MLL Associates with Telomeres and Regulates Telomeric Repeat-Containing RNA Transcription

Abstract

Mammalian telomeres consist of TTAGGG repeats organized in nucleosomes and associated with a sixprotein complex known as shelterin, which preserves telomere structure and protects chromosome ends from the cellular DNA damage response. Recent studies have found that telomeres are transcribed into telomeric UUAGGG repeat-containing RNA (TERRA) starting from subtelomeric regions. TERRA binding at telomeres appears to be involved in cis-based mechanisms of telomeric chromatin organization and maintenance. A number of histone methyltransferases (HMTs) are known to influence telomeric chromatin status; however, the regulatory mechanisms of telomere transcription are poorly understood. Here, we show that the histone 3/lysine 4 (H3/K4) HMT and the transcriptional regulator MLL associate with telomeres and contribute to their H3/K4 methylation and transcription in a telomere length-dependent manner. In human diploid fibroblasts, RNA interference-mediated MLL depletion affects telomere chromatin modification and transcription and induces the telomere damage response. Telomere uncapping through either TRF2 shelterin protein knockdown or exposure to telomere G-strand DNA oligonucleotides significantly increases the transcription of TERRA, an effect mediated by the functional cooperation between MLL and the tumor suppressor p53. In total, our findings identify a previously unrecognized role of MLL in modifying telomeric chromatin and provide evidence for the functional interaction between MLL, p53, and the shelterin complex in the regulation of telomeric transcription and stability.Telomeric chromatin consists of TTAGGG repeats organized in nucleosomes that are associated with a complex of six proteins, named shelterin, which plays a fundamental role in telomere protection and homeostasis (11). Adjacent subtelomeric chromatin is also enriched in repetitive DNA organized in nucleosomes and typically has a low gene density. Both subtelomeric and telomeric chromatins show histone modifications characteristic of heterochromatin, and subtelomeric DNA is also methylated (6). Along with DNA and histone modifications, noncoding RNA molecules are now known to play a fundamental cis-acting role in the organization and maintenance of heterochromatic structures (5). The recent discovery of the RNA polymerase II-dependent transcription of mammalian subtelomeric and telomeric DNAs into telomeric repeat-containing RNA (TERRA), RNA transcripts containing UUAGGG repeats, and the localization of TERRA at telomeres of both interphase and metaphase chromosomes suggest that these noncoding RNAs exert a cis function at telomeric chromatin (2, 37).SET domain-containing proteins with histone methyltransferase (HMT) activity are involved in telomeric chromatin modification and function. To date, Suv3-9h1, Suv3-9h2, Suv4-20h1, and Suv4-20h2 HMTs have been linked directly with telomere maintenance. Mouse embryonic fibroblasts (MEFs) null for both the Suv3-9h1 and Suv3-9h2 or Suv4-20h1 and Suv4-20h2 genes show abnormally long telomeres compared with no...

Telomeric repeat-containing RNA (TERRA) constitutes a nucleoprotein component of extracellular inflammatory exosomes

“…TERRA expression can be regulated by developmental and stress-related signals, including DNA damage and viral infection (7,(45)(46)(47). Telomere shortening may also increase TERRA expression (48), but it is not clear that senescent cells show a global increase in TERRA levels.…”

A role for p53 in telomere protection

“…1 TERRA induction can result from DNA damage, and a requirement for p53 (TP53) has been reported. 3 The p53 protein is a DNA-binding transcription factor with potent tumor suppressor properties. 5 In response to cell stress, p53 protein accumulates and binds to degenerate consensus sequences throughout the genome, activating the transcription of many target genes involved in DNA damage repair, apoptosis, and other cellular processes.…”

Transcriptional regulation of telomeric non-coding RNA: Implications on telomere biology, replicative senescence and cancer

“…[3][4][5] Recent studies have also found overall acetylation of H3 and H4 histone tails, H3K4 di-and trimethylation and H3K79 dimethylation at telomeres, which are permissive histone marks in defining transcribed chromatin regions and their promoter sequences, and in maintaining a transcriptionally active state. 6,7 The heterochromatic nature of telomeres, the low subtelomeric gene density, as well as the transcriptional silencing exerted Transcriptional regulation of telomeric non-coding RNA Implications on telomere biology, replicative senescence and cancer Corrado Caslini Departments of Pathology; University of Michigan Medical School; Ann Arbor, MI USA PoINt oF VIew di-and trimethylation. 7 Thus, it appears that the telomere length is dominant over telomere epigenetic state in modulating TERRA levels, suggesting a telomere length-dependent transcriptional regulation of TERRA.…”

“…6,7 The heterochromatic nature of telomeres, the low subtelomeric gene density, as well as the transcriptional silencing exerted Transcriptional regulation of telomeric non-coding RNA Implications on telomere biology, replicative senescence and cancer Corrado Caslini Departments of Pathology; University of Michigan Medical School; Ann Arbor, MI USA PoINt oF VIew di-and trimethylation. 7 Thus, it appears that the telomere length is dominant over telomere epigenetic state in modulating TERRA levels, suggesting a telomere length-dependent transcriptional regulation of TERRA.…”

“…1). 7 The MLL protein is involved in regulation of RNA pol II-dependent transcription of thousands of target loci including the HOX, CDK inhibitors, and E2F-regulated genes. MLL is the founding member of a family of H3K4 HMTs (MLL2, MLL3, SET1A, SET1B, ASH1L) identified in mammalian cells, all sharing association with a protein complex evolutionarily conserved up to Saccharomyces cerevisiae where Set1, the MLL counterpart and hypothetical regulator of TERRA in yeast, is present in such a complex called COMPASS (complex of proteins associated with Set1).…”