HOT-SPOT MUTATIONS IN ADENOSINE-DEAMINASE DEFICIENCY

We have previously characterized mutant adenosine deaminase (ADA; adenosine aminohydrolase, EC 3.5.4.4) enzymes in seven children with partial ADA deficiency. Six children shared common origins, suggesting a common progenitor. However, we found evidence for multiple phenotypically different mutant enzymes. We hypothesized that many of the mutations would be at CpG dinucleotides, hot spots at which spontaneous deamination of 5-methylcytosine results in C to T or G to A transitions. Digestion of DNA from these children with Msp I and Taq I, enzymes recognizing CpG dinucleotides, identified three different mutations, each correlaging with expression of a different mutant enzyme. Sequencing of cDNA clones and genomic DNA amplified by polymerase chain reaction confirmed the presence of C to T or G to A transitions at CpG dinucleotides (C226 to T, G446 to A, and C821 to T, resulting in Arg76 to Trp, Arg149 to Gln, and Pro274 to Leu). A 'null' mutation, also found in two ADA-deficient severe combined immunodeficient children, was serendipitously detected as gain of a site for Msp I. Simultaneous loss of a site of Bal I defined the precise base substitution (T320 to C, Leu107 to Pro), confirmed by sequence analysis. To determine the true frequency of hot spot mutation in these children, consecutively ascertained through a newborn screening program, we sequenced cDNA from the remaining alleles. Two others were hot spot mutations (C631 to T and G643 to A, resulting in Arg211 to Cys and Ala215 to Thr), each again resulting in expression of a phenotypically different mutant enzyme. Only one additional mutation (previously identified by us) is not in a hot spot. These seven mutations account for all 14 chromosomes in these children. There is thus a very high frequency of hot spot mutations in partial ADA deficiency. Most of these children carry two different mutant alleles. We were able to correlate genotype and phenotype and to dissect the activity of individual mutant alleles.