Transcriptional silencing of fetal hemoglobin expression by NonO

Human fetal globin (gamma-globin) genes are developmentally silenced after birth, and reactivation of gamma-globin expression in adulthood ameliorates symptoms of hemoglobin disorders, such as sickle cell disease (SCD) and beta-thalassemia. However, the mechanisms by which gamma-globin expression is precisely regulated are still incompletely understood. Here, we found that NonO (non-POU domain-containing octamer-binding protein) interacted directly with SOX6, and repressed the expression of gamma-globin gene in human erythroid cells. We showed that NonO bound to the octamer binding motif, ATGCAAAT, of the gamma-globin proximal promoter, resulting in inhibition of gamma-globin transcription. Depletion of NonO resulted in significant activation of gamma-globin expression in K562, HUDEP-2, and primary human erythroid progenitor cells. To confirm the role of NonO in vivo, we further generated a conditional knockout of NonO by using IFN-inducible Mx1-Cre transgenic mice. We found that induced NonO deletion reactivated murine embryonic globin and human gamma-globin gene expression in adult beta-YAC mice, suggesting a conserved role for NonO during mammalian evolution. Thus, our data indicate that NonO acts as a novel transcriptional repressor of gamma-globin gene expression through direct promoter binding, and is essential for gamma-globin gene silencing.