Methods to Increase the Efficiency of Knock-in of a Construct Encoding the HIV-1 Fusion Inhibitor, MT-C34 Peptide, into the CXCR4 Locus in the CEM/R5 T Cell Line

The low knock-in efficiency, especially in primary human cells, limits the use of the genome editing technology for therapeutic purposes, rendering it important to develop approaches for increasing the knock-in levels. In this work, the efficiencies of several approaches were studied using a model of knock-in of a construct coding for the peptide HIV fusion inhibitor MT-C34 into the human CXCR4 locus in the CEM/R5 T cell line. First, donor DNA modification was evaluated as a means to improve the efficiency of plasmid transport into the nucleus. The donor plasmid was modified to include the simian virus 40 (SV40) DNA nuclear targeting sequence (DTS) or binding sites for the transcription factor NF-κB, whose effects on the knock-in levels have not been described. The modification was ineffective in the model of MT-C34 knock-in into the CXCR4 locus. A second approach consisted in modification of Cas9 nuclease by introducing two additional nuclear localization signals (NLSs) and increased the knock-in level by 30%. Finally, blocking DNA repair via the nonhomologous end joining (NHEJ) pathway with DNA-dependent protein kinase inhibitors caused a 1.8-fold increase in knock-in. A combination of the last two approaches caused an additive effect. Thus, increasing the number of NLSs in the Cas9 protein and inhibiting DNA repair via the NHEJ pathway significantly increased the level of knock-in of the HIV-1 fusion inhibitory peptide into the clinically relevant locus CXCR4. The finding can be used to develop effective gene therapy approaches for treating HIV infection.