TR4 and BCL11A Repress γ-globin Transcription via Independent Mechanisms

Nuclear receptor TR4 was previously shown to bind to the -117 position of the -globin gene promoters in vitro, which overlaps the more recently described BCL11A binding site. The role of TR4 in human -globin gene repression has not been extensively characterized in vivo, while any relationship between TR4 and BCL11A regulation through the -globin promoters is unclear at present. We show here that TR4 and BCL11A competitively bind in vitro to distinct, overlapping sequences, including positions overlapping -117 of the -globin promoter. We found that TR4 represses -globin transcription and HbF accumulation in vivo in a BCL11A-independent manner. Finally, examination of the chromatin occupancy of TR4 within the -globin locus, when compared to BCL11A, shows that both bind avidly to the locus control region and other sites, but that only BCL11A binds to the -globin promoters at statistically significant frequency. These data resolve an important discrepancy in the literature, and thus clarify possible approaches to the treatment of sickle cell disease and -thalassaemia.