Hidden treasures in unspliced EST data

Several classes of exclusively—or at least predominantly—unspliced non-coding RNAs have been described in the last years, including totally and partially intronic transcripts and long intergenic RNAs. Functionally, they appear to be involved in regulating gene expression, at least in part by associating with the chromatin. Intron-less transcripts have received little attention, even though recent findings indicate that intron-less protein-coding genes have several features that set them apart from the more abundant and much better understood spliced mRNAs. Even less is known about unspliced non-coding transcripts. Thus we systematically analyze the distribution of unspliced ESTs in the human genome. These form a large source of transcriptomic data that is almost always excluded from detailed studies. Most unspliced ESTs appear in clusters overlapping, or located in the close vicinity of, annotated RefSeq genes. Partially intronic unspliced ESTs show complex patterns of overlap with the intron/exon structure of the RefSeq gene. Distinctive patterns of CAGE tags indicate that a large class of unspliced EST clusters is forming long extensions of 3′UTRs, at least several hundreds of which probably appear also as independent 3′UTR-associated RNAs.