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A mutation in the E1 alpha subunit of pyruvate dehydrogenase associated with variable expression of pyruvate
I D Wexler1, S G Hemalatha, T C Liu
1Department of Biochemistry, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106.
Insights
Pyruvate dehydrogenase complex deficiency can stem from posttranscriptional defects. A specific gene mutation causes variable enzyme activity, potentially due to protein instability in certain tissues.
Area of Science:
- Biochemistry
- Genetics
- Molecular Biology
Background:
- Pyruvate dehydrogenase complex (PDC) deficiency is a group of inherited metabolic disorders.
- Defects in pyruvate dehydrogenase (PDH), a key component of PDC, are the most common cause of PDC deficiency.
- Previous studies identified a family with variable PDC deficiency and normal fibroblast activity but reduced activity in other tissues.
Purpose of the Study:
- To investigate the molecular basis of variable PDC deficiency in the described family.
- To determine if the defect is transcriptional or posttranscriptional.
- To identify the specific genetic mutation responsible for the observed phenotype.
Main Methods:
- Enzymatic activity assays and immunoblot studies were performed on various tissues and cultured fibroblasts.
- Sequencing of the coding region of the alpha-subunit of pyruvate dehydrogenase was conducted.
- Family members were studied to assess inheritance patterns.
Main Results:
- Enzymatic and immunoblot studies confirmed that pyruvate dehydrogenase was affected.
- Evidence indicated a posttranscriptional defect in PDC activity.
- A point mutation (Glycine to Arginine substitution at amino acid 234) was identified in the PDH alpha-subunit gene.
- The mutation was found to be inherited in a sex-linked manner.
- The mutation is located in a highly conserved region of the gene.
Conclusions:
- The identified point mutation in the pyruvate dehydrogenase alpha-subunit gene is responsible for the variable PDC deficiency in this family.
- The posttranscriptional nature of the defect and its location in a conserved region suggest a significant impact on protein function.
- Variable expression may result from tissue-specific instability of the pyruvate dehydrogenase heterotetramer due to disrupted subunit interactions.
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 fibroblasts 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 alpha-subunit of pyruvate dehydrogenase revealed a point mutation in the codon for amino acid 234 resulting in a substitution of glycine for arginine. Study 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 alpha-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.