Differential repression of alternative transcripts: a screen for miRNA targets

Alternative polyadenylation sites produce transcript isoforms with 3′ untranslated regions (UTRs) of different lengths. If a microRNA (miRNA) target is present in the UTR, then only those target-containing isoforms should be sensitive to control by a cognate miRNA. We carried out a systematic examination of 3′ UTRs containing multiple poly(A) sites and putative miRNA targets. Based on expressed sequence tag (EST) counts and EST library information, we observed that levels of isoforms containing targets for miR-1 or miR-124, two miRNAs causing downregulation of transcript levels, were reduced in tissues expressing the corresponding miRNA. This analysis was repeated for all conserved 7-mers in 3′ UTRs, resulting in a selection of 312 motifs. We show that this set is significantly enriched in known miRNA targets and mRNA-destabilizing elements, which validates our initial hypothesis. We scanned the human genome for possible cognate miRNAs and identified phylogenetically conserved precursors matching our motifs. This analysis can help identify target-miRNA couples that went undetected in previous screens, but it may also reveal targets for other types of regulatory factors.
Synopsis MicroRNAs (miRNAs) are short RNA molecules that recognize specific target sequences in the 3′ region of mRNAs. These miRNAs can then specifically keep the mRNAs from being expressed, or translated into proteins. In this article, the authors ask what happens when a targeted mRNA has several forms differing by their 3′ regions. Such 3′ variations are very common. If two or more variations are present in a single mRNA, the result is two or more mRNAs with 3′ ends of different lengths. If an miRNA target is located between the two sites of variability, the shorter transcript should be target free and should escape miRNA-mediated inhibition, while longer transcripts should be inhibited. To test this hypothesis, the authors looked at mRNAs that had these variable 3′ ends. Variants containing targets for certain miRNAs appeared to be specifically underrepresented in tissues where these particular miRNAs are found. This principle was used to find other sequence patterns in 3′ regions that had a similar effect, and a list of 312 significant patterns was obtained. The authors then scanned genome sequences and identified possible cognate miRNAs for these patterns. This new knowledge will help further an understanding of how genes are controlled.