The human telomere-associated protein TIN2 stimulates interactions between telomeric DNA tracts in vitro

Assembly path dependence of telomeric DNA compaction by TRF1, TIN2, and SA1

Telomeres, complexes of DNA and proteins, protect ends of linear chromosomes. In humans, the two shelterin proteins TRF1 and TIN2, along with cohesin subunit SA1, were proposed to mediate telomere cohesion. Although the ability of the TRF1-TIN2 and TRF1-SA1 systems to compact telomeric DNA by DNA-DNA bridging has been reported, the function of the full ternary TRF1-TIN2-SA1 system has not been explored in detail. Here, we quantify the compaction of nanochannel-stretched DNA by the ternary system, as well as its constituents, and obtain estimates of the relative impact of its constituents and their interactions. We find that TRF1, TIN2, and SA1 work synergistically to cause a compaction of the DNA substrate, and that maximal compaction occurs if all three proteins are present. By altering the sequence with which DNA substrates are exposed to proteins, we establish that compaction by TRF1 and TIN2 can proceed through binding of TRF1 to DNA, followed by compaction as TIN2 recognizes the previously bound TRF1. We further establish that SA1 alone can also lead to a compaction, and that compaction in a combined system of all three proteins can be understood as an additive effect of TRF1-TIN2 and SA1-based compaction. Atomic force microscopy of intermolecular aggregation confirms that a combination of TRF1, TIN2, and SA1 together drive strong intermolecular aggregation as it would be required during chromosome cohesion.

Structure, dynamics, and regulation of TRF1-TIN2-mediated trans- and cis-interactions on telomeric DNA

TIN2 is a core component of the shelterin complex linking double-stranded telomeric DNA-binding proteins (TRF1 and TRF2) and single-strand overhang-binding proteins (TPP1-POT1). In vivo, the large majority of TRF1 and TRF2 exist in complexes containing TIN2 but lacking TPP1/POT1; however, the role of TRF1-TIN2 interactions in mediating interactions with telomeric DNA is unclear. Here, we investigated DNA molecular structures promoted by TRF1-TIN2 interaction using atomic force microscopy (AFM), total internal reflection fluorescence microscopy (TIRFM), and the DNA tightrope assay. We demonstrate that the short (TIN2S) and long (TIN2L) isoforms of TIN2 facilitate TRF1-mediated DNA compaction (cis-interactions) and DNA-DNA bridging (trans-interactions) in a telomeric sequence- and length-dependent manner. On the short telomeric DNA substrate (six TTAGGG repeats), the majority of TRF1-mediated telomeric DNA-DNA bridging events are transient with a lifetime of ~1.95 s. On longer DNA substrates (270 TTAGGG repeats), TIN2 forms multiprotein complexes with TRF1 and stabilizes TRF1-mediated DNA-DNA bridging events that last on the order of minutes. Preincubation of TRF1 with its regulator protein Tankyrase 1 and the cofactor NAD+ significantly reduced TRF1-TIN2 mediated DNA-DNA bridging, whereas TIN2 protected the disassembly of TRF1-TIN2 mediated DNA-DNA bridging upon Tankyrase 1 addition. Furthermore, we showed that TPP1 inhibits TRF1-TIN2L-mediated DNA-DNA bridging. Our study, together with previous findings, supports a molecular model in which protein assemblies at telomeres are heterogeneous with distinct subcomplexes and full shelterin complexes playing distinct roles in telomere protection and elongation.

Non-canonical roles of canonical telomere binding proteins in cancers

Reactivation of telomerase is a major hallmark observed in 90% of all cancers. Yet paradoxically, enhanced telomerase activity does not correlate with telomere length and cancers often possess short telomeres; suggestive of supplementary non-canonical roles that telomerase might play in the development of cancer. Moreover, studies have shown that aberrant expression of shelterin proteins coupled with their release from shortening telomeres can further promote cancer by mechanisms independent of their telomeric role. While targeting telomerase activity appears to be an attractive therapeutic option, this approach has failed in clinical trials due to undesirable cytotoxic effects on stem cells. To circumvent this concern, an alternative strategy could be to target the molecules involved in the non-canonical functions of telomeric proteins. In this review, we will focus on emerging evidence that has demonstrated the non-canonical roles of telomeric proteins and their impact on tumorigenesis. Furthermore, we aim to address current knowledge gaps in telomeric protein functions and propose future research approaches that can be undertaken to achieve this.

A Truncating Germline Mutation of TINF2 in Individuals with Thyroid Cancer or Melanoma Results in Longer Telomeres

Background: Our genome sequencing analysis revealed a frameshift mutation in the shelterin gene TINF2 in a large family with individuals affected with papillary thyroid carcinoma (PTC) and melanoma. Here, we further characterized the mutation and screened for coding variants in the 6 shelterin genes in 24 families. Methods: Sanger sequencing was performed to screen for the TINF2 mutation in the key family. Quantitative reverse transcription-polymerase chain reaction (PCR) was used for TINF2 gene expression analysis. Exogenous expression and co-immunoprecipitation techniques were used for assessing TINF2 binding to TERF1. Relative telomere length (RTL) was quantified in DNAs from lymphocytes by using quantitative real-time PCR. Whole exome sequencing (WES) was performed in seven families with individuals affected with PTC and other cancer types. Screening for DNA variants in shelterin genes was performed by using whole genome sequencing data from 17 families and WES data from 7 further families. Results: The TINF2 mutation (TINF2 p.Trp198fs) showed complete co-segregation with PTC and melanoma in the key family. The mutation is not reported in databases and not identified in 23 other families we screened. The expression of TINF2 was borderline reduced in individuals with the mutation. The truncated TINF2 protein showed abolished binding to TERF1. The RTL in the individuals with the mutation was significantly longer when compared with those without the mutation from the same family as well as compared with 62 healthy controls. Among the 24 families, we identified 3 missense and 1 synonymous variant(s) in 2 shelterin genes (TINF2 and ACD). Conclusions: The rare frameshift mutation in the TINF2 gene and the associated longer telomere length suggest that dysregulated telomeres could be a mechanism predisposing to PTC and melanoma. DNA coding variants in shelterin genes are rare. Further studies are required to evaluate the roles of variants in shelterin genes in thyroid cancer and melanoma.

The Role of Ubiquitination and SUMOylation in Telomere Biology

Telomeres are a unique structure of DNA repeats covered by proteins at the ends of the chromosomes that protect the coding regions of the genome and function as a biological clock. They require a tight regulation of the factors covering and protecting their structure, as they are shortened with each cell division to limit the ability of cells to replicate uncontrollably. Additionally, they protect the chromosome ends from DNA damage responses and thereby, prevent genomic instability. Telomere dysfunction can lead to chromosomal abnormalities and cancer. Therefore, dysregulation of any of the factors that regulate the integrity of the telomeres will have implications to chromosomal stability, replicative lifespan and may lead to cell transformation. This review will cover the main factors participating in the normal function of the telomeres and how these are regulated by the ubiquitin and SUMO systems. Accumulating evidence indicate that the ubiquitin and SUMO pathways are significant regulators of the shelterin complex and other chromatin modifiers, which are important for telomere structure integrity. Furthermore, the crosstalk between these two pathways has been reported in telomeric DNA repair. A better understanding of the factors contributing to telomere biology, and how they are regulated, is important for the design of new strategies for cancer therapies and regenerative medicine.

Molecular Architecture of Full-length TRF1 Favors Its Interaction with DNA

Telomeres are specific DNA-protein structures found at both ends of eukaryotic chromosomes that protect the genome from degradation and from being recognized as double-stranded breaks. In vertebrates, telomeres are composed of tandem repeats of the TTAGGG sequence that are bound by a six-subunit complex called shelterin. Molecular mechanisms of telomere functions remain unknown in large part due to lack of structural data on shelterins, shelterin complex, and its interaction with the telomeric DNA repeats. TRF1 is one of the best studied shelterin components; however, the molecular architecture of the full-length protein remains unknown. We have used single-particle electron microscopy to elucidate the structure of TRF1 and its interaction with telomeric DNA sequence. Our results demonstrate that full-length TRF1 presents a molecular architecture that assists its interaction with telometic DNA and at the same time makes TRFH domains accessible to other TRF1 binding partners. Furthermore, our studies suggest hypothetical models on how other proteins as TIN2 and tankyrase contribute to regulate TRF1 function.

Telomere length maintenance, shortening, and lengthening

Telomeres maintain chromosome stability and cell replicative capacity. Telomere shortening occurs concomitant with aging. Short telomeres are associated with some diseases, such as dyskeratosis congenita, idiopathic pulmonary fibrosis, and aplastic anemia. Telomeres are longer in pluripotent stem cells than in somatic cells and lengthen significantly during preimplantation development. Furthermore, telomere elongation during somatic cell reprogramming is of great importance in the acquisition of authentic pluripotency. This review focuses primarily on regulatory mechanisms of telomere length maintenance in pluripotent cells, telomere length extension in early embryo development, and also telomere rejuvenation in somatic cell reprogramming. Telomere related diseases are also discussed in this review.

Resolution of telomere associations by TRF1 cleavage in mouse embryonic stem cells

Telomere associations have been observed during key cellular processes such as mitosis, meiosis, and carcinogenesis and must be resolved before cell division to prevent genome instability. Here we establish that telomeric repeat-binding factor 1 (TRF1), a core component of the telomere protein complex, is a mediator of telomere associations in mammalian cells. Using live-cell imaging, we show that expression of TRF1 or yellow fluorescent protein (YFP)-TRF1 fusion protein above endogenous levels prevents proper telomere resolution during mitosis. TRF1 overexpression results in telomere anaphase bridges and aggregates containing TRF1 protein and telomeric DNA. Site-specific protein cleavage of YFP-TRF1 by tobacco etch virus protease resolves telomere aggregates, indicating that telomere associations are mediated by TRF1. This study provides novel insight into the formation and resolution of telomere associations.

Influence of mindfulness-based stress reduction (MBSR) on telomerase activity in women with breast cancer (BC)

Mindfulness-based stress reduction (MBSR) reduces symptoms of depression, anxiety, and fear of recurrence among breast cancer (BC) survivors. However, the effects of MBSR (BC) on telomere length (TL) and telomerase activity (TA), known markers of cellular aging, psychological stress, and disease risk, are not known. This randomized, wait-listed, controlled study, nested within a larger trial, investigated the effects of MBSR (BC) on TL and TA. BC patients (142) with Stages 0-III cancer who had completed adjuvant treatment with radiation and/or chemotherapy at least 2 weeks prior to enrollment and within 2 years of completion of treatment with lumpectomy and/or mastectomy were randomly assigned to either a 6-week MBSR for BC program or a usual care. Assessments of TA and TL were obtained along with psychological measurements at baseline, 6 weeks, and 12 weeks after completing the MBSR(BC) program. The mean age of 142 participants was 55.3 years; 72% were non-Hispanic White; 78% had Stage I or II cancer; and 36% received both chemotherapy and radiation. In analyses adjusted for baseline TA and psychological status, TA increased steadily over 12 weeks in the MBSR(BC) group (approximately 17%) compared to essentially no increase in the control group (approximately 3%, p < .01). In contrast, no between-group difference was observed for TL (p = .92). These results provide preliminary evidence that MBSR(BC) increases TA in peripheral blood mononuclear cells from BC patients and have implications for understanding how MBSR(BC) may extend cell longevity at the cellular level.

Structural and functional association of androgen receptor with telomeres in prostate cancer cells

Telomeres protect the ends of linear chromosomes from being recognized as damaged DNA, and telomere stability is required for genome stability. Here we demonstrate that telomere stability in androgen receptor (AR)-positive LNCaP human prostate cancer cells is dependent on AR and androgen, as AR inactivation by AR antagonist bicalutamide (Casodex), AR-knockdown, or androgen-depletion caused telomere dysfunction, and the effect of androgen-depletion or Casodex was blocked by the addition of androgen. Notably, neither actinomycin D nor cycloheximide blocked the DNA damage response to Casodex, indicating that the role of AR in telomere stability is independent of its role in transcription. We also demonstrate that AR is a component of telomeres, as AR-bound chromatin contains telomeric DNA, and telomeric chromatin contains AR. Importantly, AR inactivation by Casodex caused telomere aberrations, including multiple abnormal telomere signals, remindful of a fragile telomere phenotype that has been described previously to result from defective telomere DNA replication. We suggest that AR plays an important role in telomere stability and replication of telomere DNA in prostate cancer cells, and that AR inactivation-mediated telomere dysfunction may contribute to genomic instability and progression of prostate cancer cells.

Telomeres and the nucleus

Telomeres are crucial for the maintenance of genome stability through "capping" of chromosome ends to prevent their recognition as double-strand breaks, thus avoiding end-to-end fusions or illegitimate recombination [1-3]. Similar to other genomic regions, telomeres participate to the nuclear architecture while being highly mobile. The interaction of telomeres with nuclear domains or compartments greatly differs not only between organisms but also between cells within the same organism. It is also expected that biological processes like replication, repair or telomere elongation impact the distribution of chromosome extremities within the nucleus, as they probably do with other regions of the genome. Pathological processes such as cancer induce profound changes in the nuclear architecture, which also affects telomere dynamics and spatial organization. Here we will expose our present knowledge on the relationship between telomeres and nuclear architecture and on how this relationship is affected by normal or abnormal telomere metabolisms.

Function, replication and structure of the mammalian telomere

Telomeres are specialized structures at the ends of linear chromosomes that were originally defined functionally based on observations first by Muller (1938) and subsequently by McClintock (1941) that naturally occurring chromosome ends do not behave as double-stranded DNA breaks, in spite of the fact that they are the physical end of a linear, duplex DNA molecule. Double-stranded DNA breaks are highly unstable entities, being susceptible to nucleolytic attack and giving rise to chromosome rearrangements through end-to-end fusions and recombination events. In contrast, telomeres confer stability upon chromosome termini, as evidenced by the fact that chromosomes are extraordinarily stable through multiple cell divisions and even across evolutionary time. This protective function of telomeres is due to the formation of a nucleoprotein complex that sequesters the end of the DNA molecule, rendering it inaccessible to nucleases and recombinases as well as preventing the telomere from activating the DNA damage checkpoint pathways. The capacity of a functional end-protective complex to form is dependent upon maintenance of sufficient telomeric DNA. We have learned a great deal about telomere structure and how this specialized nucleoprotein complex confers stability on chromosome ends since the original observations that defined telomeres were made. This review summarizes our current understanding of mammalian telomere replication, structure and function.

Intensive meditation training, immune cell telomerase activity, and psychological mediators

BACKGROUND

Telomerase activity is a predictor of long-term cellular viability, which decreases with chronic psychological distress (Epel et al., 2004). Buddhist traditions claim that meditation decreases psychological distress and promotes well-being (e.g., Dalai Lama and Cutler, 2009). Therefore, we investigated the effects of a 3-month meditation retreat on telomerase activity and two major contributors to the experience of stress: Perceived Control (associated with decreased stress) and Neuroticism (associated with increased subjective distress). We used mediation models to test whether changes in Perceived Control and Neuroticism explained meditation retreat effects on telomerase activity. In addition, we investigated whether two qualities developed by meditative practice, increased Mindfulness and Purpose in Life, accounted for retreat-related changes in the two stress-related variables and in telomerase activity.

METHODS

Retreat participants (n=30) meditated for ∼6 h daily for 3 months and were compared with a wait-list control group (n=30) matched for age, sex, body mass index, and prior meditation experience. Retreat participants received instruction in concentrative meditation techniques and complementary practices used to cultivate benevolent states of mind (Wallace, 2006). Psychological measures were assessed pre- and post-retreat. Peripheral blood mononuclear cell samples were collected post-retreat for telomerase activity. Because there were clear, a priori hypotheses, 1-tailed significance criteria were used throughout.

RESULTS

Telomerase activity was significantly greater in retreat participants than in controls at the end of the retreat (p<0.05). Increases in Perceived Control, decreases in Neuroticism, and increases in both Mindfulness and Purpose in Life were greater in the retreat group (p<0.01). Mediation analyses indicated that the effect of the retreat on telomerase was mediated by increased Perceived Control and decreased Neuroticism. In turn, changes in Perceived Control and Neuroticism were both partially mediated by increased Mindfulness and Purpose in Life. Additionally, increases in Purpose in Life directly mediated the telomerase group difference, whereas increases in Mindfulness did not.

CONCLUSIONS

This is the first study to link meditation and positive psychological change with telomerase activity. Although we did not measure baseline telomerase activity, the data suggest that increases in perceived control and decreases in negative affectivity contributed to an increase in telomerase activity, with implications for telomere length and immune cell longevity. Further, Purpose in Life is influenced by meditative practice and directly affects both perceived control and negative emotionality, affecting telomerase activity directly as well as indirectly.

Dynamics of telomeric chromatin at the crossroads of aging and cancer

Telomeres are nucleoprotein structures that protect the ends of human chromosomes through the formation of a 'cap', thus preventing exonucleolytic degradation, inter- and intra-chromosomal fusion, and subsequent chromosomal instability. During aging, telomere shortening correlates with tissue dysfunction and loss of renewal capacity. In human cancer, telomere dysfunction is involved in early chromosome instability, long-term cellular proliferation, and possibly other processes related to cell survival and microenvironment. Telomeres constitute an attractive target for the development of novel small-molecule anti-cancer drugs. In particular, individual protein components of the core telomere higher-order chromatin structure (known as the telosome or 'shelterin' complex) are promising candidate targets for cancer therapy.

Role of shelterin in cancer and aging

Mammalian telomeres are formed by tandem repeats of the TTAGGG sequence bound by a specialized six-protein complex known as shelterin, which has fundamental roles in the regulation of telomere length and telomere capping. In the past, the study of mice genetically modified for telomerase components has been instrumental to demonstrate the role of telomere length in cancer and aging. Recent studies using genetically modified mice for shelterin proteins have highlighted an equally important role of telomere-bound proteins in cancer and aging, even in the presence of proficient telomerase activity and normal telomere length. In this review, we will focus on recent findings, suggesting a role of shelterin components in cancer and aging.

Structural identity of telomeric complexes

A major issue in telomere research is to understand how the integrity of chromosome ends is controlled. Although several nucleoprotein complexes have been described at the telomeres of different organisms, it is still unclear how they confer a structural identity to chromosome ends in order to mask them from DNA repair and to ensure their proper replication. In this review, we describe how telomeric nucleoprotein complexes are structured, comparing different organisms and trying to link these structures to telomere biology. It emerges that telomeres are formed by a complex and specific network of interactions between DNA, RNA and proteins. The fact that these interactions and associated activities are reinforcing each other might help to guaranty the robustness of telomeric functions across the cell cycle and in the event of cellular perturbations. We propose that telomeric nucleoprotein complexes orient cell fate through dynamic transitions in their structures and their organization.

Telomeric armor: the layers of end protection

The linear nature of eukaryotic chromosomes necessitates protection of their physical ends, the telomeres, because the DNA-repair machinery can misconstrue the ends as double-stranded DNA breaks. Thus, protection is crucial for avoiding an unwarranted DNA-damage response that could have catastrophic ramifications for the integrity and stability of the linear genome. In this Commentary, we attempt to define what is currently understood by the term ;telomere protection'. Delineating the defining boundaries of chromosome-end protection is important now more than ever, as it is becoming increasingly evident that, although unwanted DNA repair at telomeres must be avoided at all costs, the molecular players involved in recognition, signaling and repair of DNA damage might also serve to protect telomeres.

A novel form of the telomere-associated protein TIN2 localizes to the nuclear matrix

Telomeres are specialized heterochromatin at the ends of linear chromosomes. Telomeres are crucial for maintaining genome stability and play important roles in cellular senescence and tumor biology. Six core proteins-TRF1, TRF2, TIN2, POT1, TPP1 and Rap1 (termed the telosome or shelterin complex)-regulate telomere structure and function. One of these proteins, TIN2, regulates telomere length and structure indirectly by interacting with TRF1, TRF2 and TPP1, but no direct function has been attributed to TIN2. Here we present evidence for a TIN2 isoform (TIN2L) that differs from the originally described TIN2 isoform (TIN2S) in two ways: TIN2L contains an additional 97 amino acids, and TIN2L associates strongly with the nuclear matrix. Stringent salt and detergent conditions failed to extract TIN2L from the nuclear matrix, despite removing other telomere components, including TIN2S. In human mammary epithelial cells, each isoform showed a distinct nuclear distribution both as a function of cell cycle position and telomere length. Our results suggest a dual role for TIN2 in mediating the function of the shelterin complex and tethering telomeres to the nuclear matrix.

How telomerase reaches its end: mechanism of telomerase regulation by the telomeric complex

The telomerase enzyme, which synthesizes telomeric DNA repeats, is regulated in cis at individual chromosome ends by the telomeric protein/DNA complex in a manner dependent on telomere repeat-array length. A dynamic interplay between telomerase-inhibiting factors bound at duplex DNA repeats and telomerase-promoting ones bound at single-stranded terminal DNA overhangs appears to modulate telomerase activity and to be directly related to the transient deprotection of telomeres. We discuss recent advances on the mechanism of telomerase regulation at chromosome ends in both yeast and mammalian systems.

Distinct roles of TRF1 in the regulation of telomere structure and lengthening

The telomere is a functional chromatin structure that consists of G-rich repetitive sequences and various associated proteins. Telomeres protect chromosomal ends from degradation, provide escape from the DNA damage response, and regulate telomere lengthening by telomerase. Multiple proteins that localize at telomeres form a complex called shelterin/telosome. One component, TRF1, is a double-stranded telomeric DNA binding protein. Inactivation of TRF1 disrupts telomeric localization of other shelterin components and induces chromosomal instability. Here, we examined how the telomeric localization of shelterin components is crucial for TRF1-mediated telomere-associated functions. We found that many of the mTRF1 deficient phenotypes, including chromosomal instability, growth defects, and dysfunctional telomere damage response, were suppressed by the telomere localization of shelterin components in the absence of functional mTRF1. However, abnormal telomere signals and telomere elongation phenotypes were either not rescued or only partially rescued, respectively. These data suggest that TRF1 regulates telomere length and function by at least two mechanisms; in one TRF1 acts through the recruiting/tethering of other shelterin components to telomeres, and in the other TRF1 seems to play a more direct role.

Telomere dysfunction and cell survival: roles for distinct TIN2-containing complexes

Telomeres are maintained by three DNA-binding proteins (telomeric repeat binding factor 1 [TRF1], TRF2, and protector of telomeres 1 [POT1]) and several associated factors. One factor, TRF1-interacting protein 2 (TIN2), binds TRF1 and TRF2 directly and POT1 indirectly. Along with two other proteins, TPP1 and hRap1, these form a soluble complex that may be the core telomere maintenance complex. It is not clear whether subcomplexes also exist in vivo. We provide evidence for two TIN2 subcomplexes with distinct functions in human cells. We isolated these two TIN2 subcomplexes from nuclear lysates of unperturbed cells and cells expressing TIN2 mutants TIN2-13 and TIN2-15C, which cannot bind TRF2 or TRF1, respectively. In cells with wild-type p53 function, TIN2-15C was more potent than TIN2-13 in causing telomere uncapping and eventual growth arrest. In cells lacking p53 function, TIN2-15C was more potent than TIN2-13 in causing telomere dysfunction and cell death. Our findings suggest that distinct TIN2 complexes exist and that TIN2-15C–sensitive subcomplexes are particularly important for cell survival in the absence of functional p53.

The expression of gene transcripts of telomere-associated genes in human breast cancer: correlation with clinico-pathological parameters and clinical outcome

BACKGROUND

Telomerase is a ribonucleoprotein enzyme that synthesises telomeres in human germ cells, embryogenesis and in cancer, maintaining chromosomal length, stability and cellular immortality. The hTERT gene is the rate-limiting determinant of telomerase reactivation during immortalization and malignant transformation. Telomeric DNA-binding proteins have been attracting increasing interest due to their essential role in the regulation of telomeric DNA length and in protecting against chromosomal end-to-end fusion. These proteins include hTR, TRF1, TRF2, TANK1, TANK2, POT1, TIN2, EST1, and TEP. This study represents the first comprehensive investigation of the mRNA expression of key telomere-related genes in human breast cancer.

METHODS

One hundred and twenty seven tumour tissues and 33 normal tissues were analyzed. Levels of transcription of hTERT, hTR, TRF1, TRF2, TANK1, TANK2, POT1, TIN2, EST1, and TEP1 were determined using real-time quantitative PCR. The mRNA expression of these genes was normalized against CK19 and was then analyzed against the pathological parameters and clinical outcome over a 10 year follow up period.

RESULTS

The mRNA expressions of hTERT, hTR, TANK1, EST1, and TEP1 were higher in tumour samples compared with normal breast tissue. This reached statistical significance for EST1 when comparing good prognosis tumours with normal breast tissue (means=11013 vs 1160, P=0.05). Both hTERT and TEP1 levels significantly predicted overall survival (P=0.012 and 0.005 respectively) and disease-free survival (P=0.0011 and 0.01 respectively). The mRNA levels of TANK2 and POT1 were lower in malignant tissues compared with non-malignant breast tissues and this difference reached statistical significance when comparing the levels in normal tissues with those in advanced tumours (P=0.0008 and P=0.038 respectively). Their levels fell further with increasing tumour's stage and were higher in tumours from patients who remained disease free compared with those who developed local recurrence or distant metastasis or died from breast cancer.TRF2 showed a trend similar to that of TANK2 and POT1. Furthermore, there was a highly significant correlation between TANK1 expression and that of hTERT, hTR, TRF1, TRF2 and EST1, (r=0.533, 0.586, 0.608, 0.644 and 0.551 respectively, P<0.001).

CONCLUSIONS

Genes encoding telomere-associated proteins display different patterns of mRNA expression in human breast cancer, and in normal breast tissue, suggesting different and sometimes opposing roles in mammary carcinogenesis. hTERT, hTR, TANK1, EST1 and TEP1 seem to be up-regulated, with hTERT and TEP1 correlating with clinical outcome. Conversely, TANK2 and POT1 transcription levels demonstrate a compelling trend to be lower in malignant tissues and lower still in those patients who develop recurrent disease suggesting that TANK2 and POT1 may act as tumour suppressor genes possibly by negatively regulating telomerase activity.

A critical role for TPP1 and TIN2 interaction in high-order telomeric complex assembly

Mammalian telomeric proteins function through dynamic interactions with each other and telomere DNA. We previously reported the formation of a high-molecular-mass telomeric complex (the mammalian telosome) that contains the six core proteins TRF1, TRF2, RAP1, TIN2, POT1, and TPP1 (formerly named PTOP/PIP1/TINT1) and mediates telomere end-capping and length control. In this report, we sought to elucidate the mechanism of six-protein complex (or shelterin) formation and the function of this complex. Through reconstitution experiments, we demonstrate here that TIN2 and TPP1 are key components in mediating the six-protein complex assembly. We demonstrate that not only TIN2 but also TPP1 are required to bridge the TRF1 and TRF2 subcomplexes. Specifically, TPP1 helps to stabilize the TRF1-TIN2-TRF2 interaction and promote six-protein complex formation. Consistent with this model, overexpression of TPP1 enhanced TIN2-TRF2 association. Conversely, knocking down TPP1 reduced the ability of endogenous TRF1 to associate with the TRF2 complex. Our results suggest that coordinated interactions among TPP1, TIN2, TRF1, and TRF2 may ensure robust assembly of the telosome, telomere targeting of its subunits, and, ultimately, regulated telomere maintenance.

Shelterin: the protein complex that shapes and safeguards human telomeres

Added by telomerase, arrays of TTAGGG repeats specify the ends of human chromosomes. A complex formed by six telomere-specific proteins associates with this sequence and protects chromosome ends. By analogy to other chromosomal protein complexes such as condensin and cohesin, I will refer to this complex as shelterin. Three shelterin subunits, TRF1, TRF2, and POT1 directly recognize TTAGGG repeats. They are interconnected by three additional shelterin proteins, TIN2, TPP1, and Rap1, forming a complex that allows cells to distinguish telomeres from sites of DNA damage. Without the protective activity of shelterin, telomeres are no longer hidden from the DNA damage surveillance and chromosome ends are inappropriately processed by DNA repair pathways. How does shelterin avert these events? The current data argue that shelterin is not a static structural component of the telomere. Instead, shelterin is emerging as a protein complex with DNA remodeling activity that acts together with several associated DNA repair factors to change the structure of the telomeric DNA, thereby protecting chromosome ends. Six shelterin subunits: TRF1, TRF2, TIN2, Rap1, TPP1, and POT1.

Telomerase: regulation, function and transformation

Work from several laboratories over the past decade indicates that the acquisition of constitutive telomerase expression is a critical step during the malignant transformation of human cells. Normal human cells transiently express low levels of telomerase, the ribonucleoprotein responsible for extending and maintaining telomeres, and exhibit telomere shortening after extended passage, whereas most cancers exhibit constitutive telomerase expression and maintain telomeres at stable lengths. These observations establish a direct connection between immortalization and stabilization of telomere structure. However, recent work suggests that telomerase also contributes to cancer development beyond its role in maintaining stable telomere lengths. In this review, we summarize recent observations that support the concept that telomerase plays multiple roles in facilitating human cell transformation.

Cancer and aging: the importance of telomeres in genome maintenance

Telomeres are the specialized DNA-protein structures that cap the ends of linear chromosomes, thereby protecting them from degradation and fusion by cellular DNA repair processes. In vertebrate cells, telomeres consist of several kilobase pairs of DNA having the sequence TTAGGG, a few hundred base pairs of single-stranded DNA at the 3' end of the telomeric DNA tract, and a host of proteins that organize the telomeric double and single-stranded DNA into a protective structure. Functional telomeres are essential for maintaining the integrity and stability of genomes. When combined with loss of cell cycle checkpoint controls, telomere dysfunction can lead to genomic instability, a common cause and hallmark of cancer. Consequently, normal mammalian cells respond to dysfunctional telomeres by undergoing apoptosis (programmed cell death) or cellular senescence (permanent cell cycle arrest), two cellular tumor suppressor mechanisms. These tumor suppressor mechanisms are potent suppressors of cancer, but recent evidence suggests that they can antagonistically also contribute to aging phenotypes. Here, we review what is known about the structure and function of telomeres in mammalian cells, particularly human cells, and how telomere dysfunction may arise and contribute to cancer and aging phenotypes.

Up-regulation of telomere-binding proteins, TRF1, TRF2, and TIN2 is related to telomere shortening during human multistep hepatocarcinogenesis

The telomeric repeat-binding factor 1 (TRF1), TRF2, and the TRF1-interacting nuclear protein 2 (TIN2) are involved in telomere maintenance. We describe the regulation of expression of these genes along with their relationship to telomere length in hepatocarcinogenesis. The transcriptional expression of these genes, TRF1 protein, and telomere length was examined in 9 normal livers, 14 chronic hepatitis, 24 liver cirrhosis, 5 large regenerative nodules, 14 low-grade dysplastic nodules (DNs), 7 high-grade DNs, 10 DNs with hepatocellular carcinoma (HCC) foci, and 31 HCCs. The expression of TRF1, TRF2, TIN2 mRNA, and TRF1 protein was gradually increased according to the progression of hepatocarcinogenesis with a marked increase in high-grade DNs and DNs with HCC foci and a further increase in HCCs. There was a gradual shortening of telomere during hepatocarcinogenesis with a significant reduction in length in DNs. Most nodular lesions (52 of 67) had shorter telomeres than their adjacent chronic hepatitis or liver cirrhosis, and the telomere lengths were inversely correlated with the mRNA level of these genes (P </= 0.001). This was more evident in DNs and DNs with HCC foci. In conclusion, TRF1, TRF2, and TIN2 might be involved in multistep hepatocarcinogenesis by playing crucial roles in telomere shortening.

Higher-order nuclear organization in growth arrest of human mammary epithelial cells: a novel role for telomere-associated protein TIN2

Nuclear organization, such as the formation of specific nuclear subdomains, is generally thought to be involved in the control of cellular phenotype; however, there are relatively few specific examples of how mammalian nuclei organize during radical changes in phenotype, such as those occurring during differentiation and growth arrest. Using human mammary epithelial cells in which growth arrest is essential for morphological differentiation, we show that the arrest of cell proliferation is accompanied by a reorganization of the telomere-associated protein, TIN2, into one to three large nuclear subdomains. The large TIN2 domains do not contain telomeres and occur concomitant with the continued presence of TIN2 at telomeres. The TIN2 domains were sensitive to DNase, but not RNase, occurred frequently, but not exclusively near nucleoli, and overlapped often with dense domains containing heterochromatin protein 1gamma. Expression of truncated forms of TIN2 simultaneously prevented the formation of TIN2 domains and relaxed the stringent morphogenesis-induced growth arrest in human mammary epithelial cells. Here we show that a novel extra-telomeric organization of TIN2 is associated with the control of cell proliferation and identify TIN2 as an important regulator of mammary epithelial differentiation.

The telomerase cycle: normal and pathological aspects

Telomeres are nucleoprotein complexes that cap the end of eukaryotic chromosomes and are essential for their function and stability. Telomerase, a reverse transcriptase that extends the single-stranded G-rich 3' protruding ends of chromosomes, stabilizes telomere length in germ line cells and regenerative tissues as well as in tumor cells. In the absence of telomerase telomeres shorten with cell division, a process able to trigger cell growth arrest. When telomerase is present in the cell, its activity is tightly regulated at its site of action by factors specifically bound to the telomeric DNA. Recent data indicate that telomeres reorganize during the cell cycle. This review summarizes our current knowledge on how telomeres are dynamically organized and remodeled during cell cycle and stress response, pointing out the conservation and the difference between yeast and human. We then focus on the cellular consequences of telomere modifications in normal and cancer cells. This leads to a discussion of the different roles, seemingly contradictory, of telomeres and telomerase during the initiation and the progression of a cancer.

Telomere biology in mammalian germ cells and during development

The development of an organism is a strictly regulated program in which controlled gene expression guarantees the establishment of a specific phenotype. The chromosome termini or so-called telomeres preserve the integrity of the genome within developing cells. In the germline, during early development, and in highly proliferative organs, human telomeres are balanced between shortening processes with each cell division and elongation by telomerase, but once terminally differentiated or mature the equilibrium is shifted to gradual shortening by repression of the telomerase enzyme. Telomere length is to a large extent genetically determined and the neonatal telomere length equilibrium is, in fact, a matter of evolution. Gradual telomere shortening in normal human somatic cells during consecutive rounds of replication eventually leads to critically short telomeres that induce replicative senescence in vitro and probably in vivo. Hence, a molecular clock is set during development, which determines the replicative potential of cells during extrauterine life. Telomeres might be directly or indirectly implicated in longevity determination in vivo, and information on telomere length setting in utero and beyond should help elucidate presumed causal connections between early growth and aging disorders later in life. Only limited information exists concerning the mechanisms underlying overall telomere length regulation in the germline and during early development, especially in humans. The intent of this review is to focus on recent advances in our understanding of telomere biology in germline cells as well as during development (pre- and postimplantation periods) in an attempt to summarize our knowledge about telomere length determination and its importance for normal development in utero and the occurrence of the aging and abnormal phenotype later on.

TIN2 Mediates Functions of TRF2 at Human Telomeres

Telomeres are protective structures at chromosome ends and are crucial for genomic stability. Mammalian TRF1 and TRF2 bind the double-stranded telomeric repeat sequence and in turn are bound by TIN2, TANK1, TANK2, and hRAP1. TRF1 is a negative regulator of telomere length in telomerase-positive cells, whereas TRF2 is important for telomere capping. TIN2 was identified as a TRF1-interacting protein that mediates TRF1 function. We show here that TIN2 also interacts with TRF2 in vitro and in yeast and mammalian cells. TIN2 mutants defective in binding of TRF1 or TRF2 induce a DNA damage response and destabilize TRF1 and TRF2 at telomeres in human cells. Our findings suggest that the functions of TRF1 and TRF2 are linked by TIN2.

Telomere-associated protein TIN2 is essential for early embryonic development through a telomerase-independent pathway

TIN2 is a negative regulator of telomere elongation that interacts with telomeric DNA repeat binding factor 1 (TRF1) and affects telomere length by a telomerase-dependent mechanism. Here we show that inactivation of the mouse TRF1-interacting protein 2 (TIN2) gene results in early embryonic lethality. We further observed that the embryonic lethality of TIN2 mutant mice was not affected by inactivation of the telomerase reverse transcriptase gene, indicating that embryonic lethality is not the result of telomerase-dependent changes in telomere length or function. Our findings suggest that TIN2 has a role independent of telomere length regulation that is essential for embryonic development and cell viability.

Telomere attachment, meiotic chromosome condensation, pairing, and bouquet stage duration are modified in spermatocytes lacking axial elements

During the extended prophase to the meiosis I division, chromosomes assemble axial elements (AE) along replicated sister chromatids whose ends attach to the inner nuclear membrane (NM) via a specialized conical thickening. Here, we show at the EM level that in Sycp3(-/-) spermatocyte chromosomes lack the AE and the conical end thickening, but still they attach their telomeres to the inner NM with an electron-dense plate that contains T(2)AG(3) repeats. Immunofluorescence detected telomere proteins, SCP2, and the meiosis-specific cohesin STAG3 at the Sycp3(-/-) telomere. Bouquet stage spermatocytes were approximately threefold enriched, and the number of telomere but not centromere signals was reduced to the haploid in advanced Sycp3(-/-) spermatocytes, which indicates a special mode of homolog pairing at the mammalian telomere. Fluorescence in situ hybridization with mouse chromosome 8- and 12-specific subsatellite probes uncovered reduced levels of regional homolog pairing, whereas painting of chromosomes 13 revealed partial or complete juxtapositioning of homologs; however, condensation of Sycp3(-/-) bivalents was defective. Electron microscopic analysis of AE-deficient spermatocytes revealed that transverse filaments formed short structures reminiscent of the synaptonemal complex central region, which likely mediate stable homolog pairing. It appears that the AE is required for chromosome condensation, rapid exit from the bouquet stage, and fine-tuning of homolog pairing.