Genetic Vaccines and Therapy (2003)

DNA vaccination is a relatively recent development in vaccine methodology. It is now possible to undertake a rational step-by-step approach to DNA vaccine design. Strategies may include the incorporation of immunostimulatory sequences in the backbone of the plasmid, co-expression of stimulatory molecules, utilisation of localisation/secretory signals, and utilisation of the appropriate delivery system, for example. However, another important consideration is the utilisation of methods designed to optimise transgene expression. In this review we discuss the importance of regulatory elements, kozak sequences and codon optimisation in transgene expression. Review In 1990, the direct gene transfer of plasmid DNA into mouse muscle in vivo without the need for a special delivery system was demonstrated [1]. Furthermore, intramuscular inoculation with plasmid DNA encoding reporter genes induced protein expression within the muscle cells. This study provided evidence for the idea that naked DNA could be delivered in vivo to direct protein expression. Subsequently, a further study reported the gene expression a year or more after intramuscular injection of plasmid DNA [2]. Since these initial studies, many more experiments have been carried out to evaluate different factors that determine the efficiency of gene transfer and immunogenicity of plasmid DNA. Furthermore, plasmid DNA has been used to immunise against a variety of diseases (known as DNA vaccination). Alternatively, plasmid DNA has been used to treat genetic diseases and similar factors may affect the efficacy of this gene therapy. DNA vaccines usually consist of plasmid vectors (derived from bacteria) that contain heterologous genes (transgenes) inserted under the control of a eukaryotic promoter, allowing protein expression in mammalian cells [3]. An important consideration when optimising the efficacy of DNA vaccines is the appropriate choice of plasmid vector. The basic requirements for the backbone of a plasmid DNA vector are a eukaryotic promoter, a cloning site, a polyadenylation sequence, a selectable marker and a bacterial origin of replication [4]. A strong promoter may be required for optimal expression in mammalian cells. For this, some promoters derived from viruses such as cytomegalovirus (CMV) or simian virus 40 (SV40) have been used. A cloning site downstream of the promoter should be provided for insertion of heterologous genes, and inclusion of a polyadenylation (polyA) sequence such as the bovine growth hormone (BGH) or SV40 polyadenylation sequence provides stabilisation of mRNA transcripts. The most commonly used selectable markers are bacterial antibiotic resistance genes, such as the ampicillin resistance gene. However, since the ampicillin resistance gene is precluded for use in humans, a kanamycin resistance gene is often used. Finally, the Escherichia coli ColE1 origin of replication, which is found in plasmids such as those in the pUC series, is most often used in DNA Published: 16 September 2003 Genetic Vaccines and Therapy 2003, 1:2 Received: 04 August 2003 Accepted: 16 September 2003 This article is available from: http://www.gvt-journal.com/content/1/1/2 © 2003 Garmory et al; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media for any purpose, provided this notice is preserved along with the article's original URL.