# Engineered Human Argonaute 2 for Targeted C-to-U RNA Base Editing Repurposing human AGO2 as a modular RNA-targeting scaffold reduces immunogenicity compared to Cas-based systems. - **Source:** [Nature Biotechnology](http://feeds.nature.com/nbt/rss/current) - **Published:** 2026-10-20 - **Topic:** [Data & AI](https://brifing.si/c/data-ai) - **Original article:** - **On Brifing:** https://brifing.si/s/1d946cbf-94d8-4c6a-9d0b-6609ec3dbaa9 ## Limitations of Foreign Proteins Current RNA manipulation tools rely heavily on nonhuman proteins like Cas13. These foreign scaffolds often present significant challenges in therapeutic translation, including unintended cellular effects and unknown interactions with regulatory machinery. Replacing these with engineered human proteins offers a pathway to bypass the immunogenic liabilities associated with prokaryotic-derived systems in mammalian cells. ## Introducing ALTER Mechanism Researchers developed AGO-led targeted editing of RNA (ALTER) by repurposing human Argonaute 2 (AGO2). By fusing AGO2 to a C-to-U deaminase, the system enables precise RNA editing. This modular approach leverages the natural RNA-binding capabilities of AGO2, creating a programmable scaffold that avoids the typical nuclease-driven degradation associated with native AGO2 functions. ## Superior Editing Performance In HEK293T cell experiments, ALTER demonstrated a 1.9-fold higher C-to-U editing efficiency on mRNA targets compared to existing Cas13-based editors like CURE and RESCUE. This performance gain is achieved while maintaining similar or lower levels of off-target editing, as confirmed by comprehensive transcriptome-wide RNA-seq analysis across the tested experimental conditions. ## Reduced Immunogenic Profile Machine learning models predict that ALTER generates significantly fewer immunogenic peptides than traditional Cas-based editors. The system is also compatible with validated artificial intelligence-directed deimmunization workflows. This reduction in potential immune response is critical for therapeutic applications, where repeat dosing may be required due to the rapid turnover of target RNA. ## Leveraging Native AGO2 Kinetics Human AGO2 proteins exhibit enhanced binding kinetics and affinity compared to Cas13 or free RNA base pairing. By utilizing these evolutionarily optimized properties, ALTER achieves efficient target sampling without needing to build a synthetic chassis. This integration allows the system to function effectively within the complex environment of human cells. ## Nondegradative Binding Strategy The design of ALTER relies on guide RNAs that induce a loop in the target RNA, preventing cleavage while enabling deaminase activity. By introducing strategic mismatches in the central region of the guide RNA, the system binds targets without triggering the RNA interference pathway that typically leads to transcript degradation and downregulation. ## Anatomy of Guide RNAs The guide RNA anatomy is divided into five segments, with the seed region (g2–g8) being critical for binding. By inducing a specific loop size (x) and positioning the target base (y) within this loop, researchers create a favorable substrate for the A3A deaminase enzyme, ensuring high-affinity binding without inducing unwanted target cleavage. ## AlphaFold3 Structural Modeling AlphaFold3 modeling confirmed the correct folding of the ALTER-1 fusion construct and the association of the A3A deaminase with the unbound portion of the RNA target. This structural insight was vital for optimizing the loop-inducing mechanism, ensuring that the target C is accessible for editing while avoiding steric shielding by AGO2. ## Optimization of Loop Parameters Experimental testing revealed that loop size significantly impacts editing efficiency. Smaller loops often resulted in no editing, while 20-nucleotide loops reduced efficiency. Optimal results were achieved with a 14-nucleotide loop, where the target base was positioned at specific locations to facilitate the deamination reaction while maintaining the overall stability of the complex. ## Versatile Delivery Methods ALTER-1 editing guides are highly versatile and can be delivered effectively as either plasmid-encoded shRNAs or exogenous siRNA duplexes. This flexibility simplifies the experimental workflow in HEK293T cells and suggests potential for various therapeutic delivery platforms, allowing researchers to choose the method best suited for specific clinical or laboratory applications. ## Reporter-Based Validation Using an EGFP Y66H reporter system, researchers validated the editing mechanism by observing the rescue of blue fluorescence. Controls, including nontargeting guides and the exclusion of individual components like AGO2 or A3A, confirmed that the observed editing was specifically dependent on the ALTER-1 construct and the designed loop-inducing guide RNA. ## Future Therapeutic Implications The development of ALTER represents a shift toward reprogramming endogenous mammalian machinery for therapeutic goals. By minimizing foreign components, this platform addresses key hurdles in RNA-based medicine. Future work focusing on domain truncation and further nuclease ablation could refine this system, potentially leading to more efficient and safer gene-editing therapies. ## Sources covering this story - https://www.nature.com/articles/s41587-026-03359-y — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._