Bidirectional promoter interference between two widely used internal heterologous promoters in a late-generation lentiviral construct

Gene transfer vectors encoding two or more genes are potentially powerful research tools and are poised to play an increasingly important role in gene therapy applications. Common strategies employed to express more than one transgene per vector include the use of multiple promoters, internal ribosome entry site (IRES) elements, splicing signals and fusion proteins. Of these, the IRES elements and multiple promoters have been most widely used. The use of multiple promoters, however, may be compromised by interference between promoters, promoter silencing and vector rearrangements or deletions. In this study, we demonstrate promoter interference between two internal heterologous promoters in the context of a late-generation lentiviral vector. The interference, involving the human cytomegalovirus-immediate-early promoter and human elongation-factor-1 alpha promoter, occurred bidirectionally with both promoters markedly impairing expression of the adjacent transcription unit. The data presented not only highlight the potential for interference between these widely-used promoters, but also the value of a sequential approach to vector construction that allows such effects to be recognized.