DNA Methylation and Globin Gene Expression.

Understanding how the human γ-globin gene is regulated has important clinical implications because increased levels of fetal hemoglobin (HbF) are beneficial to patients with sickle cell disease and β-thalassemia. DNA methylation is strongly implicated in developmental silencing of the γ-globin gene based on:

1. an inverse correlation between DNA methylation of the γ-globin gene and its expression,

2. the acquisition of CpG residues within the γ-globin 5’ region during evolution as the γ-globin gene was recruited to fetal stage expression, and

3. the ability of pharmacological inhibitors of DNA methyltransferase (5-azacytidine; decitabine) to reactivate high-level expression of the γ-globin gene in experimental primates that led to clinical trials demonstrating that decitabine increased HbF to therapeutic levels in patients with sickle cell disease.

Decitabine treatment in vivo decreases DNA methylation of the γ-globin gene and increases association of RNA polymerase II, acetyl Histone H3 and H4, and Histone H3 (lys4) trimethyl with the γ-globin gene, strongly suggesting that decitabine increases γ-globin gene transcription. These results are consistent with the hypothesis that γ-globin expression in adults is repressed by the binding of methyl DNA binding proteins to the methylated γ-globin promoter with subsequent recruitment of co-repressor complexes that actively repress γ-globin transcription. The reduction of γ-globin gene DNA methylation induced pharmacologically in adults by decitabine is linked to high level γ-globin expression, as is the complete loss of γ-globin gene methylation attained physiologically during erythroid differentiation of fetal liver hematopoietic progenitor cells. The mechanism of action of the drug has not been definitively established, however, and the role of DNA methylation in regulation of γ-globin gene expression remains an active area of investigation. High level γ-globin expression in baboon erythroid progenitor cell cultures without a reduction of γ-globin gene DNA methylation suggests the existence of alternative mechanisms of activation. In addition to reducing DNA methylation, decitabine activates the p38 MAP kinase pathway, increases p21WAF, and accelerates terminal erythroid differentiation. The role of these effects remains to be investigated. Increased understanding of the role of DNA methylation in γ-globin gene regulation is likely to impact the design of future therapies to increase HbF levels.