In Vivo Silencing of the Human γ-Globin Gene in Murine Erythroid Cells Following Retroviral Transduction

Increased expression of fetal hemoglobin can ameliorate the clinical severity of sickle cell disease. Whereas temporary induction of fetal hemoglobin can be achieved by pharmacologic therapy, gene transfer resulting in high-level expression of the fetal γ-globin gene may provide a permanent cure for sickle cell disease. We had previously developed a high-titer, genetically stable retroviral vector in which the human γ-globin gene was linked to HS-40, the major regulatory element of the human α-globin gene cluster. Based on experience in transgenic mice, the truncated promoter of the γ-globin gene of this vector should be active in adult erythroid cells. Our earlier studies demonstrated that this retroviral vector can give rise to high-level expression of the human γ-globin gene in murine erythroleukemia (MEL) cells. We have now utilized this vector to transduce murine bone marrow cells that were transplanted into W/Wv recipient mice. Analysis of transduction of murine BFU-eʹs in vitro and peripheral blood cells from transplanted mice in vivo demonstrated efficient transfer of the human γ-globin gene. However, in contrast to the high level of expression of the human γ-globin gene of this vector in MEL cells, the gene was completely silent in vivo in all transplanted mice. These observations confirm that all the necessary regulatory elements responsible for the developmental stage-specific expression of the human γ-globin gene reside in its proximal sequences. They also emphasize the differences between gene regulation in MEL cells, transgenic mice, and retroviral gene transfer vectors. For this form of globin gene therapy to succeed, the proximal regulatory elements of the human γ-globin gene may have to be replaced with different regulatory elements that allow the expression of the γ-globin coding sequences in adult red cells in vivo.