Reactivation of γ-globin expression using a minicircle DNA system to treat β-thalassemia

•Plasmid-length negatively impacted on the electroporation efficiency of transgenes in HSPCs.•Bacterial backbone-free MC DNA plasmids had advantages over conventional plasmids on gene expression and electroporation efficiency.•γ-globin expression was reactivated in HSPCs by MC SaCas9-sgRNA plasmids.•MC DNA as gene therapy vectors had low insertion frequencies and genotoxicity. Reactivation of fetal hemoglobin by editing the B-cell lymphoma/leukemia 11A (BCL11A) erythroid enhancer is an effective gene therapy for β-thalassemia. Using the CRISPR/Cas9 system, fetal γ-globin expression can be robustly reactivated to mitigate the clinical course of β-thalassemia. In our study, we found that the transfection efficiencies of CD34+ hematopoietic stem/progenitor cells (HSPCs) were significantly and negatively correlated with the length of plasmids and greatly affected by the linearization of plasmids. Furthermore, the transgene expression of minicircles (MC) without plasmid backbone sequences was better both in vitro and in vivo compared with conventional plasmids. Thus, MC DNA was used to deliver the cassette of Staphylococcus aureus Cas9 (SaCas9) into HSPCs, and a single-guide RNA targeting the erythroid enhancer region of BCL11A was selected. After electroporation with MC DNA, an evident efficiency of gene editing and reactivation of γ-globin expression in erythroblasts derived from unsorted HSPCs was acquired. No significant off-target effects were found by deep sequencing. Furthermore, fragments derived from lentiviral vectors, but not MC DNA, were highly enriched in promoter, exon, intron, distal-intergenic, and cancer-associated genes, indicating that MC DNA provided a relatively safe and efficient vector for delivering transgenes. The developed MC DNA vector provided a potential approach for the delivery of SaCas9 cassette and the reactivation of γ-globin expression for ameliorating syndromes of β-thalassemia.