From IscB to Cas9: Engineering and advances in the next generation of miniature gene editing tools

The CRISPR-Cas system, distinguished by its inherent modularity and broad programmability, has catalyzed a paradigm shift in genome engineering due to its unprecedented accuracy, specificity, and on-target efficiency, now serving as the cornerstone of modern genome manipulation. The efficient delivery of gene editing tools remains a major technical hurdle to clinical application, primarily due to the lack of compact editors. The recent identification of the transposon-associated nuclease IscB as an evolutionary ancestor of Cas9 has provided important insights into the molecular evolution of the CRISPR-Cas9 system. Notably, IscB is a highly compact nuclease, approximately one-third the size of Cas9, capable of precise nucleic acid cleavage in eukaryotic cells under the guidance of ωRNA. These features make it a promising candidate for the development of next-generation miniaturized genome editors. However, natural IscB exhibits limited editing performance in eukaryotic systems. This review first outlines the biochemical function of the transposon IscB and briefly traces the evolutionary origin of the Cas9 system. It then describes and compares the structural characteristics and cleavage mechanisms of OgeuIscB and Cas9. Subsequent sections summarize various engineering strategies for current IscB systems, including the development of base editors and recent advances in their application. Finally, the limitations of existing systems are discussed, and potential directions for future optimization are proposed, aiming to provide new insights and facilitate the advancement of IscB-based miniaturized editors.