Current efforts in nonviral gene therapy are plagued by a pervasive difficulty in sustaining therapeutic levels of delivered transgenes. Minicircles (plasmid derivatives with the same expression cassette but lacking a bacterial backbone) show sustained expression and hold promise for therapeutic use where persistent transgene expression is required. To characterize the widely-observed silencing process affecting expression of foreign DNA in mammals, we used a system in which mouse liver presented with either plasmid or minicircle consistently silences plasmid but not minicircle expression. We found that preferential silencing of plasmid DNA occurs at a nuclear stage that precedes transport of mRNA to the cytoplasm, evident from a consistent >25-fold minicircle/plasmid transcript difference observed in both nuclear and total RNA. Among possible mechanisms of nuclear silencing, our data favor chromatin-linked transcriptional blockage rather than targeted degradation, aberrant processing, or compromised mRNA transport. In particular, we observe dramatic enrichment of H3K27 trimethylation on plasmid sequences. Also, it appears that Pol II can engage the modified plasmid chromatin, potentially in a manner that is not productive in the synthesis of high levels of new transcript. We outline a scenario in which sustained differences at the chromatin level cooperate to determine the activity of foreign DNA.

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Ke X, McKnight RA, Maniar LE, Sun Y, Callaway CW, Majnik A, Lane RH, Cohen SS. IUGR increases chromatin-remodeling factor Brg1 expression and binding to GR exon 1.7 promoter in newborn male rat hippocampus. Am J Physiol Regul Integr Comp Physiol 309: R119 -R127, 2015. First published May 13, 2015 doi:10.1152/ajpregu.00495.2014.-Intrauterine growth restriction (IUGR) increases the risk for neurodevelopment delay and neuroendocrine reprogramming in both humans and rats. Neuroendocrine reprogramming involves the glucocorticoid receptor (GR) gene that is epigenetically regulated in the hippocampus. Using a wellcharacterized rodent model, we have previously shown that IUGR increases GR exon 1.7 mRNA variant and total GR expressions in male rat pup hippocampus. Epigenetic regulation of GR transcription may involve chromatin remodeling of the GR gene. A key chromatin remodeler is Brahma-related gene-1(Brg1), a member of the ATPdependent SWItch/Sucrose NonFermentable (SWI/SNF) chromatin remodeling complex. Brg1 regulates gene expression by affecting nucleosome repositioning and recruiting transcriptional components to target promoters. We hypothesized that IUGR would increase hippocampal Brg1 expression and binding to GR exon 1.7 promoter, as well as alter nucleosome positioning over GR promoters in newborn male pups. Further, we hypothesized that IUGR would lead to accumulation of specificity protein 1 (Sp1) and RNA pol II at GR exon 1.7 promoter. Indeed, we found that IUGR increased Brg1 expression and binding to GR exon 1.7 promoter. We also found that increased Brg1 binding to GR exon 1.7 promoter was associated with accumulation of Sp1 and RNA pol II carboxy terminal domain pSer-5 (a marker of active transcription). Furthermore, the transcription start site of GR exon 1.7 was located within a nucleosome-depleted region. We speculate that changes in hippocampal Brg1 expression mediate GR expression and subsequently trigger neuroendocrine reprogramming in male IUGR rats.intrauterine growth restriction; Brahma-related gene 1; glucocorticoid receptor; hippocampus UTEROPLACENTAL INSUFFICIENCY (UPI) leads to intrauterine growth restriction (IUGR) and complicates up to 4 -6% of all pregnancies in developed countries (13). IUGR predisposes affected newborns toward poor neurological outcomes and impaired neuroendocrine programming (18,20,46,67,71). Neuroendocrine programming of the hypothalamo-pituitary axis (HPA) axis is regulated, in part, by the hippocampus (24,40). Multiple studies suggest that IUGR-induced neuroendocrine reprogramming involves altered glucocorticoid receptor (GR) expression in the hippocampus and dysregulation of HPA axis reactivity (24,34,42,43,74,75,80