Hens as protein bioreactors

Egg whites—naturally fat-free, low in calories, and high in protein—may soon add therapeutic proteins to their litany of health benefits. Researchers have created genetically modified hens that lay eggs containing high levels of these functional therapeutic proteins in the egg whites, potentially laying the groundwork for the development of a faster and cheaper protein production method.

Therapeutic proteins, such as monoclonal antibodies and enzymes, are useful for treating diseases like cancer and diabetes. The current production methods of choice for these molecules are bacterial fermentation and mammalian cell culture, techniques that are extremely time-consuming and expensive, prompting researchers to seek alternative protein production platforms. One promising approach to this problem is creating transgenic animals that produce the proteins of interest. To date, researchers have produced transgenic goats, sheep, and cattle that secrete therapeutic proteins in their milk. These animals, however, are expensive to house and have long generation times.

Recent advances in avian transgenesis have made it theoretically possible to use transgenic chickens as 'protein bioreactors.' The shorter generation time, lower maintenance costs, and the potential to produce proteins toxic to mammalian cells are among the projected advantages of transgenic chickens over their mammalian barnyard mates.

Now, Helen M. Sang of the Roslin Institute (Midlothian, Scotland) and coworkers report successfully transfecting chicken embryos with a lentiviral vector (Proc. Natl. Acad. Sci. USA, 6 February). The vector, equine infectious anemia virus, had been loaded with transgenes encoding miR24 (a miniantibody for treating malignant melanoma) and human IFN-β-1a (hIFNβ1a, an antiviral cytokine) under the control of regulatory sequences that restrict expression to the oviducts, where the egg whites are formed.

Sang and her associates then crossed transgenic cockerels with stock hens and analyzed the offspring for the presence of the transgene. ELISA testing of egg whites from eggs laid by G₁ and G₂ hens confirmed the presence of recombinant protein in all of the eggs assayed and protein levels remained consistent over time. Later, testing of the recombinant hIFNβ1a in a cytopathic effect assay demonstrated that the protein was functional. Moreover, northern blot analysis of various tissue types confirmed that expression of the transgenes was limited to the oviduct—an important consideration since certain therapeutic proteins could be toxic to the chicken if produced throughout the body.

Sang's team will continue to analyze transgene expression levels to determine whether transgene silencing occurs in subsequent generations. The recombinant proteins have yet to be tested in humans, meaning that clinical application of this research is probably still several years away.