The Tet Dioxygenase Co-Factor Ascorbic Acid Reduces DNA Methylation and Increases Expression of the γ-Globin Gene and Acts in a Combinatorial Manner with HbF-Inducing Drugs Targeting Repressive Epigenetic Modifications

Increased levels of fetal hemoglobin (HbF) lessen the severity of symptoms and increase the life span of patients with sickle cell disease (SCD). Differences in DNA methylation of the γ-globin gene promoter between adult and fetal liver erythroid cells are highly associated with developmental differences in γ-globin expression. Mechanisms that establish and/or modulate DNA methylation of the γ-globin promoter during adult and fetal erythroid differentiation are important in the regulation of γ-globin expression. Pharmacological manipulation of DNA methylation increases HbF in nonhuman primates and SCD patients. Decitabine, a DNA methyltransferase inhibitor that demethylates DNA and increases HbF, is currently in clinical trials. Recent studies have shown that 5-hydroxymethylcytosine (5-hmC), an oxidative product of 5-methylcytosine (5-mC) catalyzed by activity of the TET dioxygenase family, is an intermediate in developmental processes that demethylate DNA. Previously we showed that the γ-globin gene promoter was demethylated during fetal liver erythroid differentiation and to a lesser extent during adult bone marrow (BM) erythroid differentiation. We have investigated the role of 5-hmC in the mechanism of γ-globin gene demethylation by analyzing 5-hmC levels at the HpaII site located at position -51 5’ to the γ-globin transcription start site using a T4-MspI assay in DNA isolated from FACS-purified subpopulations of baboon BM cells enriched for different stages of erythroid lineage differentiation. Levels of 5-hmC were >3 fold higher (p<0.001) in the CD117+CD36+ subpopulation enriched in CFUe (7.15+1.34%) compared to the terminal erythroid precursors (2.33+0.84%) showing that 5-hmC levels are dynamically regulated during erythroid differentiation. Although baboon BM erythroid subpopulations express both TET2 and TET3, higher levels of TET3 were observed in terminal erythroid precursors than in the more primitive CD117+CD36+ subpopulation. High levels of TET3 were also observed in FACS-purified erythroid cells derived from cultured CD34+ baboon BM, human peripheral blood, and human cord blood cells suggesting a role for TET3 in erythroid differentiation. To investigate the relationship between 5-hmC, 5-mC, and γ-globin expression, levels of γ-globin promoter 5-hmC and 5-mC were determined in purified erythroid cells derived from baboon BM CD34+ erythroid progenitors grown in culture conditions resulting in either high (liquid culture) or low (AFT024 murine fetal liver stromal cell line co-culture) levels of γ-globin expression. Levels of γ-globin promoter 5-hmC (mean difference 4.93% total cytosine; p<0.005) and γ-globin chain expression (mean difference γ/γ+β=0.44; p<0.001) were higher and γ-globin promoter 5-mC levels lower (mean difference -25.2% total cytosine; p<0.01) in erythroid progenitors grown in liquid cultures compared to stromal cell line co-cultures. Supplementation of culture media with ascorbic acid, a co-factor of the TET dioxygenases, increased γ-globin expression (mean difference γ/γ+β=0.12; p<0.005) and reduced the level of γ-globin promoter DNA methylation (mean difference -29.0% total cytosine; p<0.001) in baboon BM erythroid progenitors grown in both liquid and co-cultures compared to untreated controls. Ascorbic acid also increased γ-globin expression in cultures derived from human peripheral blood CD34+ progenitors (mean difference γ/γ+β=0.08; p<0.05). In addition, in baboon BM erythroid progenitor cultures ascorbic acid increased γ-globin expression in an additive manner in combination with either the DNA methyltransferase inhibitor decitabine (p<0.001) or the LSD1 inhibitor tranylcypromine (p<0.001) compared to either drug alone, while no combinatorial effects on γ-globin expression were observed with hydroxyurea. These results demonstrate that ascorbic acid is a DNA hypomethylating agent that increases γ-globin gene expression and are consistent with a role for the TET-mediated 5-hmC pathway in the regulation of DNA methylation and expression of the γ-globin gene. Furthermore, these results suggest that vitamin C deficiency, observed in approximately 50% of patients with sickle cell disease, may limit HbF induction by drugs that target epigenetic silencing mechanisms.