Mi2β Is Required for γ-Globin Gene Silencing: Temporal Assembly of a GATA-1-FOG-1-Mi2 Repressor Complex in β-YAC Transgenic Mice

Activation of gamma-globin gene expression in adults is known to be therapeutic for sickle cell disease. Thus, it follows that the converse, alleviation of repression, would be equally effective, since the net result would be the same: an increase in fetal hemoglobin. A GATA-1-FOG-1-Mi2 repressor complex was recently demonstrated to be recruited to the -566 GATA motif of the (A)gamma-globin gene. We show that Mi2 beta is essential for gamma-globin gene silencing using Mi2 beta conditional knockout beta-YAC transgenic mice. In addition, increased expression of (A)gamma-globin was detected in adult blood from beta-YAC transgenic mice containing a T>G HPFH point mutation at the -566 GATA silencer site. ChIP experiments demonstrated that GATA-1 is recruited to this silencer at day E16, followed by recruitment of FOG-1 and Mi2 at day E17 in wild-type beta-YAC transgenic mice. Recruitment of the GATA-1-mediated repressor complex was disrupted by the -566 HPFH mutation at developmental stages when it normally binds. Our data suggest that a temporal repression mechanism is operative in the silencing of gamma-globin gene expression and that either a trans-acting Mi2 beta knockout deletion mutation or the cis-acting -566 (A)gamma-globin HPFH point mutation disrupts establishment of repression, resulting in continued gamma-globin gene transcription during adult definitive erythropoiesis.