Exploiting internal ribosome entry sites in gene therapy vector design

Efficient and regulated co-expression of multiple genes is an important consideration in the design of gene therapy vectors. While the augmentation of a single therapeutic gene is often sufficient for gene therapy of simple mendelian disorders, strategies for the treatment of complex disorders and infectious diseases necessitate the introduction of multiple genes into the cell. Complex disorders such as cancer often involve mutations in multiple genes and a combination strategy targeting different defective genes simultaneously are often more effective than any single strategy. Likewise, approaches for treating infectious diseases such as HIV-1 (human immunodeficiency virus) often involve the blocking of multiple steps of the viral replication pathway simultaneously to prevent the emergence of resistant strains of the virus. Even for the treatment of single gene defects, the additional incorporation of a selectable marker gene is often necessary to achieve sustained expression of the therapeutic gene in the cells. Among the several different strategies to co-express multiple genes, the incorporation of an IRES (internal ribosome entry site) into gene therapy vector design represents one of the more promising strategies. IRES functions as a ribosome-landing pad for the efficient internal initiation of translation ensuring coordinate expression of several genes and are located at the 5'UTR (5' untranslated regions) of these genes. Currently, the most popular IRES utilized for gene therapy is the IRES from the EMCV (encephalomyocarditis virus). However, the major caveat with present vector systems utilizing this IRES is that the expression of the downstream gene is significantly less efficient than the upstream gene. This review will examine the growing list of naturally occurring and synthetic IRESes and how they can be exploited for human gene therapy.