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Somatic cell nuclear transfer

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In genetics and developmental biology, somatic cell nuclear transfer (SCNT) is a technique for creating an ovum with a donor nucleus (see process below) . It can be used in embryonic stem cell research, or in regenerative medicine where it is sometimes referred to "therapeutic cloning." It can also be used as the first step in the process of reproductive cloning.

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[edit] The process

Image:Cloning-cell.jpg

In SCNT the nucleus, which contains the organism's DNA, of a somatic cell (a body cell other than a sperm or egg cell) is removed and the rest of the cell is discarded. At the same time, the nucleus of an egg cell is removed. The nucleus of the somatic cell is then inserted into the enucleated egg cell. After being inserted into the egg, the somatic cell nucleus is reprogrammed by the host cell. The egg, now containing the nucleus of a somatic cell, is stimulated in such a way that it begins to divide. After many mitotic divisions in culture, this single cell forms a blastocyst (an early stage embryo with about 100 cells) with almost identical DNA to the original organism.

[edit] SCNT in stem cell research

Some researchers use SCNT in stem cell research. The aim of carrying out this procedure is to obtain stem cells that are genetically matched to the donor organism. Presently, no human stem cell lines have been derived from SCNT research.


Embryonic stem cells of a mouse.

A potential use of genetically-customized stem cells would be to create cell lines that have any genes linked to a particular disease. For example, if a person with Parkinson's disease donated his or her somatic cells, then the stem cells resulting SCNT would have any genes which contribute to Parkinson's disease. In this scenario, the disease-specific stem cell lines would be studied in order to better understand the disease.<ref name=Semb>Semb H. "Human embryonic stem cells: origin, properties and applications." APMIS. 2005 Nov-Dec;113(11-12):743-50. PMID 16480446</ref>

In another scenario, genetically-customized stem cell lines would be generated for cell-based therapies to transplant to the patient. The resulting cells would be genetically identical to the somatic cell donor, thus avoiding any complications from immune system rejection.<ref name=Semb/><ref>Hadjantonakis AK, Papaioannou VE. "Can mammalian cloning combined with embryonic stem cell technologies be used to treat human diseases? Genome Biol. 2002 Jul 30;3(8):REVIEWS1023. PMID 12186652</ref>

Only a handful of the labs in the world are currently using SCNT techniques in human stem cell research. In the United States, scientists at the Harvard University Stem Cell Institute, the University of California San Francisco, and possibly Advanced Cell Technology are currently researching a technique to use somatic cell nuclear transfer to produce embryonic stem cells.<ref name="weise">Elizabeth Weise, "Cloning race is on again," USA Today (January 17, 2006, retrieved october 6, 2006)</ref> In the United Kingdom, the Human Fertilisation and Embryology Authority has granted permission to research groups at the Roslin Institute and the Newcastle Centre for Life.<ref name="bbc">"Dolly scientists' human clone bid," BBC News (September 28, 2004, retrieved October 6, 2006)</ref> SCNT may also be occurring in China.<ref name="mann">Charles C. Mann, "The First Cloning Superpower," Wired (January 2003, retrieved October 6, 2006)</ref>

In 2005, a South Korean research team led by Professor Hwang Woo-suk, published claims to have derived stem cell lines via SCNT,<ref>Hwang WS, Roh SI, Lee BC, Kang SK, Kwon DK, Kim S, Kim SJ, Park SW, Kwon HS, Lee CK, Lee JB, Kim JM, Ahn C, Paek SH, Chang SS, Koo JJ, Yoon HS, Hwang JH, Hwang YY, Park YS, Oh SK, Kim HS, Park JH, Moon SY, Schatten G. "Patient-specific embryonic stem cells derived from human SCNT blastocysts." Science. 2005 Jun 17;308(5729):1777-83. PMID 15905366</ref> but supported those claims with fabricated data.<ref>Kennedy D. "Editorial retraction. of Hwang WS et. al." Science. 2006 Jan 20;311(5759):335. PMID 16410485</ref>

[edit] SCNT in reproductive cloning

Main article: cloning

This technique is currently the basis for cloning animals (such as the famous Dolly the sheep)<ref name=Campbell>Campbell KH, McWhir J, Ritchie WA, Wilmut I. "Sheep cloned by nuclear transfer from a cultured cell line." Nature. 1996 Mar 7;380(6569):64-6. PMID 8598906</ref>, and in theory could be used to clone humans. However, most researchers believe that in the foreseeable future it will not be possible to use this technique to produce a human clone that will develop to term.<ref>Revel M. "Research on animal cloning technologies and their implications in medical ethics: an update." Med Law. 2000;19(3):527-43. PMID 11143888</ref><ref>Rhind SM, Taylor JE, De Sousa PA, King TJ, McGarry M, Wilmut I. "Human cloning: can it be made safe?" Nat Rev Genet. 2003 Nov;4(11):855-64. PMID 14634633</ref>

[edit] Limitations

The stresses placed on both the egg cell and the introduced nucleus are enormous, leading to a high loss in resulting cells. For example, Dolly the sheep was born after 277 eggs were used for SCNT, which created 29 viable embryos, which only produced three lambs at birth, only one of which lived.<ref name=Campbell/> As the procedure currently cannot be automated, but has to be performed surprisingly manually under a microscope, SCNT is very resource intensive. The biochemistry involved in reprogramming the differentiated somatic cell nucleus and activating the recipient egg is far from understood.

In SCNT, not all of the donor cell's genetic information is transferred, as the DNA mitochondria is left behind, with the resulting hybrid cells retaining those structures which originally belonged to the egg. As a consquence, clones such as Dolly that are born from SCNT are not perfect copies of the donor of the nucleus.

[edit] Controversy

Main article: stem cell controversy
Blastocyst. See also: Blastocyst in utero

Proposals to use Nucleus Transfer techniques in human stem cell research raise a set of concerns beyond the moral status of any created embryo. These have led to some individuals and organizations who are also opposed to human embryonic stem cell research to be concerned about, or opposed to, SCNT research.

One concern is that blastula creation in human stem cell research will lead to the reproductive cloning of humans. Both processes use the same first step: the creation of a nuclear transferred embryo, most likely via SCNT. Those who hold this concern often advocate for strong regulation of SCNT to preclude implantation of any derived products for the intention of human reproduction. <ref name="Regulation">Lori B. Andrews et al. (March 19, 2002)."Open Letter to US Senators on Human Cloning and Eugenic Engineering." Retrieved on August 7, 2006</ref>, or its prohibition.<ref name="reifkin">Jeremy Rifkin. (February 18, 2002). "Fusion Biopolitics." The Nation. Retrieved on August 7, 2006.</ref>

A second concern is the appropriate sourcing of the eggs that are needed. SCNT requires human eggs, which can only be obtained from women. The most common source of these eggs today are eggs that are produced and in excess of the clinical need during IVF treatment. This is a minimally invasive procedure, but it does carry some health risks, such as ovarian hyperstimulation syndrome, and in very rare instances even death.

[edit] Policies

SCNT is currently legal for research purposes in the United Kingdom, having been incorporated into the 1990 Human Fertilisation and Embryology Act in 2001.<ref name="coghlan">Andy Coghlan, "Cloning opponents fear loopholes in new UK law," New Scientist (November 23, 2001, retrieved October 6, 2006)</ref> Permission must be obtained from the Human Fertilisation and Embryology Authority in order to perform or attempt SCNT.

In the United States, the practice remains legal, as it has not been addressed by federal law.<ref name="NBAC">"Chapter 5: Legal and Policy Considerations. Cloning Human Beings" Report and Recommendations of the National Bioethics Advisory Commission, June 1997. Accessed 21 Oct 06</ref>

In 2005, the United Nations adopted a proposal submitted by Costa Rica, calling on member states to "prohibit all forms of human cloning inasmuch as they are incompatible with human dignity and the protection of human life." <ref name="un">United Nations, "General Assembly Adopts United Nations Declaration on Human Cloning By Vote of 84-34-37," press release (August 3, 2005, retrieved October 6, 2006)</ref> This phrase may include SCNT, depending on interpretation.

The Council of Europe's Convention on Human Rights and Biomedicine and its Additional Protocol to the Convention for the Protection of Human Rights and Dignity of the Human Being with regard to the Application of Biology and Medicine, on the Prohibition of Cloning Human Being appear to ban SCNT. Of the Council's 45 member states, the Convention has been signed by 31 and ratified by 18. The Additional Protocol has been signed by 29 member nations and ratified by 14.<ref name="coe">Council of Europe, Convention for the Protection of Human Rights and Dignity of the Human Being with regard to the Application of Biology and Medicine: Convention on Human Rights and Biomedicine (April 4, 1997, retrieved October 6, 2006); Council of Europe, Additional Protocol to the Convention for the Protection of Human Rights and Dignity of the Human Being with regard to the Application of Biology and Medicine, on the Prohibition of Cloning Human Being] (January 12, 1998, retrieved October 6, 2006)</ref>

[edit] See also


[edit] References

<references/>de:Zellkerntransfer

da:Terapeutisk kloning fr:Clonage thérapeutique ru:Терапевтическое клонирование

[edit] Further reading

[edit] External links

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