Polly the sheep

Polly (Born 1997 - died unknown), a ewe, was the first mammal to have been successfully cloned from an adult somatic cell and to be a transgenic animal at the same time. This is not to be confused with Dolly the Sheep which was the first animal to be successfully cloned from an adult somatic cell where there was no genetic manipulation carried out on the adult donor nucleus. Polly the Sheep, like Dolly the Sheep was cloned at the Roslin Institute in Edinburgh, Scotland.

The creation of Polly was building on the experiments of Somatic Nuclear Transfer that had lead to the cloning of Dolly the Sheep. But the crucial difference was that in creating Polly, scientists injected in her DNA a new gene. The gene chosen was of a therapeutic value to Humans to demonstrate the potential of such Recombinant DNA technology combined with Animal Cloning to produce pharmacological and therapeutic proteins to treat Human Diseases. The protein in question was the human blood clotting factor IX. Another difference to Dolly the Sheep was the source cell type of the nucleus that was transferred. In the case of Dolly the Sheep, the nucleus that was transferred came from mammary gland cells from a 6 year old ewe but in the case of Polly the nucleus of Fibroblast cells were used.

Background
Prior to the production of Polly, the only demonstrated way to produce a transgenic animal was by Microinjection of DNA into the pronucleii of fertilized oocytes. However, only a small proportion of the animails will integrate the injected DNA into their genome and in the rare cases that they do integrate this new genetic information, the pattern of expression of the injected transgene's protein due to the random integration is very variable. As the aim of such research is to produce an animal that expresses a particular protein in high levels in, for example, its milk, Microinjection is a very costly procedure that does not produce the desired animal.

In mice, there is an additional option for genetic transfer that is not available in animals. Embryonic stem cells provide a means to transfer new DNA into the germline. They also allow precise genetic modifications by gene targeting. Modified embryonic stem cells can be selected in vitro before the experiment moves on further for the production of an animal. Embryonic stem cells capable of contributing to the germline of livestock species such as sheep have not been isolated.

The production of Dolly the Sheep and also the Megan and Morag, the two sheep that led to the production of Dolly, demonstrated that viable sheep can be produced by nuclear transfer from a variety of somatic cell types which have been cultured in vitro. Polly the sheep was the further step where by somatic cells which were cultured in vitro, just as in the case with the previous sheep, but in this case they were transfected with foreign DNA, and transfected cells which stably integrated this new piece of genetic information were selected. The nucleii of these somatic cells was then transferred into an empty oocyte, as in the procedure of nuclear transfer and this was used to produce a transgenic animal, Polly the sheep.

Transgene
The transgene that was inserted in the donor somatic cells was designed to express human clotting factor IX protein in the milk of sheep. This protein plays an essential role in blood coagulation and deficiency leads to the disease Haemophilia B of which treatment requires intravenous infusion of factor IX. The production of this protein in livestock milk, a process known as pharming, would provide a source of this therapeutic protein that would reduce the cost and also would be free of potential infectious risk associated with the current source of this protein (human blood).