Each cell has 1000 to 2500 mitochondria in its cytoplasm. Mitochondria contain DNA in a single circular chromosome containing 37 genes that code for 13 proteins, various RNAs, and several regulating enzymes. However, > 90% of mitochondrial proteins are coded by nuclear genes. For practical purposes, all mitochondria are inherited from the cytoplasm of the egg; thus, mitochondrial DNA comes only from the mother.
Mitochondrial disorders (see also Mitochondrial Oxidative Phosphorylation Disorders) can be due to mitochondrial or nuclear DNA abnormalities (eg, deletions, duplications, mutations). High-energy tissues (eg, muscle, heart, brain) are particularly at risk of malfunction due to mitochondrial abnormalities. Particular mitochondrial DNA abnormalities result in characteristic manifestations (see table Some Mitochondrial Disorders). Mitochondrial disorders are equally common among males and females.
(See also Overview of Genetics.)
Pearls & Pitfalls
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Mitochondrial abnormalities may occur in many common disorders such as some types of Parkinson disease (which may involve large mitochondrial deletions in the cells of the basal ganglia) and many types of muscle disorders.
Some Mitochondrial Disorders
Disorder | Description |
|---|---|
Progressive paralysis of the extraocular muscles usually preceded by bilateral, symmetric, progressive ptosis that begins months to years earlier | |
A multisystem variant of chronic progressive external ophthalmoplegia that also includes heart block, retinitis pigmentosa, and central nervous system degeneration | |
Variable but often devastating bilateral vision loss that often occurs in adolescents and that is due to a point mutation in mitochondrial DNA | |
Myoclonic epilepsy with ragged-red fibers (MERRF) syndrome | Progressive myoclonus and seizures, dementia, ataxia, and myopathy |
Mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome | MELAS episodes |
Pearson syndrome | Sideroblastic anemia, pancreatic insufficiency, and progressive liver disease that begins in the first few months of life and is frequently fatal in infants |
Maternal inheritance patterns characterize abnormalities of mitochondrial DNA. Thus, all offspring of an affected female are at risk of inheriting the abnormality, but no offspring of an affected male are at risk. Variability in clinical manifestations is the rule, and these abnormalities mimic a broad range of disorders, which often makes diagnosis extremely difficult. Variability may be due in part to variable mixtures of inherited mutant and normal mitochondrial genomes within cells and tissues.
Key Points
Mitochondrial disorders have maternal inheritance patterns.
Because clinical manifestations can be subtle and variable, diagnosis can be very difficult.
