Conserved telomere maintenance component 1 interacts with STN1 and maintains chromosome ends in higher eukaryotes

Dynamic Interactions of Arabidopsis TEN1: Stabilizing Telomeres in Response to Heat Stress

Telomeres are the essential nucleoprotein structures that provide a physical cap for the ends of linear chromosomes. The highly conserved CST (CTC1/STN1/TEN1) protein complex facilitates telomeric DNA replication and promotes telomere stability. Here we report three unexpected properties of Arabidopsis thaliana TEN1 that indicate it possesses functions distinct from other previously characterized telomere proteins. First, we show that telomeres in ten1 mutants are highly sensitive to thermal stress. Heat shock causes abrupt and dramatic loss of telomeric DNA in ten1 plants, likely via deletional recombination. Second, we show that AtTEN1 has the properties of a heat-shock induced molecular chaperone. At elevated temperature, AtTEN1 rapidly assembles into high molecular weight homo-oligomeric complexes that efficiently suppress heat-induced aggregation of model protein substrates in vitro. Finally, we report that AtTEN1 specifically protects CTC1 from heat-induced aggregation in vitro, and from heat-induced protein degradation and loss of telomere association in vivo. Collectively, these observations define Arabidopsis TEN1 as a highly dynamic protein that works in concert with CTC1 to preserve telomere integrity in response to environmental stress.

Quantitative telomeric chromatin isolation protocol for human cells

The ends of eukaryotic chromosomes, known as telomeres, consist of repetitive DNA sequences, multiple proteins and noncoding RNAs. Telomeres are dynamic structures that play crucial roles as guardians of genome stability and tumor suppressors. Defects in telomere length or protein composition can accelerate aging and are seen in telomere syndromes, which affect various proliferative tissues such as the bone marrow or the lungs. One of the biggest challenges in the telomere field is to identify the molecular changes at telomeres that occur during normal development, in cancer and in telomere syndromes. To tackle this problem, our laboratory has established a quantitative telomeric chromatin isolation protocol (QTIP) for human cells, in which chromatin is cross-linked, immunopurified and analyzed by mass spectrometry. QTIP involves stable isotope labeling by amino acids in cell culture (SILAC) to compare and identify quantitative differences in telomere protein composition of cells from various states.

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

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

Mechanisms Used by Plants to Cope with DNA Damage

Because the genome stores all genetic information required for growth and development, it is of pivotal importance to maintain DNA integrity, especially during cell division, when the genome is prone to replication errors and damage. Although over the last two decades it has become evident that the basic cell cycle toolbox of plants shares several similarities with those of fungi and mammals, plants appear to have evolved a set of distinct checkpoint regulators in response to different types of DNA stress. This might be a consequence of plants' sessile lifestyle, which exposes them to a set of unique DNA damage-inducing conditions. In this review, we highlight the types of DNA stress that plants typically experience and describe the plant-specific molecular mechanisms that control cell division in response to these stresses.

A POT1 mutation implicates defective telomere end fill-in and telomere truncations in Coats plus

Coats plus (CP) can be caused by mutations in the CTC1 component of CST, which promotes polymerase α (polα)/primase-dependent fill-in throughout the genome and at telomeres. The cellular pathology relating to CP has not been established. We identified a homozygous POT1 S322L substitution (POT1(CP)) in two siblings with CP. POT1(CP)induced a proliferative arrest that could be bypassed by telomerase. POT1(CP)was expressed at normal levels, bound TPP1 and telomeres, and blocked ATR signaling. POT1(CP)was defective in regulating telomerase, leading to telomere elongation rather than the telomere shortening observed in other telomeropathies. POT1(CP)was also defective in the maintenance of the telomeric C strand, causing extended 3' overhangs and stochastic telomere truncations that could be healed by telomerase. Consistent with shortening of the telomeric C strand, metaphase chromosomes showed loss of telomeres synthesized by leading strand DNA synthesis. We propose that CP is caused by a defect in POT1/CST-dependent telomere fill-in. We further propose that deficiency in the fill-in step generates truncated telomeres that halt proliferation in cells lacking telomerase, whereas, in tissues expressing telomerase (e.g., bone marrow), the truncations are healed. The proposed etiology can explain why CP presents with features distinct from those associated with telomerase defects (e.g., dyskeratosis congenita).

Identification of Genes That Modulate Susceptibility to Formaldehyde and Imatinib by Functional Genomic Screening in Human Haploid KBM7 Cells

Though current functional genomic screening systems are useful for investigating human susceptibility to chemical toxicity, they have limitations. Well-established, high-throughput yeast mutant screens identify only evolutionarily conserved processes. RNA interference can be applied in human cells but is limited by incomplete gene knockout and off-target effects. Human haploid cell screening is advantageous as it requires knockdown of only a single copy of each gene. A human haploid cell mutant library (KBM7-Mu), derived from a chronic myeloid leukemia (CML) patient, was recently developed and has been used to identify genes that modulate sensitivity to infectious agents and pharmaceutical drugs. Here, we sought to improve the KBM7-Mu screening process to enable efficient screening of environmental chemicals. We developed a semi-solid medium based screening approach that cultures individual mutant colonies from chemically resistant cells, faster (by 2-3 weeks) and with less labor than the original liquid medium-based approach. As proof of principle, we identified genetic mutants that confer resistance to the carcinogen formaldehyde (FA, 12 genes, 18 hits) and the CML chemotherapeutic agent imatinib (6 genes, 13 hits). Validation experiments conducted on KBM7 mutants lacking each of the 18 genes confirmed resistance of 6 FA mutants (CTC1, FCRLA, GOT1, LPR5, M1AP, and MAP2K5) and 1 imatinib-resistant mutant (LYRM9). Despite the improvements to the method, it remains technically challenging to limit false positive findings. Nonetheless, our findings demonstrate the broad applicability of this optimized haploid approach to screen toxic chemicals to identify novel susceptibility genes and gain insight into potential mechanisms of toxicity.

Telomeres and telomerase as therapeutic targets to prevent and treat age-related diseases

Telomeres, the protective ends of linear chromosomes, shorten throughout an individual's lifetime. Telomere shortening is a hallmark of molecular aging and is associated with premature appearance of diseases associated with aging. Here, we discuss the role of telomere shortening as a direct cause for aging and age-related diseases. In particular, we draw attention to the fact that telomere length influences longevity. Furthermore, we discuss intrinsic and environmental factors that can impact on human telomere erosion. Finally, we highlight recent advances in telomerase-based therapeutic strategies for the treatment of diseases associated with extremely short telomeres owing to mutations in telomerase, as well as age-related diseases, and ultimately aging itself.

Using Centromere Mediated Genome Elimination to Elucidate the Functional Redundancy of Candidate Telomere Binding Proteins in Arabidopsis thaliana

Proteins that bind to telomeric DNA form the key structural and functional constituents of telomeres. While telomere binding proteins have been described in the majority of organisms, their identity in plants remains unknown. Several protein families containing a telomere binding motif known as the telobox have been previously described in Arabidopsis thaliana. Nonetheless, functional evidence for their involvement at telomeres has not been obtained, likely due to functional redundancy. Here we performed genetic analysis on the TRF-like family consisting of six proteins (TRB1, TRP1, TRFL1, TRFL2, TRFL4, and TRF9) which have previously shown to bind telomeric DNA in vitro. We used haploid genetics to create multiple knock-out plants deficient for all six proteins of this gene family. These plants did not exhibit changes in telomere length, or phenotypes associated with telomere dysfunction. This data demonstrates that this telobox protein family is not involved in telomere maintenance in Arabidopsis. Phylogenetic analysis in major plant lineages revealed early diversification of telobox proteins families indicating that telomere function may be associated with other telobox proteins.

Telomere- and Telomerase-Associated Proteins and Their Functions in the Plant Cell

Telomeres, as physical ends of linear chromosomes, are targets of a number of specific proteins, including primarily telomerase reverse transcriptase. Access of proteins to the telomere may be affected by a number of diverse factors, e.g., protein interaction partners, local DNA or chromatin structures, subcellular localization/trafficking, or simply protein modification. Knowledge of composition of the functional nucleoprotein complex of plant telomeres is only fragmentary. Moreover, the plant telomeric repeat binding proteins that were characterized recently appear to also be involved in non-telomeric processes, e.g., ribosome biogenesis. This interesting finding was not totally unexpected since non-telomeric functions of yeast or animal telomeric proteins, as well as of telomerase subunits, have been reported for almost a decade. Here we summarize known facts about the architecture of plant telomeres and compare them with the well-described composition of telomeres in other organisms.

The molecular genetics of the telomere biology disorders

The importance of telomere function for human health is exemplified by a collection of Mendelian disorders referred to as the telomere biology disorders (TBDs), telomeropathies, or syndromes of telomere shortening. Collectively, the TBDs cover a spectrum of conditions from multisystem disease presenting in infancy to isolated disease presentations in adulthood, most notably idiopathic pulmonary fibrosis. Eleven genes have been found mutated in the TBDs to date, each of which is linked to some aspect of telomere maintenance. This review summarizes the molecular defects that result from mutations in these genes, highlighting recent advances, including the addition of PARN to the TBD gene family and the discovery of heterozygous mutations in RTEL1 as a cause of familial pulmonary fibrosis.

Human CST abundance determines recovery from diverse forms of DNA damage and replication stress

Mammalian CST (CTC1-STN1-TEN1) is a telomere-associated complex that functions in telomere duplex replication and fill-in synthesis of the telomeric C-strand following telomerase action. CST also facilitates genome-wide replication recovery after HU-induced fork stalling by increasing origin firing. CTC1 and STN1 were originally isolated as a DNA polymerase α stimulatory factor. Here we explore how CST abundance affects recovery from drugs that cause different types of DNA damage and replication stress. We show that recovery from HU and aphidicolin induced replication stress is increased by CST over-expression. Elevated CST increases dNTP incorporation and origin firing after HU release and decreases the incidence of anaphase bridges and micronuclei after aphidicolin removal. While the frequency of origin firing after HU release is proportional to CST abundance, the number of cells entering S-phase to initiate replication is unchanged by CST overexpression or STN1 depletion. Instead the CST-related changes in origin firing take place in cells that were already in S-phase at the time of HU addition, indicating that CST modulates firing of late or dormant origins. CST abundance also influences cell viability after treatment with HU, aphidicolin, MMS and camptothecin. Viability is increased by elevated CST and decreased by STN1 depletion, indicating that endogenous CST levels are limiting. However, CST abundance does not affect viability after MMC treatment. Thus, CST facilitates recovery from many, but not all, forms of exogenous DNA damage. Overall our results suggest that CST is needed in stoichiometric amounts to facilitate re-initiation of DNA replication at repaired forks and/or dormant origins.

Evolution of TERT-interacting lncRNAs: expanding the regulatory landscape of telomerase

Long non-coding RNAs (lncRNAs) evolve rapidly and are functionally diverse. The emergence of new lncRNAs is driven by genome disturbance events, including whole genome duplication, and transposition. One of the few lncRNAs with a conserved role throughout eukaryotes is the telomerase RNA, TER. TER works in concert with the telomerase reverse transcriptase (TERT) to maintain telomeres. Here we discuss recent findings from Arabidopsis thaliana and its relatives illustrating the remarkable evolutionary flexibility within TER and the potential for non-canonical TERT-lncRNA interactions. We highlight the two TERs in A. thaliana. One is a conventional telomerase template. The other lncRNA negatively regulates telomerase activity in response to DNA damage, a function mediated by co-option of a transposable element. In addition, we discuss evidence for multiple independent TER loci throughout the plant family Brassicaceae, and how these loci not only reflect rapid convergent evolution, but also the flexibility of having a lncRNA at the core of telomerase. Lastly, we discuss the propensity for TERT to bind a suite of non-templating lncRNAs, and how such RNAs may facilitate telomerase regulation and off-telomere functions.

Replicating through telomeres: a means to an end

Proper replication of the telomeric DNA at chromosome ends is critical for preserving genome integrity. Yet, telomeres present challenges for the replication machinery, such as their repetitive and heterochromatic nature and their potential to form non-Watson-Crick structures as well as the fact that they are transcribed. Numerous telomere-bound proteins are required to facilitate progression of the replication fork throughout telomeric DNA. In particular, shelterin plays crucial functions in telomere length regulation, protection of telomeres from nuclease degradation, control of DNA damage response at telomeres, and the recruitment of associated factors required for telomere DNA processing and replication. In this review we discuss the recently uncovered functions of mammalian telomere-specific and telomere-associated proteins that facilitate proper telomere replication.

Chromatin dynamics of plant telomeres and ribosomal genes

Telomeres and genes encoding 45S ribosomal RNA (rDNA) are frequently located adjacent to each other on eukaryotic chromosomes. Although their primary roles are different, they show striking similarities with respect to their features and additional functions. Both genome domains have remarkably dynamic chromatin structures. Both are hypersensitive to dysfunctional histone chaperones, responding at the genomic and epigenomic levels. Both generate non-coding transcripts that, in addition to their epigenetic roles, may induce gross chromosomal rearrangements. Both give rise to chromosomal fragile sites, as their replication is intrinsically problematic. However, at the same time, both are essential for maintenance of genomic stability and integrity. Here we discuss the structural and functional inter-connectivity of telomeres and rDNA, with a focus on recent results obtained in plants.

AKTIP/Ft1, a New Shelterin-Interacting Factor Required for Telomere Maintenance

Telomeres are nucleoprotein complexes that protect the ends of linear chromosomes from incomplete replication, degradation and detection as DNA breaks. Mammalian telomeres are protected by shelterin, a multiprotein complex that binds the TTAGGG telomeric repeats and recruits a series of additional factors that are essential for telomere function. Although many shelterin-associated proteins have been so far identified, the inventory of shelterin-interacting factors required for telomere maintenance is still largely incomplete. Here, we characterize AKTIP/Ft1 (human AKTIP and mouse Ft1 are orthologous), a novel mammalian shelterin-bound factor identified on the basis of its homology with the Drosophila telomere protein Pendolino. AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes and has been previously implicated in the control of apoptosis and in vesicle trafficking. RNAi-mediated depletion of AKTIP results in formation of telomere dysfunction foci (TIFs). Consistent with these results, AKTIP interacts with telomeric DNA and binds the shelterin components TRF1 and TRF2 both in vivo and in vitro. Analysis of AKTIP- depleted human primary fibroblasts showed that they are defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint. Accordingly, AKTIP physically interacts with PCNA and the RPA70 DNA replication factor. Ft1-depleted p53-/- MEFs did not arrest in the S phase but displayed significant increases in multiple telomeric signals (MTS) and sister telomere associations (STAs), two hallmarks of defective telomere replication. In addition, we found an epistatic relation for MST formation between Ft1 and TRF1, which has been previously shown to be required for replication fork progression through telomeric DNA. Ch-IP experiments further suggested that in AKTIP-depleted cells undergoing the S phase, TRF1 is less tightly bound to telomeric DNA than in controls. Thus, our results collectively suggest that AKTIP/Ft1 works in concert with TRF1 to facilitate telomeric DNA replication.

Stn1 is critical for telomere maintenance and long-term viability of somatic human cells

Disruption of telomere maintenance pathways leads to accelerated entry into cellular senescence, a stable proliferative arrest that promotes aging-associated disorders in some mammals. The budding yeast CST complex, comprising Cdc13, Stn1, and Ctc1, is critical for telomere replication, length regulation, and end protection. Although mammalian homologues of CST have been identified recently, their role and function for telomere maintenance in normal somatic human cells are still incompletely understood. Here, we characterize the function of human Stn1 in cultured human fibroblasts and demonstrate its critical role in telomere replication, length regulation, and function. In the absence of high telomerase activity, shRNA-mediated knockdown of hStn1 resulted in aberrant and fragile telomeric structures, stochastic telomere attrition, increased telomere erosion rates, telomere dysfunction, and consequently accelerated entry into cellular senescence. Oxidative stress augmented the defects caused by Stn1 knockdown leading to almost immediate cessation of cell proliferation. In contrast, overexpression of hTERT suppressed some of the defects caused by hStn1 knockdown suggesting that telomerase can partially compensate for hStn1 loss. Our findings reveal a critical role for human Stn1 in telomere length maintenance and function, supporting the model that efficient replication of telomeric repeats is critical for long-term viability of normal somatic mammalian cells.

Telomere DNA recognition in Saccharomycotina yeast: potential lessons for the co-evolution of ssDNA and dsDNA-binding proteins and their target sites

In principle, alterations in the telomere repeat sequence would be expected to disrupt the protective nucleoprotein complexes that confer stability to chromosome ends, and hence relatively rare events in evolution. Indeed, numerous organisms in diverse phyla share a canonical 6 bp telomere repeat unit (5'-TTAGGG-3'/5'-CCCTAA-3'), suggesting common descent from an ancestor that carries this particular repeat. All the more remarkable, then, are the extraordinarily divergent telomere sequences that populate the Saccharomycotina subphylum of budding yeast. These sequences are distinguished from the canonical telomere repeat in being long, occasionally degenerate, and frequently non-G/C-rich. Despite the divergent telomere repeat sequences, studies to date indicate that the same families of single-strand and double-strand telomere binding proteins (i.e., the Cdc13 and Rap1 families) are responsible for telomere protection in Saccharomycotina yeast. The recognition mechanisms of the protein family members therefore offer an informative paradigm for understanding the co-evolution of DNA-binding proteins and the cognate target sequences. Existing data suggest three potential, inter-related solutions to the DNA recognition problem: (i) duplication of the recognition protein and functional modification; (ii) combinatorial recognition of target site; and (iii) flexibility of the recognition surfaces of the DNA-binding proteins to adopt alternative conformations. Evidence in support of these solutions and the relevance of these solutions to other DNA-protein regulatory systems are discussed.

Single-cell telomere-length quantification couples telomere length to meristem activity and stem cell development in Arabidopsis

Telomeres are specialized nucleoprotein caps that protect chromosome ends assuring cell division. Single-cell telomere quantification in animals established a critical role for telomerase in stem cells, yet, in plants, telomere-length quantification has been reported only at the organ level. Here, a quantitative analysis of telomere length of single cells in Arabidopsis root apex uncovered a heterogeneous telomere-length distribution of different cell lineages showing the longest telomeres at the stem cells. The defects in meristem and stem cell renewal observed in tert mutants demonstrate that telomere lengthening by TERT sets a replicative limit in the root meristem. Conversely, the long telomeres of the columella cells and the premature stem cell differentiation plt1,2 mutants suggest that differentiation can prevent telomere erosion. Overall, our results indicate that telomere dynamics are coupled to meristem activity and continuous growth, disclosing a critical association between telomere length, stem cell function, and the extended lifespan of plants.

Functional Proteomic Analysis of Repressive Histone Methyltransferase Complexes Reveals ZNF518B as a G9A Regulator

Cell-type specific gene silencing by histone H3 lysine 27 and lysine 9 methyltransferase complexes PRC2 and G9A-GLP is crucial both during development and to maintain cell identity. Although studying their interaction partners has yielded valuable insight into their functions, how these factors are regulated on a network level remains incompletely understood. Here, we present a new approach that combines quantitative interaction proteomics with global chromatin profiling to functionally characterize repressive chromatin modifying protein complexes in embryonic stem cells. We define binding stoichiometries of 9 new and 12 known interaction partners of PRC2 and 10 known and 29 new interaction partners of G9A-GLP, respectively. We demonstrate that PRC2 and G9A-GLP interact physically and share several interaction partners, including the zinc finger proteins ZNF518A and ZNF518B. Using global chromatin profiling by targeted mass spectrometry, we discover that even sub-stoichiometric binding partners such as ZNF518B can positively regulate global H3K9me2 levels. Biochemical analysis reveals that ZNF518B directly interacts with EZH2 and G9A. Our systematic analysis suggests that ZNF518B may mediate the structural association between PRC2 and G9A-GLP histone methyltransferases and additionally regulates the activity of G9A-GLP.

Extending the model of Arabidopsis telomere length and composition across Brassicaceae

Telomeres are repetitive TG-rich DNA elements essential for maintaining the stability of genomes and replicative capacity of cells in almost all eukaryotes. Most of what is known about telomeres in plants comes from the angiosperm Arabidopsis thaliana, which has become an important comparative model for telomere biology. Arabidopsis tolerates numerous insults to its genome, many of which are catastrophic or lethal in other eukaryotic systems such as yeast and vertebrates. Despite the importance of Arabidopsis in establishing a model for the structure and regulation of plant telomeres, only a handful of studies have used this information to assay components of telomeres from across land plants, or even among the closest relatives of Arabidopsis in the plant family Brassicaceae. Here, we determined how well Arabidopsis represents Brassicaceae by comparing multiple aspects of telomere biology in species that represent major clades in the family tree. Specifically, we determined the telomeric repeat sequence, measured bulk telomere length, and analyzed variation in telomere length on syntenic chromosome arms. In addition, we used a phylogenetic approach to infer the evolutionary history of putative telomere-binding proteins, CTC1, STN1, TEN1 (CST), telomere repeat-binding factor like (TRFL), and single Myb histone (SMH). Our analyses revealed conservation of the telomeric DNA repeat sequence, but considerable variation in telomere length among the sampled species, even in comparisons of syntenic chromosome arms. We also found that the single-stranded and double-stranded telomeric DNA-binding complexes CST and TRFL, respectively, differ in their pattern of gene duplication and loss. The TRFL and SMH gene families have undergone numerous duplication events, and these duplicate copies are often retained in the genome. In contrast, CST components occur as single-copy genes in all sampled genomes, even in species that experienced recent whole genome duplication events. Taken together, our results place the Arabidopsis model in the context of other species in Brassicaceae, making the family the best characterized plant group in regard to telomere architecture.

Genetic architecture of natural variation of telomere length in Arabidopsis thaliana

Telomeres represent the repetitive sequences that cap chromosome ends and are essential for their protection. Telomere length is known to be highly heritable and is derived from a homeostatic balance between telomeric lengthening and shortening activities. Specific loci that form the genetic framework underlying telomere length homeostasis, however, are not well understood. To investigate the extent of natural variation of telomere length in Arabidopsis thaliana, we examined 229 worldwide accessions by terminal restriction fragment analysis. The results showed a wide range of telomere lengths that are specific to individual accessions. To identify loci that are responsible for this variation, we adopted a quantitative trait loci (QTL) mapping approach with multiple recombinant inbred line (RIL) populations. A doubled haploid RIL population was first produced using centromere-mediated genome elimination between accessions with long (Pro-0) and intermediate (Col-0) telomere lengths. Composite interval mapping analysis of this population along with two established RIL populations (Ler-2/Cvi-0 and Est-1/Col-0) revealed a number of shared and unique QTL. QTL detected in the Ler-2/Cvi-0 population were examined using near isogenic lines that confirmed causative regions on chromosomes 1 and 2. In conclusion, this work describes the extent of natural variation of telomere length in A. thaliana, identifies a network of QTL that influence telomere length homeostasis, examines telomere length dynamics in plants with hybrid backgrounds, and shows the effects of two identified regions on telomere length regulation.

DNA-Directed Polymerase Subunits Play a Vital Role in Human Telomeric Overhang Processing

Telomeres consist of TTAGGG repeats bound by the shelterin complex and end with a 3' overhang. In humans, telomeres shorten at each cell division, unless telomerase (TERT) is expressed and able to add telomeric repeats. For effective telomere maintenance, the DNA strand complementary to that made by telomerase must be synthesized. Recent studies have discovered a link between different activities necessary to process telomeres in the S phase of the cell cycle to reform a proper overhang. Notably, the human CST complex (CTC1/STN1/TEN1), known to interact functionally with the polymerase complex (POLA/primase), was shown to be important for telomere processing. Here, focus was paid to the catalytic (POLA1/p180) and accessory (POLA2/p68) subunits of the polymerase, and their mechanistic roles at telomeres. We were able to detect p68 and p180 at telomeres in S-phase using chromatin immunoprecipitation. We could also show that the CST, shelterin, and polymerase complexes interact, revealing contacts occurring at telomeres. We found that the polymerase complex could associate with telomerase activity. Finally, depletion of p180 by siRNA led to increased overhang amounts at telomeres. These data support a model in which the polymerase complex is important for proper telomeric overhang processing through fill-in synthesis, during S phase. These results shed light on important events necessary for efficient telomere maintenance and protection.

IMPLICATIONS

This study describes the interplay between DNA replication components with proteins that associate with chromosome ends, and telomerase. These interactions are proposed to be important for the processing and protection of chromosome ends.

The CDC13-STN1-TEN1 complex stimulates Pol α activity by promoting RNA priming and primase-to-polymerase switch

Emerging evidence suggests that Cdc13-Stn1-Ten1 (CST), an RPA-like ssDNA-binding complex, may regulate primase-Pol α (PP) activity at telomeres constitutively, and at other genomic locations under conditions of replication stress. Here we examine the mechanisms of PP stimulation by CST using purified complexes derived from Candida glabrata. While CST does not enhance isolated DNA polymerase activity, it substantially augments both primase activity and primase-to-polymerase switching. CST also simultaneously shortens the RNA and lengthens the DNA in the chimeric products. Stn1, the most conserved subunit of CST, is alone capable of PP stimulation. Both the N-terminal OB fold and the C-terminal winged-helix domains of Stn1 can bind to the Pol12 subunit of the PP complex, and stimulate PP activity. Our findings provide mechanistic insights on a well-conserved pathway of PP regulation that is critical for genome stability.

Telomere stability and development of ctc1 mutants are rescued by inhibition of EJ recombination pathways in a telomerase-dependent manner

The telomeres of linear eukaryotic chromosomes are protected by caps consisting of evolutionarily conserved nucleoprotein complexes. Telomere dysfunction leads to recombination of chromosome ends and this can result in fusions which initiate chromosomal breakage-fusion-bridge cycles, causing genomic instability and potentially cell death or cancer. We hypothesize that in the absence of the recombination pathways implicated in these fusions, deprotected chromosome ends will instead be eroded by nucleases, also leading to the loss of genes and cell death. In this work, we set out to specifically test this hypothesis in the plant, Arabidopsis. Telomere protection in Arabidopsis implicates KU and CST and their absence leads to chromosome fusions, severe genomic instability and dramatic developmental defects. We have analysed the involvement of end-joining recombination pathways in telomere fusions and the consequences of this on genomic instability and growth. Strikingly, the absence of the multiple end-joining pathways eliminates chromosome fusion and restores normal growth and development to cst ku80 mutant plants. It is thus the chromosomal fusions, per se, which are the underlying cause of the severe developmental defects. This rescue is mediated by telomerase-dependent telomere extension, revealing a competition between telomerase and end-joining recombination proteins for access to deprotected telomeres.

POT1a and components of CST engage telomerase and regulate its activity in Arabidopsis

Protection of Telomeres 1 (POT1) is a conserved nucleic acid binding protein implicated in both telomere replication and chromosome end protection. We previously showed that Arabidopsis thaliana POT1a associates with the TER1 telomerase RNP, and is required for telomere length maintenance in vivo. Here we further dissect the function of POT1a and explore its interplay with the CST (CTC1/STN1/TEN1) telomere complex. Analysis of pot1a null mutants revealed that POT1a is not required for telomerase recruitment to telomeres, but is required for telomerase to maintain telomere tracts. We show that POT1a stimulates the synthesis of long telomere repeat arrays by telomerase, likely by enhancing repeat addition processivity. We demonstrate that POT1a binds STN1 and CTC1 in vitro, and further STN1 and CTC1, like POT1a, associate with enzymatically active telomerase in vivo. Unexpectedly, the in vitro interaction of STN1 with TEN1 and POT1a was mutually exclusive, indicating that POT1a and TEN1 may compete for the same binding site on STN1 in vivo. Finally, unlike CTC1 and STN1, TEN1 was not associated with active telomerase in vivo, consistent with our previous data showing that TEN1 negatively regulates telomerase enzyme activity. Altogether, our data support a two-state model in which POT1a promotes an extendable telomere state via contacts with the telomerase RNP as well as STN1 and CTC1, while TEN1 opposes these functions.

Role of STN1 and DNA polymerase α in telomere stability and genome-wide replication in Arabidopsis

The CST (Cdc13/CTC1-STN1-TEN1) complex was proposed to have evolved kingdom specific roles in telomere capping and replication. To shed light on its evolutionary conserved function, we examined the effect of STN1 dysfunction on telomere structure in plants. STN1 inactivation in Arabidopsis leads to a progressive loss of telomeric DNA and the onset of telomeric defects depends on the initial telomere size. While EXO1 aggravates defects associated with STN1 dysfunction, it does not contribute to the formation of long G-overhangs. Instead, these G-overhangs arise, at least partially, from telomerase-mediated telomere extension indicating a deficiency in C-strand fill-in synthesis. Analysis of hypomorphic DNA polymerase α mutants revealed that the impaired function of a general replication factor mimics the telomeric defects associated with CST dysfunction. Furthermore, we show that STN1-deficiency hinders re-replication of heterochromatic regions to a similar extent as polymerase α mutations. This comparative analysis of stn1 and pol α mutants suggests that STN1 plays a genome-wide role in DNA replication and that chromosome-end deprotection in stn1 mutants may represent a manifestation of aberrant replication through telomeres.

Telomeres form an elaborate nucleoprotein structure that may represent an obstacle for replication machinery and renders this region prone to fork stalling. CST is an evolutionary conserved complex that was originally discovered to specifically act at telomeres. Interestingly, the function of CST seems to have diverged in the course of evolution; in yeast it is required for telomere protection, while in mammals it was proposed to facilitate replication through telomeres. In plants, inactivation of CST leads to telomere deprotection and genome instability. Here we show that the telomere deprotection in Arabidopsis deficient in STN1, one of the CST components, is consistent with defects in telomere replication and that STN1 phenotypes can be partially phenocopied by an impairment of a general replication factor, DNA polymerase α. In addition, we provide evidence that STN1 facilitates re-replication at non-telomeric loci. This suggests a more general role of CST in genome maintenance and further infers that its seemingly specific function(s) in telomere protection may rather represent unique requirements for efficient replication of telomeric DNA.

The role of double-strand break repair pathways at functional and dysfunctional telomeres

Telomeres have evolved to protect the ends of linear chromosomes from the myriad of threats posed by the cellular DNA damage signaling and repair pathways. Mammalian telomeres have to block nonhomologous end joining (NHEJ), thus preventing chromosome fusions; they need to control homologous recombination (HR), which could change telomere lengths; they have to avoid activating the ATM (ataxia telangiectasia mutated) and ATR (ATM- and RAD3-related) kinase pathways, which could induce cell cycle arrest; and they have to protect chromosome ends from hyperresection. Recent studies of telomeres have provided insights into the mechanisms of NHEJ and HR, how these double-strand break (DSB) repair pathways can be thwarted, and how telomeres have co-opted DNA repair factors to help in the protection of chromosome ends. These aspects of telomere biology are reviewed here with particular emphasis on recombination, the main focus of this collection.

Tpz1TPP1 SUMOylation reveals evolutionary conservation of SUMO-dependent Stn1 telomere association

Elongation of the telomeric overhang by telomerase is counteracted by synthesis of the complementary strand by the CST complex, CTC1(Cdc13)/Stn1/Ten1. Interaction of budding yeast Stn1 with overhang-binding Cdc13 is increased by Cdc13 SUMOylation. Human and fission yeast CST instead interact with overhang-binding TPP1/POT1. We show that the fission yeast TPP1 ortholog, Tpz1, is SUMOylated. Tpz1 SUMOylation restricts telomere elongation and promotes Stn1/Ten1 telomere association, and a SUMO-Tpz1 fusion protein has increased affinity for Stn1. Our data suggest that SUMO inhibits telomerase through stimulation of Stn1/Ten1 action by Tpz1, highlighting the evolutionary conservation of the regulation of CST function by SUMOylation.

Interplay between nonsense-mediated mRNA decay and DNA damage response pathways reveals that Stn1 and Ten1 are the key CST telomere-cap components

A large and diverse set of proteins, including CST complex, nonsense mediated decay (NMD), and DNA damage response (DDR) proteins, play important roles at the telomere in mammals and yeast. Here, we report that NMD, like the DDR, affects single-stranded DNA (ssDNA) production at uncapped telomeres. Remarkably, we find that the requirement for Cdc13, one of the components of CST, can be efficiently bypassed when aspects of DDR and NMD pathways are inactivated. However, identical genetic interventions do not bypass the need for Stn1 and Ten1, the partners of Cdc13. We show that disabling NMD alters the stoichiometry of CST components at telomeres and permits Stn1 to bind telomeres in the absence of Cdc13. Our data support a model that Stn1 and Ten1 can function in a Cdc13-independent manner and have implications for the function of CST components across eukaryotes.

Elucidation of the DNA end-replication problem in Saccharomyces cerevisiae

The model for telomere shortening at each replication cycle is currently incomplete, and the exact contribution of the telomeric 3' overhang to the shortening rate remains unclear. Here, we demonstrate key steps of the mechanism of telomere replication in Saccharomyces cerevisiae. By following the dynamics of telomeres during replication at near-nucleotide resolution, we find that the leading-strand synthesis generates blunt-end intermediates before being 5'-resected and filled in. Importantly, the shortening rate is set by positioning the last Okazaki fragments at the very ends of the chromosome. Thus, telomeres shorten in direct proportion to the 3' overhang lengths of 5-10 nucleotides that are present in parental templates. Furthermore, the telomeric protein Cdc13 coordinates leading- and lagging-strand syntheses. Taken together, our data unravel a precise choreography of telomere replication elucidating the DNA end-replication problem and provide a framework to understand the control of the cell proliferation potential.

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

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

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

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

Telomere repeat binding proteins are functional components of Arabidopsis telomeres and interact with telomerase

Although telomere-binding proteins constitute an essential part of telomeres, in vivo data indicating the existence of a structure similar to mammalian shelterin complex in plants are limited. Partial characterization of a number of candidate proteins has not identified true components of plant shelterin or elucidated their functional mechanisms. Telomere repeat binding (TRB) proteins from Arabidopsis thaliana bind plant telomeric repeats through a Myb domain of the telobox type in vitro, and have been shown to interact with POT1b (Protection of telomeres 1). Here we demonstrate co-localization of TRB1 protein with telomeres in situ using fluorescence microscopy, as well as in vivo interaction using chromatin immunoprecipitation. Classification of the TRB1 protein as a component of plant telomeres is further confirmed by the observation of shortening of telomeres in knockout mutants of the trb1 gene. Moreover, TRB proteins physically interact with plant telomerase catalytic subunits. These findings integrate TRB proteins into the telomeric interactome of A. thaliana.

Mammalian E-type cyclins control chromosome pairing, telomere stability and CDK2 localization in male meiosis

Loss of function of cyclin E1 or E2, important regulators of the mitotic cell cycle, yields viable mice, but E2-deficient males display reduced fertility. To elucidate the role of E-type cyclins during spermatogenesis, we characterized their expression patterns and produced additional deletions of Ccne1 and Ccne2 alleles in the germline, revealing unexpected meiotic functions. While Ccne2 mRNA and protein are abundantly expressed in spermatocytes, Ccne1 mRNA is present but its protein is detected only at low levels. However, abundant levels of cyclin E1 protein are detected in spermatocytes deficient in cyclin E2 protein. Additional depletion of E-type cyclins in the germline resulted in increasingly enhanced spermatogenic abnormalities and corresponding decreased fertility and loss of germ cells by apoptosis. Profound meiotic defects were observed in spermatocytes, including abnormal pairing and synapsis of homologous chromosomes, heterologous chromosome associations, unrepaired double-strand DNA breaks, disruptions in telomeric structure and defects in cyclin-dependent-kinase 2 localization. These results highlight a new role for E-type cyclins as important regulators of male meiosis.

Understanding the control of meiosis is fundamental to deciphering the origin of male infertility. Although the mechanisms controlling meiosis are poorly understood, key regulators of mitosis, such as cyclins, appear to be critical. In this regard, male mice deficient for cyclin E2 exhibit subfertility and defects in spermatogenesis; however, neither the stages of germ cell differentiation affected nor the responsible mechanisms are known. We investigated how E-type cyclins control male meiosis by examining their expression in spermatogenesis and the consequences that multiple deletions of Ccne1 and Ccne2 alleles produce. Loss of Ccne2 expression increases cyclin E1 levels as a compensatory effect, but there are still meiotic defects and subfertility. Further, loss of one Ccne1 allele in the absence of cyclin E2 results in infertility as does loss of the remaining Ccne1 allele, but with even more severe meiotic abnormalities. We further found that cyclin E1 is involved in sex chromosome synapsis while E2 is involved with homologous pairing and chromosome and telomere integrity. These processes and structures were severely disrupted in absence of both cyclin E1 and E2, uncovering new roles for the E-type cyclins in regulating male meiosis.

Analysis of poly(ADP-Ribose) polymerases in Arabidopsis telomere biology

Maintaining the length of the telomere tract at chromosome ends is a complex process vital to normal cell division. Telomere length is controlled through the action of telomerase as well as a cadre of telomere-associated proteins that facilitate replication of the chromosome end and protect it from eliciting a DNA damage response. In vertebrates, multiple poly(ADP-ribose) polymerases (PARPs) have been implicated in the regulation of telomere length, telomerase activity and chromosome end protection. Here we investigate the role of PARPs in plant telomere biology. We analyzed Arabidopsis thaliana mutants null for PARP1 and PARP2 as well as plants treated with the PARP competitive inhibitor 3-AB. Plants deficient in PARP were hypersensitive to genotoxic stress, and expression of PARP1 and PARP2 mRNA was elevated in response to MMS or zeocin treatment or by the loss of telomerase. Additionally, PARP1 mRNA was induced in parp2 mutants, and conversely, PARP2 mRNA was induced in parp1 mutants. PARP3 mRNA, by contrast, was elevated in both parp1 and parp2 mutants, but not in seedlings treated with 3-AB or zeocin. PARP mutants and 3-AB treated plants displayed robust telomerase activity, no significant changes in telomere length, and no end-to-end chromosome fusions. Although there remains a possibility that PARPs play a role in Arabidopsis telomere biology, these findings argue that the contribution is a minor one.

Responses to telomere erosion in plants

In striking contrast to animals, plants are able to develop and reproduce in the presence of significant levels of genome damage. This is seen clearly in both the viability of plants carrying knockouts for key recombination and DNA repair genes, which are lethal in vertebrates, and in the impact of telomere dysfunction. Telomerase knockout mice show accelerated ageing and severe developmental phenotypes, with effects on both highly proliferative and on more quiescent tissues, while cell death in Arabidopsis tert mutants is mostly restricted to actively dividing meristematic cells. Through phenotypic and whole-transcriptome RNAseq studies, we present here an analysis of the response of Arabidopsis plants to the continued presence of telomere damage. Comparison of second-generation and seventh-generation tert mutant plants has permitted separation of the effects of the absence of the telomerase enzyme and the ensuing chromosome damage. In addition to identifying a large number of genes affected by telomere damage, many of which are of unknown function, the striking conclusion of this study is the clear difference observed at both cellular and transcriptome levels between the ways in which mammals and plants respond to chronic telomeric damage.

TeloTool: a new tool for telomere length measurement from terminal restriction fragment analysis with improved probe intensity correction

Telomeres comprise the protective caps of natural chromosome ends and function in the suppression of DNA damage signaling and cellular senescence. Therefore, techniques used to determine telomere length are important in a number of studies, ranging from those investigating telomeric structure to effects on human disease. Terminal restriction fragment (TRF) analysis has for a long time shown to be one of the most accurate methods for quantification of absolute telomere length and range from a number of species. As this technique centers on standard Southern blotting, telomeric DNA is observed on resulting autoradiograms as a heterogeneous smear. Methods to accurately determine telomere length from telomeric smears have proven problematic, and no reliable technique has been suggested to obtain mean telomere length values. Here, we present TeloTool, a new program allowing thorough statistical analysis of TRF data. Using this new method, a number of methodical biases are removed from previously stated techniques, including assumptions based on probe intensity corrections. This program provides a standardized mean for quick and reliable extraction of quantitative data from TRF autoradiograms; its wide application will allow accurate comparison between datasets generated in different laboratories.

Short telomeres: from dyskeratosis congenita to sporadic aplastic anemia and malignancy

Telomeres are DNA-protein structures that form a protective cap on chromosome ends. As such, they prevent the natural ends of linear chromosomes from being subjected to DNA repair activities that would result in telomere fusion, degradation, or recombination. Both the DNA and protein components of the telomere are required for this essential function, because insufficient telomeric DNA length, loss of the terminal telomeric DNA structure, or deficiency of key telomere-associated factors may elicit a DNA damage response and result in cellular senescence or apoptosis. In the setting of failed checkpoint mechanisms, such DNA-protein defects can also lead to genomic instability through telomere fusions or recombination. Thus, as shown in both model systems and in humans, defects in telomere biology are implicated in cellular and organismal aging as well as in tumorigenesis. Bone marrow failure and malignancy are 2 life-threatening disease manifestations in the inherited telomere biology disorder dyskeratosis congenita. We provide an overview of basic telomere structure and maintenance. We outline the telomere biology defects observed in dyskeratosis congenita, focusing on recent discoveries in this field. Last, we review the evidence of how telomere biology may impact sporadic aplastic anemia and the risk for various cancers.

Functional characterization of human CTC1 mutations reveals novel mechanisms responsible for the pathogenesis of the telomere disease Coats plus

Coats plus is a rare recessive disorder characterized by intracranial calcifications, hematological abnormalities, and retinal vascular defects. This disease results from mutations in CTC1, a member of the CTC1-STN1-TEN1 (CST) complex critical for telomere replication. Telomeres are specialized DNA/protein structures essential for the maintenance of genome stability. Several patients with Coats plus display critically shortened telomeres, suggesting that telomere dysfunction plays an important role in disease pathogenesis. These patients inherit CTC1 mutations in a compound heterozygous manner, with one allele encoding a frameshift mutant and the other a missense mutant. How these mutations impact upon telomere function is unknown. We report here the first biochemical characterization of human CTC1 mutations. We found that all CTC1 frameshift mutations generated truncated or unstable protein products, none of which were able to form a complex with STN1-TEN1 on telomeres, resulting in progressive telomere shortening and formation of fused chromosomes. Missense mutations are able to form the CST complex at telomeres, but their expression levels are often repressed by the frameshift mutants. Our results also demonstrate for the first time that CTC1 mutations promote telomere dysfunction by decreasing the stability of STN1 to reduce its ability to interact with DNA Polα, thus highlighting a previously unknown mechanism to induce telomere dysfunction.

Molecular basis of telomere syndrome caused by CTC1 mutations

Mutations in CTC1 lead to the telomere syndromes Coats Plus and dyskeratosis congenita (DC), but the molecular mechanisms involved remain unknown. CTC1 forms with STN1 and TEN1 a trimeric complex termed CST, which binds ssDNA, promotes telomere DNA synthesis, and inhibits telomerase-mediated telomere elongation. Here we identify CTC1 disease mutations that disrupt CST complex formation, the physical interaction with DNA polymerase α-primase (polα-primase), telomeric ssDNA binding in vitro, accumulation in the nucleus, and/or telomere association in vivo. While having diverse molecular defects, CTC1 mutations commonly lead to the accumulation of internal single-stranded gaps of telomeric DNA, suggesting telomere DNA replication defects as a primary cause of the disease. Strikingly, mutations in CTC1 may also unleash telomerase repression and telomere length control. Hence, the telomere defect initiated by CTC1 mutations is distinct from the telomerase insufficiencies seen in classical forms of telomere syndromes, which cause short telomeres due to reduced maintenance of distal telomeric ends by telomerase. Our analysis provides molecular evidence that CST collaborates with DNA polα-primase to promote faithful telomere DNA replication.

Fission yeast shelterin regulates DNA polymerases and Rad3(ATR) kinase to limit telomere extension

Studies in fission yeast have previously identified evolutionarily conserved shelterin and Stn1-Ten1 complexes, and established Rad3^ATR/Tel1^ATM-dependent phosphorylation of the shelterin subunit Ccq1 at Thr93 as the critical post-translational modification for telomerase recruitment to telomeres. Furthermore, shelterin subunits Poz1, Rap1 and Taz1 have been identified as negative regulators of Thr93 phosphorylation and telomerase recruitment. However, it remained unclear how telomere maintenance is dynamically regulated during the cell cycle. Thus, we investigated how loss of Poz1, Rap1 and Taz1 affects cell cycle regulation of Ccq1 Thr93 phosphorylation and telomere association of telomerase (Trt1^TERT), DNA polymerases, Replication Protein A (RPA) complex, Rad3^ATR-Rad26^ATRIP checkpoint kinase complex, Tel1^ATM kinase, shelterin subunits (Tpz1, Ccq1 and Poz1) and Stn1. We further investigated how telomere shortening, caused by trt1Δ or catalytically dead Trt1-D743A, affects cell cycle-regulated telomere association of telomerase and DNA polymerases. These analyses established that fission yeast shelterin maintains telomere length homeostasis by coordinating the differential arrival of leading (Polε) and lagging (Polα) strand DNA polymerases at telomeres to modulate Rad3^ATR association, Ccq1 Thr93 phosphorylation and telomerase recruitment.

Stable maintenance of telomeres is critical to maintain a stable genome and to prevent accumulation of undesired mutations that may lead to formation of tumors. Telomere dysfunction can also lead to premature aging due to depletion of the stem cell population, highlighting the importance of understanding the regulatory mechanisms that ensure stable telomere maintenance. Based on careful analysis of cell cycle-regulated changes in telomere association of telomerase, DNA polymerases, Replication Protein A, checkpoint kinases, telomere protection complex shelterin, and Stn1-Ten1 complex, we will provide here a new and dynamic model of telomere length regulation in fission yeast, which suggests that shelterin-dependent regulation of differential arrival of leading and lagging strand DNA polymerase at telomeres is responsible for modulating Rad3^ATR checkpoint kinase accumulation and Rad3^ATR-dependent phosphorylation of shelterin subunit Ccq1 to control telomerase recruitment to telomeres.

Cdk1 regulates the temporal recruitment of telomerase and Cdc13-Stn1-Ten1 complex for telomere replication

In budding yeast (Saccharomyces cerevisiae), the cell cycle-dependent telomere elongation by telomerase is controlled by the cyclin-dependent kinase 1 (Cdk1). The telomere length homeostasis is balanced between telomerase-unextendable and telomerase-extendable states that both require Cdc13. The recruitment of telomerase complex by Cdc13 promotes telomere elongation, while the formation of Cdc13-Stn1-Ten1 (CST) complex at the telomere blocks telomere elongation by telomerase. However, the cellular signaling that regulates the timing of the telomerase-extendable and telomerase-unextendable states is largely unknown. Phosphorylation of Cdc13 by Cdk1 promotes the interaction between Cdc13 and Est1 and hence telomere elongation. Here, we show that Cdk1 also phosphorylates Stn1 at threonine 223 and serine 250 both in vitro and in vivo, and these phosphorylation events are essential for the stability of the CST complexes at the telomeres. By controlling the timing of Cdc13 and Stn1 phosphorylations during cell cycle progression, Cdk1 regulates the temporal recruitment of telomerase complexes and CST complexes to the telomeres to facilitate telomere maintenance.

Signaling of double strand breaks and deprotected telomeres in Arabidopsis

Failure to repair DNA double strand breaks (DSB) can lead to chromosomal rearrangements and eventually to cancer or cell death. Radiation and environmental pollutants induce DSB and this is of particular relevance to plants due to their sessile life style. DSB also occur naturally in cells during DNA replication and programmed induction of DSB initiates the meiotic recombination essential for gametogenesis in most eukaryotes. The linear nature of most eukaryotic chromosomes means that each chromosome has two "broken" ends. Chromosome ends, or telomeres, are protected by nucleoprotein caps which avoid their recognition as DSB by the cellular DNA repair machinery. Deprotected telomeres are recognized as DSB and become substrates for recombination leading to chromosome fusions, the "bridge-breakage-fusion" cycle, genome rearrangements and cell death. The importance of repair of DSB and the severity of the consequences of their misrepair have led to the presence of multiple, robust mechanisms for their detection and repair. After a brief overview of DSB repair pathways to set the context, we present here an update of current understanding of the detection and signaling of DSB in the plant, Arabidopsis thaliana.

Telomere-end processing: mechanisms and regulation

Telomeres are specialized nucleoprotein complexes that provide protection to the ends of eukaryotic chromosomes. Telomeric DNA consists of tandemly repeated G-rich sequences that terminate with a 3' single-stranded overhang, which is important for telomere extension by the telomerase enzyme. This structure, as well as most of the proteins that specifically bind double and single-stranded telomeric DNA, are conserved from yeast to humans, suggesting that the mechanisms underlying telomere identity are based on common principles. The telomeric 3' overhang is generated by different events depending on whether the newly synthesized strand is the product of leading- or lagging-strand synthesis. Here, we review the mechanisms that regulate these processes at Saccharomyces cerevisiae and mammalian telomeres.

Human TEN1 maintains telomere integrity and functions in genome-wide replication restart

TEN1 is a component of the mammalian CTC1-STN1-TEN1 complex. CTC1 and/or STN1 functions in telomere duplex replication, C-strand fill-in, and genome-wide restart of replication following fork stalling. Here we examine the role of human TEN1 and ask whether it also functions as a specialized replication factor. TEN1 depletion causes an increase in multitelomere fluorescent in situ hybridization (FISH) signals similar to that observed after CTC1 or STN1 depletion. However, TEN1 depletion also results in increased telomere loss. This loss is not accompanied by increased telomere deprotection, recombination, or T-circle release. Thus, it appears that both the multiple telomere signals and telomere loss stem from problems in telomere duplex replication. TEN1 depletion can also affect telomere length, but whether telomeres lengthen or shorten is cell line-dependent. Like CTC1 and STN1, TEN1 is needed for G-overhang processing. Depletion of TEN1 does not effect overhang elongation in mid-S phase, but it delays overhang shortening in late S/G2. These results indicate a role for TEN1 in C-strand fill-in but do not support a direct role in telomerase regulation. Finally, TEN1 depletion causes a decrease in genome-wide replication restart following fork stalling similar to that observed after STN1 depletion. However, anaphase bridge formation is more severe than with CTC1 or STN1 depletion. Our findings indicate that TEN1 likely functions in conjunction with CTC1 and STN1 at the telomere and elsewhere in the genome. They also raise the possibility that TEN1 has additional roles and indicate that TEN1/CTC1-STN1-TEN1 helps solve a wide range of challenges to the replication machinery.

CST for the grand finale of telomere replication

Telomeric DNA at eukaryotic chromosome ends terminates with single stranded 3' G-rich overhangs. The overhang is generated by the interplay of several dynamic processes including semiconservative DNA replication, 3' end elongation by telomerase, C-strand fill-in synthesis and nucleolytic processing. The mammalian CST (CTC1-STN1-TEN1) complex is directly involved at several stages of telomere end formation. Elucidation of its structural organization and identification of interaction partners support the notion that mammalian CST is, as its yeast counterpart, a RPA-like complex. CST binding at mammalian telomere 3' overhangs increases upon their elongation by telomerase. Formation of a trimeric CST complex at telomeric 3'overhangs leads to telomerase inhibition and at the same time mediates a physical interaction with DNA polymerase-α. Thus CST seems to play critical roles in coordinating telomerase elongation and fill-in synthesis to complete telomere replication.

Structure of the human telomeric Stn1-Ten1 capping complex

The identification of the human homologue of the yeast CST in 2009 posed a new challenge in our understanding of the mechanism of telomere capping in higher eukaryotes. The high-resolution structure of the human Stn1-Ten1 (hStn1-Ten1) complex presented here reveals that hStn1 consists of an OB domain and tandem C-terminal wHTH motifs, while hTen1 consists of a single OB fold. Contacts between the OB domains facilitate formation of a complex that is strikingly similar to the replication protein A (RPA) and yeast Stn1-Ten1 (Ten1) complexes. The hStn1-Ten1 complex exhibits non-specific single-stranded DNA activity that is primarily dependent on hStn1. Cells expressing hStn1 mutants defective for dimerization with hTen1 display elongated telomeres and telomere defects associated with telomere uncapping, suggesting that the telomeric function of hCST is hTen1 dependent. Taken together the data presented here show that the structure of the hStn1-Ten1 subcomplex is conserved across species. Cell based assays indicate that hTen1 is critical for the telomeric function of hCST, both in telomere protection and downregulation of telomerase function.

DNA repair at telomeres: keeping the ends intact

The molecular era of telomere biology began with the discovery that telomeres usually consist of G-rich simple repeats and end with 3' single-stranded tails. Enormous progress has been made in identifying the mechanisms that maintain and replenish telomeric DNA and the proteins that protect them from degradation, fusions, and checkpoint activation. Although telomeres in different organisms (or even in the same organism under different conditions) are maintained by different mechanisms, the disparate processes have the common goals of repairing defects caused by semiconservative replication through G-rich DNA, countering the shortening caused by incomplete replication, and postreplication regeneration of G tails. In addition, standard DNA repair mechanisms must be suppressed or modified at telomeres to prevent their being recognized and processed as DNA double-strand breaks. Here, we discuss the players and processes that maintain and regenerate telomere structure.

Replication of telomeres and the regulation of telomerase

Telomeres are the physical ends of eukaryotic chromosomes. They protect chromosome ends from DNA degradation, recombination, and DNA end fusions, and they are important for nuclear architecture. Telomeres provide a mechanism for their replication by semiconservative DNA replication and length maintenance by telomerase. Through telomerase repression and induced telomere shortening, telomeres provide the means to regulate cellular life span. In this review, we introduce the current knowledge on telomere composition and structure. We then discuss in depth the current understanding of how telomere components mediate their function during semiconservative DNA replication and how telomerase is regulated at the end of the chromosome. We focus our discussion on the telomeres from mammals and the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe.