Telomeres Acquire Distinct Heterochromatin Characteristics during siRNA-Induced RNA Interference in Mouse Cells

Department of Anatomical and Cellular Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong SAR, The People's Republic of China.
Current Biology (Impact Factor: 9.57). 03/2008; 18(3):183-7. DOI: 10.1016/j.cub.2007.12.059
Source: PubMed


Telomeres are protective structures present at the ends of linear chromosomes and consist of simple repeating-DNA sequences and specialized proteins [1, 2]. Integrity of the telomeres is important in maintaining genome stability[1-6]. RNA interference(RNAi) involves short double-stranded RNA (21-23 nucleotides long), termed short interference RNA(siRNA), resulting in the downregulation of genes with cognate sequences [7-9]. During transient siRNA-induced RNAi in mouse fibroblast cultures, we found significant reversible changes related to the telomeres. Telomeres acquired distinct heterochromatin features. There were increased bindings of Argonaute-1 (AGO1), telomeric repeat-binding factor 1(TERF1), and heterochromatin protein 1beta (HP1beta) on the telomeres. Histone H3 (lysine 9) was hypermethylated at the telomeres. The chromosome ends also were associated with an unidentified RNA. During RNAi, expression of a transgene inserted adjacent to the telomere was downregulated. In addition, the concentration of a group of heterogeneous high-molecular-weight RNA containing telomeric repeat sequences was increased, and this RNA formed a small number of transient, discrete nuclear foci. Our findings suggest that telomeres participate actively in the siRNA-induced RNAi process. These responses of telomeres to the RNAi process might partially account for the off-target effects of RNAi.

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Available from: John Murnane, Jul 21, 2014
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    • "Transfection of siRNAs that were designed to target TERRA decreased telomere-association of ORC2 [56]. However, in the Deng et al. paper, the TERRA signal on the Northern blot was smaller than 1 kb which is considerably smaller than what has been reported for TERRA in studies from several other laboratories [7] [8] [30] [35] [57]. Thus, it seems that a different RNA population may have been assessed by Deng et al. "
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    ABSTRACT: Telomeres are heterochromatic structures at the ends of eukaryotic chromosomes. As other heterochromatin regions, telomeres are transcribed, from the subtelomeric region towards chromosome ends into the long non-coding RNA TERRA. Telomere transcription is a widespread phenomenon as it has been observed in species belonging to several kingdoms of the eukaryotic domain. TERRA is part of telomeric heterochromatin in addition to being present in the nucleoplasm. Here, we review the current knowledge of TERRA structure, biogenesis and turnover. In addition, we discuss presumed roles of this RNA during replication of telomeric DNA, heterochromatin formation and the regulation of telomerase.
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    • "Our discoveries define human telomeres as components of integral ''genic'' units and make TERRA rise above the transcriptional noise associated with the human genome, supporting the idea that TERRA might exert important functions in telomere biology. It has been proposed that TERRA could repress telomerase activity at chromosome ends by base-pairing with the template sequence in the telomerase RNA moiety (Luke et al. 2008; Schoeftner and Blasco 2008). In this scenario, methylation-mediated transcriptional repression of TERRA CpG-island promoters may be part of an epigenetic tumor suppressor genesilencing program accompanying cell transformation (Esteller 2007; Suzuki and Bird 2008). "
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    ABSTRACT: 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.
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    • "It was reported that such transcription occurred at specific stages of the cell cycle in yeast (5,6) and mammalian cells (7). The transcript was processed to activate the RNAi mechanism (8,9), which in turn heterochromatinated and silenced the cognate DNA sequence. Interestingly, the inactivated X chromosome in mammalian female cells has been shown to periodically visit the nucleolus in order to maintain the heterochromatic silent state; however, the relationship between nucleolar localization and transcription was not clear (10). "
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    ABSTRACT: Heterochromatin is critical for genome integrity, and recent studies have suggested the importance of transcription in heterochromatin for maintaining its silent state. We previously developed a method to generate a large homogeneously staining region (HSR) composed of tandem plasmid sequences in human cells that showed typical heterochromatin characteristics. In this study, we examined transcription in the HSR. We found that transcription of genes downstream to no-inducible SRα promoter was restricted to a few specific points inside the large HSR domain. Furthermore, the HSR localized to either to the surface or to the interior of the nucleolus, where it was more actively transcribed. The perinucleolar or intranucleolar locations were biased to late or early S-phase, and the location depended on either RNA polymerase II/III or I transcription, respectively. Strong activation of the inducible TRE promoter resulted in the reversible loosening of the HSR domain and the appearance of transcripts downstream of not only the TRE promoters, but also the SRα promoters. During this process, detection of HP1α or H3K9Me3 suggested that transcription was activated at many specific points dispersed inside large heterochromatin. The transcriptional rules obtained from studying artificial heterochromatin should be useful for understanding natural heterochromatin.
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