f2: TERRA/TelRNAs associate to telomeric chromatin and may be involved in regulation of telomere length. Model for a role of telomeric RNAs in the regulation of telomere length. TERRA/TelRNA acts as a potent inhibitor of telomerase activity in vitro, possibly by formation of RNA:RNA hybrids with the template region of the telomerase RNA component.

Mentions: On account of their compact heterochromatic structure, telomeres were not regarded to be permissive for transcription. However, other heterochromatic domains in the genome, such as mouse major satellite or human heterochromatic satellite III repeats, were already shown to be efficiently transcribed by RNA polymerase II, giving rise to non-coding RNAs (Lehnertz et al, 2003; Jolly et al, 2004; Rizzi et al, 2004). Recently, two independent reports showed that the telomeric C-rich strand is frequently transcribed by RNA polymerase II, giving rise to UUAGGG-repeat containing non-coding RNAs (TERRA or TelRNA) (Azzalin et al, 2007; Schoeftner and Blasco, 2008). Although formal evidence is still missing, the detection of subtelomeric sequences in TelRNA/TERRA molecules strongly suggests the existence of transcriptional control elements at subtelomeres (Azzalin et al, 2007). Up to date, transcripts containing telomeric repeats have been described in Mus musculus, Homo sapiens, S. cerevisiae and Danio rerio (Azzalin et al, 2007; Luke et al, 2008; Schoeftner and Blasco, 2008). The fact that retrotransposition events at HTT arrays of D. melanogaster also depend on transcription suggests that transcription is a universal process occurring at the ends of linear, eukaryotic chromosomes. Importantly, telomeric RNAs can be detected at telomeres by RNA-FISH techniques, suggesting that TERRA/TelRNAs can associate with telomeric chromatin in cis, a feature reported earlier for the non-coding XIST RNA that controls mammalian dosage compensation (Azzalin et al, 2007; Payer and Lee, 2008; Schoeftner and Blasco, 2008). Interestingly, in a panel of female mouse cell lines, TERRA/TelRNA form accumulations (Tacs) in the immediate vicinity of the territory of inactive X chromosome (Xi), suggesting an involvement of TERRA/TelRNA in the biology of X inactivation (Schoeftner and Blasco, 2008). TERRA/TelRNA molecules range between ca 100 bp and >9 kb in length and were reported to form intermolecular G-quadruplex structure with single-stranded telomeric DNA, but can also fold into a compact repeated structure containing G-quartets (Azzalin et al, 2007; Schoeftner and Blasco, 2008; Xu et al, 2008; Martadinata and Phan, 2009; Randall and Griffith, 2009). Several lines of evidence exist implicating TelRNA/TERRA in the negative control of telomere length (Schoeftner and Blasco, 2008). Increased TelRNA/TERRA levels by interfering with TelRNA/TERRA decay, such as the impairment of non-sense-mediated RNA decay in human cells or by deletion of the 5′–3′exonuclease Rat1p in S. cerevisiae, are associated with a loss of telomere reserve (Azzalin et al, 2007; Luke et al, 2008). Current models propose a role for TelRNA/TERRA in controlling telomerase activity. In yeast, the formation of a DNA/RNA hybrid between TelRNA/TERRA and telomeres is thought to inhibit elongation by telomerase, whereas in mammals, TelRNA/TERRA was shown to efficiently inhibit telomerase activity in vitro, presumably by base pairing with the template region of the RNA component of telomerase (TERC) (Luke et al, 2008; Schoeftner and Blasco, 2008) (Figure 2). These working models are supported by expression data showing low TelRNA/TERRA levels during mouse embryogenesis and in cancer cells—two biological conditions that are characterized by rapid cell proliferation and dependence on high telomerase activity (Schoeftner and Blasco, 2008). On the other hand, accumulation of TelRNA/TERRA in adult tissues could be coupled to telomerase inhibition and ageing (Schoeftner and Blasco, 2008). Importantly, in immortal cell lines, as well as during nuclear reprogramming, TelRNA/TERRA levels correlate with the average telomere reserve (Schoeftner and Blasco, 2008; Marion et al, 2009). Together with the fact that TelRNA/TERRA can be localized to telomeric DNA repeats this suggests that TelRNA/TERRA could locally control telomerase activity in cis, a mechanism that could explain the preferential elongation of the shortest telomere in yeast and mammals on the molecular level (Marcand et al, 1999; Hemann et al, 2001; Samper et al, 2001; Teixeira et al, 2004; Schoeftner and Blasco, 2008). In addition, this mechanism would also preclude excessive telomere elongation by telomerase (i.e. telomere elongation during nuclear reprogramming, Marion et al, 2009), a condition that was found to be associated with impaired female fertility and fecundity in D. melanogaster (Walter et al, 2007). However, until formal evidence for a direct role of TERRA in telomerase inhibition has been presented, a speculative role of telomerase recruitment by TelRNA/TERRA should be considered (Figure 2).