Читать книгу The SAGE Encyclopedia of Stem Cell Research - Группа авторов - Страница 98

Deoxyribonucleic acid, or DNA, is a self-replicating polymer present in most live organisms. It contains all of the information necessary for the development of live beings. The term DNA cloning, as well as molecular cloning, recombinant DNA technology, and gene cloning, refer to a set of methods that transfer DNA from one organism to a self-replicating genetic element such as a bacterial plasmid or a virus. The DNA is then taken from the plasmid and propagated in a foreign host cell. This technology, developed in the 1970s, is commonplace in molecular biology laboratories today.

Reproductive cloning is a technology used to generate an animal that has the same nuclear DNA as another currently or previously existing animal. Scientist Ian Wilmut and his team deeply revolutionized the scientific world at large in 1997 when they created Dolly, a sheep cloned from a mammary cell. Cloning today is used to, among other things, produce stem cells that offer the possibility of more effective medical treatments than ever before. Dolly’s birth also caused widespread concern about the eventual possibility of cloning human beings, a possibility strongly opposed by the creators of this technology. Even though they opposed cloning of the human species, however, scientists who created the first cloned mammal argued that science should eventually be allowed to combine the technologies of cloning human embryos with that of genetic modification. This could allow science to discover cures for people suffering from serious hereditary diseases. On the other hand, others have argued that this would mean the creation of genetically altered humans.

Dolly was created by reproductive cloning technology through a process known as somatic cell nuclear transfer. In order to produce Dolly, the scientific team used the nucleus extracted from an udder of an adult Finn Dorset white sheep. Most of a cell’s genes are held in the nucleus of a cell. Scientists had to figure out how to reprogram the cells, that is, to keep the cells alive while at the same time, stopping their growth. They achieved this by making changes in the growth medium and injecting the cell into a Blackface sheep’s unfertilized egg cell, from which the nucleus had been previously removed. Afterward, the cells were fused through electrical pulses. Once scientists had fused the nucleus from the white sheep cell with the egg cell from the black-faced sheep, they cultured it for approximately a week to ensure that it divided and developed properly, before implanting it into the uterus of a surrogate mother, another Blackface sheep. Dolly, however, was born with a white face. In order to achieve Dolly’s birth, the researchers had produced close to 300 cell fusions, from which 29 embryos resulted. These were subsequently implanted into 13 surrogate mothers. Only the pregnancy that produced Dolly, however, developed to full term.

The principal motive for the scientist team to clone sheep was because they wanted to clone mammals that could produce medication in their milk. Since then, scientists have achieved the transfer of human genes into those of sheep, cows, and other species. The purpose of these cross-species experiments was to produce medications that could treat cystic fibrosis and other diseases, or a blood-clotting agent known as factor IX to treat hemophilia, for example. Moreover, it has been found recently that embryonic cells taken from mice can help mend the damaged hearts of sheep. This was one step closer to discovering if human embryonic stem cells could heal the damage caused by heart attacks. There are many other medical possibilities for cloned animals.

Another important medical purpose for animal cloning would be xenotransplantation, the generation of organs to transplant into human organisms and save human lives. In 2000, scientists achieved the combination of genetic technology with cloning techniques for pigs. Human use of these pig-produced organs was impeded because of the presence of the alpha gal antigen in the pig cells. The alpha gal antigen, which can be found in most mammals but not in primates, can trigger allergic reactions in human beings, which can range from mild to severe. By 2002, however, scientists had succeeded in doing away with the gene that produces it. Cloned pigs with this genetic modification could be bred to produce a consistent supply of viable donor organs for human transplant. However, as with any new medical technology, there are still concerns hampering its further development and implementation, such as viral transmission. These risks apply to animals bred for food as well, according to some activists, despite reassurances by oversight institutions.

Research of cloned cells and cloned animals can lead to a greater understanding of the natural development of animal and human life, from how embryos develop to issues about aging and age-related diseases. Cloned animals often develop symptoms such as obesity, even though their natural offspring do not. Cloning could lead to a better understanding and treatment of those diseases. Today, some pharmaceutical companies already produce large herds of transgenic and nontransgenic cloned cattle from different cell lines—cells from adult and embryo animals—for the production of varied pharmaceutical protein products.

Some pharmaceutical companies have already undertaken the project of marketing pharmaceutical products produced in the milk of transgenic animals. Another important commercial purpose of this technology would be to create herds of mammals traditionally bred for food. Large herds of cloned animals could be developed by agro-industrial companies for egg, meat, and milk consumption. This technology could allow producers to tailor the production of food and dairy with characteristics that would be even more palatable to consumers’ tastes and preference. Cloned animals can be bred naturally, that is, sexually. Elite herd and flock farm animals could eventually be cloned to produce hardier stock more resistant to disease or with a higher productivity rate compared to traditional stock.

Scientists also argue that animal cloning could enrich biodiversity by ensuring the survival and flourishing of rare breeds, endangered species, and even the recovery of extinct species. Already some endangered species, including the African wildcat, the Asian banteng, and a species of European wild sheep have been cloned in past years. Some zoos maintain frozen samples of DNA from animals they want to preserve or reproduce. However, critics warn that it is very unlikely that a viable and sustainable population could be produced from cloned animals, because not only is their production costly and unstable, but also because of the loss of genetic diversity necessary for animal species to flourish in the wild.

Young male calf Starbuck II, clone of the CIAQ sire Hanoverhill Starbuck, was born on September 7, 2000, in Saint-Hyacinthe, Quebec. The clone was the result of the combined efforts of CIAQ, L’Alliance Boviteq Inc., and the Faculté de médecine vétérinaire de l’Université de Montréal. Genomics arrived in the breeding industry in 2009, and upon Starbuck II’s death in September 2010, the era of cloning had ended. (Flickr)

Cloning technology has also been used to clone pet animals, with the first pet cloned in 2004. By 2008, a United States company offered a dog cloning service, and announced the successful cloning of a pet dog named Missy. In the same year, the National University of Seoul in Korea produced five clones for a California pet owner. In order to clone a pet, scientists take a small tissue fragment from the still living or very recently deceased pet, and freeze the cells. As in the case of Dolly the sheep, another dog supplies the egg. The DNA in the egg is then replaced for that of the frozen sample and the cloned embryo is inserted into a surrogate mother dog that will birth and suckle the newborn puppy. The surrogate dog does not need to be of the same breed. As with related therapies, the viability of the egg is poor and requires dozens of attempts before reaching success. Also, pet cloning raises controversial issues related to the boundaries between science, technology, commercial interests, and ethics.

Some important problems persist in reference to the production of cloned animals. First, for any of the cloning technologies commonly used today, the success rate in the live birth of animals containing the transferred gene remains very low. Second, even though clones are genetically identical, physical characteristics such as weight and size, as well as behavior characteristics, may not be the same because the DNA has been modified during the cloning process. These issues undeniably affect the expression and activity of some genes in hard to predict ways. Third, the technology necessary for animal cloning is still very costly. Finally, consumer advocates and other activists question the safety of cloned animals for human consumption and the ethics related to cloning technology. Health problems in cloned animals, such as weaknesses, abnormalities, and inbreeding, have arisen, which many feel have not yet been adequately addressed by scientists. Others argue that cloning would create a monoculture of animal breeding vulnerable to diseases due to a lack of genetic variation. Cloned animals should be grown and observed for yet a longer period of time in order to assess possible problems that develop in time. The cloning of some animals risks possible contagion of pathogens. For example, some technologies use viruses as a culture that could be expressed in hosts, human or nonhuman. And yet others question the moral viability of some of the cloning processes and issues, such as those pertaining to the manmade generation and destruction of animal and human life, even if in early embryo form.