Base Editing: Chemistry on a Target Nucleotide in the Genome of Living Cells

Point mutations represent the majority of known human genetic variants associated with disease but are difficult to correct cleanly and efficiently using standard genome editing methods. In this lecture I will describe the development, application, and evolution of base editing, a novel approach to genome editing that directly converts a target base pair to another base pair in living cells without DNA backbone cleavage and without requiring donor DNA templates or homologous-directed repair processes that are inefficient in non-dividing cells. Through a combination of protein engineering and protein evolution, we recently developed two classes of base editors (BE4 and ABE) that together enable all four types of transition mutations (C to T, T to C, A to G, and G to A) to be efficiently and cleanly installed at target positions in genomic DNA. The four transition mutations collectively account for most known human pathogenic point mutations. Base editing has been successfully performed in a wide range of organisms including bacteria, fungi, plants, fish, frogs, mammals, and even human embryos. Recently we have substantially improved key features of both classes of base editors, including their efficiency, product purity, genome-targeting scope, and DNA specificity. We have also observed base editing in vivo in post-mitotic cells that do not support efficient homology-directed repair. Base editing can be used to correct pathogenic point mutations, introduce disease-suppressing mutations, and create cell and animal models of human disease. These studies also suggest the potential of base editing to serve as therapeutics for some diseases with a genetic component.