Dissecting the mechanisms underlying the accumulation of mitochondrial DNA deletions in human skeletal muscle.
Bottom Line: Large-scale mitochondrial DNA (mtDNA) deletions are an important cause of mitochondrial disease, while somatic mtDNA deletions cause focal respiratory chain deficiency associated with ageing and neurodegenerative disorders.No significant difference was observed in individual patients or in the total dataset (small fibre regions mean 6.59 kb--large fibre regions mean 6.51 kb).Thus no difference existed in the rate of clonal expansion throughout muscle fibres between mtDNA deletions of different sizes; smaller mitochondrial genomes therefore do not appear to have an inherent replicative advantage in human muscle.
Affiliation: Wellcome Trust Centre for Mitochondrial Research, and Newcastle University Centre for Brain Ageing and Vitality, Institute for Ageing and Health, Newcastle University, Newcastle upon Tyne, UK.Show MeSH
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Mentions: Longitudinal sections of muscle biopsy underwent histochemical assessment of COX activity; COX-deficient regions of muscle fibres were targeted for investigation in this study, representing fibre areas where mitochondrial activity has been disrupted to a sufficient degree by mtDNA deletion accumulation to cause a biochemical defect. Two categories of fibre were selected for investigation; ‘large’ (over 500 μm in length) and ‘small’ (under 200 μm in length) regions of COX-deficiency (Fig. 2). Values were chosen based on a maximum cutting length on the microscope/LMD system of 650 μm. The area (µm2) of single muscle fibres isolated by laser microdissection was determined at the time of cutting and was found to be a more variable measurement than fibre length when extracting fibres. However, no overlap was observed between the two groups in terms of COX-deficient fibre area (P < 0.0001, Mann–Whitney test); small COX-deficient fibre regions had a mean area of 5725 ± 3194 µm2 (range 1302–12 800 µm2) compared with 32 120 ± 10 536 µm2 (range 19 412–57 289 µm2) for large COX-deficient regions (Fig. 3A). Although large and small COX-deficient muscle fibre regions were selected based upon the length of the fibre region affected by the biochemical defect, we decided that total fibre area (µm2) isolated by laser microdissection would be a more appropriate and reliable measure. The area of COX-deficiency is a better measure of the spread and accumulation of the mtDNA deletion species responsible for the biochemical defect since measurements based on length fail to take into consideration fibre-width variation.Figure 2.
Affiliation: Wellcome Trust Centre for Mitochondrial Research, and Newcastle University Centre for Brain Ageing and Vitality, Institute for Ageing and Health, Newcastle University, Newcastle upon Tyne, UK.