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 (∼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.

MCEE, Methylmalonic aciduria, Methylmalonyl-CoA epimerase
dx.doi.org/10.1002/humu.20373, hdl.handle.net/1765/66888
Human Mutation
Department of Pediatric Surgery

Bikker, H, Bakker, H.D, Abeling, N.G.G.M, Poll-The, B.T, Kleijer, W.J, Rosenblatt, D.S, … Duran, M. (2006). A homozygous nonsense mutation in the methylmalonyl-CoA epimerase gene (MCEE) results in mild methylmalonic aciduria. Human Mutation, 27(7), 640–643. doi:10.1002/humu.20373