High-precision base editors for site-specific single nucleotide conversion

The CRISPR-Cas system is a powerful tool for site-specific genome editing. The Cas9endonuclease is capable of introducing DNA double strand breaks at a desired location withhigh precision. The lesion is repaired either in an errorprone process leading to gene inactivation or in a highly accurate way relying on an external template. Modification of the template may serve to introduce selected mutations, modified sequencesorentirely new genes.The accurate pathway comes with low endogenous efficiency. Most hereditary diseasesin humans are caused bysingle point mutations, the correction of which requires only subtle changes to the DNA. Recently, base editors were developedthat allow the introduction of selective nucleotide substitutions.A mutant Cas9enzyme that does not induce double strand breaks is fused to a nucleobase deaminase catalysingC-to-T mutations (by C-to-U deamination) or A-to-G mutations (by A-to-I deamination). Such base editors have enormous potential in genome editing, gene therapy and precision breeding. Yet, current editors suffer from limited specificity in that theyedit different and/or multiple nucleobases within a larger sequence window. For broadapplication, especially in human therapies, improved systems will be needed that provide extraordinary accuracy of site-specific editing, ideally without any off-target effects.