Biopharming in Transgenic Farm Animals
For decades simple recombinant proteins, such as insulin and human growth hormone, have been produced in genetically engineered bacteria and yeast. But more complex proteins, such as blood clotting factors and monoclonal antibodies, cannot be produced in this manner because they have complex folding patterns and require the addition of sugar molecules to become biologically active. Only mammalian cells are capable of performing these sophisticated modifications.
Although some complex therapeutic proteins can be manufactured in mammalian cell culture (e.g., Chinese hamster ovary cells), producing these substances in transgenic farm animals offers some advantages. Lactating female cows and goats can be induced to secrete the recombinant protein into their milk, from which it is readily purified.16 Examples of therapeutic proteins produced in the milk of farm animals include blood clotting factors, fibrinogen, and alpha-1-antitrypsin.
Thousands of years of animal breeding have yielded domesticated varieties of cows, sheep, and goats that produce prodigious volumes of milk. Dairy cows, for example, have a yearly milk output of about 10,000 liters, making it possible for a single lactating cow to produce tens of kilograms of therapeutic proteins. Relatively small herds of a few hundred lactating transgenic cows or goats can produce several hundred kilograms of purified protein per year.