Biol. Pharm. Bull. 29(6) 1294—1296 (2006)

gene therapy due to their excellent safety profile, albeit their low transgene expression efficiency in comparison to viral vectors. It is obvious that the intracellular DNA trafficking, particularly the nuclear entry, is a very important issue for high transgene expression. Moreover, we previously proposed that the ‘controlled intranuclear disposition’ of the delivered DNA would also be highly important for achieving practical gene therapy. For example, transgene expression with nonviral vectors is generally transient. Thus, the intranuclear disposition of the exogenous DNA is of great interest. Recently, we examined the intranuclear disposition of plasmid DNA delivered into mouse liver by a hydrodynamics-based injection. We have demonstrated that expression efficiency from one copy of the exogenous DNA dramatically decreases over time (‘silencing’) without increased promoter methylation. These findings raise the question of whether silencing of exogenous DNA occurs similarly in cultured cells. In this study, we transfected naked luciferase-plasmid DNA into HeLa cells by electroporation, and the intranuclear disposition of the plasmid DNA was examined. Our results revealed relatively slow silencing of exogenous DNA in cultured cells.