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Prionless Cows could be on the Way

By Pippa Wysong, Access Excellence


Blacksburg, VA (01/19/04)-- Researchers are proposing to fight bovine spongiform encephalopathy (BSE) in a novel way -- by genetically engineering cows so they are unable to produce prions.

It is not known whether the approach will work, or even if cattle can survive without prions, but the idea is partly supported by the fact that a few years ago researchers elsewhere created mice that lacked prions, said Dr. Will Eyestone, PhD. He is a research associate professor of biotechnology at Virginia Tech, and his department was awarded $300,000 in funding from the National Institutes of Health to give it a try.

Prions are protein fragments that appear naturally in the nerve and brain cells of mammals, but nobody knows what their role is or even if they are really needed for survival in higher mammals. The fact that prionless mice survived in experiments, and appeared to behave normally suggest the proteins may not even be very important -- at least in mice. Whether this is true in either humans or cattle is not known though some research suggests prions may play a role in memory.

In spongiform encephalopathy diseases such as BSE (commonly called Mad Cow disease) the brain develops a sponge-like texture throughout and the animal dies. The change in the brain tissue is caused by once healthy prions bending or folding out of shape.

Little is understood about where prion diseases originated or why certain types seem to be transmissible and others not. Several varieties occur spontaneously in nature affecting animals as diverse as elk, deer and even humans. In humans the disease is called Creutzfeld-Jacob Disease (CJD) and the spontaneous version of the disease is rare.

In sheep the disease is called scrapie, and is the type many scientists believe is transmitted to cattle. The route of transmission is thought to be a result of cattle eating a type of feed that incorporates the remains of sheep; the problem being that some of the sheep may have been infected with scrapie. The practice of mixing the remains of sheep into cattle feed was a common practice in the UK but not in the US.

There is still uncertainty about the real origins of BSE, Dr. Eyestone said. Indeed, it wasn't until the 1950s that it was discovered that any of the spongiform encephalopathies were transmissible. This came with the discovery of a form of the brain disease known as kuru. It occurred in cannibals who had eaten the brain of a person who had been infected with the disease, he said.

It is believed that the new variant CJD (vCJD) in humans appeared as the result of eating beef infected with BSE. However, there are still more unanswered questions than answered ones in the scientific community. For one, if eating BSE infected beef was sufficient for people to develop vCJD, why haven't larger populations been infected? It is possible some people may be genetically susceptible to the disease while others have a natural protection.

Dr. Eyestone's project is to try to create a cow that is genetically incapable of producing prions. This will be done by using genetic material from cow tissue and "knocking out" the gene responsible for encoding for prion proteins. He will use a process called homologous recombination.

The first steps in the work are to obtain skin cells from cattle, culture then in the lab, then alter the prion gene so it will no longer cause prions to be produced. Dr. Eyestone likens the process of altering the gene sequence to searching for a word in a word processor and replacing it with a different word.

The next step is to "remove the native chromosomes from a cow egg and replace them with the nucleus of a cell in which the prion gene has been modified," he said. The researchers will then "transfer this egg to a cow for the balance of gestation. The resulting calf will bear the same modification to the prion gene that was made in the cultured skin cell," he said.

The altered egg will then be stimulated to grow using the same cloning technique that was used for the production of Dolly the sheep, the first large mammal to be developed with this type of cloning technique.

If the prionless cow survives, researchers will then do tests to confirm that it would not be susceptible to BSE. This type of cow might be of use in the beef and dairy industries. If, however, the cow does not survive, researchers will learn that healthy prions are needed for the survival of cows.

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