New Vectors for Stable and Safe Gene Modification

Stable gene modification without affecting normal cellular function is a main goal both for basic and applied science. Current strategies to achieve stable gene modification are either very inefficient (homologous recombination) or very genotoxic (retroviral gene transfer). Retroviral vectors derived from oncoretroviruses (murine leukaemia virus MLV) were the first to be widely used for stable gene modifications for gene therapy strategies. They are efficient, poorly immunogenic and derived from a non-pathogenic virus. MLV-based vectors were also the first ones to show real therapeutic effect by correcting the immune system of about 50 patients with four different severe immunodeficiencies X-linked Severe Combined Immunodeficiency (SCID-X1), SCID-Adenosine Deaminase Deficiency (SCIDADA), Wiskott-Aldrich Syndrome (WAS) and Chronic Granulomatous Disease (CGD)(Fischer et al.). However, for most applications therapeutic efficacy were incomplete or come together with serious adverse effects such as cell transformation(Neven et al., 2009). Different studies have demonstrated that the enhancer elements present in the old generation retroviral vectors are important in cell transformation(Montini et al., 2009). Therefore, safer integrative vectors are required to replace MLV-based retroviral vectors when stable gene modification is required. New vectors for stable expression must consider three aspects: 1efficiency, 2ectopic or unregulated expression of the transgene and 3genotoxicity (genomic alterations due to vector integrations). Probably the most important feature for a vector to reach clinic is efficiency. Gene Therapy vectors must be able to efficiently transduce their target cells in order to reach therapeutic benefits. High efficiency is especially relevant when the therapeutic protein does not confer any positive advantage to the target cells. It is therefore important to keep in mind that very safe vectors are of no use if the efficiency does not reach a minimum. We can say that, at least for some applications, efficiency is no longer a limitation, and that safety is now the main concern. The ectopic or unregulated expression of the transgene is other aspect to take into consideration. As conventional therapeutic agents, transgenes have a window where and when to exert their function. In many diseases, the expression of the affected gene is restricted to a particular tissue, to a particular stage of the development and/or in response to environmental conditions. The expression of the transgene in non-target cells as well as the expression of non-physiological transgene levels may cause toxic or deleterious effects.