Abstract
Recent transcriptome studies suggest that long noncoding RNAs (lncRNAs) are key components of the mammalian genome, and their study has become a new frontier in biomedical research. In fact, lncRNAs in the mammalian genome were identified and studied at particular epigenetic loci, including imprinted loci and X-chromosome inactivation center, at least two decades ago—long before development of high throughput sequencing technology. Since then, researchers have found that lncRNAs play essential roles in various biological processes, mostly during development. Since much of our understanding of lncRNAs originates from our knowledge of these well-established lncRNAs, in this review we will focus on lncRNAs from the X-chromosome inactivation center and the Dlk1-Dio3 imprinted cluster as examples of lncRNA mechanisms functioning in the epigenetic regulation of mammalian genes.
Highlights
Long non-coding RNAs are defined as RNAs over 200 nt in length that do not encode proteins
Those topics are reviewed elsewhere [5,6,7,8,9]; here, we focus on the role played by long noncoding RNAs (lncRNAs) in X-chromosome inactivation (XCI)
Studies of Xist RNA in the past two decades have greatly advanced our understanding of how lncRNAs regulate gene expression epigenetically
Summary
Long non-coding RNAs are defined as RNAs over 200 nt in length that do not encode proteins. In 2002, following largescale sequencing of mouse cDNA libraries, Okazaki et al revealed that a huge proportion of the mammalian transcriptome does not code for proteins and defined lncRNAs as a significant transcript class [1]. The recent ENCODE (Encyclopedia of DNA Elements) study reported over 9640 lncRNA loci in the human genome, roughly half the number of protein-coding genes [2]. These studies have changed our view of the mammalian genome and underscored the importance of understanding lncRNA function at a mechanistic level. The most studied lncRNAs are perhaps those transcribed from the X-chromosome inactivation center and from imprinted loci
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