Why Clone Mammals?

Given that we already knew from amphibian studies in the 1960s that nuclei were pluripotent, why clone mammals? Many of the reasons are medical and commercial, and there are good reasons why these techniques were first developed by pharmaceutical companies rather than at universities. Cloning is of interest to some developmental biologists who study the relationships between the nucleus and cytoplasm during fertilization or who study aging (and the loss of totipotency that appears to accompany it), but cloned mammals are of special interest to those people concerned with protein pharmaceuticals. Protein drugs such as human insulin, protease inhibitor, and clotting factors are difficult to manufacture. Due to immunological rejection problems, the human proteins are usually much better tolerated by patients than proteins from other animals. So the problem becomes how to obtain large amounts of the human protein. One of the most efficient ways of producing these proteins is to insert the human genes encoding them into the oocyte DNA of sheep, goats, or cows. Animals containing a gene from another individual (often of a different species) a transgene are called transgenic animals. A transgenic female sheep or cow might not only contain the gene for the human protein, but might also be able to express the gene in her mammary tissue and thereby secrete the protein in her milk. Thus, shortly after the announcement of Dolly, the same laboratory announced the birth of Polly (Schnieke et al. 1997). Polly was cloned from transgenic fetal sheep fibroblasts that contained the gene for human clotting factor IX, a gene whose function is deficient in hereditary hemophilia.


Producing transgenic sheep, cows, or goats is not an efficient undertaking. Only 20% of the treated eggs survive the technique. Of these, only about 5% express the human gene. And of those transgenic animals expressing the human gene, only half are female, and only a small percentage of these actually secrete a high level of the protein into their milk. (And it often takes years for them to first produce milk). Moreover, after several years of milk production, they die, and their offspring are usually not as good at secreting the human protein as the originals. Cloning would enable pharmaceutical companies to make numerous copies of such an "elite transgenic animal," all of which should produce high yields of the human protein in their milk. The medical importance of such a technology would be great, since such proteins could become much cheaper for the patients who need them for survival. The economic incentives for cloning are therefore enormous (Meade 1997).