Selective DNA Release from DQAsome/DNA Complexes at Mitochondria-Like Membranes

The number of diseases found to be associated with defects of the mitochondrial genome has grown significantly over the past decade (Wallace 1999). Despite major advances in understanding mtDNA defects at the genetic and biochemical level, there is no satisfactory treatment available for the vast majority of patients and the exploration of gene therapeutic approaches is highly warranted. However, mitochondrial gene therapy still appears only theoretical and speculative. Any possibility for gene replacement depends on the use of a yet unavailable mitochondria-specific transfection vector. Mitochondria-specific vectors must posses two properties: they have to transport DNA to the side of mitochondria; they must not release DNA during endocytosis. Amphiphile compounds with delocalized cationic charge centers such as rhodamine 123 and the bola-amphiphile dequalinium have long been known to accumulate in mitochondria. Sufficient lipophilicity combined with delocalization of the positive charge to reduce the free energy change when moving from an aqueous to a hydrophobic environment are believed to be prerequisite for mitochondrial accumulation in response to the mitochondrial membrane potential. We have recently succeeded in preparing cationic vesicles made of dequalinium that we termed DQAsomes (Weissig et al. 1998a). We have shown that DQAsomes bind and protect DNA against DNase activity (Lasch et al. 1999). Based on the intrinsic property of dequalinium to preferentially accumulate in mitochondria in response to the electrochemical gradient at the mitochondrial membrane, we believe that DQAsomes can serve as a vector to deliver DNA to mitochondria in living cells. As a first step in the development of mitochondria-specific DNA delivery systems, we report here that DQAsome/DNA complexes selectively release DNA at cardiolipin-rich liposomes mimicking both the inner and the outer mitochondrial membrane. We demonstrate that DNA remains tightly associated with DQAsomes in the presence of an excess of anionic lipids other than cardiolipin.