Embryonic and fetal β-globin gene repression by the orphan nuclear receptors, TR2 and TR4

Abstract

The TR2 and TR4 orphan nuclear receptors comprise the DNA-binding core of direct repeat erythroid definitive, a protein complex that binds to direct repeat elements in the embryonic and fetal beta-type globin gene promoters. Silencing of both the embryonic and fetal beta-type globin genes is delayed in definitive erythroid cells of Tr2 and Tr4 null mutant mice, whereas in transgenic mice that express dominant-negative TR4 (dnTR4), human embryonic epsilon-globin is activated in primitive and definitive erythroid cells. In contrast, human fetal gamma-globin is activated by dnTR4 only in definitive, but not in primitive, erythroid cells, implicating TR2/TR4 as a stage-selective repressor. Forced expression of wild-type TR2 and TR4 leads to precocious repression of epsilon-globin, but in contrast to induction of gamma-globin in definitive erythroid cells. These temporally specific, gene-selective alterations in epsilon- and gamma-globin gene expression by gain and loss of TR2/TR4 function provide the first genetic evidence for a role for these nuclear receptors in sequential, gene-autonomous silencing of the epsilon- and gamma-globin genes during development, and suggest that their differential utilization controls stage-specific repression of the human epsilon- and gamma-globin genes.

The LSD1 inhibitor RN-1 recapitulates the fetal pattern of hemoglobin synthesis in baboons (P. anubis)

“…It is possible that this difference is dose-related and that the e-globin gene is less sensitive to de-repression by RN-1 because the e-globin promoter contains two DR elements that bind DRED with higher affinity than the single DR1 element in the γ-globin promoter. [22][23][24] LSD1 demethylates other proteins including DNMT1. Demethylation of DNMT1 by LSD1 increases DNMT1 stability while targeted deletion of LSD1 results in loss of DNMT1 and DNA demethylation in ES cells.…”

“…22 Recognition of the role of Direct Repeat (DR) elements in the e-and γ-globin gene promoters in repression of these respective genes led to the identification of the TR2 and TR4 non-steroidal nuclear receptor proteins that specifically bind these elements and subsequently recruit a multi-protein co-repressor complex that includes DNA methyltransferase 1 (DNMT1), lysine specific demethylase 1 (LSD1), and histone deacetylases (HDACs) to repress gene expression. [23][24][25] Both DNMT1 and LSD1 have also been identified as components of co-repressor complexes also recruited by Bcl11a. 26,27 LSD1, the first histone demethylase identified, demethylates lysine 4 (H3K4) and lysine 9 (H3K9) residues of histone H3 and represses gene expression during hematopoietic differentiation.…”

The Switch from Fetal to Adult Hemoglobin

“…By examining proteins that bind the direct repeat elements found within the promoter of the g-globin genes, the orphan nuclear hormone receptors TR2 and TR4 have been suggested to play a role as repressors of the expression of the g-globin genes (Tanabe et al 2002(Tanabe et al , 2007Cui et al 2011). Paradoxically, overexpression of TR2 and TR4 in transgenic mouse models results in elevated expression of g-globin and when overexpressed in sickle cell disease mouse models can result in partial amelioration of hematologic and pathologic symptoms of these mice (Campbell et al 2011).…”

Fetal hemoglobin chemical inducers for treatment of hemoglobinopathies

“…The DRED complex binds to DR sequences observed in the promoter of γ-and ɛ-globin genes. Silencing of fetal β-type globin genes is delayed in definitive erythroid cells of TR2 and TR4 null mutant mice, whereas transgenic mice that express dominant-negative TR4, human fetal γ-globin is activated in definitive, but not in primitive erythroid cells [60]. These findings strongly suggest that TR2/TR4 acts as a stage-selective repressor.…”