Targeting Cas9 to Perform Rescue Experiments in Stably Transduced Knockout HEK293 Cells

CRISPR-Cas9 gene editing technology has revolutionized molecular biology. Often, this technology is employed to delete a gene encoding a protein of interest. The resulting phenotype provides valuable insight into the protein's function. The functional importance of the target protein can be confirmed by reintroducing the protein to restore the lost function (rescue). This is typically accomplished by introducing the protein-coding cDNA in trans using an expression vector. However, in knockout cell lines that stably express the CRISPR-Cas9 system, the newly introduced expression plasmid may also be cleaved by Cas9. The protocol presented here provides a strategy to circumvent this potential barrier to rescue experiments. This approach is demonstrated using HEK293 cells in which the gene encoding the E3 ubiquitin ligase scaffold protein CUL4B was disrupted by CRISPR-Cas9. Transduction of these cells with a guide RNA (gRNA) targeting the integrated Cas9 transgene resulted in the loss of detectable Cas9 protein. Cas9 ablation enabled restoration of CUL4B expression and function following introduction of a CUL4B expression plasmid. These results provide proof of concept for a broadly applicable approach to studying protein function through rescue experiments.