TRANSGENIC FARM ANIMALS

A transgenic animal is an individual in which a gene (or genes) from another individual has been artificially inserted with the new genetic information forming a permanent part of the genome and being passed on to their offspring according to the Mendelian laws of inheritance. So far, transgenic cattle, sheep, pigs, rabbits, and chicken have been produced.

After fertilization a mammalian embryo passes through many stages of development such as formation of pronuclei, morula formation, blastocyst formation, implantation etc. Using these early development stages of mammalian embryos, genetic engineers have used different methods to introduce foreign genes into the genome.

Table A list of transgenic animals produced with their promoter, enhancer and structural transgenes

ANIMAL	GENES TANSFERRED (PROMOTER OR ENHANCER/STRUCTURAL TRANSGENES)
Goat	A variant of tPA gene (LAtPA)
Sheep	mMT/hGH, mMT/TK, mMT/bGH, mMT/hGRF, oBLG/hFIX, oBLG/alpha1AT, oMT/oGH
Rabbit	mMT/hGH, hMT/hGH, rbEu/rb,c-myc
Pig	mMT/hGH, mMT/bGH, hMT/pGH, MLV/rGH, MLV/rGH, bPRL/bGH
Fish	hGH, mMT/hGh, mMT/bGal, cd-crystallin SV/hygro, AFP
Cow	BPV, lactoferrin
Chicken	ALV, REV
Mouse	mMT/rGH, mMT/bGH, mMT/oGH, mMT/hGh, mMT/hGRF, mMT/hFIX

Methods to induce foreign genes into the genome

a) Micro-injection in to a pronucleus

A fertilized egg is held in a pipette by suction and several copies of the foreign genes are injected into one of the pronuclei via a micropipette. Although this has been the most widely used and most successful method, 30% of the treated embryos degenerate and die with in a few hours.

b) Using retroviruses as vectors to carry foreign DNA into morulas.

A morula is placed on a culture of fibroblasts infected with retrovirus after removing the
zona pellucida layer. The virus cannot penetrate this layer therefore it is essential to get rid of it. The retrovirus, genetically engineered to carry foreign DNA, infect the cells of the morula as they are shed from the fibroblast. This technique often creates ‘mosaic offspring’, where some cells contain the foreign gene while others do not.

One of the limitations of this technique is that the modified virus cannot leave the transgenic species and under certain conditions it could mutate regaining its ability to cause disease in the tissues where it occurs.

c) Retroviral infection of the stem cells, which are then injected into the cavity of another blastocyst.

In this method, stem cells, from the inner cell mass of an embryo, are infected with genetically-engineered retroviruses, then injected into the central cavity of a different blastocyst. The injected cells colonise the new embryo and participate in the formation of all the tissues, including the ovaries and testes from which cells are formed. This technique has been used to produce transgenic mice and hamsters.
The first transgenic animals produced were mice, into which a gene cloning for rat growth hormone was inserted by microinjection. Two techniques were used to introduce the new gene into the mouse genome:

a) As many as 10000 copies of the rat gene were injected into a pronucleus of a fertilized mouse egg, using a microneedle.

b) After treatment with calcium chloride, to make them more permeable, the rat DNA was applied to the outside of a 2-8- celled mouse embryo in culture.

 

The treated eggs, or embryos were then transplanted into foster mothers. Embryos with the inserted genes were identified by DNA hybridization with small samples of DNA from white blood cells or skin cells. Transformed adult mice produced the growth hormone mainly in their livers rather than pituitary gland where it is normally synthesized.

This technique can be used successfully to transfer other genes such as those affecting the properties of hair, hides, cold tolerance, disease-resistance and milk production which might help to produce strains of farm animals with economic advantage over existing breeds and stocks. In this regard, research is going on to use techniques which can improve the quality of the farm animals. In this regard, following two techniques are of great importance.