期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2015
卷号：112
期号：11
页码：3445-3450
DOI：10.1073/pnas.1502849112
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceAlternative splicing (AS) plays an important role in the mammalian brain, but our atlas of AS events is incomplete. Here, we conducted comprehensive analysis of deep RNA-Seq data of mouse cortex to identify new AS events and evaluate their functionality. We expanded the number of annotated AS events more than 10-fold and demonstrated that, like many known events, thousands of newly discovered events are regulated, conserved, and likely functional. In particular, some can regulate gene expression levels through nonsense-mediated decay, a known mechanism for RNA binding protein autoregulation. Surprisingly, we discovered a number of chromatin regulators as novel targets of this mechanism, revealing a new regulatory link between epigenetics and AS that primarily emerged in the mammalian lineage. Alternative splicing (AS) dramatically expands the complexity of the mammalian brain transcriptome, but its atlas remains incomplete. Here we performed deep mRNA sequencing of mouse cortex to discover and characterize alternative exons with potential functional significance. Our analysis expands the list of AS events over 10-fold compared with previous annotations, demonstrating that 72% of multiexon genes express multiple splice variants in this single tissue. To evaluate functionality of the newly discovered AS events, we conducted comprehensive analyses on central nervous system (CNS) cell type-specific splicing, targets of tissue- or cell type-specific RNA binding proteins (RBPs), evolutionary selection pressure, and coupling of AS with nonsense-mediated decay (AS-NMD). We show that newly discovered events account for 23-42% of all cassette exons under tissue- or cell type-specific regulation. Furthermore, over 7,000 cassette exons are under evolutionary selection for regulated AS in mammals, 70% of which are new. Among these are 3,058 highly conserved cassette exons, including 1,014 NMD exons that may function directly to control gene expression levels. These NMD exons are particularly enriched in RBPs including splicing factors and interestingly also regulators for other steps of RNA metabolism. Unexpectedly, a second group of NMD exons reside in genes encoding chromatin regulators. Although the conservation of NMD exons in RBPs frequently extends into lower vertebrates, NMD exons in chromatin regulators are introduced later into the mammalian lineage, implying the emergence of a novel mechanism coupling AS and epigenetics. Our results highlight previously uncharacterized complexity and evolution in the mammalian brain transcriptome.
