Mitochondrial threshold effect is a phenomenon where the number of mutatedmtDNA has surpassed a certain threshold which causes theelectron transport chain and ATP synthesis of amitochondrion to fail.[1] There isn't a set number that needs to be surpassed, however, it is associated with an increase of the number of mutatedmtDNA. When there is 60-80% of mutatedmtDNA present, that is said to be the threshold level.[1] While 60-80% is the general threshold level, this is also dependent on the individual, the specific organ in question and what the specific mutation is. There are three specific types of mitochondrial threshold effects: phenotypic threshold effect, biochemical threshold effect and translational threshold effect.[citation needed]
Threshold expression is a phenomenon in which phenotypic expression of amitochondrial disease within an organ system occurs when the severity of the mutation, relative number of mutantmtDNA, and reliance of the organ system onoxidative phosphorylation combine in such a way that ATP production of the tissue falls below the level required by the tissue. Thephenotype may be expressed even if the percentage of mutant mtDNA is below 50% if the mutation is severe enough.[citation needed]
Phenotypic threshold effect is when there is a certain amount of wild-typemtDNA present in themitochondrion which is able to balance out the number of mutatedmtDNA.[2] As a result, thephenotype is normal. However, if the number of wild-typemtDNA decreases and the number of mutantmtDNA increases, resulting in an imbalance between the two, the threshold level has been altered which causes complications. This occurs because the wild-typemtDNA present are able to keep theelectron transport chain and ATP synthesis functioning despite there being a few number of them present. They are able to counterbalance the mutatedmtDNA, however, when the number drops below threshold level the mutantmtDNA take over.[2]
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