Engineering Miniature CRISPR-Cas Un1Cas12f1 for Efficient Base Editing

Adeno-associated virus (AAV) is a relatively safe and efficient vector for gene therapy. However, due to its 4.7-kilobase limit of cargo, SpCas9-mediated base editors can't be packaged into a single AAV vector, which hinders its clinical application. The development of efficient miniature base editors becomes an urgent need. Un1Cas12f1 is a class II V-F type CRISPR-Cas protein with only 529 amino acids. Although Un1Cas12f1 has been engineered to be a base editor in mammalian cells, the base editing efficiency is less than 10%, which limits its therapeutic applications. Here, we developed hypercompact and high-efficiency base editors by engineering Un1Cas12f1, fusing non-specific DNA binding protein Sso7d and truncating sgRNA, termed as STUminiBEs. We demonstrated the robust A-to-G conversion (54% on average) by STUminiABE or C-to-T conversion (45% on average) by STUminiCBE. We packaged STUminiCBE into AAVs and successfully introduced a premature stop codon on the PCSK9 gene in mammalian cells. In sum, STUminiBEs are efficient miniature base editors and could readily be packaged into AAVs for biological research or biomedical applications.