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haematologica | 2019; 104(12)
from experimental results showing a correlation between high levels of g-globin expression and the lack of DNA
methylation in the 5’ g-globin promoter region. Subsequently, high levels of HbF were observed following treatment of baboons with 5-azacytidine (5-aza), an inhibitor of DNA methylation.9 The Ginder laboratory, working in the chicken system, showed that inhibitors of two different epigenetic-modifying enzymes (DNMT1 and HDAC) in combination increased the expression of develop- mentally silenced globin genes.10 Numerous clinical studies have now confirmed the ability of pharmacological DNMT1 inhibitors, (5-aza and decitabine) to increase HbF in patients with β-thalassemia and SCD.3 In recent years great progress has been made to increase our understanding of the mecha- nism responsible for developmental g-globin silencing by the discovery of three trans-acting, site-specific DNA binding proteins (BCL11A, TR2/TR4, and ZBTB7A) that recognize and bind to specific sequences within the g-globin promot- er.11,12 Critical to the repressive activity of these proteins is their ability to recruit multiprotein co-repressors containing epigenetic-modifying enzymes (DNMT1, HDAC, LSD1, G9A) whose activities directly establish the repressive chro- matin environment silencing g-globin expression.13,14 Pharmacological inhibitors of these enzymes increase g-glo- bin expression in various cell culture, mouse, and non- human primate model systems, often to impressive levels that would be predicted to provide therapeutic benefits to SCD and β-thalassemia patients.3-5 The major issue hindering use of these drugs in patients are dose-limiting hematologic side-effects that include neutropenia, thrombocytopenia, or thrombophilia.
The methylated DNA binding protein family includes the founding member MeCP2 and at least six additional proteins (MBD1-6) identified by homology searches. MeCP2, MBD1, MBD2, and MBD3 each contain a methylated DNA binding domain (MBD) that binds specifically to methylated CpG residues in vitro.15 MBD2 and MBD3 are closely related and are >80% homologous outside the MBD.16 MBD2 and MBD3 each contain a C terminal coiled-coiled (CC) domain that mediates protein-protein interactions. Additional domains present in MBD2 but absent in MBD3 include an N terminal glycine-arginine (GR) rich domain, a transcriptional repressor (TRD) domain that overlaps with the MBD and is essential for interaction with the NURD complex, and an intrinsically disordered region (IDR) important for binding to methylated DNA and recruitment of the NURD co-repressor protein complex. Isoforms of MBD2 lacking one or more of these domains have been identified.
The Ginder laboratory previously identified MBD2 as a repressor of g-globin expression through experiments in MBD2-/- human β-globin YAC mice17 and isolated MBD2 as a component of a purified multi-protein complex that bound the methylated and developmentally silenced σ-globin gene
EDITORIALS
A new target for fetal hemoglobin reactivation
Angela Rivers,1,2 Robert Molokie,2,3 and Donald Lavelle2,3
1Department of Pediatrics, University of Illinois at Chicago; 2Jesse Brown VA Medical Center and 3Department of Medicine, University of Illinois at Chicago, Chicago, IL, USA
E-mail: DONALD LAVELLE - dlavelle@uic.edu doi:10.3324/haematol.2019.230904
Inherited hemoglobinopathies, including sickle cell dis- ease (SCD) and the thalassemias, are the most common human monogenic diseases and represent a highly signif- icant global health problem.1,2 SCD drastically impacts the quality of life and reduces the life span of approximately 100,000 patients in the US and millions worldwide. Over 300,000 individuals are born with the disease each year with the vast majority in the developing nations of sub Saharan Africa where most of them fail to reach their fifth birthday. Hematopoietic stem cell (HSC) transplantation can be cura- tive, but over 80% of patients lack a suitable donor. Gene therapy and gene editing technologies also offer the possibil- ity of a cure, but require myeloablative drug-conditioning regimens for successful transplantation of the edited autolo- gous HSC population. The technological and hospital infra- structure required to implement these advanced methods are beyond the resources available in the locations where most patients reside. The only currently available treatment option for patients with β-thalassemia major is chronic transfusion therapy. Elevated levels of fetal hemoglobin (HbF; a2g2) reduce the severity of symptoms and lengthen the life span of SCD patients by inhibiting deoxy HbS poly- merization, while in β-thalassemia patients, increased HbF alleviates the lack of β-globin production. Hydroxyurea (HU), a drug approved by the US Food and Drug Administration (FDA) that can increase HbF in SCD, is not effective in a large subset of patients and, importantly, the increased HbF is heterogeneously distributed within the ery- throcyte population resulting in a large fraction of erythro- cytes lacking protective levels. Effective treatment of the large numbers of patients projected worldwide in the com- ing years would be best accomplished with an affordable, easily-administered, orally-available drug designed to achieve effective increases in HbF levels. A logical approach to increase HbF for therapy of the hemoglobinopathies is to intervene with the epigenetic repression mechanism that executes the switch from HbF to adult hemoglobin (HbA; a2g2).3-5 In this issue, the Ginder laboratory has identified a specific co-repressor, MBD2-NURD, that is responsible for silencing g-globin expression in adult erythroid cells and has delineated critical amino acid residues within the MBD2 protein that recruit the co-repressor containing the epigenet- ic-modifying enzymes that mediate silencing.6 The identifi- cation of these sites of recruitment should allow the identi- fication and development of new drugs that interfere with these interactions to alleviate gene repression and increase g- globin expression in adult erythroid cells and that, due to the mild phenotype of MBD2-/- mice,7 would be expected to have acceptable side-effects in patients.
The switch from HbF to HbA expression occurs in late gestation and involves the acquisition of repressive epigenet- ic marks at the g-globin promoter. The first evidence that an epigenetic mechanism might be involved in this switch arose
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