BioWorld International Correspondent

LONDON - Human proteins destined for use as pharmaceuticals could be produced in hen's eggs in the future. The proteins would be manufactured along with the avian protein ovalbumin, which makes up more than half of the white of a hen's egg, and later separated out.

Researchers in the UK, with collaborators at Viragen (Scotland) Ltd., of Penicuik, UK, have shown that it is possible to make transgenic chickens that produce human proteins in their eggs, and that subsequent generations of the transgenic birds also produce such eggs.

Harry Griffin, director of the Roslin Institute in Midlothian, UK, told Bioworld International, "We have described a significant step toward a robust system for protein production that has practical application. The biological activity of the proteins we have made has been demonstrated in vitro, although proving that these molecules are effective in vivo, and meeting the regulatory hurdles, is several years down the road."

A team from the Roslin Institute, led by Helen Sang, with collaborators from Viragen and from Oxford Biomedica Ltd, of Oxford, UK, report their work in a paper published in the Jan. 15 Proceedings of the National Academy of Sciences. Its title is "Oviduct-specific expression of two therapeutic proteins in transgenic hens." The first author is Simon Lillico.

Griffin, who is not one of the authors, predicted that the genetically modified chickens could help speed the production of human antibodies and other proteins for use in cancer treatments.

"There are over 200 anticancer treatments that are under development that make use of human antibodies, but current methods for producing these are expensive and their capacity is limited," he said. "The objective of our work is to use the chicken as a bioreactor - the input is chicken feed, and the output would be eggs containing valuable human proteins, which can be separated out from the egg whites."

One of the advantages of chickens, he added, is that their generation time is only 6 months, so that a sizeable flock of the transgenic birds can be obtained in only 18 months.

Karen Jervis, vice president and managing director of Viragen, told BioWorld International that the company is hoping to find a commercial partner interested in producing a protein that way. Viragen currently has a further two proteins in the pipeline that it plans to manufacture in hens' eggs. "We want to prove the technology with as many proteins as possible," she said.

Sang and her colleagues set out to develop a way to make transgenic chickens by injecting newly laid eggs with a viral vector that was linked to additional genes. At the time a hen's egg is laid, about 24 hours after its fertilization in the bird's oviduct, the embryo consists of a disc of about 60,000 cells.

The group used a lentiviral vector, from which all viral coding sequences had been removed, coupled to the promoter of the ovalbumin protein, which in turn was coupled to the coding sequence for either human beta-interferon, or an antibody called miR24. Human beta-interferon currently is used to treat multiple sclerosis, while miR24 is an experimental treatment for malignant melanoma.

When the researchers injected one of those constructs into newly laid fertile hens' eggs, they found that the genetic material was taken up by some of the cells of the embryo. When that happened, the resulting chicks were chimeras - part wild type and part genetically modified.

Griffin said: "The PNAS paper describes one cockerel obtained in this way, which produced sperm that was 5 per cent transgenic - and as you would expect, 5 per cent of this bird's offspring were also transgenic."

The promoter to which the genes of interest were attached ensured, Griffin explained, that the transgene was expressed only in the oviduct, so that the only tissue containing the human protein was the bird's egg white.

"This is important, because you don't want a bioactive protein, such as beta-interferon, circulating in the chickens," Griffin said.

When the researchers allowed the transgenic birds to breed and produce offspring, they found that there was no fall-off in the level of production of the transgenic proteins in subsequent generations.

Griffin concluded: "The researchers have shown that this is a robust and efficient way of producing transgenic chickens. Secondly, they have developed a genetic construct that ensures that expression of the transgene is restricted to the oviduct. Thirdly, they have shown that this system provides stable inheritance of the transgene from one generation to the next."

Work on improved ways of purifying the proteins produced from the egg white will continue, as will progress toward meeting the requirements of drug regulatory agencies.