Editing Somatic cell nuclear transfer (section)

==Limitations==
{{More citations needed section|date=August 2016}}
Somatic cell nuclear transfer (SCNT) can be inefficient due to stresses placed on both the egg cell and the introduced nucleus. This can result in a low percentage of successfully reprogrammed cells. For example, in 1996 Dolly the sheep was born after 277 eggs were used for SCNT, which created 29 viable embryos, giving it a measly 0.3% efficiency.<ref name="Edwards 2003 113–123">{{Cite journal|last1=Edwards|first1=J. L.|last2=Schrick|first2=F. N.|last3=McCracken|first3=M. D.|last4=Amstel|first4=S. R. Van|last5=Hopkins|first5=F. M.|last6=Welborn|first6=M. G.|last7=Davies|first7=C. J.|date=2003|title=Cloning Adult Farm Animals: A Review of the Possibilities and Problems Associated with Somatic Cell Nuclear Transfer|url=https://onlinelibrary.wiley.com/doi/abs/10.1034/j.1600-0897.2003.00064.x|journal=American Journal of Reproductive Immunology|language=en|volume=50|issue=2|pages=113–123|doi=10.1034/j.1600-0897.2003.00064.x|pmid=12846674|s2cid=25230664|issn=1600-0897}}</ref> Only three of these embryos survived until birth, and only one survived to adulthood.<ref name=Campbell/> Millie, the offspring that survived, took 95 attempts to produce.<ref name="Edwards 2003 113–123"/> Because the procedure was not automated and had to be performed manually under a [[microscope]], SCNT was very resource intensive. Another reason why there is such high mortality rate with the cloned offspring is due to the fetus being larger than even other large offspring, resulting in death soon after birth.<ref name="Edwards 2003 113–123"/> The [[biochemistry]] involved in reprogramming the [[cellular differentiation|differentiated]] somatic cell nucleus and activating the recipient egg was also far from understood. Another limitation is trying to use one-cell embryos during the SCNT. When using just one-cell cloned embryos, the experiment has a 65% chance to fail in the process of making morula or blastocyst. The biochemistry also has to be extremely precise, as most late term cloned fetus deaths are the result of inadequate placentation.<ref name="Edwards 2003 113–123"/> However, by 2014, researchers were reporting success rates of 70-80% with cloning pigs<ref>Shukman, David (14 January 2014) [https://www.bbc.co.uk/news/science-environment-25576718 China cloning on an 'industrial scale'] BBC News Science and Environment, Retrieved 10 April 2014</ref> and in 2016 a Korean company, Sooam Biotech, was reported to be producing 500 cloned embryos a day.<ref name="NewScientist500">{{cite magazine|last1=Zastrow|first1=Mark|title=Inside the cloning factory that creates 500 new animals a day|url=https://www.newscientist.com/article/2076681-inside-the-cloning-factory-that-creates-500-new-animals-a-day/|access-date=23 February 2016|magazine=New Scientist|date=8 February 2016}}</ref>

In SCNT, not all of the donor cell's genetic information is transferred, as the donor cell's [[mitochondria]] that contain their own [[mitochondrial DNA]] are left behind. The resulting hybrid cells retain those mitochondrial structures which originally belonged to the egg. As a consequence, clones such as Dolly that are born from SCNT are not perfect copies of the donor of the nucleus. This fact may also hamper the potential benefits of SCNT-derived tissues and organs for therapy, as there may be an immuno-response to the non-self mtDNA after transplant. Additionally, the genes found in the mitochondria’s genome need to communicate with the cell’s genome and a failure of somatic cell nuclear reprogramming can lead to non communication to the cell’s genome causing SCNT to fail.<ref>{{Cite journal |last1=Czernik |first1=Marta |last2=Anzalone |first2=Debora A. |last3=Palazzese |first3=Luca |last4=Oikawa |first4=Mami |last5=Loi |first5=Pasqualino |date=2019-04-16 |title=Somatic cell nuclear transfer: failures, successes and the challenges ahead |url=https://ijdb.ehu.eus/article/180324mc |journal=The International Journal of Developmental Biology |language=en |volume=63 |issue=3–4–5 |pages=123–130 |doi=10.1387/ijdb.180324mc |pmid=31058291 |issn=0214-6282}}</ref> 

Epigenetic factors play an important role in the success or failure of SCNT attempts. The varying gene expression of a previously activated cell and its mRNAs may lead to overexpression, underexpression, or in some cases non functional genes which will affect the developing fetus.<ref>{{Cite journal |last1=Malin |first1=Katarzyna |last2=Witkowska-Piłaszewicz |first2=Olga |last3=Papis |first3=Krzysztof |date=September 2022 |title=The many problems of somatic cell nuclear transfer in reproductive cloning of mammals |url=http://dx.doi.org/10.1016/j.theriogenology.2022.06.030 |journal=Theriogenology |volume=189 |pages=246–254 |doi=10.1016/j.theriogenology.2022.06.030 |pmid=35809358 |issn=0093-691X}}</ref> One such example of epigenetic limitations to SCNT is regulating histone methylation. Differing regulation of these histone methylation genes can directly affect the transcription of the developing genome, causing failure of the SCNT.<ref name=":1">{{Cite journal |last1=Srirattana |first1=Kanokwan |last2=Kaneda |first2=Masahiro |last3=Parnpai |first3=Rangsun |date=2022-02-10 |title=Strategies to Improve the Efficiency of Somatic Cell Nuclear Transfer |journal=International Journal of Molecular Sciences |volume=23 |issue=4 |pages=1969 |doi=10.3390/ijms23041969 |doi-access=free |pmid=35216087 |pmc=8879641 |issn=1422-0067}}</ref> Another contributing factor to failure of SCNT includes the X chromosome inactivation in early development of the embryo. A non coding gene called XIST is responsible for inactivating one X chromosome during development, however in SCNT this gene can have abnormal regulation causing mortality to the developing fetus.<ref name=":1" />