similarities for human and Tetrahymena thermophila telomerase as well as the species specificity of human and mouse telomerase for their cognate TPP1 partners. We show that swapping in the telomerase essential N-terminal (TEN) and insertions in fingers domain (IFD)-TRAP regions of the human telomerase catalytic protein subunit TERT into the mouse TERT backbone is sufficient to bias the species specificity toward human TPP1. Employing a structural homology-based mutagenesis screen focused on surface residues of the TEN and IFD regions, we identified TERT residues that are critical for contacting TPP1 but dispensable for other aspects of telomerase structure or function. We present a functionally validated structural model for how human telomerase engages TPP1 at telomeres, setting the stage for a high-resolution structure of this interface.

Nopp140-mediated concentration of telomerase in Cajal bodies regulates telomere length

Cajal bodies (CBs) are nuclear organelles concentrating two kinds of RNA–­protein complexes (RNPs), spliceosomal small nuclear (sn), and small CB-specific (sca)RNPs. Whereas the CB marker protein coilin is responsible for retaining snRNPs, the tether for scaRNPs is not known. Here we show that Nopp140, an intrinsically disordered CB phosphoprotein, is required to recruit and retain all scaRNPs in CBs. Knockdown (KD) of Nopp140 releases all scaRNPs leading to an unprecedented reduction in size of CB granules, hallmarks of CB ultrastructure. The CB-localizing protein WDR79 (aka TCAB1), which is mutated in the inherited bone marrow failure syndrome dyskeratosis congenita, is a specific component of all scaRNPs, including telomerase. Whereas mislocalization of telomerase by mutation of WDR79 leads to critically shortened telomeres, mislocalization of telomerase by Nopp140 KD leads to gradual extension of telomeres. Our studies suggest that the dynamic distribution of telomerase between CBs and nucleoplasm uniquely impacts telomere length maintenance and identify Nopp140 as a novel player in telomere biology.

Human RTEL1 stabilizes long G-overhangs allowing telomerase-dependent over-extension

Telomere maintenance protects the cell against genome instability and senescence. Accelerated telomere attrition is a characteristic of premature aging syndromes including Dyskeratosis congenita (DC). Mutations in hRTEL1 are associated with a severe form of DC called Hoyeraal-Hreidarsson syndrome (HHS). HHS patients carry short telomeres and HHS cells display telomere damage. Here we investigated how hRTEL1 contributes to telomere maintenance in human primary as well as tumor cells. Transient depletion of hRTEL1 resulted in rapid telomere shortening only in the context of telomerase-positive cells with very long telomeres and high levels of telomerase. The effect of hRTEL1 on telomere length is telomerase dependent without impacting telomerase biogenesis or targeting of the enzyme to telomeres. Instead, RTEL1 depletion led to a decrease in both G-overhang content and POT1 association with telomeres with limited telomere uncapping. Strikingly, overexpression of POT1 restored telomere length but not the overhang, demonstrating that G-overhang loss is the primary defect caused by RTEL1 depletion. We propose that hRTEL1 contributes to the maintenance of long telomeres by preserving long G-overhangs, thereby facilitating POT1 binding and elongation by telomerase.

Structure and function of the N-terminal domain of the yeast telomerase reverse transcriptase

The elongation of single-stranded DNA repeats at the 3′-ends of chromosomes by telomerase is a key process in maintaining genome integrity in eukaryotes. Abnormal activation of telomerase leads to uncontrolled cell division, whereas its down-regulation is attributed to ageing and several pathologies related to early cell death. Telomerase function is based on the dynamic interactions of its catalytic subunit (TERT) with nucleic acids—telomerase RNA, telomeric DNA and the DNA/RNA heteroduplex. Here, we present the crystallographic and NMR structures of the N-terminal (TEN) domain of TERT from the thermotolerant yeast Hansenula polymorpha and demonstrate the structural conservation of the core motif in evolutionarily divergent organisms. We identify the TEN residues that are involved in interactions with the telomerase RNA and in the recognition of the ‘fork’ at the distal end of the DNA product/RNA template heteroduplex. We propose that the TEN domain assists telomerase biological function and is involved in restricting the size of the heteroduplex during telomere repeat synthesis.

Impaired telomere integrity and rRNA biogenesis in PARN-deficient patients and knock-out models

PARN, poly(A)-specific ribonuclease, regulates the turnover of mRNAs and the maturation and stabilization of the hTR RNA component of telomerase. Biallelic PARN mutations were associated with Høyeraal-Hreidarsson (HH) syndrome, a rare telomere biology disorder that, because of its severity, is likely not exclusively due to hTR down-regulation. Whether PARN deficiency was affecting the expression of telomere-related genes was still unclear. Using cells from two unrelated HH individuals carrying novel PARN mutations and a human PARN knock-out (KO) cell line with inducible PARN complementation, we found that PARN deficiency affects both telomere length and stability and down-regulates the expression of TRF1, TRF2, TPP1, RAP1, and POT1 shelterin transcripts. Down-regulation of dyskerin-encoding DKC1 mRNA was also observed and found to result from p53 activation in PARN-deficient cells. We further showed that PARN deficiency compromises ribosomal RNA biogenesis in patients' fibroblasts and cells from heterozygous Parn KO mice. Homozygous Parn KO however resulted in early embryonic lethality that was not overcome by p53 KO. Our results refine our knowledge on the pleiotropic cellular consequences of PARN deficiency.

The PinX1/NPM interaction associates with hTERT in early-S phase and facilitates telomerase activation

Background

Telomere maintenance is an important factor in tumorigenesis. PinX1 is a potent telomerase regulator which also involves in telomerase loading to telomeres. Nucleophosmin (NPM) can partially attenuate PinX1 inhibition of telomerase activity and NPM loading to hTERT requires PinX1. However, the role of the PinX1/NPM interaction in telomerase activity is not fully understood.

Method

The long-term effects of PinX1 and NPM down-regulation on telomere length were investigated by TRF assay. The localization of the PinX1/NPM association and the NPM/PinX1/hTERT complex formation were examined by immunofluorescence studies.

Results

Concurrent long-term down-regulation of PinX1 and NPM led to a substantial decrease in telomere length. The interaction with PinX1 was crucial in NPM localization in the nucleolus during the S phase. PinX1 and NPM associated throughout S phase and the NPM/PinX1/hTERT complex formation peaked during the early-S phase. The PinX1/NPM interaction was shown to localize away from Cajal Bodies at the start of S phase.

Conclusion

PinX1/NPM interaction is important in telomerase regulation during catalysis. NPM is recruited to hTERT by PinX1 and is required in the proposed telomerase modulating unit to activate telomerase when telomere extension occurs during S phase.

Electronic supplementary material

The online version of this article (10.1186/s13578-019-0306-y) contains supplementary material, which is available to authorized users.

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.

Oncogenic Activity of Wrap53 in Human Colorectal Cancer In Vitro and in Nude Mouse Xenografts

Background

WD40-encoding RNA antisense to p53 (Wrap53) has been implicated in cancer development. However, the role of Wrap53 remains unknown in colorectal cancer. The aim of this study was to elucidate the function of Wrap53 in colorectal cancer tumorigenesis and development.

Material/Methods

This study analyzed Wrap53 expression in colorectal cancer tissue specimens using The Cancer Genome Atlas data and tumor cell lines and assessed the effects of Wrap53 knockdown on regulation of cancer cell malignant phenotypes in vitro and in nude mouse xenografts.

Results

Wrap53 expression was upregulated in colorectal cancer tissue specimens and cell lines. Knockdown of Wrap53 expression induced colorectal cancer cell line apoptosis and cell cycle arrest in the G1 phase, but reduced tumor cell line proliferation and invasion in vitro. Knockdown of Wrap53 in a nude mouse xenograft assay inhibited tumor cell line xenograft formation and growth.

Conclusions

Wrap53 is likely a potential oncogene or possesses oncogenic activity in colorectal cancer, promoting colorectal tumorigenesis. Targeting Wrap53 expression may represent a novel strategy for the control of colorectal cancer.

DNA Methylation of Telomere-Related Genes and Cancer Risk

Researchers hypothesized that telomere shortening facilitates carcinogenesis. Previous studies found inconsistent associations between blood leukocyte telomere length (LTL) and cancer. Epigenetic reprogramming of telomere maintenance mechanisms may help explain this inconsistency. We examined associations between DNA methylation in telomere-related genes (TRG) and cancer. We analyzed 475 participants providing 889 samples 1 to 3 times (median follow-up, 10.1 years) from 1999 to 2013 in the Normative Aging Study. All participants were cancer-free at each visit and blood leukocytes profiled using the Illumina 450K array. Of 121 participants who developed cancer, 34 had prostate cancer, 10 melanoma, 34 unknown skin malignancies, and 43 another cancer. We examined 2,651 CpGs from 80 TRGs and applied a combination of Cox and mixed models to identify CpGs prospectively associated with cancer (at FDR < 0.05). We also explored trajectories of DNA methylation, logistic regression stratified by time to diagnosis/censoring, and cross-sectional models of LTL at first blood draw. We identified 30 CpGs on 23 TRGs whose methylation was positively associated with cancer incidence (β = 1.0-6.93) and one protective CpG in MAD1L1 (β = -0.65), of which 87% were located in TRG promoters. Methylation trajectories of 21 CpGs increased in cancer cases relative to controls; at 4 to 8 years prediagnosis/censoring, 17 CpGs were positively associated with cancer. Three CpGs were cross-sectionally associated with LTL. TRG methylation may be a mechanism through which LTL dynamics reflect cancer risk. Future research should confirm these findings and explore potential mechanisms underlying these findings, including telomere maintenance and DNA repair dysfunction. Cancer Prev Res; 11(8); 511-22. ©2018 AACR.

An Activity Switch in Human Telomerase Based on RNA Conformation and Shaped by TCAB1

Ribonucleoprotein enzymes require dynamic conformations of their RNA constituents for regulated catalysis. Human telomerase employs a non-coding RNA (hTR) with a bipartite arrangement of domains-a template-containing core and a distal three-way junction (CR4/5) that stimulates catalysis through unknown means. Here, we show that telomerase activity unexpectedly depends upon the holoenzyme protein TCAB1, which in turn controls conformation of CR4/5. Cells lacking TCAB1 exhibit a marked reduction in telomerase catalysis without affecting enzyme assembly. Instead, TCAB1 inactivation causes unfolding of CR4/5 helices that are required for catalysis and for association with the telomerase reverse-transcriptase (TERT). CR4/5 mutations derived from patients with telomere biology disorders provoke defects in catalysis and TERT binding similar to TCAB1 inactivation. These findings reveal a conformational "activity switch" in human telomerase RNA controlling catalysis and TERT engagement. The identification of two discrete catalytic states for telomerase suggests an intramolecular means for controlling telomerase in cancers and progenitor cells.

WDR79 mediates the proliferation of non-small cell lung cancer cells by regulating the stability of UHRF1

WD repeat protein 79 (WDR79) is a member of the WD-repeat protein family characterized by the presence of a series of WD-repeat domains and is a scaffold protein that participates in telomerase assembly, Cajal body formation and DNA double strand break repair. Although previous studies have revealed that WDR79 is frequently overexpressed in non-small cell lung cancer (NSCLC) and promotes the proliferation of NSCLC cells, the underlying mechanism responsible for WDR79-mediated NSCLC proliferation is not fully understood. In this study, we report a novel molecular function of WDR79 that mediates NSCLC cell proliferation by controlling the stability of UHRF1. In the nucleus, WDR79 colocalized and interacted with UHRF1. As a result, overexpression of WDR79 stabilized UHRF1, whereas ablation of WDR79 decreased the level of UHRF1. Meanwhile, we showed that WDR79 can protect UHRF1 from poly-ubiquitination-mediated proteolysis, which facilitated the stabilization of UHRF1. We further demonstrated that WDR79 exerts a proliferation effect on NSCLC cells by stabilizing UHRF1. These findings reveal that WDR79 is a novel UHRF1 regulator by maintaining UHRF1 stability, and they also provide a clue as to how to explore WDR79 for potential therapeutic application in NSCLC.

Telomerase RNA Imaging in Budding Yeast and Human Cells by Fluorescent In Situ Hybridization

Telomerase, the enzyme that elongates telomeres in most eukaryotes, is a ribonucleoprotein complex composed of a reverse transcriptase catalytic subunit (TERT in human, Est2 in the budding yeast S. cerevisiae), regulatory factors and a noncoding RNA called hTERC (in human) or TLC1 (in budding yeast). Telomerase trafficking is a major process in the biogenesis and regulation of telomerase action at telomeres. Due to its higher signal-to-noise ratio, imaging of the telomerase RNA moiety is frequently used to determine telomerase intracellular localization. Here we describe how to image telomerase RNA in human and yeast cells using fluorescence in situ hybridization.

Epstein-Barr virus-induced up-regulation of TCAB1 is involved in the DNA damage response in nasopharyngeal carcinoma

Telomerase Cajal body protein 1 (TCAB1), which is involved in Cajal body maintenance, telomere elongation and ribonucleoprotein biogenesis, has been linked to cancer predisposition, including nasopharyngeal carcinoma (NPC), due to its oncogenic properties. However, there are no specific reports to date on the functional relevance of TCAB1 and Epstein–Barr virus (EBV), which is considered to be a risk factor for NPC. In this study, we first examined NPC clinical tissues and found a notable overexpression of TCAB1 in EBV-positive specimens. Secondly, on a cellular level, we also observed that TCAB1 expression rose gradually along with the increased duration of EBV exposure in NPC cell lines. Additionally, EBV infection promoted cell proliferation and telomerase activity, but the activation was significantly inhibited after TCAB1 knockdown. Moreover, depletion of TCAB1 caused both cell cycle arrest and apoptosis, and suppressed the activation of ataxia telangiectasia and Rad3 related protein (ATR) induced by EBV, resulting in accumulation of DNA damage. Taken together, we here demonstrate that up-regulated expression of TCAB1, induced by EBV in the development of NPC, is involved in stimulating telomerase activity and regulating the DNA damage response within the context of EBV infection.

Specific genomic cues regulate Cajal body assembly

The assembly of specialized sub-nuclear microenvironments known as nuclear bodies (NBs) is important for promoting efficient nuclear function. In particular, the Cajal body (CB), a prominent NB that facilitates spliceosomal snRNP biogenesis, assembles in response to genomic cues. Here, we detail the factors that regulate CB assembly and structural maintenance. These include the importance of transcription at nucleating gene loci, the grouping of these genes on human chromosomes 1, 6 and 17, as well as cell cycle and biochemical regulation of CB protein function. We also speculate on the correlation between CB formation and RNA splicing levels in neurons and cancer. The timing and location of these specific molecular events is critical to CB assembly and its contribution to genome function. However, further work is required to explore the emerging biophysical characteristics of CB assembly and the impact upon subsequent genome reorganization.

Assembly and trafficking of box C/D and H/ACA snoRNPs

Box C/D and box H/ACA snoRNAs are abundant non-coding RNAs that localize in the nucleolus and mostly function as guides for nucleotide modifications. While a large pool of snoRNAs modifies rRNAs, an increasing number of snoRNAs could also potentially target mRNAs. ScaRNAs belong to a family of specific RNAs that localize in Cajal bodies and that are structurally similar to snoRNAs. Most scaRNAs are involved in snRNA modification, while telomerase RNA, which contains H/ACA motifs, functions in telomeric DNA synthesis. In this review, we describe how box C/D and H/ACA snoRNAs are processed and assembled with core proteins to form functional RNP particles. Their biogenesis involve several transport factors that first direct pre-snoRNPs to Cajal bodies, where some processing steps are believed to take place, and then to nucleoli. Assembly of core proteins involves the HSP90/R2TP chaperone-cochaperone system for both box C/D and H/ACA RNAs, but also several factors specific for each family. These assembly factors chaperone unassembled core proteins, regulate the formation and disassembly of pre-snoRNP intermediates, and control the activity of immature particles. The AAA+ ATPase RUVBL1 and RUVBL2 belong to the R2TP co-chaperones and play essential roles in snoRNP biogenesis, as well as in the formation of other macro-molecular complexes. Despite intensive research, their mechanisms of action are still incompletely understood.

WDR79/TCAB1 plays a conserved role in the control of locomotion and ameliorates phenotypic defects in SMA models

SMN (Survival Motor Neuron) deficiency is the predominant cause of spinal muscular atrophy (SMA), a severe neurodegenerative disorder that can lead to progressive paralysis and death. Although SMN is required in every cell for proper RNA metabolism, the reason why its loss is especially critical in the motor system is still unclear. SMA genetic models have been employed to identify several modifiers that can ameliorate the deficits induced by SMN depletion. Here we focus on WDR79/TCAB1, a protein important for the biogenesis of several RNA species that has been shown to physically interact with SMN in human cells. We show that WDR79 depletion results in locomotion defects in both Drosophila and Caenorhabditis elegans similar to those elicited by SMN depletion. Consistent with this observation, we find that SMN overexpression rescues the WDR79 loss-of-function phenotype in flies. Most importantly, we also found that WDR79 overexpression ameliorates the locomotion defects induced by SMN depletion in both flies and worms. Our results collectively suggest that WDR79 and SMN play evolutionarily conserved cooperative functions in the nervous system and suggest that WDR79/TCAB1 may have the potential to modify SMA pathogenesis.

WDR79 promotes the proliferation of non-small cell lung cancer cells via USP7-mediated regulation of the Mdm2-p53 pathway

WD repeat protein 79 (WDR79) is a member of the WD-repeat protein family and functions as a scaffold protein during telomerase assembly, Cajal body formation and DNA double strand break repair. We have previously shown that WDR79 is frequently overexpressed in cell lines and tissues derived from non-small cell lung cancer (NSCLC) and it accelerates cell proliferation in NSCLC. However, the detailed mechanism underlying the role of WDR79 in the proliferation of NSCLC cells remains unclear. Here, we report the discovery of a molecular interaction between WDR79 and USP7 and show its functional significance in linking the Mdm2-p53 pathway to the proliferation of NSCLC cells. We found that WDR79 colocalized and interacted with USP7 in the nucleus of NSCLC cells. This event, in turn, reduced the ubiquitination of Mdm2 and p53, thereby increasing the stability and extending the half-life of the two proteins. We further found that the functional effects of WDR79 depended upon USP7, because the knockdown of USP7 resulted in their attenuation. Finally, we demonstrated that WDR79 promoted the proliferation of NSCLC cells via USP7. Taken together, our findings reveal a novel molecular function of WDR79 and may lead to broadly applicable and innovative therapeutic avenues for NSCLC.

Life and Death of Yeast Telomerase RNA

Telomerase reverse transcriptase elongates telomeres to overcome their natural attrition and allow unlimited cellular proliferation, a characteristic shared by stem cells and the majority of malignant cancerous cells. The telomerase holoenzyme comprises a core RNA molecule, a catalytic protein subunit, and other accessory proteins. Malfunction of certain telomerase components can cause serious genetic disorders including dyskeratosis congenita and aplastic anaemia. A hierarchy of tightly regulated steps constitutes the process of telomerase biogenesis, which, if interrupted or misregulated, can impede the production of a functional enzyme and severely affect telomere maintenance. Here, we take a closer look at the budding yeast telomerase RNA component, TLC1, in its long lifetime journey around the cell. We review the extensive knowledge on TLC1 transcription and processing. We focus on exciting recent studies on telomerase assembly, trafficking, and nuclear dynamics, which for the first time unveil striking similarities between the yeast and human telomerase ribonucleoproteins. Finally, we identify questions yet to be answered and new directions to be followed, which, in the future, might improve our knowledge of telomerase biology and trigger the development of new therapies against cancer and other telomerase-related diseases.

Endogenous Telomerase Reverse Transcriptase N-Terminal Tagging Affects Human Telomerase Function at Telomeres In Vivo

Telomerase action at telomeres is essential for the immortal phenotype of stem cells and the aberrant proliferative potential of cancer cells. Insufficient telomere maintenance can cause stem cell and tissue failure syndromes, while increased telomerase levels are associated with tumorigenesis. Both pathologies can arise from only small perturbation of telomerase function. To analyze telomerase at its low endogenous expression level, we genetically engineered human pluripotent stem cells (hPSCs) to express various N-terminal fusion proteins of the telomerase reverse transcriptase from its endogenous locus. Using this approach, we found that these modifications can perturb telomerase function in hPSCs and cancer cells, resulting in telomere length defects. Biochemical analysis suggests that this defect is multileveled, including changes in expression and activity. These findings highlight the unknown complexity of telomerase structural requirements for expression and function in vivo.

The Cajal body and the nucleolus: "In a relationship" or "It's complicated"?

From their initial identification as 'nucleolar accessory bodies' more than a century ago, the relationship between Cajal bodies and nucleoli has been a subject of interest and controversy. In this review, we seek to place recent developments in the understanding of the physical and functional relationships between the 2 structures in the context of historical observations. Biophysical models of nuclear body formation, the molecular nature of CB/nucleolus interactions and the increasing list of joint roles for CBs and nucleoli, predominantly in assembling ribonucleoprotein (RNP) complexes, are discussed.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.