Retrovirus-mediated transfer of a hygromycin phosphotransferase-thymidine kinase fusion gene into human CD34⁺ bone marrow cells

Retrovirus-mediated gene transfer into human hematopoietic stem cells has been proposed as a means of therapy for various inherited diseases and as a method of gene marking. The transduction efficiency of an amphotropic retroviral vector (PA317/HyTK) containing a hygromycin phosphotransferase-thymidine kinase fusion gene was examined with human CD34⁺ bone marrow cells in the presence of interleukin-3 (IL-3), interleukin-6 (IL-6), and stem cell factor. Transduction efficiencies determined from the ability of transduced granulocyte-macrophage colony forming units (CFU-GM) to grow in hygromycin B and from polymerase chain reaction analysis of individual transduced CFU-GM growing in the presence of hygromycin B were 0.3-3.0% (mean ± S.D., 1.1 ± 0.9%) and 0.1-1.2% (mean ± S.D., 0.5 ± 0.4%), respectively. Ganciclovir at a dose of ∼ 1 μM reduced the number of CFU-GM derived from vector-infected CD34⁺ cells by 50%. These findings demonstrate that human hematopoietic stem cells infected with this retroviral vector are susceptible to ganciclovir, offering the potential to control transduced gene expression in vivo.