Gene therapy for chronic myelogenous leukemia (CML): A retroviral vector that renders hematopoietic progenitors methotrexate-resistant and CML progenitors functionally normal and nontumorigenic in vivo

Chronic myelogenous leukemia (CML) is a malignant disease of the human hematopoietic stem cell caused by the BCR/ABL gene rearrangement. The only curative therapy is allogeneic transplantation. Although autologous transplants may prolong survival, most patients relapse because of disease persisting in the host and in the graft. Continued administration of chemotherapy after transplant could reduce the incidence of relapse provided that the autograft can be protected by transfer of a drug-resistance gene. However, CML autografts will almost certainly contain malignant stem cells that will also be rendered drug-resistant. The presence of the BCR/ABL oncoprotein is necessary and sufficient for malignant transformation seen in CML. We thus hypothesized that transfer of a vector that combines a drug- resistance gene with anti-BCR/ABL antisense (AS) sequences may allow for posttransplant chemotherapy to decrease persistent disease while rendering inadvertently transduced CML stem and progenitor cells functionally normal. We constructed a retroviral vector, LasBD, that combines the methotrexate (MTX)-resistant tyrosine-22 dihydrofolate-reductase (tyr22-DHFR) gene and AS sequences directed at the b3a2 BCR/ABL breakpoint. b3a2 BCR/ABL containing 32D and MO7s cells were transduced with LasBD and selected in MTX for 14 days. Expression of the AS sequences reduced BCR/ABL mRNA and p210(scr/abl) protein levels by 6- to 10-fold in most cells. This subsequently led to the restoration of normal function of BCR/ABL cDNA⁺cells: they grew significantly slower in the presence of interleukin-3 (IL-3); they underwent apoptotic cell death when cultured without Il-3; and they had restored expression and function of adhesion receptors. These effects p190(scr/abl)- containing cells. LasBD also rendered 20% to 30% of primary Ph^- and Ph⁺ CD34⁺ cells MTX-resistant and decreased BCR/ABL MRNA levels in MTX resistant Ph⁺CD34⁺ cells by 10-fold. Expression of the MTX-resistant DHFR gene and the AS sequences has been stable for at least 1 year in vitro and for more than 70 days in vivo. Finally, LasBD decreased tumorigenicity of 32D(scr/abl) cells in vivo by 3 to 4 logs. In conclusion, the tyr22-DHFR gene in the LasBD vector can protect normal hematopoietic cells from MTX-mediated toxicity, whereas the AS sequences in LasBD can suppress expression of the BCR/ABL gene and restore normal function of BCR/ABL cDNA-containing cells. The LasBD vector may therefore prove to be an extremely useful adjunct in autologous transplantation for CML.