Full genetic rescue of adenosine deaminase-deficient mice through introduction of the human gene.

We have shown recently that adenosine deaminase (ADA)-deficient mice die perinatally with severe liver cell degeneration. In addition to enzyme substitution, we report the restoration of viability through introduction of the human ADA gene. The ADA gene is subject to complex developmental and tissue-specific regulation. To include the cis-regulatory elements necessary for correct regulation of the human ADA gene, a large transgenic locus constituting the human ADA gene with 10 kb of 5' and 4 kb of 3' flanking sequences was generated by co-injection of two overlapping DNA fragments into murine zygotes. Probably as a result of extrachromosomal (homologous) recombination between the fragments, one of the two transgenic lines contained a reconstituted, functional human ADA gene. As in man, human ADA expression generally was low in these transgenic mice, but high in the thymus, spleen and gastro-duodenal part of the gut. Apparently, all cis-regulatory elements essential for a human expression pattern were incorporated in the transgene and were functional in the murine background. Similarly to man, the upper alimentary tract of the transgenic mice revealed low human ADA activity in contrast to extremely high levels of murine ADA. The human gene probably lacks the cis-regulatory elements that target high level murine ADA expression to the murine upper alimentary tract. ADA-deficient mice rescued by introduction of the human ADA transgene appeared histologically and immunologically normal. Apparently, human ADA can complement murine ADA in all tissues, even in the epithelium of the upper alimentary tract where human ADA activity is as much as 70-fold lower than murine ADA activity in wild-type mice. Clearly, the lethal phenotype of ADA-deficient mice is due to the absence of ADA.