Engineered mRNA-packaging virus-like particles enable efficient and precise mitochondrial base editing in vitro and in vivo

Mitochondrial DNA (mtDNA) base editors (mtBEs) are promising tools for studying and correcting pathogenic mtDNA mutations. However, the application of mtBEs is often hindered by substantial mtDNA-wide off-target editing that accumulates with prolonged mtBE expression resulting from plasmid DNA or viral delivery. To overcome this limitation, we developed mtBE-VLPs, a virus-like-particle (VLP) platform for transiently delivering mtBEs in vitro and in vivo with minimal off-target effects. While canonical protein-packaging VLPs yielded undetectable mtDNA base editing, engineered mRNA-packaging VLPs enabled efficient but transient mtBE cargo delivery. By iteratively engineering mRNA-packaging VLP compositions, we improved mtBE-VLP-mediated base editing efficiencies from less than 1% to greater than 50% across multiple target mtDNA loci in bulk populations of treated cultured cells. To further improve the packaging and delivery of certain mtBEs, we developed a strategy to selectively reduce mtBE cargo expression in VLP producer cells but not transduced cells, which increased VLP-mediated base editing efficiencies for certain mtBEs by 7.1-fold. Compared to plasmid DNA delivery in vitro, mtBE-VLPs exhibited equivalent on-target editing but 23- to 41-fold lower mtDNA-wide off-target editing, highlighting the improved editing specificity offered by transient mtBE delivery. mtBE-VLPs efficiently installed a pathogenic mtDNA mutation in mouse embryonic fibroblasts (88% efficiency) and corrected a pathogenic mtDNA mutation in a human patient-derived cell line (41% efficiency). Additionally, a single subretinal injection of mtBE-VLPs yielded 6-20% (average 12%) mtDNA base editing in the mouse retinal pigment epithelium with minimal mtDNA-wide off-target editing. These findings establish mRNA-packaging mtBE-VLPs as promising vehicles for efficient and precise mitochondrial base editing via transient mtBE delivery.