Functional requirements for phenotypic correction of murine beta-thalassemia: implications for human gene therapy

As initial human gene therapy trials for beta -thalassemia are contemplated , 2 critical questions important to trial design and planning have emerged. First, what proportion of genetically corrected hematopoietic stem cells ( HSCs) will be needed to achieve a therapeutic benefit? Second, what level o f expression of a transferred globin gene will be required to improve beta -thalassemic erythropoiesis? These questions were directly addressed by mea ns of a murine model of severe beta -thalassemia. Generation of beta -thala ssemic mice chimeric for a minority proportion of genetically normal HSCs d emonstrated that normal HSC chimerism levels as low as 10% to 20% resulted in significant increases in hemoglobin (Hb) level and diminished extramedul lary erythropoiesis. A large majority of the peripheral red cells in these mice were derived from the small minority of normal HSCs. In a separate set of independent experiments, beta -thalassemic mice were bred with transgen ic mice that expressed different levels of human globins. Human gamma -glob in messenger RNA (mRNA) expression at 7% of the level of total endogenous a lpha -globin mRNA in thalassemic erythroid cells resulted in improved red c ell morphology, a greater than 2-g/dL increase in Hb, and diminished reticu locytosis and extramedullary erythropoiesis. Furthermore, gamma -globin mRN A expression at 13% resulted in a 3-g/dL increase in Hb and nearly complete correction of red cell morphology and other indices of inefficient erythro poiesis. These data indicate that a significant therapeutic benefit could b e achieved with expression of a transferred globin gene at about 15% of the level of total alpha -globin mRNA in patients with severe beta -thalassemi a in whom 20% of erythroid precursors express the vector genome. (Blood. 20 01;97:3275-3282) (C) 2001 by The American Society of Hematology.