Genetic expression in partial adenosine deaminase deficiency. mRNA levels and protein turnover for the enzyme variants in human B-lymphoblast cell lines.

A severe genetic deficiency of adenosine deaminase is causally associated with an autosomal recessive form of severe combined immunodeficiency disease, while subjects with absent erythrocyte but partial lymphocyte enzyme activity remain immunocompetent. The genetic expression of adenosine deaminase in B-lymphoblast cell lines derived from four unrelated subjects with the "partial" enzyme deficiency was examined. Enzymatic activity among these cell lines ranged from 5 to 50% of normal with the level of immunoreactive adenosine deaminase protein either proportional to enzyme activity or elevated in two of the cases. Northern blot analysis using a cDNA probe showed that adenosine deaminase mRNA in each of these cell lines was of normal expected size (1.6-1.8 kilobases) and was present in normal to above normal amounts. Rates of enzyme synthesis varied from 165 to 15% of normal. Adenosine deaminase protein degradation rates in these cell lines were 1.5 to almost 3 times faster than normal, consistent with the observed absence of the enzyme in erythrocytes. From these analyses apparent abnormalities in mRNA regulation, translation, and protein degradation can be identified among the partially adenosine deaminase-deficient cell lines studied. Ultimately, it will be essential to determine the nature of the protein mutation and the gene defect to define the structural alterations and functional abnormalities of enzyme variants isolated from subjects with partial adenosine deaminase deficiency.