No abstract

ABSTRACT. Defects in pyruvate dehydrogenase, the first catalytic component of the pyruvate dehydrogenase complex, are the most common cause of pyruvate dehydrogenase complex deficiency. A family with variable pyruvate dehydrogenase complex deficiency had been described in which cultured skin fibroblast s of affected family members had normal pyruvate dehydrogenase complex activity, but different tissues and blood lymphocytes had significantly diminished activities. Enzymatic activity and immunoblot studies indicated that pyruvate dehydrogenase was affected. Further evidence is presented here showing that the defect affecting pyruvate dehydrogenase complex activity is posttranscriptional. Sequencing of the coding region of the a -subunit of pyruvate dehydrogenase revealed a point mutation in the codon for amino acid 234 resulting in a substitution of glycine for arginine. Stud y of other members of the family suggested that this mutation is inherited in a sex-linked mode. The point mutation is located in a highly conserved region of the pyruvate dehydrogenase a-subunit gene that contains both hydrophobic and positively charged amino acid residues. Variable expression of pyruvate dehydrogenase complex deficiency in this case may be due to instability of the pyruvate dehydrogenase heterotetramer in specific tissues because of a disruption in subunit-subunit interaction. (Pediatr Res 32: 169-174,1992) Abbreviations Ell pyruvate dehydrogenase E1a , a -subunit of E. E 1,8 , ,8-subunit of E 1 E3, dihydrolipoamide dehydrogenase PDC, pyruvate dehydrogenase complex PCR, polymerase chain reaction Th e PDC is a nuclear-encoded mitochondrial enzyme complex t hat catalyzes the conversion of pyruvate to acetyl-CoA. PDC consists of thre e catalytic components: E 1 (pyruvate:lipoam ide 2-oxidoreductase, EC 1.2.4.1.), dihydrolipoamide acetyltransferase, and E 3 as well as a prot ein X that forms part of the comp lex. The E 1 com ponent consists of two subunits encoded by different genes, a and (3, which com bine as a heterotetramer. Regulation of the complex is via phosphorylation by Ei-kinase and dephosphorylation by phospho E j-phosphatase of th ree specific serine residues on Ela (1,2).PDC deficiency, an inborn error of pyruvate metabolism, is manifested by elevated serum pyruvat e and lactate. Individuals with PDC deficiency have varying levels of neur ologic dysfunction ranging from mild ataxia to neuroanatom icallesions incompatible with life (3, 4). Most PDC mutations affect the E1 component of PDC (5, 6). Recently, identification of mut ations affecting Ela has been facilitated by the cloning and charac terization of cDNA (7-10) and genomic DNA for Eta (1 1, 12). By in situ hybridization, the Ela gene has been localized to the X chromosome (13). Mutations involving deletions or single base changes have been found on the a -subunit (14-17).Previously, we described a family with PDC deficiency in which variable PDC enzymatic activity was observed in different tissues (18). The proband was found to have minimal PDC activi...