Vertebrate Telomere Research Articles

A meta-analysis on the heritability of vertebrate telomere length.

Telomere dynamics are linked with both cellular and organismal senescence, and life history, individual quality and health. Telomere dynamics, particularly telomere length, have therefore garnered much research interest in evolutionary biology. To examine the evolution of telomere length, it is important to quantify its heritability, the proportion of total variation explained by additive genetic effects. Many studies have quantified telomere length heritability, but estimates are varied, and no general conclusion has been drawn. Additionally, it is unclear whether biological and methodological factors influence telomere length heritability estimates. We present the first meta-analysis of telomere length heritability, using 104 estimates from 43 studies over 18 vertebrate species. We calculated an overall mean heritability and examined how estimates varied by study, phylogeny, species-specific ecology, environmental setting, age at sampling, laboratory methods, statistical methods, sex and repeated measurements. Overall heritability was moderate (44.9%, 95% CI: 25.2-64.7%), and there was considerable heterogeneity in heritability estimates, in particular among studies and estimates. Laboratory method influenced heritability estimates, with in-gel hybridization TRF yielding higher heritabilities than qPCR and Southern blot TRF. There was also an effect from statistical method, with twin-based and SNP-based estimates lower than correlation-based or pedigree-based estimates. Our results highlight an overall heritable basis of telomere length, and we recommend future research on a wider range of taxa, and the use of variance-partitioning methods with relatedness or SNP data over correlation methods to minimize heritability estimation bias.

Telomere organization and the interstitial telomeric sites involvement in insects and vertebrates chromosome evolution

Telomere has a central role in chromosomal stability events. Chromosome ends organized in telomere-loop prevent activation of DNA damage response (DDR) mechanisms, thus keeping the chromosome structure organized. On the other hand, free chromosome ends, dysfunctional telomeres, and interstitial telomeric sequences (ITS) can trigger chromosome rearrangements. Here, the telomere organization, function, and maintenance mechanisms, in addition to ITS types and their involvement in chromosome changes, were revisited. Despite a general (TTAGGG)n sequence being present in vertebrate telomeres, insects show more diversification of their telomere motif. The relation between ITS and chromosome rearrangements was observed in insects and vertebrates, demonstrating different types of genome organization and distribution. Some ITS cannot be considered relicts of chromosome rearrangements because probable they were inserted during a double-strand break repair mechanism. On the other hand, the involvement of telomere sequences participating or triggering chromosome rearrangements or organizing satellite DNA components in several species groups is evident. The genomic assembling advances and applying other methodologies over ITS, and their flanking regions, can help to understand the telomere participation in the chromosomal evolution in species groups with highly diversified karyotypes.

Telomere length is highly heritable and independent of growth rate manipulated by temperature in field crickets.

Many organisms are capable of growing faster than they do. Restrained growth rate has functionally been explained by negative effects on lifespan of accelerated growth. However, the underlying mechanisms remain elusive. Telomere attrition has been proposed as a causal agent and has been mostly studied in endothermic vertebrates. We established that telomeres exist as chromosomal-ends in a model insect, the field cricket Gryllus campestris, using terminal restriction fragment and Bal 31 methods. Telomeres comprised TTAGGn repeats of 38kb on average, more than four times longer than the telomeres of human infants. Bal 31 assays confirmed that telomeric repeats were located at the chromosome-ends. We tested whether rapid growth between day 1, day 65, day 85, and day 125 is achieved at the expense of telomere length by comparing nymphs reared at 23°C with their siblings reared at 28°C, which grew three times faster in the initial 65days. Surprisingly, neither temperature treatment nor age affected average telomere length. Concomitantly, the broad sense heritability of telomere length was remarkably high at ~100%. Despite high heritability, the evolvability (a mean-standardized measure of genetic variance) was low relative to that of body mass. We discuss our findings in the context of telomere evolution. Some important features of vertebrate telomere biology are evident in an insect species dating back to the Triassic. The apparent lack of an effect of growth rate on telomere length is puzzling, suggesting strong telomere length maintenance during the growth phase. Whether such maintenance of telomere length is adaptive remains elusive and requires further study investigating the links with fitness in the wild.

ZBTB10 binds the telomeric variant repeat TTGGGG and interacts with TRF2.

Telomeres are nucleoprotein structures at the ends of linear chromosomes and present an essential feature for genome integrity. Vertebrate telomeres usually consist of hexameric TTAGGG repeats, however, in cells that use the alternative lengthening of telomeres (ALT) mechanism, variant repeat sequences are interspersed throughout telomeres. Previously, it was shown that NR2C/F transcription factors bind to TCAGGG variant repeats and contribute to telomere maintenance in ALT cells. While specific binders to other variant repeat sequences have been lacking to date, we here identify ZBTB10 as the first TTGGGG-binding protein and demonstrate direct binding via the two zinc fingers with affinity in the nanomolar range. Concomitantly, ZBTB10 co-localizes with a subset of telomeres in ALT-positive U2OS cells and interacts with TRF2/RAP1 via the N-terminal region of TRF2. Our data establishes ZBTB10 as a novel variant repeat binding protein at ALT telomeres.

Genomic in situ hybridization in interspecific hybrids of scallops (Bivalvia, Pectinidae) and localization of the satellite DNA Cf303, and the vertebrate telomeric sequences (TTAGGG)n on chromosomes of scallop Chlamys farreri (Jones & Preston, 1904).

Mitotic chromosome preparations of the interspecific hybrids Chlamys farreri (Jones & Preston, 1904) × Patinopecten yessoensis (Jay, 1857), C. farreri × Argopecten irradians (Lamarck, 1819) and C. farreri × Mimachlamys nobilis (Reeve, 1852) were used to compare two different scallop genomes in a single slide. Although genomic in situ hybridization (GISH) using genomic DNA from each scallop species as probe painted mitotic chromosomes of the interspecific hybrids, the painting results were not uniform; instead it showed species-specific distribution patterns of fluorescent signals among the chromosomes. The most prominent GISH-bands were mainly located at centromeric or telomeric regions of scallop chromosomes. In order to illustrate the sequence constitution of the GISH-bands, the satellite Cf303 sequences of C. farreri and the vertebrate telomeric (TTAGGG)n sequences were used to map mitotic chromosomes of C. farreri by fluorescence in situ hybridization (FISH). The results indicated that the GISH-banding pattern presented by the chromosomes of C. farreri is mainly due to the distribution of the satellite Cf303 DNA, therefore suggesting that the GISH-banding patterns found in the other three scallops could also be the result of the chromosomal distribution of other species-specific satellite DNAs.

Are the TTAGG and TTAGGG telomeric repeats phylogenetically conserved in aculeate Hymenoptera?

Despite the (TTAGG)n telomeric repeat supposed being the ancestral DNA motif of telomeres in insects, it was repeatedly lost within some insect orders. Notably, parasitoid hymenopterans and the social wasp Metapolybia decorata (Gribodo) lack the (TTAGG)n sequence, but in other representatives of Hymenoptera, this motif was noticed, such as different ant species and the honeybee. These findings raise the question of whether the insect telomeric repeat is or not phylogenetically predominant in Hymenoptera. Thus, we evaluated the occurrence of both the (TTAGG)n sequence and the vertebrate telomere sequence (TTAGGG)n using dot-blotting hybridization in 25 aculeate species of Hymenoptera. Our results revealed the absence of (TTAGG)n sequence in all tested species, elevating the number of hymenopteran families lacking this telomeric sequence to 13 out of the 15 tested families so far. The (TTAGGG)n was not observed in any tested species. Based on our data and compiled information, we suggest that the (TTAGG)n sequence was putatively lost in the ancestor of Apocrita with at least two subsequent independent regains (in Formicidae and Apidae).

Stabilization of Telomeric I-Motif Structures by (2'S)-2'-Deoxy-2'-C-Methylcytidine Residues.

G-quadruplexes and i-motifs are tetraplex structures present in telomeres and the promoter regions of oncogenes. The possibility of producing nanodevices with pH-sensitive functions has triggered interest in modified oligonucleotides with improved structural properties. We synthesized C-rich oligonucleotides carrying conformationally restricted (2'S)-2'-deoxy-2'-C-methyl-cytidine units. The effect of this modified nucleoside on the stability of intramolecular i-motifs from the vertebrate telomere was investigated by UV, CD, and NMR spectroscopy. The replacement of selected positions of the C-core with C-modified residues induced the formation of stable intercalated tetraplexes at near-neutral pH. This study demonstrates the possibility of enhancing the stability of the i-motif by chemical modification.

Editorial: The Evolving Telomeres.

Editorial: The Evolving Telomeres.

In situ Localization of (GATA)n and (TTAGGG)n Repeated DNAs and W Sex Chromosome Differentiation in Parodontidae (Actinopterygii: Characiformes)

The family Parodontidae presents a conserved diploid number of 54 chromosomes and different stages associated with ZW sex chromosome differentiation. For the great majority of species in this family it was proposed that the karyotypic diversification is mostly due to repetitive DNA mobility and accumulation. In this study, 2 repetitive probes, (GATA)n and (TTAGGG)n, were used to assess probable mechanisms of chromosome diversification, especially those related to molecular differentiation of the W chromosome. Results showed that the (GATA)n sequence is involved in the differentiation of the W chromosome female-specific region of Parodontidae and that it is accumulated in diverse autosomes. The (TTAGGG)n repeat is part of the vertebrate telomere, and the presence of interstitial telomeric sites may help to identify chromosome rearrangements. However, in Parodontidae, no interstitial telomeric sites were detected. This study shows plasticity in the amount of the (GATA)n repeat in Parodontidae that may be involved in chromatin modifications and transcriptional control of the W chromosome, and the role of repetitive DNAs in genomic diversification in this fish family is discussed.

TRF2 Recruits RTEL1 to Telomeres in S Phase to Promote T-Loop Unwinding

SummaryThe helicase RTEL1 promotes t-loop unwinding and suppresses telomere fragility to maintain the integrity of vertebrate telomeres. An interaction between RTEL1 and PCNA is important to prevent telomere fragility, but how RTEL1 engages with the telomere to promote t-loop unwinding is unclear. Here, we establish that the shelterin protein TRF2 recruits RTEL1 to telomeres in S phase, which is required to prevent catastrophic t-loop processing by structure-specific nucleases. We show that the TRF2-RTEL1 interaction is mediated by a metal-coordinating C4C4 motif in RTEL1, which is compromised by the Hoyeraal-Hreidarsson syndrome (HHS) mutation, RTEL1R1264H. Conversely, we define a TRF2I124D substitution mutation within the TRFH domain of TRF2, which eliminates RTEL1 binding and phenocopies the RTEL1R1264H mutation, giving rise to aberrant t-loop excision, telomere length heterogeneity, and loss of the telomere as a circle. These results implicate TRF2 in the recruitment of RTEL1 to facilitate t-loop disassembly at telomeres in S phase.

TRNA-derived G-quadruplex protects motor neurons.

Although not generally appreciated, the first 3D visualization of a nucleic acid was provided by the crystal structures of yeast phenylalanine transfer RNA (tRNA) (1, 2). [The DNA structure determined in the 1950s was a low-resolution fiber-diffraction–based model (3).] It was interesting to see that the L-shaped tRNA molecule is maintained by both Watson–Crick and non-Watson–Crick types of base interactions. These types include Hoogsteen base pairs that use the N7-C6 face of a purine to make hydrogen-bonding interactions. The scheme of the Hoogsteen base pairing also allows the formation of a planar interaction with four guanine residues, known as G-tetrad or G-quartet (Fig. 1A). Despite the fact that the G-tetrad interaction was first proposed over 50 y ago (4), demonstration of its biological significance was much later and not until it was found in the structure of telomeres (5⇓–7). The 3′-end overhang of vertebrate telomeres are rich in guanines and consist of many TTAGGG repeats that can spontaneously fold into four-stranded DNA structures with two or more G-tetrads stacking on top of each other. Functionally, formation of this unique structure—named G-quadruplex—plays a major regulatory role in telomere maintenance (8). Since then, G-quadruplex structures became widely noted to be present throughout the genome and …

Organismal stress, telomeres and life histories

Most organisms, including ourselves, are exposed to environmental stressors at various points during life, and responses to such stressors have been optimised by evolution to give the best fitness outcomes. It is expected that environmental change will substantially increase long-term stress exposure in many animal groups in the coming decades. A major challenge for biologists is to understand and predict how this will influence individuals, populations and ecosystems, and over what time scale such effects will occur. This requires a multi-disciplinary approach, combining studies of mechanisms with studies of fitness consequences for individuals and their descendants. In this review, I discuss the positive and negative fitness consequences of responses to stressful environments, particularly during early life, and with an emphasis on studies in birds. As many of the mechanisms underlying stress responses are highly conserved across the vertebrate groups, the findings from these studies have general applicability when interpreted in a life history context. One important route that has recently been identified whereby chronic stress exposure can affect health and longevity over long time frames is via effects on telomere dynamics. Much of this work has so far been done on humans, and is correlational in nature, but studies on other taxa, and experimental work, are increasing. I summarise the relevant aspects of vertebrate telomere biology and critically appraise our current knowledge with a view to pointing out important future research directions for our understanding of how stress exposure influences life histories.

Myb-domain protein Teb1 controls histone levels and centromere assembly in fission yeast

The TTAGGG motif is common to two seemingly unrelated dimensions of chromatin function-the vertebrate telomere repeat and the promoter regions of many Schizosaccharomyces pombe genes, including all of those encoding canonical histones. The essential S. pombe protein Teb1 contains two Myb-like DNA binding domains related to those found in telomere proteins and binds the human telomere repeat sequence TTAGGG. Here, we analyse Teb1 binding throughout the genome and the consequences of reduced Teb1 function. Chromatin immunoprecipitation (ChIP)-on-chip analysis reveals robust Teb1 binding at many promoters, notably including all of those controlling canonical histone gene expression. A hypomorphic allele, teb1-1, confers reduced binding and reduced levels of histone transcripts. Prompted by previously suggested connections between histone expression and centromere identity, we examined localization of the centromeric histone H3 variant Cnp1 and found reduced centromeric binding along with reduced centromeric silencing. These data identify Teb1 as a master regulator of histone levels and centromere identity.

Telomeres, a busy platform for cell signaling.

Telomeres are the terminal structures at the ends of linear chromosomes that represent a solution to the end replication problem. Specific binding of the six-protein subunit complex shelterin to telomeric, repetitive TTAGGG DNA sequences contributes to the stable architecture and maintenance of telomeres. Proteins involved in the DNA damage response are also localized at telomeres, and play a role in the surveillance and maintenance of telomere integrity. The enzyme responsible for telomere extension is telomerase, a ribonucleoprotein with reverse transcriptase activity. In the absence of telomerase, telomeres shorten to a length threshold that triggers the DNA damage response and replicative senescence. Here, we will summarize the latest findings concerning vertebrate telomere structure and epigenetics, and we present data regarding the impact of short telomeres upon cell signaling. In particular, in murine embryonic stem cells lacking telomerase, we found that distribution of cytosolic/nuclear β-catenin, a key component of the Wnt signaling pathway, changes when telomeres become critically short. We discuss implications and future perspectives of the effect of epigenetic modifications and/or conformational changes of telomeres on cell metabolism and signaling networks. Such an analysis may unveil potential therapeutic targets for pathologies like cancer, where the integrity of telomeres is altered.

Conservation and characterization of unique porcine interstitial telomeric sequences

Telomeres are composed of TTAGGG repeats and located at the ends of chromosomes. Telomeres protect chromosomes from instability in mammals, including mice and humans. Repetitive TTAGGG sequences are also found at intrachromosomal sites, where they are named as interstitial telomeric sequences (ITSs). Aberrant ITSs are implicated in chromosomal instability and found in cancer cells. Interestingly, in pigs, vertebrate telomere sequences TTAGGG (vITSs) are also localized at the centromeric region of chromosome 6, in addition to the end of all chromosomes. Surprisingly, we found that botanic telomere sequences, TTTAGGG (bITSs), also localize with vITSs at the centromeric regions of pig chromosome 6 using telomere fluorescence in situ hybridization (FISH) and by comparisons between several species. Furthermore, the average lengths of vITSs are highly correlated with those of the terminal telomeres (TTS). Also, pig ITSs show a high incidence of telomere doublets, suggesting that pig ITSs might be unstable and dynamic. Together, our results show that pig cells maintain the conserved telomere sequences that are found at the ITSs from of plants and other vertebrates. Further understanding of the function and regulation of pig ITSs may provide new clues for evolution and chromosomal instability.

ATR cooperates with CTC1 and STN1 to maintain telomeres and genome integrity inArabidopsis

The CTC1/STN1/TEN1 (CST) complex is an essential constituent of plant and vertebrate telomeres. Here we show that CST and ATR (ataxia telangiectasia mutated [ATM] and Rad3-related) act synergistically to maintain telomere length and genome stability in Arabidopsis. Inactivation of ATR, but not ATM, temporarily rescued severe morphological phenotypes associated with ctc1 or stn1. Unexpectedly, telomere shortening accelerated in plants lacking CST and ATR. In first-generation (G1) ctc1 atr mutants, enhanced telomere attrition was modest, but in G2 ctc1 atr, telomeres shortened precipitously, and this loss coincided with a dramatic decrease in telomerase activity in G2 atr mutants. Zeocin treatment also triggered a reduction in telomerase activity, suggesting that the prolonged absence of ATR leads to a hitherto-unrecognized DNA damage response (DDR). Finally, our data indicate that ATR modulates DDR in CST mutants by limiting chromosome fusions and transcription of DNA repair genes and also by promoting programmed cell death in stem cells. We conclude that the absence of CST in Arabidopsis triggers a multifaceted ATR-dependent response to facilitate maintenance of critically shortened telomeres and eliminate cells with severe telomere dysfunction.

Surprises from the Chromosome Front: Lessons from Arabidopsis on Telomeres and Telomerase

Telomeres serve two vital functions: They act as a buffer against the end-replication problem, and they prevent chromosome ends from being recognized as double-strand DNA (dsDNA) breaks. These functions are orchestrated by the telomerase reverse transcriptase and a variety of telomere protein complexes. Here, we discuss our recent studies with Arabidopsis thaliana that uncovered a new and highly conserved telomere complex called CST (Cdc13/CTC1, STN1, TEN1). Formerly believed to be yeast specific, CST has now been identified as a key component of both plant and vertebrate telomeres, which is essential for genome integrity and stem cell viability. We also describe the unexpected discovery of alternative telomerase ribonucleoprotein complexes in Arabidopsis. Fueled by duplication and diversification of the telomerase RNA subunit and telomerase accessory proteins, these telomerase complexes act in concert to maintain genome stability. In addition to the canonical telomerase enzyme, one of two alternative telomerase ribonucleoprotein (RNP) complexes functions as a novel negative regulator of enzyme activity in response to genotoxic stress. These contributions highlight the immense potential of Arabidopsis in probing the depths of the chromosome end.

Kinetic and Thermodynamic Control of G‐Quadruplex Folding

A matter of speed: when allowed to fold in a K(+)/poly(ethylene glycol) solution, the guanine (G)-rich strand of vertebrate telomere DNA forms a parallel/antiparallel G-quadruplex, which is a (3+1) hybrid, within microseconds before slowly transforming into the parallel one within hours. Thus, the conformation that a G-quadruplex initially adopts under physiological conditions may not be the one it adopts at the equilibrium state.

Age Prediction in the Chickens Using Telomere Quantity by Quantitative Fluorescence In situ Hybridization Technique

Telomeres are special structures at the ends of eukaryotic chromosomes. Vertebrate telomeres consist of tandem repeats of conserved TTAGGG sequence and associated proteins. Birds are interesting models for molecular studies on aging and cellular senescence because of their slow aging rates and longer life spans for their body size. In this longitudinal study, we explored the possibility of using telomeres as an age-marker to predict age in Single Comb White Leghorn layer chickens. We quantified the relative amount of telomeric DNA in isolated peripheral blood lymphocytes by the Quantitative Fluorescence in situ Hybridization technique on interphase nuclei (IQ FISH) using telomere-specific DNA probes. We found that the amount of telomeric DNA (ATD) reduced significantly with an increase in chronological age of the chicken. Especially, the telomere shortening rates are greatly increased in growing individuals compared to laying and old-aged individuals. Therefore, using the ATD values obtained by IQ FISH we established the possibility of age prediction in chickens based on the telomere theory of aging. By regression analysis of the ATD values at each age interval, we formulated an equation to predict the age of chickens. In conclusion, the telomeric DNA values by IQ FISH analyses can be used as an effective age-marker in predicting the chronological age of chickens. The study has implications in the breeding and population genetics of poultry, especially the reproductive potential.

Genome-wide YFP Fluorescence Complementation Screen Identifies New Regulators for Telomere Signaling in Human Cells

Detection of low-affinity or transient interactions can be a bottleneck in our understanding of signaling networks. To address this problem, we developed an arrayed screening strategy based on protein complementation to systematically investigate protein-protein interactions in live human cells, and performed a large-scale screen for regulators of telomeres. Maintenance of vertebrate telomeres requires the concerted action of members of the Telomere Interactome, built upon the six core telomeric proteins TRF1, TRF2, RAP1, TIN2, TPP1, and POT1. Of the ∼12,000 human proteins examined, we identified over 300 proteins that associated with the six core telomeric proteins. The majority of the identified proteins have not been previously linked to telomere biology, including regulators of post-translational modifications such as protein kinases and ubiquitin E3 ligases. Results from this study shed light on the molecular niche that is fundamental to telomere regulation in humans, and provide a valuable tool to investigate signaling pathways in mammalian cells.