Mechanisms of expression of pyruvate dehydrogenase deficiency caused by an E1alpha subunit mutation.

[pyruvate dehydrogenase deficiency]

To characterize the biochemical mechanisms of expression of the pyruvate dehydrogenase (PDH ) E 1 alpha subunit exon 10 R 302 C missense mutation .Mutations in the X-linked E 1 alpha subunit gene are responsible for most cases of PDH deficiency , an important cause of neurodevelopmental defects and neurodegeneration with primary lactic acidemia . Although the disease shows extreme allelic heterogeneity , the R 302 C mutation has been defined in several unrelated cases .Cell lines expressing selectively either the mutant or wild-type E 1 alpha alleles against identical genetic backgrounds were generated from the fibroblasts of a female heterozygous for the R 302 C mutation . Enzyme activity , mRNA , polypeptide expression , and turnover were studied in each .The residual PDH activity was below measurable levels in the cell line (B 5 ) expressing only the mutant allele and normal in the wild-type polypeptide expressing (A 10 ) cell line , confirming that the R 302 C mutation alone is sufficient to cause a severe PDH deficiency . The mutant polypeptide was less stable than the wild-type polypeptide , but the steady-state level of the mutant E 1 alpha protein was reduced only two - to threefold .The primary mechanism of expression of the R 302 C mutation must be limitation of catalytic efficiency . We speculate that catalysis may be inhibited in the mutant polypeptide because conformational changes are induced near serine 300 , a residue that is particularly important as a regulatory phosphorylation site in the wild-type polypeptide .