Transgenic animals: how they are created and what's their purpose - Printable Version
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Transgenic animals: how they are created and what's their purpose - BojanaL - 10-14-2012
Transgenic or genetically modified animals are created when foreign DNA sequence is incorporated in their genome. Main purpose is to provide more information on human and/or animal related diseases, to enhance novel therapeutics development, to assess environmental pollution….
Whatever the purpose, first part of the experiment is always the same: DNA piece need to be inserted into animal’s genome. Mouse is often used for all kind of genetic experiments. Transgenic mouse can be created using two methods: transforming embryonic stem cell or by injecting DNA sequence into pronucleus of fertilized egg.
Mouse blastocyst derived stem cells are transfected with DNA sequence of interest. Sequence is attached to a vector, promoter and enhancer, to ensure proper functioning of the gene in the host’s genome. Successfully transfected embryonic stem cells are injected back into mouse blastocyst. Before implanting embryo in mouse, female is mating with sterile male to ensure hormonal changes necessary for uterus to accept embryo (mating is a trigger). 1 out of 3 implanted embryos survive until birth. After they are born, tissue sample will show if animal is carrying a gene of interest. Less than 20% of offspring will be positive for the gene tested and they will be heterozygous for that gene (present in only one copy of the gene). Mating two heterozygous animals will result in 25% percent of homozygous offspring. They have two copies of desired gene (both from mother and father) and that is the moment when new, transgenic, animal is created. Gene of interest starts to express on a regular basis.
Second method uses sperm head for incorporating sequence of interest. Prepared DNA piece (with vector, promoter…) is injected into male pronucleus before he fuses with egg’s pronucleus. At the stage of 2 cells, embryo will be implanted into female’s uterus (prepared the same way like in stem cell method) for further development.
These methods are applicable to a lot of animals and can be beneficial in medicine (to heal different protein deficiencies). For example, Alpha1-Antitrypsin Deficiency is genetic disorder resulting in lung damage. Transgenic goats carrying alpha1-antitrypsin gene were successfully created but high expenses of protein extraction and purification from goat’s milk prevented company to produce it on a large scale. One other company managed to overcome these difficulties and as from 2006 human antithrombin expressed in goat’s milk is widely used to prevent blood clotting during surgery.
Severe combined immunodeficiency disorder (SCID), known as bubble boy disease, is genetic disorder resulting in inability of organism to fight even slight infections. A lot of babies die in the first year of life due to severe and recurring infections. So far, bone marrow transplantation is only solution and it needs to be provided in the first few month of baby’s life of even in utero (before baby is born). Other solution that proved to be effective (at least at the beginning) was gene therapy using viral vectors “equipped” with gene of interest. Hematopoietic stem cells “enriched” with missing gene helped 4-year old girl to cope with SCID. Later studies showed that this kind of therapy could induce leukemia as retrovirus carrying a sequence of interest could trigger expression of the nearby oncogene as well. Today, efforts are made to produce viral vector that will not affect oncogenesis. A list of disorders that could be treated with gene therapy is long (muscular dystrophy, Parkinson disease, cystic fibrosis…) and even thought solutions are not perfect yet - this medical field is constantly improving.
Besides using transgenic animals or cells for therapeutic purposes, they could serve as model organisms for studying various disorders: triggering genes responsible for disease, mechanism of illness could be observed. Knock out animals are carrying dysfunctional genes that are not expressing proteins. They are designed to detect role and value of different proteins, enzymes, hormones… in the body. When protein expression is lacking, affected biochemical process and/or subsequent disorder will be easily recognized.
Some pets are hypo-allergic - genetically modified to prevent allergic reaction. Farm animals could be “improved” to grow faster or to digest some food that normally wouldn’t be able to do. Green fluorescent protein (GFP) can be perfect marker of environmental pollution once incorporated in zebrafish genome. It is also massively used as a marker of genetic expression, for analysis of neuronal activity, as a viability assay (in cryobiology)... Insects can be genetically altered as well: development of mosquitoes resistant to malaria and incorporation of lethal genes in male mosquitoes responsible for Dengue fever help combat these issues in affected areas.
When it comes to transgenic animals, my emotions are mixed: I encourage getting new knowledge and providing solutions that could help planet as a whole to become a better place, but idea that so many animals are sacrificed for that purpose is making me so sad. I hope that we'll find out that more benefits than damage is made, when we summarize everything we've done.
RE: Transgenic animals: how they are created and what's their purpose - adimed - 10-07-2013
The transgenesis of animals began in 1980. Animal transgenesis applications that are directly or indirectly related to health of human beings can be majorly divided into three categories: improving the products of animals for consumption by humans, to obtain value products such as xeno organs and recombinant proteins, to acquire information regarding gene regulation and function as well as on diseases of humans. Transgenic technology holds enormous potential in the field of medicine, agriculture and industrial applications.
Transgenesis will facilitate bigger herds with specific traits – To produce animals with desired traits farmers have always utilized selective breeding (for example, high growth rate, elevated milk production). The method of traditional breeding may be time consuming and difficult. By genetic engineering techniques it is possible to generate animals with desired traits in a short time span and also with greater precision.
Researchers can improve the quality of livestock genetically – Transgenic cattle that produce more milk or milk with lower cholesterol or lactose levels exist. Similarly sheep with more wool and pigs with more meet are also produced by transgenesis.
Disease-resistant livestock – Researchers are investigating on producing animals that are disease resistant such as pigs that are influenza resistant and currently this knowledge is very limited with regard to farm animals.
Xenotransplantation - Transplant organs may be shortly available from transgenic animals (transgenic pigs may provide transplant organs and is currently under investigation).
Milk-producing transgenic animals are especially useful for nutritional supplements - In the year 1997, Rosie, the first transgenic cow produced human protein-enriched milk (due to human gene alpha-lactcalumin) at 2.4 g/ litre. This transgenic milk has a nutritional value compared to natural cow milk and could be offered to babies or old ones with special digestive or nutritional needs.
Utilization of transgenic animals in industry include material fabrication and safety tests of chemicals - In the year 2001, two researchers at Canada (Nexia Biotechnologies) spliced genes of spider into the cells of lactating goats to produce spider silk. Transgenic animals with toxic sensitivity have been raised for safety testing of chemicals.