Locus control region elements HS1 and HS4 enhance the therapeutic efficacy of globin gene transfer in β-thalassemic mice

Globin gene transfer in autologous hematopoietic stem cells is a promising therapeutic option for subjects with beta-thalassemia major. In this approach, high level, erythroid-specific globin transgene expression should correct ineffective erythropoiesis and hemolytic anemia following the delivery of only 1 to 2 vector copies per cell. The generation of vectors that provide high-level globin expression and require low vector copy (VC) integration is therefore essential for both safety and efficacy. We show here the major roles played by 2 lesser-known locus control region elements, termed HS1 and HS4. Partial deletions within HS4 markedly reduce in vivo globin expression requiring multiple VC per cell to correct the anemia. Most strikingly, addition of HS1 to HS2-3-4 increases globin expression by 52%, yielding 9 g Hb/VC in beta-thalassemic mice. Thus, while vectors encoding HS2-3-4 provide curative levels of hemoglobin at 1 to 2 copies per cell, adding HS1 is a promising alternative strategy if upcoming clinical trials prove higher levels of expression to be necessary.