A single-base-pair substitution abolishes D-amino-acid oxidase activity in the mouse

Mutant ddY/DAO- mice lacking D-amino-acid oxidase (DAO) activity were examined for the cause of their lack of enzyme activity. Total RNA was extracted from the kidney of the ddY/DAO- mice and cDNA was synthesized. After cDNA encoding DAO was amplified by the polymerase chain reaction it was cloned into a plasmid and sequenced. Comparison of the DAO cDNA sequence with that of normal BALB/c mice revealed the presence of a single-base substitution (G----A) which causes a Gly-181----Arg substitution in the middle of the enzyme molecule. The mutant DAO cDNA was inserted into an expression vector and was expressed in transfected COS-1 cells. The transfected cells synthesized the DAO mutant protein, but they did not show DAO activity. In contrast, when cells were transfected with an expression vector carrying wild-type DAO cDNA, where the substituted base-pair was replaced by a normal base-pair, they showed DAO activity. These results indicate that the single base-pair substitution is the cause of the loss of DAO activity in the ddY/DAO- mice.