Editing Episome

{{Short description|Type of plasmid}}
An '''episome''' is a special type of [[plasmid]], which remains as a part of the eukaryotic genome without integration. Episomes manage this by replicating together with the rest of the genome and subsequently associating with [[metaphase]] chromosomes during mitosis. Episomes do not degrade, unlike standard plasmids, and can be designed so that they are not epigenetically silenced inside the eukaryotic cell nucleus.<ref>{{Cite journal |last1=Van Craenenbroeck |first1=Kathleen |last2=Vanhoenacker |first2=Peter |last3=Haegeman |first3=Guy |date=September 2000 |title=Episomal vectors for gene expression in mammalian cells: Episomal vectors for eukaryotic gene expression |url=http://doi.wiley.com/10.1046/j.1432-1327.2000.01645.x |journal=European Journal of Biochemistry |language=en |volume=267 |issue=18 |pages=5665–5678 |doi=10.1046/j.1432-1327.2000.01645.x|pmid=10971576 }}</ref> Episomes can be observed in nature in certain types of long-term infection by [[adeno-associated virus]] or [[Epstein–Barr virus|Epstein-Barr virus]]. In 2004, it was proposed that non-viral episomes might be used in [[Gene therapy|genetic therapy]] for long-term change in [[gene expression]].<ref>{{cite journal | vauthors = Conese M, Auriche C, Ascenzioni F | title = Gene therapy progress and prospects: episomally maintained self-replicating systems | journal = Gene Therapy | volume = 11 | issue = 24 | pages = 1735–1741 | date = December 2004 | pmid = 15385951 | doi = 10.1038/sj.gt.3302362 | s2cid = 34565556 }}</ref>

As of 1999, there were many known sequences of [[DNA]] (deoxyribonucleic acid) that allow a standard plasmid to become episomally retained. One example is the [[Scaffold/matrix attachment region|S/MAR]] sequence.<ref>{{cite journal | vauthors = Piechaczek C, Fetzer C, Baiker A, Bode J, Lipps HJ | title = A vector based on the SV40 origin of replication and chromosomal S/MARs replicates episomally in CHO cells | journal = Nucleic Acids Research | volume = 27 | issue = 2 | pages = 426–428 | date = January 1999 | pmid = 9862961 | pmc = 148196 | doi = 10.1093/nar/27.2.426 }}</ref>

The length of episomal retention is fairly variable between different genetic constructs and there are many known features in the sequence of an episome which will affect the length and stability of genetic expression of the carried transgene. Among these features is the number of [[CpG site]]s which contribute to epigenetic silencing of the transgene carried by the episome.<ref>{{cite journal | vauthors = Haase R, Argyros O, Wong SP, Harbottle RP, Lipps HJ, Ogris M, Magnusson T, Vizoso Pinto MG, Haas J, Baiker A | display-authors = 6 | title = pEPito: a significantly improved non-viral episomal expression vector for mammalian cells | journal = BMC Biotechnology | volume = 10 | issue = 1 | pages = 20 | date = March 2010 | pmid = 20230618 | pmc = 2847955 | doi = 10.1186/1472-6750-10-20 | doi-access = free }}</ref>

== Mechanism of episomal retention ==
The mechanism behind episomal retention in the case of S/MAR episomes is generally still uncertain. 
As of 1985, in the case of latent Epstein-Barr virus infection, episomes seemed to be associated with nuclear proteins of the host cell through a set of viral proteins.<ref>{{cite journal | vauthors = Rawlins DR, Milman G, Hayward SD, Hayward GS | title = Sequence-specific DNA binding of the Epstein-Barr virus nuclear antigen (EBNA-1) to clustered sites in the plasmid maintenance region | language = English | journal = Cell | volume = 42 | issue = 3 | pages = 859–868 | date = October 1985 | pmid = 2996781 | doi = 10.1016/0092-8674(85)90282-X | s2cid = 9342392 }}</ref>

== Episomes in prokaryotes ==
Episomes in prokaryotes are special sequences which can divide either separate from or integrated into the prokaryotic chromosome.<ref>{{Citation |last=Campbell |first=Allan |title=Episomes |date=1974 |url=https://doi.org/10.1007/978-1-4899-1710-2_18 |work=Bacteria, Bacteriophages, and Fungi: Volume 1 |pages=295–307 |editor-last=King |editor-first=Robert C. |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4899-1710-2_18 |isbn=978-1-4899-1710-2 |access-date=2022-06-25}}</ref>

== References ==
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[[Category:Molecular biology]]