Increasing the minicircle DNA purity using an enhanced triplex DNA technology to eliminate DNA contaminants

DNA vectors for human gene therapy have to meet the efficacy and safety requirements. Minicircles (MCs), a class of optimized DNA vectors free of plasmid backbone (PB) DNAs, have emerged as promising candidates because of their superior transgene expression profiles. However, the existence of impure DNAs, including the unrecombined MC producing plasmid (PP) and PB circle, in the MC products made using the current technologies exceed the safety limit. Here, we report the development of an enhanced triplex DNA (TriD) technology to eliminate almost all the impure DNAs from the MC products. To do this, a pair of optimized TriD forming sequences was placed to flank the kanamycin resistance gene in the PP. The MC products were incubated with a biotinylated TriD forming DNA oligonucleotide (olig), and the resulted TriDs were removed by binding to streptovidin-coated magnetic beads. Consequently, the residual impure DNAs were 0.03% or less in the final MC products. The reproducibility of this technique was confirmed with MCs of various transgene expression cassettes, sizes, and quantities, suggesting its great potential in making high quality MC for human gene therapy.