|Classification and external resources|
Mitochondrial diseases are a group of disorders relating to the mitochondria, the organelles that are the "powerhouses" of the eukaryotic cells that compose higher-order life-forms (including humans). The mitochondria convert the energy of food molecules into the ATP that powers most cell functions.
Mitochondrial diseases comprise those disorders that in one way or another affect the function of the mitochondria or are due to mitochondrial DNA. Mitochondrial diseases take on unique characteristics both because of the way the diseases are often inherited and because mitochondria are so critical to cell function. The subclass of these diseases that have neuromuscular disease symptoms are often referred to as a mitochondrial myopathy.
In addition to the Mitochondrial myopathies, other examples include:
The Symptoms include poor growth, loss of muscle coordination, muscle weakness, visual problems, hearing problems, learning disabilities, mental retardation, heart disease, liver disease, kidney disease, gastrointestinal disorders, respiratory disorders, neurological problems, and dementia.
The effects of mitochondrial disease can be quite varied. Since the distribution of the defective mitochondrial DNA may vary from organ to organ within the body, and each mutation is modulated by other genome variants, the mutation that in one individual may cause liver disease might in another person cause a brain disorder. The severity of the specific defect may also be great or small. Some minor defects cause only "exercise intolerance", with no serious illness or disability. Defects often affect the operation of the mitochondria and multiple tissues more severely, leading to multi-system diseases.
Although mitochondrial diseases vary greatly in presentation from person to person, several major clinical categories of these conditions have been defined, based on the most common phenotypic features, symptoms, and signs associated with the particular mutations that tend to cause them.
An outstanding question and area of research is whether ATP depletion or reactive oxygen species are in fact responsible for the observed phenotypic consequences.
Mitochondrial disorders may be caused by mutations, acquired or inherited, in mitochondrial DNA (mtDNA) or in nuclear genes that code for mitochondrial components. They may also be the result of acquired mitochondrial dysfunction due to adverse effects of drugs, infections, or other environmental causes (see MeSH).
Mitochondrial DNA inheritance behaves differently from autosomal and sex-linked inheritance. Nuclear DNA has two copies per cell (except for sperm and egg cells), and one copy is inherited from the father and the other from the mother. Mitochondrial DNA, however, is strictly inherited from the mother and each mitochondrial organelle typically contains multiple mtDNA copies (see Heteroplasmy). During cell division the mitochondrial DNA copies segregate randomly between the two new mitochondria, and then those new mitochondria make more copies. If only a few of the mtDNA copies inherited from the mother are defective, mitochondrial division may cause most of the defective copies to end up in just one of the new mitochondria (for more detailed inheritance patterns, see Human mitochondrial genetics). Mitochondrial disease may become clinically apparent once the number of affected mitochondria reaches a certain level; this phenomenon is called 'threshold expression'.
Mitochondrial DNA mutations occur frequently, due to the lack of the error checking capability that nuclear DNA has (see Mutation rate). This means that mitochondrial DNA disorders may occur spontaneously and relatively often. Defects in enzymes that control mitochondrial DNA replication (all of which are encoded for by genes in the nuclear DNA) may also cause mitochondrial DNA mutations.
Most mitochondrial function and biogenesis is controlled by nuclear DNA. Human mitochondrial DNA encodes only 13 proteins of the respiratory chain, while most of the estimated 1,500 proteins and components targeted to mitochondria are nuclear-encoded. Defects in nuclear-encoded mitochondrial genes are associated with hundreds of clinical disease phenotypes including anemia, dementia, hypertension, lymphoma, retinopathy, seizures, and neurodevelopmental disorders .
Although research is ongoing, treatment options are currently limited; vitamins are frequently prescribed, though the evidence for their effectiveness is limited.. Pyruvate has been proposed recently as a treatment option.
Spindle transfer, where the nuclear DNA is transferred to another healthy egg cell leaving the defective mitochondrial DNA behind, is a potential treatment procedure that has been successfully carried out on monkeys. 
Embryonic mitochondrial transplant and protofection have been proposed as a possible treatment for inherited mitochondrial disease, and allotopic expression of mitochondrial proteins as a radical treatment for mtDNA mutation load.
About 1 in 4,000 children in the United States will develop mitochondrial disease by the age of 10 years. Up to 4,000 children per year in the United States are born with a type of mitochondrial disease.
Many diseases of aging are caused by defects in mitochondrial function. Since the mitochondria are responsible for processing oxygen and converting substances from the foods we eat into energy for essential cellular functions, if there are problems with the mitochondria, it can lead to many defects for adults. These include Type 2 diabetes, Parkinson's disease, atherosclerotic heart disease, stroke, Alzheimer's disease, and cancer. Many medicines can also injure the mitochondria.
Notable people who suffered from mitochondrial disease include: