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Phenotype-genotype correlations in skeletal muscle of patients with mtDNA deletions
C T Moraes1, M Sciacco, E Ricci
1Department of Neurology, University of Miami, FL 33136, USA.
Abstract:
Large-scale deletions of mitochondrial DNA (mtDNA) have been associated with a subgroup of mitochondrial encephalomyopathies, usually characterized by progressive external ophthalmoplegia (PEO) and mitochondrial proliferation in muscle fibers. We and others have shown that muscle from patients with mtDNA deletions have variable cytochrome c oxidase (COX) deficiency and reduction of mitochondrially-synthesized polypeptides in affected muscle fibers. The present work summarizes the phenotype-genotype correlations observed in patients' muscle. In situ hybridization revealed that, while most COX-deficient fibers had increased levels of mutant mtDNA, they almost invariably had reduced levels of normal mtDNA. PCR quantitation of both deleted and wild-type mtDNAs in normal and respiration-deficient muscle fibers from patients with the "common deletion" showed that deleted mtDNAs were present in normal fibers (31 +/- 26%), but their percentages were much higher in affected fibers (95% +/- 2%). Absolute levels of deleted mtDNA were also increased in affected fibers, whereas absolute levels of wild-type mtDNA were significantly reduced. Taken together, our results suggest that although a specific ratio between mutant and wild-type mitochondrial genomes is probably the major determinant of the respiratory chain deficiency associated with mtDNA deletions, the reduction in the absolute amounts of wild-type mtDNA may also play a significant pathogenetic role.
Insights
Large-scale mitochondrial DNA (mtDNA) deletions cause mitochondrial encephalomyopathies. Muscle fibers with these deletions show reduced normal mtDNA, impacting cellular respiration and disease progression.
Area of Science:
- Biochemistry
- Genetics
- Cell Biology
Background:
- Large-scale mitochondrial DNA (mtDNA) deletions are linked to mitochondrial encephalomyopathies, often presenting as progressive external ophthalmoplegia (PEO).
- Affected muscle fibers typically exhibit mitochondrial proliferation, variable cytochrome c oxidase (COX) deficiency, and reduced mitochondrially-synthesized polypeptides.
Purpose of the Study:
- To investigate phenotype-genotype correlations in patients with mtDNA deletions.
- To elucidate the role of mutant and wild-type mtDNA levels in COX deficiency.
Main Methods:
- In situ hybridization to assess mtDNA levels in muscle fibers.
- Polymerase chain reaction (PCR) quantitation of deleted and wild-type mtDNA in normal and respiration-deficient muscle fibers.
Main Results:
- COX-deficient fibers showed increased mutant mtDNA but reduced normal mtDNA.
- Deleted mtDNA constituted 95% of mtDNA in affected fibers, compared to 31% in normal fibers.
- Absolute levels of wild-type mtDNA were significantly reduced in affected fibers.
Conclusions:
- The ratio of mutant to wild-type mtDNA is a key factor in respiratory chain deficiency.
- Reduced absolute amounts of wild-type mtDNA also contribute significantly to the pathogenesis of mitochondrial encephalomyopathies associated with mtDNA deletions.