The functional role of S/MARs in episomal vectors as defined by the stress-induced destabilization profile of the vector sequences.

The scaffold/matrix attachment regions (S/MARs) are chromosomal elements that participate in the formation of chromatin domains and have origin of replication support functions. Because of all these functions, in recent years, they have been used as part of episomal vectors for gene transfer. The S/MAR of the human beta-interferon gene has been shown to support efficient episome retention and transgene expression in various mammalian cells. In Jurkat and other cells, DNA plasmid vectors containing Epstein-Barr virus origin of replication (EBV OriP) and the EBV nuclear antigen-1 gene mediate prolonged episome retention in the host cell nucleus, which, however, diminishes over time. In order to enhance retention, we combined this system with an S/MAR element. Unexpectedly, this completely eliminated the capacity of episomes to replicate. Calculation of the stress-induced DNA duplex destabilization profile of the vectors suggested that the S/MAR element had created an increase in molecular stability at the OriP site that may have disturbed replicative potential. In contrast, introduction of an alternative initiation of replication region from the beta-globin gene locus, instead of EBV OriP and the EBV nuclear antigen-1 gene, restored replicative capacity and enhanced episome retention mediated by the S/MAR. These effects were associated with a destabilization profile at the initiation of replication region. These data demonstrate a correlation between S/MAR-mediated vector retention and the presence of an unstable duplex at a replication origin, in this particular setting. We consider that the calculation of stress-induced duplex destabilization may be an informative first step in the design of units that replicate extrachromosomally, particularly as the latter present a safer and, therefore, attractive alternative to integrating viral vectors for gene therapy applications.