title: Strict control of telomerase activation using Cre-mediated inversion.
abstract: BACKGROUND
  Human cells appear exquisitely sensitive to the levels of hTERT expression, the telomerase reverse transcriptase. In primary cells that do not express hTERT, telomeres erode with each successive cell division, leading to the eventual loss of telomere DNA, an induction of a telomere DNA damage response, and the onset of cellular senescence or crisis. In some instances, an average of less than one appropriately spliced hTERT transcript per cell appears sufficient to restore telomerase activity and telomere maintenance, and overcome finite replicative capacity.
  
  
  RESULTS
  To underscore this sensitivity, we showed that a widely used system of transcriptional induction involving ecdysone (muristerone) led to sufficient expression of hTERT to immortalize human fibroblasts, even in the absence of induction. To permit tightly regulated expression of hTERT, or any other gene of interest, we developed a method of transcriptional control using an invertible expression cassette flanked by antiparallel loxP recombination sites. When introduced into human fibroblasts with the hTERT cDNA positioned in the opposite orientation relative to a constitutively active promoter, no telomerase activity was detected, and the cell population retained a mortal phenotype. Upon inversion of the hTERT cDNA to a transcriptionally competent orientation via the action of Cre recombinase, cells acquired telomerase activity, telomere DNA was replenished, and the population was immortalized. Further, using expression of a fluorescent protein marker, we demonstrated the ability to repeatedly invert specific transcripts between an active and inactive state in an otherwise isogenic cell background.
  
  
  CONCLUSIONS
  This binary expression system thus provides a useful genetic means to strictly regulate the expression of a given gene, or to control the expression of at least two different genes in a mutually exclusive manner.
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, Terrence Donnelly Centre for Cellular and Biomolecular Research, Toronto, Ontario, M5S 3E1. mark.ungrin@utoronto.ca
authors.forename: Mark D
authors.initials: MD
authors.lastname: Ungrin
authors.forename: Lea
authors.initials: L
authors.lastname: Harrington
mesh_terms: D002460:Cell Line; D007446:Chromosome Inversion / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics* / Q000378:metabolism*; D004789:Enzyme Activation; D005347:Fibroblasts / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic / Q000502:physiology; D006801:Humans; D019426:Integrases / Q000235:genetics*; D015202:Protein Engineering / Q000379:methods*; D011994:Recombinant Proteins / Q000378:metabolism; D019098:Telomerase / Q000235:genetics* / Q000378:metabolism; D015533:Transcriptional Activation / Q000502:physiology; D014162:Transfection / Q000379:methods*; D014764:Viral Proteins / Q000235:genetics*
chemical_list: D004268:DNA-Binding Proteins; D011994:Recombinant Proteins; D014764:Viral Proteins; C045073:Cre recombinase; D019426:Integrases; D019098:Telomerase