A Homozygous Nonsense Mutation in the methylmalonyl-CoA Epimerase Gene (MCEE) Results in Mild Methylmalonic Aciduria

Hum Mutat. 2006 Jul;27(7):640-3. doi: 10.1002/humu.20373.


Methylmalonic aciduria (MMA-uria) is an autosomal recessive inborn error of amino acid metabolism, involving valine, threonine, isoleucine, and methionine. This organic aciduria may present in the neonatal period with life-threatening metabolic acidosis, hyperammonemia, feeding difficulties, pancytopenia, and coma. Most affected patients have mutations in the methylmalonyl-coenzyme A (methylmalonyl-CoA) mutase gene. Mildly affected patients may present in childhood with failure to thrive and recurrent attacks of metabolic acidosis. Both a higher residual activity of methylmalonyl-CoA mutase as well as the vitamin B12-responsive defects (cblA and cblB) may form the basis of the mild disorder. A few patients with moderate MMA-uria are known in whom no defect could be identified. Here we present a 16-year-old female patient with persisting moderate MMA-uria (approximately 50 mmol/mol creatinine). She was born to consanguineous Caucasian parents. Her fibroblast mutase activity was normal and no effect of vitamin B12 supplementation could be established. Reduced incorporation of 14C-propionate into macromolecules suggested a defect in the propionate-to-succinate pathway. We found a homozygous nonsense mutation (c.139C>T) in the methylmalonyl-CoA epimerase gene (MCEE), resulting in an early terminating signal (p.R47X). Both parents were heterozygous for this mutation; they were found to excrete normal amounts of methylmalonic acid (MMA). This is the first report of methylmalonyl-CoA epimerase deficiency, thereby unequivocally demonstrating the biochemical role of this enzyme in human metabolism.

Publication types

  • Case Reports

MeSH terms

  • Amino Acid Metabolism, Inborn Errors / diagnosis
  • Amino Acid Metabolism, Inborn Errors / genetics*
  • Child, Preschool
  • Codon, Nonsense*
  • Consanguinity
  • DNA Mutational Analysis
  • Female
  • Homozygote*
  • Humans
  • Methylmalonic Acid / urine*
  • Racemases and Epimerases / genetics*


  • Codon, Nonsense
  • Methylmalonic Acid
  • Racemases and Epimerases
  • methylmalonyl-coenzyme A racemase