Loss of Human TGS1 Hypermethylase Promotes Increased Telomerase RNA and Telomere Elongation

TERRA transcripts localize at long telomeres to regulate telomerase access to chromosome ends

The function of TERRA in the regulation of telomerase in human cells is still debated. While TERRA interacts with telomerase, how it regulates telomerase function remains unknown. Here, we show that TERRA colocalizes with the telomerase RNA subunit hTR in the nucleoplasm and at telomeres during different phases of the cell cycle. We report that TERRA transcripts relocate away from chromosome ends during telomere lengthening, leading to a reduced number of telomeric TERRA-hTR molecules and consequent increase in "TERRA-free" telomerase molecules at telomeres. Using live-cell imaging and super-resolution microscopy, we show that upon transcription, TERRA relocates from its telomere of origin to long chromosome ends. Furthermore, TERRA depletion by antisense oligonucleotides promoted hTR localization to telomeres, leading to increased residence time and extended half-life of hTR molecules at telomeres. Overall, our findings indicate that telomeric TERRA transcripts inhibit telomere elongation by telomerase acting in trans, impairing telomerase access to telomeres that are different from their chromosome end of origin.

Sm complex assembly and 5' cap trimethylation promote selective processing of snRNAs by the 3' exonuclease TOE1

Competing exonucleases that promote 3' end maturation or degradation direct quality control of small non-coding RNAs, but how these enzymes distinguish normal from aberrant RNAs is poorly understood. The Pontocerebellar Hypoplasia 7 (PCH7)-associated 3' exonuclease TOE1 promotes maturation of canonical small nuclear RNAs (snRNAs). Here, we demonstrate that TOE1 achieves specificity toward canonical snRNAs through their Sm complex assembly and cap trimethylation, two features that distinguish snRNAs undergoing correct biogenesis from other small non-coding RNAs. Indeed, disruption of Sm complex assembly via snRNA mutations or protein depletions obstructs snRNA processing by TOE1, and in vitro snRNA processing by TOE1 is stimulated by a trimethylated cap. An unstable snRNA variant that normally fails to undergo maturation becomes fully processed by TOE1 when its degenerate Sm binding motif is converted into a canonical one. Our findings uncover the molecular basis for how TOE1 distinguishes snRNAs from other small non-coding RNAs and explain how TOE1 promotes maturation specifically of canonical snRNAs undergoing proper processing.

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.

TRIMETHYLGUANOSINE SYNTHASE1 mutations decanalize female germline development in Arabidopsis

Here, we report the characterization of a plant RNA methyltransferase, orthologous to yeast trimethylguanosine synthase1 (Tgs1p) and whose downregulation was associated with apomixis in Paspalum grasses. Using phylogenetic analyses and yeast complementation, we determined that land plant genomes all encode a conserved, specific TGS1 protein. Next, we studied the role of TGS1 in female reproduction using reporter lines and loss-of-function mutants in Arabidopsis thaliana. pAtTGS1:AtTGS1 reporters showed a dynamic expression pattern. They were highly active in the placenta and ovule primordia at emergence but, subsequently, showed weak signals in the nucellus. Although expressed throughout gametophyte development, activity became restricted to the female gamete and was also detected after fertilization during embryogenesis. TGS1 depletion altered the specification of the precursor cells that give rise to the female gametophytic generation and to the sporophyte, resulting in the formation of a functional aposporous-like lineage. Our results indicate that TGS1 participates in the mechanisms restricting cell fate acquisition to a single cell at critical transitions throughout the female reproductive lineage and, thus, expand our current knowledge of the mechanisms governing female reproductive fate in plants.

DNAzyme-Mediated Biodeposition Coupling Adjustable Cascade Electric Fields for Photoelectrochemical Telomerase Activity Monitoring

Telomerase, as a specialized reverse transcriptase, plays a vital role in early cancer diagnostics and prognosis; thus, developing efficient sensing technologies is of vital importance. Herein, an innovative "signal-on-off" photoelectrochemical (PEC) sensing platform was developed for ultrasensitive evaluation of telomerase activity based on an electron-transfer tunneling distance regulation strategy and DNAzyme-triggerable biocatalytic precipitation. Concretely, cascade internal electric fields between CuInS2 quantum dots (QDs), graphitic carbon nitride nanosheets (g-C3N4 NSs), and TiO2 nanorod arrays (NRAs) were developed to realize cascade electron extraction and hole transfer. Enabled by such a design, an effective "signal-on" state to gain a progressively enhanced PEC output was designed by suppressing the photogenerated electron-hole pair recombination. With the introduction of hairpin probe H2 and the subsequent extension of the primer sequence driven by the target telomerase, the CuInS2 QDs labeled with hairpin probe H1 were programmatically unfolded, resulting in CuInS2 QDs' close proximity to the working electrode away from the cascade interface, accompanied by the formation of G-quadruplex/hemin complexes. The gradual undermining of tunneling distance and implantation of DNAzyme-initiating biocatalytic precipitation tremendously induced the sluggish migration kinetics of the photoinduced charge, accompanied by the photocurrent intensity decrement, leading to the "signal-off" state. Under optimized conditions, the as-prepared PEC biosensor realizes ultrasensitive detection of telomerase activity from 10 to 105 cell·mL-1 with a detection limitation of 3 cells·mL-1. As a proof of concept, this well-designed method provides new insights into signal amplification for telomerase activity evaluation and also presents promising potential for further development in drug screening, healthcare diagnostics, and biological assays.

The 3' exonuclease TOE1 selectively processes snRNAs through recognition of Sm complex assembly and 5' cap trimethylation

Competing exonucleases that promote 3' end maturation or degradation direct quality control of small non-coding RNAs, but how these enzymes distinguish normal from aberrant RNAs is poorly understood. The Pontocerebellar Hypoplasia 7 (PCH7)-associated 3' exonuclease TOE1 promotes maturation of canonical small nuclear RNAs (snRNAs). Here, we demonstrate that TOE1 achieves specificity towards canonical snRNAs by recognizing Sm complex assembly and cap trimethylation, two features that distinguish snRNAs undergoing correct biogenesis from other small non-coding RNAs. Indeed, disruption of Sm complex assembly via snRNA mutations or protein depletions obstructs snRNA processing by TOE1, and in vitro snRNA processing by TOE1 is stimulated by a trimethylated cap. An unstable snRNA variant that normally fails to undergo maturation becomes fully processed by TOE1 when its degenerate Sm binding motif is converted into a canonical one. Our findings uncover the molecular basis for how TOE1 distinguishes snRNAs from other small non-coding RNAs and explain how TOE1 promotes maturation specifically of canonical snRNAs undergoing proper processing.

CRISPR-based m6A modification and its potential applications in telomerase regulation

Telomerase determines cell lifespan by controlling chromosome stability and cell viability, m⁶A epigenetic modification plays an important role in the regulation of telomerase activity. Using CRISPR epigenome editing to analyze specific m⁶A modification sites in telomerase will provide an important tool for analyzing the molecular mechanism of m⁶A modification regulating telomerase activity. In this review, we clarified the relevant applications of CRISPR system, paid special attention to the regulation of m⁶A modification in stem cells and cancer cells based on CRISPR system, emphasized the regulation of m⁶A modification on telomerase activity, pointed out that m⁶A modification sites regulate telomerase activity, and discussed strategies based on telomerase activity and disease treatment, which are helpful to promote the research of anti-aging and tumor related diseases.

Compilation and functional classification of telomere length-associated genes in humans and other animal species

Telomeres are the terminal regions of chromosomes that ensure their stability while cell division. Telomere shortening initiates cellular senescence, which can lead to degeneration and atrophy of tissues, so the process is associated with a reduction in life expectancy and predisposition to a number of diseases. An accelerated rate of telomere attrition can serve as a predictor of life expectancy and health status of an individual. Telomere length is a complex phenotypic trait that is determined by many factors, including the genetic ones. Numerous studies (including genome-wide association studies, GWAS) indicate the polygenic nature of telomere length control. The objective of the present study was to characterize the genetic basis of the telomere length regulation using the GWAS data obtained during the studies of various human and other animal populations. To do so, a compilation of the genes associated with telomere length in GWAS experiments was collected, which included information on 270 human genes, as well as 23, 22, and 9 genes identified in the cattle, sparrow, and nematode, respectively. Among them were two orthologous genes encoding a shelterin protein (POT1 in humans and pot-2 in C. elegans). Functional analysis has shown that telomere length can be influenced by genetic variants in the genes encoding: (1) structural components of telomerase; (2) the protein components of telomeric regions (shelterin and CST complexes); (3) the proteins involved in telomerase biogenesis and regulating its activity; (4) the proteins that regulate the functional activity of the shelterin components; (5) the proteins involved in telomere replication and/or capping; (6) the proteins involved in the alternative telomere lengthening; (7) the proteins that respond to DNA damage and are responsible for DNA repair; (8) RNA-exosome components. The human genes identified by several research groups in populations of different ethnic origins are the genes encoding telomerase components such as TERC and TERT as well as STN1 encoding the CST complex component. Apparently, the polymorphic loci affecting the functions of these genes may be the most reliable susceptibility markers for telomere-related diseases. The systematized data about the genes and their functions can serve as a basis for the development of prognostic criteria for telomere length-associated diseases in humans. Information about the genes and processes that control telomere length can be used for marker-assisted and genomic selection in the farm animals, aimed at increasing the duration of their productive lifetime.

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.

Postmenopausal osteoporosis: Effect of moderate-intensity treadmill exercise on bone proteomics in ovariectomized rats

Objectives

This study aimed to identify the key proteins in the bone mass of ovariectomized (OVX) rats after a period of regular moderate-intensity treadmill exercise and to investigate their effects using tag mass spectrometry and quantitative proteomics with a view to improving the understanding and treatment of postmenopausal osteoporosis.

Methods

Sixty three-month-old female Sprague-Dawley tats of specific-pathogen-free grade were randomly and equally divided into a sham operation group, ovariectomized group (OVX) and ovariectomized combined exercise (OVX + EX) group, and the latter took moderate-intensity treadmill exercise for 17 weeks. After this period of time, body composition and bone density were measured using dual-energy x-ray absorptiometry, and serum bone metabolism indicators were measured using an enzyme immunoassay. In addition, the bone microstructure was examined using micro-computed tomography and scanning of the femur, and femur proteins were subject to proteomic analysis.

Results

Compared with the rats in the OVX group, the bone metabolism indicators in the OVX + EX group decreased significantly, femur bone density increased significantly, the number of the trabeculae increased, and continuity was higher. In the OVX + EX group, 17 proteins were significantly upregulated and 33 significantly downregulated. The main gene ontology and signaling pathways enriched by the proteins were identified as the tumor necrosis factor-mediated signaling pathways. The protein-protein interaction network identified the key proteins, and the correlation analysis of these proteins and the bone parameters found histone deacetylase 8(HDAC8) and leucine-rich transmembrane and O-methyltransferase domain containing (LRTOMT) and trimethylguanosine synthase 1(TGS1) and ankyrin repeat domain 46(ANKRD46) to be the key targets of exercise in relation to postmenopausal osteoporosis.

Conclusion

Moderate-intensity treadmill exercise significantly improved the bone mass of OVX rats, and differentially expressed proteins, such as HDAC8 and LRTOMT and TGS1 and ANKRD46, could be the target of moderate-intensity treadmill exercise.

Measuring the tail: Methods for poly(A) tail profiling

The 3'-end poly(A) tail is an important and potent feature of most mRNA molecules that affects mRNA fate and translation efficiency. Polyadenylation is a posttranscriptional process that occurs in the nucleus by canonical poly(A) polymerases (PAPs). In some specific instances, the poly(A) tail can also be extended in the cytoplasm by noncanonical poly(A) polymerases (ncPAPs). This epitranscriptomic regulation of mRNA recently became one of the most interesting aspects in the field. Advances in RNA sequencing technologies and software development have allowed the precise measurement of poly(A) tails, identification of new ncPAPs, expansion of the function of known enzymes, discovery and a better understanding of the physiological role of tail heterogeneity, and recognition of a correlation between tail length and RNA translatability. Here, we summarize the development of polyadenylation research methods, including classic low-throughput approaches, Illumina-based genome-wide analysis, and advanced state-of-art techniques that utilize long-read third-generation sequencing with Pacific Biosciences and Oxford Nanopore Technologies platforms. A boost in technical opportunities over recent decades has allowed a better understanding of the regulation of gene expression at the mRNA level. This article is categorized under: RNA Methods > RNA Analyses In Vitro and In Silico.

Biogenesis of telomerase RNA from a protein-coding mRNA precursor

Telomerase is a eukaryotic ribonucleoprotein (RNP) enzyme that adds DNA repeats onto chromosome ends to maintain genomic stability and confer cellular immortality in cancer and stem cells. The telomerase RNA (TER) component is essential for telomerase catalytic activity and provides the template for telomeric DNA synthesis. The biogenesis of TERs is extremely divergent across eukaryotic kingdoms, employing distinct types of transcription machinery and processing pathways. In ciliates and plants, TERs are transcribed by RNA polymerase III (Pol III), while animal and ascomycete fungal TERs are transcribed by RNA Pol II and share biogenesis pathways with small nucleolar RNA (snoRNA) and small nuclear RNA (snRNA), respectively. Here, we report an unprecedented messenger RNA (mRNA)-derived biogenesis pathway for the 1,291 nucleotide TER from the basidiomycete fungus Ustilago maydis. The U. maydis TER (UmTER) contains a 5'-monophosphate, distinct from the 5' 2,2,7-trimethylguanosine (TMG) cap common to animal and ascomycete fungal TERs. The mature UmTER is processed from the 3'-untranslated region (3'-UTR) of a larger RNA precursor that possesses characteristics of mRNA including a 5' 7-methyl-guanosine (m7G) cap, alternative splicing of introns, and a poly(A) tail. Moreover, this mRNA transcript encodes a protein called Early meiotic induction protein 1 (Emi1) that is conserved across dikaryotic fungi. A recombinant UmTER precursor expressed from an mRNA promoter is processed correctly to yield mature UmTER, confirming an mRNA-processing pathway for producing TER. Our findings expand the plethora of TER biogenesis mechanisms and demonstrate a pathway for producing a functional long noncoding RNA from a protein-coding mRNA precursor.

Telomere biology disorders: time for moving towards the clinic?

Telomere biology disorders (TBDs) are a group of rare diseases caused by mutations that impair telomere maintenance. Mutations that cause reduced levels of TERC/hTR, the telomerase RNA component, are found in most TBD patients and include loss-of-function mutations in hTR itself, in hTR-binding proteins [NOP10, NHP2, NAF1, ZCCHC8, and dyskerin (DKC1)], and in proteins required for hTR processing (PARN). These patients show diverse clinical presentations that most commonly include bone marrow failure (BMF)/aplastic anemia (AA), pulmonary fibrosis, and liver cirrhosis. There are no curative therapies for TBD patients. An understanding of hTR biogenesis, maturation, and degradation has identified pathways and pharmacological agents targeting the poly(A) polymerase PAPD5, which adds 3'-oligoadenosine tails to hTR to promote hTR degradation, and TGS1, which modifies the 5'-cap structure of hTR to enhance degradation, as possible therapeutic approaches. Critical next steps will be clinical trials to establish the effectiveness and potential side effects of these compounds in TBD patients.

TGS1 impacts snRNA 3'-end processing, ameliorates survival motor neuron-dependent neurological phenotypes in vivo and prevents neurodegeneration

Trimethylguanosine synthase 1 (TGS1) is a highly conserved enzyme that converts the 5'-monomethylguanosine cap of small nuclear RNAs (snRNAs) to a trimethylguanosine cap. Here, we show that loss of TGS1 in Caenorhabditis elegans, Drosophila melanogaster and Danio rerio results in neurological phenotypes similar to those caused by survival motor neuron (SMN) deficiency. Importantly, expression of human TGS1 ameliorates the SMN-dependent neurological phenotypes in both flies and worms, revealing that TGS1 can partly counteract the effects of SMN deficiency. TGS1 loss in HeLa cells leads to the accumulation of immature U2 and U4atac snRNAs with long 3' tails that are often uridylated. snRNAs with defective 3' terminations also accumulate in Drosophila Tgs1 mutants. Consistent with defective snRNA maturation, TGS1 and SMN mutant cells also exhibit partially overlapping transcriptome alterations that include aberrantly spliced and readthrough transcripts. Together, these results identify a neuroprotective function for TGS1 and reinforce the view that defective snRNA maturation affects neuronal viability and function.

How Structural Features Define Biogenesis and Function of Human Telomerase RNA Primary Transcript

Telomerase RNA has been uncovered as a component of the telomerase enzyme, which acts as a reverse transcriptase and maintains the length of telomeres in proliferated eukaryotic cells. Telomerase RNA is considered to have major functions as a template for telomeric repeat synthesis and as a structural scaffold for telomerase. However, investigations of its biogenesis and turnover, as well as structural data, have provided evidence of functions of telomerase RNA that are not associated with telomerase activity. The primary transcript produced from the human telomerase RNA gene encodes for the hTERP protein, which presents regulatory functions related to autophagy, cellular proliferation, and metabolism. This review focuses on the specific features relating to the biogenesis and structure of human telomerase RNA that support the existence of an isoform suitable for functioning as an mRNA. We believe that further investigation into human telomerase RNA biogenesis mechanisms will provide more levels for manipulating cellular homeostasis, survival, and transformation mechanisms, and may contribute to a deeper understanding of the mechanisms of aging.

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.

Telomerase in Cancer: Function, Regulation, and Clinical Translation

Simple Summary

Cells undergoing malignant transformation must circumvent replicative senescence and eventual cell death associated with progressive telomere shortening that occurs through successive cell division. To do so, malignant cells reactivate telomerase to extend their telomeres and achieve cellular immortality, which is a “Hallmark of Cancer”. Here we review the telomere-dependent and -independent functions of telomerase in cancer, as well as its potential as a biomarker and therapeutic target to diagnose and treat cancer patients.

Abstract

During the process of malignant transformation, cells undergo a series of genetic, epigenetic, and phenotypic alterations, including the acquisition and propagation of genomic aberrations that impart survival and proliferative advantages. These changes are mediated in part by the induction of replicative immortality that is accompanied by active telomere elongation. Indeed, telomeres undergo dynamic changes to their lengths and higher-order structures throughout tumor formation and progression, processes overseen in most cancers by telomerase. Telomerase is a multimeric enzyme whose function is exquisitely regulated through diverse transcriptional, post-transcriptional, and post-translational mechanisms to facilitate telomere extension. In turn, telomerase function depends not only on its core components, but also on a suite of binding partners, transcription factors, and intra- and extracellular signaling effectors. Additionally, telomerase exhibits telomere-independent regulation of cancer cell growth by participating directly in cellular metabolism, signal transduction, and the regulation of gene expression in ways that are critical for tumorigenesis. In this review, we summarize the complex mechanisms underlying telomere maintenance, with a particular focus on both the telomeric and extratelomeric functions of telomerase. We also explore the clinical utility of telomeres and telomerase in the diagnosis, prognosis, and development of targeted therapies for primary, metastatic, and recurrent cancers.

Telomerase ribonucleoprotein and genome integrity-An emerging connection in protozoan parasites

Telomerase has an established role in telomere maintenance in eukaryotes. However, recent studies have begun to implicate telomerase in cellular roles beyond telomere maintenance. Specifically, evidence is emerging of cross-talks between telomerase mediated telomere homeostasis and DNA repair pathways. Telomere shortening due to the end replication problem is a constant threat to genome integrity in eukaryotic cells. This poses a particular problem in unicellular parasitic protists because their major virulence genes are located at the subtelomeric loci. Although telomerase is the major regulator of telomere lengthening in eukaryotes, it is less studied in the ancient eukaryotes, including clinically important human pathogens. Recent research is highlighting interplay between telomerase and the DNA damage response in human parasites. The importance of this interplay in pathogen virulence is only beginning to be illuminated, including the potential to highlight novel developmental regulation of telomerase in parasites who transition between multiple developmental stages throughout their life cycle. In this review, we will discuss the telomerase ribonucleoprotein enzyme and DNA repair pathways with emerging views in human parasites to give a broader perspective of the possible connection of telomere, telomerase, and DNA repair pathways across eukaryotic lineages and highlight their potential role in pathogen virulence. This article is categorized under: RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

The S-adenosylmethionine analog sinefungin inhibits the trimethylguanosine synthase TGS1 to promote telomerase activity and telomere lengthening

Mutations in many genes that control the expression, the function, or the stability of telomerase cause telomere biology disorders (TBDs), such as dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia. Mutations in a subset of the genes associated with TBDs cause reductions of the telomerase RNA moiety hTR, thus limiting telomerase activity. We have recently found that loss of the trimethylguanosine synthase TGS1 increases both hTR abundance and telomerase activity and leads to telomere elongation. Here, we show that treatment with the S-adenosylmethionine analog sinefungin inhibits TGS1 activity, increases the hTR levels, and promotes telomere lengthening in different cell types. Our results hold promise for restoring telomere length in stem and progenitor cells from TBD patients with reduced hTR levels.

A Trimethylguanosine Synthase1-like (TGS1) homologue is implicated in vernalisation and flowering time control

KEY MESSAGE

A plant-specific Trimethylguanosine Synthase1-like homologue was identified as a candidate gene for the efl mutation in narrow-leafed lupin, which alters phenology by reducing vernalisation requirement. The vernalisation pathway is a key component of flowering time control in plants from temperate regions but is not well understood in the legume family. Here we examined vernalisation control in the temperate grain legume species, narrow-leafed lupin (Lupinus angustifolius L.), and discovered a candidate gene for an ethylene imine mutation (efl). The efl mutation changes phenology from late to mid-season flowering and additionally causes transformation from obligate to facultative vernalisation requirement. The efl locus was mapped to pseudochromosome NLL-10 in a recombinant inbred line (RIL) mapping population developed by accelerated single seed descent. Candidate genes were identified in the reference genome, and a diverse panel of narrow-leafed lupins was screened to validate mutations specific to accessions with efl. A non-synonymous SNP mutation within an S-adenosyl-L-methionine-dependent methyltransferase protein domain of a Trimethylguanosine Synthase1-like (TGS1) orthologue was identified as the candidate mutation giving rise to efl. This mutation caused substitution of an amino acid within an established motif at a position that is otherwise highly conserved in several plant families and was perfectly correlated with the efl phenotype in F2 and F6 genetic population and a panel of diverse accessions, including the original efl mutant. Expression of the TGS1 homologue did not differ between wild-type and efl genotypes, supporting altered functional activity of the gene product. This is the first time a TGS1 orthologue has been associated with vernalisation response and flowering time control in any plant species.

Landscape of functional interactions of human processive ribonucleases revealed by high-throughput siRNA screenings

Processive exoribonucleases are executors of RNA decay. In humans, their physical but not functional interactions were thoughtfully investigated. Here we have screened cells deficient in DIS3, XRN2, EXOSC10, DIS3L, and DIS3L2 with a custom siRNA library and determined their genetic interactions (GIs) with diverse pathways of RNA metabolism. We uncovered a complex network of positive interactions that buffer alterations in RNA degradation and reveal reciprocal cooperation with genes involved in transcription, RNA export, and splicing. Further, we evaluated the functional distinctness of nuclear DIS3 and cytoplasmic DIS3L using a library of all known genes associated with RNA metabolism. Our analysis revealed that DIS3 mutation suppresses RNA splicing deficiency, while DIS3L GIs disclose the interplay of cytoplasmic RNA degradation with nuclear RNA processing. Finally, genome-wide DIS3 GI map uncovered relations with genes not directly involved in RNA metabolism, like microtubule organization or regulation of telomerase activity.

Maturation and shuttling of the yeast telomerase RNP: assembling something new using recycled parts

As the limiting component of the budding yeast telomerase, the Tlc1 RNA must undergo multiple consecutive modifications and rigorous quality checks throughout its lifecycle. These steps will ensure that only correctly processed and matured molecules are assembled into telomerase complexes that subsequently act at telomeres. The complex pathway of Tlc1 RNA maturation, involving 5'- and 3'-end processing, stabilisation and assembly with the protein subunits, requires at least one nucleo-cytoplasmic passage. Furthermore, it appears that the pathway is tightly coordinated with the association of various and changing proteins, including the export factor Xpo1, the Mex67/Mtr2 complex, the Kap122 importin, the Sm₇ ring and possibly the CBC and TREX-1 complexes. Although many of these maturation processes also affect other RNA species, the Tlc1 RNA exploits them in a new combination and, therefore, ultimately follows its own and unique pathway. In this review, we highlight recent new insights in maturation and subcellular shuttling of the budding yeast telomerase RNA and discuss how these events may be fine-tuned by the biochemical characteristics of the varying processing and transport factors as well as the final telomerase components. Finally, we indicate outstanding questions that we feel are important to be addressed for a complete understanding of the telomerase RNA lifecycle and that could have implications for the human telomerase as well.

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).

Regulation of human telomerase RNA biogenesis and localization

Maintenance of telomeres is essential for genome integrity and replicative capacity in eukaryotic cells. Telomerase, the ribonucleoprotein complex that catalyses telomere synthesis is minimally composed of a reverse transcriptase and an RNA component. The sequence and structural domains of human telomerase RNA (hTR) have been extensively characterized, while the regulation of hTR transcription, maturation, and localization, is not fully understood. Here, we provide an up-to-date review of hTR, with an emphasis on current breakthroughs uncovering the mechanisms of hTR maturation and localization.

Polygenic basis and biomedical consequences of telomere length variation

Telomeres, the end fragments of chromosomes, play key roles in cellular proliferation and senescence. Here we characterize the genetic architecture of naturally occurring variation in leukocyte telomere length (LTL) and identify causal links between LTL and biomedical phenotypes in 472,174 well-characterized UK Biobank participants. We identified 197 independent sentinel variants associated with LTL at 138 genomic loci (108 new). Genetically determined differences in LTL were associated with multiple biological traits, ranging from height to bone marrow function, as well as several diseases spanning neoplastic, vascular and inflammatory pathologies. Finally, we estimated that, at the age of 40 years, people with an LTL >1 s.d. shorter than the population mean had a 2.5-year-lower life expectancy compared with the group with ≥1 s.d. longer LDL. Overall, we furnish new insights into the genetic regulation of LTL, reveal wide-ranging influences of LTL on physiological traits, diseases and longevity, and provide a powerful resource available to the global research community.

Composition and Function of Telomerase-A Polymerase Associated with the Origin of Eukaryotes

The canonical DNA polymerases involved in the replication of the genome are unable to fully replicate the physical ends of linear chromosomes, called telomeres. Chromosomal termini thus become shortened in each cell cycle. The maintenance of telomeres requires telomerase—a specific RNA-dependent DNA polymerase enzyme complex that carries its own RNA template and adds telomeric repeats to the ends of chromosomes using a reverse transcription mechanism. Both core subunits of telomerase—its catalytic telomerase reverse transcriptase (TERT) subunit and telomerase RNA (TR) component—were identified in quick succession in Tetrahymena more than 30 years ago. Since then, both telomerase subunits have been described in various organisms including yeasts, mammals, birds, reptiles and fish. Despite the fact that telomerase activity in plants was described 25 years ago and the TERT subunit four years later, a genuine plant TR has only recently been identified by our group. In this review, we focus on the structure, composition and function of telomerases. In addition, we discuss the origin and phylogenetic divergence of this unique RNA-dependent DNA polymerase as a witness of early eukaryotic evolution. Specifically, we discuss the latest information regarding the recently discovered TR component in plants, its conservation and its structural features.

Human Telomerase RNA: Telomerase Component or More?

Telomerase is a ribonucleoprotein complex that maintains the lengths of telomeres. Most studies of telomerase function have focused on the involvement of telomerase activation in the immortalization of cancer cells and cellular rejuvenation. However, some studies demonstrated that the results do not meet expectations for telomerase action in telomere maintenance. Recent results give reason to think that major telomerase components-the reverse transcriptase protein subunit and telomerase RNA-may participate in many cellular processes, including the regulation of apoptosis and autophagy, cell survival, pro-proliferative effects, regulation of gene expression, and protection against oxidative stress. However, the difficulties faced by scientist when researching telomerase component functions often reduce confidence in the minor effects observed in experiments. In this review, we focus on the analysis of the functions of telomerase components (paying more attention to the telomerase RNA component), both as a complex and as independent components, providing effects that are not associated with telomerase activity and telomere length maintenance. Despite the fact that the data on alternative roles of telomerase components look illusory, it would be wrong to completely reject the possibility of their involvement in other biological processes excluded from research/discussion. Investigations to improve the understanding of every aspect of the functioning of telomerase components will provide the basis for a more precise development of approaches to regulate cellular homeostasis, which is important for carcinogenesis and aging.

Intimate functional interactions between TGS1 and the Smn complex revealed by an analysis of the Drosophila eye development

Trimethylguanosine synthase 1 (TGS1) is a conserved enzyme that mediates formation of the trimethylguanosine cap on several RNAs, including snRNAs and telomerase RNA. Previous studies have shown that TGS1 binds the Survival Motor Neuron (SMN) protein, whose deficiency causes spinal muscular atrophy (SMA). Here, we analyzed the roles of the Drosophila orthologs of the human TGS1 and SMN genes. We show that the Drosophila TGS1 protein (dTgs1) physically interacts with all subunits of the Drosophila Smn complex (Smn, Gem2, Gem3, Gem4 and Gem5), and that a human TGS1 transgene rescues the mutant phenotype caused by dTgs1 loss. We demonstrate that both dTgs1 and Smn are required for viability of retinal progenitor cells and that downregulation of these genes leads to a reduced eye size. Importantly, overexpression of dTgs1 partially rescues the eye defects caused by Smn depletion, and vice versa. These results suggest that the Drosophila eye model can be exploited for screens aimed at the identification of genes and drugs that modify the phenotypes elicited by Tgs1 and Smn deficiency. These modifiers could help to understand the molecular mechanisms underlying SMA pathogenesis and devise new therapies for this genetic disease.

We explored the functional relationships between TGS1 and SMN using Drosophila as model organism. TGS1 is an enzyme that modifies the structure of the 5’-end of several RNAs, including telomerase RNA and the small nuclear RNAs (snRNAs) that are required for messenger RNA maturation. The SMN protein regulates snRNAs biogenesis and mutations in human SMN cause Spinal Muscular Atrophy (SMA), a devastating disorder characterized by neurodegeneration, progressive paralysis and death. We show that mutations in the Drosophila TGS1 (dTgs1) gene cause lethality, which is rescued by a human TGS1 transgene. We also show that the dTgs1 protein physically interacts with all subunits of the Smn complex, and that downregulation of either dTgs1 or Smn leads to a reduced Drosophila eye size. Notably, overexpression of dTgs1 partially rescues the eye defects caused by Smn knockdown, and vice versa, indicating that these genes cooperate in eye development. These results suggest that the eye model can be exploited for screens aimed at detection of chemical and genetic modifiers of the eye mutant phenotype elicited by dTgs1 and Smn deficiency, providing new clues about SMA pathogenesis and potential therapies.

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.