A screening protocol for identification of functional mutants of RNA editing adenosine deaminases

Library Screening Reveals Sequence Motifs That Enable ADAR2 Editing at Recalcitrant Sites

Adenosine deaminases acting on RNA (ADARs) catalyze the hydrolytic deamination of adenosine to inosine in duplex RNA. The inosine product preferentially base pairs with cytidine resulting in an effective A-to-G edit in RNA. ADAR editing can result in a recoding event alongside other alterations to RNA function. A consequence of ADARs' selective activity on duplex RNA is that guide RNAs (gRNAs) can be designed to target an adenosine of interest and promote a desired recoding event. One of ADAR's main limitations is its preference to edit adenosines with specific 5' and 3' nearest neighbor nucleotides (e.g., 5' U, 3' G). Current rational design approaches are well-suited for this ideal sequence context, but limited when applied to difficult-to-edit sites. Here we describe a strategy for the in vitro evaluation of very large libraries of ADAR substrates (En Masse Evaluation of RNA Guides, EMERGe). EMERGe allows for a comprehensive screening of ADAR substrate RNAs that complements current design approaches. We used this approach to identify sequence motifs for gRNAs that enable editing in otherwise difficult-to-edit target sites. A guide RNA bearing one of these sequence motifs enabled the cellular repair of a premature termination codon arising from mutation of the MECP2 gene associated with Rett Syndrome. EMERGe provides an advancement in screening that not only allows for novel gRNA design, but also furthers our understanding of ADARs' specific RNA-protein interactions.

Sensitive ADAR editing reporter in cancer cells enables high-throughput screening of small molecule libraries

Adenosine to inosine editing is common in the human transcriptome and changes of this essential activity is associated with disease. Children with ADAR1 mutations develop fatal Aicardi-Goutières syndrome characterized by aberrant interferon expression. In contrast, ADAR1 overexpression is associated with increased malignancy of breast, lung and liver cancer. ADAR1 silencing in breast cancer cells leads to increased apoptosis, suggesting an anti-apoptotic function that promotes cancer progression. Yet, suitable high-throughput editing assays are needed to efficiently screen chemical libraries for modifiers of ADAR1 activity. We describe the development of a bioluminescent reporter system that facilitates rapid and accurate determination of endogenous editing activity. The system is based on the highly sensitive and quantitative Nanoluciferase that is conditionally expressed upon reporter-transcript editing. Stably introduced into cancer cell lines, the system reports on elevated endogenous ADAR1 editing activity induced by interferon as well as knockdown of ADAR1 and ADAR2. In a single-well setup we used the reporter in HeLa cells to screen a small molecule library of 33 000 compounds. This yielded a primary hit rate of 0.9% at 70% inhibition of editing. Thus, we provide a key tool for high-throughput identification of modifiers of A-to-I editing activity in cancer cells.

RNA-Seq analysis identifies a novel set of editing substrates for human ADAR2 present in Saccharomyces cerevisiae

ADAR2 is a member of a family of RNA editing enzymes found in metazoa that bind double helical RNAs and deaminate select adenosines. We find that when human ADAR2 is overexpressed in the budding yeast Saccharomyces cerevisiae it substantially reduces the rate of cell growth. This effect is dependent on the deaminase activity of the enzyme, suggesting yeast transcripts are edited by ADAR2. Characterization of this novel set of RNA substrates provided a unique opportunity to gain insight into ADAR2's site selectivity. We used RNA-Seq. to identify transcripts present in S. cerevisiae subject to ADAR2-catalyzed editing. From this analysis, we identified 17 adenosines present in yeast RNAs that satisfied our criteria for candidate editing sites. Substrates identified include both coding and noncoding RNAs. Subsequent Sanger sequencing of RT-PCR products from yeast total RNA confirmed efficient editing at a subset of the candidate sites including BDF2 mRNA, RL28 intron RNA, HAC1 3'UTR RNA, 25S rRNA, U1 snRNA, and U2 snRNA. Two adenosines within the U1 snRNA sequence not identified as substrates during the original RNA-Seq. screen were shown to be deaminated by ADAR2 during the follow-up analysis. In addition, examination of the RNA sequence surrounding each edited adenosine in this novel group of ADAR2 sites revealed a previously unrecognized sequence preference. Remarkably, rapid deamination at one of these sites (BDF2 mRNA) does not require ADAR2's dsRNA-binding domains (dsRBDs). Human glioma-associated oncogene 1 (GLI1) mRNA is a known ADAR2 substrate with similar flanking sequence and secondary structure to the yeast BDF2 site discovered here. As observed with the BDF2 site, rapid deamination at the GLI1 site does not require ADAR2's dsRBDs.