Development of a self-assembling nuclear targeting vector system based on the tetracycline repressor protein

The ultimate destination for most gene therapy vectors is the nucleus and nuclear import of potentially therapeutic DNA is one of the major barriers for nonviral vectors. We have developed a novel approach of attaching a nuclear localization sequence (NLS) peptide to DNA in a non-essential position, by generating a fusion between the tetracycline repressor protein TetR and the SV40-derived NLS peptide. The high affinity and specificity of TetR for the short DNA sequence tetO was used in these studies to bind the NLS to DNA as demonstrated by the reduced electrophoretic mobility of the TetR.tetO-DNA complexes. The protein TetR-NLS, but not control protein TetR, specifically enhances gene expression from lipofected tetO-containing DNA between 4- and 16-fold. The specific enhancement is observed in a variety of cell types, including primary and growth-arrested cells. Intracellular trafficking studies demonstrate an increased accumulation of fluorescence labeled DNA in the nucleus after TetR-NLS binding. In comparison, binding studies using the similar fusion of peptide nucleic acid (PNA) with NLS peptide, demonstrate specific binding of PNA to plasmid DNA. However, although we observed a 2-8.5-fold increase in plasmid-mediated luciferase activity with bis-PNA-NLS, control bis-PNA without an NLS sequence gave a similar increase, suggesting that the effect may not be because of a specific bis-PNA-NLS-mediated enhancement of nuclear transfer of the plasmid. Overall, we found TetRNLS-enhanced plasmid-mediated transgene expression at a similar level to that by bis-PNA-NLS or bis-PNA alone but specific to nuclear uptake and significantly more reliable and reproducible.