MOLECULAR-GENETIC CHARACTERIZATION AND URINARY-EXCRETION PATTERN OF METABOLITES IN 2 FAMILIES WITH MCAD DEFICIENCY DUE TO COMPOUND HETEROZYGOSITY WITH A 13 BASE-PAIR INSERTION IN ONE ALLELE

Two families with medium-chain acyl-CoA dehydrogenase (MCAD) deficienc y due to compound heterozygosity are described. All patients have a 13 bp insertion in exon 11 of one allele at the MCAD gene locus. In the other allele patients in one of the families harbour the prevalent G98 5 mutation, and the other family possess an unidentified mutation caus ing reduced levels of MCAD mRNA. We demonstrate that the disease in th ese families is inherited as an autosomal recessive trait. Individuals heterozygous for the mutations show heterozygous/control levels of be ta-oxidation activities in cultured fibroblasts (9.1-16.3 pmol/min per mg protein; control 10-17 pmol/min per mg protein), and in the excret ion of the 'beta-oxidation metabolites', hexanoylglycine (< 2 mu mol/m mol creatinine), suberylglycine (< mu mol/mmol creatinine) and phenylp ropionylglycine (< 2 mu mol/mmol creatinine). This shows that there is no 'negative dominance' from the mutant monomeric protein onto the no rmal ones, in accordance with the finding of low levels of MCAD mRNA f rom the allele harbouring the 13 bp insertion as well as the allele wi th the unidentified mutation, and the low steady-state level of enzyme protein expressed from the G985-bearing allele. In the family possess ing the G985 and the 13 bp insertion mutations, two asymptomatic compo und heterozygous individuals were detected. They exhibited elevated ex cretion of hexanoylglycine (5-15 mu mol/mmol creatinine) and suberylgl ycine (4-13 mu mol/mmol creatinine), together with beta-oxidation acti vity in fibroblasts in the homozygous range (2.9 pmol/min per mg prote in), showing a lack of correlation between the genotype, some biochemi cal parameters and the clinical phenotype.