Caenorhabditis elegans telomere-binding proteins TEBP-1 and TEBP-2 adapt the Myb module to dimerize and bind telomeric DNA

Protecting chromosome ends from misrecognition as double-stranded (ds) DNA breaks is fundamental to eukaryotic viability. The protein complex shelterin prevents a DNA damage response at mammalian telomeres. Mammalian shelterin proteins TRF1 and TRF2 and their homologs in yeast and protozoa protect telomeric dsDNA. N-terminal homodimerization and C-terminal Myb-domain-mediated dsDNA binding are two structural hallmarks of end protection by TRF homologs. Yet our understanding of how Caenorhabditis elegans protects its telomeric dsDNA is limited. Recently identified C. elegans proteins TEBP-1 (also called DTN-1) and TEBP-2 (also called DTN-2) are functional homologs of TRF proteins, but how they bind DNA and whether or how they dimerize is not known. TEBP-1 and TEBP-2 harbor three Myb-containing domains (MCDs) and no obvious dimerization domain. We demonstrate biochemically that only the third MCD binds DNA. We solve the X-ray crystal structure of TEBP-2 MCD3 with telomeric dsDNA to reveal the structural mechanism of telomeric dsDNA protection in C. elegans. Mutagenesis of the DNA-binding site of TEBP-1 and TEBP-2 compromises DNA binding in vitro, and increases DNA damage signaling, lengthens telomeres, and decreases brood size in vivo. Via an X-ray crystal structure, biochemical validation of the dimerization interface, and SEC-MALS analysis, we demonstrate that MCD1 and MCD2 form a composite dimerization module that facilitates not only TEBP-1 and TEBP-2 homodimerization but also heterodimerization. These findings provide fundamental insights into C. elegans telomeric dsDNA protection and highlight how different eukaryotes have evolved distinct strategies to solve the chromosome end protection problem.

Exploring TRF2-Dependent DNA Distortion Through Single-DNA Manipulation Studies

TRF2 is a component of shelterin, a telomere-specific protein complex that protects the ends of mammalian chromosomes from DNA damage signaling and improper repair. TRF2 functions as a homodimer and its interaction with telomeric DNA has been studied, but its full-length DNA-binding properties are unknown. This study examines TRF2's interaction with single-DNA strands and focuses on the conformation of the TRF2-DNA complex and TRF2's preference for DNA chirality. The results show that TRF2-DNA can switch between extended and compact conformations, indicating multiple DNA-binding modes, and TRF2's binding does not have a strong preference for DNA supercoiling chirality when DNA is under low tension. Instead, TRF2 induces DNA bending under tension. Furthermore, both the N-terminal domain of TRF2 and the Myb domain enhance its affinity for the telomere sequence, highlighting the crucial role of multivalent DNA binding in enhancing its affinity and specificity for telomere sequence. These discoveries offer unique insights into TRF2's interaction with telomeric DNA.

The Altered Functions of Shelterin Components in ALT Cells

Telomeres are nucleoprotein complexes that cap the ends of eukaryotic linear chromosomes. Telomeric DNA is bound by shelterin protein complex to prevent telomeric chromosome ends from being recognized as damaged sites for abnormal repair. To overcome the end replication problem, cancer cells mostly preserve their telomeres by reactivating telomerase, but a minority (10-15%) of cancer cells use a homologous recombination-based pathway called alternative lengthening of telomeres (ALT). Recent studies have found that shelterin components play an important role in the ALT mechanism. The binding of TRF1, TRF2, and RAP1 to telomeres attenuates ALT activation, while the maintenance of ALT telomere requires TRF1 and TRF2. POT1 and TPP1 can also influence the occurrence of ALT. The elucidation of how shelterin regulates the initiation of ALT remains elusive. This review presents a comprehensive overview of the current findings on the regulation of ALT by shelterin components, aiming to enhance the insight into the altered functions of shelterin components in ALT cells and to identify potential targets for the treatment of ALT tumor cells.

Modified Peptide Molecules As Potential Modulators of Shelterin Protein Functions; TRF1

In this work, we present studies on relatively new and still not well-explored potential anticancer targets which are shelterin proteins, in particular the TRF1 protein can be blocked by in silico designed "peptidomimetic" molecules. TRF1 interacts directly with the TIN2 protein, and this protein-protein interaction is crucial for the proper functioning of telomere, which could be blocked by our novel modified peptide molecules. Our chemotherapeutic approach is based on assumption that modulation of TRF1-TIN2 interaction may be more harmful for cancer cells as cancer telomeres are more fragile than in normal cells. We have shown in vitro within SPR experiments that our modified peptide PEP1 molecule interacts with TRF1, presumably at the site originally occupied by the TIN2 protein. Disturbance of the shelterin complex by studied molecule may not in short term lead to cytotoxic effects, however blocking TRF1-TIN2 resulted in cellular senescence in cellular breast cancer lines used as a cancer model. Thus, our compounds appeared useful as starting model compounds for precise blockage of TRF proteins.

Phosphorylation of TRF2 promotes its interaction with TIN2 and regulates DNA damage response at telomeres

Protein phosphatase magnesium-dependent 1 delta (PPM1D) terminates the cell cycle checkpoint by dephosphorylating the tumour suppressor protein p53. By targeting additional substrates at chromatin, PPM1D contributes to the control of DNA damage response and DNA repair. Using proximity biotinylation followed by proteomic analysis, we identified a novel interaction between PPM1D and the shelterin complex that protects telomeric DNA. In addition, confocal microscopy revealed that endogenous PPM1D localises at telomeres. Further, we found that ATR phosphorylated TRF2 at S410 after induction of DNA double strand breaks at telomeres and this modification increased after inhibition or loss of PPM1D. TRF2 phosphorylation stimulated its interaction with TIN2 both in vitro and at telomeres. Conversely, induced expression of PPM1D impaired localisation of TIN2 and TPP1 at telomeres. Finally, recruitment of the DNA repair factor 53BP1 to the telomeric breaks was strongly reduced after inhibition of PPM1D and was rescued by the expression of TRF2-S410A mutant. Our results suggest that TRF2 phosphorylation promotes the association of TIN2 within the shelterin complex and regulates DNA repair at telomeres.

Increased amounts and stability of telomeric repeat-containing RNA (TERRA) following DNA damage induced by etoposide

Telomeric repeat-containing RNAs (TERRAs) are long noncoding RNAs transcribed from subtelomeres toward telomeric repeat tracts, which have been implicated in telomere protection and heterochromatin formation. Genotoxic stress leads to upregulation of TERRAs. However, the mechanism of DNA damage-mediated TERRA induction remains elusive. Here, we treated HeLa cells with etoposide, a DNA double-strand break-generating agent, for various times and monitored the levels of TERRAs. Etoposide treatment led to a gradual time-dependent increase in TERRAs. Etoposide-mediated induction was evident in many TERRAs arising from various chromosome loci, including 20q and XpYp. Chromatin immunoprecipitation assays revealed no significant changes in the occupancy of RNA polymerase II at telomeres upon etoposide treatment. Interestingly, TERRAs arising from 20q, XpYp, 10q, and 13q degraded at slower rates in cells treated with etoposide, while degradation rates of TERRAs from many loci tested were nearly identical in both etoposide- and mock-treated cells. Telomere damage occurred from early time points of etoposide treatment, but telomere lengths and abundance of telomeric repeat-binding factor 2 (TRF2) at telomeres remained unchanged. In summary, etoposide treatment led to telomere damage and TERRA accumulation, but telomere lengths and TRF2-mediated telomere integrity were maintained. Etoposide-mediated TERRA accumulation could be attributed partly to RNA stabilization. These findings may provide insight into the post-transcriptional regulation of TERRAs in response to DNA damage.

Genetic polymorphisms, mRNA expression levels of telomere-binding proteins, and associates with telomere damage in PAHs-Exposure workers

Coke oven emissions (COEs), confirmed human carcinogens, are mainly composed of polycyclic aromatic hydrocarbons (PAHs). Telomere shortening in blood leukocytes has been associated with COEs, and polymorphisms in metabolic enzymes. However, the relationship between polymorphisms in telomere related genes and telomere shortening in COEs exposed workers has never been evaluated. Therefore, we measured telomere length and mRNA expression levels of telomere-binding proteins (TBPs) by qPCR method in leucocyte from 544 COEs exposed workers and 238 office staffs (referents). Flight mass spectrometry was used to perform the genotyping of selected functional and susceptible SNPs. The results showed that the telomere length in the exposure group 0.75(0.51,1.08) was significantly shorter than that in the control group 1.05(0.76,1.44) (P < 0.001). The mRNA expression levels of TPP1, TERF1 and TERF2 genes in the exposure group were significantly lower than those in the control group (P < 0.05), the mRNA expression level of POT1 in the exposure group was significantly higher than that in the control group (P < 0.05). We used the wild homozygous genotype as a reference, subjects carrying TERT rs2736109 AA, TERT rs3215401 CC and TERT rs2736100 GT + GG genotypes had significantly longer telomere length in the exposure group (P < 0.05). In conclusion, the workers exposed to COEs had shorter telomere length, which was regulated by the TPP1, TERF1, TERF2 and POT1 genes expression levels, and the gene polymorphisms of TERT gene were associated with the telomere length among PAHs-exposure workers.

Genetic variation in five genes important in telomere biology and risk for breast cancer

Telomeres, consisting of TTAGGG nucleotide repeats and a protein complex at chromosome ends, are critical for maintaining chromosomal stability. Genomic instability, following telomere crisis, may contribute to breast cancer pathogenesis. Many genes critical in telomere biology have limited nucleotide diversity, thus, single nucleotide polymorphisms (SNPs) in this pathway could contribute to breast cancer risk. In a population-based study of 1995 breast cancer cases and 2296 controls from Poland, 24 SNPs representing common variation in POT1, TEP1, TERF1, TERF2 and TERT were genotyped. We did not identify any significant associations between individual SNPs or haplotypes and breast cancer risk; however, data suggested that three correlated SNPs in TERT (−1381C>T, −244C>T, and Ex2-659G>A) may be associated with reduced risk of breast cancer among individuals with a family history of breast cancer (odds ratios 0.73, 0.66, and 0.57, 95% confidence intervals 0.53–1.00, 0.46–0.95 and 0.39–0.84, respectively). In conclusion, our data do not support substantial overall associations between SNPs in telomere pathway genes and breast cancer risk. Intriguing associations with variants in TERT among women with a family history of breast cancer warrant follow-up in independent studies.

TRF2 is required for repair of nontelomeric DNA double-strand breaks by homologous recombination

TRF2 (telomeric repeat binding factor 2) is an essential component of the telomeric cap, where it forms and stabilizes the T-loop junctions. TRF2 forms the T-loops by stimulating strand invasion of the 3' overhang into duplex DNA. TRF2 also has been shown to localize to nontelomeric DNA double-strand breaks, but its functional role in DNA repair has not been examined. Here, we present evidence that TRF2 is involved in homologous recombination (HR) repair of nontelomeric double-strand breaks. Depletion of TRF2 strongly inhibited HR and delayed the formation of Rad51 foci after gamma-irradiation, whereas overexpression of TRF2 stimulated HR. Depletion of TRF2 had no effect on nonhomologous end-joining, and overexpression of TRF2 inhibited nonhomologous end-joining. We propose, based on our results and on the ability of TRF2 to mediate strand invasion, that TRF2 plays an essential role in HR by facilitating the formation of early recombination intermediates.

Telomere protection by mammalian Pot1 requires interaction with Tpp1

The shelterin complex at mammalian telomeres contains the single-stranded DNA-binding protein Pot1, which regulates telomere length and protects chromosome ends. Pot1 binds Tpp1, the shelterin component that connects Pot1 to the duplex telomeric DNA-binding proteins Trf1 and Trf2. Control of telomere length requires that Pot1 binds Tpp1 as well as the single-stranded telomeric DNA, but it is not known whether the protective function of Pot1 depends on Tpp1. Alternatively, Pot1 might function similarly to the Pot1-like proteins of budding and fission yeast, which have no known Tpp1-like connection to the duplex telomeric DNA. Using mutant mouse cells with diminished Tpp1 levels, RNA interference directed to mouse Tpp1 and Pot1, and complementation of mouse Pot1 knockout cells with human and mouse Pot1 variants, we show here that Tpp1 is required for the protective function of mammalian Pot1 proteins.

A RAP1/TRF2 complex inhibits nonhomologous end-joining at human telomeric DNA ends

The mechanisms by which telomeres are distinguished from DNA double-strand breaks are poorly understood. Here we have defined the minimal requirements for the protection of telomeric DNA ends from nonhomologous end-joining (NHEJ). Neither long, single-stranded overhangs nor t loop formation is essential to prevent NHEJ-mediated ligation of telomeric ends in vitro. Instead, a tandem array of 12 telomeric repeats is sufficient to impede illegitimate repair in a highly directional manner at nearby DNA ends. The polarity of end protection is consistent with the orientation of naturally occurring telomeres and is well suited to minimize interference between chromosome capping and the repair of DNA double-strand breaks in subtelomeric sequences. Biochemical fractionation and reconstitution revealed that telomere protection is mediated by a RAP1/TRF2 complex, providing evidence for a direct role for human RAP1 in the protection of telomeric DNA from NHEJ.

TRF2 overexpression diminishes repair of telomeric single-strand breaks and accelerates telomere shortening in human fibroblasts

Repair of single strand breaks in telomeric DNA is less efficient than in other genomic regions. This leads to an increased vulnerability of telomeric DNA towards damage induced by reactive oxygen species (ROS) and to accelerated telomere shortening under oxidative stress. The causes for the diminished repair efficacy in telomeres are unknown. We show here that overexpression of the telomere-binding protein TRF2 further reduces telomeric, but not genomic, single strand break repair. This suggests the possibility of strand break repair in telomeres being sterically hindered by the three-dimensional structure of the telomere DNA-protein complex and explains the effect of TRF2 on telomere shortening rates in telomerase-negative cells.

DNA damage responses in neural cells: Focus on the telomere

Postmitotic neurons must survive for the entire life of the organism and be able to respond adaptively to adverse conditions of oxidative and genotoxic stress. Unrepaired DNA damage can trigger apoptosis of neurons which is typically mediated by the ataxia telangiectasia mutated (ATM)-p53 pathway. As in all mammalian cells, telomeres in neurons consist of TTAGGG DNA repeats and several associated proteins that form a nucleoprotein complex that prevents chromosome ends from being recognized as double strand breaks. Proteins that stabilize telomeres include TRF1 and TRF2, and proteins known to play important roles in DNA damage responses and DNA repair including ATM, Werner and the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs). We have been performing studies of developing and adult neurons aimed at understanding the effects of global and telomere-directed DNA damage responses in neuronal plasticity and survival in the contexts of aging and neurodegenerative disorders. Deficits in specific DNA repair proteins, including DNA-PKcs and uracil DNA glycosylase (UDG), render neurons vulnerable to adverse conditions of relevance to the pathogenesis of neurodegenerative disorders such as Alzheimer's disease and stroke. Similarly, early postmitotic neurons with reduced telomerase activity exhibit accentuated responses to DNA damage and are prone to apoptosis demonstrating a pivotal role for telomere maintenance in both mitotic cells and postmitotic neurons. Our recent findings suggest key roles for TRF2 in regulating the differentiation and survival of neurons. TRF2 affects cell survival and differentiation by modulating DNA damage pathways, and gene expression. A better understanding of the molecular mechanisms by which neurons respond to global and telomere-specific DNA damage may reveal novel strategies for prevention and treatment of neurodegenerative disorders. Indeed, work in this and other laboratories has shown that dietary folic acid can protect neurons against Alzheimer's disease by keeping homocysteine levels low and thereby minimizing the misincorporation of uracil into DNA in neurons.

The basic domain of TRF2 directs binding to DNA junctions irrespective of the presence of TTAGGG repeats

The replication of long tracts of telomeric repeats may require specific factors to avoid fork regression (Fouché, N., Ozgür, S., Roy, D., and Griffith, J. (2006) Nucleic Acids Res., in press). Here we show that TRF2 binds to model replication forks and four-way junctions in vitro in a structure-specific but sequence-independent manner. A synthetic peptide encompassing the TRF2 basic domain also binds to DNA four-way junctions, whereas the TRF2 truncation mutant (TRF2(DeltaB)) and a mutant basic domain peptide do not. In the absence of the basic domain, the ability of TRF2 to localize to model telomere ends and facilitate t-loop formation in vitro is diminished. We propose that TRF2 plays a key role during telomere replication in binding chickenfoot intermediates of telomere replication fork regression. Junction-specific binding would also allow TRF2 to stabilize a strand invasion structure that is thought to exist at the strand invasion site of the t-loop.

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.

The G-quadruplex Ligand Telomestatin Inhibits POT1 Binding to Telomeric Sequences In vitro and Induces GFP-POT1 Dissociation from Telomeres in Human Cells

Telomestatin is a potent G-quadruplex ligand that specifically interacts with the 3' telomeric overhang, leading to its degradation and that induces a delayed senescence and apoptosis of cancer cells. Protection of Telomere 1 (POT1) was recently identified as a specific single-stranded telomere-binding protein involved in telomere capping and T-loop maintenance. We showed here that a telomestatin treatment inhibits POT1 binding to the telomeric overhang in vitro. The treatment of human EcR293 cells by telomestatin induces a dramatic and rapid delocalization of POT1 from its normal telomere sites but does not affect the telomere localization of the double-stranded telomere-binding protein TRF2. Thus, we propose that G-quadruplex stabilization at telomeric G-overhang inactivates POT1 telomeric function, generating a telomere dysfunction in which chromosome ends are no longer properly protected.

Telomere Protection: An Act of God

Telomeres are nucleoprotein structures that shelter the ends of linear chromosomes from being inappropriately recognized as DNA double-strand breaks. New work has revealed that Apollo, a nuclease previously implicated in DNA repair, also has a role in safeguarding telomeres during S phase.

ORC binding to TRF2 stimulates OriP replication

In higher eukaryotes, the origin recognition complex (ORC) lacks sequence-specific DNA binding, and it remains unclear what other factors specify an origin of DNA replication. The Epstein-Barr virus origin of plasmid replication (OriP) recruits ORC, but the precise mechanism of ORC recruitment and origin activation is not clear. We now show that ORC is recruited selectively to the dyad symmetry (DS) region of OriP as a consequence of direct interactions with telomere repeat factor 2 (TRF2) and ORC1. TRF-binding sites within DS stimulate replication initiation and facilitate ORC recruitment in vitro and in vivo. TRF2, but not TRF1 or hRap1, recruits ORC from nuclear extracts. The amino-terminal domain of TRF2 associated with a specific region of ORC1 and was necessary for stimulation of DNA replication. These results support a model in which TRF2 stimulates OriP replication activity by direct binding with ORC subunits.

hSnm1B Is a Novel Telomere-associated Protein

Artemis, a member of the beta-CASP family, has been implicated in the regulation of both telomere stability and length. Prompted by this, we examined whether the other two putative DNA-binding members of this family, hSnm1A and hSnm1B, may associate with telomeres. hSnm1A was found to not interact with the telomere. Conversely, hSnm1B was found to associate with telomeres in vivo by both immunofluorescence and chromatin immunoprecipitation. Furthermore, the C terminus of hSnm1B was shown to interact with the TRF homology domain of TRF2 indicating that hSnm1B is likely recruited to the telomere via interaction with the double-stranded telomere-binding protein TRF2.

Mammalian Rad9 plays a role in telomere stability, S- and G2-phase-specific cell survival, and homologous recombinational repair

The protein products of several rad checkpoint genes of Schizosaccharomyces pombe (rad1+, rad3+, rad9+, rad17+, rad26+, and hus1+) play crucial roles in sensing changes in DNA structure, and several function in the maintenance of telomeres. When the mammalian homologue of S. pombe Rad9 was inactivated, increases in chromosome end-to-end associations and frequency of telomere loss were observed. This telomere instability correlated with enhanced S- and G2-phase-specific cell killing, delayed kinetics of gamma-H2AX focus appearance and disappearance, and reduced chromosomal repair after ionizing radiation (IR) exposure, suggesting that Rad9 plays a role in cell cycle phase-specific DNA damage repair. Furthermore, mammalian Rad9 interacted with Rad51, and inactivation of mammalian Rad9 also resulted in decreased homologous recombinational (HR) repair, which occurs predominantly in the S and G2 phases of the cell cycle. Together, these findings provide evidence of roles for mammalian Rad9 in telomere stability and HR repair as a mechanism for promoting cell survival after IR exposure.

Critically short telomeres in acute myeloid leukemia with loss or gain of parts of chromosomes

Telomeres, nucleoprotein complexes at chromosome ends, protect chromosomes against end-to-end fusion. Previous in vitro studies in human fibroblast models indicated that telomere dysfunction results in chromosome instability. Loss of telomere function can result either from critical shortening of telomeric DNA or from loss of distinct telomere-capping proteins. It is less clear whether telomere dysfunction has an important role in human cancer development in vivo. Acute myeloid leukemia (AML) is a good model to study mechanisms that generate chromosome instability in human cancer development because distinct groups of AML are characterized either by aberrations that theoretically could result from telomere dysfunction (terminal deletions, gains/losses of chromosome parts, nonreciprocal translocations), or aberrations that are unlikely to result from telomere dysfunction (e.g., reciprocal translocations or inversions). Here we demonstrate that AML with multiple chromosome aberrations that theoretically could result from telomere dysfunction is invariably characterized by critically short telomeres. Short telomeres in this group are not associated with low telomerase activity or decreased expression of essential telomeric capping proteins TRF2 and POT1. In contrast, telomerase activity levels are significantly higher in AML with short telomeres. Notably, short telomeres in the presence of high telomerase may relate to significantly higher expression of TRF1, a negative regulator of telomere length. Our observations suggest that, consistent with previous in vitro fibroblast models, age-related critical telomere shortening may have a role in generating chromosome instability in human AML development.

TRF2 dysfunction elicits DNA damage responses associated with senescence in proliferating neural cells and differentiation of neurons

Telomeres are specialized structures at the ends of chromosomes that consist of tandem repeats of the DNA sequence TTAGGG and several proteins that protect the DNA and regulate the plasticity of the telomeres. The telomere-associated protein TRF2 (telomeric repeat binding factor 2) is critical for the control of telomere structure and function; TRF2 dysfunction results in the exposure of the telomere ends and activation of ATM (ataxia telangiectasin mutated)-mediated DNA damage response. Recent findings suggest that telomere attrition can cause senescence or apoptosis of mitotic cells, but the function of telomeres in differentiated neurons is unknown. Here, we examined the impact of telomere dysfunction via TRF2 inhibition in neurons (primary embryonic hippocampal neurons) and mitotic neural cells (astrocytes and neuroblastoma cells). We demonstrate that telomere dysfunction induced by adenovirus-mediated expression of dominant-negative TRF2 (DN-TRF2) triggers a DNA damage response involving the formation of nuclear foci containing phosphorylated histone H2AX and activated ATM in each cell type. In mitotic neural cells DN-TRF2 induced activation of both p53 and p21 and senescence (as indicated by an up-regulation of beta-galactosidase). In contrast, in neurons DN-TRF2 increased p21, but neither p53 nor beta-galactosidase was induced. In addition, TRF2 inhibition enhanced the morphological, molecular and biophysical differentiation of hippocampal neurons. These findings demonstrate divergent molecular and physiological responses to telomere dysfunction in mitotic neural cells and neurons, indicate a role for TRF2 in regulating neuronal differentiation, and suggest a potential therapeutic application of inhibition of TRF2 function in the treatment of neural tumors.

G-Quadruplex stabilization by telomestatin induces TRF2 protein dissociation from telomeres and anaphase bridge formation accompanied by loss of the 3′ telomeric overhang in cancer cells

Inhibition of telomerase activity by telomerase inhibitors induces a gradual loss of telomeres, and this in turn causes cancer cells to enter to a crisis stage. Here, we report the telomerase inhibitor telomestatin, which is known to stabilize G-quadruplex structures at 3' single-stranded telomeric overhangs (G-tails), rapidly dissociates TRF2 from telomeres in cancer cells within a week, when given at a concentration that does not cause normal cells to die. The G-tails were dramatically reduced upon short-term treatment with the drug in cancer cell lines, but not in normal fibroblasts and epithelial cells. In addition, telomestatin also induced anaphase bridge formation in cancer cell lines. These effects of telomestatin were similar to those of dominant negative TRF2, which also causes a prompt loss of the telomeric G-tails and induces an anaphase bridge. These results indicate that telomestatin exerts its anticancer effect not only through inhibiting telomere elongation, but also by rapidly disrupting the capping function at the very ends of telomeres. Unlike conventional telomerase inhibitors that require long-term treatments, the G-quadruplex stabilizer telomestatin induced prompt cell death, and it was selectively effective in cancer cells. This study also identifies the TRF2 protein as a therapeutic target for treating many types of cancer which have the TRF2 protein at caps of the telomere DNA of each chromosome.

Telomere length homeostasis requires that telomerase levels are limiting

Stabilization of telomere length in germline and highly proliferative human cells is required for long-term survival and for the immortal phenotype of cancer-derived cells. This is achieved through expression of telomerase reverse transcriptase (TERT), which synthesizes telomeric repeats through reverse transcription of its tightly associated RNA template (TR). The telomeric repeat binding factor TRF1 inhibits telomerase at telomeres in cis in a length-dependent manner to achieve telomere length homeostasis. Here we manipulate telomerase activity over a wide range in cancer and primary cells. Concomitant overexpression of TERT and TR was necessary and sufficient to substantially increase telomerase activity. Upon overexpression, more telomerase associated with telomeres and telomeres elongated at a constant rate (up to 0.8 kb/population doubling (PD)) in a length-independent manner. Thus, in less than 50 PDs, the length of telomeres increased 3-8-fold beyond physiological size, while telomere-bound TRF1 and TRF2 increased proportionally to telomere length. Thus, long telomeres do not permanently adopt a structural state that is non-extendible. A low cellular concentration of telomerase is critical to achieve preferential elongation of short telomeres and telomere length homeostasis.

Top-down analysis of basic proteins by microchip capillary electrophoresis mass spectrometry

A system of microchip capillary electrophoresis/electrospray ionization mass spectrometry (microchip-CE/ESI-MS) for rapid characterization of proteins has been developed. Capillary electrophoresis (CE) enables rapid analysis of a sample present in very small quantity, such as at femtomole levels, at high resolution. Faster CE/MS analysis is expected by downsizing the normal capillary to the microchip (microchip) capillary. Although rapidity and high resolution are advantages of CE separation, electroosmotic flow (EOF) instability caused by the interaction between proteins and the microchannel surface results in low reproducibility in the analysis of basic proteins under neutral pH conditions. By coating the microchannel surface with a basic polymer, polyE-323, basic proteins, which have pI values of over 7.5, could be separated and detected by microchip-CE/MS on quadrupole (Q) and time-of-flight (TOF) hybrid instruments. By increasing the cone and collision voltages during the analysis by microchip-CE/ESI-MS of a small protein, some product ions, which contain the sequence information, could also be obtained, i.e., 'top-down' analysis of the protein could be accomplished with this microchip-CE/MS system. To our knowledge, this is the first report of 'top-down' analysis of a protein by microchip-CE/MS. Since it requires a much shorter time and a smaller sample amount for analysis than the conventional liquid chromatography (LC)/ESI-MS method, microchip-CE/MS promises to be suitable for the high-throughput characterization of proteins.

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.

DNA damage-induced phosphorylation of the human telomere-associated protein TRF2

Several protein kinases from diverse eukaryotes known to perform important roles in DNA repair have also been shown to play critical roles in telomere maintenance. Here, we report that the human telomere-associated protein TRF2 is rapidly phosphorylated in response to DNA damage. We find that the phosphorylated form of TRF2 is not bound to telomeric DNA, as is the ground form of TRF2, and is rapidly localized to damage sites. Our results suggest that the ataxia-telangiectasia-mutated (ATM) protein kinase signal-transduction pathway is primarily responsible for the DNA damage-induced phosphorylation of TRF2. Unlike DNA damage-induced phosphorylation of other ATM targets, the phosphorylated form of TRF2 is transient, being detected rapidly at DNA damage sites postirradiation, but largely dissipated by 2 hours. In addition, we report that the phosphorylated form of TRF2 is present at telomeres in cell types undergoing telomere-based crisis and a recombination-driven, telomerase-independent, alternative lengthening of telomeres (ALT) pathway, likely as a consequence of a telomere-based DNA damage response. Our results link the induction of TRF2 phosphorylation to the DNA damage-response system, providing an example of direct cross-talk via a signaling pathway between these two major cellular processes essential for genomic stability, telomere maintenance, and DNA repair.

Inhibition of 6A8 alpha-mannosidase gene expression resulted in telomere length shortening in nasopharyngeal carcinoma cell CNE-2L2

Telomere length shortening was observed in the nasopharyngeal carcinoma cell CNE-2L2 when 6A8 alpha-mannosidase expression was inhibited by antisense 6A8 DNA. Transduction with mock or an irrelevant DNA did not affect the telomere length in the carcinoma cells. Telomerase activity and mRNA transcription of TRF 1 and 2 were not changed in the cells treated with antisense 6A8. The Con A binding test showed an enhancement on the proteins isolated from the cells treated with antisense 6A8, but not on those from mock- or irrelevant DNA-treated cells. The data imply an association between glycosylation modification with telomere shortening in antisense 6A8-treated cells.

Cell-specific nucleolar localization of TBP-related factor 2

TATA-binding protein (TBP)-related factor 2 (TRF2) is one of four closely related RNA polymerase II transcription factors. We compared the intracellular localizations of TBP and TRF2 during the cell cycle and mitosis in HeLa cells. We show that during interphase, endogenous or exogenously expressed TRF2 is located almost exclusively in the nucleolus in HeLa or Cos cells. TRF2 localization is not affected by stress or mitotic stimuli, but TRF2 is rapidly released from the nucleolus upon inhibition of pol I transcription or treatment by RNase. These results suggest that localization of HeLa TRF2 requires a nucleolar-associated RNA species. In contrast, in 3T3 fibroblast cells, exogenously expressed TRF2 localizes to the nucleoplasm. Constitutive expression of ectopic TRF2 in 3T3 cells leads to a prolonged S phase of the cell cycle and reduced proliferation. Together with previous data, our results highlight the cell-specific localization and functions of TRF2. Furthermore, we show that during cell division, HeLa TRF2 and TBP are localized in the mitotic cytoplasm and TRF2 relocalizes into the nascent nucleoli immediately after mitosis, whereas TBP reassociates with the chromatin. Although partially contradictory results have been reported, our data are consistent with a model where only small proportion of the cellular TBP remains associated with specific promoter loci during mitosis.

The telomeric protein TRF2 binds the ATM kinase and can inhibit the ATM-dependent DNA damage response

The telomeric protein TRF2 is required to prevent mammalian telomeres from activating DNA damage checkpoints. Here we show that overexpression of TRF2 affects the response of the ATM kinase to DNA damage. Overexpression of TRF2 abrogated the cell cycle arrest after ionizing radiation and diminished several other readouts of the DNA damage response, including phosphorylation of Nbs1, induction of p53, and upregulation of p53 targets. TRF2 inhibited autophosphorylation of ATM on S1981, an early step in the activation of this kinase. A region of ATM containing S1981 was found to directly interact with TRF2 in vitro, and ATM immunoprecipitates contained TRF2. We propose that TRF2 has the ability to inhibit ATM activation at telomeres. Because TRF2 is abundant at chromosome ends but not elsewhere in the nucleus, this mechanism of checkpoint control could specifically block a DNA damage response at telomeres without affecting the surveillance of chromosome internal damage.

Different telomere damage signaling pathways in human and mouse cells

Programmed telomere shortening in human somatic cells is thought to act as a tumor suppressor pathway, limiting the replicative potential of developing tumor cells. Critically short human telomeres induce senescence either by activating p53 or by inducing the p16/RB pathway, and suppression of both pathways is required to suppress senescence of aged human cells. Here we report that removal of TRF2 from human telomeres and the ensuing de-protection of chromosome ends induced immediate premature senescence. Although the telomeric tracts remained intact, the TRF2(DeltaBDeltaM)-induced premature senescence was indistinguishable from replicative senescence and could be mediated by either the p53 or the p16/RB pathway. Telomere de-protection also induced a growth arrest and senescent morphology in mouse cells. However, in this setting the loss of p53 function was sufficient to completely abrogate the arrest, indicating that the p16/RB response to telomere dysfunction is not active in mouse cells. These findings reveal a fundamental difference in telomere damage signaling in human and mouse cells that bears on the use of mouse models for the telomere tumor suppressor pathway.

[Identification of genes associated with human osteosarcoma metastasis suppression using suppression subtractive hybridization]

OBJECTIVE

To identify genes associated with metastasis suppression and to investigate the molecular mechanism of osteosarcoma metastasis.

METHODS

The subtracted cDNA library of low metastatic human osteosarcoma cell line SOSP-9607 was constructed using suppression subtractive hybridization. Partial clones were sequenced. The acquired sequence data were aligned against the GenBank nucleotide database using Blastn to search for sequence matches. The interested clone was used to perform Northern blot and reverse transcriptase-PCR (RT-PCR) analysis on mRNA isolated from low metastatic cell line SOSP-9607 and OS-9901, high metastatic cell line SOSP-M and three pulmonic metastatic nodules of nude mice.

RESULTS

A cDNA clone from low metastatic cell line SOSP-9607 subtracted cDNA library was identified as telomeric repeat binding factor 2 (TERF2) by sequence analysis and Blastn search. Northern blot and RT-PCR analysis demonstrated that TERF2 expressed highly in low metastatic cell line SOSP-9607 and OS-9901, but not in high metastatic cell line SOSP-M and three pulmonic metastatic nodules.

CONCLUSION

TERF2 may be important for suppressing metastasis of osteosarcoma.

Targeting assay to study the cis functions of human telomeric proteins: evidence for inhibition of telomerase by TRF1 and for activation of telomere degradation by TRF2

We investigated the control of telomere length by the human telomeric proteins TRF1 and TRF2. To this end, we established telomerase-positive cell lines in which the targeting of these telomeric proteins to specific telomeres could be induced. We demonstrate that their targeting leads to telomere shortening. This indicates that these proteins act in cis to repress telomere elongation. Inhibition of telomerase activity by a modified oligonucleotide did not further increase the pace of telomere erosion caused by TRF1 targeting, suggesting that telomerase itself is the target of TRF1 regulation. In contrast, TRF2 targeting and telomerase inhibition have additive effects. The possibility that TRF2 can activate a telomeric degradation pathway was directly tested in human primary cells that do not express telomerase. In these cells, overexpression of full-length TRF2 leads to an increased rate of telomere shortening.

p53 binds telomeric single strand overhangs and t-loop junctions in vitro

The interaction of p53 with a human model telomere in vitro was examined by electron microscopy. p53 demonstrated a sequence-independent affinity for telomeric DNA in vitro, localizing to the 3' single strand overhang and the t-loop junction both in the presence and absence of associated TRF2. Binding was not observed above background along the duplex telomeric repeats. However, the efficiency of TRF2-catalyzed t-loop formation on the model DNA was increased 2-fold in the presence of p53 although a variety of single strand or Holliday junction-binding proteins did not facilitate t-loop formation. These results suggest that p53 has an active role in telomere maintenance and structure through association with the t-loop junction.

Senescence induced by altered telomere state, not telomere loss

Primary human cells in culture invariably stop dividing and enter a state of growth arrest called replicative senescence. This transition is induced by programmed telomere shortening, but the underlying mechanisms are unclear. Here, we report that overexpression of TRF2, a telomeric DNA binding protein, increased the rate of telomere shortening in primary cells without accelerating senescence. TRF2 reduced the senescence setpoint, defined as telomere length at senescence, from 7 to 4 kilobases. TRF2 protected critically short telomeres from fusion and repressed chromosome-end fusions in presenescent cultures, which explains the ability of TRF2 to delay senescence. Thus, replicative senescence is induced by a change in the protected status of shortened telomeres rather than by a complete loss of telomeric DNA.

Gene expression for suppressors of telomerase activity (telomeric-repeat binding factors) in breast cancer

Mechanisms regulating telomerase activity and telomere length remain incompletely understood in human breast cancer. We therefore studied gene expression for telomeric-repeat binding factors (TRFs) in relation to telomerase activity, telomere length, and clinicopathologic factors in human breast cancer. Telomerase activity was detected in 65.8% of 38 breast cancers, but none of 16 noncancerous samples. Terminal restriction fragments were longer in noncancerous than in cancerous tissues, but not significantly. Among 8 patients with both cancer and paired noncancerous tissue available for terminal restriction fragments length assay, terminal restriction fragments were shorter in cancers than in paired noncancerous samples in all but one. Significantly more mRNA encoding TRF1 and 2 was detected in noncancerous than in cancer tissues. Additionally, expression of TRF1 and 2 mRNA was significantly higher in cancers without detectable telomerase activity than in cancers showing activity. Expression of these genes tended to show a negative correlation with terminal restriction fragments length, but this was not statistically significant. No correlation was seen between TRF1 or 2 mRNA expression, and clinicopathologic factors except for TRF1 with respect to tumor size and progesterone receptor status. In addition to reactivation of telomerase activity, escape from negative regulation of this activity is needed to maintain telomere length during cell proliferation in breast cancer. Genes encoding telomerase inhibitors might be of value in gene therapy against human breast cancer.

Telomeric proteins regulate episomal maintenance of Epstein-Barr virus origin of plasmid replication

Episomal maintenance and DNA replication of EBV origin of plasmid replication (OriP) plasmid maintenance is mediated by the viral encoded origin binding protein, EBNA1, and unknown cellular factors. We found that telomeric repeat binding factor 2 (TRF2), TRF2-interacting protein hRap1, and the telomere-associated poly(ADP-ribose) polymerase (Tankyrase) bound to the dyad symmetry (DS) element of OriP in an EBNA1-dependent manner. TRF2 bound cooperatively with EBNA1 to the three nonamer sites (TTAGGGTTA), which resemble telomeric repeats. Mutagenesis of the nonamers reduced plasmid maintenance function and increased plasmid sensitivity to genotoxic stress. DS affinity-purified proteins possessed poly(ADP-ribose) polymerase (PARP) activity, and EBNA1 was subject to NAD-dependent posttranslational modification in vitro. OriP plasmid maintenance was sensitive to changes in cellular PARP/Tankyrase activity. These findings imply that telomere-associated proteins regulate OriP plasmid maintenance by PAR-dependent modifications.

Breaks at telomeres and TRF2-independent end fusions in Fanconi anemia

Fanconi anemia (FA) is a rare genetic disease characterized by chromosome instability, progressive pancytopenia and cancer susceptibility. Telomeres are intimately related to chromosome stability and play an important role in organismal viability at the hematological level. Since previous works suggested an accelerated shortening of telomeres in FA, we have studied several markers of telomere integrity and function in FA patients and age-matched controls to get insights into the mechanisms and consequences of telomere erosion in FA. A higher frequency of extra-chromosomic TTAGGG signals and of chromosome ends with undetectable TTAGGG repeats was observed in FA cells by fluorescence in situ hybridization (FISH), suggesting intensive breakage at telomeric sequences. This was proven by measuring the frequency of excess of telomeric signals per cell, which was 2.8-fold higher in FA. Consistent with previous reports, quantitative FISH analysis showed an accelerated telomere shortening of 0.68 kb in FA, which occurred concurrently in both chromosome arms in a similar magnitude. Our data therefore suggest that the telomere erosion in FA is caused by a higher rate of breakage at TTAGGG sequences in vivo in differentiated cells, in addition to mere replicative shortening during lymphocyte proliferation. Consistent with impaired telomeres in FA patients, we observed a >10-fold increase in chromosome end fusions in FA compared to normal controls. This observation was independent of TRF2, a telomere binding factor that protects human telomeres from end fusions, since immunohistochemistry studies in FA cell lines and corrected counterparts by retrovirus-mediated transfer of FANCA and FANCD2 cDNA showed that a functional FA pathway is not required for telomere binding of TRF2.

A human interstitial telomere associates in vivo with specific TRF2 and TIN2 proteins

Mammalian telomeres are composed of long arrays of TTAGGG repeats that form a nucleoprotein complex which protects the chromosome ends. Human telomere function is known to require two TTAGGG repeat factors, TRF1 and TRF2, and several interacting proteins, but the mechanism by which the DNA/protein complex prevents end to end fusion in vivo has not been elucidated. In order to better understand the role of specific telomere-associated proteins in the organisation of chromosome ends, we have studied a patient with a rare chromosome rearrangement that has given rise to an interstitial telomere. Using specific antibodies and immuno-FISH on unfixed metaphase chromosomes, we show that the proteins TRF2 and TIN2 (TIN2 interacts with TRF1) co-localise with the interstitial TTAGGG repeats. Our results demonstrate, for the first time in humans, that TRF2 and TIN2 proteins associate with interstitial duplex TTAGGG repeats, in vivo. They confirm that double stranded-telomeric repeats, even when complexed with specific proteins, are not sufficient to create a functional telomere. Finally, they suggest a possible role for proteins in stabilising interstitial TTAGGG repeats.

Protection of mammalian telomeres

Telomeres allow cells to distinguish natural chromosome ends from damaged DNA. When telomere function is disrupted, a potentially lethal DNA damage response can ensue, DNA repair activities threaten the integrity of chromosome ends, and extensive genome instability can arise. It is not clear exactly how the structure of telomere ends differs from sites of DNA damage and how telomeres protect chromosome ends from DNA repair activities. What are the defining structural features of telomeres and through which mechanisms do they ensure chromosome end protection? What is the molecular basis of the telomeric cap and how does it act to sequester the chromosome end? Here I discuss data gathered in the last few years, suggesting that the protection of human chromosome ends primarily depends on the telomeric protein TRF2 and that telomere capping involves the formation of a higher order structure, the telomeric loop or t-loop.

T-loop assembly in vitro involves binding of TRF2 near the 3' telomeric overhang

Mammalian telomeres contain a duplex TTAGGG-repeat tract terminating in a 3' single-stranded overhang. TRF2 protein has been implicated in remodeling telomeres into duplex lariats, termed t-loops, in vitro and t-loops have been isolated from cells in vivo. To examine the features of the telomeric DNA essential for TRF2-promoted looping, model templates containing a 500 bp double-stranded TTAGGG tract and ending in different single-stranded overhangs were constructed. As assayed by electron microscopy, looped molecules containing most of the telomeric tract are observed with TRF2 at the loop junction. A TTAGGG-3' overhang of at least six nucleotides is required for loop formation. Termini with 5' overhangs, blunt ends or 3' termini with non-telomeric sequences at the junction are deficient in loop formation. Addition of non-telomeric sequences to the distal portion of a 3' overhang beginning with TTAGGG repeats only modestly diminishes looping. TRF2 preferentially localizes to the junction between the duplex repeats and the single-stranded overhang. Based on these findings we suggest a model for the mechanism by which TRF2 remodels telomeres into t-loops.

Strand-specific postreplicative processing of mammalian telomeres

Telomeres are specialized nucleoprotein structures that stabilize the ends of linear eukaryotic chromosomes. In mammalian cells, abrogation of telomeric repeat binding factor TRF2 or DNA-dependent protein kinase (DNA-PK) activity causes end-to-end chromosomal fusion, thus establishing an essential role for these proteins in telomere function. Here we show that TRF2-mediated end-capping occurs after telomere replication. The postreplicative requirement for TRF2 and DNA-PKcs, the catalytic subunit of DNA-PK, is confined to only half of the telomeres, namely, those that were produced by leading-strand DNA synthesis. These results demonstrate a crucial difference in postreplicative processing of telomeres that is linked to their mode of replication.

Telomere maintenance by telomerase and by recombination can coexist in human cells

Immortal human cells maintain their telomeres by two independent mechanisms, a prevalent one dependent on de novo synthesis of telomeric DNA by telomerase, and a rarer one based on telomere recombination [alternative lengthening of telomeres (ALT)]. Studies with yeast have indicated that expression of telomerase inhibits telomere recombination. In the present study, we have investigated whether expression of telomerase in cells that use ALT would similarly reveal dominance of telomere elongation by telomerase over telomere recombination. Telomerase-negative WI38 VA13/2RA ALT cells were reconstituted for telomerase activity through ectopic expression of the enzyme subunits, hTERT and hTR, and the presence and function of telomerase and ALT were monitored during long term cell growth by enzymatic assays, detection of the ALT-associated PML bodies (APBs) and analysis of telomere dynamics. Our results indicate that telomerase activity and APBs persisted in the cells over at least 90 population doublings. The activity of both pathways on telomeres was determined by analysis of telomere length versus time by gel electrophoresis and in situ hybridization. ALT cells are characterized by very heterogeneous telomeres with a much longer average size than the telomeres of telomerase-positive cells. Telomere dynamics in our cells were compatible with both ALT and telomerase being biologically active since the long telomeres typical of ALT were maintained, while short telomeres, thought to be the preferential substrate of telomerase, were elongated. These findings, indicating that human cells may be capable of concomitantly utilizing both mechanisms of telomere maintenance without effects on their growth and viability, have implications for cancer therapy.

Expression of telomeric repeat binding factor 1 and 2 and TRF1-interacting nuclear protein 2 in human gastric carcinomas

Telomeric repeat binding factor 1 (TRF1) and 2 (TRF2) may play key roles in the maintenance of telomere function. TRF1 negatively regulates telomere elongation, while TRF2 protects the chromosome ends by inhibiting end-to-end fusions. We examined the expression of TRF1 and TRF2 in 20 gastric carcinomas by reverse transcription polymerase chain reaction and then analyzed the relation with telomerase activity and other telomerase components such as human telomerase reverse transcriptase (TERT), human telomerase RNA component (hTR), human telomerase-associated protein (TEP1) and TRF1-interacting, ankyrin-related ADP-ribose polymerase (tankyrase) as well as TRF1-interacting nuclear protein 2 (TIN2). Of 20 gastric carcinomas examined, 10 (50%) and 12 (60%) expressed TRF1 and TRF2 at higher levels than did non-neoplastic mucosa, respectively. No obvious correlation was observed between TRF1 expression and telomerase activity or expression of TERT, hTR and TEP1. Carcinomas with high TRF1 expression expressed significantly higher levels of tankyrase and TIN2 than did those with low TRF2 expression (p<0.05). The telomerase activities and the levels of TERT, hTR and TEP1 showed tendency to be lower in tumors expressing TRF1 at low levels, although it was not significant. On the other hand, carcinomas with short telomere length (shorter than 2 Kbp) expressed significantly stronger telomerase activities and higher TRF1 expression (p<0.05) and tended to express TRF2 and TIN2 at higher levels than those with long telomere length. The results suggest that gastric carcinomas with short telomeres need high levels of telomerase activity and large quantity of TRFs and TIN2, whereas those with long telomeres do not require high levels of telomerase activity and telomere associated proteins.

DNA damage-induced cell cycle checkpoints involve both p53-dependent and -independent pathways: role of telomere repeat binding factor 2

Treatment of colon cancer cells with MNNG causes DNA damage with reduced telomeric signals in a p53-dependent manner, but increased cell cycle arrest in S-G(2)/M by both p53-dependent and independent mechanisms. Results also indicate that cellular levels of TRF2 may play a critical role in MNNG-induced cell cycle arrest and apoptosis of colon cancer cells.

Structure of the TRFH dimerization domain of the human telomeric proteins TRF1 and TRF2

TRF1 and TRF2 are key components of vertebrate telomeres. They bind to double-stranded telomeric DNA as homodimers. Dimerization involves the TRF homology (TRFH) domain, which also mediates interactions with other telomeric proteins. The crystal structures of the dimerization domains from human TRF1 and TRF2 were determined at 2.9 and 2.2 A resolution, respectively. Despite a modest sequence identity, the two TRFH domains have the same entirely alpha-helical architecture, resembling a twisted horseshoe. The dimerization interfaces feature unique interactions that prevent heterodimerization. Mutational analysis of TRF1 corroborates the structural data and underscores the importance of the TRFH domain in dimerization, DNA binding, and telomere localization. A possible structural homology between the TRFH domain of fission yeast telomeric protein Taz1 with those of the vertebrate TRFs is suggested.

Infection of synoviocytes with HTLV-I induces telomerase activity

To investigate the mechanism of synovial hyperplasia by human T-lymphotropic virus type I (HTLV-I) infection, the enzymatic activity of telomerase and expression of telomerase-related factors in HTLV-I infected synoviocytes were examined. Cultured synoviocytes obtained from four patients with osteoarthritis (OA) and four with traumatic joint disease (TJD) were infected by HTLV-I. Telomerase activity was detected by telomeric repeat amplification protocol (TRAP) assay. Expression of telomerase-related mRNAs such as telomerase reverse transcriptase (hTERT), telomerase RNA component (hTERC), and telomeric repeat binding factor 2 (TRF2) were also examined. Telomerase activity was detected in all HTLV-I-infected synoviocytes but not in uninfected synoviocytes. A remarkable induction of hTERT mRNA was observed in four of eight HTLV-I-infected synoviocytes, whereas expressions of hTERC, TRF2, and TEP-1 mRNAs were not changed. Our results clearly demonstrate that HTLV-I upregulates telomerase activity in synoviocytes probably via upregulation of hTERT activity. These findings suggest that telomerase activation in synoviocytes has an important role in upregulated proliferative activity of HAAP synoviocytes.

Spermiogenesis deficiency in mice lacking the Trf2 gene

The discovery of TATA-binding protein-related factors (TRFs) has suggested alternative mechanisms for gene-specific transcriptional regulation and raised interest in their biological functions. In contrast to recent observations of an embryonic lethal phenotype for TRF2 inactivation in Caenorhabditis elegans and Xenopus laevis, we found that Trf2-deficient mice are viable. However, Trf2-/- mice are sterile because of a severe defect in spermiogenesis. Postmeiotic round spermatids advance at most to step 7 of differentiation but fail to progress to the elongated form, and gene-specific transcription deficiencies were identified. We speculate that mammals may have evolved more specialized TRF2 functions in the testis that involve transcriptional regulation of genes essential for spermiogenesis.