The degree of phenotypic correction of murine β-thalassemia intermedia following lentiviral-mediated transfer of a human γ-globin gene is influenced by chromosomal position effects and vector copy number

Abstract

Increased fetal hemoglobin (HbF) levels diminish the clinical severity of β-thalassemia and sickle cell anemia. A treatment strategy using autologous stem cell–targeted gene transfer of a γ-globin gene may therefore have therapeutic potential. We evaluated oncoretroviral- and lentiviral-based γ-globin vectors for expression in transduced erythroid cell lines. Compared with γ-globin, oncoretroviral vectors containing either a β-spectrin or β-globin promoter and the α-globin HS40 element, a γ-globin lentiviral vector utilizing the β-globin promoter and elements from the β-globin locus control region demonstrated a higher probability of expression. This lentiviral vector design was evaluated in lethally irradiated mice that received transplants of transduced bone marrow cells. Long-term, stable erythroid expression of human γ-globin was observed with levels of vector-encoded γ-globin mRNA ranging from 9% to 19% of total murine α-globin mRNA. The therapeutic efficacy of the vector was subsequently evaluated in a murine model of β-thalassemia intermedia. The majority of mice that underwent transplantation expressed significant levels of chimeric mα2hγ2molecules (termed HbF), the amount of which correlated with the degree of phenotypic improvement. A group of animals with a mean HbF level of 21% displayed a 2.5 g/dL (25 g/L) improvement in Hb concentration and normalization of erythrocyte morphology relative to control animals. γ-Globin expression and phenotypic improvement was variably lower in other animals due to differences in vector copy number and chromosomal position effects. These data establish the potential of using a γ-globin lentiviral vector for gene therapy of β-thalassemia.