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Deregulated Expression of Mammalian lncRNA through Loss of SPT6 Induces R-Loop Formation, Replication Stress, and Cellular Senescence

ABSTRACT: RNA polymerase II (Pol II) generates interspersed protein-coding and long noncoding (lnc) transcripts across the mammalian genome. We recently established that lncRNAs are transcribed by different Pol II CTD isoforms correlating with reduced RNA processing efficiency and increased RNA turnover. How protein-coding and lncRNA transcription units are distinguished during gene expression remains unclear. We now show that H3K36me3 chromatin is a predominant epigenetic mark for active protein-coding but not lncRNA genes. Surprisingly depletion of the Pol II associated transcription elongation factor SPT6, causes redistribution of H3K36me3 histone marks to lncRNA genes which correlates with increased lncRNA transcription. SPT6 knockdown also reduces recruitment of Integrator complex to chromatin resulting in a transcription termination defect for lncRNA genes. Notably these extended lncRNAs are both chromatin-restricted and form increased levels of RNA:DNA hybrid (R-loops) with associated DNA damage. This results in a cellular senescence phenotype and underlies the importance of restricting lncRNA expression.

ORGANISM(S): Homo sapiens

PROVIDER: GSE110028 | GEO | 2018/11/21

REPOSITORIES: GEO

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Project description:Pervasive transcription in the mammalian genome produces thousands of long noncoding RNAs (lncRNAs) and promoter- or enhancer-associated unstable transcripts. They preferentially locate to chromatin, at which some regulate chromatin structure, transcription and RNA processing. While several RNA sequences responsible for nuclear localization have been identified, such as repeats in the lncRNA Xist and Alu-like elements for long RNAs, how lncRNAs as a class are enriched on chromatin remains elusive. To screen for cis-elements that contribute to RNA-chromatin localization, we developed a high-throughput method named RNA elements for subcellular localization by sequencing (REL-seq), and discovered a U1 small nuclear ribonucleoprotein (snRNP)-recognition motif being critical for chromatin localization of reporter RNAs. Across the genome, chromatin-bound lncRNAs, which are enriched with 5’ splice sites and depleted of 3’ splice sites, exhibit high levels of U1 snRNA binding compared to cytoplasm-localized protein-coding mRNAs. Acute depletion of U1 snRNA, or U1 snRNP protein component SNRNP70, drastically reduces the chromatin association of hundreds of lncRNAs and unstable transcripts without altering the overall transcription rate in cells. In addition, rapid degradation of SNRNP70 reduces the localization of both nascent and polyadenylated lncRNA transcripts to chromatin, and disrupts the nuclear-speckles and genome-wide localization of Malat1, a highly conserved and abundant lncRNA. Moreover, chromatin-bound U1 snRNP interacts with transcriptionally engaged RNA polymerase (Pol) II. Together, these results demonstrate that U1 snRNP acts widely to tether and mobilize lncRNAs to chromatin in a Pol II transcription-dependent manner. Our findings uncover a novel role of U1 snRNP beyond pre-mRNA processing and provide molecular insights into how lncRNAs are recruited to Pol II-transcribed genes and have a propensity for chromatin-associated functions.

Project description:RSC (remodels the structure of chromatin) is an essential ATP-dependent chromatin remodeling complex in Saccharomyces cerevisiae. The catalytic subunit of RSC, Sth1 uses its ATPase activity to slide or remove nucleosomes. RSC has been shown to regulate the width of the nucleosome-depleted regions (NDRs) by sliding the flanking nucleosomes away from NDRs. As such the nucleosomes encroach NDRs when RSC is depleted and leads to transcription initiation defects. In this study, we examined the effects of the catalytic-dead Sth1 on transcription and compared them to the effects observed during acute and rapid Sth1 depletion by auxin-induced degron strategy. We found that rapid depletion of Sth1 reduces recruitment of TBP and Pol II in highly transcribed genes, as would be expected considering its role in regulating chromatin structure at promoters. In contrast, cells harboring the catalytic-dead Sth1 exhibited a severe reduction in TBP binding, but surprisingly, also displayed a substantial accumulation in Pol II occupancies within coding regions. After depleting endogenous Sth1 in the catalytic dead mutant, we observed a further increase in Pol II occupancies, suggesting that the inactive Sth1 contributed to the observed accumulation of Pol II in coding regions. Notwithstanding the Pol II increase, the ORF occupancies of histone chaperones FACT and Spt6 were significantly reduced in the mutant. These results suggest a potential role for RSC in recruiting/retaining these chaperones in coding regions. Pol II accumulation despite substantial reductions in TBP, FACT, and Spt6 occupancies in the catalytic-dead mutant could be indicative of severe transcription elongation and termination defects. Such defects would be consistent with studies showing that RSC is recruited to coding regions in a transcription-dependent manner. Thus, these findings imply a role for RSC in transcription elongation and termination processes, in addition to its established role in transcription initiation.

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Deregulated Expression of Mammalian lncRNA through Loss of SPT6 Induces R-Loop Formation, Replication Stress, and Cellular Senescence

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