Genetically modified sheep goats and pigs
Most of the transgenic studies on sheep and goats have focused on the use of their mammary glands as bioreactors for the production of pharmaceutical proteins. Although sheep and goats have lower milk production than cows, they can produce hundreds of liters a year. At the same time, genetically modified sheep are more easily obtained than genetically modified cattle. Some people have used a method similar to the production of transgenic mice to construct a vector encoding a human gene driven by a mammary gland-specific promoter. The obtained transgenic sheep and goats can both secrete the expressed human protein into milk. The protein encoded by the transgenic sheep expressed by the transgenic sheep is glycosylated, and theoretically it should be as biologically active as the protein extracted from the human body. However, to determine whether the proteins produced by the mammary glands of transgenic animals are identical to the native forms of human proteins, more in-depth studies are needed. The earliest pharmaceutical proteins expressed in transgenic mice include human growth hormone (hGH), human tissue plasminogen activator (htPA), etc., but such pharmaceutical proteins used in humans are obtained from the milk of rats. On the one hand, the yield is low. On the other hand, it is psychologically difficult to accept rat milk. Considering people's psychological needs and the cost and difficulty of the production of transgenic animals, sheep can be regarded as the best choice. The pharmaceutical proteins currently expressed in livestock are basically obtained in transgenic sheep. As early as 1991, White et al. first used the goat lactoglobulin promoter to drive the antitrypsin (ATT) gene to form the transfer gene. Then, the gene was transferred to sheep by microinjection. Four female and one male and five transgenic sheep were finally obtained. Four female sheep all gave birth to heterozygous lambs. After further mating, homozygous transgenic sheep can be obtained. It has been proved that ATT is contained in the milk produced by 4 ewes during the milking period, and the content is as high as 1-35 g/L, while each sheep can produce 250-800 L of milk during the milk production period. Therefore, the use of transgenic animals can be seen. The enormous potential of producing medicinal proteins, produced ATT can be used to treat hereditary ATT deficiency and emphysema. In the same way, people have obtained goats that express htPA in milk, and the yield has also reached several htPA to 25g per liter of milk. Some pharmaceutical proteins such as various cytokines, if constitutively expressed in large quantities will jeopardize the development of the host animal, then the use of inducible promoters and mammary gland-specific promoters should be considered, so that foreign genes can only be induced. After the expression. The metallothionein gene promoter is one of the commonly used promoters for inducing expression. After obtaining the transgenic sheep, when an exogenous gene needs to be expressed, as long as an appropriate amount of Cd2+ is added to the feed of the transgenic sheep, Fe2+ can induce the expression of the foreign gene. . Using this method, one has constructed a human growth hormone (hGH) transfer gene and transferred it to sheep to obtain a transgenic sheep that can express human growth hormone in milk under induction. At present, various pharmaceutical proteins such as coagulation factor IX, interleukin 2, and urokinase have been successfully expressed in the milk of transgenic sheep. Sheep and goats expressing the transgene in the mammary gland do not have an adverse effect on the mother of the sheep. However, when the bovine growth hormone gene driven by the metallothionein gene promoter was introduced into pigs, the adverse effects of the transferred gene on the transgenic pigs were observed. Although transgenic pigs express different levels of bovine somatotropin, their overall efficiency in converting feed into tadpoles increases. Unfortunately, this positive effect is offset by other physiological negative effects. These negative effects include Gastric ulcers, loss of kidney function, cramps, swollen joints, pneumonia, etc. The cause of these symptoms is still unclear, perhaps because of the long-term growth hormone. The pig's transgenic experiment was the most successful in transforming human hemoglobin genes. Specifically, the human β-globin gene regulatory region was linked to two human α1 globins and one human βa globin gene to construct a vector. The resulting transgenic pigs express human hemoglobin in the blood, and they have been tested according to various chemical standards and found that they are identical in nature to natural human hemoglobin. Maybe in the near future, people can use human hemoglobin produced by genetically modified animals to assist transfusions.
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