MOLECULAR-BASIS OF MAPLE-SYRUP-URINE-DISEASE - NOVEL MUTATIONS AT THEE1-ALPHA LOCUS THAT IMPAIR E1(ALPHA(2)BETA(2)) ASSEMBLY OR DECREASE STEADY-STATE E1-ALPHA MESSENGER-RNA LEVELS OF BRANCHED-CHAIN ALPHA-KETOACID DEHYDROGENASE COMPLEX

We report the occurrence of three novel mutations in the Ela (BCKDHA) locus of the branched-chain alpha-keto acid dehydrogenase (BCKAD) comp lex that cause maple syrup urine disease (MSUD). An 8-bp deletion in e xon 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 o f an intermediate-MSUD patient (Lo). The second allele of patient Lo c arries an A-to-G transition in exon 9 of the E1 alpha gene. This misse nse mutation changes Tyr-368 to Cys (Y368C) in the E1 alpha subunit. B oth the 8-bp deletion and the single C insertion generate a downstream nonsense codon. Both mutations appear to be associated with a low abu ndance of the mutant E1 alpha mRNA, as determined by allele-specific o ligonucleotide probing. Transfection studies strongly suggest that the Y368C substitution in the E1 alpha subunit impairs its proper assembl y with the normal E1 beta. Unassembled as well as misassembled E1 alph a and E1 beta subunits are degraded in the cell.