[Heterogeneity of leucinosis. Correlations between clinical manifestations, protein tolerance and enzyme deficiency]

Maple syrup urine disease (MSUD) is an inborn error of branched-chain aminoacid (BCAA) catabolism due to a defect of BC ketoacid decarboxylation. Beside the classical form of the disease, general variant forms have been recently reported. From our personal experience in 21 patients consisting of 14 patients with classical MSUD and 7 patients with "variant" forms (1 intermittent, 2 subacute forms, 1 lipoamide dehydrogenase deficiency and 3 composite heterozygotes), we tried to correlate clinical features with protein tolerance and enzyme activity. All classical forms share an acute neonatal presentation, with a low leucine tolerance (m +/- SD = 465 +/- 88 mg per day), and a very low enzyme activity (m +/- SD = 2.5 +/- 1.5% of controls), mild variations being consistent with a possible polyallelism within this group. All "variant" forms share a late onset with a reasonable leucine tolerance (2-3 grams per day) and a fair enzyme activity, ranging from 7 to 20% of controls. However, no strict correlation could be found between the severity of the outcome and the extent of the residual enzyme activity, since acute episodes or chronic deterioration occurred even in "variants", irrespective of the level of their enzyme activity. Finally, our data suggest that variant forms could result from a composite heterozygotism, combining heterozygotism for the gene of classical MSUD and heterozygotism for a "petite" mutation, undetectable when isolated. The occurrence of classical MSUD and "variant" forms of the disease within a single family of our series further supports this hypothesis.