Retroviral-mediated in vivo gene transfer into muscle cells and synthesis of human factor IX in mice

Muscle cells, when transduced with a factor IX (FIX) expression vector, are capable of producing FIX protein and secreting it into the blood. These cells can thus serve as appropriate targets for gene transfer in hemophilia B patients. In this work, we studied the potential of retrovirus-mediated in vivo gene transfer of the FIX gene into muscle cells. Producer cells, which produce recombinant retroviruses containing the human FIX cDNA under the transcriptional control of the chicken beta-actin promoter, were treated with the cytostatic drug mitomycin C (MC), and injected into the rectus femoris muscle of 6-week-old CB-17/1cr severe combined immune-deficient (SCID) mice. As a control, SCID mice were injected with NIH 3T3 cells, which were stably transformed by the retroviral vector described above and therefore secreted FIX, but did not produce retroviruses. The injected MC-treated cells did not cause any tumors to develop in the mice, and the amount of human FIX in the plasma of mice injected with the producer cells was significantly higher, at any of the time points measured beyond 10 days postinjection, than the amount of human FIX in the plasma of mice injected with FIX-secreting 3T3 cells. These results indicate that muscle cells transduced by retroviruses produced human FIX and secreted it into the blood. In vivo gene transfer to muscle cells by injection of retrovirus-producing cells, or by direct injection of recombinant retroviruses, could prove to be important for the gene therapy of hemophilia B.