Telosome, a mammalian telomere-associated complex formed by multiple telomeric proteins

Initiation of Pulmonary Fibrosis after Silica Inhalation in Rats is linked with Dysfunctional Shelterin Complex and DNA Damage Response

Occupational exposure to silica has been observed to cause pulmonary fibrosis and lung cancer through complex mechanisms. Telomeres, the nucleoprotein structures with repetitive (TTAGGG) sequences at the end of chromosomes, are a molecular "clock of life", and alterations are associated with chronic disease. The shelterin complex (POT1, TRF1, TRF2, Tin2, Rap1, and POT1 and TPP1) plays an important role in maintaining telomere length and integrity, and any alteration in telomeres may activate DNA damage response (DDR) machinery resulting in telomere attrition. The goal of this study was to assess the effect of silica exposure on the regulation of the shelterin complex in an animal model. Male Fisher 344 rats were exposed by inhalation to Min-U-Sil 5 silica for 3, 6, or 12 wk at a concentration of 15 mg/m3 for 6 hr/d for 5 consecutive d/wk. Expression of shelterin complex genes was assessed in the lungs at 16 hr after the end of each exposure. Also, the relationship between increased DNA damage protein (γH2AX) and expression of silica-induced fibrotic marker, αSMA, was evaluated. Our findings reveal new information about the dysregulation of shelterin complex after silica inhalation in rats, and how this pathway may lead to the initiation of silica-induced pulmonary fibrosis.

The Genomic Health of Human Pluripotent Stem Cells: Genomic Instability and the Consequences on Nuclear Organization

Human pluripotent stem cells (hPSCs) are increasingly used for cell-based regenerative therapies worldwide, with embryonic and induced pluripotent stem cells as potential treatments for debilitating and chronic conditions, such as age-related macular degeneration, Parkinson's disease, spinal cord injuries, and type 1 diabetes. However, with the level of genomic anomalies stem cells generate in culture, their safety may be in question. Specifically, hPSCs frequently acquire chromosomal abnormalities, often with gains or losses of whole chromosomes. This review discusses how important it is to efficiently and sensitively detect hPSC aneuploidies, to understand how these aneuploidies arise, consider the consequences for the cell, and indeed the individual to whom aneuploid cells may be administered.

Targeting Telomerase and ATRX/DAXX Inducing Tumor Senescence and Apoptosis in the Malignant Glioma

Glioblastoma multiforme (GBM) is a type of brain tumor that is notorious for its aggressiveness and invasiveness, and the complete removal of GBM is still not possible, even with advanced diagnostic strategies and extensive therapeutic plans. Its dismal prognosis and short survival time after diagnosis make it a crucial public health issue. Understanding the molecular mechanisms underlying GBM may inspire novel and effective treatments against this type of cancer. At a molecular level, almost all tumor cells exhibit telomerase activity (TA), which is a major means by which they achieve immortalization. Further studies show that promoter mutations are associated with increased TA and stable telomere length. Moreover, some tumors and immortalized cells maintain their telomeres with a telomerase-independent mechanism termed the “alternative lengthening of telomeres” (ALT), which relates to the mutations of the α-thalassemia/mental retardation syndrome X-linked protein (ATRX), the death-domain associated protein (DAXX) and H3.3. By means of the mutations of the telomerase reverse transcriptase (TERT) promoter and ATRX/DAXX, cancers can immortalize and escape cell senescence and apoptosis. In this article, we review the evidence for triggering GBM cell death by targeting telomerase and the ALT pathway, with an extra focus on a plant-derived compound, butylidene phthalide (BP), which may be a promising novel anticancer compound with good potential for clinical applications.

High-throughput gene expression analysis identifies p53-dependent and -independent pathways contributing to the adrenocortical dysplasia (acd) phenotype

In mammalian cells TPP1, encoded by the Acd gene, is a key component of the shelterin complex, which is required for telomere length maintenance and telomere protection. In mice, a hypomorphic mutation in Acd causes the adrenocortical dysplasia (acd) phenotype, which includes limb and body axis anomalies, and perinatal lethality. p53 deficiency partially rescues limb and body axis anomalies in acd mutant embryos, but not perinatal lethality, implicating p53-independent mechanisms in the acd phenotype. Loss of function of most shelterin proteins results in early embryonic lethality. Thus, study of the hypomorphic acd allele provides a unique opportunity to understand telomere dysfunction at an organismal level. The aim of this study was to identify transcriptome alterations in acd mutant and acd, p53 double mutant embryos to understand the p53-dependent and -independent factors that contribute to the mutant phenotypes in the context of the whole organism. Genes involved in developmental processes, cell cycle, metabolic pathways, tight junctions, axon guidance and signaling pathways were regulated by p53-driven mechanisms in acd mutant embryos, while genes functioning in immune response, and RNA processing were altered independently of p53 in acd, p53 double mutant embryos. To our best of knowledge, this is the first study revealing detailed transcriptomic alterations, reflecting novel p53-dependent and -independent pathways contributing to the acd phenotype. Our data confirm the importance of cell cycle and DNA repair pathways, and suggest novel links between telomere dysfunction and immune system regulation and the splicing machinery. Given the broad applicability of telomere maintenance in growth, development, and genome stability, our data will also provide a rich resource for others studying telomere maintenance and DNA damage responses in mammalian model systems.

Biphasic recruitment of TRF2 to DNA damage sites promotes non-sister chromatid homologous recombination repair

TRF2 (TERF2) binds to telomeric repeats and is critical for telomere integrity. Evidence suggests that it also localizes to non-telomeric DNA damage sites. However, this recruitment appears to be precarious and functionally controversial. We find that TRF2 recruitment to damage sites occurs by a two-step mechanism: the initial rapid recruitment (phase I), and stable and prolonged association with damage sites (phase II). Phase I is poly(ADP-ribose) polymerase (PARP)-dependent and requires the N-terminal basic domain. The phase II recruitment requires the C-terminal MYB/SANT domain and the iDDR region in the hinge domain, which is mediated by the MRE11 complex and is stimulated by TERT. PARP-dependent recruitment of intrinsically disordered proteins contributes to transient displacement of TRF2 that separates two phases. TRF2 binds to I-PpoI-induced DNA double-strand break sites, which is enhanced by the presence of complex damage and is dependent on PARP and the MRE11 complex. TRF2 depletion affects non-sister chromatid homologous recombination repair, but not homologous recombination between sister chromatids or non-homologous end-joining pathways. Our results demonstrate a unique recruitment mechanism and function of TRF2 at non-telomeric DNA damage sites.

Shelterin-Mediated Telomere Protection

For more than a decade, it has been known that mammalian cells use shelterin to protect chromosome ends. Much progress has been made on the mechanism by which shelterin prevents telomeres from inadvertently activating DNA damage signaling and double-strand break (DSB) repair pathways. Shelterin averts activation of three DNA damage response enzymes [the ataxia-telangiectasia-mutated (ATM) and ataxia telangiectasia and Rad3-related (ATR) kinases and poly(ADP-ribose) polymerase 1 (PARP1)], blocks three DSB repair pathways [classical nonhomologous end joining (c-NHEJ), alternative (alt)-NHEJ, and homology-directed repair (HDR)], and prevents hyper-resection at telomeres. For several of these functions, mechanistic insights have emerged. In addition, much has been learned about how shelterin maintains the telomeric 3' overhang, forms and protects the t-loop structure, and promotes replication through telomeres. These studies revealed that shelterin is compartmentalized, with individual subunits dedicated to distinct aspects of the end-protection problem. This review focuses on the current knowledge of shelterin-mediated telomere protection, highlights differences between human and mouse shelterin, and discusses some of the questions that remain.

Interstitial telomeric loops and implications of the interaction between TRF2 and lamin A/C

The protein-DNA complexes that compose the end of mammalian chromosomes-telomeres-serve to stabilize linear genomic DNA and are involved in cellular and organismal aging. One mechanism that protects telomeres from premature degradation is the formation of structures called t-loops, in which the single-stranded 3' overhang present at the terminal end of telomeres loops back and invades medial double-stranded telomeric DNA. We identified looped structures formed between terminal chromosome ends and interstitial telomeric sequences (ITSs), which are found throughout the human genome, that we have termed interstitial telomeric loops (ITLs). While they form in a TRF2-dependent manner similar to t-loops, ITLs further require the physical interaction of TRF2 with the nuclear intermediate filament protein lamin A/C. Our findings suggest that interactions between telomeres and the nucleoskeleton broadly impact genomic integrity, including telomere stability, chromosome structure, and chromosome fragility. Here, we review the roles of TRF2 and lamin A/C in telomere biology and consider how their interaction may relate telomere homeostasis to cellular and organismal aging.

The C-Terminal Extension Unique to the Long Isoform of the Shelterin Component TIN2 Enhances Its Interaction with TRF2 in a Phosphorylation- and Dyskeratosis Congenita Cluster-Dependent Fashion

TIN2 is central to the shelterin complex, linking the telomeric proteins TRF1 and TRF2 with TPP1/POT1. Mutations in TINF2, which encodes TIN2, that are found in dyskeratosis congenita (DC) result in very short telomeres and cluster in a region shared by the two TIN2 isoforms, TIN2S (short) and TIN2L (long). Here we show that TIN2L, but not TIN2S, is phosphorylated. TRF2 interacts more with TIN2L than TIN2S, and both the DC cluster and phosphorylation promote this enhanced interaction. The binding of TIN2L, but not TIN2S, is affected by TRF2-F120, which is also required for TRF2's interaction with end processing factors such as Apollo. Conversely, TRF1 interacts more with TIN2S than with TIN2L. A DC-associated mutation further reduces TIN2L-TRF1, but not TIN2S-TRF1, interaction. Cells overexpressing TIN2L or phosphomimetic TIN2L are permissive to telomere elongation, whereas cells overexpressing TIN2S or phosphodead TIN2L are not. Telomere lengths are unchanged in cell lines in which TIN2L expression has been eliminated by clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated mutation. These results indicate that TIN2 isoforms are biochemically and functionally distinguishable and that shelterin composition could be fundamentally altered in patients with TINF2 mutations.

The expression of telomere-related proteins and DNA damage response and their association with telomere length in colorectal cancer in Saudi patients

Background

Colorectal cancer is the leading cause of cancer-related deaths in Saudi Arabia. Cancer has a multifactorial nature and can be described as a disease of altered gene expression. The profiling of gene expression has been used to identify cancer subtypes and to predict patients’ responsiveness. Telomere-associated proteins that regulate telomere biology are essential molecules in cancer development. Thus, the present study examined their contributions to colorectal cancer progression in Saudi patients.

Methods

The expression of hTERT, TRF1, TRF2, POT1, ATR, ATM, Chk1 and Chk2 were measured via real-time PCR in matched cancerous and adjacent tissues of CRC patients. The protein level of hTERT, TRF1, TRF2, ATR, ATM, Chk1 and Chk2 were measured using immunohistochemistry. A region of hTERT core promoter was sequenced via Sanger sequencing. Methylation of CTCF binding site was examined via methylation-specific PCR. Finally, the length of telomere was estimated using q-PCR.

Results

Our results showed that POT1, ATR, Chk1 and Chk2 show increased expression in CRC relative to the adjacent mucosa. The expression levels of each gene were associated with clinicopathological characteristics of patients with CRC. There was a positive correlation between the age of the patients and hTERT expression. Regarding tumor site, telomere length, ATR, ATM and Chk1 were shown to be altered. No somatic mutation was detected in hTERT core promoter, and no differences in methylation patterns at CTCF binding site in the promoter between normal and cancer tissues.

Conclusion

Analysis of targeted genes expression in colorectal cancer based on the clinical variables revealed that tumor location and age could have a role in gene expression and telomere length variations and this could be taken under consideration during CRC diagnosis and therapy. Other epigenetic mechanisms could influence hTERT expression in cancers. Our findings warrant further validation through experiments involving a larger number of patients.

What I got wrong about shelterin

The ASBMB 2018 Bert and Natalie Vallee award in Biomedical Sciences honors our work on shelterin, a protein complex that helps cells distinguish the chromosome ends from sites of DNA damage. Shelterin protects telomeres from all aspects of the DNA damage response, including ATM and ATR serine/threonine kinase signaling and several forms of double-strand break repair. Today, this six-subunit protein complex could easily be identified in one single proteomics step. But, it took us more than 15 years to piece together the entire shelterin complex, one protein at a time. Although we did a lot of things right, here I tell the story of shelterin's discovery with an emphasis on the things that I got wrong along the way.

Shwachman-Diamond Syndrome Protein SBDS Maintains Human Telomeres by Regulating Telomerase Recruitment

Shwachman-Diamond syndrome (SDS) is a rare pediatric disease characterized by various systemic disorders, including hematopoietic dysfunction. The mutation of Shwachman-Bodian-Diamond syndrome (SBDS) gene has been proposed to be a major causative reason for SDS. Although SBDS patients were reported to have shorter telomere length in granulocytes, the underlying mechanism is still unclear. Here we provide data to elucidate the role of SBDS in telomere protection. We demonstrate that SBDS deficiency leads to telomere shortening. We found that overexpression of disease-associated SBDS mutants or knockdown of SBDS hampered the recruitment of telomerase onto telomeres, while the overall reverse transcriptase activity of telomerase remained unaffected. Moreover, we show that SBDS could specifically bind to TPP1 during the S phase of cell cycle, likely functioning as a stabilizer for TPP1-telomerase interaction. Our findings suggest that SBDS is a telomere-protecting protein that participates in regulating telomerase recruitment.

The structurally similar TRFH domain of TRF1 and TRF2 dimers shows distinct behaviour towards TIN2

The telomere repeat binding-factor 1 and 2 (TRF1 and TRF2) proteins of the shelterin complex bind to duplex telomeric DNA as homodimers, and the homodimerization is mediated by their TRFH (TRF-homology) domains. We performed molecular dynamic (MD) simulations of the dimer forms of TRF1_TRFH and TRF2_TRFH in the presence/absence of the TIN2_TBM (TIN2, TRF-interacting nuclear protein 2, TBM, TRF-binding motif) peptide. The MD results suggest that TIN2_TBM is necessary to ensure the stability of TRF1_TRFH homodimer but not the TRF2_TRFH homodimer. In TRF1-TIN2-TRF2 complex, the peptide enhances the protein-protein interactions to yield a stable heterodimer. Both monomers in TRF1_TRFH homodimer interact almost equally with the peptide, whereas in TRF2_TRFH homodimer, monomer TRF2_TRFH(M1) exhibits more dominant interactions than the TRF2_TRFH(M2). The common residues of TRF1/2_TRFH(M1) that form interactions with TIN2_TBM in all peptide-bound systems originate from the H3 (helix) and L3 (loop) regions. Additionally, in the homodimer systems, residues of TRF1/2_TRFH(M2) also interact with the peptide. The residue pair E71-K213 is responsible for different conformations of TRF1_TRFH homodimers; specifically, this residue pair enhances the protein-peptide/protein interactions in peptide-bound/unbound systems, respectively. TRF1_TRFH and TRF2_TRFH proteins have a conserved but different interface responsible for the protein-protein/peptide interactions that exist in the corresponding dimers.

Functional Analysis of Human Hub Proteins and Their Interactors Involved in the Intrinsic Disorder-Enriched Interactions

Some of the intrinsically disordered proteins and protein regions are promiscuous interactors that are involved in one-to-many and many-to-one binding. Several studies have analyzed enrichment of intrinsic disorder among the promiscuous hub proteins. We extended these works by providing a detailed functional characterization of the disorder-enriched hub protein-protein interactions (PPIs), including both hubs and their interactors, and by analyzing their enrichment among disease-associated proteins. We focused on the human interactome, given its high degree of completeness and relevance to the analysis of the disease-linked proteins. We quantified and investigated numerous functional and structural characteristics of the disorder-enriched hub PPIs, including protein binding, structural stability, evolutionary conservation, several categories of functional sites, and presence of over twenty types of posttranslational modifications (PTMs). We showed that the disorder-enriched hub PPIs have a significantly enlarged number of disordered protein binding regions and long intrinsically disordered regions. They also include high numbers of targeting, catalytic, and many types of PTM sites. We empirically demonstrated that these hub PPIs are significantly enriched among 11 out of 18 considered classes of human diseases that are associated with at least 100 human proteins. Finally, we also illustrated how over a dozen specific human hubs utilize intrinsic disorder for their promiscuous PPIs.

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.

Inhibition of TRF1 Telomere Protein Impairs Tumor Initiation and Progression in Glioblastoma Mouse Models and Patient-Derived Xenografts

Glioblastoma multiforme (GBM) is a deadly and common brain tumor. Poor prognosis is linked to high proliferation and cell heterogeneity, including glioma stem cells (GSCs). Telomere genes are frequently mutated. The telomere binding protein TRF1 is essential for telomere protection, and for adult and pluripotent stem cells. Here, we find TRF1 upregulation in mouse and human GBM. Brain-specific Trf1 genetic deletion in GBM mouse models inhibited GBM initiation and progression, increasing survival. Trf1 deletion increased telomeric DNA damage and reduced proliferation and stemness. TRF1 chemical inhibitors mimicked these effects in human GBM cells and also blocked tumor sphere formation and tumor growth in xenografts from patient-derived primary GSCs. Thus, targeting telomeres throughout TRF1 inhibition is an effective therapeutic strategy for GBM.

Telomere length heterogeneity in placenta revealed with high-resolution telomere length analysis

INTRODUCTION

Telomeres, are composed of tandem repeat sequences located at the ends of chromosomes and are required to maintain genomic stability. Telomeres can become shorter due to cell division and specific lifestyle factors. Critically shortened telomeres are linked to cellular dysfunction, senescence and aging. A number of studies have used low resolution techniques to assess telomere length in the placenta. In this study, we applied Single Telomere Length Analysis (STELA) which provides high-resolution chromosome specific telomere length profiles to ask whether we could obtain more detailed information on the length of individual telomeres in the placenta.

METHODS

Term placentas (37-42 weeks) were collected from women delivering at University Hospital of Wales or Royal Gwent Hospital within 2 h of delivery. Multiple telomere-length distributions were determined using STELA. Intraplacental variation of telomere length was analysed (N = 5). Telomere length distributions were compared between labouring (N = 10) and non-labouring (N = 11) participants. Finally, telomere length was compared between female (N = 17) and male (N = 20) placenta.

RESULTS

There were no significant influences of sampling site, mode of delivery or foetal sex on the telomere-length distributions obtained. The mean telomere length was 7.7 kb ranging from 5.0 kb to 11.7 kb across all samples (N = 42) and longer compared with other human tissues at birth. STELA also revealed considerable telomere length heterogeneity within samples.

CONCLUSIONS

We have shown that STELA can be used to study telomere length homeostasis in the placenta regardless of sampling site, mode of delivery and foetal sex. Moreover, as each amplicon is derived from a single telomeric molecule, from a single cell, STELA can reveal the full detail of telomere-length distributions, including telomeres within the length ranges observed in senescent cells. STELA thus provides a new tool to interrogate the relationship between telomere length and pregnancy complications linked to placental dysfunction.

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.

Telomere biology in aging and cancer: early history and perspectives

The ends of eukaryotic linear chromosomes are protected from undesired enzymatic activities by a nucleoprotein complex called the telomere. Expanding evidence indicates that telomeres have central functions in human aging and tumorigenesis. While it is undoubtedly important to follow current advances in telomere biology, it is also fruitful to be well informed in seminal historical studies for a comprehensive understanding of telomere biology, and for the anticipation of future directions. With this in mind, I here summarize the early history of telomere biology and current advances in the field, mostly focusing on mammalian studies relevant to aging and cancer.

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.

Polymorphisms in human telomerase reverse transcriptase (hTERT) gene, gene- gene and gene-smoking interaction with susceptibility to gastric cancer in Chinese Han population

Aims

To investigate the association of telomerase reverse transcriptase (TERT) gene polymorphisms and additional gene-gene and gene- environment interaction with gastric cancer (GC) risk.

Results

GC risk was significantly higher in carriers of G allele of rs2736100 than those with TT genotype (TG+ GG versus TT), adjusted OR (95%CI) =1.68 (1.26-2.17), and higher in carriers of G allele of rs2853669 than those with AA genotype (AG+ GG versus AA), adjusted OR (95%CI) = 1.72 (1.19-2.33). We also found that interaction between rs2736100 and smoking was associated with higher GC risk. Smokers with TG or GG of rs2736100 genotype have elevated GC risk, compared to never- smokers with TT of rs2736100 genotype, OR (95%CI) = 3.12 (1.82 -4.61). Pairwise linkage equilibrium (LD) analysis between SNPs was measured and the D’ value between rs2736100 and rs2736109 was more than 0.8. A haplotype containing the rs2736100- G and rs2736109- A alleles was associated with a statistically increased GC risk (OR= 2.66, 95%CI= 1.28 – 4.12, p<0.0001).

Materials and Methods

A total of 1088 participants (686 males, 402 females) were selected, including 360 GC patients and 728 normal participants. Logistic regression was performed to investigate association between single nucleotide polymorphisms (SNPs) within TERT gene and GC susceptibility. Generalized multifactor dimensionality reduction (GMDR) model was used to screen gene- gene and gene- environment interaction combinations.

Conclusions

We found that G allele of rs2736100 and G allele of rs2853669 in TERT gene, interaction between rs2736100 and smoking, and haplotype containing the rs2736100- G and rs2736109- A alleles were all associated with increased GC risk.

Expression of miR-23a induces telomere shortening and is associated with poor clinical outcomes in patients with coronary artery disease

Telomeric repeat binding factor (TRF) 2 (TRF2) plays an important role in telomere maintenance. miR-23a may directly inhibit TRF2 expression, thereby, inducing telomere shortening and cellular senescence. The present study aimed to determine whether miR-23a and TRF2 are expressed in patients with coronary artery disease (CAD), and whether pitavastatin might affect these levels. The present study included 104 patients with CAD and 50 controls. Patients with CAD were randomly divided into two subgroups (a moderate lipid lowering therapy (LLT) group and an aggressive LLT group). Peripheral blood mononuclear cells (PBMCs) were taken from patients with CAD and from controls at baseline and after 12 months. Levels of miR-23a were higher in the CAD group than in the controls. Levels of TRF2 protein were lower in the CAD group than in the controls. Our randomized clinical study showed that aggressive LLT decreased miR-23a and increased TRF2 levels, whereas moderate LLT generated no change in these levels. Our transfected cell model showed that miR-23a controlled TRF2 expression. After a mean follow-up of 339 days, cardiovascular events were associated with high miR-23a, low TRF2 or low relative telomere length. Multivariate analysis showed that levels of miR-23a (RR: 4.9, 95% CI: 1.9-14.3) were a strong predictor of cardiovascular events after adjustment for baseline characteristics. In conclusion, elevated levels of miR-23a play an important role in coronary atherosclerosis via down-regulated TRF2, and may provide important prognostic information in patients with CAD. Additionally, aggressive LLT may prevent telomere erosion via down-regulated miR-23a.

Mechanisms of DNA damage, repair, and mutagenesis

Living organisms are continuously exposed to a myriad of DNA damaging agents that can impact health and modulate disease-states. However, robust DNA repair and damage-bypass mechanisms faithfully protect the DNA by either removing or tolerating the damage to ensure an overall survival. Deviations in this fine-tuning are known to destabilize cellular metabolic homeostasis, as exemplified in diverse cancers where disruption or deregulation of DNA repair pathways results in genome instability. Because routinely used biological, physical and chemical agents impact human health, testing their genotoxicity and regulating their use have become important. In this introductory review, we will delineate mechanisms of DNA damage and the counteracting repair/tolerance pathways to provide insights into the molecular basis of genotoxicity in cells that lays the foundation for subsequent articles in this issue. Environ. Mol. Mutagen. 58:235-263, 2017. © 2017 Wiley Periodicals, Inc.

Structural and functional analysis of the human POT1-TPP1 telomeric complex

POT1 and TPP1 are part of the shelterin complex and are essential for telomere length regulation and maintenance. Naturally occurring mutations of the telomeric POT1-TPP1 complex are implicated in familial glioma, melanoma and chronic lymphocytic leukaemia. Here we report the atomic structure of the interacting portion of the human telomeric POT1-TPP1 complex and suggest how several of these mutations contribute to malignant cancer. The POT1 C-terminus (POT1C) forms a bilobal structure consisting of an OB-fold and a holiday junction resolvase domain. TPP1 consists of several loops and helices involved in extensive interactions with POT1C. Biochemical data shows that several of the cancer-associated mutations, partially disrupt the POT1-TPP1 complex, which affects its ability to bind telomeric DNA efficiently. A defective POT1-TPP1 complex leads to longer and fragile telomeres, which in turn promotes genomic instability and cancer.

Analysis of Telomere Proteins by Chromatin Immunoprecipitation (ChIP)

Telomere chromatin immunoprecipitation (ChIP) is an experimental method used to determine whether proteins are associated with telomere DNA inside the nuclei of cells and tissues. Telomere-associated proteins are first covalently crosslinked to telomere DNA, and then immunoprecipitated using an antibody specific for the protein of interest. This method has become one of the most indispensable tools for investigating the protein complexes that associate with telomeres.

Telomere Length Analysis by Quantitative Fluorescent in Situ Hybridization (Q-FISH)

Length is a functional parameter of telomeres, the nucleoprotein structures that protect chromosome ends. The availability of highly specific, high affinity probes for telomeric repeat sequences allowed the development of quantitative approaches aimed at measuring telomere length directly on chromosomes or in interphase nuclei. Here, we describe a general method for telomere quantitative FISH on metaphase chromosomes and discuss its most common applications in research.

TRF1 phosphorylation on T271 modulates telomerase-dependent telomere length maintenance as well as the formation of ALT-associated PML bodies

TRF1, a component of the shelterin complex, plays a key role in both telomerase-dependent telomere maintenance and alternative lengthening of telomeres, the latter also known as ALT. Characteristics of ALT cells include C-circles and ALT-associated PML bodies, referred to as APBs. The function of TRF1 is tightly regulated by post-translational modification including phosphorylation, however TRF1 phosphorylation sites have yet to be fully characterized. Here we report a novel TRF1 phosphorylation site threonine 271. We show that a nonphosphorylatable mutation of T271A impairs TRF1 binding to telomeric DNA in vivo and renders TRF1 defective in inhibiting telomerase-dependent telomere elongation. On the other hand, TRF1 carrying a phosphomimic mutation of T271D is competent in not only binding to telomeric DNA but also inhibiting telomerase-mediated telomere lengthening. These results suggest that TRF1 phosphorylation on T271 negatively regulates telomerase-mediated telomere maintenance. We find that in telomerase-negative ALT cells, TRF1 carrying either a T271A or T271D mutation is able to promote C-circle production but fails to support APB formation. These results suggest that TRF1 phosphorylation on T271 is necessary for APB formation but dispensable for C-circle production. These results further imply that APB formation can be mechanistically separated from C-circle production.

Determination of local chromatin composition by CasID

Chromatin structure and function are determined by a plethora of proteins whose genome-wide distribution is typically assessed by immunoprecipitation (ChIP). Here, we developed a novel tool to investigate the local chromatin environment at specific DNA sequences. We combined the programmable DNA binding of dCas9 with the promiscuous biotin ligase BirA* (CasID) to biotinylate proteins in the direct vicinity of specific loci. Subsequent streptavidin-mediated precipitation and mass spectrometry identified both known and previously unknown chromatin factors associated with repetitive telomeric, major satellite and minor satellite DNA. With super-resolution microscopy, we confirmed the localization of the putative transcription factor ZNF512 at chromocenters. The versatility of CasID facilitates the systematic elucidation of functional protein complexes and locus-specific chromatin composition.

Heregulin, a new regulator of telomere length in human cells

The growth factor heregulin (HRG) promotes breast cancer (BC) tumorigenesis and metastasis and differentially modulates BC cell responses to DNA-damaging agents via its dual extracellular and nuclear localization. Given the central role of telomere dysfunction to drive carcinogenesis and to alter the chemotherapeutic profile of transformed cells, we hypothesized that an unanticipated nuclear function of HRG might be to regulate telomere length. Engineered overexpression of the HRGβ2 isoform in non-aggressive, HRG-negative MCF-7 BC cells resulted in a significant shortening of telomeres (up to 1.3 kb) as measured by Southern blotting of telomere terminal restriction fragments. Conversely, antisense-mediated suppression of HRGβ2 in highly aggressive, HRG-overexpressing MDA-MB-231 and Hs578T cells increased telomere length up to 3.0 kb. HRGβ2 overexpression promoted a marked upregulation of telomere-binding protein 2 (TRF2) protein expression, whereas its knockdown profoundly decreased TRF2 expression. Double staining of endogenous HRGβ2 with telomere-specific peptide nucleic acid probe/fluorescence in situ hybridization (PNA/FISH) revealed the partial localization of HRG at the chromosome ends. Moreover, a predominantly nucleoplasmic staining pattern of endogenous HRGβ2 appeared to co-localize with TRF2 and, concomitantly with RAP1, a telomere regulator that specifically interacts with TRF2. Small interfering RNA-mediated knockdown of HRG decreased the expression of TRF2 and RAP1, decreased their presence at chromosome ends, and coincidentally resulted in the formation of longer telomeres. This study uncovers a new function for HRGβ2 in controlling telomere length, in part due to its ability to regulate and interact with the telomere-associated proteins TRF2 and RAP1.

Heregulin, a new interactor of the telosome/shelterin complex in human telomeres

Telomere length, shape and function depend on a complex of six core telomere-associated proteins referred to as the telosome or shelterin complex. We here demonstrate that the isoform β2 of the heregulin family of growth factors (HRGβ2) is a novel interactor of the telosome/shelterin complex in human telomeres. Analysis of protein-protein interactions using a high-throughput yeast two-hybrid (Y2H) screen identified RAP1, the only telomere protein that is conserved from yeasts to mammals, as a novel interacting partner of HRGβ2. Deletion analysis of RAP1 revealed that the linker domain, a region previously suggested to recruit negative regulators of telomere length, interacts specifically with HRGβ2. Co-immunoprecipitation and imaging experiments demonstrated that, in addition to RAP1, HRGβ2 could associate with the RAP1-associated telomeric repeat binding factor 2 (TRF2). Deletion analysis of HRGβ2 confirmed that a putative nuclear localization signal (NLS) was necessary for nuclear HRGβ2 to exert a negative regulation of telomere length whereas the N-terminus (extracellular) amino acids of HRGβ2 were sufficient to interact with RAP1/TRF2 and promote telomere shortening. Taken together, our studies identify nuclear HRGβ2 as one of the previously unknown regulators predicted to be recruited by the RAP1 linker domain to negatively regulate telomere length in human cells. Our current findings reveal that a new, but likely not the last, unexpected visitor has arrived to the “telosome/shelterin town”.

Dynamic peptides of human TPP1 fulfill diverse functions in telomere maintenance

Telomeres are specialized nucleoprotein complexes that comprise the ends of linear chromosomes. Human telomeres end in a short, single-stranded DNA (ssDNA) overhang that is recognized and bound by two telomere proteins, POT1 and TPP1. Whereas POT1 binds directly to telomere ssDNA, its interaction with TPP1 is essential for localization of POT1 to the telomere. TPP1 also provides enhanced binding and sequence discrimination that regulates POT1-TPP1 interactions exclusively with telomere ssDNA. Finally, TPP1 recruits telomerase, the enzyme responsible for synthesis of telomere DNA, to the telomere. While the oligosaccharide-oligonucleotide-binding (OB)-fold domain of TPP1 has been solved by X-ray crystallography, the molecular interactions within the POT1-TPP1-ssDNA ternary complex and the conformational changes that contribute to its diverse functions remain ambiguous. We employed hydrogen/deuterium exchange combined with mass spectrometry to identify three peptides, all residing within the OB-fold of TPP1, that exhibit altered exchange rates upon complex formation or ssDNA binding. Mutation of these regions combined with functional assays revealed the diverse contributions of each moiety in protein-protein interactions, regulating telomerase activity or DNA-binding. Together, these functional data combined with biophysical analyses and homology modeling provide a molecular understanding of the diverse contributions of TPP1 in telomere maintenance.

Expression of Telomere Binding Proteins (RAP1 and POT1) in Renal Cell Carcinoma and Their Correlation with Clinicopathological Parameters

Telomere stability is indispensable for continuous proliferation of cells. Telomere structure is maintained by group of six proteins termed as shelterin. RAP1 and POT1 proteins are significant members of shelterin complex. Expression of RAP1 and POT1 are crucial for telomere maintenance and hence uncontrolled division of cells. Notably, expression of RAP1 and POT1 is unknown in renal cell carcinoma (RCC). In view of these facts, the present study was initiated to investigate the expression of RAP1 and POT1 in RCC and their relationships with clinicopathological features. In total 65 histopathologically confirmed RCC cases and their adjacent normal renal parenchyma were analyzed for gene expression. The mRNA expression of telomere binding proteins RAP1 and POT1 were measured using RT-PCR. Expression of RAP1 was observed to be significantly increased in tumour tissues as compared to corresponding normal renal tissues (P = 0.004). The gene expression of RAP1 was documented to be related to grades of RCC (P = 0.002) and subtypes of RCC (P = 0.01). Although, POT1 expression was up-regulated in RCC tissue, however it was not statistically significant. Also, POT1 expression was not related to grades, stages and subtypes of RCC. This is the first study which shows correlation RAP1 with grades and subtypes of RCC.

Ruthenium(II) polypyridyl complexes as dual inhibitors of telomerase and topoisomerase

One novel ruthenium polypyridyl complex, [Ru(bpy)2(icip)](2+) (1), and two previously reported ruthenium polypyridyl complexes, [Ru(bpy)2(pdppz)](2+) ()2 and [Ru(bpy)2(tactp)](2+) (3) (bpy = 2,2'-bipyridine, icip = 2-(indeno[2,1-b]chromen-6-yl)-1H-imidazo[4,5-f][1,10]phenanthroline, pdppz = phenanthro[4,5-abc]dipyrido[3,2-h:2',3'-j]phenazine, tactp = 4,5,9,18-tetraazachryseno[9,10-b]-triphenylene), have been synthesised. As expected, these complexes show inhibition towards telomerase by inducing and stabilising the G-quadruplex structure, and behave as topoisomerase I/II poisons at the same time. Additionally, the acute and chronic cytotoxicities of the complexes are considered. Furthermore, cell apoptosis experiments are used to briefly study the mechanism. Because studies involving multi-target inhibition towards topoisomerase and telomerase of Ru(II) complexes have not been reported previously, the present research may help to develop innovative chemical strategies and therapies.

Postnatal telomere dysfunction induces cardiomyocyte cell-cycle arrest through p21 activation

The molecular mechanisms that drive mammalian cardiomyocytes out of the cell cycle soon after birth remain largely unknown. Here, we identify telomere dysfunction as a critical physiological signal for cardiomyocyte cell-cycle arrest. We show that telomerase activity and cardiomyocyte telomere length decrease sharply in wild-type mouse hearts after birth, resulting in cardiomyocytes with dysfunctional telomeres and anaphase bridges and positive for the cell-cycle arrest protein p21. We further show that premature telomere dysfunction pushes cardiomyocytes out of the cell cycle. Cardiomyocytes from telomerase-deficient mice with dysfunctional telomeres (G3 Terc(-/-)) show precocious development of anaphase-bridge formation, p21 up-regulation, and binucleation. In line with these findings, the cardiomyocyte proliferative response after cardiac injury was lost in G3 Terc(-/-) newborns but rescued in G3 Terc(-/-)/p21(-/-) mice. These results reveal telomere dysfunction as a crucial signal for cardiomyocyte cell-cycle arrest after birth and suggest interventions to augment the regeneration capacity of mammalian hearts.

Cdk-dependent phosphorylation regulates TRF1 recruitment to PML bodies and promotes C-circle production in ALT cells

TRF1, a duplex telomeric DNA binding protein, is implicated in homologous-recombination-based alternative lengthening of telomeres, known as ALT. However, how TRF1 promotes ALT activity has yet to be fully characterized. Here we report that Cdk-dependent TRF1 phosphorylation on T371 acts as a switch to create a pool of TRF1, referred to as (pT371)TRF1, which is recruited to ALT-associated PML bodies (APBs) in S and G2 phases independently of its binding to telomeric DNA. We find that phosphorylation of T371 is essential for APB formation and C-circle production, both of which are hallmarks of ALT. We show that the interaction of (pT371)TRF1 with APBs is dependent upon ATM and homologous-recombination-promoting factors Mre11 and BRCA1. In addition, (pT371)TRF1 interaction with APBs is sensitive to transcription inhibition, which also reduces DNA damage at telomeres. Furthermore, overexpression of RNaseH1 impairs (pT371)TRF1 recruitment to APBs in the presence of campothecin, an inhibitor that prevents topoisomerase I from resolving RNA-DNA hybrids. These results suggest that transcription-associated DNA damage, perhaps arising from processing RNA-DNA hybrids at telomeres, triggers (pT371)TRF1 recruitment to APBs to facilitate ALT activity.

Stop pulling my strings - what telomeres taught us about the DNA damage response

Mammalian cells have evolved specialized mechanisms to sense and repair double-strand breaks (DSBs) to maintain genomic stability. However, in certain cases, the activity of these pathways can lead to aberrant DNA repair, genomic instability and tumorigenesis. One such case is DNA repair at the natural ends of linear chromosomes, known as telomeres, which can lead to chromosome-end fusions. Here, we review data obtained over the past decade and discuss the mechanisms that protect mammalian chromosome ends from the DNA damage response. We also discuss how telomere research has helped to uncover key steps in DSB repair. Last, we summarize how dysfunctional telomeres and the ensuing genomic instability drive the progression of cancer.

POT1-TPP1 Binding and Unfolding of Telomere DNA Discriminates against Structural Polymorphism

Telomeres are nucleoprotein complexes that reside at the ends of linear chromosomes and help maintain genomic integrity. Protection of telomeres 1 (POT1) and TPP1 are telomere-specific proteins that bind as a heterodimer to single-stranded telomere DNA to prevent illicit DNA damage responses and to enhance telomerase-mediated telomere extension. Telomere DNA is guanosine rich and, as such, can form highly stable secondary structures including G-quadruplexes. G-quadruplex DNA folds into different topologies that are determined by several factors including monovalent ion composition and the precise sequence and length of the DNA. Here, we explore the influence of DNA secondary structure on POT1-TPP1 binding. Equilibrium binding assays reveal that the POT1-TPP1 complex binds G-quadruplex structures formed in buffers containing Na(+) with an affinity that is fivefold higher than for G-quadruplex structures formed in the presence of K(+). However, the binding of the second heterodimer is insensitive to DNA secondary structure, presumably due to unfolding resulting from binding of the first POT1-TPP1. We further show that the rate constant for POT1-TPP1-induced unfolding of DNA secondary structure is substantially faster for G-quadruplex topologies formed in the presence of Na(+) ions. When bound to DNA, POT1-TPP1 forms complexes with similar CD spectra and enhances telomerase activity for all DNA substrates tested, regardless of the substrate secondary structure or solution monovalent ion composition. Together, these data indicate that binding of POT1-TPP1 unfolds telomere secondary structure to assist loading of additional heterodimers and to ensure efficient promotion of telomerase-mediated extension.

Polymorphisms in human telomerase reverse transcriptase (hTERT) gene and susceptibility to gastric cancer in a Turkish population: Hospital-based case-control study

Erosion of telomeres, tandem nucleotide repeats (TTAGGG)n that cap the end of eukaryotic chromosomes, has been related with carcinogenesis. The human telomerase reverse transcriptase (hTERT) gene is encoded the rate-limiting catalytic subunit of the telomerase complexes, which is essential for the protection of telomeric DNA length and chromosomal stability. The purpose of this study was to examine the effect of four functional single nucleotide polymorphisms (SNPs) of hTERT (rs2736109 G>A, rs2735940 T>C, rs2853669 A>G and rs2736100 T>G) on susceptibility to gastric cancer (GC) in Turkish population. The genotype frequency of hTERT rs2736109 G>A, rs2735940 T>C, rs2853669 A>G and rs2736100 T>G polymorphisms were determined by using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and TaqMan methods in 104 subjects with GC and 209 healthy control subjects. We found that hTERT rs2736109 G>A (AA+AG vs. GG OR=1.68 95% CI=1.01-2.81, P=0.04), rs2735940 T>C (CC vs. CT+TT: OR=2.53 95% CI=1.01-6.13, P=0.03), and rs2736100 T>G (TT vs. TG+GG: OR=2.27 95% CI=1.23-4.17, P=0.006) polymorphisms were associated with risk of GC. In the haplotype analysis, hTERT Grs2736109/Trs2735940/Ars2853669/Grs2736100 haplotype was also related with an increased risk of GC (OR=1.75; 95% CI: 1.05-2.93, P=0.03). Because this is the first study regarding the hTERT rs2736109 G>A, rs2735940 T>C, rs2853669 A>G and rs2736100 T>G polymorphisms and the risk of GC susceptibility in the literature, further independent studies are needed to verify our results in a larger sample sizes, as well as in patients of different populations.

Effects of PCB126 and PCB153 on telomerase activity and telomere length in undifferentiated and differentiated HL-60 cells

PCBs are persistent organic pollutants that are carcinogenic and immunotoxic and have developmental toxicity. This suggests that they may interfere with normal cell maturation. Cancer and stem/progenitor cells have telomerase activity to maintain and protect the chromosome ends, but lose this activity during differentiation. We hypothesized that PCBs interfere with telomerase activity and the telomere complex, thereby disturbing cell differentiation and stem/progenitor cell function. HL-60 cells are cancer cells that can differentiated into granulocytes and monocytes. We exposed HL-60 cells to PCB126 (dioxin-like) and PCB153 (nondioxin-like) 6 days before and during 3 days of differentiation. The differentiated cells showed G0/G1 phase arrest and very low telomerase activity. hTERT and hTR, two telomerase-related genes, were downregulated. The telomere shelterins TRF1, TRF2, and POT1 were upregulated in granulocytes, and TRF2 was upregulated and POT1 downregulated in monocytes. Both PCBs further reduced telomerase activity in differentiated cells, but had only small effects on the differentiation and telomere-related genes. Treatment of undifferentiated HL-60 cells for 30 days with PCB126 produced a downregulation of telomerase activity and a decrease of hTERT, hTR, TRF1, and POT1 gene expression. With PCB153, the effects were less pronounced and some shelterin genes were increased after 30 days of exposure. With each PCB, no differentiation of cells was observed and cells continued to proliferate despite reduced telomerase activity, resulting in shortened telomeres after 30 days of exposure. These results indicate cell-type and PCB congener-specific effects on telomere/telomerase-related genes. Although PCBs do not seem to strongly affect differentiation, they may influence stem or progenitor cells through telomere attrition with potential long-term consequences for health.

A higher-order entity formed by the flexible assembly of RAP1 with TRF2

Telomere integrity is essential to maintain genome stability, and telomeric dysfunctions are associated with cancer and aging pathologies. In human, the shelterin complex binds TTAGGG DNA repeats and provides capping to chromosome ends. Within shelterin, RAP1 is recruited through its interaction with TRF2, and TRF2 is required for telomere protection through a network of nucleic acid and protein interactions. RAP1 is one of the most conserved shelterin proteins although one unresolved question is how its interaction may influence TRF2 properties and regulate its capacity to bind multiple proteins. Through a combination of biochemical, biophysical and structural approaches, we unveiled a unique mode of assembly between RAP1 and TRF2. The complete interaction scheme between the full-length proteins involves a complex biphasic interaction of RAP1 that directly affects the binding properties of the assembly. These results reveal how a non-DNA binding protein can influence the properties of a DNA-binding partner by mutual conformational adjustments.

Differential senescence capacities in meibomian gland carcinoma and basal cell carcinoma

Meibomian gland carcinoma (MGC) and basal cell carcinoma (BCC) are common eyelid carcinomas that exhibit highly dissimilar degrees of proliferation and prognoses. We address here the question of the differential mechanisms between these two eyelid cancers that explain their different outcome. A total of 102 confirmed MGC and 175 diagnosed BCC cases were analyzed. Twenty confirmed MGC and twenty diagnosed BCC cases were collected to determine the telomere length, the presence of senescent cells, and the expression levels of the telomere capping shelterin complex, P53, and the E3 ubiquitin ligase Siah1. Decreased protein levels of the shelterin subunits, shortened telomere length, over-expressed Ki-67, and Bcl2 as well as mutations in P53 were detected both in MGC and BCC. It suggests that the decreased protein levels of the shelterin complex and the shortened telomere length contribute to the tumorigenesis of MGC and BCC. However, several parameters distinguish MGC from BCC samples: (i) the mRNA level of the shelterin subunits decreased in MGC but it increased in BCC; (ii) P53 was more highly mutated in MGC; (iii) Siah1 mRNA was over-expressed in BCC; (iv) BCC samples contain a higher level of senescent cells; (v) Ki-67 and Bcl2 expression were lower in BCC. These results support a model where a preserved P53 checkpoint in BCC leads to cellular senescence and reduced tumor proliferation as compared to MGC.

PIAS1-mediated sumoylation promotes STUbL-dependent proteasomal degradation of the human telomeric protein TRF2

The human telomeric protein TRF2 protects chromosome ends by facilitating their organization into the protective capping structure. Here we show that the stability of TRF2 is regulated via modification by the small ubiquitin-like modifiers (SUMO). TRF2 specifically interacts with and is sumoylated by PIAS1 in mammalian cells. The proteasome inhibitor stabilizes SUMO-conjugated TRF2 without affecting the level of unmodified TRF2, suggesting that SUMO conjugation is required for proteasomal degradation of TRF2. We also show that RNF4, a mammalian SUMO-targeted ubiquitin ligase, interacts with TRF2 in a SUMO-dependent manner and preferentially targets SUMO-conjugated TRF2 for ubiquitination. Collectively, our data demonstrate that the PIAS1-mediated sumoylation status of TRF2 serves as a molecular switch that controls the level of TRF2 at telomeres.

Involvement of SRSF11 in cell cycle-specific recruitment of telomerase to telomeres at nuclear speckles

Telomerase, a unique ribonucleoprotein complex that contains the telomerase reverse transcriptase (TERT), the telomerase RNA component (TERC) and the TERC-binding protein dyskerin, is required for continued cell proliferation in stem cells and cancer cells. Here we identify SRSF11 as a novel TERC-binding protein that localizes to nuclear speckles, subnuclear structures that are enriched in pre-messenger RNA splicing factors. SRSF11 associates with active telomerase enzyme through an interaction with TERC and directs it to nuclear speckles specifically during S phase of the cell cycle. On the other hand, a subset of telomeres is shown to be constitutively present at nuclear speckles irrespective of cell cycle phase, suggesting that nuclear speckles could be the nuclear sites for telomerase recruitment to telomeres. SRSF11 also associates with telomeres through an interaction with TRF2, which facilitates translocation of telomerase to telomeres. Depletion of SRSF11 prevents telomerase from associating with nuclear speckles and disrupts telomerase recruitment to telomeres, thereby abrogating telomere elongation by telomerase. These findings suggest that SRSF11 acts as a nuclear speckle-targeting factor that is essential for telomerase association with telomeres through the interactions with TERC and TRF2, and provides a potential target for modulating telomerase activity in cancer.

Expression of human protection of telomere 1 correlates with telomere length and radiosensitivity in the human laryngeal cancer Hep-2 cell line

The close association between telomere length and radiosensitivity has been established by several studies. There is also a hypothesis that telomere length may be regulated by human protection of telomere 1 (hPOT1) in human carcinoma cells. In the present study, the hPOT1 level between the radioresistant Hep-2R cells and the wild-type were compared, and the results showed that the hPOT1 gene was upregulated in the radioresistant Hep-2R cell lines compared with the wild-type. This suggested that the expression level of hPOT1 correlates with radiosensitivity. Additionally, an hPOT1-directed short hairpin (sh)RNA plasmid was constructed and transferred into the Hep-2R cells, which lead to telomere shortening, an increase in apoptosis and markedly decreased growth of the RNAi-Hep-2R cell line. These results demonstrate that hPOT1-directed shRNAs are associated with telomere length and radiosensitivity, and maybe a potent sensitizer for laryngeal cancer radiotherapy.

Mir-23a induces telomere dysfunction and cellular senescence by inhibiting TRF2 expression

Telomeric repeat binding factor 2 (TRF2) is essential for telomere maintenance and has been implicated in DNA damage response and aging. Telomere dysfunction induced by TRF2 inhibition can accelerate cellular senescence in human fibroblasts. While previous work has demonstrated that a variety of factors can regulate TRF2 expression transcriptionally and post-translationally, whether microRNAs (miRNAs) also participate in post-transcriptionally modulating TRF2 levels remains largely unknown. To better understand the regulatory pathways that control TRF2, we carried out a large-scale luciferase reporter screen using a miRNA expression library and identified four miRNAs that could target human TRF2 and significantly reduce the level of endogenous TRF2 proteins. In particular, our data revealed that miR-23a could directly target the 3′ untranslated region (3′UTR) of TRF2. Overexpression of miR-23a not only reduced telomere-bound TRF2 and increased telomere dysfunction-induced foci (TIFs), but also accelerated senescence of human fibroblast cells, which could be rescued by ectopically expressed TRF2. Our findings demonstrate that TRF2 is a specific target of miR-23a, and uncover a previously unknown role for miR-23a in telomere regulation and cellular senescence.

Unraveling the pathogenesis of Hoyeraal-Hreidarsson syndrome, a complex telomere biology disorder

Hoyeraal-Hreidarsson (HH) syndrome is a multisystem genetic disorder characterized by very short telomeres and considered a clinically severe variant of dyskeratosis congenita. The main cause of mortality, usually in early childhood, is bone marrow failure. Mutations in several telomere biology genes have been reported to cause HH in about 60% of the HH patients, but the genetic defects in the rest of the patients are still unknown. Understanding the aetiology of HH and its diverse manifestations is challenging because of the complexity of telomere biology and the multiple telomeric and non-telomeric functions played by telomere-associated proteins in processes such as telomere replication, telomere protection, DNA damage response and ribosome and spliceosome assembly. Here we review the known clinical complications, molecular defects and germline mutations associated with HH, and elucidate possible mechanistic explanations and remaining questions in our understanding of the disease.

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.