title: CTCF-dependent enhancer-blocking by alternative chromatin loop formation.
abstract: The mechanism underlying enhancer-blocking by insulators is unclear. We explored the activity of human beta-globin HS5, the orthologue of the CTCF-dependent chicken HS4 insulator. An extra copy of HS5 placed between the beta-globin locus control region (LCR) and downstream genes on a transgene fulfills the classic predictions for an enhancer-blocker. Ectopic HS5 does not perturb the LCR but blocks gene activation by interfering with RNA pol II, activator and coactivator recruitment, and epigenetic modification at the downstream beta-globin gene. Underlying these effects, ectopic HS5 disrupts chromatin loop formation between beta-globin and the LCR, and instead forms a new loop with endogenous HS5 that topologically isolates the LCR. Both enhancer-blocking and insulator-loop formation depend on an intact CTCF site in ectopic HS5 and are sensitive to knock-down of the CTCF protein by siRNA. Thus, intrinsic looping activity of CTCF sites can nullify LCR function.
authors.affiliation: Laboratory of Cellular and Developmental Biology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
authors.forename: Chunhui
authors.initials: C
authors.lastname: Hou
authors.forename: Hui
authors.initials: H
authors.lastname: Zhao
authors.forename: Keiji
authors.initials: K
authors.lastname: Tanimoto
authors.forename: Ann
authors.initials: A
authors.lastname: Dean
mesh_terms: D000076246:CCCTC-Binding Factor; D002843:Chromatin / Q000235:genetics / Q000648:ultrastructure*; D004268:DNA-Binding Proteins / Q000502:physiology*; D044127:Epigenesis, Genetic; D005786:Gene Expression Regulation; D006801:Humans; D038101:Insulator Elements; D019901:Locus Control Region; D012097:Repressor Proteins / Q000502:physiology*; D019076:Transgenes; D055544:beta-Globins
chemical_list: D000076246:CCCTC-Binding Factor; C000619697:CTCF protein, human; D002843:Chromatin; D004268:DNA-Binding Proteins; D012097:Repressor Proteins; D055544:beta-Globins