Anepisome is a special type ofplasmid, 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 withmetaphase 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.[1] Episomes can be observed in nature in certain types of long-term infection byadeno-associated virus orEpstein-Barr virus. In 2004, it was proposed that non-viral episomes might be used ingenetic therapy for long-term change ingene expression.[2]
As of 1999, there were many known sequences ofDNA (deoxyribonucleic acid) that allow a standard plasmid to become episomally retained. One example is theS/MAR sequence.[3]
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 ofCpG sites which contribute to epigenetic silencing of the transgene carried by the episome.[4]
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.[5]
^Rawlins DR, Milman G, Hayward SD, Hayward GS (October 1985). "Sequence-specific DNA binding of the Epstein-Barr virus nuclear antigen (EBNA-1) to clustered sites in the plasmid maintenance region".Cell.42 (3):859–868.doi:10.1016/0092-8674(85)90282-X.PMID2996781.S2CID9342392.
