Maple syrup urine disease: alpha-ketoisocaproate decarboxylation activity in different types of cultured amniotic fluid cells

Variant maple syrup urine disease (MSUD)--the entire spectrum

BACKGROUND

In the rare inborn autosomal recessive disorder maple syrup urine disease (MSUD) the accumulation of the branched-chain amino acids (BCAAs) and their metabolic products results in acute and chronic brain dysfunction. About 20% of the patients suffer from non-classic variant forms of MSUD of different clinical severity.

AIM

Up to now variant cases have mostly been published as individual case reports; the aim of this study was to give a comparative description of 16 individuals (aged 6-30 years) with different forms of variant MSUD.

METHODS

Laboratory data, information on clinical course and treatment as well as aspects of developmental, intellectual and social outcome were obtained retrospectively. Data from in vitro and in vivo methods measuring the degree of enzyme deficiency were included.

RESULTS

In addition to a mild phenotype, which fits well into the so-called intermittent variant, and a more severe phenotype with a wider range from a mild variant to an almost classic form, which fits well into the so-called intermediate variant, we assume the existence of an asymptomatic, non-disease variant of MSUD. These clinical phenotypes are not unambiguously differentiable on the basis of biochemical parameters.

CONCLUSION

A continuum of clinical severity from asymptomatic to very severe (border to classic) exists in variant MSUD. Apart from newborns with classic MSUD, also those with variant forms benefit from early diagnosis and start of adequate treatment.

Analysis of maple syrup urine disease in cell culture: use of substrates

Branched-chain 2-oxo acid dehydrogenase activity in human skin fibroblasts against L-leucine, L-valine, L-isoleucine and derived 2-oxo acids was compared in incubations with 1 mmol/l of 1-14C-labelled substrate. The results suggested that the amino acids are the more suitable substrates for an estimation of decarboxylation activity in intact cells. In control cell lines (n = 12), 14CO2 release from amino acids was highest for valine and least for leucine. In a representative number of fibroblast strains of patients with different forms of MSUD (n = 11; residual decarboxylation activity 2-60% of the controls), 14CO2 release from the different amino acids was reduced to a similar degree. Additional measurement of 2-oxo[1-14C]acid release suggested that substrate supply to the branched-chain 2-oxo acid dehydrogenase complex was not rate limiting in the cell lines under investigation.

Prenatal diagnosis of the organic acidurias

Prenatal diagnosis for the genetic counselling of families at risk for having children with the life-threatening organic acidurias is advancing rapidly. The two major approaches to prenatal diagnosis are the assay for deficient activity of the enzymes in cultured amniocytes and the measurement of increased concentrations of the organic acids in the amniotic fluid. The latter, when done by stable isotope dilution analysis, is rapid, relatively inexpensive and very reliable.