Precision A3G base editors for disease modeling and correction

Cytosine base editors (CBEs) enable efficient cytosine-to-thymine substitutions at targeted genomic loci without introducing double-stranded breaks. Among CBEs, APOBEC3G BEs (A3G-BEs) preferentially edit the second cytosine within a 5'-CC-3' motif in human cells, reducing potential bystander editing. However, A3G-BEs often unintentionally edit multiple CC motifs within their editing window and are limited by protospacer adjacent motif (PAM) constraints imposed by SpCas9, which restricts their applicability. Here, we engineered A3G-BE variants through linker optimization, rational mutagenesis, and the integration of SpG and SpRY Cas9 effectors with relaxed PAM constraints. These improvements enhanced the precision of single-cytosine editing within CC motifs and broadened the targeting scope to previously inaccessible genomic sites. We then validated the engineered A3G-BE variants by precisely installing and correcting cystic fibrosis-causing mutations in HEK293T cells. When applied to 16HBE14o-human bronchial epithelial cells, precise editing modulated cystic fibrosis transmembrane conductance regulator mRNA levels, protein expression, and channel function, establishing precision A3G-BE variants as powerful tools for modeling and treating cystic fibrosis and other human diseases.