Molecular basis for paradoxical carriers of adenosine deaminase (ADA) deficiency that show extremely low levels of ADA activity in peripheral blood cells without immunodeficiency

Adenosine deaminase (ADA) deficiency causes an autosomal recessive form of severe combined immunodeficiency and also less severe phenotypes, depending to a large degree on genotype, In general, ADA activity in cells of carrie rs: is approximately half-normal. Unexpectedly, healthy first-degree relati ves of two unrelated ADA-deficient severe combined immunodeficient patients (mother and brother in family I; mother in family II) had only 1-2% of nor mal ADA activity in PBMC, lower than has previously been found in PBMC of h ealthy individuals with so-called "partial ADA deficiency." The level of de oxyadenosine nucleotides in erythrocytes of these paradoxical carriers was slightly elevated, but much lower than levels found in immunodeficient pati ents with ADA deficiency. ADA activity in EBV-lymphoblastoid cell lines (LC L) and T cell lines established from these carriers was 10-20% of normal. E ach of these carriers possessed two mutated ADA alleles, Expression of clon ed mutant ADA cDNAs in an ADA-deletion strain of Escherichia coil indicated that the novel mutations G239S and M310T were responsible for the residual ADA activity. ADA activity in EBV-LCL extracts of the paradoxical carriers was much more labile than ADA from normal EBV-LCL. Immunoblotting suggeste d that this lability was due to denaturation rather than to degradation of the mutant protein. These results further define the threshold level of ADA activity necessary for sustaining immune function.