Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most commonly recognized defect of mitochondrial beta-oxidation. It is potentially fatal, but shows a wide clinical spectrum. The aim of the present study was to investigate whether any correlation exists between MCAD genotype and disease phenotype. We determined the prevalence of the 14 known and seven previously unknown non-G985 mutations in 52 families with MCAD deficiency not caused by homozygosity for the prevalent G985 mutation. This showed that none of the non-G985 mutations are prevalent, and led to the identification of both disease-causing mutations in 14 families in whom both mutations had not previously been reported. We then evaluated the severity of the mutations identified in these 14 families. Using expression of mutant MCAD in Escherichia coli with or without co-overexpression of the molecular chaperonins GroESL we showed that five of the missense mutations affect the folding and/or stability of the protein, and that the residual enzyme activity of some of them could be modulated to a different extent depending on the amounts of available chaperonins. Thus, some of the missense mutations may result in relatively high levels of residual enzyme activity, whereas the mutations leading to premature stop codons will result in no residual enzyme activity. By correlating the observed types of mutations identified to the clinical/biochemical data in the 14 patients in whom we identified both disease-causing mutations, we show that a genotype/phenotype correlation in MCAD deficiency is not straightforward. Different mutations may contribute with different susceptibilities for disease precipitation, when the patient is subjected to metabolic stress, but other genetic and environmental factors may play an equally important role.

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Evidence that inherited disorders of mitochondrial fatty acid β-oxidation can cause sudden infant death syndrome (SIDS) comes from case reports, systematic autopsy studies, and family studies. Family studies are important when no pediatric autopsy has been done, which is still frequent. After reviewing the fatty acid β-oxidation, and its pathophysiology, we present the results of our metabolic study on 189 siblings of SIDS victims, and on 84 ‘near-miss’ infants. We have found evidence for a disorder of fat oxidation in 28 (15%) infants in the first group, and in 14 (17%) infants in the second group. Diagnosing and treating such disorders early in infancy may prevent some cases of SIDS to occur.