MicroRNAs (miRNAs) help orchestrate cellular growth and survival through post-transcriptional mechanisms. The dysregulation of miRNA biogenesis can lead to cellular growth defects, chemotherapeutic resistance and plays a direct role in the development of many chronic diseases. Among these RNAs, miR-21 is consistently overexpressed in most human cancers leading to the down regulation of key tumor suppressing and pro-apoptotic factors; suggesting that inhibition of miR-21 biogenesis could reverse these negative effects. However, targeted inhibition of miR-21 using small molecules has had limited success. To overcome difficulties in targeting RNA secondary structure with small molecules, we developed a class of cyclic β-hairpin peptidomimetics which binds to RNA stem loop structures, such as miRNA precursors, with potent affinity and specificity. We screened an existing cyclic peptide library and discovered a lead structure which binds to pre-miR21 with a KD=200nM and prefers it over other pre-miRNAs. The NMR structure of the complex shows the peptide recognizes the Dicer cleavage site and alters processing of the precursor to the mature miRNA in vitro and in cultured cells. The structure provides a rational for the peptide binding activity and clear guidance for further improvements in affinity and targeting.

Rbfox proteins regulate tissue-specific splicing by targeting a conserved GCAUG sequence within pre-mRNAs. We report here that sequence-specific binding of the conserved Rbfox RRM to miRNA precursors containing the same sequence motif in their terminal loops, including miR-20b and miR-107, suppresses their nuclear processing. The structure of the complex between precursor miR-20b and Rbfox RRM shows the molecular basis for recognition, and reveals changes in the stem-loop upon protein binding. In mammalian cells, Rbfox2 downregulates mature miR-20b and miR-107 levels and increases the expression of their downstream targets PTEN and Dicer, respectively, suggesting that Rbfox2 indirectly regulates many more cellular miRNAs. Thus, some of the widespread cellular functions of Rbfox2 protein are attributable to regulation of miRNA biogenesis, and might include the mis-regulation of miR-20b and miR-107 in cancer and neurodegeneration.

The RNA Recognition Motif (RRM) is the largest family of eukaryotic RNA-binding proteins. Engineered RRMs with new specificity would provide valuable tools and an exacting test of our understanding of specificity. We have achieved the first successful re-design of the specificity of an RRM using rational methods and demonstrated re-targeting of activity in cells. We engineered the conserved RRM of human Rbfox proteins to specifically bind to the terminal loop of miR-21 precursor with high affinity and inhibit its processing by Drosha and Dicer. We further engineered Giardia Dicer by replacing its PAZ domain with the designed RRM. The reprogrammed enzyme degrades pre-miR-21 specifically in vitro and suppresses mature miR-21 levels in cells, which results in increased expression of PDCD4 and significantly decreased viability for cancer cells. The results demonstrate the feasibility of engineering the sequence-specificity of RRMs and of using this ubiquitous platform for diverse biological applications.