Mitochondrial myopathies and muscle stem cells

Abstract

Mitochondrial myopathies are a group of progressive muscle disorders caused by mutations in the mitochondrial genome (mtDNA) for which there is no effective treatment. Culturing of myoblasts from patients with sporadically occurring mitochondrial diseases has suggested that mtDNA mutations may be lower or absent in muscle stem cells (satellite cells). The activation of muscle satellite cells and subsequent repair of muscle fibres may favourably shift the balance of delete to wild-type (WT) mtDNA, thereby decreasing mtDNA mutation load in affected muscle. This research has investigated muscle precursor cells from patients with mitochondrial myopathy due to sporadically occurring mtDNA deletions. This was to determine if they will benefit from attempts to “gene shift” the balance of WT and mutated mtDNA in their muscles using high intensity resistance training. Fluorescently Activated Cell Sorting (FACS) on the basis of CD56 (NCam) was used to isolate satellite cells and real time PCR to analyse them. In all eight patients investigated mtDNA deletions were detected in satellite cells at levels similar to mature muscle. In most of these patients the mtDNA deletions were lost during the culturing of their myoblasts. In some patients, however, the mutation was maintained, although there was a gradual decline in mutation load as the myoblasts headed towards differentiation. It was hypothesised that this difference between patients in the maintenance or loss of mutations in their myoblasts was attributable to an mtDNA bottleneck effect at the point of satellite cell activation. A second selection point occurred during the process of myoblast proliferation, possibly mediated by segregation of WT and delete mtDNA after cell division. Daughter cells that inherit large amounts of delete mtDNA will be unable to continue to proliferate. If efforts to “gene shift” in these patients will involve the activation of satellite cells to repair damaged muscle, it is paramount that this process does not exhaust the muscle stem cell pool. Satellite cell numbers have been determined in patients harbouring sporadically occurring mtDNA deletions, who will be considered potential beneficiaries of exercise based interventions. No significant difference was observed in satellite cell numbers when patients were compared to controls. In addition, a single patient was examined for satellite cell numbers over eleven years and no reduction in numbers was found. Given that the large majority of single deletion patients will lose their mtDNA mutation during the process of muscle regeneration and that they will not suffer from an exhaustion of the satellite cell pool, “gene shifting” remains a viable therapy in these patients. However, the mechanisms behind the process are somewhat different to what was originally hypothesised.