Characterization of the DNA binding specificity of Shelterin complexes

Slow G-Quadruplex Conformation Rearrangement and Accessibility Change Induced by Potassium in Human Telomeric Single-Stranded DNA

The guanine-rich telomeric repeats can form G-quadruplexes (G4s) that alter the accessibility of the single-stranded telomeric overhang. In this study, we investigated the effects of Na+ and K+ on G4 folding and accessibility through cation introduction and exchange. We combined differential scanning calorimetry (DSC), circular dichroism (CD), and single molecule Förster resonance energy transfer (smFRET) to monitor the stability, conformational dynamics, and complementary strand binding accessibility of G4 formed by single-stranded telomeric DNA. Our data showed that G4 formed through heating and slow cooling in K+ solution exhibited fewer conformational dynamics than G4 formed in Na+ solution, which is consistent with the higher thermal stability of G4 in K+. Monitoring cation exchange with real time smFRET at room temperature shows that Na+ and K+ can replace each other in G4. When encountering high K+ at room or body temperature, G4 undergoes a slow conformational rearrangement process which is mostly complete by 2 h. The slow conformational rearrangement ends with a stable G4 that is unable to be unfolded by a complementary strand. This study provides new insights into the accessibility of G4 forming sequences at different time points after introduction to a high K+ environment in cells, which may affect how the nascent telomeric overhang interacts with proteins and telomerase.

Thioredoxin (Trx): A redox target and modulator of cellular senescence and aging-related diseases

Thioredoxin (Trx) is a compact redox-regulatory protein that modulates cellular redox state by reducing oxidized proteins. Trx exhibits dual functionality as an antioxidant and a cofactor for diverse enzymes and transcription factors, thereby exerting influence over their activity and function. Trx has emerged as a pivotal biomarker for various diseases, particularly those associated with oxidative stress, inflammation, and aging. Recent clinical investigations have underscored the significance of Trx in disease diagnosis, treatment, and mechanistic elucidation. Despite its paramount importance, the intricate interplay between Trx and cellular senescence-a condition characterized by irreversible growth arrest induced by multiple aging stimuli-remains inadequately understood. In this review, our objective is to present a comprehensive and up-to-date overview of the structure and function of Trx, its involvement in redox signaling pathways and cellular senescence, its association with aging and age-related diseases, as well as its potential as a therapeutic target. Our review aims to elucidate the novel and extensive role of Trx in senescence while highlighting its implications for aging and age-related diseases.

Shelterin Components Modulate Nucleic Acids Condensation and Phase Separation in the Context of Telomeric DNA

Telomeres are nucleoprotein complexes that protect the ends of chromosomes and are essential for chromosome stability in Eukaryotes. In cells, individual telomeres form distinct globules of finite size that appear to be smaller than expected for bare DNA. Moreover, telomeres can cluster together, form telomere-induced-foci or co-localize with promyelocytic leukemia (PML) nuclear bodies. The physical basis for collapse of individual telomeres and coalescence of multiple ones remains unclear, as does the relationship between these two phenomena. By combining single-molecule force spectroscopy measurements, optical microscopy, turbidity assays, and simulations, we show that the telomere scaffolding protein TRF2 can condense individual DNA chains and drives coalescence of multiple DNA molecules, leading to phase separation and the formation of liquid-like droplets. Addition of the TRF2 binding protein hRap1 modulates phase boundaries and tunes the specificity of solution demixing while simultaneously altering the degree of DNA compaction. Our results suggest that the condensation of single telomeres and formation of biomolecular condensates containing multiple telomeres are two different outcomes driven by the same set of molecular interactions. Moreover, binding partners, such as other telomere components, can alter those interactions to promote single-chain DNA compaction over multiple-chain phase separation.

A GO catalogue of human DNA-binding transcription factors

To control gene transcription, DNA-binding transcription factors recognise specific sequence motifs in gene regulatory regions. A complete and reliable GO annotation of all DNA-binding transcription factors is key to investigating the delicate balance of gene regulation in response to environmental and developmental stimuli. The need for such information is demonstrated by the many lists of transcription factors that have been produced over the past decade. The COST Action Gene Regulation Ensemble Effort for the Knowledge Commons (GREEKC) Consortium brought together experts in the field of transcription with the aim of providing high quality and interoperable gene regulatory data. The Gene Ontology (GO) Consortium provides strict definitions for gene product function, including factors that regulate transcription. The collaboration between the GREEKC and GO Consortia has enabled the application of those definitions to produce a new curated catalogue of over 1400 human DNA-binding transcription factors, that can be accessed at https://www.ebi.ac.uk/QuickGO/targetset/dbTF. This catalogue has facilitated an improvement in the GO annotation of human DNA-binding transcription factors and led to the GO annotation of almost sixty thousand DNA-binding transcription factors in over a hundred species. Thus, this work will aid researchers investigating the regulation of transcription in both biomedical and basic science.

TAZ maintains telomere length in TNBC cells by mediating Rad51C expression

Background

Telomere maintenance is crucial for the unlimited proliferation of cancer cells and essential for the “stemness” of multiple cancer cells. TAZ is more extensively expressed in triple negative breast cancers (TNBC) than in other types of breast cancers, and promotes proliferation, transformation and EMT of cancer cells. It was reported that TAZ renders breast cancer cells with cancer stem cell features. However, whether TAZ regulates telomeres is still unclear. In this study, we explored the roles of TAZ in the regulation of telomere maintenance in TNBC cells.

Methods

siRNA and shRNA was used to generate TAZ-depleted TNBC cell lines. qPCR and Southern analysis of terminal restriction fragments techniques were used to test telomere length. Co-immunoprecipitation, Western blotting, immunofluorescence, Luciferase reporter assay and Chromatin-IP were conducted to investigate the underlying mechanism.

Results

By knocking down the expression of TAZ in TNBC cells, we found, for the first time, that TAZ is essential for the maintenance of telomeres in TNBC cells. Moreover, loss of TAZ causes senescence phenotype of TNBC cells. The observed extremely shortened telomeres in late passages of TAZ knocked down cells correlate with an elevated hTERT expression, reductions of shelterin proteins, and an activated DNA damage response pathway. Our data also showed that depletion of TAZ results in overexpression of TERRAs, which are a group of telomeric repeat‐containing RNAs and regulate telomere length and integrity. Furthermore, we discovered that TAZ maintains telomere length of TNBC cells likely by facilitating the expression of Rad51C, a crucial element of homologous recombination pathway that promotes telomere replication.

Conclusions

This study supports the notion that TAZ is an oncogenic factor in TNBC, and further reveals a novel telomere-related pathway that is employed by TAZ to regulate TNBC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01466-z.

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.

Shaping human telomeres: from shelterin and CST complexes to telomeric chromatin organization

The regulation of telomere length in mammals is crucial for chromosome end-capping and thus for maintaining genome stability and cellular lifespan. This process requires coordination between telomeric protein complexes and the ribonucleoprotein telomerase, which extends the telomeric DNA. Telomeric proteins modulate telomere architecture, recruit telomerase to accessible telomeres and orchestrate the conversion of the newly synthesized telomeric single-stranded DNA tail into double-stranded DNA. Dysfunctional telomere maintenance leads to telomere shortening, which causes human diseases including bone marrow failure, premature ageing and cancer. Recent studies provide new insights into telomerase-related interactions (the 'telomere replisome') and reveal new challenges for future telomere structural biology endeavours owing to the dynamic nature of telomere architecture and the great number of structures that telomeres form. In this Review, we discuss recently determined structures of the shelterin and CTC1-STN1-TEN1 (CST) complexes, how they may participate in the regulation of telomere replication and chromosome end-capping, and how disease-causing mutations in their encoding genes may affect specific functions. Major outstanding questions in the field include how all of the telomere components assemble relative to each other and how the switching between different telomere structures is achieved.

Pancreatic adenocarcinoma: molecular drivers and the role of targeted therapy

Prognosis from pancreatic ductal adenocarcinoma (PDAC) continues to be poor despite the many efforts channeled to improve its management. Although the mainstay treatment is still traditional chemotherapy, recent advances highlighted a promising potential for targeted therapy in the management of this disease. Those advances emphasize the significance of timely genomic profiling of tumor tissue as well as germline testing of patients to identify potential markers of targeted therapy. While targeted therapy is reserved for a relatively small subset of patients with PDAC, ongoing research is uncovering additional markers, and targeted agents, that will hopefully translate to better outcomes for patients.

Long interspersed element-1 ribonucleoprotein particles protect telomeric ends in alternative lengthening of telomeres dependent cells

Malignant cells ensure telomere maintenance by the alternative lengthening of telomeres (ALT) in the absence of telomerase activity (TA). The retrotransposons "long interspersed nuclear element-1" (LINE-1, L1) are expressed in malignant cells and are primarily known to contribute to complex karyotypes. Here we demonstrate that LINE-1 ribonucleoprotein particles (L1-RNPs) expression is significantly higher in ALT⁺- versus in TA⁺-human glioma. Analyzing a role of L1-RNP in ALT, we show that L1-RNPs bind to telomeric repeat containing RNA (TERRA), which is critical for telomere stabilization and which is overexpressed in ALT⁺ cells. In turn, L1-RNP knockdown (KD) abrogated the nuclear retention of TERRA, resulted in increased telomeric DNA damage, decreased cell growth and reduced expression of ALT characteristics such as c-circles and PML-bodies. L1-RNP KD also decreased the expression of Shelterin- and the ALT-regulating protein Topoisomerase IIIα (TopoIIIα) indicating a more general role of L1-RNPs in supporting telomeric integrity in ALT. Our findings suggest an impact of L1-RNP on telomere stability in ALT⁺ dependent tumor cells. As L1-RNPs are rarely expressed in normal adult human tissue those elements might serve as a novel target for tumor ablative therapy.

Association of newly identified genetic variant rs2853677 of TERT with non-small cell lung cancer and leukemia in population of Jammu and Kashmir, India

BACKGROUND

Telomere genetics has recently been emerged as an important field in molecular oncology. Various genome-wide association studies in different population groups have revealed that polymorphisms in Telomere maintenance gene (TERT) gene located on 5p15.33 is associated with susceptibility to leukemia and lung cancer risk. However, association of TERT with leukemia and lung cancer risk in north Indian population groups is still unknown. This study observed the association between genetic variant rs2853677 of TERT and leukemia and lung cancer in the state of Jammu and Kashmir, India.

METHODS

A total of 781 subjects, out of which 381 cases (203 leukemic patients and 178 non-small cell lung cancer patients NSCLC) and 400 healthy controls were recruited for the study. Genetic variant rs2853677of TERT was detected using the real-time and Taqman Chemistry. Hardy-Weinberg Equilibrium was assessed using the chi square test. The allele and genotype- specific risks were estimated as odds ratio with 95% confidence interval.

RESULTS

We observed that variant rs2853677 was strongly associated with lung cancer and leukemia risk with an odds ratio (OR) =1.8 (1.03-3.2 at 95% CI); p value (adjusted) = 0.03; odds ratio (OR) =2.9 (1.4-5.5.at 95% CI); p value (adjusted) = 0.002, respectively.

CONCLUSION

The results of this study suggested that rs2853677 of TERT signifies association in multiple cancers and suggests that it can become potential marker for diagnosis of non-small cell lung cancer and leukemia. The study will provide an insight in understanding the genetic etiology and highlights the role of telomere-associated pathways in non-small cell lung cancer and leukemia. However, it would be quite interesting to explore the contribution of this variant in other cancers as well.

Emerging role of nutrition and the non-coding landscape in type 2 diabetes mellitus: A review of literature

With the advent of recent advances in molecular techniques and whole genome sequencing, we have come to know that the non-coding landscape (including non-coding RNAs, tRNAs and even telomeres) plays a major role in the regulation of cellular processes. Furthermore, the deregulation of this landscape has been found to contribute to and even bring about the pathogenesis of a large number of diseases. One of such diseases is diabetes mellitus (type 2 specifically) whose incidence rate and global burden is constantly increasing. Nutrition has been proven to be a key player in the development, onset and control of type 2 diabetes mellitus. Additionally, non-coding DNA based molecular markers are emerging as biomarkers of T2D, susceptibility, and perhaps dietary supplements can modulate non-coding DNA based markers expression and function in T2D management. In this review, we provide a brief overview of the developmental origins and genetics of type 2 diabetes mellitus, how each component of the non-coding landscape contributes to the development and progression of the disease and finally we discuss how dietary interventions modulate the non-coding landscape in T2D.

Oligomer formation and G-quadruplex binding by purified murine Rif1 protein, a key organizer of higher-order chromatin architecture

Rap1-interacting protein 1 (Rif1) regulates telomere length in budding yeast. We previously reported that, in metazoans and fission yeast, Rif1 also plays pivotal roles in controlling genome-wide DNA replication timing. We proposed that Rif1 may assemble chromatin compartments that contain specific replication-timing domains by promoting chromatin loop formation. Rif1 also is involved in DNA lesion repair, restart after replication fork collapse, anti-apoptosis activities, replicative senescence, and transcriptional regulation. Although multiple physiological functions of Rif1 have been characterized, biochemical and structural information on mammalian Rif1 is limited, mainly because of difficulties in purifying the full-length protein. Here, we expressed and purified the 2418-amino-acid-long, full-length murine Rif1 as well as its partially truncated variants in human 293T cells. Hydrodynamic analyses indicated that Rif1 forms elongated or extended homo-oligomers in solution, consistent with the presence of a HEAT-type helical repeat segment known to adopt an elongated shape. We also observed that the purified murine Rif1 bound G-quadruplex (G4) DNA with high specificity and affinity, as was previously shown for Rif1 from fission yeast. Both the N-terminal (HEAT-repeat) and C-terminal segments were involved in oligomer formation and specifically bound G4 DNA, and the central intrinsically disordered polypeptide segment increased the affinity for G4. Of note, pulldown assays revealed that Rif1 simultaneously binds multiple G4 molecules. Our findings support a model in which Rif1 modulates chromatin loop structures through binding to multiple G4 assemblies and by holding chromatin fibers together.

Telomere Biology and Thoracic Aortic Aneurysm

Ascending aortic aneurysms are mostly asymptomatic and present a great risk of aortic dissection or perforation. Consequently, ascending aortic aneurysms are a source of lethality with increased age. Biological aging results in progressive attrition of telomeres, which are the repetitive DNA sequences at the end of chromosomes. These telomeres play an important role in protection of genomic DNA from end-to-end fusions. Telomere maintenance and telomere attrition-associated senescence of endothelial and smooth muscle cells have been indicated to be part of the pathogenesis of degenerative vascular diseases. This systematic review provides an overview of telomeres, telomere-associated proteins and telomerase to the formation and progression of aneurysms of the thoracic ascending aorta. A better understanding of telomere regulation in the vascular pathology might provide new therapeutic approaches. Measurements of telomere length and telomerase activity could be potential prognostic biomarkers for increased risk of death in elderly patients suffering from an aortic aneurysm.

PRL-3 promotes telomere deprotection and chromosomal instability

Phosphatase of regenerating liver (PRL-3) promotes cell invasiveness, but its role in genomic integrity remains unknown. We report here that shelterin component RAP1 mediates association between PRL-3 and TRF2. In addition, TRF2 and RAP1 assist recruitment of PRL-3 to telomeric DNA. Silencing of PRL-3 in colon cancer cells does not affect telomere integrity or chromosomal stability, but induces reactive oxygen species-dependent DNA damage response and senescence. However, overexpression of PRL-3 in colon cancer cells and primary fibroblasts promotes structural abnormalities of telomeres, telomere deprotection, DNA damage response, chromosomal instability and senescence. Furthermore, PRL-3 dissociates RAP1 and TRF2 from telomeric DNA in vitro and in cells. PRL-3-promoted telomere deprotection, DNA damage response and senescence are counteracted by disruption of PRL-3–RAP1 complex or expression of ectopic TRF2. Examination of clinical samples showed that PRL-3 status positively correlates with telomere deprotection and senescence. PRL-3 transgenic mice exhibit hallmarks of telomere deprotection and senescence and are susceptible to dextran sodium sulfate-induced colon malignancy. Our results uncover a novel role of PRL-3 in tumor development through its adverse impact on telomere homeostasis.

Telomere Recognition and Assembly Mechanism of Mammalian Shelterin

Shelterin is a six-subunit protein complex that plays crucial roles in telomere length regulation, protection, and maintenance. Although several shelterin subunits have been studied in vitro, the biochemical properties of the fully assembled shelterin complex are not well defined. Here, we characterize shelterin using ensemble biochemical methods, electron microscopy, and single-molecule imaging to determine how shelterin recognizes and assembles onto telomeric repeats. We show that shelterin complexes can exist in solution and primarily locate telomeric DNA through a three-dimensional diffusive search. Shelterin can diffuse along non-telomeric DNA but is impeded by nucleosomes, arguing against extensive one-dimensional diffusion as a viable assembly mechanism. Our work supports a model in which individual shelterin complexes rapidly bind to telomeric repeats as independent functional units, which do not alter the DNA-binding mode of neighboring complexes but, rather, occupy telomeric DNA in a "beads on a string" configuration.

Reconstitution of human shelterin complexes reveals unexpected stoichiometry and dual pathways to enhance telomerase processivity

The human shelterin proteins associate with telomeric DNA to confer telomere protection and length regulation. They are thought to form higher-order protein complexes for their functions, but studies of shelterin proteins have been mostly limited to pairs of proteins. Here we co-express various human shelterin proteins and find that they form defined multi-subunit complexes. A complex harboring both TRF2 and POT1 has the strongest binding affinity to telomeric DNA substrates comprised of double-stranded DNA with a 3′ single-stranded extension. TRF2 interacts with TIN2 with an unexpected 2:1 stoichiometry in the context of shelterin (RAP1₂:TRF2₂:TIN2₁:TPP1₁:POT1₁). Tethering of TPP1 to the telomere either via TRF2–TIN2 or via POT1 gives equivalent enhancement of telomerase processivity. We also identify a peptide region from TPP1 that is both critical and sufficient for TIN2 interaction. Our findings reveal new information about the architecture of human shelterin and how it performs its functions at telomeres.

The human shelterin complex protects telomere ends from being recognized as damaged DNA sites and regulates telomere length in conjunction with telomerase. Here the authors establish the stoichiometries of human shelterin complexes of various compositions and show shelterin provides dual pathways to stimulate telomerase processivity.

Inhibitors of telomerase and poly(ADP-ribose) polymerases synergize to limit the lifespan of pancreatic cancer cells

Imetelstat (GRN163L) is a potent and selective inhibitor of telomerase. We have previously reported that GRN163L could shorten telomeres and limit the lifespan of CD18/HPAF and CAPAN1 pancreatic cancer cells. Here, we examined the effects of GRN163L on two other pancreatic cancer cell lines: AsPC1 and L3.6pl. In both lines, chronic exposure to GRN163L led to an initial shortening of telomeres followed by a stabilization of extremely short telomeres. In AsPC1 cells, telomere attrition eventually led to the induction of crisis and the loss of the treated population. In L3.6pl cells, crisis was transient and followed by the emergence of GRN163L-resistant cells, which could grow at increasing concentrations of GRN163L. The Shelterin complex is a telomere-associated complex that limits the access of telomerase to telomeres. The telomerase inhibitory function of this complex can be enhanced by drugs that block the poly(ADP-ribosyl)ation of its TRF1 and/or TRF2 subunits. Combined treatment of the GRN163L-resistant L3.6pl cells with GRN163L and 3-aminobenzamide (3AB), a general inhibitor of poly(ADP-ribose) polymerases, led to additional telomere shortening and limited the lifespan of the resistant cells. Results from this work suggest that inhibitors of telomerase and poly(ADP-ribose) polymerases can cooperate to limit the lifespan of pancreatic cancer cells.

Role of telomeres and associated maintenance genes in Type 2 Diabetes Mellitus: A review

Type 2 Diabetes Mellitus (T2DM), a multifactorial complex disorder, is emerging as a major cause of morbidity, mortality and socio-economic burden across the world. Despite huge efforts in understanding genetics of T2DM, only ∼10% of the genetic factors have been identified so far. Telomere attrition, a natural phenomenon has recently emerged in understanding the pathophysiology of T2DM. It has been indicated that Telomeres and associated pathways might be the critical components in the disease etiology, though the mechanism(s) involved are not clear. Recent Genome Wide (GWAS) and Candidate Gene Case-Control Association Studies have also indicated an association of Telomere and associated pathways related genes with T2DM. Single Nucleotide Polymorphisms (SNPs) in the telomere maintenance genes: TERT, TERC, TNKS, CSNK2A2, TEP1, ACD, TRF1 and TRF2, have shown strong association with telomere attrition in T2DM and its pathophysiology, in these studies. However, the assessment has been made within limited ethnicities (Caucasians, Han Chinese cohort and Punjabi Sikhs from South Asia), warranting the study of such associations in different ethnic groups. Here, we propose the possible mechanisms, in the light of existing knowledge, to understand the association of T2DM with telomeres and associated pathways.

Influence of exposure to pesticides on telomere length in tobacco farmers: A biology system approach

Various pesticides in the form of mixtures must be used to keep tobacco crops pest-free. Recent studies have shown a link between occupational exposure to pesticides in tobacco crops and increased damage to the DNA, mononuclei, nuclear buds and binucleated cells in buccal cells as well as micronuclei in lymphocytes. Furthermore, pesticides used specifically for tobacco crops shorten telomere length (TL) significantly. However, the molecular mechanism of pesticide action on telomere length is not fully understood. Our study evaluated the interaction between a complex mixture of chemical compounds (tobacco cultivation pesticides plus nicotine) and proteins associated with maintaining TL, as well as the biological processes involved in this exposure by System Biology tools to provide insight regarding the influence of pesticide exposure on TL maintenance in tobacco farmers. Our analysis showed that one cluster was associated with TL proteins that act in bioprocesses such as (i) telomere maintenance via telomere lengthening; (ii) senescence; (iii) age-dependent telomere shortening; (iv) DNA repair (v) cellular response to stress and (vi) regulation of proteasome ubiquitin-dependent protein catabolic process. We also describe how pesticides and nicotine regulate telomere length. In addition, pesticides inhibit the ubiquitin proteasome system (UPS) and consequently increase proteins of the shelterin complex, avoiding the access of telomerase in telomere and, nicotine activates UPS mechanisms and promotes the degradation of human telomerase reverse transcriptase (hTERT), decreasing telomerase activity.

Downregulation of Ubiquitin-conjugating Enzyme UBE2D3 Promotes Telomere Maintenance and Radioresistance of Eca-109 Human Esophageal Carcinoma Cells

Ubiquitin-conjugating enzyme UBE2D3 is an important member of the ubiquitin-proteasome pathways. Our previous study showed that the expression of UBE2D3 was negatively related to human telomerase reverse transcriptase (hTERT) and radioresistance in human breast cancer cells. However, in esophageal carcinoma, the exact effects and mechanisms of UBE2D3 in radioresistance remain unclear. This study shows that UBE2D3 knockdown was associated with significant increases in radioresistance to X-rays, telomerase activity, telomere length, and telomere shelterins. UBE2D3 knockdown-mediated radioresistance was related to a decrease in the spontaneous and ionizing radiation-induced apoptosis, resulting from a decrease in the Bax/Bcl-2 ratio. Furthermore, UBE2D3 downregulation was associated with increased G1-S phase transition and prolonged IR-induced G2/M arrest through over expression of cyclin D1, decrease of CDC25A expression and promotion of the ATM/ATR-Chk1-CDC25C pathway. Moreover, UBE2D3 downregulation reduced spontaneous DNA double-strand breaks and accelerated the repair of DNA damage induced by IR. The current data thus demonstrate that UBE2D3 downregulation enhances radioresistance by increased telomere homeostasis and prolonged IR-induced G2/M arrest, but decreases the IR-induced apoptosis and the number of DNA damage foci. These results suggest that UBE2D3 might be a potential molecular target to improve radiotherapy effects in esophageal carcinoma.

The OB-fold domain 1 of human POT1 recognizes both telomeric and non-telomeric DNA motifs

The POT1 protein plays a critical role in telomere protection and telomerase regulation. POT1 binds single-stranded 5'-TTAGGGTTAG-3' and forms a dimer with the TPP1 protein. The dimer is recruited to telomeres, either directly or as part of the Shelterin complex. Human POT1 contains two Oligonucleotide/Oligosaccharide Binding (OB) fold domains, OB1 and OB2, which make physical contact with the DNA. OB1 recognizes 5'-TTAGGG whereas OB2 binds to the downstream TTAG-3'. Studies of POT1 proteins from other species have shown that some of these proteins are able to recognize a broader variety of DNA ligands than expected. To explore this possibility in humans, we have used SELEX to reexamine the sequence-specificity of the protein. Using human POT1 as a selection matrix, high-affinity DNA ligands were selected from a pool of randomized single-stranded oligonucleotides. After six successive rounds of selection, two classes of high-affinity targets were obtained. The first class was composed of oligonucleotides containing a cognate POT1 binding sites (5'-TTAGGGTTAG-3'). The second and more abundant class was made of molecules that carried a novel non-telomeric consensus: 5'-TNCANNAGKKKTTAGG-3' (where K = G/T and N = any base). Binding studies showed that these non-telomeric sites were made of an OB1-binding motif (TTAGG) and a non-telomeric motif (NT motif), with the two motifs recognized by distinct regions of the OB1 domain. POT1 interacted with these non-telomeric binding sites with high affinity and specificity, even when bound to its dimerization partner TPP1. This intrinsic ability of POT1 to recognize NT motifs raises the possibility that the protein may fulfill additional functions at certain non-telomeric locations of the genome, in perhaps gene transcription, replication, or repair.

LINE-1 induces hTERT and ensures telomere maintenance in tumour cell lines

A hallmark of cancer cells is an activated telomere maintenance mechanism, which allows prolonged survival of the malignant cells. In more than 80% of tumours, telomeres are elongated by the enzyme telomerase, which adds de novo telomere repeats to the ends of chromosomes. Cancer cells are also characterized by expression of active LINE-1 elements (L1s, long interspersed nuclear elements-1). L1 elements are abundant retrotransposons in the eukaryotic genome that are primarily known for facilitating aberrant recombination. Using L1-knockdown (KD), we show for the first time that L1 is critical for telomere maintenance in telomerase-positive tumour cells. The reduced length of telomeres in the L1-KD-treated cells correlated with an increased rate of telomere dysfunction foci, a reduced expression of shelterin proteins and an increased rate of anaphase bridges. The decreased telomere length was associated with a decreased telomerase activity and decreased telomerase mRNA level; the latter was increased upon L1 overexpression. L1-KD also led to a decrease in mRNA and protein expression of cMyc and KLF-4, two main transcription factors of telomerase and altered mRNA levels of other stem-cell-associated proteins such as CD44 and hMyb, as well as a corresponding reduced growth of spheroids. The KD of KLF-4 or cMyc decreased the level of L1-ORF1 mRNA, suggesting a specific reciprocal regulation with L1. Thus, our findings contribute to the understanding of L1 as a pathogenicity factor in cancer cells. As L1 is only expressed in pathophysiological conditions, L1 now appears to be target in the rational treatment of telomerase-positive cancer.

Telomerase inhibitor Imetelstat (GRN163L) limits the lifespan of human pancreatic cancer cells

Telomerase is required for the unlimited lifespan of cancer cells. The vast majority of pancreatic adenocarcinomas overexpress telomerase activity and blocking telomerase could limit their lifespan. GRN163L (Imetelstat) is a lipid-conjugated N3'→P5' thio-phosphoramidate oligonucleotide that blocks the template region of telomerase. The aim of this study was to define the effects of long-term GRN163L exposure on the maintenance of telomeres and lifespan of pancreatic cancer cells. Telomere size, telomerase activity, and telomerase inhibition response to GRN163L were measured in a panel of 10 pancreatic cancer cell lines. The cell lines exhibited large differences in levels of telomerase activity (46-fold variation), but most lines had very short telomeres (2-3 kb in size). GRN163L inhibited telomerase in all 10 pancreatic cancer cell lines, with IC50 ranging from 50 nM to 200 nM. Continuous GRN163L exposure of CAPAN1 (IC50 = 75 nM) and CD18 cells (IC50 = 204 nM) resulted in an initial rapid shortening of the telomeres followed by the maintenance of extremely short but stable telomeres. Continuous exposure to the drug eventually led to crisis and to a complete loss of viability after 47 (CAPAN1) and 69 (CD18) doublings. Crisis In these cells was accompanied by activation of a DNA damage response (γ-H2AX) and evidence of both senescence (SA-β-galactosidase activity) and apoptosis (sub-G1 DNA content, PARP cleavage). Removal of the drug after long-term GRN163L exposure led to a reactivation of telomerase and re-elongation of telomeres in the third week of cultivation without GRN163L. These findings show that the lifespan of pancreatic cancer cells can be limited by continuous telomerase inhibition. These results should facilitate the design of future clinical trials of GRN163L in patients with pancreatic cancer.

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.

G-quadruplexes incorporating modified constituents: a review

This review summarizes the results of structural studies carried out with analogs of G-quadruplexes built from natural nucleotides. Several dozens of base-, sugar-, and phosphate derivatives of the biological building blocks have been incorporated into more than 50 potentially quadruplex forming DNA and RNA oligonucleotides and the stability and folding topology of the resultant intramolecular, bimolecular and tetramolecular architectures characterized. The TG4T, TG5T, the 15 nucleotide-long thrombin binding aptamer, and the human telomere repeat AG3(TTAG3)3 sequences were modified in most cases, and four guanine analogs can be noted as being particularly useful in structural studies. These are the fluorescent 2-aminopurine, the 8-bromo-, and 8-methylguanines, and the hypoxanthine. The latter three analogs stabilize a given fold in a mixture of structures making possible accurate structural determinations by circular dichroism and nuclear magnetic resonance measurements.