ADAR-mediated RNA editing in CNS disorders: from pathogenic mechanisms to therapeutic opportunities

The adenosine deaminases acting on RNA (ADAR) family of enzymes (ADAR1 and ADAR2) catalyze adenosine-to-inosine (A-to-I) RNA editing. This post-transcriptional change is remarkably prevalent in the central nervous system (CNS). ADAR-mediated editing is critical for proper brain development, synaptic plasticity, and immunological homeostasis in the central nervous system (CNS) via recoding neurotransmitter receptors and ion channels. Conversely, a wide range of CNS disorders, such as neurodegenerative diseases (Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis), neuropsychiatric conditions (schizophrenia, bipolar disorder, and major depression), cerebrovascular diseases, and gliomas, are now linked to dysregulation of ADAR activity, whether through loss-of-function mutations, altered expression, or mislocalization. To address the main question of whether altered RNA editing is a fundamental driver of pathogenesis, a compensatory response, or a context-dependent modulator, this review critically synthesizes existing evidence. The basic processes of ADAR enzymes and their regulation throughout neurodevelopment are first described. Next, we thoroughly assess the unique molecular fingerprints of ADAR dysregulation across several CNS disorders, emphasizing recurring themes such as Alu RNA hypo-editing, induction of innate immunity, and GRIA2 editing, which can cause excitotoxicity. Lastly, we examine new treatment approaches that use or reinstate ADAR activity, such as small-molecule modulators and site-directed RNA editing tools (leveraging endogenous ADAR for programmable editing of RNA [LEAPER], clustered ADAR-recruiting guide RNAs (gRNAs) for effective RNA editing [CLUSTER], and mimicking inverted repeats to recruit ADARs using engineered oligoribonucleotides [MIRROR]). We summarize by reviewing key obstacles to clinical translation, including crossing the blood-brain barrier, the risks of off-target editing, and the challenges of achieving spatiotemporal accuracy. We also list important open topics for further investigation.