Transgenic cows making therapeutic proteins
Scientists at AgResearch have generated transgenic cows that produce therapeutic proteins in their milk. Here, we explain the advantages and disadvantages of using transgenic technologies to produce proteins.
What are therapeutic proteins?
Therapeutic proteins are used to treat human diseases. They include hormones, antibodies, vaccines, growth factors and blood clotting factors.
Making therapeutic proteins
Therapeutic proteins are usually synthesised in cell-based systems. In these systems, the desired gene is added to a cell, and the cell’s own machinery makes the protein. The 4 systems currently used to make therapeutic proteins are:
The cell-based system used for making a particular therapeutic protein depends upon the size and structure of the protein, as well as the amount of protein required.
Small, simple proteins (like insulin, human growth hormone and interferon) are usually produced in genetically engineered microorganisms. Human insulin, for example, is a small protein consisting of 51 amino acids. Human insulin was first made synthetically in the bacteria E. coli in the late 1970s. It was approved by the USA Federal Drug Administration (FDA) in 1982 and has been available for diabetics since that time.
Larger, more complex proteins (like antibodies) are usually produced in mammalian cells. Herceptin, the breast cancer-treating antibody, is an example of a large complex protein that is synthesised in mammalian cell culture. To produce this protein, mammalian cells are grown in large sterile vats in nutrient-rich media under strict manufacturing standards. This type of production can be expensive to set up and maintain, making the end product costly. For example, the Herceptin antibody is expensive, with a 12-month course costing up to NZ$100,000.
The number of therapeutic proteins being developed by the biomedical industry is growing rapidly, and some scientists are predicting that demand for proteins will soon exceed their ability to supply them. Transgenic animals could provide an alternative source of therapeutic proteins to help meet these demands.
Producing therapeutic proteins in transgenic farm animals is sometimes called ‘biopharming’. Currently, research groups around the world are investigating whether transgenic animals such as goats, cattle, pigs, rabbits and chickens can be used to produce therapeutic proteins. The animals are used as sterile bioreactors to produce large, complex proteins or proteins that can’t be made in other cell systems.
Making therapeutic proteins in transgenic cows
Scientists at AgResearch have generated transgenic cows that produce myelin basic protein (MBP) in their milk. MBP is part of the insulating layer that surrounds nerves. In patients with multiple sclerosis, this insulating layer is gradually destroyed, which prevents the nerves from communicating. Treatment with human MBP may help reduce symptoms of multiple sclerosis.
Advantages and disadvantages of making proteins in cows
Dairy cows produce large amounts of protein naturally in their milk. Their protein-making machinery makes an excellent ‘biofactory’.
The main advantages of making therapeutic proteins in transgenic cows are:
- capacity – large amounts of protein are produced – the average dairy cow can produce more therapeutic proteins (about 5–10g/L) than mammalian cells in culture (about 1g/L)
- cost – transgenesis is cheaper than mammalian cell culture
- scale – it is easy to scale production up or down to meet demand
- function – proteins are folded and fully functional
- harvesting – proteins can readily be purified from milk.
The main disadvantages of making therapeutic proteins in cows are:
- it takes a long time to generate and validate transgenic cows
- proteins can’t be harvested until lactation begins
- the transgene may affect the cow, for example, as the transgene inserts randomly into a chromosome, it may disrupt a gene required for an important function.
The first international biopharming success
In June 2006, the first human therapeutic protein made in a transgenic animal was approved for use in Europe and the USA. ATryn®, an antithrombin, is made in transgenic goats. It is used to prevent blood clots in patients with a rare blood disease. A relatively small herd of goats (about 80) can produce enough protein to supply all of Europe.
Transgenic research in New Zealand
In April 2010, AgResearch was granted approval from ERMA (the Environmental Risk Management Authority) to continue developing transgenic animals, including goats, sheep and cattle that produce therapeutic proteins in their milk.
- 24 February 2010