An adenosine deaminase (ADA) allele contains two newly identified deleterious mutations (Y97C and L106V) that interact to abolish enzyme activity

Genetic deficiency of the purine salvage enzyme adenosine deaminase (ADA)results in varying degrees of immunodeficiency, ranging from neonatal onsetSevere Combined Immunodeficiency (SCID) to an adult onset immunodeficiencydisorder. Multiple different mutations have now been identified in theseimmunodeficient patients. Additional mutations, initially identified inhealthy individuals, abolish ADA in erythrocytes but retain 10-80% ofactivity in non-erythroid cells ('partial deficiency mutations'). Ingeneral, severity of disease correlates inversely with the amount ofresidual ADA expressed by the mutant enzymes and directly with theaccumulation of the toxic metabolites deoxyATP and deoxyadenosine. Wereport two newly identified mutations (Y97C and L106V), both carried on thesame allele of an immunodeficient patient who was diagnosed prenatally andsuccessfully transplanted with haploidentical bone marrow. Based on theability of mutant cDNAs to express ADA in vitro , the L106V mutationresulted in activity similar to 'partial' mutations (30% of normal) whilethe Y97C mutation resulted in detectable but markedly reduced activity(1.5% of normal). However, the presence of both mutations on the sameallele virtually abolished detectable enzyme activity. Analysis of thecrystallographic structure of ADA to understand the marked deleteriouseffect of the Y97C mutation suggested a previously unappreciated role ofsalt bridges in the catalytic mechanism of ADA. The patient was alsoheteroallelic for a previously described deletion of the promoter and exon1. Testing of additional patients in whom we had not identified a mutationon the second allele revealed presence of this deletion in three of fourpatients tested. This deletion is therefore relatively common, accountingfor 10% of almost 100 chromosomes studied by this and other laboratories,but is easily missed by currently used methods of mutation detection.Lastly, the finding of two mutations on the same allele that interact toreduce residual enzyme function emphasizes hazards in evaluating potentialgenotype-phenotype correlations in individuals analyzed only for thepresence of single specific mutations.