Transgenic Mammals and Gene Targeting – Biology Research Paper
Molecular biotechnology has been one of the most prospective fields of science for more than three decades. Its applications are numerous and have not even come close to reaching the limit. Since discovery of genes scientists have been researching a way to temper with them to achieve a desirable effect. In last two decades technology became available which allows
introduction of specific genetic material from a donor into a host organism. Organisms which have altered or foreign genetic material, are now being referred to as transgenic. (Wang, 2003) Several methods have been devised for introduction or modification of genetic material. Most common and most controversial is embryonic stem-cell mediated gene transfer. It involves introduction of modified DNA into embryonic stem-cells, which will take up the DNA and hopefully express the desired genes. Application of this kind of technology is practically limitless. It scope spans across multiple industries, varying from agricultural uses to medical applications. (Westphal, 1989)
Latest achievements involve production of medically important proteins like anti-clotting factors through expression of the gene in lactating cows, goats and pigs. (William 1997)
Industries like agriculture have been desperate to express desired traits in livestock in order to increase production and the quality of the product in order to meet the demand of growing population. The most common fields in transgenic livestock that are being researched are: growth, pathogen resistance and quality of meat products. Many more applications of transgenic technology are now being considered as the field expands almost exponentially.
However the most important application of this technology so far is identification of gene function. Function of the gene can be found by “turning it off” and studying the effect on the organism. This approach is known as gene targeting or gene knockout. (Capecchi, 1994)
Background:
Genomes of several mammals have been sequenced so far and few more are under way. Knowing the base pair composition of the genome is only half the task done, the other half would be the identification of gene function. Most common genes are present in all species, thus they are identified with relative ease. Genes that are completely unknown are much harder to identify as very little is known about them. Gene targeting is a most common technique being used to identify the function of the gene. It involves introduction of a mutation into the “unknown” gene which will suppress its expression. Usually it is a mutation of the start codon or promoter region, however many more possibilities are available. The genome with inactivated gene is introduced into embryo and then selected for. Embryo that has taken up the DNA is then introduced into surrogate mother who will give berth to a transgenic offspring. Offspring is then mated with each other in order to create a homozygote of the mutation. Homozygotes are then studied for the effect of the mutation on the functioning of the organism. (Capecchi, 1994)
DNA microinjections is a common technique used for gene targeting as it involves manipulation and reintroduction of the genetic material into the same species.
Many genes have been identified through loss of function. For example, recently it was found that a transcription factor is responsible for mammalian cold sensation. Transgenic mice that were created that had a mutation in the gene which codes for the transcription factor (NGFIB). They were unable to respond to cold stimuli which provided strong evidence that a transcription factor is indeed required in the cold response pathway. (Kim, 2005)
A discrepancy in the metabolic pathway of the organism would indicate that a gene coding for a “missing factor” isn’t functioning thus will allow its isolation and identification.
Major Topic:
First transgenic experiments were carried out on mice. In 1981 first transgenic mammal was created. The animal had a copy of human growth hormone integrated into its genome. It was hypothesized that a growth hormone gene would speed up and increase overall growth of the mice. The outcome was as expected, a mouse which grew quicker and was much bigger in size.
Today application of the technology is expanding. Human growth hormone was also introduced to pigs. (Westphal, 1989) The experiment was a success as produced pigs had larger carcasses and less body fat. However there was a downside to the outcome, transgenic pigs were infertile. However research is being carried out to solve the infertility problem. There also was a huge public opinion formed on transgenic animals. Public view of transgenic animals is that they maybe harmful to both humans and the environment. Some critics say that scientist accelerate “selection” through transgenic technologies. In the end all comes down to ethics, which will have a final word on use of transgenic animals.
One aspect of technology that have bypassed both ethics and public opinion was use of transgenic mice of identify unknown genes. Mice are perfect for study of gene function. They have high reproduction rate, a lot is known about mouse genome and they are relatively easy to handle.
Gene targeting allowed identification of the function of genes which were previously unknown. In early 1990s research was conducted on function of Insulin-like growth factors. Knockout mice were created with a mutation in the insulin-like growth factor gene. Produced mice showed delayed bone development, retarded post-natal growth, growth deficiency and infertility. (FBS) Hence it proved that insulin-like growth factor plays an important role in fetal development. (DeChiara, 1990; Baker J, 1993) IGF2 gene is located on chromosome 11, p15.5 in mice. Further knockout experiments proved that IGF2 is jointly regulated with H19 and INS2 genes and all theses genes play vital role in fetal development. (Strachan, T 1999)
In 2004 group of Japanese and Korean scientist were successful in creating a parthenogenetic mice which survived and grew into adulthood. They were successful through careful manipulation of IGF2, H19 and INS2 genes. These genes are imprinted during early embryonic development as on copy comes from a father and one from a mother. Through manual imprinting of the gene (introduction of the deletion in the promoter region) in prepared oocyte, they were successful in creating an offspring without a physical father. (Kono, 2004)
This emphasizes the result gene knockout technique can have on the outcome of the study. Knowledge of a single gene function contributed to discovery which had a world wide impact.
Discussion:
Transgenic animals so far have proven to benefit humanity in every way. Medical applications have proved to be a great benefit and thus created a positive public opinion. On other hand use of transgenic animals in agriculture is considered dangerous to us and the environment. Agricultural applications of transgenic animals would be a great asset to the industry as costs will be cut and product produced will have higher quality. Industrial uses are not under direct criticism as they are mostly controlled by the private sector. The private applications are have lesser exposure. For example in Canada, scientists in private sector have introduced silk spider genes into lactating goats. The experiment was a success as goats were producing silk from mammary glands. The company, Nexia Biotechnologies also has claimed that they are aiming to produce other industrially important products from lactating goats (Nexia).
General public doesn’t really have an opinion on use of this controversial technology as its opinion is force fed by individuals who obviously don’t like the idea.
Many topics have been brought up as scientists and governments have made ground rules on what is allowed and what’s not. Topics like ethics, morals, god and human responsibility as only morally capable and accountable creatures on this planet have been the major weapon of hypocrites against technology that will be saving lives and providing us with better quality products at reduced cost. (Outka, 2002)
Future applications of the technology would range from production of important proteins to release of wide range of transgenic farming animals. Research is being carried out on pigs to create spare organs that will be compatible with human physiology. Research is also underway to manufacture milk through transgenesis for treatment of debilitating diseases such as phenylketonuria (PKU), hereditary emphysema, and cystic fibrosis. (Margawati, 2003)
Conclusion:
Insertion or manipulation of genetic material is a system that gives rise to transgenic animals. Several methods have been devised for introduction of the genetic material into the host. Most commonly DNA is directly inserted into the pronuclei of the egg through the technique called microinjection. The following egg is introduced into the surrogate mother. Identification of the offspring that have taken up the DNA is a next logical step. (Capecchi, 1994) Using the following technique scientists have been able to create “super” animals which have traits that are superior to other animals. The technology has unlimited amount of applications in fields of medicine, agriculture and industrial biotechnology. Transgenic mammals are widely found in agriculture where traits like increased carcass size, product quality and disease resistance are in high demand. Medical applications of the technology involve production of proteins which are essential for survival of people with blood clotting disorders. (Westphal, 1989)
Identification of gene function is probably the most important exploit of the transgenic technology. Genomes of several animals have been successfully sequenced, but function of the identified genes is at most a guess. Gene targeting is a technique in which the gene under question is disabled through various means and then introduced into the pronucleus of the egg. The egg is then inserted into surrogate mother who will hopefully give birth to the offspring lacking the functional gene. The identified mutant is then studied, whether it’s looking for discrepancy in the metabolic pathway or the gene end product. (Capecchi, 1994)
Mice have been used extensively in gene targeting as they produce a lot of progeny, easy to maintain and handle. Today many previously unknown genes have been identified using gene targeting technique. Technology is on the rise as more genomes are being sequenced and more genes are found. Uses of transgenic animals are vast and are proving to benefit the humanity greatly.
References:
Baker J, Liu JP, Robertson EJ, Efstratiadis A: Role of insulin-like growth factors in embryonic and postnatal growth. Cell 1993 Oct 8;75(1):73-82
Strachan, T. Read, 1999 A.P. Human molecular genetics. John Wiley & Sons, Singapore, pp. 198-200
Kono et al. 2004, Birth of parthenogenetic mice that can develop to adulthood. Nature 428: 860-864
DeChiara TM, Efstratiadis A, Robertson EJ: A growth-deficiency phenotype in heterozygous mice carrying an insulin-like growth factor II gene disrupted by targeting.
Nature 1990 May 3;345(6270):78-80
Gene Outka, 2002, The ethics of stem cell research, Kennedy Institute of Ethics Journal, vol. 12, No. 2 (2002), pp. 175-213
S.J. Kim, 2005, A transcription factor for cold sensation!, Molecular Pain (Short Report) 1:11
Capecchi, Mario R. “Targeted Gene Replacement” Scientific American, March 1994:52-59
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Nexia Biotechnologies, www.nexiabiotechnologies.com
E. T. Margawati, 2003 Transgenic Animals: Their Benefits to Human Welfare, An ActionBioscience.org original article