SRP488031 PRJNA1068267 Efficient C-to-U Transitions In Vivo Using Engineered APOBEC with the gRNA-free base editing systems. Base editing emerges as a groundbreaking approach for targeted genetic modifications, offering vast opportunities for fundamental research and therapeutic applications in genetic diseases. While CRISPR-based DNA and RNA base editing systems capitalize on sgRNA-guided specificity and diverse deaminase functionalities, achieving efficient C-to-U RNA editing has remained a challenge due to intrinsic limitations of cytidine deaminases. In this work, we unveil engineered APOBEC enzymes, refined through systematic enhancements and AI-facilitated protein engineering. These modified enzymes demonstrate unprecedented catalytic versatility across various contexts, especially when integrated with PUF10 RNA recognition module. Crucially, our work presents the successful application of novel RNA base editors for in vivo RNA editing within the brains of Mef2c mutant mice, an established autism spectrum disorder model, utilizing AAV for delivery. The application of this AAV-mediated RNA cytosine base editing technique not only rectifies aberrant protein expression but also significantly mitigates autistic-like behaviors observed in the model mice. This work introduces a pioneering collection of RNA base editing instruments and highlights their therapeutic potential in addressing neurological disorders.