Gene therapy for adenosine deaminase deficiency

In the last decade, gene transfer into hematopoietic cells has evolved from an experimental procedure which resulted in successfully transduced in vitro hematopoietic colonies to the first clinical trials in patients suffering from severe combined immunodeficiency disease caused by the absence of functional adenosine deaminase. Significant in vivo expression of the newly introduced gene encoding human adenosine deaminase has been observed in descendents of murine and rhesus monkey hematopoietic stem cells following retrovirus mediated gene transfer. So far, 10 patients have received genetically repaired T-cells, hematopoietic stem cells or both without the appearance of any side effect. The clinical bone marrow gene transfer studies differ largely from the monkey studies with respect to myeloablation, which was applied in the monkey studies, but not in the patient studies. Ongoing studies in patients show that the introduced gene is present in circulating blood cells. In the initial phase of the trial, the frequency of transduced circulating blood cells is lower than in rhesus monkey studies. This difference may be contributed to the fact that conditioning was not performed in the patients.