Sources and upstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 114

Table 5.6 Proteins of actual/potential therapeutic use that have been
produced in the milk of transgenic animals

Protein Animal species Expression level in milk (mg l 1 )

tPA Goat 6000

IL-2 Rabbit 0.5

Factor VIII Pig 3

Factor IX Sheep 1000

α 1 -Antitrypsin Goat 20 000

Fibrinogen Sheep 5000

EPO Rabbit 50

Antithrombin III Goat 14 000

Human α-lactalbumin Cow 2500

IGF-I Rabbit 1000

Protein C Pig 1000

GH Rabbit 50

the genetic complement of the cell. After fertilization, the ova may be implanted into a surrogate
mother. Each cell of the resultant transgenic animal will harbour a copy of the transferred DNA.
As this includes the animal's germ cells, the novel genetic information introduced can be passed
on from one generation to the next.
A transgenic animal harbouring a gene coding for a pharmaceutically useful protein could
become a live bioreactor-producing the protein of interest on an ongoing basis. In order to render
such a system practically useful, the recombinant protein must be easily removable from the ani-
mal, in a manner which would not be injurious to the animal (or the protein). A simple way of
achieving this is to target protein production to the mammary gland. Harvesting of the protein
thus simply requires the animal to be milked.
Mammary-specifi c expression can be achieved by fusing the gene of interest with the promoter-
containing regulatory sequence of a gene coding for a milk-specifi c protein. Regulatory sequences of
the whey acid protein (WAP), -lactoglobulin genes have all been used to date to

promote production of various pharmaceutical proteins in the milk of transgenic animals (Table 5.6).
One of the earliest successes in this regard entailed the production of human tPA in the milk of
transgenic mice. The tPA gene was fused to the upstream regulatory sequence of the mouse WAP,
the most abundant protein found in mouse milk. More practical from a production point of view, was
the subsequent production of tPA in the milk of transgenic goats, again using the murine WAP gene
regulatory sequence to drive expression ( Figure 5.2). Goats and sheep have proven to be the most
attractive host systems, as they exhibit a combination of attractive characteristics. These include:

β -casein and α - and β

• high milk production capacities (Table 5.7);

• ease of handling and breeding, coupled to well-established animal husbandry techniques.

A number of additional general characteristics may be cited that render attractive the production
of pharmaceutical proteins in the milk of transgenic farm animals. These include:

• Ease of harvesting of crude product, which simply requires the animal to be milked.