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Genetically modified pigs - Production and Application for medicine and agriculture
The use of swine in biomedical research has gained much importance as they have always been considered excellent models for the studies related to atherosclerosis, various cardiovascular diseases, cutaneous pharmacology, diabetes, cancer biology, ophthalmology and toxicology, lipoprotein metabolism, pathobiology of intestinal transport, injury and repair, repair and healing of wounds, etc. They have also been considered for being potential source of different organs for the xenotransplantation as can be seen in the heart transplantation studies.

While the different biological systems of pig have similarities with the corresponding human biological systems, there are a number of advantages offered by the study of pig as research model such as short generation interval, short gestation period being around 114 days and the production of multiple offspring at a time. The complete sequencing of the genome of a particular species of animal is essential for basic research involving it, otherwise the species is considered second-class species. Phylogenetic approaches have shown the presence of high rate similarity between pig and human genome compared to that of mouse. Moreover, the availability of inbred strains of pig facilitates the use of pigs in different research studies.

There are a number of methods to carry out the genetic modification of the animals.

a) Injection of DNA construct directly into the pronuclei of zygotes, first tested in mice has been applied in mammals.

b) Oocyte transduction, whereby the replication defective retrovirus is integrated into the chromosomes of the oocyte after its injection between the zona pellucida and the plasma membrane of oocyte that is arrested in the metaphase II of meiosis. It was carried out in pigs after application in cattles.

c) Sperm-mediated gene transfer, a method highly efficient for the transgenic pig creation, whereby the in-vitro fertilization or insemination of the pigs was carried out with sperm previously mixed with DNA construct of interest.

As targeted integration is not achieved properly in other methods, it makes way for the development of homologous recombination method or use of Embryonic stem cells (ES cell technology). However, this method has been successfully applied only in mice and for other species, a true ES cell that goes with germline is yet to be developed. Presently, the method utilised for the introduction of targeted modifications, knock-ins or knock-outs, is via the cloning of modified somatic cells. Due to advancement of technology, cloning is being developed using different stem cell lines, which has been proved successful by the production of cloned pigs from the stem cell line developed from mesenchymal stem cells and skin-derived stem cells. The introduction of genetic modification using zinc finger nucleases in combination with the donor stem cells may prove to be a highly efficient method for the genetic modification of swine.

The different applications of genetically modified pigs in medical field can be summarized as follows:

1) The α-1, 3-galactose cell surface epitope produced by the GGTA1 gene in pigs causes the production of the anti-gal antibodies in humans leading to hyperacute rejection (HAR) of the transplanted organs. Hence, the GGTA1- gene knockout pigs would offer successful transplantation of the organs without evoking any immunological response within the human body.

2) The production of human haemoglobin in the blood of transgenic pigs for isolation and treatment of trauma patients is one of the interesting applications being studied. The production of Protein C, in-activator of certain human coagulation factors Va and VIIIa in the milk of pigs has been studied. It has been found that the mammary epithelial cells of the pigs are capable of making the coagulation factors VIII and IX biologically active due to post-translational modifications.

3) The transgenic pigs can be used as better models for different diseases such as Retinitis pigmentosa, cardiovascular diseases: Fat-1, Diabetes, Alzheimer’s disease, cystic fibrosis, Huntington’s disease by the introduction of different mutations in the genes involved in the pathophysiology of the diseases.

4) The transgenic pigs can be used for cell tracking with the introduction of genes expressing different fluorescent proteins into the pigs. The stem cells expressing fluorescent proteins isolated from these transgenic pigs can be used as molecular markers for the tracking of various biological mechanisms.

5) Production of human and pig hybrid organs is a very interesting application that needs further in-depth study. The production of human hepatocytes in transgenic pigs to help in the transplantation of the regenerated human hepatocytes to patients of liver failure from the transgenic pigs shows great promise.

6) Transgenic porcine livers expressing albumin gene are being studied for use as bio-artificial liver support system as a bridge to human liver transplantation.

Transgenic pigs also have application in agriculture in the production and growth of pigs whose meat are safe environmentally, lean and healthier for human consumption by the introduction of different genes expressing growth hormones and to reduce pollution by alteration in the composition of the carcass.

The in-depth study of genetically modified pigs has met with remarkable advancement in biotechnology and can be utilised for various therapeutic applications in near future for the benefit of mankind and community.
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The pigs are considered to be worth studying subjects with regard to genetic engineering. Though, different experiments have so far been conducted yet the study is passing through the evolution process. The pigs altered through the genetic engineering are supposed to be more human familiar than the originals. Yes, they had the disorder to be friendly with humans but the modified pigs are highly beneficial to mankind.

The following benefits can be observed in the modified pigs:
1. xenotransplantation into humans
2. Highly familiar with humans
3. Help in production of medicine
4. Developing models for humans
5. Agricultural benefits for prevention of diseases
6. Change in composition of carcas
7. Amelioration in pigs to face heat stress
8. Highly protection of environment
9. Change in pigs behavior

The modification in the animals have been rightly cited in your article. You are absolutely right in saying that the meat consumed by the humans of pig , would be of finest quality if they are modified. I do agree with your another point that the meat would also be healthier for the people who consume pig meat a lot.

Exactly, the biotechnology has revolutionized the world through science and technology. The change in behavior of modified pigs clearly implies its importance. Besides, pigs are psysiologically analogus to mankind and anatomical in other way. I second your opinion that experiments had been firstly started from mice and now they are being carried over the mammals.

Importantly, transgenic pigs are donor organism for xenotransplantation into humans because they have many same characteristics of humans. hyperacute rejection is considered to be the biggest barrier in the xenotransplanation. Though, acute humoral xenograft rejection is also considered to be a barrier. Scientists are working in making the pigs as the best donor for transplantation of organs. They hold the view that the modification of multi transgenic pigs for clinical xenotransplantation would be the biggest challenge for them.
Sasa Milosevic
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