Small-molecule inhibitors of proteasome increase CjCas9 protein stability

The small size of CjCas9 can make easier its vectorization for in vivo gene therapy. However, compared to the SpCas9, the CjCas9 is, in general, less efficient to generate indels in target genes. The factors that affect its efficacity are not yet determined. We observed that the CjCas9 protein expressed in HEK293T cells after transfection of this transgene under a CMV promoter was much lower than the SpCas9 protein in the same conditions. We thus evaluated the effect of proteasome inhibitors on CjCas9 protein stability and its efficiency on FXN gene editing. Western blotting showed that the addition of MG132 or bortezomib, significantly increased CjCas9 protein levels in HEK293T and HeLa cells. Moreover, bortezomib increased the level of CjCas9 protein expressed under promoters weaker than CMV such as CBH or EFS but which are specific for certain tissues. Finally, ddPCR quantification showed that bortezomib treatment enhanced CjCas9 efficiency to delete GAA repeat region of FXN gene in HEK293T cells. The improvement of CjCas9 protein stability would facilitate its used in CRISPR/Cas system.