Gene Therapy for beta-thalassemia.

Gene transfer for beta-thalassemia requires gene transfer into hematopoietic stem cells using integrating vectors that direct regulated expression of beta globin at therapeutic levels. Among integrating vectors, oncoretroviral vectors carrying the human beta-globin gene and portions of the locus control region (LCR) have suffered from problems of vector instability, low titers and variable expression. In recent studies, human immunodeficiency virus-based lentiviral (LV) vectors were shown to stably transmit the human beta-globin gene and a large LCR element, resulting in correction of beta-thalassemia intermedia in mice. Several groups have since demonstrated correction of the mouse thalassemia intermedia phenotype, with variable levels of beta-globin expression. These levels of expression were insufficient to fully correct the anemia in thalassemia major mouse model. Insertion of a chicken hypersensitive site-4 chicken insulator element (cHS4) in self-inactivating (SIN) LV vectors resulted in higher and less variable expression of human beta-globin, similar to the observations with cHS4-containing retroviral vectors carrying the human gamma-globin gene. The levels of beta-globin expression achieved from insulated SIN-LV vectors were sufficient to phenotypically correct the thalassemia phenotype from 4 patients with human thalassemia major in vitro, and this correction persisted long term for up to 4 months, in xeno-transplanted mice in vivo. In summary, LV vectors have paved the way for clinical gene therapy trials for Cooley's anemia and other beta-globin disorders. SIN-LV vectors address several safety concerns of randomly integrating viral vectors by removing viral transcriptional elements and providing lineage-restricted expression. Flanking the proviral cassette with chromatin insulator elements, which additionally have enhancer-blocking properties, may further improve SIN-LV vector safety.