A rare disease-associated mutation in the medium-chain acyl-CoA dehydrogenase (MCAD) gene changes a conserved arginine, previously shown to be functionally essential in short-chain acyl-CoA dehydrogenase (SCAD)

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is a serious and potentially fatal inherited defect in the β-oxidation of fatty acids. Approximately 80% of patients with MCAD deficiency are homozygous for a single disease-causing mutation (G985). The remaining patients (except for a few cases worldwide) are compound heterozygous with G985 in one allele. By sequencing of cloned PCR-amplified MCAD cDNA from a G985 compound heterozygous patient, we identified a C-to-T transition at position 157 as the only change in the entire coding sequence of the non-G985 allele. The presence of the T157 mutation was verified in genomic DNA from the patient and her mother by a PCR-based assay. The mutation changes a conserved arginine at position 28 (R28C) of the mature MCAD protein. The effect of the T157 mutation on MCAD protein was investigated by expression of mutant MCAD cDNA in COS-7 cells. On the basis of knowledge about the three-dimensional structure of the MCAD protein, we suggest that the mutation destroys a salt bridge between arginine²⁸ and glutamate⁸⁶, thereby affecting the formation of enzymatically active protein. Twenty-two additional families with compound heterozygous patients were tested in the PCR-based assay. The T157 mutation was identified in one of these families, which had an MCAD-deficient child who died unexpectedly in infancy. Our results indicate that the mutation is rare. It is, however, noteworthy that a homologous mutation has previously been identified in the short-chain acyl-CoA dehydrogenase (SCAD) gene of a patient with SCAD deficiency, suggesting that the conserved arginine is crucial for formation of active enzyme in the straight-chain acyl-CoA dehydrogenases.