AN ASP8ASN SUBSTITUTION RESULTS IN THE ADENOSINE-DEAMINASE (ADA) GENETIC-POLYMORPHISM (ADA-2 ALLOZYME) - OCCURRENCE ON DIFFERENT CHROMOSOMAL BACKGROUNDS AND APPARENT INTRAGENIC CROSSOVER

We have now determined the molecular genetic basis for the common bioc hemical polymorphism at the adenosine deaminase (ADA) locus. The ADA2 allele contains a G to A transition at nt22 (relative to the ATG) tha t results in substitution of asparagine for aspartic acid at codon 8 ( Asp8Asn). Introduction of the nucleotide substitution into an ADA 1. c DNA and transfection into monkey kidney (Cos) cells confirmed that the mutation resulted in expression of an enzyme that comigrated with the naturally occurring ADA 2 allozyme. The substitution of neutral aspar agine for anionic aspartic acid is consistent with the more cathodal e lectrophoretic migration of ADA 2 as compared with ADA 1. The nucleoti de substitution was found on at least two different genetic background s, suggesting independent recurrence of the mutation. Consistent with independent recurrence, the G to A transition is at a CpG dinucleotide and represents a type of mutation that occurs with high frequency. We have also unexpectedly identified a probable intragenic crossover in the very large first intron that is rich in repetitive DNA sequences.