Editing Stem cell (section)

=== Cell cycle control ===
{{Further|Cell cycle}}
Embryonic stem cells (ESCs) have the ability to divide indefinitely while keeping their [[pluripotency]], which is made possible through specialized mechanisms of [[cell cycle]] control.<ref name=":0">{{cite journal | vauthors = Koledova Z, Krämer A, Kafkova LR, Divoky V | title = Cell-cycle regulation in embryonic stem cells: centrosomal decisions on self-renewal | journal = Stem Cells and Development | volume = 19 | issue = 11 | pages = 1663–1678 | date = November 2010 | pmid = 20594031 | doi = 10.1089/scd.2010.0136 }}</ref> Compared to proliferating [[somatic cell]]s, ESCs have unique cell cycle characteristics—such as rapid cell division caused by shortened [[G1 phase]], absent [[G2 phase|G0 phase]], and modifications in [[cell cycle checkpoint]]s—which leaves the cells mostly in [[S phase]] at any given time.<ref name=":0" /><ref name=":1">{{cite journal | vauthors = Barta T, Dolezalova D, Holubcova Z, Hampl A | title = Cell cycle regulation in human embryonic stem cells: links to adaptation to cell culture | journal = Experimental Biology and Medicine | volume = 238 | issue = 3 | pages = 271–275 | date = March 2013 | pmid = 23598972 | doi = 10.1177/1535370213480711 | s2cid = 2028793 }}</ref> ESCs' rapid division is demonstrated by their short doubling time, which ranges from 8 to 10 hours, whereas somatic cells have doubling time of approximately 20 hours or longer.<ref name=":2">{{cite journal | vauthors = Zaveri L, Dhawan J | title = Cycling to Meet Fate: Connecting Pluripotency to the Cell Cycle | language = en | journal = Frontiers in Cell and Developmental Biology | volume = 6 | pages = 57 | date = 2018 | pmid = 29974052 | pmc = 6020794 | doi = 10.3389/fcell.2018.00057 | doi-access = free }}</ref> As cells differentiate, these properties change: G1 and G2 phases lengthen, leading to longer cell division cycles. This suggests that a specific cell cycle structure may contribute to the establishment of pluripotency.<ref name=":0" />

Particularly because G1 phase is the phase in which cells have increased sensitivity to differentiation, shortened G1 is one of the key characteristics of ESCs and plays an important role in maintaining undifferentiated [[phenotype]]. Although the exact molecular mechanism remains only partially understood, several studies have shown insight on how ESCs progress through G1—and &nbsp;potentially other phases—so rapidly.<ref name=":1" />

The cell cycle is regulated by complex network of [[cyclin]]s, [[cyclin-dependent kinase]]s (Cdk), [[cyclin-dependent kinase inhibitor]]s (Cdkn), pocket proteins of the retinoblastoma (Rb) family, and other accessory factors.<ref name=":2" /> Foundational insight into the distinctive regulation of ESC cell cycle was gained by studies on mouse ESCs (mESCs).<ref name=":1" /> mESCs showed a cell cycle with highly abbreviated G1 phase, which enabled cells to rapidly alternate between M phase and S phase. In a somatic cell cycle, oscillatory activity of Cyclin-Cdk complexes is observed in sequential action, which controls crucial regulators of the cell cycle to induce unidirectional transitions between phases: [[Cyclin D]] and Cdk4/6 are active in the G1 phase, while [[Cyclin E]] and [[Cyclin-dependent kinase 2|Cdk2]] are active during the late G1 phase and S phase; and [[Cyclin A]] and Cdk2 are active in the S phase and G2, while [[Cyclin B]] and [[Cyclin-dependent kinase 1|Cdk1]] are active in G2 and M phase.<ref name=":2" /> However, in mESCs, this typically ordered and oscillatory activity of Cyclin-Cdk complexes is absent. Rather, the Cyclin E/Cdk2 complex is constitutively active throughout the cycle, keeping [[retinoblastoma protein]] (pRb) [[hyperphosphorylated]] and thus inactive. This allows for direct transition from M phase to the late G1 phase, leading to absence of D-type cyclins and therefore a shortened G1 phase.<ref name=":1" /> Cdk2 activity is crucial for both cell cycle regulation and cell-fate decisions in mESCs; downregulation of Cdk2 activity prolongs G1 phase progression, establishes a somatic cell-like cell cycle, and induces expression of differentiation markers.<ref>{{cite journal | vauthors = Koledova Z, Kafkova LR, Calabkova L, Krystof V, Dolezel P, Divoky V | title = Cdk2 inhibition prolongs G1 phase progression in mouse embryonic stem cells | journal = Stem Cells and Development | volume = 19 | issue = 2 | pages = 181–194 | date = February 2010 | pmid = 19737069 | doi = 10.1089/scd.2009.0065 }}</ref>

In human ESCs (hESCs), the duration of G1 is dramatically shortened. This has been attributed to high mRNA levels of G1-related Cyclin D2 and Cdk4 genes and low levels of cell cycle regulatory proteins that inhibit cell cycle progression at G1, such as [[P21Cip1|p21<sup>CipP1</sup>]], [[p27Kip1|p27<sup>Kip1</sup>]], and p57<sup>Kip2</sup>.<ref name=":0" /><ref>{{cite journal | vauthors = Becker KA, Ghule PN, Therrien JA, Lian JB, Stein JL, van Wijnen AJ, Stein GS | title = Self-renewal of human embryonic stem cells is supported by a shortened G1 cell cycle phase | journal = Journal of Cellular Physiology | volume = 209 | issue = 3 | pages = 883–893 | date = December 2006 | pmid = 16972248 | doi = 10.1002/jcp.20776 | s2cid = 24908771 }}</ref> Furthermore, regulators of Cdk4 and Cdk6 activity, such as members of the Ink family of inhibitors (p15, p16, p18, and p19), are expressed at low levels or not at all. Thus, similar to mESCs, hESCs show high Cdk activity, with Cdk2 exhibiting the highest kinase activity. Also similar to mESCs, hESCs demonstrate the importance of Cdk2 in G1 phase regulation by showing that G1 to S transition is delayed when Cdk2 activity is inhibited and G1 is arrest when Cdk2 is knocked down.<ref name=":0" /> However unlike mESCs, hESCs have a functional G1 phase. hESCs show that the activities of Cyclin E/Cdk2 and Cyclin A/Cdk2 complexes are cell cycle-dependent and the Rb checkpoint in G1 is functional.<ref name=":2" />

ESCs are also characterized by G1 checkpoint non-functionality, even though the G1 checkpoint is crucial for maintaining genomic stability. In response to [[DNA damage]], ESCs do not stop in G1 to repair DNA damages but instead, depend on S and G2/M checkpoints or undergo apoptosis. The absence of G1 checkpoint in ESCs allows for the removal of cells with damaged DNA, hence avoiding potential mutations from inaccurate DNA repair.<ref name=":0" /> Consistent with this idea, ESCs are hypersensitive to DNA damage to minimize mutations passed onto the next generation.<ref name=":2" />