Mutations in mitochondrial DNA as a cause of exercise intolerance

Exercise intolerance is a common presentation of metabolic myopathies, espe cially of congenital errors of glycogen and lipid metabolism. Recently, how ever, exercise intolerance has been associated with specific defects in pro tein-coding genes of mitochondrial DNA (mtDNA), including mutations in gene s for complex I, complex III, and complex IV. Contrary to the general rules of mitochondrial genetics, all patients were sporadic cases and all mutati ons were restricted to skeletal muscle, suggesting that they were somatic m utations not affecting the germ line.