REPAIRx, a specific yet highly efficient programmable A>I RNA base editor

Programmable A>I RNA editing is valuable for basic research and medicine. A variety of editors have been created, but a genetically encoded editor that is both precise and efficient remains elusive, as exemplified in the editor REPAIR comprising the ADAR2 deaminase domain fused to dCas13b. REPAIR is highly efficient, but also causes massive off-target effects. Mutations that weaken the deaminase domain can minimize the undesirable effects, but only at the expense of on-target editing. We have circumvented this dilemma using a multipronged approach: choosing a different Cas protein (CasRx), inserting the deaminase domain into the middle of CasRx, and directing the editor to the nucleus. The new editor (REPAIRx) is precise yet highly efficient, outperforming all major rivals at both mRNA and nuclear RNA targets. Thus, Vx markedly expands the RNA editing toolkit and illustrates a novel strategy for base editor optimization.