Basic defect in the expression of adenosine deaminase in ADA-SCID disease. II. Deficiency of ADA-CRM detected in heterozygote human-Chinese hamster cell hybrids

Adenosine deaminase (ADA) deficiency in cells derived from humans with severe combined immunodeficiency is due to an aberration of the ADA protein

In order to determine the molecular basis of adenosine deaminase (ADA) deficiency in cells derived from patients with severe combined immunodeficiency (SCID) disease, we used a human ADA cDNA clone (1) to analyse the organization and transcription of the ADA gene in both normal and ADA-SCID cells. In five lymphoblastoid ADA-SCID cell lines we could detect no deletions or rearrangements in the ADA gene and its flanking sequences. Furthermore, synthesis and processing of ADA mRNA appeared to be normal in the ADA-SCID cells, and ADA-specific mRNA from two ADA-SCID cells could be translated in vitro into a protein with the molecular weight of normal ADA; this protein, however, could hardly be precipitated with an ADA antiserum. The results indicate that in these two ADA-SCID cell lines, the lack of ADA activity is not due to transcriptional or translational defects, but to subtle changes in the configuration of the protein affecting both its enzymatic and immunological characteristics.

Isolation of cDNA clones for human adenosine deaminase

Clones encoding human adenosine deaminase (ADA) were isolated from a cDNA library made from the lymphoblastoid cell line MOLT-4. The isolation procedure was based on the selection of clones hybridizing with a radioactive probe complementary to an RNA preparation, which had been highly enriched in ADA-specific mRNA. The latter RNA preparation was obtained by size-fractionating MOLT-4 RNA and selecting fractions that were translatable into ADA. The assay for the presence of ADA in the in vitro translation products, was based on immunoprecipitation with a specific anti-ADA serum. The antiserum used was shown to precipitate a 42-kDal protein with the properties of ADA. Positive clones were further screened by means of hybrid-released in vitro translation assays. Two clones were obtained which were able to select mRNA that could be translated into a 42-kDal protein immunoprecipitable with the ADA-antiserum. By use of Southern blots containing DNA from somatic cell hybrids, one of these ADA cDNA clones was assigned to the human chromosome 20 known to contain the ADA gene.