Abstract
An important diagnostic muscle biopsy finding in patients with mitochondrial DNA disease is the presence of respiratory-chain deficient fibres. These fibres are detected as cytochrome c oxidase-deficient following a sequential cytochrome c oxidase-succinate dehydrogenase reaction, often in a mosaic pattern within a population of cytochrome c oxidase-normal fibres. Detailed analysis of muscle biopsies from patients with various mitochondrial DNA defects shows that a spectrum of deficiency exists, as there are a large number of fibres which do not correspond to being either completely cytochrome c oxidase-normal (brown staining) or cytochrome c oxidase-deficient (blue staining). We have used a combination of histochemical and immunocytochemical techniques to show that a population of cytochrome c oxidase-intermediate reacting fibres are a gradation between normal and deficient fibres. We show that cytochrome c oxidase-intermediate fibres also have different genetic characteristics in terms of amount of mutated and wild-type mtDNA, and as such, may represent an important transition between respiratory normal and deficient fibres. Assessing changes in intermediate fibres will be crucial to evaluating the responses to treatment and in particular to exercise training regimes in patients with mitochondrial DNA disease.
Highlights
A central finding in the investigation and diagnosis of patients with mitochondrial myopathies is the presence of respiratory deficient muscle fibres in skeletal muscle
Subsequent studies have extensively used this technique to explore the molecular mechanisms involved in mitochondrial myopathies, and the COX deficient fibres show high levels of mutated mtDNA associated with low levels of wild-type mtDNA [4,5]
We showed that there was a significant difference in the percentage level of deleted mtDNA between COX-normal (n = 28) and COX-intermediate fibres (n = 23) (p < 0.0001) and between COX-intermediate fibres and COX-deficient fibres (n = 25) (p < 0.0001), we did not observe a significant difference in the total mtDNA copy number per lm3 (MTND1 probe) between COX-normal and COX-intermediate fibres (p = 0.098) or between COX-intermediate and COX-deficient fibres (P = 0.98)
Summary
A central finding in the investigation and diagnosis of patients with mitochondrial myopathies is the presence of respiratory deficient muscle fibres in skeletal muscle. These fibres are identified using a histochemical assay for cytochrome c oxidase (COX) activity, with COX-deficient. J.L. Murphy et al / Neuromuscular Disorders 22 (2012) 690–698 resulting from a disorder of mtDNA maintenance) or reflect a generalised mitochondrial translation deficiency. Murphy et al / Neuromuscular Disorders 22 (2012) 690–698 resulting from a disorder of mtDNA maintenance) or reflect a generalised mitochondrial translation deficiency In these patients there is typically a mosaic pattern of enzyme deficiency with both COX-normal and COX-deficient fibres present in the same biopsy. Subsequent studies have extensively used this technique to explore the molecular mechanisms involved in mitochondrial myopathies, and the COX deficient fibres show high levels of mutated mtDNA (in patients with heteroplasmic mtDNA defects) associated with low levels of wild-type mtDNA [4,5]
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