title: Reversal of the silencing of tetracycline-controlled genes requires the coordinate action of distinctly acting transcription factors.
abstract: BACKGROUND
  Regulation of genes transferred to eukaryotic organisms is often limited by the lack of consistent expression levels in all transduced cells, which may result in part from epigenetic gene silencing effects. This reduces the efficacy of ligand-controlled gene switches designed for somatic gene transfers such as gene therapy.
  
  
  METHODS
  A doxycycline-controlled transgene was stably introduced in human cells, and clones were screened for epigenetic silencing of the transgene. Various regulatory proteins were targeted to the silent transgene, to identify those that would mediate regulation by doxycycline.
  
  
  RESULTS
  A doxycycline-controlled minimal promoter was found to be prone to gene silencing, which prevents activation by a fusion of the bacterial TetR DNA-binding domain with the VP16 activator. DNA modification studies indicated that the silenced transgene adopts a poorly accessible chromatin structure. Several cellular transcriptional activators were found to restore an accessible DNA structure when targeted to the silent transgene, and they cooperated with Tet-VP16 to mediate regulation by doxycycline.
  
  
  CONCLUSIONS
  Reversal of the silencing of a tetracycline-regulated minimal promoter requires a chromatin-remodeling activity for subsequent promoter activation by the Tet-VP16 fusion protein. Thus, distinct regulatory elements may be combined to obtain long-term regulation and persistent expression of exogenous genes in eukaryotic cells.
authors.affiliation: Laboratory of Molecular Biotechnology, Institute of Biotechnology and Center for Biotechnology UNIL-EPFL, University of Lausanne, 1015 Lausanne, Switzerland.
authors.forename: Renata
authors.initials: R
authors.lastname: Pankiewicz
authors.forename: Yann
authors.initials: Y
authors.lastname: Karlen
authors.forename: Markus O
authors.initials: MO
authors.lastname: Imhof
authors.forename: Nicolas
authors.initials: N
authors.lastname: Mermod
mesh_terms: D002460:Cell Line; D002843:Chromatin / Q000378:metabolism; D002999:Clone Cells; D019175:DNA Methylation; D044127:Epigenesis, Genetic / Q000187:drug effects; D005786:Gene Expression Regulation / Q000187:drug effects*; D020868:Gene Silencing; D017930:Genes, Reporter; D015316:Genetic Therapy; D006801:Humans; D008156:Luciferases / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013752:Tetracycline / Q000494:pharmacology*; D015534:Trans-Activators / Q000235:genetics*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D014162:Transfection; D019076:Transgenes; D001616:beta-Galactosidase / Q000032:analysis / Q000378:metabolism
chemical_list: D002843:Chromatin; D011993:Recombinant Fusion Proteins; D015534:Trans-Activators; D014157:Transcription Factors; D008156:Luciferases; D001616:beta-Galactosidase; D013752:Tetracycline