CRISPR-Cas12a nucleases have expanded the toolbox for targeted genome engineering in a broad range of organisms. Here, using a high-throughput engineering approach, we explored the potential of a novel CRISPR-MAD7 system for genome editing in human cells. We evaluated several thousand optimization conditions and demonstrated accurate genome reprogramming with modified MAD7. We identified crRNAs that allow for ≤95% non-homologous end joining (NHEJ) and 66% frameshift mutations in various genes and observed the high-cleavage fidelity of MAD7 resulting in undetectable off-target activity. We explored the dsDNA delivery efficiency of CRISPR-MAD7, and by using our optimized transfection protocol, we obtained ≤85% chimeric antigen receptor (CAR) insertions in primary T cells, thus exceeding the baseline integration efficiencies of therapeutically relevant transgenes using currently available virus-free technologies. Finally, we evaluated multiplex editing efficiency with CRISPR-MAD7 and demonstrated simultaneous ≤35% CAR transgene insertions and ≤80% gene disruption efficiencies. Both the platform and our transfection procedure are easily adaptable for further preclinical studies and could potentially be used for clinical manufacturing of CAR T cells.
Keywords: CAR T-cells; CRISPR; HDR; MAD7; NHEJ; frameshift mutations.