[Maple syrup urine disease].

Branched chain a-ketoacid dehydrogenase (BCKDH) deficiency results in maple syrup urine disease (MSUD). We examined the molecular basis of familial cases ofMSUD by analyzing the activity, subunit structure, mRNA sequence, and genome structure of the affected enzyme. The BCKDH activity in the proband with MSUD was 6% ofthe normal control level. Immunoblot analysis revealed that the ElB subunit ofBCKDH was absent and that the Ela subunit of BCKDH was markedly reduced. We amplified the cDNAs of the Ela subunit and the Elf subunit of the BCKDH complex obtained from cells of the patient, using the polymerase chain reaction method, then sequenced the amplified cDNAs. The deduced amino acid sequence for the Ela subunit of the patient's cell was normal. An 1 1-bp deletion was identified in the region that encoded the mitochondrial targeting leader peptide in the Elft cDNA. This 11-bp sequence is found in the first exon of the BCKDH-Elf# gene, as a direct tandem repeat. Amplification ofgenomic DNA revealed that the consanguineous parents were heterozygous for this mutant allele, and sister and brother of the patient with the disease were homozygous for this mutant allele. This 11-bp deletion mutation caused a change in the reading frame and the mature El: protein was defective. These observations show the biological importance of the El,@ subunit of BCKDH to maintain normal function of the enzyme activity. The absence of the ElfI subunit results in instability of the Ela subunit. (J. Clin. Invest. 1991. 87:1862-1866.)