Molecular Basis of Maple Syrup Urine Disease: Novel Mutations at the E1α Locus That Impair E1 (α₂β₂) Assembly or Decrease Steady-State E1α mRN A Levels of Branched-Chain α-Keto Acid Dehydrogenase Complex

We report the occurrence of three novel mutations in the E1α (BCKDHA) locus of the branched-chain α-keto acid dehydrogenase (BCKAD) complex that cause maple syrup urine disease (MSUD). An 8-bp deletion in exon 7 is present in one allele of a compound-heterozygous patient (GM-649). A single C nucleotide insertion in exon 2 occurs in one allele of an intermediate-MSUD patient (Lo). The second allele of patient Lo carries an A-to-G transition in exon 9 of the E1α gene. This missense mutation changes Tyr-368 to Cys (Y368C) in the E1α subunit. Both the 8-bp deletion and the single C insertion generate a downstream nonsense codon. Both mutations appear to be associated with a low abundance of the mutant E1α mRNA, as determined by allele-specific oligonucleotide probing. Transfection studies strongly suggest that the Y368C substitution in the E1α subunit impairs its proper assembly with the normal E1β. Unassembled as well as misassembled E1α and E1β subunits are degraded in the cell.