A random shRNA-encoding library for phenotypic selection and hit-optimization.

RNA interference (RNAi) is a mechanism for inhibiting gene expression through the action of small, non-coding RNAs. Most existing RNAi libraries target single genes through canonical pathways. Endogenous microRNAs (miRNAs), however, often target multiple genes and can act through non-canonical pathways, including pathways that activate gene expression. To interrogate all possible functions, we designed, synthesized, and validated the first shRNA-encoding library that is completely random at the nucleotide level. Screening in an IL3-dependent cell line, FL5.12, yielded shRNA-encoding sequences that double cell survival upon IL3 withdrawal. Using random mutagenesis and re-screening under more stringent IL3-starvation conditions, we hit-optimized one of the sequences; a specific nucleotide change and the creation of a mismatch between the two halves of the stem both contributed to the improved potency. Our library allows unbiased selection and optimization of shRNA-encoding sequences that confer phenotypes of interest, and could be used for the development of therapeutics and tools in many fields of biology.