Episomal S/MAR-based replicons do not alter expression profile of host genome. Homo sapiens

Methods: Autonomously replicating vectors represent a simple and versatile model system for genetic modifications, but their localisation in the nucleus is largely unknown. Using circular chromosome conformation capture we mapped genomic contact sites of S/MAR-based replicons in HeLa cells. The influence of cis-active sequences on genomic localisation was assessed using replicons containing either an insulator sequence or an intron Results: While the original and the insulator-containing replicons displayed distinct contact sites, the intron-containing replicon showed a rather broad genomic contact pattern. Our results indicate a preference for certain chromatin structures and a rather non-dynamic behaviour during mitosis. Independent of inserted cis-active elements established vector molecules reside preferentially within actively transcribed regions, especially within promoter sequences and transcription start sites. Conclusions: S/MAR-based episomal replicons have a limited number of preferential contact sites and seem to be fairly non-dynamic during mitosis. We show that cis-acting elements do have an impact on the chormosomal localisation of episomal replicons, even though the epigenetic signatue of these contact sites are similar. Independently of the inserted cis-acting element, these contact sites are preferentially located within actively transcribed regions, especially promoter sites. Knowledge of preferred contact sites of exogenous DNA, e.g. viral or non-viral episomes, contribute to our understanding of episome behaviour in the nucleus and can be used for vector improvement and guiding of DNA sequences to specific subnuclear sites. Overall design: Genome-wide mapping of chromosomal contact sites of S/MAR-based replicons