A novel mutation of delta-aminolaevulinate dehydratase in a healthy child with 12% erythrocyte enzyme activity.

Cloning, expression and phenotype studies of the defective gene for delta-aminolaevulinate dehydratase (ALAD) in a family with an asymptomatic girl who had ALAD deficiency were carried out. The proband was identified by neonatal ALAD screening, and had erythrocyte ALAD activity at 12% of the normal control. She was heterozygous for ALAD deficiency, which was inherited from her father. Nucleotide sequence analysis of the cloned ALAD cDNA revealed C36 to G and T168 to C mutations on the same allele. The former mutation resulted in F12L substitution, whereas the latter was a silent mutation. All family members who had decreased ALAD activity had the same mutation. Expression of the mutant ALAD cDNA in Chinese hamster ovary cells produced an ALAD protein without significant enzyme activity. Additionally, the mutant ALAD cDNA which encodes F12L substitution produced an aberrant migration pattern in polyacrylamide gel electrophoresis under denaturing conditions. This finding probably reflects an abnormal folding of the F12L protein, since the mutation occurred in the alpha1 helix of the N-terminal arm of the enzyme, which is involved in the extensive quaternary interactions among the subunits. This is also the first report of ALAD gene mutation in an asymptomatic subject.