Pot1 and telomere maintenance

Canine sperm motility is associated with telomere shortening and changes in expression of shelterin genes

BACKGROUND

Motion quality is a critical property for essential functions. Several endogenous and exogenous factors are involved in sperm motility. Here, we measured the relative telomere length and evaluated the gene expression of its binding-proteins, shelterin complex (TRF1, TRF2, RAP1, POT1, TIN2, and TPP1) in sperm of dogs using relative quantitative real-time PCR. We compared them between two sperm subpopulations with poor and good motion qualities (separated by swim-up method). Telomere shortening and alterations of shelterin gene expression result from ROS, genotoxic insults, and genetic predisposition.

RESULTS

Sperm kinematic parameters were measured in two subpopulations and then telomeric index of each parameter was calculated. Telomeric index for linearity, VSL, VCL, STR, BCF, and ALH were significantly higher in sperms with good motion quality than in sperms with poor quality. We demonstrated that poor motion quality is associated with shorter telomere, higher expression of TRF2, POT1, and TIN2 genes, and lower expression of the RAP1 gene in dog sperm. The levels of TRF1 and TPP1 gene expression remained consistent despite variations in sperm quality and telomere length.

CONCLUSION

Data provided evidence that there are considerable changes in gene expression of many shelterin components (TRF2, TIN2, POT1and RAP1) associated with shortening telomere in the spermatozoa with poor motion quality. Possibly, the poor motion quality is the result of defects in the shelterin complex and telomere length. Our data suggests a new approach in the semen assessment and etiologic investigations of subfertility or infertility in male animals.

Epigenetic mechanism of Gtl2-miRNAs causes the primitive sheep characteristics found in purebred Merino sheep

BACKGROUND

It is not uncommon for some individuals to retain certain primitive characteristics even after domestication or long-term intensive selection. Wild ancestors or original varieties of animals typically possess strong adaptability to environmental preservation, a trait that is often lacking in highly artificially selected populations. In the case of the Merino population, a world-renowned fine wool sheep breed, a phenotype with primitive coarse wool characteristic has re-emerged. It is currently unclear whether this characteristic is detrimental to the production of fine wool or whether it is linked to the adaptability of sheep. The underlying genetic/epigenetic mechanisms behind this trait are also poorly understood.

RESULTS

This study identified lambs with an ancestral-like coarse (ALC) wool type that emerged during the purebred breeding of Merino fine wool sheep. The presence of this primitive sheep characteristic resulted in better environmental adaptability in lambs, as well as improved fine wool yield in adulthood. Reciprocal cross experiments revealed that the ALC phenotype exhibited maternal genetic characteristics. Transcriptomic SNP analysis indicated that the ALC phenotype was localized to the imprinted Gtl2-miRNAs locus, and a significant correlation was found between the ALC wool type and a newly identified short Interstitial Telomeric Sequences (s-ITSs) at this locus. We further confirmed that a novel 38-nt small RNA transcribed from these s-ITSs, in combination with the previously reported 22-nt small RNAs cluster from the Gtl2-miRNAs locus, synergistically inhibited PI3K/AKT/Metabolic/Oxidative stress and subsequent apoptotic pathways in wool follicle stem cells, resulting in the ALC wool type. The necessity of Gtl2-miRNAs in controlling primary hair follicle morphogenesis, as well as the wool follicle type for ALC wool lambs, was verified using intergenic differentially methylated region-knockout mice.

CONCLUSION

The ALC wool type of Merino sheep, which does not reduce wool quality but increases yield and adaptability, is regulated by epigenetic mechanisms in the imprinted Gtl2-miRNAs region on sheep chromosome 18, with the maternally expressed imprinted gene responsible for the ALC phenotype. This study highlights the significance of epigenetic regulation during embryonic and juvenile stages and emphasizes the advantages of early adaptation breeding for maternal parents in enhancing the overall performance of their offspring.

Telomerase Inhibition by MST-312 Sensitizes Breast Cancer Cells to the Anti-cancer Properties of Plumbagin

Breast cancer is the most common cause of malignancy and the second most common cause of death due to cancer in women. This heterogeneous disease is currently broadly classified as estrogen receptor (ER), progesterone receptor (PR) positive luminal tumors, human epidermal growth factor receptor 2 (HER2) amplified tumors and triple-negative breast cancers (TNBC). Phytochemicals are proven to be promising anti-cancer chemotherapeutics agents with minimal cytotoxic effects on normal cells. Plumbagin (5-hydroxy-2-methyl-1, 4-naphthoquinone) is a phytochemical derived from the roots of Plumbago zeylanica and it is known to possess anti-cancer properties similar to other compounds of naphthoquinones. In about 90% of cancer cells, the telomerase enzyme activity is revived to add telomeric repeats to evade apoptosis. In this study, a combinatorial approach of combining the anti-cancer compound plumbagin to induce genotoxicity and a potent telomerase inhibitor, MST-312 (synthetic derivative of tea catechins), was used to determine the combinational treatment-induced lethality in breast cancer cells such as MDA-MB-231 (TNBC) and MCF-7 (lumina) cells. MDA-MB-231 cells were responsive to combination treatment in both short-term (48 h) and long-term treatment (14 days) in a synergistic manner, whereas in MCF-7, the combination treatment was more effective in the long-term regimen. Furthermore, the cytotoxic effects of the plumbagin and MST-312 combination treatment were not recoverable after the short-term treatment. In conclusion, a combination treatment of MST-312 and plumbagin is proven to be more effective than a single plumbagin compound treatment in inducing DNA damage and telomere dysfunction leading to greater genome instability, cell cycle arrest and eventually cell death in cancer cells.

Measuring telomerase activity using TRAP assays

Telomerase is a reverse transcriptase that consists of the telomerase reverse transcriptase (TERT) protein and the telomerase RNA component TERC which also harbors the template region for telomere synthesis. In its canonical function the enzyme adds single-stranded telomeric hexanucleotides de novo to the ends of linear chromosomes, telomeres, in telomerase-positive cells such as germline, stem- and cancer cells. This potential biochemical activity of telomerase can be measured with the help of a telomerase repeat amplification protocol (TRAP) which often includes a PCR amplification due to the low abundance of telomerase in most cells and tissues. The current chapter describes various TRAP methods to detect telomerase activity (TA) using gel-based methods, its advantages and deficits, how to perform an ELISA-based TRAP assay and how best to interpret its results. Since development of the TRAP assay in 1994, there have been numerous modifications and adaptations of the method from real-time PCR analysis, isothermal amplification and nanotechnology to CRISPR/Cas-based methods which will be briefly mentioned. However, it is not possible to cover all different TRAP methods and thus there is no comprehensiveness claimed by this chapter. Instead, the author describes various aspects of using TRAP assays including required controls, sample preparation, etc. in order to avoid pitfalls and set-backs in applying this rather complex and demanding technique. The TRAP assay is particularly important to support clinical diagnosis of cancer, analyze tumor therapy as well as to evaluate various approaches to inhibit TA as a form of anti-cancer therapy.

POT-3 preferentially binds the terminal DNA-repeat on the telomeric G-overhang

Eukaryotic chromosomes typically end in 3' telomeric overhangs. The safeguarding of telomeric single-stranded DNA overhangs is carried out by factors related to the protection of telomeres 1 (POT1) protein in humans. Of the three POT1-like proteins in Caenorhabditis elegans, POT-3 was the only member thought to not play a role at telomeres. Here, we provide evidence that POT-3 is a bona fide telomere-binding protein. Using a new loss-of-function mutant, we show that the absence of POT-3 causes telomere lengthening and increased levels of telomeric C-circles. We find that POT-3 directly binds the telomeric G-strand in vitro and map its minimal DNA binding site to the six-nucleotide motif, GCTTAG. We further show that the closely related POT-2 protein binds the same motif, but that POT-3 shows higher sequence selectivity. Crucially, in contrast to POT-2, POT-3 prefers binding sites immediately adjacent to the 3' end of DNA. These differences are significant as genetic analyses reveal that pot-2 and pot-3 do not function redundantly with each other in vivo. Our work highlights the rapid evolution and specialisation of telomere binding proteins and places POT-3 in a unique position to influence activities that control telomere length.

Telomeres and Mitochondrial Metabolism: Implications for Cellular Senescence and Age-related Diseases

Cellular senescence is an irreversible cell arrest process, which is determined by a variety of complicated mechanisms, including telomere attrition, mitochondrial dysfunction, metabolic disorders, loss of protein homeostasis, epigenetic changes, etc. Cellular senescence is causally related to the occurrence and development of age-related disease. The elderly is liable to suffer from disorders such as neurodegenerative diseases, cancer, and diabetes. Therefore, it is increasingly imperative to explore specific countermeasures for the treatment of age-related diseases. Numerous studies on humans and mice emphasize the significance of metabolic imbalance caused by short telomeres and mitochondrial damages in the onset of age-related diseases. Although the experimental data are relatively independent, more and more evidences have shown that there is mutual crosstalk between telomeres and mitochondrial metabolism in the process of cellular senescence. This review systematically discusses the relationship between telomere length, mitochondrial metabolic disorder, as well as their underlying mechanisms for cellular senescence and age-related diseases. Future studies on telomere and mitochondrial metabolism may shed light on potential therapeutic strategies for age-related diseases. Graphical Abstract The characteristics of cellular senescence mainly include mitochondrial dysfunction and telomere attrition. Mitochondrial dysfunction will cause mitochondrial metabolic disorders, including decreased ATP production, increased ROS production, as well as enhanced cellular apoptosis. While oxidative stress reaction to produce ROS, leads to DNA damage, and eventually influences telomere length. Under the stimulation of oxidative stress, telomerase catalytic subunit TERT mainly plays an inhibitory role on oxidative stress, reduces the production of ROS and protects telomere function. Concurrently, mitochondrial dysfunction and telomere attrition eventually induce a range of age-related diseases, such as T2DM, osteoporosis, AD, etc. :increase; :reduce;⟝:inhibition.

Nematode chromosomes

The nematode Caenorhabditis elegans has shed light on many aspects of eukaryotic biology, including genetics, development, cell biology, and genomics. A major factor in the success of C. elegans as a model organism has been the availability, since the late 1990s, of an essentially gap-free and well-annotated nuclear genome sequence, divided among 6 chromosomes. In this review, we discuss the structure, function, and biology of C. elegans chromosomes and then provide a general perspective on chromosome biology in other diverse nematode species. We highlight malleable chromosome features including centromeres, telomeres, and repetitive elements, as well as the remarkable process of programmed DNA elimination (historically described as chromatin diminution) that induces loss of portions of the genome in somatic cells of a handful of nematode species. An exciting future prospect is that nematode species may enable experimental approaches to study chromosome features and to test models of chromosome evolution. In the long term, fundamental insights regarding how speciation is integrated with chromosome biology may be revealed.

Telomeres and Cancer

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

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.

Telomere associated gene expression as well as TERT protein level and telomerase activity are altered in the ovarian follicles of aged mice

Telomeres cap the ends of eukaryotic chromosomes to maintain genomic stability and integrity during an organism’s lifespan. The length of telomeres inevitably shortens due to DNA replication, genotoxic agents, and biological aging. A limited number of cell types, e.g., stem cells, germline cells, and early embryos can elongate shortened telomeres via the enzymatic action of telomerase, which is composed of telomerase reverse transcriptase (TERT) and telomerase RNA component (Terc). Additionally, telomere-associated proteins including telomeric repeat binding factor 1 (TRF1) and 2 (TRF2), as well as protection of telomeres 1a (POT1a), bind to telomeres to maintain their structural integrity and length. During ovarian aging in mammals, telomeres progressively shorten, accompanied by fertility loss; however, the molecular mechanism underlying this attrition during follicle development remains unclear. In this study, the primary, secondary, preantral, and antral follicles were obtained either from 6-week-old adult (n = 19) or 52-week-old aged (n = 12) mice. We revealed that the Tert, Terc, Trf1, Trf2, and Pot1a gene expression (P < 0.001) and TERT protein (P < 0.01) levels significantly decreased in certain ovarian follicles of the aged group when compared to those of the adult group. Also, telomerase activity exhibited remarkable changes in the follicles of both groups. Consequently, altered telomere-associated gene expression and reduced TERT protein levels in the follicles of aged mice may be a determinant of telomere shortening during ovarian aging, and infertility appearing in the later decades of reproductive lifespan. Further investigations are required to determine the molecular mechanisms underlying these alterations in the follicles during ovarian aging.

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

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

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

Simple Summary

A link between telomere length and some age-related diseases has been identified, including metabolic syndrome. So far, there is no mechanism to explain the origin or cause of telomere shortening in this syndrome; however, oxidative stress is a constant factor. Therefore, we reviewed scientific evidence that supported the association between oxidative stress and telomere length dynamics, also examining how each of the metabolic syndrome components individually affects the length. In this regard, there is strong scientific evidence that an increase in the number of metabolic syndrome components is associated with a shorter telomere length, oxidative damage at the lipid and DNA level, and inflammation, as well as its other components, such as obesity, hyperglycemia, and hypertension, while for dyslipidemia, there is a little more discrepancy. The difficulty for the correct treatment of metabolic syndrome lies in its multifactorial nature. Hence, there is a need to carry out more studies on healthy lifestyles during aging to prevent and reduce oxidative damage and telomere wear during aging, and consequently the progression of chronic degenerative diseases, thus improving the living conditions of older people.

Abstract

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

Shelterin Complex at Telomeres: Implications in Ageing

Different factors influence the development‎ and control of ageing. It is well known that progressive telomere shorting is one of the molecular mechanisms underlying ageing. The shelterin complex consists of six telomere-specific proteins which are involved in the protection of chromosome ends. More particularly, this vital complex protects the telomeres from degradation, prevents from activation of unwanted repair systems, regulates the activity of telomerase, and has a crucial role in cellular senescent and ageing-related pathologies. This review explores the organization and function of telomeric DNA along with the mechanism of telomeres during ageing, followed by a discussion of the critical role of shelterin components and their changes during ageing.

Frequent Mutations of POT1 Distinguish Pulmonary Sarcomatoid Carcinoma From Other Lung Cancer Histologies

INTRODUCTION

Pulmonary sarcomatoid carcinoma (PSC) is a rare subtype of non-small-cell lung cancer (NSCLC) harboring mutations in many canonical NSCLC-driver genes (eg, TP53, KRAS, MET). Protection of telomeres 1 (POT1) mutations are observed in angiosarcoma and chronic lymphocytic leukemia, but their frequency in other solid tumors, including NSCLC subtypes, has not been rigorously explored.

MATERIALS AND METHODS

We analyzed next-generation sequencing data from 62,368 tumors, including 11,134 NSCLCs and 100 PSCs. We performed logistic regression to identify associations between POT1 mutation frequency and tumor histology across 184 tumor categories, adjusting for tumor mutational burden. We further explored co-occurring gene mutations in genes previously reported to underlie PSC tumorigenesis.

RESULTS

Across 184 tumor categories, POT1 mutations were most frequent in PSC and were 14 times more common in PSC (28%) than in other tumor types (P = 1.23 × 10^-31) and 6.7 times more common in PSC than other NSCLCs (P = 5.1 × 10^-17). PSCs harboring KRAS mutations were significantly more likely to harbor POT1 mutations (P = 1.3 × 10^-3), whereas those with TP53 mutations were less likely to harbor POT1 mutations (P = .037). One-fourth of POT1-mutated PSCs harbored a second POT1 mutation. Across all PSCs, 83% of POT1 mutations were in the OB1/OB2 (DNA-binding) domain (P = 1.5 × 10^-5), an enrichment not observed in other tumor types.

CONCLUSION

We report an unanticipated association between POT1 mutation and PSC. Unlike other molecular alterations that are frequent across NSCLC subtypes, POT1 mutations are largely unique to PSC. This finding may help to develop disease-defining molecular subgroups within PSC and presents opportunities for molecularly stratified prognostication and therapy.

Administration of a Nucleoside Analog Promotes Cancer Cell Death in a Telomerase-Dependent Manner

Telomerase, the end-replication enzyme, is reactivated in malignant cancers to drive cellular immortality. While this distinction makes telomerase an attractive target for anti-cancer therapies, most approaches for inhibiting its activity have been clinically ineffective. As opposed to inhibiting telomerase, we use its activity to selectively promote cytotoxicity in cancer cells. We show that several nucleotide analogs, including 5-fluoro-2'-deoxyuridine (5-FdU) triphosphate, are effectively incorporated by telomerase into a telomere DNA product. Administration of 5-FdU results in an increased number of telomere-induced foci, impedes binding of telomere proteins, activates the ATR-related DNA-damage response, and promotes cell death in a telomerase-dependent manner. Collectively, our data indicate that telomerase activity can be exploited as a putative anti-cancer strategy.

Telomere length and telomerase activity during folliculogenesis in mammals

Telomeres are repetitive non-coding DNA sequences located at the ends of chromosomes in eukaryotic cells. Their most important function is to protect chromosome ends from being recognized as DNA damage. They are also implicated in meiosis and synapse formation. The length of telomeres inevitably shortens at the end of each round of DNA replication and, also, as a consequence of the exposure to oxidative stress and/or genotoxic agents. The enzyme telomerase contributes to telomere lengthening. It has been reported that telomerase is exclusively expressed in germ cells, granulosa cells, early embryos, stem cells, and various types of cancerous cells. Granulosa cells undergo many mitotic divisions and either granulosa cells or oocytes are exposed to a variety of genotoxic agents throughout folliculogenesis; thus, telomerase plays an important role in the maintenance of telomere length. In this review article, we have comprehensively evaluated the studies focusing on the regulation of telomerase expression and activity, as well as telomere length, during folliculogenesis from primordial to antral follicles, in several mammalian species including mice, bovines, and humans. Also, the possible relationships between female infertility caused by follicular development defects and alterations in the telomeres and/or telomerase activity are discussed.

The shieldin complex mediates 53BP1-dependent DNA repair

53BP1 is a chromatin-binding protein that regulates the repair of DNA double-strand breaks by suppressing the nucleolytic resection of DNA termini1,2. This function of 53BP1 requires interactions with PTIP3 and RIF14-9, the latter of which recruits REV7 (also known as MAD2L2) to break sites10,11. How 53BP1-pathway proteins shield DNA ends is currently unknown, but there are two models that provide the best potential explanation of their action. In one model the 53BP1 complex strengthens the nucleosomal barrier to end-resection nucleases12,13, and in the other 53BP1 recruits effector proteins with end-protection activity. Here we identify a 53BP1 effector complex, shieldin, that includes C20orf196 (also known as SHLD1), FAM35A (SHLD2), CTC-534A2.2 (SHLD3) and REV7. Shieldin localizes to double-strand-break sites in a 53BP1- and RIF1-dependent manner, and its SHLD2 subunit binds to single-stranded DNA via OB-fold domains that are analogous to those of RPA1 and POT1. Loss of shieldin impairs non-homologous end-joining, leads to defective immunoglobulin class switching and causes hyper-resection. Mutations in genes that encode shieldin subunits also cause resistance to poly(ADP-ribose) polymerase inhibition in BRCA1-deficient cells and tumours, owing to restoration of homologous recombination. Finally, we show that binding of single-stranded DNA by SHLD2 is critical for shieldin function, consistent with a model in which shieldin protects DNA ends to mediate 53BP1-dependent DNA repair.

Systematic analysis of human telomeric dysfunction using inducible telosome/shelterin CRISPR/Cas9 knockout cells

CRISPR/Cas9 technology enables efficient loss-of-function analysis of human genes using somatic cells. Studies of essential genes, however, require conditional knockout (KO) cells. Here, we describe the generation of inducible CRISPR KO human cell lines for the subunits of the telosome/shelterin complex, TRF1, TRF2, RAP1, TIN2, TPP1 and POT1, which directly interact with telomeres or can bind to telomeres through association with other subunits. Homozygous inactivation of several subunits is lethal in mice, and most loss-of-function studies of human telomere regulators have relied on RNA interference-mediated gene knockdown, which suffers its own limitations. Our inducible CRISPR approach has allowed us to more expediently obtain large numbers of KO cells in which essential telomere regulators have been inactivated for biochemical and molecular studies. Our systematic analysis revealed functional differences between human and mouse telomeric proteins in DNA damage responses, telomere length and metabolic control, providing new insights into how human telomeres are maintained.

A calmodulin-like protein (LCALA) is a new Leishmania amazonensis candidate for telomere end-binding protein

BACKGROUND

Leishmania spp. telomeres are composed of 5'-TTAGGG-3' repeats associated with proteins. We have previously identified LaRbp38 and LaRPA-1 as proteins that bind the G-rich telomeric strand. At that time, we had also partially characterized a protein: DNA complex, named LaGT1, but we could not identify its protein component.

METHODS AND RESULTS

Using protein-DNA interaction and competition assays, we confirmed that LaGT1 is highly specific to the G-rich telomeric single-stranded DNA. Three protein bands, with LaGT1 activity, were isolated from affinity-purified protein extracts in-gel digested, and sequenced de novo using mass spectrometry analysis. In silico analysis of the digested peptide identified them as a putative calmodulin with sequences identical to the T. cruzi calmodulin. In the Leishmania genome, the calmodulin ortholog is present in three identical copies. We cloned and sequenced one of the gene copies, named it LCalA, and obtained the recombinant protein. Multiple sequence alignment and molecular modeling showed that LCalA shares homology to most eukaryotes calmodulin. In addition, we demonstrated that LCalA is nuclear, partially co-localizes with telomeres and binds in vivo the G-rich telomeric strand. Recombinant LCalA can bind specifically and with relative affinity to the G-rich telomeric single-strand and to a 3'G-overhang, and DNA binding is calcium dependent.

CONCLUSIONS

We have described a novel candidate component of Leishmania telomeres, LCalA, a nuclear calmodulin that binds the G-rich telomeric strand with high specificity and relative affinity, in a calcium-dependent manner.

GENERAL SIGNIFICANCE

LCalA is the first reported calmodulin that binds in vivo telomeric DNA.

Implications of telomeres and telomerase in endometrial pathology

BACKGROUND

Eukaryotic chromosomal ends are linear and are protected by nucleoprotein complexes known as telomeres. The complex structural anatomy and the diverse functions of telomeres as well as the unique reverse transcriptase enzyme, telomerase that maintains telomeres are under intensive scientific scrutiny. Both are involved in many human diseases including cancer, but also in ageing and chronic disease such as diabetes. Their intricate involvement in many cellular processes and pathways is being dynamically deciphered in many organs including the endometrium. This review summarizes our current knowledge on the topic of telomeres and telomerase and their potential role in providing plausible explanations for endometrial aberrations related to common gynaecological pathologies.

OBJECTIVE AND RATIONALE

This review outlines the recent major findings in telomere and telomerase functions in the context of endometrial biology. It highlights the contemporary discoveries in hormonal regulation, normal endometrial regeneration, stem cells and common gynaecological diseases such as endometriosis, infertility, recurrent reproductive failure and endometrial cancer (EC).

SEARCH METHODS

The authors carried out systematic PubMed (Medline) and Ovid searches using the key words: telomerase, telomeres, telomere length, human telomerase reverse transcriptase, telomeric RNA component, with endometrium, hormonal regulation, endometrial stem/progenitor cells, endometrial regeneration, endometriosis, recurrent miscarriage, infertility, endometrial hyperplasia, EC and uterine cancer. Publications used in this review date from 1995 until 31st June 2016.

OUTCOMES

The human endometrium is a unique somatic organ, which displays dynamic telomerase activity (TA) related to the menstrual cycle. Telomerase is implicated in almost all endometrial pathologies and appears to be crucial to endometrial stem cells. In particular, it is vital for normal endometrial regeneration, providing a distinct route to formulate possible curative, non-hormonal therapies to treat chronic endometrial conditions. Furthermore, our current understanding of telomere maintenance in EC is incomplete. Data derived from other malignancies on the role of telomerase in carcinogenesis cannot be extrapolated to EC because unlike in other cancers, TA is already present in proliferating healthy endometrial cells.

WIDER IMPLICATIONS

Since telomerase is pivotal to endometrial regeneration, further studies elucidating the role of telomeres, telomerase, their associated proteins and their regulation in normal endometrial regeneration as well as their role in endometrial pathologies are essential. This approach may allow future development of novel treatment strategies that are not only non-hormonal but also potentially curative.

Telomerase Mechanism of Telomere Synthesis

Telomerase is the essential reverse transcriptase required for linear chromosome maintenance in most eukaryotes. Telomerase supplements the tandem array of simple-sequence repeats at chromosome ends to compensate for the DNA erosion inherent in genome replication. The template for telomerase reverse transcriptase is within the RNA subunit of the ribonucleoprotein complex, which in cells contains additional telomerase holoenzyme proteins that assemble the active ribonucleoprotein and promote its function at telomeres. Telomerase is distinct among polymerases in its reiterative reuse of an internal template. The template is precisely defined, processively copied, and regenerated by release of single-stranded product DNA. New specificities of nucleic acid handling that underlie the catalytic cycle of repeat synthesis derive from both active site specialization and new motif elaborations in protein and RNA subunits. Studies of telomerase provide unique insights into cellular requirements for genome stability, tissue renewal, and tumorigenesis as well as new perspectives on dynamic ribonucleoprotein machines.

Shared Subunits of Tetrahymena Telomerase Holoenzyme and Replication Protein A Have Different Functions in Different Cellular Complexes

In most eukaryotes, telomere maintenance relies on telomeric repeat synthesis by a reverse transcriptase named telomerase. To synthesize telomeric repeats, the catalytic subunit telomerase reverse transcriptase (TERT) uses the RNA subunit (TER) as a template. In the ciliate Tetrahymena thermophila, the telomerase holoenzyme consists of TER, TERT, and eight additional proteins, including the telomeric repeat single-stranded DNA-binding protein Teb1 and its heterotrimer partners Teb2 and Teb3. Teb1 is paralogous to the large subunit of the general single-stranded DNA binding heterotrimer replication protein A (RPA). Little is known about the function of Teb2 and Teb3, which are structurally homologous to the RPA middle and small subunits, respectively. Here, epitope-tagging Teb2 and Teb3 expressed at their endogenous gene loci enabled affinity purifications that revealed that, unlike other Tetrahymena telomerase holoenzyme subunits, Teb2 and Teb3 are not telomerase-specific. Teb2 and Teb3 assembled into other heterotrimer complexes, which when recombinantly expressed had the general single-stranded DNA binding activity of RPA complexes, unlike the telomere-specific DNA binding of Teb1 or the TEB heterotrimer of Teb1, Teb2, and Teb3. TEB had no more DNA binding affinity than Teb1 alone. In contrast, heterotrimers reconstituted with Teb2 and Teb3 and two other Tetrahymena RPA large subunit paralogs had higher DNA binding affinity than their large subunit alone. Teb1 and TEB, but not RPA, increased telomerase processivity. We conclude that in the telomerase holoenzyme, instead of binding DNA, Teb2 and Teb3 are Teb1 assembly factors. These findings demonstrate that Tetrahymena telomerase holoenzyme and RPA complexes share subunits and that RPA subunits have distinct functions in different heterotrimer assemblies.

Telomerase Regulation from Beginning to the End

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

A mutation in the POT1 gene is responsible for cardiac angiosarcoma in TP53-negative Li-Fraumeni-like families

Cardiac angiosarcoma (CAS) is a rare malignant tumour whose genetic basis is unknown. Here we show, by whole-exome sequencing of a TP53-negative Li-Fraumeni-like (LFL) family including CAS cases, that a missense variant (p.R117C) in POT1 (protection of telomeres 1) gene is responsible for CAS. The same gene alteration is found in two other LFL families with CAS, supporting the causal effect of the identified mutation. We extend the analysis to TP53-negative LFL families with no CAS and find the same mutation in a breast AS family. The mutation is recently found once in 121,324 studied alleles in ExAC server but it is not described in any other database or found in 1,520 Spanish controls. In silico structural analysis suggests how the mutation disrupts POT1 structure. Functional and in vitro studies demonstrate that carriers of the mutation show reduced telomere-bound POT1 levels, abnormally long telomeres and increased telomere fragility.

Formation and stabilization of the telomeric antiparallel G-quadruplex and inhibition of telomerase by novel benzothioxanthene derivatives with anti-tumor activity

G-quadruplexes formed in telomeric DNA sequences at human chromosome ends can be a novel target for the development of therapeutics for the treatment of cancer patients. Herein, we examined the ability of six novel benzothioxanthene derivatives S1–S6 to induce the formation of and stabilize an antiparallel G-quadruplex by EMSA, UV-melting and CD techniques and the influence of S1–S6 on A549 and SGC7901 cells through real-time cell analysis, wound healing, trap assay methods. Results show that six compounds could differentially induce 26 nt G-rich oligonucleotides to form the G-quadruplex with high selectivity vs C-rich DNA, mutated DNA and double-stranded DNA, stabilize it with high affinity, promote apoptosis and inhibit mobility and telomerase activity of A549 cells and SGC7901 cells. Especially, S1, S3, S4 displayed stronger abilities, of which S3 was the most optimal with the maximum ΔT_m value being up to 29.8 °C for G-quadruplex, the minimum IC₅₀ value being 0.53 μM and the maximum cell inhibitory rate being up to 97.2%. This study suggests that this type of compounds that induce the formation of and stabilize the telomeric antiparallel G-quadruplex, and consequently inhibit telomerase activity, leading to cell apoptosis, can be screened for the discovery of novel antitumor therapeutics.

Combinatorial recognition of a complex telomere repeat sequence by the Candida parapsilosis Cdc13AB heterodimer

The telomere repeat units of Candida species are substantially longer and more complex than those in other organisms, raising interesting questions concerning the recognition mechanisms of telomere-binding proteins. Herein we characterized the properties of Candida parapsilosis Cdc13A and Cdc13B, two paralogs that are responsible for binding and protecting the telomere G-strand tails. We found that Cdc13A and Cdc13B can each form complexes with itself and a heterodimeric complex with each other. However, only the heterodimer exhibits high-affinity and sequence-specific binding to the telomere G-tail. EMSA and crosslinking analysis revealed a combinatorial mechanism of DNA recognition, which entails the A and B subunit making contacts to the 3′ and 5′ region of the repeat unit. While both the DBD and OB4 domain of Cdc13A can bind to the equivalent domain in Cdc13B, only the OB4 complex behaves as a stable heterodimer. The unstable Cdc13AB_DBD complex binds G-strand with greatly reduced affinity but the same sequence specificity. Thus the OB4 domains evidently contribute to binding by promoting dimerization of the DBDs. Our investigation reveals a rare example of combinatorial recognition of single-stranded DNA and offers insights into the co-evolution of telomere DNA and cognate binding proteins.

Germline mutations in shelterin complex genes are associated with familial glioma

Gliomas are the most common brain tumor, with several histological subtypes of various malignancy grade. The genetic contribution to familial glioma is not well understood. Using whole exome sequencing of 90 individuals from 55 families, we identified two families with mutations in POT1 (p.G95C, p.E450X), a member of the telomere shelterin complex, shared by both affected individuals in each family and predicted to impact DNA binding and TPP1 binding, respectively. Validation in a separate cohort of 264 individuals from 246 families identified an additional mutation in POT1 (p.D617Efs), also predicted to disrupt TPP1 binding. All families with POT1 mutations had affected members with oligodendroglioma, a specific subtype of glioma more sensitive to irradiation. These findings are important for understanding the origin of glioma and could have importance for the future diagnostics and treatment of glioma.

Genetic and molecular identification of three human TPP1 functions in telomerase action: recruitment, activation, and homeostasis set point regulation

Telomere length homeostasis is essential for the long-term survival of stem cells, and its set point determines the proliferative capacity of differentiated cell lineages by restricting the reservoir of telomeric repeats. Knockdown and overexpression studies in human tumor cells showed that the shelterin subunit TPP1 recruits telomerase to telomeres through a region termed the TEL patch. However, these studies do not resolve whether the TPP1 TEL patch is the only mechanism for telomerase recruitment and whether telomerase regulation studied in tumor cells is representative of nontransformed cells such as stem cells. Using genome engineering of human embryonic stem cells, which have physiological telomere length homeostasis, we establish that the TPP1 TEL patch is genetically essential for telomere elongation and thus long-term cell viability. Furthermore, genetic bypass, protein fusion, and intragenic complementation assays define two distinct additional mechanisms of TPP1 involvement in telomerase action at telomeres. We demonstrate that TPP1 provides an essential step of telomerase activation as well as feedback regulation of telomerase by telomere length, which is necessary to determine the appropriate telomere length set point in human embryonic stem cells. These studies reveal and resolve multiple TPP1 roles in telomere elongation and stem cell telomere length homeostasis.

The telomeric protein Pot1 from Schizosaccharomyces pombe binds ssDNA in two modes with differing 3' end availability

Telomere protection and length regulation are important processes for aging, cancer and several other diseases. At the heart of these processes lies the single-stranded DNA (ssDNA)-binding protein Pot1, a component of the telomere maintenance complex shelterin, which is present in species ranging from fission yeast to humans. Pot1 contains a dual OB-fold DNA-binding domain (DBD) that fully confers its high affinity for telomeric ssDNA. Studies of S. pombe Pot1-DBD and its individual OB-fold domains revealed a complex non-additive behavior of the two OB-folds in the context of the complete Pot1 protein. This behavior includes the use of multiple distinct binding modes and an ability to form higher order complexes. Here we use NMR and biochemical techniques to investigate the structural features of the complete Pot1-DBD. These experiments reveal one binding mode characterized by only subtle alternations to the individual OB-fold subdomain structures, resulting in an inaccessible 3′ end of the ssDNA. The second binding mode, which has equivalent affinity, interacts differently with the 3′ end, rendering it available for interaction with other proteins. These findings suggest a structural switch that contributes to telomere end-protection and length regulation.

Novel telomere-anchored PCR approach for studying sexual stage telomeres in Aspergillus nidulans

Telomere length varies between germline and somatic cells of the same organism, leading to the hypothesis that telomeres are lengthened during meiosis. However, little is known about the meiotic telomere length in many organisms. In the filamentous fungus Aspergillus nidulans, the telomere lengths in hyphae and asexual spores are invariant. No study using existing techniques has determined the telomere length of the sexual ascospores due to the relatively low abundance of pure meiotic cells in A. nidulans and the small quantity of DNA present. To address this, we developed a simple and sensitive PCR strategy to measure the telomere length of A. nidulans meiotic cells. This novel technique, termed “telomere-anchored PCR,” measures the length of the telomere on chromosome II-L using a small fraction of the DNA required for the traditional terminal restriction fragment (TRF) Southern analysis. Using this approach, we determined that the A. nidulans ascospore telomere length is virtually identical to telomeres of other cell types from this organism, approximately 110 bp, indicating that a surprisingly strict telomere length regulation exists in the major cell types of A. nidulans. When the hyphal telomeres were measured in a telomerase reverse transcriptase (TERT) knockout strain, small decreases in length were readily detected. Thus, this technique can detect telomeres in relatively rare cell types and is particularly sensitive in measuring exceptionally short telomeres. This rapid and inexpensive telomere-anchored PCR method potentially can be utilized in other filamentous fungi and types of organisms.

Expression of Shelterin component POT1 is associated with decreased telomere length and immunity condition in humans with severe aplastic anemia

Abnormal telomere attrition has been found to be closely related to patients with SAA in recent years. To identify the incidence of telomere attrition in SAA patients and investigate the relationship of telomere length with clinical parameters, SAA patients (n=27) and healthy controls (n=15) were enrolled in this study. Telomere length of PWBCs was significantly shorter in SAA patients than in controls. Analysis of gene expression of Shelterin complex revealed markedly low levels of POT1 expression in SAA groups relative to controls. No differences in the gene expression of the other Shelterin components-TRF1, TRF2, TIN2, TPP1, and RAP1-were identified. Addition of IFN-γ to culture media induced a similar fall in POT1 expression in bone marrow cells to that observed in cells cultured in the presence of SAA serum, suggesting IFN-γ is the agent responsible for this effect of SAA serum. Furthermore, ATR, phosphorylated ATR, and phosphorylated ATM/ATR substrate were all found similarly increased in bone marrow cells exposed to SAA serum, TNF-α, or IFN-γ. In summary, SAA patients have short telomeres and decreased POT1 expression. TNF-α and IFN-γ are found at high concentrations in SAA patients and may be the effectors that trigger apoptosis through POT1 and ATR.

CTC1 increases the radioresistance of human melanoma cells by inhibiting telomere shortening and apoptosis

Melanoma has traditionally been viewed as a radioresistant cancer. However, recent studies suggest that under certain clinical circumstances, radiotherapy may play a significant role in the treatment of melanoma. Previous studies have demonstrated that telomere length is a hallmark of radiosensitivity. The newly discovered mammalian CTC1-STN1-TEN1 (CST) complex has been demonstrated to be an important telomere maintenance factor. In this study, by establishing a radiosensitive/radioresistant human melanoma cell model, MDA-MB-435/MDA-MB-435R, we aimed to investigate the association of CTC1 expression with radiosensitivity in human melanoma cell lines, and to elucidate the possible underlying mechanisms. We found that CTC1 mRNA and protein levels were markedly increased in the MDA-MB-435R cells compared with the MDA-MB-435 cells. Moreover, the downregulation of CTC1 enhanced radiosensitivity, induced DNA damage and promoted telomere shortening and apoptosis in both cell lines. Taken together, our findings suggest that CTC1 increases the radioresistance of human melanoma cells by inhibiting telomere shortening and apoptosis. Thus, CTC1 may be an attractive target gene for the treatment of human melanoma.

T-oligo as an anticancer agent in colorectal cancer

In the United States, there will be an estimated 96,830 new cases of colorectal cancer (CRC) and 50,310 deaths in 2014. CRC is often detected at late stages of the disease, at which point there is no effective chemotherapy. Thus, there is an urgent need for effective novel therapies that have minimal effects on normal cells. T-oligo, an oligonucleotide homologous to the 3'-telomere overhang, induces potent DNA damage responses in multiple malignant cell types, however, its efficacy in CRC has not been studied. This is the first investigation demonstrating T-oligo-induced anticancer effects in two CRC cell lines, HT-29 and LoVo, which are highly resistant to conventional chemotherapies. In this investigation, we show that T-oligo may mediate its DNA damage responses through the p53/p73 pathway, thereby inhibiting cellular proliferation and inducing apoptosis or senescence. Additionally, upregulation of downstream DNA damage response proteins, including E2F1, p53 or p73, was observed. In LoVo cells, T-oligo induced senescence, decreased clonogenicity, and increased expression of senescence associated proteins p21, p27, and p53. In addition, downregulation of POT1 and TRF2, two components of the shelterin protein complex which protects telomeric ends, was observed. Moreover, we studied the antiproliferative effects of T-oligo in combination with an EGFR tyrosine kinase inhibitor, Gefitinib, which resulted in an additive inhibitory effect on cellular proliferation. Collectively, these data provide evidence that T-oligo alone, or in combination with other molecularly targeted therapies, has potential as an anti-cancer agent in CRC.

If the cap fits, wear it: an overview of telomeric structures over evolution

Genome organization into linear chromosomes likely represents an important evolutionary innovation that has permitted the development of the sexual life cycle; this process has consequently advanced nuclear expansion and increased complexity of eukaryotic genomes. Chromosome linearity, however, poses a major challenge to the internal cellular machinery. The need to efficiently recognize and repair DNA double-strand breaks that occur as a consequence of DNA damage presents a constant threat to native chromosome ends known as telomeres. In this review, we present a comparative survey of various solutions to the end protection problem, maintaining an emphasis on DNA structure. This begins with telomeric structures derived from a subset of prokaryotes, mitochondria, and viruses, and will progress into the typical telomere structure exhibited by higher organisms containing TTAGG-like tandem sequences. We next examine non-canonical telomeres from Drosophila melanogaster, which comprise arrays of retrotransposons. Finally, we discuss telomeric structures in evolution and possible switches between canonical and non-canonical solutions to chromosome end protection.

Coordinated interactions of multiple POT1-TPP1 proteins with telomere DNA

Telomeres are macromolecular nucleoprotein complexes that protect the ends of eukaryotic chromosomes from degradation, end-to-end fusion events, and from engaging the DNA damage response. However, the assembly of this essential DNA-protein complex is poorly understood. Telomere DNA consists of the repeated double-stranded sequence 5'-TTAGGG-3' in vertebrates, followed by a single-stranded DNA overhang with the same sequence. Both double- and single-stranded regions are coated with high specificity by telomere end-binding proteins, including POT1 and TPP1, that bind as a heterodimer to single-stranded telomeric DNA. Multiple POT1-TPP1 proteins must fully coat the single-stranded telomere DNA to form a functional telomere. To better understand the mechanism of multiple binding, we mutated or deleted the two guanosine nucleotides residing between adjacent POT1-TPP1 recognition sites in single-stranded telomere DNA that are not required for multiple POT1-TPP1 binding events. Circular dichroism demonstrated that spectra from the native telomere sequence are characteristic of a G-quadruplex secondary structure, whereas the altered telomere sequences were devoid of these signatures. The altered telomere strands, however, facilitated more cooperative loading of multiple POT1-TPP1 proteins compared with the wild-type telomere sequence. Finally, we show that a 48-nucleotide DNA with a telomere sequence is more susceptible to nuclease digestion when coated with POT1-TPP1 proteins than when it is left uncoated. Together, these data suggest that POT1-TPP1 binds telomeric DNA in a coordinated manner to facilitate assembly of the nucleoprotein complexes into a state that is more accessible to enzymatic activity.

Chromatin structure in telomere dynamics

The establishment of a specific nucleoprotein structure, the telomere, is required to ensure the protection of chromosome ends from being recognized as DNA damage sites. Telomere shortening below a critical length triggers a DNA damage response that leads to replicative senescence. In normal human somatic cells, characterized by telomere shortening with each cell division, telomere uncapping is a regulated process associated with cell turnover. Nevertheless, telomere dysfunction has also been associated with genomic instability, cell transformation, and cancer. Despite the essential role telomeres play in chromosome protection and in tumorigenesis, our knowledge of the chromatin structure involved in telomere maintenance is still limited. Here we review the recent findings on chromatin modifications associated with the dynamic changes of telomeres from protected to deprotected state and their role in telomere functions.

Fission yeast RecQ helicase Rqh1 is required for the maintenance of circular chromosomes

Protection of telomeres protein 1 (Pot1) binds to single-stranded telomere overhangs and protects chromosome ends. RecQ helicases regulate homologous recombination at multiple stages, including resection, strand displacement, and resolution. Fission yeast pot1 and RecQ helicase rqh1 double mutants are synthetically lethal, but the mechanism is not fully understood. Here, we show that the synthetic lethality of pot1Δ rqh1Δ double mutants is due to inappropriate homologous recombination, as it is suppressed by the deletion of rad51(+). The expression of Rad51 in the pot1Δ rqh1Δ rad51Δ triple mutant, which has circular chromosomes, is lethal. Reduction of the expression of Rqh1 in a pot1 disruptant with circular chromosomes caused chromosome missegregation, and this defect was partially suppressed by the deletion of rad51(+). Taken together, our results suggest that Rqh1 is required for the maintenance of circular chromosomes when homologous recombination is active. Crossovers between circular monomeric chromosomes generate dimers that cannot segregate properly in Escherichia coli. We propose that Rqh1 inhibits crossovers between circular monomeric chromosomes to suppress the generation of circular dimers.

Towards physiological complexity with in vitro single-molecule biophysics

Single-molecule biology has matured in recent years, driven to greater sophistication by the development of increasingly advanced experimental techniques. A progressive appreciation for its unique strengths is attracting research that spans an exceptionally broad swath of physiological phenomena--from the function of nucleosomes to protein diffusion in the cell membrane. Newfound enthusiasm notwithstanding, the single-molecule approach is limited to an intrinsically defined set of biological questions; such limitation applies to all experimental approaches, and an explicit statement of the boundaries delineating each set offers a guide to most fruitfully orienting in vitro single-molecule research in the future. Here, we briefly describe a simple conceptual framework to categorize how submolecular, molecular and intracellular processes are studied. We highlight the domain of single-molecule biology in this scheme, with an emphasis on its ability to probe various forms of heterogeneity inherent to populations of discrete biological macromolecules. We then give a general overview of our high-throughput DNA curtain methodology for studying protein-nucleic acid interactions, and by contextualizing it within this framework, we explore what might be the most enticing avenues of future research. We anticipate that a focus on single-molecule biology's unique strengths will suggest a new generation of experiments with greater complexity and more immediately translatable physiological relevance.

An alternative telomerase RNA in Arabidopsis modulates enzyme activity in response to DNA damage

Telomerase replenishes telomere tracts by reiteratively copying its RNA template, TER. Unlike other model organisms, Arabidopsis thaliana harbors two divergent TER genes. However, only TER1 is required for telomere maintenance. Here we examine the function of TER2. We show that TER2 is spliced and its 3' end is truncated in vivo to generate a third TER isoform, TER2(S). TERT preferentially associates with TER2 > TER1 > TER2(S). Moreover, TER2 and TER2(S) assemble with Ku and POT1b (protection of telomeres), forming RNP (ribonucleoprotein) complexes distinct from TER1 RNP. Plants null for TER2 display increased telomerase enzyme activity, while TER2 overexpression inhibits telomere synthesis from TER1 and leads to telomere shortening. These findings argue that TER2 negatively regulates telomerase by sequestering TERT in a nonproductive RNP complex. Introduction of DNA double-strand breaks by zeocin leads to an immediate and specific spike in TER2 and a concomitant decrease in telomerase enzyme activity. This response is not triggered by replication stress or telomere dysfunction and is abrogated in ter2 mutants. We conclude that Arabidopsis telomerase is modulated by TER2, a novel DNA damage-induced noncoding RNA that works in concert with the canonical TER to promote genome integrity.

Telomere structure and telomerase in health and disease (review)

Telomerase is the enzyme responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences. Telomerase activity is exhibited in gametes and stem and tumor cells. In human somatic cells, proliferation potential is strictly limited and senescence follows approximately 50–70 cell divisions. In most tumor cells, on the contrary, replication potential is unlimited. The key role in this process of the system of the telomere length maintenance with involvement of telomerase is still poorly studied. Undoubtedly, DNA polymerase is not capable of completely copying DNA at the very ends of chromosomes; therefore, approximately 50 nucleotides are lost during each cell cycle, which results in gradual telomere length shortening. Critically short telomeres cause senescence, following crisis and cell death. However, in tumor cells the system of telomere length maintenance is activated. Much work has been done regarding the complex telomere/telomerase as a unique target, highly specific in cancer cells. Telomeres have additional proteins that regulate the binding of telomerase. Telomerase, also associates with a number of proteins forming the sheltering complex having a central role in telomerase activity. This review focuses on the structure and function of the telomere/telomerase complex and its altered behavior leading to disease, mainly cancer. Although telomerase therapeutics are not approved yet for clinical use, we can assume that based on the promising in vitro and in vivo results and successful clinical trials, it can be predicted that telomerase therapeutics will be utilized soon in the combat against malignancies and degenerative diseases. The active search for modulators is justified, because the telomere/telomerase system is an extremely promising target offering possibilities to decrease or increase the viability of the cell for therapeutic purposes.

Kinetics of DNA replication and telomerase reaction at a single-seeded telomere in human cells

In most cancer cells, telomerase is activated to elongate telomere DNA, thereby ensuring numerous rounds of cell divisions. It is thus important to understand how telomerase and the replication fork react with telomeres in human cells. However, the highly polymorphic and repetitive nature of the nucleotide sequences in human subtelomeric regions hampers the precise analysis of sequential events taking place at telomeres in S phase. Here, we have established HeLa cells harboring a single-seeded telomere abutted by a unique subtelomere DNA sequence, which has enabled us to specifically focus on the seeded telomere. We have also developed a modified chromatin immunoprecipitation (ChIP) method that uses restriction digestion instead of sonication to fragment chromatin DNA (RES-ChIP), and a method for immunoprecipitating 5-bromo-2'-deoxyuridine (BrdU)-labeled single-stranded DNA by incubating DNA with anti-BrdU antibody in the nondenaturing condition. We have shown that DNA replication of the seeded telomere takes place during a relatively narrow time window in S phase, and telomerase synthesizes telomere DNA after the replication. Moreover, we have demonstrated that the telomerase catalytic subunit TERT associates with telomeres before telomere DNA replication. These results provide a temporal and spatial framework for understanding DNA replication and telomerase reaction at human telomeres.

Maintaining the end: roles of telomere proteins in end-protection, telomere replication and length regulation

Chromosome end protection is essential to protect genome integrity. Telomeres, tracts of repetitive DNA sequence and associated proteins located at the chromosomal terminus, serve to safeguard the ends from degradation and unwanted double strand break repair. Due to the essential nature of telomeres in protecting the genome, a number of unique proteins have evolved to ensure that telomere length and structure are preserved. The inability to properly maintain telomeres can lead to diseases such as dyskeratosis congenita, pulmonary fibrosis and cancer. In this review, we will discuss the known functions of mammalian telomere-associated proteins, their role in telomere replication and length regulation and how these processes relate to genome instability and human disease.

Telomere protection by TPP1/POT1 requires tethering to TIN2

To prevent ATR activation, telomeres deploy the single-stranded DNA binding activity of TPP1/POT1a. POT1a blocks the binding of RPA to telomeres, suggesting that ATR is repressed through RPA exclusion. However, comparison of the DNA binding affinities and abundance of TPP1/POT1a and RPA indicates that TPP1/POT1a by itself is unlikely to exclude RPA. We therefore analyzed the central shelterin protein TIN2, which links TPP1/POT1a (and POT1b) to TRF1 and TRF2 on the double-stranded telomeric DNA. Upon TIN2 deletion, telomeres lost TPP1/POT1a, accumulated RPA, elicited an ATR signal, and showed all other phenotypes of POT1a/b deletion. TIN2 also affected the TRF2-dependent repression of ATM kinase signaling but not to TRF2-mediated inhibition of telomere fusions. Thus, while TIN2 has a minor contribution to the repression of ATM by TRF2, its major role is to stabilize TPP1/POT1a on the ss telomeric DNA, thereby allowing effective exclusion of RPA and repression of ATR signaling.

Telomerase and telomere-associated proteins: structural insights into mechanism and evolution

Recent advances in our structural understanding of telomerase and telomere-associated proteins have contributed significantly to elucidating the molecular mechanisms of telomere maintenance. The structures of telomerase TERT domains have provided valuable insights into how experimentally identified conserved motifs contribute to the telomerase reverse transcriptase reaction. Additionally, structures of telomere-associated proteins in a variety of organisms have revealed that, across evolution, telomere-maintenance mechanisms employ common structural elements. For example, the single-stranded 3' overhang of telomeric DNA is specifically and tightly bound by an OB-fold in nearly all species, including ciliates (TEBP and Pot1a), fission yeast (SpPot1), budding yeast (Cdc13), and humans (hPOT1). Structures of the yeast Cdc13, Stn1, and Ten1 proteins demonstrated that telomere maintenance is regulated by a complex that bears significant similarity to the RPA heterotrimer. Similarly, proteins that specifically bind double-stranded telomeric DNA in divergent species use homeodomains to execute their functions (human TRF1 and TRF2 and budding yeast ScRap1). Likewise, the conserved protein Rap1, which is found in budding yeast, fission yeast, and humans, contains a structural motif that is known to be critical for protein-protein interaction. In addition to revealing the common underlying themes of telomere maintenance, structures have also elucidated the specific mechanisms by which many of these proteins function, including identifying a telomere-specific domain in Stn1 and how the human TRF proteins avoid heterodimerization. In this review, we summarize the high-resolution structures of telomerase and telomere-associated proteins and discuss the emergent common structural themes among these proteins. We also address how these high-resolution structures complement biochemical and cellular studies to enhance our understanding of telomere maintenance and function.

Telomere protection by TPP1/POT1 requires tethering to TIN2

To prevent ATR activation, telomeres deploy the single-stranded DNA binding activity of TPP1/POT1a. POT1a blocks the binding of RPA to telomeres, suggesting that ATR is repressed through RPA exclusion. However, comparison of the DNA binding affinities and abundance of TPP1/POT1a and RPA indicates that TPP1/POT1a by itself is unlikely to exclude RPA. We therefore analyzed the central shelterin protein TIN2, which links TPP1/POT1a (and POT1b) to TRF1 and TRF2 on the double-stranded telomeric DNA. Upon TIN2 deletion, telomeres lost TPP1/POT1a, accumulated RPA, elicited an ATR signal, and showed all other phenotypes of POT1a/b deletion. TIN2 also affected the TRF2-dependent repression of ATM kinase signaling but not to TRF2-mediated inhibition of telomere fusions. Thus, while TIN2 has a minor contribution to the repression of ATM by TRF2, its major role is to stabilize TPP1/POT1a on the ss telomeric DNA, thereby allowing effective exclusion of RPA and repression of ATR signaling.

Xenopus laevis Ctc1-Stn1-Ten1 (xCST) protein complex is involved in priming DNA synthesis on single-stranded DNA template in Xenopus egg extract

The Ctc1-Stn1-Ten1 (CST) complex is an RPA (replication protein A)-like protein complex that binds to single-stranded (ss) DNA. It localizes at telomeres and is involved in telomere end protection in mammals and plants. It is also known to stimulate DNA polymerase α-primase in vitro. However, it is not known how CST accomplishes these functions in vivo. Here, we report the identification and characterization of Xenopus laevis CST complex (xCST). xCST showed ssDNA binding activity with moderate preference for G (guanine)-rich sequences. xStn1-immunodepleted Xenopus egg extracts supported chromosomal DNA replication in in vitro reconstituted sperm nuclei, suggesting that xCST is not a general replication factor. However, the immunodepletion or neutralization of xStn1 compromised DNA synthesis on ssDNA template. Because primed ssDNA template was replicated in xStn1-immunodepleted extracts as efficiently as in control ones, we conclude that xCST is involved in the priming step on ssDNA template. These results are consistent with the current model that CST is involved in telomeric C-strand synthesis through the regulation of DNA polymerase α-primase.

The Pot1a-associated proteins Tpt1 and Pat1 coordinate telomere protection and length regulation in Tetrahymena

We have identified two new Pot1a-associated telomere proteins, Pat1 and Tpt1, from Tetrahymena. Tpt1 is required to prevent telomere elongation and appears to be the Tetrahymena equivalent of vertebrate TPP1. Pat1 depletion causes gradual telomere shortening, indicating that it is needed for telomerase to gain access to the DNA terminus.

We have identified two new telomere proteins, Tpt1 and Pat1, from the ciliate Tetrahymena thermophila. Although Tetrahymena telomerase is well characterized, only one telomere protein had previously been identified. This was the G-overhang binding-protein Pot1a. Tpt1 and Pat1 were isolated as Pot1a binding partners and shown to localize to telomeres. As Tpt1 and Pat1 were both found to be essential, conditional cell lines were generated to explore their function. Tpt1 depletion caused a rapid growth arrest and telomere elongation in the absence of cell division. The phenotype was similar to that seen after Pot1a depletion suggesting that Tpt1 and Pot1a function together to regulate telomere length and prevent telomere deprotection. In contrast, Pat1 depletion had a modest effect on cell growth but caused progressive telomere shortening similar to that observed upon TERT depletion. Thus Pat1 appears to be needed for telomerase to maintain the chromosome terminus. Analysis of Pot1a-Tpt1-Pat1 complex formation using purified proteins indicated that Tpt1 interacts directly with Pot1a while Pat1 interacts with Tpt1. Our results indicate that Tpt1 is the Tetrahymena equivalent of mammalian TPP1, Schizosaccharomyces pombe Tpz1, and Oxytricha nova TEBPβ.