Abstract 2065: Development of a genetically validated, cell-based reporter assay for ADAR1 editing activity

Abstract Adenosine Deaminase Acting on RNA (ADAR) enzymes perform adenosine to inosine base editing in RNA, particularly targeting adenosines located within a specific double-stranded stem-loop motif. The ADAR1 gene encodes both a universally expressed isoform (p110) and an interferon-inducible isoform (p150), which plays a role in innate immunity by mediating interferon signaling. In the context of healthy, uninfected cells, ADAR1 performs A-to-I editing on endogenous double-stranded RNA to prevent it from activating downstream dsRNA sensors RIG-I and MDA5, which in-turn activate a pro-inflammatory response. Loss of function mutations in ADAR1 result in aberrant activation of the dsRNA sensors and are involved in autoimmune disorders. ADAR1 dysfunction also impacts cancer cell growth, proliferation, and response to immunotherapy. ADAR1 expression is increased in several types of cancer and ADAR1 knock-out has been demonstrated to improve the response to certain immunotherapies such PD-1/PD-L1 blockade and to circumvent tumor immunotherapy resistance mechanisms, making ADAR1 an attractive target for therapeutic development. We have developed and validated a high-throughput, cell-based assay for measuring ADAR1 editing activity. Several ADAR editing reporter constructs were designed and evaluated for their relative response to ADAR1 over-expression. They all feature an ADAR1 hairpin target with a stop codon (UAG) which is susceptible to ADAR1-mediated editing to a tryptophan (UUG), located upstream of a firefly luciferase gene. We show that increasing transient expression of ADAR1 led to proportional increased activity of the downstream firefly luciferase, indicating dose-dependent transcript editing by ADAR1. Selected reporters were used in conjunction with ADAR2 overexpression to identify an ADAR1-biased reporter. Thus, a reporter with low luciferase activity in response to ADAR2 expression was chosen to establish stable HEK293 cell lines (which express low levels of ADAR1) with and without constitutively expressed ADAR1 protein. ADAR1-overexpressing HEK293 cells displayed high luciferase activity, in accordance with high ADAR1-mediated editing of the reporter transcript, which was decreased in response to siRNA-mediated knockdown of ADAR1 expression or pharmacological inhibition of ADAR1 activity. Sequencing of the reporter RNA confirmed that changes in reporter activity were correlated with editing of the RNA transcript. Finally, these validated ADAR1 editing reporter cell lines were further engineered to constitutively express Renilla luciferase under the control of a CMV promoter, serving as an internal control for the toxicity of ADAR1 inhibitors. In conclusion, our robust cell-based assay provides a well-validated resource to identify modulators of ADAR1 editing activity in a cellular context. Citation Format: Valerie Sapp, Jim Bilakovics, Ava Burr, Fernando Martins, Pavel Shashkin, Veronique Baron, Henry Zhu, Junguk Park. Development of a genetically validated, cell-based reporter assay for ADAR1 editing activity abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 2065.