Methyl binding domain protein 2 mediates -globin gene silencing in adult human YAC transgenic mice

The genes of the vertebrate -globin locus undergo a switch in expression during erythroid development whereby embryonic fetal genes of the cluster are sequentially silenced and adult genes are activated. We describe here a role for DNA methylation and MBD2 in the silencing of the human fetal -globin gene. The -globin gene is reactivated upon treatment with the DNA methyltransferase inhibitor 5-azacytidine in the context of a mouse containing the entire human -globin locus as a yeast artificial chromosome ( YAC) transgene. To elucidate the mechanism through which DNA methylation represses the -globin gene in adult erythroid cells, YAC MBD2 / mice were generated by breeding YAC mice with MBD2 mice. Adult YAC MBD2 / mice continue to express the -globin gene at a level commensurate with 5-azacytidine treatment, 10to 20-fold over that observed with 1-acetyl-2-phenylhydrazine treatment alone. In addition, the level of -globin expression is consistently higher in MBD2 mice in 14.5and 16.5-days postcoitus fetal liver erythroblasts suggesting a role for MBD2 in embryonic fetal erythroid development. DNA methylation levels are modestly decreased in MBD2 mice. MBD2 does not bind to the -globin promoter region to maintain -globin silencing. Finally, treatment of MBD2null mice with 5-azacytidine induces only a small, nonadditive induction of -globin mRNA, signifying that DNA methylation acts primarily through MBD2 to maintain -globin suppression in adult erythroid cells.