We study how enhancers activate transcription in the chromatin environment of eukaryotic cells. The human beta-globin genes are activated by an enhancer/locus control region that lies between 6 and 60 Kb distant from the genes themselves. How the strong enhancer activity of the distant LCR becomes manifest at the promoters of these genes is not well understood, but it encompasses altering chromatin structure. Two types of complexes have been described that carry out chromatin modifications to relieve nucleosomal repression: SWI/SNF complexes that alter nucleosome stability or position (mobilization), and histone acetyltransferase (HAT) complexes that acetylate histone N-terminal tails. To investigate how the LCR changes chromatin and activates transcription, we have studied a model 6 Kb globin locus on viral episomes in human erythroid K562 cells. The locus contains a complete human epsilon-globin gene linked to the strong beta-globin LCR HS2 enhancer. We have compared aspects of structure and function of this enhancer dependent gene to an enhancerless gene, or one linked to an inactivated HS2. Using probes of chromatin structure and quantitative real time PCR, we determined that enhancer dependent nucleosome mobilization due to a Brg1 containing SWI/SNF complex likely preceeds histone hyperacetylation at this promoter. In addition, these studies provided evidence for proximity of the enhancer and gene promoter when transcription is active, as would be predicted by the looping model of enhancer action. However, other studies indicated that histone acetylation and RNA pol II were detected throughout sequences intervening between the enhancer and promoter, suggesting that histone acetylation may spread through the locus from the enhancer, and that at least some polymerase molecules may access the promoter from the enhancer by moving through the locus, as a tracking model of enhancer action would predict. Interposition of an chromatin insulator between the enhancer and gene supported this latter conclusion, as we saw an apparant block to acetylation and polymerase movement and accumulation of polymerase at the enhancer. Taken together, these results are most consistent with a facilitated tracking model of enhancer action. In this view, an enhancer recruits activators, remodeling complexes and polymerase which then move to the promoter without loosing contact with the enhancer. This movement eventually results in loop formation. We continue to investigate enhancer activated gene transcription in our model locus and in the endogenous human chromosome.

Agency
National Institute of Health (NIH)
Institute
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Type
Intramural Research (Z01)
Project #
1Z01DK015508-15
Application #
6809848
Study Section
(LCDB)
Project Start
Project End
Budget Start
Budget End
Support Year
15
Fiscal Year
2003
Total Cost
Indirect Cost
Name
U.S. National Inst Diabetes/Digst/Kidney
Department
Type
DUNS #
City
State
Country
United States
Zip Code
Lee, Jongjoo; Krivega, Ivan; Dale, Ryan K et al. (2017) The LDB1 Complex Co-opts CTCF for Erythroid Lineage-Specific Long-Range Enhancer Interactions. Cell Rep 19:2490-2502
Krivega, Ivan; Dean, Ann (2016) Chromatin looping as a target for altering erythroid gene expression. Ann N Y Acad Sci 1368:31-9
Deng, Wulan; Rupon, Jeremy W; Krivega, Ivan et al. (2014) Reactivation of developmentally silenced globin genes by forced chromatin looping. Cell 158:849-860
Song, Sang-Hyun; Hou, Chunhui; Dean, Ann (2007) A positive role for NLI/Ldb1 in long-range beta-globin locus control region function. Mol Cell 28:810-22
Zhao, Hui; Kim, Aeri; Song, Sang-Hyun et al. (2006) Enhancer blocking by chicken beta-globin 5'-HS4: role of enhancer strength and insulator nucleosome depletion. J Biol Chem 281:30573-80
Dean, Ann (2006) On a chromosome far, far away: LCRs and gene expression. Trends Genet 22:38-45
Zhao, Hui; Dean, Ann (2005) Organizing the genome: enhancers and insulators. Biochem Cell Biol 83:516-24
Dean, Ann (2004) Chromatin remodelling and the interaction between enhancers and promoters in the beta-globin locus. Brief Funct Genomic Proteomic 2:344-54
Kim, AeRi; Dean, Ann (2004) Developmental stage differences in chromatin subdomains of the beta-globin locus. Proc Natl Acad Sci U S A 101:7028-33
Zhao, Hui; Dean, Ann (2004) An insulator blocks spreading of histone acetylation and interferes with RNA polymerase II transfer between an enhancer and gene. Nucleic Acids Res 32:4903-19

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