The H/ACA complex disrupts triplex in hTR precursor to permit processing by RRP6 and PARN

Telomerase RNA structural heterogeneity in living human cells detected by DMS-MaPseq

Telomerase is a specialized reverse transcriptase that uses an intrinsic RNA subunit as the template for telomeric DNA synthesis. Biogenesis of human telomerase requires its RNA subunit (hTR) to fold into a multi-domain architecture that includes the template-containing pseudoknot (t/PK) and the three-way junction (CR4/5). These two hTR domains bind the telomerase reverse transcriptase (hTERT) protein and are thus essential for telomerase catalytic activity. Here, we probe the structure of hTR in living cells using dimethyl sulfate mutational profiling with sequencing (DMS-MaPseq) and ensemble deconvolution analysis. Unexpectedly, approximately 15% of the steady state population of hTR has a CR4/5 conformation lacking features thought to be required for hTERT binding. The proportion of hTR CR4/5 that is folded into the primary functional conformation does not require hTERT expression and the fraction of hTR that assumes a misfolded CR4/5 domain is not refolded by overexpression of its hTERT binding partner. This result suggests a functional role for an RNA folding cofactor other than hTERT during telomerase biogenesis. Mutagenesis demonstrates that stabilization of the alternative CR4/5 conformation is detrimental to telomerase assembly and activity. Moreover, the alternative CR4/5 conformation is not found in telomerase RNP complexes purified from cells via an epitope tag on hTERT, supporting the hypothesis that only the major CR4/5 conformer is active. We propose that this misfolded portion of the cellular hTR pool is either slowly refolded or degraded. Thus, kinetic traps for RNA folding that have been so well-studied in vitro may also present barriers for assembly of ribonucleoprotein complexes in vivo.

LARP3, LARP7, and MePCE are involved in the early stage of human telomerase RNA biogenesis

Human telomerase assembly is a highly dynamic process. Using biochemical approaches, we find that LARP3 and LARP7/MePCE are involved in the early stage of human telomerase RNA (hTR) and that their binding to RNA is destabilized when the mature form is produced. LARP3 plays a negative role in preventing the processing of the 3'-extended long (exL) form and the binding of LARP7 and MePCE. Interestingly, the tertiary structure of the exL form prevents LARP3 binding and facilitates hTR biogenesis. Furthermore, low levels of LARP3 promote hTR maturation, increase telomerase activity, and elongate telomeres. LARP7 and MePCE depletion inhibits the conversion of the 3'-extended short (exS) form into mature hTR and the cytoplasmic accumulation of hTR, resulting in telomere shortening. Taken together our data suggest that LARP3 and LARP7/MePCE mediate the processing of hTR precursors and regulate the production of functional telomerase.

The C-terminal extension of dyskerin is a dyskeratosis congenita mutational hotspot that modulates interaction with telomerase RNA and subcellular localization

Dyskerin is a component of the human telomerase complex and is involved in stabilizing the human telomerase RNA (hTR). Many mutations in the DKC1 gene encoding dyskerin are found in X-linked dyskeratosis congenita (X-DC), a premature aging disorder and other related diseases. The C-terminal extension (CTE) of dyskerin contributes to its interaction with the molecular chaperone SHQ1 during the early stage of telomerase biogenesis. Disease mutations in this region were proposed to disrupt dyskerin-SHQ1 interaction and destabilize dyskerin, reducing hTR levels indirectly. However, biochemical evidence supporting this hypothesis is still lacking. In addition, the effects of many CTE disease mutations on hTR have not been examined. In this study, we tested eight dyskerin CTE variants and showed that they failed to maintain hTR levels. These mutants showed slightly reduced but not abolished interaction with SHQ1, and caused defective binding to hTR. Deletion of the CTE further reduced binding to hTR, and perturbed localization of dyskerin to the Cajal bodies and the nucleolus, and the interaction with TCAB1 as well as GAR1. Our findings suggest impaired dyskerin-hTR interaction in cells as a previously overlooked mechanism through which dyskerin CTE mutations cause X-DC and related telomere syndromes.

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.

The fission yeast methyl phosphate capping enzyme Bmc1 guides 2'-O-methylation of the U6 snRNA

Splicing requires the tight coordination of dynamic spliceosomal RNAs and proteins. U6 is the only spliceosomal RNA transcribed by RNA Polymerase III and undergoes an extensive maturation process. In humans and fission yeast, this includes addition of a 5' γ-monomethyl phosphate cap by members of the Bin3/MePCE family as well as snoRNA guided 2'-O-methylation. Previously, we have shown that the Bin3/MePCE homolog Bmc1 is recruited to the S. pombe telomerase holoenzyme by the LARP7 family protein Pof8, where it acts in a catalytic-independent manner to protect the telomerase RNA and facilitate holoenzyme assembly. Here, we show that Bmc1 and Pof8 are required for the formation of a distinct U6 snRNP that promotes 2'-O-methylation of U6, and identify a non-canonical snoRNA that guides this methylation. We also show that the 5' γ-monomethyl phosphate capping activity of Bmc1 is not required for its role in promoting snoRNA guided 2'-O-methylation, and that this role relies on different regions of Pof8 from those required for Pof8 function in telomerase. Our results are consistent with a novel role for Bmc1/MePCE family members in stimulating 2'-O-methylation and a more general role for Bmc1 and Pof8 in guiding noncoding RNP assembly beyond the telomerase RNP.

TERRA stability is regulated by RALY and polyadenylation in a telomere-specific manner

Telomeric repeat-containing RNA (TERRA) is a long non-coding RNA transcribed from telomeres that plays key roles in telomere maintenance. A fraction of TERRA is polyadenylated, and the presence of the poly(A) tail influences TERRA localization and stability. However, the mechanisms of TERRA biogenesis remain mostly elusive. Here, we show that the stability of TERRA transcripts is regulated by the RNA-binding protein associated with lethal yellow mutation (RALY). RALY depletion results in lower TERRA levels, impaired localization of TERRA at telomeres, and ultimately telomere damage. Importantly, we show that TERRA polyadenylation is telomere specific and that RALY preferentially stabilizes non-polyadenylated TERRA transcripts. Finally, we report that TERRA interacts with the poly(A)-binding protein nuclear 1 (PABPN1). Altogether, our results indicate that TERRA stability is regulated by the interplay between RALY and PABPN1, defined by the TERRA polyadenylation state. Our findings also suggest that different telomeres may trigger distinct TERRA-mediated responses.

A missense variant in the nuclear localization signal of DKC1 causes Hoyeraal-Hreidarsson syndrome

Hoyeraal-Hreidarsson syndrome (HHS) is the most severe form of dyskeratosis congenita (DC) and is caused by mutations in genes involved in telomere maintenance. Here, we identified male siblings from a family with HHS carrying a hemizygous mutation (c.1345C > G, p.R449G), located in the C-terminal nuclear localization signal (NLS) of the DKC1 gene. These patients exhibit progressive cerebellar hypoplasia, recurrent infections, pancytopenia due to bone marrow failure, and short leukocyte telomere lengths. Single-cell RNA sequencing analysis suggested defects in the NLRP3 inflammasome in monocytes and the activation and maturation of NK cells and B cells. In experiments using induced pluripotent stem cells (iPSCs) from patients, DKC1_R449G iPSCs had short telomere lengths due to reduced levels of human telomerase RNA (hTR) and increased cytosolic proportions of DKC1. Treatment with dihydroquinolizinone RG7834 and 3'deoxyanosine cordycepin rescued telomere length in patient-derived iPSCs. Together, our findings not only provide new insights into immunodeficiency in DC patients but also provide treatment options for telomerase insufficiency disorders.

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.

Domain specific mutations in dyskerin disrupt 3' end processing of scaRNA13

Mutations in DKC1 (encoding dyskerin) cause telomere diseases including dyskeratosis congenita (DC) by decreasing steady-state levels of TERC, the non-coding RNA component of telomerase. How DKC1 mutations variably impact numerous other snoRNAs remains unclear, which is a barrier to understanding disease mechanisms in DC beyond impaired telomere maintenance. Here, using DC patient iPSCs, we show that mutations in the dyskerin N-terminal extension domain (NTE) dysregulate scaRNA13. In iPSCs carrying the del37L NTE mutation or engineered to carry NTE mutations via CRISPR/Cas9, but not in those with C-terminal mutations, we found scaRNA13 transcripts with aberrant 3' extensions, as seen when the exoribonuclease PARN is mutated in DC. Biogenesis of scaRNA13 was rescued by repair of the del37L DKC1 mutation by genome-editing, or genetic or pharmacological inactivation of the polymerase PAPD5, which counteracts PARN. Inspection of the human telomerase cryo-EM structure revealed that in addition to mediating intermolecular dyskerin interactions, the NTE interacts with terminal residues of the associated snoRNA, indicating a role for this domain in 3' end definition. Our results provide mechanistic insights into the interplay of dyskerin and the PARN/PAPD5 axis in the biogenesis and accumulation of snoRNAs beyond TERC, broadening our understanding of ncRNA dysregulation in human diseases.

MPP6 stimulates both RRP6 and DIS3 to degrade a specified subset of MTR4-sensitive substrates in the human nucleus

Recent in vitro reconstitution analyses have proven that the physical interaction between the exosome core and MTR4 helicase, which promotes the exosome activity, is maintained by either MPP6 or RRP6. However, knowledge regarding the function of MPP6 with respect to in vivo exosome activity remains scarce. Here, we demonstrate a facilitative function of MPP6 that composes a specific part of MTR4-dependent substrate decay by the human exosome. Using RNA polymerase II-transcribed poly(A)+ substrate accumulation as an indicator of a perturbed exosome, we found functional redundancy between RRP6 and MPP6 in the decay of these poly(A)+ transcripts. MTR4 binding to the exosome core via MPP6 was essential for MPP6 to exert its redundancy with RRP6. However, at least for the decay of our identified exosome substrates, MTR4 recruitment by MPP6 was not functionally equivalent to recruitment by RRP6. Genome-wide classification of substrates based on their sensitivity to each exosome component revealed that MPP6 deals with a specific range of substrates and highlights the importance of MTR4 for their decay. Considering recent findings of competitive binding to the exosome between auxiliary complexes, our results suggest that the MPP6-incorporated MTR4-exosome complex is one of the multiple alternative complexes rather than the prevailing one.

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.

The methyl phosphate capping enzyme Bmc1/Bin3 is a stable component of the fission yeast telomerase holoenzyme

The telomerase holoenzyme is critical for maintaining eukaryotic genome integrity. In addition to a reverse transcriptase and an RNA template, telomerase contains additional proteins that protect the telomerase RNA and promote holoenzyme assembly. Here we report that the methyl phosphate capping enzyme (MePCE) Bmc1/Bin3 is a stable component of the S. pombe telomerase holoenzyme. Bmc1 associates with the telomerase holoenzyme and U6 snRNA through an interaction with the recently described LARP7 family member Pof8, and we demonstrate that these two factors are evolutionarily linked in fungi. Our data suggest that the association of Bmc1 with telomerase is independent of its methyltransferase activity, but rather that Bmc1 functions in telomerase holoenzyme assembly by promoting TER1 accumulation and Pof8 recruitment to TER1. Taken together, this work yields new insight into the composition, assembly, and regulation of the telomerase holoenzyme in fission yeast as well as the breadth of its evolutionary conservation.

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.

Rapid factor depletion highlights intricacies of nucleoplasmic RNA degradation

Turnover of nucleoplasmic transcripts by the mammalian multi-subunit RNA exosome is mediated by two adaptors: the Nuclear EXosome Targeting (NEXT) complex and the Poly(A) tail eXosome Targeting (PAXT) connection. Functional analyses of NEXT and PAXT have largely utilized long-term factor depletion strategies, facilitating the appearance of indirect phenotypes. Here, we rapidly deplete NEXT, PAXT and core exosome components, uncovering the direct consequences of their acute losses. Generally, proteome changes are sparse and largely dominated by co-depletion of other exosome and adaptor subunits, reflecting possible subcomplex compositions. While parallel high-resolution 3′ end sequencing of newly synthesized RNA confirms previously established factor specificities, it concomitantly demonstrates an inflation of long-term depletion datasets by secondary effects. Most strikingly, a general intron degradation phenotype, observed in long-term NEXT depletion samples, is undetectable upon short-term depletion, which instead emphasizes NEXT targeting of snoRNA-hosting introns. Further analysis of these introns uncovers an unusual mode of core exosome-independent RNA decay. Our study highlights the accumulation of RNAs as an indirect result of long-term decay factor depletion, which we speculate is, at least partly, due to the exhaustion of alternative RNA decay pathways.

RNA-binding protein Mub1 and the nuclear RNA exosome act to fine-tune environmental stress response

Comparative RNA interactome capture identifies potential regulators of RNA metabolism in fission yeast and reveals RNA exosome–dependent buffering of stress-responsive gene expression networks.

The nuclear RNA exosome plays a key role in controlling the levels of multiple protein-coding and non-coding RNAs. Recruitment of the exosome to specific RNA substrates is mediated by RNA-binding co-factors. The transient interaction between co-factors and the exosome as well as the rapid decay of RNA substrates make identification of exosome co-factors challenging. Here, we use comparative poly(A)+ RNA interactome capture in fission yeast expressing three different mutants of the exosome to identify proteins that interact with poly(A)+ RNA in an exosome-dependent manner. Our analyses identify multiple RNA-binding proteins whose association with RNA is altered in exosome mutants, including the zinc-finger protein Mub1. Mub1 is required to maintain the levels of a subset of exosome RNA substrates including mRNAs encoding for stress-responsive proteins. Removal of the zinc-finger domain leads to loss of RNA suppression under non-stressed conditions, altered expression of heat shock genes in response to stress, and reduced growth at elevated temperature. These findings highlight the importance of exosome-dependent mRNA degradation in buffering gene expression networks to mediate cellular adaptation to stress.

Poly (A)-specific ribonuclease (PARN): More than just "mRNA stock clearing"

In eukaryotic cells, the balance between the synthesis and the degradation decides the steady-state levels of messenger RNAs (mRNA). The removal of adenosine residues from the poly(A) tail, called deadenylation, is the first and the most crucial step in the process of mRNA degradation. Poly (A)-specific ribonuclease (PARN) is one such enzyme that catalyses the process of deadenylation. Although PARN has been primarily known as the regulator of the mRNA stability, recent evidence clearly suggests several other functions of PARN, including a role in embryogenesis, oocyte maturation, cell-cycle progression, telomere biology, non-coding RNA maturation and ribosome biogenesis. Also, deregulated PARN activity is shown to be a hallmark of specific disease conditions. Pathogenic variants in the PARN gene have been observed in various cancers and inherited bone marrow failure syndromes. The focus in this review is to highlight the emerging functions of PARN, particularly in the context of human diseases.

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.

Arabidopsis retains vertebrate-type telomerase accessory proteins via a plant-specific assembly

The recent discovery of the bona-fide telomerase RNA (TR) from plants reveals conserved and unique secondary structure elements and the opportunity for new insight into the telomerase RNP. Here we examine how two highly conserved proteins previously implicated in Arabidopsis telomere maintenance, AtPOT1a and AtNAP57 (dyskerin), engage plant telomerase. We report that AtPOT1a associates with Arabidopsis telomerase via interaction with TERT. While loss of AtPOT1a does not impact AtTR stability, the templating domain is more accessible in pot1a mutants, supporting the conclusion that AtPOT1a stimulates telomerase activity but does not facilitate telomerase RNP assembly. We also show, that despite the absence of a canonical H/ACA binding motif within AtTR, dyskerin binds AtTR with high affinity and specificity in vitro via a plant specific three-way junction (TWJ). A core element of the TWJ is the P1a stem, which unites the 5′ and 3′ ends of AtTR. P1a is required for dyskerin-mediated stimulation of telomerase repeat addition processivity in vitro, and for AtTR accumulation and telomerase activity in vivo. The deployment of vertebrate-like accessory proteins and unique RNA structural elements by Arabidopsis telomerase provides a new platform for exploring telomerase biogenesis and evolution.

Monophyletic Origin and Divergent Evolution of Animal Telomerase RNA

Telomerase RNA (TR) is a noncoding RNA essential for the function of telomerase ribonucleoprotein. TRs from vertebrates, fungi, ciliates, and plants exhibit extreme diversity in size, sequence, secondary structure, and biogenesis pathway. However, the evolutionary pathways leading to such unusual diversity among eukaryotic kingdoms remain elusive. Within the metazoan kingdom, the study of TR has been limited to vertebrates and echinoderms. To understand the origin and evolution of TR across the animal kingdom, we employed a phylogeny-guided, structure-based bioinformatics approach to identify 82 novel TRs from eight previously unexplored metazoan phyla, including the basal-branching sponges. Synthetic TRs from two representative species, a hemichordate and a mollusk, reconstitute active telomerase in vitro with their corresponding telomerase reverse transcriptase components, confirming that they are authentic TRs. Comparative analysis shows that three functional domains, template-pseudoknot (T-PK), CR4/5, and box H/ACA, are conserved between vertebrate and the basal metazoan lineages, indicating a monophyletic origin of the animal TRs with a snoRNA-related biogenesis mechanism. Nonetheless, TRs along separate animal lineages evolved with divergent structural elements in the T-PK and CR4/5 domains. For example, TRs from echinoderms and protostomes lack the canonical CR4/5 and have independently evolved functionally equivalent domains with different secondary structures. In the T-PK domain, a P1.1 stem common in most metazoan clades defines the template boundary, which is replaced by a P1-defined boundary in vertebrates. This study provides unprecedented insight into the divergent evolution of detailed TR secondary structures across broad metazoan lineages, revealing ancestral and later-diversified elements.

SUMOylation- and GAR1-Dependent Regulation of Dyskerin Nuclear and Subnuclear Localization

The nuclear and subnuclear compartmentalization of the telomerase-associated protein and H/ACA ribonucleoprotein component dyskerin is an important although incompletely understood aspect of H/ACA ribonucleoprotein function. Four SUMOylation sites were previously identified in the C-terminal nuclear/nucleolar localization signal (N/NoLS) of dyskerin. We found that a cytoplasmic localized C-terminal truncation variant of dyskerin lacking most of the C-terminal N/NoLS represents an under-SUMOylated variant of dyskerin compared to wild-type dyskerin. We demonstrate that mimicking constitutive SUMOylation of dyskerin using a SUMO3 fusion construct can drive nuclear accumulation of this variant and that the SUMO site K467 in this N/NoLS is particularly important for the subnuclear localization of dyskerin to the nucleolus in a mature H/ACA complex assembly- and SUMO-dependent manner. We also characterize a novel SUMO-interacting motif in the mature H/ACA complex component GAR1 that mediates the interaction between dyskerin and GAR1. Mislocalization of dyskerin, either in the cytoplasm or excluded from the nucleolus, disrupts dyskerin function and leads to reduced interaction of dyskerin with the telomerase RNA. These data indicate a role for dyskerin C-terminal N/NoLS SUMOylation in regulating the nuclear and subnuclear localization of dyskerin, which is essential for dyskerin function as both a telomerase-associated protein and as an H/ACA ribonucleoprotein.

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

Biogenesis of the human telomerase RNA (hTR) involves a complex series of posttranscriptional modifications, including hypermethylation of the 5' mono-methylguanosine cap to a tri-methylguanosine cap (TMG). How the TMG cap affects hTR maturation is unknown. Here, we show that depletion of trimethylguanosine synthase 1 (TGS1), the enzyme responsible for cap hypermethylation, increases levels of hTR and telomerase. Diminished trimethylation increases hTR association with the cap-binding complex (CBC) and with Sm chaperone proteins. Loss of TGS1 causes an increase in accumulation of mature hTR in both the nucleus and the cytoplasm compared with controls. In TGS1 mutant cells, increased hTR assembles with telomerase reverse transcriptase (TERT) protein to yield elevated active telomerase complexes and increased telomerase activity, resulting in telomere elongation in cultured human cells. Our results show that TGS1-mediated hypermethylation of the hTR cap inhibits hTR accumulation, restrains levels of assembled telomerase, and limits telomere elongation.

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.

Quality-Control Mechanism for Telomerase RNA Folding in the Cell

Long non-coding RNAs can often fold into different conformations. Telomerase RNA, an essential component of the telomerase ribonucleoprotein (RNP) enzyme, must fold into a defined structure to fulfill its function with the protein catalytic subunit (TERT) and other accessory factors. However, the mechanism by which the correct folding of telomerase RNA is warranted in a cell is still unknown. Here we show that La-related protein Pof8 specifically recognizes the conserved pseudoknot region of telomerase RNA and instructs the binding of the Lsm2-8 complex to its mature 3' end, thus selectively protecting the correctly folded RNA from exonucleolytic degradation. In the absence of Pof8, TERT assembles with misfolded RNA and produces little telomerase activity. Therefore, Pof8 plays a key role in telomerase RNA folding quality control, ensuring that TERT only assembles with functional telomerase RNA to form active telomerase. Our finding reveals a mechanism for non-coding RNA folding quality control.

Telomerase RNA processing: Implications for human health and disease

Telomeres are composed of repetitive DNA sequences that are replenished by the enzyme telomerase to maintain the self-renewal capacity of stem cells. The RNA component of human telomerase (TERC) is the essential template for repeat addition by the telomerase reverse transcriptase (TERT), and also serves as a scaffold for several factors comprising the telomerase ribonucleoprotein (RNP). Unique features of TERC regulation and function have been informed not only through biochemical studies but also through human genetics. Disease-causing mutations impact TERC biogenesis at several levels including RNA transcription, post-transcriptional processing, folding, RNP assembly, and trafficking. Defects in TERC reduce telomerase activity and impair telomere maintenance, thereby causing a spectrum of degenerative diseases called telomere biology disorders (TBDs). Deciphering mechanisms of TERC dysregulation have led to a broader understanding of noncoding RNA biology, and more recently points to new therapeutic strategies for TBDs. In this review, we summarize over two decades of work revealing mechanisms of human telomerase RNA biogenesis, and how its disruption causes human diseases.

EXOSC10 sculpts the transcriptome during the growth-to-maturation transition in mouse oocytes

Growing mammalian oocytes accumulate substantial amounts of RNA, most of which is degraded during subsequent meiotic maturation. The growth-to-maturation transition begins with germinal vesicle or nuclear envelope breakdown (GVBD) and is critical for oocyte quality and early development. The molecular machinery responsible for the oocyte transcriptome transition remains unclear. Here, we report that an exosome-associated RNase, EXOSC10, sculpts the transcriptome to facilitate the growth-to-maturation transition of mouse oocytes. We establish an oocyte-specific conditional knockout of Exosc10 in mice using CRISPR/Cas9 which results in female subfertility due to delayed GVBD. By performing multiple single oocyte RNA-seq, we document dysregulation of several types of RNA, and the mRNAs that encode proteins important for endomembrane trafficking and meiotic cell cycle. As expected, EXOSC10-depleted oocytes have impaired endomembrane components including endosomes, lysosomes, endoplasmic reticulum and Golgi. In addition, CDK1 fails to activate, possibly due to persistent WEE1 activity, which blocks lamina phosphorylation and disassembly. Moreover, we identified rRNA processing defects that cause higher percentage of developmentally incompetent oocytes after EXOSC10 depletion. Collectively, we propose that EXOSC10 promotes normal growth-to-maturation transition in mouse oocytes by sculpting the transcriptome to degrade RNAs encoding growth-phase factors and, thus, support the maturation phase of oogenesis.

Unraveling the structure and biological functions of RNA triple helices

It has been nearly 63 years since the first characterization of an RNA triple helix in vitro by Gary Felsenfeld, David Davies, and Alexander Rich. An RNA triple helix consists of three strands: A Watson–Crick RNA double helix whose major‐groove establishes hydrogen bonds with the so‐called “third strand”. In the past 15 years, it has been recognized that these major‐groove RNA triple helices, like single‐stranded and double‐stranded RNA, also mediate prominent biological roles inside cells. Thus far, these triple helices are known to mediate catalysis during telomere synthesis and RNA splicing, bind to ligands and ions so that metabolite‐sensing riboswitches can regulate gene expression, and provide a clever strategy to protect the 3′ end of RNA from degradation. Because RNA triple helices play important roles in biology, there is a renewed interest in better understanding the fundamental properties of RNA triple helices and developing methods for their high‐throughput discovery. This review provides an overview of the fundamental biochemical and structural properties of major‐groove RNA triple helices, summarizes the structure and function of naturally occurring RNA triple helices, and describes prospective strategies to isolate RNA triple helices as a means to establish the “triplexome”.

This article is categorized under:

RNA Structure and Dynamics > RNA Structure and Dynamics

RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry

RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems

Small-Molecule PAPD5 Inhibitors Restore Telomerase Activity in Patient Stem Cells

Genetic lesions that reduce telomerase activity inhibit stem cell replication and cause a range of incurable diseases, including dyskeratosis congenita (DC) and pulmonary fibrosis (PF). Modalities to restore telomerase in stem cells throughout the body remain unclear. Here, we describe small-molecule PAPD5 inhibitors that demonstrate telomere restoration in vitro, in stem cell models, and in vivo. PAPD5 is a non-canonical polymerase that oligoadenylates and destabilizes telomerase RNA component (TERC). We identified BCH001, a specific PAPD5 inhibitor that restored telomerase activity and telomere length in DC patient induced pluripotent stem cells. When human blood stem cells engineered to carry DC-causing PARN mutations were xenotransplanted into immunodeficient mice, oral treatment with a repurposed PAPD5 inhibitor, the dihydroquinolizinone RG7834, rescued TERC 3' end maturation and telomere length. These findings pave the way for developing systemic telomere therapeutics to counteract stem cell exhaustion in DC, PF, and possibly other aging-related diseases.

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.

The conserved structure of plant telomerase RNA provides the missing link for an evolutionary pathway from ciliates to humans

Telomerase is essential for maintaining telomere integrity. Although telomerase function is widely conserved, the integral telomerase RNA (TR) that provides a template for telomeric DNA synthesis has diverged dramatically. Nevertheless, TR molecules retain 2 highly conserved structural domains critical for catalysis: a template-proximal pseudoknot (PK) structure and a downstream stem-loop structure. Here we introduce the authentic TR from the plant Arabidopsis thaliana, called AtTR, identified through next-generation sequencing of RNAs copurifying with Arabidopsis TERT. This RNA is distinct from the RNA previously described as the templating telomerase RNA, AtTER1. AtTR is a 268-nt Pol III transcript necessary for telomere maintenance in vivo and sufficient with TERT to reconstitute telomerase activity in vitro. Bioinformatics analysis identified 85 AtTR orthologs from 3 major clades of plants: angiosperms, gymnosperms, and lycophytes. Through phylogenetic comparisons, a secondary structure model conserved among plant TRs was inferred and verified using in vitro and in vivo chemical probing. The conserved plant TR structure contains a template-PK core domain enclosed by a P1 stem and a 3' long-stem P4/5/6, both of which resemble a corresponding structural element in ciliate and vertebrate TRs. However, the plant TR contains additional stems and linkers within the template-PK core, allowing for expansion of PK structure from the simple PK in the smaller ciliate TR during evolution. Thus, the plant TR provides an evolutionary bridge that unites the disparate structures of previously characterized TRs from ciliates and vertebrates.

N-terminal residues of human dyskerin are required for interactions with telomerase RNA that prevent RNA degradation

The telomerase holoenzyme responsible for maintaining telomeres in vertebrates requires many components in vivo, including dyskerin. Dyskerin binds and regulates the accumulation of the human telomerase RNA, hTR, as well as other non-coding RNAs that share the conserved H/ACA box motif. The precise mechanism by which dyskerin controls hTR levels is unknown, but is evidenced by defective hTR accumulation caused by substitutions in dyskerin, that are observed in the X-linked telomere biology disorder dyskeratosis congenita (X-DC). To understand the role of dyskerin in hTR accumulation, we analyzed X-DC substitutions K39E and K43E in the poorly characterized dyskerin N-terminus, and A353V within the canonical RNA binding domain (the PUA). These variants exhibited impaired binding to hTR and polyadenylated hTR species, while interactions with other H/ACA RNAs appear largely unperturbed by the N-terminal substitutions. hTR accumulation and telomerase activity defects of dyskerin-deficient cells were rescued by wildtype dyskerin but not the variants. hTR 3′ extended or polyadenylated species did not accumulate, suggesting hTR precursor degradation occurs upstream of mature complex assembly in the absence of dyskerin binding. Our findings demonstrate that the dyskerin-hTR interaction mediated by PUA and N-terminal residues of dyskerin is crucial to prevent unchecked hTR degradation.

How RNAi machinery enters the world of telomerase

Human telomerase holoenzyme consists of the catalytic component TERT and the template RNA TERC. However, a network of accessory proteins plays key roles in its assembly, localization and stability. Defects in genes involved in telomerase biology affect the renewal of critical stem cell populations and cause disorders such as telomeropathies. Moreover, activation of telomerase in somatic cells allows neoplastic cells to proliferate indefinitely, thus contributing to tumorigenesis. For these reasons, identification of new players involved in telomerase regulation is crucial for the determination of novel therapeutic targets and biomarkers. In the very last years, increasing evidence describes components of the RNAi machinery as a new layer of complexity in human telomerase activity. In this review, we will discuss how AGO2 and other proteins which collaborate with AGO2 in RNAi pathway play a pivotal role in TERC stability and function.

Disruption of Telomerase RNA Maturation Kinetics Precipitates Disease

Mutations in RNA-processing enzymes are increasingly linked to human disease. Telomerase RNA and related noncoding RNAs require 3' end-processing steps, including oligoadenylation. Germline mutations in poly(A)ribonuclease (PARN) cause accumulation of extended human telomerase RNA (hTR) species and precipitate dyskeratosis congenita and pulmonary fibrosis. Here, we develop nascent RNAend-seq to measure processing rates of RNA precursors. We find that mature hTR derives from extended precursors but that in PARN-mutant cells hTR maturation kinetically stalls and unprocessed precursors are degraded. Loss of poly(A)polymerase PAPD5 in PARN-mutant cells accelerates hTR maturation and restores hTR processing, indicating that oligoadenylation and deadenylation set rates of hTR maturation. The H/ACA domain mediates hTR maturation by precisely defining the 3' end, recruiting poly(A)polymerase activity, and conferring sensitivity to PARN regulation. These data reveal a feedforward circuit in which post-transcriptional oligoadenylation controls RNA maturation kinetics. Similar alterations in RNA processing rates may contribute to mechanisms of RNA-based human disease.