SMC Backgrounder: Cloning – beyond Dolly the sheep

Since the creation of the first cloned vertebrate in 1962 [1] and the first mammal in 1996 [2], cloning, in various forms, has become a widely used and relatively successful technique.

Cloning technologies are at present used primarily for reproductive cloning in agriculture and research, or for the harvesting of embryonic stem cells for research into methods of therapeutic cloning. However, anxiety and misunderstanding surround the field and some feel there is a sinister edge to cloning.

This article will outline the different types of cloning technologies with clear examples of each, and highlight the scientific research being undertaken in New Zealand in the field of cloning.

Reproductive cloning – Somatic cell nuclear transfer (SCNT)

Making new animals from old ones – SCNT is the technique that was used to create Dolly the sheep at the Roslin Institute in Edinburgh. The nucleus of an adult animal cell, say a skin cell or a muscle cell, is injected into a donor egg that has had its nucleus, and therefore all its genetic material, removed. Cell division is stimulated by electrical impulses or chemicals and once the cell has divided to a certain stage it is implanted into the uterus of a female surrogate [3]. This technique is so scientifically significant because it takes an adult fully differentiated cell (one that is fixed to only ever be one cell type) and stimulates it to behave as a stem cell, or one that has the potential to differentiate into any cell type in the body. The cell has been, in effect, reprogrammed to its initial state. However, with this technique comes high rates of death and deformity of cloned animals, which scientists believe is a result of imperfect cellular reprogramming, leading to detrimental changes in gene expression.

How has reproductive cloning been used?

As with Dolly, cloning has been used to create copies of whole animals, and is mainly used for breeding purposes, allowing the rapid multiplication of animals with desirable characteristics and could provide an alternative to artificial insemination. For example, Brophy and colleagues at AgResearch [4] have cloned cattle to improve the functional properties of their milk, more specifically, the milk produces proteins that make cheese-making easier. AgResearch holds MoRST approval to clone sheep, cattle and deer using SCNT with various cell types to develop advanced options for future farming. For example, the technology could be used to produce cloned animals that are specialised dairy herds which produce human pharmaceuticals in their milk, or herds with improved disease resistance and muscle composition [5].

Cloned animals are expensive to produce but they can be bred with conventional animals to produce offspring with the desired characteristics. Commercially, Ambreed NZ [6] has a joint venture with Clone International [7] to clone NZ’s two most valuable dairy cows and use the calves to produce semen to keep up with increasing overseas demand.

Reproductive cloning has also been used in NZ to save endangered species from extinction. The last of the Enderby Island cattle breed was cloned with a view to artificially inseminating the calf at a later date with frozen sperm [8]. Further animals have since been born and as of 2002 the number totalled seven animals [9].

Cloned animals as food

The New Zealand Food Safety Authority states that there is no accepted scientific evidence that food from cloned animals is any less safe than foods from non-cloned animals, and believes that there is no need for specific regulation of such foods, other than that which already exists for non-cloned animals. However, a survey from the European Union suggests most people are opposed to cloning animals for human consumption, and Food Standards Australia suggests that any cloning of animals for food would need a conservative regulatory approach as there exists in the public mind close links between cloning and genetic modification. There are currently no cloned animals in the food chain in New Zealand [10].

Therapeutic Cloning

Another branch of cloning is ’embryo cloning’, the production of embryos for use in research. These embryos are not used to clone humans or animals, they are used to harvest stem cells from for use in human health and disease research. Stem cells have the capacity to be stimulated to differentiate into any cell in the body and it is hoped that by creating, for example, brain cells from stem cells, these brain cells could be used to treat Alzheimer’s patients by returning them to the patient and replacing their damaged brain cells. However, the harvesting of stem cells results in the destruction of the human embryo, hence the reason why embryonic stem cells as a source for therapy is so contentious. No New Zealand institution is permitted to produce embryonic stem cells, however they can be imported for research purposes.

Where else can you get stem cells from?

A new technique has been developed that allows one cell to be removed from an 8-cell stage mouse embryo (the very first stage of development) which can be used to produce a stem cell line, but at the same time maintains the developmental potential of the embryo and allows the mouse to develop normally [11]. This would avoid any issues relating to the loss of the embryo from stem cell harvesting. Another group were able to extract adult spermatogonial cells from adult human testis and it was found that these cells behaved very similarly to embryonic stem cells [12]. These cells may likewise provide a non-controversial source of stem cells, with a view to providing individual stem cell therapy for degenerative diseases or cancer, for example, in the same way that embryonic stem cells could be used.

What has therapeutic cloning been successfully used for?

The most successful use of therapeutic cloning so far is the alleviation of Parkinson’s symptoms in mice after embryonic stem cells from cloned mice embryos were matured into brain cells and transplanted back into the diseased mouse strain [13]. The hope is that this technology will one day be transferable to humans.

Will we ever be able to clone humans?

A survey of 192 countries and 8 major intergovernmental organisations indicates that there is a widespread and strong opposition to human reproductive cloning [14]. In New Zealand, public consultation by the Bioethics Council [15] and found widespread mistrust of scientists working in the field of human reproductive technology and high levels of anxiety amongst the public regarding the cloning of human embryos. The report notes that for most people, cloning equals Dolly the sheep which is (erroneously) seen as the goal of human embryo research. The Human Assisted Reproductive Technology Act (2004) [16] lists prohibited actions by scientists, including the prohibition of the genetic modification of embryos for reproductive purposes, effectively barring the production of human clones.

This paper was reviewed by Associate Professor Martin Kennedy, of the University of Otago’s Carny Centre for Pharmacogenomics, Christchurch.

References and Resources

1. Gurdon, J., The developmental capacity of nuclei taken from intestinal epithelium cells of feeding tadpoles. J Embryol Exp Morphol, 1962. 10(622-40).

2. Wilmut, I., et al., Viable offspring derived from fetal and adult mammalian cells. Nature, 1997. 385(6619): p. 810-3.

3. Cloning Factsheet.

4. Brophy, B., et al., Cloned transgenic cattle produce milk with higher levels of beta-casein and kappa-casein. Nature Biotechnology, 2003. 21: p. 157-162.

5. Gene Technology in New Zealand: scientific issues and implications.

6. Ambreed NZ.

7. Clone International.

8. Wells, D.N., et al., Adult somatic cell nuclear transfer is used to preserve the last surviving cow of the Enderby Island cattle breed. Reproduction, Fertility and Development, 1998. 10(4): p. 369 – 378.

9. Enderby Island Cattle.

10. NZFSA – FAQ Cloned Animals.

11. Chung, Y., et al., Embryonic and extraembryonic stem cell lines dervied from single mouse blastomeres. Nature, 2006. 439: p. 216-219.

12. Conrad, S., et al., Generation of pluripotent stem cells from adult human testis. 2008. 456(7220): p. 344-349.

13. Tabar, V., et al., Therapeutic cloning in individual parkinsonian mice. Nature Medicine, 2008. 14(4): p. 379-381.

14. Hayes, R.,

15. Toi te Taiao – The Bioethics Council.

16. Human Assisted Reproductive Technology Act 2004.