Inhibition of microRNA function by antimiR oligonucleotides

MicroRNAs (miRNAs) are an abundant class of small (approximately 22 nt) endogenous non-coding RNAs that direct post-transcriptional regulation of gene expression. Metazoan miRNAs regulate a wide range of biological processes, including developmental timing, apoptosis, differentiation, cell proliferation and metabolism [1-6]. Moreover, there is ample evidence that dysregulation of individual or entire families of miRNAs is associated with the pathogenesis of human diseases, such as cancer, CNS disorders, viral infections, cardiovascular and metabolic diseases [7-12].The first miRNA genes, lin-4 and let-7, were discovered in C. elegans by Victor Ambros and Gary Ruvkun, and shown to base-pair imperfectly to 3' untranslated regions (UTRs) of heterochronic genes, thereby controlling timing of larval development in the worm [13-15]. To date 18,226 miRNAs have been annotated in animals, plants and viruses, including 1,527 miRNAs encoded in the human genome [16]. miRNAs are either expressed from independent transcriptional units or derive from introns of protein-coding genes or exons or introns of long ncRNAs. Approximately 50% of the mammalian miRNAs are located within introns of protein-coding genes [17,18]. The primary transcripts of miRNA genes, termed pri-miRNAs, are usually several kilobases long and possess a 5' CAP and a poly(A) tail [19,20]. Pri-miRNAs are processed in the nucleus to approximately 70 nt hairpin-structures, known as pre-miRNAs (Figure 1), by the nuclear Microprocessor complex, consisting of DGCR8 and the RNase III enzyme Drosha [21-23]. Pre-miRNAs are exported to the cytoplasm by Exportin-5 [24-27] and processed further by Dicer, to approximately 22 nt double-stranded miRNA duplexes (Figure 1) [28-32]. The miRNA duplexes are loaded into an Argonaute protein in the miRNA-induced silencing complex (miRISC) and rapidly unwound. During this process the mature miRNA is retained in the miRISC, whereas the complementary strand, known as the miRNA star (miR