The -566 ^Aγ-Globin HPFH Mutation Disrupts Temporal Repression of Fetal Hemoglobin Synthesis During Fetal Liver Definitive Erythropoiesis.

Abstract 2019

Poster Board I-1041

Hereditary persistence of fetal hemoglobin (HPFH) is a condition associated with continued fetal hemoglobin (HbF) production in adults, where normally only very low levels of HbF are found. Sickle cell disease (SCD) patients are phenotypically normal if they carry a compensatory HPFH mutation due to the high levels of HbF. Understanding the molecular mechanisms leading to reactivation or derepression of γ-globin gene expression will lead to the development of new or better therapies to treat SCD patients. In our long-established and highly-characterized model system, transgenic mice carrying wild-type human β-globin locus yeast artificial chromosomes (β-YACs) express predominantly γ-globin and a lesser amount of γ-globin in the primitive erythroid cells of the yolk sac, mostly β-globin and some γ-globin in the definitive erythroid cells of the fetal liver and nearly exclusively β-globin in the adult definitive red blood cells, as measured both at the transcript and protein levels. We recently identified a novel ^Aγ-globin gene silencer motif located at -566 relative to the mRNA CAP site in a GATA motif. Repression is mediated by binding a GATA-1-FOG-1-Mi2 protein complex. Since our initial studies of this GATA-1 repressor complex were performed using β-YAC transgenic mice in which a second copy of the ^Aγ-globin gene was introduced between the locus control region (LCR) and the γ-globin gene, our first goal was to test if this mutation was functional at the normally-located ^Aγ-globin globin gene. β-YAC transgenic mice were produced with the T>G HPFH point mutation at the -566 GATA site of this gene. These mice display a mild HPFH phenotype during adult definitive erythropoiesis; γ-globin gene expression levels were increased approximately 3% compared to wild-type β-YAC mice. Expression of γ-globin is also elevated relative to wild-type β-YAC controls during primitive erythropoiesis in the embryonic yolk sac and definitive erythropoiesis in the fetal liver. Chromatin immunoprecipitation (ChIP) experiments using day E12 to E18 post-conception fetal liver samples from wild type β-YAC transgenic mice demonstrate that GATA-1 is recruited to this GATA silencer first at day E16, followed by recruitment of FOG-1 and Mi2 at day E17. In addition, ChIP experiments performed with day E18 samples from the -566 HPFH mice demonstrate that this point mutation disrupts the recruitment of GATA-1 to this site at a developmental stage when it normally binds as a repressor in wild-type β-YAC transgenic samples. GATA-2 does not bind at the -566 GATA motif when γ-globin is actively transcribed. Thus, GATA-2/GATA-1 competition does not play a role in the function of this silencer or the mechanism of HPFH at this site. In addition, BCL11A does not appear to be a component of this GATA-1 repressor complex. Taken together our data indicate that a temporal repression mechanism is operative in the silencing of γ-globin gene expression and that the presence of the -566 ^Aγ-globin HPFH mutation disrupts establishment of repression, resulting in continued γ-globin gene transcription during adult definitive erythropoiesis.