Structures of lipid-DNA complexes: supramolecular assembly and gene delivery

Recently, there has been a flurry of experimental work on understanding the supramolecular assemblies that are formed when cationic liposomes (CLs) are mixed with DNA. From a biomedical point of view, CLs (vesicles) are empirically known to be carriers of genes (sections of DNA) in nonviral gene delivery applications. Although viral-based carriers of DNA are presently the most common method of gene delivery, nonviral synthetic methods are rapidly emerging as alternative carriers, because of their ease of production and nonimmunogenicity (viral carriers very often evoke an undesirable and potentially lethal immune response). At the moment, cationic-lipid-based carriers have emerged as the most popular nonviral method to deliver genes in therapeutic applications, for example, CL carriers are used extensively in clinical trials worldwide. However, because the mechanism of transfection (the transfer of DNA into cells by CL carriers, followed by expression) of CL-DNA complexes remains largely unknown, the measured efficiencies are, at present, very low. The low transfection efficiencies of current nonviral gene delivery methods are the result of poorly understood transfection-related mechanisms at the molecular and self-assembled levels. Recently, work has been carried out on determining the supramolecular structures of CL-DNA complexes by the quantitative technique of synchrotron X-ray diffraction. When DNA is mixed with CLs (composed of mixtures of cationic DOTAP and neutral DOPC lipids), the resulting CL-DNA complex consists of a multilamellar structure (L-alpha(C)) comprising DNA monolayers sandwiched between lipid bilayers. The existence of a different columnar inverted hexagonal (H-II(C)) phase in CL-DNA complexes was also demonstrated using synchrotron X-ray diffraction. Ongoing functional studies and optical imaging of cells are expected to clarify the relationship between the supramolecular structures of CL-DNA complexes and transfection efficiency.