Dilution of reporter gene with stuffer DNA does not alter the transfection efficiency of polyethylenimines

BACKGROUND: Polyethylenimines (PEIs) are among the most efficient non-viral gene transfer agents developed so far. However, transfections with these polymers were shown to require a very high copy number of plasmid DNA per cell to achieve gene expression. Here, we investigate whether it is possible to reduce the amount of plasmid DNA while keeping a high transfection efficiency. METHODS: Transfection experiments were performed under various conditions in order to study the interdependence between the amount of reporter DNA, the amine-to-phosphate ratio and the transfection efficiency. RESULTS: When suboptimal amounts of linear PEI 22 kDa/DNA complexes were used for transfection, a severe reduction in reporter gene expression was observed. On the other hand, for optimal amounts of PEI/DNA complexes more than half of the reporter gene can be replaced by carrier DNA or polyglutamic acid without substantially decreasing the transfection efficiency of the polymer both in cultured cells and after systemic administration in mice. When used under the same in vitro experimental conditions, the lipospermine DOGS, but not the monocationic lipid DOTAP, gave similar results. CONCLUSIONS: Taken together, our data suggest that the activity of compounds with endosome-buffering capacities, such as PEIs and lipospermines, requires a threshold amount of transfection agent. In addition, our results indicate that, in many gene transfer situations, it will be possible to lower the dose of active plasmid thus reducing costs and the risk of immune stimulation triggered by bacterial DNA.