Abstract B130: Elevated cancer-intrinsic type I interferon signaling confers a dependency on the RNA editor ADAR1

The deamination of adenosine to inosine (known as A-to-I editing) is one of the most prevalent RNA modifications that occurs in metazoans and is mediated by the Adenosine Deaminase Acting on RNA (ADAR) family of enzymes. The enzyme ADAR/ADAR1 catalyzes the majority of A-to-I editing where it has been demonstrated to effect coding sequence, miRNA function and silencing of Alu repetitive elements1. A critical function of ADAR1 is to edit double stranded RNA (dsRNA) structures that can activate the cytoplasmic nucleoside sensors MDA5 and PKR, preventing aberrant activation of an innate immune type I interferon (IFN) response2,3. Consistent with this hypothesis, mutations in ADAR1 and other enzymes involved in nucleoside metabolism and/or sensing are found in Aicardi-Goutieres Syndrome (AGS), an interferonopathy associated with spontaneous interferon production4. Through analysis of pooled CRISPR and shRNA screening data, we have identified that cancer cell lines with elevated expression of specific type I interferon stimulated genes (ISGs) are dependent on ADAR1 for their survival. Tumor cells can display elevated intrinsic type I interferon signaling and associated dsRNA burden due to multiple factors, including chronic cytoplasmic DNA activation of STING, oncovirus infection and other proinflammatory signals. We have identified a core subset of 26 ISGs that predict a cellular dependency on ADAR1 and demonstrate elevated expression of these ISGs in a diverse range of tumor types including head and neck squamous cell carcinoma, breast cancer, esophageal cancer, and lung cancer. Utilizing an isogenic cell line with low basal type I interferon signaling and knockout of ADAR1, we demonstrate that we can induce cell death through exogenous treatment with the dsRNA mimetic poly-(I:C) or type I interferons (IFNα or IFNβ) but not the type II interferon IFNγ. We believe that chronic ISG expression and dsRNA burden creates a dependency on ADAR1 to prevent activation of MDA5 and PKR. These data suggest an ADAR1 inhibitor could be beneficial for the treatment of tumors with elevated ISG expression. Eisenberg et al., Nature Review Genetics (2018) Ahmad et al., Cell (2018) Chung et al., Cell (2018) Crow et al., Nature Review Immunology (2015) Citation Format: Alexandra K. Gardino, Cindy Collins, Maureen S. Lynes, P. Ann Boriack-Sjodin, Robert A. Copeland, Scott A. Ribich. Elevated cancer-intrinsic type I interferon signaling confers a dependency on the RNA editor ADAR1 [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference on Molecular Targets and Cancer Therapeutics; 2019 Oct 26-30; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2019;18(12 Suppl):Abstract nr B130. doi:10.1158/1535-7163.TARG-19-B130