Molecular Insights into Kcnq1ot1 Noncoding Antisense RNA Mediated Long Range Transcriptional Gene Silencing
Abstract: Non-coding antisense RNAs have been implicated in the epigenetic silencing of individual gene as well as chromosomal domains. While silencing of the overlapping gene by antisense RNAs has been well investigated, their functional role in silencing of chromosomal domains remains enigmatic. To elucidate mechanisms underlying the non-coding RNA mediated epigenetic silencing of chromosomal domains, we have chosen an antisense non-coding RNA, Kcnq1ot1, as a model system. Previously, a functional role of Kcnq1ot1 RNA and/or its transcriptional process has been implicated in silencing of multiple genes in the Kcnq1 imprinted cluster. However, these studies could not rule out the mechanisms involving other than Kcnq1ot1 RNA. Furthermore, it was also unclear how the Kcnq1ot1 promoter escapes silencing when its encoded RNA is capable of silencing flanking genes in cis.We have shown that NF-Y transcription factor plays a central role in the Kcnq1ot1 promoter activity, and that mutation of the NF-Y binding sites not only resulted in loss of silencing of flanking genes but also the ability of the Kcnq1ot1 promoter to protect against repressive chromatin marks, indicating that NF-Y maintains transcription-competent chromatin at the promoter through resisting the strong silencing effects of Kcnq1ot1 RNA.The Kcnq1ot1 RNA is an RNA Polymerase II encoded 91 kb long moderately stable nuclear transcript. We have demonstrated that it is the RNA not the act of transcription responsible for silencing and that the degree of silencing was proportional to the length of Kcnq1ot1 RNA. The kinetics of heterochromatin formation in relation to Kcnq1ot1 transcription revealed that overlapping gene was silenced initially by occlusion of basal transcription machinery and heterochromatin formation, whereas nonoverlapping gene was silenced subsequently by Kcnq1ot1-mediated heterochromatin spreading. This transcriptional silencing by Kcnq1ot1 RNA is mediated by an 890 bp region through promoting its interaction with the chromatin. Interestingly, we show that Kcnq1ot1 RNA establishes heterochromatin structures in a lineage-specific fashion by interacting with chromatin and chromatin remodelling complexes such as G9a and PRC2 complexes. More importantly, one of the parental chromosomes comprising Kcnq1 domain always found in the vicinity of perinucleolar region. Based on these data we proposed a mechanism whereby Kcnq1ot1 RNA establishes transcriptional silencing through recruitment of chromatin remodelling machinery and the maintenance of silencing achieved via targeting to the perinucleolar region.
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