Type and position of promoter elements in retroviral vectors have substantial effects on the expression level of an enhanced green fluorescent protein reporter gene

Although gene transfer with retroviral vectors has already been applied to patients as part of clinical protocols, low expression of transgenes in target cells still remains a problem. Therefore, we compared various retroviral vectors using different promoters and backbones for expression of the enhanced green fluorescent protein (EGFP) reporter gene in fibroblasts and CD34(+) cells. Methods: The N2A retroviral vector was used to test expression from the herpes simplex virus thymidine kinase promoter (vector N2A-TK-EGFP), a human phosphoglycerate kinase promoter (vector N2A-PGK-EGFP), and the SV40 promoter (vector N2A-SV-EGFP). Additional constructs used the spleen focus-forming virus (SFFV) long terminal repeat (LTR) as promoter and expressed EGFP alone (vector SF beta 1-EGFP) or EGFP and a downstream (vector SF beta 1-EGFP-IRES) or upstream (vector SF beta 1-IRES-EGFP) internal ribosomal entry site. Results: For NIH 3T3 cells the fluorescence-activated cell sorting analysis revealed that the most active internal promoter was the SV40 promoter in the vector N2A-SV-EGFP (mean fluorescence intensity, MFI, 66.7 +/- 0.4), followed by N2A-PGK-EGFP (26.3 +/- 1.8 MFI), and N2A-TK-EGFP (4.8 +/- 0.1 MFI). Expression from the SF beta 1-EGFP vector (82.6 +/- 6.7 MFI) and the SF beta 1-EGFP-IRES vector (102.8 +/- 6.2 MFI) was higher than from SF beta 1-IRES-EGFP vector (15.5 +/- 1.8 MFI). In human CD34-positive cells, the EGFP expression from all vectors was considerably lower than in fibroblasts with the SF beta 1-EGFP vector still being four- to fivefold more active than the internal promoters tested. Conclusion: The SFFV LTR seems to allow a high expression of transgenes, as long as the transgene is not expressed downstream of an internal ribosomal entry site. Internal promoters may be useful for targeted gene expression in specific cell types, but the reduced level of expression from some internal promoters has to be taken into consideration.