Editing a γ-Globin Repressor Binding Site Restores Fetal Hemoglobin Synthesis and Corrects the Phenotype of Sickle Cell Disease Erythrocytes

Sickle cell disease (SCD) is a severe, life-threatening disorder caused by a single amino acid change (β6Glu→Val) in the adult hemoglobin (Hb) β-chain that causes Hb polymerization with consequent red blood cell (RBC) rigidity, anemia, vaso-occlusive crises, organ damage and reduced life expectancy. The co-inheritance of genetic mutations causing a sustained fetal γ-globin chain production in adult life (hereditary persistence of fetal hemoglobin, HPFH) decreases sickling and significantly reduces the clinical severity of SCD. HPFH mutations cluster at several loci in the promoters of the two γ-globin genes, HBG1 and HBG2, and, disrupt binding sites for transcriptional repressors (e.g., BCL11A and LRF), leading to elevated levels of fetal hemoglobin (HbF) representing up to 40% of total hemoglobin tetramers. In addition, SNPs at position -158 of both γ-globin promoters are associated with enhanced γ-globin expression and may identify a putative transcriptional repressor binding site.