Просмотр кода страницы Exon skipping

{{short description|Form of RNA splicing used to cause cells to "skip" over faulty sections of genetic code}}
In [[molecular biology]], '''exon skipping''' is a form of [[RNA splicing]] used to cause cells to “skip” over faulty or misaligned sections ([[exon]]s) of a gene, leading to a truncated but still functional protein despite the genetic mutation.

== Mechanism ==
{{See also|RNA splicing}}
Exon skipping is used to restore the [[reading frame]] within a gene. [[Gene]]s are the genetic instructions for creating a protein, and are composed of [[intron]]s and [[exon]]s. Exons are the sections of DNA that contain the instruction set for generating a protein; they are interspersed with non-coding regions called introns. The introns are later removed before the protein is made, leaving only the coding exon regions.

Splicing naturally occurs in [[pre-mRNA]] when introns are being removed to form mature-mRNA that consists solely of exons. Starting in the late 1990s, scientists realized they could take advantage of this naturally occurring cellular splicing to downplay genetic [[mutations]] into less harmful ones.<ref name = "wahl">{{cite web | vauthors = Wahl M | url = http://quest.mda.org/article/exon-skipping-dmd-what-it-and-whom-can-it-help | title = Exon Skipping in DMD: What Is It and Whom Can It Help? | work = Quest Magazine Online | date = 1 October 2011 }}</ref><ref name=goyenvalle2004>{{cite journal | vauthors = Goyenvalle A, Vulin A, Fougerousse F, Leturcq F, Kaplan JC, Garcia L, Danos O | title = Rescue of dystrophic muscle through U7 snRNA-mediated exon skipping | journal = Science | volume = 306 | issue = 5702 | pages = 1796–9 | date = December 2004 | pmid = 15528407 | doi = 10.1126/science.1104297 | bibcode = 2004Sci...306.1796G | s2cid = 9359783 | doi-access = free }}</ref>

The mechanism behind exon skipping is a mutation specific [[antisense oligonucleotide]] (AON). An antisense oligonucleotide is a synthesized short nucleic acid polymer, typically fifty or fewer base pairs in length that will bind to the mutation site in the pre-messenger RNA, to induce exon skipping.<ref name = "hard">{{cite journal | vauthors = Harding PL, Fall AM, Honeyman K, Fletcher S, Wilton SD | title = The influence of antisense oligonucleotide length on dystrophin exon skipping | journal = Molecular Therapy | volume = 15 | issue = 1 | pages = 157–66 | date = January 2007 | pmid = 17164787 | doi = 10.1038/sj.mt.6300006 | doi-access = free }}</ref> The AON binds to the mutated exon, so that when the gene is then translated from the mature mRNA, it is “skipped” over, thus restoring the disrupted reading frame.<ref name = "hard"/> This allows for the generation of an internally deleted, but largely functional protein.

Some mutations require exon skipping at multiple sites, sometimes adjacent to one another, in order to restore the reading frame. Multiple exon skipping has successfully been carried out using a combination of AONs that target multiple exons.<ref name = "aartsma1">{{cite journal | vauthors = Aartsma-Rus A, Fokkema I, Verschuuren J, Ginjaar I, van Deutekom J, van Ommen GJ, den Dunnen JT | title = Theoretic applicability of antisense-mediated exon skipping for Duchenne muscular dystrophy mutations | journal = Human Mutation | volume = 30 | issue = 3 | pages = 293–9 | date = March 2009 | pmid = 19156838 | doi = 10.1002/humu.20918 | s2cid = 45979175 | doi-access = free }}</ref>

== As a treatment for Duchenne muscular dystrophy ==

Exon skipping is being heavily researched for the treatment of [[Duchenne muscular dystrophy]] (DMD), where the muscular protein [[dystrophin]] is prematurely truncated, which leads to a non-functioning protein. Successful treatment by way of exon skipping could lead to a mostly functional dystrophin protein, and create a phenotype similar to the less severe [[Becker muscular dystrophy]] (BMD).<ref name = "wahl"/><ref name="what">[http://www.muscular-dystrophy.org/about_muscular_dystrophy/research_faqs/612_what_is_exon_skipping_and_how_does_it_work What Is Exon Skipping and How Does It Work?] {{Webarchive|url=https://web.archive.org/web/20141208190454/http://www.muscular-dystrophy.org/about_muscular_dystrophy/research_faqs/612_what_is_exon_skipping_and_how_does_it_work |date=2014-12-08 }} Muscular Dystrophy Campaign. N.p., 11 July 2009. Web. 05 Nov. 2012.</ref>

In the case of Duchenne muscular dystrophy, the protein that becomes compromised is dystrophin.<ref name="what"/> The dystrophin protein has two essential functional domains that flank a central rod domain consisting of repetitive and partially dispensable segments.<ref name = "aartsma">{{cite journal | vauthors = Aartsma-Rus A, van Ommen GJ | title = Antisense-mediated exon skipping: a versatile tool with therapeutic and research applications | journal = RNA | volume = 13 | issue = 10 | pages = 1609–24 | date = October 2007 | pmid = 17684229 | pmc = 1986821 | doi = 10.1261/rna.653607 }}</ref> Dystrophin’s function is to maintain muscle fiber stability during contraction by linking the extra cellular matrix to the cytoskeleton. Mutations that disrupt the [[open reading frame]] within dystrophin create prematurely truncated proteins that are unable to perform their job. Such mutations lead to muscle fiber damage, replacement of muscle tissue by fat and fibrotic tissue, and premature death typically occurring in the early twenties of DMD patients.<ref name = "aartsma"/> Comparatively, mutations that do not upset the open reading frame, lead to a dystrophin protein that is internally deleted and shorter than normal, but still partially functional. Such mutations are associated with the much milder Becker muscular dystrophy. Mildly affected BMD patients carrying deletions that involve over two thirds of the central rod domain have been described, suggesting that this domain is largely dispensable.

Dystrophin can maintain a large degree of functionality so long as the essential terminal domains are unaffected, and exon skipping only occurs within the central rod domain. Given these parameters, exon skipping can be used to restore an open reading frame by inducing a deletion of one or several exons within the central rod domain, and thus converting a DMD phenotype into a BMD phenotype.	

The genetic mutation that leads to Becker muscular dystrophy is an [[in-frame deletion]]. This means that, out of the 79 exons that code for dystrophin, one or several in the middle may be removed, without affecting the exons that follow the deletion. This allows for a shorter-than-normal dystrophin protein that maintains a degree of functionality. In Duchenne muscular dystrophy, the genetic mutation is out-of-frame. Out-of-frame mutations cause a premature stop in protein generation - the ribosome is unable to “read” the RNA past the point of initial error - leading to a severely shortened and completely non-functional dystrophin protein.<ref name = "aartsma" />

The goal of exon skipping is to manipulate the splicing pattern so that an out-of-frame mutation becomes an in-frame mutation, thus changing a severe DMD mutation into a less harmful in-frame BMD mutation.

One exon-skipping drug was approved in 2016, by the US FDA: [[eteplirsen]] (ExonDys51), a [[Morpholino]] oligo from [[Sarepta Therapeutics]] targeting exon 51 of human dystrophin. Another exon-skipping Morpholino, [[golodirsen]] (Vyondys 53) (targeting dystrophin exon 53), was approved in the United States in December 2019.<ref name="Golodirsen PR">{{cite press release | title=FDA grants accelerated approval to first targeted treatment for rare Duchenne muscular dystrophy mutation | website=U.S. [[Food and Drug Administration]] (FDA) | date=12 December 2019 | url=https://www.fda.gov/news-events/press-announcements/fda-grants-accelerated-approval-first-targeted-treatment-rare-duchenne-muscular-dystrophy-mutation | archive-url=https://web.archive.org/web/20191213043443/https://www.fda.gov/news-events/press-announcements/fda-grants-accelerated-approval-first-targeted-treatment-rare-duchenne-muscular-dystrophy-mutation | archive-date=13 December 2019 | url-status=dead | access-date=12 December 2019}} {{PD-notice}}</ref> A third [[antisense oligonucleotide]], [[viltolarsen]] (Viltepso), targeting dystrophin exon 53 was approved for medical use in the United States in August 2020.<ref name="Viltolarsen PR">{{cite press release | title=FDA Approves Targeted Treatment for Rare Duchenne Muscular Dystrophy Mutation | website=U.S. [[Food and Drug Administration]] (FDA) | date=12 August 2020 | url=https://www.fda.gov/news-events/press-announcements/fda-approves-targeted-treatment-rare-duchenne-muscular-dystrophy-mutation | archive-url=https://web.archive.org/web/20200820041846/https://www.fda.gov/news-events/press-announcements/fda-approves-targeted-treatment-rare-duchenne-muscular-dystrophy-mutation | url-status=dead | archive-date=August 20, 2020 | access-date=12 August 2020}} {{PD-notice}}</ref>

{| class="wikitable"
|+ exon skipping drugs approved for DMD
|-
! drug !! exon!! company !! UD FDA approval
|-
| eteplirsen || 51 || Sarepta || September 2016
|-
| golodirsen || 53 || Sarepta || December 2019
|-
| viltolarsen || 53 || NS Pharma || August 2020
|-
| casimerson || 45 || Sarepta || March 2021
|}

[[Genetic testing]], usually from blood samples, can be used to determine the precise nature and location of the DMD mutation in the dystrophin gene. It is known that these mutations cluster in areas known as the 'hot spot' regions — primarily in exons 45–53 and to a lesser extent exons 2–20.<ref name = "aartsma1"/> As the majority of DMD mutations occur in these 'hot spot' regions, a treatment which causes these exons to be skipped could be used to treat up to 50% of DMD patients.<ref name = "aartsma1"/><ref name="what"/><ref>{{cite journal | vauthors = van Deutekom JC, van Ommen GJ | title = Advances in Duchenne muscular dystrophy gene therapy | journal = Nature Reviews. Genetics | volume = 4 | issue = 10 | pages = 774–83 | date = October 2003 | pmid = 14526374 | doi = 10.1038/nrg1180 | s2cid = 207859539 }}</ref>

== See also ==
*[[Antisense therapy]]
*[[Poison exon]]

== References ==
{{Reflist|30em}}

[[Category:Genetics]]