Mitochondrial diseases are a group of disorders relating to the mitochondria, the organelles that are the "powerhouses" of the eukaryotic cells that comprise higher-order lifeforms (including humans). The mitochondria convert the energy of food molecules into the ATP that powers most cell functions. more...
Mitochondrial diseases comprise those disorders that in one way or another affect the function of the mitochondria and/or are due to mitochondrial DNA. Mitochondrial diseases take on unique characteristics both because of the way the diseases are often inherited and because that 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.
Mitochondrial inheritance behaves differently from the sort of inheritance that we are most familiar with. Regular nuclear DNA has two copies per cell (except for sperm and egg cells). One copy is inherited from the father and the other from the mother. Mitochondria, however, contain their own DNA, and contain typically from five to ten copies, all inherited from the mother (for more detailed inheritance patterns, see mitochondrial genetics). When mitochondria divide, the copies of DNA present are divided randomly between the two new mitochondria, and then those new mitochondria make more copies. As a result, if only a few of the DNA 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. Once more than half of the DNA copies are defective, mitochondrial disease begins to become apparent, this phenomenon is called 'threshold expression'.
It should be noted, however, that not all of the enzymes and other components necessary for proper mitochondrial function are encoded in the mitochondrial DNA. Most mitochondrial function is controlled by nuclear DNA instead.
To make things even more confusing, mutations to mitochondrial DNA occur frequently, due to the lack of the error checking capability that nuclear DNA has. This means that a mitochondrial disorder can occur spontaneously rather than be inherited. Further, sometimes the enzymes that control mitochondrial DNA duplication (and which are encoded for by genes in the nuclear DNA) are defective, causing mitochondrial DNA mutations to occur at a rapid rate.
Defects and symptoms
The effects of mitochondrial disease can be quite varied. Since the distribution of defective DNA may vary from organ to organ within the body, the mutation that in one person may cause liver disease might in another person cause a brain disorder. In addition, the severity of the defect may be great or small. Some minor defects cause only "exercise intolerance", with no serious illness or disability. Other defects can more severely affect the operation of the mitochondria and can cause severe body-wide impacts. As a general rule, mitochondrial diseases are worst when the defective mitochondria are present in the muscles or nerves, because these are the most energy-hungry cells of the body.
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