Decrease of α-Chains in β-Thalassemia

In the pathophysiology of β-thalassemia, globin chain imbalance plays a central role in predicting red blood cell (RBC) life span and disease severity. Strategies to improve globin chain imbalance are therefore a legitimate target in the management of this incurable genetic disorder. Classical gene addition with the available retroviral vectors can alter one of the two variables while combined reduction of achains could provide a more potent therapeutic effect. We developed foamy virus (FV) vectors for the production of β-globin and vectors targeting the a-globin transcript using the shRNA technology. Using FVderived vectors, we expressed human anti-a-globin short hairpin RNAs, off a potent PolIII promoter (H1); of the 4 different shRNAs tested, α-globin mRNA reduction varied from 6.3 to 54% of the control CD34+ cells. Similarly, vectors developed for the mouse α-globin, resulted in a significant reduction (range 15-28% of the control) of aglobin in erythroid colonies of Lin- cells. To assay vector performance in vivo, we investigated the hematological parameters in thal3+/- mice transpalnted with FV-transduced Lin- cells, transduced with anti-alpha-globin shRNA. Despite low chimerism and low vector copy numbers (<0.5 per cell), we observed a 10% reduction in red cell distribution width, a marker for distorted erythropoiesis. We finally developed a combination FV vector expressing β-globin off a HS40 enhancer and anti-α-globin shRNA and tested its performance in human CD34+ cells from a thalassemic patient. Globin chain imbalance was ameliorated from a β/α ratio of 0.12 to the level of 0.5, clearly indicating a therapeutic benefit. Overall, shRNA control of α-globin excess is a feasible target but requires improvements since the RNAi effect is tough to predict and should ideally be combined with controllable elements.