Improved design of hammerhead ribozyme for selective digestion of target RNA through recognition of site-specific adenosine-to-inosine RNA editing

Programmable technologies to manipulate gene expression at the RNA level

RNA has long been an enticing therapeutic target, but is now garnering increased attention, largely driven by clinical successes of RNA interference-based drugs. While gene knockdown by well-established RNA interference- and other oligonucleotide-based strategies continues to advance in the clinic, the repertoire of targetable effectors capable of altering gene expression at the RNA level is also rapidly expanding. In this review, we focus on several recently developed bifunctional molecular technologies that both interact with and act upon a target RNA. These new approaches for programmable RNA knockdown, editing, splicing, translation, and chemical modifications stand to provide impactful new modalities for therapeutic development in the coming decades.

Intracellular selection of trans-cleaving hammerhead ribozymes

Hammerhead ribozyme is the smallest and best characterized catalytic RNA-cleaving ribozyme. It has been reported as potential therapeutic tools to manipulate the expression of target genes. However, most of naturally occurring hammerhead ribozymes process self-cleavage rather than cleave substrate RNA in trans, and its high intracellular activity relies on the tertiary interaction of Loop II and steam I bulge, resulting in decreased performance as applied in gene silencing. We described a direct intracellular selection method to evolve hammerhead variants based on trans-cleavage mode via using a toxin gene as the reporter. And a dual fluorescence proteins system has also been established to quantitatively evaluate the efficiency of selected ribozymes in the cell. Based on this selection strategy, we obtained three mutants with enhanced intracellular cleaving activity compared to wide type hammerhead ribozyme. The best one, TX-2 was revealed to possess better and consistent gene knockdown ability at different positions on diverse targeted mRNA either in prokaryotic or eukaryotic cells than wild-type hammerhead ribozyme. These observations imply the efficiency of the intracellular selection method of the trans-acting ribozyme and the potentials of improved ribozyme variants for research and therapeutic purposes.

Construction of a guide-RNA for site-directed RNA mutagenesis utilising intracellular A-to-I RNA editing

As an alternative to DNA mutagenesis, RNA mutagenesis can potentially become a powerful gene-regulation method for fundamental research and applied life sciences. Adenosine-to-inosine (A-to-I) RNA editing alters genetic information at the transcript level and is an important biological process that is commonly conserved in metazoans. Therefore, a versatile RNA-mutagenesis method can be achieved by utilising the intracellular RNA-editing mechanism. Here, we report novel guide RNAs capable of inducing A-to-I mutations by guiding the editing enzyme, human adenosine deaminase acting on RNA (ADAR). These guide RNAs successfully introduced A-to-I mutations into the target-site, which was determined by the reprogrammable antisense region. In ADAR2-over expressing cells, site-directed RNA editing could also be performed by simply introducing the guide RNA. Our guide RNA framework provides basic insights into establishing a generally applicable RNA-mutagenesis method.