Editing Regulatory sequence (section)

====Super-enhancer====
{{main|Super-enhancer}}
[[File:Super-enhancer.png|thumb|435x435px|A super-enhancer is a cluster of typical enhancers that drives a high level of transcription of a target gene]]

While enhancers are needed for transcription of genes in a cell above low levels, a cluster of enhancers, known as a super-enhancer, can cause transcription of a target gene at even higher levels. Super-enhancers usually drive genes needed for cell identity to express at high levels.<ref name=Hnisz2013>{{cite journal |vauthors=Hnisz D, Abraham BJ, Lee TI, Lau A, Saint-André V, Sigova AA, Hoke HA, Young RA |title=Super-enhancers in the control of cell identity and disease |journal=Cell |volume=155 |issue=4 |pages=934–47 |date=November 2013 |pmid=24119843 |pmc=3841062 |doi=10.1016/j.cell.2013.09.053 |url=}}</ref><ref name=Hnisz2015>{{cite journal |vauthors=Hnisz D, Schuijers J, Lin CY, Weintraub AS, Abraham BJ, Lee TI, Bradner JE, Young RA |title=Convergence of developmental and oncogenic signaling pathways at transcriptional super-enhancers |journal=Mol Cell |volume=58 |issue=2 |pages=362–70 |date=April 2015 |pmid=25801169 |pmc=4402134 |doi=10.1016/j.molcel.2015.02.014 |url=}}</ref> In cancers, a super-enhancer may also drive a particular oncogene to express at a high level.<ref name=Hnisz2013 /><ref name=Hnisz2015 />

A super-enhancer is defined as a cluster of typical enhancers in close genomic proximity (within about 9,000<ref name=Hnisz2013 /> to 22,0000<ref name="pmid30169995">{{cite journal |vauthors=Khan A, Mathelier A, Zhang X |title=Super-enhancers are transcriptionally more active and cell type-specific than stretch enhancers |journal=Epigenetics |volume=13 |issue=9 |pages=910–922 |date=2018 |pmid=30169995 |pmc=6284781 |doi=10.1080/15592294.2018.1514231 |url=}}</ref> base pairs in length) that, all together, regulate the expression of a target gene.<ref name=Sengupta>{{cite journal |vauthors=Sengupta S, George RE |title=Super-Enhancer-Driven Transcriptional Dependencies in Cancer |journal=Trends Cancer |volume=3 |issue=4 |pages=269–281 |date=April 2017 |pmid=28718439 |pmc=5546010 |doi=10.1016/j.trecan.2017.03.006 |url=}}</ref>  Super-enhancer-driven genes are expressed at significantly higher levels than the expression of genes under the control of typical enhancers.<ref name=Sengupta />

A diagram of a super-enhancer is shown in the Figure in this section. In this Figure, the super-enhancer is 12,000 nucleotides long and has four typical enhancers within its length.  Each of the typical enhancers simultaneously contacts the [[Promoter (genetics)|promoter]] region of the same target gene.  Each typical enhancer within the super-enhancer has multiple [[DNA binding site|DNA motifs]] to which transcription factors bind. Each typical enhancer is also bound to a 26-component [[Mediator (coactivator)|mediator complex]] which transmits the signals from the transcription factors bound to the enhancer to the promoter of their joint target gene.  The protein [[BRD4]] forms a complex with each typical enhancer in the super-enhancer and helps to stabilizes the super-enhancer structure.<ref name="pmid29930091">{{cite journal |vauthors=Sabari BR, Dall'Agnese A, Boija A, Klein IA, Coffey EL, Shrinivas K, Abraham BJ, Hannett NM, Zamudio AV, Manteiga JC, Li CH, Guo YE, Day DS, Schuijers J, Vasile E, Malik S, Hnisz D, Lee TI, Cisse II, Roeder RG, Sharp PA, Chakraborty AK, Young RA |title=Coactivator condensation at super-enhancers links phase separation and gene control |journal=Science |volume=361 |issue=6400 |pages= |date=July 2018 |pmid=29930091 |pmc=6092193 |doi=10.1126/science.aar3958 |url=}}</ref>  In addition, the architectural protein YY1 (indicated by paired red zigzags) helps keep the loops together that bring the typical enhancers to their target gene in the super-enhancer.<ref name="pmid29224777">{{cite journal |vauthors=Weintraub AS, Li CH, Zamudio AV, Sigova AA, Hannett NM, Day DS, Abraham BJ, Cohen MA, Nabet B, Buckley DL, Guo YE, Hnisz D, Jaenisch R, Bradner JE, Gray NS, Young RA |title=YY1 Is a Structural Regulator of Enhancer-Promoter Loops |journal=Cell |volume=171 |issue=7 |pages=1573–1588.e28 |date=December 2017 |pmid=29224777 |pmc=5785279 |doi=10.1016/j.cell.2017.11.008 |url=}}</ref> Therefore, there are many proteins in close association at a super-enhancer. These proteins generally have a structured domain as well as a tail with an [[Intrinsically disordered proteins|intrinsically disordered region]] (IDR).<ref name=Boija>{{cite journal |vauthors=Boija A, Klein IA, Sabari BR, Dall'Agnese A, Coffey EL, Zamudio AV, Li CH, Shrinivas K, Manteiga JC, Hannett NM, Abraham BJ, Afeyan LK, Guo YE, Rimel JK, Fant CB, Schuijers J, Lee TI, Taatjes DJ, Young RA |title=Transcription Factors Activate Genes through the Phase-Separation Capacity of Their Activation Domains |journal=Cell |volume=175 |issue=7 |pages=1842–1855.e16 |date=December 2018 |pmid=30449618 |pmc=6295254 |doi=10.1016/j.cell.2018.10.042 |url=}}</ref> Many of the IDRs of these proteins interact with each other, thereby forming a water-excluding gel or [[biomolecular condensate|phase-separated condensate]] around the super-enhancer.<ref name=Boija />

Some super-enhancers induce very high levels of transcription such as the mouse α-globin super-enhancer<ref name=Blayney>{{cite journal |vauthors=Blayney JW, Francis H, Rampasekova A, Camellato B, Mitchell L, Stolper R, Cornell L, Babbs C, Boeke JD, Higgs DR, Kassouf M |title=Super-enhancers include classical enhancers and facilitators to fully activate gene expression |journal=Cell |volume=186 |issue=26 |pages=5826–5839.e18 |date=December 2023 |pmid=38101409 |pmc=10858684 |doi=10.1016/j.cell.2023.11.030 |url=}}</ref> and the Wap super-enhancer.<ref name=Shin>{{cite journal |vauthors=Shin HY, Willi M, HyunYoo K, Zeng X, Wang C, Metser G, Hennighausen L |title=Hierarchy within the mammary STAT5-driven Wap super-enhancer |journal=Nat Genet |volume=48 |issue=8 |pages=904–911 |date=August 2016 |pmid=27376239 |pmc=4963296 |doi=10.1038/ng.3606 |url=}}</ref>  The mouse α-globin super-enhancer has five typical enhancers within the super-enhancer. Only when acting together, they increase transcription of the ''α-globin gene'' by 450-fold.<ref name=Blayney />  In another example, the mouse Wap super-enhancer includes three typical enhancers.  Only when the three typical enhancers act together do they increase transcription of the ''Wap'' gene by 1000-fold.<ref name=Shin /> 

The enhancers within the super-enhancers described above act synergistically.  However, in a second type of super-enhancer, the component enhancers act additively. In a third group, super-enhancers appear to act “logistically” where promoter activity reaches a limit.  One study examined 773 target genes that were paired with near-by groups of possible super-enhancers (with 2–20 enhancers in close proximity likely acting as super-enhancers).  In this study it appeared that 277, 92, and 250 of the likely super-enhancers acted by the additive, synergistic, and logistic models.<ref name="pmid33770473">{{cite journal |vauthors=Choi J, Lysakovskaia K, Stik G, Demel C, Söding J, Tian TV, Graf T, Cramer P |title=Evidence for additive and synergistic action of mammalian enhancers during cell fate determination |journal=eLife |volume=10 |issue= |pages= |date=March 2021 |pmid=33770473 |pmc=8004103 |doi=10.7554/eLife.65381 |doi-access=free |url=}}</ref> 

Super-enhancers may occupy regions of the genome about 10,000 to 60,000 nucleotides long.<ref name="pmid31724731">{{cite journal |vauthors=Wang X, Cairns MJ, Yan J |title=Super-enhancers in transcriptional regulation and genome organization |journal=Nucleic Acids Res |volume=47 |issue=22 |pages=11481–11496 |date=December 2019 |pmid=31724731 |pmc=7145697 |doi=10.1093/nar/gkz1038 |url=}}</ref> while typical enhancers are each about 204 base pairs long.<ref name="pmid32728217">{{cite journal |vauthors=Meuleman W, Muratov A, Rynes E, Halow J, Lee K, Bates D, Diegel M, Dunn D, Neri F, Teodosiadis A, Reynolds A, Haugen E, Nelson J, Johnson A, Frerker M, Buckley M, Sandstrom R, Vierstra J, Kaul R, Stamatoyannopoulos J |title=Index and biological spectrum of human DNase I hypersensitive sites |journal=Nature |volume=584 |issue=7820 |pages=244–251 |date=August 2020 |pmid=32728217 |pmc=7422677 |doi=10.1038/s41586-020-2559-3 |bibcode=2020Natur.584..244M |url=}}</ref> When 8 types of cells were evaluated, super-enhancers constituted between 2.5% to 10.9% of the enhancers driving transcription while typical enhancers were the majority of enhancers driving transcription.  There were between 257 and 1,099 super-enhancers in these eight cell types and between 5,512 and 23,869 typical enhancers.<ref name="pmid26569311">{{cite journal |vauthors=Niederriter AR, Varshney A, Parker SC, Martin DM |title=Super Enhancers in Cancers, Complex Disease, and Developmental Disorders |journal=Genes (Basel) |volume=6 |issue=4 |pages=1183–200 |date=November 2015 |pmid=26569311 |pmc=4690034 |doi=10.3390/genes6041183 |doi-access=free |url=}}</ref>

While super-enhancers are only active at about 2.5% – 10.9 % of actively transcribed sites in a cell, they recruit transcription machinery more actively than at typical single enhancers. The super-enhancers in a cell utilize about 12% to 36% of the RNA polymerases, mediator proteins, BRD4 proteins, and other transcription machinery of the cell.<ref name=Hnisz2013 />