Factors affecting the performance of different long terminal repeats in the retroviral vector

The long terminal repeat (LTR) of retrovirus contains the nucleotide sequences that control gene expression. Although several different LTRs have been used in the context of retroviral vector, the activity of the various LTRs has not yet been systematically compared for their level of gene expression. We evaluated the effect of four different LTRs on gene expression using luciferase, stem cell factor, and enhanced green fluorescence protein as reporter genes. LTRs tested in this study were derived from Moloney murine leukemia virus, myeloproliferative sarcoma virus, murine stem cell virus, and spleen focus-forming virus. It was found that the level of gene expression is affected by not only LTRs but also the transgenes and the cell types in which gene expression occurs. Furthermore, the presence of other nucleotide sequences such as the internal ribosome entry site (IRES)-neo cassette could also significantly affect gene expression. Our results suggested that the LTR should be chosen carefully, more or less on an empirical basis.