Advances in high-capacity extrachromosomal vector technology: episomal maintenance, vector delivery, and transgene expression.

Recent developments in extrachromosomal vector technology have offered new ways of designing safer, physiologically regulated vectors for gene therapy. Extrachromosomal, or episomal, persistence in the nucleus of transduced cells offers a safer alternative to integrating vectors which have become the subject of safety concerns following serious adverse events in recent clinical trials. Extrachromosomal vectors do not cause physical disruption in the host genome, making these vectors safe and suitable tools for several gene therapy targets, including stem cells. Moreover, the high insert capacity of extrachromosomal vectors allows expression of a therapeutic transgene from the context of its genomic DNA sequence, providing an elegant way to express normal splice variants and achieve physiologically regulated levels of expression. Here, we describe past and recent advances in the development of several different extrachromosomal systems, discuss their retention mechanisms, and evaluate their use as expression vectors to deliver and express genomic DNA loci. We also discuss a variety of delivery systems, viral and nonviral, which have been used to deliver episomal vectors to target cells in vitro and in vivo. Finally, we explore the potential for the delivery and expression of extrachromosomal transgenes in stem cells. The long-term persistence of extrachromosomal vectors combined with the potential for stem cell proliferation and differentiation into a wide range of cell types offers an exciting prospect for therapeutic interventions.