Telomeric RNAs Research Articles

Telomeric RNAs as a novel player in telomeric integrity

Telomeres protect linear chromosome ends from being recognized and processed as double-strand breaks by DNA repair activities. This protective function of telomeres is essential for chromosome stability. Until recently, telomeres have been considered to be transcriptionally silent. This notion was overturned in a series of recent papers that describe the existence of telomeric repeat-containing RNAs (TERRAs) in vertebrates and yeast. Here, we summarize recent developments in this field of telomere research, in particular the possible mechanisms that control TERRA expression.

The middle and the end

Welcome to ‘The Middle & The End’, an EMBO Journal focus review series on centromere and telomere biology. The molecular biology of DNA and chromosomes has always been a centre of attention for readers and contributors of The EMBO Journal alike since its inception 27 years ago. Those years have seen amazing progress in our understanding of these notable chromosomal regions, yet even in the light of most recent advances it is clear that a lot more is yet to be deciphered—figuratively speaking, we are still far from the ‘end’ in terms of learning and understanding them, but we may well be in the very ‘middle’ of things by now. Having a middle and an end is a privilege of eukaryotic chromosomes, which—in contrast to the circular DNA of typical prokaryotes—are usually linear, a feature that endows them with characteristic properties as well as problems. For one, ends need to be protected and maintained, despite the intricacies of semi‐conservative replication and in the face of DNA break surveillance mechanisms liable to attempt their repair. The middle, on the other hand, is commonly the place for spindle microtubule attachment and thus important for chromosome segregation. In light of these peculiarities, centromeres and telomeres have become highly specialized compared with the bulk of eukaryotic chromosomes, which fulfills the more intuitive (and most fundamental) role of DNA, to encode genes for proteins or RNAs and to regulate their expression. This specialization of centromeres and telomeres is reflected at numerous levels, starting with DNA sequence through chromatin structure to the recruitment and function of specialized structural and signalling …

A ‘higher order’ of telomere regulation: telomere heterochromatin and telomeric RNAs

Protection of chromosome ends from DNA repair and degradation activities is mediated by specialized protein complexes bound to telomere repeats. Recently, it has become apparent that epigenetic regulation of the telomric chromatin template critically impacts on telomere function and telomere-length homeostasis from yeast to man. Across all species, telomeric repeats as well as the adjacent subtelomeric regions carry features of repressive chromatin. Disruption of this silent chromatin environment results in loss of telomere-length control and increased telomere recombination. In turn, progressive telomere loss reduces chromatin compaction at telomeric and subtelomeric domains. The recent discoveries of telomere chromatin regulation during early mammalian development, as well as during nuclear reprogramming, further highlights a central role of telomere chromatin changes in ontogenesis. In addition, telomeres were recently shown to generate long, non-coding RNAs that remain associated to telomeric chromatin and will provide new insights into the regulation of telomere length and telomere chromatin. In this review, we will discuss the epigenetic regulation of telomeres across species, with special emphasis on mammalian telomeres. We will also discuss the links between epigenetic alterations at mammalian telomeres and telomere-associated diseases.

Telomeric RNAs Mark Sex Chromosomes in Stem Cells

Telomeric regions are known to be transcribed in several organisms. Although originally reported to be transcribed from all chromosomes with enrichment near the inactive X of female cells, we show that telomeric RNAs in fact are enriched on both sex chromosomes of the mouse in a developmentally specific manner. In female stem cells, both active Xs are marked by the RNAs. In male stem cells, both the X and the Y accumulate telomeric RNA. Distribution of telomeric RNAs changes during cell differentiation, after which they associate only with the heterochromatic sex chromosomes of each sex. FISH mapping suggests that accumulated telomeric RNAs localize at the distal telomeric end. Interestingly, telomeric expression changes in cancer and during cellular stress. Furthermore, RNA accumulation increases in Dicer-deficient stem cells, suggesting direct or indirect links to RNAi. We propose that telomeric RNAs are tied to cell differentiation and may be used to mark pluripotency and disease.

Role of TRF2 in the assembly of telomeric chromatin

Telomeric chromatin is composed of TTAGGG repeats bound by the shelterin complex. Mammalian telomeres also contain arrays of nucleosomes enriched in histone heterochromatic marks, as well as are associated by long non-coding telomeric RNAs. In addition, the adjacent subtelomeric DNA repeats are heavily methylated. This repressive chromatin environment at telomeres is thought to suppress the expression of closely located genes, a phenomenon known as "telomere position effect" (TPE), which can be influenced by telomere length and the telomeric chromatin. Here, we address the role of TRF2, a major telomere component, in the epigenetic regulation of telomeres. To this end, we took advantage of transgenic mice that over express TRF2 under the keratin 5 (K5) promoter (K5TRF2 mice). K5TRF2 mice have very short telomeres and an increased susceptibility to develop skin cancer. Interestingly, K5TRF2 cells show an aberrant nucleosomal organization at telomeres. In particular, K5TRF2 cells display a loss of heterochromatin marks at telomeric and subtelomeric repeats, which is concomitant with decreased abundance of core histones H3 and H4 and with an increased nucleosomal spacing at telomeres. Telomere shortening in K5TRF2 mice is also accompanied by decreased abundance of telomeric transcripts. Together, these findings indicate a previously unnoticed role for TRF2 in the assembly of telomeric chromatin.

When flies and mice develop cancer

![][1] The Spanish National Cancer Institute (CNIO) Meeting on Development and Cancer took place between 4 and 6 February 2008 in Madrid, Spain, and was organized by K. Basler, G. Morata, E. Moreno and M. Torres. ![][2] Developmental biologists rightly argue that genes regulating developmental pathways are reiterated in cancer development. At the recent Spanish National Cancer Institute (CNIO) Meeting on Development and Cancer, novel concepts and processes that are considered to be common to both embryonic and cancer development were discussed. It is worth pointing out that tumour formation should be considered to be a form of aberrant organ development, in which many developmental pathways that control proliferation, differentiation and growth or morphogenesis are affected. This report is structured in three parts, describing the work presented at the meeting according to the stages of tumour development to which it pertains. First, the normal behaviour of cell populations that are the targets of transformation, in particular adult stem cells and the pathways that regulate their behaviour. Second, processes that occur at the early stages of cancer development before morphological malformations are detectable, in particular the process that is best known from studies in flies as ‘cell competition’. Third, developmental processes and pathways that are active in the growing tumour—which we consider to be an extrinsic organ—for example, the recruitment of blood vessels or the role of morphogens in controlling the size and shape of tumours, and the progression from hyperplasia to invasive tumours (Fig 1). Figure 1. Tumour development and genes involved in the different stages. Schematic representation of how stem cells (A) , by acquiring a mutation through cell competition ( B ) and by the selection of additional mutations, can give rise to the ‘tumour organ’ ( C ). The genes involved in these processes are listed on the right‐hand side, and most of them … [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif

Developmentally regulated transcription of mammalian telomeres by DNA-dependent RNA polymerase II

Mammalian telomeres consist of non-coding TTAGGG repeats that are bound by the multi-protein complex 'shelterin', thus protecting chromosome ends from DNA repair mechanisms and degradation. Mammalian telomeric chromatin is enriched for the constitutive heterochromatin marks H3K9me3, H4K20me3 and HP1 (refs 2, 3, 4, 5, 6, 7). Similar to pericentric heterochromatin, telomeric heterochromatin is thought to be fundamental for the maintenance of chromosomal integrity. Here, we report that telomeric repeats are transcribed by DNA-dependent RNA polymerase II, which, in turn, interacts with the TRF1 shelterin protein. Telomeric RNAs (TelRNAs) contain UUAGGG repeats, are polyadenylated and are transcribed from the telomeric C-rich strand. Transcription of mammalian telomeres is regulated by several mechanisms, including developmental status, telomere length, cellular stress, tumour stage and chromatin structure. Using RNA-flourescent in situ hybridization (FISH), we show that TelRNAs are novel structural components of telomeric chromatin. Importantly, we provide evidence that TelRNAs block the activity of telomerase in vitro, suggesting that TelRNAs may regulate telomerase activity at chromosome ends. Our results indicate that TelRNAs are novel components of mammalian telomeres, which are anticipated to be fundamental for understanding telomere biology and telomere-related diseases, such as cancer and ageing.