The interaction of plasmid DNA with polyamidoamine dendrimers: mechanism of complex formation and analysis of alterations induced in nuclease sensitivity and transcriptional activity of the complexed DNA.

The application of synthetic vectors for gene transfer has potential advantages over virus-based systems. However, little is known about the mechanisms involved in binding of synthetic materials to DNA and the nature of the DNA complexes that result from this interaction. Polyamidoamine (PAMAM) dendrimers are unique polymers with defined spherical structure. Dendrimers bind DNA to form complexes that efficiently transfect cells in vitro. We examined the formation of DNA/dendrimer complexes and found it based entirely on charge interaction. Electronmicroscopic examination of the complexes indicated that the majority of the plasmid DNA is contracted into isolated toroids, but also revealed larger, irregular aggregates of polymer and DNA. The binding of plasmid DNA to dendrimer appears to alter the secondary and tertiary structure, but does not fragment the DNA or alter its primary structure. Complexed DNA is protected against degradation by either specific nucleases or cellular extracts containing nuclease activity. While the initiation of transcription in vitro from promoters (for either T7 polymerase or eukaryotic RNA polymerase II) in dendrimer-complexed DNA is inhibited, elongation of the RNA transcript and translation do not appear to be affected. These resemble alterations of the DNA function when complexed with naturally-occurring polycations like non-acetylated histones. However, DNA complexed to dendrimer appears to maintain transcriptional activity while histone complexes at similar charge ratios do not. These results elucidate some aspects of the interaction between PAMAM dendritic polymers and DNA, and could lead to improvements in the design of polymers or formation of DNA complexes that will increase the efficiency of non-viral gene transfer.