| The greatest obstacle to gene therapy lies in development of efficient gene delivery vectors. Currently, the most efficient vectors are derived from viruses. Even though viruses have, over millions of years, evolved to become very efficient at delivering genes as part of their life cycles, their gene transfer properties are not necessary optimized for therapeutic application. In addition, research in the past two decades had made apparent that there is not a "magical" gene delivery system that is suitable for all applications. Therefore, recent efforts have emphasized tailoring of specific viral vectors to satisfy particular therapeutic needs. The efficiency of gene delivery vectors is greatly dependent on overcoming certain barriers to the gene transfer process, and the goal of this research is to engineer specific properties into viral vectors to circumvent key gene delivery barriers.;Alleviating transcriptional silencing of MoMLV in mouse stem cells. The combined application of gene therapy and stem cell engineering for tissue regeneration has recently drawn immense interest. In recent clinical trials, several patients who suffered from different forms of severe combined immunodeficiency syndrome (SCID) were treated with Moloney Murine Leukemia (MoMLV) vectors in which corrective genes were introduced into the patients' blood stem cells. This marked the first successful cure in a gene therapy clinical trial. However, the vectors require further engineering to improve the gene expression and safety of retroviral gene delivery to stem cells. Among their limitations, MoMLV vectors suffer from transcriptional silencing in many stem cells, both in vitro and in vivo. Enhancing gene expression after vector delivery could improve therapeutic efficacy, as well as improve safety by lowering the necessary dosage. So far, five different regions within the retroviral genome have been identified as recognized targets that promote silencing, and most of which lie within the U3 region of the viral Long Terminal Repeat (LTR). The effect of silencing can be reduced by modification and/or deletions of these regions; however, complete relief has never been achieved. Furthermore, improvements in long-term expression by these LTR modifications usually come at the cost of significant reductions in vector production and infectivity. (Abstract shortened by UMI.). |
