Production of Recombinant Therapeutic Proteins in the Milk of Transgenic Animals - - BioPharm International

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Production of Recombinant Therapeutic Proteins in the Milk of Transgenic Animals


BioPharm International
Volume 19, Issue 8


Figure 2. Schematic representation of the somatic cell nuclear nuclear transfer process employed for the production of transgenic animals used for the production of recombinant proteins.
The targeting of a recombinant protein to the milk of a transgenic animal (Figure 1) is achieved by first generating an expression vector containing the gene encoding the protein of interest fused to milk-specific regulatory elements. This transgene is then introduced in the germline of the chosen production species. Pronuclear microinjection of one-cell embryos (Figure 1) or, alternatively, transfection into a primary cell population suitable for somatic cell nuclear transfer (Figure 2) have both been used to generate transgenic founders.


Figure 3. Timeline associated with the creation of a herd of transgenic goats producing recombinant proteins in their milk.
Following germline integration, mammary gland-specific transgenes are predictably inherited by the offspring of the founder animal. The expression level of the protein(s) of interest is variable. Concentrations surpassing 1 g/L are attained routinely and levels of up to 20 g/L have also been achieved. Expression levels are dependent on the mammary-specific regulatory sequences employed, the gene expressed, and the integration site of the transgene. Milk can easily be obtained using established large-scale technologies of the dairy industry, and is an excellent starting material from which recombinant therapeutic proteins can be purified. The choice of the production species is largely driven by the expected quantity of the therapeutic protein needed. There is usually a trade-off between milk yield and time to natural lactation. Another consideration may be a species-specific ability to perform specialized post-translational modifications more efficiently.

SCREENING THE MAMMALS

Transgenic mice have mainly been used for the testing of expression constructs prior to or concomitant with the generation of larger founder transgenic animals. This model allows the relatively inexpensive and rapid evaluation and optimization of transgene constructs and has proven crucial to the development of milk expression technology. The model allows the definition of regulatory sequences that efficiently target expression of heterologous genes to the mammary gland.7 Obviously, the very limited milk yield from transgenic mice restricts expression of recombinant proteins to small amounts. But this can be sufficient to obtain meaningful data on the protein of interest. As an example, it was possible to purify enough Malaria antigen MSP142 from transgenic mouse milk to test for immune protection in a primate model.8

The generation of transgenic rabbits by pronuclear microinjection is straightforward and inexpensive. Relative to ruminants, rabbits have a short gestation interval that allows up to eight lactations per year. However, only 1.5 L of milk can be obtained per lactation, and this limits the value of this expression system to products with a commercial scale in the low-kilogram range;9–12 Labor-intensive milking and high husbandry costs could become prohibitive for larger quantities of purified proteins.

Recombinant protein production in the milk of transgenic sows has been reported for human Protein C,13 factor VIII,14 and factor IX.15 Lactating sows can yield a surprising amount of milk (100–200 L) and it has been reported that the porcine mammary gland cells can carry out the complex post-translational modifications (γ-carboxylation, proteolytic processing) on factor IX and Protein C at rates higher than those encountered with mammalian cell and transgenic mouse milk systems.16


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