Genomic Context and Insert Orientation in the Regulation of Transgene Expression in Adenoviral Vectors

Adenoviral vectors are among the most efficient platforms for gene delivery; however, the level and pattern of transgene expression in these vectors are largely shaped by the viral genomic context. This review discusses the mechanisms of adenoviral transcription and alternative splicing and how they influence the expression of inserted expression cassettes. Particular attention is given to the role of insertion orientation and transgene placement within the E1 and E3 regions, as well as to the effects of viral regulatory elements, including the E1A enhancer. We analyze evidence on the use of insulating sequences to reduce nonspecific activation and improve the controllability of transgene expression. We also consider the use of endogenous adenoviral promoters-the major late promoter (MLP) and the E3 region promoter-and their contribution to enhanced transgene expression through late viral transcription. Overall, these findings support principles for the rational design of adenoviral vectors, both for high-level protein production and for building systems with regulated or tissue-specific expression.