Identification of a Point Mutation Resulting in a Heat-labile Adenosine Deaminase (ADA) in Two Unrelated Children with Partial ADA Deficiency

We have determined the mutation in a child with partial adenosine deaminase (ADA) deficiency who is phenotypically homozygous for a mutant ADA gene encoding a heat-labile enzyme (Am. J. Hum. Genet. 38: 13-25). Sequencing of cDNA demonstrated a C to A transversion that results in the replacement of a proline by a glutamine residue at codon 297. As this mutation generated a new recognition site in exon 10 of genomic DNA for the enzyme Alu I, Southern blot analysis was used to establish that this child was indeed homozygous for the mutation. The abnormal restriction fragment generated by this mutation was also found in a second partially ADA-deficient patient who phenotypically is a genetic compound and also expresses a heat-labile ADA (in addition to a more acidic than normal ADA) (Am. J. Hum. Genet. 38: 13-25). Sequencing of cDNA clones from the second patient established the identical codon 297 mutation. Transfection of the mutant cDNA into heterologous cells resulted in expression of a heat-labile ADA of normal electrophoretic mobility and isoelectric point, properties exhibited by the ADA in the patients' cells.