Directed Evolution of Adenine Base Editors with Increased Activity and Therapeutic Application
The foundational adenine base editors (e.g. ABE7.10) enable programmable C:G to T:A point mutations but editing efficiencies can be low at challenging loci in primary human cells. Here we further evolve ABE7.10 using a library of adenosine deaminase variants to create ABE8s. At NGG PAM sites, ABE8s result in ~1.5x higher editing at protospacer positions A5-A7 and ~3.2x higher editing at positions A3-A4 and A8-A10 compared with ABE7.10. Non-NGG PAM variants have a ~4.2-fold overall higher on-target editing efficiency than ABE7.10. In human CD34+ cells, ABE8 can recreate a natural allele at the promoter of the gamma-globin genes HBG1 and HBG2, with up to 60% efficiency, causing persistence of fetal hemoglobin. In primary human T cells, ABE8s achieve 98-99% target modification which is maintained when multiplexed across three loci. Delivered as mRNA, ABE8s induce no significant levels of sgRNA-independent off-target adenine deamination in genomic DNA and very low levels of adenine deamination in cellular mRNA.