Transcriptional regulation of fetal to adult hemoglobin switching: new therapeutic opportunities

Abstract

In humans, embryonic, fetal, and adult hemoglobins are sequentially expressed in developing erythroblasts during ontogeny. For the past 40 years, this process has been the subject of intensive study because of its value to enlighten the biology of developmental gene regulation and because fetal hemoglobin can significantly ameliorate the clinical manifestations of both sickle cell disease and ␤-thalassemia. Understanding the normal process of loss of fetal globin expression and activation of adult globin expression could potentially lead to new therapeutic approaches for these hemoglobin disorders. Herein, we briefly review the history of the study of hemoglobin switching and then focus on recent discoveries in the field that now make new therapeutic approaches seem feasible in the future. Erythroid-specific knockdown of fetal gene repressors or enforced expression of fetal gene activators may provide clinically applicable approaches for genetic treatment of hemoglobin disorders that would benefit from increased fetal hemoglobin levels. (Blood. 2011;117(15):3945-3953) IntroductionFetal hemoglobin (HbF, ␣ 2 ␥ 2 ) was first distinguished from adult hemoglobin (␣ 2 ␤ 2 ) by its resistance to alkaline denaturation by Korber 1 and von Kruger 2 almost 150 years ago. In 1934, Brinkman et al described experiments using alkaline denaturation, as measured by a spectrophotometer coupled to a photoelectric output, to differentiate fetal and adult hemoglobin by their dissimilar reaction kinetics. 3 The differences in susceptibility to alkaline denaturation, along with what was known about the consistency of the heme moiety between umbilical cord and adult blood, led the authors to conclude that the difference between the entities must be attributable to the "globin" portion of the molecule. 3 Now, nearly 80 years later, the field is making substantial progress in uncovering the molecular mechanisms that regulate the developmental expression of the fetal and adult globin genes. In the past several years, genetic approaches have helped identify 3 loci that may account for up to 50% of the variance in HbF expression in certain human populations. [4][5][6] In addition, experimentation facilitated by murine transgenic, knockout, and vector knockdown technology has contributed to the identification of other potentially important regulators of fetal and adult globin gene expression. 7,8 Herein, we will focus on the recent discoveries that have advanced the field. A synopsis of hemoglobin switching: 1970 to 2000In their classic paper, Brinkman et al noted that "unsolved is the question whether human hemoglobin is changing from one form to the other, or whether there are 2, originally different forms (eg, the resistant fetal type and the adult form)." 3 Much has been learned since that time, including the structure and sequence of both the ␣-and ␤-globin loci [9][10][11][12] (Figure 1), the transcriptional regulation of the genes contained within these loci during development, and the clinical diseases that result from genetic change...

Estimation of the difference in HbF expression due to loss of the 5' -globin BCL11A binding region

“…Our study, together with recent publications, demonstrate that its binding domain, located 5' to the dgene, is a major player in the complex network of factors involved in HbF expression. We found that the loss of this region is associated with an increase of 2.7 g/dL of HbF, which would be sufficient to compensate the Hb lack in bthal major patients 13 and would be sufficient to prevent sickle cell anemia complications. These findings would help to focus efforts towards defining new therapeutic strategies through BCL11A silencing or preventing its binding for HbF reactivation.…”

Development of gene therapy for blood disorders: an update

“…90,91 Efforts in this area were advanced with the discovery that BCL11A is a major regulator of developmentally stage-specific repression of the g-globin gene. 92 Genetic deletion of BCL11A has been used to correct sickle cell disease in adult mice by interference with fetal hemoglobin silencing.…”

EHMT1 and EHMT2 inhibition induces fetal hemoglobin expression

“…[9][10][11] The silencing of HbF in adults is also mediated by epigenetic changes, including DNA methylation and histone modifications, and several chromatin-modifying enzymes, including DNA methyltransferases, 12 HDACs, 8 PRMT5, 13 and LSD1, 12,14 have been shown to contribute to this process. However, it is likely that other epigenetic regulators remain to be identified.…”