Genetic modification can be used to increase pig production efficiency, improve carcass traits, as well as enhance animal health and disease resistance. It also can generate novel genotypes that are capable of producing proteins with pharmaceutical applications in human medicine.

Transgenic pigs have traditionally been produced by injecting one-cell embryos (produced in vitro or in vivo) witDNA of the gene of interest, notes Wayne Singleton, swine reproductive specialist at Purdue University.

This DNA is hopefully integrated into the embryonic genome at the first cleavage division, producing an animal that has the transgene in all the cells of its body, including the germ cells. This transgenic animal will, hopefully, express the gene of interest in the correct manner.

One of the first genes transferred into pigs was the growth hormone gene, notes Singleton. Although the gene was successfully integrated and expressed, controlling time and place of gene expression was a major problem.

Transgenic pigs have found a place in the developing pharmaceutical industry. For example, pigs have been produced carrying genes involved in the blood-clotting cascade in humans. “These proteins are produced in the sow’s milk, which is collected and the protein purified,” says Singleton. “The protein may then be used for treatment of human blood clotting disorders.”

It has become possible to target where transgenes are expressed (for example, in the mammary gland), as well as when they are expressed by using specific leader sequences of DNA that target certain tissues or are controllable by nutrients in the diet.

Pigs transgenic for growth hormone have been produced that exhibit improved weight gain, feed efficiency and carcass composition. However, long-term expression of the transgene remains detrimental to the animal’s health, notes Singleton. Transgenic pigs also have been produced that are capable of secreting large quantities of human proteins in their milk. These pigs are healthy, although some detrimental effects on lactation have surfaced. Proteins produced by transgenic pigs as well as other domestic farm animals are now at the clinical-trial stage.

Although this technique holds much promise, it is still inefficient and requires significant resources. Many of the embryos injected do not integrate the new gene. Recently, the possibility of producing pigs using methods other than microinjection has become possible. “Porcine primordial germ cells have been successfully cultured in vitro and have contributed to the development of a live piglet,” says Singleton. “If cells can be transfected with the transgene and identified as positive before being integrated into an embryo, the percentage of transgenic offspring born would increase considerably, raising the efficiency of the entire procedure.

Cloning is another alternative. “If the cells used to clone an embryo were stably transfected with the transgene, many copies of identical transgenic clones could be produced,” says Singleton.

This and many other technologies are unfolding in the reproductive side of pork production today.