Computational design of a high-precision mitochondrial DNA cytosine base editor.

Bystander editing remains a major limitation of current base editors, hindering their precision and therapeutic potential. Here, we present a de novo protein design strategy that creates a structurally rigid interface between a DNA-binding TALE domain and a cytosine deaminase, forming a unified editing module termed TALE-oriented deaminase (TOD). Cryo-EM analysis of TOD-DNA complexes confirms that this precise spatial architecture tightly restricts the deaminase activity window, thereby minimizing unwanted deamination. To further enhance editing specificity, we develop a split version, termed DdCBE-TOD, which virtually eliminates off-target editing. As a proof of concept, we apply DdCBE-TOD to generate a mitochondrial disease mouse model and to correct a pathogenic mutation associated with MERRF syndrome in patient-derived cells, achieving single-nucleotide precision. This work introduces a generalizable and computationally guided approach for ultra-precise base editing, offering a promising platform for both mechanistic studies and therapeutic correction of single-nucleotide mutations.