Activation of δ-globin gene expression by erythroid Krupple-like factor: A potential approach for gene therapy of sickle cell disease

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Hemoglobin A2 (HbA2; α2δ2) is a powerful inhibitor of HbS (α2β2/(S) polymerization. However, HbA2 levels are normally low in sickle cell patients. We show that a major reason for low δ-globin gene expression is the defective CACCC box at -90 in the δ-globin promoter. When the CACCC box defect in δ is corrected, expression of an HS2 δ/Luciferase reporter is equivalent to HS2 β/Luciferase. Erythroid Krupple-like factor (EKLF), which binds to the CACCC box of the β-globin gene and activates high-level expression, does not bind to the normal δ-globin promoter. Our goal is to design a modified EKLF that binds to the defective δ-globin promoter and enhances δ-globin gene expression. To test the feasibility of this strategy, we inserted the β-globin CACCC box at -90 of the δ-globin gene promoter to produce an HS2 δ(CAC)-β construct and quantitated human δ- and β-globin mRNA in stably transformed murine erythroleukemia (MEL) cells. δ-Globin mRNA in these cells was 22.0% ± 9.0% of total human globin mRNA (δ/δ + β) as compared with 3.0% ± 1.3% in the HS2 δ-β control. In a second set of experiments a GAL4 DNA-binding site was inserted at -90 of the δ-globin gene to produce an HS2 δ(GAL4)-β construct. This construct and a GAL4((1-147))/EKLF expression vector were stably transfected into MEL cells. δ-Globin mRNA in these cells was 27.8% ± 7.1% of total human globin mRNA as compared with 9.9% ± 2.5% in the HS2 δ(GAL4)-β plus GAL4((1-147)) control. These results show that δ-globin gene expression can be significantly increased by a modified EKLF. Based on these results, we suggest that modified EKLFs, which contain zinc fingers designed to bind specifically to the defective δ-globin CACCC box, may be useful in gene therapy approaches to increase HbA2 levels and inhibit HbS polymerization.

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