Expression Pattern of Mitochondrial Respiratory Chain Enzymes in Skeletal Muscle of Patients Harboring the A3243G Point Mutation or Large-Scale Deletions of Mitochondrial DNA

Abstract

To assess the detailed expression pattern of mitochondrial-encoded proteins in skeletal muscle of patients with mitochondrial diseases we performed determinations of cytochrome content and enzyme activities of respiratory chain complexes of 12 patients harboring large-scale deletions and of 10 patients harboring the A3243G mutation. For large-scale deletions we observed a mutation gene dose-dependent linear decline of cytochrome aa3 content, cytochrome c oxidase (COX) activity, and complex I activity. The content of cytochromes b and the complex III activity was either not affected or only weakly affected by the deletion mutation and did not correlate to the degree of heteroplasmy. In contrast, in skeletal muscle harboring the A3243G mutation all investigated enzymes containing mitochondrial-encoded subunits were equally affected by the mutation, but we observed milder enzyme deficiencies at a comparable mutation gene dose. The results of single fiber analysis of selected biopsies supported these findings but revealed differences in the distribution of COX deficiency. Whereas predominantly type I fibers were affected in A3243G and deletion CPEO biopsies, we observed in MELAS and KSS biopsies higher quantities of COX-deficient type 2 fibers. Our findings indicate different pathomechanisms of deletion and A3243G mutations.

Muscle 3243A→G mutation load and capacity of the mitochondrial energy‐generating system

“…Complex I deficiency is most frequently found in patients harboring the 3243A→G mutation, but deficiencies of complex III and IV and combined deficiencies have also been described to varying degrees 20, 39, 40. However, in two of these studies,39, 40 no complex III activity but succinate:cytochrome c oxidoreductase activity was measured, which mainly is a measure for complex II activity (our own observations).…”

“…The m.tRNA Leu(CUN) catalyzes the incorporation of the remaining leucines 5. In muscle tissue of patients with the 3243A→G mutation, decreased activities of respiratory chain enzyme complexes I, III, and IV were measured, and mutation load correlated with the cytochrome b and aa3 content in muscle 20. Investigation of OXPHOS complexes in muscle specimens of patients with the 3243A→G mutation by blue native two‐dimensional electrophoresis showed almost complete absence of complex I, partial reduction of complexes III and IV, and normal amounts of complexes II and V 21.…”

Sensory Ataxic Neuropathy in Golden Retriever Dogs Is Caused by a Deletion in the Mitochondrial tRNATyr Gene

“…The affected animals showed a bluish staining, indicating a shift towards a lower COX activity compared to the SDH activity. Fibers totally lacking COX-activity and ragged red fibers are common findings in humans harboring large scale deletions and tRNA mutations in mtDNA [13],[14],[17]. Strikingly, no such fibers were found in muscle samples from the dogs.…”

Mitochondrial Diseases ‐ An Expanding Spectrum of Disorders and Affected Genes

“…Therefore, large-scale deletions have, at a comparable degree of heteroplasmy, a much more deleterious effect (cf. also Vielhaber et al 2002). This difference has been attributed to the involvement of several different tRNA genes (Chomyn, 1998).…”