Polymerization defects within human telomerase are distinct from telomerase RNA and TEP1 binding

A role for a novel 'trans-pseudoknot' RNA-RNA interaction in the functional dimerization of human telomerase

The integral RNA (hTER) of the human telomerase ribonucleoprotein has a conserved secondary structure that contains a potential pseudoknot. Here we examine the role of an intermolecular hTER-hTER interaction in the previously reported functional dimerization of telomerase. We provide evidence that the two conserved, complementary sequences of one stem of the hTER pseudoknot domain can pair intermolecularly in vitro, and that formation of this stem as part of a novel "trans-pseudoknot" is required for telomerase to be active in its dimeric form. Such RNA-RNA interaction mirrors a known property of retroviral reverse transcriptases, which use homodimeric viral genomic RNA substrates.

A human TERT C-terminal polypeptide sensitizes HeLa cells to H2O2-induced senescence without affecting telomerase enzymatic activity

We have constructed a 27-kDa hTERT C-terminal polypeptide (hTERTC27) devoid of domains required for telomerase activity and demonstrated that it is capable of nuclear translocation/telomere-end targeting. Here we showed that expression of a low level of hTERTC27 renders hTERT positive HeLa cells sensitive to H(2)O(2)-induced oxidative stress and subsequent cell senescence. The senescence-associated gene, the cyclin/cdk inhibitor p21(Waf1), was up-regulated. This occurs without changing the expression of endogenous hTERT, causing significant telomere shortening or inhibiting telomerase activity. Results from this study suggest for the first time that in addition to telomerase activity, the C-terminus of hTERT also plays a role in hTERT-mediated cellular resistance to oxidative stress.

N-terminal domain of yeast telomerase reverse transcriptase: recruitment of Est3p to the telomerase complex

Telomerase is a reverse transcriptase that maintains chromosome ends. The N-terminal half of the catalytic protein subunit (TERT) contains three functional domains (I, II, and III) that are conserved among TERTs but not found in other reverse transcriptases. Guided by an amino acid sequence alignment of nine TERT proteins, mutations were introduced into yeast TERT (Est2p). In support of the proposed alignment, mutation of virtually all conserved residues resulted in loss-of-function or temperature sensitivity, accompanied by telomere shortening. Overexpression of telomerase component Est3p led to allele-specific suppression of the temperature-sensitive mutations in region I, suggesting that Est3p interacts with this protein domain. As predicted by the genetic results, a lethal mutation in region I resulted in loss of Est3p from the telomerase complex. We conclude that Est2p region I is required for the recruitment of Est3p to yeast telomerase. Given the phylogenetic conservation of region I of TERT, this protein domain may provide the equivalent function in all telomerases.

Telomerase: biochemical considerations for enzyme and substrate

Telomerase extends chromosome ends by iterative reverse transcription of its RNA template. Following the addition of each telomeric repeat, the RNA template and the telomeric substrate reset their relative position in the active site provided by the telomerase reverse transcriptase (TERT). This step might require the formation of guanine-rich secondary structures in the nascent product. Results from numerous studies begin to delineate TERT sub-domains that orchestrate these events and support the model of cooperative action between distinct active sites within telomerase multimers. Natural telomere substrates are protein-DNA complexes that show an asymmetry between the two ends of a chromosome, possibly reflecting their differential mode of replication.

Minimum length requirement of the alignment domain of human telomerase RNA to sustain catalytic activity in vitro

Telomeres are essential for genomic stability and cell viability. Telomerase, the enzyme responsible for telomere maintenance, is composed of a reverse transcriptase protein subunit and an integral RNA component which contains the templating domain. In human telomerase, the template region consists of 11 nt (3'-rCAAUCCCAAUC-5') and comprises an alignment domain (italicised) plus a template sequence encoding the telomeric repeat d(GGT TAG). In this study, the alignment domain of human telomerase was systematically reduced from the 3' end and the resultant recombinant enzyme activity was evaluated in vitro. Deletion or substitution of one or two residues from the 3' end of the alignment domain caused only a slight reduction in overall catalytic activity and did not alter the processivity of the enzyme. Deletion or substitution of three or more residues from the 3' end of the alignment domain resulted in total loss of catalytic activity. These results suggest that the two most 3' terminal RNA residues are relevant but not essential for overall activity and that the minimal length requirement of the alignment domain is 3 nt. Furthermore, base pairing between the 3' end of the primer substrate and the first two residues of the alignment domain is also not an absolute requirement for processive synthesis.

Stabilization of telomere length and karyotypic stability are directly correlated with the level of hTERT gene expression in primary fibroblasts

Telomere shortening and lack of telomerase activity have been implicated in cellular senescence in human fibroblasts. Expression of the human telomerase (hTERT) gene in sheep fibroblasts reconstitutes telomerase activity and extends their lifespan. However, telomere length is not maintained in all cell lines, even though in vitro telomerase activity is restored in all of them. Cell lines expressing higher levels of hTERT mRNA do not exhibit telomere erosion or genomic instability. By contrast, fibroblasts expressing lower levels of hTERT do exhibit telomere shortening, although the telomeres eventually stabilize at a shorter length. The shorter telomere lengths and the extent of karyotypic abnormalities are both functions of hTERT expression level. We conclude that telomerase activity is required to bypass senescence but is not sufficient to prevent telomere erosion and genomic instability at lower levels of expression.

Functional analysis of the C-terminal extension of telomerase reverse transcriptase. A putative "thumb" domain

Telomerase is an RNA-protein complex responsible for the extension of one strand of telomere terminal repeats. The catalytic protein subunit of telomerase, known generically as telomerase reverse transcriptase (TERT), exhibits significant homology to reverse transcriptases (RTs) encoded by retroviruses and retroelements. The mechanisms of telomerase may therefore be similar to those of the conventional reverse transcriptases. In this report, we explore potential similarity between these two classes of proteins in a region with no evident sequence similarity. Previous analysis has implicated a C-terminal domain of retroviral RTs (known as the "thumb" domain) in template-primer binding and in processivity control. The equivalent region of TERTs, although similar to one another, does not exhibit significant sequence homology to retroviral RTs. However, we found that removal of this region of yeast TERT similarly resulted in a decrease in the stability of telomerase-DNA complex and in the processivity of telomerase-mediated nucleotide addition. Moreover, the C-terminal domain of TERT exhibits a nucleic acid binding activity when recombinantly expressed and purified. Finally, amino acid substitutions of conserved residues in this region of TERT were found to impair telomerase activity and processivity. We suggest that mechanistic similarity between telomerase and retroviral RTs may extend beyond the regions with apparent sequence similarity.

Human telomerase and its regulation

The telomere is a special functional complex at the end of linear eukaryotic chromosomes, consisting of tandem repeat DNA sequences and associated proteins. It is essential for maintaining the integrity and stability of linear eukaryotic genomes. Telomere length regulation and maintenance contribute to normal human cellular aging and human diseases. The synthesis of telomeres is mainly achieved by the cellular reverse transcriptase telomerase, an RNA-dependent DNA polymerase that adds telomeric DNA to telomeres. Expression of telomerase is usually required for cell immortalization and long-term tumor growth. In humans, telomerase activity is tightly regulated during development and oncogenesis. The modulation of telomerase activity may therefore have important implications in antiaging and anticancer therapy. This review describes the currently known components of the telomerase complex and attempts to provide an update on the molecular mechanisms of human telomerase regulation.

C-terminal regions of the human telomerase catalytic subunit essential for in vivo enzyme activity

Most human cancer cells are thought to acquire the ability to divide beyond the capacity of normal somatic cells through illegitimately activating the gene hTERT, which encodes the catalytic subunit of telomerase. While telomerase reverse transcriptase (TERT) is conserved in most eukaryotes, mounting evidence suggests that the C terminus of the human protein may have functions unique to higher eukaryotes. To search for domains responsible for such functions, we assayed a panel of tandem substitution mutations encompassing this region of human TERT for in vitro and in vivo functionality. We found four clusters of mutations that inactivated the biochemical and biological functions of telomerase, separated by mutations that had little or no effect on enzyme activity. We also identified a region where mutations generate catalytically active but biologically inert proteins. This C-terminal region that dissociates activities of telomerase (C-DAT) does not appear to be involved in nuclear localization or protein multimerization. Instead, it appears that the C-DAT region is involved in a step of in vivo telomere synthesis after the assembly of a catalytically active enzyme. Intriguingly, all of the described regions reside in a portion of TERT that is dispensable for cellular viability in yeast, arguing for a divergent role of the C terminus in higher eukaryotes.

Nucleolar localization of hTERT protein is associated with telomerase function

Telomerase is a ribonucleoprotein (RNP) complex that prevents telomeric erosion in eukaryotic cells. Although there are also other associated proteins in this complex, the catalytic activity of this complex is composed of two components. One is a reverse transcriptase subunit, TERT (telomerase reverse transcriptase); another is an RNA template subunit, TR (telomerase RNA). However, where these two parts are assembled in mammalian cells is unclear. In the present study, we investigated the intracellular distribution of human TERT (hTERT) protein and observed that hTERT protein in individual cells could concentrate in or be excluded from the nucleolus. Further we have identified a nucleolar targeting signal in the hTERT protein. Point mutations that disrupted this signal region interrupted telomerase RNP complex formation, decreased telomerase activity, and caused telomere shortening in cells transfected with mutated hTERT. Our results indicate that the amino acid sequence of the extreme N-terminus (1-15) of hTERT, which targets nucleolar localization of the protein, is required for full telomerase function.

The nucleolar localization domain of the catalytic subunit of human telomerase

Telomerase is the enzyme essential to complete the replication of the terminal DNA of most eukaryotic chromosomes. In humans, this enzyme is composed of the telomerase reverse transcriptase (hTERT) and telomerase RNA (hTR) subunits. hTR has been found in the nucleolus, a site of assembly of ribosomes as well as other ribonucleoproteins (RNPs). We therefore tested whether the hTERT component is also found in the nucleolus, where it could complex with the hTR RNA to form a functional enzyme. We report here that hTERT does indeed localize to the nucleolus, and we mapped the domain responsible for this localization to the hTR-binding region of the protein by deletion analysis. Substitution mutations in two of the three conserved hTR-binding domains in this nucleolar localization domain (NoLD) abolished nucleolar localization. However, another mutation that impeded hTR binding did not alter this subcellular localization. Additionally, wild type hTERT was detected in the nucleolus of cells that failed to express hTR. Taken together, we propose that the nucleolar localization of hTERT involves more than just the association with the hTR subunit. Furthermore, the coincidental targeting of both the hTR and hTERT subunits to the nucleolus supports the premise that the assembly of telomerase occurs in the nucleolus.

Two independent regions of human telomerase reverse transcriptase are important for its oligomerization and telomerase activity

Human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, contains motifs conserved among reverse transcriptases. Several nucleic acid-dependent polymerases that share a "fingers, palm, and thumb substructure" were shown to oligomerize. Here we demonstrate that hTERT also has this ability using partially purified recombinant hTERTs and mammalian cells co-expressing differently tagged hTERTs. Human template RNA (hTR), by contrast, has no effect on the structural oligomerization of hTERTs. Therefore, hTERT has an intrinsic ability of oligomerization in the absence of hTR. We identified two separate regions as essential for the oligomerization. The regions, amino acids 301-538 (amino-terminal region) and amino acids 914-928 (carboxyl-terminal region), are outside the fingers and palm substructure covering motif T to D and interact with each other in vivo. A substituted mutant of hTERT, hTERT-D712A-V713I, which was reported as a dominant negative form of hTERT, bound to the wild-type hTERT and inhibited its telomerase activity transiently expressed in telomerase-negative finite normal human fibroblast. The truncated forms of hTERT containing the binding region to the wild-type hTERT partially inhibited the telomerase activity, probably by preventing the wild-type hTERT from forming an oligomer. Taken together, the oligomerization of hTERT is an important step for telomerase activity.

Functional multimerization of human telomerase requires an RNA interaction domain in the N terminus of the catalytic subunit

Functional human telomerase complexes are minimally composed of the human telomerase RNA (hTR) and a catalytic subunit (human telomerase reverse transcriptase [hTERT]) containing reverse transcriptase (RT)-like motifs. The N terminus of TERT proteins is unique to the telomerase family and has been implicated in catalysis, telomerase RNA binding, and telomerase multimerization, and conserved motifs have been identified by alignment of TERT sequences from multiple organisms. We studied hTERT proteins containing N-terminal deletions or substitutions to identify and characterize hTERT domains mediating telomerase catalytic activity, hTR binding, and hTERT multimerization. Using multiple sequence alignment, we identified two vertebrate-conserved TERT N-terminal regions containing vertebrate-specific residues that were required for human telomerase activity. We identified two RNA interaction domains, RID1 and RID2, the latter containing a vertebrate-specific RNA binding motif. Mutations in RID2 reduced the association of hTR with hTERT by 50 to 70%. Inactive mutants defective in RID2-mediated hTR binding failed to complement an inactive hTERT mutant containing an RT motif substitution to reconstitute activity. Our results suggest that functional hTERT complementation requires intact RID2 and RT domains on the same hTERT molecule and is dependent on hTR and the N terminus.

A critical stem-loop structure in the CR4-CR5 domain of mammalian telomerase RNA

Telomerase is an enzyme that maintains telomere length by adding telomeric sequence repeats onto chromosome ends. The telomerase ribonucleoprotein complex consists of two essential components, a reverse transcriptase and an RNA molecule that provides the template for telomeric repeat synthesis. A common secondary structure of vertebrate telomerase RNA has been proposed based on a phylogenetic comparative analysis of 35 sequences. Here we report the identification of an additional essential base-paired region in the CR4-CR5 domain of mammalian telomerase RNA, termed P6.1. Mouse telomerase RNAs with mutations that disrupted base pairings in the P6.1 helix were unable to reconstitute telomerase activity in vivo. In contrast, an RNA mutant with compensatory mutations that restored base pairings in the P6.1 helix restored telomerase activity. In an in vitro reconstitution system stable base pairing of the P6.1 stem was required for the RNA-protein interaction between the CR4-CR5 domain and the telomerase reverse transcriptase (TERT) protein. Interestingly, two RNA mutations, one that extends the P6.1 stem and one that alters the conserved nucleotides of the L6.1 loop, allowed RNA-protein binding but significantly impaired telomerase activity. These data establish the presence of the P6.1 stem-loop and its importance for the assembly and enzymatic activity of the mammalian telomerase complex.

Regulation of telomerase activity by alternate splicing of human telomerase reverse transcriptase mRNA in a subset of neuroblastomas

It has been proposed that the regulation of telomerase takes place at the transcriptional level, the expression of the catalytic subunit human telomerase reverse transcriptase (hTERT) being crucial for telomerase activity (TA). Recently, differential splicing of hTERT mRNA has been demonstrated in various tissues during embryonal development, and it has been suggested that only full-length transcripts translate into functionally active telomerase. With this in view, we analyzed the different hTERT transcripts by reverse transcriptase-polymerase chain reaction in neuroblastic tumors and compared the results with the TA, the tumor growth fraction, and the MYCN status. In a series of 38 neuroblastic tumors, high TA and full-length hTERT transcripts were found in nine samples, whereas nine samples showed absence of both enzymatic activity and hTERT transcripts. Interestingly, in another eight samples, low or absent TA coincided with a lack of full-length hTERT transcripts. Eleven samples contained hTERT transcripts with low or undetectable TA and one sample had low TA but no hTERT transcripts. TA correlated with MYCN amplification and was weakly associated with the proliferative activity. Moreover, a significant correlation with tumor progression was observed. Our findings point at a posttranscriptional regulation of TA in a subset of neuroblastic tumors. Because high TA was detected only in tumors with full-length hTERT transcripts, reverse transcriptase-polymerase chain reaction analysis of archival neuroblastic tumor samples might help to appraise the malignant potential in individual cases.

N-terminal domains of the human telomerase catalytic subunit required for enzyme activity in vivo

Most tumor cells depend upon activation of the ribonucleoprotein enzyme telomerase for telomere maintenance and continual proliferation. The catalytic activity of this enzyme can be reconstituted in vitro with the RNA (hTR) and catalytic (hTERT) subunits. However, catalytic activity alone is insufficient for the full in vivo function of the enzyme. In addition, the enzyme must localize to the nucleus, recognize chromosome ends, and orchestrate telomere elongation in a highly regulated fashion. To identify domains of hTERT involved in these biological functions, we introduced a panel of 90 N-terminal hTERT substitution mutants into telomerase-negative cells and assayed the resulting cells for catalytic activity and, as a marker of in vivo function, for cellular proliferation. We found four domains to be essential for in vitro and in vivo enzyme activity, two of which were required for hTR binding. These domains map to regions defined by sequence alignments and mutational analysis in yeast, indicating that the N terminus has also been functionally conserved throughout evolution. Additionally, we discovered a novel domain, DAT, that "dissociates activities of telomerase," where mutations left the enzyme catalytically active, but was unable to function in vivo. Since mutations in this domain had no measurable effect on hTERT homomultimerization, hTR binding, or nuclear targeting, we propose that this domain is involved in other aspects of in vivo telomere elongation. The discovery of these domains provides the first step in dissecting the biological functions of human telomerase, with the ultimate goal of targeting this enzyme for the treatment of human cancers.

Functional multimerization of the human telomerase reverse transcriptase

The telomerase enzyme exists as a large complex (approximately 1,000 kDa) in mammals and at minimum is composed of the telomerase RNA and the catalytic subunit telomerase reverse transcriptase (TERT). In Saccharomyces cerevisiae, telomerase appears to function as an interdependent dimer or multimer in vivo (J. Prescott and E. H. Blackburn, Genes Dev. 11:2790-2800, 1997). However, the requirements for multimerization are not known, and it remained unclear whether telomerase exists as a multimer in other organisms. We show here that human TERT (hTERT) forms a functional multimer in a rabbit reticulocyte lysate reconstitution assay and in human cell extracts. Two separate, catalytically inactive TERT proteins can complement each other in trans to reconstitute catalytic activity. This complementation requires the amino terminus of one hTERT and the reverse transcriptase and C-terminal domains of the second hTERT. The telomerase RNA must associate with only the latter hTERT for reconstitution of telomerase activity to occur. Multimerization of telomerase also facilitates the recognition and elongation of substrates in vitro and in vivo. These data suggest that the catalytic core of human telomerase may exist as a functionally cooperative dimer or multimer in vivo.

Telomerase activation, cellular immortalization and cancer

The maintenance of specialized nucleoprotein structures termed telomeres is essential for chromosome stability. Without new synthesis of telomeres at chromosome ends the chromosomes shorten with progressive cell division, eventually triggering either replicative senescence or apoptosis when telomere length becomes critically short. The regulation of telomerase activity in human cells plays a significant role in the development of cancer. Telomerase is tightly repressed in the vast majority of normal human somatic cells but becomes activated during cellular immortalization and in cancers. While the mechanisms for telomerase activation in cancers have not been fully defined, they include telomerase catalytic subunit gene (hTERT) amplification and trans-activation of the hTERT promoter by the myc oncogene product. Ectopic expression of hTERT is sufficient to restore telomerase activity in cells that lack the enzyme and can immortalize many cell types. Understanding telomerase biology will eventually lead to several clinically relevant telomerase-based therapies. These applications include inhibiting or targeting telomerase as a novel antineoplastic strategy and using cells immortalized by telomerase for therapeutic applications.