Rapid Publication Maple Syrup Urine Disease Complete Defect of the E1,f Subunit of the Branched Chain a-Ketoacid Dehydrogenase Complex Due to a Deletion of an 11-bp Repeat Sequence which Encodes a Mitochondrial Targeting Leader Peptide in a Family with th

Branched chain a-ketoacid dehydrogenase (BCKDH) defi-ciency results in maple syrup urine disease (MSUD). We exam-ined the molecular basis of familial cases ofMSUD by analyz-ing the activity, subunit structure, mRNA sequence, and ge-nome 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 se-quenced the amplified cDNAs. The deduced amino acid se-quence 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 main-tain 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.) Key words: gene mutation * com-plex enzyme- protein degradation * enzyme comple