Fetal hemoglobin (Hb F) levels increase in most patients with sickle cell disease following intermittent butyrate therapy. Although the full effects of butyrate on Hb F levels usually require multiple treatment cycles, in some patients a peak level is achieved after a few days of butyrate therapy. Our investigation of the mechanism(s) responsible for this rapid induction of Hb F by butyrate showed that reticulocyte ␥-globin chain synthesis markedly increased within 24 hours of butyrate exposure, without concomitant changes in reticulocyte ␥-globin mRNA levels. This suggests that butyrate might induce Hb F by increasing the efficiency of translation of ␥-globin mRNA.

Butyrate is a prototype of histone deacetylase inhibitors that is believed to reactivate silent genes by inducing epigenetic modifications. Although butyrate was shown to induce fetal hemoglobin (HbF) production in patients with hemoglobin disorders, the mechanism of this induction has not been fully elucidated. Our studies of the epigenetic configuration of the ␤-globin cluster suggest that DNA methylation and histone H3 acetylation are important for the regulation of developmental stage-specific expression of the ␤-like globin genes, whereas acetylation of both histones H3 and H4 seem to be important for the regulation of tissuespecific expression. These studies suggest that DNA methylation may be important for the silencing of the ␤-like globin genes in nonerythroid hematopoietic cells but may not be necessary for their silencing in nonhematopoietic cells. Furthermore, our studies demonstrate that butyrate exposure results in a true reversal of the normal developmental switch from ␥-to ␤-globin expression. This is associated with increased histone acetylation and decreased DNA methylation of the ␥-globin genes, with opposite changes in the ␤-globin gene. These studies provide strong support for the role of epigenetic modifications in the normal developmental and tissue-specific regulation of globin gene expression and in the butyratemediated pharmacologic induction of HbF production.