Looping, Linking, and Chromatin Activity: New Insights into β-globin Locus Regulation

Given our association with the looping and competition models (Choi and Engel 1988xChoi, O and Engel, J.D. Cell. 1988; 55: 17–26Abstract | Full Text PDF | PubMed | Scopus (166)See all ReferencesChoi and Engel 1988), we can readily incorporate these new data into predictive corollaries to the competition hypothesis; however, we note that these predictions can be applied to support linking as well. If the human locus is defined by intergenic elements that promote or restrict the ability of the LCR to activate transcription of genes within subdomains (Figure 2Figure 2), a primary layer of chromatin differentiation could serve to augment the positive or negative regulatory function of discrete transcriptional cis elements neighboring the genes. For example, we showed that deletion of the e-globin gene silencer from a YAC has very modest effects on e-globin gene expression in adult erythroid cells (Liu et al. 1997xLiu, Q, Bungert, J, and Engel, J.D. Proc. Natl. Acad. Sci. USA. 1997; 94: 169–174Crossref | PubMed | Scopus (43)See all ReferencesLiu et al. 1997), while the effect of its deletion in transgenic mice bearing only fragments of the locus is quite dramatic (Li et al. 1998xLi, Q, Blau, C.A, Clegg, C.H, Rohde, A, and Stamatoyannopoulos, G. J. Biol. Chem. 1998; 273: 17361–17367Crossref | PubMed | Scopus (19)See all ReferencesLi et al. 1998). We originally proposed that functional redundancy of cis elements could account for the lack of effects detected when the small e-globin gene silencer was deleted from a large transgenic YAC. The report of Gribnau et al. 2000xGribnau, J, Diderich, K, Pruzina, S, Calzolari, R, and Fraser, P. Mol. Cell. 2000; 5: 377–386Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesGribnau et al. 2000 support an additional alternative: that discrete cis element mutations are insufficient to confer full expressivity of local “silencer” or “enhancer” activities, and that these gene proximal elements can only act at the proper developmental stage when they do so within the context of regionally imposed (metastable) chromatin constraints. Thus, deletion of the e-globin silencer may not confer a significant positive transcriptional effect on the e-globin gene when examined in the context of the whole locus because the chromatin subdomain harboring e-globin is suppressed in adult erythroid cells (i.e., the primary effect of chromatin suppression dominates a secondary effect of silencer deletion).Figure 2Stage-Specific Activation within Subdomains of the β-globin Locus by an LCR HolocomplexChromatinized DNA status (either relatively open or closed, which is meant to reflect general sensitivity to DNase I) is depicted as spring shapes with the more condensed chromatin represented as tighter coiling. Individual LCR HS sites and genes are shown as spheres (shaded from yellow to red) and cylinders (blue), respectively. Purple lines show possible structural constraints generated by linkage of two putative boundary elements (purple spheres) and how this might contribute to the formation of chromosomal subdomains. The architectural constraints within the locus and/or the LCR holocomplex configuration may also influence which gene(s) can be activated by LCR at specific developmental stages (green arrows representing the domains that are rendered more accessible after differential orientation of the LCR in the primitive and definitive erythroid stages).View Large Image | View Hi-Res Image | Download PowerPoint SlideClearly there are a host of challenges remaining in this field, not the least of which concerns our ability to clearly define the caveats and possible limitations in interpreting each new study within the context of a huge body of existing literature. Lest we become complacent, it's important to remember that each new year for the past three decades has been accompanied by often startling revelations about how globin diseases arise, how the locus is regulated, how LCRs function, and, most recently, how chromatin structure impinges on differential modulation of the genes. The formidable tasks of devising clear distinctions among the possible models, as well as executing defining experiments to distinguish among them, is still a challenge we face daily, and often optimistically.*E-mail: d-engel@nwu.edu.