TR4 and BCL11A repress γ-globin transcription via independent mechanisms.

Nuclear receptor TR4 (NR2C2) was previously shown to bind to the -117 position of the γ-globin gene promoters in vitro, which overlaps the more recently described BCL11 transcription factor A (BCL11A) binding site. The role of TR4 in human γ-globin gene repression has not been extensively characterized in vivo, whereas any relationship between TR4 and BCL11A regulation through the γ-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 γ-globin promoter. We found that TR4 represses γ-globin transcription and fetal hemoglobin accumulation in vivo in a BCL11A-independent manner. Finally, examination of the chromatin occupancy of TR4 within the β-globin locus, compared with BCL11A, shows that both bind avidly to the locus control region and other sites, but only BCL11A binds to the γ-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 β-thalassaemia.