Laboratory mice have been genetically altered so that their milk produces a human protein that soon may be widely used to prevent heart attacks, scientists announced yesterday.

Using a novel genetic engineering technique, researchers at the National Institutes of Health and at Integrated Genetics, a private biotechnology company in Boston, injected the human gene for tissue-type plasminogen activator, TPA, into the fertilized eggs of mice. The female mouse pups grew up to produce TPA in their breast milk but nowhere else in their bodies.

The Food and Drug Administration is expected within the next several weeks to approve the use of TPA for dissolving blood clots, the major cause of heart attacks. The protein is currently manufactured by Genentech Inc., a biotechnology company in South San Francisco, Calif., using traditional gene-engineering techniques that place the human TPA gene in bacteria, an expensive and complicated process.

The new approach may make it possible to inexpensively produce large amounts of several different protein-based drugs such as Factor VIII, a drug injected by hemophiliacs to prevent bleeding, in animals such as cows and goats.

The NIH-Integrated Genetics team is the first research group to report using so-called transgenic techniques to manufacture a human protein.

To create a transgenic animal, scientists insert genes into a fertilized egg and the genes become a permanent part of the animal's genetic makeup that can be passed on to the next generation. Previous applications of transgenic technology, which was first developed about seven years ago, have created unique animals, including giant mice injected with genes for a rat growth factor.

The transgenic approach appears to be significantly more efficient than bacterial fermentation. The transgenic mice produce grams to tens of grams of human protein for every liter of mouse milk, compared with milligrams (thousandths of a gram) of human protein for every liter of fermentation broth, said Alan Smith, vice president and scientific director of Integrated Genetics. "That is a 1,000-fold difference."

"It is possible to produce the world supply of {a needed protein drug such as} Factor VIII with a small herd of 100 cows," said Katherine Gordon, head of the transgenic program at Integrated Genetics.

Using conventional genetic engineering techniques, companies such as Genentech manufactured proteins similar to TPA by inserting its gene into a genetic carrier that can exist inside bacteria without actually becoming part of the bacteria's genes.

The bacteria are then grown in large vats continuously supplied with nutrient-containing solutions. The bacteria excrete the protein into the solution from which it is later purified.

In the transgenic technique, the gene is inserted directly into the chromosomes of a fertilized egg. As the egg divides and grows into a mature animal, the gene is present in every cell in the body.

In this case, the researchers hooked the TPA gene to a genetic control unit that allows the gene

to be turned on only in the

mammary tissue that produces breast milk, from which it can be purified.

Instead of having to supply sophisticated solutions to keep the TPA-producing cells alive in a laboratory culture, the NIH-Integrated Genetics researchers need only to feed the genetically altered ani- mals.

Integrated Genetics officials say they believe this technique can be applied to a number of proteins useful as drugs. They predicted that the transgenic animals could become a billion-dollar annual industry.

Financial analysts already predict that Genentech's TPA could produce sales of between $300 million to $1 billion a year. Integrated Genetics said that its TPA will not be available for at least five years.

The ability to selectively produce human TPA in breast milk was achieved after years of trying to understand how genes turn themselves on and off in different tissues of the body, said Lothar Hennighauser, a researcher at the National Institute of Diabetes and Digestive and Kidney Diseases who has worked on gene regulation of breast milk proteins for eight

years.

"It shows how esoteric research in something like milk gland proteins can pay off." (ILLUSTRATION)

MAKING HUMAN PROTEINS IN MOUSE MILK

1. Scientists take a

segment of a mouse gene that is only activated in mammary tissue and fuse it with a human gene that directs the production of tissue-type Plasminogen Activator (TPA).

2. The scientists inject this hybrid gene into a newly fertilized mouse egg. The hybrid gene becomes a permanent part of the egg's genetic structure.

3. The egg is placed in a mouse "foster mother" to develop; a genetically altered mouse pup will be born.

4. As a genetically

altered female pup matures, the hybrid gene "turns on" in the mammary glands and produces human TPA. The TPA, purified and concentrated from mouse milk, can be used to treat heart attacks.

5. Mice carrying the

hybrid gene can be mated to produce succeeding generations of TPA-producing mice.