Read this article to learn about the process of trans-genesis in large animals.
In general, trans-genesis in large animals is more difficult than with mice. There are several factors for the lower efficiency of trans-genesis in large animals.
These include less number of eggs they produce and technical difficulties in handling, besides long gestational periods to get the offspring (It takes about 2 years to produce a calf from a fertilized egg).
Some of the early experiments to produce transgenic large animals were far from satisfactory. For instance, transgenic sheep overproducing growth hormone grow leaner with increased feed efficiency. But they are more susceptible to infection, become infertile and tend to die at young age. All this might be due to ineffective control of gene regulation. Several improvements have been made to produce transgenic animals with desirable characters.
Biotechnologists are particularly interested to improve the quality of animals, with improved resistance to diseases, besides enhancing their ability produce foods. ‘Building a better animal’, being the motto. Further, production of commercial and pharmaceutical compounds by transgenic animals is also gaining importance in recent years. The protocol adopted for producing other transgenic animals is comparable with that already described for transgenic mice, with certain modifications.
Transgenic Cattle:
The mammary gland of the dairy cattle is an ideal bioreactor for producing several new proteins (of pharmaceutical importance), besides improving the quality and quantity of the existing ones. For instance, a transgenic cow, with an over-expressed casein transgene, can give milk with higher content of casein.
If lactase transgene is introduced and expressed in the mammary gland, milk free from lactose will be secreted. Such a milk will be a boon for lactose intolerant people who experience indigestion and other complications, after consuming normal milk and milk products.
Some success has been achieved in creating transgenic cattle with improved resistance to viral, bacterial and parasitic diseases. However, this is not an easy job due to the complexicity of genetic control to combat the disease-producing organisms.
Attempts have been made in recent years to produce cattle with inherited immunological protection by trans-genesis. Introduction of genes that code for heavy and light chains of monoclonal antibodies has met with some success in this direction.
In vivo immunization of an animal although not yet fully successful, is ideal for disease protection. In vivo immunization primarily involves the insertion of a transgene for an antibody that specifically binds to an antigen.
Transgenic Sheep and Goats:
Trans-genesis experiments in sheep and goats mostly involve the development of mammary glands as bioreactors for the production of proteins for pharmaceutical use. This is possible despite the fact that quantity of milk produced by sheep and goats is less than that of dairy cattle (cow, buffalo). Some proteins produced by sheep and goats have good pharmaceutical use (Table 41.2).
Transgenic sheep with increased wool production:
Keratin is the wool protein with highly cross- linked disulfide bridges. For good production of quality wool, the amino acid cysteine (or its precursor methionine) is required in large quantities. However, cysteine supply to sheep is always inadequate, since the microbes harboring the rumen utilize it and release in the form of sulfide.
This problem can be overcome by producing transgenic sheep containing bacterial genes for the synthesis of cysteine. The two enzymes, synthesized by the transgenes, are capable of trapping the hydrogen sulfide liberated in the intestine to produce cysteine. Thus, good supply of cysteine to the sheep improves the quality and quantity of wool.
Transgenic Pigs:
Transgenic pigs that can produce human hemoglobin have been successfully developed. This human hemoglobin can be separated from pig hemoglobin by simple analytical techniques. Hemoglobin, the oxygen carrying protein of RBC, can be used as a substitute in blood transfusion experiments.
In fact, hemoglobin can be stored for longer period (a few months) than whole blood (weeks only). Further, there is no problem of contamination (like HIV) as is the case with whole blood. However, the free hemoglobin (naked hemoglobin) cannot transport oxygen as effectively as the hemoglobin of RBC.
In addition, naked hemoglobin is easily degraded and the breakdown products cause damage to kidney. There also exists a risk of contamination by pig viruses and other compounds to cause allergic reactions. With these limitations, the initial enthusiasm for substituting blood transfusion with free hemoglobin has remained short-lived. It is now advised not to use naked hemoglobin for transfusion, when there is a heavy blood loss. However, it can be used during major surgeries for supplementing the whole blood transfusion.
Pig in organ farms:
The human organs such as heart, liver, pancreas, kidney and lungs are in great demand for transplantation surgery. The shortage of these transplantable organs can be overcome by developing them in animals. Pig is a favorite animal for harvesting human organs. This is because the physiology of pigs is close to that of humans.
Further, pigs do not carry any major infectious diseases transmissible to humans. The use of pigs in organ farming is still at the experimental stages. In the preliminary experiments, organ transplantation from transgenic pigs into primates showed some promising results. The day may not be very far for utilizing transgenic pigs as donors of human organs.
Transgenic Chickens:
The production of transgenic chickens (or other birds) is rather complicated. This is mainly because during fertilization in chickens, several sperms enter the ovum instead of one. This is in contrast to mammals where usually only one sperm enters the egg. The identification of male pronuclei that will fuse with female pronuclei is quite difficult. Further, embryonic stem (ES) cells have not been identified in chicken. Despite all these limitations, transgenic chickens have been developed.
The blastoderm cells (from an egg) can be removed from a donor chicken. They are transfected with transgenes (usually by lipofection with liposomes). The so modified blastoderm cells are reintroduced into the sub-germinal space of irradiated blastoderm of freshly laid eggs. Some of the resulting chickens may carry the transgene. Transgenic lines of chickens can be established.
Trans-genesis in chicken can be used to develop low fat and cholesterol, and high protein containing eggs. Transgenic chickens that are resistant to viral and bacterial diseases have also been developed. Some attempts have also been made to develop pharmaceutical proteins in the eggs of transgenic chickens.
Transgenic Fish:
Several transgenic fish (catfish, salmon, trout etc.) have been developed with increase in their growth and size. This was carried out by introducing growth hormone transgene (by microinjection or electroporation). The fertilized eggs with inserted transgene are incubated in temperature-regulated holding tanks. (Note: The fish egg development is external in contrast to the mammalian embryogenesis). The efficiency of fish trans-genesis is as high as 70%. It was found that the transgenic salmon fish (with growth hormone transgene) were 10 times heavier than the normal ones, at the end of one year.