The short duration of transgene expression is likely related to the rapid silencing of the CMV promoter.
For the latter cell line rapid silencing of exogenous plasmid DNA by histone deacetylation has been frequently observed [ 42, 43].
Moreover, rapid silencing and blockade of viral protein expression by modified RNAi was observed within one-two hours post inoculation [ 37].
A range of studies in allopolyploid plant genomes have revealed rapid epigenetic changes, including alteration in cytosine methylation patterns, rapid silencing of ribosomal RNA and protein-coding genes, and de-repression of dormant transposable elements.
As the constitutive control, the EF1α promoter was selected, which leads to long lasting transgene expression within pEPito vectors and is not prone to rapid silencing like the CMV immediate early promoter element [ 17, 36].
siRNAs can be introduced into mammalian cells by in vitro transcription or by transfection with chemically synthesized siRNAs to achieve effective and very rapid silencing of a target gene, although this approach is limited to cells that can be transfected at high rates, and the effects are transient [ 8].
Hirochika et al. have also demonstrated that Tto1, a tobacco TE, can transpose in a heterologous systems such as rice or Arabidopsis thaliana even if the copy number is positively correlated with DNA methylation, leading to rapid silencing of the element [ 43].
The LCR included four distal DHSs that were required for perforin expression from its natural locus, and their engineered deletion from the PRF1 BAC transgene abolished LCR function and led to rapid gene silencing.
Phenotypic instability and rapid gene silencing, accompanied by DNA methylation re-patterning and transcriptional changes, were found in newly formed Arabidopsis thaliana x Arabidopsis arenosa allopolyploids [5], [6].
Several laboratories have reported that 5-aza-dC reactivated promoters exhibit rapid re-silencing [29], [30]; however these experiments could not use selection to maintain expression.
