FULL GENETIC RESCUE OF ADENOSINE DEAMINASE-DEFICIENT MICE THROUGH INTRODUCTION OF THE HUMAN GENE

We have shown recently that adenosine deaminase (ADA)-deficient mice d ie perinatally with severe liver cell degeneration. In addition to enz yme substitution, we report the restoration of viability through intro duction of the human ADA gene, The ADA gene is subject to complex deve lopmental and tissue-specific regulation, To include the cis-regulator y elements necessary for correct regulation of the human ADA gene, a l arge 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 tw o 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 t hese transgenic mice, but high in the thymus, spleen and gastro-duoden al 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 activit y in contrast to extremely high levels of murine ADA, The human gene p robably lacks the cis-regulatory elements that target high level murin e ADA expression to the murine upper alimentary tract, ADA-deficient m ice rescued by introduction of the human ADA transgene appeared histol ogically and immunologically normal. Apparently human ADA can compleme nt murine ADA in all tissues, even in the epithelium of the upper alim entary 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.