Fetal Globin Gene Repressors as Drug Targets for Molecular Therapies To Treat the β-Globinopathies

Abstract

The human ␤-globin locus is comprised of embryonic, fetal, and adult globin genes that are expressed in a developmental stagespecific manner. Mutations in the globin locus give rise to the ␤-globinopathies, ␤-thalassemia and sickle cell disease, which begin to manifest symptoms around the time of birth. Although the fetal globin genes are autonomously silenced in adult-stage erythroid cells, mutations lying both within and outside the locus lead to natural variations in the level of fetal globin gene expression, and some of these significantly ameliorate the clinical symptoms of the ␤-globinopathies. Multiple reports have now identified several transcription factors that are involved in fetal globin gene repression in definitive (adult)-stage erythroid cells (the TR2/TR4 heterodimer, MYB, KLFs, BCL11A, and SOX6). To carry out their repression functions, chromatin-modifying enzymes (such as DNA methyltransferase, histone deacetylases, and lysine-specific histone demethylase 1) are additionally involved as a consequence of forming large macromolecular complexes with the DNA-binding subunits of these cellular machines. This review focuses on the molecular mechanisms underlying fetal globin gene silencing and possible near-future molecularly targeted therapies for treating the ␤-globinopathies.

Inhibition of G9a methyltransferase stimulates fetal hemoglobin production by facilitating LCR/γ-globin looping

“…42 Our data suggest that histone-modifying effects of G9a, as well as its functional interplay with other proteins present at the globin locus, should be explored in the context of HbF expression. [43][44][45] An understanding of how the methyltransferase-dependent and methyltransferase-independent functions of G9a are linked at the level of chromatin to influence globin locus epigenetics can further advance efforts toward the development of HbF-inducing drugs.…”

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.…”

Compound loss of function of nuclear receptors Tr2 and Tr4 leads to induction of murine embryonic β-type globin genes

“…This suggests that depletion of LSD1 or DNMT1 alone cannot overcome the repressed state of the bh1-globin gene and, therefore, that additional corepressors are involved in repression of this gene at the adult stage. 50 Alternatively, the residual LSD1 or DNMT1 mRNA (25%) may be sufficient to maintain the repression of bh1-globin transcription.…”