Long-Term Expression of Human Adenosine Deaminase in Mice after Transplantation of Bone Marrow Infected with Amphotropic Retroviral Vectors

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Abstract
Three retroviral vectors, containing a human adenosine deaminase (ADA) cDNA linked to either the simian virus 40 (SV40) early promoter, the human cytomegalovirus (CMV) immediate early promoter, or the Moloney murine leukemia virus (MoMLV) promoter, were tested for their ability to express ADA following infection and transplantation of murine bone marrow. Virus was produced by using PA317 amphotropic retrovirus packaging cells. The titer of each of the vectors was similar and no helper virus was detected. Human ADA was expressed in the blood of some animals for 6 months after transplantation of infected marrow, and vector DNA was found in the spleen and in bone marrow from these animals. The percentage of animals expressing human ADA (33%) and the amount of human ADA in blood (1–5% of total ADA) was similar for each of the vectors. These results show that amphotropic vectors are capable of infecting pluripotent hematopoietic stem cells having long-term repopulating ability, and that a variety of promoters allow gene expression following differentiation of these early cells. The first genetic disease treated by human gene therapy will probably be severe combined immunodeficiency caused by deficiency of the enzyme for adenosine deaminase (ADA). Osborne et al. demonstrate that retroviral-mediated gene transfer using a murine amphotropic vector can be used to insert a human ADA gene into pluripotent stem cells of mouse bone marrow. Circulating blood cells of the treated mice were still expressing human ADA 6 months after transfer.