Editing Repeated sequence (DNA) (section)

== Repeated sequences in human disease ==
For humans, some repeated DNA sequences are associated with diseases. Specifically, tandem repeat sequences, underlie several [[Trinucleotide repeat disorder|human disease conditions]], particularly trinucleotide repeat diseases such as [[Huntington's disease]], [[fragile X syndrome]], several [[spinocerebellar ataxia]]s, [[myotonic dystrophy]] and [[Friedreich ataxia|Friedreich's ataxia]].<ref name="pmid25608779">{{cite journal | vauthors = Usdin K, House NC, Freudenreich CH | title = Repeat instability during DNA repair: Insights from model systems | journal = Critical Reviews in Biochemistry and Molecular Biology | volume = 50 | issue = 2 | pages = 142–167 | date = 22 January 2015 | pmid = 25608779 | pmc = 4454471 | doi = 10.3109/10409238.2014.999192 }}</ref> [[Trinucleotide repeat expansion]]s in the [[germline]] over successive generations can lead to increasingly severe manifestations of the disease. [[Trinucleotide repeat expansion|These trinucleotide repeat expansions]] may occur through [[Slipped strand mispairing|strand slippage]] during [[DNA replication]] or during [[DNA repair]] synthesis.<ref name="pmid25608779" /> It has been noted that [[gene]]s containing pathogenic CAG repeats often encode proteins that themselves have a role in the [[DNA damage (naturally occurring)|DNA damage]] response and that repeat expansions may impair specific DNA repair pathways.<ref name="Massey2018">{{cite journal | vauthors = Massey TH, Jones L | title = The central role of DNA damage and repair in CAG repeat diseases | journal = Disease Models & Mechanisms | volume = 11 | issue = 1 | pages = dmm031930 | date = January 2018 | pmid = 29419417 | pmc = 5818082 | doi = 10.1242/dmm.031930 }}</ref> Faulty repair of DNA damages in repeat sequences may cause further expansion of these sequences, thus setting up a vicious cycle of pathology.<ref name="Massey2018" />

=== Huntington's disease ===
[[File:Huntington's_disease_(5880985560).jpg|thumb|280x280px|Image of the repeated DNA sequence in Huntington's disease.]]
[[Huntington's disease]] is a neurodegenerative disorder which is due to the expansion of repeated trinucleotide sequence CAG in [[exon]] 1 of the ''[[huntingtin]]'' gene (''HTT''). This gene is responsible for encoding the protein huntingtin which plays a role in preventing apoptosis,<ref>{{cite journal | vauthors = Cattaneo E, Zuccato C, Tartari M | title = Normal huntingtin function: an alternative approach to Huntington's disease | journal = Nature Reviews. Neuroscience | volume = 6 | issue = 12 | pages = 919–930 | date = December 2005 | pmid = 16288298 | doi = 10.1038/nrn1806 | s2cid = 10119487 }}</ref> otherwise known as cell death, and [[DNA repair|repair of oxidative DNA damage]].<ref>{{cite journal | vauthors = Maiuri T, Mocle AJ, Hung CL, Xia J, van Roon-Mom WM, Truant R | title = Huntingtin is a scaffolding protein in the ATM oxidative DNA damage response complex | journal = Human Molecular Genetics | volume = 26 | issue = 2 | pages = 395–406 | date = January 2017 | pmid = 28017939 | doi = 10.1093/hmg/ddw395 | doi-access = free }}</ref>  In Huntington's disease the expansion of the trinucleotide sequence CAG encodes for a mutant huntingtin protein with an expanded polyglutamine domain.<ref>{{cite journal | vauthors = Schulte J, Littleton JT | title = The biological function of the Huntingtin protein and its relevance to Huntington's Disease pathology | journal = Current Trends in Neurology | volume = 5 | pages = 65–78 | date = January 2011 | pmid = 22180703 | pmc = 3237673 }}</ref> This domain causes the protein to form aggregates in nerve cells preventing normal cellular function and resulting in neurodegeneration.
[[File:Fragile_X_Chromosomal_Differences.png|thumb|280x280px|Fragile X repeated CCG DNA sequence in comparison to a normal X chromosome.]]

=== Fragile X syndrome ===
[[Fragile X syndrome]] is caused by the expansion of the DNA sequence CCG in the ''FMR1'' gene on the X chromosome.<ref>{{cite journal | vauthors = Penagarikano O, Mulle JG, Warren ST | title = The pathophysiology of fragile x syndrome | journal = Annual Review of Genomics and Human Genetics | volume = 8 | issue = 1 | pages = 109–129 | date = 2007-09-01 | pmid = 17477822 | doi = 10.1146/annurev.genom.8.080706.092249 }}</ref> This gene produces the RNA-binding protein FMRP. In the case of Fragile X syndrome the repeated sequence makes the gene unstable and therefore silences the gene ''FMR1.''<ref>{{cite journal | vauthors = Hagerman RJ, Berry-Kravis E, Hazlett HC, Bailey DB, Moine H, Kooy RF, Tassone F, Gantois I, Sonenberg N, Mandel JL, Hagerman PJ | display-authors = 6 | title = Fragile X syndrome | journal = Nature Reviews. Disease Primers | volume = 3 | issue = 1 | pages = 17065 | date = September 2017 | pmid = 28960184 | doi = 10.1038/nrdp.2017.65 | s2cid = 583204 }}</ref> Because the gene resides on the X chromosome, females who have two X chromosomes are less effected than males who only have on X chromosome and one Y chromosome because the second X chromosome can compensate for the silencing of the gene on the other X chromosome.

=== Spinocerebellar ataxias ===
The disease [[spinocerebellar ataxia]]s has CAG [[Trinucleotide repeat disorder|trinucleotide repeat sequences that underlie several types of spinocerebellar ataxias]] (SCAs-[[Spinocerebellar ataxia type 1|SCA1]]; [[Spinocerebellar ataxia|SCA2; SCA3; SCA6; SCA7; SCA12; SCA17]]).<ref name="Abugable2019">{{cite journal | vauthors = Abugable AA, Morris JL, Palminha NM, Zaksauskaite R, Ray S, El-Khamisy SF | title = DNA repair and neurological disease: From molecular understanding to the development of diagnostics and model organisms | journal = DNA Repair | volume = 81 | pages = 102669 | date = September 2019 | pmid = 31331820 | doi = 10.1016/j.dnarep.2019.102669 | doi-access = free }}</ref> Similar to Huntington's disease, the polyglutamine tail created due to this trinucleotide expansion causes aggregation of proteins, preventing normal cellular function and causing neurodegeneration.<ref>{{cite journal | vauthors = Honti V, Vécsei L | title = Genetic and molecular aspects of spinocerebellar ataxias | journal = Neuropsychiatric Disease and Treatment | volume = 1 | issue = 2 | pages = 125–133 | date = June 2005 | pmid = 18568057 | pmc = 2413192 | doi = 10.2147/nedt.1.2.125.61044 | doi-access = free }}</ref>

=== Friedreich's Ataxia ===
[[Friedreich's ataxia]] is a type of ataxia that has an expanded repeat sequence GAA in the frataxin gene.<ref>{{cite journal | vauthors = Bürk K | title = Friedreich Ataxia: current status and future prospects | journal = Cerebellum & Ataxias | volume = 4 | issue = 1 | pages = 4 | date = 2017 | pmid = 28405347 | pmc = 5383992 | doi = 10.1186/s40673-017-0062-x | doi-access = free }}</ref> The frataxin gene is responsible for producing the frataxin protein, which is a mitochondrial protein involved in energy production and cellular respiration.<ref>{{cite journal | vauthors = Mazzara PG, Muggeo S, Luoni M, Massimino L, Zaghi M, Valverde PT, Brusco S, Marzi MJ, Palma C, Colasante G, Iannielli A, Paulis M, Cordiglieri C, Giannelli SG, Podini P, Gellera C, Taroni F, Nicassio F, Rasponi M, Broccoli V | display-authors = 6 | title = Frataxin gene editing rescues Friedreich's ataxia pathology in dorsal root ganglia organoid-derived sensory neurons | journal = Nature Communications | volume = 11 | issue = 1 | pages = 4178 | date = August 2020 | pmid = 32826895 | pmc = 7442818 | doi = 10.1038/s41467-020-17954-3 | bibcode = 2020NatCo..11.4178M }}</ref> The expanded GAA sequence results in the silencing of the first intron resulting in loss of function in the frataxin protein. The loss of a functional ''FXN'' gene leads to issues with mitochondrial functioning as a whole and can present phenotypically in patients as difficulty walking.

=== Myotonic dystrophy ===
[[Myotonic dystrophy]] is a disorder that presents as muscle weakness and consists of two main types: DM1 and DM2.<ref>{{cite journal | vauthors = Hahn C, Salajegheh MK | title = Myotonic disorders: A review article | journal = Iranian Journal of Neurology | volume = 15 | issue = 1 | pages = 46–53 | date = January 2016 | pmid = 27141276 | pmc = 4852070 }}</ref> Both types of myotonic dystrophy are due to expanded DNA sequences. In DM1 the DNA sequence that is expanded is CTG while in DM2 it is CCTG. These two sequences are found on different genes with the expanded sequence in DM2 being found on the ''ZNF9'' gene and the expanded sequence in DM1 found on the ''DMPK'' gene. The two genes don't encode for proteins unlike other disorders like Huntington's disease or Fragile X syndrome. It has been shown, however, that there is a link between RNA toxicity and the repeat sequences in DM1 and DM2.

=== Amyotrophic lateral sclerosis and Frontotemporal dementia ===
Not all diseases caused by repeated DNA sequences are trinucleotide repeat diseases. The diseases [[amyotrophic lateral sclerosis]] and [[frontotemporal dementia]] are caused by hexanucleotide GGGGCC repeat sequences in the ''[[C9orf72]]'' gene, causing RNA toxicity that leads to neurodegeneration.<ref>{{cite journal | vauthors = van Blitterswijk M, DeJesus-Hernandez M, Rademakers R | title = How do C9ORF72 repeat expansions cause amyotrophic lateral sclerosis and frontotemporal dementia: can we learn from other noncoding repeat expansion disorders? | journal = Current Opinion in Neurology | volume = 25 | issue = 6 | pages = 689–700 | date = December 2012 | pmid = 23160421 | pmc = 3923493 | doi = 10.1097/WCO.0b013e32835a3efb }}</ref><ref name="Abugable2019" />