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==Causes== Mitochondrial disorders may be caused by [[mutations]] (acquired or inherited), in [[mitochondrial DNA]] (mtDNA), or in [[nuclear gene]]s 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.<ref>{{Cite web|url=https://meshb.nlm.nih.gov/record/ui?ui=D028361|title=Mitochondrial diseases|website=MeSH|access-date=2 August 2019}}</ref> [[File:Maternal Inheritance - mitochondrial DNA.png|thumb|Example of a pedigree for a genetic trait inherited by mitochondrial DNA in animals and humans. Offspring of the males with the trait don't inherit the trait. Offspring of the females with the trait always inherit the trait (independently from their own gender).]] [[Nuclear DNA]] has two copies per cell (except for sperm and egg cells), one copy being inherited from the father and the other from the mother. Mitochondrial DNA, however, is inherited from the mother only (with [[Paternal mtDNA transmission|some exceptions]]) and each [[mitochondrion]] typically contains between 2 and 10 [[mtDNA]] copies. During [[cell division]] the mitochondria segregate randomly between the two new cells. Those mitochondria make more copies, normally reaching 500 mitochondria per cell. As mtDNA is copied when mitochondria proliferate, they can accumulate random mutations, a phenomenon called [[heteroplasmy]]. 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]]". Mitochondria possess many of the same DNA repair pathways as nuclei doβbut not all of them;<ref>{{cite journal | vauthors = Alexeyev M, Shokolenko I, Wilson G, LeDoux S | title = The maintenance of mitochondrial DNA integrity--critical analysis and update | journal = Cold Spring Harbor Perspectives in Biology | volume = 5 | issue = 5 | pages = a012641 | date = May 2013 | pmid = 23637283 | pmc = 3632056 | doi = 10.1101/cshperspect.a012641 }}</ref> therefore, mutations occur more frequently in mitochondrial DNA than in nuclear DNA (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 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]].<ref>{{cite journal |author2-link=Helen H. Lu |vauthors = Scharfe C, Lu HH, Neuenburg JK, Allen EA, Li GC, Klopstock T, Cowan TM, Enns GM, Davis RW |title = Mapping gene associations in human mitochondria using clinical disease phenotypes |journal = PLOS Comput Biol |year = 2009 |pmid = 19390613 |volume = 5 |issue = 4 |pages = e1000374 |doi = 10.1371/journal.pcbi.1000374 |pmc = 2668170 |veditors = Rzhetsky A|bibcode = 2009PLSCB...5E0374S |doi-access = free }}</ref> A study by Yale University researchers (published in the February 12, 2004, issue of the ''[[New England Journal of Medicine]]'') explored the role of mitochondria in insulin resistance among the offspring of patients with type 2 diabetes.<ref name="Petersen et al.">{{cite journal |last1=Petersen |first1=Kitt Falk |last2=Dufour |first2=Sylvie |last3=Befroy |first3=Douglas |last4=Garcia |first4=Rina |last5=Shulman |first5=Gerald I. |title=Impaired Mitochondrial Activity in the Insulin-Resistant Offspring of Patients with Type 2 Diabetes |journal=New England Journal of Medicine |date=12 February 2004 |volume=350 |issue=7 |pages=664β671 |doi=10.1056/NEJMoa031314 |pmid=14960743 |pmc=2995502 }}</ref> Other studies have shown that the mechanism may involve the interruption of the mitochondrial signaling process in body cells ([[intramyocellular lipids]]). A study conducted at the Pennington Biomedical Research Center in Baton Rouge, Louisiana<ref>{{cite journal |last1=Sparks |first1=Lauren M. |last2=Xie |first2=Hui |last3=Koza |first3=Robert A. |last4=Mynatt |first4=Randall |last5=Hulver |first5=Matthew W. |last6=Bray |first6=George A. |last7=Smith |first7=Steven R. |title=A High-Fat Diet Coordinately Downregulates Genes Required for Mitochondrial Oxidative Phosphorylation in Skeletal Muscle |journal=Diabetes |date=July 2005 |volume=54 |issue=7 |pages=1926β1933 |id={{Gale|A134380159}} {{ProQuest|216493144}} |doi=10.2337/diabetes.54.7.1926 |pmid=15983191 }}</ref> showed that this, in turn, partially disables the genes that produce mitochondria.
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