title: Optimizing retroviral gene expression for effective therapies.
abstract: With their ability to integrate their genetic material into the target cell genome, retroviral vectors (RV) of both the gamma-retroviral (γ-RV) and lentiviral vector (LV) classes currently remain the most efficient and thus the system of choice for achieving transgene retention and therefore potentially long-term expression and therapeutic benefit. However, γ-RV and LV integration comes at a cost in that transcription units will be present within a native chromatin environment and thus be subject to epigenetic effects (DNA methylation, histone modifications) that can negatively impact on their function. Indeed, highly variable expression and silencing of γ-RV and LV transgenes especially resulting from promoter DNA methylation is well documented and was the cause of the failure of gene therapy in a clinical trial for X-linked chronic granulomatous disease. This review will critically explore the use of different classes of genetic control elements that can in principle reduce vector insertion site position effects and epigenetic-mediated silencing. These transcriptional regulatory elements broadly divide themselves into either those with a chromatin boundary or border function (scaffold/matrix attachment regions, insulators) or those with a dominant chromatin remodeling and transcriptional activating capability (locus control regions,, ubiquitous chromatin opening elements). All these types of elements have their strengths and weaknesses within the constraints of a γ-RV and LV backbone, showing varying degrees of efficacy in improving reproducibility and stability of transgene function. Combinations of boundary and chromatin remodeling; transcriptional activating elements, which do not impede vector production; transduction efficiency; and stability are most likely to meet the requirements within a gene therapy context especially when targeting a stem cell population.
authors.affiliation: Gene Expression and Therapy Group, King's College London School of Medicine, Department of Medical and Molecular Genetics, Guy's Hospital, London, SE1 9RT, United Kingdom. michael.antoniou@kcl.ac.uk
authors.forename: Michael N
authors.initials: MN
authors.lastname: Antoniou
authors.forename: Kristian Alsbjerg
authors.initials: KA
authors.lastname: Skipper
authors.forename: Omer
authors.initials: O
authors.lastname: Anakok
mesh_terms: D000818:Animals; D002843:Chromatin / Q000378:metabolism; D019175:DNA Methylation; D015870:Gene Expression; D015316:Genetic Therapy / Q000379:methods*; D005822:Genetic Vectors; D006801:Humans; D016086:Lentivirus / Q000235:genetics; D019901:Locus Control Region; D050436:Regulatory Elements, Transcriptional; D012190:Retroviridae / Q000235:genetics*; D019076:Transgenes / Q000235:genetics
chemical_list: D002843:Chromatin