Lela Khoriauli

CpG-island promoters drive transcription of human telomeres

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.

Uncoupling of Satellite DNA and Centromeric Function in the Genus Equus

In a previous study, we showed that centromere repositioning, that is the shift along the chromosome of the centromeric function without DNA sequence rearrangement, has occurred frequently during the evolution of the genus Equus. In this work, the analysis of the chromosomal distribution of satellite tandem repeats in Equus caballus, E. asinus, E. grevyi, and E. burchelli highlighted two atypical features: 1) several centromeres, including the previously described evolutionary new centromeres (ENCs), seem to be devoid of satellite DNA, and 2) satellite repeats are often present at non-centromeric termini, probably corresponding to relics of ancestral now inactive centromeres. Immuno-FISH experiments using satellite DNA and antibodies against the kinetochore protein CENP-A demonstrated that satellite-less primary constrictions are actually endowed with centromeric function. The phylogenetic reconstruction of centromere repositioning events demonstrates that the acquisition of satellite DNA occurs after the formation of the centromere during evolution and that centromeres can function over millions of years and many generations without detectable satellite DNA. The rapidly evolving Equus species gave us the opportunity to identify different intermediate steps along the full maturation of ENCs.

TERRA Expression Levels Do Not Correlate with Telomere Length and Radiation Sensitivity in Human Cancer Cell Lines

Mammalian telomeres are transcribed into long non-coding telomeric repeat-containing RNA (TERRA) molecules 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). 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.

Early-Life Telomere Dynamics Differ between the Sexes and Predict Growth in the Barn Swallow (Hirundo rustica)

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.

Brood size, telomere length, and parent-offspring color signaling in barn swallows

Trade-offs select for optimal allocation of resources among competing functions. Parents are selected to maximize production of viable offspring by balancing between progeny number and “quality.” Telomeres are nucleoproteins, at the ends of eukaryotic chromosomes, that shorten when cells divide. Because shortening below a certain threshold depresses organismal functioning and rate of shortening depends on environmental conditions, telomeres are good candidates as mediators of trade-offs. We altered brood size of barn swallow Hirundo rustica and found that brood enlargement caused a reduction in relative telomere length (RTL). Reliable signals of offspring quality should evolve that mediate adaptive parental care allocation. Because nestlings with darker coloration receive more care, we analyzed the covariation between RTL and coloration and found that RTL increased with plumage darkness, both within and between broods. Hence, we provide unprecedented evidence that signals relevant to parent-offspring communication reflect telomere length and thus offspring reproductive value.

Genome-wide evolutionary and functional analysis of the Equine Repetitive Element 1: an insertion in the myostatin promoter affects gene expression

BackgroundIn mammals, an important source of genomic variation is insertion polymorphism of retrotransposons. These may acquire a functional role when inserted inside genes or in their proximity. The aim of this work was to carry out a genome wide analysis of ERE1 retrotransposons in the horse and to analyze insertion polymorphism in relation to evolution and function. The effect of an ERE1 insertion in the promoter of the myostatin gene, which is involved in muscle development, was also investigated.ResultsIn the horse population, the fraction of ERE1 polymorphic loci is related to the degree of similarity to their consensus sequence. Through the analysis of ERE1 conservation in seven equid species, we established that the level of identity to their consensus is indicative of evolutionary age of insertion. The position of ERE1s relative to genes suggests that some elements have acquired a functional role. Reporter gene assays showed that the ERE1 insertion within the horse myostatin promoter affects gene expression. The frequency of this variant promoter correlates with sport aptitude and racing performance.ConclusionsSequence conservation and insertion polymorphism of ERE1 elements are related to the time of their appearance in the horse lineage, therefore, ERE1s are a useful tool for evolutionary and population studies. Our results suggest that the ERE1 insertion at the myostatin locus has been unwittingly selected by breeders to obtain horses with specific racing abilities. Although a complex combination of environmental and genetic factors contributes to athletic performance, breeding schemes may take into account ERE1 insertion polymorphism at the myostatin promoter.

More on the lack of correlation between TERRA expression and telomere length

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 (<5.3 kb); rather, in cells with elongated telomeres, the signal corresponding to shorter molecules is stronger compared to parental cells. Therefore, in cells overexpressing telomerase, the total number of TERRA molecules, detected by northern blotting, appears greater than in the parental cells. The discrepancy of the results obtained with the two techniques may be the consequence of differential post-transcriptional processing of telomeric transcripts in the two cell lines. In our previous report we measured telomere length, by Southern blotting, and TERRA levels both by northern blotting and by qRT-PCR, in independent clones isolated from HeLa cells. Van Beneden et al. (1) claim that, by carefully analyzing our qRT-PCR results [Figure 3 in Ref. (2)], an inverse correlation between TERRA expression and mean telomere length seems to emerge. We performed linear regression analysis on the data previously presented [Figure 3 in Ref. (2)] and found a mild inverse correlation between TERRA expression and mean telomere length for both 15q and XpYp subtelomeres (r = −0.56 and −0.54, respectively), but the correlation coefficients were not statistically significant (p = 0.23 and 0.26). To better characterize the eight HeLa clones described in our previous report [Figures 2A,B in Ref. (2)], we carried out qRT-PCR reactions using four additional primer pairs for the following subtelomeres: 10q (13), 12q, 17q, and XqYq (15). As shown in Figure ​Figure1A,1A, no significant correlation between average telomere length and subtelomere-specific TERRA levels was found. Figure 1 (A) Scatter plot of mean telomere length, measured by Southern blotting, versus TERRA expression determined by qRT-PCR of the same HeLa clones from Smirnova et al. (2) using four primer pairs: 10q (blue diamonds) hybridizing with 3 subtelomeres (10q, ... We also isolated seven new HeLa clones; we then analyzed their average telomere length and measured TERRA expression by qRT PCR with the six primer pairs mentioned above. Again, we did not observe any significant inverse correlation between telomere length and TERRA levels (Figure ​(Figure11B). Van Beneden et al. (1) also argue that our TERRA level comparison in HeLa cells with short or artificially elongated telomeres (HeLa hTR/hTERT) is misleading, due to the inefficient transfer of high molecular weight RNA in standard northern blotting (2). To avoid any bias due to the transfer method, we quantified TERRA expression levels by qRT-PCR using our independent six primer pairs (Figure ​(Figure1C).1C). The results show that there is a two to eightfold increase in TERRA expression in the cell line with longer telomeres compared to the parental cell line with shorter telomeres. The discrepancy between qRT-PCR and northern blot data could be due to different factors, including the presence of long molecules undetected by northern blotting [Figure 1B in Ref. (2)]; in addition, as discussed above, these two techniques recognize different regions on TERRA molecules and possibly different states of TERRA post-transcriptional processing. In conclusion, contrarily to what was proposed by Van Beneden et al. (1), we strongly believe that a perfect method to measure TERRA expression is not available at the moment. The debate raised on our article highlights the importance of reviewing critically the techniques to evaluate telomere transcription and the need of new methodological improvements together with a better understanding of TERRA biogenesis and post-transcriptional processing. This discussion has also allowed us to provide new data further strengthening our previous conclusion that a correlation between TERRA cellular levels and telomere length is not a general feature in human cancer cells.

Enhanced gene amplification in human cells knocked down for DNA-PKcs.

Gene amplification, a key mechanism for oncogene activation and drug resistance in tumour cells, involves the generation and joining of DNA double-strand breaks. Amplified DNA can be carried either on intra-chromosomal arrays or on extra-chromosomal elements (double minutes). We previously showed that, in rodent cells deficient in DNA-PKcs, intra-chromosomal amplification is significantly enhanced. In the present work, we studied gene amplification in human HeLa cell lines in which the expression of the DNA-PKcs gene was constitutively inhibited by shRNAs. These cell lines showed an increased sensitivity to ionizing radiations, an enhanced frequency of chromosomal aberrations and an increased rate of occurrence of methotrexate resistant colonies compared to the control cell lines (6-18 times). The main mechanism of resistance to methotrexate was extra-chromosomal amplification of the dihydrofolate reductase gene. These results indicate that, in human cells, inhibition of DNA-PKcs gene expression favours gene amplification occurring via the production of double minutes. In addition, they show that cell lines constitutively expressing shRNAs are good model systems to study the role of specific functions in gene amplification.

Yolk vitamin E prevents oxidative damage in gull hatchlings

Oxidative stress experienced during early development can negatively affect diverse life-history traits, and organisms have evolved complex defence systems against its detrimental effects. Bird eggs contain maternally derived exogenous antioxidants that play a major role in embryo protection from oxidative damage, including the negative effects on telomere dynamics. In this study on the yellow-legged gull (Larus michahellis), we manipulated the concentration of vitamin E (VE) in the egg yolk and analysed the consequences on oxidative status markers and telomere length in the hatchlings. This study provides the first experimental evidence that, contrary to the expectation, a physiological increase in yolk VE concentration boosted total antioxidant capacity and reduced the concentration of pro-oxidant molecules in the plasma, but did not reduce telomere attrition or ameliorate oxidative damage to proteins and lipids in the early postnatal period.

Telomere length is reflected by plumage coloration and predicts seasonal reproductive success in the barn swallow

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.