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At a glance

It is a progressive multiorgan disease. The mitochondrial myopathies are a rare group of conditions affecting the respiratory chain and oxidative phosphorylation. A total of five proteins complexes make up the mitochondrial electron transport chain (see Complex Diseases). Clinically, this medical condition affects every organ of the body and the clinical symptoms include poor growth, loss of muscle coordination, muscle weakness, visual problems, hearing problems, learning disabilities, heart disease, liver disease, kidney disease, gastrointestinal disorders, respiratory disorders, neurological problems, autonomic dysfunction, and dementia.

Synonyms

Mitochondrial Cytopathy; Mitochondrial Myopathy.

History

The first pathogenic mutation in mitochondrial DNA was identified in 1988 and since this initial identification and 2016, 275 other disease-causing mutations were identified.

Nature

Mitochondria are the main source of adenosine tri-phosphate (ATP) in the cell. They depend on several metabolic pathways to supply ATP during varying cellular conditions: glycolysis is the main pathway during nutritional repletion state, and fatty acid oxidation is the main pathway during fasting. ATP is generated by five protein complexes, called the respiratory chain, contained in the inner mitochondrial membrane: NADH from the tricarboxylic cycle is a substrate for complex I, and the reduced form of flavin adenine dinucleotide (FADH2) from fatty acid oxidation is a substrate for complex II. Strictly speaking, disorders of pyruvate metabolism, fatty acid oxidation, ketogenesis, or ketolysis, and defects of the urea cycle or of the respiratory chain, are mitochondrial pathologies; however, the term mitochondrial disease is usually a synonym for respiratory chain or oxidative phosphorylation anomaly.

Incidence

In the United States, it is estimated that one in 4,000 children will develop mitochondrial disease by the age of 10 years. The prevalence of the disease at birth is 1:4,000 to have a type of mitochondrial disease in the United States. Because mitochondrial disorders contain many variations and subsets, some particular mitochondrial disorders are very rare. The average number of births per year among women at risk for transmitting mtDNA disease is estimated to approximately 150 in the United Kingdom and 800 in the United States.

Genetic inheritance

Any mode of inheritance may be observed: autosomal recessive, dominant, X-linked, maternal, or sporadic. This is a consequence of the high number of genes encoding respiratory chain proteins: most are located in the cell DNA but 26 are in the mtDNA. Consequently, mutations, depletion, deletions, or duplications can occur in both genetic materials. Transmission of mitochondrial diseases is complex because mtDNA is maternally inherited, has a different structure than complementary DNA, and is more subject to spontaneous mutations. Therefore, many mtDNA mutations accumulate with age and are probably the cause of some diseases (eg, Parkinson, diabetes). Moreover, there are many mitochondria in each cell, and they are randomly partitioned between daughter cells during mitosis. Thus, if ...

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