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Interstitial telomeric sequences (ITSs) are stretches of telomeric-like repeats located at internal chromosomal sites. We previously demonstrated that ITSs have been inserted during the repair of DNA double-strand breaks in the course of evolution and that some rodent ITSs, called TERC-ITSs, are flanked by fragments retrotranscribed from the telomerase RNA component (TERC). In this work, we carried out an extensive search of TERC-ITSs in 30 vertebrate genomes and identified 41 such loci in 22 species, including in humans and other primates. The fragment retrotranscribed from the TERC RNA varies in different lineages and its sequence seems to be related to the organization of TERC. Through comparative analysis of TERC-ITSs with orthologous empty loci, we demonstrated that, at each locus, the TERC-like sequence and the ITS have been inserted in one step in the course of evolution. Our findings suggest that telomerase participated in a peculiar pathway of DNA double-strand break repair involving retrotranscription of its RNA component and that this mechanism may be active in all vertebrate species. These results add new evidence to the hypothesis that RNA-templated DNA repair mechanisms are active in vertebrate cells.

Interstitial telomeric sequences (ITSs) are short stretches of telomeric-like repeats (TTAGGG)n at nonterminal chromosomal sites. We previously demonstrated that, in the genomes of primates and rodents, ITSs were inserted during the repair of DNA double-strand breaks. These conclusions were derived from sequence comparisons of ITS-containing loci and ITS-less orthologous loci in different species. To our knowledge, insertion polymorphism of ITSs, i.e., the presence of an ITS-containing allele and an ITS-less allele in the same species, has not been described. In this work, we carried out a genome-wide analysis of 2504 human genomic sequences retrieved from the 1000 Genomes Project and a PCR-based analysis of 209 human DNA samples. In spite of the large number of individual genomes analyzed we did not find any evidence of insertion polymorphism in the human population. On the contrary, the analysis of ITS loci in the genome of a single horse individual, the reference genome, allowed us to identify five heterozygous ITS loci, suggesting that insertion polymorphism of ITSs is an important source of genetic variability in this species. Finally, following a comparative sequence analysis of horse ITSs and of their orthologous empty loci in other Perissodactyla, we propose models for the mechanism of ITS insertion during the evolution of this order.

Individuals differ in realized fitness but the genetic/phenotypic traits that underpin such variation are often unknown. Telomere dynamics may be a major source of variation in fitness traits because physiological telomere shortening depends on environmental and genetic factors and may impair individual performance. Here, we showed that, in a population of a socially monogamous, biparental passerine bird, the barn swallow (Hirundo rustica), breeding in northern Italy, telomere length (TL) of both adult males and females positively correlated with seasonal reproductive and fledging success, as expected because long telomeres are supposed to boost performance. Telomere length was correlated with sexually dimorphic coloration in both sexes, showing for the first time in any species that coloration reliably reflects TL and may mediate mutual mate choice, leading to the observed positive assortative mating for TL in the barn swallow. Thus, TL appears to be associated with variation in a major fitness trait and may be an ultimate target of mate choice, as individuals of both sexes can use coloration to adaptively choose high-quality mates that possess long telomeres.

Telomeres are transcribed into noncoding telomeric repeat-containing RNAs (TERRA), which are essential for telomere maintenance. Deregulation of TERRA transcription impairs telomere metabolism and a role in tumorigenesis has been proposed. Head and neck cancer (HNC) is one of the most frequent cancers worldwide, with head and neck squamous cell carcinoma (HNSCC) being the predominant type. Since HNSCC patients are characterized by altered telomere maintenance, a dysfunction in telomere transcription can be hypothesized. In this prospective study, we compared TERRA levels in the tumor and matched normal tissue from 23 HNSCC patients. We then classified patients in two categories according to the level of TERRA expression in the tumor compared to the normal tissue: (1) lower expression in the tumor, (2) higher or similar expression in tumor. A significant proportion of patients in the first group died of the disease within less than 34 months postsurgery, while the majority of patients in the second group were alive and disease-free. Our results highlight a striking correlation between TERRA expression and tumor aggressiveness in HNSCC suggesting that TERRA levels may be proposed as a novel molecular prognostic marker for HNSCC.

The composition of breeding pairs can deviate from random, resulting in a positive association between homologous traits between mates. Assortative mating can arise either as a by-product of constraints imposed on random pairing by variation in individual life histories or because of mutual sex preferences. Because individuals that prefer high-quality mates gain fitness advantages, mate choice should result in positive assortative mating for traits that are associated with individual quality. Telomeres are nucleoprotein complexes that function to maintain chromosome integrity. Telomeres shorten at each cell division, but the rate of shortening depends on exogenous and endogenous effects, including antioxidant defense mechanisms that reduce the negative impact of pro-oxidants on telomere length. Once a critical length is attained, cells enter either senescence or apoptosis, causing a decline in tissue renewal and ultimately in individual performance. Assortative mating is therefore expected both for telomere length and for antioxidant defense because both males and females may accrue benefits from choosing a mate with long telomeres and efficient antioxidant defense. Consistent with this prediction, in the barn swallow (Hirundo rustica), we found a positive correlation between telomere length (RTL) and total antioxidant capacity (TAC) of social mates which was not the spurious result of age or breeding date. Because parents with relatively long telomeres produce offspring which also have relatively long telomeres, assortative mating seems to be adaptive. The evidence of assortative mating for telomere length and antioxidant capacity suggests that epigamic signals exist that reliably reflect these traits and may mediate mutual mating preferences. Telomeres protect chromosome integrity but shorten at each cell division, impairing organismal functions. Because individuals vary in telomere length and rate of shortening, as well as in antioxidant defenses reducing the negative impact of pro-oxidants on telomeres, adaptive mate choice should result in assortative mating for telomere length and antioxidants. Consistent with this prediction, in barn swallows, telomere length and antioxidant capacity were positively correlated between social mates.

The longstanding dogma that telomeres, the heterochromatic extremities of linear eukaryotic chromosomes, are transcriptionally silent was overturned by the discovery that DNA-dependent RNA polymerase II (RNAPII) transcribes telomeric DNA into telomeric repeat-containing RNA (TERRA). Here, we show that CpG dinucleotide-rich DNA islands, shared among multiple human chromosome ends, promote transcription of TERRA molecules. TERRA promoters sustain cellular expression of reporter genes, are located immediately upstream of TERRA transcription start sites, and are bound by active RNAPII in vivo. Finally, the identified promoter CpG dinucleotides are methylated in vivo, and cytosine methylation negatively regulates TERRA abundance. The existence of subtelomeric promoters, driving TERRA transcription from independent chromosome ends, supports the idea that TERRA exerts fundamental functions in the context of telomere biology.

In addition to their location at terminal positions, telomeric-like repeats are also present at internal sites of the chromosomes (intrachromosomal or interstitial telomeric sequences, ITSs). According to their sequence organization and genomic location, two different kinds of ITSs can be identified: (1) heterochromatic ITSs (het-ITSs), large (up to hundreds of kb) stretches of telomeric-like DNA localized mainly at centromeres, and (2) short ITSs (s-ITSs), short stretches of telomeric hexamers distributed at internal sites of the chromosomes. Het-ITSs have been only described in some vertebrate species and they probably represent the remnants of evolutionary chromosomal rearrangements. On the contrary, s-ITSs are probably present in all mammalian genomes although they have been described in detail only in some completely sequenced genomes. Sequence database analysis revealed the presence of 83, 79, 244 and 250 such s-ITSs in the human, chimpanzee, mouse and rat genomes, respectively. Analysis of the flanking sequences suggested that s-ITSs were inserted during the repair of DNA double-strand breaks that occurred in the course of evolution. An extensive comparative analysis of the s-ITS loci and their orthologous ‘empty’ loci confirmed this hypothesis and suggested that the repair event involved the direct action of telomerase. Whereas het-ITSs seem to be intrinsically prone to breakage, the instability of s-ITSs is more controversial. This observation is consistent with the hypothesis that s-ITSs are probably not themselves prone to breakage but represent ‘scars’ of ancient breakage that may have occurred within fragile regions. This study will review the current knowledge on these two types of ITS, their molecular organization, how they arose during evolution, their implications for chromosomal instability and their potential applications as phylogenetic/forensic markers.

Mammalian telomeres are transcribed into long non-coding telomeric RNA molecules (TERRA) that seem to play a role in the maintenance of telomere stability. In human cells, CpG island promoters drive TERRA transcription and are regulated by methylation. It was suggested that the amount of TERRA may be related to telomere length. To test this hypothesis we measured telomere length and TERRA levels in single clones isolated from five human cell lines: HeLa (cervical carcinoma), BRC-230 (breast cancer), AKG and GK2 (gastric cancers) and GM847 (SV40 immortalized skin fibroblasts). We observed great clonal heterogeneity both in TRF (Terminal Restriction Fragment) length and in TERRA levels. However, these two parameters did not correlate with each other. Moreover, cell survival to γ-rays did not show a significant variation among the clones, suggesting that, in this cellular system, the intra-population variability in telomere length and TERRA levels does not influence sensitivity to ionizing radiation. This conclusion was supported by the observation that in a cell line in which telomeres were greatly elongated by the ectopic expression of telomerase, TERRA expression levels and radiation sensitivity were similar to the parental HeLa cell line.

We appreciated the commentary of Van Beneden et al. (1) on our article (2) because is giving us the opportunity to discuss the advantages and limitations of the various methods currently in use to measure TElomeric Repeat containing RNA (TERRA) cellular levels and to present new data supporting our previous conclusions. Which is the most appropriate method to analyze TERRA levels? With qRT-PCR, specific primer pairs are used to amplify reverse-transcribed fragments complementary to a portion of the subtelomeric region adjacent to the telomere; the number of transcripts containing subtelomeric fragments is measured while no information on the number of UUAGGG repeats within TERRA molecules is obtained. The qRT-PCR method has been extensively used by several groups, including ours; however, we can identify several limitations: (1) primers are constructed on subtelomeric sequences (3), thus, very short and possibly functionally irrelevant RNA molecules containing only a few UUAGGG repeats are detected together with molecules containing large numbers of repeats. However, the mechanisms of TERRA processing and the structure of physiologically relevant molecules have not been clarified yet. Most likely the function of TERRA is related to the UUAGGG repeats for the following reasons: (i) the subtelomeric tract contained in TERRA molecules is relatively short while the UUAGGG repeats can reach several kilobases; in particular, in TERRA molecules transcribed from the XqYq human subtelomere, the distance between the transcription start site and the first telomeric repeat is 257 nt (4, 5); (ii) UUAGGG oligonucleotides interact with several telomere associated proteins, including TRF1 and TRF2 (6). Using mass spectrometry, it was demonstrated that different members of the heterogeneous nuclear ribonucleoprotein family bind abundantly to TERRA repeats (7, 8) and, more recently, 115 proteins, specifically binding to UUAGGG repeats, were identified (9). (iii) The UUAGGG repeats of TERRA molecules are able to fold into G-quadruplex structures (10) that are required for the binding of TERRA to chromatin (11). (iv) TERRA repeats can inhibit the telomerase enzymatic activity in vitro (12) through base pairing with the telomeric repeat template but their role in the regulation of telomerase in vivo is more controversial (8, 13). (2) Due to the repetitive nature of subtelomeric sequences, it has been often impossible to design primers specific for single subtelomeres; indeed it should be kept in mind that most primer pairs used so far amplify fragments from more than one subtelomere (3, 14, 15). (3) Not all human subtelomeric sequences have been fully assembled (15, 16) and to specifically analyze their transcription remains a challenge; therefore, until we have specific primer sets for each subtelomere, quantification of TERRA molecules by qRT-PCR will not reflect the whole TERRA transcriptome. (4) TERRA promoters and putative promoter start sites have been identified only on 20 human subtelomeres (4) and the transcriptional regulation of the remaining subtelomeres still needs to be elucidated. (5) Quantification of TERRA expression using qRT-PCR on transformed heteroploid cell lines may be biased by variations in the number of chromosome ends recognized by each primer pair. (6) The contribution of each subtelomere to total TERRA is variable, depending on its transcription efficiency (2, 15). It is also important to point out that, since telomere length (17) and TERRA transcription vary from end to end, we should be able to measure both of them at single chromosome-end level to precisely define the relationship between these two parameters. Comparing average telomere length with the expression of a few subtelomeric regions may be misleading. Regarding northern blotting, as clearly shown by the Decottignies group (1, 14) the visualization of high molecular weight RNA molecules is favored by alkaline treatment of the gels. Using this approach, these authors observed the appearance of high molecular weight TERRA molecules in cell lines in which telomeres were artificially hyper-elongated by ectopic expression of telomerase holoenzyme; in parallel, measuring TERRA levels by qRT-PCR, they concluded that, in the cell lines with longer telomeres, TERRA expression was reduced to 50%. However, the comparison between the qRT-PCR and the northern blotting results is confusing [Figures 1B,E in Ref. (1)]. In the northern blots, the appearance of a high molecular weight (>5.3 kb) TERRA fraction in cells with long telomeres does not parallel a loss of the lower molecular weight molecules (

Many human primary somatic cells can be immortalized by inducing telomerase activity through the exogenous expression of the human telomerase catalytic subunit (hTERT). This approach has been extended to the immortalization of cell lines from several mammals. Here, we show that hTERT expression is not sufficient to immortalize primary fibroblasts from three equid species, namely donkey, Burchelli’s zebra and Grevy’s zebra. In vitro analysis of a reconstituted telomerase composed by hTERT and an equid RNA component of telomerase (TERC) revealed a low activity of this enzyme compared to human telomerase, suggesting a low compatibility of equid and human telomerase subunits. This conclusion was also strengthened by comparison of human and equid TERC sequences, which revealed nucleotide differences in key regions for TERC and TERT interaction. We then succeeded in immortalizing equid fibroblasts by expressing hTERT and hTERC concomitantly. Expression of both human telomerase subunits led to telomerase activity and telomere elongation, indicating that human telomerase is compatible with the other equid telomerase subunits and proteins involved in telomere metabolism. The immortalization procedure described herein could be extended to primary cells from other mammals. The availability of immortal cells from endangered species could be particularly useful for obtaining new information on the organization and function of their genomes, which is relevant for their preservation.

Telomeres are conserved DNA-protein structures at the termini of eukaryotic chromosomes which contribute to maintenance of genome integrity, and their shortening leads to cell senescence, with negative consequences for organismal functions. Because telomere erosion is influenced by extrinsic and endogenous factors, telomere dynamics may provide a mechanistic basis for evolutionary and physiological trade-offs. Yet, knowledge of fundamental aspects of telomere biology under natural selection regimes, including sex- and context-dependent variation in early-life, and the covariation between telomere dynamics and growth, is scant. In this study of barn swallows (Hirundo rustica) we investigated the sex-dependent telomere erosion during nestling period, and the covariation between relative telomere length and body and plumage growth. Finally, we tested whether any covariation between growth traits and relative telomere length depends on the social environment, as influenced by sibling sex ratio. Relative telomere length declined on average over the period of nestling maximal growth rate (between 7 and 16 days of age) and differently covaried with initial relative telomere length in either sex. The frequency distribution of changes in relative telomere length was bimodal, with most nestlings decreasing and some increasing relative telomere length, but none of the offspring traits predicted the a posteriori identified group to which individual nestlings belonged. Tail and wing length increased with relative telomere length, but more steeply in males than females, and this relationship held both at the within- and among-broods levels. Moreover, the increase in plumage phenotypic values was steeper when the sex ratio of an individual’s siblings was female-biased. Our study provides evidence for telomere shortening during early life according to subtly different dynamics in either sex. Furthermore, it shows that the positive covariation between growth and relative telomere length depends on sex as well as social environment, in terms of sibling sex ratio.

Short blocks of telomeric-like DNA (Interstitial Telomeric Sequences, ITSs) are found far from chromosome ends. We addressed the question as to how such sequences arise by comparing the loci of 10 human ITSs with their genomic orthologs in 12 primate species. The ITSs did not derive from expansion of pre-existing TTAGGG units, as described for other microsatellites, but appeared suddenly during evolution. Nine insertion events were dated along the primate evolutionary tree, the dates ranging between 40 and 6 million years ago. Sequence comparisons suggest that in each case the block of (TTAGGG)n DNA arose as a result of double-strand break repair. In fact, ancestral sequences were either interrupted precisely by the tract of telomeric-like repeats or showed the typical modifications observed at double-strand break repair sites such as short deletions, addition of random sequences, or duplications. Similar conclusions were drawn from the analysis of a chimpanzee-specific ITS. We propose that telomeric sequences were inserted by the capture of a telomeric DNA fragment at the break site or by the telomerase enzyme. Our conclusions indicate that human ITSs are relics of ancient breakage rather than fragile sites themselves, as previously suggested.

The intrachromosomal location of (T2AG3)n telomeric sequences has been reported in several species. It was proposed that interstitial telomeres (ITs) originated through telomeric fusion of ancestral chromosomes. However, the data so far obtained derive mainly from cytogenetic observations. Cloning and database searching of human IT sequences allowed us to identify three classes: (i) short ITs, composed of few, essentially exact T2AG3 units; (ii) subtelomeric ITs, composed of larger arrays (several hundred base pairs) including many degenerate units within subtelomeric domains; (iii) fusion ITs, in which two extended stretches of telomeric repeats are oriented head-to-head. The number of short ITs is over 50 and subtelomeric ITs are probably present at all chromosomal ends. Surprisingly, the telomeric sequence in 2q13 remains the only fusion IT so far characterized, and evidence presented here suggests that another member of this class may be present in 1q41. Different molecular mechanisms generated the three classes. In particular, several short ITs interrupt precisely repetitive elements or are flanked by direct repeats of 10–41 bp, and are conserved in gorilla and chimpanzee. These features strongly suggest that telomeric repeats were inserted at intrachromosomal sites through the repair of double-strand breaks that occurred in the germline during evolution.

A comparative analysis of two primate and two rodent genomes suggests that telomerase was utilized, in some instances, for the repair of DNA double-strand breaks during mammalian evolution., Background In vertebrates, tandem arrays of TTAGGG hexamers are present at both telomeres and intrachromosomal sites (interstitial telomeric sequences (ITSs)). We previously showed that, in primates, ITSs were inserted during the repair of DNA double-strand breaks and proposed that they could arise from either the capture of telomeric fragments or the action of telomerase. Results An extensive comparative analysis of two primate (Homo sapiens and Pan troglodytes) and two rodent (Mus musculus and Rattus norvegicus) genomes allowed us to describe organization and insertion mechanisms of all the informative ITSs present in the four species. Two novel observations support the hypothesis of telomerase involvement in ITS insertion: in a highly significant fraction of informative loci, the ITSs were introduced at break sites where a few nucleotides homologous to the telomeric hexamer were exposed; in the rodent genomes, complex ITS loci are present in which a retrotranscribed fragment of the telomerase RNA, far away from the canonical template, was inserted together with the telomeric repeats. Moreover, mutational analysis of the TTAGGG arrays in the different species suggests that they were inserted as exact telomeric hexamers, further supporting the participation of telomerase in ITS formation. Conclusion These results strongly suggest that telomerase was utilized, in some instances, for the repair of DNA double-strand breaks occurring in the genomes of rodents and primates during evolution. The presence, in the rodent genomes, of sequences retrotranscribed from the telomerase RNA strengthens the hypothesis of the origin of telomerase from an ancient retrotransposon.