Efficient genome editing in medaka () using a codon-optimized SaCas9 system.

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) system, belonging to the type II CRISPR/Cas system, is an effective gene-editing tool widely used in different organisms, but the size of Cas9 (SpCas9) is quite large (4.3 kb), which is not convenient for vector delivery. In this study, we used a codon-optimized Cas9 (SaCas9) system to edit the tyrosinase (, oculocutaneous albinism II (), and paired box 6.1 () genes in the fish model medaka(), in which the size of SaCas9 (3.3 kb) is much smaller and the necessary protospacer-adjacent motif (PAM) sequence is 5'-NNGRRT-3'. We also used a transfer RNA (tRNA)‍-single-guide RNA (sgRNA) system to express the functional sgRNA by transcription eitherin vivo or in vitro, and the combination of SaCas9 and tRNA-sgRNA was used to edit the gene in the medaka genome. The SaCas9/sgRNA and SaCas9/tRNA-sgRNA systems were shown to edit the medaka genome effectively, while the PAM sequence is an essential part for the efficiency of editing. Besides, tRNA can improve the flexibility of the system by enabling the sgRNA to be controlled by a common promoter such as cytomegalovirus. Moreover, the all-in-one cassette cytomegalovirus (CMV)‍-SaCas9-tRNA-sgRNA-tRNA is functional in medaka gene editing. Taken together, the codon-optimized SaCas9 system provides an alternative and smaller tool to edit the medaka genome and potentially other fish genomes.