title: In vitro and in vivo consequences of variant medium-chain acyl-CoA dehydrogenase genotypes.
abstract: BACKGROUND
  Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency is the most common inherited disorder of the mitochondrial fatty acid oxidation, caused by mutations in the ACADM gene. Since the introduction of neonatal screening for MCAD deficiency, a subgroup of newborns have been identified with variant ACADM genotypes that had never been identified before in clinically ascertained patients. In vitro residual MCAD enzyme activity has been found to facilitate risk-stratification. In this study we integrated results of in vitro (residual MCAD enzyme activities) and in vivo (clinical fasting tolerance tests, and phenylpropionic acid loading tests) tests in this subgroup of newborns, defining the consequences of variant ACADM genotypes.
  
  
  METHODS
  Enzyme analyses were performed in leukocytes with: hexanoyl-CoA (C6-CoA) +/- butyryl-CoA (C4-CoA), and phenylpropionyl-CoA (PP-CoA). In vitro studies were performed in 9 subjects with variant ACADM genotypes, in vivo functional tests in 6 of these subjects.
  
  
  RESULTS
  Enzyme analyses with C6-CoA, C6-CoA + C4-CoA, and PP-CoA identified significantly higher residual MCAD enzyme activities in subjects with variant ACADM genotypes when compared to patients with classical ACADM genotypes.After prolonged fasting (range 15-18.5 hours) no hypoglycaemia was observed. Increasing concentrations of free fatty acids indicated lipolysis, and ketone body concentrations were sufficient for blood glucose concentrations in 5 out of 6 subjects. Phenylpropionic acid loading clearly demonstrated in vivo residual MCAD enzyme activity in all studied subjects.
  
  
  CONCLUSIONS
  Subjects with variant ACADM genotypes and residual MCAD enzyme activities >10% display residual MCAD enzyme activities in vitro and in vivo. Our findings support the hypothesis that the guidelines on maximal duration of fasting might be abandoned in subjects with residual MCAD enzyme activities >10% under normal conditions. An emergency regimen and parental instructions remain necessary in all subjects with MCAD deficiency, regardless of residual MCAD enzyme activity.
authors.affiliation: Section of Metabolic Diseases, Beatrix Children's Hospital, University of Groningen, University Medical Centre, Groningen, The Netherlands. n.touw@umcg.nl
authors.forename: Catharina M L
authors.initials: CM
authors.lastname: Touw
authors.forename: G Peter A
authors.initials: GP
authors.lastname: Smit
authors.forename: Klary E
authors.initials: KE
authors.lastname: Niezen-Koning
authors.forename: Conny
authors.initials: C
authors.lastname: Bosgraaf-de Boer
authors.forename: Albert
authors.initials: A
authors.lastname: Gerding
authors.forename: Dirk-Jan
authors.initials: DJ
authors.lastname: Reijngoud
authors.forename: Terry G J
authors.initials: TG
authors.lastname: Derks
mesh_terms: D042964:Acyl-CoA Dehydrogenase / Q000235:genetics* / Q000378:metabolism; D015331:Cohort Studies; D005215:Fasting; D008401:Gas Chromatography-Mass Spectrometry; D005838:Genotype; D006801:Humans; D009154:Mutation
chemical_list: D042964:Acyl-CoA Dehydrogenase