The most common mutation causing medium-chain acyl-CoA dehydrogenase deficiency is strongly associated with a particular haplotype in the region of the gene

Inborn errors of mitochondrial fatty acid oxidation

Inborn errors of the mitochondrial beta-oxidation of long-chain fatty acids represent an evolving field of inherited metabolic disease. Fatty acid oxidation defects demonstrate an abnormal response to the process of fasting adaptation and affect those tissues that utilize fatty acids as an energy source. These tissues include cardiac and skeletal muscle and liver. Muscle directly uses fatty acids as an energy source whilst hepatic metabolism of fatty acids is mostly directed toward the synthesis of ketone bodies for energy utilization by tissues such as brain. The clinical phenotypes of fatty acid oxidation disorders include disease of one or more of these fatty acid-metabolizing tissues. In this review, we provide an overview of the pathway, discuss the disorders that are well established, and describe recent advances in the field. Currently available diagnostic procedures are critically evaluated.

Mammalian mitochondrial beta-oxidation

The enzymic stages of mammalian mitochondrial beta-oxidation were elucidated some 30-40 years ago. However, the discovery of a membrane-associated multifunctional enzyme of beta-oxidation, a membrane-associated acyl-CoA dehydrogenase and characterization of the carnitine palmitoyl transferase system at the protein and at the genetic level has demonstrated that the enzymes of the system itself are incompletely understood. Deficiencies of many of the enzymes have been recognized as important causes of disease. In addition, the study of these disorders has led to a greater understanding of the molecular mechanism of beta-oxidation and the import, processing and assembly of the beta-oxidation enzymes within the mitochondrion. The tissue-specific regulation, intramitochondrial control and supramolecular organization of the pathway is becoming better understood as sensitive analytical and molecular techniques are applied. This review aims to cover enzymological and organizational aspects of mitochondrial beta-oxidation together with the biochemical aspects of inherited disorders of beta-oxidation and the intrinsic control of beta-oxidation.

Most cases of medium-chain acyl-CoA dehydrogenase deficiency escape detection in France

DNA from 414 French blood donors from the Paris area was assessed for the A985G mutation responsible for most cases of autosomal recessive medium-chain acyl-CoA dehydrogenase (MCAD) deficiency. The mutant gene frequency averaged 1/140, predicting a frequency of mutant homozygotes of 1/19 000. Discrepancy between the numbers of expected (42 per year) and recorded cases of MCAD (6 per year) suggests that most MCAD-deficient patients escape detection in France.

The A985G mutation in the medium-chain acyl-CoA dehydrogenase gene: high prevalence in the Swiss population resident in Geneva

We have determined the frequency of the A985G mutation in the medium-chain acyl-CoA dehydrogenase (MCAD) gene in a cohort of 1142 healthy babies born in two Geneva hospitals. Among babies with at least one Swiss parent, heterozygotes were detected at a frequency of 1/52, with a 95% confidence range from 1/82 to 1/38. The high frequency of the carrier state for this mutation suggests that MCAD-deficient babies are born with a frequency of 1/10,000 in the Swiss population. This number is in sharp contrast with the low number of symptomatic MCAD-deficient patients diagnosed in this country. Thus, the fraction of homozygotes who remain asymptomatic is likely to be very high in the Swiss population, and possibly higher than in other countries of northern Europe.

Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity

Severe and prolonged impairment of mitochondrial beta-oxidation leads to microvesicular steatosis, and, in severe forms, to liver failure, coma and death. Impairment of mitochondrial beta-oxidation may be either genetic or acquired, and different causes may add their effects to inhibit beta-oxidation severely and trigger the syndrome. Drugs and some endogenous compounds can sequester coenzyme A and/or inhibit mitochondrial beta-oxidation enzymes (aspirin, valproic acid, tetracyclines, several 2-arylpropionate anti-inflammatory drugs, amineptine and tianeptine); they may inhibit both mitochondrial beta-oxidation and oxidative phosphorylation (endogenous bile acids, amiodarone, perhexiline and diethylaminoethoxyhexestrol), or they may impair mitochondrial DNA transcription (interferon-alpha), or decrease mitochondrial DNA replication (dideoxynucleoside analogues), while other compounds (ethanol, female sex hormones) act through a combination of different mechanisms. Any investigational molecule should be screened for such effects.

Molecular genetic characterization and urinary excretion pattern of metabolites in two 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) deficiency 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 G985 mutation, and the other family possess an unidentified mutation causing reduced levels of MCAD mRNA. We demonstrate that the disease in these families is inherited as an autosomal recessive trait. Individuals heterozygous for the mutations show heterozygous/control levels of beta-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 excretion of the 'beta-oxidation metabolites', hexanoylglycine (< 2 mumol/mmol creatinine), suberylglycine (< 2 mumol/mmol creatinine) and phenylpropionylglycine (< 2 mumol/mmol creatinine). This shows that there is no 'negative dominance' from the mutant monomeric protein onto the normal ones, in accordance with the finding of low levels of MCAD mRNA from the allele harbouring the 13 bp insertion as well as the allele with the unidentified mutation, and the low steady-state level of enzyme protein expressed from the G985-bearing allele. In the family possessing the G985 and the 13 bp insertion mutations, two asymptomatic compound heterozygous individuals were detected. They exhibited elevated excretion of hexanoylglycine (5-15 mumol/mmol creatinine) and suberylglycine (4-13 mumol/mmol creatinine), together with beta-oxidation activity in fibroblasts in the homozygous range (2.9 pmol/min per mg protein), showing a lack of correlation between the genotype, some biochemical parameters and the clinical phenotype.

Scottish frequency of the common G985 mutation in the medium-chain acyl-CoA dehydrogenase (MCAD) gene and the role of MCAD deficiency in sudden infant death syndrome (SIDS)

Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, is an autosomal recessive inborn error of metabolism associated with various clinical presentations, including sudden unexplained death in young children. We have determined the Scottish frequency of the common G985 mutation found in Caucasians and in samples from Scottish patients with sudden infant death syndrome (SIDS). The heterozygote frequency of the mutation was found to be 1 in 276 (95% confidence interval: 1/76-1/2279) in 552 healthy controls and 1 in 74 (95% confidence interval: 1/27-1/377) in 233 SIDS patients: a difference that was not statistically significant (Fisher's exact test; two-sided; p = 0.316). None of the SIDS samples was found to be homozygous for the G985 mutation.

Molecular basis of inherited medium-chain acyl-CoA dehydrogenase deficiency causing sudden child death

Deficiency of medium-chain acyl-CoA dehydrogenase (MCAD) is an important cause of sudden death in children. The majority of surviving individuals with MCAD deficiency studied to date are homozygous for a single point mutation at bp 985 of the MCAD mRNA (A985G). We have now identified a four-base-pair deletion in exon 11 of one allele of the MCAD gene in an American child who died of MCAD deficiency. The deletion mutation results in a frameshift and premature termination codon in the mutant MCAD mRNA. The second mutant allele contained the common point mutation A985G, and thus the proband was a compound heterozygote. Protein immunoblot analysis of the child's liver proteins revealed that the mutant MCAD proteins were barely detectable. Allele-specific oligonucleotide hybridization analysis performed on amplified exon 11 of the child's MCAD gene clearly identified both mutations. MCAD RFLP analysis of the patient's DNA revealed heterozygosity at the Taq I MCAD RFLP site, thus, the two mutations are associated with different haplotypes. Therefore, we have identified a new mutation in the MCAD gene and have developed a nucleic-acid-based screening approach which allows the post mortem identification of MCAD deficiency.

Mutations in the medium chain acyl-CoA dehydrogenase (MCAD) gene

Medium chain acyl-CoA dehydrogenase (MCAD) catalyzes the first reaction of the beta-oxidation cycle for 4-10-carbon fatty acids. MCAD deficiency is one of the most frequent inborn metabolic disorders in populations of northwestern European origin. In the compilation of data from a worldwide study of 172 unrelated patients each representing an independent pedigree, a total of 8 different mutations have been identified. Among them, a single prevalent mutation, 985A-->G, was found in 90% of 344 variant alleles. 985A-->G causes glutamate substitution for lysine-304 in the mature MCAD subunit, which causes impairment of tetramer assembly and instability of the protein. Three of 7 rarer mutations have been identified in a few unrelated patients, while the remaining 4 have each been found in only a single pedigree. In addition to tabulating the mutations, the acyl-CoA dehydrogenase gene family, the structure of the MCAD gene and the evolution of 985A-->G mutation are briefly discussed.