Abstract A122: Self-recognition of Alu duplex RNAs is the basis for MDA5-mediated interferonopathies

Abstract

Melanoma Differentiation Associated Gene-5 (MDA5) is an innate immune receptor that binds to viral double-stranded RNAs (dsRNAs) and initiates type I and III interferon signaling cascade thereby playing a key role in antiviral immune response. Recently, a number of mutations that lead to aberrant activation of MDA5 have been implicated in various autoinflammatory disorders including Aicardi-Goutieres syndrome. The mechanistic basis of this constitutive MDA5 activation, however, has remained elusive. An understanding of the subtle balance of self vs. non-self discrimination by MDA5 is important, especially in the context of recent reports demonstrating the targeted activation of MDA5 as a potential therapeutic strategy against diverse carcinoma. Our work revealed a hitherto unknown role played by the RNA-rich cellular environment in preventing aberrant MDA5 activation by imposing cooperative filament assembly on dsRNAs as a functional requirement for signal activation. We further employed a novel RNase protection-RNAseq approach to show that the disease-causing gain-of-function (GOF) mutants of MDA5 can form signaling-competent filaments on endogenous RNA populations comprising mainly Alu RNA duplexes. Strikingly, under physiologic conditions, the wild type MDA5 is not activated by Alu RNAs because of its sensitivity to structural irregularities such as bulges and mismatches commonly occurring in Alu:Alu hybrids. The GOF mutants, on the other hand, show reduced sensitivity to disruptions in duplex RNA structures as revealed by our in-depth biochemical probing. Overall, the work reveals the underlying mechanism behind MDA5-mediated inflammatory disorders. Moreover, it highlights the unique role played by Alu RNAs as an evolutionary tether on MDA5, keeping its affinity towards “self” ligands under check during the course of evolution. Citation Format: Sadeem Ahmad, Xin Mu, Sun Hur. Self-recognition of Alu duplex RNAs is the basis for MDA5-mediated interferonopathies [abstract]. In: Proceedings of the Fourth CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; Sept 30-Oct 3, 2018; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2019;7(2 Suppl):Abstract nr A122.