The beta-thalassemias and sickle cell anemia are severe congenital anemias for which there is presently no curative therapy other than allogeneic bone marrow transplantation. This therapeutic option, however, is not available to most patients due to the lack of an HLA-matched bone marrow donor. Emerging modalities based on cell engineering offer new prospects for potentially curative approaches that are applicable to more patients. The first is based on the transfer of a regulated globin gene in autologous hematopoietic stem cells (HSCs). This strategy, simple in principle, raises major challenges in terms of controlling transgene expression, which ideally should be erythroid-specific, differentiation and stage-restricted, elevated, position-independent, and sustained over time. Following the original report by May et al., several groups have reported that lentiviral vectors encoding slightly different combinations of proximal and distal transcriptional control elements of the normal human ss-globin gene permit lineage-specific and elevated ss-globin expression in vivo, resulting in therapeutic hemoglobin production and correction of anemia in ss-thalassemic mice. Clinical studies utilizing the TNS.3 vector are likely to be initiated in the US in 2009. While the addition of the wild-type ss-globin gene is naturally suited for treating ss-thalassemia, several alternatives have been proposed for the treatment of sickle cell disease, using either gamma- or mutant ss-globin gene addition, trans-splicing or RNA interference. The recent discovery that adult somatic cells can be reprogrammed to become pluripotent stem cells from which HSCs can be derived, provides yet another venue for developing stem cell engineering using either lentiviral vectors or homologous recombination techniques. Altogether, these recent advances bode well for the advent of curative stem cell-based therapies.
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