Bone marrow gene therapy for adenosine deaminase deficiency

Deficiency of adenosine deaminase (ADA) results in severe combined immunodeficiency disease (SCID). The cause for this is believed to be the accumulation of one of the substrates for ADA, 2'-deoxyadenosine to which especially T cells are hypersensitive. This disease can be treated successfully with bone marrow transplantation if a suitable donor is available. Alternatively, the human ADA gene could be introduced into the autologous bone marrow. We have generated a retroviral vector containing the human ADA gene. With this vector we were able to restore human ADA-activity in ADA-SCID T cells to normal levels resulting in a sensitivity to 2'-deoxyadenosine that is also found for T cells from a healthy donor. In murine studies we have shown that our retrovirus can infect pluripotent hemopoietic stem cells resulting in long-term (> 6 months) expression of human ADA in the hemopoietic system of transplanted animals. These results were confirmed in rhesus monkeys where we were able to detect the provirus in both peripheral blood mononuclear cells and granulocytes for as long as the animals were analyzed, i.e. up to more than 1 year post bone marrow transplantation. On the basis of these results we have proposed a clinical protocol for the treatment of ADA-SCID patients with bone marrow gene therapy.