Classical Maple Syrup Urine Disease: Cofactor resistance

The unremitting clinical course of an infant female who died on her 13th day of life with a severe form of classical Maple Syrup Urine Disease is described. Initially high urinary levels of leucine, isoleucine, and valine and their branched-chain α-keto acid derivatives returned to normal following restriction of these amino acids at 8 days of age and the administration of thiamine and pyridoxine. Methods were developed to study the genetic control of this enzyme defect and its response to cofactors in fibroblasts cultured from the skin of the proband, her parents, and normal controls. Cells cultured from the affected proband produced C¹⁴O₂ from C¹⁴-labeled leucine, isoleucine, and valine at less than 1% of the normal rate. Cells from the parents exhibited partially impaired decarboxylating activity for all three branched-chain amino acids. The paternal cell line had more decarboxylating activity than the maternal cell line over a 3-hr course of incubation and over a 600-fold range of substrate concentration. The specificity of this inborn error was confirmed in fibroblasts from the proband that oxidized pyruvate-1-C¹⁴ normally. This activity was stimulated 90% when a mixture of pyridoxal phosphate, sodium lipoate, thiamine pyrophosphate, β-nicotinamide adenine dinucleotide, coenzyme A, and flavine adenine dinucleotide was added to the buffer medium. In normal cells this mixture of coenzymes also stimulated decarboxylation of isoleucine and valine, but not of leucine. In contrast, the coenzymes failed to augment branched-chain amino acid decarboxylation by the proband's cell line. We conclude from these observations that the proband inherited a mutant gene, whose product had essentially absent branched-chain amino acid decarboxylase activity. The disease was inherited as an autosomal recessive trait, but fibroblasts from each parent carried a quantitatively different mutation for the impaired enzyme function, implying that the child was doubly heterozygous. The mutant allele(s) produced a defective apoenzyme which was common to all three branched-chain amino acids, and which was unresponsive to cofactor stimulation.