Silencing of episomal transgene expression by plasmid bacterial DNA elements in vivo

We previously demonstrated that sustainable enhanced levels of transgene products could be expressed from a bacterial DNA-free expression cassette either formed from a fragmented plasmid in mouse liver or delivered as a minicircle vector. This suggested that bacterial DNA sequences played a role in episomal transgene silencing. To further understand the silencing mechanism, we systematically altered the DNA components in both the expression cassette and the bacterial backbone, and compared the gene expression profiles from mice receiving different DNA forms. In nine vectors tested, animals that received the purified expression cassette alone always expressed persistently higher levels of transgene compared to 2fDNA groups. In contrast, animals that received linearized DNA by a single cut in the bacterial backbone had similar expression profiles to that of intact plasmid groups. All three linear DNAs formed large concatemers and small circles in mouse liver, while ccDNA remained intact. In all groups, the relative amount of vector DNA in liver remained similar. Together, these results further established that the DNA silencing effect was mediated by a covalent linkage of the expression cassette and the bacteria DNA elements.