Myopathies and mitochondrial diseases pose a major challenge in diagnosis due to the multitude of different entities and - in the case of mitochondriopathies - the possible involvement of multiple organs. Furthermore, there is broad clinical variability within particular diseases; patients with hereditary myopathy, for example, can show great phenotypic variability despite identical genetic defects. In addition to environmental factors, gender-specific influences, and the degree of heteroplasmy in mitochondrial diseases, the existence of disease-modifying genes has long been assumed as an explanation. In recent years, risk genes, which can influence the course of disease, have been identified for some myopathies and mitochondrial diseases. The precise role of these disease-modifying genes in the pathogenesis of the diseases is largely unexplained and requires further research.
Keywords: Degree of heteroplasmy; Friedreich ataxia; Multiple organ involvement; Muscular dystrophies; Phenotype.