Improvement of exogenous DNA nuclear importation by nuclear localization signal-bearing vectors: a promising way for non-viral gene therapy?

Several vectors have been developed in order to target genes to specific cells. Virus-based vectors lead to a high transfection efficiency in vitro, but display important disadvantages such as pathological risks, which they expose to patients. Plasmid-associated chemical vectors lack these disadvantages, but allow only a very low efficiency of transgene expression. Most of the non-viral-based gene transfer techniques developed until now mainly focused their efforts to overcome the problem of DNA entry into the cell. Some recent works, however, have begun to investigate the nucleus entry problem and suggest that the trafficking of DNA from cytosol to the nucleus may be improved by using the nuclear localization signal (NLS) found in some nuclear proteins. If the vector contains one or several NLS, either as covalently or non-covalently DNA-linked peptides, a competition may take place between the rate of dissociation of the DNA-vector complexes and the rate of loading of the complexes to the NLS-mediated nucleus importation machinery. This equilibrium may be displaced towards the importation pathway by the use of NLS-bearing proteins instead of peptides. The possibility of recruiting normal endogenous cellular pathways of nuclear uptake to promote entry of exogenously applied DNA through the nuclear pore complex would, thus, seem promising. Nevertheless, attempts to improve the transport of DNA to the nucleus through the use of NLSs have achieved limited success. Although these systems show improved transgene expression, little is known about how they function in transfected cells, and the optimal formulation for gene expression is yet to be determined.