Hybrid AAV6/2-Mediated Human β-Globin Expression in β-Thalassemia Patient Hematopoietic Stem Cells Transplanted Into Irradiated BALB/c Nude Mouse

Abstract 4715

β -Thalassemia is one of the most common worldwide monogenic human diseases,caused by molecular defects in the human β -globin gene cluster leading to decrease or absence of β-globin. Loss of β -globin chains causes ineffective production of oxygen-carrying hemoglobin and therefore results in severe anemia. The treatment for β -Thalassemia major usually includes lifelong blood transfusions but chronic blood transfusion often causes iron overload, and accumulated iron produces tissue damage in multiple organs, so that iron chelating treatment is also needed. Bone marrow transplantation is another effective therapy, which can eliminate a patient's dependence on blood transfusions, however, it is difficult to find a matching donor for most patients; therefore it is only available for a minority of patients.

Gene therapy is one potential novel therapy for treatment of inherited monogenic disorders. The long–term therapeutic strategy for this disease is to replace the defective β-globin gene via introduction of a functional gene into hematopoietic stem cells (HSCs). Adeno-associated virus type 2 (AAV), a nonpathogenic human parvovirus, has gained attention as a potentially useful vector for human gene therapy. AAV can infect both dividing and non-dividing cells and wild AAV integrates preferentially at a specific site on human chromosome 19. In the absence of helper virus, recombinant AAV will stably integrate into the host cell genome, mediating long-term and stable expression of the transgene.

In this study, we used a hybrid rAAV6/2 vector carrying the human β-globin gene to transduce HSCs from a β -Thalassemia patient, followed by transplantation into irradiated BALB/c nude mice. One month post-transplantation, Hb was prepared from peripheral blood and analyzed by Western Blot and HPLC respectively. RNA and DNA were isolated from bone marrow cells (BMCs) from recipient mice transplanted with mock-infected or hybrid rAAV–globin-infected cells and analyzed by RT-PCR and PCR respectively. The results showed: 1. Human β-actin and β-globin transcripts were detected by RT-PCR in BMCs from all recipient mice, indicating that human HSCs were successfully transplanted in these mice and that the human β-globin gene was transcriptionally active in the donor cells. 2. The level of human hemoglobin expressed in peripheral red blood cells of recipient mice as measured by HPLC (ratio of β/α) was increased to 0.3 from 0.05 of pre-transplantation levels. Expression of human β-globin was also confirmed in recipient mice by Western Blot; a 2–3-fold increase compared with that of controls.

Our results indicate that human HSCs from a β-Thalassemia patient can be efficiently transduced by a hybrid rAAV6/2-β-globin vector followed by expression of normal human β-globin protein. This study provides a proof-of-concept that rAAV6/2-mediated gene transfer into human HSCs might be a potential approach for gene therapy of β-Thalassemia.